TR-105852-V1

EPRI • 3412 Hillview Avenue, Palo Alto, California 94304 • PO Box 10412, Palo Alto, California 94303 • USA800.313.3774 • 650.855.2121 • [email protected] • www.epri.com

Valve Application, Maintenance, andRepair GuideVolume 1

TR-105852v1

Final Report, February 1999

EPRI Project ManagerV. Varma

Effective December 6, 2006, this report has been made publicly available in accordance with Section 734.3(b)(3) and published in accordance with Section 734.7 of the U.S. Export Administration Regulations. As a result of this publication, this report is subject to only copyright protection and does not require any license agreement from EPRI. This notice supersedes the export control restrictions and any proprietary licensed material notices embedded in the document prior to publication.

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DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES

THIS PACKAGE WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OFWORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC.(EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) NAMEDBELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITHRESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEMDISCLOSED IN THIS PACKAGE, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULARPURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNEDRIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS PACKAGE ISSUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR

(B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDINGANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISEDOF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THISPACKAGE OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED INTHIS PACKAGE.

ORGANIZATION(S) THAT PREPARED THIS PACKAGE

Kalsi Engineering, Inc.

ORDERING INFORMATION

Requests for copies of this package should be directed to the EPRI Distribution Center, 207 Coggins Drive, P.O.Box 23205, Pleasant Hill, CA 94523, (925) 934-4212.

Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc.EPRI. POWERING PROGRESS is a service mark of the Electric Power Research Institute, Inc.

Copyright © 1999 EPRI, Inc. All rights reserved.

iii

CITATION

This report was prepared by

Kalsi Engineering, Inc.745 Park Two Dr.Sugarland, TX 77478

Principal InvestigatorsBahir H. EldiwanyDaniel Alvarez

and

EPRI Nuclear Maintenance Applications Center (NMAC)1300 W.T. Harris Blvd.Charlotte, NC 28262

This report describes research sponsored by EPRI. The report is a corporate documentthat should be cited in the literature in the following manner:

Valve Application, Maintenance, and Repair Guide, Volume 1, EPRI, Palo Alto, CA: 1998.TR-105852-V1.

v

REPORT SUMMARY

The Valve Application, Maintenance, and Repair Guide is a two-volume series that providesa generic overview of valve application, selection, maintenance, and repair. Volume 1of the series is a comprehensive reference on the application and use of valves thatprovides guidance on the selection of specific types of valves on the basis of functionaland system requirements. This document is based on an earlier EPRI document (NP-6516, Guide for the Application and Use of Valves in Power Plant Systems). Extensiveillustrations and sample calculations make the guide useful to a wide range ofpersonnel. This volume has been expanded to include general maintenancerequirements and diagnostics for different valve types.

Information on valves and valve operators, where other comprehensive NMACdocuments are available (such as Air Operated Valves, Solenoid Valves, Check Valves, Safetyand Relief Valves, and the Technical Repair Guide series on Limitorque operators), havebeen referenced without duplicating the contents in this volume.

Background

The improper application, incorrect use, and ineffective maintenance of valves inpower plant systems cause significant losses in plant availability. Over the last severalyears, EPRI, the U.S. NRC, and the electric utilities have conducted many valve andactuator research projects to improve plant safety and availability by reducing valveand actuator problems. These projects resulted in many proprietary and non-proprietary documents that deal with the various specialized areas of valve/actuatorsizing, performance characteristics, maintenance, repair, testing, and diagnostictechniques. However, information to aid plant personnel in resolving these problems isdifficult to glean from scattered sources, and access may be restricted by proprietaryconsiderations.

Objective

To provide a comprehensive and authoritative guidebook on the application, use, andmaintenance of valves, in which information is readily accessible and understandableby a wide range of plant personnel.

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Approach

The project team was selected from a group of specialists who were heavily involved inrecent valve and actuator research projects. The project team determined the scope ofthis project using the initial release of this guide (EPRI NP-6516) and all of thesignificant reports from the recent research projects. This guide outline was revised toeliminate topics that were either irrelevant or covered in greater depth elsewhere. Thescope of this guide was expanded to include maintenance, troubleshooting, anddiagnostic equipment. An overview of other key documents is provided to assist thereader in quickly finding sources of additional information. Numerous illustrationsand examples of applications, valve sizing, and strategies for use and maintenancewere incorporated to make the guide easier to use.

Results

The guide contains a thorough treatment of the application of valves on the basis oftheir functional requirements. It covers gate, globe, butterfly, ball, plug, and diaphragmvalves and manual, hydraulic, and electro-hydraulic actuators, including theirinstallation, operation, maintenance, and most common problems. For other types ofvalves and actuators not covered in this guide, references to pertinent EPRI/NMACdocuments are given. The guide presents information in a clear and understandablemanner to those with little knowledge of the factors involved in successful valveapplications. For those who have extensive experience with valves and actuators, thisguide provides easy access to specific information that is pertinent to specific needswith references.

EPRI Perspective

Although the information contained in the guide focuses on the application andmaintenance of valves in power plant systems, it is also directly applicable tocomparable system applications in the chemical, petroleum, marine, and similarindustries. The intended audience of the guide includes system designers; engineerswho establish specification requirements for valves; personnel who install, operate,maintain, and repair valves; plant training instructors; and others for whom a more in-depth knowledge of valves could lead to improved valve performance. The guide willbe helpful in evaluating valve/actuator applications in existing systems, selecting newand replacement valves/actuators, and developing/updating valve maintenanceprograms and procedures.

Interest Categories

ValvesPlant Support Engineering

EPRI Licensed Material

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ACKNOWLEDGMENTS

The original Guide for the Application and Use of Valves in Nuclear Power Plant Systems(NP-6516), published by EPRI in 1990, was developed by Stone & Webster EngineeringCorporation of Massachusetts and Kalsi Engineering, Inc., of Texas. They received widecooperation from experienced nuclear utility personnel and service industries. Thisrevision was created on the solid framework of the earlier publication.

We wish to extend our thanks to the individuals who spent many hours performingdetailed reviews of this revision, so necessary to produce a quality document. Inparticular, we thank Kenneth Hart of Pennsylvania Power & Light for his extensivecomments and input on valve packing and maintenance program issues. Otherreviewers include Chris Hansen of Vermont Yankee, Greg Harttraft of GPU, JohnHolstrom of Duke Engineering Services, Eric Cartwright of PECO, and Jim Wilson andEugene Phillips of Wisconsin Electric Co.


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CONTENTS

1 INTRODUCTION/SUMMARY HOW TO USE THE GUIDEBOOK ....................................... 1-1

1.1 Introduction................................................................................................................... 1-1

1.2 Summary/How to Use the Guidebook........................................................................... 1-2

1.2.1 General .................................................................................................................. 1-2

1.2.2 Valve Functions...................................................................................................... 1-3

1.2.3 Specific Valve Types by Function .......................................................................... 1-4

1.2.4 Actuator Types....................................................................................................... 1-5

1.2.5 General Design Requirements for Valves and Actuators ....................................... 1-6

1.2.6 Valve Pressure Boundary and Structural Integrity.................................................. 1-6

1.2.7 Valve Maintenance and Inspection Programs........................................................ 1-6

1.2.8 Troubleshooting and Recommended Corrective Actions ....................................... 1-7

1.2.9 Installation, Testing, and Maintenance Requirements............................................ 1-7

1.2.10 Diagnostic Equipment and Methods..................................................................... 1-7

1.2.11 Valve Selection Chart........................................................................................... 1-7

1.2.12 References and Bibliography ............................................................................... 1-8

1.2.13 Appendices .......................................................................................................... 1-8

2 GENERAL VALVE DESIGN................................................................................................ 2-1

2.1 Nomenclature/Glossary of Terms ................................................................................. 2-1

2.1.1 Introduction ............................................................................................................ 2-1

2.1.2 Glossary of Terms.................................................................................................. 2-1

2.2 Common Valve Construction Features ....................................................................... 2-19

2.2.1 Body-to-Bonnet Connections ............................................................................... 2-20

2.2.2 Seat and Seat Rings ............................................................................................ 2-23

2.2.3 Disc-to-Stem Connection ..................................................................................... 2-34

2.2.4 Disc/Stem Guide Arrangements........................................................................... 2-35


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2.3 Accessories and Special Features.............................................................................. 2-37

2.3.1 Manual Override Handwheels or Levers .............................................................. 2-37

2.3.2 Stem Leak-Off Connection................................................................................... 2-39

2.3.3 Limit Switch .......................................................................................................... 2-40

2.3.4 Internal and External Bypass ............................................................................... 2-40

2.3.5 Remote Position Sensor ...................................................................................... 2-41

2.3.6 Bonnet Extension................................................................................................. 2-41

2.3.7 Impact, Hammerblow, and Chain-Operated Handwheels .................................... 2-42

2.3.8 Stem Backseating Feature................................................................................... 2-42

2.3.9 Fire Safety Feature .............................................................................................. 2-43

2.4 Valve Trim................................................................................................................... 2-43

2.4.1 Trim Components and Materials .......................................................................... 2-43

2.4.2 Design Practices to Minimize Corrosion............................................................... 2-45

2.4.3 Design Practices to Minimize Erosion .................................................................. 2-47

2.4.4 Design Practices to Minimize Wear and Galling................................................... 2-49

2.4.5 Cobalt-Free Alloys for Hard-Surfacing of Trim...................................................... 2-52

2.4.6 Design Practices to Minimize the Effects of Temperature .................................... 2-54

2.5 Valve Stem Seals ....................................................................................................... 2-55

2.5.1 Flexible Metal Seals ............................................................................................. 2-56

2.5.2 Valve Stem Packings ........................................................................................... 2-59

2.6 Gasket Types and Materials ....................................................................................... 2-77

2.6.1 Gasket Types....................................................................................................... 2-77

2.6.2 Flat Metal Gaskets ............................................................................................... 2-81

2.6.3 Flat Non-Metallic and Metal Clad Gaskets ........................................................... 2-81

2.6.4 Spiral Wound Gaskets ......................................................................................... 2-81

3 FUNCTIONAL REQUIREMENTS OF VALVES................................................................... 3-1

3.1 General ......................................................................................................................... 3-1

3.2 Isolation Valves............................................................................................................. 3-3

3.3 Modulating/Throttling Valves......................................................................................... 3-5

3.4 Pressure Relief Valves.................................................................................................. 3-8

3.5 Check Valves.............................................................................................................. 3-10

4 GATE VALVES .................................................................................................................. . 4-1


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4.1 Introduction and Application ......................................................................................... 4-1

4.2 Design........................................................................................................................... 4-1

4.2.1 General .................................................................................................................. 4-1

4.2.2 Solid Wedge........................................................................................................... 4-3

4.2.3 Flexible Wedge ...................................................................................................... 4-5

4.2.4 Split Wedge............................................................................................................ 4-6

4.2.5 Parallel-Expanding Gate ........................................................................................ 4-8

4.2.6 Parallel Slide Double-Disc.................................................................................... 4-11

4.2.7 Westinghouse Flexible Wedge............................................................................. 4-13

4.2.8 Slab Gate ............................................................................................................. 4-15

4.2.9 Pressure Locking in Gate Valves ......................................................................... 4-17

4.2.10 Options to Mitigate Pressure Locking in Gate Valves ........................................ 4-21

4.2.11 Thermal Binding in Wedge Gate Valves ............................................................ 4-21

4.3 Installation Practices................................................................................................... 4-23

4.4 Operation Practices and Precautions ......................................................................... 4-24

4.5 Common Problems ..................................................................................................... 4-24

4.6 Maintenance Methods ................................................................................................ 4-27

4.7 Recent Improvements in Flexible Wedge Gate Valve Designs................................... 4-28

5 GLOBE VALVES—ISOLATION FUNCTION....................................................................... 5-1


5.2 Design........................................................................................................................... 5-1

5.3 Installation Practices..................................................................................................... 5-5

5.4 Operation Practices and Precautions ........................................................................... 5-5

5.5 Common Problems ....................................................................................................... 5-5

5.6 Maintenance Methods .................................................................................................. 5-6

6 GLOBE VALVES—MODULATING/THROTTLING FUNCTION .......................................... 6-1


6.1.1 General .................................................................................................................. 6-1

6.1.2 System Differential Pressure versus Control Valve Differential Pressure............... 6-2

6.1.3 High Pressure Drop Applications ........................................................................... 6-8

6.2 Design........................................................................................................................... 6-8

6.2.1 General .................................................................................................................. 6-8


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6.2.2 Single-Port (Single-Seated) Valves........................................................................ 6-8

6.2.3 Double-Port (Double-Seated) Valves ................................................................... 6-10

6.2.4 Cage-Style Valves: Balanced and Unbalanced.................................................... 6-12

6.2.5 Angle Valves ........................................................................................................ 6-13

6.2.6 Y-Style Valves...................................................................................................... 6-13

6.2.7 Three-Way Valves................................................................................................ 6-14

6.2.8 High Pressure Drop Service Control Valves......................................................... 6-15

6.2.9 Flow Characteristics ............................................................................................. 6-18

6.2.10 Rangeability ....................................................................................................... 6-27

6.2.11 Stability .............................................................................................................. 6-28



6.5 Common Problems ..................................................................................................... 6-31


7 BUTTERFLY VALVES—ISOLATION FUNCTION .............................................................. 7-1


7.2 Design........................................................................................................................... 7-4

7.2.1 General .................................................................................................................. 7-4

7.2.2 Symmetric (Lens Type) Disc with Concentric Shaft................................................ 7-7

7.2.3 Nonsymmetric Disc with Single Offset Shaft .......................................................... 7-9

7.2.4 Nonsymmetric Disc with Double Offset Shaft ....................................................... 7-11

7.2.5 Nonsymmetric Disc with Triple Offset Design....................................................... 7-11

7.2.6 Special Disc ......................................................................................................... 7-12

7.2.7 Valve Shaft, Shaft Connections, and Seal ........................................................... 7-13

7.2.8 Valve Bearings..................................................................................................... 7-14

7.2.9 Valve Seats.......................................................................................................... 7-15


7.3.1 Valve-to-Pipe Connections................................................................................... 7-19

7.3.2 Valve Orientation.................................................................................................. 7-19

7.3.3 Valve Location ..................................................................................................... 7-19

7.3.4 Shaft Orientation .................................................................................................. 7-21


7.5 Common Problems ..................................................................................................... 7-22


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8 BUTTERFLY VALVES—MODULATING/THROTTLING FUNCTION.................................. 8-1


8.2 Hydrodynamic Torque Characteristics .......................................................................... 8-2

8.3 Effect of Hydraulic System Characteristics on Peak Hydrodynamic Torque ................ 8-3

8.4 Torque Characteristics of Butterfly Valves .................................................................... 8-5

8.5 Common Problems ....................................................................................................... 8-7


9 BALL VALVES—ISOLATION FUNCTION.......................................................................... 9-1


9.2 Design and Materials .................................................................................................... 9-1

9.2.1 General .................................................................................................................. 9-1

9.2.2 Floating Ball ........................................................................................................... 9-2

9.2.3 Trunnion Mounted Ball ........................................................................................... 9-4

9.2.4 Wedged Ball........................................................................................................... 9-6

9.3 Installation Practices..................................................................................................... 9-8

9.4 Operation Practices and Precautions ........................................................................... 9-8

9.5 Common Problems ....................................................................................................... 9-8


10 BALL VALVES—MODULATING/THROTTLING FUNCTION ......................................... 10-1

10.1 Introduction and Application ..................................................................................... 10-1

10.2 Design....................................................................................................................... 10-1

10.3 Installation Practices................................................................................................. 10-4

10.4 Operation Practices and Precautions ....................................................................... 10-4

10.5 Common Problems ................................................................................................... 10-5

10.6 Maintenance Methods .............................................................................................. 10-5

11 PLUG VALVES ............................................................................................................... 11 -1


11.2 Design....................................................................................................................... 11-1




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11.5 Common Problems ................................................................................................... 11-4


12 DIAPHRAGM VALVES—ISOLATION FUNCTION ......................................................... 12-1


12.2 Design....................................................................................................................... 12-1



12.5 Common Problems ................................................................................................... 12-4


13 VALVE ACTUATORS—GENERAL INFORMATION....................................................... 13-1

13.1 General ..................................................................................................................... 13-1

13.2 Actuator Types.......................................................................................................... 13-4

13.2.1 Manual Actuators ............................................................................................... 13-4

13.2.2 Motorized Actuators ........................................................................................... 13-4

13.2.3 Pneumatic Actuator............................................................................................ 13-7

13.2.4 Hydraulic Actuators ............................................................................................ 13-8

13.2.5 Electrohydraulic Actuators................................................................................ 13-11

13.2.6 Solenoid Actuator............................................................................................. 13-11

13.2.7 Process Medium Actuators .............................................................................. 13-13

13.3 Considerations in Actuator Selection ...................................................................... 13-13

14 MANUAL ACTUATORS .................................................................................................. 14-1


14.2 Design Considerations.............................................................................................. 14-3

14.2.1 Operating Force ................................................................................................. 14-3

14.2.2 Lever Position Control........................................................................................ 14-3

14.2.3 Chain-Wheel Operators...................................................................................... 14-3

14.2.4 Hammerblow or Impact Handwheels.................................................................. 14-4

14.2.5 Gear Operators .................................................................................................. 14-4



14.5 Common Problems ................................................................................................... 14-5



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15 GENERAL DESIGN REQUIREMENTS FOR VALVES AND ACTUATORS.................... 15-1

15.1 Introduction............................................................................................................... 15-1

15.2 Fluid Parameters....................................................................................................... 15-2

15.2.1 Introduction ........................................................................................................ 15-2

15.2.2 Flow Media......................................................................................................... 15-2

15.2.3 Pressure/Temperature ....................................................................................... 15-3

15.2.4 Velocity .............................................................................................................. 15-3

15.2.5 Viscosity ............................................................................................................. 15-4

15.2.6 Density, Specific Gravity .................................................................................... 15-4

15.2.7 Radiation............................................................................................................ 15-4

15.2.8 System Contaminants ........................................................................................ 15-4

15.3 Operating Modes and Transients.............................................................................. 15-5

15.3.1 Introduction ........................................................................................................ 15-5

15.3.2 Plant Condition................................................................................................... 15-5

15.3.3 System Condition ............................................................................................... 15-7

15.4 Fluid Transients ........................................................................................................ 15-9

15.4.1 General .............................................................................................................. 15-9

15.4.2 System Fluid Transients..................................................................................... 15-9

15.4.3 Fluid Transients Caused by Valves.................................................................. 15-11

15.5 Environmental Considerations and Natural Hazards .............................................. 15-13

15.5.1 Introduction ...................................................................................................... 15-13

15.5.2 Environmental Conditions ................................................................................ 15-14

15.6 Valve Performance Requirements .......................................................................... 15-17

15.6.1 Introduction ...................................................................................................... 15-17

15.6.2 Speed of Operation or Stroke Time.................................................................. 15-17

15.6.3 Flow Rate and Pressure Drop .......................................................................... 15-18

15.6.4 Leak Rate......................................................................................................... 15-18

15.6.5 Frequency of Operation ................................................................................... 15-19

15.6.6 Nuclear Valve Qualification .............................................................................. 15-19

16 PRESSURE CONTAINMENT AND STRUCTURAL INTEGRITY REQUIREMENTS....... 16-1

16.1 Introduction............................................................................................................... 16-1

16.2 Codes and Standards ............................................................................................... 16-1

16.2.1 General .............................................................................................................. 16-1


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16.2.2 Pressure/Temperature Ratings .......................................................................... 16-5

16.2.3 Codes and Standards for Pressure Relief Valves ............................................ 16-11

16.3 Materials ................................................................................................................. 16-12

16.3.1 Material Compatibility ....................................................................................... 16-12

16.3.2 General Discussion of Pressure Boundary Materials ....................................... 16-12

16.3.3 Body Materials ................................................................................................. 16-15

16.3.4 Special Considerations for Material Selection for Valves in Raw Water,Especially Seawater.................................................................................................... 16-17

16.4 Corrosion Allowance ............................................................................................... 16-19

16.5 Valve End Connections .......................................................................................... 16-22

16.5.1 General ............................................................................................................ 16-22

16.5.2 Threaded Ends ................................................................................................ 16-22

16.5.3 Welding Ends................................................................................................... 16-23

16.5.4 Brazing Ends.................................................................................................... 16-25

16.5.5 Solder Ends ..................................................................................................... 16-25

16.5.6 Flanged Ends................................................................................................... 16-25

16.5.7 Flared Ends...................................................................................................... 16-27

16.5.8 Hub Ends (Bell and Spigot).............................................................................. 16-27

16.6 System/Valve Interactions ...................................................................................... 16-27

16.6.1 General ............................................................................................................ 16-27

16.6.2 Pipeline End Loads .......................................................................................... 16-27

16.6.3 Leakage ........................................................................................................... 16-28

16.6.4 Vibration........................................................................................................... 16-28

16.7 Shop Tests.............................................................................................................. 16-29

16.8 Structural Integrity and Valve Operability................................................................ 16-30

17 VALVE MAINTENANCE AND INSPECTION PROGRAMS ............................................ 17-1

17.1 Introduction............................................................................................................... 17-1

17.2 Definitions ................................................................................................................. 17-2

17.3 Objective and Scope of Valve Maintenance Programs............................................. 17-2

17.3.1 Objective and Maintenance Philosophy ............................................................. 17-3

17.3.2 The Maintenance Rule (MR) .............................................................................. 17-3

Methodology to Select Plant SCCs to Be in the MR Scope ...................................... 17-4

Establishing Criteria and Goals ................................................................................. 17-4


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Maintenance Preventable Functional Failures (MPFFs)............................................ 17-5

Controlling Equipment Removal of Service ............................................................... 17-5

Periodic Effectiveness Assessment........................................................................... 17-5

17.3.3 Scope................................................................................................................. 17-5

17.4 Valve Maintenance Group ........................................................................................ 17-6

17.5 Valve Categorization and Prioritization ..................................................................... 17-7

17.6 Coordination between Maintenance Group and Other Groups................................. 17-9

17.7 Involvement of Valve Maintenance Group with Other Activities................................ 17-9

17.8 Inspection Frequency and Scope ........................................................................... 17-10

17.9 Maintenance Schedule ........................................................................................... 17-10

17.10 Spare Parts Inventory and Control........................................................................ 17-11

18 TROUBLESHOOTING AND RECOMMENDED CORRECTIVE ACTIONS ..................... 18-1

18.1 Introduction............................................................................................................... 18-1

18.2 Gate Valve Problems................................................................................................ 18-3

18.2.1 Solid, Flex, and Split Wedge Gate Valve Problems ........................................... 18-3

18.2.1.1 Excessive Packing Leaks............................................................................ 18-3

18.2.1.2 Valve Will Not Respond to the Actuation Signal.......................................... 18-4

18.2.1.3 Valve Will Not Fully Open............................................................................ 18-6

18.2.1.4 Valve Will Not Fully Close or Properly Seat................................................. 18-6

18.2.1.5 Excessive Flange Leaks.............................................................................. 18-7

18.2.2 Double-Disc Gate Valve Problems..................................................................... 18-8

18.2.2.1 Excessive Packing Leaks............................................................................ 18-8

18.2.2.2 Valve Will Not Respond to the Actuation Signal.......................................... 18-8

18.2.2.3 Valve Will Not Fully Open............................................................................ 18-9

18.2.2.4 Valve Will Not Fully Close or Properly Seat................................................. 18-9

18.2.2.5 Excessive Flange Leaks.............................................................................. 18-9

18.2.3 Westinghouse Gate Valve Problems.................................................................. 18-9

18.3 Globe Valve Problems ............................................................................................ 18-10

18.3.1 Excessive Packing Leaks................................................................................. 18-10

18.3.2 Valve Will Not Respond to the Actuation Signal............................................... 18-10

18.3.3 Valve Will Not Fully Open................................................................................. 18-10

18.3.4 Valve Will Not Fully Close or Properly Seat...................................................... 18-11

18.3.5 Excessive Flange Leaks .................................................................................. 18-11


xviii

18.4 Butterfly and Ball Valve Problems........................................................................... 18-11

18.4.1 Excessive Packing Leaks................................................................................. 18-11

18.4.2 Valve Will Not Respond to the Actuation Signal............................................... 18-12

18.4.3 Valve Will Not Fully Open................................................................................. 18-13

18.4.4 Valve Will Not Fully Close or Properly Seat...................................................... 18-13

18.4.5 Excessive Flange Leaks .................................................................................. 18-14

18.5 Plug Valve Problems............................................................................................... 18-14

18.6 Diaphragm Valve Problems .................................................................................... 18-15

18.7 Inspection and Repair Checklists:........................................................................... 18-15

19 INSTALLATION, TESTING, AND MAINTENANCE REQUIREMENTS........................... 19-1

19.1 Introduction............................................................................................................... 19-1

19.2 Installation Requirements ......................................................................................... 19-1

19.2.1 General Valve Installation Requirements ........................................................... 19-1

19.2.2 Bypasses ........................................................................................................... 19-3

19.3 Testing and Inspection Considerations..................................................................... 19-5

19.3.1 Shop Performance Testing ................................................................................ 19-5

19.3.2 Pre-Operational Tests ........................................................................................ 19-6

19.3.3 In-Service Test Requirements............................................................................ 19-6

19.4 Maintenance Requirements .................................................................................... 19-15

19.4.1 Separation and Maintenance ........................................................................... 19-15

19.4.2 General Good Maintenance Practices ............................................................. 19-22

20 DIAGNOSTIC EQUIPMENT AND METHODS................................................................. 20-1

20.1 Introduction............................................................................................................... 20-1

20.2 Equipment................................................................................................................. 20-2

20.2.1 Boroscopes ........................................................................................................ 20-2

20.2.2 Radiography....................................................................................................... 20-2

20.2.3 Acoustics............................................................................................................ 20-2

20.2.4 Temperature Monitoring..................................................................................... 20-3

20.2.5 Ultrasonics ......................................................................................................... 20-3

20.2.6 Stem Thrust/Torque Measurement Devices....................................................... 20-4

20.3 Methods for Measuring Stem Thrust/Torque ............................................................ 20-4

20.3.1 Spring Pack Displacement ................................................................................. 20-4


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20.3.2 Strain Measurement of the Yoke Legs............................................................... 20-5

20.3.3 Strain Measurement of the Stem........................................................................ 20-5

20.3.4 Load Measurement at the Actuator Base........................................................... 20-6

20.3.5 Electric Motor Power Monitor ............................................................................. 20-7

20.3.6 Diaphragm/Piston Pressure ............................................................................... 20-7

20.3.7 Data Acquisition ................................................................................................. 20-7

20.4 Summary .................................................................................................................. 20-8

21 VALVE SELECTION GUIDELINE CHARTS ................................................................... 21-1

22 REFERENCES AND BIBLIOGRAPHY............................................................................ 22-1

22.1 EPRI / NMAC Reports .............................................................................................. 22-1

22.2 Proprietary Documents Developed under EPRI MOV Performance PredictionProgram............................................................................................................................ 22-3

22.3 Proprietary Documents Developed under Utility-Sponsored Generic Thrust andTorque Overload Qualification Program for Limitorque Actuators.................................... 22-4

22.4 NRC Generic Letters, Information Notices, and Related References ....................... 22-6

22.5 Books, Magazines, Technical Meetings, and Journal Articles .................................. 22-8

22.6 Codes and Standards ............................................................................................. 22-13

23 APPENDIX A: RECENT ADVANCES IN VALVE AND ACTUATOR TECHNOLOGY..... 23-1

23.1 Introduction............................................................................................................... 23-1

23.2 Background............................................................................................................... 23-1

23.3 Motor-Operated Valve Performance Prediction Methodology................................... 23-2

23.3.1 System Flow Model ............................................................................................ 23-3

23.3.2 Solid and Flex Wedge Gate Valve Model........................................................... 23-3

23.3.3 Methodologies for Special Design Gate Valves ................................................. 23-6

23.3.4 Butterfly Valve Model ......................................................................................... 23-6

23.3.5 Globe Valve Model............................................................................................. 23-7

23.4 EPRI/NMAC Application and Maintenance Guides................................................... 23-7

23.5 Generic Thrust and Torque Qualification Program for Limitorque Actuators.......... 23-14

23.5.1 Background...................................................................................................... 23-14

23.5.2 Technical Approach ......................................................................................... 23-15

23.5.3 Highlights of Results and Conclusions ............................................................. 23-16


xx

24 APPENDIX B: CONTROL VALVE SIZING METHODS AND EXAMPLES...................... 24-1

24.1 General Methods, Definitions, and Evaluation.......................................................... 24-1

24.1.1 Introduction to Control Valve Specification, Sizing, and Selection ..................... 24-1

24.1.2 Definitions .......................................................................................................... 24-2

24.1.3 Sizing Formulas and Procedures for Liquid Flow ............................................... 24-9

24.1.4 Sizing Formulas and Procedures for Gas Flow ................................................ 24-29

24.2 Examples of Sizing for Special High Pressure Drop Applications ........................... 24-42

24.2.1 Feedwater Recirculation................................................................................... 24-42

24.2.2 Atmospheric Steam Dump and Turbine Bypass............................................... 24-47

24.2.3 Attemperator Spray Control.............................................................................. 24-50

24.2.4 Deaerator Level Control ................................................................................... 24-52

24.2.5 Feedwater Pump Flow Control......................................................................... 24-56

25 APPENDIX C: VALVE PROCUREMENT SPECIFICATION............................................ 25-1

25.1 General ..................................................................................................................... 25-1

25.2 Specific Elements ..................................................................................................... 25-2

25.3 Data Sheets .............................................................................................................. 25-6


xxi

LIST OF FIGURES

Figure 2-1 Globe Valve Typical Valve Nomenclature.............................................................. 2-2

Figure 2-2 Gate Valve Typical Valve Nomenclature ............................................................... 2-3

Figure 2-3 Screwed Bonnet .................................................................................................. 2-20

Figure 2-4 Flanged (Bolted) Bonnet...................................................................................... 2-21

Figure 2-5 Welded Bonnet.................................................................................................... 2-22

Figure 2-6 Pressure-Sealed Bonnet ..................................................................................... 2-22

Figure 2-7 Seat Joint Mating Surfaces (Lay of Roughness Concentric) ............................... 2-23

Figure 2-8 Seat Plane Distortion under Vertical and Horizontal Bending Moments .............. 2-24

Figure 2-9 Typical Globe Valve Seating Configurations ....................................................... 2-27

Figure 2-10 Cross Ring Indentation ...................................................................................... 2-28

Figure 2-11 Soft Seat Retention Methods............................................................................. 2-29

Figure 2-12 Methods for Attaching Seat to Body .................................................................. 2-31

Figure 2-13 Flexible Seat...................................................................................................... 2-32

Figure 2-14 Floating Seat ..................................................................................................... 2-32

Figure 2-15 Spring-Loaded Packing Seals ........................................................................... 2-33

Figure 2-16 Stem Connections ............................................................................................. 2-34

Figure 2-17 Gate Valve Gate Guide ..................................................................................... 2-36

Figure 2-18 Manual Override Lever on Pressure-Relief Valve.............................................. 2-38

Figure 2-19 Manual Override Handwheel on Motor-Operated Valve .................................... 2-38

Figure 2-20 Steam Leak-Off Connection .............................................................................. 2-39

Figure 2-21 External Bypass................................................................................................. 2-41

Figure 2-22 Bonnet Extension .............................................................................................. 2-42

Figure 2-23 Trim Components .............................................................................................. 2-44

Figure 2-24 Bellows Seal ...................................................................................................... 2-56

Figure 2-25 Bellows on Butterfly Valve ................................................................................. 2-57

Figure 2-26 Metal Diaphragm Stem Seal.............................................................................. 2-58

Figure 2-27 Basic Types of Stem Seals................................................................................ 2-60

Figure 2-28 Packing Gland Details ....................................................................................... 2-62

Figure 2-29 Distribution of Stresses in the Packing and Location of Actual Sealing Point.... 2-63

Figure 2-30 Live Loading of Valve Packing Using Disc Springs ........................................... 2-73


xxii

Figure 2-31 Packing Compressive Stress Versus Consolidation .......................................... 2-74

Figure 2-32 Lantern Ring / Stem Leakoff Connection........................................................... 2-76

Figure 3-1 Valve Classification by Function ............................................................................ 3-2

Figure 4-1 Inside Screw Stem Thread Configurations ............................................................ 4-2

Figure 4-2 Rising Stem Design, Outside Screw...................................................................... 4-2

Figure 4-3 Wedge Gate Valve ................................................................................................ 4-4

Figure 4-4 Anchor/Darling Double-Disc Gate Valve................................................................ 4-8

Figure 4-5 W-K-M Through-Conduit Double-Wedge Parallel Expanding Gate Valve ............. 4-9

Figure 4-6 Parallel Slide Double-Disc Gate Valve................................................................. 4-11

Figure 4-7 Through-Conduit Parallel Slide Double-Disc Gate Valve..................................... 4-12

Figure 4-8 Westinghouse Flexible Wedge Gate Valve ......................................................... 4-14

Figure 4-9 Slab Gate Valve .................................................................................................. 4-16

Figure 4-10 Gate Valve Bonnet Overpressurization ............................................................. 4-18

Figure 4-11 Typical Seat and Guide Damage Locations in Conventional Flexible WedgeGate Valves Under High Flow Conditions ..................................................................... 4-25

Figure 5-1 T-Pattern Globe Valve ........................................................................................... 5-2

Figure 5-2 Angle-Pattern Globe Valve .................................................................................... 5-2

Figure 5-3 Y-Pattern Globe Valve........................................................................................... 5-3

Figure 5-4 Velan 2" (5.1 cm), 1500# Globe Valve (Guide-Based) Model: Figure No.137132............................................................................................................................ 5-4

Figure 6-1 Pressure Drop Through a Control Valve at Minimum, Design, and MaximumSystem Flows.................................................................................................................. 6-2

Figure 6-2 Control Valve Sizing Example ............................................................................... 6-5

Figure 6-3 Single-Port Control Valve ...................................................................................... 6-9

Figure 6-4 Double-Seated Globe Valve ................................................................................ 6-11

Figure 6-5 Balanced Disc Cage Style Valve ......................................................................... 6-13

Figure 6-6 Y-Style Body Valve.............................................................................................. 6-14

Figure 6-7 Three-Way Valve for Flow Diverting Service Unbalanced Disc ........................... 6-14

Figure 6-8 Three-Way Valve, Balanced Plug........................................................................ 6-15

Figure 6-9 Low Noise, Anti-Cavitation Trim........................................................................... 6-16

Figure 6-10 High Pressure Drop Multiple Step Plug and Cage............................................. 6-17

Figure 6-11 High Pressure Drop Control Valve, Labyrinth Design........................................ 6-18

Figure 6-12 Inherent Flow Curves for Various Valve Plugs with Constant Delta P Acrossthe Valve ....................................................................................................................... 6-19

Figure 6-13 Comparison of Installed Characteristics versus Inherent Characteristics .......... 6-20

Figure 6-14 Typical Pump Characteristics ............................................................................ 6-22

Figure 6-15 Flow Schematic without Piping Losses.............................................................. 6-22

Figure 6-16 Installed Characteristics without Piping Losses ................................................. 6-24


xxiii

Figure 6-17 Flow Schematic with Piping Losses................................................................... 6-25

Figure 6-18 Installed Characteristics with Piping Losses ...................................................... 6-27

Figure 6-19 Force Balance Diagram for Control Valves........................................................ 6-29

Figure 7-1 Typical Motor-Operated Butterfly Valve................................................................. 7-2

Figure 7-2 Most Common Butterfly Valve Disc Shapes Used in Nuclear Power Plants .......... 7-5

Figure 7-3 Typical Variations in Butterfly Disc Designs........................................................... 7-6

Figure 7-4 Typical Symmetric Disc Design with Elastomer Lined Body .................................. 7-8

Figure 7-5 Cross-Section of a Typical Nonsymmetric Butterfly Valve ................................... 7-10

Figure 7-6 Valve Disc Flow Orientation Terminology ............................................................ 7-11

Figure 7-7 Triple Offset Butterfly Valve................................................................................. 7-12

Figure 7-8 Fishtail Disc ......................................................................................................... 7-13

Figure 7-9 Special Disc Design for Noise and Cavitation Reduction..................................... 7-13

Figure 7-10 Typical Seat Designs......................................................................................... 7-16

Figure 7-11 Inflatable Seat Butterfly Valve ........................................................................... 7-17

Figure 7-12 Effect of Upstream Disturbance, Shaft Orientation, and Disc OpeningDirection on Hydrodynamic Torque............................................................................... 7-20

Figure 7-13 Hydrostatic Torque Component in a Horizontal Shaft Installation...................... 7-21

Figure 8-1 Flow Through a Symmetric Disc Butterfly Valve .................................................... 8-2

Figure 8-2 Variation in Location of Peak Hydrodynamic Torque for Constant Head andPumped Systems ............................................................................................................ 8-4

Figure 8-3 Typical Opening Torque Characteristics of a Symmetric Disc Butterfly Valveunder High Flow Conditions ............................................................................................ 8-6

Figure 9-1 Floating Ball........................................................................................................... 9-4

Figure 9-2 Trunnion-Mounted Ball .......................................................................................... 9-5

Figure 9-3 Wedged Ball Design .............................................................................................. 9-7

Figure 10-1 Eccentric Rotating Plug/Ball Control Valve........................................................ 10-2

Figure 10-2 Segmented Ball with Tubular Resistance Trim .................................................. 10-3

Figure 10-3 Multistage Anticavitation Ball Valve ................................................................... 10-4

Figure 11-1 Nonlubricated Plug Valve .................................................................................. 11-2

Figure 11-2 Lubricated Plug Valve........................................................................................ 11-2

Figure 11-3 Lubricated Tapered Plug Valve ......................................................................... 11-3

Figure 12-1 Saunders Pattern Flexible Diaphragm Valve..................................................... 12-2

Figure 12-2 Straightway Flexible Diaphragm Valve .............................................................. 12-3

Figure 12-3 Full Bore Body Flexible Diaphragm Valve ......................................................... 12-3

Figure 13-1 Types of Valve Actuators................................................................................... 13-2

Figure 13-2 Limitorque SMB-0 Motor Operator Cutaway View ............................................. 13-5

Figure 13-3 Simplified Motor Operator.................................................................................. 13-6

Figure 13-4 Hydraulic Actuator with Fail-Safe Operation Using a Mechanical Spring........... 13-9


xxiv

Figure 13-5 Hydraulic Actuator with Fail-Safe Operation Using a Gas Spring .................... 13-10

Figure 13-6 Solenoid Actuator ............................................................................................ 13-12

Figure 14-1 Manual Lever..................................................................................................... 14-1

Figure 14-2 Worm Gear Actuator.......................................................................................... 14-2

Figure 16-1 Butt Weld End Connection .............................................................................. 16-24

Figure 16-2 Socket Weld End Connection.......................................................................... 16-24

Figure 16-3 Butterfly Valve End Connections ..................................................................... 16-26

Figure 19-1 Test Valve Arrangement for Maintained Flowrate Test...................................... 19-9

Figure 19-2 Globe Valve Reverse Air Test (Test Pressure Under Seat)............................. 19-10

Figure 19-3 Globe Valve Reverse Air Test (Test Pressure Above Seat) ............................ 19-11

Figure 19-4 Gate Valve Reverse Air Test (With Body Vent Test Connection) .................... 19-12

Figure 19-5 Gate Valve Through Body Air Test (LOCA pushes disc toward outboardseat. Through body pressurization measures leakage by both seats.) ....................... 19-12

Figure 19-6 Required Valve Maintenance Clearance for Typical Installation...................... 19-19

Figure 19-7 Required Maintenance Clearance for Chain-Operated Valve.......................... 19-20

Figure 19-8 Human Factors Clearance-General ................................................................. 19-21

Figure 21-1 Valve Selection Chart (This figure is located in a pouch inside the backcover of this report.) ...................................................................................................... 21-1

Figure 23-1 Tilted Disc Contact Mode Resulting in Point Contact with the DownstreamSeat............................................................................................................................... 23-5

Figure 23-2 Limitorque Actuator Test Fixture...................................................................... 23-15

Figure 24-1 Pressure Profile of Fluid Passing through a Valve............................................. 24-3

Figure 24-2 Pressure Profile through Restriction .................................................................. 24-4

Figure 24-3 Effects of Vaporization....................................................................................... 24-5

Figure 24-4 Globe Valve FL Values..................................................................................... 24-11

Figure 24-5 High Performance Butterfly/Ball FL Values....................................................... 24-12

Figure 24-6 Liquid Critical Pressure Ratio Factor Curve..................................................... 24-13

Figure 24-7 Globe Valve Liquid Incipient Cavitation Factor (Fi) Values .............................. 24-17

Figure 24-8 Reynolds Number Factor................................................................................. 24-18

Figure 24-9 Compressibility Factors for Gases with Reduced Pressures from 0 to 40 ....... 24-34

Figure 24-10 Compressibility Factors for Gases with Reduced Pressures from 0 to 6 ....... 24-35

Figure 24-11 Conventional Method of Recirculation Control: Control Valve (On-Off) inSeries with a Breakdown Orifice ................................................................................. 24-44

Figure 24-12 Method of Recirculation Control Using High Pressure, Modulating Anti-Cavitation Valve .......................................................................................................... 24-44

Figure 24-13 Globe Angle Control Valve with Anti-Cavitation Trim..................................... 24-45

Figure 24-14 Globe Control Valve with Low Noise Trim ..................................................... 24-48

Figure 24-15 Typical Condensate System.......................................................................... 24-53

Figure 24-16 Typical Condensate System Curve ............................................................... 24-54


xxv

Figure 24-17 Globe Control Valve with Anti-Cavitation Variable Resistance Trim .............. 24-54

Figure 24-18 Main Feedwater System................................................................................ 24-57

Figure 25-1 Suggested Manual Valve Data Sheet by Purchaser.......................................... 25-8

Figure 25-2 Suggested Manual Valve Data Sheet by Bidder/Seller ................................... 25-11

Figure 25-3 Suggested Motor-Operated Valve Data Sheet by Purchaser .......................... 25-13

Figure 25-4 Suggested Motor-Operated Valve Data Sheet by Bidder/Seller ...................... 25-17

Figure 25-5 Control Valve Data Sheet ................................................................................ 25-20

Figure 25-6 Relief Valve Data Sheet .................................................................................. 25-24

Figure 25-7 Rupture Disc Data Sheet................................................................................. 25-26


xxvii

LIST OF TABLES

Table 2-1 Corrosion Ranking for Materials Selection............................................................ 2-46

Table 2-2 Critical Variables for Accelerated Erosion-Corrosion ............................................ 2-49

Table 2-3 Chart of Wear and Galling Resistance of Material Combinations ......................... 2-52

Table 2-4 Typical Properties of Plastics and Elastomers Used in Valves for Soft Seats,Seals, and Gaskets ...................................................................................................... 2-68

Table 2-5 Typical Radiation Resistance of Plastics .............................................................. 2-70

Table 2-6 Gasket Materials and Contact Facings, Gasket Factors M for OperatingConditions, and Minimum Design Seating Stress y...................................................... 2-79

Table 3-1 Control Valve Seat Leakage Classifications (In Accordance with ANSI/FCI70-2-1976........................................................................................................................ 3-6

Table 3-2 Seat Leakage Criteria ............................................................................................. 3-7

Table 6-1 Valve Cv and Pressure as a Function of Flow Rate without Line Losses .............. 6-23

Table 6-2 Valve Cv and Pressure as a Function of Flow Rate with Line Losses ................... 6-26

Table 13-1 Normal Application of Power Actuators for Valves.............................................. 13-3

Table 14-1 Maximum Recommended Rim Pull as a Function of Handwheel Diameter ........ 14-3

Table 16-1 Valve Design Codes ........................................................................................... 16-2

Table 16-2 Typical Valve Standards ..................................................................................... 16-3

Table 16-3 Safety Classes and Applicable Standards.......................................................... 16-5

Table 16-4 Pressure/Temperature Ratings for Steel Valves. Source: ANSI B 16.34 -1981.............................................................................................................................. 16-6

Table 16-5 Cast Iron Gate Valve Ratings Source: MSS-SP-70 ............................................ 16-8

Table 16-6 Bronze Gate, Globe, and Check Valve Ratings Source: MSS-SP-80................. 16-9

Table 16-7 Commonly Used Pressure Boundary Materials ................................................ 16-13

Table 18-1 Inspection Checklist for Solid and Flexible Wedge Gate Valves....................... 18-17

Table 18-2 Inspection Checklist for Butterfly Valves........................................................... 18-25

Table 19-1 Valve Maintenance Clearance Data ................................................................. 19-16

Table 20-1 Comparison of Selected Diagnostic Methods ..................................................... 20-9

Table 21-1 Valve Selection Matrix ........................................................................................ 21-2

Table 24-1 Typical Valve Recovery Coefficients (FL) and Incipient Cavitation Factors (F

i) . 24-10

Table 24-2 Typical Critical Pressure Values ....................................................................... 24-14

Table 24-3 Typical Values of Cv: Globe Valve, Flow over the Seat..................................... 24-20


xxviii

Table 24-4 Typical Values of Cv: Globe Valve, Flow under the Seat .................................. 24-21

Table 24-5 Typical Piping Geometry Factors, Fp : Valve with both Reducer and

Expander..................................................................................................................... 24-22

Table 24-6 Typical Piping Geometry Factors, Fp: Valve with Outlet Expander Only ........... 24-23

Table 24-7 Terminal Pressure Drop Ratios (xT)................................................................... 24-31

Table 24-8 Gas Physical Data ............................................................................................ 24-32


1-1

1 INTRODUCTION/SUMMARY HOW TO USE THE

GUIDEBOOK

1.1 Introduction

The purpose of this guide is to present, in a comprehensive manner, information andmethods that have been successfully utilized in the application, use, maintenance, andrepair of valves in power plant systems. The information presented in this guideprovides state-of-the-art valve and actuator technology in use in U.S. power plants,including:

• The latest advances in the application, use, and maintenance of valves and actuators

• Current techniques used for both in situ and off-line repairs

• Guidelines for troubleshooting valve and actuator problems

• New and emerging technologies for diagnostic systems and equipment

• Requirements for valve maintenance programs that provide significantimprovements in valve reliability and plant availability

• Recent regulatory issues concerning the performance of valves and actuators innuclear power plant applications

Over the last several years, EPRI, the U.S. NRC, and electric utilities have conductedmany research projects to improve plant safety and availability by reducing valve andactuator problems. These projects resulted in many proprietary and nonproprietarydocuments, which deal with various specialized areas of valve/actuator sizing,performance characteristics, valve and actuator maintenance/repair as well as testingand diagnostic technologies. However, information to aid plant personnel in resolvingthese problems is difficult to glean from scattered sources, and access may be restrictedby proprietary consideration. Brief summaries along with a comprehensive listing ofkey documents are included in this guide to assist the reader to quickly find additionalsources of information.


Introduction/Summary How to Use the Guidebook

1-2

This is Volume 1 of a two-volume guide. In this volume, the focus is on the application,use, maintenance, and troubleshooting of gate, globe, butterfly, plug, and diaphragmvalves in power plant applications. Volume 1 is a revision of NMAC NP-6516, issued inAugust 1990. Apart from the technical update (which is very extensive), several topicswere eliminated from this revision because they are covered in great depth in otherrecent EPRI/NMAC publications. For example, check valves are not discussed in thisrevision because they are covered in two very detailed documents [1.20,1 1.21]. Air-operated valves and solenoid valves are also omitted because they are covered inReferences 1.2 and 1.7 respectively. Only minimum discussions of motor operators areincluded because detailed discussions are given in other EPRI documents [1.22, 1.23,1.24, 1.25, and 1.26].

Volume 2 of this guide [1.1] provides detailed discussions about most current valverepair techniques both in situ and off-line for gate, globe, and check valves. Thediscussions in Volume 2 cover component repair, flaw removal techniques, materialselection, machining, welding, heat treatment guidelines, final inspection and testingrequirements, which are also applicable to other valve types.

This guide was developed for persons who prepare valve specifications, install andoperate valves in various applications, and perform required valve maintenance andrepairs. The guide will also be useful to system designers, plant management,engineers, and others who need in-depth understanding of the capabilities andlimitations of valves that affect performance and system availability. For readers withlittle valve background, the guide is intended to provide basic understanding of valvetechnology. For readers with extensive valve experience, the guide is a reference book,which provides easy access to specific valve information as well as guidance to othersources of specialized areas.

1.2 Summary/How to Use the Guidebook

1.2.1 General

This section provides the reader with a “road map” to the information presented in thisguide and to facilitate easy access to it. The Table of Contents provides a fairlydescriptive title for each section. Section 2 provides the nomenclature and glossary ofterms that are common in the industry and used throughout the text. Aspects ofcomponent construction common to several different types of valves and actuators arediscussed in Section 2. Figures are used extensively to illustrate the different types ofvalves and specific component details and features.

1 Numbers in brackets denote technical references given in Section 22.



1-3

1.2.2 Valve Functions

Section 3 provides the basic valve functions and the features necessary to perform thesefunctions. These functions generally fall into one of the following four categories:

Isolation. The valve is used to isolate portions of a system, an entire system from othersystems, or a given piece of equipment (such as a heat exchanger) within a system. Toachieve isolation, the valve is typically closed and is expected to exhibit a very low seatleakage.

Modulating/Throttling. In performing a modulating function, the position of the valveclosure element (gate, plug, disc, or ball) is varied between the fully open and the fullyclosed positions. The position of the closure element is controlled by an actuator that isan integral part of the valve or is attached to the valve stem. The position of the valveclosure element is automatically controlled by a feedback signal to the actuator toachieve a desired condition (for example, flow rate, fluid level, temperature, pressure)within the system. Modulating valves are used where automatic, repeatable, andaccurate control of a system fluid parameter is required.

A throttling function is similar to the modulating function except that the position ofthe valve closure element is manually controlled either locally or remotely (using apower source to the actuator). The valve closure element is positioned at a fixedpercentage of valve opening to satisfy a specific system flow requirement. The valvethen provides a constant hydraulic resistance to achieve a fixed pressure drop at agiven system flow rate. When the system flow requirement changes, the valve ismanually repositioned to provide the necessary hydraulic resistance and pressure drop.

In this guide, the discussions of air-operated valves and solenoid valves are kept to aminimum because these valves are discussed in great detail in References 1.2 and 1.7respectively.

Check (Non-Return). Check valves are located in a hydraulic system to ensure that theprocess medium flows in one direction only. A common application for check valves isat the discharge of multiple pumps in parallel that provide flow and pressure head to acommon manifold. In the event that one of the pumps ceases to produce flow andpressure head, a check valve located in its discharge line prevents a flow reversalthrough the non-operating pump caused by the pressure head produced by theoperating pump(s). Another typical application is at system interfaces where the intentis to allow flow in one direction only from one system into another. Check valves arenot normally considered isolation valves because they may exhibit higher leakage ratesthan usually required for isolation applications.



1-4

In this volume of the guide, the discussion of check valve application, use andmaintenance is kept to a minimum because these subjects are discussed in great detailin References 1.20 and 1.21. Volume 2 of this guide provides detailed guidance forcheck valve repair.

Pressure-Relief. Pressure-relief valves are used to protect piping systems andcomponents from overpressurization by dissipating excess system pressure to apressure suppression system or to the atmosphere. Pressure relief is performed in anumber of ways including:

• The valve opens automatically to discharge system media when pressure at valveinlet (acting directly on valve disc) exceeds a predetermined level. No externalpower source is needed.

• A pilot valve opens automatically when pressure at the inlet of the pilot valveexceeds a predetermined level. The opening of the pilot valve subsequently opensthe main valve. Alternatively, the pilot valve may be opened at any inlet pressureby the application of an external power source.

• The valve opens when the actuator power source receives a signal that the valveinlet pressure exceeds a predetermined level.

• The valve opens when the actuator’s power source receives a signal that othersystem conditions or events have occurred that will cause a pressure rise to occur(for example, power failure to a pump or the sub-normal pressure preceding apressure surge or water hammer).

In this guide, the discussions of pressure relief valves are eliminated because thesevalves are discussed in great detail in Reference 1.4.

1.2.3 Specific Valve Types by Function

Sections 4 through 12 provide information on specific types of valves commonly usedto perform isolation and modulating/throttling functions. The specific types addressedare gate, globe, butterfly, ball, plug, and diaphragm valves. The information providedfocuses on a number of areas pertinent to the application of each specific valve type.These are as follows:

Introduction and Application. Performance features and capabilities of the specific valvetype are discussed with respect to the stated function, together with other applicationconsiderations. For example, for flow isolation, fully open gate valves offer minimalflow resistances and pressure drops (thus reducing pumping costs). However, gatevalves require a relatively long stem travel to open and close. Therefore, stroke times



1-5

for gate valves are relatively longer than for globe valves, which could adversely affectthe system performance. On the other hand, globe valves, while satisfying stroke timerequirements, introduce high flow resistances and pressure drops, which may beunacceptable in some applications.

Design. Using a valve cross-sectional drawing, the design features of the specific valvetype are discussed.

The effect of different variants of the valve type (for example, solid wedge versusflexible wedge gate valves) on valve performance is noted. The advantages anddisadvantages of the variants are discussed.

Installation Practices. The proximity of other components (pumps, piping connections,etc.) may affect valve performance. Installation configuration, direction of flow, forces,and moments applied to the valve by the connecting pipe, orientation to vertical, andaccumulation of debris/biological growth inside the valve are typical installationconsiderations. These are discussed as they apply to each specific valve type andfunction, and an assessment is provided where a particular sensitivity to any of theseexists. General guidelines for valve installation are given in Section 19.

Operation Practices and Precautions. Methods to improve the functional reliability ofvalves through correct operational practices are discussed. Practices that may adverselyaffect the performance of valves are presented. Such practices include applyingexcessive actuator loading thrust to reduce seat leakage and using of valves for otherthan the intended function (for example, long-term throttling with a gate valve).

Common Problems. For each valve type, a section is devoted to provide a concise list ofthe common valve problems and malfunctions. Wherever possible, suggested correctiveand preventive actions are given. Detailed repair procedures are given in Volume 2[1.1].

Maintenance. General discussions of maintenance methods and practices for specificvalves are provided. The focus is on areas that are considered critical to achievesatisfactory valve performance. General discussions of other valve maintenance issuesincluding programmatic consideration, troubleshooting, corrective action, maintenancerequirements, and diagnostic equipment are given in Sections 17 through 20.

1.2.4 Actuator Types

Section 13 provides a general introduction to the different types of valve actuators.Section 14 is dedicated to manual actuators. For other types of actuators, the reader isreferred to other EPRI documents [1.2, 1.4, 1.7, and 1.22 through 1.26].



1-6

1.2.5 General Design Requirements for Valves and Actuators

Deficient performance and valve failures result from the use of valves under operatingconditions for which they were not intended. A complete knowledge of all of theconditions to which the valve will be subjected is extremely valuable in avoidingproblems. This includes system start-up, shutdown, and anticipated transientconditions. All verified pertinent valve data should be recorded and filed for futurereference. Section 15 provides detailed discussion of general design requirements thatneed to be defined and applied to valves during the original or replacementprocurement cycle.

1.2.6 Valve Pressure Boundary and Structural Integrity

The valve is an integral part of the system pressure boundary and must be designed sothat the integrity of the system is maintained. Section 16 discusses pressure boundaryand structural integrity requirements including:

• Applicable codes and standards

• Pressure temperature ratings

• Materials and material compatibility

• Pressure boundary materials and their proper selection

• Corrosion allowance

• Valve end connections

• Pipeline loads and vibrations

• Leakage, and shop hydrostatic testing

• Structural integrity and valve operability

1.2.7 Valve Maintenance and Inspection Programs

In the last few years, there has been ever-increasing pressure on the electric powerindustry to improve plant efficiency, shorten plant outages, and cut costs. Under thisenvironment, valve maintenance groups are required to improve the efficiency of valverepairs and reliability. Section 17 discusses the different factors that affect valvemaintenance and have direct impact on valve reliability and plant availability.Recommendations to enhance maintenance programs and procedures are also includedin Section 17.



1-7

1.2.8 Troubleshooting and Recommended Corrective Actions

One of the most important responsibilities of plant maintenance and operationpersonnel is to quickly identify valve problems and determine the necessary correctiveactions. In many cases, the root cause is simple but not obvious. Section 18 providesguidance on troubleshooting and recommended corrective actions for gate, globe,butterfly, ball, plug, and diaphragm valves. The use of checklists can improve thequality and the effectiveness of the maintenance activities and are recommended in thisguide (see Section 18 for sample checklists).

1.2.9 Installation, Testing, and Maintenance Requirements

Valve installation, testing, and maintenance must meet certain code and regulatoryrequirements. For nuclear power plants, these requirements are more stringent than inany other application. Section 19 provides a detailed discussion of these requirementsand identifies the governing codes that should be reviewed for additional information.

1.2.10 Diagnostic Equipment and Methods

Recent advances in computers and measurement equipment coupled with innovativesolutions for measurement problems resulted in a surge in valve diagnostic equipmentand methods. Section 20 provides a summary of the state of the art of valve andactuator diagnostic equipment and methods. It is expected that these advances willcontinue and new equipment will be developed while existing equipment will befurther refined. Thus, the reader is encouraged to continue to obtain new informationfrom diagnostic equipment vendors and service companies that develop and maintainthe equipment. However, the information provided in Section 20 can be used as astarting point to identify the specific plant needs.

1.2.11 Valve Selection Chart

Section 21 provides information on using the Valve Selection Chart shown in Figure 21-1. The chart is in the form of an algorithm and is provided for use as a wall chart. Itprovides a structured path of the mental process of selecting a new valve or evaluatingan existing valve. Caution should be exercised in using the chart because it is not a“go/no-go” device, but rather one that suggests options to be evaluated and points tothe direction of needed additional investigation. Some of the options shown may notalways be available to the user. Decisions such as the type of valve end connections,valve body/bonnet material, etc., may be mandated by overall system considerations.Several typical valve applications are presented in the text to assist the reader in the useof the Valve Selection Chart.



1-8

1.2.12 References and Bibliography

As mentioned above, the vast amount of information/documents developed over thelast few years makes it difficult for plant personnel to locate the applicable documentsfor a particular need. In this guide, a listing of the key references, codes, and standardsare provided to enable the reader to locate additional documents for further study. InSection 22, the references and bibliography are listed according to their categories in sixdifferent groups. Proprietary documents (available only to certain programparticipants) are included in separate sections and clearly identified. Most of thesereferences provide additional references for specific information such as valve testreports and friction coefficient data.

1.2.13 Appendices

Appendices are provided to broaden the scope of knowledge presented in the text.References in the text are made to specific appendices where additional information isgiven on the subject being discussed.

Section 23 provides a brief discussion of recent advances in the valve and actuatorstechnology along with latest regulatory requirements. Section 23 also provides a briefsummary of some key EPRI/NMAC documents that are believed to be of particularinterest to the reader.

Section 24 provides a brief discussion of control valve sizing methods based on theInstrument Society of America (ISA) approach. Several examples are provided tofurther clarify the methods used and to understand their limitations. It should be notedthat several computer programs have been developed by valve manufacturers andothers to perform control valve sizing calculations. Evaluation and discussion of thesecomputer programs are outside the scope of this guide. It is recommended, however,that the reader seek information about such software from the developingorganizations.

Section 25 provides valve procurement specifications. Suggested data sheets for use bythe purchaser and bidder/seller are included for convenience.

Finally, complete reading of the entire guide, including the appendices, should providethe reader with an overall view of the current state of the art of valve and actuatortechnology.


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2 GENERAL VALVE DESIGN

2.1 Nomenclature/Glossary of Terms

2.1.1 Introduction

This section covers commonly used valve terminology and nomenclature. As anexample, Figures 2-1 and 2-2 show a globe and a gate valve along with typicalnomenclature used for these valve types. Reference is given, where appropriate, tofigures found in later sections which depict the term being defined. Many terms used inthis document are defined in the following standards and technical textbooks.

• Glossary of Valves Terms, Grove Valve Regulator Company, Oakland, CA, 1980.

• ASME Standard 112, Diaphragm Actuated Control Valve Terminology, AmericanSociety of Mechanical Engineers, New York, NY.

• ISA Handbook of Control Valves, Second Edition, Instrument Society of America, 1976.

• Control Valve Handbook, Second Edition, Fisher Control Company, Marshalltown,Iowa, 1977.

• ANSI B95.1, Terminology for Pressure Relief Devices.

2.1.2 Glossary of Terms

Active Valve

A valve that is required to change obturator position to accomplish its requiredfunction(s).

Actual Discharge Area

The minimum net area that determines the flow through a valve.


General Valve Design

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Actuator Spring (Diaphragm Actuator) (Figure 2-1)

A spring that moves the actuator stem in a direction opposite to the direction created bydiaphragm pressure.

Actuator Stem (Diaphragm Actuator) (Figure 2-1)

A rod-like extension of the diaphragm plate to permit convenient external connection(usually to the valve stem).

Figure 2-1Globe Valve Typical Valve Nomenclature



2-3

Figure 2-2Gate Valve Typical Valve Nomenclature

Backpressure

Pressure on the downstream side of the valve.

Backseat (Figure 2-1)

A shoulder on the stem disc of a valve that seals against a mating surface inside thebonnet to act as a back-up seal to the packing to limit stem seal leakage.



2-4

Belleville Spring

A cone-shaped washer/disc spring used where small deflections and relatively highloads are required.

Bellows Seal Bonnet (Figure 2-24)

A bonnet that uses metal bellows for sealing against leakage of controlled fluid aroundthe valve stem.

Block and Bleed

The capability of obtaining a pressure seal across the upstream and downstream seatsof a valve, usually a gate valve, when the body pressure is bled off to the atmospherethrough blowdown valves or vent plugs. This is useful in testing the integrity of seatshut-off and in accomplishing minor repairs under line pressure. It is also useful inkeeping different process fluids separated. See Double Block and Bleed.

Body (Figures 2-1 and 2-2)

The principal pressure-containing part of a valve where the closure element and seatsare located.

Bonnet (Figures 2-1 and 2-2)

• The separable portion of the valve pressure boundary that permits access to theinternals.

• The major part of the bonnet assembly, excluding the sealing means.

• The top pressure-containing part of a valve, attached to the body, that guides thestem and adapts to extensions or operators.

Bonnet Assembly

An assembly that includes the part through which a valve plug stem moves and ameans for sealing against leakage around the stem. It usually provides a means formounting the actuator.

Bore (or Port)

The inside diameter, or other control configuration, of the flow passage through a valve(for example, the diameter of the hole in the ball of a ball valve, the inside diameter of



2-5

seat rings). The bore is usually the minimum flow area when the disc is in the fullyopen position.

Boss (Figure 2-1)

A localized projection on a valve surface provided for various purposes, such asattachment of drain connections or other accessories.

Breaking Pin

See Shear Pin.

Breaking Pin Device

See Shear Pin Device.

Breaking Pressure

The value of inlet static pressure at which a breaking pin or shear pin device functions.Terms such as “breaking pressure,” “force,” “load,” or “torque” are used to identify theload for which the intentional section of weakness is designed to fail.

Bubble-Tight Shut-Off

A phrase used in describing the sealing ability of a valve. During air pressure testing ofa valve in the closed position, leakage past the seats is bubbled through water. Toqualify as “bubble-tight,” no bubbles should be observed in a prescribed time span.

Burst Pressure

The value of inlet static pressure at which a rupture disc functions.

Bypass (Figure 2-21)

A system of pipes and valves intended to permit the diversion of flow or pressurearound a line valve or to communicate the body cavity to either the upstream ordownstream side.

Cage. (Figure 6-9A)

A hollow cylindrical trim element that is a guide to align the movement of a valve discwith a seat ring and also to retain the seat ring in the valve body. Often the walls of thecage contain openings that determine the flow characteristics of a control valve.



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Capacity

Rate of flow through a valve under stated conditions of pressure drop and fluiddensity.

Chatter

Rapid reciprocating or vibrating motion of the valve disc during which the disccontacts the seat. In mid-stroke, a valve may chatter on its guides or cage withouttouching the seat.

Closing Pressure

The value of the decreasing inlet static pressure at which the valve disc of a safety valvere-established contact with the seat or at which lift becomes zero.

Closure Element (Figures 2-1 and 2-2)

The moving part of a valve, positioned in the flow stream, that controls flow throughthe valve. Ball, gate, plug, clapper, disc, etc., are specific names for closure elements.

Coefficient of Discharge

The ratio of the measured flow capacity to the theoretical flow capacity.

Control Valve

A power-operated device that modifies the fluid flow rate in a process control system.It consists of a valve connected to an actuator mechanism that is capable of changingthe position of a flow-controlling element in the valve in response to a signal from thecontrolling system.

Cv (Valve Flow Coefficient)

The number of gallons of water at 60°F (15.6°C) that will flow through a given valvewithin 1 minute, with a pressure drop (loss) of 1 psi (6.9 kPa).

Dead Band (Diaphragm Actuator)

The amount that the actuating pressure on the diaphragm can be varied withoutinitiating valve disc motion.



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Design Pressure

The pressure used in the design of a valve and other pressure-retaining components forthe purpose of determining the minimum permissible wall thickness. When applicable,static head should be added to the design pressure to determine the thickness of thepressure-retaining components. There are slight differences in the exact definition ofthe design pressure used by different codes; therefore, the definition from theapplicable code, such as ASME, must be used.

Design Temperature

The temperature that is used to determine allowable stresses for the purpose of designcalculations. Generally, the design temperature is set at a value higher (or further fromambient) than the operating temperature and includes allowances for upsets andvariation in operating conditions.

Diaphragm (Figure 2-1)

A flexible pressure responsive element that transmits force to the diaphragm plate.

Diaphragm Actuator (Figure 2-1)

An assembly utilizing fluid pressure acting on a diaphragm to develop a force to movethe actuator stem. It may or may not have a spring for positioning and return of theactuator stem.

Diaphragm Pressure Span (or Range)

Difference between the high and low values of the diaphragm pressure range. Thismay be stated as an inherent or installed characteristic.

Direct Acting Actuator (Figure 6-19)

A diaphragm actuator in which the actuator stem extends with increasing diaphragmpressure.

Disc (Figure 2-1 and 2-2)

The closure element of a gate, globe, check, butterfly, safety, or relief valve. The disc indifferent valve designs may be referred to as gate, wedge, poppet, or plug.

Discharge Area

See Actual Discharge Area.



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Double Block and Bleed

The capability of a valve to isolate the body cavity from line pressure when the valve isin either the fully closed or fully open position. (See Block and Bleed for this operationwith the valve in only the closed position.) In open position, pressure energized seat-ball valves and through-conduct gate valves can effectively shut off the system pressurefrom entering the valve body cavity from either the upstream or downstream side,permitting the integrity of the seats to be checked with the closure member in the openposition.

Dynamic Unbalance

The net force produced on the valve disc in any stated open position by the fluidpressure acting upon it.

Effective Area

In a diaphragm actuator, the effective area is that part of the diaphragm area that iseffective in producing a stem force. (The effective area of a diaphragm may change as itis stroked, usually being maximum at the start and minimum at the end of the travelrange. Molded diaphragms that incorporate convolutions have less change in effectivearea than flat sheet diaphragms.)

Equal Percentage Flow Characteristic

An inherent flow characteristic that, for an equal increment of rated travel, will ideallygive an equal percentage change of the flow coefficient.

Explosion Rupture Disc Device

A type of rupture disc device designed for use at high rates of pressure rise.

Extension Bonnet (Figure 2-22)

A bonnet with an extension between the packing box assembly and bonnet flange tothermally isolate the stem packing from the process fluid.



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Fail-As-Is1

A characteristic of a particular type of actuator that, upon loss of power supply, willcause the valve plug, ball, or disc to remain in the position attained at the time of theloss of external actuating power.

Fail-Closed1

A condition wherein the valve disc will move to the closed position upon loss ofexternal actuating power.

Fail-Indeterminate1

A characteristic of a particular type of actuator that, upon loss of power supply, canmove to any undefined position.

Fail-Open1

A condition wherein the valve disc will move to the open position upon loss of externalactuating power.

Fail-Safe1

The selection of fail-as-is, fail-closed, or fail-open action that avoids an undesirableconsequence in a fluid system.

Field Serviceable

A statement indicating that normal repair of the valve or replacement of operatingparts can be accomplished in the field without return to the manufacturer.

Fire Safe

A statement associated with a valve design that is capable of passing certain specifiedleakage and operational tests during and after exposure to fire of specified conditions.

1 In addition to the loss of actuator power, a loss of actuator signal should be considered in determining the failureposition of the valve disc.



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Flow Characteristic

Relationship between flow through the valve and percent rated travel as the latter isvaried from 0 to 100%. This is a special term. It should always be designated as eitherinherent flow characteristic or installed flow characteristic.

Flow Coefficient

See Cv.

Flow Rating Pressure

The inlet static pressure at which the relieving capacity of a pressure relief device ismeasured for rating purposes.

Flutter

Rapid reciprocating motions of the valve disc during which the disc does not contactthe seat or body.

Fusible Plug Device

A type of non-reclosing pressure relief device designed to function by yielding ormelting a plug of suitable melting temperature material.

Gate (Figure 2-2)

The closure element of a gate valve.

Globe Valve (Figure 2-1)

A basic control valve type that gets its name from the globular shape of its body. Itnormally uses the basic valve disc as its valve closure member.

Hard Facing

A surface preparation in which an alloy is deposited on a critical valve surface (forexample, seat, guide, disc), usually by weld overlay or spray coating techniques, toincrease resistance to wear, galling, abrasion, and corrosion.

High-Recovery Valve

A valve design that dissipates relatively little flow stream energy due to streamlinedinternal contours and minimal flow turbulence. Therefore, pressure downstream of the



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valve vena contracta recovers to a high percentage of its inlet value. (Straight-throughflow valves, such as rotary-shaft ball valves, are typically high-recovery valves.)

Inherent Diaphragm Pressure Span (or Range)

The high and low values of pressure applied to the diaphragm to produce rated valveplug travel with atmospheric pressure in the valve body. (This range is often referred toas a “bench set” range since it is the range over which the valve will stroke when it isset on the work bench.)

Inherent Flow Characteristic

Flow characteristic when constant pressure drop is maintained across the valve.

Inherent Rangeability

Ratio of maximum to minimum flow coefficient within which deviation from thespecified inherent characteristic does not exceed some stated limit.

Inlet Size

The nominal pipe size of the inlet of a valve, unless otherwise designated.

Installed Diaphragm Pressure Span (or Range)

The high and low values of pressure applied to the diaphragm to produce rated valveplug travel with stated conditions in the valve body. (It is because of forces acting onthe valve plug that the installed diaphragm pressure range can differ from the inherentdiaphragm pressure range.)

Installed Flow Characteristic

Flow characteristic, when pressure drop across the valve varies, as dictated by flow andrelated conditions in the system in which the valve is installed.

Lantern Ring (Figure 2-20)

A spacer installed between packing sets to permit injection of sealant or lubricant intothe packing area, or as a leak-off collection chamber from which leakage past the firstset is piped to a safe location.



2-12

Leak Test Pressure

The specified inlet static pressure at which a quantitative seat leakage test is performedin accordance with a standard procedure.

Leakage

Quantity of fluid passing through an assembled valve when the valve is in the closedposition under stated closure forces with pressure differential [6.12].

Linear Flow Characteristic

An inherent flow characteristic that can be represented ideally by a straight line on arectangular plot of percent of related flow coefficient (Cv) versus percent rated travel.(Equal increments of travel yield equal increments of flow at a constant pressure drop.)

Live Loading (Figure 2-30)

A term used in reference to stem packing stuffing box arrangements to denote that thepacking gland follower is loaded through springs in order to minimize loss of packingload due to packing consolidation and wear.

Lock-Up Valves

A device used to retain air pressure on a pneumatic actuator or chamber upon loss ofair supply, causing the valve to fail as is.

Low-Recovery Valve

A valve design that dissipates a considerable amount of flow stream energy due toturbulence created by the contours of the flow path. Consequently, pressuredownstream of the valve vena contracta recovers to a lesser percentage of its inlet valuethan is the case with a valve having more streamline flow path. (Although individualdesigns vary, conventional globe-style valves generally have low pressure recoverycapability.)

Lower Valve Body

A half housing for internal valve parts having one flow connection. For example, thehalf housing of a split body valve.



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Maximum Allowable Working Pressure (MAWP)

The maximum pressure permissible in a pressure-retaining component at a designatedtemperature. This pressure is based on the nominal thickness of the component,exclusive of allowances for corrosion and thickness required for loadings other thanpressure. Maximum allowable working pressure is also used as the basis for thepressure setting of the pressure relieving devices protecting the component.

Maximum Allowable Pressure Drop

The maximum flowing or shutoff pressure drop that a valve can withstand. While themaximum inlet pressure is commonly dictated by the valve body, maximum allowablepressure drop is generally limited by the internal controlling components (plug, stem,disc, shaft, bearings, and seals).

Non-Rising Stem Gate Valves (Figure 4-1B)

A gate valve having its stem threaded into the gate. As the stem turns, the gate moves(for example, from the closed to the opened position), but the stem does not rise. Stemthreads are exposed to line fluids.

Outlet Size

The nominal pipe size of the outlet of a valve, unless otherwise designated.

Outside Screw And Yoke (OS&Y) (Figure 4-2)

A valve in which the fluid does not come in contact with the stem threads. The stemsealing element is between the valve body and the stem threads.

Packing (Stuffing) Box Assembly (Figure 2-28)

The part of the bonnet assembly used to seal against leakage around the valve stem,including various combinations of all or part of the following: packing gland, packingnut, gland follower, lantern ring, packing spring, packing flange, packing flange studsor bolts, packing flange nuts, packing ring, packing wiper ring, and felt wiper ring.

Packing Gland (Figure 2-28)

The piece that compresses the packing.



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Passive Valve

A valve that maintains obturator position and is not required to change obturatorposition to accomplish its intended function(s).

Piston Actuator

A fluid pressure operated piston and cylinder assembly for positioning the actuatorstem in relation to the operating fluid pressure or pressures.

Pilot Valve

An auxiliary valve that, when actuated, causes the actuation of a main valve.

Plug

See Closure Element.

Port

The flow control orifice of a control valve. It is also used to refer to the inlet or outletopenings of a valve.

Port Guided (Figures 5-1, 5-2)

A design in which the valve plug is aligned by the body port or ports only.

Pressure-Containing Member

A part of the component that is in actual contact with the pressure media.

Pressure-Retaining Member

A part of the component that is stressed due to its function in holding one or morepressure-containing members in position.

Push-Down-to-Close Construction

A globe-style valve construction in which the valve plug is located between theactuator and the seat ring, so that extension of the actuator stem moves the valve plugtoward the seat ring, finally closing the valve.



2-15

Push-Down-to-Open Construction

A globe-style valve construction in which the seat ring is located between the actuatorand the valve plug, so that extension of the actuator stem moves the valve plug awayfrom the seat ring, opening the valve.

Quick Opening Flow Characteristic

An inherent flow characteristic in which there is maximum change in flow coefficientwith minimum stem travel.

Rangeability

Ratio of maximum to minimum flow coefficient (Cv) within which the deviation fromthe specified flow characteristics does not exceed stated limits.

Rated Cv

The value of Cv at the rated full-open position.

Rated Lift

The design lift at which a valve attains its rated flow capacity.

Rated Travel

Linear movement of the valve plug from the closed position to the rated full-openposition. (The rated full-open position is the maximum opening recommended by themanufacturer.)

Reseating Pressure

The pressure at which the pressure relief valve reseats after discharge.

Reverse-Actuating Actuator

A diaphragm actuator in which the actuator stem retracts to the actuator withincreasing diaphragm pressure.

Rising Stem (Figure 4-1A)

A valve stem that rises as the valve is opened.



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Seat (Figures 2-1 and 2-2)

That portion of the valve internals contacted by a valve closure member to achieve ashutoff.

Seat Angle (Figure 2-9)

The angle between the axis of the valve stem and the seating surface. A flat seatedvalve has a seat angle of 90°.

Seat Area

The area determined by the inside and outside diameters of the seat.

Seat Diameter

The smallest diameter of contact between the fixed and moving portions of the pressurecontaining element of a valve.

Seat Load

The contact force between the seat and the valve plug.

Seat Ring (Figures 2-1 and 2-2)

A separate piece inserted in a valve body to form a valve seat.

Secondary Orifice

The ring-shaped opening at the exit of the huddling chamber of a safety valve.

Separable Flange

A removable flange that fits over a valve body flow connection, generally held in placeby a retaining ring.

Set Pressure

The value of increasing inlet static pressure at which a pressure relief valve displaysone of the operational characteristics as defined under opening pressure, poppingpressure, or start-to-discharge pressure, depending on service or as designated by theapplicable code or regulation. It is one value of pressure stamped on the pressure reliefvalve.



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Shear Pin

The load-carrying element of a shear pin device. It is an intentional section of weaknessor minimum strength used to protect other valve or actuator components. It should beeasily identifiable and replaceable with minimum effort.

Shear Pin Device

A type of non-reclosing pressure relief device actuated by inlet static pressure anddesigned to function by shearing a load-carrying pin that supports a pressure-containing member.

Static Unbalance

The net force produced on the valve disc in its closed position by the fluid pressureacting upon it.

Stem (Figures 2-1 and 2-2)

A rod or shaft transmitting force/torque from an operator to the closure element of avalve to change its position.

Stem Connector (Figure 2-1)

A fitting to connect the actuator stem to the valve stem.

Stem Guided (Figure 2-1)

A special case of top guided construction in which the valve disc is aligned by a guideacting on the valve stem.

Stem Unbalance, Stem Rejection Force, or Piston Effect

The net force produced on the valve disc stem in any position by the fluid pressureacting upon it.

Stuffing Box (Figure 2-28)

The annular chamber provided around a valve stem in a sealing system into whichdeformable packing is introduced.



2-18

Through Conduit

An expression characterizing valves that, in the open position, present a smoothuninterrupted interior surface across the seat rings and through the valve port, thusaffording minimum pressure drop. There are no cavities or large gaps in the borebetween seat rings and body closures or between seat rings and ball/gate.

Top Guided (Figure 2-1)

A design in which the valve plug is aligned by a single guide in the body, adjacent tothe bonnet or in the bonnet.

Top and Bottom Guided (Figure 6-4)

A design in which the valve plug is aligned by guides in the body or in the bonnet, andin the bottom flange. The plug is guided above and below the seat.

Top and Port Guided

A design in which the valve plug is aligned by a guide in the bonnet or body, and thebody port.

Trim

The internal parts of a valve that are in contact with line fluid other than the body andbonnet (usually consisting of the seat ring, valve plug, stem, valve plug guide, guidebushing, and cage.)

Trunnion

A trunnion is a reinforced area, similar to a boss, that houses opposing pivots, journals,and other mechanical devices (for example, packing), generally cylindrical in shapeand projecting from the exterior of each side of the piece. In butterfly, ball, and plugvalve bodies, trunnions provide the support for the shaft journal bearings, thrustbearings, packing, and actuator mounting. (The ball in a ball valve may have trunnionsthat mate with the sleeve bearings).

Upper Valve Body

A half housing for a split-body type valve.

Valve Body Assembly

An assembly of a body, bonnet assembly, and bottom flange.



2-19

Valve Plug (Figure 2-1)

A movable part that provides a variable restriction in a port.

Valve Plug Guide (Figure 6-5)

That portion of a valve plug that aligns its movement in either a seat ring, bonnet,bottom flange, or any two of these.

Valve Plug Stem (Figures 2-1 and 2-2)

A rod extending through the bonnet assembly to permit positioning the valve plug.

Vena Contracta

The location where the cross-sectional area of the flow stream is at its minimum size,where fluid velocity is at its highest level, and fluid pressure is at its lowest level.

Wire Drawing

Erosion caused by small high velocity jets in closely spaced surfaces, or by cavitation orliquid droplet impingement. Usually occurs when the disc is closed, but someunintentional gap due to local damage or particulates causes the surfaces to not be inintimate contact.

Yield Temperature

The temperature at which the fusible material of a fusible plug device becomessufficiently soft to extrude from its holder and relieve pressure.

Yoke (Figures 2-1 and 2-2)

A structure by which the valve actuator assembly is supported rigidly on the bonnetassembly.

2.2 Common Valve Construction Features

Details of construction common to most valves are related to the minimum requiredcomponents to achieve pressure and seating integrity and to actuate the valve.Although some variance may be found between manufacturers, these commonconstruction feature serve the same basic functions of connecting the body and bonnet,shutting off pressure, connecting the stem to the disc, and sealing around the movablestem.



2-20

2.2.1 Body-to-Bonnet Connections

The bonnet can be a removable portion of a valve connected to the body by screwing,flange bolting, welding, or a pressure sealing mechanism. In some cases, the bonnetmay be an integral part of the valve body. Removal of the bonnet generally providesaccess to the valve trim, except in end-entry valves such as ball and butterfly valves.For the end-entry valves, access to the trim is through the inlet or outlet ports orthrough the body joint.

Screwed Bonnet: The screwed-in bonnet type valve, shown in Figure 2-3, is one of thesimplest and least expensive constructions; it is commonly limited to valve sizes up to 3inches. In valves larger than 3 inches, the tools and torque required to tighten the jointbecome too cumbersome. Threaded joints should be avoided where thread corrosion orgalling can make disassembly difficult. There are two variations of the threaded joint:one where the bonnet is screwed directly onto the body, the other where a union isused. Screwing the bonnet directly onto the body requires that the gasket or groundjoint accommodate itself to rotating faces, and frequent unscrewing the bonnet maydamage the joint faces. Another disadvantage of these joints is the variability in thecircumferential alignment between the bonnet and body, because the final assemblyposition is dependent on the number of turns required during threading. Threadedjoints, however, offer the advantage of being easily seal welded to provide a redundantseal or to eliminate the joint seal altogether.

