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Edison Electric

Institute

Electric Power

Research lnstitute

Topics:

Feedwater heaters

Heat exchangers

Operation

Maintenance

Reliability

Performance

EPRl 3-3239

Project 1887-3

Final Report

September 19

Recommended uidelines for

the

Operation and Maintenanceof

eedwater Heaters

Prepared

by

International Energy Associates Limited

Washington D C

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Recommended Guidelines for the

Operation and Maintenance of

Feedwater Heaters

CS 3239

Research Project 1887 3

Final Report September 1983

Prepared by

INTERNATIONAL ENERGY ASSOCIATES LIMITED

600 New Ham pshire Avenue N.W.

Washington D.C. 2003 7

Principal Investigators

F. L. Wadsworth

T. J.

Kielar

Subcontractor

POWERFECT INC.

53 East Cedar Street

Livingston New Jersey 07039

Principal Investigator

M.

C. Catapano

Consultant

R

R Noe

Prepared for

Edison Electr ic Institute

1111

19th Street N.W.

Washington D.C. 20036

and

Electr ic Power Research Institute

3412 Hil lview Avenue

Palo Alto California 9430 4

EPRl Project Manager

I

A. Diaz-Tous

Availabil i ty and Performance Program

Coal Combustion Systems Division

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ORDERING INFORMATION

Requests for copies of this report should be directed to Research Reports Center

RRC), Box 50490, Palo Alto, CA 94303, 415) 965-4081. There is no charge for reports

requested by EPRI member utilities and affiliates,

U S

utility associations,

U S

government

agencies federal, state, and local), media, and foreign organizations with which EPRI has an

information exchange agreement. On request, RRC will send a catalog of EPRI reports.

Copyr~ght

1983

Electrlc Power Research Institute, Inc. All rights reserved

NOTICE

This report was prepared by the organ~zat~on s)amed below as an account of work sponsored by the Electr~c

Power Research

Institute,

Inc. EPRI) and the Ed~so n lectric Institute EEI). Neither EPRI. EEI, members of EPRI,

the organizationis) named below, nor any person acting on behalf of any of

them:

a) makes any warranty,

express or impl~ed, ith respect to the use of any information, apparatus, method, or process d~sclosedn ths

report or that such use may not infringe privately owned rights; or

b)

assumes any l~abil~t~es~ t hespect to the

use of, or for damages resulting from the use of, any ~n forr nato n, pparatus, method, or process disclosed in

th~ s eport.

Prepared by

Internatonal Energy Assoc~ates ~miled

Washington,

D C

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BSTR CT

Previous

EPRI

surveys, studies, and workshops have identified feedwater heater FWH)

problems as having a significant impact on fossil plant performance and availabil-

ity. One of the root causes of these problems is the current lack of comprehensive

standards, guidelines and procedures for assisting utility personnel in the opera-

tion and maintenance of their FWH systems. The guidelines in this publication have

been developed to help correct the problem by providing utility personnel with ex-

planations of the principal failures experienced in the past, their symptoms, prac-

tical techniques to avoid or minimize the problems, and other recommendations for

improving operation, maintenance, and management of WH systems. The guidelines are

essentially a collation of the experiences of those utilities and individuals who

have experienced some success

in coping with

WH

problems. Comments and suggestions

from users are solicited to help EPRI make future edition s) of these guidelines

more complete and more beneficial tc the utilities.

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EPRI PERSPECTIVE

PROJECT DESCRIPTION

Failure of feedwater heaters FWHs) has a significantly adverse effect on the avail-

ability and thermal efficiency of both fossil fueled and nuclear power plants, with

especially severe financial consequences for baseloaded units. EPRI Final Report

CS-1776, Failure Cause Analysis--Feedwater Heaters April 1981), identifies six

major categories of WH problems in fossil fueled plants and recommends approaches

to reducing their severity. EPRI Final Report CS-3184, Corrosion-Related Failures

in Feedwater Heaters July 1983), provides further information into generic failure

modes of commonly used materials in FWHs and how to control them. A growing body

of knowledge on nuclear plant FWHs complements these findings with data on their

specific operating conditions.

Utility personnel need guidelines to assist them to apply this information to their

specific requirements for operation, maintenance, and repair or replacement of FWHs.

This final report for RP1887-3 addresses those needs. It will be complemented with

the results from RP1887-1, Recommended Design and Procurement Guidelines for Feed-

water Heaters in Large Power Generating Units; these results will be published later

this year.