Joining the bonnet to the body using a union ring offers the advantage of preventingmotion between the joint faces as the two are being made up, thereby permittingrepeated unscrewing of the bonnet without damaging the joint faces or seals. Unionsalso prevent accidental unscrewing of the bonnet by the operator.

Figure 2-3Screwed Bonnet



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Flanged (Bolted) Bonnet: Flanged bonnet joints, such as those found in valves shown inFigure 2-4, have an advantage over the screwed joint in that smaller tools and lowertorque are required to tighten the joint. Flanged joints can be used on any size valve,under any operating pressure, but they become very bulky and heavy when used onvery large valves and under high operating pressures. At temperatures above 650°F(343°C), creep relaxation can, in time, noticeably lower the bolt load and allow the jointto leak. If the application is critical, the flanged joint can be seal welded.

Figure 2-4Flanged (Bolted) Bonnet

Welded Bonnet: Welding the bonnet to the body effectively provides a very economicaland long-term seal regardless of size, operating pressure, and temperature. Thisarrangement can be used to achieve both a sealing function and a load carryingfunction, as shown in Figure 2-5. When coupled with screwed or flanged joints, theweld joint is designed to seal only against pressure and requires minimal weldmaterial. Except for cast iron, welding can be performed on most materials.

This arrangement is used where the valve is expected to be maintenance-free for longperiods, where the valve is a throw-away design due to its relative cost to replaceversus repair, or where the required sealing reliability of the valve far outweighs thedifficulty of gaining access to valve internals, such as in bellows-sealed stem valves.



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Figure 2-5Welded Bonnet

Pressure Sealed Bonnet: The pressure sealed bonnet design, shown in Figure 2-6,provides the advantage of reduced weight and size over flanged connections andallows the internal bonnet pressure to increase the joint sealing contact stress instead ofunloading it as in bolted designs. This joint is most attractive in larger valves and highpressure applications where the pressure forces are high enough to generate therequired contact stress to seal at the metal-to-metal joint. This type of bonnet seal isusually available only on valves of pressure class 600 or higher. It is particularly suitedto high temperature applications (660°F or 348.9°C) where bolted bonnet joints canloosen due to bolt creep. One of the disadvantages of this type of bonnet joint is that itprovides no positive mechanical location between the bonnet and body and oftenallows misalignment to occur, which can cause stem binding. Binding can lead to stemgalling, leakage through stem packing, and potential valve inoperability. In largevalves, proper assembly of the bonnet usually requires the valve to be installed withthe stem vertical and pointing upward.

Figure 2-6Pressure-Sealed Bonnet



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Another drawback of the pressure seal bonnet joint is that it can start to leak inapplications where frequent pressure or temperature fluctuations are experienced;therefore, the bonnet cannot be safely tightened under pressure when a leak occursbecause of the possibility of making the leakage more severe when attemptingcorrective action. In addition, if a leak should occur, it is more difficult to repair andreassemble the valve than with a bolted bonnet due to the required careful alignmentand tightening sequence procedures during assembly.

Graphite pressure seals have seen wide acceptance in the valve industry because theycan eliminate many of the problems associated with metal pressure seals. Most majorvalve manufacturers offer graphite pressure seals for their product lines. Utilizinggraphite pressure seals requires special precautions to prevent extrusion and to ensureadequate loads to effect a seal.

2.2.2 Seat and Seat Rings

The seat is the fixed pressure-containing portion of a valve that comes in contact withthe closure member of the valve. The valve seat material(s) must be consistent with anymaterial restrictions for the valve application. The seat can be all metal construction ormay incorporate soft conforming seat inserts, such as elastomers or plastics, to make atighter seal or to reduce the required load to seal.

For seat tightness, the objective is to block off, or minimize, the path formed by the“valleys” on the seating surface. An enlarged view of the valleys in mating surfaces isshown in Figure 2-7. Filling in the valleys requires that the compressive stresses in themating surfaces be of sufficient magnitude to elastically or plastically deform themating surfaces until the leak path is blocked off.

Figure 2-7Seat Joint Mating Surfaces (Lay of Roughness Concentric)



2-24

In addition to the basic design of the seat itself, other factors that directly affect seatingand operability are distortions that can occur at the disc/seat interface due to pressure,thermal gradients, and mechanical loads transmitted to the valve body by the adjacentpiping. As shown in Figure 2-8, applied bending moments on gate valve bodies causethe seat plane to tilt and distort, which can result in leakage and gate pinching inwedge-type valves. Gate pinching can also be caused by thermally induced deflections(see Section 4.2.10). In globe valves, body distortions produce ovality in the seat, whichleads to mismatch with the circular seating area on the tapered seated plugs.Distortions caused by line loads become more severe when venturi-type valves orvalves that are smaller than the pipeline size are installed with upstream anddownstream reducers.

Figure 2-8Seat Plane Distortion under Vertical and Horizontal Bending Moments

To avoid leakage or binding problems caused by line loads, valves should not belocated at points of large line loads. Also, the section modulus of the valve body shouldbe significantly greater than the pipe to keep the stresses and distortions withinacceptable limits. Axisymmetric type valves, such as ball and butterfly, tend to bestiffer and are less sensitive to line loads.



2-25

Metal-to-Metal Seating: When using metal-to-metal seating, the high compressivestresses required to produce surface conformance between the two seating surfaces areachieved by making narrow line-to-line contact between the disc or plug and the seat.Narrow line-to-line contact should provide for a certain minimum width in order toestablish a tight seal and prevent indentation type of damage caused by the plug on theseat. In addition, the seat should have enough base width to provide adequate backupcross-section capable of supporting the high compressive stress at the disc-seat interfacewithout yielding the base material. In control valves, seat loading is usually expressedas pounds of force per linear inch of mean seat joint circumference. For globe typecontrol valves using line-to-line contact, loading may vary from 25 to 600 pounds perinch (4.4 to 105 N/mm), and most manufacturers rely on their own tests to developspecific magnitudes. Based on the ISA Handbook of Control Valves [5.1], typical valuesare:

1. 25 pounds per inch (4.4 N/mm) - Low pressure drop service, leak-tight shut-off isnot required.

2. 50 pounds per inch (8.8 N/mm) - Moderate pressure drop service, slight leakageexpected (0.1% Cv maximum).

3. 100 pounds per inch (17.5 N/mm) - Nearly drop-tight service (0.1% Cv maximum)will seal 3,000 psi (20.7 MPa) pressure drop on 0.015 inch (0.381 mm) width, 30°joint of 316 SS.

4. 300 pounds per inch (52.5 N/mm) - Drop tight service (will seal 6,000 psi (41.4 MPa)on 0.025 inch (0.635 mm) width, 20° joint of AISI 440-C SS, hardness 55 Rc).

5. 600 pounds per inch (105 N/mm) - Pressure service greater than 6,000 psi (41.4MPa).

Although the apparent average compressive seating stress on Items 3 and 4 is 13,000 psi(89.6 MPa) and 35,000 psi (241.3 MPa), which is less than the yield strength of thematerial, localized contact stresses at the peaks of the surface irregularities are muchhigher, thus providing the surface yielding needed to accomplish a seal.

The required degree of seat tightness and accompanying stem thrust should bereasonably selected. Specifying high seat tightness increases the size and cost of theactuator needed to develop the higher loads.

As an alternative to using high contact forces, the mating surfaces can be“superfinished” to achieve a good seal. However, this superfinish can degrade quicklyin applications where fluid contaminants are present that can get trapped between themating surfaces during opening and closing action. Another common method used to



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accomplish a seal is to lap the disc and seat during assembly; however, lapping thesurfaces should be limited so that a wide contact band does not develop.

Developing high compressive stress to achieve good seating should be weighed againstpotential damage due to galling or gross surface yielding. Surfaces that slide underload, such as the disc of a gate valve, should be sized so that contact stress ismaintained below the galling threshold for the material combination. Depending on themating materials and the details of the actual geometry, the calculated average contactstress to gall can vary from as low as 2,000 psi (13.8 MPa) for some stainless steels to ashigh as 47,000 psi (324 MPa) for cobalt-based materials such as Stellite 6 [5.39]. Inreality, the contact stress at the surface is not uniform due to the irregular and unevenloading encountered in actual application; therefore, the average contact stress shouldbe limited to lower values. Typically, the contact stress for Stellite 6 is limited to 20,000psi (137.9 MPa) to avoid galling in sliding applications.

In gate valves, the sliding surfaces may encounter one or more of the following contactmodes during an opening or a closing stroke: flat-on-flat, edge-on-flat, edge-on-edge(nonscissoring), and edge-on-edge (scissoring) [2.1, 2.2, 2.9, 2.10]. The contact modedepends primarily on the edge geometry of the seats and guides, and on the length andlocation of the body guides with respect to the disc guides. The magnitude of contactstress and associated wear/damage is proportional to the valve internal clearances (forexample, guide rail-to-guide slot clearance and stem head-to-gate clearance) and to thevalve operating conditions. Some contact modes may cause severe damage to theseating surfaces and result in seat leakage.

Typical seating configurations employed in globe valves are shown in Figure 2-9. Theseat design shown in Figure 2-9A, used in low pressure globe applications, providesthe advantage of not requiring precise alignment between the disc and seat, and iteliminates galling because the surfaces move normal to each other during loading. Theseat design shown in Figures 2-9B, 2-9C, and 2-9D allows higher contact stress to bedeveloped due to the narrower contact band between the mating surfaces; however, itrequires better control of alignment between the disc and seat. As shown in Figures 2-9C and 2-9D, taper angles (half-cone angle) between 15° and 45° are in use in variousdisc designs. Even though small taper angles (as low as 15°) have been used in somevalves, they should be avoided because it has been found that for reliable nonstickingoperation of the disc, magnitudes of 30° and higher should be used.



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Figure 2-9Typical Globe Valve Seating Configurations

When selecting the disc/seat contact geometry and materials, the potential for crossring indentation type damage should also be considered. Seat ring indentation, asshown in Figure 2-10, is caused when a hard narrow surface and a soft wider surfacecontact, and the softer material yields. Indentation left on the softer component cancreate leakage during subsequent shut-off if the normal clearances present in theassembly of the plug-to-seat components allow the new seating band to cross theprevious indentation. Cross ring indentation damage and its adverse effect on shutoffcan be prevented by making the narrower component of a softer material than thewider component.



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Figure 2-10Cross Ring Indentation

Soft Seating: Soft seats are used to accomplish good seating with much lower contactforce than in metal-to-metal seats. It is easier to deform the softer materials and fill outthe valleys in the mating surfaces with considerably lower forces. In most designs, thesoft seat rings provide the primary seating with metal-to-metal closure acting as asecondary seal in case of damage or failure of the soft seal material. This secondarymetal-to-metal contact also makes the seats fire safe and allows some degree of seattightness should the resilient seat ring fail. Whenever the temperature, radiation, andpressure environment permit, soft seals should be strongly considered because of theease in accomplishing good seating with low contact force.

Since soft seat ring materials do not have the required strength and stiffness to resistrupture against pressure and blowout against differential pressure, they must besecurely clamped in the seat. Several methods of restraining the seat ring in globevalves are shown in Figure 2-11. Similar restraint methods are employed in some gateand butterfly valves.



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Figure 2-11Soft Seat Retention Methods

At operating temperatures, the material properties to be considered in the selection of asoft seat ring are:

1. Fluid compatibility including chemical reaction, swelling, loss of hardness,permeability, degradation

2. Room for thermal expansion



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3. Hardness

4. Permanent set and extrusion under load

5. Rate of recovery upon removal of the load

6. Tensile, compressive, and tear strength

7. Radiation resistance

8. Abrasion resistance

9. Wear resistance

10. Thermal resistance

The material properties of seals, soft seats, and gasket materials are discussed in detailin Section 2.5.

Seat Attachment: The method of attachment and sealing of the leakage path between theseat and the body is as important as the seat itself. Methods of attaching fixed seats tothe body (Figure 2-12) include screwing, welding, interference fitting the seat ring intothe body or seat pocket using press or shrink fits, bolting, clamping between twopieces, and welding and machining the seat face into the body. Sealing at the body isachieved using elastomers, gaskets, soft metals, metal-to-metal sealing by interference,and seal welding.



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Figure 2-12Methods for Attaching Seat to Body

The seat-to-body restraint should be independent of the seat loading and should notdepend on the seat load to achieve a seal. Inherent in fixed seat designs is the problemthat body distortions, caused by pressure, thermal gradients, and line loads, aretransmitted directly to the seat. These distortions create leakage paths between the disc-to-seat mating surfaces in metal-to-metal seating unless some flexibility is designed intothe disc (as in flex disc gate valves) or globe valve seat, as shown in Figure 2-13. Thetype of attachment to the body should also consider maintenance that may be requiredon the seat.



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Figure 2-13Flexible Seat

When using gaskets, the seat should incorporate a metal-to-metal stop as shown inFigure 2-11B to limit the amount of compression applied to the gasket since repeatedstress cycling of the gasket will lead to relaxation of the joint seal and eventual leakage.Metal type gaskets should not be reused unless explicitly permitted by the gasketvendor.

Floating seats, such as those used in trunnion-mounted ball valves, do not requireindependent restraints but are held in place by the ball itself. Sealing of the floatingseat in the body is accomplished using elastomeric or packing seals, as shown in Figure2-14.

Figure 2-14Floating Seat



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One ball valve design for high temperature service applications uses spring-loadedpacking seals as shown in Figure 2-15.

Figure 2-15Spring-Loaded Packing Seals



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2.2.3 Disc-to-Stem Connection

The disc-to-stem connection joint, which transmits the load from the actuator to thedisc, should be designed to have equal or greater strength than the stem itself.However, only the American Petroleum Institute (API) Code imposes this requirement,but several valve manufacturers supplying valves to the power industry do not followthis guideline. Depending on valve type, the joint can be fixed, be free to rotate, orallow freedom for the disc to float laterally, as shown in Figure 2-16.

Figure 2-16Stem Connections



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Fixed Joints: Typical fixed joint disc-to-stem connections commonly used are integral,welded, and screwed. These connections are normally used in non-rotating stemapplications where the stem and seat maintain their axial alignment. When this joint isused in a globe valve application, an external means to prevent stem rotation should beprovided. This is necessary to prevent galling at the disc-seat interface. When used inwedging-type gate valves, precautions should be taken to ensure that excessive lateraldisplacement of the stem would not cause binding of the stem in the bonnet stuffingbox area.

Free to Rotate Connections: Free to rotate connections should be used in rotating stemapplications or when disc-to-seat rotation is undesirable. These joints are frequentlyfound in globe valves and non-rising stem gate valves. In non-rising stem gate valves,the disc-to-stem joint is threaded so that the rotation of the stem in the disc opens andcloses the valve (Figure 4-1). Most free to rotate connections provide some limitedlateral disc displacement to prevent stem binding and allow the disc to align itself withrespect to the seat face. Free to rotate connections should incorporate some means ofpreventing the disc from spinning. Asymmetric flow created by multiple elbowsupstream can cause the disc to spin; in fact, this has occurred in some swing checkvalve designs [1.20]. Spinning discs can damage the disc and seat upon contact, and cancause premature failure of the connection due to excessive wear.

Laterally Floating Connections:. Floating connections are generally T-slot designs thatpermit assembly of the joint by simply sliding the parts together in a lateral direction.The T-slot is usually oriented in the direction of the flow (that is, in line with theexpected disc displacement) to permit sliding to occur without causing stem binding.

These joints are most commonly found in gate valve applications where the gatereceives its alignment from guides in the body during the complete open to close cycle.These joints incorporate an anti-rotation type feature, such as a square head, to preventstem rotation.

As shown in Figure 2-16C, another type of design found in power plants uses a doublearticulated link type stem-to-disc connection to allow the wedge to float freely in thelateral direction.

2.2.4 Disc/Stem Guide Arrangements

Guides are required for certain valve types to provide proper alignment of movingparts to prevent poor valve performance or inoperability. These guides aremanufactured from soft materials or from very hard anti-wear or anti-galling materials,depending on the application and service.



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Wedge Gate Guides: Gate guides (Figure 2-17) are provided specifically on wedge gatevalves to keep the gate away from the seat faces, except for a small distance very nearthe fully-closed position, so as to minimize wear on the seating faces. The disc can slideon the guide in either flat or tipped orientation, depending upon the details of the valveinternal geometry (for example, guide length and guide clearance), the severity of the∆P load across the disc at mid-stroke positions, and the magnitude of the frictioncoefficient at the sliding interfaces. Typically, the sliding surfaces on the gate and guideare overlaid with hard-facing materials to prevent galling of the sliding interfaces, dueto the load generated by the differential pressure acting across the gate as the valve isbeing closed or opened. Under certain conditions, the localized guide stresses can causeplastic deformation as well as galling/gouging of the sliding surfaces. In some extremecases, the guide rail may break and cause the gate to stick in midstroke.

Figure 2-17Gate Valve Gate Guide



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Stem Guides: Stem guides (Figure 2-23) are most commonly found in globe valves. Stemguides, which provide alignment for the plug, are typically manufactured from softermaterials to provide some lubricity and to prevent galling of the stem. Stem guidesshould be provided where significant side loads on the plug are present. These forcescan be generated by side discharge such as in angle globe valves. Stem guides are alsoprovided when the stem is relatively long and flexible, such as in extended bonnetglobe valves.

Disc Guides: Disc guides (Figures 6-7, 6-8, and 6-9) are most commonly found in controlvalves and relief and safety valves. These guides provide alignment between the discand seats and offer lateral support for uneven fluid discharge forces.

2.3 Accessories and Special Features

The selection of accessories and special features for a valve can be as important as thevalve itself and, in some cases, actually controls the type of valve selected. Controlvalves have a larger selection of accessories and options available because they areoften placed in special service. Although some accessories can be used in any type ofvalve, they are suitable only for certain applications.

Accessories common to valves of all types are discussed below.

2.3.1 Manual Override Handwheels or Levers

Handwheels and levers are treated as accessories when their function is not necessaryto the normal operation of the valve. They are provided as alternatives to the normalmeans of actuation, whether it is self-actuation (Figure 2-18) or power actuation (Figure2-19). These accessories provide the means to locally actuate the valve during abnormalvalve operation, when the actuator malfunctions, or during valve testing ormaintenance. Manual handwheels or levers, whether used as an accessory or as theprimary means of actuating the valve, should be sized so that no more than 150 pounds(667 N) of force is required during any period of the actuation. Manual handwheels orlevers should be sized so that normal access to the valve is not hindered. Handwheelrim force limit, as a function of the handwheel size, is given in Table 14-1 in Section 14.Section 14 also provides further guidance on manual actuator sizes and accessrequirements.



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Figure 2-18Manual Override Lever on Pressure-Relief Valve

Figure 2-19Manual Override Handwheel on Motor-Operated Valve



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2.3.2 Stem Leak-Off Connection

Stem leak-off connections (Figure 2-20) are used when it is necessary to keep the systemfluid, such as radioactive contaminated water, from leaking into the atmosphere. Insome applications, stem leak-off connections are used in reverse direction to providevacuum seal for valves connected to the condenser by injecting water into the lanternring. A fitting is provided adjacent to the lantern ring location, between the upper andlower packing sets. Fluid leaking past the lower packing set is captured and piped offto a collection reservoir before having a chance to escape to the ambient environment.The leak-off connection is also used in some applications to periodically check packingleakage. In these applications, the upper packing set is designed to the samerequirements as the lower set but is usually not expected to seal against full bonnetgage pressure under normal operation.

Figure 2-20Steam Leak-Off Connection



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As discussed in Section 2.5.2, recent advances in valve packing technology show thatthe presence of lantern rings doubles the required number of packing rings and thepacking thrust/torque. Furthermore, leak-free packing with lower packingthrust/torque can be achieved by eliminating the lantern ring (see Section 2.5.2 fordetails).

2.3.3 Limit Switch

Externally mounted-limit switches are installed on manually operated valves toprovide an indirect indication of the open/closed position of the stem and may be usedto provide a signal for alarms, relays, and/or indicating lights. In addition to the abovefunctions, the limit switches on valves with pneumatic or hydraulic actuators are usedto control the stem travel. In motor-operated valves, the limit switches are internallymounted within the actuator [1.22, 1.23, 1.24, 1.25, and 1.26].

2.3.4 Internal and External Bypass

Bypasses, whether internal or external, perform the function of equalizing pressure.Bypasses can be installed from the upstream side of the valve to the downstream side.Bypasses can be installed between the body and the upstream or downstream side toprevent pressure locking (see Section 4.2.8).

External bypasses can be specified to include manual valves, remotely actuated valves,relief valves, or check valves (Figure 2-21). When bypasses are installed internally, theyperform the function of communicating the body cavity pressure to either the upstreamor downstream side of the disc. Internal bypasses are sometimes equipped with reliefvalves or check valves to accomplish specific functions. In some gate valves, a hole isdrilled in the upstream (or downstream) side of the gate to equalize the body pressureto the upstream (or downstream) pressure, thus eliminating pressure lockingconditions [4.2, 5.30]. Bypasses can result in valve leakage in one direction, thusrendering the valve unidirectional.



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Figure 2-21External Bypass

2.3.5 Remote Position Sensor

Remote position sensors are typically displacement transducers consisting of linearvoltage differential transformer (LVDT) or potentiometer devices, which remotelyindicate the open or closed position of the valve closure member. These sensors areused where a positive indication of the control valve stem position is required inresponse to a command signal. Various other electronic (digital/analog) and pneumaticdevices are also used.

2.3.6 Bonnet Extension

Bonnet extensions (Figure 2-22) are most commonly used when the stem packingrequires easier access or when the system temperature is very cold or very hot. Bonnetextension, when used in extreme temperature applications, provides a thermal barrierso the packing can perform under more suitable temperature conditions. Bonnetextensions are also used to locate the actuator at a location further removed from thevalve or the local environment, which may be difficult to reach, hazardous, oruncomfortable to the operator.



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Figure 2-22Bonnet Extension

2.3.7 Impact, Hammerblow, and Chain-Operated Handwheels

Impact handwheels are used to create higher starting torques than can be achieved by agradual application of force. Handwheels incorporate slack in the drive mechanism topermit some initial velocity to develop and to cause an impact upon engaging the stemnut. The effective impact force can be as much as two to four times the normallyapplied force. This hammerblow action can sometimes eliminate the need for reductiongears on valves. This action can be taken advantage of in both the opening and closingdirection of the valve. Chain-operated handwheels are used primarily where access tothe actuator is difficult or hazardous, and where operation of the valve is infrequent.

2.3.8 Stem Backseating Feature

Rising stem valves may have a backseat that can be used to seal the stem to the bonnetwhen the valve is in the fully open position. The backseat is provided for maintenancepurposes and should not be relied on to fulfill safety functions. In general, valvesshould not be backseated with power because the backseat might not be designed towithstand high stem thrust and damage may occur.



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The valve is opened until a shoulder on the stem or disc bears firmly against aprepared beveled surface below the packing, provided on the underside of the bonnet.This provides a metal-to-metal seal against leakage through the stem. Stem backseatingis available on both rotating and nonrotating stem valves and is commonly found ongate and globe valves. This feature is not found on 90° turn valves such as plug, ball,and butterfly.

2.3.9 Fire Safety Feature

Fire safety is a special feature that is available in many types of valves, including thosewhich use resilient materials. “Fire safety” means that the valve provides limitedsealing and seating in the event of fire for a period of time sufficient to permitemergency shutdown of the system. In power plants, most valves that use metal-to-metal seats and have high temperature stem packing materials, such as graphite, arefire safe. Where resilient seats are used, there is a backup metal-to-metal seat that takesover when the resilient member is consumed by the fire.

2.4 Valve Trim

2.4.1 Trim Components and Materials

Components of the valve considered to be trim consist of the removable or in-linerepairable internal parts contacting the flowing fluid. For example, in a globe valve theplug or disc, seats, stem, guides, bushings, and cages are trim components (Figures 2-23and 6-5). Other components considered as trim but that do not come into direct contactwith the fluid are components making up the stuffing box: packing gland, spring,lantern ring, and packing retainer ring. Secondary trim parts are stem-to-discattachment, seat retaining rings, seat-to-body seals, spacers, etc. Parts not included astrim are components that define the valve pressure boundary: body, bonnet, bodyclosures, and bonnet and body bolts and nuts.



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Figure 2-23Trim Components

Closure gaskets and seals are neither pressure boundary components nor trimcomponents but maintain leak-tightness integrity of the valve.

Stainless steels 316 SS, 410 SS, and 17-4 PH SS are the most commonly used materialsfor valve stems and other valve trim materials. Although not as corrosion resistant as316 SS, the higher strength and correspondingly higher allowable stresses make the 410SS and 17-4 PH materials much more attractive for larger sizes and higher pressurerated valves since smaller diameter stems can be used. Cobalt-free trim materials arediscussed in Section 2.4.5.

In general, trim material selection should consider all of the important factors discussedbelow, in addition to mechanical strength considerations. Additional discussions areprovided in Section 15.



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2.4.2 Design Practices to Minimize Corrosion

Corrosion in valves can be minimized or eliminated by selecting materials that do notreact with the fluid or with the material around them. There are different types ofcorrosion, and the corrosion type will dictate the selection of material required.Corrosion control is especially important in valves that are subjected to fluids that posea hazard if allowed to leak into the environment.

Corrosion is the deterioration of a metal by reaction with the environment. Corrosion isgenerally controlled by selecting corrosion resistant materials. Corrosion resistance of acomponent can be improved by plating, cladding, overlaying, or heat-treating of thewetted surfaces. The rate of corrosion is influenced by the fluid velocity media andtemperature. Table 2-1 lists some commonly used trim materials and their suitabilityfor power plant applications.

Galvanic corrosion is often found in valves due to the use of dissimilar materials for thebody and trim. Listed below is the relative galvanic series of materials, presented in theorder of most corroded (anodic) to least corroded (cathodic). Copper and platinummaterials are included in this series for reference.

• Carbon steel

• Cast iron

• Ni-resist

• Type 440-C SS

• 17-4 PH SS

• Type 316 SS

• Stellite and Colmonoy

• Nickel

• Inconel

• Copper Bronze

• Monel

• Platinum



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Table 2-1Corrosion Ranking for Materials Selection(Condensed from Reference 5.1)

Boric Acid C C A A A A A A A A A B B X X X X

Chlorine Gas A A B B A A A A A C A C C C

Chlorine Liquid C C C C B C C A B C B C C C

Freon, Wet B B B A A A A A A A A X X X

Freon, Dry B B A A A A A A A A A X X X

Hydrogen A A A A A A A A A A A A A A

Oxygen A A A A A A A A A A A A A A

Sodium Chloride C C B B A A A A A A A B B B

Sodium Chromate A A A A A A A A A A A A A A

Sodium Hydroxide A A A A C A A A A A A B B A

Sodium Hypochloride C C C C B-C B-C C A B A X C C X

Water, Boiler Feed B C A A C A A A A A A B A A

Water, Distilled A A A A A A A A A A A B B X

Water, Sea B B B B A A A A A A A C C A

Key: A – Can be or is being successfully usedB – Proceed with cautionC – Should not be usedX – Information lacking

In the above listing the electrolytic potential and the rate of galvanic corrosion betweentrim and body material is proportional to their separation.

Area differences also affect galvanic corrosion. A larger anodic area, compared to thecathodic area, is preferred because it reduces the amount of corrosion. As an example, astainless steel bolt in carbon steel body will usually cause the carbon steel to corrode atonly a slightly increased rate, whereas a carbon steel bolt in a stainless steel body willcorrode at a rapid rate because the stainless steel acts as a large cathode.



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2.4.3 Design Practices to Minimize Erosion

Erosion is wear damage in which loss of material occurs due to the action of movingparticles carried in a fluid stream. This action is most severe when the velocity of thefluid is high, such as during valve throttling or closing and opening under highpressure drops. Entrained sand, slurries, catalyst fines, and liquid particles in flashingflow are sometimes associated with this type of wear. The selection of materials for thepressure containment parts (that is, the body and bonnet) is rather limited from thestandpoint of their ability to withstand erosion; and the use of sacrificial liners at theareas of impingement has been successful. There are four principal types of erosion:

• Abrasive particle

• Cavitation

• High liquid velocity impingement

• Erosive-corrosive

These types of erosion and specific guidance regarding how to improve the resistanceof the trim materials to their effects are discussed in the following paragraphs.

Abrasive Erosion: In abrasive erosion, small particles which are harder than the trimsurface are carried at high velocity in the fluid stream and impinge upon and scouraway the trim metal. Resistance of materials to impingement erosion varies with theangle of impingement. At low impingement angles (<15° with respect to the surface),hard-facing materials with large amounts of carbides, such as Stellite 1, arerecommended. At high impingement angles (>80°), hard-facing alloys with largeamounts of relatively ductile matrix material, such as cobalt in Stellite 21, arerecommended.

Stellite 6, however, has been found to provide the best combination of erosionresistance and wear resistance as a trim material for the widest range of valvegeometries that have large variations in impingement angles. However, Stellites withtheir cobalt content will be activated if they are in fluids that are transported throughthe reactor core region, thus creating a radiation concern in that the cobalt may plateout on the interior walls of a piping system or be captured in crevices. Cobalt-facealternatives are discussed in Section 2.4.5.

Cavitation Erosion: Cavitation occurs as the result of vapor bubbles forming when thepressure of a liquid flowing in the restricted passages of a valve becomes less than thevapor pressure of the liquid at that temperature. The bubbles then collapse as the flowarea enlarges and the pressure recovers. The implosion of bubbles produces shock



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waves and very high localized stresses at the surface of the metal, causing the materialto fail and detach from the surface.

Since no known material can withstand continuous severe cavitation service withoutfailure, ease of trim replacement should be a strong consideration for service incavitating conditions. Cobalt-based hard-facing alloys, such as Stellite 6 and Stellite 21,have found extensive use for resisting cavitation erosion. Other materials used are Type440C, No. 6 Colmonoy, hardened tool-steel, Deloro 50, NOREM 02, NOREM B4, andsintered tungsten carbide with a nickel binder.

Cavitation erosion may be reduced by system design or by selecting the hardest trimmaterial that will not crack from the impact of repeated valve closure and thermalshock; using multiple valves to distribute the total pressure drop by providing backpressure; and using valves that incorporate multiple pressure drop stages designed toprevent cavitation through any one stage.

High Velocity Fluid Impingement Erosion: High velocity fluid impingement erosion occurswhen extremely high velocity fluid jets turn abruptly, bouncing off one surface toimpinge and erode the adjacent part. Impingement erosion may be a form of erosion-corrosion, whereby the high velocity fluid jet blasts away the protective surface coatingas rapidly as it forms.

Fluid impingement erosion can be prevented or reduced using the same techniques andmaterials for improving resistance to abrasive erosion and cavitation erosion.

Combined Erosion and Corrosion: Both erosion and corrosion may occur in a pipingsystem, although not simultaneously. The erosion strips away the protective coating ofcorrosion, thus allowing additional corrosion to occur by repeating the cycle.Accelerated failures of carbon steel piping and fittings have occurred in feedwaterservice due to a combined erosion-corrosion phenomenon. Valves and othercomponents installed in these systems are subjected to the same degradation mode. Thefailures are attributed to a single phase erosion-corrosion phenomenon that occurs toplain carbon steel when exposed to flowing water having a low dissolved oxygencontent (less than 10 ppb) in combination with a pH value less than about 9.3.

As reported in References 1.10 and 1.20, erosion-corrosion is essentially a flow-assisteddissolution process of the magnetite corrosion film normally present underdeoxygenated feedwater conditions. This phenomenon results in much higher metalcorrosion rates than would normally be encountered. Loss rates can be greater than0.040 inch (about 1 mm) per year in severe cases. The worst attack occurs in areas of thefeedwater system where temperatures are between 260°F and 400°F (125°C and 200°C).

The phenomenon is critically dependent on a number of variables, particularly flowvelocity, temperature, pH and oxygen content of the feedwater, and the elemental



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composition of the steel. A comparison of the critical operational variables to typicalPWR feedwater conditions is provided in Table 2-2.

Table 2-2Critical Variables for Accelerated Erosion-Corrosion

Critical Operational Variables forAccelerated Erosion-Corrosion

Typical PWRFeedwater Condition

pH less than 9.3 pH between 8.8 and 9.6

Temperatures between 212°F and 525°F

(100°C and 275°C), with worst attack between

260°F and 400°F (125°C and 200°C)

Varies depending on location in system, typicallybetween 100°F and 450°F (35°C and 230°C)

Dissolved oxygen content less than 10 ppb Dissolved oxygen content less than 5 ppb in hotstandby, and less than 3 ppb in power operation

Turbulent hydrodynamic conditions (high fluidflow velocities)

Fluid flow velocity varies throughout the system.High localized velocities in fittings and valves arecommon.

Valve parts intended for PWR feedwater applications, particularly parts exposed tohighly turbulent flow, should not be constructed of plain carbon steel. The use of lowalloy steel, with at least 0.5% chrome, has been shown to significantly reduce erosion-corrosion attack and should be used as a replacement material whenever possible.Typical replacement materials would be 1/2 Cr-1/2 Mo Plate (A-387, Type 2), 1/2 Cr-1/2 Mo Plate (A-387, Type 12), 1/2 Cr-1/2 Mo Forging (A-182, Type F12 and A-336,Class F12), 1-1/4 Cr-1/2 Mo Casting (A-217, Type WC6) and 1-1/4 Cr-1/2 Mo Bar (A-739, Type Bll).

2.4.4 Design Practices to Minimize Wear and Galling

Wear and galling of materials is responsible for many valve problems, especiallyinvolving operability. Depending on the extent of the damage, the valve may requiremore force than normal to actuate it or damage may even make the valve inoperable. Insome cases, the damage may be so severe that the structural integrity of the valve iscompromised.

Galling is a condition that occurs on the rubbing surfaces of mating parts wherematerial transfer results in localized cold welding, with subsequent spalling and afurther roughening of the surfaces. Galling causes the upset material to:

• Jam the valve during stroking

• Ruin the seat joint



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• Increase the operating force

• In the worst case, make the valve inoperable

Factors affecting galling include the type of materials in contact, temperature, surfacefinish, hardness, contact pressure, and the line fluid. Higher temperature will generallyanneal or soften the metals, increasing their galling potential.

Test data show that hardness is the most significant factor affecting wear; the harderthe material, the less the wear.

Galling, like wearing, can be prevented by:

• Using hard materials.

• Selecting pairs of material with low galling potential (Table 2-3). Using differentmaterials for components in contact rather than the same material.

• Assuring a 5- to l0-Rc difference in the hardness of the materials.

• Designing a reasonable loading. As a rule of thumb, 10,000 psi (70 MPa) averagecontact stress provides adequate margin against galling when using Stellite materialpairs.

• Designing adequate operating clearances.

• Using an appropriate lubricant between the sliding surfaces, breaking in thesecomponents by cycling under low loads before subjecting them to the full loads.

One of the most common methods used to prevent wear and galling is hard-facing.Hard-facing is the process of applying—by welding, plasma spraying, or flamespraying—a layer, edge, or point of wear-resistant metal onto another metal to increaseits resistance to abrasion, erosion, or galling. In a few cases hardfacing is applied toimpart some corrosion resistance to the base metal. It is used when external lubricationis not feasible or is inadequate to give the desired service life, and is usually appliedonly to the critical surfaces. As opposed to heat treatment to achieve high surfacehardness, hard-facing can be used effectively in very large components where thecontact area is small and heat treatment of the entire component would be impractical.Also, because hard-facing is a welding technique, it can be used for in-line repair or torefurbish large components without dismantling. No particular restrictions areimposed when using a base metal of carbon steel, but there are some restrictions whenusing other metals, including stainless steels. For most metals it is desirable to preheatthe base metal to prevent cracking of the hard-facing, as well as the base metal, ascooling occurs.



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Stellite, Colmonoy, and tungsten carbide are the principal materials used in hard-facing; however, tungsten carbide has limited corrosion resistance and is subject tothermal shock failure. Hard-facing on valves is typically used on the plug or disc-to-seat joint to maintain a tight seal. Other areas that are overlaid include the stem,bushing, and disc or plug guides.

The most popular hard-face materials are Stellites, which are patented alloys of hardtungsten and chromium particles in a softer cobalt matrix. Stellite 6 is used on valveseats, while the slightly harder, but more brittle, Stellite 12 may be used on plugs. Forfield repair of worn surfaces, Stellite 21 offers more ductility and lower crackingtendencies, making its use more practical, even though its wear properties are not asgood as Stellite 6. The erosion resistance of Stellites is higher than indicated by theirsurface hardness, which is a measure of the matrix rather than micro particle hardness.For smaller valve parts, the disc or plug and seats may be made of solid Stellitematerial.

In contrast to Stellite, Colmonoy and tungsten carbides are usually applied to all trimshapes by the spray welding process and are then fused to give a non-porous surface.Colmonoy has high hot-hardness and holds this hardness with thermal cycling. Whenusing tungsten carbide, service temperature and thermal shock must be given carefulconsideration. Loading the valve seat must be uniform, and impact forces duringclosure should be low to prevent cracking of tungsten carbide.

For wear resistance, hardness is required only on the surface of the metal. Additionally,hard facing may be achieved by case hardening techniques such as carburizing andnitriding. These superficial hardness treatments usually produce case depths of lessthan 0.025 inch (0.635 mm) that are normally not detected by conventional hardnessmeasurement such as Brinell and Rockwell tests but require microhardness testingmethods.

Table 2-3 lists the wear and galling resistance of various combinations of materials. Inaddition to these materials, plastic lined bushings have been found to be effective whenservice conditions permit their usage. More quantitative information on wear andgalling can be found in References 5.15 and 5.40.



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Table 2-3Chart of Wear and Galling Resistance of Material Combinations (Reference 5.1)

304 ss P P F P P P F P P P F F F F F F F

316 ss P P F P P P F P P P F F F F F F F

Bronze F F S S S S S S S S F F F F F F F

Inconel P P S P P P F P F F F F F F F F S

Monel P P S P P P F F F F F F F S F F S

Hastelloy “B" P P S P P P F F S F F F F S F S S

Hastelloy "C" F F S F F F F F F F F F F S F S S

Titanium 75A P P S P F F F P F F F F F S F F S

Nickel P P S F F S F F P P F F F S F F S

Alloy 20 P P S F F F F F P P F F F S F F S

Type 416 Hard F F F F F F F F F F F F F S S S S

Type 440 Hard F F F F F F F F F F S F S S S S S

17-4 Ph F F F F F F F F F F F S P S S S S

Alloy 6 (co-cr) F F F F S S S S S S S S S F S S S

ENC* F F F F F F F F F F S S S S P S S

Cr Plate F F F F F S S F F F S S S S S P S

AL Bronze F F F S S S S S S S S S S S S S P

Key: S – SatisfactoryF – FairP – Poor* – Electroless Nickel coating

2.4.5 Cobalt-Free Alloys for Hard-Surfacing of Trim

Cobalt-60 has been identified as the principal isotope responsible for out-of-coreradiation contamination problems plaguing the nuclear power industry. Cobalt-60 is anactivation product of natural cobalt, which is found in cobalt-based alloys. Cobalt-based alloys, such as Stellite, are used as hardfacing material on valves, mostly on seats,but also for disc guide surfaces and gate faces. These surfaces wear over time. Inaddition, valve repair, such as seat lapping to improve seat leakage performance, hasbeen identified as producing significant amounts of cobalt grinding debris.



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Natural cobalt in these alloys is 100% cobalt-59, having a thermal neutron absorption of34 barns and, if it is in a pathway to the reactor vessel, will pass through the core, beexposed to thermal neutron flux, and be activated to cobalt-60, having a half life of 5.25years and emitting 1.3 meV gamma rays. These small particles accumulate in the pipingsystem in crevices and cracks where the flow velocity is small and in stagnant pocketsor “crud traps,” which are inherent in the design of some valve bodies. The strength ofthese radioactive sources thus grows with time and becomes a major hindrance toaccess for maintenance work.

Several years ago, primarily because of high price and uncertain availability, there hadbeen some effort by manufacturers to develop hardsurfacing using cobalt-free alloyssuch as ASTM A565, Gr616, Deloro-Cabot 40 and 50, and Colmonoy 5. Because thesealloys did not exhibit the same mechanical and corrosion-resistant attributes ofStellite™ and because the price and source of cobalt stabilized, most of these effortswere discontinued.

Recently, there have been renewed efforts to develop low-cobalt or cobalt-free alloys toreplace cobalt-base alloys to reduce the exposure of service personnel to radiation dueto cobalt-60. Several EPRI-sponsored efforts have been conducted to evaluate therelease of cobalt from PWR valves and from valve repair, evaluate low-cobalt alloy orcobalt-free hardfacing, and to develop cobalt-free alloys as a valid alternative to cobaltalloys for hardfacing.

A family of cobalt-free alloys named NOREM™ emerged as a good candidate forfurther evaluation and testing. For nuclear power plant applications, cobalt-free alloysshould meet several requirements including:

1. Material should have high resistances to erosion, corrosion, wear, and galling undertypical plant conditions which may include high flow velocities,cavitation/flashing, high contact stresses, and large temperature variations.

2. Material should have multi-layer hardfacing deposit capability for various basematerials typically used in power plant applications. The hardfacing should behomogenous, not subject to cracking, and capable of being applied with little or nopreheat. The deposits should be economical to apply using existingequipment/machinery and should be repairable on a localized basis.

3. Material should be available in different forms such that it can be used in spareparts and other repairs.

Extensive testing and evaluation by EPRI [5.27] and some utilities showed that some ofthe NOREM alloys meet the above requirements and in general are equivalent to orbetter than those of the cobalt-base Stellite. Several utilities and manufacturers arecurrently using NOREM in field repairs and as replacement of the Stellite hardfacing



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material. EPRI's Welding Repair Guide [1.1] provides detailed discussion for the selectionand application of welding filler materials including NOREM alloys. Interested valveusers should consult the manufacturers for the latest technical information and testresults.

2.4.6 Design Practices to Minimize the Effects of Temperature

High or low temperature cycling can have a detrimental effect on operation of the valvedue to differential expansion between various parts of the valve, but problemsassociated with high temperatures are more common in power plant applications. Thematerial property most affected by low temperatures is impact strength, but, in therange of low temperature service expected in power plants, it is usually of no concern.Trim materials begin to lose impact strength below 0°F (-17.8°C), with most otherproperties remaining about the same. Low and moderate temperature applications,however, do permit the use of plastics and other nonmetals for soft seat inserts andseat-to-body seals not possible in high temperature service.

Geometry-dependent design features in thermal cycling service include ensuringadequate clearances between moving parts; preventing loosening of interference fittedparts caused by differential rate of thermal expansion of their respective materials; andgalling of material related to temperature, loading, and degree of contact, such asrepeated impact from closing or vibration. Other factors to be considered are thermalcycling effects on valve sealing, seat gasket sealing, and loosening of components suchas guides, bushings, and seats during service.

The material properties considered for establishing high temperature operating limitsare tensile, yield, creep and rupture, hot hardness, impact strength, and aging. Equallyimportant in high temperature service is oxidation resistance, heat treatingtemperature, and galling resistance of the trim materials at operating temperatures. Ingeneral, yield, tensile, and compressive strengths decrease when temperature isincreased. Above 800°F (427°C), creep and rupture also become important factors inmaterial selection. In high temperature service, trim undergoes an initial elasticdeformation and then continues to deform or creep with time under load. Hot hardnessis necessary to prevent damage to seating surfaces, to prevent galling, and to minimizewear. Scaling resistance is the ability of a material to withstand oxidation on thermalcycling without repeated scaling or flaking of the surface.

Other aspects of temperature effects are pressure locking, thermal binding, and discpinching in gate valves. Section 4 provides detailed descriptions of these phenomenaalong with design practices to minimize or eliminate their effects on valve performance.

Finally, changes in the stem diameter due to temperature changes affect stem sealingand packing performance, as will be discussed in the next section.



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2.5 Valve Stem Seals

This section presents the different types of stem seal arrangements typically used innuclear power plants. It also presents the major factors that affect stem sealperformance.

The stem seal performance discussion is included here for the following reasons:

1. Stem leakage is a common problem in all valve designs discussed in the followingsections.

2. Stem leakage is one of the major factors that affect equipment reliability and plantavailability/productivity.

3. Recent advances in valve packing technology resulted in eliminating many valveleakage problems. However, solving stem leakage problems requires a goodunderstanding of the stem sealing mechanism and correction of the misconceptionscarried over from the older and obsolete technology.

There are two basic ways of sealing the fluid around the stem: using flexible seals suchas diaphragms or bellows, or using packings. Flexible seals experience no slidingbetween the stem and the seal and depend on flexure of the sealing member toaccommodate the stem movement. Packings allow the stem to slide through them anddepend on radial pressure between the packing and the stem to achieve a seal.

Flexible seals are available in either elastomers or metals, depending on the fluid mediaand pressure to be sealed, and are either a diaphragm or bellows design. Flexible sealsprovide better sealing and are used where external leakage or periodic maintenance isnot permissible. Flexible seals are available in either elastomers or plastic and metal,but the pressure and temperature limitations of the elastomeric and plastic sealsprevent them from being used to any great extent in power plant applications. Theflexible elastomeric seals are found primarily in diaphragm valves (see Section 12).

Packings are found most commonly in valve applications because they caneconomically seal against some of the harshest environments and can permit virtuallyunlimited axial, as well as rotational, movement of the stem. Packings are typicallymade of flexible materials, which can be compressed to generate the required radialpressure to seal against the stem. Unlike metal bellows and diaphragms, packingsrequire periodic maintenance and replacement to maintain their effectiveness.



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2.5.1 Flexible Metal Seals

The two basic shapes of flexible metal seals used for providing zero external leakageare the bellows and metal diaphragm. Bellows seals are used in valves where longerstrokes and larger flow capacity are required, whereas metal diaphragms are used invalves that have very limited stroke and flow capacity.

Bellows Seals: A bellows seal (Figure 2-24) consists of multiple convolutions in a thinmetal sheet that surrounds the valve stem and forms a complete seal between themoving valve stem and the stationary valve body. This multiple convolutionconstruction is obtained by either hydroforming a thin metal sheet or by welding theedges of flat circular sheets, which are called leaf-type bellows.

Figure 2-24Bellows Seal

Bellows seals are of either internal pressurized or external pressurized design. In theinternal pressurized version, only the inner surface of the bellows is subjected to thefluid pressure, thus permitting the use of non-corrosion resistance materials for theouter extension bonnet. A well-designed bellows seal has an anti-rotation devicepreventing torsion-induced damage during operation, assembly, or disassembly.



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Single-ply bellows are limited to low internal pressure, and multilayer designs increasethe useable range to as high as 3,000 psi (20.7 MPa) differential pressure.

Bellows are ideal for service where the fluids are highly toxic, radioactive, volatile, orextremely expensive and external leakage cannot be tolerated. Another example wherebellow applications are particularly suited is in borated water where even a minuteleakage of fluid past a seal can result in the formation of abrasive crystals. Conventionalpacking rings are rapidly worn away in such applications. For high temperatureapplications, bellows are seal welded onto the stem and bonnet, thus eliminating theneed for elastomeric or plastic seals at these joints.

Bellows seals are relatively expensive, require a long length, have limited fatigue life,and have a shorter stroke, which restricts their use to services that cannot be served byconventional seals. Since bellows do have a finite life, they should be inspectedfrequently. A conventional packing is customarily used as backup to prevent externalvalve leakage in case of bellows failure.

Bellows are normally used on valves having non-rotating rising stems, such as globevalves, gate valves, and safety valves, where the stem travel is relatively short. Recentlybellows have been used in quarter-turn valves (Figure 2-25) in low pressure andvacuum service.

Figure 2-25Bellows on Butterfly Valve



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Metal Diaphragm Seals: Metal diaphragm seals (Figure 2-26) are either single- ormultiple-ply thin flexible members installed between the stem and the valve plug toseal the system fluid. The diaphragm is attached to the bonnet by either clamping orseal welding. Movement of the plug in the closed direction is provided by the stemforce acting through the diaphragm, and the return stroke is provided by the systempressure and/or springs.

Figure 2-26Metal Diaphragm Stem Seal

Metal diaphragm seals are used for the same applications as bellows seals but have thedisadvantages of having a much shorter stroke and no physical connection between thestem and plug. Lack of a physical connection between the stem and plug preventspositive indication of the plug position and mechanical operation of the valve in theopening direction. Valves using this type stem seal rely on the fluid pressure to open



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the plug and, therefore, can be used only in the fluid flow to open orientation, andpreferably with the stem vertical. Since metal diaphragms have no physical connectionto the stem, they cannot be mechanically pulled open. Metal diaphragms should not beused in throttling applications because they can flutter, due to fluid/structureexcitation, which can cause rapid fatigue failure.

Metal diaphragms have a finite life and should be inspected or replaced at regularintervals. Conventional packing should be installed as backup in the event ofdiaphragm failure.

2.5.2 Valve Stem Packings

In many nuclear power plants, valve stem leakage continues to be a major problem thatcontributes to high maintenance cost, low reliability, and loss of plant availability.

A basic understanding of the packing system’s sealing function is crucial to its properapplication and reliable performance as a valve stem seal. Even though the stuffing boxdesign is simple and has been used as a valve stem seal for decades, its principle ofoperation is not adequately understood by many valve users and even somemanufacturers. Over the years, this lack of understanding has led to several variationsin packing designs and gland configurations which, in some cases, even degrade theperformance instead of providing the anticipated improvement.

The following sections present a summary of the fundamentals of sealing mechanism instem packing glands, a historical review of the important research, and a discussion ofthe new guidelines that have resulted from stem packing improvement programs.

Basic Types of Stem Packings: There are three basic types of stem packings (Figure2-27), which rely on soft sealing material for their sealing action but are fundamentallydifferent in their principles of operation:

• Compression packings or jam-type packings

• Lip-type, pressure-energized packings

• Interference-type seals (O-rings)



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Figure 2-27Basic Types of Stem Seals



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The most commonly used packing in valve stems is compression-type packing rings ofbraided or precompressed flexible sealing material, usually of square or rectangularcross section, which are placed into a packing box and compressed by a packing gland.This type of packing relies on externally applied compressive force to achieve a seal.Such packings require periodic adjustment or some other means (discussed later) tocontinue to supply the necessary packing pressure to maintain a seal.

Lip-type packings, usually called V-packing or chevron type, rely on a relatively lowexternal force to effect an initial seal and a pressure energizing action due to their cross-sectional shape. As the system pressure increases, the force at the sealing edgeincreases, thus maintaining a positive seal. Such seals usually require little or noadjustment during operation. In order to prevent binding and over-adjustment, acompression stop ledge is often used to limit the minimum packing height.

The interference type of seal (for example, O-rings) relies on the radial cross-sectionalsqueeze and system pressure to effect a seal and on the elasticity of the seal material tomaintain the sealing preload. This type of stem seal also requires no adjustments inservice.

Packing Gland Construction and Sealing Mechanism: Figure 2-28 shows a cross-sectionalview of a typical packing gland design. The assembly consists of a packing glandflange, gland follower, and a number of packing rings. The packing flange transmitsthe applied bolt force through a spherical contact surface to the follower, which, inturn, axially compresses the packing rings. The spherical contact interface prevents sideloading of the follower against the stem under the unavoidable misalignment of thegland flange during tightening the bolts.



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Figure 2-28Packing Gland Details

The axial compressive load transmitted to the packing rings tends to expand them inthe lateral, or radial, direction. This lateral expansion tendency of the packing cross-section is prevented by its confinement against the stem and the stuffing box insidewall. This causes a radial contact pressure to be developed between the packing andstem interface, as well as the packing and stuffing box wall. Friction losses in the upperpacking rings (due to friction with the stem and the stuffing box wall) reduce the axialcompression load on the lower rings, which in turn results in a decrease in the radialcontact pressure between the lower packing rings and the stem.

Fluid pressure can migrate between the packing rings and the stem up to a point whereits magnitude exceeds the radial pressure between the packing and the stem. Sealing isachieved at a point where the radial packing pressure just exceeds the fluid pressuretrying to force its way across this interface. All the packing rings below the sealingpoint are essentially ineffective in providing a seal around the stem. However, manyvalve manufacturers have employed deep stuffing boxes in their designs in the past.

The first and most significant documented research towards understanding the sealingmechanism of flexible packings was conducted by White and Denny under thesponsorship of the British Ministry of Supply during the war and published in 1947[5.41]. One of the important contributions from their work was a simple apparatus that



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allowed the radial contact pressure distribution between the packing and the stem to bedetermined. The apparatus used a small diameter radial hole in the stem, throughwhich an externally adjustable pressure could be introduced at the sealing surface. Thispressure was gradually increased until it reached the magnitude necessary to overcomethe contact packing pressure between the packing and the stem. By positioning thestem with the balancing hole at various locations along the packing length, detailedstatic pressure distributions were obtained for a number of packing configurations.

Two other notable fundamental research contributions that led to furtherunderstanding of contact pressure distribution in packings under static as well asdynamic conditions were made by Turnbull [5.42] and Denny and Turnbull [5.43] in1958 and 1960, respectively. The major finding from these research studies was that thepacking ring closest to the gland follower has the highest radial pressure, and thisradial pressure decays exponentially as the distance from the gland increases (Figure 2-29). They also found that under dynamic conditions this radial pressure tends toredistribute itself, tending to decrease in the packing rings farther away from the glandfollower and concentrating near the top rings.

Figure 2-29Distribution of Stresses in the Packing and Location of Actual Sealing Point



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For nuclear power applications, considerable research has been done by many differentorganizations, including EPRI, valve manufacturers, packing manufacturers, materialsuppliers, research institutes, and nuclear power utilities in the USA, Canada, U.K., andFrance.

In recent years [5.44 through 5.50], the performance of valve stem packing has beenenhanced as a result of:

• Improvements in gland loading arrangements that eliminate the need for packingadjustments in service. “Live loading” has become an accepted term for such anarrangement.

• Development of improved packing materials, compositions, and various forms inwhich they are manufactured including flexible graphite rings precompressed bydie molding in square, chevron V-packing, or wedge shapes; braided graphite; andbraided carbon.

• Extensive testing of several arrangements of flexible graphite in combination withbraided graphite or braided carbon rings stacked in different sequences in thestuffing box.

• Testing to determine the optimum number of packing rings and range of glandloads for different applications. Comparative testing of various shapes of packingrings such as square, chevron, and wedge cross-section.

• Development of better corrosion inhibitors to eliminate stem pitting, which cancause rapid degradation of stem packing.

• Collection of a vast amount of valve diagnostic data over a long period of time withdifferent valve designs and under different operating conditions.

Common Packing Materials: Common packing materials include:

Asbestos. In the past, the most commonly used packing material in power plants wasbraided asbestos with impregnated graphite or mica to provide lubrication attemperatures up to 1,000°F (540°C). One of the most popular braided asbestos packingmaterial used was John Crane 187I, which is reinforced with Inconel wire mesh forhigh pressure, high temperature strength, and contains a zinc inhibitor to prevent stemcorrosion. However, in high temperature service and sometimes even in storage, thebraided material hardens due to loss of volatile binder material in packing. The loss ofresiliency causes leakage and prevents further adjustment. Additionally, the braidedasbestos material swells when exposed to process fluid and shrinks when drying up.These volume changes cause premature stem leakage and require frequent packingadjustment. Because of health hazards posed to the public, asbestos is generally



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prohibited and is presently being eliminated as a packing material. Most manufacturershave stopped manufacturing asbestos-based packings in the United States.

Graphite. In recent years an intensive effort has gone into the development of suitablealternatives to asbestos packing and gasket materials. At a conference organized byValve Manufacturers Association of America (VMA) in 1986, several material suppliers,gasket/packing manufacturers, valve companies, petrochemical industry groups, andpower industry groups concluded that graphite is the only acceptable substitute forasbestos for high temperature applications.

In the 1970s, Union Carbide developed flexible graphite using a process that introducesno organic or inorganic binders, additives, fillers, or other potentially fugitiveingredients [1.15]. The process employs a high quality particulate graphite, chemicaltreatment, and rapid heating to produce flexible graphite sheets (or tapes). Packingrings die-formed from flexible graphite tape have become the preferred graphitepacking. Graphite material is now widely used in pressure seals, spiral wound gaskets,and other metal-clad gasket configurations. Die-formed flexible graphite packing offersthe following advantages over asbestos-based packing rings:

• Low coefficient of friction (less than 0.1)

• Self-lubricating

• Contains no binders, fillers, or resins

• Impermeable to gases and fluids

• Flexible, yet free of cold flow or high temperature flow problems (low creeprelaxation)

• Corrosion resistant

• Excellent resistance to temperature changes

• Anisotropic, having high thermal conductivity along the plane of the sheet

• Suitable for temperatures to 1,000°F (540°C) in oxidizing environment and to5,500°F (3,000°C) in inert or reducing environment

• Asbestos free

• High chemical resistance - operates in fluid pH range 1 through 14

• Nuclear grade with typical leachable chloride content of less than 50 ppm available



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• Available in high density (70 to 110 lb/ft3

; 1,120 to 1,760 kg/m3

) die-formed rings(which have excellent flexibility) or ribbon form

• Highest radiation resistance of all packing materials

• Available with passivating corrosion inhibitors which prevent stem pitting withoutloss of packing stress, as encountered with sacrificial inhibitors

The above factors, along with misconceptions carried over from the era of asbestos-based packings, resulted in many packing problems in the early stages of employingflexible graphite packing. Reference 5.44 provides an excellent discussion of thesemisconceptions and shows that proper application of graphite packing can eliminatemany of the packing problems. These misconceptions, along with recommendations forproper application of flexible graphite packing, are summarized as follows:

1. Myth: Valves require periodic repacking.Asbestos packing required periodic repacking (at some periodic frequency) becauseof the loss of packing flexibility and elasticity that is caused by the depletion of thevarious binders and fillers under pressure and temperature. Flexible graphite, onthe other hand, does not contain binders or fillers and tends to maintain its elasticitythroughout its life. However, flexible graphite packing must be contained withupper and lower anti-extrusion rings to prevent extrusion outside the packing box.In the absence of live loading, retorquing may be occasionally required tocompensate for packing consolidation.

2. Myth: Valve sealing is accomplished by pressure breakdown mechanisms.With asbestos packing, it was assumed that sealing is accomplished by a series ofpressure breakdowns, similar to the labyrinth seal design. This assumption led todeep stuffing box designs to accommodate a large number of packing rings,especially for higher pressure systems. Testing has shown that only one die-formedgraphite ring is required to provide adequate sealing. However, to ensure backupprotection, the graphite packing set typically includes several die-formed rings inaddition to anti-extrusion rings on the top and bottom. Graphite bushings are alsoused to fill the space previously occupied by excessive packing rings.

3. Myth: Valve packing will leak.Because asbestos packing is harder to consolidate (due to higher friction and stifferInconel reinforced rings), packing leakage at start-up was considered normal. Withproper installation and adequate consolidation of flexible graphite packing, valvepacking will not leak.

4. Myth: Lantern ring prevents packing leakage.Lantern rings do not serve a good function in modern packing designs. They cancorrode to the stuffing box, damage the stem and at best require an additional set of



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packing rings. With lantern rings, it is extremely difficult (if not impossible) toadequately load and consolidate the lower rings. Utilization of lantern rings alsodoubles the packing drag on the stem.

5. Myth: Every mechanic knows the right amount of torque to tighten a packing.Packing loads depend on several factors including system pressure, frictioncoefficient, stem diameter, packing height, and the ability of the packing material totransfer axial load to radial pressure. The 1,000-pounds-per-inch of stem diameterrule does not differentiate between different applications or operating conditions.Packing loads should be predicted using analytical or empirical formulas thataccount for all packing parameters as well as operating conditions.

6. Myth: Packing is not part of the valve pressure boundary.Although the ASME Pressure Vessel Code views valve packing as outside the scopeof the valve pressure boundary, packing failure can have a significant impact onpersonnel/plant safety and on the environment. Recent advances in packingtechnology will eventually lead to leak-free packing designs. Additional researchwill be needed if valve packing is to be considered part of the valve pressureboundary.

Plastics and Elastomers. Several plastics and elastomers are used in valve stempackings for temperatures lower than those requiring graphite. Teflon in virgin form oras filler materials is used extensively in valves because of its low coefficient of frictionand excellent chemical resistance for temperatures up to 400°F (200°C). Teflon’s mainlimitation is lower radiation resistance (maximum 104 rads) than other plastics andelastomers used as packing, gasket, and soft seating insert materials. However, Teflonis used in many applications in power plants where radiation levels and temperaturesare low.

An important development in elastomers came from the Department of Energy’sfunding to develop high temperature elastomers for geothermal applications. Oneformulation of EPDM (ethylene propylene family) capable of withstanding 600°F(315°C) water or steam environment has been developed and is in commercial use.EPDM’s main limitation is its inability to tolerate any exposure to petroleum-basedfluids, which cause excessive swelling, degeneration, and sticking to metal surfaces,especially copper alloys. EPDM is particularly unsuited for solenoid-operated valves inair systems, which invariably transmit some lubricant mist. In nuclear powerapplications, some grades of EPDM are likely to make strong inroads and extend thetemperature limits of soft seating materials. EPDM is commercially supplied by severalseal manufacturers in O-ring, chevron, V-packing, or other special forms.



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Tables 2-4 and 2-5 give a summary comparison of properties of various plastics andelastomers used as gaskets and seals.

Table 2-4Typical Properties of Plastics and Elastomers Used in Valves for Soft Seats, Seals,and Gaskets(Source: Reference 5.51)

TEFLON(Halon, TFE, Fluon)

• Radiation resistance - maximum 104 rads

• Low coefficient of friction

• High chemical resistance

• Temperature limit of 400°F (200°C)

• Susceptible to abrasion

TEFLON(Glass Filled)

• Radiation resistance - maximum 104 rads

• Low to moderate coefficient of friction



• Susceptible to abrasion, but better than unfilled Teflon

NYLON(Zytel, Nypel, Fosta)

• Radiation resistance - 106 rads

• Moderate coefficient of friction

• Moderate to low chemical resistance


• Not susceptible to abrasion

KEL-F(CTFE)



• Good chemical resistance


• Susceptible to abrasion

TEFZEL • Radiation resistance - 107 rads




• Moderate resistance to abrasion

POLYETHYLENE • Radiation resistance - 108 rads

• Low to moderate coefficient of friction




NATURAL GUM RUBBER • Radiation resistance - 107 rads

• High coefficient of friction






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Table 2-4 (cont.)Typical Properties of Plastics and Elastomers Used in Valves for Soft Seats, Seals,and Gaskets(Source: Reference 5.51)

BUNA-N • Radiation resistance - 106 rads



• High resistance to petroleum products



VITON • Radiation resistance - 107 rads


• Good chemical resistance



ETHYLENE,PROPYLENE,TERPOLYMER





• (Has been placed in valve service

with temperature of 400–450°F (200–230°C)but no operating data available as yet)


• See discussion on high temperature



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Table 2-5Typical Radiation Resistance of Plastics(Source: Reference 5.51)

GROUP 1 Plastics retaining satisfactory properties after exposure to 1010 rads

• Phenolic, glass laminate

• Phenolic, asbestos filled

• Polyurethane


• Epoxy, aromatic curing agent

• Furane resin (Duralon)

• Polyester, glass filled

• Polyester, mineral filled

• Polystyrene (Amphenol, Styron)

• Polyvinyl carbazole (Polectron)

• Silicone, glass filled

• Silicone, mineral filled


• Polyethylene

• Polyester film, unfilled (Mylar)

• Polyvinyl chloride* (PVC, Tygon, Pliovac)

• Polyvinyl formal (Formvar)

• Silicone, unfilled

• Polypropylene


• Aniline - formaldehyde (Cibanite)

• Cellulose acetate (Tenite, Celanese)

• Melamine - formaldehyde (Melmac)

• Monochlorotrifluoroethylene* (Kel-F, Polyfluoron Fluorothen)

• Phenol formaldehyde, fabric filler (Bakelite)

• Phenolic, unfilled

• Polycarbonate (Lexan, Merlon)

• Polyvinylidene chloride* (Saran)

• Urea - formaldehyde

• PVF (Polyvinyl fluoride)

• PVDF (Polyvinyl difluoride)


• Polyamide (Nylon, Zytel)

• Polyester, unfilled

• Polyformaldehyde (Delrin, Celcon)

• Polymethyl alpha - chloracrylate (Gafite)

• Vinyl chloride - acetate


• Tetrafluoroethylene* (Teflon)

* Tests have shown these materials to evolve halogenated gases due to radiation exposure, possibly at lower doses thanindicated here; their use should be restricted.



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Guidelines for Stem Packings: One of the most significant activities in the United States inthis area was EPRI research project RP2233-3, “Valve Stem Packing Improvements,”initiated in early 1984. The work performed under this project culminated in thedevelopment of specific guidelines for improving the stem packing performance innuclear power plants. EPRI Report NP-5697 [1.15] provides a comprehensivedescription of effort under this project. Subsequent testing and field experience byseveral utilities [5.44] and packing companies [5.45] provided new insights intounderstanding and predicting valve packing performance. These efforts resulted in thedevelopment of effective valve packing programs and prediction models thateliminated many packing problems [5.44, 5.45, 5.46]. This in turn resulted in:

• Eliminating periodic valve repacking

• Achieving leak-free valve operation

• Reducing radiation exposure

• Improving valve reliability and plant availability

• Relaxing post-maintenance test requirements

• Savings in maintenance costs and work load

• Minimizing packing loads

• Maximizing operational margins for power-operated valves

Stem sealing problems are generally caused by either packing-related problems orother valve problems outside the packing area (such as bent stem or disc-to-seatmisalignment). Thus, if unexpected stem leakage occurs, it is necessary to determinethe root cause before retorquing or repacking the valve.

The major factors that can affect the performance of valve stem packings are:

• Misalignment

• Packing gland pressure level and distribution

• Packing composition and configuration

• Surface finish of stem and stuffing box

• Radial stem guidance

• Stem taper and dimensional variation (including temperature effects)

• System pressure, temperature, and fluid medium



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• Frequency of stem movement

• Stuffing box depth and diameter

• Valve stem orientation (horizontal or vertical)

• Vibration

• Installation practices and procedures

In the following sections, some of the important findings from recent developments aresummarized. However, the reader is encouraged to refer to the original references formore detailed discussions.

Packing Assembly and Consolidation [5.46]: Proper packing assembly and consolidation isa determinant factor in obtaining leak-free packing. Packing consolidation should beperformed by retorquing and stroking to ensure uniform radial loading of the entirevalve packing set. Better load transfer is obtained by reversing stem direction betweenretorquing. Recommendations of packing manufacturers should be followed.

Break-Away vs. Running Packing Friction [5.50]: Field and laboratory testing show thatbreak-away packing friction can be as low as 5% higher than running friction and ashigh as two times higher than running friction. In designing or modifying a valvepacking, the break-away packing friction should be kept as close as possible to therunning friction.

Higher break-away packing friction reduces operating margins in MOVs and causescontrol problems, especially in air-operated valves. Packing material, stem finish, andtemperature are the main factors affecting the ratio of break-away packing friction torunning packing friction.

Maintaining Gland Load by “Live Loading”: Conventional packings progressivelyconsolidate and wear in service, thereby causing a loss of gland load which is initiallyapplied to achieve a good seal. Eventually, this leads to leakage when the radialpacking stress due to reduced gland load falls below the fluid pressure to be sealed.Periodic adjustment of the packing gland has been an accepted practice in conventionalapplications to maintain adequate gland loading and to prevent leakage. Some utilitieshave found that periodic retorque every three to four years provides good packingperformance. However, this approach is not preferred in some nuclear powerapplications due to higher reliability requirements and the additional radiationexposure that complicates maintenance activities. The regulatory aspects of plantoperation may dictate plant shutdown or reduced power operation to correct leakage ifoperating limits are exceeded.



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By applying load to the packing rings through a spring-loaded gland followerarrangement (see Figure 2-30 for typical arrangement), the loss of gland load with theconsolidation or wear of the packing is minimized. Figure 2-31 shows the performanceadvantage of a live-loaded packing over the conventional packing arrangements.Without live loading, a very small amount of consolidation results in a large reductionin the packing compressive stress, which quickly reduces to a level below which aneffective stem seal can be maintained. With live loading, the magnitude of theconsolidation that can be tolerated without leakage can be increased by a factor of 15 to20 in most applications. An additional advantage of the live-loading arrangement isthat it minimizes the potential problem of inadvertently creating high stem friction andmaking the valve inoperable by over-tightening the bolts in conventional packingglands that require manual adjustment.

Figure 2-30Live Loading of Valve Packing Using Disc Springs



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Figure 2-31Packing Compressive Stress Versus Consolidation

Improved Packing Composition and Configuration: The improved packing gland designutilizes a square cross-section, precompressed flexible graphite material packing ringsin the middle of the packing box, in combination with braided graphite or braidedcarbon anti-extrusion rings at the top and bottom. It has been found that anarrangement consisting of three or four flexible packing rings and one end ring at topand bottom locations gives good performance. The anti-extrusion rings preventextrusion of the flexible graphite material into the relatively large radial clearanceusually present between the stem and gland follower. The anti-extrusion rings also actas scraper rings, preventing the loss of graphite material, which has a tendency toadhere to the stem, during cycling.

It may be possible to adjust packing configuration to improve performance. Forexample, running load may be reduced by using two die-formed rings (instead of threeor four) or by using narrow rings instead of the square design. Another packingconfiguration utilizing wedge rings that can convert axially applied gland load moreefficiently in the radial sealing direction has also been developed as a result of thisEPRI project. Instead of relying upon the gland load alone, this configuration alsoutilizes the system pressure to increase the radial sealing load as pressure is increased.Specific values of gland compression details for various types of packings should beobtained from the valve/packing manufacturer.

Retrofit Considerations: To install the live-loading as a retrofit to existing valves requirescareful evaluation of several factors. The space needed to incorporate a spring stackthat provides sufficient force throughout the anticipated range of deflection is limited



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in both the vertical and radial clearances in the valve stuffing box area. This can makeretrofitting particularly difficult in the smaller valves. New and longer gland studs areusually needed to install the spring stack, and adequate guiding of the individualBelleville spring must be provided to ensure proper performance. To get the properbenefits from the live loading arrangement, the guidelines established in EPRI ReportNP-5697 [1.15] should be followed. Several manufacturers have utilized guidelines tomake recommendations for specific installations with proper consideration to themaximum and minimum gland loads that can be achieved within the space constraintsand the expected in-service packing consolidation that can be tolerated withoutleakage.

Stuffing Box Spacers: For many years, some valve designers have erroneously considereddeep stuffing boxes to be more effective in controlling stem leakage; stuffing boxes withtwelve packing rings were not uncommon. As discussed earlier, since the glandpressure decays exponentially as a function of distance away from the follower, it hasbeen found that only three to four packing rings are essential to accomplish a good seal,and the use of additional rings contributes to an unnecessary increase in stem friction.

One of the primary reasons for packing leakage is inadequate gland load. As packingwears and consolidates with usage and time, the gland load decays, which eventuallyresults in leakage. Deeper stuffing boxes with a larger number of packing rings resultsin more consolidation of packing and a greater loss of gland load. Deep stuffing boxesare also more difficult to clean and repack.

To overcome these deficiencies, metal or carbon spacers can be installed in the bottomof the stuffing box to reduce the number of packing rings. A set of five rings, consistingof three die-formed graphite packing rings (which accomplish the sealing function) andtwo braided graphite end rings (which confine the loose graphite particles within thesealed gland), has been found to work well (5.44, 5.45, 5.46). Hardened carbon spacerson the top and bottom of the packing set can also improve stem alignment and provideadditional radial support.

Lantern Ring/Stem Leak-Off Connection: Deep stuffing box designs were inherited fromother industries, and they usually employ lantern rings in the center. In petrochemicaland other non-nuclear power applications, lantern rings are used to allow injection of agrease or sealant material through an external connection in the middle of the packingring stack to provide a secondary backup seal (Figure 2-32). Lantern rings can be usedto effectively seal off a leaking stem when additional gland load cannot successfullyovercome the leakage. Relatively high viscosity sealants, capable of performing at hightemperatures, are available. Since the pressure sealing capability using viscous sealantis increased by an increase in the length of the resistance path, deep stuffing boxes doprovide an advantage when sealant injection is permissible. However, in most nuclearpower applications, this is not acceptable.



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Figure 2-32Lantern Ring / Stem Leakoff Connection

In some nuclear power plant applications, the lantern ring and associated leakoffconnection, in conjunction with double packing arrangement, is used to collect leakageof contaminated water past the lower packing ring set and allow contaminated water tobe piped off to a remote location. It should be pointed out that in double-packingarrangements, the gland load has to be high enough (typically higher than in a singlepacking arrangement) to transmit sufficient compressive load to the lower packing setto achieve a seal.

Stem Corrosion and Use of Inhibitors: Stem corrosion and pitting can cause quick failure ofthe packing by abrading away the packing material. It has been found that lowchromium content stainless steel stem materials (400 series) are more prone to pittingcorrosion than high chromium materials (300 series steel) in the presence of moisture.In general, properly heat-treated 17-4 PH material has been found to be very resistantto pitting corrosion.

When the valve is hydrostatically tested with water and then stored without takingprecautions to avoid corrosion, stem packing failures are often encountered during the



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first cycle of operation due to severe pitting of the stem during storage. Both asbestosand graphite, the most commonly used packing materials, can cause corrosion of thestem, even though graphite packing material has less corrosion-inducing impuritiesand is, therefore, less aggressive than asbestos. One of the practices used to avoid stemcorrosion during storage is to specify that valves be shipped with packing removed.

Historically, most valve manufacturers use generous quantities of grease wheninstalling stem packings for many non-nuclear applications. The greases used providereasonable protection against pitting corrosion of the stem during storage bypreventing intrusion of water or moisture into the packing material during hydrostatictesting, especially at the interface between the stem and packing. The level of protectionprovided by this approach is not very reliable. In nuclear power plants, the use ofgrease is unacceptable in most stem packing applications.

Corrosion inhibitors are employed in stem packings to provide a positive protectionagainst stem pitting. The most frequently used inhibitors have been sacrificial typeswhich undergo oxidation corrosion instead of allowing the stem material to beattacked. This effectively prevents pitting corrosion of the stem, while the sacrificialanode is consumed. Zinc and aluminum in various forms have been the mostcommonly employed sacrificial corrosion inhibitors. Zinc has been found to be moreeffective than aluminum. Both have been used in the form of washers in the packingset, as well as in a powder form uniformly distributed in the packing material itself.The protection obtained by using the powdered form, even with best attempts toachieve uniform dispersion and the use of binding or tacking agents to keep thepowder particles in place, has been found inadequate. Solid zinc washers have been thepreferred sacrificial corrosion inhibitor by most users.

One recent development has been the use of passivating corrosion inhibitors that forma protective film on the stem that inhibits corrosion. Some manufacturers offer flexiblegraphite packings impregnated with barium molybdate; other manufacturers areoffering nonmetallic, inorganic inhibitors that are an integral part of the graphite sheetitself. An advantage of the passivating type of inhibitor over the sacrificial type is thatthere is no loss or increase of material in the packing box; therefore, there is no changeof packing compressive stress due to the inhibitor material being consumed.

2.6 Gasket Types and Materials

2.6.1 Gasket Types

As contrasted to stem packings which provide a dynamic seal, gaskets are used forstatic sealing applications. Several types of gasket designs are used in valveconstruction, many of which are of the same type as those used at flanged ends.Classification of the most commonly used gasket materials and types is shown in



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Table 2-6, taken from ASME Section VIII, Division 1, Appendix 2, which also providesdesign rules for bolted flange connections using gaskets. The differences between theactual gasket width furnished and the effective width recommended should be takeninto account, as discussed in these standards, to avoid either over-stressing the gasketor having insufficient stress, causing leakage.

Table 2-6 lists the types of gaskets in the order of increasing minimum design seatingstress required. The Code-suggested design values of the gasket factors (m) (m is themultiple of pressure to develop sufficient compression load to ensure a tight joint) andthe minimum design seating stress (y) are not mandatory. Gasket manufacturers cansuggest lower values which still provide a satisfactory static seal at lower bolt loads.There are other commercially available gasket materials that are not included in theASME Pressure Vessel Code Table 2-6. For gasket materials other than those given inthis table, the supplier should be contacted to obtain y and m values.



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Table 2-6Gasket Materials and Contact Facings, Gasket Factors M for Operating Conditions,and Minimum Design Seating Stress y(Extracted from the ASME Pressure Vessel Code, Section VIII Division 1)



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Table 2-6 (cont.)Gasket Materials and Contact Facings, Gasket Factors M for Operating Conditions,and Minimum Design Seating Stress y(Extracted from the ASME Pressure Vessel Code, Section VIII Division 1)



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Types of gaskets most frequently used in valves are described below.

2.6.2 Flat Metal Gaskets

Soft metals (for example, soft aluminum, copper, brass, iron, or stainless steel) can beused as solid flat metal gaskets. These gaskets are designed to deform plastically andconform to the irregularities of the sealing surface during installation. Therefore, thesegaskets require relatively high unit seating stress and correspondingly high bolt loads.

2.6.3 Flat Non-Metallic and Metal Clad Gaskets

For lower seating stress, rubber or polymeric material gaskets can be used, but they arenot suitable for high pressures and temperatures. They are typically limited to ANSIClass 300 valve ratings and temperatures of 250°F (120°C) or less. For higher pressuresand temperatures, metal-clad asbestos substitute material is used.

2.6.4 Spiral Wound Gaskets

Spiral wound gaskets have the distinct advantage of high elastic deformability oversimple flat gaskets. This makes them suitable for use in sealing joints betweencomponents that can have differential thermal expansion where the spiral wound typegaskets can elastically accommodate the changes in dimensions. Spiral wound gasketsare made out of a V-shaped metal strip that is spirally wound on edge and a soft fillermaterial such as graphite, Teflon or asbestos substitute, or rubber is inlaid between thelaminations. Such gaskets are best used in construction in which the compression of thegasket is controlled to within the manufacturer’s recommendations by solid metal-to-metal contact between the mating parts.

Some gaskets are supplied with an integral ring of a specific thickness on either theoutside diameter or the inside diameter to limit the compression of the resilient metalV-shaped rings to the desired limits, thus preventing crushing of the gasket. The fillermaterials used limit the temperatures to about 450°F (230°C) with Teflon. These gasketsare also available with flexible graphite as a filler, which can be used in applicationsexceeding 1,000°F (540°C).


3-1

3 FUNCTIONAL REQUIREMENTS OF VALVES

3.1 General

Many different types of valves are used in power plants to perform various functions.Valves are used for on-off service, modulating/throttling service, to protectcomponents against overpressure, and to prevent backflow from occurring. Manydifferent types of valve designs and valve body styles can be used to perform thesefunctions. Some of the valve types in common use are:

• Gate valves

• Ball valves

• Butterfly valves

• Globe valves

• Check valves

• Plug valves

In general, valves may be categorized within the following four groups (Figure 3.1):

• Isolation Valves: Used for on-off service (including throttled position) with local orremote actuation. Depending on the particular application and operating conditions,isolation valves can be either gate, globe, butterfly, ball, plug, or diaphragm valves.

• Control Valves: Used for modulating or throttling service. Their operation isautomatic in response to continuous monitoring of some parameter in the controlledsystem. In general, control valves require no manual operator action. A controlvalve functions as a variable resistance in a pipeline.

• Pressure Relief Valves: Used to provide protection against excessive pressure. Thevalve opens automatically when pressure exceeds a preset level and closes afterpressure recedes below a preset level. Power-operated relief valves that open orclose in response to command signals are also utilized.


Functional Requirements of Valves

3-2

• Check Valves: Used to allow flow in the normal flow direction and to prevent flowin the opposite flow direction (reverse flow). Check valves are typically opened andclosed by the flow forces.

Figure 3-1Valve Classification by Function

Some types of valves are capable of performing an on-off function as well as amodulating function, making them suitable for either line valve or control valveapplications. Globe valves, ball valves, and butterfly valves are examples of this type ofvalve.

Another distinction that can be made between valves, based on their principle ofoperation, is that some require an external supply of power to actuate them, and othersare self-contained. Check valves, pressure relief valves, and self-regulating valves areexamples of the self-contained type; whereas gate valves and control valves are not self-contained.

Most valve applications can be satisfied by more than one type of valve. Successfulvalve selection requires a thorough review and analysis of the functions the valve isrequired to perform and its suitability over the entire range of operating conditions.



3-3

Proper selection of a valve is complex and requires consideration of such factors assafety function, design, operating method (for example, manual or remote, includingelectric, air, and hydraulic), reliability, space limitations, ease of installation,maintenance, installed life, and cost. Proper selection requires knowledge of the varioustypes of valves that are available, differences in their design and principles ofoperation, their advantages, and their limitations. In the past, the final selection mayhave been personal preference, based on satisfactory past experience with a certain typeof valve or manufacturer, since two or more valve styles may satisfy the particularapplication.

With the objective of familiarizing the reader with the various functions that valvesmust perform, this section introduces four major functional categories (Figure 3-1).

3.2 Isolation Valves

Valves are categorized as isolation valves when their function requires them to beeither closed or open (including partially open positions). Isolation valves, both manualand power operated, are typically used to isolate a system component or a section ofthe piping system for:

• Maintenance

• Testing (for example, hydrostatic, pneumatic, operational, or functional)

• Diversion of flow from one system component or piping section to another tofacilitate load adjustments and/or to balance equipment duty hours

When an isolation valve is fully closed, it normally exhibits a very low leakage rateacross its closed port(s).

Isolation valves may be required to perform some safety functions such as shuttingdown the plant and maintaining the plant in a safe shut-down condition under designbasis conditions. Containment isolation valves are a special subset of isolation valvesused in nuclear power plants. The selection of containment isolation valves must meetthe following requirements:

• Operating/design fluid conditions

• Periodic performance of a low pressure air leak rate test

• U.S. Code of Federal Regulations 10CFR50, General Design Criteria 54, 55, 56,and 57



3-4

For containment isolation applications, manufacturer’s tests should include, in additionto those required by the applicable codes, seat leak tests representing the accidentconditions against which the valves must isolate. The manufacturer’s test pressuresshould be the same as the pressures at which the valves will be periodically tested atthe plant.

The selection, design, and installation of isolation valves should take into account thefollowing considerations:

• Resilient seats should be mechanically retained (instead of vulcanized or epoxybonded) to ensure better seat reliability and to facilitate seat replacement.

• Selection of metal seated valves should consider the ease of seat repair. For example,in situ repair of globe valve seats is easier than the repair of gate valve seats.

• In valve selection, attention should be given to design features which provide betterleak-tightness. For example, some pressure-energized seat designs and triple-offsetdisc butterfly valves can provide better leak-tightness as compared to conventionaldisc butterfly valves.

• Where off-line maintenance is expected, flanged-end valves should be used.

• Globe and nonsymmetric-disc butterfly valves used as containment isolation valvesshould be installed such that the packing is on the side of the seat away from thepenetration. With this orientation, the valve seat provides the primary seal, and thevalve packing is not required to seal against containment pressure.

• The valve and actuator should have adequate access to facilitate maintenance andrepair activities.

• Some valve designs (such as double disc gate valves) must be installed with stemvertical.

• Valve installation with stem vertical and up significantly facilitates in-linemaintenance and/or repair.

• Local drains should be a Y-pattern globe, gate, ball, plug, or straightway diaphragmvalve in the system pressure/temperature rating to make provision for cleaning outthe drain.

• Root connections for flow, pressure, or differential pressure instruments should beglobe or diaphragm valves to avoid rapid application of pressure to the instrument.For standpipes and liquid level gauges, other valve types may be used. (Note: Gatevalves are not readily available in sizes 2 inch (51 mm) and smaller in ANSI ratingsabove 1,500 pounds).



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• It is suggested that instrument root valves be 3/4-inch (19-mm) valves, except fororifice flanges.

3.3 Modulating/Throttling Control Valves

A modulating control valve is a device used to automatically throttle process fluids asrequired by changes in a variable such as pressure, temperature, flow, or liquid level.

Modulating control valves differ from isolation valves in that modulating controlvalves must continuously modulate to control fluid flow at precise, intermediateopenings. The control valve must be vibration-free and reliable under a wide range ofoperating conditions.

In a modulating service, the valve is usually partially open, and the position of thevalve disc is varied between the open and closed position. Modulation of the disc isachieved by an actuator, which is either mounted to or integral with the valve body.Valve position is proportional to a signal or a condition to achieve a desired systemparameter (for example, flow rate, temperature, pressure).

A throttling function is similar to the modulating function, except that the valve isusually positioned at a fixed percentage open, but may require periodic manualrepositioning of the valve, either directly using manual handwheels or remotely using apower actuator to meet system requirements. This type of throttling occurs inapplications where variations in the parameters of concern are not critical and maypermit a long period of adjustment, or they may be adjusted to suit seasonal changes intemperatures.

Typical of a rough throttling application would be cooling water flow to a coil in the oilsump of a ring-oiled bearing. The bearing must be kept warm, neither too hot nor toocold, but the allowable range is fairly wide.

Leakage for control valves in the fully closed position varies with the construction type.The leakage class achievable for various types of control valves can be defined inaccordance with ANSI/FCI 70-2-1976 [6.12]. Tables 3-1 and 3-2 summarize controlvalve seat leakage classification from this standard.



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Table 3-1Control Valve Seat Leakage Classifications(In Accordance with ANSI/FCI 70-2-1976 [6.12]

MaximumClass

LeakageAllowable

TestMedium Test Pressures

TestingProcedures

Required forEstablishing

Rating

I No test required, provideduser and supplier so agree

II 0.5% of ratedcapacity

Air or waterat 50–125°F(10–52°C)

45–60 psig (310–414kPa) or max.operating differ-ential, whicheveris lower

Pressure applied to valveinlet, with outlet open toatmosphere or connected toa low head loss measuringdevice, full normal closingthrust provided by actuator

III 0.1% of ratedcapacity

As above As above As above

IV 0.01% of ratedcapacity

As above As above As above

V 0.0005 ml perminute of waterper inch of portdiameter per psidifferential

Water at50–125°F(10–52°C)

Max. servicepressure drop acrossvalve plug, not toexceed ANSI bodyrating. (100 psi/690kPa pressure dropmin.)

Pressure applied to valveinlet after filling entire bodycavity and connected pipingwith water and strokingvalve plug closed. Use netspecified max. actuatorthrust, but no more, even ifavailable during test. Allowtime for leakage flow tostabilize.

VI Not to exceedamounts shown inTable 3-2 basedon port diameter

Air or nitrogenat 50–125°F(10–52°C)

50 psig (345 kPa) ormax. rateddifferential pressureacross valve plug,which ever is lower

Actuator should be adjustedto operating conditionsspecified with full normalclosing thrust applied tovalve plug seat. Allow timefor leakage flow to stabilizeand use suitable measuringdevice.



3-7

Table 3-2Seat Leakage Criteria

Nominal Port Diameter ml per Bubbles per

Millimeters Inches Minute Minute*

25 1 0.15 1

38 1-1/2 0.30 2

51 2 0.45 3

64 2-1/2 0.60 4

76 3 0.90 6

102 4 1.70 11

152 6 4.00 27

203 8 6.75 45

* Bubbles per minute as tabulated are a suggested alternative based on a suitablecalibrated measuring device; in this case, a 1/4-in. (6.3-mm) OD x 0.032-in. (0.8-mm) wall tube submerged in water to a depth of from 1/8 to 1/4 in. (3 to 6 mm). Thetube end shall be cut square and smooth with no chamfers or burrs, and the tube axisshall be perpendicular to the surface of the water. Other apparatus may beconstructed and the number of bubbles per minute may differ from those shown aslong as they correctly indicate the flow in ml per minute. Provisions should be madeto avoid overpressuring of measuring devices resulting from inadvertent opening ofthe valve plug.

A control valve assembly consists of a valve body subassembly and an actuator sub-assembly. Many different styles of control valve bodies are in common use, each havingcertain advantages and limitations for a given service requirement.

Valve styles typically used in control valve service include globe valves, ball valves,plug valves, butterfly valves, and diaphragm valves. Variations of these styles are usedto provide a higher degree of accuracy, as well as linear flow to valve positionindication. These variations are discussed in the section dealing with a particular valveand function.

There has been a growing trend in recent years toward the use of rotary valves incontrol applications. The major reasons for this are that rotary valves:

• Require less space

• Provide high flow capacity with low pressure drop

• Can provide good throttling control, especially with special shaped or contouredclosure elements

• Are very economical, particularly in larger sizes



3-8

Leading control valve manufacturers estimate that most existing control valveapplication problems can be resolved and could have been averted if accurateapplication data and operating conditions had been provided prior to the selection andsizing of the valve. Recent studies [1.6, 5.38] show that an accurate prediction of valveperformance requires a detailed study of the entire hydraulic system includingpressure/flow sources (for example, pumps, upstream tanks/reservoirs, surge tanks,accumulators); flow resistances in the hydraulic system (such as heat exchangers,strainers, other valves, orifice plates, pipes, elbows, tees); piping layout (single-lineflow or parallel-line flows); fluid type (water, steam, air, nitrogen); and operatingconditions.

The reason is that, as the control valve disc position changes, the total system flowresistance changes. In a pumped system for example, the pump operating point on thepump curve will also change to a new equilibrium point where the total systempressure drop at the new flow rate matches the head developed by the pump at thenew operating point. Thus, analyzing a control valve problem should involveexamination of the entire hydraulic system (see Reference 1.6 for additional discussion).

3.4 Pressure Relief Valves

Pressure relief valves are discussed in great detail in Reference 1.4 and are only brieflydiscussed here.

Valves provided to function as pressure relief devices are used to dissipate excessivesystem pressure to a pressure suppression system or to the atmosphere, thus avoidingoverpressurization of the protected system. This pressure relief function can beperformed by:

• Installation of a valve that opens automatically to discharge system media whenpressure at the inlet of the valve, acting directly on the main valve disc, exceeds apredetermined level. No external power source is needed.

• Use of a pilot valve that opens automatically when pressure at the inlet of the valveexceeds a predetermined level. The opening of the pilot valve subsequently causesthe main valve disc to open by action of the inlet pressure. The pilot valve mayalternately be provided with means to be opened at any inlet pressure by theapplication of an external power source.

• Installation of a power-operated valve where the main valve disc is opened by theapplication of external power to the actuator.

The term “pressure relief valve” encompasses relief valves, safety valves, and safety-relief valves.



3-9

Relief, safety, and safety-relief valves are used to provide protection for both systemcomponents and operating personnel. These valves were originally designed usingweights mounted on the valve stem. The weights established the set point at which thevalve would automatically open to protect against overpressure. The valve would thenclose automatically when pressure dropped below the set point. This design has thedisadvantage of being sensitive to system vibration, as well as lacking proper enclosureto provide protection of valve components.

The disadvantages associated with the use of weights led to the use of springs forcontrolling system pressure. Springs resulted in a more compact design, which ishighly desirable in large volume, high pressure applications. Although the use ofsprings to control force on the valve disc is preferable to weights, springs aresusceptible to changes in force applied as system temperature is elevated. As analternative to the spring-type relief device, pressure may be controlled using pilot-operated valves.

Pilot-operated relief valves are a type of pressure relief valve that utilizes either theprocess pressure or an external power source through a pilot mechanism to actuate thevalve. Since the valve operating mechanisms (pilots) have static and moving seals withsmall clearances, the process fluid must be extremely clean. Some of these pilot-operated valves offer the advantage of allowing independent adjustment of bothaccumulation and blowdown external to the valve.

A rupture disc is a unique type of overpressure protection device, consisting of amembrane held between flanges, which is designed to burst at a predeterminedpressure. The major difference between rupture discs and pressure relief valves is thatthe rupture disc does not re-close. It will remain tight until it bursts, at which time itmust be replaced. Since the rupture disc is operated by a pressure differential, it issensitive to back pressure; therefore, the burst pressure of a rupture disc will vary asthe back pressure or downstream pressure varies. Rupture discs can be used tosupplement relief valves. They can also be used at the inlet of a pressure relief valve toprotect the valve from the corrosive effects of the process fluid.

Power-operated relief valves (PORVs) are used in conjunction with spring-loaded reliefvalves. PORVs are actuated at a system pressure well below the set point of spring-loaded valves to eliminate unnecessary operation of spring-loaded valves that oftenleak after reseating.

Proper selection of pressure relief valves requires an understanding of the relievingrequirements of the system or component that is to be protected and the environmentalconditions associated with that installation. The system relieving requirements includeconsiderations such as response time and discharge capacity.



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Typical process areas that must be protected are low-pressure storage tanks, pressurevessels, heat exchangers, pumps or compressors, and piping systems.

Low-pressure storage tanks must be protected when liquid is pumped into or out of thetank. This is required to prevent overpressurizing or collapsing the tank when liquid isbeing moved from or to the tank.

Heat exchangers that have valves on both the inlet and outlet can be isolated if bothvalves are shut. Safety/relief valves should be provided to protect the heat exchangerfrom the effects of thermal expansion of the liquids that may be isolated within the heatexchanger. Consideration should also be given to protection of equipment on the lowpressure side if a tube within the heat exchanger should rupture.

Positive displacement pumps and reciprocating compressors should have pressurerelief valves on their discharges to relieve the fluid if the discharge should be blocked.

3.5 Check Valves

This section provides an overview of check valves (also called non-return valves).Detailed information on selection, installation, troubleshooting, and maintenance ofcheck valves is given in EPRI’s Application Guide for Check Valves [1.20] and EPRI’s CheckValve Maintenance Guide [1.21].

Check valves are self-actuated valves whose functions include:

• Prevention of reverse flow

• Keeping lines full of fluid

• Prevention of loss of fluid when the system is not in operation

• Prevention of reverse rotation of pumps

• Prevention of outflow of fluid from vessel

• Prevention of water column separation

Check functions are generally satisfied by using lift, swing, tilting disc, double disc, orsilent (nozzle) check valves. These valves are best installed in a horizontal line and areopened by the velocity head of the flowing fluid in the normal flow direction. In almostall cases, the impetus to close the disc is initiated by the weight of the valve disc or bysprings with the primary seating force generated by the system differential pressure. Insome instances, auxiliary external weights, springs, dashpots, or other actuation means



3-11

are used to aid closing, decrease slamming action against the seats, or to preventclosing when servicing.

Check valves are used to ensure that the process medium flows in one direction only.Typical applications are at the discharge of multiple pumps that provide flow andpressure head to a common manifold. In the event that one of the pumps ceases toproduce flow and pressure head, the check valve at its discharge prevents a flowreversal through that pump caused by the pressure head produced by the otherpump(s). Other applications include feedwater lines to boilers and, in general, a meansto minimize the loss of process media in the event of a pipe line rupture.

Even though good shut-off can be provided by some check valves (especially when softseats are used), the main function of these valves should be to prevent flow reversal.Check valves should not normally be considered as a suitable replacement for isolationvalves.

Check valves should not be oversized and should be located a safe distance from anyflow disturbance (such as pumps, elbows, tees, or other valves). Over-sizing andturbulence caused by upstream flow disturbances create instability of the closuremember and may result in premature degradation or failure of the valve. Some designsare available in right angle patterns. Almost all check valves are top entry designs andallow servicing without removal from the line.

Various manufacturers, architect engineers, nuclear steam supply system suppliers, andusers have developed their own criteria for selecting the type of check valve to be usedin a particular service. In general, the selection criteria have been qualitative, and morethan one type of check valve can be chosen to successfully meet the requirements of agiven application, provided all the important technical factors are properly taken intoconsideration during sizing and selection. Therefore, it is not uncommon to see swing,tilting disc, or lift checks being used in similar applications and performingsuccessfully at different plants and sometimes even within the same plant. Theapplication and use of check valves in power plants has been the subject of acomprehensive study, the results of which are documented in Reference 1.20. Thisstudy was prompted by the unexpected failure of several check valves in nuclearpower plants, which resulted in significant loss of plant availability, as well asequipment damage.

Recently, EPRI published the Check Valve Maintenance Guide [1.21] to provide nuclearutilities with detailed discussions of check valve maintenance issues. Check valves arenot discussed further in this Guide, and the reader is referred to References 1.20 and1.21.


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4 GATE VALVES

4.1 Introduction and Application

Gate valves, the most commonly used valves in power plants, are primarily used foron-off service. Gate valves are available in a variety of materials; therefore, they aresuitable for the toughest applications in high pressure and high temperature systems.Gate valves, ranging in size from 1/4 inch (6 mm) to sizes exceeding 48 inches (1200mm), offer the lowest pressure drop during fluid flow conditions, approaching that ofan equal length of straight pipe when fully open, but tend to have more operabilityproblems and a higher seat leakage rate than globe valves.

4.2 Design

This section provides general descriptions, advantages, and disadvantages of most gatevalve designs in nuclear power plants. Other gate value designs not commonly used innuclear power plants (such as knife gate valves) are not discussed here.

4.2.1 General

Gate valves can be either rising stem or non-rising stem design (Figure 4-1). Rising stemdesigns, utilizing an outside screw and yoke (OS&Y) (Figure 4-2), provide theadvantage of having the power threads outside the fluid, thus minimizing threaddamage from exposure to the fluid. Rising stem action allows the incorporation of anoptional backseating feature to assist in isolating the packing from the process fluid bypulling the stem up against the inside of the bonnet (see Section 2.3.8).


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Figure 4-1Inside Screw Stem Thread Configurations

Figure 4-2Rising Stem Design, Outside Screw

Another rising stem option utilizes a power screw inside the valve body that exposesthe threads to the fluid. A non-rising stem configuration requires the power screw to beinside the valve disc or wedge. Since the stem rotates in the packing without axialmotion, packing wear and damage resulting from abrasive contaminants andundesirable materials being dragged across the packing is minimized. Thedisadvantages of the non-rising stem are that the threads are exposed to the fluid, the


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stem cannot be backseated, and the disc position cannot be judged by the stem positionfrom the outside. Additionally, inside screw and non-rising stem configurations areusually limited to low pressure and low temperature applications. These designs aregenerally used in fluids with good lubrication properties and without abrasivecomponents because the working fluid lubricates the stem threads.

Most gate valve designs are offered with metal-to-metal seating at the gate-to-seatinterface. Metal-to-metal seating allows valves to operate at a much higher temperaturethan would be possible with elastomeric or polymeric-type seat materials. Metal-to-metal seating also makes the valve fire safe for most applications. The types of gatevalves available include:

• Solid wedge

• Split wedge

• Flexible wedge

• Parallel expanding

• Parallel slide double-disc

• Slab

• Knife

4.2.2 Solid Wedge

Solid wedge gates (Figure 4-3B) are of a simple, one-piece construction characterized bya V-shaped wedge that converts the axial stem thrust to a high seat load, normal to theseat faces. The seats can be separate pieces held firmly in the valve body by pressfitting, welding, or threading; or they can be machined into the body. Typically, thebody of a wedge gate valve has gate guides on the sides (as shown in Figure 2-16) thatmate with guide slots on the sides of the disc. These guides support the load caused bydifferential pressure across the wedge and keep the wedge away from the seat faces,except for a small distance very near the fully closed position, so as to minimize seatwear.

In small valves, the seat loading caused by the closing force applied through the stemto the gate is much higher than the seat loading created by the pressure differentialacross the gate. Therefore, seating effectiveness is not significantly increased byincreasing the differential pressure across the gate. In larger valves, the differentialpressure acting on the gate provides the primary load against the seat, and themechanical force from the stem is used to enhance the seating action.


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Figure 4-3Wedge Gate Valve

The solid wedge design is preferred in smaller sizes where the stiffness of valve bodyand disc is much higher than that of the adjacent pipe. The increased stiffnessminimizes seat distortion which can increase seat leakage or gate pinching, due to pipeloads transmitted to the valve ends. The solid wedge gate design is not suited for largevalves, especially in high temperature applications where differential expansion anddistortion of the gate, body, and seats, due to mechanical and thermal loads, can causeloss of seat tightness and/or binding of the gate, which can either increase theoperating thrust required or, in some cases, cause complete inoperability.


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The following list identifies some of the advantages and disadvantages of the solidwedge gate.

Advantages:

• Sealing can be improved by increasing stem force.

• Simple construction.

• Bi-directional operation due to symmetrical construction.

Disadvantages:

• Sensitive to line loads: bending moment, torsion, and axial loads that aretransmitted by the adjacent pipe to the valve ends. The sensitivity increases with thesize of the valve.

• Seating is sensitive to thermal distortions because the solid wedge gate does nothave the ability to easily conform to the seat face plane distortion.

• Lack of disc flexibility makes solid wedge gate valves more susceptible to thermalbinding (see Section 4.2.10).

• Difficult to perform in-line repair because of the difficulty in achieving accuratematching of seat angles during lapping.

• Depending on the clearance in the gate area, the gate could tilt under flow forcesand create galling or high wear at the disc/seat faces.

4.2.3 Flexible Wedge

Flexible wedge gate design (Figure 4-3A) was introduced to minimize leakage or gatebinding and sticking problems caused by distortion of the valve body due to thermaland pipeline stresses transmitted to the valve ends. The flexible wedge design, a simplevariation of the solid wedge, is constructed in one piece composed of two discsconnected with an integral boss that permits independent flexure of the discs. Becausethe flexible wedge is simple and has no separate components that could become loosein service, it is widely used in power plants.

The following list identifies some of the advantages and disadvantages of the flexiblewedge design.


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Advantages:

• Better immunity to line loads than solid wedge design, minimizing sticking andleakage.

• Simpler in construction than the split wedge.


• Simultaneous seating of both discs can be used to check body seat integrity, withoutline pressure, by pressurizing the body between the seats.

• Easier to repair seat faces in line than the solid wedge since the flexible wedgedesign can tolerate more angular mismatch.

• Flexible wedge gate valves are less susceptible to thermal binding than solid wedgegate valves.

Disadvantages:

• Both wedge pieces can independently seat simultaneously, thus trapping pressurein the body. This can cause inadvertent overpressure in the body during pressure orthermal transients and an increase in thrust required to open the valve due to thecombined friction from the two wedge pieces, in some cases rendering the valveinoperable. This condition is often referred to as “pressure locking” or “double-discdrag” (see Section 4.2.9).

• Depending on the clearance in the gate guide area, it is possible for the gate to tiltunder flow forces and create galling or high wear at the disc/seat faces.

4.2.4 Split Wedge

Split wedge gates (Figure 4-3C) are composed of two separate pieces. The split wedgeconstruction permits the gate assembly to more easily tolerate line loads andtemperature transients by allowing each wedge piece to align with its mating seat. Thisfeature is used in larger gate valves to overcome sticking problems encountered withthe solid wedge. Because of the ability of each gate wedge to align itself independentlyagainst its respective seat, this type of construction allows both wedge pieces to seatsimultaneously; consequently, fluid pressure can be trapped in the body. Under atemperature increase, the thermal expansion of this trapped fluid can cause very highpressures in the body, which can damage the pressure boundary. The trapped fluidincreases the thrust required to open the valve (also called pressure locking), andoccasionally results in complete inoperability. Provisions to relieve the body pressuremust be made in such valves to eliminate these problems (see Sections 4.2.9 and 4.2.10for details).


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The following list identifies the advantages and disadvantages of split wedge type gateconstruction.

Advantages:

• Better immunity to line loads than solid wedge design, minimizing sticking andleakage.

• Can provide simultaneous shut-off against pressure on both the upstream and thedownstream seats (block and bleed).


• Simultaneous seating of both discs can be used to check body seat integrity withoutline pressure by pressurizing the body between the seats.

• Easier to repair seat faces in line than the solid wedge because the split wedgedesign can tolerate more angular mismatch.

Disadvantages:

• Both wedge pieces can independently seat simultaneously, thus trapping pressurein the body. This can cause inadvertent overpressure in the body during pressure orthermal transients and an increase in thrust required to open the valve due to thecombined friction from the two wedge pieces, in some cases rendering the valveinoperable. This condition is often referred to as “pressure locking” or “double-discdrag.”

• The two-piece construction is more expensive and somewhat more complex than asolid wedge. It also has the potential for allowing disengagement between the gatepieces and the stem.

• Depending on the clearance in the gate guide area, it is possible for the gate to tiltunder flow forces and create galling or high wear at the disc/seat faces.

• Valve cannot be used for throttling, and disc assembly cannot be left in midstrokeposition for any extended period of time.


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4.2.5 Parallel-Expanding Gate

Parallel-expanding gate valves are of multiple-piece construction with the faces of thegate pieces that contact the seat parallel to each other. Two different designs arediscussed in this section:

• The Anchor/Darling double-disc gate valve, shown in Figure 4-4

• The W-K-M parallel expanding gate valve, shown in Figure 4-5

Figure 4-4Anchor/Darling Double-Disc Gate Valve


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Figure 4-5W-K-M Through-Conduit Double-Wedge Parallel Expanding Gate Valve

When going from open to closed position, the wedge pieces move down together, as anassembly, without any relative motion between them until, at the very end of thestroke, one of the pieces contacts the bottom stop. Continued motion of the stem afterthis contact causes a climbing action of one wedge piece on the other at the inclinedplane interface between them, which in turn expands them laterally against theirrespective seats. Parallel-expanding gate valve designs can provide simultaneousseating against both the upstream and downstream pressures. This can be an advantagebecause of the redundancy in seating available in such design. However, the doubleseating feature can also be a disadvantage because the body can trap fluid, which cancause inadvertent high pressure during thermal transients.

The W-K-M parallel-expanding gate valve designs employ special mechanisms thatkinematically prevent premature gate expansion when the gate assembly is in the mid-travel position. Expansion of the gate before reaching the end of the stroke may preventthe gate from closing completely. This design is also made in a through-conduitdouble-wedge arrangement that permits expansion of the gate in the open position aswell as the closed position (Figure 4-5). The valve preferred flow direction is with gatedownstream (segment upstream).


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As part of EPRI’s Performance Prediction Mehodology (PPM), the required thrustprediction methodologies were developed for the Anchor/Darling double-disc gatevalves (see Reference 2.14) and for the W-K-M parallel expanding gate valves (seeReference 2.17). The methodology for the W-K-M valve shown in Figure 4-5 shows that(under certain conditions) premature wedging in the closing direction can occur in thenon-preferred orientation (or under reverse flow conditions with the valve in thepreferred orientation). Premature wedging may prevent the valve from achieving fullflow isolation.

The following list identifies some of the advantages and disadvantages of parallelexpanding gates.

Advantages:

• Can provide a positive, simultaneous shut-off against pressure on both theupstream and the downstream seats.

• Through-conduit double-wedge design can double block and bleed, that is, provideblock and bleed in closed position and also prevent the line pressure from enteringthe body cavity through both seats simultaneously in the open position.


• Double-disc seating can be used to check integrity of both seats simultaneously bypressurizing the body between the seats.

Disadvantages:

• Depending on the actual construction and stiffness of the gate, the parallel-expanding gate design can be very intolerant of line loads and thermal transients.

• Normally unidirectional or has a preferred flow direction for best performance. Thetwo wedge pieces are usually asymmetrical, and one of the two pieces has betterability to self-align with respect to the seat face.

• Both wedge pieces can independently seat simultaneously, thus trapping pressurein the body. This can cause inadvertent overpressure in the body during pressure orthermal transients and an increase in thrust required to open the valve due to thecombined friction from the two wedge pieces, in some cases rendering the valveinoperable. This condition is often referred to as “pressure locking” or “double-discdrag.”

• More complex and requires special mechanism to prevent inadvertent mid-travelexpanding movement of the discs toward the seating surfaces.


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• Depending on the clearance in the gate guide area, it is possible for the non-through-conduit gate designs to tilt under flow forces and create galling or highwear at the disc/seat faces. The through-conduit gate design, shown in Figure 4-5, isnot susceptible to this problem.

• Valve cannot be used for throttling, and disc assembly cannot be left in midstrokeposition for an extended period of time.

4.2.6 Parallel Slide Double-Disc

The parallel slide double-disc gate, also called a parallel expanding double-disc gate(Figures 4-6 and 4-7), is constructed in two pieces, with each disc allowed to floatindependently and mate with its seat. The individual pieces are not mechanicallywedged against their respective seat but are preloaded by a spring between them thatprovides initial seating force. The flexibility of the spring allows distortion and changesin dimensions between the seat faces to be easily accommodated without pinching thegate, which provides complete immunity from sticking and binding under line loadsand thermal transients. The pressure differential across the gate increases thedownstream seat contact force and provides a tighter seal. Parallel slide double-disctypes of gates can provide only a downstream seal and are most effective in the largervalve sizes in applications where at least moderate differential pressure exists. The self-wiping action of the gate against the seat during operation deeps the seat face clean ofany foreign material and provides good sealing action over a long time, especially inclean fluid service.

Figure 4-6Parallel Slide Double-Disc Gate Valve


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Figure 4-7Through-Conduit Parallel Slide Double-Disc Gate Valve

The following list identifies some of the advantages and disadvantages of the parallelslide double-disc gate valve.

Advantages:

• Of all the gate valve designs discussed, parallel slide double-discs are most tolerantof, and virtually immune to, line loads due to the ability of the spring between thegate pieces to absorb large seat deflections with virtually no change in seat contactforce.


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• They are tolerant of temperature changes during operation. The gate will not binddue to differential thermal expansion effects because of the resilient spring betweenthe discs.

• They are tolerant of a lack of parallelism between the two seat faces because of theability of the two independent gate pieces to align themselves. This feature alsoprovides a good shut-off under bending moments transmitted to the valve ends bythe adjacent pipe, which causes tilting of the seat faces. The ability to absorb largevariations in dimensions between the seat faces, without any adverse effect on thevalve performance, allows more economical fabrication tolerances to be used than inwedge gate valves.

• Less tendency to galling due to smaller changes in seat loading under line loads andthermal transients.

• Can be used bidirectionally due to symmetrical design.

• Double-disc seating can be used to check body seat integrity without line pressureby pressurizing the body between the seats.

Disadvantages:

• Sealing cannot be improved by increasing stem force as in wedge gate valves.

• Downstream sealing only, upstream disc does not seal against line pressure.

• Floating gate pieces can trap body pressure and effect double-disc seating, allowinginadvertent overpressure in the body during pressure or thermal transients.

• Gate has constant spring load over entire stroke creating nominally higher runningtorque.

• Depending on the clearance in the gate guide area, it is possible for the non-through-conduit gate designs to tilt under flow forces and create galling or highwear at the disc/seat faces. The through-conduit gate design, shown in Figure 4-7, isnot susceptible to this problem.

4.2.7 Westinghouse Flexible Wedge

The features of the Westinghouse flexible wedge gate valve design that make it uniquefrom other flexible wedge gate valve designs are the stem, disc assembly, and guiderails. The stem and disc assembly (Figure 4-8) includes the stem, double-pinnedlinkage, and flexible wedge. The upper portion of the stem is threaded with ACMEthreads that engage mating threads of the operator nut. The bottom of the stem is a


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clevis-type stem head to which the linkage system is connected. The double-pinnedlinkage allows the disc to translate relative to the stem in a direction parallel to fluidflow. The upper portion of the disc contains a keystone-shaped slot that retains thebearing block of the stem-to-disc connection. The disc is a one-piece flexible wedgewith hardfaced sealing surfaces and guide slot surfaces.

Figure 4-8Westinghouse Flexible Wedge Gate Valve


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Two guide rails are installed in parallel slots in the body cavity to guide the disc duringopening and closing strokes. The upper portion of the guide rails is wider than thelower portion with a tapered transition between the two portions. This design results ina smaller disc-to-guide clearance in the vicinity of the fully open position that keeps thedisc from rattling under flow turbulence. The larger disc-to-guide clearance in thevicinity of the fully closed position allows the disc to contact the downstream seatearlier during the closing stroke than a valve with tighter clearance.

As part of EPRI’s PPM, the required thrust prediction methodology was developed forthe Westinghouse flexible wedge gate valves [2.15]. Apart from the complexity of theconnection between the stem and the disc assembly, the advantages and disadvantagesof the typical flexible wedge gate valve design given in Section 4.2.3 also apply to theWestinghouse design.

4.2.8 Slab Gate

The slab gate design features a very simple one-piece parallel gate (Figure 4-9), whichis matched flat on both sides. The slab type of gate requires axially movable seatsbetween the seat and body and a soft-type seat insert to allow seating without the highcontact stresses required in metal-to-metal seats. Seating between the gate and seatfaces is accomplished in both upstream and downstream locations. Both seats aredesigned to float freely in their respective seat pockets which are machined into thevalve body, and are forced against the gate by springs. When the gate is closed, theupstream seat is axially forced against the gate by the springs, and the differentialpressure is acting on the unbalanced annular area of the seat. Downstream seating isachieved by floating the gate against the downstream seat, due to the differentialpressure acting across the entire area defined by the seat bore.


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Figure 4-9Slab Gate Valve


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The following list identifies some of the advantages and disadvantages of slab-stylegate valves.

Advantages:

• Can tolerate line loads better than wedging type gate valves without binding or seatdegradation. Virtually immune to line loads if sufficient clearances are present, dueto the spring-loaded seat design that can absorb large changes in dimensions of theseat pocket area caused by line loads transmitted to the valve ends by the adjacentpipe.

• Easy to maintain because of the removable seat design.

• Will self-relieve body overpressure to the high pressure side by pushing the springenergized seats away from the gate. This eliminates the high pressure build-up inthe body cavity associated with most of the other gate valves under temperatureincreases.

Disadvantages:

• Seating effectiveness cannot be increased by the application of additional force tothe stem as in wedge gate valves.

• Service conditions limited to 400°F (200°C) with conventional soft seating materialsmade of elastomers and plastics. Some designs utilize higher temperature seatmaterials, for example, carbon or graphite for higher temperature applications.

• Where applicable, cannot compete with butterfly valve in size or cost.

4.2.9 Pressure Locking in Gate Valves

Normal operation of most gate valves requires that the force needed to actuate thevalve consider only the effects of single seating where the primary seat load andassociated friction occur at the downstream seat. In some instances, due to valveconstruction and operating procedures, double seating (on both the upstream anddownstream seats) can occur, thus increasing the force required to actuate the valve.Pressure locking in gate valves is associated with the increase in the required openingthrust due to trapped body pressure. The double seating occurs when the pressuretrapped inside the valve body (or bonnet) exceeds the upstream and downstreampressures and can be hydraulically induced or thermally induced (see References 4.2,5.30, 5.52 and 5.53 for detailed discussions).


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Thermally induced pressure locking occurs when the temperature of the liquid trappedin the valve body increases above its initial value. Hydraulically induced pressurelocking occurs when the upstream and downstream pressures drop, leaving thetrapped body pressure at its initial level.

Hydraulically induced pressure locking can also be caused by system pressuresurges/transients that increase the body pressure. Pressure locking is not limited toliquid flow and can occur in steam applications where valve configuration permitscondensate to collect and enter the bonnet.

Double seating or pressure locking is most common in double-disc gate and flexiblewedge gate valves, where each side of the gate can make contact with its respectiveseat, as shown in Figure 4-10. Operating problems associated with this phenomenonprompted the U.S. NRC to issue Generic Letter 95-07 [4.2], requesting nuclear utilitiesto review safety-related, power-operated (including motor-, air-, and hydraulicallyoperated) gate valves for susceptibility to pressure locking and thermal binding.Concerns about pressure locking and fluid entrapment in the valve bonnet have beenrecognized for over 20 years [4.27].

Figure 4-10Gate Valve Bonnet Overpressurization


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Recent research identified the major factors that affect required opening thrust underpressure locking conditions. Some limitations in the previous pressure lockingcalculation methodologies developed by Commonwealth Edison [5.53] and EntergyOperations [5.54] have been addressed in a later study [5.30]. It should be noted that acalculation methodology to predict required opening thrust under pressure lockingconditions is needed to solve pressure locking problems when physical valvemodifications cannot be performed due to operational or time constraints. The mainfactors that affect the required opening thrust under pressure locking conditions aresummarized as follows [5.30]:

Disc Flexibility: Disc flexibility is one of the major factors responsible for increasedthrust requirements under pressure locking conditions. In double-disc designs (such assplit wedge and parallel-expanding disc designs), the trapped bonnet pressure acts onthe entire upstream and downstream disc areas and results in increased seat contactforce and friction. In flexible wedge disc designs, the trapped bonnet pressure acts oneach disc less the disc hub area and results in pressure, bending, and shear deflectionsthat are resisted by the body seats.

Valve Body Flexibility: Valve body flexibility is another major factor that contributes topressure locking, especially for low pressure class valves. For a solid wedge gate valvewhere the disc is relatively rigid, body flexibility is the main factor that causes the seatload changes under varying pressure conditions.

Strain Energy in Stem and Valve Topworks: A self-locking stem and gear train can store asignificant amount of elastic strain energy in the stem and valve topworks duringwedging. Spring-loaded actuators (such as Limitorque models SB and SBD) can storeeven more strain energy due to their higher flexibility. The stored strain energy candrive the disc deeper into the seat when the valve body expands under pressure. Asubsequent pressure drop will cause disc pinching and an increase in unwedgingthrust.

Sequence of Pressure Changes: The actual sequence of pressure changes (including shortduration pressure surges) that occur in the bonnet, upstream pipes, or downstreampipes when the valve is closed can result in hydraulically induced pressure locking,which affects the opening thrust (see Reference 5.30).

Thermally Induced Pressure Locking: Idaho National Engineering and EnvironmentalLaboratory (INEEL) performed pressure locking tests to investigate the effect oftemperature on bonnet pressure and opening thrust [5.52]. INEEL found pressure toincrease rapidly with temperature in a water-solid bonnet. The tests show thatpressurization of the bonnet might not occur if seat leakage is high. However, suchleakage is not reliable in preventing pressurization. These tests also showed that theopening thrust increases linearly with bonnet pressure.


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Bonnet Entrapped Air: The INEEL testing mentioned above showed that pressurizationcurves of 0.5, 1.0, and 2.0% of entrapped air by volume are similar to those with noentrapped air, except that initial pressurization begins at higher temperatures.Furthermore, the temperature at which initial pressurization begins increases with theincrease of percentage entrapped air (up to 2%). Subsequent pressurization followingdepressurization occurs immediately, as in the case of tests with no entrapped air. Thefact that the presence of an air pocket delays first pressurization but not subsequentpressurization suggests that the air pocket is either collapsed or forced into solution bythe first pressurization cycle.

Packing Leakage: Packing leakage can prevent pressure locking from occurring.However, as the bonnet pressure increases, the packing pressure increases and sealingcapability tends to improve. Furthermore, an increase in bonnet pressure and packingcontact pressure will likely increase the packing force, which will further increase theopening thrust.

Even though a lack of bubble-tight seating would reduce the amount of pressuregenerated in the body, there are cases of catastrophic failure of the valve body orbonnet due to pressure locking. Provisions must be made to eliminate the possibility ofthis excessive pressure build-up in the body cavity to avoid structural damage to thepressure boundary.

When a relief valve is used to prevent overpressure in the body cavity, the actualpressure in the body may still be higher than in the upstream or downstream piping.This condition should be considered when sizing actuators. The differential pressureacross each seat must be considered to arrive at the total frictional resistance.

The required operating thrust/torque can also increase under another scenario thatinvolves thermally induced pressurization in the piping between two valves with tightseats [4.30]. The heating of water-filled piping between two closed block or isolationvalves can increase both the trapped pressure between the two valves and the requiredopening thrust/torque for both valves. This phenomenon can occur inside or outsidethe containment and is not limited to gate valves. In some applications, the installationof a pressure relief valve in the piping between the two valves may be required toprevent fluid pressurization.


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4.2.10 Options to Mitigate Pressure Locking in Gate Valves

One or more of the methods itemized below can be used to prevent trapping bodypressure:

• Drill a hole in the upstream disc.

• Install external bypasses to connect the body cavity to the upstream or downstreampipe or nozzle. These bypasses often incorporate manually operated valves that canbe used during valve or system testing.

• Incorporate external bypass in the body to communicate the body cavity to theupstream or downstream conduit.

• Install a non-functioning upstream seat (for example, with a notch across the face).

• Install a relief valve to vent excessive pressure from the body cavity.

• Install an internal relief valve in the upstream disc to limit the amount of differentialpressure between the body and the upstream side.

• Implement administrative controls to relieve pressure in the body cavity by openinga remotely actuated valve before opening the valve.

The method selected will depend on the desired end result and on the particularfunction of the valve. External or internal bypasses on the body make the valve bodyunidirectional and, when connected to the downstream side, require that the valve becapable of achieving upstream seating. Modifying the upstream disc makes the gateunidirectional, and special attention needs to be given when initially installing thevalve and when reassembling after maintenance. Relief valves installed in the upstreamdisc may corrode, leak, or get stuck open or closed with line debris, thus renderingthem ineffective. Block valves that isolate two fluid media are better served by usingadministrative controls to energize power-operated relief valves to vent the pressure inthe body before actuation begins.

4.2.11 Thermal Binding in Wedge Gate Valves

Thermal binding is generally associated with a wedge gate valve that is closed whilethe system is hot and then is allowed to cool before attempting to open the valve.Mechanical interference occurs because of different expansion and contractioncharacteristics of the valve body and disc. Thus, reopening the valve might beprevented until the valve and disc are reheated. Solid wedge gate valves are mostsusceptible to thermal binding. However, flexible wedge gate valves experiencingsignificant temperature changes or operating with significant upstream anddownstream temperature differences may also thermally bind. Some parallel disc gatevalve designs are not susceptible to thermal binding (see Figure 4.6 for an example).


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Differential thermal expansion of components should be carefully considered in hightemperature valve design. Problems created by differential thermal expansion can becaused by both temperature differences and coefficient of thermal expansiondifferences.

Thermal binding refers to an increase in wedge gate valve opening thrust when thetemperature distribution in the valve during opening is different from that when thevalve was closed. The increase in opening thrust can cause the valve to fail to openbecause the required stem thrust exceeds the actuator’s capability or the structuralstrength of the valve/actuator weak link. Because of disc stiffness, solid wedge gatevalves are more susceptible to thermal binding than flexible wedge gate valves.Thermal binding in gate valves can be caused by many factors including:

• The coefficient of thermal expansion of the gate material (αgate) is different from thecoefficient of thermal expansion of the valve body material (αbody). The openingthrust tends to increase when:

— The valve temperature during opening is lower than the valve temperatureduring closing and αgate < α body.

— The valve temperature during opening is higher than the valve temperatureduring closing and αgate > αbody.

Under either of these conditions, the change in temperature tends to increase thedisc-to-seat interference and opening thrust.

• The average temperature of the gate is less than the average temperature of thebody. As the temperature of the gate increases after closing, the gate expandscausing additional gate-to-seat interference, which increases the opening thrust.

• The stem temperature during closing is less than the stem temperature after closing.As the stem temperature increases, the stem compressive force increases, and thedisc is forced deeper into the seat.

When hot fluid enters a cold valve, it immediately surrounds the valve trim. The trimexpands quickly, causing differential thermal expansion between the trim and bodydue to the relatively lower mass of some of the trim components. For moving parts, thisexpansion results in reduced working clearances causing accelerated wear, highactuation forces, or binding and galling. For interference fit parts, such as seats andbushings or guides, thermal cycling can cause loosening. These parts must be screwed,welded, or brazed in place in applications where high thermal gradients and cyclingare present.


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EPRI is currently conducting a research project to develop a mathematical model toquantify the opening thrust requirements under thermal binding conditions. Thisproject may also provide some recommendations for means to mitigate the effects ofthermal binding. In the meantime, the following suggestions may be useful inaddressing thermal binding:

• For existing valves, Attachment 2 of the NRC Generic Letter 95-07 [4.2] providespotential resolutions for gate valves susceptible to thermal binding which include:

— Replacing a wedge gate valve with a parallel-disc gate valve.

— When allowed, procedure modifications to prevent thermal binding from takingplace. This may include using limit switch control in the closing direction orcracking open the disc before a large temperature change takes place.

• For new valves, the valve operating conditions should be considered in the materialselection of valve components and in disc shape design. Detailed finite elementanalyses may be performed to ensure that thermal binding will not occur under anyplant operating conditions. Alternatively, parallel-disc gate valves may be used inapplications where thermal binding is a concern.

4.3 Installation Practices

General valve installation practices are discussed in Section 19. The ideal orientation forany valve is to have the stem (or bonnet) vertically up in a horizontal run of piping toobtain optimal wear characteristics and operability. This orientation cannot always beaccommodated when considering overall plant design and arrangements. There arealso precautions that should be taken with certain valves relative to proximity of otherequipment or pipe fittings. Valves should always be installed in an orientation in whichthey are seismically qualified, when seismic qualification is required.

Regardless of the type of gate valve used, with the stem in other than the verticalupright orientation, uneven, unpredictable wear can occur on guides, guiding surfaces,stems, T-slots, and seats, and packing life can be shortened. In addition, testing,disassembly, and maintenance become more difficult.

If the stem (and thereby the bonnet) is oriented at an angle below the horizontal, therelatively large volume of the bonnet acts as an unflushable accumulator and traps anyinsoluble material passing through the valve. The bonnet can also act as a cold trap(that is, it can precipitate out material that is held in solution by the heat of the fluid).This precipitate material can shorten valve life, packing life, and can hamperdisassembly. If the material is radioactive, the disassembly procedure becomes evenmore difficult.


Gate Valves

4-24

Valves that require manual operation or frequent maintenance and repair should bereadily accessible. Valves and actuators should have sufficient clearance for repair orremoval and reinstallation. Large and heavy valve/actuator assemblies may requireadditional supports. Power supply equipment (such as power cables or air lines)should be routed in such a way as to not interfere with maintenance activities.Valve/actuator assemblies should be protected from corrosive drippings from otherequipment and should not be subjected to extreme temperatures and/or radiationbeyond design limits.

4.4 Operation Practices and Precautions

Because of the potential for seat erosion and/or damage to the gate, gate valves shouldnot be used for long term throttling unless specifically designed for that service.Double-disc valves should never be used for throttling.

Never use anything other than the handwheel to operate a manual valve. For example,the use of a cheater bar to open or close a valve may damage the operator or the valveinternals. On a motor-operated valve where the handwheel is used, care should betaken to limit the amount of handwheel torque or force to avoid damage to the valve oractuator.

Care must be used when backseating a valve to reduce packing leakage, especially in ahot valve. If the line is hot, the portion of the stem that is withdrawn from the body willcool to ambient temperature, causing the stem to shrink and possibly causing damageto the stem and/or backseat. When backseating to reduce packing leakage, avoidbackseating a valve with motor operation, because damage to the backseat, stem, ormotor might occur. Instead, carefully use the handwheel. (See Section 2.3.8 for cautionson backseating.)

4.5 Common Problems

• In conventional wedge gate valves, fluid force during intermediate disc travelimposes a moment on the disc that tends to cause disc tipping, which in turn isresponsible for high edge loading and damage to the disc and seat faces as well asthe lower guide surfaces as shown in Figure 4-11. The fluid-induced moment on thedisc for any given flow and ∆P condition is zero in the fully open and fully closedpositions with a maxima at an intermediate disc travel position. The magnitude ofthe fluid-induced moment on the disc and the potential for damage increases withan increase in flow velocity. Under high energy blowdown conditions, damage tothe disc and seat faces and/or the guide surfaces has been observed with manyconventional wedge gate valve designs and parallel disc designs [5.55].


Gate Valves

4-25

Figure 4-11Typical Seat and Guide Damage Locations in Conventional Flexible Wedge GateValves Under High Flow Conditions

• Normally open valves with high turbulence flows (such as downstream of pumps,control valves, orifices, strainers, and elbows) may be subject to high wear rates dueto the turbulence-induced motion between internal components. In solid andflexible disc valves (with single-piece discs), wear typically occurs at two locations:(1) stem head and disc T-slot and (2) body and disc guides.

In other gate valves with multiple-piece discs, additional wear can occur betweenthe disc components. Excessive wear can cause valve failure, including stemseparation from disc and disc sticking at an intermediate position.

• Seat leakage is a common problem in gate valves and can be caused by severalfactors:

— Insufficient wedging loads

— Sediment or scale in the seat area

— Disc and seat erosion

— Wire drawings or steam cuttings caused by high flow velocities between the discand seat

— High pipe loads and moments, especially with low pressure class valves

— Excessive wedging forces which cause high deformations


Gate Valves

4-26

— Reversed installation of unidirectional valves (for example, double-disc valvesinstalled with flow in the non-preferred direction)

• Stem packing leakage is a common problem.

• Leakage through the bonnet flange is also a frequent problem. Body-to-bonnet jointsutilizing a spiral wound gasket may exhibit leakage if gasket surfaces are notcleaned properly or have not been properly dressed. Occasionally, the dimensionsof the joints do not provide for proper gasket compression, or the bolts are nottorqued properly.

• The increase in the required opening thrust under pressure locking and/or thermalbinding conditions can cause the valve to fail to open. The common modes of failureto open under pressure locking and/or thermal binding conditions includeinsufficient actuator output and failure of the weak link (in the valve or theactuator).

• Stem thrust may become smaller under higher disc friction loads due to increase instem factor or stem coefficient of friction. This phenomenon (called rate of loadingeffect or load sensitive behavior) was observed during EPRI’s testing, NRC-sponsoredtesting at INEEL, and valve operations in nuclear plants. These tests show thatconversion of actuator output torque to stem thrust became less efficient at higherthrust levels. For the same actuator torque switch setting, the stem output thrustunder high ∆P condition can be lower by as much as 25% of its value under staticconditions. The rate of loading effect must be accounted for in evaluating requiredstem thrust under load.

• Static and fatigue failures in internal valve and actuator components can occur dueto excessive stem thrust values. In particular, during programs to verify MOVdesign basis capability, the opening/closing thrust levels for many valves had to beincreased significantly to ensure MOV capability. During in situ testing and controlswitch activities, some valves and actuators were inadvertently overloaded beyondtheir thrust ratings. Failures include broken stems, stripped stem threads, broken orseverely deformed gate T-slots, and bent or broken guide rails. Such failures can beprevented by appropriate stress/fatigue analyses of the weak link components.

• In some service situations, isolation valves may stay in one position for long periodsof time. If left in the open position for a long time, deposits and particulates canaccumulate in the gate guides and recesses of the valve, preventing full closure andpossibly resulting in damage to the disc or seat if the valve is forced closed.

• Threads often bind due to corrosion and foreign matter, especially in gate valveswith inside threads. Outside threads also become corroded and crusted with


Gate Valves

4-27

deposits, but the deposits are easily seen and can be removed to make the valveoperable.

• The use of gate valves in throttling service is a basic misapplication, and thispractice usually leads to damage of the valve or the valve seats.

• Some valve vendors underpredicted thrust and torque requirements for some gatevalves by underestimating friction coefficients, flow effects, and metal-to-metalinteractions.

• Some MOV problems are caused by overestimation of motor actuator outputtorque/thrust capability.

• Weak link failure can be caused by under-predicting the actuator output thrust,which can be caused by overpredicting stem friction coefficient (stem factor) orignoring inertia overshoot.

• Many valve problems are caused by improper maintenance and/or repairs. Forexample, elastomeric and non-metallic components can be damaged by impropersolvents and cleaners. The use of counterfeit and low-quality, commercial gradespare parts can also cause valve failures.

4.6 Maintenance Methods

General good maintenance practices are discussed in Section 17. Always follow themanufacturer’s maintenance instructions. Valves should be stroked at least once everysix months to ensure proper operation and to detect internal or external leakage such asin the seat, packing, or bonnet-gasket.

Lapping or grinding of disc and seats are the most common corrective maintenanceactions taken (see Reference 1.1 for detailed discussion). When these operations aredone with a lapping machine or lapping plate, ensure that the original angle of thesealing surface is maintained.

During disassembly of the valve, match mark the bolted joint or pressure seal pressureboundary parts to assure proper orientation upon reassembly. Improper orientation ofpressure boundary parts will often result in joint leakage or malfunction due tomisalignment upon return to service.

In body-to-bonnet joints, ensure that gasket surfaces are cleaned properly and properlydressed and that the bolts are torqued properly.

A procedure for valve assembly should include proper cleaning of the sealing surfaces,a check of the dimensions and surface finish of joints, and require that bolts be torqued


Gate Valves

4-28

to a given value, with proper sequencing and in at least three increments.Manufacturer’s recommended torque values should be followed unless other valuescan be justified.

For pressure seal bonnet valves, the use of a detailed procedure is recommended. Thebody and bonnet surfaces, where the seal ring contacts them, require an extremelygood surface finish and must be free of corrosion products. After reassembly and initialpressure buildup, the bonnet should be retightened.

Threaded-in seat rings that do not have a provision for seat welding tend to loosen,causing seat leakage. Extreme caution must be used to avoid thread damage whilereinstalling the seats.

Gaskets or seals should not be re-used unless specifically permitted by themanufacturer.

Practice good cleanliness, and remove all debris and foreign materials from the valveafter performing maintenance. Allowing flow through a valve with the gate in the near-closed position helps to flush debris and foreign particles from the valve body cavity.

4.7 Recent Improvements in Flexible Wedge Gate Valve Designs

The research activities conducted in response to the U.S. NRC GL 89-10 revealedseveral problems with many flexible wedge gate valves from different valvemanufacturers. These problems relate to lack of reliable operation under design basisconditions including higher than anticipated stem thrust requirements, unpredictablevalve behavior, damage to the valve seats and guides under blowdown/high flowconditions, failure of some internal components (weak links) under high thrustconditions, significant degradation of performance when cycled under ∆P and flow,thermal binding and pressure locking. With good understanding of these problems andtheir causes, many valve manufacturers started implementing design changes in orderto minimize and hopefully eliminate these problems. For example, in a joint effortbetween General Electric Company and Kalsi Engineering, Inc., an improved flexiblewedge gate valve design was developed [5.55]. The new design (called “SentinelValve”) incorporates several features to address known problems such as pressurelocking, thermal binding, seat leakage, disc/guide gouging, structural strengthmargins, and fatigue life.


5-1

5 GLOBE VALVES—ISOLATION FUNCTION


A globe valve can be used full open, full closed, or for throttling within limits of thedisc and seat configuration.

Globe valves are normally metal or hard-seated, but they can be furnished withresilient disc inserts or seats that are suited for compressed air, compressed gas, orfluids that contain small particles of foreign material.

Globe valves have fewer operability problems as compared to gate valves and normallyprovide excellent seat leak tightness, but they do so at the penalty of a higher pressuredrop. Globe valves also require large actuators.

5.2 Design

There are three basic body shapes in globe valves:

• Standard pattern (also called T-pattern), which is the most common shape(Figure 5-1).

• Angle pattern (Figure 5-2).

• Y-pattern (Figure 5-3), in which the stem is inclined at an angle (for example, 45°)with respect to the pipe axis. The Y-pattern body is designed to reduce the flowresistance of the globe valve. The flow resistance of the angle valve is between thatof the standard globe and Y-pattern.


Globe Valves—Isolation Function

5-2

Figure 5-1T-Pattern Globe Valve

Figure 5-2Angle-Pattern Globe Valve



5-3

Figure 5-3Y-Pattern Globe Valve

In contrast to the gate valve’s disc-to-seat sealing action (which is accompanied bysliding and friction), the globe valve plug or disc approaches or moves away from theseat in a direction perpendicular to the seat plane without sliding. Thus, relatively highseat contact stresses can be developed to get very tight shut-off without galling theseating surfaces.

Globe valve stems are either a rising and nonrotating design or a rising and rotatingdesign. Some rising and rotating stem globe valves have an integral stem-to-discconnection that causes sliding at the seat face during the final closing action.

Globe valves are available in a wide variety of materials with both metal-to-metalseating components and soft seating options. Due to its relatively short stroke toachieve the full open position (as compared to gate valves), globe valves can easilyincorporate diaphragm or bellows-type stem seals to provide zero external leakage.

Other options available in globe valves are dual and balanced plug designs to reduceactuator force requirements, cage guiding, and anti-cavitation and noise control trimsfor high pressure drop applications and gas services. Special details pertinent to controlapplications are discussed further in Section 6.

Guidance of the plug in the mid-travel position can be achieved by either a stem guideor plug guide, as discussed in Section 2.2.4. Plug guidance is preferred for larger valvesto avoid stem/plug vibrations due to fluid dynamic forces.

For globe valves with unbalanced discs, the major component of required stem thrust isthe differential pressure load on the disc, which in turn depends on the differential



5-4

pressure area. Testing has shown that, for common valve designs under incompressibleflow conditions, there are two possible areas that need to be considered: (1) the areabased on disc seating diameter (seat-based valves), and (2) the area based on disc guidediameter (guide-based valves).

The required thrust prediction methodology for globe valves is given in Reference 2.3along with a detailed criterion for determining whether a valve is seat-based or guide-based. Figures 5-1, 5-2, and 5-3 show typical seat-based designs. A guide-based designis shown in Figure 5-4.

Figure 5-4Velan 2" (5.1 cm), 1500# Globe Valve (Guide-Based)Model: Figure No. 137132



5-5


Installation practices noted for gate valves in Section 4.3 apply to globe valves as well.Additionally, installation of globe valves with the stems vertically upright is even morecritical than for gate valves, as proper guidance of the disc into the seat is needed toachieve tight closure. If the stem and valve body are other than vertical, the disc tendsto cock or go off center, and the disc seating surface, when going into the seat, will havelittle tendency to compensate to effect the correct seating angle. Y-pattern globe valvesare normally provided with improved guidance to permit valve operation with thestem at an angle to vertical.

Flow Direction

Globe valves are normally installed so that flow is from under the seat. In someapplications, however, it is more important that stem packing be isolated from pressureor vacuum from the downstream side when the valve is closed. For example, a valveused as a containment isolation valve in a line with normal flow into the containment,but with containment accident flow out of the containment, should be installed withnormal flow over the seat. In addition, valves connected directly to a vacuumcondenser should be installed so that the packing is not exposed to vacuum when thevalve is shut.


The operation practices and precautions for gate valves indicated in Section 4.4 alsoapply to globe valves.

Globe valves may be used for rough throttling. However, if the valve was not specifiedfor throttling, cavitation, chattering, and vibration may occur in the throttled position.

5.5 Common Problems

Most problems noted for gate valves in Section 4.6 also apply to globe valves.

• Globe valves improperly applied for throttling will cause damage to the valveand/or adjacent piping (for example, erosion, cavitation damage, flow-inducedvibration, and high wear).

• Thrust requirements for some globe valves may be greater than predicted by valvevendors. The required thrust for globe valves can be alternatively calculated usingEPRI’s PPM [2.1, 2.3].



5-6

• Some globe valve designs may experience stem guide or plug damage due to highside loads caused by high midstroke differential pressures. This most often occursunder high flow rates such as during blowdown events.

• Typical problems of isolation globe valves include stem/disc separation, stickingsolenoids, pneumatic system failures, and seat leakage.

• Antirotation arms in globe valves cause several problems including binding,rotating, and breaking.

• Similar to gate valves, globe valves operated by motor actuators are susceptible toload-sensitive behavior (see Section 4.5).

• Metal diaphragm sealed globe valves (Figure 2-25) do not have their stem connecteddirectly to their disc and rely on a spring to open the valve when the stem iswithdrawn. Therefore, these valves should not be used as throttle/control valves,nor should they be used in dirty service.


Always follow the manufacturer’s instructions.

The maintenance methods for gate valves discussed in Section 4.6 generally apply toglobe valves.

Threaded-in seat rings that do not have a provision for seal welding tend to loosen,causing seat leakage. Extreme caution should be used to avoid thread damage while re-installing the seats.

Re-surfacing the seats of a large Y-pattern globe whose stem is not vertical is verydifficult due to gravity effects causing tool misalignment and setup difficulty. Useextreme care and proper tooling when performing this maintenance activity (seeReference 1.1 for detailed guidance on globe valve repair).


6-1

6 GLOBE VALVES—MODULATING/THROTTLING

FUNCTION


6.1.1 General

Globe valves are the most extensively used valves for modulating service, due in partto the adaptability of the basic design to accommodate difficult conditions such as highpressure, temperature, and differential pressure applications. Compared to ball andbutterfly valves, globe valves present a higher flow resistance. The flow capacity ofglobe valves is about one-third that of low-resistance valves such as ball and butterfly.However, as flow capacity decreases, resistance to cavitation and noise increases.

This section presents a general discussion of globe valve designs used in modulatingand throttling applications along with their performance characteristics and limitations.As will be shown, most control valve problems are caused by improper selection,sizing, and/or installation. Leading control valve manufacturers estimate that mostexisting control valve application problems can be resolved and could have beenavoided if accurate application data and operating conditions were established andprovided before selecting and sizing the valve.

Technical papers and standards have been published by control valve manufacturers,individuals, and organizations such as the Instrument Society of America (ISA) to aidthe user in the sizing, specification, selection, and testing of control valves. ISA-S75.01[6.37] provides flow equations for sizing control valves. Derivation of the variousfactors that appear in the sizing formulas, as well as representative values of valvecapacity factors, are included in this standard. Alternatively, these factors can bemeasured using the control valve test procedures given in ISA-S75.02 [6.38].

Additional information can be found in other EPRI documents (see Sections 22.1 and22.2). The following information highlights areas that cause recurrent control valvesizing problems.


Globe Valves—Modulating/Throttling Function

6-2

6.1.2 System Differential Pressure versus Control Valve Differential Pressure

Control valve ∆P is the difference between the pressure at the control valve inlet andthe pressure at the control valve outlet for a given flow rate. The control valve pressuredrop varies with flow rate. Inlet pressure is the pressure available after piping andequipment resistance losses between the source and the control valve are subtractedfrom the source pressure at a given flow rate. Outlet pressure is the pressure whichresults after piping and equipment resistance losses between the receiver (final elementin the loop) and the control valve are added to the receiver pressure at a given flowrate. This is depicted in Figure 6-1.

Figure 6-1Pressure Drop Through a Control Valve at Minimum, Design, and MaximumSystem Flows

The control valve represents a variable flow resistance in the hydraulic system. Acontrol valve will change the total flow resistance of the entire hydraulic system untilthe total system pressure drop is equal to the system head imparted by the pressuresource (for example, centrifugal pump) at a given flow rate.



6-3

Moore [5.57, 5.1] has developed guidelines for the allocation of pressure drop across thecontrol valve, acknowledging that “the choice of pressure drop is a complex problemwhich cannot be defined by a set of numerical rules.” The guidelines should thus beused more as benchmarks than design criteria. They are as follows:

1. In a pumped circuit, the pressure drop allocated to the control valve should beequal to 33% of the dynamic loss in the system at the rated flow or 15 psi, whicheveris greater.

2. The pressure drop allocated to a control valve in the suction or discharge line of acentrifugal compressor should be 5% of the suction absolute pressure or 50% of thedynamic losses of the system, whichever is greater.

3. In a system where static pressure moves liquid from one pressure vessel to another,the pressure drop allocated to the valve should be 10% of the lower terminal vesselpressure, or 50% of the system dynamic losses, whichever is greater.

4. Valves in steam lines to turbines, reboilers, and process vessels should be allocated10% of the design absolute pressure for the steam system or 5 psi, whichever isgreater.

Some confusion exists in differentiating between assigned ∆P and actual ∆P. There is atendency to assume that because a ∆P is assigned to a valve, the valve creates that ∆P;however, this is not the case. The assigned pressure drop is the pressure that is addedto the system resistance to ensure that sufficient ∆P is available to permit the controlvalve to perform its function. This assigned ∆P is a design number, necessary indetermining requirements for motive force, such as the pump in a liquid handlingsystem. Thus, assigned ∆P has no significance under actual operating conditions.Under actual operating conditions, the control valve is throttled to dissipate energydeveloped in excess of system equipment losses (actual system ∆P).

Most often, the size of the control valve is too large for the application, which results incontrol problems including instability (Section 6.2.11). Furthermore, oversized controlvalves may have to be throttled to small openings, which can result in cavitation,flashing, and/or choking.

Control valves are, by design, capable of controlling over a wide range of conditions.However, a sizing error usually results in higher energy consumption. Controlrequirements, both minimum and maximum, are usually met.

Under some circumstances, however, the following factors may result in an installed oractual ∆P so high that control under minimum flow requirements may becomeimpossible:



6-4

• Conservative published manufacturer’s data.

Example: A pump with flow capacity greater than that required will develop a higher discharge head than anticipated for a given flow.

Result: Control valve pressure drop is higher than expected, and flow control at low flow rates becomes difficult.

• Conservative (high) surface roughness used for determining friction losses in pipe.

Example: The use of high friction factors in the formula for determining line lossin a piping system will indicate a friction head loss of as high as twice the lossthrough clean new pipe. Losses through pipe and fitting, when the installation isnew, could be only half of that anticipated after a period of service (that is, actualpiping causes lower pressure loss at a given flow).

Result: Control valve pressure drop is higher than expected, and flow control atlow flow rates becomes difficult.

• The design margin imposed on system head to compensate for anticipated piping orequipment additions that never materialize will increase actual ∆P across the controlvalve.

• Addition of safety factors to flow requirements will increase actual ∆P to the system.

In summary, the control valve does not dictate the ∆P in a fluid handling system, but itprovides a variable restriction to dissipate the difference between the system head (by apump or upstream tank) and the system head loss (other than the control valve) at agiven flow rate. Thus, for a given flow rate, the pressure drop across the control valveshould satisfy the following equation:

∆Pcontrol valve = ∆Psource - ∆Psystem

If the actual pressure drop across the control valve (at a given disc opening) is smallerthan the above value, the valve is oversized.

If the ∆P assigned for the control valve during system design is less than the ∆Pavailable in the actual installation, the valve could be oversized. Control valveoversizing, which results in oversizing the pressure source (for example, pump), canresult in a considerable waste of energy over the life of the system.

In the illustration shown in Figure 6-2, there are two sets of conditions given: calculatedand actual. The calculated inlet and outlet pressures are the result of applying linelosses based on old pipe. Actual inlet and outlet pressures are based on conditions that



6-5

could exist in a new installation. That is, they exclude the increase in the friction lossesassociated with old pipe, resulting in a higher ∆P across the control valve.

CALCULATED ACTUAL

FLOW Max Nor Min Max Nor Min Start-Up

Pi psia 620 640 672 683 687 739 748Pf psia 387 15Lto in ft 200 111

Lfrom in ft 132 70

P1 psia 600 625 671 672 679 738 748P2 psia 400 397 388 394 392 387 15

∆P psid 200 228 283 278 287 351 733

Q gpm 3000 2500 600 3000 2500 600 200T °F 350 100Specific Gravity 0.89 0.99Cv 200 156 34 170 137 30 8

Figure 6-2Control Valve Sizing Example



6-6

The normal and maximum flow rates remain unchanged; however, minimum flow ratefor the actual condition is lowered to 200 gpm. This is not unusual, particularly in theearly stages of a project when production is low or under test or startup conditions.

Finally, should temperatures differ from those anticipated, serious complications couldresult. In this case, cavitation would occur at the “startup/test” condition. This problemmay not be worth designing for if the conditions that result in cavitation are short term;however, cavitation should be considered in the application. Valve styles other thanglobe could and should be considered for this application, but for purposes of thisillustration, the discussion is limited to globe style valves.

The valve flow coefficient (Cv in gpm/ psi) is defined as the flow rate in U.S. gallonsper minute of 60°F water that flows through a valve with a pressure drop of one psi.The valve flow coefficient for each flow rate is calculated as follows (see Cv results inthe table of Figure 6-2):

( )4.62QCv ∆Ρ

ρ=

where

Q = Flow rate, gpm

∆P = Valve pressure drop, psi

ρ = Fluid density at operating temperature, lb/ft3

Referring to the percent travel versus Cv curves, shown in Figure 6-2, for typical 4- and6-inch valves, the Cv of 200 required for the maximum flow rate of 3000 gpm exceedsthe capacity of the 4-inch valve, indicating that the 6-inch valve will be required. Thiswould seem to satisfy the capacity requirements in that the 6-inch valve will bethrottled from between approximately 8% travel to 50% travel under the calculatedconditions.

If, however, the installed valve is to operate under the conditions marked “startup,” aCv requirement of 8 falls somewhat below the point of control in the 6-inch valve.

For the 6-inch valve, the Cv at 10% travel is 40, and the valve will have a Cv of 8 at atravel of somewhere between 10 and 0%. This point is difficult to predict and will notbe repeatable below approximately 2% travel. Refer to Section 6.2.10 for a detaileddiscussion on rangeability.



6-7

One approach that should be considered in applications such as that illustrated inFigure 6-2 is to confirm that meeting the specified flow is an absolute necessity.Occasionally, maximum conditions are based on an arbitrary value, which is somewhatflexible, or on one that must be passed through the valve, but not controlled. In eithercase, the conditions should be reconciled between the specifying engineer and thesystem design engineer to ensure the best possible selection.

If the maximum flow of 3000 gpm in the previous example is somewhat arbitrary andrepresents an approximate uncontrolled maximum (valve full open) and if a somewhatlower flow could be accepted, then the 4-inch valve could be a better selection. The 4-inch valve would throttle the normal flow at between 60 and 80% travel and wouldmeet the minimum flow requirement at between 2 and 25% travel. A maximum fullopen flow of approximately 2900 gpm could be expected.

If a maximum uncontrolled flow (valve full open) of no less than 3000 gpm is required,then the 6-inch valve is a necessity, but, to ensure control at the possible minimum flowcondition of 200 gpm, a 6-inch valve with reduced trim could be specified, resulting ina Cv versus stroke curve that approximates the 4-inch full area curve. The maximum Cv

would be somewhat higher than the 4-inch valve, while minimum Cv would beapproximately the same as the 4-inch valve.

If it is required that 3000 gpm be a controlled flow, the 6-inch full area trim must beused. Control under minimum conditions must be handled in some other manner, suchas by adding a second but smaller valve in parallel to the 6-inch valve to control thelow flows. The important point is that all options will have been considered and allparties concerned are aware of the various options available.

Another point to consider is the selection of the ∆P which was made in specifying thepump in Figure 6-2. A ∆P of 200 for this case is excessive by whatever criteria forselection is used. A more reasonable selection of 50 to 75 ∆P would result in amaximum required Cv of 200 to 325 and, by applying the same criteria as above in thedetermination of worst-case minimum conditions of 600 to 200 gpm, the minimum Cv

required of 34 to 14 would fall within the range of the 6-inch valve.

The selection of the control valve and the pump motive requirements can be optimizedusing a detailed system analysis. Computer programs can provide detailed flow results(including flow rates, pressure drops, cavitation/choking status, etc.) throughout thecontrol valve stroke using the detailed hydraulic system resistances and the systemhead data. Valve and pump manufacturers can provide recommendations for controlvalve sizing, selection, and installation.



6-8

6.1.3 High Pressure Drop Applications

The following applications, common to many power plants, illustrate the use of some ofthe more specialized control valves used for high pressure drop applications:

• Feedwater recirculation

• Atmospheric steam dump and turbine bypass

• Attemperator spray control

• Deaerator level control

• Feedwater pump flow control

Examples of control valve sizing for these applications are shown in Section 24.

6.2 Design

6.2.1 General

In order to meet the ever changing requirements of fluid flow control, several styles ofglobe valves have been developed. This section describes the available styles.

6.2.2 Single-Port (Single-Seated) Valves

The simplest and the most commonly used control valve body style is a globe valve,shown in Figure 6-3. This figure shows a single-port, top-guided design in which thevalve disc is guided within the lower portion of the valve bonnet. This single-port valveis generally specified for applications where tight shutoff is required.



6-9

Figure 6-3Single-Port Control Valve



6-10

Since the high pressure fluid acts across the entire area of the disc defined by the seatport diameter, the resultant unbalanced force on the disc can be quite large and is thedominant component in sizing the actuator. Because of relatively high actuator thrustrequirements, single-port globe valves are most commonly used in 3-inch (75-mm) andsmaller valves, even though they may also be used in 4-inch to 8-inch (100-mm to 200-mm) sizes with high thrust actuators.

The flow direction in globe valves can be either over the plug or under the plug. Thesedifferent flow directions produce different plug force and actuating force requirements.The plug force versus travel curve, as the plug is moved away from the fully closedposition, plays an important role in determining the stability of valve operation at anygiven position. A flow over the plug control valve configuration exhibits the highestdegree of control instability when operating near the fully closed position, due to arelatively steep negative plug force versus travel gradient in this type of construction.See Section 6.2.11 for a discussion on valve stability.

6.2.3 Double-Port (Double-Seated) Valves

Double-port valve bodies shown in Figure 6-4 are used to balance the forces acting onthe disc as high pressure fluid tends to exert opening force on one seat and closingforce on the other. The net force is lower than in single-port valves, which permits asmaller actuator to be used for a given size valve. The smaller actuator also provides formore stable control operation due to the absence of large plug force versus travelgradients. Double-port valves are most commonly used in sizes 6 inches (150 mm) orlarger and are generally of top- and bottom-guided construction. Since it is difficult toclose the two seats simultaneously, particularly due to differential thermal expansioneffects in operation, double-port valves should not be required to perform a tightshutoff function.



6-11

Figure 6-4Double-Seated Globe Valve

It should be pointed out that the double-seated valves use slightly different diametersfor the top and bottom seat to allow assembly and removal of the smaller disc throughthe larger seat. This difference in seat areas contributes to some unbalanced force on thedisc. Additionally, complete cancellation of forces on the disc when it is off the seat isnot possible because of the difference in fluid dynamic forces for flow under the discversus flow over the disc in the two ports. The total imbalance forces can reach as highas 40% of the equivalent single-port valve value in some designs.



6-12

6.2.4 Cage-Style Valves: Balanced and Unbalanced

The valve disc is closely guided inside a cylindrical cage in cage-style valveconstruction, which has the advantage of easy trim removal and maintenance. Thecage-style construction offers the choice of using balanced or unbalanced disc designand easy interchangeability of internal parts to provide special low noise, highdifferential pressure, or anticavitation cage trims.

The cage has a number of specially shaped flow ports that uniformly distribute flowaround the disc and also serve to provide the desired flow characteristics. Uniformdistribution of the flow around the disc tends to balance horizontal side loads. The discis guided at a large diameter, which makes it more suitable for high pressure dropservice without causing lateral disc vibrations in throttling positions, which can occurin some top-guided designs.

Balanced-disc cage-style valves provide a good choice in many applications, providingthe advantages of a balanced disc that are otherwise available only with more bulkyand complex double-port bodies. The cage-type trim provides valve disc guiding, seatring retention, and flow characterization through specially shaped ports in the cage.The main difference between the unbalanced and balanced disc cage valves is the useof balancing holes that equalize pressure above and below the disc area, therebynullifying most of the static imbalance forces.

A sliding piston ring-type seal between the upper portion of the valve disc and theinside of the cage wall is required to prevent leakage of upstream high pressure fluidinto the lower pressure region on the downstream side. For service temperatures of400°F (200°C) or less, a variety of elastomeric and polymeric materials can be used forthis sliding seal application. Because these piston seals cannot be mechanically loadedto compensate for wear and to improve their seal tightness, piston seals often permitsome leakage past the plug and should not be called upon to provide the tight shutoffthat single-seat valves provide.

As shown in Figure 6-5, the piston sliding-seal diameter is usually a little larger thanthe sealing diameter at the seat port, which results in a plug area that is not 100%balanced when the disc is in the closed position. This should be taken into accountwhen calculating the net static unbalanced forces in sizing the actuators. Reducedunbalanced force across the disc permits the use of smaller actuators than necessary forconventional single-port (unbalanced disc) bodies. The net axial force due to flowdynamic effects and its variation as a function of valve travel is also much lower than inconventional single- or double-port valves, enabling cage style valves to provide a verystable operation, even under very high differential pressures.



6-13

Figure 6-5Balanced Disc Cage Style Valve

The standard direction of flow is through the cage openings and down through the seatring. Standard shutoff performance meets ANSI/FCI-70-2 Class III requirements, andClass IV or better shutoff is offered in some designs (see Table 3-1). Due to reduced discforces and smaller actuator requirements, these valves are used in sizes up to 16 inches(400 mm) and pressure ratings up to ANSI Class 2500.

6.2.5 Angle Valves

Angle valves are single-seated valves having a body configuration in which the axes ofthe valve inlet and outlet connections are at 90° to each other (Figure 5-2). Angle valvesoffer an advantage of lower pressure drop than a standard T-pattern valve (Figure 5-1).Both top-guided and cage-guided body constructions are available. Cage-guidedbalanced disc designs permit the angle valve body to be used in large sizes and highpressure service. Angle valves can also serve the purpose of elbows.

6.2.6 Y-Style Valves

The Y-style body construction shown in Figure 6-6 offers the advantage of higher flowcapacity than the T-pattern globe or angle body styles. A disadvantage of Y-style valvesis that they have a high side thrust component on the valve disc due to a non-uniformfluid flow, especially at low lift and in high pressure drop applications. Special designfeatures have been incorporated by various manufacturers to reduce the side thrust bypreventing the fluid from flowing behind the disc. In addition to its use as a controlvalve, the Y-pattern design has been widely used for main steam isolation service,providing low pressure drop capability under full flow condition.



6-14

Figure 6-6Y-Style Body Valve

6.2.7 Three-Way Valves

Three-way valves use a double-port body construction for diverting or mixing serviceand require three pipeline connections. Since the pressure differentials across the twoseats are different, actuator selection requires careful consideration, especially whenunbalanced valve construction, as shown in Figure 6-7, is used. For higher pressuresand larger sizes, another option is to utilize cage-style trim, shown in Figure 6-8, formore positive disc guiding and to keep the actuator size small.

Figure 6-7Three-Way Valve for Flow Diverting Service Unbalanced Disc



6-15

Figure 6-8Three-Way Valve, Balanced Plug

6.2.8 High Pressure Drop Service Control Valves

Cage-guided construction lends itself to adaptation of the special features necessary tohandle high pressure drops across the valve without causing cavitation and noise inliquid service and high aerodynamic noise in steam or gas service.

In general, all of the designs for high pressure drop service employ a series of tortuouspaths for the fluid flow, which creates a high head loss. This irrecoverable pressurehead loss reduces the final velocity at each stage. Due to lower velocity, the fluidpressure is never allowed to drop below the vapor pressure of the liquid at theoperating temperature, thus greatly reducing the possibility of cavitation or flashing.This principle has been employed in many design variations that effectively handlepressure drops as high as 3,000 psi (20.7 MPa) without cavitation and noise damage.

Figure 6-9 shows some of the typical designs utilizing a high pressure drop cagecartridge. In general for liquid service, the flow can be from inside the cage to theoutside or vice versa, whereas in compressible fluid service, such as steam, the usualarrangement is to have the flow from the inside of the cage to the outside toaccommodate the larger increase in volume associated with the compressible media asit goes through pressure reduction.



6-16

Another type of disc and seat design, which does not utilize a cage-type constructionbut is also suitable for high pressure drop service, is shown in Figures 6-10 and 6-11.This design consists of a series of expansion chambers along the length of the disc thatact as a labyrinth passage from the high pressure side to the low pressure side.

Figure 6-9Low Noise, Anti-Cavitation Trim



6-17

Figure 6-10High Pressure Drop Multiple Step Plug and Cage



6-18

Figure 6-11High Pressure Drop Control Valve, Labyrinth Design

6.2.9 Flow Characteristics

Flow characteristics, the relationship between flow coefficient and valve stroke, dependon the shape of the disc/plug as well as the valve’s internal geometry. The three mostcommon types of flow characteristics are equal percentage, linear, and quick opening[5.1, 5.2]. Figure 6-12 shows the ideal inherent characteristic curve for each of the flowcharacteristics. These characteristics can be approximated by contouring the plug.However, the real curves often deviate considerably from these ideal characteristicsbecause there are body effects and other uncontrollable factors, in addition to the needfor maximizing the flow capacity for a particular valve.



6-19

Figure 6-12Inherent Flow Curves for Various Valve Plugs with Constant Delta P Across theValve

A brief synopsis of each of the three flow characteristics is given below:

• Equal Percentage. Equal percentage is the characteristic most commonly used inprocess control. The change in flow per unit of valve stroke is directly proportionalto the flow occurring just before the change is made. While the flow characteristic ofthe valve itself may be equal percentage, most control loops will produce aninstalled characteristic approaching linear when the overall system pressure drop islarge relative to that across the valve.

• Linear. An inherently linear characteristic produces equal changes in flow per unitof valve stroke, regardless of plug position. Linear plugs are used on those systemswhere the valve pressure drop is a major portion of the total system drop.

• Quick Open. Quick open plugs are used for on-off applications designed to producemaximum flow quickly.

Inherent versus Installed Characteristics: When a constant pressure drop is maintainedacross the valve, the characteristic of the valve alone controls the flow; thischaracteristic is referred to as the inherent flow characteristic. Installed characteristicsinclude both the valve and pipeline effects. The difference can best be understood byexamining an entire system.



6-20

When placed into service in actual systems and the pump characteristics and pipingloss are accounted for, equal percentage, linear, and quick open inherent flowcharacteristics change significantly to what is referred to as installed characteristics. Thedeviation of the installed characteristics from the inherent characteristics depends onthe system flow resistance and system head source. In systems with very small flowresistance and constant head (such as between constant pressure upstream anddownstream reservoirs), the difference between installed characteristics and inherentcharacteristics is small.

A typical example is containment isolation valves where the containment represents aninfinite reservoir. In systems with high flow resistance and variable head source (suchas a centrifugal pump), the difference between installed characteristics and inherentcharacteristics can be very significant (see Figure 6-13 for a typical example). In Figure6-13, the inherent equal percentage trim exhibits a nearly linear installed characteristic,while the inherent linear trim appears to be almost quick opening when installed.

Figure 6-13 contrasts inherent characteristics with installed characteristics. The curvesin Figure 6-13 show, from the standpoint of proportional band, that in the low flowoperating region, for a given flow change, a very small change in lift is required for thelinear trim, compared with the equal percentage trim. Thus, the flow rate is sensitive tovalve opening in the low flow rate region.

Figure 6-13Comparison of Installed Characteristics versus Inherent Characteristics



6-21

Operating in the higher flow region, the opposite is true; that is, a larger change in liftis required for the same change in flow for the equal percent trim, while the linear trimrequires an even higher change in lift. Consequently, overall sensitivity will bedecreased for both trims. The equal percentage trim would exhibit an almost constantsensitivity over the entire operating range, thus requiring only one proportional bandsetting in the controller. Because the linear trim does exhibit a nonlinear change in flow,as a function of lift, it would require several proportional bands.

In deciding whether an inherent linear characteristic or an inherent equal percentagecharacteristic should be chosen, the general rule is that if the valve is the primarypressure loss mechanism and the inlet pressure is constant, the linear characteristicshould be chosen. However, such a system (having very little system pressure lossand/or constant inlet pressure) is not typical. On the other hand, if pipe and fittingresistance are major factors in the system, equal percentage would be the appropriatechoice (which is the case in the majority of applications).

In actual practice, control instruments can be adjusted to handle normally anticipatedflow changes without having to be readjusted. It is difficult to determine from controlperformance whether the valve has linear or equal percentage trim, unless manualcontrol is required, then there will be a tremendous difference.

To illustrate the above flow characteristics, assume that a centrifugal pump supplieswater to a system in which a control valve is used to maintain the downstream pressureat 80 psig. The pump characteristics and system flow schematic for this set ofconditions are given in Figures 6-14 and 6-15, respectively. Assuming a maximum flowrate of 200 gpm with a pump discharge pressure (P1) of 100 psig and that pipe frictionlosses are negligible, the flow coefficient (Cv) can be determined to be 45, using the ISAliquid sizing formula (see Section 24). A 2-inch valve would provide this flow capacity.