PROJECT OBJECTIVE

The specific objective of this study was to prepare guidelines that utility person-

nel can apply to develop detailed procedures and policies to meet their specific

requirements for operation, maintenance, and replacement of closed FWHs.

PROJECT RESULTS

These guidelines contain four main sections. The first describes the general con-

figuration of closed FWHs The second section addresses the six problem categories

identified by PRI CS-1776 in terms of causes, symptoms, operating practices to

correct or minimize the causes and symptoms, and recommendations for alleviating

them.

The analysis required to limit WH and plant operations whenever one or more

FWHs are out of service is described in the third section, and the final section

gives a basis for assessing a repair or replacement decision for a FWH.

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The scope of this publication is limited for several reasons. There is too much

detailed information available to include all that is pertinent. Detailed proce-

dures must be prepared for each plant depending on its equipment, configuration, and

operating requirements. Operators and maintenance personnel must obtain accurate

current information from vendors for this purpose.

This document will be of interest to utility engineers and plant operators who

are

responsible for planning and conducting the operation, maintenance, and repair of

closed FWHs in all types of plants. This preliminary guide is promulgated with the

intention to improve it on the basis of user comments as well as new developments.

Isidro

A.

~iaz-TOUS, roject Manager

Coal Combustion Systems Division

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ACKNOWLEDGMENT

~t is a pleasure to acknowledge the valuable contribution provided to this work by

many individuals in the electric utility industry and in engineering and manufac-

turing firms, In addition to conducting two comprehensive reviews of drafts of this

document, the members of the I Prime Movers Feedwater Beater Task Force provided

encouragement, examples of their experience, and constructive suggestions throughout

the effort. Special thanks are expressed to Tom Haynes of Duke Power Company who

served with dedication on the Task Force and spent numerous additional hours helping

the authors improve the substance o the guide while also contributing ideas to EPRI

for improvement

to

future editions.

v

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CONTENTS

Se c t ion

1 GENERAL DE SCR IPT ION OF CLOSED FEEDWATER HEATERS

1 . 1 G e n e r a l

1 . 2 C l o s e d F e e d w a t e r H e a t e r s

2

TH

NEED OR UNDERSTANDING THE K E Y PROBLEMS

2 . 1 L e v e l C o n t r o l A n d D r a i n s C o o l e r Z o n e P r o b l e m s

2 . 1 . 1

O v e r v i e w O f M a j o r P r o b l e m A r e a s

2 . 1 . 2 E x a m p l e s O f D r a i n s C o o l e r P r o b l e m s

2 . 1 . 3 S y m p t o m s O f L e v e l C o n t r o l A n d D r a i n s C o o l e r

Z o n e P r o b l e m s

2 . 1 . 4 O p e r a t i o n a l rac t ices T o A v o i d O r M i t i g a t e

P r o b l e m s

2 . 1 . 5 Prevent ive A n d C o r r e c t i v e Maintenance

2 . 1 . 6 S y s t e m M o d i f i c a t i o n

2 . 2 T u b e V i b r a t i o n

2 .2 .1 O v e r v i e w

2 . 2 .2 S y m p t o m s O f V i b r a t i o n P r o b l e m s

2 2 3

O p e r a t i n g A n d M a i n t e n a n c e rac t i ces

To

A v o i d

O r M i t i g a t e V i b r a t i o n D a m a g e

2 . 2 . 4 S y s t e m M o d i f i c a t i o n

2 3 T u b e I n l e t E r o s i o n

2 .3 .1 O v e r v i e w

2 . 3 . 2 D e s i g n C o n s i d e r a t i o n s

2 . 3 . 3 S y m p t o m s A n d D e t e c t i o n O f T u b e I n l e t E r o s i o n