6-22

Figure 6-14Typical Pump Characteristics

Figure 6-15Flow Schematic without Piping Losses

To determine the plug valve characteristics that should be specified, analyze theinstalled flow characteristic of equal percentage and linear trim for this 2-inch valve.

Based on the typical pump characteristic in Figure 6-14, Table 6-1 shows several valuesof flow, the required valve Cv and the percent of maximum Cv that the valve must haveto control flow.



6-23

The inherent percentage of total valve lift for equal percentage and linear plugs can bedetermined using Figure 6-12. The installed characteristic, plotted as valve lift versusflow in gpm, is shown in Figure 6-16. A study of Figure 6-16 shows that either installedcharacteristic would provide good control for this situation.

Table 6-1Valve C v and Pressure as a Function of Flow Rate without Line Losses

Q Flow Rate(gpm)

P1 Pump DischargePressure (psig)

∆P AcrossValve (psid)

Cv

RequiredPercent of RequiredMaximum Valve Cv

200 100 20 45* 100

150 125 45 22 49

100 150 70 12 27

50 170 90 5.2 11

* Cv = 45 is assumed to be maximum Cv



6-24

Figure 6-16Installed Characteristics without Piping Losses

In the previous idealized example, the downstream pressure was held constant andpressure drop variations were due to the pump only. A more realistic installation existswhere the delivered pressure must be held constant after passing through the valvewith some line restriction (R) in series with the valve. This installation is shownschematically in Figure 6-17.



6-25

Figure 6-17Flow Schematic with Piping Losses

To find the installed characteristic of equal percentage and linear trim in a suitablysized valve, a pressure drop distribution must be determined. The pressure drop acrossthe control valve, ∆Pv, is given by:

80R1v −∆Ρ−Ρ=∆Ρ

where

P1 = Pump discharge head, psig

∆PR = Pressure drop across the restriction, R, psi

=2

RC

Q

(for water flow at room temperature)

CR = Flow coefficient of the restriction, R, gpm psi

Let CR = 50 gpm psi

At maximum pump flow rate of 200 gpm, the control valve pressure drop is given by:

psi0.4

8050

200100

2

v

=

−

−=∆Ρ



6-26

The corresponding valve flow coefficient is given by:

psi

gpm100

4

200QCv

=

=∆Ρ

=

The control valve can then be sized for the maximum required Cv of 100 gpm/ psi.

A 3-inch control valve would be chosen to handle these maximum flow conditions.Since the pressure drop across the restriction will vary with flow in accordance with thesquare root law ,CQ R ∆Ρ= the available pressure drop across the valve at variousflowing quantities can be determined, keeping in mind the pump characteristics. This isshown in Table 6-2. As before, the percent of maximum Cv that the valve must have tocontrol flow is calculated, and the installed characteristic is plotted, as shown in Figure6-18.

Table 6-2Valve C v and Pressure as a Function of Flow Rate with Line Losses

QFlow Rate

(gpm)

P1

PumpDischargePressure

(psig)

∆PR

AcrossRestriction

(psid)

∆Pv

Across Valve(psid)

Control ValveRequired Cv

psigpm/

Percent ofRequiredMaximumValve Cv

psigpm/

200 100 16 4 100* 100

150 125 9 36 25 25

100 150 4 66 12 12

50 170 1 89 5 5

*Cv = 100 is assumed to be maximum Cv.



6-27

Figure 6-18Installed Characteristics with Piping Losses

6.2.10 Rangeability

The Instrument Society of America (ISA), in Standard S75.05, “Control ValveTerminology” (6.39), defines inherent rangeability as the ratio of the largest flowcoefficient (Cv) to the smallest flow coefficient (Cv) within which the deviation from thespecified inherent flow characteristic does not exceed the stated limits.

Permissible deviation values between actual and manufacturer-specified inherent flowcharacteristics for globe and butterfly valve specimens are published in ISA S85.11,“Inherent Flow Characteristics and Rangeability of Control Valves.” These deviations(or acceptable limits) vary from approximately ±10% at 100% Cv to ±18% at 10% Cv.



6-28

A quick opening plug has a fairly linear characteristic over the first 80% of its flowrange (Figure 6-12), and the linear characteristic is maintained down to a point close toits seat. Rangeability is generally in excess of 100 to 1, with higher values observed onthe larger sizes with plugs having low seating angles.

Linear and equal percentage plugs follow their intended characteristic down to a plugposition, at which the flow is a function of the proximity of the seating surfaces ratherthan of the plug contour. This point generally occurs at around 5% flow in the smallerplugs and drops to about 1% in the larger sizes, giving rangeabilities from 20:1 to ashigh as 100:1.

This inherent rangeability should not be confused with the range of loads over which itwill operate satisfactorily in service. If, for example, a linear plug is selected with arangeability of 100:1 for a liquid pressure control application, a narrow range of loadswould be available over which optimum control could be obtained within thecapability of the controller. An equal percentage plug, even with a lower rangeabilitythan the quick opening plug, would perform well over a wide range of loads. On theother hand, a liquid level loop might operate satisfactorily over a wider range of loadswith a quick opening plug than a high rangeability equal percentage plug. Only wherethe valve characteristic is well matched to the application will the valve rangeabilitycorrespond to the range of loads (with constant relative stability) observed in service.

6.2.11 Stability

Valve stability must be given consideration while sizing a control valve actuator. Whenstability criteria for actuator sizing (discussed in Appendix D1 in Reference 1.2) are notfulfilled, certain valve/actuator combinations can lead to unstable operation. Unstableoperation is characterized by oscillations of the stem, sometimes at a very highfrequency, around the desired travel position. In addition to causing poor control (orloss of control), rapid stem cycling can cause quick degradation of the stem packing,actuator rubber diaphragm failure due to fatigue, or damage to the plug and seat areas.

Valve stability is achieved when the actuator rate of change of forces exceeds the rate ofchange of forces acting on the valve plug. Figure 6-19 shows a typical control valveforce balance diagram.



6-29

Figure 6-19Force Balance Diagram for Control Valves

Criteria for stability have been well established for different types of valve internaldesigns and actuators. In general, increasing the actuator spring stiffness to well abovethe force gradients, due to fluid forces across the plug, eliminates instability problems.See Appendix D1 in Reference 1.2 for more detailed quantitative criteria specific to thevalve and actuator combination of interest.



6-30


Valve sizing coefficients are usually determined by the manufacturer from tests withthe valve mounted in a straight run of pipe that is the same diameter as the valve body.If the installed process piping configurations are different from the standard testmanifold, the valve capacity is changed.

Control valves are often smaller than the line size in which they are installed, and careshould be exercised to ensure proper alignment in the pipeline to avoid overstressingthe valve.

Care should be used when laying out piping adjacent to control valves to avoidinterference between the control valve operator and the piping.

The valve should be installed with the stem vertical and up. With the stem in other thanthe vertical orientation, uneven and unpredictable wear can occur on the guides,guiding surfaces, and seats. The stem packing life will also be shortened. In addition,maintenance becomes more difficult with the stem shifted from the vertical.

Long air lines leading to the air-operated actuator may result in poor control andresponse.

Changes from the installation design should not be made without first ascertaining thatthe change will not have an adverse impact on valve operation or seismic integrity,where applicable.


Unlike most isolation valve operations, control valve operation is automatic andrequires no special instructions to the operator. Theoretically, all of the operatingparameters are addressed at the outset and are incorporated into the selection andspecification so that the final installation will function in a satisfactory manner with noadditional operator intervention required.

Unfortunately, there are occasions when, due to improper or incomplete specification,control valves cannot meet the actual system requirements and must be manuallyoperated until such time as replacement parts or a replacement valve can besubstituted. Under these conditions, concerns should be for proper system operation,with the understanding that there is no automatic compensation for process deviationsin the control process parameter.



6-31

6.5 Common Problems

Common problems encountered with improperly sized and/or specified control valvesinclude:

• Erosion resulting from excessive flow velocities and cavitation.

• Wire drawing caused by operating the plug close to the seat over extended periods.This most often is the result of oversized trim in the valve.

• Broken, worn parts resulting from excessive vibration.

• Malfunctioning positioners.

• Instability, which may result in poor control, high packing wear, and actuatorcomponent wear. See Appendix D1 in Reference 1.2 for a detailed discussion ofvalve stability.

• Chattering and seat damage when throttling near the seat.

All of the above problems can be the result of operation of the valve beyond theconditions for which it was designed. This may be due to changes made to the system,incomplete specification data, poor communication between designers and suppliers,or a combination of the above.


For a general discussion of good maintenance practices, see Sections 17, 18, and 19.

Most control valve manufacturers have highly skilled service engineers available formaintenance and repair of their valves and actuators. Many recurring valve problemsare the result of improper maintenance and/or the use of substandard or counterfeitparts. It is recommended that all service and maintenance work be performed byqualified personnel, using authorized parts furnished by the control valvemanufacturer. Training programs are available through the manufacturer to trainpersonnel in the operation, maintenance, and service of the equipment. When orderingreplacement parts, always include the model and serial number of the valve beingrepaired. Periodic inspections should be made to ensure that biasing springs have notvibrated out of adjustment.


7-1

7 BUTTERFLY VALVES—ISOLATION FUNCTION


Butterfly valves are high pressure recovery valves (also called high capacity and higharea ratio valves) with relatively small overall pressure drop in the fully open positionas compared to globe valves. Butterfly valves are used for both isolation and throttlingservice. This section provides general discussions of butterfly valves in typical nuclearpower plant applications. Special considerations related to butterfly valves inmodulating/throttling service are given in the next section. The Butterfly MOVApplication Guide [1.6] provides detailed discussions for the design, installation,operation, and torque requirements for butterfly valves in nuclear power plants.Reference 1.6 should be consulted for additional details not covered here.

Figure 7-1 shows an overall assembly of a motor-operated butterfly valve in which thefollowing principal components are identified:

• Butterfly valve

• Limitorque HBC gear operator for quarter-turn operations

• Limitorque SMB actuator

• Motor

• Switch compartment


Butterfly Valves—Isolation Function

7-2

Figure 7-1Typical Motor-Operated Butterfly Valve

Butterfly valves offer several advantages over other types of valves, especially inapplications where soft seats are acceptable. Their advantages include:

• Reduced initial installation cost, weight, and space requirements, particularly inlarge sizes

• Reduced operating energy cost because of high flow capacity (Cv) and low pressuredrop in the full open position



7-3

• Reduced maintenance cost even when handling dirty fluids and fluids withsuspended solids (for example, in service water applications)

• Improved sealing capability with seat tightness up to Class VI [6.12], particularlywith high performance valve designs

• Versatility in material selection, which extends butterfly valve applications to higheroperating pressures (typically up to Class 600) and temperatures (typically up to400°F; 200°C) and lower leakage (typically up to Class VI) requirements

• Generally self-closing hydrodynamic torque characteristics that make somebutterfly valves a good choice for fail-close operation

• Flow characteristics that make butterfly valves well suited for throttling service

Proper sizing, selection, and installation techniques result in years of trouble-freebutterfly valve service. Most problems with butterfly valves in nuclear power plantsystems result from misapplication and improper sizing rather than deficiencies invalve or actuator designs.

Butterfly valves use circular, flat discs that can be rotated approximately 90° from fullyclosed to fully open positions. The disc rotation of some valves is limited to 60° to 70°.Some “angle seated” butterfly valve designs close at angles other than 0°. The disc isattached to a shaft that extends outside the body and can be rotated by an actuator.

In nuclear power plants, butterfly valves are most commonly used in low pressure andlow temperature water systems and in containment purge and venting systems.Although they are most often used in pressure service of ANSI Class 300 or less, higherpressure designs are available up to ANSI Class 1500 in smaller sizes. Sealing thesevalves is accomplished by rotating the valve’s flat disc into the flow stream until it isapproximately perpendicular to the flow axis of the connecting pipe, thus effectivelyblocking the flow area. Butterfly valves are available in a variety of materials and endconnections, but are generally limited to 400°F (200°C) because of the soft seatscommonly used to achieve an effective seal.

Some butterfly valve designs accomplish a metal-to-metal seat along a tapered seatingsurface, making these valves suitable for high temperature service. Butterfly valves arecompact, lightweight, and relatively inexpensive, and they are available in sizesexceeding 72 inches (1,800 mm).

The pressure drop across butterfly valves is small, but not as small as in gate valves orball valves which have no obstruction in the flow stream when in the wide-openposition. Because the butterfly disc is always in the flow stream, erosion of the discmust be considered.



7-4

7.2 Design

7.2.1 General

Butterfly valves are typically installed as line size valves where valves and inlet/outletpipes have the same nominal diameters. Alternatively, butterfly valves may beinstalled in larger diameter pipes using inlet reducers and outlet increasers in order toenhance the low-flow/throttling characteristics and to reduce the cost of the MOV andits installation.

The overall population of butterfly valves in a U.S. nuclear power plant are dividedinto two broad categories:

AWWA Design Butterfly Valves. A large population of ASME Class 2 and 3 nuclearsafety-related as well as nonsafety-related valves in U.S. nuclear power plants arelimited to maximum shutoff differential pressure of 200 psi or less (1,379 kPa), amaximum normal service temperature of 300°F (150°C), and a one-time faultedtemperature capability of 350°F (175°C). Many valves for these service conditions arebasically designed in accordance with requirements of ANSI/AWWA Standard forRubber-Seated Butterfly Valves [6.36]. Henry Pratt, Fisher, Allis-Chalmers, and BIF are themajor suppliers of this type of design.

High Performance Butterfly Valves. In the 1960s, a new class of butterfly valvesemerged with a higher pressure/temperature envelope and shutoff capabilityconforming to the full pressure ratings of ANSI B16.34 Class 150, 300, and 600 [6.24].This class of valves is now commonly referred to as “high performance butterflyvalves.” Posi-Seal, Rockwell (McCanna), and Jamesbury are the major suppliers of highperformance butterfly valves to U.S. nuclear power plants.1

Butterfly valve bodies are generally very stiff in comparison to the adjacent piping,making them virtually immune to line loads (axial, bending moment, or torsion). Theyare also insensitive to thermal gradients through the body due to their symmetric axisand stiff construction.

The torque required to fully seat the disc can be minimized by using pressure-energized seats (see Section 7.2.9). Butterfly valves have no body cavities that can trapsolids or contaminants. Servicing any of the major components of the valve requiresremoval of the valve from the line. Because the shaft rotates without axial motion, thebutterfly valve shaft cannot be backseated. However, some designs can be furnished

1 It should be noted that some manufacturers provide both American Water Works Association

(AWWA) and high performance designs.



7-5

with secondary shaft seals inboard of the shaft bearing to protect the bearing fromcontamination.

The most common butterfly valve disc shapes used in U.S. nuclear power plants areshown in Figure 7-2, and can be divided into two basic disc designs: conventionalsymmetric (concentric) disc and nonsymmetric disc designs (Figure 7-3).

Figure 7-2Most Common Butterfly Valve Disc Shapes Used in Nuclear Power Plants



7-6

Figure 7-3Typical Variations in Butterfly Disc Designs



7-7

7.2.2 Symmetric (Lens Type) Disc with Concentric Shaft

The symmetric disc type design (Figure 7-3a) is generally referred to as the standarddisc, conventional disc, or lenticular disc. Flow and torque characteristics of asymmetric disc valve do not depend on the flow direction, and the valve has nopreferred flow direction. Symmetric disc design is typically furnished with a rubber-lined body to provide a seal in the fully closed position, as shown in Figure 7-4. In thisdesign, the shaft penetrates the rubber liner, and an enlarged hub area around the shaftis provided with interference against the rubber liner to prevent leakage around theshaft in the fully closed position. The disc hub area maintains a continuous contactagainst the body liner throughout the disc rotation, which has a tendency to causehigher wear in this region.

The main advantages and the disadvantage of the use of symmetric disc butterflyvalves are summarized below.

Advantages:

• Simple and compact construction compared to nonsymmetric disc butterfly valves

• Suitable for bi-directional service due to the symmetric disc shape.

• Requires smaller dynamic torque in the closing direction than in the openingdirection because the hydrodynamic torque is typically self-closing [1.6]. This isparticularly beneficial in applications where the isolation valve is required to close.

Disadvantage:

• Sliding action under interference between the seat and disc causes higher seat wearthan in nonsymmetric disc valves.



7-8

Figure 7-4Typical Symmetric Disc Design with Elastomer Lined Body



7-9

7.2.3 Nonsymmetric Disc with Single Offset Shaft

In the single offset nonsymmetric disc design (Figure 7-3b), the shaft centerline (and thecenter of disc rotation) is offset axially from the plane of the valve seat along the pipecenterline. In this shaft/disc design, the valve seat is continuous, and the shaft does notpenetrate the seat as shown in Figure 7-5. This design is available in resilient as well asmetal-to-metal seat. The disc face away from the shaft is typically flat or has a smallcurvature and is commonly referred to as the flat face. The other disc face is generallyconvex and contoured to accommodate the shaft. This face is generally referred to asthe curved face.

Flow and torque characteristics of the valve depend on the flow direction with respectto the disc. When the shaft is on the downstream side (or the flat face of the disc is onthe upstream side) of the flow direction, the installation is commonly referred to as shaftdownstream or flat face forward (Figure 7-6). Similarly, when the shaft is on the upstreamside (or the curved face of the disc faces the upstream side), the installation is referredto as shaft upstream or curved face forward.



7-10

Figure 7-5Cross-Section of a Typical Nonsymmetric Butterfly Valve



7-11

Figure 7-6Valve Disc Flow Orientation Terminology

7.2.4 Nonsymmetric Disc with Double Offset Shaft

In this disc design, the shaft has a seat offset (similar to the single offset design) and arelatively small lateral shaft offset (Figure 7-3c). The magnitude of the shaft offset intypical high performance valves varies from 1/32 inch to 1/8 inch (0.794 mm to 3.175mm). This design is available with resilient seats as well as metal seats. The doubleoffset provides a cam-like action that is claimed to reduce seat wear and enhancesealing capability in certain applications. In double offset design, the resultant forcedue to differential pressure across the disc in the closed position does not pass throughthe shaft centerline. An external torque may be required to prevent disc opening due todifferential pressure in double offset designs that have large disc offset.

7.2.5 Nonsymmetric Disc with Triple Offset Design

Another variation in disc design that is relatively uncommon in U.S. nuclear powerplants is the triple offset seat design, shown in Figure 7-7. The main feature of thisdesign is that, in addition to the seat and shaft offsets described above, the shaft has anadditional (third) offset with respect to the disc centerline. This geometry provides astronger camming action between the disc and seat, which provides a tight metal-to-metal seal, even in large valves. The triple offset design is torque seated, as contrastedto the other three disc designs shown in Figure 7-3, which are position seated.



7-12

Figure 7-7Triple Offset Butterfly Valve

7.2.6 Special Disc

Butterfly valves with special disc designs are used primarily in throttling service or todecrease the torque required to actuate the valve. These special designs include fishtaildiscs (Figure 7-8) for torque control and serrated edge or orifice discs (Figure 7-9) forflow, noise, and cavitation control. In general, these valves are not required to give zeroleakage in the fully closed position and are used primarily in a fully open or partiallyopen position. In most cases, the shape of the disc makes the valve unidirectional.



7-13

Figure 7-8Fishtail Disc

Figure 7-9Special Disc Design for Noise and Cavitation Reduction

7.2.7 Valve Shaft, Shaft Connections, and Seal

Butterfly valve shafts are designed to transmit actuator output torque to the valve discand to support the disc against fluid-induced forces. The valve shaft may be a one-piecedesign extending through the valve disc, which is commonly found in small valves. Atwo-piece shaft design (stub-shaft type) is used in some designs. The engagementlength of the shaft-to-disc connection is approximately 1.5 shaft diameters in two-pieceshaft designs.



7-14

As shown in Figure 7-5, the shaft is supported on both sides of the disc by sleevebearings and is connected to the disc by dowel pins, taper pins, or other means. Theother end of the shaft is connected to the actuator by a single key, double key, spline,square head connection, or other special design.

Sealing of the shaft in butterfly valves (and quarter-turn valves in general) is relativelyeasier than that for rising shaft valves such as gate and globe valves. This is due to thefact that the rotary shaft motion does not have a tendency to transfer and create a lossof the packing material to the environment outside the packing box area. The mostcommonly used butterfly valve shaft seals are:

• Pull-down packing gland (stuffing box). Both live loaded packing (for example, withBelleville springs) and conventional bolt torque preloaded pull-down packingglands (as shown in Figure 7-5) are used with butterfly valves. The stuffing box isusually designed to accept a minimum of four packing rings. Flexible graphitepacking rings with composite carbon/graphite end rings are commonly used in thistype of design.

• V-type (self-adjusting chevron type) packing. V-type packing is well suited for quarter-turn valves in general. Line pressure acts on the inside surface of the V-rings toeffect a seal across the shaft; therefore, correct orientation of the V-rings in thepacking cavity is required. Ethylene propylene terepolymer (EPT), rubber, andcomposite Teflon are the typical packing materials for this type of packing design.

The shaft seal design for butterfly valve applications should allow for easy packingreplacement and in-service adjustment. One of the less commonly used shaft seals is aconventional O-ring, which is not suited for easy maintenance or replacement inservice.

7.2.8 Valve Bearings

As shown in Figures 7-4 and 7-5, sleeve-type bearings are used to support the valveshaft against forces due to differential pressure across the disc assembly. Thesebearings are installed in the valve body hubs. Corrosion resistant and self-lubricatingbearing materials (such as solid bronze, graphite-impregnated bronze, and Teflon-impregnated fabric with stainless steel backing) are commonly used. Stainless steelbearings (often with some surface treatment) are also used in some applications. Metaltype sleeve bearings are typically designed such that the shaft-to-bearing contactstresses do not exceed one-fifth the compressive strength of the bearing or shaftmaterial at operating temperature [6.36]. Operating experience shows that thebearing/shaft coefficient of friction does not exceed 0.25 throughout the design life ofnon-stainless steel bearings in clean water service. For stainless steel bearings, thecoefficient of friction can be as high as 0.60. Lower coefficients of friction (0.15 or even



7-15

less) may be obtained with Teflon and other self-lubricating reinforced plastic bearingmaterials in clean fluid applications. The valve manufacturers should be consulted forthe bearing material coefficient of friction or the bearing friction factor applicable totheir specific valve designs.

In addition to the sleeve bearings, which carry the forces induced by differentialpressure across the disc, valves larger than 20 inches (500 mm) are typically equippedwith one or two thrust bearings to support the weight of the disc assembly and to keepthe disc centered with respect to the seat [6.36]. The torque contribution from thesethrust bearings to the total operating torque requirements is negligible.

7.2.9 Valve Seats

A large number of combinations of valve seat designs and materials are available tomeet the variety of applications and operating conditions. Figures 7-4, 7-5, and 7-10show the variations most commonly found in nuclear power plant applications ofbutterfly valves. Based on seat leakage and seat torque requirements, valve seats maybe divided into nominal leakage seats, low leakage seats, and tight shutoff seats. Exceptfor externally pressurized elastomer seats of inflatable designs (Figure 7-11) which arenot commonly used in nuclear power plants, tight shutoff seats require higherseating/unseating torque than the nominal leakage seat designs. In some throttling andmodulating applications in which small leakage is tolerable, a clearance is providedbetween the disc OD and the seat ID. In such designs, the seat does not add anycomponent to the total torque requirements.



7-16

Figure 7-10Typical Seat Designs



7-17

Figure 7-11Inflatable Seat Butterfly Valve

The elastomeric seat may be disc-mounted or body-mounted. Seat design may bemetal-to-metal seal or soft seal using elastomers or plastics against metal. Stainless steelor nickel-copper alloy seating surfaces are recommended for frequently operated valves(more than once a month). Even though a desirable feature for any size valve, theANSI/AWWA standard for rubber-seated butterfly valves [6.36] requires that rubberseats be replaceable at the installation site for valves 30 inches (750 mm) and larger.Rubber seats should be resistant to microbiological attack and ozone attack. Provisionsshould be made for ease of maintenance, for example, adjustment or replacement ofseats, by providing proper access to the valve.

Sealing in the fully closed position is achieved by intimate contact between the sealingsurfaces on the disc and body. The most commonly used methods of effecting thisintimate contact are described below:

• Interference type seats. In interference type seats, sealing is achieved by elasticallydeforming the seat. The amount of interference is preset to ensure sealing under thedesign differential pressure. Figure 7-4 shows the elastomer-lined body design inwhich the liner also acts as the interference type seat. This is the most commonlyused design in symmetric disc butterfly valves. Figures 7-10a and 7-10b show theadjustable type (typically elastomeric material) and lip type (typically plastic



7-18

material) variation of the interference seat designs, respectively. The adjustable typeseat design requires a controlled magnitude of torque (specified by the valvemanufacturer) on the seat retainer ring adjustment screws. The amount of seatadjustment varies with seal tightness and required seating torque. Over-adjustmentof the seat increases required seating/unseating torque and reduces seal life due toincreased wear rate. This seat design offers the advantage of being easilyreplaceable in the field.

• Pressure-energized or self-energized type seats. In pressure-energized type seats, the linedifferential pressure at the fully closed position is used to generate intimate contactbetween the seat and its mating surface. Figures 7-10c and 7-10d show twocommonly used designs of pressure-energized seats. The seal ring is typically madeof reinforced Teflon or other composite plastic material and has a shape that permitsthe upstream high pressure fluid to increase the contact pressure at the sealinginterface. Figure 7-10d shows a pressure-energized metal seat design that provides atight shutoff and meets the fire safe sealing requirements.

• Inflatable type seat. In inflatable type seat designs, external pressure is applied to theresilient seat member after seating and removed before unseating the disc. Figure 7-11 shows an inflatable type elastomer seat design for a symmetric disc valve.

Typical materials used for the seal ring in the elastomer type seal design are ethylenepropylene terepolymer (EPT) or nitrile. Maximum temperature for EPT material usedin the seats is 300°F (149°C) for normal conditions and 350°F (177°C) for faultedconditions.

Rubber seats in butterfly valves are usually made of 40 to 80 durometer hardness(based on shutoff pressure requirements); 65 to 70 durometer is the most typical range.Continuous exposure to high temperature and/or certain fluid environments can causehardening of the rubber material with age, thus causing an increase inseating/unseating torque. Valve manufacturers provide recommendations for the seatreplacement frequency to ensure satisfactory seal performance.

The valve seat design or the actuator torque requirements may dictate a preferred flowdirection for the valve. The shaft upstream (seat ring downstream) is the preferreddirection from a sealing standpoint because elastic deflections due to the differentialpressure across the disc tend to close up the clearances between the disc and seatmating surfaces, thus providing a tighter seal. The shaft downstream direction exhibitslower dynamic torque and is the preferred direction from an actuator size standpoint.Another advantage for shaft downstream installation is that the shaft packing is on thelow pressure (downstream) side of the seat and the potential for packing leakage ismostly eliminated. This feature can be particularly important for some applications (forexample, containment isolation) where the valve safety function is to isolate and remainclosed for an extended period of time such as in post-LOCA conditions.



7-19

Because of the large variation in seat designs and materials, the required seat torquevalue or calculation procedures should be obtained from the valve manufacturer orfrom in situ testing.


This section presents the main factors to be considered during the selection andinstallation of butterfly valves. (See Section 8 for additional discussion.)

7.3.1 Valve-to-Pipe Connections

Valves with welded-end connections are not typically recommended for nuclear powerplant applications because the welded ends have to be cut to allow for internalinspection and routine maintenance and repair (including replacement of elastomericseats and shaft bearings). Butterfly valves, in particular, are typically accessed throughthe ends (except for packing adjustment and replacement). As mentioned above,butterfly valves are typically used in low pressure systems and come in a variety offlanged and flangeless end connections that allow quick removal and installation in thepipe line. Only flanged end valves should be used (with a blind flange) for end-of-lineapplications. Both the upstream and downstream pipes must be empty beforeperforming any maintenance activity on wafer and lugged design valves.

7.3.2 Valve Orientation

Symmetric disc butterfly valves are bidirectional and, in general, can be installed withflow in either direction. Nonsymmetric disc valves have a preferred flow direction andshould be installed according to manufacturer’s recommendations. It should be notedthat the required actuation torque with shaft upstream orientation (see Figure 7-6) canbe more than twice that with shaft downstream [1.6].

7.3.3 Valve Location

Ideally, butterfly valves should be installed in straight pipe runs with a minimum ofeight pipe diameters of straight upstream pipes. However, in typical applications,butterfly valves are installed within short distances of other piping components thatcan have a significant effect on valve performance, especially in high flow rateapplications. Upstream elbows, tees, pumps, and other valves result in velocity skewsand turbulence, which can significantly increase the required dynamic torque. Figure 7-12 shows three different valve orientations with respect to an upstream elbow.



7-20

Figure 7-12Effect of Upstream Disturbance, Shaft Orientation, and Disc Opening Direction onHydrodynamic Torque

In Configuration 1, the velocity skew tends to assist valve closing. In Configuration 2,the velocity skew tends to oppose valve closing. In Configuration 3, the velocity skewhas small effect on the valve because the flow is nearly symmetric around the disc.



7-21

Configuration 3 (where the valve shaft and the elbow are in the same plane) is typicallyrecommended because it has the least effect on the valve performance in both theclosing and opening direction.

An upstream elbow model [1.6, 2.4] has been developed to bound torque requirementswith an upstream elbow in a given orientation and proximity from valve inlet. Theupstream elbow model can be used to estimate the effect of other upstream pipingcomponents on butterfly valve torque requirements. The effect of an upstreamdisturbance diminishes after 8 to 10 pipe diameters. Two out-of-plane elbows producea swirl that can persist for more than 20 pipe diameters.

7.3.4 Shaft Orientation

Although butterfly valves can be installed in almost any orientation, the vertical shaftwith actuator on top is the preferred orientation. In applications where the valve shaftis horizontal, the hydrostatic torque component results from the variation in the statichead of the process fluid from the top to the bottom of the valve disc due to gravity(Figure 7-13). Depending upon the direction of the hydrostatic torque and the directionof shaft rotation to open or close the valve, this torque component may assist or opposethe actuator in the seating direction. In general, the hydrostatic torque may beneglected except for very large valves, 30 inches (750 mm) and larger.

Figure 7-13Hydrostatic Torque Component in a Horizontal Shaft Installation

The hydrostatic torque becomes zero (or negligibly small) under any one of thefollowing conditions:



7-22

• Valve shaft is vertical. This orientation results in a zero moment arm for symmetricand single offset discs, and a negligible moment arm for a majority of the doubleoffset disc designs.

• Liquid levels in both the upstream and downstream pipes are the same (either full,empty, or partially full).

• Process fluid is air, gas, or steam.

In some large valves, the magnitude of the hydrostatic torque component can be highenough to overcome the total seating/unseating torque. In the absence of valveoperator resistance, the valve may open by itself under hydrostatic torque.

For double and triple offset disc designs (where the shaft is offset from the pipecenterline), the pressure drop across the valve disc gives another hydrostatic torquecomponent, which is referred to as ∆P-induced hydrostatic torque. This hydrostatictorque component can be very significant, especially for large valves under highpressure drop.

Caution: Both symmetric and nonsymmetric disc butterfly valves can experience veryhigh unseating torque requirements if an incompressible fluid is trapped between twotight seal valves. Increase in the pressure of the trapped liquid or water (such as byheating) can lead to a pressure locking scenario. In addition to the increase in thebearing torque, double and triple offset disc valves will have a ∆P-induced hydrostatictorque component.


Butterfly valves, depending on the design and direction of flow, may open or close bythemselves under flow conditions. Therefore, care should be used when operating alever-operated manual butterfly valve to prevent personnel injury. Most worm gearoperators have self-locking gear trains to prevent the valve disc from drifting [1.6].

Although the use of standard design butterfly valves is usually restricted to isolationservice, throttling with these valves can be tolerated if the valve is no less than 20%open, and the design limits are not exceeded [1.6].

7.5 Common Problems

• Valve disc does not reach the fully open or fully closed position due to improperlyset limit switches.



7-23

• Liner deterioration due to chemical attack on lined butterfly valves can occur if theliner material is not selected properly. Liner deterioration can also occur due to highfluid velocity or improper use as a throttling valve.

• Elastomeric seat materials require periodic inspection or replacement. Dependingon the valve service, elastomeric seats should be replaced every 5 to 10 years.

• Some valve vendors/suppliers have underpredicted torque requirements for somebutterfly valves.

• Continuous operation downstream from flow disturbance sources such as elbows,pumps, or other valves may increase dynamic torque requirements.

• High flow turbulence may cause disc vibrations and high bearing and packingwear.

• Butterfly valves used for containment isolation valves frequently fail the annual leakrate tests because the resilient seat material dries and/or hardens between tests.Liner hardening can also increase the torque required to operate the valve.

• For service water butterfly valve applications, the presence of solid particles andbiological growth can cause several valve problems, including:

— Accelerated erosion and corrosion (including galvanic corrosion, especially insalt water systems)

— Accelerated degradation of the seat material (especially elastomeric materials)

— Accelerated wear of the bearing

— Increased bearing friction coefficient

— Increased seating/unseating torque requirements

• Some butterfly MOV problems are caused by overestimation of output torque ofmotor actuators.

• Disc erosion may be a problem in normally open applications with high flowvelocities (for example, over 16 ft/sec or 5 m/sec) because the disc is always in theflow stream. The presence of solid particles in the fluid will increase disc and bodyerosion.

Many service water valve problems can be eliminated by proper material selection andadequate maintenance.



7-24


Removal of the valve is normally required to perform maintenance, except for packingadjustment/replacement. However, some maintenance, such as seat replacement, canbe performed on a lugged or flanged butterfly valve with offset discs by removing thepiping on the seat side of the valve.

Always follow the manufacturer’s maintenance recommendations.


8-1

8 BUTTERFLY VALVES—MODULATING/THROTTLING

FUNCTION


Butterfly valves have unique flow and torque characteristics that can cause valveinstability in some throttling applications. The Butterfly MOV Application Guide [1.6]and the Butterfly Performance Prediction Methodology [2.4] provide detailed discussions ofthese characteristics and their effect on valve performance. In this section, some of thebutterfly valve’s key characteristics are discussed, and the reader is referred toReferences 1.6 and 2.4 for comprehensive discussions. Information given in Section 7for isolation functions also applies (for the most part) to modulating/throttlingfunctions.

Butterfly valves have high pressure recovery factors [1.6, 5.1, and 6.37] and tend tocavitate and choke at low disc opening angles. Thus, butterfly valves are notrecommended for throttling/modulating near the fully closed position. For example,butterfly valves are not recommended for some service water systems where water haslarge seasonal temperature variations. Although a service water butterfly valve mayprovide acceptable performance during the hot weather season, it may cavitate duringthe cold weather when the valve is throttled at low disc angles to satisfy systemrequirements. It should be noted that some butterfly valves have special disc designsthat reduce flow cavitation and noise (see Section 7.2.6).

Under certain flow conditions, some butterfly valves with single offset disc designsmay experience dynamic torque reversal at midstroke positions when installed with theshaft on the downstream side [1.6, 2.1, 2.4 and 2.11]. Dynamic torque reversal maycause instability, vibrations, and high bearing wear. Thus, for throttling/modulatingservice, single offset disc valves should not be installed with shaft downstream unless itcan be shown that the valve is not susceptible to dynamic torque reversal underoperating conditions.


Butterfly Valves—Modulating/Throttling Function

8-2

8.2 Hydrodynamic Torque Characteristics

Flow around a butterfly valve disc produces both lift and drag forces similar to theforces acting on an airplane wing. The non-uniform pressure distributions on theupstream and downstream faces of the disc have a resultant force that does not passthrough the shaft axis, as shown in Figure 8-1. The product of this resultant force on thedisc and its moment arm to the center of disc rotation is the hydrodynamic torquecomponent, Thyd. For a given disc shape, the hydrodynamic torque is proportional to thevalve pressure drop, ∆Pv, and disc diameter, ddisc, raised to the third power. Theconstant of proportionality, Ct, called hydrodynamic torque coefficient, varies as a functionof disc opening angle. For a given disc shape at a fixed disc angle, the hydrodynamictorque, Thyd, is given by:

lbftdC12

1T v

3discthyd −∆Ρ= (U.S. Customary Units)

mNdC10T v3disct

6hyd −∆Ρ= − (SI Units)

where Ct is dimensionless, ddisc is in inches or millimeters and ∆Pv is in psi or kPa. Ingeneral, ∆Pv is limited to the valve pressure drop at the onset of choking (see References1.6 and 2.4 for detailed discussions).

Figure 8-1Flow Through a Symmetric Disc Butterfly Valve

Most manufacturers determine torque (and flow) coefficients by performing flow looptests on full-size valves of selected sizes and pressure ratings, or on precisely scaledmodels of their valve product line. Tests are typically performed under fully turbulent,non-choked flow conditions using water or with air at low pressure drop ratio(maximum flow velocity is well below the speed of sound) to simulate nearlyincompressible flow.



8-3

8.3 Effect of Hydraulic System Characteristics on Peak Hydrodynamic Torque

The hydrodynamic torque coefficient, Ct, curve has a peak at around 70° to 80° discopening for most disc designs. In actual valve installations, the peak in thehydrodynamic torque does not necessarily occur at the location where thehydrodynamic coefficient, Ct, has a peak. This is due to the fact that the pressure dropacross the valve, ∆Pv, typically changes with the disc opening. The amount of change in∆Pv across the valve depends upon the valve flow characteristics and the characteristicsof the hydraulic system in which it is installed. Since both Ct and ∆Pv depend upon thedisc opening angle, the actual peak in hydrodynamic torque occurs at a disc positionwhere the product of these two quantities reaches a maximum value. The followingtwo cases illustrate this effect.

Case 1: Nearly constant pressure drop across the valve

Figure 8-2a shows a hydraulic system in which the differential pressure between thetwo reservoirs is constant and the total resistance of the piping is low. Pressure dropacross the valve, ∆Pv, decreases only slightly near the full open position due to therelatively small amount of pressure loss to overcome the piping resistance. Thus, thevalve has nearly a constant pressure drop regardless of the disc opening angle. Thismeans that the hydrodynamic torque will reach a maximum at nearly the same discopening where the hydrodynamic torque coefficient peaks.

Case 2: Variable pressure drop across the valve

In pumped systems and/or in systems having high piping resistance, the pressure dropacross the valve, ∆Pv, can change significantly as a function of disc opening, in amanner similar to that shown in Figure 8-2b. The large variation in ∆Pv is caused by thecombined effect of pump characteristics (discharge pressure drops with increasing flowrates), system resistance (pressure drop across the piping resistance increases as asquare function of the flow rate), and the valve flow characteristics (flow resistancecoefficient decreases with increasing disc angles).

The effect of decreasing ∆Pv with increasing disc opening on the hydrodynamic torqueis shown in Figure 8-2b. The peak in the torque curve has shifted toward a lower discopening angle because the product of the torque coefficient, Ct, and ∆Pv reaches amaximum at this location. It should also be noted that the magnitude of the torque atthe peak location will be different because it depends on the actual ∆Pv at that discopening angle.



8-4

Figure 8-2Variation in Location of Peak Hydrodynamic Torque for Constant Headand Pumped Systems

In closing, it should be noted that the discussion here focused on the hydrodynamictorque component only. The total dynamic torque curve exhibits a similar, but notexactly the same, behavior. The difference is due to the contribution of the bearing



8-5

torque component, Tb, which is also dependent upon the differential pressure acrossthe valve. The relative contribution of each of these components determines the actuallocation of peak total dynamic torque. The calculation procedures to determine themagnitude and location of the peak dynamic torque are described in References 1.6 and2.4.

8.4 Torque Characteristics of Butterfly Valves

An important consideration in determining the operating torque requirements ofbutterfly valves is that the maximum torque may be dictated by the dynamic torquerequirements at some intermediate disc position (for example, between 10° and 80°opening) rather than the seating/unseating torque requirements. The magnitude of thedynamic torque is strongly dependent upon valve size, total pressure drop, and massflow rate through the valve.

Whether the maximum torque requirements are governed by the dynamic torque or bythe seating/unseating torque for a valve depends upon its size, design, and actualapplication conditions. For example, dynamic torque values for valve sizes smaller than20 inches (500 mm) operating with water at flow velocities of 16 ft/sec (4.877 m/sec) orless (AWWA Class “B” maximum velocity limit) are typically bounded by theseating/unseating torque values for tight shut-off seat designs provided by most valvemanufacturers. However, if design basis conditions include pipe rupture, velocitieswell above 16 ft/sec (4.877 m/sec) may be encountered. Under these higher velocities,dynamic torques can exceed the seating/unseating torque requirements, even for valvesizes smaller than 20 inches (500 mm). Therefore, the evaluation of butterfly valvetorque requirements should include analyses of both:

• Total seating/unseating torque, TTS

• Total dynamic torque, TTD

The required actuator torque is the larger of these two torque requirements. Figure 8-3shows a typical opening torque curve for a symmetric disc butterfly valve along withvarious torque components in a high flow application. The actuator torque required toopen the valve in this example is determined by the total dynamic torque, TTD, ratherthan the total seating/unseating torque, TTS. A good knowledge of the behavior ofvarious torque components is required to determine the total seating torquerequirements as well as the total dynamic torque requirements.



8-6

Figure 8-3Typical Opening Torque Characteristics of a Symmetric Disc Butterfly Valve underHigh Flow Conditions

Hydrodynamic torque can be very high in applications with high flow velocities. Whenthe hydrodynamic torque assists disc rotation and the sum of the frictional torques(bearing, packing, and hub seal) is relatively small, the actuator will apply a restrainingtorque to prevent the disc from slamming shut. Under these conditions, the concernwould be the structural strength of the shaft and its connections to the actuator and tothe disc rather than the actuator motive torque. To account for these conditions, themaximum transmitted torque, TTR, is defined as the maximum motive or restrainingtorque applied by the actuator to the valve shaft during a valve stroke under thespecified flow conditions and is equal to the largest of:

• Seating/unseating torque

• Maximum total dynamic torque

• Maximum hydrodynamic torque component

The maximum transmitted torque excludes any additional actuator torque caused bydisc obstruction before reaching a specified disc position as set by the actuator. Themaximum transmitted torque is used to evaluate the structural strength of the keycomponents within the torque train of a butterfly valve.



8-7

8.5 Common Problems

In addition to the information given in Section 7, the following considerations apply tobutterfly valve installations for modulating/throttling service:

• Butterfly valves may cause noise, cavitation, choking or/and flashing whensubjected to high pressure drops especially in applications where valve inletpressure and temperature are near saturation conditions. Extended operation underthese conditions may result in damage to the downstream piping as well as to thevalve. Thus, it is particularly important to carefully evaluate valve characteristics atoperating conditions.

• The location of the peak total dynamic torque depends on the system resistancesand the pressure source (see Figure 8-2). Operating near the peak total dynamictorque may result in unstable operation.

• Nonsymmetric disc valves with shaft downstream orientation may have torquereversal at midstroke. Operation near torque reversal may result in unstableoperation.

• Evaluation of the butterfly valve performance in a particular installation should bebased on the valve installed characteristics and not on the inherent characteristics(provided by the valve manufacturer).

• Butterfly valves are rather sensitive to upstream flow disturbances such as pumps,elbows, and other valves. These flow disturbance sources cause velocity skews andhigh turbulence that can affect the performance of butterfly valves and increase thebearing and packing wear.


Information given in Section 7 for isolation butterfly valves also applies to modulatingservice.


9-1

9 BALL VALVES—ISOLATION FUNCTION


Ball valves are quarter-turn valves, occupy less vertical space than rising stem valves,and can be installed in almost any orientation. Ball valves are bi-directional except forsome eccentric or wedged ball designs. With elastomeric or plastic seat designs, ballvalves are normally limited to 400°F (200°C) service. With metal seats and hightemperature packing materials, they can be used in higher temperature applications.Due to their basic spherical-shaped sealing members and stiff body design, ball valvescan tolerate high pipe bending moments and thermal gradients without affecting theirseating or operating.

Ball valves (like butterfly valves) are high pressure recovery valves and are susceptibleto cavitation, choking, and flashing. The pressure drop across a full-bore ball valve inthe fully open position is nearly equal to the pressure drop across an equal length ofstraight pipe. Reduced and venturi bore valves are used in low flow velocityapplications where some pressure drop is acceptable or desirable.

9.2 Design and Materials

9.2.1 General

Ball valve bodies are available in two- or three-piece designs with either end-entry, ortop-entry (of the ball) construction. Body pieces are joined by welding, flange bolting,or threading, and may incorporate multiple ports. Figures 9-1 and 9-2 show end-entrydesigns with two- and three-piece construction respectively. Some designs, such as thetop-entry (Figure 9-3) and bolted three-piece swing out type body designs, allow thevalve to be serviced without completely removing it from the line. Ball valves do notnormally incorporate stem backseating since the valve stem only rotates without axialmovement.

The body cavity can trap crud and foreign materials because in the fully open positionthe valve seats isolate the body cavity from the main flow. Body drains are usuallyprovided to flush the body. Special features available in some designs include rotating


Ball Valves—Isolation Function

9-2

seats to provide uniform seat wear, pressure energized seats to improve sealing, andcoatings to provide corrosion resistance. In addition to the typical solid spherical shape,balls can use ribbed, tubular, or hollow construction to minimize weight, especially inlarger sizes.

Ball valves are grouped into two basic types: floating ball and trunnion-mounted ball.A variation of the trunnion-mounted is the wedged ball design that allows mechanicalloading of the seat. The selection of a particular ball valve design depends on the size ofthe valve and the application.

9.2.2 Floating Ball

In the floating ball design (Figure 9-1), the ball is supported by the seats and is allowedto move axially between them. To assist in low pressure seating, sufficient preload isprovided to keep the ball in contact with the seats at all times. This preload isaccomplished by a slight amount of designed interference between the ball and seatsduring assembly. As the valve is closed, differential pressure forces the balldownstream, causing it to bear against the downstream seat without losing contactwith the upstream seat.

Some of the advantages and disadvantages of the floating ball valve design areidentified below.

Advantages:

• Simple and compact construction.

• Economical.

• Can easily be made fire safe by use of fire-safe seat materials and stem packingmaterials.

• No stem bearings required.

• Can be easily coated to improve corrosion resistance.

• Single stem penetration in the body.

• Absence of bearings and nonfloating seats make floating ball valves more suitablefor dirty service than trunnion-mounted valves.



9-3

Disadvantages:

• Even though 1-inch and smaller size floating ball valve designs are suitable for highpressure service (up to ANSI Class 1500), the high torques associated with thisdesign limit use of larger sizes to lower pressures (up to ANSI Class 300). For sizeslarger than 12 inches, floating ball designs are typically not recommended oravailable due to the fact that the force caused by differential pressure across the ballacts on the downstream seat instead of the bearings in a trunnion-mounted balldesign. Torque created by the seat friction in a floating ball is much higher becauseof its larger effective radius compared to the radius of the bearings used intrunnion-mounted balls.

• Simultaneous seating against both upstream and downstream pressure is notpossible; only the downstream seat is effective in providing shut-off.

• Pressure can be trapped in body/ball cavity.

• Body cavity acts as a crud or containment trap.

• Seating action cannot be mechanically enhanced by the application of additionaltorque to the stem.



9-4

Figure 9-1Floating Ball

9.2.3 Trunnion Mounted Ball

In the trunnion-mounted ball design (Figure 9-2), the ball is prevented from floatingdownstream by bearing-supported trunnions. Since the load due to differentialpressure across the ball is carried by trunnions with a smaller radius than the ball itself,the trunnion-mounted ball valve design has a lower operating torque than the floatingball design. For this reason, higher pressure and larger size ball valves utilize thetrunnion-mounted design. Seating is achieved by allowing the upstream seat to floatand load against the ball. The floating seat consists of a metal ring that carries anarrower width polymeric seat ring that does the sealing. The seating force is providedby differential pressure acting on the unbalanced annular area of the seat, with springsproviding the initial load to keep the seat against the ball. The springs are sized toprovide sufficient preload to achieve a low pressure seat.



9-5

Figure 9-2Trunnion-Mounted Ball

Some of the advantages and disadvantages of the trunnion-mounted ball valve designare identified below.

Advantages:

• Lower operating torque than floating ball.

• Relieves body over-pressure to low pressure side of the system by pushing thefloating seat away from the ball.

• Suitable for higher pressure service than floating ball design, especially in largersizes.



9-6

• Ball weight can be supported by the thrust bearings instead of the seats as in thefloating ball design, thus providing more uniform seating load and wear.

Disadvantages:

• More expensive than floating ball design.

• Bearings can experience high wear if abrasive solids are present in the fluid whichcan cause the torque to increase, making it unsuitable for fluids contaminated withsolids.

• Due to the non-floating action of the ball, fire safety is more difficult to achieve.

9.2.4 Wedged Ball

The wedged ball design, shown in Figure 9-3, is similar in construction to the trunnion-mounted valve, except the stem forces the ball into the downstream seat at the end ofits rotation during closing. While opening, the reverse action takes place, that is, theball is moved away in a direction normal to the seat first and then rotated, resulting inlower operating torque to open and close the valve under pressure and less damage tothe seat due to the absence of sliding and scraping action. To achieve this mechanicalseating action, the basic construction is a little more complex than the trunnion-mounted design. The wedged ball design has only one seat, does not offer as smooth abore as the floating ball or trunnion mounted ball valve, and therefore has a slightlyhigher pressure drop under fluid flow conditions.



9-7

Figure 9-3Wedged Ball Design

Some of the advantages and disadvantages of the wedged ball valve design areidentified below.

Advantages:

• Mechanically loads the seat to achieve seating.

• Seating is aided by differential pressure.

• Torque is minimized since the ball does not drag on the seat during turning.

• Fire safe.

• Seating action is less affected by seat and ball wear.

Disadvantages:

• Unidirectional.

• Uses multiple turns of the handwheel or actuator to achieve 90° rotation of the ball.

• Higher pressure drop than conventional ball valves.



9-8


The performance and reliability of ball valves are relatively unaffected by orientation.However, orientation that places the body shaft penetration at the low point of thevalve should be avoided to minimize the effect of debris on the packing system.

For other than flanged or screwed end valves, care must be taken to avoid overheatingor burning the seats and seals when welding or brazing into the line. Manufacturer’sinstallation instructions must be followed. Some designs may require removal of theseats and seals prior to installation.


Although the use of standard design ball valves is normally restricted to isolationservice, rough throttling with these valves can be tolerated if the valve is no less than20% open and the manufacturer’s design limits are not exceeded.

The internal body/ball configuration of ball valves is such that there are inaccessibleareas behind the ball where suspended solids in the fluid can be trapped. If the solidsare not tightly adhering or do not coagulate, removal of the solids from inaccessibleareas can be accomplished by putting the valve in the partially open position. Thisresults in internal turbulence and eddies that tend to scour out the valve. Thisprocedure is especially critical in radioactive service. Depending on the design andflow direction, ball valves may open or close by themselves under flowing conditions,especially in larger sizes. Therefore, care should be used when operating a lever-operated manual ball valve to prevent personal injury.

The correct size valve wrench must be used to open or close a manual valve. Exercisecaution against the use of excessive leverage on the wrench. Do not use a pipe wrench.

9.5 Common Problems

• The primary problems with ball valve seats are damage by debris and wear of theelastomer or plastic.

• If a ball valve has not been operated for an extended period of time, the initialbreakaway torque can be two to three times the normal operating torque (for someseat designs). Accumulation of debris and foreign materials in the valve body cavitymay also interfere with valve operation.

• In MOVs, the ball may not reach the fully open or fully closed position because thelimit switches are not properly set. When used, torque switches may also trip beforereaching the fully closed position.



9-9

• Sometimes, the ball assembly in a large trunnion valve will shift during transit,making the valve inoperable after installation.


Maintenance methods discussed in Section 4.6 generally apply here.

Always follow the manufacturer’s maintenance recommendations.

Ensure that the valve is depressurized before disassembling. Particular care should betaken that there is no residual pressure in the area behind the ball (that is, between theseats).


10-1

10 BALL VALVES—MODULATING/THROTTLING

FUNCTION


Standard ball valve designs are not generally well suited for control valve servicebecause of the possibility of erosion damage to the seats. However, special seat designshave been specifically developed for control applications. The valve ball remains incontact with the seat during rotation, which creates a shearing effect and keeps theseating surfaces clean. For high temperature applications, metal seats are typicallyutilized. To obtain the desired flow characteristics, some ball designs have a contouredV-notch shape that provides control, even in the low travel positions, whilemaintaining a high rangeability. Rangeability is defined as a ratio of maximum tominimum flow within which the deviation from the specified flow characteristic doesnot exceed stated limits. Since the closure member is not in the flow stream when thevalve is fully open, ball valves have less pressure drop than butterfly valves, especiallyin high pressure ratings.

The standard ball valve is a high recovery valve. However, flow disturbances causedby upstream and downstream piping components (such as reducers and elbows) canaffect the valve flow and torque coefficients.

Ball valves provide a wide range of continuous flow rate. Although ball valves can beoperated near the seat, continuous operation near the fully closed position is notrecommended because of cavitation and choking concerns.

10.2 Design

Conventional ball valves are used primarily for isolation service. Like butterfly valves,conventional ball valves cannot provide fine control, and they also experience erosion,cavitation, or noise when operated near the fully closed position. For modulatingservice, special features are incorporated into the design, as shown in Figures 10-1, 10-2, and 10-3.


Ball Valves—Modulating/Throttling Function

10-2

The special cam type (partial ball) rotary valve (Figure 10-1) is particularly useful insituations where particulates are present and finer control is required. This rotary valveoffers good control and metal-to-metal seating. It uses a cam-shaped eccentricallymounted disc connected to the shaft by arms that can flex slightly to provide a tightshut-off without requiring high closing forces.

Figure 10-1Eccentric Rotating Plug/Ball Control Valve

Figure 10-2 shows a U-shaped ball design that provides finer control near the fullyclosed position. The tube bundle (immediately downstream of the ball) preventsexcessive pressure drop across the ball itself, thus limiting valve cavitation. This design,however, is less cavitation resistant than the design shown in Figure 10-3.



10-3

Figure 10-2Segmented Ball with Tubular Resistance Trim

The ball design shown in Figure 10-3 incorporates a multistage pressure drop pathwhen the ball is in the mid-travel position. The orifices in this ball design provide finercontrol and limit the pressure drop across any one stage, which prevents cavitation.This ball valve design is being successfully used in low pressure throttling applicationsand is much better suited for dirty service than are globe-type control valves. Anotheradvantage to this design is its high rangeability with relatively low pressure drop whenin the fully opened position.



10-4

Figure 10-3Multistage Anticavitation Ball Valve


Installation practices discussed in Sections 9.3 and 6.3 apply to ball valves inmodulating service.


The operating practices and precautions discussed in Sections 9.4 and 6.4 apply to ballvalves being used in modulating service.



10-5

10.5 Common Problems

As with other high pressure recovery valves, cavitation is one of the main ball valveproblems, especially in modulating/throttling service. In conventional ball valves,cavitation will occur at a lower valve pressure drop, ∆Pv, than it would in a globe-typevalve. For example, a ball valve with 100°F (38°C) water and 100 psia (689.5 kPa) inletpressure would cavitate at a ∆Pv of about 35 psid (241 kPa), while a globe valve couldtolerate up to 80 psid (552 kPa) before cavitation would occur. Thus, careful evaluationof valve cavitation must be performed before specifying ball valves (or any highpressure recovery valve in general). The problems discussed in Sections 9.5 and 6.5 alsoapply.


Maintenance practices discussed in Sections 9.6 and 6.6 apply to ball valves being usedin modulating service.


11-1

11 PLUG VALVES


Plug valves are used primarily for isolation service and are available in lubricated andnonlubricated designs. Lubricated plug valves can cause lubricant contamination to theprocess fluid and should not be used where process fluid contamination is nottolerable. Nonlubricated (sleeved) plug valves are suitable for use in liquid radwastesystems because of the absence of crud pockets in the valve body. However,nonlubricated plug valves require high torques to operate and are difficult to maintain.Plug valves tend to be less expensive than ball valves.

11.2 Design

Like the ball valve, the plug valve is a quarter-turn valve. The plug valve is compactand simple in construction. It uses a cylindrical or conical-shaped closure memberinstead of the spherical shape used in the ball valve. The plug valve is basically an on-off service valve, but can be used for throttling if precise control is not required.

The two basic designs of plug valves are the nonlubricated type (Figure 11-1) in whicha metal plug is either surrounded by a resilient sleeve or fits between resilient seats andthe lubricated plug (Figure 11-2) in which sealant or lubricant is injected between theplug and body seating surface to achieve a tight seat. An all-metal constructionvariation of the nonlubricated plug valve uses a lift-turn-reseat motion of the plug.

The lubricated design is available in both cylindrical plug (Figure 11-2) and taperedplug (Figure 11-3) types. Tapered plug valves are more widely used than thecylindrical designs because the plug can be adjusted within the body (by an externaladjustment screw) to compensate for wear, thus providing better shutoff duringservice. However, conventional tapered plug valves are prone to being wedged into thebody due to hydraulic pressure imbalances that exist above and below the plug endsduring rapid hydraulic transients. This wedging problem is commonly referred to as“taper locking” and results in a substantial increase in operating torque. Somemanufacturers have incorporated special patented design features to eliminate thistaper locking problem.


Plug Valves

11-2

Nonlubricated plug valves require a higher torque to operate than ball valves becauseplug valves have a larger area constantly in contact with the plug sleeve, which acts asa seat.

Figure 11-1Nonlubricated Plug Valve

Figure 11-2Lubricated Plug Valve


Plug Valves

11-3

Figure 11-3Lubricated Tapered Plug Valve

Plug valves feature either a top- or bottom-entry design, both of which can be readilyserviced in line. Application of conventional lubricated plug valves is generally limitedto temperatures not exceeding 250°F (120°C) and where slight contamination of theprocess fluid by the sealant is acceptable. At higher temperatures, the asymmetricconstruction of the valve body leads to significant distortion of the body seatingsurfaces which increases the seating gap, making it harder to seat. The nonlubricateddesign is more tolerant of temperature, but is still limited to 400°F (200°C) by theresilient material used. The all-metal variation is suitable for higher temperatureservice.


Plug Valves

11-4


Plug valves are relatively insensitive to stem/shaft orientation. However, orientationthat places the stem at the low point of the valve should be avoided because debrisaccumulation can cause problems with shaft sealing.

Lubricated plugs have small clearances between the body and plug, and are susceptibleto binding due to distortion resulting from piping loads. Care should be used wheninstalling piping on upstream and downstream nozzles to ensure that piping loads donot distort the valve body or cause plug binding.


The operational precautions described in Section 9.4 for ball valves are generallyapplicable to plug valves.

Lubricated plug valves must be lubricated to operate freely. Infrequent use and lack oflubrication can cause binding.

Nonlubricated plug valves, like ball valves, will require higher torque to operate if theyhave been idle for a prolonged period of time.

Although the use of the standard design plug valve is usually restricted to isolationservice, rough throttling with plug valves can be tolerated if the valve is more than 20%open and the manufacturer’s design limits are not exceeded.


Plug seizing in place due to infrequent use is a major problem. Leakage past the seatingsurface due to wear and infrequent attention to lubrication is a problem in lubricatedplugs.

Tapered plugs may become locked into position under pressure transients. Thiscondition is commonly referred to as taper locking.


Plug Valves

11-5


Be sure that the valve is depressurized before disassembling. Particular care should betaken to ensure that there is no residual pressure locked in the area behind the plug(that is, between the upstream and downstream sealing area).

Precautions for bolted bonnet gate valves, given in Section 4.6, generally apply to plugvalves also.

Always follow the manufacturer’s instructions.

Ensure that the special tools frequently required for resleeving a nonlubricated plugvalve are available and used.


12-1

12 DIAPHRAGM VALVES—ISOLATION FUNCTION


Probably the most reliable valve for flow isolation in low pressure and low temperatureservice is the flexible diaphragm valve because it is extremely simple and requires littlemaintenance. Diaphragm valves are particularly suited for radioactive service, tightclosure service, fluid service where the fluid contains grit or suspended solids, andfluid service where the fluid is corrosive or scale forming. The use of diaphragm valvesis limited in pressure and temperature use. The use of a diaphragm valve in safety-related systems has some restrictions imposed for nuclear use by ASME III and theNRC in Regulatory Guide 1.84.

12.2 Design

The diaphragm valve is comprised of a bonnet, body, and flexible-sealing member. Theflexible sealing member is available in a variety of materials such as Buna-N, Viton,TFE, polyethylene, or neoprene. This valve is particularly suited for corrosive fluid,slurries, scale-forming service, and where zero stem leakage is mandatory. The bodymay also be fully lined to accommodate these services.

Although diaphragm valves have been tested and operated satisfactorily for over50,000 cycles, design life of the diaphragm should be limited to 20,000 cycles or 10years, whichever occurs first. The valves are available in sizes from 1/2 inch to 16 inch(12 mm to 400 mm); but, due to their large overall size, they are not generallyrecommended larger than an 8-inch size. Because of their materials of construction,flexible diaphragm valves are limited to temperatures less than 300°F (150°C).

Flexible diaphragm valves are available in three basic body configurations (Figures 12-1, 12-2, and 12-3):

• Saunders pattern flexible diaphragm valve

• Straightway flexible diaphragm valve

• Full-bore body flexible diaphragm valve


Diaphragm Valves—Isolation Function

12-2

Depending on the valve design and installation, there is a potential for trapping fluid inthe valve and upstream or downstream piping.

The Saunders pattern or conventional weir design (Figure 12-1) is the most commonlyused diaphragm valve. The valve is self-draining when installed in horizontal pipingwith the stem axis oriented to approximately 20° above horizontal.

The straightway design (Figure 12-2) has no weir but incorporates a straight-throughflow path. However, the pressure-temperature rating for the diaphragm in this valve isless than the rating for the diaphragm found in a Saunders pattern valve.

The full-bore type (Figure 12-3) provides a full rounded bore and streamlined flow.This type also has a weir, but the weir height is considerably less than the weir heightin the Saunders pattern valve.

Figure 12-1Saunders Pattern Flexible Diaphragm Valve



12-3

Figure 12-2Straightway Flexible Diaphragm Valve

Figure 12-3Full Bore Body Flexible Diaphragm Valve



12-4


When installed in a horizontal pipe with the stem vertical, the weir in a Saunderspattern valve prevents full draining of the attached piping and can become a source fortrapping crud. However, the valve is self-draining when the stem forms an angle ofapproximately 20° above horizontal.

Diaphragm valves are often used in boric acid systems. These systems are normallyheat traced to keep the boric acid solution above the boric acid crystallizationtemperature. Care should be taken not to install the heat tracing or insulation above thebody flange because overheating and damage to the diaphragm will occur.


Over-tightening the handwheel on the valve will cause damage to the diaphragm.Never use a larger size handwheel than the handwheel provided with the valve. If thevalve is provided with a travel stop, set the travel stop so that the valve will shut tightlywithout over-torquing.


• As mentioned in Section 12.3, overheating or damage to the diaphragm can occur ifimproperly heat traced.

• Damage to the diaphragm can occur if over-tightened.

• Damage to the lining of a lined diaphragm will occur if the lining material is notcompatible with the fluid chemistry. In a corrosive service, body corrosion can occurif the lining is damaged and the process fluid has leaked through the lining.


The maintenance methods discussed in Section 4.6 are generally applicable todiaphragm valves.

Replacing the diaphragm due to damage or leakage is the most frequent maintenanceaction required. To speed replacement, a spare bonnet for each size and materialshould be available. A new diaphragm should be placed on the spare bonnet and theentire bonnet replaced on the valve requiring a new diaphragm.


13-1

13 VALVE ACTUATORS—GENERAL INFORMATION

13.1 General

Actuators are devices installed on valves to permit control of the closure member. Theactuator can be either locally or remotely controlled to open, close, change, or maintaina position. The basic types of actuators are:

• Manual

• Electric motor

• Solenoid

• Pneumatic

• Hydraulic

• Electrohydraulic

• A combination of these types

Figure 13-1 provides a brief summary of the most typical actuator types, and Table 13-1presents features, capabilities, and suitable areas of application for power actuators.Other conditions which should be considered in actuator selection are stabilityrequirements for the application, temperature, and fail-safe operation.

Most valves can operate by means of a handwheel or lever supplied with the valve.Various accessories can be adapted to fit most types of valves to permit valve operationunder the following conditions:

• Remote or inaccessible location

• Insufficient handwheel output torque or thrust

• Longer or shorter valve stroke time


Valve Actuators—General Information

13-2

Figure 13-1Types of Valve Actuators



13-3

Table 13-1Normal Application of Power Actuators for Valves

Actuator Type

Feature Electric Motor/ Gear Box Drive Pneumatic Hydraulic

Output thrustor torque

Up to 500,000 lb or 60,000 ft-lb(2,224 kN or 81 kN-m)

Up to 23,000 1b(102 kN)*

Virtually unlimited

Stroke length Unlimited Diaphragm type: limited toshort stroke

Piston type: unlimited

Unlimited

Availablestarting torque/thrust

High Low High

Valve types thatoperator can be usedwith

All Globe, diaphragm, ball,butterfly, plug: not normallyused with gate valves

All

Operating speed andstroke time

Normally the slowest of the threeactuator types. Can be providedfor fast actuation. Due toincreased size and weightrequired, and inherent operatinginertia, careful selection isrequired.

Fast (5 sec or less): speedcontrol can be provided onthe actuator

Fast (5 sec or less)

Normalspeeds

Gate: stem moves at 12 inch/min(305 mm/min)

Globe: stem moves at4 inch/min(102 mm/min)

Small ball, butterfly, and plugs:5-10 sec/90°

Large ball, butterfly, and plugs:30-60 sec/90°

Failure mode Fails as-is if self-locking geartrain is used.

Any required position canbe accommodated

Any requiredposition can beaccommodated

Source of energy tooperate

Station electrical power (orinstrumentation back-up power)

Station compressed air,accumulators, springs; oneor combination thereof

Station compressedair, accumulators,electric power,springs, dedicatedpressurized hydraulicsystem; one orcombination thereof

Suitable for throttling Yes Best Yes

* Special pneumatic actuators have been developed providing thrust up to 100,000 pounds (444.8 kN).



13-4

The Electric Power Research Institute (EPRI) has published several application guidesto address sizing, installation, operation, maintenance, and repair of the mostcommonly used electric motor actuators in U.S. nuclear power plants (see References1.5, 1.6, 1.22, 1.23, 1.24, 1.25, and 1.26). EPRI has also published several otherdocuments to address special types of valves, such as air-operated valves [1.2], safetyand relief valves [1.4], solenoid valves [1.7], and main steam isolation valves [1.27 and1.28]. The actuators and devices used to operate these valves are discussed within eachdocument, and the reader is referred to these documents for detailed information. Thediscussion in this document is limited to the actuator types and their selections fornuclear power plant applications. Manual actuators are also discussed.

13.2 Actuator Types

13.2.1 Manual Actuators

The most common manual actuators are the handwheel and the lever. Torque androtation, applied to the rim of the handwheel, are translated to stem force. The stemtravels through a screw-threaded connection between the valve stem and the yoke nutto which the handwheel is affixed. The screw-threaded connection is normally self-locking so that the valve stem will remain in the position in which it is left. Gearing canbe utilized to change the plane of rotation between the handwheel and the valve stem(for example, from horizontal to vertical). Gearing is also used to increase (or decrease)the output torque or thrust to the valve stem. The number of handwheel turns neededto achieve full valve stroke depends on the gear ratio of the gear set used.

Simple lever actuators are often used with small quarter-turn valves such as ball, plug,and butterfly valves. Some lever designs are available with self-locking features for usein throttling applications or to keep the valve stem in the as-left position. Manualactuators are further discussed in Section 14.

13.2.2 Motorized Actuators

Motor-operated valves (MOVs) are provided with reversible ac or dc electric motoractuators. The electric motors are normally 15-minute duty motors and rarely arecontinuous duty motors. Typically, the motor delivers torque and rotation through areduction gear arrangement to turn the stem nut, which causes the threaded valve stemto move to either open or close the valve. Figure 13-2 is a cutaway view of an electricmotor actuator, which shows the electric motor, the reduction gears, the stem nut andthe valve stem. Figure 13-3 shows a simplified schematic of the operation of an electricmotor actuator. In gate valves, the stem speed is generally 12 inches per minute (305mm/min). In globe valves, the stem speed is generally 4 inches per minute (102mm/min).



13-5

Figure 13-2Limitorque SMB-0 Motor Operator Cutaway View



13-6

Figure 13-3Simplified Motor Operator

To convert the higher speed, multiturn motion from the motor actuator to a slowerquarter-turn motion required for the operation of rotary valves (such as butterfly, balland plug valves), a gear reducer with position stops is utilized between the actuatorand the valve stem (Figure 7-1). The stroke time for quarter-turn valves varies from lessthan 5 seconds to over 60 seconds.

Control devices (such as limit and torque switches in Figure 13-2) are used to sense theposition of the valve stem and/or the amount of applied torque and to shut off themotor power supply once the required limit is reached. The reduction gears are usuallyself-locking such that the valve stem position is maintained without the continuedapplication of an external power source. Thus, the actuator can be used to position thevalve at an intermediate position, as in throttling service.

Motor operators are available for modulating and throttling service. When motor-operated valves are ordered for infrequent (or non-modulating) throttling service, it isessential that all the conditions are given to the manufacturer, including the number oftimes the valve is going to be positioned per hour and the approximate range ofmovement of the valve stem. This information is required to ensure that a continuousduty motor (as opposed to the normally provided 15-minute duty motor) is suppliedand that the motor actuator sizing and selection is correct.



13-7

The motor operator has an automatic transfer mechanism to switch from the manualmode to the electric power mode and visa versa. This mechanism uses a pawl-clutcharrangement, which is subject to wear and possible failure when used frequently. Amotor-operated valve should not be specified when it is intended that the valve bethrottled manually.

Electric motor sizing calculations, installation, maintenance and repair procedures aretypically provided by the actuator manufacturer. EPRI has published several technicalrepair guides for Limitorque and Rotork electric motor actuators, which are the mostcommon in U.S. nuclear power plants (see References 1.22, 1.23, 1.24, 1.25, and 1.26).Motor actuator sizing calculations are provided in EPRI’s MOV guides [1.5, 1.6]. Thesedocuments provide detailed discussions and data, and can be reviewed for in-depthinformation.

13.2.3 Pneumatic Actuator

Pneumatic actuators are generally provided as diaphragm type, piston type, or vanetype. The diaphragm actuator uses a circular diaphragm sealed at its perimeter, whichis normally pressurized on one side, with the other side vented to the atmosphere.Applied air pressure in the range of 20 to 50 psig (138 to 345 kPa) develops a forcewhich is transmitted to the valve stem through a large circular plate. A spring can beincorporated into the actuator to provide force in a direction opposite to that developedby the applied air pressure. The amount of travel is limited by the proportions of thediaphragm, which maintains a static seal throughout the actuator travel. This type ofactuator is most commonly used with modulating control valves.

Piston-type actuators use a piston with sliding seals. Actuating air pressure in the rangeof 100 to 150 psi (690 to 1,034 kPa) is commonly used, and the length of actuator travelcan be larger than diaphragm actuators. As in the diaphragm actuator, biasing springscan be used with the cylinder actuator to provide a fail-open or fail-closed action onloss of air pressure. Alternatively, the cylinder operator can be double acting, with airpressure applied to either open or close the valve. Fail-in-last-position on loss of airpressure can be achieved by trapping air on both sides of the piston. However, thisapproach is not reliable because of potential air leakage.

Vane-type actuators also use sliding seals and utilize air pressure in the range of 100 to150 psig (690 to 1,034 kPa). These actuators are normally used to provide quarter-turnmotion for actuating ball valves or butterfly valves. Springs can be provided to movethe valve to its fail-safe position on loss of actuating air.

EPRI’s Air-Operated Valve Maintenance Guide [1.2] provides a comprehensive discussionof various aspects of air-operated valves including application, operation, trouble



13-8

shooting, maintenance, and repair. Reference 1.2 should be reviewed for in-depthdiscussions of air actuators.

13.2.4 Hydraulic Actuators

Hydraulic actuators utilize a high pressure hydraulic fluid (in the range of 1,000 to over3,000 psi or 6,900 to over 20,700 kPa) to provide high stem force or torque with arelatively small actuator. A common application of this type of actuator is in turbinestop service where the hydraulic pressure holds the valve open and spring actionrapidly closes the valve when the hydraulic pressure is released.

Hydraulic actuators are used as a substitute for pneumatic actuators when high forcesare required or higher overall actuator stiffness is desired. Actuator stiffness is a majorconsideration in the stability of the control valve/actuator system. Hydraulic actuatorsare used in quarter-turn valves where high volumes of oil are not required andactuation speeds are relatively slow. The hydraulic actuator‘s major disadvantage isthat it requires a high pressure supply module, which can be bulky.

Hydraulic actuators are normally limited to an actuating hydraulic fluid pressure of3,000 psi (20,700 kPa), and a temperature not exceeding 350°F (177°C), due to the typesof elastomeric or polymeric seals used. Process fluid temperature does not normallybecome a constraint in the hydraulic actuator selection because the temperature of thehydraulic fluid is typically much lower than the process fluid temperature. Someactuators, however, can be equipped with metal-seal rings, which allow them tooperate at high temperatures.

Depending on the service, hydraulic actuators can be combined with springs orpneumatics to provide fail-safe operation in either the fail-open or fail-closed position.This approach has been used in main steam isolation valves and turbine stop valveswhere the hydraulic pressure is used to open the valve and, at the same time,compresses a mechanical or gas spring used to achieve a fast fail-closed operation(Figures 13-4 and 13-5). When a signal is given to close, the hydraulic fluid isdischarged, allowing the valve to close. The speed of closing is controlled not by thesupply of fluid, but by its exhaust, thus providing extremely fast actuation.



13-9

Figure 13-4Hydraulic Actuator with Fail-Safe Operation Using a Mechanical Spring



13-10

Figure 13-5Hydraulic Actuator with Fail-Safe Operation Using a Gas Spring

Hydraulic operators are not widely used in the nuclear power industry. They have tobe provided with an integral hydraulic system, normally supplied by the valvemanufacturer. Control of leakage and particulate content in these systems is of primaryimportance since small bleed orifices and clearances are often used.



13-11

13.2.5 Electrohydraulic Actuators

There are two variations of the electrohydraulic actuator. One design incorporates adedicated electric motor-driven pumping system mounted directly on the actuator orvalve. The other design uses a low-level electric power-operated coil to control the flowof oil to a hydraulic cylinder and the position of the piston. The coil is attached to apivoted nozzle, through which high pressure hydraulic oil flows. A control signalcauses the coil to move within a permanent magnet and varies the flow to either side ofa hydraulic piston, causing the valve to either open or close. This type ofelectrohydraulic actuator is used primarily on control valves and offers the advantagethat the actuator can be operated remotely from an instrument, if there is no otherauxiliary pressure (such as a pneumatic pressure) available to operate a valve.

Electrohydraulic actuators are not widely used because:

• They are expensive relative to a diaphragm-actuated control valve with atransducer.

• Electrohydraulic actuators require a constant source of pressure, which in turnrequires a constant use of electric power to pump the hydraulic fluid.

• The operating speeds of electrohydraulic actuators are sometimes lower than can beobtained with a diaphragm actuator.

• Their maximum stem thrust is somewhat lower than can be obtained with largediaphragm actuators or high pressure cylinder actuators.

13.2.6 Solenoid Actuator

Solenoid actuators are usually limited to applications involving short travel and lowstem thrust requirements. Solenoid actuators are generally furnished as an actuator-valve assembly or solenoid valve. Valve actuation occurs when a coil (see Figure 13-6)is energized with ac or dc power. The resulting electromagnetic force lifts a moveablesolenoid core or plunger, together with the valve stem and valve disc, opening thevalve. Spring action is used to return the valve to its original position when the coil isde-energized.



13-12

Figure 13-6Solenoid Actuator

The valves can be direct acting, where the plunger is connected directly to the maindisc of the valve, or pilot operated, where the plunger opens a small pilot valve thatallows system pressure to act on components of the valve to open the main disc.Actuation time of solenoid valves is very rapid. Solenoid valves can be obtained in two-way, three-way, or four-way design. A common application is in directing the flow ofcompressed air or hydraulic fluid to larger actuators.

Solenoid actuators, like electric motor actuators, require an electric power source.However, the use of solenoid actuators as a direct method of actuating valves is limitedbecause of their relatively low output force. Solenoid actuators are used extensively toactuate small pilot valves in remote-controlled pneumatic and hydraulic systems.Solenoids are used in actuating valves up to 8 inch, class 2500, when provided with apilot arrangement. Normally solenoid valves can only seal in the flow-to-closedirection. Solenoid valves without pilot operation are generally limited to 2-inch andsmaller sizes. Multiple solenoids can be supplied to provide more than one direction ormode of operation, such as in three- and four-way valves.

Solenoid valves should not be used where foreign magnetic material can be attracted tothe operating mechanism. When solenoid actuators are specified, both the minimumand maximum operating differential pressure should be specified. This allows themanufacturer to determine the force required to actuate or prevent actuation of thevalve.

EPRI’s Solenoid Valve Maintenance and Application Guide [1.7] provides a comprehensivediscussion of various aspects of solenoid valves including application, operation,trouble shooting, maintenance and repair. Reference 1.7 should be reviewed for in-depth discussions of solenoid actuators.



13-13

13.2.7 Process Medium Actuators

Actuation by process medium consists of using the process fluid to provide pressure ona diaphragm or cylinder actuator to generate the force required to close the valve.Process medium actuators have found very limited use and are primarily found onmain steam isolation valves where extremely high actuation speeds are required in thefail-closed position. In this application, system steam, normally from the upstream sideof the valve, is piped to the top of a piston actuator. The steam in these actuatorsreplaces the compressed gas or springs used in similar type actuators.

These actuators provide no distinct advantage over the more conventional fail-closedspring or compressed gas actuators and require that the process fluid be a clean fluid toavoid corrosion, wear, and sticking of the actuator. Typically, these actuators areconstructed from corrosion-resistant materials, such as stainless steel, bronze, ornonmetallics.

13.3 Considerations in Actuator Selection

Actuator selection involves evaluation of numerous factors including the following:

• Valve type

• Type of service

• Available source of energy

• Availability of backup power

• Thrust or torque requirements in both the opening and closing directions

• Temperature limitations

• Ionizing radiation

• Performance under design basis conditions

• On-off or modulating

• Duty cycle

• Stability

• Remote actuation or computer control

• Fail-safe operation



13-14

• Override requirements

• Dynamic performance; actuator stiffness against movement by pressure or flow

• Weight

• Location, space and accessibility

• Maintenance requirements

• Cost

• Availability

When selecting manual actuators, such as levers and handwheels, excessive protrusionincreases the risk of injury or accidental change of the valve setting. When usinghandwheels or levers, consideration should be given to the selection of methods forlocking the position of the closure member.

Particularly important in power plants is the fail-safe operation of the actuator. Thevalve may be required to fail-open, fail-closed, or fail-in-last-position. The fail-safemode may be provided by springs, weights, gas pressure, or gears. Mechanical springs,weights, or gas springs can provide the fail-open and fail-closed modes. Gear actuatorsare usually incapable of providing a fail-open or fail-closed action and are suitable onlyfor fail-in-last-position applications.

When procuring an isolation valve, it must be specified if the valve must operateagainst a high differential pressure in an “off-normal” condition so that the operatorcan be properly sized. This “off-normal” condition is often overlooked during valveprocurement.


14-1

14 MANUAL ACTUATORS


The simplest forms of the manual actuator are the hand (or manual) lever, as shown inFigure 14-1, and the handwheel, as shown in Figure 14-2. Levers are normally used inquarter-turn valves, such as ball and butterfly valves, and handwheels are used wheremultiple turns are required to actuate the valve, such as in gate and globe valves.

Figure 14-1Manual Lever


Manual Actuators

14-2

Figure 14-2Worm Gear Actuator

The use of the manual actuator for normal operation is limited by the amount offorce/torque required to actuate the valve and the stroke time. In larger valves, manualactuators are coupled with gears (see Figure 14-2) to produce the required force.Manual actuators on large valves are normally provided for manual override andemergency operation. Manual actuators are most frequently used in small valves andcontrol equipment. These actuators should not require more than 60 pounds (0.27 kN)of force during the majority of the travel, and 150 pounds (0.67 kN) of peak force tofully operate the valve (see Table 14-1).

The effort that can be exerted by an average person depends upon the size of thehandwheel and the orientation of the handwheel relative to the person. Hammerblowor impact handwheels are used to create higher starting torques than can be achievedby a gradual application of effort, as discussed in Section 2.3.7.


Manual Actuators

14-3

14.2 Design Considerations

14.2.1 Operating Force

Manual handwheels and levers should be sized so that no more that 150 pounds (0.67kN) of force is required to actuate the valve under maximum operating conditions.They should be of a size that does not hinder normal access to the valve. Furtherguidance on access requirements is given in Section 17.

Table 14-1 provides the average tangential force and the corresponding torque valuesas functions of the handwheel diameter. Table 14-1 is based upon tests performed bythe U.S. Navy and represents the handwheel rim pull achievable by an average person.Lever length is equal to one-half the handwheel diameter.

Table 14-1Maximum Recommended Rim Pull as a Function of Handwheel Diameter

Achievable HandwheelDiameter inch (mm)

Average Tangential Forcelb (N)

Resulting Torqueft-lb (N-m)

Below 4 (100) 50 (220) 8 or less (10.8)

4 to 6 (100 to 150) 60 (270) 10 to 15 (13.6 to 20.3)

7 to 9 (180 to 230) 100 (440) 29 to 38 (39.3 to 51.5)

10 to 14 (250 to 360) 125 (550) 52 to 72 (70.5 to 97.6)

15 to 23 (380 to 580) 145 (640) 90 to 136 (122 to 184)

24 (610) and above 150 (670) 150 (203) and higher

14.2.2 Lever Position Control

Levers can be supplied with locking devices (Figure 14-1) to maintain the closuremember at any discrete position. These devices are required when the fluidhydrodynamic forces tend to close or open the closure member in the midtravelposition, such as in ball and butterfly valves. Padlocks can be also be attached to thesedevices to secure the position of the closure member in order to prevent unauthorizedor inadvertent operation of the valve.