2 3 4

O p e r a t i o n a l rac t i ces T o A v o i d O r M i t i g a t e

D a m a g e

age

1 1

1 1

1 1

2 1

2 1

2 1

2 1 4

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S e c t i o n

2 .3 .5 S y s te m M o d i f i c a t i o n s

2 .3 .6 M a i nt e n an c e P r a c t i c e s

2 .4 Wate r Chemis t ry And Co r ros ion

2.4.1 Overview

2.4.2 The Feed wate r Heater Environment

2 .4 .3 De tec t io n Of Wate r Chemis t ry Re la t ed

Problems

2.4.4 Op er at i on s And Maintenance

2.4.5 C h e m i s t r y C o n s i d e r a t i o n s R e l a t e d To

Design And Sys tem Mo di f i ca t i ons

2.5 Steam Impingem ent

2.5.1 Overview

2.5.2 Symptoms Of Impin gem ent A tt a ck

2.5.3

O p e r a t i o n a l P r a c t i c e s T o Av oi d O r

M i t i g a t e

Steam Impingement Damage

2.5.4 P r ev en t iv e And Co r r ec t i ve Main tenance

2.5.5

Sys tem Des ign And Modi f i ca t ion s To Reduce

Impingement Damage

2 .6 P rob lems As soc ia t e d Wi th Tube P lu gg i ng

2.6.1 Overview

2 .6.2 Tube P lugg ing Tech n ique s And L i m i t a t io ns

2.6.3 P r i n c i p a l P r ob le m s And P o t e n t i a l S o l u t i o n s

2 .7 Misce l l aneous

2.7.1 The Tubesheet/Channel Barrel T r a n s i t i o n

R a d i u s

2.7.2 W eld ed V e r s u s B o l t e d P a r t i t i o n P l a t e s

2.7.3 Channel

c c e s s

Cover

METHODICAL APPROACH TO OPERATION OF FEEDWATER HEATERS UNDER

DEGRADED CONDITIONS

3.1 Overview: he Need To Dev elop Syst ems Approach

3.2 P r a c t i c a l Examples Of An Approach Dete rmin in g

O p e r a t i o n a l L i m i t a t i o n s

age

2 46

2 47

2 48

2 48

2 51

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S e c t i o n

4

THE

REPAIR

OR

REPLACE DECISION

PROCESS

4.1 Overview

4.2

C o s t s Of Re p ea te d Fe ed wa te r He a t e r F a i l u r e s

4.3

Age Cons ide ra t ions

4.4

Mechanical. Design f The Feedwater Heater

4.5

C o n s t r u c t i o n M a t e r i a l s

4.6

Mechan ica l Cond i t ion

4.7

Regu la to ry And Fi s ca l Cl ima te

age

4 1

4

1

4 2

4 14

4 14

4 14

4 1

4 15

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ILLUSTRATIONS

Fi gu r e

1-1

Ty pic al Two-Zone H or izo nta l Feedwater Heate r

Condensing And Sub coo l ing Zones)

1-2 Ty pi ca l Three-Zone Ho r iz on tal Feedwater Heater

Des upe rheat in g, Condensing, And Sub coo l ing Zones)

1 -3 T yp i ca l V e r t i c a l Feedw a te r H ea t e r H i gh -P r e ssu r e,

Th ree-Z one , Head Down)

1-4 Typ ica l Channel Co nf ig ura t ion s

2-1 Dr ain s Coo ler Shroud ing Designs

2-2 Compari son Of Feedwater He ater Cap aci t an ce

2-3

B e l l y B and M od i f i ca t i on To V er t i c a l Feedw a te r H ea t e r Sh e l l

2-4 Dra in s Cool ing Zone Shroud Mo di f i ca t ion

2-5

Damage From Fl as h in g t The Entrance To The Drains Cooler Zone

2-6 V e r t ic a l Channel Down Feedwater He ater With Welded B l i s t e r

2-7 Feedwater He ater Te s t She et

2-8 Unit

W

Load

Vs

Feedw a te r H ea t e r Sh e l l O pe r a t i ng P r e s su r e

Gen er ic Example)