14.2.3 Chain-Wheel Operators

Chain-wheel operators are generally used when the handwheel is 7 feet (2.1 m) orhigher above the floor or platform level and in an inaccessible or hazardous area.Chain-wheel operators are attached to the rim or the spokes of the valve handwheel.


Manual Actuators

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14.2.4 Hammerblow or Impact Handwheels

The hammerblow handwheel sometimes eliminates the need for reduction gears onvalves by providing higher torques than is otherwise possible. It typically has from 30°to 330° of slack in its rotation and is purposely very heavy in order to provide aflywheel effect. The hammerblow handwheel is rotated in the slack area, then slammedinto the driving lugs on the stem to obtain a tight seat or deliver a high opening force tounseat.

Hammerblow handwheels increase the effective torque/thrust by a factor of 2 to 4,compared to the values shown in Table 14-1.

14.2.5 Gear Operators

Gear operators reduce the handwheel effort by a factor of 3:1 to 70:1 in most cases. Themost common are the worm and bevel gear types. Gear operators are also used tochange the orientation of the handwheel with respect to the stem.


The use of chain-wheel operators should be kept to a minimum, and they should beinstalled so they are not blocking personnel passage. Note that if a chain wheel isnecessary, the valve is likely to have limited access for maintenance and repair.

The routing of reach rods and extension stems should be as direct as possible from theoperating station to the valves to minimize the number of auxiliary devices, such asgear boxes and universal joints, and to make the system more efficient.

When using flexible cable as part of a remote operating system, the applied torque andthe minimum radius to which the cable can be bent must stay within themanufacturer’s limits. A remote operating system must be supported in accordancewith the manufacturer’s recommendations.

When installing a remote operating system initially and after maintenance, ensure thatall parts are in their proper position. Mispositioning certain parts may cause difficultiesin operation and inaccuracy in valve position at the remote station.


Manual Actuators

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Operating manual actuators under high pressure and flow can be dangerous topersonnel. Hydrodynamic forces and torques can be very high and may slam the valvein the closing or opening direction unless self-locking gears are used. Thus, care shouldbe taken when manually operating a valve under high pressure and flow.

Operating a valve that was left in the same position for an extended period of time maycause the valve packing to leak. At-the-valve manual operation may expose plantpersonnel to process fluid through packing leakage. Safety precautions must befollowed to prevent personnel exposure to packing leakage.

When operating a valve, do not over-torque the stem, or use cheater bars on thehandwheel to increase the rim pull force. Particular care should be used with ahammerblow handwheel.

To minimize the possibility of over-torquing a valve, use the same diameter handwheelthat was supplied with the valve.


• The most common problem with remote operating systems is lack of properlubrication (including grease aging, hardening, and contamination), which canmake the system difficult to operate.

• Solid shafting, gearing, and flexible shafting, if not sized properly, will result indifficult operation and sometimes failure of a component.

• Inadequate maintenance may result in loosening of nuts and bolts and may causepersonnel injury.


Periodically check installations for proper tightness of nuts and bolts and properalignment of all parts. Ensure that there is proper lubrication on the stem, in gear boxes,and on universal joints. Apply proper lubricants and coatings to prevent corrosion.


15-1

15 GENERAL DESIGN REQUIREMENTS FOR VALVES

AND ACTUATORS

15.1 Introduction

Many valves perform critical functions in a power plant. Besides effectively meeting therequirements of normal system operating conditions, many valves, particularly those innuclear safety-related systems, must perform their functions, often under degradedconditions and in a harsh environment. Sometimes the valve‘s function is merely to fail-as-is and to retain its pressure boundary. At other times, it may be required to open,close, or modulate while in a harsh environment, such as saturated steam, extreme heat,high radiation, or full submergence, often concurrently with loss of power or loss ofinstrument air.

In order to establish performance requirements and to properly specify a valve, it isnecessary to determine the fluid parameters being contained by the valve and toconsider other factors that could affect valve operation. Considerations include

• Flow conditions, for example, turbulent, laminar, flashing, cavitating, or two-phaseflow

• Pipe orientation, for example, horizontal or vertical

• Valve stem, handwheel, and operator orientation

• Anticipated localized conditions, such as water trapped in the valve bonnet, thermaloverpressurization due to inadvertent line isolation, or inadvertent linepressurization due to seat leakage of a pressure boundary valve

This determination is made by first identifying all the system operating conditions suchas normal, startup, shutdown, standby, abnormal/upset, emergency, faulted, and test.It is also important to identify which plant operating conditions apply to each of theabove since the requirements imposed on the systems and their valves may vary byplant operating and environmental conditions.


General Design Requirements for Valves and Actuators

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Once the system and plant operating conditions are identified, the required design flowrate, differential pressure, system pressure, and temperature are determined. Otherrequirements that do not apply to the particular application can be eliminated. Thisapproach will narrow down the types of valves and operators that may be acceptable.The available space for installation, operation, and maintenance is also evaluated inorder to further narrow down the valve selection.

In addition to safety and environmental requirements imposed by the applicable codes,all valves should undergo a commercial evaluation to determine the reliability andredundancy requirements to adequately fulfill their functions. For example, a largevalve in a power production system that could cause loss of production should it fail orhave to be removed from service frequently for repair would have more stringentdesign and maintenance requirements than a small valve in a system that would notadversely affect power production should it be out of service.

15.2 Fluid Parameters

15.2.1 Introduction

Fluid parameters are important factors in selecting the best valve and actuator for theapplication. These parameters will influence the choice of the valve type, size, andmaterials for the body, disc, seat, and packing. Fluid parameters also influence therequirements for special features, valve accessories, and spare parts.

15.2.2 Flow Media

The chemistry of the flow media will determine the materials required for the valvebody, disc, and other wetted parts of the valve. A highly corrosive media will requirecareful consideration of the valve type and materials used for valve internals. This istrue for metal parts as well as for synthetics and elastomers used for valve seat andpacking materials. Some valve manufacturers provide material compatibility tables intheir catalogs as an application guide in the selection process.

Biofouling may be a serious problem in some water systems. Given the properconditions, marine organisms attach themselves to wetted surfaces and grow. Theirpresence on valve internal surfaces may prevent the valve from performing its function.They can also accelerate corrosion attack on some alloys used for valve parts bycreating local shielding of the metal surface from oxygen required to preserve passivity.

At nuclear power plants, the potential for leakage from a valve with radioactive fluidor a combustible gas (such as hydrogen) requires special consideration in the valveselection and application. For example, the potential for leakage through the valve



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packing can be reduced by use of a stem leak-off connection, by use of diaphragmvalves, or by use of a packless valve design (metal diaphragm or metal bellows), asdiscussed in Section 2.5.

15.2.3 Pressure/Temperature

System design pressure, temperature, and differential pressure across the valve arebased on the most severe pressure and temperature combination expected duringsystem operation or under design basis conditions. The design pressure andtemperature are used to determine the primary pressure rating of the valve. Theadjusted pressure/temperature rating tables given in ANSI B16.34, B16.5, and MSSstandards (for non-ferrous valves) list the maximum allowable working pressures for agiven temperature. The maximum operating temperature may be limited to atemperature lower than that specified in the pressure/temperature rating table ifelastomers are used for valve seats and seals. This restriction is described in theapplicable code or is sometimes available in manufacturers’ catalogs in the form oftables or charts, and is typical for diaphragm, butterfly, ball, or plug valves. Section16.2 discusses pressure/temperature ratings in greater detail.

15.2.4 Velocity

Flow velocity is determined by flow rate and pipe size. Mean pipe velocity in the rangeof 5–15 ft/sec (1.5–4.5 m/sec) for water and 100–300 ft/sec (30–90 m/sec) for saturatedor super heated steam is not generally a concern for on/off valve applications.However, excessive flow velocities (such as during blowdown conditions) can causetipping of the valve disc in some gate valve designs and might result in galling orgouging of the guides, guide slots, disc, and seating surfaces, which in turn can lead toexcessively high thrust/torque requirements.

In throttling and modulating applications, flow rate is used in the control valve sizingcalculation and will influence the choice of valve size and type. Control valvecavitation, flashing, and choking are of particular concern especially in high flowvelocity applications (see Section 15.4.2). Accurate evaluation of valve cavitation andchoking is particularly important for throttling/modulating service where continuousoperation under cavitation causes severe damage to the valve and downstream pipingcomponents. Such evaluation should include the entire hydraulic system including thepressure source (for example, pump) and the upstream and downstream flowresistances. Some valve manufacturers and engineering companies have computersoftware to perform such evaluation throughout the valve stroke.



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15.2.5 Viscosity

Most valve applications involve a turbulent flow where the valve resistance coefficient(Kv) is independent of viscosity. The valve flow resistance coefficient (Kv) is used insizing valves for water flow and for flow of other liquids that behave like water. If theliquid is extremely viscous and viscosity is ignored, significant sizing errors may occur.

It is important to note that because fluid viscosity depends on the operatingtemperature, the valve performance can change with the operating temperature,especially for very viscous fluids.

15.2.6 Density, Specific Gravity

Density or specific gravity of the fluid must be taken into consideration if the pressuredrop is calculated using the Cv coefficient (which is based on flow of water at 60°F or15.6°C). The density of liquid changes with temperature but very little with pressure,unless very high pressures are being considered. The densities of gases and vapors,however, are greatly affected by pressure changes.

At power plants, commonly encountered liquids that have densities different fromwater are sea water, borated water, and oils.

15.2.7 Radiation

Elastomers and synthetics are commonly used in valve construction as seats, seals,liners, and sleeves. They have a lower radiation resistance than metals. Therefore, ifelastomers or synthetics are considered for handling of radioactive flow media or iftheir location would expose them to radiation, the total radiation dose must bespecified, including the design basis accident dose, to establish the design basis for thevalve over its specified design life. This integrated total radiation dose, together withtemperature and flow media chemistry, will determine the type of elastomer to be usedin the valve and the frequency with which it must be replaced. Sections 2.5 and 2.6provide more detail on elastomeric materials.

15.2.8 System Contaminants

Particle contaminants, such as dirt and grinding dust, should be avoided in valveapplications. Contaminants may cause the following conditions:

• Seat leakage, by preventing the valve from being fully seated if dirt accumulates onthe seat



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• Damage to the valve seat or disc, through erosion or abrasion of the seat/discmaterial

• Damage to the valve bearing (such as in butterfly and trunnion-mounted ballvalves) which can significantly increase required actuation forces and torques

Some systems are required to handle abrasive fluids/slurries. In these systems,contaminants cannot be avoided, and their presence should be identified to the valvemanufacturer. The presence of particle contamination eliminates the use of a gate valve.For globe valve applications, a hard-faced disc and seat is required. A diaphragm orplug valve may be considered as an alternative for systems handling abrasive fluids.

15.3 Operating Modes and Transients

15.3.1 Introduction

This section discusses plant, system, and valve conditions. It is very important toassociate a plant condition with each system condition because various nuclear safety,personnel safety, plant availability, and investment protection requirements areassociated with each plant condition. For example, minor leakage into the containmentatmosphere of radioactive fluid through the packing of a valve located inside thecontainment is of no consequence following a loss-of-coolant accident (LOCA), sincethere will be several feet of radioactive water on the floor.

15.3.2 Plant Condition

Plant condition is the status of the plant as a whole resulting from a postulated event.The number of postulated plant conditions that a unit could experience over itsoperational life is infinite. Because of this, a list of conditions to be used as a designbasis for the unit for each of the plant operating and test conditions should be made.The listing should be selected based on judgment and experience and should besufficiently severe and diverse to provide an adequately conservative design basis thatenvelopes all credible plant conditions that can be imposed on the valve. All ASMESection III safety class systems and valves must meet the resulting design basis. Thedesign basis for other systems and valves must be developed from the aboveconditions, based on applicable code, availability, and investment protectionrequirements.

A nuclear plant condition is further described by one of the following categories:

• Normal conditions (including performance testing)

• Upset conditions



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• Emergency conditions

• Faulted conditions

These conditions correspond to service limit levels A through D, as defined in ASMESection III NCA-2142. The plant licensing documents contain the design basis used tosatisfy emergency and faulted conditions. Non-nuclear power plants do not use thesedefinitions but should be evaluated considering the normal and abnormal conditionsthat they must operate under.

Normal Conditions: Normal conditions define the plant status in the course of systemstartup, operation in the design power range, hot standby, and system shutdown,including refueling. Normal conditions include reactor coolant system heatup andcooldown, large step load increases or decreases, and steady state fluctuations.

Upset Conditions (Incidents of Moderate Frequency): An upset condition is any deviationfrom normal conditions that is anticipated to occur often enough that the design shouldinclude a capability to withstand the condition without operational impairment. Upsetconditions include those transients that result from any single operator error or controlmalfunction, transients caused by a fault in a system component requiring its isolationfrom the system, turbine trip from full power, lifting of relief valves, loss of normalfeedwater, minor secondary system leakage that would not prevent an orderlyshutdown or cooldown assuming normal makeup, and transients due to loss of load orpower. Upset conditions include any abnormal incidents not resulting in a forcedoutage and also forced outages for which the corrective action does not include anyrepair of mechanical damage.

Emergency Conditions (Infrequent Incidents): Emergency conditions include thosedeviations from normal conditions that may occur during the operational life of theplant and that require shutdown for correction of the conditions or repair of damage inthe system. Included in this category would be a small loss-of-coolant accident and asmall steam line break. These types of conditions have a low probability of occurrencebut are included to provide assurance that no gross loss of structural integrity willresult as a concurrent effect of any damage developed in the system.

Faulted Conditions (Limiting Faults): Faulted conditions are those combinations ofconditions associated with extremely low probability postulated events. They are notanticipated to occur during the operational life of the plant, but their consequenceswould be such that if they did occur, the integrity and operability of the system couldbe impaired to the extent that considerations of public health and safety would beinvolved. Such considerations require compliance with safety criteria that may bespecified by jurisdictional authorities. Postulated faulted conditions would include alarge loss-of-coolant accident, large steam line break, large feedwater line break, and asteam generator tube rupture.



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Plant Test Conditions: Plant test conditions include hydrostatic, pneumatic, and leaktests specified by plant requirements. Other types of tests are performance testsclassified as normal conditions.

15.3.3 System Condition

System condition is the status of a particular system during the postulated plantcondition. Whenever “system condition” is used in this document it also means“system portion condition,” as applicable. Seldom is it possible to establish a single setof conditions that satisfactorily covers a complete fluid system. Internal conditions,such as temperature, pressure, and sometimes chemistry, vary in different portions ofthe system. Also, external environmental conditions vary, depending on the areas inwhich the system is located.

In order to satisfactorily address unique system conditions that may vary from plantconditions, several categories of system conditions are identified.

Systems Utilized during Multiple Modes of Operation: Systems may have several differentmodes of operation and could be exposed to different conditions, depending uponsystem alignments and the phase of plant operation. Systems and valves that mustoperate under multiple modes of operation during the various plant conditions must bedesigned to meet their functional requirements under all of these conditions and modesof operation. For example, during system operation, rapid realignment of a pumpsuction to an alternate supply of cooling water may be required, introducing thermalshock to the system valves. If the valves must continue to function, this thermal shockshould be accounted for in their design.

System in Normal Standby or Normal Shutdown: Systems or portions of systems may notoperate for extended periods of time during plant operation. This includes both nuclearand non-nuclear and redundant trains of operating systems that are placed on standby.Valves in these systems may be subjected to more severe conditions of service than asystem that is in continuous operation. While the valves are idle, corrosion (bothinternal and external) may build up if materials are improperly specified. Valve stemsmay pit, and the valve may become difficult to stroke. The valves or parts of the valvesmay be subject to inadvertent overpressurization. Foreign material may build up inpockets and prevent the valve (particularly a gate valve) from fully closing. Lubricantsmay leak into electrical areas of motor operators, or humidity may build up insidemotors. It is important that these conditions be addressed and that the valves beperiodically maintained, inspected, observed, and stroked to the extent practicable, toensure proper operation.



15-8

Operating Transients Created during System Startup, Shutdown, or Realignment: Systemstartup, shutdown, or realignment can introduce transients to the system and valves.Fluid transient pressure surges resulting from pump starts and stops, rapid valveclosing or opening, and/or discharge into an open system are further addressed inSections 15.4 and 15.5. Thermal stresses can result from hot fluids entering stagnantlines or cold liquids entering hot lines, and can cause valve binding or bent stemsresulting in failure of the valve to operate. Failures could also result from excessivetorque being applied to a valve, if the motor operator is too large and/or limit, ortorque switches are improperly applied or adjusted. The effect of these transients canoften be minimized by proper valve design and operating procedures. For example,some valves have additional design features that allow for a breakaway torquecondition, so that the tendency to overpower the valve with over-sized operators ortorque switch adjustments is reduced. A “hammerblow” capability is typical of such adesign feature.

Overpressurization Potential during System Portion Isolation and Maintenance: Generally,isolation valves should be provided to isolate portions of systems or equipment from apressure source. These isolation valves are required during maintenance that requiresopening of the system pressure boundary. Care should be taken in choosing the type ofisolation valve, so that it meets the safety requirements of the maintenance worker, aswell as the normal system requirements. Vents and drains are usually required formost applications to facilitate draining for maintenance and filling prior to return toservice. Large systems may need a bypass line around the upstream isolation valve toprovide warming, filling, and/or pressurization before opening the isolation valves.

Operating and maintenance procedures must clearly state the sequence of closing andopening isolation valves to prevent inadvertent overpressurization of a system portion.For example, if two pumps discharge into a common header, the discharge isolationvalve of the pump being removed from service must be closed first to protect the pumpsuction piping from being overpressurized. This overpressurization potential is causedby back leakage through the pump discharge check valve when the pump suctionisolation valve is closed first. For another example, during maintenance and isolation ofa feedwater heater, the heat source (such as extraction steam) is required to be isolatedbefore the heated fluid system (condensate side of a feedwater heater) to preventoverpressurization. The reverse is true when bringing the heater back into service.

Unique Alignments during System Flushing and Performance Testing: During this testing,the unit is considered to be in a normal condition. An example is a system alignmentthat may only be used to test pump performance. The test mode may impose additionalfunctional requirements on the valves in the affected system portion. Chemical cleaningof a system is also in this category.



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15.4 Fluid Transients

15.4.1 General

Fluid transients are conditions that result from altering the system from its steady statecondition. Fluid transients occur when systems are started or stopped, flow is increasedor decreased, portions of system are realigned, components malfunction, or thermalconditions change. Of concern are thermal shock initiated from operation of either thevalve or the system and dynamic fluid effects such as water hammer, flashing, andcavitation.

Many fluid transients can be eliminated or minimized by system design and/oroperating procedures. To properly analyze whether transient analysis is required of asystem, the various transients must be listed and the applicability to each systemevaluated. The applicability evaluation should explain under which plant conditionsthe transient is applicable or, if not applicable, the reasons or precautions that make itnot applicable. Once listed, the transients that envelope other transients can bedetermined, and the listing can be used as input to specifications and operating andmaintenance instructions. System-caused transients can cause valve operabilityproblems; valve-caused transients can affect system operation.

15.4.2 System Fluid Transients

System-induced fluid transients, discussed below, that can affect valves include waterhammer, cavitation and flashing, column separation, and thermal shock.

Water Hammer (Steam Hammer): Water hammer is the dynamic effect caused by therapid acceleration, deceleration, or flow reversal of a mass of liquid. Severe waterhammers may be caused by:

• Condensation collapse in steam pipe after initially injecting steam into subcooledwater (water cannon)

• Steam and subcooled water interactions in horizontal and near horizontal pipes

• Subcooled water flow into a vertical initially steam filled pipe

• Hot water entering lower pressure line with subsequent flashed steam bubblecollapse

• Steam-propelled water slug flow



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• Rapid valve operation (for example, < 1.0 second for every 200 feet (61 m) ofconnecting pipe)

• Water column separation and rejoining

Normal water hammer conditions that should be accounted for in the design include:

• Pump start with inadvertently voided discharge lines

• Expected flow discharge into initially empty lines

• Rapid valve opening, closing, or instability

• Check valve delayed opening or closing, then “popping” open

• Water entrainment in steam lines caused by factors such as steam line control valvesor relief valves opening with a loop seal or condensed steam in the line

Although not strictly a water hammer, improper selection and installation of a checkvalve on the discharge of a reciprocating pump can chatter between pulsations anddamage the valve seat.

Cavitation: Cavitation is a phenomenon that usually occurs in systems where liquidvelocities are high and pressures are near the saturation pressures of the liquid in thesystem. When the velocity of the liquid increases at sudden changes of pipe crosssection, at sharp bends, at throttle valves, or in other similar situations, the localizedliquid pressure drops below the vapor pressure of the liquid, and the liquid will flash.The flowing stream now consists of liquid plus pockets of vapor. As the liquid flowsback to regions of higher pressure, the pockets of vapor collapse (cavitate). It is thecollapse of the vapor pockets that causes the damage. Although mild cavitation is oflittle concern, severe cavitation can destroy valves and piping and must be considered.

If cavitation is long term rather than transient, valve design/sizing factors discussed inAppendix B should be considered. For example, cavitation can occur in a control valveunder conditions where relatively cold water is reduced in pressure to just below thevapor pressure. If this condition is common during normal operation of the valve, not atransient condition, then cavitation should be a design consideration.

Flashing: Flashing occurs when a liquid is reduced in pressure below its vapor pressure.Flashing may occur in a valve when the liquid passes through a restriction and thenexpands again. When this condition exists, vapor bubbles form and the result is a two-phase flow consisting of the liquid and its vapor.



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Thermal Shock: Thermal shock results from the rapid heating or cooling of metals.Thermal shock is usually the result of rapid realignment of systems or portions ofsystems, disturbance of thermal stratification, or operation of pressure relief devices.These conditions should be avoided to the extent possible while still maintaining thesystem and valve function. However, for nuclear plants, it may not be possible toeliminate thermal transients during some plant conditions because of operationalsequences that require safety systems to operate rapidly to mitigate or prevent moreserious conditions. In designing for these operating conditions, it is generally assumedthat only one thermal cycle is involved and that the system can be designed to performits safety function.

Severe thermal transients can affect valve operability by distorting the working parts,causing binding and sometimes incomplete stroking. Full torque seating during athermal transient can render a valve, particularly a gate valve, inoperable followingequilibrium temperatures because differential expansion of the gate and body canallow the gate to be too deeply seated and cause it to bind when it is cooled down. Inextreme cases of thermal shock, equipment may no longer be functional.

In order to minimize the effect of thermal shock on valves and equipment, theoperators should ensure that systems are slowly heated or cooled during manual plantstartup or shutdowns. Slow plant heatup using warming or bypass lines may berequired. Standby systems may be brought in to operation slowly to provide for mixingof fluids having different temperatures.

Column Separation: Column separation occurs in piping when the vertical water columncannot be supported by upward pressure of an idled system (approximately 30 feet(9 m) for cold water). If a pump stops and a leaky check valve at the pump dischargeallows back-leakage, the water column will separate, forming a vacuum void. Uponrestart of the pump, water hammer will occur if the system logic does not provide for aslow opening pump discharge valve to allow slow filling of the void. Another methodof solving the problem is to provide low leakage tilting disc check valves in the verticalpipe run to reduce the column length to less than a separable length.

15.4.3 Fluid Transients Caused by Valves

There are several types and applications of valves that may produce significant fluidtransients. Relief valves, check valves, and fast-acting flow control and isolation valvescan produce pipeline forces and moments that should be considered in piping systemdesign. A description of each is provided below.

Safety and Relief Valve Fluid Transients: High pressure relief valves, other than thermalrelief valves, have the potential to create significant transient loads in upstream anddownstream discharge piping systems. Relief valves often create substantial forces



15-12

upstream of the relief valve, while the downstream forces vary depending on thesystem parameters. For example, if the downstream piping is empty and the liquiddoes not flash, downstream piping segments will experience relatively small forces (F,in pounds) due to momentum change at each elbow equal to ρAV where ρ is thedensity of the liquid in slugs per cubic feet, A is the pipe area in square feet, and V isthe liquid velocity in feet per second. When the downstream piping is filled with liquidor with liquid slugs, significant downstream forces may occur. A transient analysisshould be considered for all possible significant forces, upstream or downstream.

If the relief valve has superheated liquid upstream, a special two-phase flow analysisshould be considered for the affected portions of the piping, including the valve,because these forces could become quite large.

Another special relief valve application exists when steam discharge is preceded by aloop seal water slug. This example often occurs in pressurized water reactor plantpressurizer safety valves where high pressure (2,500 psia or 17.24 MPa) steam drives asubcooled slug of water through the valve and then into the discharge piping, creatingsevere forces in the piping. Generally, this is still the ρAV case, but the velocity of theslug may approach 400 feet/second (120 m/sec).

Steam safety valves usually create modest forces in closed piping systems after flow isestablished, but will create significant discharge forces at the discharge pipe in an opensystem. This discharge force is sustained and will build up if the upstream pressureincreases due to accumulation. An open pipe inside a drip pan assembly should also betreated as an open discharge. The discharge force is the combined PA + ρAV, where Pis the pressure in the exit pipe and other variables are as noted above.

Check Valve Fluid Transients: The operating characteristics of check valves affect theirindividual response to various fluid transient conditions. For example, swing check,tilting-disc check, and double-disc check valves generally close very quickly after theflow reverses in direction. Lift check valves have a controlled closure rate, whichusually means that closure follows the flow reversal by a predictable time.

There are two applications where check valves induce significant fluid transients. Themost common application is where two or more pumps, each of which has a dischargecheck valve, combine into a common header. When one pump trips and one or morepumps continue to operate, sudden closure of the check on the discharge side of thetripped pump sends pressure waves throughout the piping system. When a check valveis closed just before or very close to the start of flow reversal, water hammer will notoccur or will be negligible. Swing check valves cause the most severe transients becauseof the relatively long distance and time to travel to the seat. Silent or lift check valvescause the least severe transients because of their relatively short distance and time totravel to the seat.



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The second application is less common but may produce a more severe transient.Typical of the second case is the use of a check valve to protect a system following apostulated pipe rupture. Designing for transients of this type is common for nuclearpower plants. For example, a postulated rupture of the main feedwater piping,upstream of a check valve in a line to the steam generator, will create severe loads onthe check valves, which close rapidly to contain the reactor or steam generatorinventory.

Other applications of single in-line check valves usually do not cause significanttransient loads when flows stop in the system. However, the system analyst should besensitive to possible transients if a particular check valve can be forced to closesuddenly due to system back pressure because the closure may create high pressureflow reversals.

Power-Operated Valve Fluid Transients: Motor-operated, air-operated, and other standardclosing valves typically do not create significant fluid transients. However, there areseveral cases that require consideration in a power plant. The main turbine tripisolation valve, which closes in approximately 100 milliseconds following a turbinetrip, can create significant fluid transients. When the turbine trips, the governor valvesclose as rapidly as possible, without dependence on any steam isolation valves. Therapid closure of the governor valves leads to extremely large loads in the main steampiping. The fast operating steam bypass valves to the condenser should be alsoevaluated.

Most isolation or control valves 6 inches (150 mm) in diameter or less, with operatingtimes greater than 5.0 seconds, will not require analysis for transient loads. Valves thatclose more quickly should be considered possible fluid transient producers.Additionally, valves that open in less than 2.0 seconds should also be evaluated.Isolation or control valves of sizes larger than 6 inches in diameter, in general, requirecareful review to determine if transient analysis is required.

15.5 Environmental Considerations and Natural Hazards

15.5.1 Introduction

Piping system components may be exposed to a range of environmental and naturalhazards that are potentially damaging. Valves important to plant safety must bequalified for any adverse environmental conditions to which they may be exposed. Foreach combination of valve and condition, the function the valve must perform inconjunction with that particular condition or any combination of conditions must bedecided. The design basis for system valves, as well as the licensing basis in the case ofa nuclear power station, should contain those particular combinations of hazards andplant conditions that are deemed credible coincident events.



15-14

The following sections identify some of the environmental and natural phenomena thatshould be considered in valve selection and system valve design.

15.5.2 Environmental Conditions

Valves important to plant safety must be qualified for any adverse environmentalconditions to which they may be exposed and under which they must function,maintain integrity, or both. Very often, the worst case of these environmentalconditions will not occur simultaneously.

External Pressure and Temperature Excursions: All valves in any piping system are subjectto external temperature and pressure variation. These variations may be minor, such asthose resulting from changes in weather conditions and ventilation, or more severe inthe case of high energy line breaks or loss of reactor coolant at a nuclear power plant.

The high energy line break (HELB) or loss-of-coolant accident (LOCA) pressure andtemperature increases associated with each of these plant conditions will be suddenand severe, potentially affecting both valve operator and valve body operability.Weather extremes and loss of ventilation are likely to be a problem only with motoroperators. However, even though the less severe condition may not affect the systempressure boundary, failure of the valve motor to operate is of equal concern.

External Contaminants: Contaminants may influence material selection of external partssuch as the valve body, bonnet, yoke, bolting, and operator. External contaminants towhich power plant valves may be exposed include:

• Chemical fluid (for example, boric acid) from leakage of nearby piping.

• Seawater environment (applicable to intake structures and yard piping).

— Salt air

— Dripping seawater

• Caustic NaOH (Valves inside containment only) - containment spray after LOCA ormain steam line break.

Humidity: Valves in high humidity areas, particularly inside the containment, shouldinclude humidity as a factor in material selection for both valves and power actuators.

Radiation Exposure: When valves are located in radiation areas, the radiation exposureexpected to be received by the valve over the life of the plant must be included whenconsidering the use of any nonmetallic parts that may deteriorate under high radiation.These nonmetallic parts may include gaskets, O-rings, packing, linings, diagrams, seals,



15-15

and soft seats. The radiation exposure to be considered in selecting a specific part is afunction of several variables. The source, source strength, distance from the source,length of exposure time, radiation type (generally only gamma and beta are ofconcern), and radiation shielding provided are generally considered.

Regardless of the radiation source, radiation level, or the material being used, safety-related valves must be designed so that their safety function is not impaired by thedegradation of nonmetallic parts over the life of the valve. Therefore, the radiationexposure used in the evaluation of the suitability of valve material must be the sum ofthe normal service design life dose plus the accident dose. The total exposure shouldconsider, as a minimum, radiation exposure from the contained fluid, direct radiationfrom adjacent radioactive lines or equipment, and external harsh environment,primarily inside the containment following a plant emergency or faulted conditions.

In selecting nonmetallic parts, it is necessary to select materials preferred under normalconditions without radiation (avoiding materials such as Teflon that have a very lowtolerance for radiation). The radiation tolerance of the properties (for example, tensilestrength, compressibility, etc.) of these materials is then compared to the total 40 yearnormal dose plus full accident dose. If the tolerance exceeds the exposure, the materialis satisfactory, and no additional evaluation is involved. Most valves located outsidethe containment will pass this evaluation if the internal fluid is not reactor coolant fromthe containment sump or some other very radioactive source.

If the evaluation of the material choices is not satisfactory for the radiation conditions,systematic evaluation of the factors that determine the actual dose to the specific valvepart will be required. It may be possible to qualify the valve for operation over the lifeof the plant by:

• Substituting a higher radiation resistant material

• Evaluating distance from the source to the part

• Modifying the required operability time following an accident

• Decreasing the time interval between replacement of the specific valve part

• Evaluating the required safety function

Invariably, a few valves (usually inside containment) will require detailed evaluationof all effects, including possible modification of valve design, use of a different type ofvalve, increased maintenance or replacement of nonmetallic parts, or relocation of thevalve to satisfy mechanical environmental qualification. An example of qualification byfunctional evaluation is the soft seat of the accumulator relief valves, which must resistharsh LOCA conditions for only a few minutes. Since the accumulators perform their



15-16

safety function (that is, they discharge) within minutes of a LOCA, the relief valve hasno further function. However, soft-seated containment isolation valves must remainleak-tight for the design basis duration of the accident, plus a 10% margin.

Where it is cost effective, the radiation concerns identified above should be applied toall valves in radiation areas.

Tornadoes, Hurricanes, and High Wind: Depending on the design of the plant, theseenvironmental hazards could affect valves in a number of ways. For example:

• If the valve is not enclosed in a missile-protected building and it is required tofunction during this event, it must be designed to survive a wind-generated missilehit.

• Rapidly lowered atmospheric pressure due to a tornado may result in a pressuredifferential across components larger than that normally experienced. This pressuredifferential will affect a valve in the following ways:

— Structural integrity.

— Motor operators could suffer from grease migration into limit switches.

— The set point of safety/relief valves will be affected by the pressure difference.

Seismic (Earthquake): Valves that are required to withstand the effects of an earthquakemust be designed to accept those forces and moments when supported by inlet andoutlet connections only and to ensure the ability of the valve and system to maintainpressure boundary and/or operability (for example, no binding). The weight of thevalve assembly, the size and configuration of the operator with respect to the valve,and the orientation of the operator are of concern to the stress engineer. A small valvewith a large and heavy operator is of special concern due to amplification of seismicloads. The use of socket-welded valves should be minimized in seismic systems due todifficulty in qualifying these joints.

An active valve that must operate during or after a seismic event is required to betested for operability under simulated seismic conditions prior to delivery. The designof the valve, yoke, and actuator assembly should exhibit a relatively high naturalfrequency (greater than 33 hertz) to avoid amplification on the seismic acceleration.

Flooding: Flooding can be caused by natural external sources (dam failure; lake, river, orcoastal flooding; extreme rainfall, tidal wave) or by internal sources, generally due toan operator error (most likely) or a line break (unlikely, but part of the design basis fora nuclear plant). The effect on valves is the same from either source. Floodingsubmergence is not likely to have an adverse short-term effect on a manual valve unless



15-17

it normally operates at a high temperature and the chill shock could damage it. If anelectric (motor- or solenoid-) or air-operated valve could be submerged, the electricaldevice and power supply or the control air circuit must be watertight. This is the caseeven if the valve is a passive valve that is required only to maintain position, ratherthan change position (otherwise, a short circuit due to flooding could cause anundesired change of position).

Regardless of the event or combination of events, a valve should be designed tomaintain its pressure boundary. Additionally, if required for reliable system operationor plant shutdown, the valve should be designed for normal operation during thedesign basis event.

Although the design requirement may not be as stringent in some cases for non-nuclear-related piping systems, the most severe operating cases expected during plantoperation should be considered in valve design and selection.

15.6 Valve Performance Requirements

15.6.1 Introduction

Performance parameters that should be considered in the design and specification of avalve include speed of operation, required flow rate and allowable pressure drop,allowable leak rate, and frequency of operation.

15.6.2 Speed of Operation or Stroke Time

The speed of operation or stroke time is an essential parameter for many power-operated valves. The valve must operate fast enough to satisfy system operatingrequirements but not so fast as to cause a system transient, such as water hammer.When it is necessary to prevent inadvertent operation of a fast operating manual valvesuch as a ball or butterfly valve, it might be appropriate to install a gear operator,regardless of the need for handwheel rim pull requirements.

High pressure manual valves sometimes require 200 or 300 turns of a handwheel tofully open a closed valve. Consideration should then be given to providing the valvewith a motor operator to reduce the stroke time (and to reduce the work of the plantoperating staff).

Control valves are relatively fast acting. However, it is generally necessary to specify tothe manufacturer the required stroke time.



15-18

Relief, safety, and safety-relief valves are inherently fast acting valves. The speed ofoperation must be compatible with system operating requirements.

Speed of operation for many valves depends on the valve load and actuator power (air,hydraulic, or electric). The speed of operation under test conditions may not be thesame as under design basis conditions. Changes in the required actuation thrust/torque(due to changes in the valve pressure drop, friction, or packing load) and changes inactuator output (due to reduced voltage, air/hydraulic pressure or gear efficiency) cansignificantly alter the stroke time.

15.6.3 Flow Rate and Pressure Drop

Piping is normally sized based on reasonable velocity, and most line valves, exceptcontrol valves, are the same size as the piping. Pressure relief valves and control valvesare specified with specific flow rates for the design, and control valves are specified forthe design pressure drop(s).

Having the line valves the same size as the piping normally provides the desired flowrate, except for check valves and control valves. Check valves should have a flowvelocity through the valve that is adequate to keep the valve fully open but that is notexcessive. The sizing of control valves is discussed in Appendix B.

The total system pressure drop analysis may influence the type of valve or valves to beused. If pressure drop is critical, a more expensive full port ball valve may have to beused instead of a valve with a standard port. Likewise, a more expensive gate valve,whose overall size is larger, may be required instead of a butterfly valve.

15.6.4 Leak Rate

Seat leakage criteria should be established for each valve, as a function of itsapplication in system design. For example, leakage past the seat of a valve that isolatesa high pressure system from a low pressure system should not exceed the capacity ofthe relief valve installed in the low pressure system.

There is no code or standard, except ASME Section XI, that specifies acceptable leakagerates after the valve is placed in service (see Section 19). There are shop test leakageacceptance criteria, if imposed when the valve is purchased, such as MSS-SP-61.However, the leakage measured in the shop tests cannot be expected to be achievedafter the valve is placed in service, due to wear of the valve parts and lack of systemcleanliness.



15-19

Some valves, particularly those in steam service, cannot tolerate any significant amountof seat leakage without possible damage to the seats due to steam cutting. Controlvalves usually do not perform an isolation function, and seat leakage is not a concern.

15.6.5 Frequency of Operation

Most valves are designed for several thousand full stroke cycles, and repeatedoperation should have no overall effect on operation. Note that valves that are idle forprolonged periods of time may accumulate debris on their inner moving parts;therefore, the valves may not operate when required or may require higher thannormal operating force/torque.

Motor operators are designed for at least 2000 cycles, but frequent short-time operationmay cause damage to the motor (see References 1.5, 1.6, 1.22, 1.23, 1.24, 1.25, and 1.26for in-depth discussion of motor operators).

Most control valves operate almost continuously and, therefore, require periodicmaintenance on items such as packing seals and air operator diaphragms.

Relief, safety, and safety-relief valves are not intended or designed for frequentoperation. Frequent operation will result in damage or a change in set point.

15.6.6 Nuclear Valve Qualification

For active nuclear safety-related valve assemblies, qualification by analysis and/or testis required to ensure that the valves will operate on demand under all conditions,including seismic loads, other dynamic loads, and adverse environmental conditions,both external and internal.

The qualification requirements are spelled out in documents such as 10CFR50, U.S.NRC Regulatory Guides, and Standard Review Plans. Several industry standards havebeen issued to address qualifications, such as IEEE-382, “Qualification of Actuators forPower Operated Valve Assemblies with Safety Related Functions in Nuclear PowerPlants”; IEEE-323, “Qualifying Class 1E Equipment for Nuclear Power GeneratingStations”; IEEE-344, “Recommended Practices for Seismic Qualification of Class 1EEquipment for Nuclear Power Generating Stations”; ANSI B16.41, “FunctionalQualification Requirements for Power Operated Active Valve Assemblies for NuclearPower Plants,” and ASME QME-1-1997, “Qualification of Active MechanicalEquipment Used in Nuclear Power Plants.” Other standards are being prepared toaddress qualification of check valves, pressure relief valves, and nonmetallic parts. Inthe past several years, particular emphasis has been placed on environmentalqualification of lubricants and nonmetallic parts used in such applications as seatinserts, gaskets, packing, O-rings, and piston rings.


16-1

16 PRESSURE CONTAINMENT AND STRUCTURAL

INTEGRITY REQUIREMENTS

16.1 Introduction

Valves must be constructed not only to provide pressure containment, but also to bestructurally secure under all loading conditions over and above internal fluid pressureloads. Industry codes and standards provide extensive design rules and guidance forvalve design (see Section 22.6). These codes and standards provide the necessary rulesfor establishing such design requirements as wall thicknesses for pressure boundaryparts, end connection configuration, and accepted materials, along with their allowablestresses. Industry codes and standards do not provide design rules for non-pressureboundary parts critical to valve operation such as valve yokes, gaskets, and packing.

16.2 Codes and Standards

16.2.1 General

A clear understanding of the applicable codes and standards that apply to a valveinstallation is essential to design, evaluate, procure, install, or modify nuclear valvesand nuclear balance-of-plant valves, as well as fossil plant valves, where ASME I andANSI B31.1 apply. The applicable edition of the code or standard should be known aswell.

There are over 70 industry documents that relate to valve requirements for design,manufacture, or testing. The most frequently used valve documents are published bythe following organizations:

• American Society of Mechanical Engineers (ASME Boiler and Pressure Vessel Code)

• American National Standards Institute (ANSI Standards)

• Manufacturers Standardization Society of the Valve and Fitting Industry (MSS-SPStandard Practices)


Pressure Containment and Structural Integrity Requirements

16-2

• American Water Works Association (AWWA Standards)

• American Petroleum Institute (API Standards)

• Underwriters Laboratory, Inc. (UL Standards)

• Instrument Society of America (ISA)

For nuclear plants, the codes and standards most frequently used for valve design arethe ASME Boiler and Pressure Vessel Code, Section III, Nuclear Power PlantComponents; and ANSI B16.34. A chronology of the development of the major codesand standards is presented below and summarized in Table 16-1.

Table 16-1Valve Design Codes

Valve Type/Function Code or Standard Applicable Dates

Alternative Guidelines for Design of

Butt Weld and Flanged Valves

MSS-SP-66 mid-60s through 1973

Design of Category I Valves ASME III

ANSI B16.5*

ANSI B16.34*

*as invoked by ASME III

(Note 1)

(Note 2)

1971 on

up to 1977

1977 on

Design of Non-Nuclear Boiler

Pressure Boundary Valves

ASME I

ANSI B16.5*

ANSI B16.34*

*as invoked by

ASME I (Note 1)

1914 on

up to 1977

1977 on

Design of Nonsafety, Nonboiler

Valves

ANSI B16.5

ANSI B16.34

Up to 1973

1973 on

Notes:

1. ANSI B16.34 provides the pressure-temperature rating, as well as requirements for minimum wall thickness, materials,marking, dimensions, and testing. ANSI B16.5 provides the pressure-temperature rating, minimum wall thickness, and outlinesthe requirements for testing.

2. ASME III applies to nuclear safety-related valves. The earlier editions of ASME III referred to ANSI B16.5 or MSS-SP-66primarily for pressure-temperature ratings and wall thickness, but retained the rules for materials, design, examination, andtesting. The current ASME III refers to a large extent to ANSI B16.34 for valve requirements, but it still retains design rules,special material requirements, and special nondestructive examination requirements.



16-3

The earlier editions of ASME III relied on ANSI B16.5 and/or MSS-SP-66 forpressure/temperature ratings and wall thickness but retained the rules for materials,design, examination, and testing. Currently, ASME III refers to ANSI B16.34 for mostvalve requirements. ASME III still retains design rules, special material requirements,and special nondestructive examination requirements.

In addition to providing rules for nuclear plant valve design, ANSI B16.34 applies tonuclear balance-of-plant valves and to fossil plants. For these applications, otherstandards also have been used for the design of valves. Table 16-2 identifies otherstandards that might be applied to the design and/or selection of valves to be used innon-nuclear valve applications.

Table 16-2Typical Valve Standards

Standard No. Title

AWWA-C504

API-602

MSS-SP-67

MSS-SP-70

MSS-SP-72

MSS-SP-80

MSS-SP-84

MSS-SP-66

MSS-SP-71

MSS-SP-85

Rubber Seated Butterfly Valves

Compact Gate Valves

Butterfly Valves

Cast Iron Gate Valves

Ball Valves

Bronze Gate, Globe, Angle, and Check Valves

Steel Valves, Socket Welding and Threaded Ends

Pressure Temperature Ratings for Steel Valves

Cast Iron Check Valves

Bronze Valves

For older plants, ANSI B16.5 provided primary guidelines, and MSS-SP-66 providedalternative guidelines for the design of butt weld end valves. When ANSI B16.34 wasissued, the thrust of MSS-SP-66 was incorporated as special class valves (that is,nondestructive examination such as radiography allowed a higher pressure for a giventemperature), and MSS-SP-66 was withdrawn.

Special consideration should be made if these or any other utilized standards are notincluded in Table 126.1 of ANSI B31.1.

The above referenced standards provide many of the design rules for valves. However,they do not address non-pressure containing functional components or internal partsfor non-nuclear valve applications. For nuclear valves, the requirements for internalparts have been given only a limited formal design approach for class 1 valves byASME III, Subarticle 3500. In order to properly address ASME code class 2 and 3



16-4

valves, ASME code case N62-4 was issued providing rules for materials, design,fabrication, inspection, and examination of internal and external valve parts. Prior touse, the code case should be consulted for the full scope of items covered. Code casesare optional. Code case rules become mandatory only if a purchaser invokes itsrequirements on a manufacturer, and then the entire code case is mandatory. Codecases are periodically reviewed, at which time they are reaffirmed or annulled. Codecases are annulled when the requirements have been incorporated into the code (that is,ASME III) or when the code case is no longer needed.

The categorization of nuclear safety-related equipment, including valves, is determinedby referring to ANSI/ANS-51.1 (formerly ANSI N18.2, Nuclear Safety Criteria for theDesign of Stationary Pressurized Water Reactor Plants), ANSI/ANS-52.1 (FormerlyANSI N2.2, Nuclear Safety Criteria for the Design of Boiling Water Reactor Plants),Code of Federal Register (10CFR50.55a), and U.S. NRC Regulatory Guide 1.26 (QualityGroup Classifications and Standards for Water, Steam, and Radioactive-Waste-Containing Components of Nuclear Power Plants). It should be noted that ANSI/ANS-51.1 and ANSI/ANS-52.1 are currently undergoing revision and will result in a new,combined standard, ANSI/ANS-50.1.

The following safety classes and the basic standards that apply to them are given inTable 16-3. Consult 10CFR50.55a, Regulatory Guide 1.26, and ANSI/ANS-51.2 and 52.1for complete definitions.

• Safety class 1 is for reactor coolant pressure boundary components.

• Safety Class 2 is for components that form part of the reactor coolant pressureboundary but may be excluded from Safety Class 1 by provisions of 10CFR50.55a,or those that are necessary for safe shutdown of the reactor or to maintain thereactor in a safe condition.

• Safety Class 3 is for systems supporting Safety Class 1 and 2 systems.



16-5

Table 16-3Safety Classes and Applicable Standards

Safety Class l0CFR50.55a Reg. Guide 1.26 Remarks

1 Reactor CoolantPressure Boundary(referred to asQuality Group A inReg. Guide 1.26)

ASME III Cl 1

–

–

–

–

2 – Quality Group BASME III Cl 2

–

3 – Quality Group CASME III Cl 3

–

NNS* – Quality Group D ANSIB31.1

For systems that contain or maycontain radioactive material, butare not in Groups A, B, or C

* Not nuclear safety-related

16.2.2 Pressure/Temperature Ratings

As previously stated, the pressure/temperature rating of a valve is provided in variouscodes and standards. The standard used depends on the materials selected and thevalve style.

Typical pressure temperature ratings are included in the following codes andstandards:

• Steel, Nickel Alloy, and Other Special Alloy Valves: ASME III, ANSI B16.34 (seeTable 16-4)

• Cast Iron Gate Valves: MSS-SP-70 (see Table 16-5)Cast Iron Check Valves: MSS-SP-71Cast Iron Globe Valves: MSS-SP-85

• Bronze Gate, Globe, and Check Valves: MSS-SP-80 (see Table 16-6)



16-6

Table 16-4Pressure/Temperature Ratings for Steel ValvesSource: ANSI B 16.34 - 1981

RATINGS FOR GROUP 1.1 MATERIALS

A 105 (a) A155-KCF70 (e) A350-LF2 (d) A516-70 (a) (g) A675-70

A 155-KC70 (e) A216WCB (a) A 515-70 (a) A537 C1.1 (d) A696 Gr.C (a)

NOTES:(a) Permissible, but not recommended for prolonged usage above about 800°F (425°C).(d) Not to be used over 650°F (340°C).(e) Not to be used over 700°F (370°C).(g) Not to be used over 850°F (450°C).

STANDARD CLASS VALVES-FLANGED AND BUTT WELDING END

Temp. °F Working Pressure by Classes, psig

150 300 400 600 900 1500 2500 4500

-20 to 100

200

300

285

260

230

740

675

655

990

900

875

1480

1350

1315

2220

2025

1970

3705

3375

3280

6170

5625

5470

11110

10120

9845

400

500

600

200

170

140

635

600

550

845

800

730

1270

1200

1095

1900

1795

1640

3170

2995

2735

5280

4990

4560

9505

8980

8210

650

700

750

125

110

95

535

535

505

715

710

670

1075

1065

1010

1610

1600

1510

2685

2665

2520

4475

4440

4200

8055

7990

7560

800

850

900

80

65

50

410

270

170

550

355

230

825

535

345

1235

805

515

2060

1340

860

3430

2230

1430

6170

4010

2570

950

1000

35

20

105

50

140

70

205

105

310

155

515

260

860

430

1545

770



16-7

SPECIAL CLASS BUTT WELDING END VALVES ONLY

Temp. °F Working Pressure by Classes, psig

150 300 400 600 900 1500 2500 4500

-20 to 100

200

300

290

290

290

750

750

750

1000

1000

1000

1500

1500

1500

2250

2250

2250

3750

3750

3750

6250

6250

6250

11250

11250

11250

400

500

600

290

290

275

750

750

715

1000

1000

950

1500

1500

1425

2250

2250

2140

3750

3750

3565

6250

6250

5940

11250

11250

10690

650

700

750

270

265

240

700

695

630

935

925

840

1400

1390

1260

2100

2080

1890

3495

3470

3150

5825

5780

5250

10485

10405

9450

800

850

900

200

130

85

515

335

215

685

445

285

1030

670

430

1545

1005

645

2570

1670

1070

4285

2785

1785

7715

5015

3215

950

1000

50

25

130

65

170

85

260

130

385

195

645

320

1070

535

1930

965



16-8

Table 16-5Cast Iron Gate Valve RatingsSource: MSS-SP-70.(Used by Permission of Manufacturers Standardization Society)

Class 125 250 800 Hyd

TempDegrees F

NPS2-12

NPS14-24

NPS30-48

NPS2-12

NPS14-24

NPS2-12

-20 to 150 200 150 150 500 300 800

200 190 135 115 460 280

225 180 130 100 440 270

250 175 125 85 415 260

275 170 120 65 395 250

300 165 110 50 375 240

325 155 105 355 230

350 150 100 335 220

375 145 315 210

400 140 290 200

425 130 270

450 125 250



16-9

Table 16-6Bronze Gate, Globe, and Check Valve RatingsSource: MSS-SP-80.(Used by Permission of Manufacturers Standardization Society)

Pressure – psi(3)

Pressure Class 125 150 200 300 350

End Connection THD THD FLG (2) THD THD (5) THD FLG (2) THD

Temp(1) MATERIAL

degrees F ASTM B-62 ASTM B-61-20 to 150 200 300 225 400 1,000 600 500 1,000

200 185 270 210 375 920 560 475 920

250 170 240 195 350 830 525 450 830

300 155 210 180 325 740 490 425 750

350 140 180 165 300 650 450 400 670

400 -- -- -- 275 560 410 375 590

406 125 150 150 -- -- -- -- --

450 120(4) 145(4) -- 250 480 375 350 510

500 -- -- -- 225 390 340 325 430

550 -- -- -- 200 300 300 300 350

Notes:

1. For lower temperatures, see Paragraph 2.5 in MSS-SP-80.

2. P-T Ratings - ANSI B16.24

3. Refer to Paragraph 2.4 for safe P-T rating for solder-joint pipe systems.

4. Some codes (that is, ASME BPVC, Section I) limit the rating temperatures of the indicated material to 406°F (208°C).

5. Alternate ratings for valve size 1/8 - 2 inches (3 - 50 mm) having threaded ends and union ring body-bonnet joints.

Prior to determining the rating of a valve, a determination of the ANSI pressure classmust be made. The class is based on the design and operating conditions of the system(that is, temperature and pressure). After the ANSI pressure class is determined, itmust be recognized that other conditions may limit the valve’s final rating. Valves withelastomeric or plastic gaskets, packing, or seating elements may not meet the entirerange of pressure-temperature conditions for their designated pressure class.

ANSI B31.1 rules for non-nuclear valves provide no specific allowance for excursions ofoperating pressure or temperature above design condition values. The maximumdesign pressures and temperatures are established by the pressure/temperature tablespreviously referenced.

The user of this document should refer to the codes or standards and addendaapplicable to the particular plant to determine the code provisions, if any, that permitallowance for variations from design conditions.



16-10

Current editions of several codes and standards now permit the operating pressure toexceed the design pressure by not more than 10% under conditions of relief or safetyvalve operation. In addition, under certain conditions ASME III permits class 2 and 3valves to operate at a higher pressure than that normally allowed for the attainedtemperature. If the ASME criteria are allowed for these occasional transients, then othersections of ASME III apply as appropriate. ANSI B16.34 also makes provisions fordeparture from the standard pressure/temperature ratings.

The applicable code or standard should always be consulted when selecting a valve toensure that the system design pressure and temperature are enveloped by the pressuretemperature rating allowed by the applicable code or standard. When selecting thepressure class of the valve, other considerations may apply such as pressure spikes dueto dynamic loads (for example, water hammer) or greater strength required to supporta heavy operator.

Special Class Valves: A special class valve is a standard class butt weld end valve forwhich additional nondestructive examination (for example, radiography) is required,thus permitting a higher pressure-temperature rating. Tables of acceptable pressureand temperature are published in ANSI B16.34 for both standard class valves andspecial class valves. For example, a class 600 carbon steel valve made from A216 WCBmay be used at 1,200 psig (8,274 kPa) at 500°F (260°C) as a standard class valve. Thesame valve, when nondestructive examination is performed to merit the rating of class600 special class, may be used at 1,500 psig (10,340 kPa) at 500°F (260°C).

This option can be valuable when the pressure and temperature allowed by B16.34standard class do not meet the system requirements, but the special class does meet thesystem requirements. A special class is sometimes cost effective and would not have thehigher fluid flow pressure drop associated with the higher pressure class valve.

Intermediate Rating Valves: ANSI B16.34 and ASME III specifies a minimum wallthickness for each standard pressure class (that is, class 150, class 300) and insidediameter of valve. When the actual wall thickness of valve exceeds the minimum wallthickness specified for the standard pressure class and inside diameter but is less thanthe specified minimum wall thickness for the next higher standard pressure class, ANSIB16.34 and ASME III make provisions and provide formulae for determining anintermediate pressure rating. This option requires higher hydrostatic test pressuresthan the next lower standard pressure class and should be exercised by or through themanufacturer.

Intermediate rating valves are used when system pressures and temperature exceedthose allowed for a standard pressure class and the wall thickness exceeds thatrequired for the standard pressure class. For example, a manufacturer may provide aclass 1878 valve for a PWR reactor coolant system where a standard class 1500 wouldnot suffice, but a standard class 2500 would far exceed the requirements.



16-11

This option is different from special class valves in that an additional wall thicknessabove the minimum is required to allow a higher pressure-temperature rating forintermediate rating versus additional nondestructive testing for special class.

16.2.3 Codes and Standards for Pressure Relief Valves

ASME Boiler and Pressure Vessel Code, Sections I, III, and VIII provide design rules forpower boilers (non-nuclear), nuclear components, and pressure vessels (non-nuclear),and their overpressure protection requirements. The types of pressure relieving devicesallowed and their design requirements are included in these sections.

ASME Section III, Sections NB, NC, and ND, contain design rules for nuclear safety-related power plant components, including pressure relief valves. SubsectionNB/NC/ND 7000, “Overpressure Protection Requirements,” addresses pressure reliefvalve operating requirements, installation provisions, capacity certificationrequirements, and shop testing requirements. Subsection NB/NC/ND 3590 containsdesign rules specifically for pressure relief valves. This section was incorporated intoSection III in the 1980 edition, summer 1982 addenda. Prior to that, the rules forpressure relief valve design were contained in the ASME code case N100, “PressureRelief Valve Design Rules.” The pressure and temperature ratings of ANSI B16.34 donot apply to pressure relief valves. The design pressure and temperature of the valveare as specified in the design specification.

ASME Section I contains design rules for pressure relief valves for overpressureprotection of power boilers, and ASME Section VIII contains design rules foroverpressure protection of unfired pressure vessels. These sections of the code containrequirements for capacity certification, operational requirements, materialrequirements, shop testing requirements, and installation provisions.

ASME Sections I, III, and VIII are specific about testing requirements for pressure reliefvalves. The hydrostatic test pressure is based on the set pressure of the valve, not the100°F (38°C) pressure rating of ANSI B16.34. For pressure relief valves, the testingprovisions of ANSI B16.34 do not apply. These sections also require set pressureverification by test and capacity certification by test.

Besides the ASME code, other standards are used for pressure relief valves.

ANSI standards such as ANSI B16.5 (for flange dimensions only), ANSI B16.34 (asspecified in Section III for minimum wall thickness requirements for a valve body), andANSI B147.1 (for seat tightness testing) are used. ANSI/ASME-PTC 25.3, PerformanceTest Codes, contains rules for conducting tests on pressure relief valves.



16-12

16.3 Materials

16.3.1 Material Compatibility

Materials must be compatible with the fluid and with each other. Wetted materialsmust be carefully considered. Bonnet bolting, for example, may be wetted by stem sealleakage. It may be false economy to use a stainless steel body to resist boric acidcorrosion, yet specify carbon steel bonnet bolting.

It is important to avoid using materials of wide electrical potential difference.However, it is not sufficient to consider only potential differences when evaluating thecorrosion rate of dissimilar metals in contact. The relative areas of dissimilar metalsmust also be considered. If the surface area of the anode is large, the current density atthe anode will be small, and corrosion due to galvanic effects will be insignificant.Thus, bronze trim in a steel valve is acceptable, in spite of the substantial potentialdifference between bronze and steel. There is not enough area of the bronze trim toaccelerate the corrosion of the large area of the anodic valve body, but on the otherhand, it would be unwise to use a steel seat in a bronze valve (see Section 2.4 foradditional discussions).

16.3.2 General Discussion of Pressure Boundary Materials

Pressure boundary parts are defined in ASME III as the body, bonnet, disc, and boltingthat join the bonnet to the body. Stems and seats are not pressure boundary parts.ASME III requires that these parts be made of an ASME III material, except for 2-inch(50-mm) and smaller line valve discs and safety valve discs and nozzles, which areinternally contained by the external body structure. However, ASME III permits use ofmaterial produced under ASTM specifications, provided the requirements of the ASTMspecification are identical to, or more stringent than, the ASME III material.

Other valve standards and codes do not specifically identify pressure boundarymaterials. However, ANSI B16.34 requires the body, bonnet, or cover and body-bonnet,or body-cover bottom to be constructed of material listed in Table 1 of ANSI B16.34.

Materials commonly used for pressure boundary parts (as defined in ASME III) fallinto three categories:

• Stainless steels or other corrosion resistant alloys

• Carbon steels

• Low-alloy steels



16-13

See Table 16-7 for commonly used pressure boundary materials.

Table 16-7Commonly Used Pressure Boundary Materials

Valve PartStainless

Steel ValvesCarbon

Steel ValvesLow Alloy

Steel Valves

Body/bonnet ASTM A351,

Gr CF8 (304 SS)

ASTM A216-WCB ASTM A217-WC6

(1-1/4 Cr, 1/2 Mo)

Disc/wedge ASTM A351,

Gr CF8M (316 SS)

ASTM A351, Gr CF3

(low carbon SS)

ASTM A216-WCC ASTM A217-WC9

(2-1/4 Cr, 1 Mo)

Castings ASTM A351, Gr CF3M

(low carbon SS)

Forgings ASTM A182-F307, F316

ASTM A182-F304L,

F316L

ASTM A105

ASTM A350-LF2

ASTM A182, F11

(1-1/4 Cr, 1/2 Mo)

ASTM A182, F22

(2-1/4 Cr, 1 Mo)

Plate ASTM A240-304, 304L

ASTM A240-316, 316L

ASTM A515GR70

ASTM A516GR70

ASTM A387-1, CL2

(1-1/4 Cr, 1/2 Mo)

ASTM A387-2, CL2

(2-1/4 Cr, 1 Mo)

Bolts, studs, andnuts

ASTM A193, Gr B7*

ASTM A194, Gr 2H*

ASTM A193, Gr B6 (410

SS)

ASTM A194, Gr 6 (410

SS)

ASTM A193, Gr B8 (304

SS)**

ASTM A193, Gr 8 (304

SS)**

ASTM A564, Gr 630

ASTM A193, Gr B7

ASTM A194, Gr 2H

ASTM A193, Gr B7

ASTM A194, Gr 2H

ASTM A193, Gr 16

ASTM A194, Gr 4

* Although sometimes provided, these materials are not appropriate for stainless steel valves due to their potential forcorrosion.

** Not recommended for threading into 304 or 316 bodies, as galling may occur.



16-14

The selection of materials is dependent on such factors as resistance to corrosionand/or erosion, and to some extent, the pressure/temperature rating for the variousmaterials. It is common practice for the valve body to match the piping material. Fluidsystem conditions, including environment, primarily dictate material selection. Forexample, the boric acid content of a pressurized water reactor coolant system leads tothe selection of stainless steel body, bonnet, and bolting. The superior erosion resistanceof stainless steel is another reason for its selection for this high-velocity system. Further,the required retention of water purity in a demineralized water system requires the useof stainless steels, where small amounts of corrosion products, which could result fromthe use of carbon steel, cannot be tolerated. Carbon and low-alloy steel valves are usedin the steam, feedwater, extraction steam, and condensate systems, where the waterchemistry can be controlled to restrict the corrosion rate.

Carbon steels and stainless steels have yield strengths about equal at room temperature;however, low-alloy steels generally have a significantly higher yield strength thancarbon or stainless steels. At the higher operating temperatures of a water-cooledreactor (500° F to 600°F; 260°C to 316°C), the yield strength of stainless steel is less thanthat of carbon steel. Carbon steel is not recommended for prolonged usage above 800°F(427°C) because of its potential graphitization damage and creep damage at elevatedtemperatures. The low-alloy steels have the highest yield strength at 500°F to 600°F(260°C to 316°C).

For valve bodies and bonnets, the same material or product form is not required to beused for both parts. The rating applied, however, must be based on the valve body withthe bonnet designed and material selected accordingly. All materials should be selectedbased on specific service conditions. For example: (a) A stainless steel valve in corrosiveservice conditions should have stainless steel bolting to preclude bolting corrosion dueto leakage. (b) For steam service, which has a high moisture content and which mightresult in erosion, 2-1/4 Cr 1Mo or 1-1/4 Cr 1/2 Mo material should be used for thevalve body and bonnet, even though the temperature would permit carbon steel. Inaddition, for high velocity service, 2-1/4 Cr 1Mo is superior to carbon steel, andstainless steel is vastly superior.

Several other materials are available for valves, such as cast iron (ASTM A-126), ductileiron (ASTM A-395), and bronze (ASTM B-62). Note that ASME III does not permit castiron or ductile iron valves. Other alloys are also used for service environments such asseawater, where aluminum bronze valves are often used.



16-15

16.3.3 Body Materials

The following common materials are available for valve bodies, with advantages anddisadvantages identified:

Cast iron - ASTM A 126, Class B

Advantages:

• Low cost

• Good for general service

Disadvantages:

• Limited pressure and temperature rating

• Brittle, can crack easily

• Not allowed by ASME III

Bronze - ASTM B61 and B62

Advantages:

• Low cost

• Good for general service (air and water)

Disadvantages:

• Limited in temperature and pressure (The limits are normally 350 psig (2,413 kPa)at 550°F (288°C), up to 1,000 psig (6,895 kPa) at 150°F (66°C), depending on the alloyused, pressure rating of the valve, and the method of installation, that is, threaded,flanged, soldered, or silver brazed.)

Carbon steel - ASTM A216, Gr WCB

Advantages:

• Widely used (available)

• Moderately priced



16-16

Disadvantages:

• Not appropriate for prolonged service over 800°F (427°C)

• Poor chemical resistance to most corrosives

• Poor resistance to erosion by high velocity vapor droplets, such as flashingcondensate or wet steam

• Should never be used for any trim parts, except base material for the disc, which isoverlaid with corrosion-resistant material at the seat

Chrome-moly - ASTM A217, Gr WC6 and WC9, are low alloy steels and not stainlesssteels.

Advantages:

• Gives additional erosion resistance and is, therefore, recommended on flashing orerosive service

• Can operate continuously at high temperatures

• Price is reasonable, considering its superior characteristics

Disadvantages:

• Welding must be followed by post-weld heat treating.

• Has about the same resistance to corrosion as carbon steel.

Stainless steel - ASTM A351, Gr CF8 (304 SS), or ASTM A351, Gr CF8M (316 SS)

Advantages:

• Good high temperature, pressure performance

• Good general corrosion resistance

• Most widely used stainless steel in the valve industry

Disadvantages:

• High initial cost



16-17

There are many more alloys available that are generally used because of their ownparticular resistance to various fluid chemistries, such as aluminum bronze (ASTM-B148), bronze (ASTM-B61), alloy 20 stainless steel (ASTM-A35a, CN7M), Monel(ASTM-A494, M-35), and Inconel (ASTM-A494, CYAO).

16.3.4 Special Considerations for Material Selection for Valves in Raw Water,Especially Seawater

This section provides an overview of material selection considerations for valves in rawwater service including candidate materials and a brief discussion of microbiologicallyinduced corrosion.

Material Selection: The service conditions that need to be identified prior to selection ofmaterials for raw water service are:

• Fluid chemistry, including bacterial analysis; and flow velocity range, includingpossible stagnant conditions

• Suspended particulate matter

• Chemical additives or treatment to control fouling and/or limit bacteria that mayresult in microbiologically induced corrosion (MIC)

• Tendencies of the fluid to deposit scale

• Compatibility of materials to preclude galvanic corrosion from use of dissimilarmetals in contact with each other

• Possible cavitation of materials from suspended matter, turbulence, or flashing

Candidate Materials: The selection of appropriate materials of construction for fluid-wetted components of the valves depends upon the design basis service conditions andthe corrosion allowance. Some candidate materials and their technical limitations orperformance concerns are:

• Carbon steel or cast iron - Corrosion rates must be determined for the site-specificapplication and integrated into the valve design as a corrosion allowance for wettedsurfaces. Trim should be corrosion resistant or coated with appropriate material.Carbon steel or cast iron is generally not suitable for seawater service.

• Lined carbon steel or cast iron - Elastomeric (natural rubber, BUNA-N, EPDM)materials or multifunctional epoxy resin (MFER) linings can be applied to allowseawater service. High maintenance costs and downtime may result from holidaysand/or pinholes in the lining and separation of the lining from valve components.



16-18

• Austenitic cast irons - Materials such as Ni-resist have improved resistance topitting attacks and are appropriate for seawater service.

• Plastics or reinforced plastics - Jointing requires special care and attention to obtainleak-tightness. Ultraviolet stabilizers are required for outdoor above-groundapplications of reinforced plastics to avoid embrittlement degradation.

• Copper alloys - Copper-nickels, brasses, and bronzes have demonstrated goodperformance in seawater applications, but they are not immune to corrosion and canbe susceptible to sulfide and bacterial attack and, in certain cases, to erosion. Brassesand aluminum bronzes can also undergo dezincification and dealumination,respectively, and consideration must be given to inhibiting this type of corrosion.

• 300 series stainless steels - 300 series stainless steels are susceptible to severelocalized corrosive attack in slow moving or stagnant fluids. Sediment deposits orbacterial colonies impede the supply of oxygen to the metal surface, which causes itto lose passivity and resistance to pitting attack. Increases in velocity above 5 ft/secresult in less fouling. Stainless steels and other nickel-bearing alloys maintain theirpassive layers and corrosion resistance at higher velocities. Crevice corrosion canalso occur in the 300 series stainless steels.

• High nickel alloys -Monels, Inconels, Incoloys, and Hastelloys are suitable forseawater applications, although they can pit under certain conditions.

• Titanium - Appropriate for seawater service but can foul without proper treatment,and can pit at temperatures above 250°F (120°C). Fabrication by welding can bedifficult. Degradation of titanium due to MIC is unreported.

• 6% molybdenum stainless steels - Used recently for replacement of 300 seriesstainless steels and some non-ferrous alloys that have suffered significant corrosiveattack in service. The temperatures at which pitting and crevice corrosion can occur(critical temperatures) have been measured to be at least 60°F (16°C) higher thanthose for 316 stainless steel. These materials are weldable and product forms areavailable for valve applications.

Microbiologically Induced Corrosion (MIC): MIC is recognized as a widespread problem inraw water systems. Although a wide range of micro-organisms is involved, most of thereported case histories have been attributed to sulfate reducing bacteria and ironoxidizing bacteria.

Most steels and alloys are susceptible to at least some form of MIC. Some materialssuch as 6-Mo alloys may be more resistant to MIC. Prevention of MIC requires a rangeof solutions including materials with higher MIC resistance, chemical treatment, andflow monitoring.



16-19

16.4 Corrosion Allowance

Corrosion allowance, as used herein, is defined as additional wall thickness over thatrequired by ANSI B16.34 to compensate for corrosion loss over the life of the valve.

Corrosion allowance should be specified when ordering valves that are cast or forged,although the casting or forging process will normally dictate that the final wallthickness of the valve will be in excess of that required by ANSI B16.34. In the smallersizes and lower pressure ratings, the required wall thickness is often far less than theminimum practical thickness of a casting. In addition, some foundries produce theircastings at least 1/8 inch (3.175 mm) thicker for each inch (25.4 mm) of metal thickness,compared to the specified wall thickness.

Certain product lines of some manufacturers were originally designed to meet therequired wall thickness of API standards. API standards require a wall thickness inexcess of ANSI B16.34, thus providing a corrosion allowance when used in ANSI B16.34applications.

It is noted that ANSI B16.34 has provided some excess in their tabulated wall thickness.When comparing these values against the required wall thickness determined bycalculation, Annex F, paragraph F1.4 of ANSI B16.34, states, in part that “…The actualvalues in Table 3 are approximately 0.1 inches (2.54 mm) heavier than those given bythe equation…” Some users take this to mean a corrosion allowance, although it doesnot specifically say this, nor should it be interpreted that way. Total compliance withANSI 16.34 would require wall thickness in accordance with Table 3 (of ANSI B16.34)for the life of the valve.

An acceptable method of determining the corrosion allowance that complies with ANSI16.34 is to use the actual design pressure and design temperature of the system and usethe rationale given in ANSI B16.34, Section 6.1.4. An example of this method, adoptedfrom EPRI report NP-5479 [1.20], is given below:

A method of calculation is as follows:

Given: System Design Pressure (Pd), System Design Temperature (Td), and Valve Pressure Class, Size (diameter), and Material

Find: Corrosion Allowance CA



16-20

Step 1

Enter the appropriate pressure-temperature rating from Table 2 of ANSI B16.34 at thedesign temperature (Td), and determine the pressure rating of the valve for the valvepressure class and the next lower valve pressure class. Call these pressures P2 and P1,respectively.

Step 2

Enter Table 3 of ANSI B16.34 at the valve diameter (d) and determine the valve bodyminimum wall thickness for the valve pressure class and the next lower valve pressureclass. Call these thicknesses t1 and t2, respectively.

Step 3

The required valve body minimum wall thickness (tm) at the design pressure (Pd) anddesign temperature (Td) may be found by interpolation from:

( )2112

1d1m tt

PP

PPtt −

−−

+=

Step 4

The corrosion allowance CA is then:

( )1212

d2m2A tt

PP

PPttC −

−−

=−=

Example. Consider a check valve at the discharge of the main feedwater pumps for aPWR. Typical valve parameters are:

Design pressure Pd = 1650 psig

Design temperature Td = 460°F

Size = 16 inch/class 900

Material = SA350-LF2

Determine the corrosion allowance CA.



16-21

Step 1

From Table 2-1.1 (ANSI B16.34)

Class 900 P2 = 1837 psig @ 460°FClass 600 P1 = 1228 psig @ 460°F

Step 2

From Table 3 (ANSI B16.34)

Class 900 t2 = 1.77 inch @ 16 inch diamClass 600 t1 = 1.18 inch @ 16 inch diam

Step 3

The valve body minimum wall thickness is

( )2112

1d1m tt

PP

PPtt −

−−

+=

( )18.177.112281837

1228165018.1t m −

−−+=

inches59.1t m =

Step 4

The corrosion allowance is

59.177.1ttC m2A −=−=

inch18.0CA =

The result in this example is that an installed valve could have a local or general loss ofwall material up to 0.180 inch and still meet code requirements. If we further postulatea loss rate of 0.02 inch per year (a high rate), the valve body would last nine years inthat particular application before the allowed code minimum wall was reached. If thisrate were known one to two years in advance of the nine-year point, the valve bodycould be replaced or repaired as a routine outage item. An alternative to this would beto include a higher rated valve or use a different alloy to extend the life of the valve tomatch the life of the plant.



16-22

16.5 Valve End Connections

16.5.1 General

Valves can be connected to pipes in several ways, including:

• Threads

• Welding

• Brazing

• Soldering

• Flanges and bolts

• Flared or hub ends

These types of end connections are most commonly used on valves, although not all ofthem are suitable for all piping materials or services.

16.5.2 Threaded Ends

This type of end connection is widely used, but not usually in nuclear service. It can beused for all materials, including plastics. Threaded end connections are limited tosmaller pipe sizes (up to 3 inches; 75 mm). The larger the pipe size, the more difficult itis to make up the screwed joint. Threaded ends are not suitable for connections thatmay experience vibration (potential for leakage) and cannot be used with bent pipe.

Piping codes (ASME III/ANSI B31.1) also restrict use of threads to certain sizes andservices. Pipe threads may be used for up to 1/2-inch (13-mm) nominal pipe size (NPS)at 5,000 psig (34,500 kPa) for certain instrument applications. For other services,threaded ends are limited to 950°F (510°C). For steam and hot water service above220°F (104°C), their use is limited to 3-inch (75-mm) NPS with the pressure limit as afunction of size.

Applicable Standards:

ANSI B2.1 Pipe threadsANSI B16.3 Malleable - Iron Thread Fittings, 150 and 300 lbANSI B16.4 Cast Iron Threaded Fitting, 125 and 250 lbANSI B16.11 Forged Steel Fittings, Socket Welding, and ThreadingANSI B16.15 Cast Bronze Threaded Fittings, 125 and 250 lb



16-23

16.5.3 Welding Ends

Welding ends are available only in steel valves. They are used mainly for highpressure-temperature services. Welding ends are recommended for lines not requiringfrequent dismantling. There are two types of welding ends: butt welding and socketwelding. Butt welding valves come in all sizes; socket welding valves are usuallylimited to smaller sizes (generally up to 2 inches; 50 mm).

A major advantage of welding over other joints, such as screwed or flanged, is thatwelding eliminates the potential for leakage during plant operation.

There are certain advantages of socket welding over butt welding and a socket weldedjoint is preferred for smaller size piping. When fatigue is not a consideration, theadvantages are as follows:

• Pipe does not have to be cut accurately.

• The joint is basically self-aligning as pipe end slips into pipe and the joint issupported by the pipe.

• Pipe does not require beveling.

• Weld spatter cannot enter the pipe.

A disadvantage to socket welds in dirty or contaminated systems is that they may trapradioactive particles. In addition, they represent a high stress concentration and maycause stress qualification problems in ASME III systems.

Figures 16-1 and 16-2 show butt weld and socket weld end configurations.


ANSI B16.25 - Butt Welding Ends

ANSI B16.11 - Forged Steel Fittings, Socket Welding, and Threading



16-24

Figure 16-1Butt Weld End Connection

Figure 16-2Socket Weld End Connection



16-25

16.5.4 Brazing Ends

Brazing end connections are available on copper alloys. The ends of valves are speciallydesigned for the use of brazing alloys to make the joint. Brazing requires temperaturesat which the filler metal is put into a liquid state, but the base metal is not. Unliketypical soldering, brazing will withstand higher temperatures because of the brazingalloy used. (Alloys used for brazing melt at temperatures higher than 1,000°F (540°C)but less than the melting temperature of the jointed parts.) NOTE: Brazing is coveredby ASME code Section IX.

16.5.5 Solder Ends

Solder joint valves are used with copper tubing. Soldering should be limited toplumbing systems only. The joint is soldered by applying heat. Because of closeclearances between the tubing and the socket of the valve, the solder flows into the jointby capillary attraction.


ANSI B16.18 - Cast Bronze Solder Joint Pressure Fittings

ANSI B16.22 - Wrought Copper and Bronze Solder Joint Pressure Fittings

16.5.6 Flanged Ends

Flanged ends are generally used for larger line sizes, although they are available insizes as small as 1/2 inch (12 mm). A flanged connection allows a valve to be removedand replaced with a minimum of work. A raised-face flange facing is the mostcommon. Other facings include flat face (used for cast iron and bronze valves), ringjoint, male-female, and tongue-groove. Tightness of the flanged connection dependsvery much on gasket selection.

Three types are available and commonly used: full-face gaskets and flat-ring gasketsfor raised and flat-face flanges; or a metal ring for ring-joint flange connection. Choosea flange type to match the piping flange. Never bolt cast iron raised face flanges to caststeel raised faces, as cracking may occur.

In addition to a standard flanged design, there are other types of end connectionsavailable only in butterfly valves. These are:

• Wafer or flangeless. The valve is held in position between the inlet/outlet pipeflanges, using through bolting.



16-26

• Lug. The same as above except that there are lugs on the valve body.

• Single flange. The same as lug type except inlet and outlet faces of body areprovided with tapped holes.

Figure 16-3 shows these types of end connections.

Figure 16-3Butterfly Valve End Connections


ANSI B16.1 - Cast Iron Pipe Flanges and Flanged Fittings

ANSI B16.5 - Steel Pipe Flanges and Flanged Fittings, 150, 300, 400, 600, 900, 1500, and 2500 lb, including reference to valves

ANSI B16.24 - Bronze Flanges and Flanged Fittings, 150 and 300 lb

MSS SP-44 - Steel Pipe Line Flanges (26 inches and larger)

Gasket types and materials are discussed in Section 2.6.



16-27

16.5.7 Flared Ends

A flared-end connection is commonly used for metal and plastic tubing up to 2 inches(50 mm) in diameter. It is used in power plants mainly in instrument hookups less thanor equal to 1/2 inch (12 mm) in diameter. The end of tubing is flared, and a ring nut isused to make a union-type joint. Various systems are available from different vendors.

16.5.8 Hub Ends (Bell and Spigot)

Hub-end connections are usually limited to domestic water and sewage piping. Thepipe is inserted in the hub end of the valve or fitting, caulked with oakum, and sealedwith molten lead.

16.6 System/Valve Interactions

16.6.1 General

Various interactions between the system and the valve may have an effect on thepressure boundary of the valve. These types of interactions include pipeline end loads,system leakage, and piping vibration.

16.6.2 Pipeline End Loads

Since valves in major industrial piping and in fossil and nuclear power plant piping areusually installed using welding ends and sometimes flanged ends in lower pressuresystems, these connections must be designed to adequately transmit all piping loadswhile maintaining pressure integrity. The appropriate industrial or ASME codes haveadequate requirements to satisfy these conditions; however, the effect of piping loadsmust also be considered on the operability of the valve itself.

The adequacy of the pressure boundary integrity of the valve and nozzles is normallyensured by verifying that the section modulus of the valve, in the approximate area ofthe intersection of the body and bonnet, is greater than the section modulus of thepiping. The code requires that, as a minimum, the modulus of the valve be at least 10%greater than that of the piping. In general practice, the modulus of the valve should besignificantly greater than that of the piping, in order to assure operability of the valve.Thus, the piping which is analyzed for loading adequacy will be assumed to fail first.

For a nuclear safety-related active valve, a specific test is normally done on a prototypevalve by imposing loading on the valve, including internal pressure loads and nozzleloads (either directly or indirectly), and operating the valve.



16-28

16.6.3 Leakage

System leakage should always be evaluated in consideration of whether the fluid ishazardous or corrosive. Flammable fluid leaks could pose a fire hazard. Boric acidleaks are of particular concern in a PWR plant because of the rapidity with which boricacid can corrode carbon steel over which it may trickle from a stem leak of a stainlesssteel valve. All reactor water in a PWR plant contains boric acid. Leakage that collectsor dribbles on a warm surface will become concentrated as the water evaporates.Concentration may increase to the point at which the boric acid precipitates as crystalson the warm surface. Corrosion will continue underneath the crystals so long asmoisture, even in the form of humid air, is present.

Leakage of radioactive fluids always presents a hazard that must be considered. If thefluid is highly radioactive, packless valves are generally used.

16.6.4 Vibration

It is prudent to consider that all valves in the plant will be subject to vibration.Vibration may be transmitted to the valve through piping connected to rotatingequipment, or it may result from hydrodynamic forces in the valve itself or in adjacentpiping. By itself, vibration of such small amplitudes is not a problem requiringcorrection, but it could cause loosening of attachments and often complete separation.Screwed connections of any kind require positive locking to prevent unscrewing orcomplete separation of the mating pieces.

A positive locking device is one that does not depend (in any way) on friction toperform its function. Thus, a split washer is not a positive locking device; a castellatednut with a split pin is a positive locking device. Taper pins are not positive lockingdevices. Small beads of weld metal intended to secure a pin can crack from vibration orthermal cycling. Upset threads depend on friction.

Many examples exist of such failures leading to valve damage or worse. This isespecially the case when the loose fastener is not observable because the valve isinaccessible (for example, inside the containment), or the fastener is inside the valvebody, or inside the housing of a valve operator.

Reference 1.20 gives a detailed discussion of check valve locking devices. Vibrationconsiderations must also include checking that the vibration frequency does not matchthe resonant frequency of the piping. Vibration can also cause fatigue failures incomponents with high stresses.



16-29

16.7 Shop Tests

When valves are initially ordered and manufactured, they are normally pressure testedto ensure structural integrity and absence of unacceptable leakage. Most of the valvecodes and standards (for example, ASME III and ANSI B16.34) require that ahydrostatic shell test be conducted at 1.5 times the 100°F (37°C) pressure rating of thevalve.

ASME III also requires a disc hydrostatic test (or closure test as it is currently described)to be conducted at either 100% of the maximum pressure allowed for the pressure classat 100°F (37°C), or 110% of the 100°F (37°C) pressure rating, depending on what editionof the code applies. No specific seat test is required, and acceptable seat leakage is notdefined in ASME III.

ANSI B16.34 has essentially the same requirements for the disc except that, for certainsizes and pressure ratings, the manufacturer has the option to perform a gas closure testat 80 psig (552 kPa).

Other standards for valves have similar requirements to ASME III and ANSI B16.34.

Seat leakage tests and acceptance criteria normally have to be specified by the user,particularly when ordering to ASME III or ANSI B16.34 requirements. The mostcommonly specified requirements are delineated in MSS-SP-61 (Pressure Testing ofSteel Valves) [6.48] for isolation valves and ANSI/FCI 70-2 (Control Valve SeatLeakage) [6.12] for control valves.

MSS-SP-61 allows seat leakage up to 10 cc per hour per inch of valve nominal size forgate and globe valves and 40 cc per hour per inch of valve nominal size for checkvalves. For critical valves, an acceptable leakage rate has been specified as 2 or 3 cc perhour per inch of valve nominal size and is sometimes called “low leakage” or“exceptional tightness.” MSS-SP-61 also has an acceptance criterion for an air seatleakage test of 0.1 cubic foot per hour per inch of valve size.

ANSI/FCI 70-2 for control valves has six classes of acceptable seat leakage rangingfrom class I, which does not require a test, to class VI, which allows 0.15 cc per minutefor a 1-inch (25-mm) valve to 6.75 cc per minute for an 8-inch (200-mm) valve.

For pressure relief valves, ASME Section III requires that the inlet portion of thepressure relief valve must be hydrostatically tested to at least 1.5 times the set pressuremarked on the valve, and for closed system applications, the outlet portion of the valvemust be hydrostatically tested to 1.5 times the design secondary pressure. ASME IIIalso requires that the valve set pressure must be verified by test.



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Seat leakage testing criteria and operability testing criteria should be specified by theuser. ASME Sections I and VIII (non-nuclear) require, on valves exceeding 1 inch (25mm) or 300 psig (2,069 kPa) set pressure, an inlet hydrostatic test of at least 1.5 timesthe design pressure and an outlet test for closed bonnet valves used on closed systemapplications at a minimum of 30 psi air (207 kPa). These sections also specify set pointtesting requirements and seat leakage testing requirements. ANSI Standard B147.1 (APIStandard RP-527) is a commonly used standard to clarify testing methods and tightnessstandards.

16.8 Structural Integrity and Valve Operability

The codes and standards applicable to valve construction focus almost exclusively onpressure containment integrity of the valve and do not address those structural featuresthat affect the capability of the valve to perform its intended function. Guidelines onvalve stem packing and gaskets are similarly absent from the codes. Several areas ofvalve design that affect valve performance are of significant concern. These are:

• Valve stem sealing configuration

• Flanged gasket seals (other than inlet/outlet flanges)

• Mechanical joining (and locking) of components (for example, disc to valve stemjoint, valve stem to actuator stem joint)

• Structural members that support and join the valve actuator to the valve proper

• Thrust/torque loading capabilities of the valve stem

Some of these areas have already been discussed in the text of this report as well as inother publications. Reports of valve malfunctions in power plants, however, continue toshow problems with stem seal leakage, bonnet gasket leakage, separation of the valvestem from the valve disc, broken yokes, and bent stems. Recent regulatory and industryefforts significantly reduced such failures.

Valve stem seals are discussed in Section 2.5.2, and the reader is encouraged to examinethis information.

Gaskets for use in circular body to bonnet or similar connections of the valve should beprovided with a gasket width comparable to those used in pipeline flanges ofcomparable size, type, and service rating. Gaskets of marginally adequate width maysuccessfully pass shop hydrostatic tests without leakage but may result in maintenanceproblems during actual service.



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Expansion of piping segments on both sides of a valve can cause binding of the valveinternals and increase the thrust/torque requirements to open/close the valve. Thus,piping forces and moments should be taken into account in procurements of newvalves and evaluations of existing valves.

The load carrying capability of mechanical joints of valve components should exceedthe capability of the components being joined.

Valves provided with power actuators, particularly those actuators of large weight,extended mass and high thrust/torque output should be evaluated to ensure theadequacy of the valve yoke to support the actuator and the maximum force it canimpose on the valve and the adequacy of the valve stem to accept this loading,particularly in a column buckling mode (for example, large gate valves). The design ofthe valve yoke should be evaluated to ensure that its natural frequency, as assembled inthe valve, exceeds 33 hertz. This evaluation can be performed using a classicalspring/mass determination of its frequency. In achieving this requirement, theproportions of the yoke should be adequate for the applied compression, tension, andshear loads. The proportions of the valve stem and the location of guides should beevaluated for column buckling.


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17 VALVE MAINTENANCE AND INSPECTION PROGRAMS

17.1 Introduction

The U.S. electric power industry is under ever-increasing pressure to improve plantefficiency, shorten plant outages, and cut costs, which means fewer people andresources. In this environment, the burden is on plant maintenance personnel toimprove the efficiency of repair and maintenance activities. Valve maintenance groups,in particular, will be under extreme pressure to keep the plant on-line.

With the recent activities to satisfy regulatory commitments such as GL 89-10, GL 95-06,GL 96-07, etc., many valves were subjected to extensive testing, increased actuatoroutput thrust/torque, and various modifications in the valves as well in the actuators.These activities will put even more pressure on the plant maintenance groups toimprove efficiency and productivity.

Furthermore, the changes in regulatory requirements, the evolution of design codesand new technologies, combined with aging of some plants will increase theresponsibilities of the valve maintenance groups in nuclear power plants. For example,in order to reduce the risk of plant personnel exposure to radiation, cobalt-free alloysare being developed to replace cobalt-based alloys (such as Stellite 6). As thistechnology matures and gets industry approval, utilities may decide to replace Stellite 6seats in many valves with the new cobalt-free material, which will add to theresponsibilities of the valve maintenance group.

The objective of this section is to address the valve maintenance programs withinelectric utilities in anticipation of the upcoming scenarios. This guide can provide onlygeneral recommendations on programmatic considerations and minimum maintenancerequirements for valves (within the scope of the guide) because most power plants havedifferent management styles and spare-part inventory requirements. This sectionprovides some recommendations for plant management and engineering to consider inupgrading or maintaining their valve maintenance and inspection programs.References 5.20 through 5.24 provide recent experiences in nuclear power plantmaintenance programs.


Valve Maintenance and Inspection Programs

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17.2 Definitions

Some of the most commonly used definitions in the maintenance field and in this guideare given below.

Maintenance. All activities performed on equipment in order to maintain or restoretheir operational functions (corrective or preventive).

Corrective Maintenance (CM). Tasks performed to restore functional capabilities offailed equipment - diagnosis and repair.

Preventive Maintenance (PM). All activities performed on equipment to avoid orreduce the probability of failure.

Periodic Preventive Maintenance. Maintenance actions initiated as a function of time,regardless of the actual condition, including life limit (discard) tasks (scheduledreplacements) and overhauls (scheduled rework).

Condition Directed Preventive Maintenance. Actions initiated as a result of equipmentcondition assessment and comparison with defined acceptance criteria. This includessurveillance tasks as in-service inspection (ISI), in-service testing (IST), and monitoringand diagnostics (predictive maintenance).

Predictive Maintenance. Assesses the status of equipment or system degradationthrough correlation with one or more parameters.

Conditional Overhaul. Restoration of equipment to a reliable condition, undertakenwhen the acceptance criteria are no longer met.

Reliability Centered Maintenance. Based on identifying equipment/system functions,functional failures, and dominant failure modes to develop or revise PM tasks.

17.3 Objective and Scope of Valve Maintenance Programs

An accurate definition of the objectives and scope of the valve maintenance programwill help in upgrading and maintaining existing programs and in assessing theeffectiveness of the maintenance program as it impacts valve reliability and overallplant availability. It also clarifies the responsibility and accountability of themaintenance personnel.



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17.3.1 Objective and Maintenance Philosophy

The objective of the valve maintenance program is to improve valve reliability, whichin turn improves the overall plant availability. Plant and personnel safety and reducingthe cost of maintenance/repair go without saying. To meet this objective, themaintenance philosophy should be based on pro-active and preventive maintenanceinstead of reactive or corrective maintenance. However, it is inevitable that instanceswill arise where corrective maintenance will be required. This maintenance philosophymay require some up-front investments, but the payback can be very handsome. Itshould be noted that:

• Small problems caused by inadequate maintenance can grow to be significant andcan force the plant to shut down.

• Ideally, valve maintenance should be performed in time to prevent damage to thesystem including the valve.

• Excessive maintenance on an individual valve or a single group of valves should beavoided because it increases the probability of causing valve problems mostly dueto human error. Excessive maintenance can also divert resources from other valvesthat may need attention.

• Maintenance planning should start during the selection of new or replacementvalves. For example, some valve designs may be avoided for certain applicationsbecause they may require high maintenance at inaccessible or high radiation areas.Valve specifications should request manufacturer’s recommendations for spareparts (if not already included). Limiting the number of valve manufacturers withinthe plant may reduce the requirements for spare-part inventory and special tools.

• Post-maintenance testing should be adequate enough to ensure valve capabilityprior to return to service.

17.3.2 The Maintenance Rule (MR)

The rule ”Requirements for Monitoring the Effectiveness of Maintenance at NuclearPower Plants” (10CFR50.65 [6.1]) was published in 1991 and became effective July 10,1996. It is a simple rule requiring that:

1. Licensees must monitor structure, systems, and components (SCCs) performance orcondition against licensee established goals and take appropriate corrective actionswhen goals are not met.

2. The above monitoring is not required where it is demonstrated that the performanceor condition is effectively controlled via an appropriate PM program.



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3. Periodic assessment (cycle) of performance and maintenance activities shall triggernecessary adjustments and balance reliability improvement efforts withmaintenance related unavailability.

Item 1 gives the scope of the rule, including most safety-related SCCs and part of thenon-safety-related SCCs.

The spirit of the rule is to use the risk contribution of the various SCCs to create ahierarchy of the attention they receive (in or out of the scope, performance criteria) andto monitor the effectiveness of the plan (its results, compared to goals) to achievereliability goals without sacrificing availability.

An Implementation Guideline (NUMARC 93-01, Reference 4.32) has been produced bythe industry under NEI supervision and endorsed by the NRC as an acceptablecompliance process.

Methodology to Select Plant SCCs to Be in the MR Scope

The guideline comments on how to interpret the categories of SCCs targeted by the ruleand directs utilities to use methodologies such as Industry Experience, EngineeringEvaluation, and PRA or IPE types of evaluations.

Establishing Criteria and Goals

All SCCs in the rule scope are evaluated against criteria. Those SCCs that do not meetthe criteria must have specific goals established and be monitored closely until theyreach them. While the difference between criteria and goals is not that clear, the spirit isthat criteria can usually cover larger groupings (like plants, systems, or train) and thatif they are met, there is no need for close monitoring (a2 SCCs). If criteria are not met(al SCCs), root cause analysis (RCA) must point to the responsible SCC, which will besubmitted to a detailed monitoring until the performance has been restored.

SCC risk significance (expressed, for instance, as a Fussell-Vessely, if more than S x 10-3)is used to define the level of monitoring (plant, system, train, or component).

Performance criteria should be SCC availability, reliability, or condition. They arespecific for risk significant SCCs and non-risk-significant SCCs that are in stand-bynormally. They can be at the plant level for the other SCCs.

Plant level performance criteria have to be chosen by the utility (under NRC watch)according to different factors:

• Design type



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• Age

• Industry

• Past plant performance

Specific criteria should be related to industry experience but consistent withassumptions used in the PRA or IPE. SCC performance by rule date (July 96) iscomputed for the last two cycles or 36 months minimum and then compared to thecriteria to be accordingly submitted or not to a specific goal and corrective plan. Goals,when necessary, are expected to be set normally at the system or train level. Ifcomponent goals are needed, they should be limited to component types (such asbreakers or check valves) or to components that have several or repetitive failures.

Maintenance Preventable Functional Failures (MPFFs)

MPFFs are failures that could have been prevented by maintenance, such as an error inprocedure implementation or a known failure mechanism that PM could havecontrolled. Design or manufacturing errors are not MPFFs the first time they occur, butthey are for subsequent failures.

Controlling Equipment Removal of Service

An important aspect of the rule is risk evaluation before removing a piece of equipmentfrom service voluntarily for PM. The guideline advises us to identify key plant safetyfunctions and the SCCs that support them and to formally assess the effect of SCCremoval on global safety before proceeding.

Periodic Effectiveness Assessment

The rule and the guideline require that each cycle’s goals and criteria be revisited toassess the performance of SCCs and the effectiveness of corrective actions.

17.3.3 Scope

The scope of a good valve maintenance program should include both safety-related andnon-safety-related valves. Valves that are not essential to plant operation and safety aretypically given lower priority and eventually may require more resources to repair. Agood maintenance program must also address other elements such as spare partsinventory, personnel training, and special tools, as discussed below. The followingdiscussions are equally applicable to valve actuators because they are essential forvalve operation.



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17.4 Valve Maintenance Group

The human factor is by far the most important factor in any maintenance program.Experience shows that a reliable and competent valve maintenance group can make abig difference in valve availability and overall plant reliability. The following factorsshould be considered in selecting and maintaining the valve group personnel:

• Individuals in the valve maintenance group (mechanics, electricians,instrumentation technicians, etc.) must be well qualified to execute maintenancetasks. Qualifications within the group should be diversified to include all aspects ofvalve and actuator maintenance, diagnostics, and repair.

• Valve maintenance groups should frequently attend in-depth technical training andshort courses (such as those offered by NMAC and equipment manufacturersincluding valves, actuators, and diagnostic equipment).

• The valve maintenance group should include:

— A group leader in charge of all maintenance activities including documentation,coordination, and updating of the maintenance program

— A technical specialist assigned to observe problems, solutions, and otherinformation from the industry, INPO, the NRC, EPRI/NMAC, NIC, MUG, AUG,EPRI PPP Users Group, manufacturers, etc.

— Spare parts specialist in charge of replenishing inventory, locating parts fromother sources (for example, other plants) in case of an emergency, andmaintenance of spare parts database and records

— Scheduling and coordination engineer who interfaces with operations and othergroups in the plant

• Assignments to the valve maintenance group should be permanent because of theaccumulation of a great amount of indispensable experience and knowledge. Somemaintenance personnel have been with their plants since construction and/orstartup, and their experiences are considered very valuable plant assets. Promotionand compensation should not cause a significant turnaround in the group.Management should ensure sufficient overlap between fresh personnel andexperienced personnel before relieving the latter from duties.

• Outside contractors and temporary task force personnel should be under directsupervision from the valve maintenance group.

• Develop and maintain ties with valve manufacturers and suppliers.



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• Valve maintenance group personnel must have accountability, ownership, and long-term commitment to their duties.

• Valve maintenance group personnel must be motivated and have goodinterpersonal relationships inside and outside their group.

• Drug and alcohol testing should be strictly enforced.

17.5 Valve Categorization and Prioritization

From a maintenance standpoint, the ranking of valves should take into account:

• The safety implications of a failure and the status of redundant systems

• The performance history of the valves or valve groups considering the application,flow conditions, media, the manufacturer, the valve design, etc.

• Performance requirements such as maximum allowable local leak rates

• Valve location and accessibility

• Valve size and type

• Availability of replacement valves and spare parts

• Recent upgrades or modifications (if any)

• Manufacturer’s maintenance recommendations

• The bases and documentation for ranking

Valves can be categorized and prioritized for maintenance and repair using variousapproaches. One approach is to divide the valve population into three groups, asfollows:

Group 1

Group 1 consists of the valves that require mandatory actions, regardless of otheractivities. This group includes valves that have special requirements for maintenancebecause of operating license, safety reasons, or government regulations, or occasionallydue to manufacturer warranty requirements (for example, turbine stop valves).Preventive maintenance or inspections must be performed on these valves. The valveswithin this group can be identified using:

• FSARs



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• Technical specifications

• Directives issued by regulatory agencies

• ASME Code requirements

• Equipment qualification reports

• In-service inspection program

• NRC bulletins

• Insurance requirements

• Significant Event Reports

• Licensing Event Reports

• Manufacturer warranties

Group 2

Group 2 consists of the valves that typically have experienced a high rate of failures,caused a loss of plant availability, have high corrective maintenance costs, or havesafety problems including release of radiation. Plant maintenance history records are agood source of information, as is experience gathered from other plants. Plant recordswould help to identify the types of failures and frequencies being experienced by theplant, and the consequences of each failure, including impact on plant powerproduction, out-of-service time, hours to repair, spare parts required, causes of failure,and the failure mechanisms involved.

Group 3

Group 3 includes valves that do not have a history of failures; however, their failurewould impact safety or significantly increase the operating cost through the loss ofplant production. Valves to be included in this group should be identified by plantengineers based on analysis of the plant piping and instrumentation diagrams or otherpertinent documents.

It is clear that a large number of valves, especially small bore manual valves, will not beincluded in these groups. They will mainly consist of vents, drains, miscellaneousvalves, and valves in systems not important from a safety and operational point ofview. Their maintenance will be basically a corrective type (that is, repair orreplacement of the affected part or a whole valve).

Another method of prioritization in use in the industry is a powerful tool forestablishing the cost consequence of valve failures. Logic models, such as those built



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with availability block diagrams and/or fault trees, enable the maintenance engineer toproperly relate the likelihood of plant availability losses to component failure, even tothe level of component failure mode. These logic models, using such availablecomputer codes as EPRI’s UNIRAM, provide a ranked criticality list based on failurerates, forced outage rate, or plant availability loss contribution. Industry typical failurerates can be used to establish an initial criticality list, which is gradually revised andsupplemented as plant-specific data become available. (It has been found, for example,that some nonsafety-related equipment is highly important to plant availability.) Thesemodel-generated criticality lists are more accurate than a simple categorization becausethey include the effects of failures on other equipment as well as their direct effect onthe unit.

17.6 Coordination between Maintenance Group and Other Groups

Coordination of tasks between the valve maintenance group, packing group, testinggroup, actuator group, outside contractors, valve/actuator vendors, and operations isone of the most important factors in reducing maintenance efforts and eliminatingunnecessary tasks and duplication of efforts. For example, it may be more efficient tomaintain the valve a few weeks ahead of schedule if the actuator has to be dismountedfor any reason.

Coordination with other groups allows for the implementation of the “one trip”approach where all valve/actuator maintenance and repair activities are performed inone trip to the valve. Reference 5.21 discusses the “whole valve” approach where theentire valve (including the actuator) is maintained at the same time, thus eliminatingunnecessary duplication of effort. Reference 5.21 shows that proper implementation ofthese concepts has been very successful for more than 10 years in some power plants.

17.7 Involvement of Valve Maintenance Group with Other Activities

The valve maintenance group must be involved in all aspects of repairs, modifications,and actuator settings. Inadequate involvement of the maintenance group with theseactivities can have serious consequences. For example, during major projects (such asGL 89-10 and GL 95-07) and special projects (for example, MSIV upgrades), a task force(using specialists from outside contractors and consultants) may be formed to work onthese projects. Even though such approaches meet the specific objectives of themoment, they tend to be extremely costly in the long run because, once the group isdisbanded, the remaining maintenance and engineering organization might be leftwithout the knowledge and rationale that was developed during these projects. Thus, itis crucial that the impact of such special projects on the maintenance program bedetermined, documented, and communicated to the valve maintenance group beforedissolving the special task force.



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17.8 Inspection Frequency and Scope

The valve categories described in Section 17.5 should be used to determine thefrequency and scope of valve inspections, which can vary from merely outside visualinspection (for example, for leaks from packing/gasket, higher noise, excessivevibration, etc.) to a full disassembly and detailed internal inspections. Prior toperforming maintenance and/or repair, the subject valve should be visually inspectedunless it is not accessible or is in a high radiation area. The use of a check list provides adocumented record for the inspection results and ensures that all of the intended tasksare performed. The use of assembly/disassembly procedures helps eliminate costlyproblems and saves time and resources in the long run.

The maintenance and inspection program must be flexible to accommodate plantexperience. For example, when a visual inspection indicates a slight packing leak, theinspection frequency should be increased to ensure that the leakage does not exceedtolerable limits without being detected.

17.9 Maintenance Schedule

An optimum maintenance schedule is one that restores the valve to a good workingcondition before causing any damage to the valve, actuator, or system and maximizesplant operation (that is, does not cause system shutdown). The following factors shouldbe considered:

• Excessive maintenance is not recommended and should be avoided for severalreasons, including potential problems due to human error, unnecessary depletion ofspare parts, and waste of manpower or resources that should be allocated to othervalves. Excessive maintenance may also impact plant outages unnecessarily.

• Spare parts, materials, tools, and procedures should be made available prior tovalve disassembly.

• If needed, diagnostic equipment and technicians should be made available prior tovalve disassembly.

• An ideal maintenance schedule would result in a reasonable and uniform work loadfor the available work force. However, this is not always possible because manymaintenance/repair activities have to be completed during plant outages.

• Scheduling should be coordinated with other groups in order to implement the onetrip concept where all activities can be performed in one trip to the valve.

• Scheduling valves with similar designs for maintenance within the same timeframemay provide significant efficiency.



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• Scheduling must account for the sequence of activities. For example, VOTES testingon the operator may require LLRT on the valve.

• The maintenance schedule must be responsive to plant requirements. For examplewhen a problem is detected with one valve, all other similar valves should bechecked promptly even if they are not within the current schedule. In some cases,routine maintenance has to be rescheduled to address an unexpected valve problem.

17.10 Spare Parts Inventory and Control

Supplying valve spare parts for nuclear safety-related applications presents severaltechnical and economic concerns and should be clearly understood and planned for.Some of the key factors are as follows [5.23]:

• Many spare parts are long-lead items and are not readily available. Theresponsibility of the spare parts specialist within the valve maintenance groupincludes identifying these items and having a contingency plan in case of anemergency need. Options include sharing parts with other plants (includingdecommissioned units), using dedicated parts, and developing and sharing sparepart electronic databases via the Internet.

• Upgrades and obsolescence do affect the spare part inventory and associated capitalinvestments.

• The evolution of design codes and changes in regulatory requirements can rendersome of the inventory obsolete (for example, asbestos packing and seals).

• Operating experience feedback should be considered in adjusting inventory.

• Inspection frequency should account for availability/procurement of parts beforecausing a valve problem.

• Spare part storage should consider the material’s shelf life and requiredenvironment.

• Control and maintenance of spare part records are key concerns for safety-relatedcomponents.

• Standardization of new/replacement valves and consolidation of valve componentscan provide significant savings in spare part inventory and associated costs.


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18 TROUBLESHOOTING AND RECOMMENDED

CORRECTIVE ACTIONS

18.1 Introduction

This section provides guidance on troubleshooting and recommended correctiveactions for gate, globe, butterfly, ball, plug, and diaphragm valves. EPRI/NMAC hasalso published several reports to address air-operated valves [1.2], safety and reliefvalves [1.4], solenoid valves [1.7], and check valves [1.20 and 1.21]. EPRI/NMAC hasalso published several other reports to address Limitorque actuators [1.22, 1.23, 1.24,and 1.25] and Rotork actuators [1.26]. Troubleshooting a valve usually involves theactuator. However, in the following, the focus will be on troubleshooting valveproblems assuming that actuator troubleshooting has been performed using theapplicable document.

Before repairing a valve, it is important to determine and eliminate the root cause of thevalve failure. For example, if a valve stem is bent (or twisted) due to accidentaloverload during testing, then stem replacement with proper measures to preventfurther overload is sufficient. However, if the stem is bent (or twisted) during normaloperation, then it is necessary to evaluate the actuator output thrust (or torque) versusrequired stem thrust (or torque) and stem strength before ordering a replacement stem.If this evaluation shows that the stem stress exceeds the allowable stress, then it may benecessary to redesign the stem with a stronger material.

A repeated valve problem indicates that the valve needs special attention. For example,repeated packing leakage may be caused by stem corrosion, bent stem, large lateralstem movement, or inadequate packing selection/design. If the packing leakage iscaused by stem corrosion, then the stem should be redesigned with a material that iscompatible with the process fluid. If the stem is bent, then the evaluation in thepreceding paragraph must be performed. If the stem has large lateral movement thatcannot be accommodated by the packing resilience, then it is necessary to determine theroot cause. For gate and globe valves, the lateral stem movement can be caused by asmall clearance between the stem head and the disc or due to misalignment betweenthe disc and seat. For butterfly and ball valves, the lateral stem movement can becaused by excessive bearing wear. The point is that even simple problems should not


Troubleshooting and Recommended Corrective Actions

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be ignored if they occur frequently because they may be symptoms of more seriousproblems.

New valves and overhauled valves should be broken in at low loads for a few cycles.Loads should be increased gradually until normal operating loads are reached.

Most valve problems require a walkdown and investigation at the valve, and in somecases internal inspection would be required. Some actuator problems can be diagnosedfrom the control room (such as instrument air pressure and control power availability).In diagnosing a valve problem, the past history of the valve, similar valves, and othervalves in the same system should be reviewed. In all cases, simple things should bechecked first. It can be extremely embarrassing and wasteful to tear down a valvesearching for a problem when the real culprit is a cocked packing gland or loss ofpower.

The cost and delivery schedule of new parts (or replacement valves) should beconsidered before authorizing repairs. Alternative solutions should be also consideredwhile scheduling for valve repair or replacement. For example, on-line leak sealing (seeReference 1.16) may be used to support continued operation until the valve is repairedduring the following outage.

It should be noted that some valve problems are caused by design/installationdeficiencies such as:

• Inadequate structural strength under seismic loads, pipe loads, etc.

• Inadequate strength to withstand missiles and flying objects under postulatedaccident conditions.

• Power sources (for example, cables, air lines, and hydraulic lines) are not protectedfrom damage under design basis conditions.

• The valve actuator is not accessible for maintenance/repairs.

Such problems may require valve/actuator replacement with major design evaluations.These problems are outside the scope of this document. As part of troubleshooting androot cause investigations, it may be necessary to calculate the required torque/thrust tooperate the valve under a given set of operating/design basis conditions. Thesecalculations typically require detailed internal dimensions, which may be obtainedfrom the valve manufacturer. Alternatively, these dimensions may be obtained duringvalve disassembly for inspection or repair. For example, detailed internal dimensionsare needed to calculate the required thrust/torque using EPRI’s Performance PredictionMethodology (PPM). References 2.1 through 2.4 and 2.14 through 2.17 provide data



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sheets and illustrations showing the dimensions needed for each valve design withinthe scope of EPRI’s PPM.

18.2 Gate Valve Problems

As discussed in Section 4, there are many gate valve designs in nuclear power plantsincluding solid wedge, flex wedge, split wedge, double disc, Westinghouse wedge gatevalves with linkage type stem-to-disc connection, and W-K-M parallel expanding discvalves. The W-K-M valves are used only in very few power plants and are notdiscussed in this report; the reader is referred to Reference 2.17 for details. Commongate valve problems and their causes are discussed in Section 4.5. Reference 1.1provides additional discussions for damage assessment and repair options for gatevalves. In this section, the most common valve problems are listed along withsuggested corrective actions.

18.2.1 Solid, Flex, and Split Wedge Gate Valve Problems

18.2.1.1 Excessive Packing Leaks

Packing leakage is one of the major problems for all types of valves. Reference 1.15provides extensive discussions of packing designs, troubleshooting, andrecommendations for solving packing leakage problems. Additional information can befound in References 5.44 through 5.50. Some of the more common packing problems aresummarized as follows:

• Insufficient packing compression.

• Improper consolidation.

• A scored or heavily pitted valve stem or stuffing box.

• Corrosion on the valve stem.

• Improper packing assembly.

• Improper stem alignment.

• Bent stem as measured by the total indicated runout (TIR). The amount of allowableTIR will vary depending on the valve size, type and manufacturer’s allowance(typically < 0.007 inch).

• Large variations in stem diameter.



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• Split ring packing improperly aligned (split-ring packing rings should be lined upwith their cuts or separations staggered).

• Excessive stem lateral movements due to small clearance between stem and disc orguide rail and guide slot. In this case, the stem lateral movement can be seen at anydisc position except in the wedged position.

• Excessive stem lateral movements due to misalignment between the disc and seat.In this case, the stem lateral movement is most pronounced as the gate wedges intoor unwedges from the seat.

• Excessive stem lateral movements due to actuator side loads on the stem.

• Improper size or type of packing.

• Loose or cocked gland.

Visual inspection and stroking the valve under some pressure can be used toinvestigate the source of the packing leakage. In many cases, it may be sufficient toincrease the packing compression to stop a packing leakage. However, it is important toverify that the margin between the available actuator thrust and the required stemthrust to operate the valve (including packing friction) under worst case flowconditions is acceptable.

18.2.1.2 Valve Will Not Respond to the Actuation Signal

The first step is to determine whether the problem is actuator related or valve related.For actuator-related problems, the following references should be consulted for rootcause and repair practices:

• For Limitorque actuators, see References 1.22 through 1.25.

• For Rotork actuators, see Reference 1.26.

• For air-operated valves, see Reference 1.2.

• For solenoid valves, see Reference 1.7.

• For manual actuators, see Section 14.



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If the problem is related to the gate valve, then it may be caused by one of the followingreasons:

• The valve/actuator assembly does not have adequate operating clearances. Directinterference from scaffolding built too close to the valve/actuator assembly hasprevented some valves from stroking.

• Packing resistance is extremely high and is locking the stem. If appropriate, loosenthe packing and check the valve operation (when there is no pressure in the system).

• If the stem does not move in the opening direction, then the valve may beexperiencing a pressure locking, thermal binding, or disc pinching condition (seeSection 4). Verify that bonnet pressure is not higher than either the upstream ordownstream pressures. It can be dangerous to plant personnel to loosen packing torelieve bonnet pressure from a valve. Permanent modification to eliminate pressurelocking and thermal binding may be required on valves that are susceptible topressure locking.

• If the stem does not move and the disc is in the wedged position, then theunwedging thrust is not sufficient to unwedge the gate. The required thrust andactuator output thrust should be evaluated for inadequate sizing.

• If the stem does not move and the disc is not in the wedged position, then the stemmay have lost engagement with the actuator. Dismounting the actuator may berequired for further investigation.

• If the stem moves but the disc does not, then the stem has lost engagement with thedisc. Possible problems are stem head broken, excessive wear between the stemhead and the gate T-slot, or T-slot ears broken or severely deformed. Internalinspection is required.

• If the stem does not move from midstroke position, the guide slots may be stuck tothe guide rails. Possible causes include guide galling, guide rail deformation, stuckanti-rotation arm, accumulation of foreign materials in the clearance between theguide rail and guide slot, or the presence of an obstruction especially in raw watersystems such as service water systems.

• If the valve initially fails to operate and then appears to operate normally, this maymask a potential problem with the valve or the actuator. In such a case, the valveand the actuator should be evaluated to determine the cause of the initial failure.

As mentioned above, simple things should be checked first. Some factors can be quicklyeliminated by visual inspection or by stroking the valve using the handwheel.



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18.2.1.3 Valve Will Not Fully Open

The first step is to determine whether the problem is actuator related or valve related.Apart from the actuator, the valve problem may be caused by one or more of thefollowing reasons:

• The valve does not have adequate operating clearances. Direct interference fromscaffolding built too close to the valve has prevented some valves from stroking.

• Obstruction inside the valve; internal inspection may be required.

• Improper stem alignment.

• Bent stem causing stem interference.

• Large variations in the stem diameter causing excessive packing resistance.

The required thrust near the fully open position is only a small percentage of theunwedging thrust. Valve failure to fully open suggests either a very simple problem(such as improper limit switch setting in the actuator) or serious damage inside thevalve as mentioned above. This problem must be corrected even if the valve is notrequired to open fully, because it may lead to additional damage and ultimatelyprevent the valve from stroking.

18.2.1.4 Valve Will Not Fully Close or Properly Seat

In addition to the applicable problems in Section 18.2.1.3, the problem may be causedby one of the following reasons:

• Insufficient stem thrust to allow adequate wedging; verify the actuator output thrustand required thrust to close and wedge the disc.

• Damaged seat or disc (which generally requires internal inspection). This may becaused by:

— Incorrectly installed disc

— Galling or gouging between the disc and seat under tilted contact mode asdiscussed in Section 4.5

— Erosion/corrosion of the disc and/or seat sealing surfaces

— Wire drawing or steam cutting (in steam service)



18-7

— Disc obstruction, including biological growth or contaminants, especially inservice water systems

— Excessive pipe loads and bending moments

• Bent stem causing stem interference.

• Excessive packing resistance.

• Damaged/worn stem threads, especially those in contact at the closed positionwhere maximum thrust is transmitted.

• Bent guide rails; this is most common with guide rails that do not have full lengthwelds

• Expansion of the discs prior to reaching the fully closed position; this occurs in splitwedge valves and can occur in flex wedge valves having T-slots perpendicular tothe flow axis. Disc expansion is caused by the stem torque prying the T-slot anddiscs apart; it can be eliminated by installing an external stem torque restraint.

• Disc and seat angles do not match; this can occur during disc and seat lapping.

• The disc seat face overlay is not large enough to accommodate the disc and seatposition variance; this may occur after the disc and seats have been resurfaced andthe disc travels too far.

After ruling out the simple causes, it may be necessary to perform internal valveinspection. Reference 1.1 provides detailed discussions for damage assessment andrepair options for gate valves.

18.2.1.5 Excessive Flange Leaks

Flange leakage can occur between the mating flanges on the piping to the valve orbetween the bonnet and valve body. These leaks can be caused by several problems,but a typical cause is one of the following:

• Gasket problems, including reuse of the old gasket, absence of the gasket, gasket ofthe wrong material or size, or improper gasket crush (see EPRI/NMAC TR-104749,Static Seals Maintenance Guide, [1.14]).

• Bolting problems, including use of the old bolts that do not tighten properly,insufficient torque for the service, or incorrect torque pattern (see EPRI/NMAC TR-104213, Bolted Joint Maintenance and Application Guide, [1.17]).



18-8

• Surfacing problems, including flange surface pitting, erosion, corrosion or beinguneven. Mating polished flanges that are even 0.001 of an inch off 90° will leak inservice. Check the installation of new parts.

• The bonnet flange design is not adequate for internal and external (actuator) forces.

• Corrosion and pitting of the pressure seal surfaces in the bonnet and body of apressure-sealed bonnet valve.

• Improper assembly of the pressure-sealed bonnet resulting in bonnet misalignmentand uneven load on the seal ring.

• Changes in piping forces and moments due to changes in operating conditions werenot accounted for in flange design. For example, if changes in operatingtemperature cause flange leaks, then it is possible that under thermal piping loads,the flange or gasket stresses exceed the allowable stress.

• Changes in piping forces and moments due to changes in operating conditions canalso cause fatigue failure to the gasket, bolts, or flanges.

• Under pressure-locking conditions [4.2, 5.30], the large increase in bonnet pressurecan cause the bonnet gasket to leak. If both the packing and bonnet do not leak, thenthe bonnet or bolt stresses can reach yield stress.

18.2.2 Double-Disc Gate Valve Problems

There are several double-disc valve designs in nuclear power plants including thosemanufactured by Anchor Darling and Aloyco. In this section, the most common valveproblems are listed along with corrective actions. In this section, only additionalproblems that pertain to double-disc designs are discussed (see Section 18.2.1 for otherproblems covered under solid and flex wedge gate valves).

18.2.2.1 Excessive Packing Leaks

Packing leakage problems summarized in Section 18.2.1.1 also apply to double-discgate valves.

18.2.2.2 Valve Will Not Respond to the Actuation Signal

In addition to the problems listed in Section 18.2.1.2, the following problems apply todouble-disc gate valves:



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• The upper disc does not break free from the lower wedge, which can increaserequired thrust significantly even when the valve is installed in the preferred flowdirection. EPRI testing shows that on one stroke the wedge did not break free [2.14].This can be caused by many factors including galling/corrosion in the valve disccomponents and accumulation of foreign materials between the moving parts in thevalve disc assembly.

18.2.2.3 Valve Will Not Fully Open

In addition to the applicable problems in Section 18.2.1.3, the problem may be causedby one of the following reasons:

• Failure of the upper wedge to unwedge from the lower wedge (see Figure 4-4).

• Excessive wear between the disc trunnions and the upper wedge. This wear mayhappen in the absence of disc anti-rotation devices (which prevent the discs fromspinning inside the upper wedge holes).

After ruling out the obvious possible causes, it is generally necessary to perform aninternal valve inspection.

18.2.2.4 Valve Will Not Fully Close or Properly Seat

In addition to the applicable problems in Section 18.2.1.4, this problem may be causedby one of the following reasons:

• Premature wedging between the upper and lower disc wedges before reaching thefully closed position.

• Accumulation of foreign materials in the body below disc assembly.

• Increase in the stiffness of the wedge spring due to hardening or accumulation offoreign materials.

18.2.2.5 Excessive Flange Leaks

See Section 18.2.1.5 for applicable reasons.

18.2.3 Westinghouse Gate Valve Problems

Westinghouse wedge gate valves have linkage-type stem to disc connections (seeFigure 4-8). Apart from problems caused by improper alignment of the linkages, allproblems are already discussed in Section 18.2.1 and 18.2.2.



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18.3 Globe Valve Problems

There are many globe valve designs in nuclear power plants including T-pattern, Y-pattern, rising stem, rising/rotating stem, unbalanced plug, and balanced plug. Themost common valve problems and their causes are discussed in Section 6.5. Reference1.1 provides additional discussions for damage assessment and repair options for globevalves. In this section, additional globe valve problems are listed along withrecommended corrective actions.

18.3.1 Excessive Packing Leaks

Packing leakage problems summarized in Section 18.2.1.1 also apply to globe valves.

18.3.2 Valve Will Not Respond to the Actuation Signal

In addition to the problems listed in Section 18.2.1.2, the following problems apply toglobe valves:

• Insufficient actuator thrust for the actual flow direction. For example, if the requiredthrust is based on flow under the plug, the valve may not open under the samepressure drop if the flow direction changes to flow over the seat (for example, dueto flow reversal). Thus, for globe valves, it is critical to verify that required thrust isbased on the worst possible combination of stroke direction, flow direction, andpressure drop.

• Insufficient actuator thrust for the applicable pressure drop area. The effectivepressure drop area in unbalanced plug globe valves can be based on either the plugseating diameter or the plug guide diameter. The EPRI Globe Valve Model Report [2.3]provides the criteria to determine whether a globe valve is seat based or guidebased. The use of the guide area will always result in conservative thrust prediction.

• Galled, corroded, or damaged stem bushings/guides.

• Galled or scored plug and/or guide sleeve.

• The operating temperature exceeds the trim design temperature, which includesgeometric characteristics (such as clearance and coefficients of thermal expansion).

18.3.3 Valve Will Not Fully Open

Most problems summarized in Section 18.2.1.3 also apply to globe valves.



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18.3.4 Valve Will Not Fully Close or Properly Seat

In addition to the applicable problems summarized in Section 18.2.1.4, the followingproblems apply to globe valves:

• Misalignment between the plug and the seat. Plug misalignment prevents propermating of seating surfaces.

• Improper mating angles between the plug and the seat. This problem usually occursafter plug and/or seat repair.

• Worn or damaged plug seal.

• Damaged seating surfaces due to excessive closing thrust.

For rising and rotating stem globe valves, the following problems apply:

• Galling at plug-to-stem interface.

• Damage in the yoke nut threads.

• Improper required thrust/torque predictions. Calculations based on rising stemglobe valves do not apply to rising and rotating stem globe valves and often yieldnonconservative thrust/torque predictions.

Additional information can be found in Reference 1.1.

18.3.5 Excessive Flange Leaks

Most problems summarized in Section 18.2.1.5 also apply to globe valves.

18.4 Butterfly and Ball Valve Problems

Butterfly and ball valves, being quarter-turn valves, have common problems asdiscussed below.

18.4.1 Excessive Packing Leaks

Packing leakage problems summarized in Section 18.2.1.1 also apply to butterfly andball valves. It should be noted that quarter-turn valves in general have fewer packingproblems as compared to rising stem valves.



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18.4.2 Valve Will Not Respond to the Actuation Signal

The first step is to determine whether the problem is actuator related or valve related. Ifthe problem is related to the valve, then it may be caused by one of the followingreasons:

• Packing resistance is extremely high and is locking the stem. If appropriate, loosenthe packing and check valve operation.

• Interference of the disc or ball with the body due to excessive wear of the sleeve orthrust bearing. Excessive sleeve bearing wear may also cause stem-to-body galling.

• If the disc or ball does not unseat, then the opening torque is not sufficient toovercome high unseating torque. The total unseating torque is the sum of the seattorque, the bearing torque, the packing torque, and the hydrostatic torque for non-vertical stem installations. An increase in the total unseating torque can be causedby:

— Degradation or contamination of the seat and/or bearing especially in servicewater applications.

— Pressure locking between the subject valve and an adjacent tight-seal closedvalve (see Section 7.3.4 for details). In this case, the trapped pressure should berelieved at the adjacent valve.

— High hydrostatic torque in nonsymmetric disc butterfly valves (see Section 7.3.4for details).

In either case, the required torque and actuator output torque should be evaluatedfor inadequate sizing.

• If the stem does not move and the disc or ball is not in the closed position, then thestem may have lost engagement with the actuator. Dismounting the actuator may berequired for further investigation.

• If the stem moves but the disc (or ball) does not, then the stem has lost engagementwith the disc (or ball). Possible causes include broken stem/key/pin due to highmaximum transmitted torque (see Section 8), wear, fatigue, or galvanic corrosionespecially in salt-water applications. Internal inspection would be required.

• If the stem does not move from the midstroke position, then the valve may have anobstruction or the hydrodynamic torque is too high. The hydrodynamic torque canbe ruled out if: a) the flow velocity is relatively low, or b) the direction of stemrotation is in the same direction in which the hydrodynamic torque acts. For



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example, if the stem would not rotate in the closing direction and the hydrodynamictorque is self closing, then the valve has an internal obstruction.

As mentioned above, simple things should be checked first. Some factors can be quicklyeliminated by visual inspection or by stroking the valve using the handwheel.

18.4.3 Valve Will Not Fully Open

The first step is to determine whether the problem is actuator related or valve related.Apart from the actuator, the valve problem may be caused by one or more of thefollowing reasons:

• High total dynamic torque. Check the valve torque requirements against theavailable input torque.

• Obstruction inside the valve. Internal inspection may be required.

• Disc/ball misalignment due to excessive bearing wear.

• Bent stem causing interference between the stem or disc/ball and valve internals.

• Disc/ball position stops improperly set.

• Disc interference with the line due to inside diameter buildup in the line. Thisproblem is peculiar to butterfly valves where the disc extends outside the valvebody near the fully open position.

Under design basis conditions (which may include blowdown), the flow velocity can berelatively high.

Caution: For butterfly valves under relatively high flow velocity conditions (suchas under blowdown conditions), the required total dynamic torque near the fully openposition can be very high (see Reference 1.6). For these cases, some valves (such ascontainment isolation valves) are limited in the open direction to about 50° open inorder to enable the valve to perform its safety function under design basis conditions.The limit switch for these valves should not be altered without proper engineeringassessment.

18.4.4 Valve Will Not Fully Close or Properly Seat

In addition to the applicable problems in Section 18.4.3, the problem may be caused byone of the following reasons:



18-14

• Insufficient stem torque to allow adequate seating. Verify the actuator output torqueand required torque to close and seat the disc/ball.

• Damaged seat or disc/ball (which generally requires internal inspection). This maybe caused by:

— The disc (or ball) is installed incorrectly.

— Galling or gouging between the disc (or ball) and seat.

— Erosion/corrosion of the disc (or ball) and/or seat sealing surfaces.

— Wire drawing or steam cutting (in steam service).

— Disc obstruction, including biological growth or contaminants especially inservice water systems.

— Soft seating material displaced from its installed location.

— Soft seating material incompatible with service conditions.

• Bent stem causing interference.

• Excessive packing resistance.

• Excessive hydrostatic torque.

• Excessive bearing wear causing disc-to-seat misalignment.

• Improper seating position. Disc/ball may be stopping outside the seating zone.

• Seat distortion due to excessive piping loads.

After ruling out the simple causes, it may be necessary to perform internal valveinspection.

18.4.5 Excessive Flange Leaks

See Section 18.2.1.5 for applicable reasons.

18.5 Plug Valve Problems

Common problems are given in Section 11.5.



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18.6 Diaphragm Valve Problems

Common problems are given in Section 12.5

18.7 Inspection and Repair Checklists:

One of the most effective ways to ensure the quality and the effectiveness of themaintenance activities is to utilize prepared checklists. The following suggestions canbe used to develop and maintain these checklists:

• The maintenance group should develop as many checklists as necessary to cover thevariety of valve/actuator types in their plant(s). Manufacturers’ instruction manualscan be used as the starting point to develop checklists.

• Different checklists can be developed for different activities such as inspection,disassembly, repair, assembly, and troubleshooting of the valve/actuator.

• If the valve is disassembled for inspection or repair, it is recommended that criticalinternal dimensions be documented for later use. References 2.1 through 2.4 and2.14 through 2.17 provide data sheets and illustrations showing the internaldimensions needed to calculate the required thrust/torque using EPRI’s PPMmethodology.

• Figures and illustrations may be included in the checklist to help documentobservations. Attaching copies of manufacturer’s drawings to the checklist may savetime in identifying part numbers and components.

• The checklist can be designed and revised to reflect the plant maintenanceexperience for each valve/actuator type in a given application. For example, thechecklist may emphasize detailed inspection of the stem-to-disc connection for signsof wear in applications with high fluid turbulence (such as in pump dischargevalves).

• The checklists should be revised as necessary to implement suggestions frommaintenance group personnel. The use of a revision number and a date will ensurethat the latest revision is used.

• The checklist should provide enough questions and blank spaces to help indocumenting observations that may shed light on unusual performance.

• The checklist should follow a logical sequence that ensures that importantinformation is captured. For example, external visual inspection should beperformed and documented before valve disassembly.



18-16

• The checklist should include both the valve and actuator (see Section 17.6 for adiscussion of the “one trip” approach).

• In addition to the checklist, photographs and/or video recordings can be veryhelpful in documenting the as-found condition of the valve and actuator. Showing ascale next to the component being photographed is an excellent way to estimate thesize of the feature(s) being documented. Showing the valve tag number, the date,and the time on photographs and videos is a good practice.

Table 18-1 is a sample checklist for performing a solid or flexible wedge gate valveinspection. It can be easily expanded to cover other types of gate and globe valves.

Table 18-2 is a sample checklist for performing a butterfly valve inspection. Similarchecklists for ball, plug, and diaphragm valves can be developed.


Table 18-1Inspection Checklist for Solid and Flexible Wedge Gate Valves

Valve Tag No. Unit

Size WO No.

Manufacturer Date

Actuator Valve Function

INSPECTION AREA(IF CHECKED)

AS FOUND CONDITION(S) ACTION REMARKS

Body External

General Condition

Anti-Rotation Arm/ Mechanism

External Bolting/Threads

Bonnet

Packing

Yoke

End Flanges/Welds


Table 18-1 (continued)Inspection Checklist for Solid and Flexible Wedge Gate Valves

Valve Tag No. Unit

Size WO No.

Manufacturer Date



Disc

General Condition

Seating Surface:

General Condition

Upstream

Downstream

Guide Slots:

General Condition

Upstream side

Downstream Side

T-Slot/Stem Connection



Valve Tag No. Unit

Size WO No.

Manufacturer Date



Stem

General Condition

Orientation from Vertical

T-Head/Disc End

Packing Area

Actuator End

Thread Surface

Thread Lubricant

Backstop Area

Total Indicated Runout:

(Note 1)

Bent or Crooked



Valve Tag No. Unit

Size WO No.

Manufacturer Date



Curved or Bowed

Tapered

Eccentric

Body Internal

General Condition

Downstream Seat:

Seat Surface

Seat Weld/Retainer

Upstream Seat:

Seat Surface

Seat Weld/Retainer



Valve Tag No. Unit

Size WO No.

Manufacturer Date



Guide Rails:

Upper Part (disc near/at openposition)

Middle Part (disc atmidstroke)

Lower Part (disc near/ atclosed position)

Gasket Sealing Area

Pressure Seal Ring Area

Pressure Seal Retainer Groove

Gasket

Pressure Seal Ring

Threads/Bolt Holes



Valve Tag No. Unit

Size WO No.

Manufacturer Date



Bonnet Internal

General Condition

Gasket Sealing Area

Pressure Seal Ring Area

Threads or Bolt Holes

Stem Backstop Area

Packing Ring Set

Packing Box Area

Packing Follower

Packing Follower Bolts

Live Load Springs



Valve Tag No. Unit

Size WO No.

Manufacturer Date



Other Components:

Comments:

Check here if complete disassembly was NOT required.Check here if continuation sheets are used. No. of sheets: ____

Inspection performed by: Date:

Final Approval: Date:

Note 1: See Machinery’s Handbook, 23rd edition (Dimensioning, Gaging And Measuring, Checking for Various Shaft Conditions; Figure 9 on page 696) for illustrations of possibleforms of runouts and methods for measuring TIRs.


Table 18-2Inspection Checklist for Butterfly Valves

Valve Tag No.

Size

Body Style: (Flanged, Wafer, Lugged or Welded)

Disc Design: (Symmetric, Single/Double/Triple Offset)

Manufacturer

Unit

WO No.

Date

Valve Function

Actuator

INSPECTION AREA

(IF CHECKED)

AS FOUND CONDITION(s) ACTION REMARKS

Body External

General Condition

External Bolting/Threads

Packing

Bonnet/Top Cover Plate

Upper Trunnion

Lower Trunnion

Bottom Cover


Table 18-2 (continued)Inspection Checklist for Butterfly Valves

Valve Tag No.

Size

Manufacturer

Unit

WO No.

Date

INSPECTION AREA

(IF CHECKED)


Upstream Flange/Weld

Downstream Flange/Weld

Disc Position Stop

Upper Bearings

General Condition

Lower Bearings

General Condition

Outboard Thrust Bearing

General Condition

Upper Thrust Bearing

General Condition



Valve Tag No.

Size

Manufacturer

Unit

WO No.

Date

INSPECTION AREA

(IF CHECKED)


Lower Thrust Bearing

General Condition

Upper Shaft

General Condition

Packing Area

Upper Bearing Area

Actuator End

Shaft-to-Actuator Connection

Shaft-to-Disc Connection

Total Indicated Runout:(1)

Lower Shaft

General Condition

Lower Bearing Area



Valve Tag No.

Size

Manufacturer

Unit

WO No.

Date

INSPECTION AREA

(IF CHECKED)


Shaft-to-Disc Connection

Total Indicated Runout:(1)

Seat

General Condition

Seat Retainer

Seat Retainer Bolts/ Screws

Disc

General Condition

Seating Edge Surface

Disc to Upper Shaft Connection

Disc to Lower Shaft Connection



Valve Tag No.

Size

Manufacturer

Unit

WO No.

Date

INSPECTION AREA

(IF CHECKED)


Body Internal

General Condition

Body Liner

Seat Area

Packing Ring Set

Packing Box Area

Packing Follower

Packing Follower Bolts

Live Load Springs

Upper Bearing Area

Lower Bearing Area

Upper Shaft Penetration

Lower Shaft Penetration



Valve Tag No.

Size

Manufacturer

Unit

WO No.

Date

INSPECTION AREA

(IF CHECKED)


Bottom Cover Seal (Gasket / O-Ring)

Disc Position Stop

Other Components

Comments:❏ Check here if complete disassembly was NOT required.❏ Check here if continuation sheets are used. No. of sheets: ______

Inspection performed by: Date:

Final Approval: Date:

Note 1: See Machinery’s Handbook, 23rd edition (Dimensioning, Gaging And Measuring, Checking for Various Shaft Conditions; Figure 9 on page 696) for illustrations of possibleforms of runouts and methods for measuring TIRs.


19-1

19 INSTALLATION, TESTING, AND MAINTENANCE

REQUIREMENTS

19.1 Introduction

A valve must be properly installed, tested, and maintained to function as it wasdesigned. This section discusses general installation, testing, and maintenancerequirements for valves. Suggested postmaintenance testing is given in Reference 1.13.

19.2 Installation Requirements

19.2.1 General Valve Installation Requirements

Installation should be preceded by a careful examination of the valve to ensure that it isin accordance with the specification, has not suffered damage, and is not dirty. Whilethorough receipt inspection procedures are desirable, justification for them is temperedby the degree of previous inspections, such as during manufacturing, and the costs ofestablishing receipt inspection procedures. Receipt inspection should include thefollowing verifications and examinations:

• Verification of appropriate certification of materials and manufacturing inspection

• Verification of external dimensions for compatibility with installation drawings

• Visual examination of exterior for damage

• Cleanliness examination

• Verification of valve operation (manual and otherwise)

• Verification that all shipping supports and/or desiccants are removed

• Verification that end connections for mating to piping system are correct


Installation, Testing, and Maintenance Requirements

19-2

Receipt inspection should be carried out in a clean area to prevent the introduction offoreign matter into the valve. Following receipt inspection, the valve should be driedout (if it has been wetted during inspection), sealed, and stored until installation.

The following preinstallation activities can be performed to ensure trouble-freeoperation after installation:

• Repack every valve before installation using plant procedures. Packing-relatedproblems (such as bad studs, corroded/damaged valve stem and/or stuffing box,and wrong packing material) are identified before installation.

• For gate and globe valves, disassemble, inspect, and blue-check valve seats prior toinstallation.

• For ball and butterfly valves, perform a quick pressure test.

• For check valves, disassemble and measure the dimensions of critical componentsfor future wear trending.

Evidence of satisfactory receipt inspection should be affixed to the valve.

Valve installation should be accomplished under conditions that give maximumassurance that no foreign matter (such as stray nuts and bolts, pieces of welding rod,etc.) is introduced into the valve. A valve serving a system important to plant operationmerits close attention to installation procedures to prevent introduction of a potentialcause of failure into the system.

If the installation is a replacement in an operating nuclear power plant, all applicableradiation procedures must be followed, and the plant lineup must ensure the safety ofpersonnel installing the valve. As a minimum, all valves isolating the work area fromthe rest of the plant should be locked shut and tagged to preclude inadvertentoperation. Where isolation valves are remotely operated, their operating circuits shouldbe deactivated and controls tagged with instructions not to operate.

The installation should be in accordance with the manufacturer’s instructions to ensurethat the physical orientation of the valve is suitable for satisfactory operation and thatthe flow orientation is proper. The space envelope (unless compromised by overallspace limitations) should be such that the valve and operator can be removed and/ordisassembled for routine maintenance, such as packing replacement, internalinspection, or operator repair.

To avoid damage, fit-up to adjacent piping should be made without forcing the pipingto the valve. Subsequent welding should be in accordance with appropriate weldingprocedures to avoid heat-induced valve damage. During welding, the disc should be



19-3

positioned (mid-position or closed) as recommended by the manufacturer. Specialwelding techniques may be required for some valves (those with limited physicalseparation between the weld and valve seat area) to limit welding-induced distortion.When welding carbon steel valves, the temperature in the seat area should not exceed500°F (260°C). For stainless steel valves, the recommended temperature limit is 350°F(177°C).

Insulation required on the system should be applied evenly and per drawingrequirements to avoid uneven thermal expansion, which can cause unpredictable stresson the valve.

The manufacturer’s standard practice for shipping valves may include dry packing,wet packing, or packing provided just for shipping the valves. Always specify that thevalves be shipped with dry packing. Ensure that at installation the valves are packedwith dry packing appropriate for each valve’s intended operating service. Section 2.5discusses valve packing in detail.

19.2.2 Bypasses

For intermittent operating systems, bypass lines for equipment and control valves arenot normally provided. Where bypass lines are provided, the bypass valve should be ofthe same material as the main valve or the equipment isolation valve, and at least thesame pressure-temperature rating as the main valve or equipment isolation valve. Thebypass valve and associated piping should also be of the same safety class and qualitygroup as the main valve or equipment being bypassed.

The bypass valve operator (whether manual or remote) should primarily bedetermined by a specifically defined system operational function and, secondarily, byvalve accessibility, either because of radiological considerations (ALARA) or physicallocation. Consideration should be given to providing clearance and accessibility to thebypass valve. Where ALARA radiation requirements are a concern, location of mainvalves, in addition to the bypass valve, must be considered. Centerline elevation andpitch (if any) of the bypass valve and piping should be the same as the main valve andpiping, except for steam lines where low point drainage of condensate is aconsideration.

For high energy systems, the bypass piping arrangement should be evaluated forproper consideration of thermal and other loading conditions.

Control Valve Bypass: Control valves should be installed with isolation (maintenance)valves and a bypass line (to provide an alternative flow path in the event of controlvalve failure, malfunction, or maintenance) only when the system is required forcontinuous plant operation and can perform its function without continuous



19-4

adjustment of the bypass valve. In general, the bypass valve and piping have the samecharacteristics as the main flow loop. Existing instrumentation should be used to themaximum to assess the effect when bypass control is used.

For safety-related applications, control valves should not be provided with manualoperators due to the possibility of manually changing the position or limiting theposition of the valve. A bypass line and valve should be used. For nonsafety-relatedapplications, control valves equipped with manual operators can be used in place of abypass line and bypass valve when control features of the control valve (highperformance trim) or actuator override provisions (valve pre-positioning) are desiredor required by service conditions.

Isolation Valve Bypass: An isolation valve may require a bypass for the followingreasons:

• To gradually warm up a steam line downstream of a closed valve, to ensure that thedownstream piping is properly drained, to minimize thermal stress to pipingand/or equipment, and to avoid water hammer from condensate

• To fill portions of empty lines or equipment to minimize water hammer

• To gradually warm up a liquid system downstream of a closed valve to minimizethermal stress effects to equipment

• To equalize pressure on both sides of a closed valve to minimize opening thrust

The requirements for a bypass should be established as part of system operation,including startup, system fill, and testing.

When a bypass is required, it is desirable that the bypass be specified as part of theisolation valve design and supplied by that valve manufacturer. However, the bypassvalve and piping should be shipped loose and installed in the field, except where thevalve manufacturer requires shop installation for testing (for example, seismic or flowtesting). The manufacturer should include the connections on the isolation valve, thebypass piping, and the bypass valve. Socket weld connections with bosses on theisolation valve body are the preferred method of attaching the bypass line where thebypass line size is 2 inches (150 mm) or smaller. The bypass line should be at leastSchedule 80 seamless pipe for structural strength (or of heavier schedule if required forpressure/temperature considerations) and of the same material as the main line, asrequired by ANSI B31.1 and/or ASME Section III.

Bypass lines with bypass valves may also be attached to the main line piping.



19-5

The recommended equipment bypass line and valve sizes are as follows:

Steam:

• 8-inch valve and below − 3/4-inch bypass

• l0-inch and larger valves − 1-inch or larger bypass

Gas or liquid:

• 4-inch valve and below − 1-inch bypass

• 6-inch to l0-inch valves − 1-1/2 inch bypass

• 12-inch and 14-inch valves − 2-inch bypass

• 16-inch to 20-inch valves − 3-inch bypass



• 88-inch to larger valves − 8-inch or larger bypass

• For 4-inch and larger valves, the bypass size is in general accordancewith MSS SP-45 [6.44].

19.3 Testing and Inspection Considerations

19.3.1 Shop Performance Testing

Line valves, particularly ASME III, may require performance tests or operability tests toensure proper, unimpeded operation. These tests require opening and closing valveswith and without differential pressure, and with and without external loading, tosimulate fluid system conditions. To verify smooth operation, ensure that the valveparts do not bind, and confirm that overall satisfactory operation takes place within acertain specified time. The differential pressure against which the valve operatesrepresents a load on a motor operator and affects the time to operate. The conditionsunder which the valve is to be tested for operability are specified by the user.

Section 16.2.3 discusses various code testing requirements imposed on relief valves.



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19.3.2 Pre-Operational Tests

All valves undergo a pressure test as part of the system hydrostatic test. In the openposition, the valves may be subjected to system tests at a pressure not to exceed thehydrostatic shell test pressure of ANSI B16.34. If the valves are closed and act as ahydrostatic test boundary, the system hydrostatic test pressure should not exceed the100°F rating of the valve. See ANSI B16.34 for more detail.

A reasonable testing program should include verifying that all valves are tagged withan identification plate, that they are properly packed with the packing gland adjustedcorrectly, and that they operate freely.

Consideration should be given to repacking valves after the system hydrostatic test.Although leakage through valve packing is not normally a cause for rejection for thesystem hydrostatic tests, packing is frequently tightened to stop all leakage and often ata pressure significantly higher than operational pressure. This can affect packingperformance when the plant goes into operation.

Motor-operated valves should be inspected to ensure proper wiring of the powersupply and the control switches. As necessary, motor-operated valves should be testedwith diagnostics (for example, torque and thrust measurements, motor parameters,switch operation, and/or stroke time) to evaluate design basis capability directly orthrough comparison to prototype test results.

Control valves should be inspected to ensure that they meet their calibration criteriaand that the power supply and air supply are properly connected. Prior to placing infull service, it should be verified that the proper input signal provides the properoutput signal. Sometimes a control valve can be operated and inspected when thesystem in which it is installed is only partially completed.

Motor-operated valves and certain manual valves under the scope of ASMESection XI are tested for leak tightness if limited seat leakage is a requirement.

Other installation tests, which are sometimes called pre-operational tests, arespecifically conducted on certain valves, depending on the type and importance toplant operation and safety. Many of these tests are formally performed anddocumented.

19.3.3 In-Service Test Requirements

After valves are placed in service, there are no code or standard requirements fortesting, except for ASME III or equivalent valves. These tests are required by ASMEBoiler and Pressure Vessel Code Section XI, article IWV, for leak tests and operability



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tests, and ASME Section XI, article IWB, IWC, and IWD for required post-disassemblyand post-repair pressure tests for ASME III, Class 1, 2, and 3 (or equivalent),respectively. There are regulatory test requirements in addition to these coderequirements, for instance, 10CFR50 Appendix J testing requirements for containmentisolation valves.

The owner must categorize valves per the definitions in ASME XI, Article IWV, andthen perform the required periodic operational tests, normally every 3 months, andrequired periodic leak tests, normally every 2 years, for line (as opposed to safety andrelief) valves. For safety and relief valves, testing is in accordance with ANSI/ASMEOM-1 (Requirements for In-Service Performance Testing of Nuclear Power PlantPressure Relief Devices). ASME -Section XI and ANSI/ASME OM-10 [6.31] provideinformation on test performance and acceptance. For example, acceptable leakage for aline valve, if not specified by the owner, is 30 cc per hour per inch of nominal valvesize, or, when tested with air, 7.5 standard cubic feet per day per inch of nominal valvesize.

Because of the exemptions, exceptions, or permitted deferrals contained therein orpermitted by the NRC, the edition of ASME XI to which the plant is committed shouldalways be consulted for the required details and valves that need to be tested. OM-10has now become ANSI/ASME OM (Part 10), and the current edition of ASME XI refersto ANSI/ASME OM (Part 10) for in-service testing of valves.

Overall Responsibility for In-Service Testing: The plant owner or agent is responsible for:

• Specifying the leakage-limiting boundaries

• Determining how the boundaries are to be tested

• Providing required test provisions in order to establish a test volume to conduct aleakage test

• Establishing maximum limiting stroke times, considering system function

Test Boundary and Connection Considerations: The following considerations are listedregarding the test boundary leakage rate testing provisions:

Direction of Testing. The leakage test should be in the same direction as theleakage that the limiting boundary would see when called upon to perform itsfunction. A reverse test (test from the opposite direction) or an alternative test (suchas a through-body test on a gate valve) may be used where proven to provideequivalent or more conservative results. A further discussion of reverse testing isgiven later in this section.



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A major reason for testing in an opposite direction is that the addition ofmaintenance stop valves and test connections inboard of the containment isolationvalves increases the number of potential leakage paths and creates an undesirableoperating situation. There would also be a reduction in system reliability. Systemsto which this criteria may apply are:

• Main steam

• Purge and exhaust lines

• Outside recirculation spray suction lines

• Containment spray lines

• Other large bore piping where additional valves create additional leakage pathsand costs

Another reason to develop alternate test methods in operating plants is toimplement the ALARA concept. In order to keep radiation doses for personnel “aslow as reasonably achievable,” revised test methods are often developed.

A third situation calling for alternative testing is when valves are water sealed, suchas suppression pool penetrations in boiling water reactor plants. Often these linescannot be easily drained and, therefore, should be designated as candidates forreverse testing.

Venting and Draining. Based on the safety function, a boundary may requiretesting with air when normally the system is filled with liquid. This would berequired if the system could rupture as a result of an accident and expose theleakage-limiting boundary to air. Sufficient system stop valves, test vents, and testdrain connections should be provided to minimize draining times and disposal ofsystem fluids in preparation for testing, especially those fluids containing chemicals.

The positioning of the test vent and drain connections should be carefullyconsidered. Often when these details are left to field construction to install, poor orinaccessible locations result, with drain connections off the side of the pipe versusthe bottom, or up on a vertical run as opposed to the lowest point, or connectionstoo small for draining the required volume in a reasonable time period.

On nuclear projects, it is sometimes necessary to install test vents or drains insidethe leakage limiting boundary, but this should generally be avoided. However, ifthis installation is necessary, the connection should consist of a double barrier (forexample, two valves in series; or one valve, a nipple, and a cap; or one valve, anipple, and a blind flange, etc.). These connections become part of the leakage



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limiting barrier, but, due to their infrequent use and multiple barriers, they do notrequire leakage testing as long as the barrier configuration is maintained using anadministrative control program.

Test Medium. The leakage limiting boundary should be tested with the fluids usedwhen performing its safety function. Some boundaries may require testing withseveral media, based on their services, unless one can be shown to be bounding.

Test Methods: The maintained flow rate test (air, water, or nitrogen) is the mostconservative test method and is often called “make-up test.” In this test, the test volumeis pressurized to the required pressure (Figure 19-1). Makeup of fluid to the test volumerequired to maintain test pressure is a direct measure of the entire boundary leakage.However, leakage in any path on the test boundary is assigned to the isolation valve;therefore, this measure is conservative for the valve. Other leakage sources (if any)should be investigated during the test.

Block Value

Inside Containment

TestVent

Test PanelConnection

Isolation Valve

Test Boundary

Outside Containment

Figure 19-1Test Valve Arrangement for Maintained Flowrate Test

Seat leakage in an isolation valve can be determined by measuring the flow rate in avent/drain line located between the test valve and the nearest downstream leak-tightvalve. Either a physical walk-down of the test boundary or an evaluation of themakeup flow is required to verify that the remaining test boundary leakage isacceptable. Due to system and piping constraints, seat leakage tests can be difficult toperform, and it is generally easier to perform the make-up test.

Alternative Testing of Globe Valves. If the leakage rate test pressure on a globevalve is under the valve disc, tending to unseat it, and if containment pressure tendsto seat the valve disc, then the reverse direction test method can be overlyconservative and may result in a high leak rate (Figure 19-2).



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Apply Test PressureIn Reverse Position

LOCAFlow

Figure 19-2Globe Valve Reverse Air Test (Test Pressure Under Seat)

If test conditions put pressure over the disc, then the design requirements of thevalve and the sizing of the valve actuator should be evaluated to demonstrate thatreverse testing of the valve provides equivalent or conservative results. It isrecommended that sizing of the valve operator be such that the operator seatingforce is at least three times the test pressure force when the valve is reverse tested(Figure 19-3). Operator seating force is the total stem load, which is equal to theseating thrust (including stem rejection thrust) plus the packing friction.



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Apply Test PressureIn Reverse PositionLOCA

Flow

Figure 19-3Globe Valve Reverse Air Test (Test Pressure Above Seat)

The test pressure on a flow-to-close valve pushes the disc onto the seat with a finiteforce, aiding in seat tightness. A seating force of three times the test pressure forceensures that there is some margin over and above the test force, should the valvehave to operate to isolate the containment. This force margin has been foundacceptable by the NRC at certain sites. However, verification of acceptability by theNRC should be made prior to the use of a flow-to-close valve.

Alternative Testing of Gate Valves. If a body vent test connection is provided onthe valve, then reverse testing can be considered conservative because test pressurepushes the disc away from the seating surface used during a LOCA. The cavitybetween the seating surfaces is vented and provides a direct measure of valveleakage (Figure 19-4). Body test connections may be added in the field after thevalve has been installed.



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LOCAFlow

Body VentTest Connection

LOCA Seat Apply TestPressure Here

DownstreamIsolationValve

Figure 19-4Gate Valve Reverse Air Test (With Body Vent Test Connection)

On split-wedge gate valves with body or bonnet test connections, a through-bodytest may be performed. Through-body tests are intended to measure containmentisolation valve leakage rates when test pressure is applied between the discs of split-wedge gate valves.

Pressurizing the body of a gate valve is a conservative test method because,regardless of the inboard valve seat, all leakage during a LOCA must pass by theoutboard valve seat or through the valve stem packing. The body test methodmeasures leakage past both valve seats and the valve stem packing (Figure 19-5).

NOTE: The valve disc should be thoroughly inspected at 10 year intervals to ensuredisc integrity.

LOCAFlow

Body Test ConnectionApply Test Pressure Here

LOCA Seat

Figure 19-5Gate Valve Through Body Air Test (LOCA pushes disc toward outboard seat.Through body pressurization measures leakage by both seats.)



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Alternative Testing of Butterfly Valves. Butterfly valves may be tested in thereverse direction if their seat construction is designed for sealing against pressureon either side. Verify that the valve stem packing is exposed to the test pressure forvalves that require their valve stem packing leakage to be detected.

Alternative Testing of Ball Valves. Ball valves need to be analyzed on anindividual basis to determine justification of reverse direction testing.

Test Documentation: To document justification for the testing method used, thefollowing test documentation should be acquired and kept on file:

• A letter of concurrence from the valve manufacturer that the proposed testing isconservative since the operator seating force is at least three times the test pressureforce

• A sectional assembly drawing of the valve in question

• Verification that the valve was installed as designed

• Appropriate correspondence with the NRC

• Appropriate surveillance programs for torque switch setting verification, discinspection, etc.

• Verification that the closing circuit uses a torque switch to close as opposed to alimit switch

Effects of Periodic Testing on Valves Normally Out of Service: Exercising test schedules neednot be maintained for valves in systems declared inoperable or not required to beoperable per OM-10 [6.31], paragraph 5217, “Valves in Systems Out of Service.”Continuing with the test schedule would require the following:

• Portion of system available for test

• Consequences of stroking the valve

• Scheduled repairs to the valve in the in-service inspection program

• System filled and vented unless valve can be stroked dry (not normallyrecommended practice)

• Power source available

• Procedure deviations



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Leakage tests are generally performed only when the system is out of service to permitthe necessary venting and draining. Leakage test frequency is generally 18 to 24months, except for certain types of valves that may require leakage tests as often asafter every stroke, as has been required for containment purge and vent valves on a fewnuclear plants.

Effect on Plant or System of Periodic Valve Testing: For those valves that operate in thecourse of plant operation at a frequency that would satisfy the code requirement,additional tests are not required, if the observations otherwise required for testing aremade and analyzed during such operation and are recorded in the plant records at therequired intervals (see OM-10, paragraph 5215) [6.31].

For valves in standby systems, the following problems can develop:

• Potential over-pressurization

• Damage due to dry stroking

• Creation of transients

• Vibration problems (check valves) due to low flow conditions

Valve Testing Systems: Because of difficulties in gaining meaningful information on theexistence of a problem or gathering data to analyze parameters to diagnose a problem ifa problem is detected, there has been extensive research and development of testingand diagnostic systems.

The most advanced types of testing and analysis systems are those available to test andanalyze motor-operated valves. There are at least five testing and analysis systemsavailable from MOVATS, Liberty Technology, Limitorque, Impell, and Wyle Labs. Theoperation of these systems varies, but all essentially provide data on thrust output ofthe operator, time-history of the operation of torque and limit switches, and motoramperage. This data can be used to determine that the operator is delivering therequired thrust, that the switches are operating in the precise sequence, as well asindication of degradation of certain elements of the assembly (for example, damage togearing and bent stem).

Other diagnostic systems, used with mixed success, utilize acoustic emission and noisetechnology to determine valve seat leakage and to establish the dynamic condition ofcheck valve intervals.

Ultrasonic techniques are being used to establish the position of the closure member ofa check valve.



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Radiography has been used to establish valve closure member position and to verifythe integrity of valve internals.

19.4 Maintenance Requirements

19.4.1 Separation and Maintenance

Although it may not be possible to attain ideal separation and maintenance conditions,proper separation of valves is important. Valves should be separated from one anotherand from other equipment and piping to ensure no interference with their movingparts. In addition, this separation should be adequate to disassemble the valve andoperator. This separation should also allow unimpeded access and egress for operation,adjustment, maintenance, repair, or examination of the valve assembly (see Table 19-1and Figures 19-6 to 19-8). Maintenance access that should be provided includes:

• Access for adjustment of packing or repacking

• Clear access to turn handwheel, including handwheel provided with a motoroperator

• Access to and clearance for swing of clutch lever for motor operators

• Access to pipe plugs on the gear case of the motor operator (to inspect for qualityand quantity of grease)

• Access to remove limit switch torque switch covers on motor operators

• Egress for removal of valve and operators for maintenance or repair



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Table 19-1Valve Maintenance Clearance Data



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Table 19-1 (Continued)Valve Maintenance Clearance Data



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Table 19-1 (Continued)Valve Maintenance Clearance Data



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Figure 19-6Required Valve Maintenance Clearance for Typical Installation



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Figure 19-7Required Maintenance Clearance for Chain-Operated Valve



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Figure 19-8Human Factors Clearance-General



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Consideration also has to be given to separation from exposure to backgroundradioactivity. Valves, particularly potentially high maintenance valves, should belocated in a low radiation area, within the bounds of reasonable plant layout orshielding should be provided. In addition, valves that, if by themselves, are lowradiation items should not be located in high radiation areas.

Preventive and corrective maintenance requires reasonable personnel access to valves,including clearances for rigging provisions for valve removal. Locating valves toprovide access from normal walkways without blocking these areas should beconsidered. Where this is not practical, platforms or ladders with a flat landing shouldbe used. Valves should not be located where access requires climbing over componentsor portions of piping systems.

The type of preventive or corrective maintenance that has to be performed should beconsidered when arranging the platform or landing. If the valve or actuator is large insize, adequate room and provision to remove heavy components, such as the actuator,valve bonnet, and valve internals, should be provided. Provision and clearances forappropriate rigging should be considered in the design layout. In addition, such itemsas a motor-operator limit switch cover can be deceptively heavy and require care whenremoving to avoid damage to the internal wiring.

The work area at a valve, as well as clear access, is very important. It has beendetermined that adequate work space can reduce maintenance time by one-third.Access and egress routes to and from the valve and work area should be adequate toget equipment in and out. In nuclear power plants, personnel access can also beadversely affected by encumbrances such as protective clothing, face shield, andbreathing apparatus.

Auxiliary services such as breathing air, compressed air, water, and other compressedgas should be readily available to perform preventive and corrective maintenance. Thestations for these services should be no further than 50 feet (15 m) from where they areneeded. Note that some valves require these services to function properly. For example,some hydraulic fail-close actuators utilize compressed nitrogen as the stored energy.This nitrogen must be periodically replenished. There should be compressed nitrogenbottles and regulators in the area to avoid having to bring this equipment into the area.

Adequate illumination and lifting pads, hoists, trolleys, rails, or other means to liftheavy equipment should also be provided.

19.4.2 General Good Maintenance Practices

Continued valve performance is best ensured by effective and efficient maintenanceand early correction of any malfunction. Thus, maintenance can be divided into



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preventive and corrective maintenance. Corrective maintenance is the more widelyunderstood and practiced since it covers the correction of established deficiencies.However, in a nuclear plant where access to equipment may be limited duringoperation because of radiation, it is desirable to conduct maintenance operations duringscheduled outages, such as a refueling period, when the plant is shut down for otherpurposes. Therefore, emphasis should be on preventive maintenance.

Preventive maintenance requires a continuing assessment of component performance.This assessment should involve an attempt to identify deficiencies so that they may becorrected during scheduled outages. As an example, assume that a refueling period,during which maintenance can be performed without entailing unscheduled downtime, is scheduled in one month. Valves that are suspected “leakers” (that is, over theleakage specification requirements but within tolerable limits) should be scheduled forseat and disc lapping during the refueling period. This will permit the correction of aminor deficiency before it becomes a deficiency that might require a plant shutdown. Ifthe normal useful life of a gasket or set of packing is to be reached shortly after ascheduled downtime, it is also desirable to perform this replacement work during thescheduled outage. Gaskets or seals should never be reused, unless specificallyrecommended by the manufacturer, particularly spiral wound metal gaskets andpressure seal bonnet seal rings. Reuse of gaskets or seals has resulted in leakage of thevalve and an unscheduled shutdown.

The valves in a system should be tested and inspected on a routine basis as part of aneffective preventive maintenance program. These examinations should be run beforescheduled outages to identify areas of potential difficulty, and their timing should beworked into the schedule of plant operations on a “not-to-delay” basis. Examples ofexaminations that might be made are:

• Leakage tests/inspections of valve seats, backseats, and packing

• Operability verification for freedom of movement, unusual noises, or vibrations

• Tests to verify that opening and/or closing times are within prescribed limits

Many valve problems can be detected during a walkdown, especially during plantstart-up and shut-down. Valve problems can be predicted by trending available data.

To augment an effective preventive maintenance program, it is desirable to maintain avalve history file containing records of corrective and preventive maintenance workperformed on all valves, so that the performance of each valve can be evaluated. Theserecords also help to develop and identify proper intervals for certain preventivemaintenance operations.



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When performing maintenance on a valve, the area around the valve should be as cleanas possible to prevent the entrance of foreign material from the surroundingenvironment into the valve. The valve should be thoroughly inspected and cleaned justprior to reassembly.

Since maintenance and plant operations may be carried out simultaneously, it isimportant that adequate safeguards be established for protection of personnel. Aspreviously stated, all work on radioactive systems must be in compliance withradiation control procedures. Administrative procedures should be developed tospecify the required degree of isolation from operating systems when maintenance isperformed.

The operations department should prepare specific instructions for each maintenanceoperation. These instructions should reflect the pressure and temperature conditions inthe operating systems from which isolation is desired and identify the valves to beshut, tagged, etc. The maintenance department should require a copy of thisinstruction, certified as completed, before maintenance is started. When themaintenance work has been completed, it should be carefully inspected and themaintenance personnel should certify to the operating personnel that the work has beencompleted, isolation of the sections can be secured, and the section re-pressurized.


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20 DIAGNOSTIC EQUIPMENT AND METHODS

20.1 Introduction

Proper assessment of valve condition or malfunction is highly dependent on the toolsused for diagnosis. Based on the diagnostic methods used, the assessment can be eitherqualitative or quantitative and either static or dynamic. Prior to the issuance of NRCBulletin 85-03 in 1985, not many tools were available that could easily quantify therequired thrust or torque to actuate a valve. Since then, several tools have beendeveloped and refined to the point where accurate quantification of required thrust ortorque is now easily achievable.

In addition to these quantitative tools, conventional tools such as boroscopes continueto be used when internal valve inspection is required or when valve disassembly is notpractical. The tools covered in this section deal primarily with the valve types coveredin this guide. Diagnostic tools for air-operated valves, safety and relief valves, solenoid,and check valves are covered respectively in EPRI documents identified in References1.2, 1.4, 1.7, 1.20, and 1.21 respectively.

Diagnostic equipment in most cases is temporarily mounted or attached to thevalve/actuator but can also be permanently mounted for continuous monitoring.Permanent monitoring is used on valves that:

• Have low operating margins

• Need to be trended

• Have high safety significance

• Are located in high radiation areas (ALARA concerns) or are difficult to access

• Require excessive maintenance or have random problems

• Are critical to power generation

• Require leakage assessment and correlation to thrust


Diagnostic Equipment and Methods

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20.2 Equipment

20.2.1 Boroscopes

The boroscope is probably one of the earliest tools used to inspect valves in line. Theboroscope can be inserted into the valve through drain penetrations in the valve bodyor through line fittings located close to the valve. Depending on the type of boroscopeused, these examinations can range from qualitative to some limited level ofquantification. Boroscopes can be equipped with a graduated reticle that can measurelinear indications, but the range of measurement is quite limited. In most cases, the useof a boroscope requires that the valve and system be depressurized and drained so thatbody or line penetrations can be removed for insertion of the boroscope.

20.2.2 Radiography

Radiography inspection is possibly the oldest method of nonintrusive valveexamination. It is the easiest to perform because it does not require that the valve orsystem be depressurized. Depending on valve location, radiography inspections can beperformed during plant operation. Images captured by radiography are not affected bythe fluid medium. Radiography examinations are usually qualitative in nature, but canprovide some reasonable accuracy in gross measurements. Radiography has theadvantage of covering a larger area than boroscope examinations, but it lacks the depthperception of the boroscope. In many cases, this examination method is used todetermine the position of the closure element or to determine if the stem hasdisengaged from the closure element.