2-9 Feedwater He ater Water Lev el Li m its

2-10 igh Sh e l l W a te r L eve l W ith S i de D r a i n s O u t l e t

2-11 Low S h e l l Water Lev el With S id e Dra ins Ou t le t

2-12 Lev el In d ic a t i on s Of V e r t ic a l Channel Down Feedwater Hea ter

2-13 Example Of Da ta Used To Check L e v el

Vs

Temperature

Per formance

2-14

ube

Vibrat ion Damage t The U-Bend

2-15 Tube Sup por t B af f l es t The U-Bend

2-16 Dra ins I n l e t B af f l e P la te /Dam

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F i g u r e

2 17 Sca l lop ed Ba ff le s And Sup ports

2 18 Tube Inlet Erosi on

2 19 Ammonia A tt ac k Of Cop per Nic kel Tub ing

2 20 E x fo li at io n Of Copper Nickel Tubing

2 21 Tube Fa i l ur e From S tr es s Corrosion

2 22 Continuous Vent O ri f i ce With St ar tu p Bypass Valve

2 23

Steam Impingement Destruction O f Tubes

2 24 Tube Damage From Steam Imp inge men t

2 25

Impingement Attac k On Small Impact Pl a te s

2 26 Impingement Ero sio n Of The Feedwater Hea ter S he ll

2 27 Fa l l en Impact P la te Within The Feedwater Heater Sh el l

2 28 Damage To Tubes From Loose Imp act P l a t e

2 29 Frequency Sp ec tra Of Feedwater Pr es su re Noise

2 30 Example Of Sig na l Tren d Fo r Tube Leak I n High Pressure

Feedwatex Heater

2 31 Ca tas tro ph ic Fa i l ur e Of Feedwater Heat er Forging

2 32 Determinat ion Of S tr e s s Con centra t ion Fa cto rs For Various

Corner Radi i

O f

C y l i n W i c a l S h e l l

2 33 W elded P a r t i t i o n P l a t e

2 34 B o l te d P a r t i t i o n P l a t e

2 35 Manway Fo r Cha nne l Ac ces s

3 1

Bo ile r Su perhe at Li m ita t io ns With Reduced Feedwater

Temperatures

3 2

O p e r a t in g L i m i t s

And

Guid e l ines For Case

3 3

Ope rat ing Lim its And Gu ide l ine s For Case 2

4 1 Ins pec t ion O f High Pressure Ho rizo nta l Feedwater Heater

age

2 37

2 42

2 56

2 57

2 58

2 62

2 67

2 68

2 69

2 70

2 7 1

2 72

2 85

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TABLES

Table

2 1 Pr op er t ies Of Sa tura te d Water Vapor And Liquid

2 2

Desired Capacitance For Typical Feedwater Heater Level

Control Systems

2-3 Tube M at er ia ls Used For Feedwater H ea ter s

And

Other

Heat Exchangers In Power Pla nt A ppl icati ons

2-4 Typ ical Con trol

i m i t s

For

V o l a t i l e Z e ro S o l i d s )

Treate d Un its Drum-Type B oi le rs )

2-5 Typ ical Con trol

i m i t s

For Low-Level Co ord ina ted

Phosphate-Treated Units

4-1 Un it Chronology And Feedw ater He ater Tube M at er ia l

4-2 Feedwater Heater M ater ia ls Inform ation

4-3

A v a i l a b i l i t y

nd

Performance C osts A ssociated W ith

Heater Outages

4-4 Performance Aspects Of Heater Tr ain s And P o te nt ia l

Ov erload s With Next Up-Stream Feedw ater H eate r Cut Out

age

2 3

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INTRODUCTION AND

SUMMARY

PURPOSE

OF

THE

GUIDE

Feedwater heater FWH) failures continue to have a significant adverse impact on

availability and thermal efficiency of

power

plants throughout the country. The

financial impact is particularly severe on baseloaded units. The Electric Power

Research Institute EPRI) has sponsored a number of surveys, studies, and workshops

in efforts to define the causes of poor

WH

performance and to initiate efforts to

assist utilities with needed improvements. During these activities, it has become

clear that the utility personnel responsible for the operation, maintenance, and

replacement of WHS need more guidance in order to improve the functions for which

they are responsible. This guide was written for those individuals and for utility

management for the purpose of providing them the benefit of lessons learned from

many FWH experiences throughout the utility industry.

LIMITATIONS OF THE GUI E

While it might seem desirable to have everything one needs to know about FWHs in

one comprehensive guide, the scope of this publication has been limited for several

reasons that are important to emphasize at the outset:

It is impossible to put a11 pertinent information in one publication

because there are so many different types and designs of FWHs and

feedwater systems in use that the detailed procedures for operation,

maintenance, and procurement must

be

tailored to the individual FWH and

to the individual plant environment.

For the same reason, it is essential that the operators and maintenance

personnel obtain from the vendors and use accurate, updated information

regarding their FWHs. In addition to vendor manuals, the utility should

have arrangement drawings of each

FWH

showing the details, dimensions,

and materials used for its internals, as well as all penetrations and

instrument connections. This

EPRI

guide should

be

helpful when used

addition t the official documentation for the individual FWH, but

utility personnel should be cautioned not to rely on generic guides or

even textbooks) in place of the official hardware-specific documentation.