Depending on valve size, this examination method can be used to determine the needfor a more detailed inspection via valve disassembly. Radiography is usually not usedon large valves because the combined wall thickness limits the clarity of images.Conventional radiographic examination records are produced by passing x-rays orgamma rays through the valve and making a permanent image on a single use film.

Newer methods of radiography permit reusable phosphor plate screens that capturethe image by an “electron trapping” method. The image on the screen can be opticallyread by scanning the screen with a focused laser beam. This information can also bedigitized for further manipulation and viewing on a computer monitor. The phosphorplate can then be erased and reused.

20.2.3 Acoustics

In the past, acoustic monitoring had limited use in power plant applications because ofthe interference caused by high ambient noise. New methods of filtering ambient noise



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have made its usage more popular. Acoustic monitoring utilizes an accelerometerattached to the body to measure acoustic emissions that are generated when solidscontact each other or when liquids or gases flow through pipes and fittings. Acousticemissions can be used to make qualitative leakage assessments by comparing theacoustic levels from the upstream side of the valve to the acoustics levels on thedownstream side of the valve.

Acoustic techniques are easy to set up and usually take only minutes since they requireno valve intrusion or adaptations. Acoustics have been used extensively in check valvenonintrusive diagnosis for monitoring disk oscillations and impacts, and in safety reliefvalves for measuring leakage and operation. Acoustic monitoring is discussed in moredetail in References 1.2, 1.20, and 1.21.

20.2.4 Temperature Monitoring

Thermocouples can also be used to detect leakage in applications where the nominaldownstream temperature is significantly different from the upstream temperature.Hot/cold fluid escaping into the downstream side of the valve would be registered as atemperature change in the localized region of the leak. Installation of thermocouplesrequires that the pressure boundary be penetrated and that the system bedepressurized.

Surface temperature monitoring using infrared thermography is used extensively insafety valves to map external surface temperatures on the valve. These temperatureprofiles are used to correlate valve performance as a function of temperature gradientson the valve body. See References 1.29 and 1.4 for detailed information about the use ofthermography.

20.2.5 Ultrasonics

Ultrasonic sensors are used to determine the relative position of internal components.This technique uses sound reflections of the internal components to characterize theirconfiguration but cannot provide accurate absolute positions. This technique is limitedto use on valves made from carbon steel or fine grain stainless steel and installed inliquid service such as water (that is, cannot be used in air or steam systems). Thistechnique is primarily used in check valves to determine the disc fluctuations as afunction of flow velocity. More detailed information can be found in References 1.20and 1.21.



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20.2.6 Stem Thrust/Torque Measurement Devices

The thrust/torque required to actuate a valve is the most important variable indetermining the health of a valve. Healthy valves exhibit smooth and repeatablethrust/torque requirements, while damaged valves exhibit erratic or nonrepeatablebehavior. Several tools are commercially available, and the selection of a specific tool tobe used depends on several factors including:

• Valve type: Gate, globe, ball, butterfly, etc.

• Feasibility: Can the sensor or instrument be easily attached on either the valve oractuator?

• Accuracy: How accurate must the measurement be?

• Schedule: Can the sensor or instrument be mounted in a reasonable time?

• Valve operability: Will the sensor or instrument impact valve operability?

• Availability: Is the sensor or instrument readily available?

• Cost: Is the cost justified?

20.3 Methods for Measuring Stem Thrust/Torque

Diagnostic methods include: (1) sensing spring pack displacement, yoke strain, andstem strain or (2) installing a load measurement device between the actuator base andthe yoke upper flange.

20.3.1 Spring Pack Displacement

Stem torque measurements and stem thrust estimates are most easily performed usingspring pack displacement. This method indirectly measures stem torque by sensingspring pack axial displacement and correlating it to the tangential force on the actuatorworm gear. Using the worm gear geometry of the actuator, this tangential force is thenconverted to stem torque.

In rising stem valves, the torque is converted to thrust using the stem thread geometryand stem-to-stem nut coefficient of friction. Using spring pack displacement to measurestem torque is limited to Limitorque actuators.

Thrust measurements using this device are not accurate because of assumptions madein the stem thread coefficient of friction, internal losses in the actuator, and the methodof calibration. Inaccuracies as high 40% have been observed in some installations. The



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method also has the disadvantage of being unable to measure loads below the initialspring pack assembly preload and can therefore not yield packing load levels in manyvalves. The spring pack displacement method is also unable to measure loads above theload required to compress the spring pack to its solid height, which in turn results inthe inability to capture maximum thrust developed. This method has severaladvantages including:

• It can be used with any type valve.

• It requires minimal actuator modifications.

• It does not require any valve modifications.

20.3.2 Strain Measurement of the Yoke Legs

With this method, stem thrust is determined indirectly by measuring the strain in theyoke legs. This is accomplished by mounting strain sensors (such as strain gauges) onthe yoke legs and then correlating the measured strain to stem thrust. No stem torquemeasurements can be made using this method, and the calibration range in the valveopening direction is limited because of the technique used to calibrate the strain in theyoke legs. Primary calibration of the yoke strain is achieved by loading the disc againstthe seat, which subjects the yoke legs to tension. In the opening direction, the amount ofload is only a fraction of the closing direction load, which limits the calibration range.

Stem thrust measurement using yoke leg strain is limited to valves installed with thestems in the vertical orientation. This measurement technique is quite sensitive tolateral load on the yoke, and care must be taken to minimize lateral load effects.Additionally, high level vibrations cause the weight of the upper works to register asstem thrust. This phantom thrust is difficult to filter from the actual thrust signatureand can lead to erroneous assessments of the actual required thrust or infer that thevalve is behaving erratically or unpredictably. This method of measuring stem loaddoes not require any modifications to the valve or actuator but does require calibrationof yoke strain before each test.

20.3.3 Strain Measurement of the Stem

Strain measurement of the stem is the most accurate and direct method of determiningstem thrust/torque. This is accomplished by directly mounting strain gauges on thestem or by attaching a strain sensing transducer to the stem. Axial strain sensors areeasily installed, require no modification to either the valve or actuator, and can beinstalled on either the smooth or threaded portion of the stem. However, axial strainsensors may affect the stroke length of the valve due to their relatively larger size.Strain sensing transducers can measure only axial strain in the stem; thus, they are



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capable only of measuring stem thrust. Strain gauges mounted directly on the stem canyield individual measurements of thrust and torque.

Two methods are used to attach strain gauges on the stem. In the first method, thestrain gauges are premounted on a strip which is then bonded to the stem. This methodallows installation on the stem without removing the stem from the valve, but it yieldsmeasurement accuracies of about +/- 5%.

In the second method, the strain gauges are bonded directly to the stem, whichtypically requires that the stem be removed from the valve. Bonding the strain gaugesdirectly on the stem yields the highest accuracy in measurement, especially when thestem is removed from the valve. Accuracies as high as +/- 0.5% are typically achievablewith directly mounted strain gauges.

The disadvantage of this method is that the strain gauges may interfere with valvestroking if there is insufficient stem length between the bottom of the actuator and thetop of the packing follower. If complete stroking is required and the smooth portion ofthe stem is not long enough, then some of the stem threads may have to be removed topermit installation of the gauges at the thread root diameter. Depending on availablespace, threads can be machined using special tools without removing the stem from thevalve.

20.3.4 Load Measurement at the Actuator Base

Stem thrust and torque can also be determined by measuring the load at the base of theactuator. This measurement is attained by mounting a precalibrated sensor between theactuator and valve yoke upper flange. Mounting of the sensor requires that the actuatorbe removed from the valve and can only be used in valves that have enough stemlength to accommodate the displaced height of the actuator. Although thrust andtorque measurement accuracies as high as +/- 0.5% can be attained using this device,displacing the actuator from its normal position subjects different stem threads to loadat the fully closed position than those in contact during normal valve operation withoutthe sensors in place. The effect of this difference is that the stem factor at the closedposition may be different, resulting in different closing thrusts for the same torqueswitch setting.

The advantage of this method is that it can be used on almost any type of valve oractuator if the stem is long enough to permit actuator engagement throughout the valvestroke. One size unit can be used on several actuator sizes by the use of simple flangeadapters. This method is commonly used to measure quarter-turn valve (butterfly andball valves) torque and to calibrate strain gauged stems and yokes



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20.3.5 Electric Motor Power Monitor

Changes in MOV performance can be measured using electric motor power traces.During initial static/dynamic valve testing, motor power traces are captured andarchived. These traces are then compared against subsequent traces to determinechanges in valve/actuator performance.

Sensors used to measure motor power require no modifications to the valve or actuator,can be easily removed or installed even during plant operation, and are relativelyinexpensive compared to other means of monitoring MOV performance.

The disadvantage of electric motor power traces is that they provide a measure ofoverall MOV performance and changes in the traces cannot be easily attributed toeither the valve or electric actuator.

20.3.6 Diaphragm/Piston Pressure

In air-operated valves (AOVs) or hydraulically operated valves, the diaphragm/pistonpressure can be measured using pressure gauges or transducers to indirectly measureactuator output. This method does not require changes to the valve and only minimalchanges, if any, to the actuator. The sensors used to measure pressure can be easilyinstalled or removed even during plant operation. They are relatively inexpensivecompared to other means of monitoring valve performance but they are not as accurate.Measuring pressure to determine actuator output can lead to false indications ofapplied thrust/torque to the stem if the actuator bottoms out at the end of its stroke.More detailed information on diagnostic equipment for AOVs is given in Reference 1.2.

20.3.7 Data Acquisition

Signals from each of the sensors described above can be captured, archived, andanalyzed using computerized portable data acquisition systems. These systemstypically acquire up to eight different signals at nominal rates of 1,000 samples persecond. Depending on the system configuration, sampling duration, and the number ofchannels acquired, these data systems can capture data at rates as high as 100,000samples per second; however, sample rates of 1,000 per second are usually fast enoughto capture transient valve and actuator characteristics. Generally, these systems areused to acquire the following signals:

• Stem thrust

• Stem torque

• Motor current



20-8

• Torque switch trip

• Spring pack displacement

• Pressure

• Flow rate

• Stem position

• Motor torque

• Motor speed

• Diaphragm/piston pressure

• Acoustic level

• Sound level

• Temperature

Data acquisition is normally initiated manually by the test engineer but can also betriggered automatically. Automatic triggering is accomplished by initiating dataacquisition when a threshold value is exceeded in the selected channels. However, falseindications can create problems with automatic triggering because data acquisition canbe initiated by spurious signal spikes. Most data acquisition systems can also export thedata for use with other data analysis software.

20.4 Summary

Significant technological advances have been made in diagnostic equipment for varioustypes of valves. These advances provide the user with more options and accuracy toassess the condition and determine the performance of the valve/actuator. A summaryof selected diagnostic methods is presented in Table 20-1. The user is encouraged toconsult equipment vendors for detailed information on these tools and for newtechnology being developed.

Permanent installation of diagnostic equipment permits continuous monitoring ofvalve/actuator performance during plant operation to verify and trend valveperformance, minimize radiation exposure, and improve plant availability.


Table 20-1Comparison of Selected Diagnostic Methods

DiagnosticMethod

Application Accuracy Limitation Advantages Disadvantages

Boroscope Internal visualsurfaceinspection (forex., seats, guides,discs, stem)

Depends onlens power.This device istypically notused for takingmeasurement.

Cannot be usedwithvalve/systemunder pressure.

Dynamic 3D visual examination. Visual examination is limited tophysically accessible locations.

Radiography Internalinspection

Qualitative. Not effective forlarge valves andthick walls.

Non-intrusive. Can be used in any typefluid.

Time consuming to set up.

Acoustics Audible signals Qualitative. Can only detectaudible signalssuch as leaks,hard contacts,tapping, etc.

Non-intrusive. Can be used in any typefluid.

Can not be used for internalexamination. Can not verify thatinternal components are atcorrect locations. Requirebaseline test data for accuratesignature analysis.

Thermocouples Leak detectionby measuringtemperaturegradient

Temperaturemeasurementis accurate butcorrelation toleakage isqualitative.

Internaltemperaturemeasurementsrequire pressureboundarypenetrationduringinstallation.

Uses conventional sensors andinstrumentation.

Thermocouples should not beleft installed permanently whensubjected to flow. Pressureboundary penetration becomespotential leak path.

Ultrasonics Identify locationof internalcomponents

+/- 10% of fullstroke length.

Fluid in valvemust be liquid;valve should beof carbon steel orfine grainstainless steel.

Non-intrusive, requires no valvemodification, results are repeatable, andsignal can be calibrated without priortesting.

Signal path may be limited byvalve geometry; entire strokepath may require multiplesetups; need as built valvedrawing and calibration beforeand after each test.

Stemthrust/torquemeasurementdevices

Accuratelymeasure appliedstemthrust/torque

From +/- 0.5%to +/-40%depending onsystem.

Requires slightmodification tothe actuator,stem, or yoke.

Can accurately measure actuator thrust/torque output and can be used with anytype valve. Can be permanently mountedfor continuous monitoring of stem load.

Depending on sensor type, itmay require valve disassembly;may limit stem stroke; can betime consuming to install.


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21 VALVE SELECTION GUIDELINE CHARTS

The valve selection chart (Figure 21-1 and Table 21-1) is intended to provide the userwith a simplified, logical way to select a valve. Before using the selection chart, it isadvantageous for the user to review the sections of this document that are pertinent tothe type of valve being selected. Selection of control and pressure relief valves is notfully covered by the selection chart. For control and pressure relief valves, selectiondepends on body type, pressure/temperature rating, and material; but since theselection of other features is more complex and requires calculations, control and safetyvalves cannot be fully covered by a logic decision chart.

Before starting the selection process, information on system, fluid, piping material,system design conditions, pipe size, and environmental conditions should be available.

The selection process should start from the top of the selection chart marked “Start” andcontinue to the end, using Table 21-1 as a source of information to answer questionsabout valve function and performance and about availability in a particular size,pressure and temperature rating, and material. Supporting charts are provided forselection of valve body material and valve actuators. Table 21-1 contains currentinformation, but valve availability in certain designs, materials, and pressure ratingsmay change with time. Cost information should be used with caution since costinformation does not take into consideration maintenance cost, which can significantlychange cost calculations.

As a result of the selection process, a ranking list can be developed that should containat least one, but most likely more than one, valve that can be successfully used. At thispoint, a top ranking valve may be chosen or, at the purchaser’s decision, other aspectsmay be considered (for example, delivery time, economic cost, etc.).

Figure 21-1Valve Selection Chart(This figure is located in a pouch inside the back cover of this report.)


Valve Selection Guideline Charts

21-2

Table 21-1Valve Selection Matrix



21-3

Table 21-1 (cont.)Valve Selection Matrix



21-4

Table 21-1 (cont.)Valve Selection Matrix



21-5

Example 1Selection of Service Water Main Pump Isolation Valve

Step 1 - Collect data in accordance with block one on the valve selection matrix (Table21-1).

1. System: Service water

2. Fluid: Seawater

3. Pipe: Cu-Ni clad pipe

4. System design condition: 100 psig, 95°F

5. Size: 30 inch

6. Environment: Salt water, no radiation

Step 2 - Determine the primary function of the valve.

The valve isolates the non-operational train; therefore, the primary function isisolation.

From Table 21-1, valves suitable for isolation are gate, globe, butterfly,diaphragm, plug, ball, sealed gate, and sealed globe.

Step 3 - Select the valve size.

The valve size will be equal to pipe nominal size (30 inches).

From Table 21-1, 30-inch size, globe, diaphragm, ball, plug, sealed gate, andsealed globe valves are not available, leaving gate and butterfly valves available.

Step 4 - Determine the valve pressure class.

Based on the design conditions, 150-pound pressure class is sufficient.

From Table 21-1, both gate and butterfly valves are available in this pressureclass.

Step 5 - Select the valve material.

Based on the material selection chart, materials suitable for this application arecopper alloys, nickel alloys, and high molybdenum austenitic steel.



21-6

Aluminum bronze is selected for the valve body material, based on previousexperience with this material.

Step 6 - Select the performance required.

1. Pressure drop in open position: low

2. Seat tightness: good

3. Maintainability: good

Based on information from Table 21-1, three ranking lists are created:

Pressure Drop Seat Tightness Maintainability

1) Butterfly 1) Butterfly 1) Butterfly

1) Gate 2) Gate 2) Gate

(no difference) (no significant difference)

Step 7 - Combine the ranking lists into one overall ranking list.

There is no significant difference between the valves for the first twocharacteristics; however, butterfly valves are somewhat easier to maintain.

Therefore, the final ranking is:

1. Butterfly

2. Gate

Step 8 - Is the valve size/weight a concern?

Valve size/weight is a concern because of limited space in the pumphouse.

From Table 21-1, butterfly valves are more compact and lighter than gate valves,and the ranking remains unchanged.

Step 9 - Secondary function.

There is a possibility that the valve may be required to throttle and, in this case, abutterfly valve is better.

The ranking remains unchanged.



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Step 10 - Final ranking.

1. Butterfly

2. Gate

Step 11 - Follow the butterfly valve-specific chart.

Step 12 - Leak tightness is required; therefore, an eccentric disc design is selected.

Steps 13 and 14 - Environmental conditions (temperature 95°F and no radiation) aresuitable for resilient seat.

Step 15 - This valve is an on-line valve; therefore, a lug or wafer design is selected.

Step 16 - Return to the main chart, and select an actuator using the actuator selectionchart. The valve is not required to fail open or closed, and fast speed is notrequired. A motor operator is selected.

Step 17 - Incorporate all data into the valve data sheet (see Section 25).



21-8

Example 2Selection of an Isolation Valve at the Inlet toMoisture Separator Reheater in the Main Steam System

Step 1 - Collect data in accordance with block one on the valve selection chart.

1. System: Main Steam

2. Fluid: Saturated steam

3. Pipe: Carbon steel


5. Size: 16 inch

6. Environment: Mild, no radiation



From Table 21-1, valves suitable for isolation are gate, globe, butterfly, diaphragm,plug, ball, sealed gate, and sealed globe.



From Table 21-1, 16-inch size globe, diaphragm, plug, sealed gate, and sealed globevalves are not available, leaving ball, gate, and butterfly valves available.



From Table 21-1, both gate and ball valves are available in this valve pressure class.Butterfly valve is deleted because it is available only as a special design.


Based on the material selection chart, a material suitable for this application iscarbon steel.



21-9


1. Pressure drop in open position: low

2. Seat tightness: good

3. Maintainability: good

Based on information from the Table 21-1, three ranking lists are created:

Pressure Drop Seat Tightness Maintainability

1) Ball 1) Ball 1) Ball

1) Gate 2) Gate 2) Gate

(no difference) (no significant difference) (no significant difference)

Step 7 - Combine the ranking lists into one overall list.

There is no significant difference in ball or gate valve characteristics.

Therefore, the final ranking is:

1. Ball

2. Gate


Valve size/weight is a concern because of limited space and the need for additionalsupports.

From Table 21-1, gate valves are more compact and lighter than ball valves.

The ranking is changed to put gate above ball valve.

Step 9 - Secondary function - None.


1. Gate

2. Ball



21-10

The ranking is based on gate valve superiority in size/weight ranking and itsavailability on the market.

Step 11 - Follow gate valve-specific chart.

Step 12 - For sizes larger than 3 inches, a bolted bonnet design is selected because thetemperature is 600°F, which is below the 700°F required for a pressure seal bonnet.

Step 13 - Outside stem and yoke design is selected since it is the only one available.

Step 14 - Select the wedge/disc type.

In the closed position, the pressure differential is expected to be high and line loadsare expected to be significant; therefore, based on the chart, the following valvesmay be used: split wedge, flexible wedge, or parallel slide double disc. The finaldecision should be based on availability, price, and plant preference.

Step 15 - Select the seat material.

Because steam is not considered a corrosive fluid and no erosion and cavitation isexpected, any manufacturer-selected material for this application is acceptable.

Step 16 - Return to the main chart, and select an actuator using the actuator selectionchart. The valve is not required to fail open or closed, and fast speed is not required.Motor operator is selected. The thrust calculated does not exceed 500,000 pounds.

Step 17 - Incorporate all the data into the valve data sheet (see Section 25).



21-11

Example 3Selection of a Manually Operated 3-inchIsolation Valve in Liquid Waste System

Step 1 - Collect data in accordance with block one on the valve selection chart.

1. System: Liquid waste

2. Fluid: Water

3. Pipe: Stainless Steel


5. Size: 3 inch

6. Environment: Radiation 108 rads/over plant life



From Table 21-1, valves suitable for isolation are gate, globe, butterfly, diaphragm,plug, ball, sealed gate, and sealed globe.





From Table 21-1, all valves are available in this pressure class.


Based on the material selection chart, a material suitable for this application isstainless steel.


1. Stem leakage - low



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2. Seat tightness - good

3. Maintainability - good

Based on information from Table 21-1, three ranking lists are created:

Stem Leakage Seat Tightness Maintainability

1. Diaphragm

2. Bellows sealed gate

3. Bellows sealed globe

4. Diaphragm sealed globe

5. Ball

6. Plug

7. Gate

8. Globe

1. Diaphragm

2. Ball

3. Plug

4. Globe



7. Gate


1. Ball

2. Diaphragm

3. Plug

4. Gate

5. Globe




Step 7 - Combine the ranking lists into one overall ranking list. After considering allaspects of the valve characteristics, the following ranking is established:

1. Diaphragm

2. Ball

3. Plug

Other valves will not be considered for the following reasons:

• Gate and globe valves create a potential for radioactive leaks.

• Diaphragm sealed globe, bellows sealed gate, and bellows sealed globe areexpensive and difficult to maintain. This application does not require such a highdegree of leak tightness.



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• A diaphragm, plug, or ball valve provides sufficient assurance of stem leaktightness in the water application.


No, valve size/weight is not a concern. In this size, there is no significant differencein weight/size between diaphragm, ball, and plug valves.

Step 9 - Secondary function - None.


1. Diaphragm

2. Ball

3. Plug

Step 11 - Follow the diaphragm valve-specific chart. A diaphragm valve is rejectedbecause it cannot be used for this radiation level based on the manufacturer’sapplication chart.

Follow the ball valve-specific chart. A ball valve cannot be used because it createsthe potential for crud traps.

Step 12 - Follow the plug valve specific chart.

Step 13 - Precise throttling is not required.

Step 14 - The fluid is water.

Step 15 - Based on information about elastomers in Section 2, polyethylene or EPT canbe used for the valve sleeve.

A plug valve is suitable for this application.

Step 16 - Return to the main chart.

Step 17 - Incorporate all the data into the valve data sheet (see Section 25).


22-1

22 REFERENCES AND BIBLIOGRAPHY

22.1 EPRI / NMAC Reports

These EPRI/NMAC reports are available only to members of the Electric PowerResearch Institute. Although some of these reports are obsolete or out of print, they areincluded to show how valve and actuator technology has evolved over the last fewyears.

1.1 In Situ State-of-the-Art Valve Welding Repair (Gate, Globe, & Check Valves), Volume 2.EPRI, Palo Alto, CA: December 1996. Report TR-105852V2.

1.2 Air-Operated Valve Maintenance Guide. EPRI, Palo Alto, CA: November 1996.Report NP-7412.

1.3 Maintenance Job Cards; Joint EPRI-CRIEPI Human Factor Studies. EPRI, Palo Alto,CA: December 1994. Report TR-104602.

1.4 Safety and Relief Valve Testing and Maintenance Guide. EPRI, Palo Alto, CA: August1996. Report TR-105872.

1.5 Application Guide for Motor-Operated Valves in Nuclear Power Plants. EPRI, PaloAlto, CA. Report TR-106563-V1. Volume 1: Gate and Globe Valves, published in1998. (This is Revision 1 of EPRI NP-6660, March 1990.)

1.6 Application Guide for Motor-Operated Valves in Nuclear Power Plants. EPRI, PaloAlto, CA. Report TR-106563-V2. Volume 2: Butterfly Valves, October 1998. (This isRevision 1 of EPRI NP-7501, January 1993.)

1.7 Solenoid Valve Maintenance and Application Guide. EPRI, Palo Alto, CA: April 1992.Report NP-7414.

1.8 Predictive Maintenance Primer. EPRI, Palo Alto, CA: April 1991. Report NP-7205.

1.9 The Maintenance Engineer Fundamentals Handbook. EPRI, Palo Alto, CA: November1996. Report TR-106853.


References and Bibliography

22-2

1.10 EPRI Workshop on Erosion-Corrosion of Carbon Steel Piping, April 14–15, 1987,Washington, DC.

1.11 Assessing Maintenance Effectiveness. EPRI, Palo Alto, CA: December 1996. ReportTR-107759.

1.12 Lubrication Guide, Revision 2. EPRI, Palo Alto, CA: February 1995. Report NP-4916.

1.13 Postmaintenance Testing, A Reference Guide. EPRI, Palo Alto, CA: April 1991. ReportNP-7213s.

1.14 Static Seals Maintenance Guide. EPRI, Palo Alto, CA: December 1994. Report TR-104749.

1.15 Valve Stem Packing Improvements. EPRI, Palo Alto, CA: March 1988. Report NP-5697.

1.16 On-Line Leak Sealing, A Guide for Nuclear Power Plant Maintenance Personnel. EPRI,Palo Alto, CA: July 1989. Report NP-6523.

1.17 Bolted Joint Maintenance & Application Guide. EPRI, Palo Alto, CA: December 1995.Report TR-104213.

1.18 How to Conduct Material Condition Inspections. EPRI, Palo Alto, CA: September1994. Report TR-104514.

1.19 Development of a Honing Tool for Main Steam Isolation Valve Seats. EPRI, Palo Alto,CA: November 1983. Report NP-3291.

1.20 Application Guide for Check Valves in Nuclear Power Plants. EPRI, Palo Alto, CA:June 1993. Report NP-5479, Revision 1.

1.21 Check Valve Maintenance Guide. EPRI, Palo Alto, CA: August 1995. Report TR-100857.

1.22 Technical Repair Guidelines for Limitorque Valve Operator Model SMB 000. EPRI, PaloAlto, CA: December 1994. Report NP-6229R1, Revision 1.

1.23 Technical Repair Guidelines for Limitorque Valve Operator Model SMB 00. EPRI, PaloAlto, CA: June 1995. Report NP-6231R1, Revision 1.

1.24 Technical Repair Guidelines for Limitorque Valve Operator Model SMB 0-4. EPRI, PaloAlto, CA: May 1993. Report NP-7214.



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1.25 Technical Repair Guidelines for Limitorque Valve Operator Model HBC 0-10. EPRI, PaloAlto, CA: December 1993. Report TR-100539.

1.26 Technical Repair Guidelines for Rotork Valve Actuators. EPRI, Palo Alto, CA:December 1995. Report TR-104884.

1.27 Anchor/Darling MSIV Guide. EPRI, Palo Alto, CA: May 1991. Report NP-7211.

1.28 Anchor/Darling MFIV Guide. EPRI, Palo Alto, CA: May 1991. Report NP-7212.

1.29 Infrared Thermography Guide. EPRI, Palo Alto, CA: December 1994. Report NP-6973,Revision 2.

22.2 Proprietary Documents Developed under EPRI MOV Performance Prediction Program

For information concerning these documents, contact the EPRI MOV PPP projectmanager, John Hosler, 3412 Hillview, Palo Alto, CA 94303; telephone: 650/855-2785;e-mail: [email protected].

2.1. EPRI MOV Performance Prediction Program: Topical Report. EPRI, Palo Alto, CA:April 1997. Report TR-103237-R2, Revision 2.

2.2 EPRI MOV Performance Prediction Program: Gate Valve Model Description Report.EPRI, Palo Alto, CA: November 1994. Report TR-103229.

2.3 EPRI MOV Performance Prediction Program: Globe Valve Model Report. EPRI, PaloAlto, CA: April 1994. Report TR-103227.

2.4 EPRI MOV Performance Prediction Program: Butterfly Valve Model DescriptionReport. EPRI, Palo Alto, CA: September 1994. Report TR-103224.

2.5 EPRI MOV Performance Prediction Program: System Flow Model Description Report.EPRI, Palo Alto, CA: June 1994. Report TR-103225.

2.6 EPRI MOV Performance Prediction Program: Assessment Report. EPRI, Palo Alto,CA: November 1994. Report TR-103231.

2.7 EPRI MOV Performance Prediction Program: Performance Prediction Methodology(PPM) Implementation Guide. EPRI, Palo Alto, CA: November 1994. Report TR-103244.



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2.8 EPRI MOV Performance Prediction Program: Methods to Address Rate-of-Loading inTorque Switch-Controlled MOVs. EPRI, Palo Alto, CA: November 1994. Report TR-103226.

2.9 EPRI MOV Performance Prediction Program: Gate Valve Design Effects TestingResults. EPRI, Palo Alto, CA: July 1994. Report TR-103255.

2.10 EPRI MOV Performance Prediction Program: Friction Separate Effects Test Report.EPRI, Palo Alto, CA: November 1993. Report TR-103119.

2.11 EPRI MOV Performance Prediction Program: Butterfly Valve Design, Elbow, andScaling Effects Report. EPRI, Palo Alto, CA: April 1994. Report TR-103257.

2.12 Review of NRC/INEL Gate Valve Test Program. EPRI, Palo Alto, CA: January 1991.Report NP-7065.

2.13 EPRI MOV Performance Prediction Program: Stem/Stem Nut Lubrication Test Report.EPRI, Palo Alto, CA. Report TR-102135.

2.14 EPRI MOV Performance Prediction Program: Stem Thrust Prediction Method forAnchor/Darling Double Disk Gate Valves. EPRI, Palo Alto, CA: November 1994.Report TR-103232.

2.15 EPRI MOV Performance Prediction Program: Stem Thrust Prediction Method forWestinghouse Flexible Wedge Gate Valves. EPRI, Palo Alto, CA: November 1995.Report TR-103233.

2.16 EPRI MOV Performance Prediction Program: Stem Thrust Prediction Method forAloyco Split Wedge Valves. EPRI, Palo Alto, CA: August 1996. Report TR-103235.

2.17 EPRI MOV Performance Prediction Program: Stem Thrust Prediction Method forW-K-M Parallel Expanding Gate Valves. EPRI, Palo Alto, CA: May 1995. Report TR-103236.

2.18 EPRI MOV Performance Prediction Program: MOV Margin Improvement Guide.EPRI, Palo Alto, CA: February 1992. Report TR-100449.

22.3 Proprietary Documents Developed under Utility-Sponsored Generic Thrust and Torque Overload Qualification Program for Limitorque Actuators

The following proprietary documents were developed by Kalsi Engineering, Inc. underthe utility-sponsored Generic Thrust and Torque Overload Qualification Program for



22-5

Limitorque Actuators. For information concerning these documents, contact the projectmanager, P. D. Alvarez of Kalsi Engineering, telephone: 281/240-6500; e-mail:[email protected].

3.1 M. S. Kalsi. Thrust Rating Increase of Limitorque Actuators. Kalsi Engineering, Inc.November 25, 1991. Document No. 1707C, Rev. 0.

3.2 G. A. Moran. Thrust Rating Increase of Limitorque SMB-000 Housing Covers. KalsiEngineering, Inc. August 5, 1992. Document No. 1752C, Rev. 0.

3.3 G. A. Moran. Fastener Analysis: Limitorque Operator Mount and Housing Cover.Kalsi Engineering, Inc. December 7, 1993. Document No. 1759C, Rev. 0.

3.4 G. A. Moran. Thrust Rating Increase of Limitorque SB-00 through SB-2 SpringCompensator Assemblies and SB-00 through SB-1 Operators. Kalsi Engineering, Inc.October 7, 1994. Document No. 1799C, Rev. 0.

3.5 P. Daniel Alvarez. Limitorque SMB-2 Actuator Overload Cycle Test Interim Report.Kalsi Engineering, Inc. June 22, 1994. Document No. 1837C, Rev. 0.

3.6 P. Daniel Alvarez. Torque Cycle Test Report for Limitorque SMB-000 Electric MotorActuator. Kalsi Engineering, Inc. December 19, 1994. Document No. 1861C,Rev. 0.

3.7 Desi Somogyi. LTAFLA (Limitorque Actuator Fatigue Life Analysis) Mathematicaland Computation Model - Predicting Fatigue Life of Limitorque Type SMB/SB/SBDActuator Torsional Components. Kalsi Engineering, Inc. October 28, 1994.Document No. 1862C, Rev. 0.

3.8 Desi Somogyi. LTAFLA User's Manual - Predicting Fatigue Life of Limitorque TypeSMB/SB/SBD Actuator Torsional Components. Kalsi Engineering, Inc. December 29,1994. Document No. 1863C, Rev. 0.

3.9 Desi Somogyi. LTAFLA Validation and Verification Manual - Predicting Fatigue Lifeof Limitorque Type SMB/SB/SBD Actuator Torsional Components. Kalsi Engineering,Inc. December 29, 1994. Document No. 1866C, Rev. 0.

3.10 P. Daniel Alvarez. Limitorque H0BC Operator Overload Cycle Test Report. KalsiEngineering, Inc. December 8, 1995, January 1996. Document No. 1860C, Rev. 0.



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22.4 NRC Generic Letters, Information Notices, and Related References

4.1 U.S. NRC Generic Letter 89-10: Safety-Related Motor-Operated Valve Testing andSurveillance, June 28, 1989, including the following supplements:

Supplement 1: Results of the Public Workshops, June 13, 1990

Supplement 2: Availability of Program Descriptions, August 3, 1990

Supplement 3: Consideration of the Results of NRC-Sponsored Testsof Motor-Operated Valves, October 25, 1990

Supplement 4: Consideration of Valve Mispositioning in BoilingWater Reactors, February 12, 1992

Supplement 5: Inaccuracy of Motor-Operated Valve DiagnosticEquipment, June 28, 1993

Supplement 6: Information on Schedule and Grouping, and StaffResponses to Additional Public Questions, March 8,1994

Supplement 7: Inadvertent Operation of MOVs, July 26, 1995

4.2 U.S. NRC Generic Letter 95-07: Pressure Locking and Thermal Binding of Safety-Related Power-Operated Gate Valves, August 17, 1995.

4.3 U.S. NRC Generic Letter 96-05: Periodic Verification of Design-Basis Capability ofSafety-Related Motor-Operated Valves, September 18, 1996.

4.4 U.S. NRC Generic Letter 89-04: Guidance on Developing Acceptable In-ServiceTesting Programs, April 3, 1989.

4.5 U.S. NRC Generic Letter 89-08: Erosion/Corrosion Induced Pipe Wall Thinning, May2, 1989.

4.6 U.S. NRC Administrator Letter 94-13, Revision 2: Access to Nuclear RegulatoryCommission Bulletin Board Systems, May 3, 1996.

4.7 U.S. NRC Information Notice 97-07: Problems Identified During Generic Letter 89-10Closeout Inspection, March 6, 1997.

4.8 U.S. NRC Information Notice 97-18: Problems Identified During Maintenance RuleBaseline Inspections, April 14, 1997.



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4.9 U.S. NRC Information Notice 96-48: Motor-Operated Valve Performance Issues,August 21, 1996; Supplement 1, July 24, 1998.

4.10 U.S. NRC Information Notice 96-30: Inaccuracy of Diagnostic Equipment for Motor-Operated Butterfly Valves, May 21, 1996.

4.11 U.S. NRC Information Notice 94-69: Potential Inadequacies in the Prediction ofTorque Requirements for and Torque Output of Motor-Operated Butterfly Valves,September 28, 1994.

4.12 U.S. NRC Information Notice 94-44: Main Steam Isolation Valve Failure to Close onDemand Because of Inadequate Maintenance and Testing, June 16, 1994.

4.13 U.S. NRC Information Notice 94-67: Problem with Henry Pratt Motor-OperatedButterfly Valves, September 26, 1994.

4.14 U.S. NRC Information Notice 94-66: Overspeed of Turbine-Driven Pumps Caused byGovernor Valve Stem Binding, September 19, 1994.

4.15 U.S. NRC Information Notice 94-61: Corrosion of William Powell Gate Valve DiscHolders, August 25, 1994.

4.16 U.S. NRC Information Notice 93-01: Accuracy of Motor-Operated Valve DiagnosticEquipment Manufactured by Liberty Technologies, January 4, 1993.

4.17 U.S. NRC Information Notice 92-60: Valve Stem Failure Caused by Embrittlement,August 20, 1992.

4.18 U.S. NRC Information Notice 92-59: Horizontally Installed Motor-Operated GateValves, August 18, 1992.

4.19 U.S. NRC Information Notice 92-56: Counterfeit Valves in the Commercial GradeSupply System, August 6, 1992.

4.20 U.S. NRC Information Notice 92-50: Cracking of Valves in the Condensate ReturnLines of a BWR Emergency Condenser System, July 2, 1992.

4.21 U.S. NRC Information Notice 92-35: Higher Than Predicted Erosion/Corrosion inUnisolable Reactor Coolant Pressure Boundary Piping Inside Containment at a BoilingWater Reactor, May 6, 1992.

4.22 U.S. NRC Information Notice 91-58: Dependency of Offset Disc Butterfly Valve'sOperation on Orientation with Respect to Flow, September 20, 1991.



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4.23 U.S. NRC Information Notice 90-73: Corrosion of Valve-to-Torque Tube Keys inSpray Pond Cross Connect Valves, November 29, 1990.

4.24 U.S. NRC Information Notice 90-21: Potential Failure of Motor-Operated ButterflyValves to Operate Because Valve Seat Friction Was Underestimated, March 22, 1990.

4.25 U.S. NRC Information Notice 88-73, Supplement 1: Direction-Dependent LeakCharacteristics of Containment Purge Valves, February 27, 1989.

4.26 U.S. NRC Information Notice 88-73: Direction-Dependent Leak Characteristics ofContainment Purge Valves, September 8, 1988.

4.27 U.S. NRC Information Notice 87-38: Inadequate or Inadvertent Blocking of ValveMovement, August 17, 1987.

4.28 U.S. NRC IE Circular 77-05: Fluid Entrapment in Valve Bonnets, March 29, 1977.

4.29 U.S. NRC Information Notice 98-24: Stem Binding in Turbine Governor Valves inReactor Core Isolation Cooling (RCIC) and Auxiliary Feedwater (AFW) Systems, June1998.

4.30 U.S. NRC Information Notice 96-49: Thermally Induced Pressurization of NuclearPower Facility Piping, August 20, 1996.

4.31 U.S. NRC Regulatory Guide 1.160: Monitoring the Effectiveness of Maintenance atNuclear Power Plants, Revision 1, January 1995.

4.32 Nuclear Energy Institute: Industry Guideline for Monitoring the Effectiveness ofMaintenance at Nuclear Power Plants, NUMARC 93-01, Revision 3, April 1996.

22.5 Books, Magazines, Technical Meetings, and Journal Articles

5.1 ISA Handbook of Control Valves, 2nd Edition. Edited by J. W. Hutchison,Instrument Society of America, 1976.

5.2 R. W. Zappe. Valve Selection Handbook, 2nd Edition. Gulf Publishing Co.,Houston, TX, 1987.

5.3 Flow of Fluids Through Valves, Fittings, and Pipe. Crane Company, 1988. CraneTechnical Paper No. 410.

5.4 J. L. Lyons. Lyon’s Valve Designer’s Handbook. Van Nostrand Reinhold Co., NewYork, NY, 1982.



22-9

5.5 Aerospace Fluid Component Designer’s Handbook, Vols. 1 and 2, Rev. D. Edited by G.W. Howell and T. M. Weathers, Technical Documentary Report No. RPL-TDR-64-25 prepared by TRW Systems Group for Air Force Rocket PropulsionLaboratory, February 1970.

5.6 R. C. Merrick. Valve Selection and Specification Guide. Van Nostrand Reinhold Co.,New York, NY, 1991.

5.7 J. P. Tullis. Hydraulics of Pipelines, Pumps, Valves, Cavitation, Transients. JohnWiley & Sons, New York, NY, 1989.

5.8 Valves, Piping and Pipelines Handbook, 2nd Edition. Compiled and published bythe Trade and Technical Press Limited, Surrey, England, 1986.

5.9 W. Ulanski. Valve and Actuator Technology. McGraw-Hill, New York, NY, 1991.

5.10 I. E. Idelchik. Handbook of Hydraulic Resistance, 2nd Edition. Translated fromRussian by G. R. Malyavskaya, sponsored by the Academy of Sciences of theUSSR; National Committee for Heat and Mass Transfer, Hemisphere PublishingCorp., Washington, DC, 1986.

5.11 A. J. Ward-Smith. Internal Fluid Flow: The Fluid Dynamics of Flow in Pipes andDucts. Clarendon Press, Oxford, 1980.

5.12 W. C. Young. Roark’s Formulas for Stress & Strain, 6th Edition. McGraw-Hill, NewYork, NY, 1989.

5.13 I. J. Karassik, W. C. Krutzcsh, W. H. Fraser, and J. P. Messina. Pump Handbook.McGraw-Hill, New York, NY, 1976.

5.14 Fluid Meters, Part 1, 6th Edition. American Society of Mechanical Engineers, NewYork, NY, 1951.

5.15 Wear Control Handbook. Edited by M. B. Peterson and W. O. Winer, sponsored bythe Research Committee on Lubrication/ASME, 1980.

5.16 Nuclear News. Published monthly by the American Nuclear Society, 555 N.Kensington Ave., LaGrange Park, IL 60526, telephone: 708/352-6611.

5.17 Valve Magazine. Published quarterly by the Valve Manufacturers Association,1050 17th Street NW, Suite 280, Washington DC 20036-5503, telephone: 202/331-8105.



22-10

5.18 T. G. Scarbrough, “NRC Regulatory Activities Regarding Performance of Safety-Related Power-Operated Valves,” presented at the Sixth EPRI Valve TechnologySymposium (July 1997).

5.19 S. Hale, “Recent Improvements in MOV Field Test Programs,” presented at theSixth EPRI Valve Technology Symposium (July 1997).

5.20 L. Larsson, “Valve Maintenance,” presented at the Sixth EPRI Valve TechnologySymposium (July 1997).

5.21 K. A. Hart, “Treating the Whole Valve to Develop Cost Effective Maintenance andInnovative Solutions to Valve Problems,” presented at the Sixth EPRI ValveTechnology Symposium (July 1997).

5.22 W. V. Fitzgerald, “Lasalle Station's Valve Maintenance Program Retrieves LostMegawatts,” presented at the Sixth EPRI Valve Technology Symposium (July1997).

5.23 M. Carnus and J. Coutier, “Replacement Parts for French PWR Valves: AnOverview of the French Practice and Experience,” presented at the Sixth EPRIValve Technology Symposium (July 1997).

5.24 D. H. Worledge, “Correlation of Air-Operated Valve Reliability with PreventiveMaintenance,” presented at the Sixth EPRI Valve Technology Symposium (July1997).

5.25 W. W. Lawrence, “Solid Particle Erosion Resistant Coatings for Steam TurbineValve Stems,” presented at the Sixth EPRI Valve Technology Symposium (July1997).

5.26 M. D. Kaveney, “In-Situ Repair of Angle Seat Valves,” presented at the SixthEPRI Valve Technology Symposium (July 1997).

5.27 M. K. Phillips, S. J. Findlan, and H. Ocken, “Arc Welding and Field Applicationsof the Iron-Base Norem Hardfacing Alloys,” presented at the Sixth EPRI ValveTechnology Symposium (July 1997).

5.28 G. Ottman, “Problems with Recurrent Leakage of a Double Disc Gate Valve inReactor Coolant Sample Service,” presented at the Sixth EPRI Valve TechnologySymposium (July 1997).

5.29 J. Persad, M. J. Scurr, S. Alikhani, and H. Miller, “Condenser Steam Dump ValveRetrofits to Solve Vibrations Problems,” presented at the Sixth EPRI ValveTechnology Symposium (July 1997).



22-11

5.30 J. K. Wang, M. S. Kalsi, and S. S. Averitt, “Enhanced Pressure LockingMethodology,” presented at the Sixth EPRI Valve Technology Symposium (July1997).

5.31 J. Polacheck and J. Quinn, Jr., “FERMI II Approach to Cobalt Reduction inValves,” presented at the Fifth Valve Technology Symposium (June 1995).

5.32 M. S. Kalsi, P. D. Alvarez, J. K. Wang, D. Somogyi, J. J. Boseman, and R. L.Hughes. An Improved Gate Valve for Critical Applications in Nuclear Power Plants,NUREG/CP-0152, July 1996.

5.33 J. C. Watkins, K. G. DeWall, and G. H. Weidenhamer. Status of Stellite 6 FrictionTesting, NUREG/CP-0152, July 1998.

5.34 B. D. Bunte. MOV Reliability Evaluation and Periodic Verification Scheduling,NUREG/CP-0152, July 1996.

5.35 W. G. Knecht. Hardfacing Materials Used in Valves for Seating and Wear Surfaces,NUREG/CP-0152, July 1996.

5.36 B. H. Eldiwany, V. Sharma, M. S. Kalsi, and K. Wolfe, “Butterfly Valve TorquePrediction Methodology, NUREG/CP-0137,” presented at the ThirdNRC/ASME Symposium on Valve and Pump Testing (July 1994).

5.37 B. H. Eldiwany, M. S. Kalsi, and V. Sharma, “Improvements in Butterfly ValveTorque Prediction Models Based on Recent Research,” presented at the JointSpecialists Meeting on Motor-Operated Valve Issues in Nuclear Power Plants,Paris (April 1994).

5.38 B. H. Eldiwany and M. S. Kalsi, “Application of Hydraulic Network Analysis toMotor-Operated Butterfly Valves in Nuclear Power Plants, NUREG/CP-0123,”presented at the Second NRC/ASME Symposium on Pump and Valve Testing(July 1992).

5.39 J. K. Wang and M. S. Kalsi. Improvements in Motor-Operated Gate Valve Design andPrediction Models for Nuclear Power Plant Systems, NUREG/CR-5807, May 1992.

5.40 Wear of Materials. American Society of Mechanical Engineers, New York, NY,1981.

5.41 G. M. White and D. F. Denny. “The Sealing Mechanism of Flexible Packings,”(British) Ministry of Supply, Memorandum No. 3/47, 1947.



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5.42 D. E. Turnbull. The Sealing Action of a Conventional Stuffing Box. BritishHydromechanics Research Association, Research Report No. 592, 1958.

5.43 D. F. Denny and D. E. Turnbull, “Sealing Characteristics of Stuffing Box Seals forRotating Shafts,” Proceedings of Institution of Mechanical Engineers. Vol. 174, No. 6(1960).

5.44 K. A. Hart, “Development of an Effective Valve Packing Program,” Proceedings ofthe Fourth NRC/ASME Symposium on Valve and Pump Testing. NUREG/CP-0152(July 1996).

5.45 D. M. VanTassell, “Argo Packing Friction Research Update,” Proceedings of theThird NRC/ASME Symposium on Valve and Pump Testing. NUREG/CP-0137 (July1994).

5.46 “Palo Verde Nuclear Station Advanced Valve Packing Training,” coursematerials for a training course offered by Argo Packing Company, August 1996.

5.47 D. M. VanTassell, “Advancements in Graphitic Pressure Seals,” presented at the1996 Winter MUG Meeting, Huntsville, AL.

5.48 M. M. Cepkauskas and C. M. Garcia, “Valve Packing Study,” Proceedings of theThird NRC/ASME Symposium on Valve and Pump Testing. NUREG/CP-0137 (July1994).

5.49 S. M. Heiman, “Packing Force Data Correlations,” Proceedings of the ThirdNRC/ASME Symposium on Valve and Pump Testing. NUREG/CP-0137 (July 1994).

5.50 D. M. VanTassell, “Evaluation of Break-Away Packing Friction and ImprovedAOV Packing Systems,” presented at the 1995 Winter Air-Operated Valves(AOV) Users Group Meeting, Clearwater, FL.

5.51 Valve Selection Guide, Revision 3. Stone and Webster Engineering Corporation,1983.

5.52 K. G. DeWall, J. C. Watkins, M. G. McKellar, and D. L. Bramwell, “LaboratoryTesting of the Pressure Locking Phenomenon,” Proceedings of the FourthNRC/ASME Symposium on Valve and Pump Testing. NUREG/CP-0152 (July 1996).

5.53 B. D. Bunte and J. F. Kelly, “Commonwealth Edison Company Pressure LockingTest Report,” Proceedings of the Fourth NRC/ASME Symposium on Valve and PumpTesting. NUREG/CP-0152 (July 1996).



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5.54 D. E. Smith. Calculations to Predict the Required Thrust to Open a Flexible Wedge GateValve Subjected to Pressure Locking, U.S. NRC NUREG/CP-0146, February 1994.

5.55 M. S. Kalsi, P. D. Alvarez, J. K. Wang, D. Somogyi, J. J. Boseman, and R. L. Hughes,“An Improved Gate Valve for Critical Applications in Nuclear Power Plants,”Proceedings of the Fourth NRC/ASME Symposium on Valve and Pump Testing.NUREG/CP-0152 (July 1996).

5.56 D. Somogyi, P. D. Alvarez, and M. S. Kalsi, “Torsional Fatigue Model for LimitorqueType SMB/SB/SBD Actuators for Motor-Operated Valves,” Proceedings of the FourthNRC/ASME Symposium on Valve and Pump Testing. NUREG/CP-0152 (July 1996).

5.57 R. W. Moore, “Allocating Pressure Drop to Control Valves,” InstrumentationTechnology, October 1970.

22.6 Codes and Standards

Section 16 provides an overview of the codes and standards applicable to nuclearpower plants.

6.1 Part 50 of the U.S. Code of Federal Regulations (10CFR50, January 1, 1996,provides the guidelines for construction, operation, and maintenance of U.S.nuclear power plants. The following sections are of particular interest here:

• Section 50.55a “Codes and Standards”

• Section 50.65 “Requirements for Monitoring the Effectiveness of Maintenance at Nuclear Power Plants”

• Section 50.70 “Inspections”

• Section 50.71 “Maintenance of Records, Making of Reports”

• Section 50.72 “Immediate Notification Requirements for Operating Nuclear Power Reactors”

• Appendix A “General Design Criteria for Nuclear Power Plants”to Part 50

• Appendix B “Quality Assurance Criteria for Nuclear Power Plants andto Part 50 Fuel Reprocessing Plants”

• Appendix J “Primary Reactor Containment Leakage Testingto Part 50 for Water Cooled Power Reactors”



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American National Standard Institute (ANSI) and the American Society of MechanicalEngineers (ASME) publish many codes which apply to nuclear power plants,including:

6.2 ASME Boiler and Pressure Vessel Code, Section II, “Materials.”

6.3 ASME Boiler and Pressure Vessel Code, Section III, “Rules for the Constructionof Nuclear Power Plant Components, Division 1.”

6.4 ASME Boiler and Pressure Vessel Code, Section V, “NondestructiveExamination.”

6.5 ASME Boiler and Pressure Vessel Code, Section VIII, “Rules for Construction ofPressure Vessels, Division 1.”

6.6 ASME Boiler and Pressure Vessel Code, Section VIII, “Rules for Construction ofPressure Vessels, Alternative Rules; Division 2.”

6.7 ASME Boiler and Pressure Vessel Code, Section IX, “Welding and BrasingQualifications.”

6.8 ASME Boiler and Pressure Vessel Code, Section XI, “Rules for In-ServiceInspection of Nuclear Power Plant Components.”

6.9 ASME III Code Case N62, “Internal and External Valve Items, Division 1, Classes1, 2, and 3.”

6.10 ASME N626.3, “Qualifications for Specialized Registered ProfessionalEngineers.”

6.11 ASME MFC-3M-1089, “Measurement of Fluid Flow in Pipes Using Orifice,Nozzle, and Venturi,” Reaffirmed 1995.

6.12 ANSI/FCI 70-2-1976, “American National Standard for Control Valve SeatLeakage,” Fluid Controls Institute, Inc./American National Standards Institute,1976.

6.13 ANSI B16.3, “Malleable Iron Threaded Fittings.”

6.14 ANSI B16.4, “Cast Iron Threaded Fittings.”

6.15 ANSI B16.5, “Pipe Flanges and Flanged Fittings.”

6.16 ANSI B16.9, “Factory-Made Wrought Steel Butt-Welding Fittings.”



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6.17 ANSI B16.10, “Face-to-Face and End-to-End Dimensions of Valves.”

6.18 ANSI B16.11, “Forged Steel Fittings, Socket Welding, and Threaded.”

6.19 ANSI B16.15, “Cast Bronze Threaded Fittings, Classes 125 and 250.“

6.20 ANSI B16.18, “Cast Copper Alloy Solder Joint Pressure Fittings.“

6.21 ANSI B16.22, “Wrought Copper and Copper Alloy Solder Joint PressureFittings.”

6.22 ANSI B16.24, “Bronze Pipe Flanges and Flanged Fittings, Classes 150 and 300.”

6.23 ANSI B16.25, “Butt-Welding Ends.”

6.24 ANSI B16.34, “Valves-Flanged and Butt-Welding End.”

6.25 ANSI B16.41 Standards, “Functional Qualification Requirements for Power-Operated Active Valve Assemblies for Nuclear Power Plants.”

6.26 ANSI B2.1, “Pipe Threads.”

6.27 ANSI/ASME Code for Pressure Piping, B31.1, “Power Piping.”

6.28 ANSI/ASME OM (Standard) - 1987, “Operation and Maintenance of Nuclear Power Plants,” 1987.

6.29 ANSI/ASME OM (Code) - 1990, “Code for Operation and Maintenance ofNuclear Power Plants,” 1990.

6.30 ANSI/ASME Omc-1990, “Addendum to ASME/ANSI OM-1987,” 1990.

6.31 ANSI/ASME S/G OM-10, “In-Service Testing of Valves in Light-Water ReactorPower Plants,” 1988.

6.32 ASME QME-1-1997, “Qualification of Active Mechanical Equipment Used inNuclear Power Plants,” 1997.

6.33 ANSI/ANS-51.1, “Nuclear Safety Criteria for the Design of StationaryPressurized Water Reactor Plants,” 1983, revised 1988.

6.34 ANSI N271-1976/ANS-56.2-1984, “Containment Isolation Provisions for FluidSystems.”



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6.35 ANSI/ANS-52.1, “Nuclear Safety Criteria for the Design of Stationary BoilingWater Reactor Plants,” 1983.

6.36 ANSI/AWWA C504-87, “AWWA Standard for Rubber-Seated Butterfly Valves.”

Instrument Society of America (ISA) and the Manufacturers Standardization Society(MSS) of the valve and fitting industry provides several standards, including:

6.37 ISA-S75.01 Standards, “Flow Equations for Sizing Control Valves.”

6.38 ISA-S75.02 Standards, “Control Valve Capacity Test Procedure.”

6.39 ISA-S75.05 Standards, “Control Valve Terminology.”

6.40 MSS SP6, “Standard Finish for Contact Faces of Pipe Flanges and ConnectingEnd Flanges of Valves and Fittings.”

6.41 MSS SP25, “Standard Marking System for Valves, Fittings, Flanges and Unions.”

6.42 MSS SP42, “Class 150 Corrosion-Resistant Gate, Globe, Angle, and Check Valveswith Flanged and Butt Weld Ends.”

6.43 MSS SP44, “Steel Pipe Line Flanges (26 Inches and Larger).”

6.44 MSS SP45, “Bypass and Drain Connections.”

6.45 MSS SP53, “Quality Standard for Steel Castings and Forgings for Valves,Flanges, and Fittings and Other Piping Components - Magnetic ParticleExamination Method.”

6.46 MSS SP54, “Quality Standard for Steel Castings for Valves, Flanges, and Fittingsand Other Piping Components - Radiographic Examination Method.”

6.47 MSS SP55, “Quality Standard for Steel Castings - Visual Methods.”

6.48 MSS SP61, “Pressure Testing of Steel Valves.”

6.49 MSS SP84, “Steel Valves - Socket Welding and Threaded Ends.”


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23 APPENDIX A: RECENT ADVANCES IN VALVE AND

ACTUATOR TECHNOLOGY

23.1 Introduction

In the last few years, many significant developments have taken place in valve andactuator technology, especially for nuclear power plant applications. Thesedevelopments resulted from extensive research programs conducted by EPRI, the U.S.Department of Energy/Nuclear Regulatory Commission (NRC), electric utilities,valve/actuator manufacturers, and service and consulting organizations.

Several groups were organized to address different industry problems and to bettercommunicate the interim results of industry research, including the MOV Users Group(MUG), EPRI MOV PPP Users Group, the Nuclear Industry Check Valve Group (NIC),the Air-Operated Valve Group, etc. The U.S. NRC has been very active with thesegroups to monitor their progress and assist them with regulatory issues.

The American Society of Mechanical Engineers (ASME) in coordination with the U.S.NRC held several pump and valve symposiums and published the proceedings inNUREGs. EPRI held several symposiums to address similar issues. The NuclearMaintenance Applications Center (NMAC) prepared several maintenance and repairguides and conducted many workshops and training courses for nuclear power plantengineers and maintenance personnel. In this section, highlights of currentdevelopments and the factors that initiated them are presented. For the latestdevelopments in valve and actuator technology, the reader is referred to nuclearindustry publications such as the ASME/NRC symposiums, NUREGs, and valve usersgroup meetings.

23.2 Background

Operating experience at nuclear power plants in the 1970s and 1980s revealedweaknesses in the performance of power-operated valves. The NRC sponsored valveand actuator tests to evaluate valve performance and disseminated the test resultsthrough public meetings and publications. On June 28, 1989, the U.S. NRC issued


Appendix A: Recent Advances in Valve and Actuator Technology

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Generic Letter 89-10 [4.1] requesting that nuclear power plant licensees verify thedesign basis capabilities of MOVs in safety-related systems. On September 18, 1996, theNRC issued Generic Letter 96-05 [4.3] requesting that nuclear power plant licenseesensure that programs are in place to periodically verify the capability of their safety-related MOVs to perform their safety functions in accordance with the licensing bases.On August 17, 1995, the NRC issued Generic Letter 95-07 [4.2] requesting that nuclearpower plant licensees ensure that safety-related power-operated gate valves susceptibleto pressure locking or thermal binding are capable of performing their safety functionswithin the current licensing bases of the facility. The NRC issued many more GenericLetters and Information Notices (see Section 22.4) to address other safety-related issuesconcerning valves and actuators.

The nuclear industry responded to these safety concerns with comprehensive programsto solve the problems. Highlights of these programs are summarized in the followingsections.

23.3 Motor-Operated Valve Performance Prediction Methodology

EPRI undertook the development of a comprehensive Performance PredictionMethodology (EPRI’s PPM) to predict the required thrusts/torques to operate gate,globe, and butterfly valves under a variety of flow conditions including design basisand blowdown [2.1 through 2.18]. This program resulted in probably the mostsignificant increase in technical knowledge in several decades regarding the ability topredict thrust/torque requirements for gate, globe, and butterfly valves for reliableperformance over a wide range of operating conditions.

EPRI MOV PPM included the development of analytical models based on firstprinciples, flow loop testing, plant in situ testing, and separate effects testing. EPRIMOV PPM resulted in a validated and computerized methodology (EPRI’s MOVPerformance Prediction Program or EPRI’s MOV PPP). The computerized methodologyconsists of a system flow model [2.5], a gate valve model [2.2], a globe valve model[2.3], and a butterfly valve model [2.4]. Hand calculation methodologies were alsodeveloped for Anchor/Darling double-disc valves [2.14], Westinghouse gate valves[2.15], Aloyco split wedge valves [2.16], and W-K-M parallel expanding gate valves[2.17]. Highlights of these models are given below.

In a safety evaluation, SER, (dated March 15, 1996) and a supplement (dated February20, 1997), the NRC accepted the EPRI methodology with certain conditions andlimitations. Reference 2.1 provides a summary of the NRC comments in this SER andEPRI’s response to these comments.



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23.3.1 System Flow Model

At the early stages of developing the EPRI PPM, it was recognized that valveperformance depends on the entire hydraulic system in which the valve is installed[5.38]. The valve pressure drop and flow rate can be determined accurately only byanalyzing the entire hydraulic system, including the flow/pressure sources and sinks(including pumps, surge tanks, and pipe elevations), significant piping resistances, andpresence of parallel pipe branches. For example, if an orifice downstream of the valveunder consideration experiences choking, the pressure drop across the valve can besignificantly less than otherwise predicted. The system flow model was designed toaccount for these factors.

The system flow model offers four options to perform the analysis (see Reference 2.5 fordetails). Guidance on reducing complicated systems to one of the model’s four basicsystems is provided by the methodology.

The system flow model within the EPRI PPM utilizes the hydraulic system data alongwith the valve flow coefficient and choking parameters versus disc positions todetermine the flow rate and pressure drop across the valve at every 1% of valve stroke(1° for butterfly valves). The accuracy of mid-stroke pressure drops is crucial, especiallyfor butterfly and gate valves under high flow rates and blowdown conditions. Forbutterfly valves, the hydrodynamic torque can exceed the seating/unseating torqueand may govern torque requirements. In mid-stroke positions, guide contact stresses(in gate valves) can exceed the material galling threshold and render the valveunpredictable and inoperable in some cases. Thus, during valve selection, operation,repair, maintenance, and testing, it is important to always consider the entire hydraulicsystem.

23.3.2 Solid and Flex Wedge Gate Valve Model

EPRI's PPP gate valve model [2.2] utilizes detailed internal valve dimensions andpressure drop (from the System Flow Model output) to calculate the disc pressure forceand moment and the equilibrium position of the gate at all disc positions from fullyclosed to fully open or vice versa. Friction coefficients at different surfaces areinterpolated from a friction algorithm at the calculated contact stresses. The disc frictionthrust component is then calculated and combined with other components (such aspacking, stem rejection force, and disc/stem weight) to calculate the required stemthrust. Some of the key findings are summarized as follows:

1. The guide rail-to-guide slot contact stress can be very high, and the resulting guidefriction thrust can govern thrust requirements. This can occur under severalconditions, including:



23-4

— The mid-stroke valve pressure drops are very high such as under blowdownconditions with little system flow resistance.

— The gate pressure force does not transfer to the seats until the valve is nearlyclosed and the valve pressure drop is almost equal to the shutoff pressure drop.This in turn can be caused by a very tight clearance between the guide rail andguide slot.

2. Figure 23-1 shows one of the important contact modes in which the disc is tilted,theoretically making a point contact at two locations against the downstream seat.Simultaneously, the upper edge of the disc guide slot makes a line contact againstthe body guide. Stresses at the contact points and contact lines depend on the localgeometry and the magnitude of the valve pressure drop at a given disc position.Under certain conditions, localized stresses can cause plastic deformation as well asgalling/gouging of the mating surfaces. In some extreme cases, the guide may breakand the gate get stuck in mid-position.

3. One of the major results from EPRI’s PPM is the development of detailed frictioncoefficient data tables for valve materials [2.1, 2.10]. The friction coefficient datatables were obtained by extensive laboratory tests using test specimens of differentgeometries and materials applicable to gate valves. The friction coefficient matrixprovides nominal and maximum (upper bounding) values and includes thefollowing:

— Different material combinations, including Stellite 6 on Stellite 6, Stellite 6 oncarbon steel, Stellite 6 on stainless steel, and carbon steel on carbon steel

— Different contact modes, including flat-on-flat, edge-on-flat, edge-on-edge (non-scissoring), and edge-on-edge (scissoring)

— Different contact stresses from less than 5 ksi to 50 ksi (34.5 MPa to 345.0 MPa)

— Different fluids including water and steam at temperatures from less than 70°Fto about 650°F (20°C to 340°C)

4. In some gate valves, the clearance between the guide rail and guide slot is verysmall. The accumulation of foreign materials in such tight clearance (from processfluid or from the guide surface) may increase the friction force and, in some cases,may lock the gate.



23-5

Figure 23-1Tilted Disc Contact Mode Resulting in Point Contact with the Downstream Seat



23-6

23.3.3 Methodologies for Special Design Gate Valves

Hand calculation methodologies are also provided for Anchor Darling double disc[2.14], Westinghouse [2.15], Aloyco split wedge [2.16], and W-K-M parallel expandinggate valves [2.17]. These models provide detailed descriptions of the valve design andoperation as well as procedures to calculate thrust requirements. For example,Reference 2.17 shows that, for typical friction coefficients, the W-K-M valve may besubject to a very high opening thrust if it is installed with the flow in the nonpreferreddirection or if the valve is subjected to reverse flow. Thus, for all practical purposes, theW-K-M parallel expanding gate valves are unidirectional and should be installed withthe gate on the downstream side. This is the manufacturer’s preferred orientation.

23.3.4 Butterfly V alve Model

EPRI’s PPP butterfly valve model [2.4] calculates the total required torque, themaximum transmitted torque (for weak link analysis), the total required dynamictorque, and the total seating/unseating torque in the direction specified by the user(either opening or closing). The model takes into account the effect of upstream flowdisturbances (such as elbows) in calculating the hydrodynamic torque component ofthe total dynamic torque. The model can be used with compressible or incompressibleflow. The model also provides recommendations for bounding bearing frictioncoefficients in clean water applications, as well as in dirty water applications (such asservice water systems).

Most of the available butterfly valve flow and torque coefficients are based on astandard test section for valve flow testing (see ISA Standard 75.02, 1988 [6.38]). In thisstandard test setup, the pressure drop is measured between two pressure taps: onelocated at two pipe diameters upstream of the test valve and the other located at sixpipe diameters downstream. One of the key concerns was the use of these flow andtorque coefficients in predicting dynamic torque requirements under downstream cleanpipe break right at the valve discharge. A blowdown test performed under EPRI’s PPMtest program [2.11] confirmed that the required dynamic torque under downstreamclean pipe break right at the valve discharge is bounded by the required dynamictorque when the downstream clean pipe break occurs at eight pipe diameters from thevalve if the valve pressure drop is the same. Thus, it is conservative to use flow andtorque coefficients from continuous pipe flow test data to predict torque requirementsunder downstream pipe rupture right at the valve discharge. This hypothesis withtechnical justification was first introduced in January 1993 in the first release of theButterfly MOV Guide [1.6].



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23.3.5 Globe Valve Model

EPRI’s PPP globe valve model [2.3] calculates the required opening and closing stemthrust for globe valves with T-pattern or Y-pattern bodies. The model is applicable tobody-guided globe valves with unbalanced or balanced discs subject to incompressibleflow. For unbalanced disc globe valves, the valve needs to be classified as seat based(differential pressure acts across disc seat area) or guide based (differential pressureacts across disc guide area) to ensure the predicted thrust is bounding. Themethodology provides guidelines to classify unbalanced disc globe valves as seat basedor guide based.

23.4 EPRI/NMAC Application and Maintenance Guides

EPRI and the Nuclear Maintenance Applications Center (NMAC) developed severaldocuments to address the (then current) valve and actuator issues (see Section 22.1).Furthermore, NMAC conducted many training seminars and workshops for plantengineers to address several of the industry concerns. The wealth of informationprovided in these documents should be utilized by plant engineers and maintenancepersonnel to address valve selection, operation, repair, maintenance, and testing. In thissection, highlights of some of the recent documents are given. For further details, thereader is encouraged to examine the original references.

EPRI NP-6516, August 1990Guide for the Application and Use of Valves in Power Plant Systems

NP-6516 was one of the first guidebooks to provide a comprehensive overview ofvalves and actuators in nuclear power plants. In the absence of other guides (at thattime) to address specific valve and actuator types (for example, check, solenoid, safety,and relief valves), this document addressed most of the common valve and actuatortypes in nuclear power plants. After the publication of NP-6516, several guides wereissued with more detailed discussions about specific valves and actuators, as shown inSection 22.1. The present publication is a revision of NP-6516 where some discussionsof valves and actuators were eliminated and references to other guides are provided.

EPRI TR-105852, Volume 2 [1.1], December 1996In Situ State-of-the-Art Valve Welding Repair (Gate, Globe & Check Valves)

This document is Volume 2 of this guide. It provides extensive guidance to the user inidentifying a specific repair issue, understanding the repair options, walking throughthe specific repair, understanding the Code requirements, and preparing the valve forsystem testing. This guide includes the following:



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• Technical descriptions for gate, globe, and check valves. Individual sections ondesign and application along with advantages and disadvantages are provided.

• Materials of construction including pressure retaining and trim materials such ascobalt-based, nickel-based, and iron-based hardfacing alloys.

• A section on each valve type, the typical repair issues, and the repair options. Therepair options direct the user to specific repair sections.

• An extensive section on specific component repairs including the component repairlist prerequisites; repair strategies; flaw removal techniques; material selection;machining, welding, and heat treatment requirements; and final inspection andtesting requirements.

• Sections on welding material selection, including detailed welding guidelines forspecific processes, base material and filler metals such as gas tungsten arc (GTA)welding of hardfacing on carbon steel substrates; and preheat and post-weld heattreatment guidelines.

• Current listing of contractors and equipment suppliers capable of providingassistance for the repair of valve components and implementation of theseguidelines.

EPRI NP-7412, November 1996 [1.2]Air-Operated Valve Maintenance Guide

NP-7412 discusses major components, such as actuators, valves, and positioners, andexplains the interrelationship of these components. Diagrams indicating the applicationand operation of various types of actuators are presented as an aid for thoroughinvestigation of malfunctioning equipment. Recent developments on diagnosticequipment for AOVs are covered in that document and measurement traces on valveswith maintenance-related problems are used to demonstrate how the diagnosticequipment can quickly solve complex valve problems. In addition, predictive andpreventive maintenance recommendations based on specific failure data are included.

The guide also includes a troubleshooting section with tables providing easilyaccessible information to minimize troubleshooting costs. Appendices augment theguide by providing a glossary of terms and various engineering schedules, includinguseful engineering parameters for the proper maintenance of air-operated valves andaccessories.



23-9

EPRI TR-104602, December 1994 [1.3]Maintenance Job Cards

TR-104602 provides general guides and information about maintenance andtroubleshooting of problems with valves and pumps. It can be used to develop moredetailed procedures within each plant.

EPRI TR-105872, August 1996 [1.4]Safety and Relief Valve Testing and Maintenance Guide

TR-105872 defines the various types of safety devices used in the nuclear industry anddetails their operating principles and applications. Specifically, the operationalcharacteristics of Crosby Valve & Gage Company, Dresser Industries, and Target RockCorporation valves used in the primary and balance-of-plant (BOP) systems of boilingwater reactor (BWR) and pressurized water reactor (PWR) power plants are covered indetail. Vacuum breakers and nonreclosing-type devices (rupture discs, fusible plugs,etc.) are not included in this document.

A failure mode and cause analysis section provides information on the reportedfailures from the Nuclear Plant Reliability Data System (NPRDS) and Licensee EventReport (LER) databases by valve types and their causes. A generic table identifies thevarious valve failure modes and probable causes.

The section on testing provides a review of ASME Code requirements along withguidelines on bench testing and testing with auxiliary lift devices (ALDs). The effect ofenvironment on the test results is highlighted.

A section on maintenance provides recommendations on predictive and preventivemaintenance. Recommended methods of disassembly, corrective repair, inspection, re-assembly, and performance monitoring are included.

In addition, the guide includes useful sections and appendices on topics like shippingand handling, valve sizing, ASME Code requirements, types of valves used in variousnuclear power plants, and manufacturers of valves and testing equipment.

EPRI TR-106563, 1998Application Guide for Motor-Operated Valves in Nuclear Power PlantsVolume 1 [1.5] Gate and Globe ValvesVolume 2 [1.6] Butterfly Valves

TR-106563 provides guidance as to the functional and design requirements for motor-operated valves in nuclear power plants. It provides methodologies for evaluatingMOV operation under various plant conditions including design basis and postulated



23-10

accident conditions. These evaluations are necessary to ensure safe plant operation andto meet regulatory requirements and various industry code requirements. Theorganization of the guide provides a framework around which a plant-specific MOVevaluation program can be developed.

The guide is published in two volumes:

• Volume 1 deals with the non-rotating, rising stem type gate and globe valves.

• Volume 2 addresses various types of butterfly valves.

Both volumes provide methodology, along with completed examples, of evaluatingvalve design features and operating conditions to determine the required operatingthrust (for gate and globe valves) and torque (for butterfly valves). Methodologies tocalculate the actuator output thrust and torque capabilities are also provided forLimitorque actuators. Even though this guide specifically addresses Limitorque valveactuators, the evaluation methodologies provided can be applied to other actuators ofsimilar design.

TR-106563 incorporates refinements and results of tests performed as part of the EPRIMOV Performance Prediction Program.

EPRI NP-7414, April 1992 [1.7]Solenoid Valve Maintenance and Application Guide

NP-7414 provides detailed information about SOV operation as well as the limitationsand design characteristics that should be considered when selecting a valve for a givenapplication. It also describes various modes of failure and evaluates industrywidefailure data. In addition, descriptions of various troubleshooting, maintenance, andrepair methods are included.

EPRI NP-7205, April 1991 [1.8]Predictive Maintenance Primer

NP-7205 provides utility plant personnel with a single-source reference to predictivemaintenance analysis methods and technologies used successfully by utilities and otherindustries. It is intended to be a ready reference for personnel considering starting,expanding, or improving a predictive maintenance program. This primer includes adiscussion of various analysis methods and how they overlap and interrelate.Additionally, 18 predictive maintenance technologies are discussed in sufficient detailfor the user to evaluate the potential of each technology for specific applications.



23-11

The Nuclear Maintenance Applications Center collected experience data from 18utilities plus other industry and government sources. They also contacted equipmentmanufacturers for information pertaining to equipment utilization, maintenance, andtechnical specifications.

The primer includes a discussion of six methods used by analysts to study predictivemaintenance data:

• Trend analysis

• Pattern recognition

• Correlation

• Test against limits or ranges

• Relative comparison data

• Statistical process analysis

Following discussions of these analysis methods are detailed descriptions of 18technologies that analysts have found useful for predictive maintenance programs atpower plants and other industrial facilities. Each technology subchapter has adescription of the operating principles involved in the technology, a listing of plantequipment where the technology can be applied, and a general description of themonitoring equipment. The descriptions also include a discussion of results obtainedfrom actual equipment users and preferred analysis techniques to be used on dataobtained from the technology.

EPRI TR-106853, November 1996 [1.9]The Maintenance Engineer Fundamentals Handbook

TR-106853 is the handbook for a maintenance course offered by EPRI. It providesdiscussions about:

• Degradation, aging, failures, and failure mechanisms

• Corrective maintenance, preventive maintenance, and modifications

• Risk-based, performance-centered predictive maintenance

• Reliability-centered maintenance

• Problem-solving approaches

• Maintenance of the maintenance program



23-12

EPRI TR-107759, December 1996 [1.11]Assessing Maintenance Effectiveness

TR-107759 provides plants with a tool for evaluating maintenance activities. Plantshave made changes to maintenance practices because of actual or perceived plantproblems without any means to measure the impact of those changes. Many changeshave been costly and have not yielded the anticipated results. This document presentssome suggested measures that can be used to evaluate maintenance practices andsuggests a basis for comparison between plants. Plants can use these measures (inwhole or in part) and may suggest others that might be useful. This document is thefirst attempt to provide a tool for industry comparison and feedback. This is animplementation document that introduces the concept of “maintenance performancemeasures” to the industry and will require revision and improvement as the industrygains experience with this concept.

EPRI NP-4916, Revision 2, February 1995 [1.12]Lubrication Guide

NP-4916 gives information from many manufacturers on lubricants suitable for variousnuclear power plant applications. Lubricant operating limits with respect totemperature and radiation dose are listed. The guide also addresses the basics of howlubricants work, how radiation affects them, and how this relates to their composition.Friction and wear are other basic topics presented, along with lubricant stress effects,shelf life, compatibility, troubleshooting, and testing. All are important maintenancetopics. Topics covered by an earlier EPRI report, Radiation Effects on Lubricants, NP-4735,have been updated and incorporated into this guide. A summary of the lubricant studyin the EPRI/Utilities Motor-Operated Valve Performance Prediction Program is alsoincluded. The guide is intended for use by power plant maintenance and engineeringpersonnel.

EPRI NP-7213s, April 1991 [1.13]Post-Maintenance Testing, A Reference Guide

NP-7213s was developed to address nuclear power industry concerns about theadequacy and consistency of any post-maintenance testing (PMT) of a component or asystem. The guide provides the user with a methodology to select the appropriatetesting activities on a consistent basis. The guide’s philosophy is to take a graduated orphased approach to testing.

The objective of the post-maintenance testing program is to ensure that the component,after any maintenance/repair has been completed, will fulfill its design function. Thetests selected must be appropriate to the maintenance or repair performed. Therefore,PMT covers aspects from visual inspection, checks, or verifications made during



23-13

maintenance work to full demonstration of a component’s ability to perform its designfunction. Most station maintenance procedures already address the inspections, checks,or verifications identified in this guide. By taking credit for the tests under variousprocedures, multiple tests on a component can be avoided. Since this guide offersextensive test matrix sets and definitions, a user can easily identify necessary testsneeded or credits recorded. Maximum benefit will be derived by using this guideduring initial maintenance planning activity.

EPRI TR-104749, December 1994 [1.14]Static Seals Maintenance Guide

TR-104749 presents information necessary for plant engineers, maintenance engineers,maintenance planners, and craft personnel to make leak-tight joints, to make repairs,and to diagnose and solve existing leakage problems. The guide provides informationdescribing the various joints in use at power plants, the function of mating parts ofvarious joint arrangements, the various gasket materials in use, and the additionalsealants and fillers used to augment the joint seal. It also addresses required surfaceconditions, seal compression, and component inspection. Finally, the guide coverswhen to look for leaks, what to consider when troubleshooting, and the temporary andpermanent repair options available when leaks are found. The guide will serve as acomprehensive reference manual for plant operations, maintenance, design,engineering, procurement, and personnel training. It will also simplify maintenanceand accelerate troubleshooting, thereby optimizing plant safety and availability.

EPRI TR-104213, December 1995 [1.17]Bolted Joint Maintenance and Application Guide

Proper design, assembly, preload, and inspection of bolted connections remainimportant activities for operators of commercial nuclear power plants. Likewise, plantleakage reduction efforts continue to receive attention at most of these generatingfacilities. TR-104213 addresses these areas of interest and represents both a majorrevision and a consolidation of several previous guidebooks dealing with general goodbolting practices and guidelines for threaded fastener usage. The guide is subdividedby major application into pressure-retaining joints, mechanical joints, and structuraljoints. Additional information on procurement and fastener receipt inspection is alsoincluded. This document will be useful to plant engineering and maintenancepersonnel responsible for procedures, assembly, inspection, and troubleshooting of thevarious types of bolted connections used in nuclear power plant applications.



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23.5 Generic Thrust and Torque Qualification Program for Limitorque Actuators

23.5.1 Background

Motor-operated valves (MOVs) can experience significant thrust overshoot after theactuator torque switch trips. The thrust overshoot is caused predominantly by theinertia of the motor and time delays in the motor current contactor dropout. Themagnitude of the thrust overshoot depends upon a number of factors including:

• Valve stiffness

• Actuator/motor size

• Motor speed

• The magnitude of differential pressure across the disc

• The match between actuator output capabilities and valve thrust requirements

With the use of diagnostic devices in recent years, inertia thrust overshoots have beenquantified. It is not uncommon to see thrust overloads of 25 to 50% above ratedcapacity of the actuators in some applications. Even higher thrusts are experienced insome MOV assemblies. During in situ testing, some actuators were inadvertentlyoverloaded beyond their thrust/torque ratings.

Recognizing this as a generic problem, Duke Power Company initiated a test programto systematically determine the capability of Limitorque actuators to withstand suchhigher overloads and to qualify them for higher thrust levels on a technically soundbasis. Duke Power was joined by more than 35 U.S. utilities in sponsoring this project.

The objective of this project was to qualify the most widely used population ofLimitorque actuators (SMB-000 through SMB-2) for higher thrusts than the publishedratings. Under the overall project objective, the specific subobjectives were:

1. To test Limitorque actuators to 200% of the rated thrust, both in the opening andclosing directions for 4,000 cycles

2. To recommend an allowable number of cycles under various levels of thrustoverloads, based on appropriate justifiable margins applied to the test results

3. To seismically qualify these actuators while being cycled under 200% of the ratedthrust in the opening and closing directions



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23.5.2 Technical Approach

The test actuators were subjected to 200% of the rated thrust both in the closing andopening direction, even though this level of thrust in the opening direction is notencountered in normal MOV applications. A test fixture (Figure 23-2) was designed tofulfill the project goals.

Seismic qualification testing of the actuators was done at Wyle Laboratories, Huntsville,Alabama. The testing was performed in accordance with IEEE Standard 344-1975requirements using sine sweep, sine beat, and triaxial random multifrequency testing.

Figure 23-2Limitorque Actuator Test Fixture



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23.5.3 Highlights of Results and Conclusions

The results and conclusions of this program are documented in the proprietary reportslisted in Section 22.3. Based on these results and their own experience, Limitorqueissued Technical Update 92-01 to allow generic use of higher thrust values on an as-needed basis (as described in their technical update). In summary, the test results showthat all of the thrust-related components of the actuators have successfully completedthe test goal of 4,000 cycles at 200% of the rated thrust in both the closing and openingdirections, 10 stall tests, and a matrix of seismic qualification tests meeting IEEEStandard 344-1975 requirements. By applying suitable margins of safety based onASME Section III (1989), Appendix II approach, these overload test cycles qualify theactuators for a number of allowable cycles at overthrust conditions.

It is important to note that certain torque-related components (for example, worm,worm gear, worm shaft, and worm shaft bushing) in some of the actuators requiredinterim replacement due to fatigue damage or excessive wear under test conditions.Failure of the torque-related components in the test fixture was caused by acombination of factors that are more severe than are likely to exist in actual MOVapplications. When utilizing the higher thrust levels, it is necessary to ensure that theactuator torque is quantified and that the existing torque ratings published byLimitorque are not exceeded without performing the appropriate stress analysis.

Since torque-related components were found to limit the overall life of the specificactuator assemblies used in the tests, the project was extended to address the fatiguelife of the torque-related components [5.56].