The same limitation applies to operating and maintenance procedures.

Practices that experience has shown to be effective are recommended

throughout this guide. However, these recommendations are submitted for

the consideration of the utilities; they should be followed only when

properly approved and promulgated by the utility.

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It is recognized that this guide would be more complete with several

sections added. One such section should be devoted to detailed advice

concerning the dos and don'ts' of FWH procurement, including the many

considerations needed in developing a good purchase specification.

Assistance in this area is badly needed by the utilities and is being

pursued in a separate EPRI project.

Recognizing the urgency to provide helpful guidance in operation and

maintenance areas, the intent was to promulgate this guide as a pre-

liminary or first edition, with plans to improve and update it based

upon comments from the users and new developments in the

FWH

field.

SCOPE ND METHODOLOGY

With the above limitations in mind, the study team first reviewed the recent EPRI

reports and seminars devoted to FWH problems as well as their many references to

find practical items suited for the purpose of this guide. Material from these

sources was combined with the extensive personal experience that several of the

contributors had accumulated in the design, operation, and repair of FWHs.

Section 1 of

this guideopresents a general description of closed FWHs and displays

several figures to identify the key components, many of which are discussed in more

detail in later sections. Section addresses each of the six recognized major FWH

problem areas* by discussing causes of the problems, symptoms,, operational prac-

tices to avoid or minimize damage, and recommended maintenance to correct the prob-

lems. While the design of FWHs is well beyond the scope of this guide, good and

bad design features are mentioned where they are necessary for proper understanding

by utility personnel and where they may be of assistance in considering modifica-

tions (a form of corrective maintenance). Subsection 2.7 also addresses several

specific problems under the miscellaneous heading. These items do not fall neat-

ly into any of the six identified major problem areas, but they were considered of

sufficient value to include in this edition of the guide. Section 3 is devoted to

the type of analysis that should be utilized in considering reasonable limitations

to be imposed upon the feedwater system and the plant when one or more FWHs are

removed from service. Section 4 discusses the principal considerations that should

be involved when the utility is faced with the decision whether to continue the

maintenance of a problem

FWH

or to replace it. Several practical examples are used

to illustrate the wide variance in the cost of

FWH

outages and the specific factors

that determine those costs.

*Based upon EPRI s 1980-1981 survey as summarized in EPRI Report CS-1776, Failure

Cause Analysis Feedwater Heaters.

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t t h e b a ck o f t h e g u i d e a r e c o p i e s o f t h e U s er F ee d ba c k F or m, w h ic h a r e p r o v id e d

t o f a c i l i t a t e f e ed ba ck fr om t h e u s e r s . T he y c a n b e f i l l e d o u t , re mo ve d, a nd m a i l e d

back t PRI t o h e l p f o c u s i mp ro ve me nt s f o r t h e n e x t e d i t i o n o f t h e g u i d e i n t h o s e

a r e a s t h a t a r e n ee de d

y

t h e u t i l i t i e s . Any p h o to g r ap h s , go od p r o c e d u r e s, o r o t h e r

i t e m s

t h a t m i gh t b e

of

i n t e r e s t t o o t h e r s w ould a l s o e welcomed.

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S e c t i o n 1

GENERAL DESCRIPTION OF CLOSED FEEDWATER HEATERS

1.1 GENERAL

~ l lo de rn , l a r g e s t e a m power p l a n t s u s e a p r o c e s s o f r e g e n e r a t i v e f e e dwa t e r h e a t -

i n g t o i n c r e a s e t h e o v e r a l l c y c l e e f f i c i e n c y o f t h e p l a n t a n d t o mi ni mi ze i nd uc ed

t h er m al s t r e s s e s i n t h e b o i l e r . T y p i c al p l a n t s u t i l i z e two ty p e s o f f e e dw at er

h ea te rs FWHs): low-p ressure and high -pressu re . Many a l s o have in te rme dia te-

p r e s s u r e

EWHs.

The low-pressure

LP)

EWHs beg in t he p r oces s by hea t ing th e subcoo led condensa te .

LP

FWHs

a r e o f t h e c l o s e d t y p e, u s i n g l ow - pr es su re t u r b i n e e x t r a c t i o n s t ea m f o r

h e a t in g . I n newer p l a n t s , t h ey a r e o f t e n p l a c ed a t t h e t u r b i n e ex h a us t t h r o a t

w i t h i n t h e c o n de n se r .

The in te rmed ia te -p ressu re IP) a n d h i g h - p r e s s u r e HP) FWHs a r e l o c a t e d a t t h e d i s -

c h a r g e o f t h e f e e d -b o o s t e r a nd t h e b o i l e r f e e d pu mps, r e s p e c t i v e l y . They a r e a l -

ways o f t h e c l o s e d t y p e a nd a r e s i m i l a r i n b a s i c d e s i g n a nd f u n c t i o n .

Some p l a n t s a l s o ha ve a d e a e r a t i n g EWH which s o f a n op en t y p e an d s e r v e s t o

remove d i s so lv ed oxygen , a s

w e l l

a s t o h ea t t h e f ee dw at er . ~ e a e r a t i n g WBs a r e

n o t w i t h i n t h e s co p e o f t h i s g u i d e.

1 .2 CLOSED FEEDWATER HEATERS

Most

I P

and HP EWHs a r e o f th e th ree -zone desup e rhea t in g , condens ing , and d r a i ns

s u b c o o l i n g z o n e s ) d e s i g n . LP

FWHs

a r e t y p i c a l l y o f t h e t h r e e -z o n e o r t wo-zo ne

condens ing and subcoo l in g zones ) des ign . The ma j o r i t y o f FWHs i n use today a r e

o f a h o r i z o n t a l c o n f i g u r a t i o n . S ee F i g u r e

1 1 and 1-2. ) However, some u t i l i t i e s

u se v e r t i c a l c o n f i g u r at i o n s, e s p e c i a l l y f o r p l a n t s t h a t h av e l i m i t e d o r e x p en s iv e )

f l o o r s p a c e . Se e F i g u r e 1 - 3. ) Th e ma jo r p a r t s o f t h e FWH a r e d i s c u s s e d b el ow:

Channel: The FWH c h an n e l p r o v i d es f o r t h e f ee d wa te r i n l e t a nd o u t l e t

n o z z l e s . T h e re a r e f o u r b a s i c c h a n n el c o n f i g u r a t i o n s . S e e F i g u r e 1 -4 .)

Ch a n ne l s a r e d e s i g n e d t o m in im iz e t h e e f f e c t s o f e r o s i o n o n t h e t u b e s h e e t

and

t o

p r o v i d e c o n v en i e n t a c c e s s f o r t u b e s h e e t p l u gg i n g a nd o t h e r r e l a t e d

maintenance .

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OPTIONAL

HEATER SUPPORT

DRAINS SUBCOOLING ZONE BAFFLES INLET F o r Ty p i c a l

ZONE BY PASS

Channel Conf

u r a t i o n s )

F i g u r e

1-1.

Typ ical Two-Zone Hor izo nta l Feedwater Hea ter Condensing and

Subcooling zon es)

Source:

HE

Standards For Closed Feedwater Hea te r s , Thi rd Edi t ion .

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OESUPEnHE TlNG

ZONE SHROUD

SHELL FEEDW TER

SKIRT

OUTLET

DESUPERHE TING

TUBE SUPPORTS ZONE B FFLES

See Figure

For Typical

HE TER SUPPORl

TIE RODS DR INS SUBCOO LING FEEDW TER

ND SP CERS

Channel Conf

ZONE

B FFLES INLET

u r a t i o n s

F i g u r e 1 2. Typical Three-Zone ~ o r i z o n t a l eedwater Heater Desuperheating,

Condensing, and Subcooling zones

Source:

HE

Standards For Closed

eedwater

Heaters , Third Edi t io n.

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IR CSE

O

S FETY V LVE

SUPPORT PL TES

CONDENS TE

W E

OP f

GLILSS

DR INS

COOLING SECTION

igure 1 3. Ty p i c a l V e r t i c a l Fe ed wa te r He a t e r

Hi gh -Pr essu re, Three-Zone, Head Down)

Source: Long Is la n d Li gh t in g Company.

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ATER

OUTLET

P SS PARTITION M NW Y C

M NW Y COVER

TUSE SHEET

P RTITION

FEEDWATER

INLET

OVER

ELLIPTICAL HEAD

H E > I I S P H E R I C \ L

HEAD

TUBE SHEET FEEDW TER I N L ~

FEEDWATER OUTLET

COVER

P SS P RTITION

P SS P RTITION

BOLTED RE MOVABLE OVER

? iS ;OV .ABLE COVER FULL OPESING

Figure 1-4. Typical Channel C o n f i g u r a t i o n s

Source:

HE

S ta n d a r d s F o r

Closed Feedwater

H e a t e r s

T h i r d E d i t i o n.