Another important outcome of the Limitorque actuator test program was thedevelopment of an actuator test stand. The test stand can be used in actuator testingand control switch setting without risking valve stem overloading. Several utilitiesprocured actuator test stands with customized features to fit their individual needs.

It should be noted that Limitorque has not increased the thrust ratings for its actuators.Furthermore, the NRC stated that users of this and other thrust limit studies areresponsible for justifying their MOV structural capability (see Enclosure 1 toSupplement 6 to Generic Letter 89-10).


24-1

24 APPENDIX B: CONTROL VALVE SIZING METHODS

AND EXAMPLES

24.1 General Methods, Definitions, and Evaluation

24.1.1 Introduction to Control Valve Specification, Sizing, and Selection

A control valve consists of four basic parts: body, bonnet, trim, and actuator. Whenspecifying the control valve body for an application, the size, style, material, rating, andconnections must be considered. Trim is discussed in Section 2. Control valve bodystyles are discussed in Sections 6, 8, and 10. Actuator types are discussed briefly inSection 13, which also provides references to other EPRI guides for more detaileddiscussions. Materials, ratings, and end connections are covered in Section 16.

Rules of Thumb for Sizing: Due to mechanical stresses, the size of a control valve body islimited by the size of the line in which the valve is installed. In order to limit stresslevels in the valve, a good rule of thumb is that a control valve should not be less thantwo nominal pipe sizes smaller than the line.

• Examples: A 6-inch (150-mm) or larger valve is required for a l0-inch (250 mm) line.A 16-inch (400-mm) or larger valve is required for a 20-inch (500 mm) line.

Another rule of thumb states that the valve should be sized to throttle the process fluidfrom between 20 and 80% of valve capacity. Some specifications even make this arequirement for the control valve vendor. While this limit may be a reasonable pointfrom which to start the sizing/selection process, the examples discussed in Sections 6.1and 6.2.9 illustrate that any serious attempt to adhere to these limits could be, at best,unnecessarily complicated and expensive, requiring the addition of a second valve or avalve that could be oversized for the application. This section is designed to expose theuser to different aspects of valve sizing. The step-by-step procedures given in Sections24.1.3 (for liquid flow) and 24.1.4 (for gas flow) are the most common methods forsizing control valves and can be applied regardless of the type of valve being sized.Several examples are provided to illustrate the use of these procedures.


Appendix B: Control Valve Sizing Methods and Examples

24-2

24.1.2 Definitions

Capacity. Flow capacity is based on the industry standard ANSI/ISA S75.01 [6.37]. Thisstandard and the corresponding measuring standards contain equations used to predictthe flow of compressible and incompressible fluids in control valves. Different forms ofthe basic equations are used for liquids and gases.

Flow Coefficient (Cv). Basic steps for sizing and selecting the correct valve includecalculating the required Cv. Equations for calculating the required Cv for both gases andliquids are given in this section.

The valve flow coefficient most commonly used as a measure of the capacity of thebody and trim of a control valve is Cv. Cv is defined as the flow of water in U.S. gallonsper minute at 60°F that will flow through a given valve producing a pressure drop of 1psi. The general equation for Cv is as follows:

drop Pressure

re temperatuflowingat gravity Specific rate Flow Cv =

P

G q C f

v ∆= (Equation 24-1)

Where:

Cv = Flow coefficient, psigpm

q = Flow rate, gpm

Gf = Specific gravity at flowing temperature

∆P = Pressure drop, psid

When selecting a control valve for an application, the calculated Cv is used to determinethe valve size and the trim sizes that will allow the valve to pass the desired flow rateand provide stable control of the process fluid.

Pressure Profile. Fluid flowing through a control valve obeys the basic laws ofconservation of mass and energy and the continuity equation. The control valve acts asa restriction in the flow stream. As the fluid stream approaches this restriction, itsvelocity increases in order for the full flow to pass through the restriction. Energy forthis increase in velocity comes from a corresponding decrease in pressure.



24-3

Maximum velocity and minimum pressure occur immediately downstream from thethrottling point at the narrowest constriction of the fluid stream, known as the venacontracta. Downstream from the vena contracta, the fluid slows and part of the energy(in the form of velocity) is converted back to pressure. A simplified profile of the fluidpressure is shown in Figures 24-1 and 24-2. The slight pressure losses in the inlet andoutlet passages are due to frictional effects. The major excursions of pressure are due tothe velocity changes in the region of the vena contracta. Detailed discussions can befound in many references including 5.1, 5.3, 5.4, 6.36, 6.38, and 1.6.

Figure 24-1Pressure Profile of Fluid Passing through a Valve



24-4

Figure 24-2Pressure Profile through Restriction

Allowable Pressure Drop. From the definition of Cv, an increase in the pressure drop for agiven Cv should result in an increase in flow rate. This occurs up to a point after whichany further increase in the pressure drop does not yield an increase in flow rate. Thispoint, called choked flow, is illustrated in Figure 24-3.



24-5

Figure 24-3Effects of Vaporization

In liquids, when the pressure at any point in the valve drops below the vapor pressureof the fluid (as shown in Figure 24-1), vapor bubbles form. These bubbles occupy morevolume than the liquid from which they were formed. As further increases in pressuredrop occur across the control valve, the proportion of bubbles to liquid increases untilthe volume of the flow is so great that the valve cannot pass additional flow. Whenadditional flow can not be passed, the pressure drop at this point is referred to as thechoked pressure drop point (see Section 8.2.3 for additional discussions).

In gases, as the downstream pressure decreases with a corresponding increase inpressure drop, the velocity of the gas across the vena contracta increases due to theincreasing volume of the gas. When the velocity reaches sonic (Mach = 1.0), any furtherincrease in the pressure drop due to decreased downstream pressure will not result inadditional flow. Sonic velocity generally occurs when the total valve pressure drop isgreater than about one half of the absolute inlet pressure (psia) but should be calculatedfor each unique situation. The pressure drop that corresponds to the sonic velocitycondition across the vena contracta is the choked (or critical flow) pressure drop (seeAppendix D in Reference 1.6 for additional discussions).

When sizing a control valve, the actual pressure drop should be compared to theallowable pressure drop, and the smaller of the two must be used in the sizingequation. This does not imply that the control valve cannot operate at the higher



24-6

pressure drop, but that only the lower pressure drop is effective in producing flowunder the stated conditions.

Cavitation. In liquids, when the pressure at the vena contracta drops below the vaporpressure of the fluid, vapor bubbles begin to form in the fluid stream. Downstreamfrom the vena contracta, the fluid decelerates with a resultant increase in pressure. Ifthis pressure is higher than the vapor pressure, the bubbles collapse (or implode) as thevapor returns to the liquid phase. This two-step mechanism, called cavitation, producesnoise and vibration and causes physical damage to the valve and downstream piping.

The onset of cavitation, known as incipient cavitation, is the point when the bubbles firstbegin to form and collapse. It can be determined from Equation 24-6. The point atwhich full or choked cavitation occurs (severe damage, vibration, and noise) can bedetermined from Equation 24-4. Under choked conditions, “allowable pressure drop”is the choked pressure drop. Continuous operation under cavitation or chokingconditions should be avoided.

Liquid Pressure Recovery Factor (FL). The liquid pressure recovery factor (FL) predicts theamount of pressure recovery that will occur between the vena contracta and the valveoutlet. FL is an experimentally determined coefficient that accounts for the influence ofthe valve’s internal geometry on the maximum capacity of the valve [6.37, 6.38].

FL also varies according to the valve type. High recovery valves, such as butterfly andball valves, have significantly lower pressures at the vena contracta and, therefore,recover more for the same pressure drop than a globe valve. Thus, butterfly and ballvalves tend to choke (or cavitate) more easily than globe valves.

Liquid Critical Pressure Ratio Factor (FF). The liquid critical pressure ratio factor (FF),multiplied by the vapor pressure, predicts the theoretical vena contracta pressure at themaximum effective (choked) pressure drop across the valve.

Flashing. Flashing occurs when the downstream pressure is equal to or less than thevapor pressure. Vapor bubbles formed at the vena contracta do not collapse, resultingin a two-phase (liquid-vapor) mixture downstream of the valve. Velocity of this two-phase flow is usually high and may erode the valve and piping components.

Choked Flow. Choked flow is a limiting, or maximum, flow rate. With fixed inlet(upstream) conditions, it is manifested by the failure of decreasing downstreampressure to increase the flow rate. With liquid flows, choking occurs as a result ofvaporization of the liquid when the pressure within the valve falls below the vaporpressure of the liquid at operating temperature. Choked flow will be accompanied byeither cavitation or flashing. If the downstream pressure is greater than the vaporpressure of the liquid, cavitation occurs. If the downstream pressure is equal to or lessthan the vapor pressure of the liquid, flashing occurs.



24-7

Choked flow occurs when the fluid velocity approaches sonic values at any point in thevalve or line. This happens in liquids when the vapor, formed as the result of pressuredrop, increases the specific volume of the fluid to the point where sonic velocity isreached. In gases, as the pressure in the downstream line is lowered, the specificvolume increases to the point where sonic velocity is reached. Lowering thedownstream pressure beyond this point in either case will not increase the flow rate.The velocity at any point in the valve or downstream piping is limited to sonic (Mach =1.0). As a result, the flow rate will be limited to an amount which yields a sonic velocityunder the specified pressure conditions.

Reynolds Number Factor (FR). Nonturbulent flow occurs at high fluid viscosities and/orlow velocities. In these circumstances, the flow rate through a valve is less than forturbulent flow, and the Reynolds number factor FR must be introduced. FR is the ratio ofnonturbulent flow rate to the turbulent flow rate predicted using Equation 24-1 (seealso Equation 24-2).

Piping Geometry Factor (Fp). Valve sizing coefficients are determined from tests run withthe valve mounted in a straight run of pipe that is the same diameter as the valve body.If the process piping configurations are different from the standard test manifold, thevalve capacity is changed. These differences can be approximated by the use of thepiping geometry factor (Fp). The effect of the piping geometry factor is significant onlyat large disc openings (see Appendix D in Reference 1.6).

Velocity. As a general rule, valve outlet velocities should be limited to the followingmaximum values:

Liquids 50 feet per second (15 m/sec)

Gases Approaching Mach 1.0

Mixed Gasesand Liquids

500 feet per second (152 m/sec)

The above values are guidelines for typical applications. In general, smaller sizedvalves can handle slightly higher velocities, and large valves can handle lowervelocities. Special applications have special velocity requirements, some of which aredescribed below.

In liquid applications where the fluid temperature is close to the saturation point, thevalve outlet velocity should be limited to 30 feet per second (9 m/sec) to avoidreducing the fluid pressure below the vapor pressure. This limit is also appropriate forapplications designed to pass the full flow rate with a minimum pressure drop acrossthe valve.



24-8

The velocity in valves in cavitating service should also be limited to 30 feet per second(9 m/sec) to minimize damage to downstream piping and to localize the pressurerecovery that causes cavitation immediately downstream from the vena contracta.

In flashing services, velocities become much higher due to the increased volumeresulting from vapor formation. For most applications, it is important to keep velocitiesbelow 500 feet per second (152 m/sec). Expanded outlet style valves help to controloutlet velocities on such applications. Erosion damage resulting from flashing can belimited by using chrome-moly body material and Stellite overlaid trim. On smallervalve applications that remain closed most of the time, such as emergency heater drainvalves, velocities of up to 1500 feet per second (457 m/sec) may be acceptable.

In gas applications where special noise attenuation trim is used, the velocitydownstream of the valve should be limited to approximately 0.33 Mach. In addition,pipe velocities downstream from the valve are critical to the overall noise level.Experimentation has shown that velocities around 0.5 Mach can create substantialnoise, even in a straight pipe. The addition of a control valve to the line will increasethe turbulence downstream, resulting in even higher noise levels. Equations to calculateMach velocities are given later in this section.

A comparison of the velocities stated above with those listed as reasonable for pipe[5.3] reveals a considerable disparity due to different considerations required for sizingpipe and sizing control valves.

The most important consideration when sizing pipe is line loss. Pipe and installationcost must be weighed against the cost of energy required to move fluid through thepiping system. Losses due to velocity in the valve body (not to be confused with totaldrop across the valve) are inconsequential compared with the piping system and,therefore, are not a factor in determining energy requirements for the system.

Expansion Factor (Y). The expansion factor (Y) accounts for the variation in specificweight as the gas passes from the valve inlet to the vena contracta. Y also accounts forthe change in cross-sectional area of the vena contracta as the pressure drop is varied.

Ratio of Specific Heats Factor (FK). The ratio of specific heats factor (FK) adjusts theequation to account for the different behavior of gases other than air.

Terminal Pressure Drop Ratio (xT). The terminal pressure drop ratio for gases (xT) is usedto predict the choking point where additional pressure drop (by lowering thedownstream pressure) will not produce additional flow due to the sonic velocitylimitation across the vena contracta. This factor is a function of the valve geometry andvaries similarly to FL, depending on the valve type.



24-9

Compressibility Factor (Z). The compressibility factor (Z) is a function of the reducedtemperature and the reduced pressure of a gas. Z is used to determine the density of agas at its actual temperature and pressure conditions.

24.1.3 Sizing Formulas and Procedures for Liquid Flow

The equation for the flow coefficient (Cv) in liquid flow is:

a

f

Rpv

P

G

FF

qC

∆= (Equation 24-2)

where

Cv = Valve flow coefficient, psigpm

Fp = Piping geometry factor

FR = Reynolds number factor

q = Flow rate, gpm

∆Pa = Allowable pressure drop across the valve in psi

Gf = Specific gravity of the flow medium at flowing temperature

The following steps should be used to compute the required Cv, body size, and trimsize:

Step 1: Calculate actual pressure drop.

The actual pressure drop across the valve (∆P) may be found using the followingequation:

21 PPP −=∆ (Equation 24-3)

Where:

P1 = Valve inlet pressure, psia

P2 = Valve outlet pressure, psia



24-10

The allowable pressure drop may be less than the actual pressure drop if the flow ischoked.

Step 2: Check for choked flow, cavitation, and flashing.

( )vF12L(choked) PFPFP −=∆ (Equation 24-4)

Where:

FL = Liquid pressure recovery factor

FF = Liquid critical pressure ratio factor

Pv = Vapor pressure of the liquid at inlet temperature, psia

See Table 24-1 and Figures 24-4 and 24-5 for FL values, and Figure 24-6 for FF

values.

Table 24-1Typical Valve Recovery Coefficients (F L) and Incipient Cavitation Factors (F i)

NOTE: Values are given for full open valves unless otherwise stated.

Valve Type Flow Direction Trim Size FL Fi

Globe Flow-to-close Full Area 0.85 0.76

Flow-to-close Reduced Area 0.80 0.72

Flow-to-open Full Area 0.90 0.81

Flow-to-open Reduced Area 0.90 0.81

Butterfly 60° Open Full 0.74 0.64

90° Open Full 0.56 0.49

Ball 90° Open Full 0.60 0.54

Multi-Stage Under Seat All 1.0 1.0



24-11

Figure 24-4Globe Valve F L Values



24-12

Figure 24-5High Performance Butterfly/Ball F L Values

The liquid critical pressure ratio factor (FF) can be found from Figure 24-6 or estimatedusing the following relationship:

P

P 0.28 0.96 F

c

vF −= (Equation 24-5)

Where:

FF = Liquid critical pressure ratio

Pv = Vapor pressure of the liquid, psia

Pc = Critical pressure of the liquid, psia (see Table 24-2 for typicalcritical pressures)



24-13

Figure 24-6Liquid Critical Pressure Ratio Factor Curve



24-14

Table 24-2Typical Critical Pressure Values

Liquid Critical Pressure(psia)

Liquid Critical Pressure(psia)

Ammonia 1,636.1 Hydrogen Chloride 1,205.4

Argon 707.0 Isobutane 529.2

Benzene 710.0 Isobutylene 529.2

Butane 551.2 Kerosene 350.0

Carbon Dioxide 1,070.2 Methane 667.3

Carbon Monoxide 507.1 Nitrogen 492.4

Chlorine 1,117.2 Nitrous Oxide 1,051.1

Dowtherm A 547.0 Oxygen 732.0

Ethane 708.5 Phosgene 823.2

Ethylene 730.5 Propane 615.9

Fuel Oil 330.0 Propylene 670.3

Fluorine 757.0 Refrigerant 11 639.4

Gasoline 410.0 Refrigerant 12 598.2

Helium 32.9 Refrigerant 22 749.7

Hydrogen 188.1 Sea Water 3,200.0

Water 3,198.7



24-15

If ∆P(choked), as calculated from Equation 24-4, is less than the actual pressure drop fromEquation 24-3, use ∆P(choked) in Equation 24-2.

It may also be useful to determine the point at which cavitation begins. If cavitation ismarginal, it may possibly be eliminated by simply turning the valve end-for-end in theline, if permitted for this application. With the direction of flow thus reversed,reconfirm the adequacy and stability of the actuator. The following equation defines thepressure drop at which cavitation begins:

)P(PFP v12i)(incipient −=∆ (Equation 24-6)

Where:

Fi = Liquid incipient cavitation factor

(Typical values for Fi are given in Table 24-1 and Figure 24-7.)

P1 = Upstream pressure, psia

Pv = Vapor pressure of the liquid, psia

The required Cv for flashing applications is determined by using the appropriateallowable differential pressure (∆P(choked)), calculated from Equation 24-4, or (∆P(actual)),whichever is less, in Equation 24-2.

Step 3: Determine specific gravity.

Specific gravity is generally available from a number of different sources for theflowing fluid at the operating temperature.

Step 4: Calculate approximate Cv Fp FR.

Generally FR can be ignored, provided the valve is not operating in a laminar flowregion due to high viscosity, very low velocity, or small Cv.

In the event there is some question, calculate the Cv Fp, assuming the Reynolds numberfactor (FR) is 1.0, and then proceed to Step 5. If the valve Reynolds number is greaterthan 2000, FR can be ignored (proceed to Step 7).

Step 5: Calculate the Reynolds number and Reynolds number factor.

To obtain the Reynolds number factor (FR), first calculate the valve Reynolds number(Rev) using the following equation, and then obtain FR from Figure 24-8:



24-16

pvLv

dv

FCF S

F q 17,300 Re = (Equation 24-7)

Where:

Rev = Valve Reynolds number

q = Fluid flow rate, gpm

Fd = Valve style factor (1.0 for globe valves, 0.71 for ball and butterflyvalves)


FL = Valve recovery coefficient

Cv = Assumed flow coefficient calculation

Sv = Kinetic viscosity of the flowing medium, centistokes



24-17

Figure 24-7Globe Valve Liquid Incipient Cavitation Factor (F i) Values



24-18

Figure 24-8Reynolds Number Factor

Step 6: Recalculate Cv Fp using the Reynolds number factor.

With the Reynolds number factor (FR), recalculate the CvFp using Equation 24-2. If theoriginal and recalculated values of CvFp are within ±10% of each other, then use therecalculated values of CvFp. If the two numbers vary by more than ±10%, then use therecalculated CvFp to calculate Rev again.

Step 7: Select the approximate body size based on CvFp.

From the Cv tables (24-3 and 24-4), select the smallest body size that will handle thecalculated CvFp.

Step 8: Calculate the piping geometry factor

If the pipe size is not given, use the approximate body size (from Step 7) to choose thecorresponding pipe size. The pipe size is used to calculate the piping geometry factor



24-19

(Fp), which can be determined from Tables 24-5 and 24-6. If the pipe diameter is thesame as the valve size, Fp is 1.

Step 9: Calculate the final Cv from Cv Fp.

Step 10: Calculate the valve exit velocity.

The following equation is used to calculate entrance or exit velocities for liquids:

A

q 0.321 V = (Equation 24-8)

Where:

q = Liquid flow rate, gpm

V = Velocity, ft/sec

A = Applicable flow area of body exit or inlet port, in2



24-20

Table 24-3Typical Values of C v: Globe Valve, Flow over the Seat

Valve Type: UnbalancedBody Rating: Class 150-600Trim Characteristics: Equal PercentageFlow Direction: Flow Over

For each valve size below, the full area values are shown on top for each size. Reducedtrim values follow, in descending order. All valve sizes in this table are in inches.



24-21

Table 24-4Typical Values of C v: Globe Valve, Flow under the Seat

Valve Type: UnbalancedBody Rating: Class 150-600Trim Characteristics: Equal PercentageFlow Direction: Flow Under

For each valve size below, the full area values are shown on top for each size. Reducedtrim values follow, in descending order. All valve sizes in this table are in inches.



24-22

Table 24-5Typical Piping Geometry Factors, F p : Valve with both Reducer and Expander

Ratio d/D

Valve Size (in.) 0.50 0.60 0.70 0.80 0.90

Globe Class 150-1500

1/2, 3/4 0.91 0.93 0.96

1-6 0.94 0.95 0.97 0.98

8-24 0.96 0.97 0.98 0.99 1.00

30-48 0.92 0.94 0.96 0.98 0.99

Class 2500

1/2-16 0.98 0.98 0.99 0.99 1.00

Butterfly/Ball

3 0.84 0.87 0.91 0.95

4, 6 0.80 0.84 0.88 0.94 0.98

8-12 0.77 0.82 0.87 0.93 0.98

14-24 0.70 0.75 0.82 0.90 0.97

Where:d = Nominal valve size in inchesD = Internal diameter of the piping in inches



24-23

Table 24-6Typical Piping Geometry Factors, F p: Valve with Outlet Expander Only

Ratio d/D

Valve Size (in.) 0.50 0.60 0.70 0.80 0.90

Globe Class 150-1500

1/2, 3/4 1.05 1.06 1.07

1-6 1.03 1.04 1.04 1.04

8-24 1.02 1.03 1.03 1.03 1.02

30-48 1.04 1.05 1.06 1.05 1.03

Class 2500

1/2-16 1.01 1.01 1.01 1.01 1.01

Butterfly/Ball

3 1.11 1.14 1.15 1.14 1.09

4, 6 1.16 1.21 1.24 1.21 1.12

8-12 1.20 1.27 1.31 1.27 1.16

14-24 1.36 1.52 1.62 1.52 1.28

Where:d = Nominal valve size in inchesD = Internal diameter of the piping in inches

The maximum effective pressure drop (∆P(choked)) may be affected by the use of reducersand expanders. This is especially true of ball and butterfly valves.

After calculating the exit velocity, compare the calculated number to the acceptablevelocity for that application. It may be necessary to go to a larger valve size.

Step 11: Recalculate Cv if the body size has changed.

Recalculate Cv if the Fp has been changed due to the selection of a larger body size.



24-24

Step 12: Select the trim size.

First, identify if the valve will be used for on/off or throttling service. Using the Cv

tables, select the appropriate trim size for the calculated Cv and the body size selected.The trim size and flow characteristic may be affected by how the valve will bethrottled.

Example One (for Liquid Sizing):

Given:

LiquidCritical Pressure (Pc)TemperatureUpstream Pressure (P1)Downstream Pressure (P2)Specific GravityValve ActionLine SizeFlow RateVapor Pressure (Pv)Kinematic Viscosity (Sv)Flow Characteristic

Water3198.7 psia250°F314.7 psia104.7 psia0.94Flow-to-open4-inch (Class 600)500 gpm30 psia0.14 centistokesEqual Percentage

Solution:

Step 1: Calculate actual pressure drop using Equation 24-3:

∆P = 314.7 psia - 104.7 = 210 psid

Step 2: Check for choked flow. Find FL using Table 24-1. Looking under “globe, flow-to-open,” find FL as 0.90. Next, estimate FF using Equation 24-5:

0.93 3,198.7

30 0.280.96 FF =−=

Insert FL and FF into Equation 24-4:

∆P(choked) = 0.902 [314.7 - (0.93) (30)] = 232.3 psid



24-25

Since the actual ∆P is less than ∆P(choked), the flow is not choked; therefore, use the actual∆P to size the valve. At this point, also check for incipient cavitation using Equation 24-6 and Table 24-1:

∆P(incipient) = 0.812 (314.7 - 30) = 187 psid

Since ∆P(actual) exceeds ∆P(incipient), cavitation is occurring, but the flow is not choked.Special attention should be paid to material selection.

Step 3: The specific gravity for water is given as 0.94.

Step 4: Calculate the approximate Cv FpFR using Equation 24-2, assuming FR and Fp are 1:

33.4 210

0.94 500 Cv ==

Step 5: Calculate the valve Reynolds number (Rev) using Equation 24-7:

6v 10x11.3

(1)(33.4)(0.90) 0.14

(1) (500) (17,300) Re ==

Then, referring to Figure 24-8, FR is 1 (since the flow is turbulent).

Step 6: Since FR is 1, the recalculated Cv Fp remains as 33.4.

Step 7: From Table 24-4 (flow-under, equal percentage, Class 600), select the smallestbody size for a Cv of 33.4, which is a 2-inch body.

Step 8: Using the 2-inch body from Step 7, determine the Fp using Table 24-5:

0.5 4

2

D

d ==

Therefore, according to Table 24-5, Fp is 0.94.

Step 9: Recalculate the final Cv from Cv Fp.

35.5 0.94

33.4 C

(final)v == psigpm/

Step 10: Using Equation 24-8, the velocity for a 2-inch body is found to be nearly 51ft/sec. Since this application is cavitating, this velocity may damage a 2-inch valve.Therefore, calculate the velocity for a 3-inch body. This choice lowers the velocity to 22



24-26

ft/sec, which is acceptable. In this example, a 4-inch valve with 2-inch trim could alsobe chosen. It may be less costly than a 3-inch valve, and the larger outlets will lower thevelocities. It may also be less costly to install a 4-inch valve in a 4-inch line.

Step 11: Since the body size has changed, recalculate the Cv by following Steps 8 and 9.The Fp is now 0.97 and the final Cv is 34.4.

Step 12: Referring to the Cv tables, for a Cv of 34, a 3-inch valve would require at least atrim size of 1.25. A trim size of 2.0 would also suffice.

Example Two (for Liquid Sizing):

Given:

LiquidCritical Pressure (Pc)TemperatureUpstream Pressure (P1)Downstream Pressure (P2)Specific GravityValve ActionLine SizeFlow RateVapor Pressure (Pv)Kinematic ViscosityFlow Characteristic

Ammonia1636.1 psia20°F149.7 psia64.4 psia0.65Flow-to-close3-inch (Class 600)850 gpm45.6 psia0.02 centistokesLinear

Solution:

Step 1: Calculate actual pressure drop using Equation 24-3.

∆P = 149.7 psia - 64.4 psia = 85.3 psid

Step 2: Check for choked flow. Find FL using Table 24-1. Looking under “globe, flow-to-close,” find FL as 0.85. Next, estimate FF using Equation 24-5:

91.01.636,1

6.4528.096.0FF =−=



24-27

Insert FL and FF into Equation 24-4:

∆P(choked) = (0.852) [149.7 - (0.91) (45.6)] = 78.2 psid

Since the actual ∆P is greater than ∆P(choked), the flow is choked and cavitating; therefore,use the ∆P(choked) to size the valve. Since the service is cavitating, special attention shouldbe given to material and trim selection.

Step 3: The specific gravity for ammonia is given as 0.65.

Step 4: Calculate the approximate Cv using Equation 24-2, assuming FR and Fp are bothequal to 1.

77.5 78.2

0.65 850 Cv == psigpm

Step 5: Calculate the valve Reynolds number (Rev) using Equation 24-7:

6v 10x90.6

(1)(77.5)(0.85) 0.02

(1) (850) (17,300) Re ==

Then, referring to Figure 24-8, FR is 1 (since the flow is turbulent).

Step 6: Since FR is 1, the recalculated Cv remains 77.5.

Step 7: From Table 24-3 (flow-over, linear, Class 600), select the smallest body size for aCv of 77.5, which is a 3-inch body.

Step 8: With the 3-inch body and 3-inch line, Fp = 1.

Step 9: With Fp = 1, the final Cv remains as 77.5.

Step 10: Using Equation 24-8, the velocity for a 3-inch body is found to beapproximately 38 ft/sec. Although this velocity is less than 50 ft/sec, cavitation maystill damage the valve. However, since the valve size cannot exceed the line size of 3inches, a larger valve size cannot be chosen to lower the velocity. Pressure cavitationdamage could be the result in this situation. A cavitation control style trim should beconsidered.

Step 11: Cv recalculation is not necessary since the body size did not change.

Step 12: Referring to Table 24-4, a 3-inch valve with a trim size of at least 2.00, or thefull size of 2.62, will furnish the required Cv of 77.5.



24-28

Flashing Liquid Velocity Calculations: When the valve outlet pressure is lower than orequal to the saturation pressure for the fluid temperature, part or all of the fluid flashesinto vapor. When flashing exists, the following equation, applicable to any fluid, can beused to determine flow velocity:

WV 100%

X V

100%

X 1

A

0.040 V g2

pf2

p

+

−= (Equation 24-9)

Or if the fluid is water, the following equation can be used:

q V100%

X V

100%

X 1

A

20 V g2

pf2

p

+


Where:

V = Velocity, ft/sec.

W = Liquid flow rate, lb/hr

q = Inlet liquid flow rate, gpm

A = Applicable flow area, in2

Vf2 = Saturated liquid specific volume at outlet pressure, ft3/lb

Vg2 = Saturated vapor specific volume at outlet pressure, ft3/lb

Xp = Liquid mass flashed to vapor, percent

Calculating Percentage Flash: The percent flash (Xp) can be calculated fromEquation 24-11:

%)100(h

hhX

fg2

f2f1p


Where:

Xp = Liquid mass flashed to vapor, percent

hf1 = Enthalpy of saturated liquid at inlet temperature, Btu/lb

hf2 = Enthalpy of saturated liquid at outlet pressure, Btu/lb

hfg2 = Enthalpy of evaporation at outlet pressure, Btu/lb



24-29

When the fluid of concern is water, the enthalpies (hf1, hf2, and hfg2) and specific volumes(Vf2 and Vg2) can be found in the saturation temperature and pressure tables and theenthalpy (hf1) in the saturation temperature tables of a set of steam tables.

Flashing Liquid Example. Assume that the water temperature at the valve inlet is350°F, and the pressure at the valve outlet is 105 psia. Referring to a set of saturatedsteam temperature tables, the saturation pressure of water at 350°F is found to be 134.5psia, which is greater than the outlet pressure of 105 psia. Therefore, the fluid isflashing. Since a portion of the liquid is flashing, Equations 24-10 and 24-11 must beused. Xp (percent flashed) can be determined using Equation 24-11 and the giveninformation:

hf1 = 321.8 Btu/lb at 350°F (from saturation temperature tables)

hf2 = 302.3 Btu/lb at 105 psia (from saturation pressure tables)

hfg2 = 886.5 Btu/lb at 105 psia (from saturation pressures tables)

2.2% (100%) 886.5

302.3 321.8 X p =−=

Therefore, the velocity can be determined from Equation 24-10 using the followinginformation:

Vf2 = 0.0178 ft3/lb at 105 psia (from saturation pressure tables)

Vg2 = 4.234 ft3/lb at 105 psia (from saturation pressure tables)

ft/sec352 )234.4( %100

%2.2 )0178.0(

%100

%2.2 1

14.3

)500()20( V =

+

−=

This velocity is within the acceptable range.

24.1.4 Sizing Formulas and Procedures for Gas Flow

Because of compressibility, gases and vapors expand as the pressure drops at the venacontracta, decreasing their specific weight. To account for the change in specific weight,an expansion factor (Y) is introduced into the valve sizing formula. The form of theequation used is one of the following, depending on the process information available:

11vp PxYCF 63.3W γ= (Equation 24-12)



24-30

ZTG

x YPCF 1,360 Q

1g1vp= (Equation 24-13)

ZT

M x YPCF 19.3 W

1

w1vp= (Equation 24-14)

ZT M

x YPCF 7,320 Q

1w1vp= (Equation 24-15)

Where:

W = Gas flow in pounds per hour


Cv = Valve sizing coefficient, psigpm

Y = Expansion factor

x = Ratio of actual pressure drop to absolute inlet pressure = (∆P/P1)

γ1 = Specific weight at inlet conditions in pounds per cubic feet

Q = Gas flow in standard cubic feet per hour (SCFH)

Gg = Specific gravity of gas relative to air at standard conditions

T1 = Absolute upstream temperature, °R = (°F + 460)

Z = Compressibility factor

Mw = Molecular weight

P1 = Upstream absolute pressure, psia

The numerical constants in the above equations are unit conversion factors.

The following steps should be used to compute the correct Cv, body size, and trim size:

Step 1: Select the appropriate equation.



24-31

Based on the information available, select one of the four equations: 24-12, 24-13,24-14, or 24-15.

Step 2: Check for choked flow.

Determine the pressure drop ratio (xT) for the valve by referring to Table 24-7.

Next, determine the ratio of specific heats factor (Fk) using Equation 24-16:

1.40

k Fk = (Equation 24-16)

Where:

Fk = Ratio of specific heats factor

k = Ratio of specific heats (taken from Table 24-8, Gas Physical Data)

Table 24-7Terminal Pressure Drop Ratios (x T)

Valve Type Flow Direction Trim Size xT

Globe Flow-to-close Full Area 0.70

Flow-to-close Reduced Area 0.70

Flow-to-open Full Area 0.75

Flow-to-open Reduced Area 0.75

Multi-Stage Under Seat All 1.00

High Performance 60° Open Full 0.46

Butterfly 90° Open Full 0.26

Ball 90° Open Full 0.25



24-32

Table 24-8Gas Physical Data

GasCritical

Pressure (psia)Critical

Temperature (°R)Molecular

Weight (Mw)Ratio of Specific

Heats (k)Air

Ammonia

Argon

Carbon Dioxide

Carbon Monoxide

Ethylene

Ethane

Helium

Hydrogen

Methane

Natural Gas

Nitrogen

Oxygen

Propane

Steam

492.4

1636.1

707.0

1070.2

507.1

730.5

708.5

32.9

188.1

667.3

667.3

492.4

732.0

615.9

3198.7

227.1

730.0

271.4

547.5

239.2

508.3

549.7

9.34

59.7

343.0

343.0

227.1

278.2

665.6

1165.5

28.9

17.0

39.9

44.0

28.0

28.0

30.0

4.00

2.01

16.04

16.04

28.0

32.0

44.0

18.02

1.4

1.31

1.67

1.29

1.4

1.40

1.19

1.66

1.4

1.31

1.31

1.4

1.4

1.13

1.33

Calculate the ratio of actual pressure drop to absolute inlet pressure (x) using Equation24-17.

1P

Px

∆= (Equation 24-17)

Where:

x = Ratio of pressure drop to absolute inlet pressure

∆P = Pressure drop (inlet pressure minus outlet pressure)

P1 = Absolute inlet pressure (psia)



24-33

Choked flow occurs when x reaches the value of FkxT. Therefore, if x is less than Fk xT,the flow is not choked. If x is greater than Fk xT, the flow is choked. If the flow is choked,then FkxT should be used in place of x (whenever it applies) in the gas sizing equations.

Step 3: Calculate the expansion factor.

The expansion factor (Y) may be expressed as:

Tk xF 3

x 1 Y −= (Equation 24-18)

If the flow is choked, use FkxT instead of x (that is, Y = 2/3 at choked flow).

Step 4: Determine the compressibility factor.

To obtain the compressibility factor (Z), first calculate the reduced pressure (Pr) and thereduced temperature (Tr):

P

P P

c

1r = (Equation 24-19)

Where:

Pr = Function of reduced pressure

P1 = Upstream pressure, psia

Pc = Critical pressure, psia (from Table 24-8)

T

T T

c

1r = (Equation 24-20)

Where:

Tr = Function of reduced temperature

T1 = Absolute upstream temperature, °R

Tc = Critical absolute temperature (from Table 24-8)

Using the factors Pr and Tr, find Z in Figure 24-9 or 24-10.



24-34

Figure 24-9Compressibility Factors for Gases with Reduced Pressures from 0 to 40



24-35

Figure 24-10Compressibility Factors for Gases with Reduced Pressures from 0 to 6

Step 5: Calculate Cv.

Using the above calculations, use one of the gas sizing equations to determine Cv

(assuming Fp = 1).

Step 6: Select approximate body size based on Cv.

From Tables 24-3 and 24-4 or manufacturer’s data, select the smallest body size thatwill provide the calculated Cv.

Step 7: Calculate the piping geometry factor (Fp).



24-36

If the pipe size is not given, use the approximate body size (from Step 6) to choose thecorresponding pipe size. The pipe size is used to calculate the piping geometry factor(Fp), which can be determined from Table 24-5 or 24-6. If the pipe diameter is the sameas the valve size, then Fp = 1.

Step 8: Calculate the final Cv.

With the calculation of Fp, determine the final Cv.

Step 9: Calculate the valve exit velocity.

Equations 24-21, 24-22, 24-23, or 24-24 are used to calculate entrance or exit velocities(in terms of the approximate Mach number). Use Equations 24-21 or 24-22 for gases,Equation 24-23 for air, and Equation 24-24 for steam.

M

Tk A5,574

Q M

w

gas = (gas flow) (Equation 24-21)

S

Tk A1,035

Q M

g

gas = (gas flow) (Equation 24-22)

T A1,225

Q M air = (air flow) (Equation 24-23)

T A1,515

S W M v

steam = (steam flow) (Equation 24-24)

Where:

M = Exit Mach number

Q = Actual flow rate in cubic feet per hour (CFH, not SCFH)

A = Applicable flow area (square inches) of body exit

T = Absolute temperature °R (= °F + 460)

W = Mass flow rate (lb/hr)

Sv = Specific volume at flow conditions (ft3/lb)



24-37

Sg = Specific gravity at standard conditions

Mw = Molecular weight

k = Ratio of specific heats

To convert SCFH to CFH, use Equation 24-25:

s

ss

a

aa

T

VP

T

VP = (Equation 24-25)

Where:

Pa = Actual operating pressure, psia

Va = Actual volume in cubic feet per hour (CFH)

Ta = Actual temperature in °R (= °F + 460)

Ps = Standard pressure, 14.1 psia

Vs = Volume in standard cubic feet per hour (SCFH)

Ts = Standard temperature, 520°R (= 60°F + 460)

After calculating the exit velocity, compare the calculated velocity to the acceptablevelocity for that application. Select a larger size valve if necessary.

Step 10: Recalculate Cv if the body size has changed.

Recalculate Cv if Fp has been changed due to the selection of a larger body size.

Step 11: Select the trim size.

Identify if the valve will be used for on-off or throttling service. Using the Cv tables,select the appropriate trim size for the Cv calculated and the body size selected. The trimsize and flow characteristic may be affected by how the valve is throttled.



24-38

Example One (for Gas Sizing)

Given:

Gas Steam

Temperature (T1) 910°R (450°F)

Upstream Pressure (P1) 139.7 psia

Downstream Pressure (P2) 49.7 psia

Mass Flow Rate (W) 10,000 lb/hr

Valve Action Flow-to-open

Critical Pressure (Pc) 3,198.7 psia

Critical Temperature (Tc) 1165.5°R (705.5°F)

Molecular Weight (Mw) 18.026

Ratio of Specific Heats (k) 1.33

Flow Characteristic Equal percentage

Line Size 2 inch (Class 600)

Specific Volume (Sv) 1.079 ft3/lb

Solution:

Step 1: Given the above information, Equation 24-14 can be used to solve for Cv.

Step 2: Referring to Table 24-7, the pressure drop ratio (xT) is 0.75. Calculate Fk usingEquation 24-16 and x using Equation 24-17:

0.95 1.40

1.33 Fk ==



24-39

64.0 139.7

49.7 139.7 x =−=

Therefore, Fk xT is 0.71 (= 0.95 x 0.75). Since x is less than Fk xT, the flow is not choked.Use x in all equations.

Step 3: Determine Y using Equation 24-18:

70.0 )71.0()3(

64.0 1 Y =−=

Step 4: Determine Z after calculating Pr and Tr using Equations 24-19 and 24-20:

04.0 7.198,3

7.139 Pr ==

0.78 460 5.705

460 0.450 Tr =

++=

Using Figure 24-9, Z is found to be 1.0.

Step 5: Determine Cv, using Equation 24-14, and assuming Fp = 1:

1.47 )026.18()64.0(

)0.1()910(

)70.0()7.139()3.19(

000,10 CF vp ==

Step 6: From Table 24-4 (flow-under, equal percentage, Class 600), select the smallestbody size for a FpCv of 47.1, which is a 2-inch body.

Steps 7 and 8: Because the pipe size is the same as the body, Fp is 1. The Cv remains 47.1.

Step 9: Since the gas is steam, calculate the velocity using Equation 24-24:

075.0 460 450 )14.3( 515,1

)079.1()000,10( M steam =

+=

Step 10: The velocity is low and does not affect the current size selection.

Step 11: Referring to Table 24-4, a Cv of 47.1 would require a 2-inch body with a trimsize of 1.62.



xl



24-40

Example Two (for Gas Sizing):

Given:

Gas Natural Gas

Temperature (T1) 525°R (65°F)

Upstream Pressure (P1) 1314.7 psia

Downstream Pressure (P2) 99.7 psia

Flow Rate 2,000,000 SCFH

Valve Action Flow-to-open

Critical Pressure (Pc) 672.92 psia

Critical Temperature (Tc) 344.2°R (-115.80°F)

Molecular Weight (Mw) 16.042

Ratio of Specific Heats (k) 1.32

Flow Characteristic Linear

Line Size Unknown (Class 600)

Specific Gravity (Sg) 0.55

Solution:

Step 1: Given the above information, Equation 24-15 can be used to solve for Cv.

Step 2: Referring to Table 24-7, the pressure drop ratio (xT) is 0.75.

Calculate Fk using Equation 24-16 and x using Equation 24-17:

0.94 1.40

1.32 F k ==



24-41

0.92 1,314.7

99.7 1,314.7 x =−=

Therefore, Fk xT is (0.94) (0.75) or 0.71. Since x is greater than Fk xT, the flow is choked.Use Fk xT in place of x in all equations.

Step 3: Determine Y using Equation 24-18:

0.67 (0.71) 3

0.71 1 Y =−=

Step 4: Determine Z by calculating Pr and Tr using Equations 24-19 and 24-20:

95.1 92.672

7.314,1 Pr ==

53.1 460 80.115

460 65 Tr =

+−+=

Using Figure 24-9, Z is found to be 0.85.

Step 5: Determine Cv, using Equation 24-15 and assuming Fp is 1:

31.1 0.71

(0.85)(525)(16.04)

(0.67)(1,314.7)(7,320)

2,000,000 Cv == psigpm/

Step 6: From Table 24-4 (flow under, linear, Class 600), select the smallest body size fora Cv of 31.1, which is a 1-1/2-inch body.

Steps 7 and 8: Since the pipe size is unknown, use 1 as the Fp factor. The Cv remains31.1.

Step 9: Since the gas is natural gas, calculate the velocity using Equation 24-22:

6.49

0.55

460)(65 (1.32) (1.23)(1,035)

*293,209 M gas =

+=

* To convert SCFH to CFH, use Equation 24-25.

Step 10: Mach numbers in excess of sonic velocity at the outlet of the valve are notpossible. A larger valve size should be selected to bring the velocity below the sonic



24-42

level. To properly size the valve, select a size to reduce the velocity to less than 1.0Mach.

Step 11: Using Equation 24-22, solve for the minimum valve area (A) required forsubsonic velocity as follows:

1.0

0.55

460)(65(1.33) A 1,035

CFH 293,209 M gas =

+=

A = 7.95 inch2

Solve for the valve diameter from the area by:

4

d A

2π=

inch 3.18

(7.95)(4)

4A d ===

ππ

Thus, a 4-inch valve is required.

Step 12: Referring to Table 24-4, a 4-inch with a trim size of 1.62 would provide therequired Cv of 31.1 psigpm/ .

24.2 Examples of Sizing for Special High Pressure Drop Applications

The following examples describe detail sizing for the high pressure drop applicationsdiscussed in Section 6.2.

24.2.1 Feedwater Recirculation

Modern high speed, high pressure feedwater pumps require protection against no-flowor low-flow conditions, which could cause overheating and resultant pump damage.Provision for recirculating the pump discharge to the deaerator or condenser allows anadequate flow of water through the pump.

One of the most difficult control valve applications in a power plant is the boilerfeedwater pump recirculation control valve. A method of recirculation control that hasbeen used in the past is to use a control valve in series with a breakdown orifice asshown in Figure 24-11. In this scheme, the control valve is utilized merely for on-offcontrol. The pressure drop through the valve is selected so that it is well below the



24-43

cavitation range, and the remaining drop is taken across the breakdown orifice. Inaddition to the cavitation problem, tight shutoff is important since even the smallestleakage across the seat will destroy the trim in a relatively short period of time underhigh pressure drop conditions. By careful selection of orifice size and sufficient seatloading, this approach has been marginally successful in the past.

In the control of high pressure reduction of liquids, conventional valves convert inletpressure to kinetic energy with corresponding excessive velocities. These highvelocities result in erosion, vibration, and usually cavitation. Control of high pressureliquids, without the erosive effects that lead to premature failure of conventionalvalves, is currently accomplished through the use of high pressure anticavitation valvesused as shown in Figure 24-12. In these valves, the fluid pressure is reduced throughmultiple steps so that each step handles only a fraction of the total pressure drop. As aresult, there is neither cavitation nor the associated phenomena of noise and trimerosion.

Use of this type of valve, shown in Figure 24-13, eliminates the need for breakdownorifices. Smooth, vibration-free operation can be achieved with the multiple-step plugand cage design and is applicable wherever there is a need for high pressure reductionof liquids, especially those that tend to cavitate in conventional valves. The multiple-step design makes it ideal for modulating applications.



24-44

Figure 24-11Conventional Method of Recirculation Control: Control Valve (On-Off) in Serieswith a Breakdown Orifice

Figure 24-12Method of Recirculation Control Using High Pressure, Modulating Anti-CavitationValve



24-45

Figure 24-13Globe Angle Control Valve with Anti-Cavitation Trim



24-46

Example:

Required: A recirculation control valve for use with a turbine-driven boilerfeedwater pump to operate under the following conditions:

Inlet Pressure 3,500 psig (2515 psia)

Mass Flow Rate 505,000 lb/hr

Fluid Temperature 335°F

Solution:

Use the inlet pressure as the sizing pressure drop since this design can handle the dropwithout cavitation. No breakdown orifice is required.

Using Equation 24-5, the liquid critical pressure ratio is found to be:

0.91 3,198.7

110 0.28 0.96 FF =−=

The vapor pressure of water at 335°F is 110 psia [5.3]. The choked differential pressureis determined from Equation 24-4:

[ ] psi415,3)110)(91.0(515,31P(choked) =−=∆

Since the choked flow differential pressure is less than the actual differential pressure,this smaller value is applied in Equation 24-2 to determine the required Cv. The flowrate is converted from lb/hr to gpm.

18 3,415

0.9 1,122 Cv == psigpm/

Thus, the required Cv = 18. From manufacturer’s data, select a 3-inch valve:

2500 lb ANSI ratingCarbon steel bodyBalanced trim440-C plugTeflon soft seat (for tight shutoff)



24-47

24.2.2 Atmospheric Steam Dump and Turbine Bypass

The turbine bypass system is designed to function during various stages of plantoperation such as start-up, transient conditions, and decay heat removal. When theplant is in the start-up period, the turbine bypass system diverts steam to thecondenser, which is used as a heat sink for the reactor. The fluctuation in turbine steamdemand is absorbed by the bypass system.

Under transient conditions, when the steam generator system temperatures andpressures increase, the turbine bypass system creates a flow path from the steamgenerator to prevent lifting the steam safety valves due to excess pressure. During aturbine or generator trip, the bypass system releases steam while the reactor output isbeing reduced. The bypass system acts to regulate the steam flow and provide a meansfor stabilization of the steam generator and steam system. On cooldown operation, thebypass system diverts steam to the condenser.

Both the turbine bypass and atmospheric dump valves have the same generalrequirements. Flow depends on the size of the power plant. Since these valves remainin the closed position for long periods of time, it is essential that they have extremelytight shutoff characteristics. Any significant seat leakage would damage the trim in arelatively short period of time.

The multistage valve shown in Figure 24-14 has the ability to handle pressure drops upto 1100 psi, with tight shutoff in a temperature range to 1050°F, with excellent stability.Seat leakage meets ANSI/FCI 70-2-1976 (R1982) [6.12], Class IV, which is 0.0005 cc/minof water per psi pressure drop per inch of seat diameter.



24-48

Figure 24-14Globe Control Valve with Low Noise Trim



24-49

Example:

Required: A main steam atmospheric dump valve to operate under the followingconditions:

Inlet Pressure 1,025 psig (1,040 psia)

Outlet pressure Atmospheric

Flow 403,000 lb/hr

Temperature 547°F

Sound pressure level Less than 90 dBA 3 feet from valve

ASME Section III Class 2

Using Equation 24-16,

0.95 1.40

1.33 Fk ==

From Equation 24-17,

0.9856 1,040

521,0 x ==

From Table 24-7,

xT = 1

Therefore,

Fk xT = 0.95

x > Fk xT; therefore, the flow is choked

From Equation 24-18,

0.6667 (1)(0.95)(3)

0.95 1 Y =−=



24-50


199 (2.34) (1,040) (0.95) (0.6667) (63.3)

403,000 Cv == psigpm/

Valve Selection:

Required Cv is 199 psigpm/

Use an 8-inch valve

Carbon steel body

600 lb ANSI

Lo-dB trim with diffuser and Lo-dB plates for SPL less than 90 dBA

Rated Cv is 275

Spring diaphragm actuator

Stroking speed - less than 3 seconds

Maximum allowable pressure drop 1,100 psi

Seat leakage - ANSI/FCI 70-2-1976 (R1982)

Meets requirements of ASME Section III, Class 2

24.2.3 Attemperator Spray Control

The use of superheat and reheat with high steam temperatures in fossil power plants(which increases efficiency in power generation) makes accurate regulation of steamtemperature vital to successful operation. Other important reasons for control of steamtemperatures are to prevent failures from overheated parts of the superheater, reheater,or turbine and to avoid erosion from excessive moisture in the last stages of the turbine.

Since the temperature of the steam is directly related to the degree of expansionthrough the turbine elements, the steam temperature must be regulated withinpermissible limits by some means of accurate control.



24-51

One of the means available for controlling steam temperature in non-nuclear plants isby attemperation. An attemperator is a device that reduces the temperature of asuperheated vapor or of a fluid passing through it.

Attemperators may be classified as of two types surface and direct contact. In thesurface type, the steam is isolated from the cooling medium by the heat exchangersurface. In the direct contact type, the steam and the cooling medium are mixed. Thedirect contact type spray attemperator has proved successful for regulating steamtemperature. Feedwater is introduced into the superheated steam line through a spraynozzle at the throat of a venturi section within the line. Because of the spray action atthe nozzle, the water quickly vaporizes, mixes with, and cools the steam. The sprayattemperator is used to control the steam temperature in the superheater and thereheater. This application is similar to the boiler feedwater pump recirculation controlvalve application in that cavitation protection and tight shutoff is required. In theprimary superheater section, the full feedwater pump discharge pressure is used as theinlet pressure to the spray valve. In the reheater section, lower steam pressures areinvolved, and the customary practice is to tap an interstage section of the feedwaterpump.

In the control of high pressure reduction of liquids, conventional valves convert inletpressure to kinetic energy, which results in excessive velocities, causing erosion,vibration, and usually cavitation. A valve such as that shown in Figure 24-13 can beused to control high pressure liquids without the erosion effects that lead to prematurefailure of conventional valves. The fluid pressure is reduced through multiple steps sothat each step handles only a fraction of the total pressure drop. As a result, there isneither cavitation nor the associated phenomena of noise and trim erosion. The use ofthis style valve results in smooth, vibration-free operation, due to the multiple-stepplug and cage design, and is applicable wherever there is a need for high pressurereduction of liquids, especially those that tend to cavitate in conventional valves. Themultiple-step design makes it ideal for throttling applications.

Example:

Required: Superheat attemperator spray control valve to operate under the followingconditions:

Inlet pressure 2,288 psig (2,303 psia)

Outlet pressure 2,273 psig (2,288 psia)

Flow 180 gpm

Temperature 365°F



24-52

Solution:

From Equation 24-5,

0.90 3,198.7

162.7 0.28 0.96 FF =−=

From Equation 24-4,

∆P(choked) = 1 [(2,303) - (0.9) (162.4)] = 2,156 psi

From Equation 24-3,

∆P = 2,303 - 2,288 = 15 psi

∆P(choked) > ∆P; therefore, the flow through the valve is not choked.

Then from Equation 24-2

44 15

0.88 180 Cv == psigpm/

Required Cv is 44 psigpm/ . From manufacturer’s data, select a 3-inch valve:

1500-1b ANSI rating

Carbon steel body

Balanced trim

440-C plug

Teflon soft seat (for tight shutoff)

24.2.4 Deaerator Level Control

The condensate system, which begins at the condenser hotwell, provides feedwater forthe boiler and requires that a large quantity of water, stored in the deaerator storagetank, be available for the feedwater pump. The function of the condensate system is tomaintain sufficient levels in the hotwell and deaerator storage tank so that flowthrough the system is stable during transient load conditions. Figure 24-15 shows a



24-53

typical condensate system with secondary items, such as alarm switches and interlocks,removed for clarity. The level in the deaerating feedwater heater storage tank ismaintained by the deaerator level control valve that throttles the condensate pumpdischarge and regulates the flow of condensate to the deaerator. The requirements ofthis valve are twofold: (1) During startup and minimum load conditions, the deaeratorlevel control valve must handle low flow at a high pressure drop. (2) While undernormal loads, it must pass a large flow with a relatively low pressure drop.

Figure 24-16 shows a typical condensate system curve and the pressure drop availablefor the deaerator level valve at various load conditions. The high pressure drop at lowflow rate induces cavitation that, if not properly controlled, can destroy the valve.Therefore, the valve trim must be specifically designed to minimize the effects ofcavitation at low flows with near pump shutoff head on the valve inlet withatmospheric outlet pressure.

The valve shown in Figure 24-17, with a special cage to provide cavitation protectionfor approximately the first 25% of stroke and ample flow area to provide the requiredcapacity for the remainder of the stroke, is well suited for this application.

Figure 24-15Typical Condensate System



24-54

Figure 24-16Typical Condensate System Curve

Figure 24-17Globe Control Valve with Anti-Cavitation Variable Resistance Trim



24-55

Typical service conditions are:

Minimum Normal Maximum

Q (gpm) 700 5,860 6,590

P1 (psig/psia) 475/490 359/374 344/359

P2 (psig/psia) 35/50 222/237 267/282

T (°F) 100 151 156

Cv 33 495 743

At minimum conditions:


0.96 3,198.7

0.95 0.28 0.96 FF =−=

From Equation 24-3,

∆P = (475 - 35) = 440 psi

From Equation 24-4,

∆P(choked) = 1 [490 - (0.96) (0.95)] = 489 psi


Then from Equation 24-2,

33 440

0.94 700 Cv == psigpm/

At normal conditions:

0.95 3,198.7

3.75 0.28 0.96 FF =−=



24-56

∆P(choked) = 1 [374 - (0.95) (3.75)] = 370 psi

∆P = 374 - 237 = 137 psi


495 137

0.98 5,860 Cv == psigpm/

At maximum conditions:

0.95 3,198.7

4.5 0.28 0.96 FF =−=

∆P(choked) = 1 [359 - (0.95) (4.5)] = 355 psi

∆P = 344 - 267 = 77 psi


743 77

0.98 6,590 Cv == psigpm/

24.2.5 Feedwater Pump Flow Control

The feedwater system, which supplies water for the boiler/steam generator, is shownin Figure 24-18. The discharge from the feedwater pumps flows through high pressureheaters into the steam generator. Feedwater flow to the steam generator is controlled bya feedwater control valve that receives its signal from the feedwater control system. Theprimary function of this system is to maintain boiler/steam generator level under alloperating conditions.

The main feedwater pump flow control valve is another difficult application in a powerplant. During the startup and minimum load conditions, this valve must handle lowflow at a high pressure drop; while under normal loads, it must pass a large flow witha relatively low pressure drop.

The high pressure reduction at low loads causes cavitation that, if not properlycontrolled, can destroy the valve. However, above 15–20% of rated flow capacity, thepressure drop is reduced to the point where cavitation is no longer prevalent. Theconventional method of feedwater flow control is to use a startup valve in conjunctionwith the main feedwater control valve. The startup valve, which in recent years



24-57

contained some form of anti-cavitation trim, is utilized to handle the low flow highpressure drop conditions. Once beyond the cavitation range, the conventional controlvalve is used to obtain the required capacity. This approach has been successful in thepast but necessitated the additional expense of a bypass system with its associatedpiping and controls.

Figure 24-18Main Feedwater System



24-58

The valve shown in Figure 24-17 represents another approach to the control of highpressure liquids and can be used quite successfully in this application. Cavitationprotection is provided through the first 20% of the rated capacity by the variableresistance trim concept. Pressure distribution reduces the fluid pressure throughmultiple stages so that each stage handles only a fraction of the total pressure drop. Asa result, there is neither cavitation nor the associated phenomena of noise and trimerosion. Above 20% of the rated capacity, the conventional cage trim provides ampleflow capacity. This concept, with features such as axial flow through segmented plates,results in a feedwater control valve with high controllability, extended valve life, andmaximum cavitation protection.

Typical service conditions are:

Minimum Maximum

Q (gpm) 900 3,300

P1 (psig/psia) 3,300/3,315 2,650/2,665

P2 (psig/psia) 105/120 2,550/2,565

T (°F) 290 350

Cv 15 311

At minimum conditions:

From Equation 24-5,

0.92 3,198.7

57 0.28 0.96 FF =−=

From Equation 24-4,

∆P(choked) = 1 [3,315 - (0.92) (57)] = 3,262 psi

From Equation 24-3,

∆P = 3,315 - 120 = 3,195 psi




24-59

Then from Equation 24-2,

15 3,195

0.92 900 Cv == psigpm/

At maximum conditions:

0.90 3,198.7

134 0.28 0.96 F F =−=

∆P(choked) = 1 [2,665 - (0.90) (134)] = 2,544 psi

∆P = 2,665 - 2,565 = 100 psi


311 100

0.89 3,300 C v == psigpm/


25-1

25 APPENDIX C: VALVE PROCUREMENT

SPECIFICATION

25.1 General

The applicable code or standard has to be established prior to procuring a valve. Fornew construction, the code or standard is established by licensing documents/permitsfor nuclear plants, by the issue of standards referenced in the governing edition ofANSI B31.1, and by Appendix F of ANSI B31.1 for non-nuclear balance-of-plant and forfossil plants.

Once a nuclear plant goes into operation, ASME XI governs for the nuclear plantequipment. ASME XI requires that replacements meet the requirements of the edition ofthe construction code to which the original component was constructed. ASME XIpermits replacements to meet the requirements of all, or a portion of, later editions ofthe construction code or ASME III if the four requirements described in ASME XI, IWA-7210 [6.8], are met. ANSI B31.1 has no similar rules or instructions for a plant in-service.

When ordering replacement nuclear valves, some utilities invoke the older codes, aspermitted by ASME XI. This can present some difficulties, particularly for older plants.Manufacturers, for the most part, are geared up to present-day codes, and materials.Manufacturers do not produce materials to older codes or standards. However,regardless of the code specified in the valve order, the valves must be manufacturedunder a Quality Assurance Program that complies with 10CFR50, Appendix B.Discussions with manufacturers indicate ordering to older codes or standards is stilloccurring, but the trend is to order using the latest code or standard. Replacementvalves for ANSI B31.1 applications should use the newer codes or standards.

When preparing specifications for valves for nuclear plants, it is recommended that thefollowing three documents be consulted for overall content:

1. Guidelines for Preparing Specifications for Nuclear Power Plant Applications (NCIG-04),EPRI Report No. NP-5638, dated April 1988, prepared for the Nuclear ConstructionIssues Group and the Electric Power Research Institute.


Appendix C: Valve Procurement Specification

25-2

2. ANSI N278.1-1975 - Self-Operated And Power-Operated Safety-Related Valves FunctionalSpecification Standard. (This document is currently undergoing revision and will bereissued as part of ANSI/ASME, QME-1).

3. ASME III, NCA-3250.

The selection of the valves to be specified should be in accordance with Section 21 ofthis report.

25.2 Specific Elements

The following specific elements should be incorporated into a nuclear or non-nuclearvalve specification, as appropriate.

Note: Asterisk items are normally required only for nuclear safety-related valves.

Applicable Code and Pressure Class. For example: ASME III, CL1, 2, or 3; ASME III, CL150,300, etc.; or ANSI B16.34, CL150. Ensure that the pressure/temperature rating of theselected pressure class envelopes the design pressure and temperature of the systeminto which it will be installed.

Materials. Specify the body, bonnet, body-to-bonnet bolting material, and the stemmaterial. Specify the hard facing material if other than the manufacturer’s standard isdesired.

Consult ANSI B16.34 [6.24] and the applicable construction code for acceptablematerials for body and bonnet and any limitations such as that for temperaturelimitations. Consider any special needs. For example, where carbon steel would beacceptable for the general service and temperature, 1-1/4 Cr, 1/2 Mo may be advisabledue to its better erosion characteristics. Stainless steel bolting should always bespecified for stainless steel valves.

Stems on carbon steel gate and globe valves are normally series 400 stainless and17-4 pH or type 316 stainless steel on stainless steel valves. 17-4 pH, because of itshigher strength, is also a common shaft material for high performance butterfly valvesand is recommended for all valves, where practical. Stem material should be specifiedand the appropriate heat treating temperature or hardness should be specified tominimize susceptibility to intergranular stress corrosion cracking. See Section 2.4 for adiscussion of valve trim materials. Stainless steel balls and stems are recommended onall steel ball valves (The stem and plug are normally integral on sleeve-lined plugvalves).



25-3

Packing material should be specified rather than determined by the vendor. Whenordering a line valve or a resilient seated valve, be specific as to what generic materialis desired (for example, natural rubber, Buna N, Teflon, etc.).

End-to-End Dimension. For replacement valves, specify the end-to-end dimension of theoriginal valve which may be in accordance with ANSI B16.10 [6.17]. If the replacementvalve is shorter than the original valve, then consideration should be given to weld thenecessary pipe length to the ends of the replacement valve, with appropriate weldpreps, in order to shorten replacement time at the plant.

For new systems, specify ANSI B16.10. ANSI B16.10 permits options on end-to-enddimensions for types within the same generic grouping (for example, pressure sealvalves can be short pattern or long pattern). For control and relief valves, themanufacturer should be consulted.

End Connection. Specify the end connection that is compatible with the installationmethod and the piping into which it will be installed. For welding end valves, specifyANSI B16.25 [6.23] and the schedule of the piping. If special welding details ordimensions are required, a weld preparation drawing should be provided.

For socket weld valves, specify ANSI B16.11 [6.18]. Also specify the required hubthickness against which face the fillet weld will be applied to ensure that the welddimension of ASME III or ANSI B31.1 can be maintained.

For flange end valves, specify ANSI B16.5 [6.15] and the pressure class.

Overpressure Protection for Gate Valves. Specify the method to be used if overpressureprotection is required, such as providing a bypass from the bonnet cavity to theupstream nozzle or providing a relief valve or a connection for a relief valve to beprovided by others. Section 4.2.9 discusses overpressure protection options.

Internal Locking Device. Specify proven locking devices, and specify that internal lockingdevices should be of corrosion-resistant material.

Handwheels/Gear Operators. Specify the limit of rim pull for manual valves and motor-operated valves. The differential pressure will have to be specified. See Section 14 forrecommended handwheel rim pull limits.

Nondestructive Examination (NDE). Specify only that NDE that is really required. NDEspecified by the applicable code or standard is normally adequate. NDE in excess of thecodes and standards is recommended for such things as:

• Visual inspection to MSS-SP-55 [6.47] for all cast steel parts that act as structuralparts (vs. pressure-retaining parts) of the assembly, such as a valve yoke.



25-4

• NDE of butt weld ends of cast valves, using the same method that will be used forthe weld that joins the valves to the piping in the plant. This NDE should detectimperfections at the manufacturer’s shop that might otherwise be detected later inthe finished installation and would have to be evaluated.

Consumable Parts and Fasteners. Require that parts such as studs, bolts, nuts, gaskets,packing, and O-rings be identified as to specific material and that the dimensions bespecified. This is to allow replacements to be ordered directly from the partmanufacturer, thereby reducing costs, improving delivery times, and permittingconsolidation of spare parts.

Hydrostatic or Air Testing. Specify a hydrostatic shell and disc or seat test for all valves.Check the specified code or standard to see if an acceptance criterion for seat leakage isdefined. Some codes or standards give options for seat leakage or give no criteria at all.Ensure that seat leakage criteria is specified precisely in some manner.

An additional air seat test should also be specified if the valve will be tested with airafter it is in service, such as a containment isolation valve. The air test pressure shouldbe the same pressure at which the valve will be tested in service.

Operational Tests. Specify operational tests for all power-operated valves and checkvalves to ensure that valves operate freely with no binding.

Review the nuclear qualification requirements, and specify any further required tests(for example, operational tests with differential pressure, operational tests with externalloads applied, etc.). Active valves may require additional testing, such as a static loaddeflection test for operability.

Environmental Conditions. Environmental conditions should be specified for all ASMEIII valves, or specify environmental conditions that envelope the group of valves beingordered.

Seismic, Environmental, and Functional Qualifications. The extent and acceptable methodsof required seismic and environmental qualifications should be specified. The seismicand environmental conditions for all power-operated valves and certain manual valveshave to be specified. See ASME III, paragraph NB-3524 [6.3], for guidance for ASME III,class 1, 2, and 3 valves.

Evaluate the need to have active valves qualified in accordance with ANSI B16.41[6.25], Functional Qualification Requirements for Power Operated Active ValveAssemblies for Nuclear Power Plants.

Note that some fossil plants require seismic qualification for valves.



25-5

Drawings. Require drawings that show installation dimensions and provide informationas to the weight and center of gravity. For nuclear valves, the weight of the valveassembly should be the actual weight, ±10%.

In addition, require that detailed dimensional drawings of the valve internals beprovided if and at such time as the manufacturer is unable or unwilling to providereplacement parts of the required quality level.

Instruction Manuals. Require that comprehensive instruction manuals be provided togive details necessary for any required preventive or corrective maintenance.Dimensions should be provided for such things as seat angles, gasket joints, and otherparts that are dimensionally critical.

Shipment. Require that all nuclear valves be weighed prior to shipment to ensure thatcorrect weights are available for any required piping stress analysis.

Motor Operators. The manufacturer or the user should demonstrate the MOV capabilityto perform its functions under worst case conditions (including design basis conditions)by testing and/or analysis. Furthermore, to help ensure that the motor operator isproperly paired with the valve, the following points should be considered:

• Require that the valve and motor operator have the ability to withstand, withoutdamage, the stall torque/thrust at the maximum voltage. If this cannot be done, theseller must make a recommendation and must reach agreement with the purchaseron the course of action to be taken. The published nominal rating (the ratingwithout safety factor) must not be used without the purchaser’s concurrence.

• Require that motor operator sizing calculations be submitted for evaluation.

• Require that the valve assembly be furnished, complete and ready for operationwith all fasteners properly installed, including locking of the stem nut locknut.

• Specify the voltage range within which the valve is to operate.

• Specify self-locking gearing to ensure that the operator will not be back driven bythe valve when the motor stops with attendant periodic operation of the torqueswitch and motor. If this cannot be accommodated, the seller should receiveconcurrence on another course of action that can be taken.

• Ensure that there are a sufficient number of limit switches to perform all therequired functions of the valve.

• Specify a handwheel to operate the assembly manually. This handwheel should notrotate during electric operation, nor should a stalled motor prevent manual



25-6

operation. When the motor is energized, the motor operator should automaticallyreturn to electric operation.

• The motor operator should have a built-in lost motion device that permits the motorto attain full speed before the load is encountered and that imparts a hammerblowto start movement of the valve disc.

Power Operators. Require that power operators, including all accessories, be solidlymounted to the valve.

Pneumatic Operators. The attached Instrument Society of America (ISA) data sheet(Figure 25-5) shows the areas that must be considered in the specification of pneumaticoperators.

25.3 Data Sheets

Figures 25-1 through 25-7 are suggested valve data sheets that can be used tosupplement the text of a specification.

The use of data sheets for manual valves is highly recommended, rather than orderingvalves as a “commodity.” Data sheets for motor-operated valves, rupture discs, controlvalves, and relief valves are considered a necessity.

During the review of quotations or development of recommendations to purchase,there are certain precautions that the engineer should be aware of before the finalselection of a valve is made. When a valve specification is submitted to themanufacturer for bids, the manufacturer has several options available. They may bid asfollows:

• Quote strictly in accordance with the specification.

• Quote valves suitable for intended service as interpreted by the manufacturer.

• Take exception to the specification and include a variety of options that covervarious application contingencies.

When a quotation is strictly in accordance with the specification, the purchaser may notbe receiving the full advantage of the manufacturer’s experience with options thatcould provide a better valve installation. The vendor is not likely to include optionsthat would improve the installation if the additional cost could jeopardize the chance ofreceiving an order, since the order may be based on the lowest quoted cost. When therequest for a quotation includes words to the effect that no exceptions to thespecification will be accepted, the manufacturer will tend not to include items thatmight improve the valve installation/application. With these additional items, the total



25-7

price of the quote may be more, hence less attractive than a competitor’s quote that wasin accordance with the specification and took no exceptions.

Two potential problem areas occur when the manufacturer interprets the specification.First, the valve application may not be as the manufacturer assumed; consequently, thewrong valve could be quoted. Second, the pricing of various quotes may not beequivalent since various manufacturers could have made different assumptions;therefore, it is difficult to compare quoted price and select the most economical quote.

When the quote contains a variety of options, the judicious acceptance of those optionsmay provide the best overall valve application/installation. However, the quotationsmay be much more difficult to evaluate, due to the deviations from the basespecification by the various manufacturers who furnished quotations.

In summary, the selection of the proper valves for the intended applications requiresthe careful evaluation of the manufacturer’s response to the specification and inclusionof those deviations from the bid specification that are suitable and economicallyjustified. The addition of a little more cost during the engineering and constructionphase of the project (due to additional evaluation/review time or inclusion of optionsthat were not in the bid specification) can often be justified by the result that costlyrepairs or loss of operating time is avoided during the life of the installation.



25-8

Note Page

Spec. No.

1 P.O.

Date

Station:

Valve Identification No. 2

Valve Description No. 3

Type

Quantity

Size

Service/Fluid 4

Applicable Code/Standard and Class 5

End Connection 6

P&ID/Line No. 7

Design Pressure 8

Maximum Differential Pressure for Operation 9

Design Differential Pressure 10

Design Temperature 8

Max/Min Flow Rate 11

Materials

Body/Bonnet 12

Disc

Stem/Shaft

Ball/Plug

Body Lining

Packing

Seat Facing

Valve Location 13

Special Requirements 14

• All parameters must be specified in U.S. Customary or S.I. units.

Figure 25-1Suggested Manual Valve Data Sheet by Purchaser

Notes for Figure 25-1:

1. Inquiry number or purchase order number.

2. Unique valve number, if assigned. This number is for information only. It is notrecommended that valve be marked with this number by the manufacturer.



25-9

3. Generic number assigned for a valve type and material together with other featuresif used.

4. Brief description of valve usage (for example, main steam, feedwater, radioactiveliquid waste, resin slurry, etc.).

5. Identification of applicable code and standard (for example, ASME III CL2 CL150,ANSI B16.34 CL600, ANSI B16.34 Special CL1500, etc.).

6. Flanged (for example, ANSI B16.5 CL150), butt weld (for example, ANSI B16.25 Sch80), socket weld, threaded, etc.

7. P&ID and line number in which valve is installed.

8. Design pressure and temperature of the piping in which the valve is installed. SeeNote 14.

9. The maximum differential pressure against which the valve must operate under allnormal or transient conditions.

10. The maximum differential pressure to which the disc/ball/plug must be designed,if different from 9. If 9 and 10 are the same, combine 9 and 10. See Note 14e.

11. Both maximum and minimum flow rates for butterfly, ball, and check valves shouldbe given. The maximum flow rate should be given for other valves such as gate andglobe.

For check valves it is also recommended that normal flows be specified, togetherwith any change in process conditions (for example, steam pressure andtemperature for extraction non-return valves). For control valves, minimum,normal, and maximum flows should be given.

12. Generic description of materials for parts (for example, carbon steel, stainless steel,EPDM, etc.). Delete or add items as applicable. If items are addressed in the text ofthe specification, refer to the text.

13. Location within the plant and environmental qualifications are required.

14. Specify any special requirements, such as:

a. Radiation, if not specified elsewhere.

b. Fluid chemistry for lined valves, or unique service.



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c. For isolation valves used also for throttling, specify fluid, temperature, ∆Pacross valve, and flow rate under throttling conditions. This is particularlyimportant for globe, butterfly, and ball valves to ensure that the correct disccharacteristics are supplied.

d. Features and accessories not specified in data sheet or in text.

e. If the piping system can experience a transient condition when the designtemperature and pressure are exceeded, explain here. This is because pipingcodes and standards have provisions for exceeding design conditions that donot apply to valves.



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Note PageSpec. No.

1 P.O.Date

Station:Valve Identification No. 2Valve Description No. 3Valve

TypeQuantitySizePressure ClassManufacturer Figure No./Model No.End-to-End DimensionHandwheel Diameter/Lever LengthWeight (including operator if required)End Connection

Gear OperatorManufacturer’s Fig. No.TypeHandwheel Diameter

Materials (include material specification or manufacturerdesignation) 4

Body 4Bonnet 4Bonnet/Cover Bolting 4Disc 4Stem/Shaft 4Ball/Plug 4Body Lining 4Seat Facing 4Packing 4Pipe Plugs 4

Performance DataCv 5

% Open for Throttling Condition 6Maximum required shaft torque at maximum differential pressure

Normal Cycling 7After prolonged idleness 7

Maximum handwheel input at maximum differential pressure


Figure 25-2Suggested Manual Valve Data Sheet by Bidder/Seller



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1. Proposal number or purchase order number.

2. Unique valve number, if assigned.

3. Generic number assigned for a valve type and material, together with otherfeatures, if assigned.

4. Material used in construction of the valves. Use items as applicable. Require thatthe material designations be given or sufficient identifications provided so thematerials can be evaluated, such as:

Body - SA 351 CF8M

Stem/Shaft - ASTM A564 Gr 630

Lining - UHMP (Ultra High Molecular Weight Polyethylene)

Seat Facing - Stellite #6

5. Flow coefficients for all valves.

6. For throttling conditions, specify approximate positions of disc/ball/plug atgiven flow rates.

7. If sleeved plug valve or ball valve is ordered, the torque required under thesetwo conditions should be specified. The required torque can increasesignificantly if the valve is not cycled frequently.



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Note Page:

Spec. No.

1 P.O.

Date:

Station:

Valve Identification No. 2

Valve Description No. 3

Valve:

Manufacturer

Model

Code (Include Class for ASME III) 4

Press. Class & Type 5

Size 6

Service/Fluid 7

Design Pressure 8

Maximum Differential Pressure for Operation 9

Design Differential Pressure 10

Design Temperature 8

Max./Min. Flow Rates 11

Materials:

Body/Bonnet 12

Disc 12

Stem/Shaft 12

Packing 12

Seat Facing 12

Guide Facing 12

Detailed Dimensions 13

End Preparation - Inlet 14

End Preparation - Outlet 14

P&ID No./Line No. 15

Valve Location 16

Required Thrust/Torque Prediction 17

Method and Parameters 18

Actuator:

Manufacturer

Figure 25-3Suggested Motor-Operated Valve Data Sheet by Purchaser



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Size

Type

Gear Ratio 19

Electric Motor:

Manufacturer

Starting Torque

Running Torque

Enclosure 20

Power - Volts/Phase/Hz 21

Insulation Class 22

Normal Ambient Temp 23

Space Heater - Location & Voltage 24

Actuator Output Prediction

Method and Parameters

Testing Requirements

Hydrostatic and Seat Leakage Testing

Dynamic Flow Testing

Stroke Time, Sec

Close, max. 25

Open, max. 25

Close, + %, - % 25

Open, + %, - % 25



Figure 25-3 (continued)Suggested Motor-Operated Valve Data Sheet by Purchaser


1. Inquiry number or purchase order number.

2. Unique valve assembly number. This number should be marked on the valve by thevalve manufacturer to ensure that the valve is installed at the location to which itsdesign parameters apply.

3. Generic valve number assigned for a valve type and material, together with otherfeatures if used.

4. Applicable code or standard (for example, ASME III CL2, ANSI B16.34).



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5. Code or standard pressure class and valve type (for example, CL600 gate valve).

6. In reduced port designs, size reductions from nominal pipe size should be limitedto no more than two sizes.

7. Brief description of valve usage (for example, main steam, feedwater, radioactiveliquid waste, etc.).

8. Design pressure and temperature of the line in which the valve is installed.

9. The maximum differential pressure against which valve must operate under normalor transient conditions.

10. The maximum differential pressure for which the disc/ball/plug must be designedif different than 8. If 8 and 9 are the same, combine 8 and 9.

11. Specify the maximum flow rate because it can affect actuator sizing, particularly forbutterfly valves.

12. Generic description of materials (for example, carbon steel, 410 stainless steel, etc.).Delete or add items as applicable. If items are addressed in the text of thespecification, refer to the text.

13. Specify or attach a description of detailed dimensions needed to perform detailedanalysis such as EPRI’s PPM.

14. Specify the type of end connection on the inlet and outlet nozzles (for example,ANSI B16.5 CL150 flange inlet, ANSI B16.5 CL300 flange outlet).

15. P&ID and line number in which the valve is installed.

16. Location within the power plant (for example, inside reactor containment, turbinebuilding). This is to correlate the valve location with the environmental conditionsgiven in the text of the specifications.

17. In the opening and/or closing direction using worst case operating conditionsincluding design basis conditions.

18. Such as EPRI’s PPM, valve factor, seat and guide friction coefficients and packingload.

19. Based on motor RPM and stroke time.

20. Type of enclosure desired (for example, totally enclosed, nonventilated [TENV]).



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21. Available power supply (for example, 575 V/3 ph/60 Hz). Specify voltage range ifnot given in the text.

22. Motor insulation class desired (for example, B, H).

23. Ambient temperature at valve location (for example, 50°C).

24. Space heaters desired (that is, in limit switch compartment and/or motor, 120 V).

25. Specify the stroke time. Always provide a reasonable range or tolerance. The stroketime should never be given as a finite time because it is virtually impossible to havea motor operator operate in a specific time repeatedly.

26. Specify any special requirements, such as:

a. Radiation if not specified elsewhere.

b. Seismic evaluation and natural frequencies/mode shapes.

c. Fluid chemistry for lined valves, or unique service.

d. Features and accessories not provided on data sheet or in text.

e. If the piping system can experience a transient condition where the designtemperature and pressure are exceeded, explain here. This is becausepiping codes and standards have provisions for exceeding designconditions that do not apply to valves.



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Note Page:

Spec. No.

1 P.O.

Date:

S &W identification No. 2

Code & Class 3

Valve-Press. Class & Type 4

Quantity

Manufacturers Model No.

Size-in.

Flow Coefficient - Cv

Stem/Shaft-Diameter 5

Thread/Pitch/Lead 5

Port/Seat Area-in. 5

Materials-(ASTM/ASME or Manufacturers Designation) 6

Body

Bonnet

Stem/Shaft

Disc & Disc Facing

Seat & Seat Facing

Yoke

Bolting

Packing Type/Make

Dimensions

End -to-End

Valve to Operator Top

Internal Dimensions 7

Total Weight incl. Operator

Torque

Required to Open/Close

Limit For Valve 8

Limit For Operator 8

Operator Output-Max. Value

Operator Output-Min. Value

Torque Switch Setting & Torque-Normal 9

Torque Switch Setting & Torque-Max. 10

Operator - Make

Type

Overall Gear Ratio

Handwheel Diameter

Figure 25-4Suggested Motor-Operated Valve Data Sheet by Bidder/Seller



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Motor-Make

Type

Rated Torque

Rated Speed-rpm

Starting Current

Full Load Current

Max. Winding Temp

Insulation Class

Stalled Torque

Overload Heater

Rating-amps

Max. Locked Rotor

Time & Rated V-sec

Rated Horsepower

Time To Close, sec (min./max.)

Time To Open, sec (min./max.)

Valve Required Thrust/Torque Prediction


Actuator Output Thrust/Torque Prediction


Test Results

Hydrostatic and Seat Leakage Testing

Dynamic Flow Testing



Figure 25-4 (Continued)Suggested Motor-Operated Valve Data Sheet by Bidder/Seller


1. Proposal number or purchase order number.

2. Unique number assigned to valve assembly.

3. Identification of code or standard (for example, ASME III CL3, ANSI B16.34).

4. Code or standard pressure class and valve type (for example, CL600 Globe).

5. These data are required to evaluate motor operator sizing.



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6. Materials used in construction of the valves. Require that the material designationsbe given or sufficient identifications provided so the material (such as disc and discfacing - SA216WCB W/Stellite #6) can be evaluated.

7. If requested, attach detailed dimensions (such as those specified in EPRI’s PPM datasheets).

8. The maximum torques the valve or operator can withstand without damage.

9. The setting of the torque switch that should operate the valve.

10. The maximum setting of the torque switch representing the limit of the valve oroperator.

11. List the purchaser’s special requirements and design features and/or data necessaryto meet these special requirements.



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Figure 25-5Control Valve Data Sheet



25-21

Figure 25-5 (continued)Control Valve Data Sheet



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25-24

Figure 25-6Relief Valve Data Sheet



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Figure 25-6 (continued)Relief Valve Data Sheet



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Figure 25-7Rupture Disc Data Sheet



25-27

Figure 25-7 (continued)Rupture Disc Data Sheet