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Desuperheating Zone: This is an enclosed portion at the outlet end

o

the tube bundle. Its purpose is to maximize the outlet feedwater temper-

ature by transferring heat from the incoming superheated extraction

steam. An impingment plate is installed below the steam inlet nozzle to

prevent impingement damage to the tubes.

Condensing Zone: This is the largest zone in the FWH. Steam exiting the

desuperheating zone is condensed as it traverses through the condensing

zone. Also, any drains from higher pressure FWHs flow into the con-

densing zone through the drains inlet nozzle. An impingement plate is

installed just inside this nozzle to protect the tubes from these

flashing drains. The condensing zone is vented continuously to remove

non-condensibles. The vent system typically consists of one or more

perforated vent pipes installed along the length of the tube bundle.

(Many other designs are also used to accomplish this function.) Non-

condensibles collect in these pipes and then pass through shell vent

connections to the deaerator or the main condenser. An orifice, in-

stalled in the vent discharge, is sized to result in a flow rate equal to

0.5

of the total steam flow entering the FWH

Drains Subcooling Zone: This zone is an enclosed portion of the inlet

end of the tube bundle. Its purpose is to maximize heat transfer from

the shellside condensate to the incoming feedwater before the condensate

exits. The condensate should be sub-cooled sufficiently to prevent

flashing as the condensate leaves the

FWH

shell through the drains outlet

nozzle.

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Section

2

THE NEED FOR UNDERSTANDING THE

KEY

PROBLEMS

Electric Power Research Institute (EPRI) surveys and workshops continue to indicate

a strong need for improving the knowledge and experience of utility personnel in-

volved in the operation, maintenance, and replacement of feedwater heaters FWHs).

It is particularly important that such personnel understand the principal problems

that have already been experienced many times throughout the industry. Only with a

good understanding of these problems will they be able to avoid, detect, and mini-

mize similar problems in their own systems. Accordingly, this section of the guide

addresses each major problem area identified by previous EPRI surveys* and provides

comments and recommendations from the combined experience of the FWH experts who

contributed to this effort. In selecting material for the following sections, the

emphasis has been to focus on those key points that experience has shown to be most

essential for proper operation and maintenance of FWH systems.

2.1 LEVEL CONTROL AND DRAINS COOLER ZONE PROBLEMS

2.1.1 Overview Of Major Problem Areas

A mistake that is often made by utility personnel is to consider the drains flowing

to the drains cooler as if they were like hot water coming from the tap in the kit-

chen sink. The failure to realize that the tap water is subcooled (by approximately

50~), whereas drains are formed within a FWH under saturated conditions, leads to a

basic misunderstanding of flashing and the need to sub-cool the drains. As these

drains travel through the heater to the drains cooling zone, the geometry of the

internals changes their direction in various flow patterns, which result in pressure

drops. saturated liquid that is subjected to a pressure drop will, of course,

flash. Flashing is similar to normal boiling in that some of the liquid is trans-

formed to steam; however, it is caused by a reduction in pressure rather than by an

addition of heat.

Few people give much thought

to

the phenomenon or realize that if a given weight of

saturated water decreased in pressure from

100

psia to - 5 psia, approximately

24

*Especially EPRI Report CS-1776, Failure Cause Analysis Feedwater Heaters.

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o f

t

w ou ld f l a s h t o v a p or nd i n c r e a s e

i n vo lu me m ore t h a n e ig h t t h o u s a n d t im e s .

T h i s e x a m p le o f p r e s s u r e loss

s

e x a g g e r a t e d a nd s h o u l d n e v e r o c c u r e x c e p t i n t h e

c a s e o f a d r a s t i c p l a n t l o a d r e d u c t i o n . H ow ever, p o o r l y d e s i g n e d c o n d en s a te f l o w

p a s s ag e s a n d d r a i n s c o o l e r e n t r a n c e a r e a s c o u l d c au s e p r e s s u r e d r o p s t h a t w ou ld

i n c r e a s e v e l o c i t i e s , d ue t o f l a s h i n g , by a s much a s a h u n d re d f o ld . F o r e ig n

m a t e r i a l s i n t h i s f l o w p a t h wo uld ha ve a s i m i l a r e f f e c t . Such e x pa n si o n ca u s e s

e x c e s s i v e v e l o c i t i e s o f t h e s t ea m /w a t er m i x t u r e , w hi ch t h e n i m p in g es upon t h e

h e a t e r i n t e r n a l s w i t h d am ag in g f o r c e.

The

e r o s i o n a nd e r o s i o n - c o r r o s i o n

a c t i o n s t h a t r e s u l t c a n d e s t r o y t u b e s , t u b e s u p p o rt s , a nd o t h e r s t r u c t u r e s i n

a s h o r t p e r i o d of t i m e

How can t h i s phenomenon, which s a m aj or c o n t r i b u t o r o f FWH f a i l u r e , b e d e a l t wi t h?

What c a n be d on e t o c o n t r o l o r e l i m i n a t e t h e f l a s h i n g ? T h i s p ro bl em c a n b e ad -

d r e s s e d i n a n o r d e r l y f a s h i o n by c o n s id e r i n g t h e s p e c i f i c s o f t h r e e b a s i c t y p e s o f

EWHs: h o r i z o n t a l , v e r t i c a l c h a n n e l down, a n d v e r t i c a l c h a n n e l u p.

2 .1.1 .1 Ho r iz on t a l Feedwate r Hea te rs . In th e ho r i zo n t a l FWH, whe ther

t s

a

t h r e e- z o ne h e a t e r w i t h d e s u p e r h e a t i n g , d r a i n s c o o l i n g , a nd c on d en s in g z o n e s o r a

tw o-zone h e a t e r w i t h c o nd e ns i ng a nd d r a i n s c o o l i n g z o n es o n l y , t h e r e a r e b a s i c a l l y

t w o

t y p e s o f d r a i n s c o o l e r d e s i g n s .

WH

m a n u fa c t u re r s r e c o gn i z e t h e s e a s s h o r t o r

p a r t i a l l e n g t h) and l on g o r f u l l l e n g t h ) d r a i n s c o o l e r s , a nd t h e d i f f e r e n c e

s

b a s i c a l l y i n t h e way t h a t t h e s h r o ud i ng r e l a t e s t o t h e t u b e s a nd b a f f l i n g a s shown

i n F ig u re 2-1. The most common

s

t h e s h o r t d r a i n s c o o l e r , w h e r e in t h e s h ro u di n g

en co mp as ses a l l o f t h e t u b e s i n t h e f i r s t f e ed w at er p a s s s t a r t i n g a t t h e t u b es h e et

a nd e n d i ng a t t h e p o i n t w h er e

t

h a s i n c or p o ra t e d a l l o f t h e s u r f a c e n ee de d t o pe r-

f or m t h e t a s k o h e a t t r a n s f e r t o d o t h e s p e c i f i e d amount o f s u bc o ol i ng . f l a t

p l a t e a t a p pr ox im a te ly t h e s h e l l m id- po in t s e r v e s a s t h e c l o s u r e o f t h e f u l l 1 80 0

a r c . T he s h r o u d in g

s

l e a k t i g h t s u c h t h a t t h e c o n de n sa t e e n t e r s a n op en in g a t t h e

b ot to m o f t h e d r a i n s c o o l e r , away f r om t h e t u b e s h e e t , a n d c o m p l e t e l y f l o o d s t h e o u t -

s i d e s u r f a c e of t h e t u be s a s t f lo w s p a s t b a f f l i n g t o t h e o u t l e t e nd , w hich i s

l o c a t e d c l o s e t o t h e ba ck o f t h e t u b e s h e e t .

I n t h e l o ng d r a i n s c o o l e r , t h e s h r o u di n g r u n s t h e f u l l l e n g t h o f t h e t u b i ng b u t

en co mp as ses o n l y a p o r t i o n of t h e t u b es i n t h e f i r s t p a s s. The f l a t p l a t e po r t i o n

o f t h e s h r o ud i n g p a s s e s b et w ee n t h e t u b e r ow s, a nd t h e a r c d e p t h v a r i e s d ep e nd i ng

u po n th e a mo un t o f t u b e s u r f a c e r e q u i r e d f o r s u b c o o l in g . T he c o n d e n s a t e e n t e r s a

l on g d r a i n s c o o l e r a t t h e en d f a r t h e s t f ro m t h e t u b e s h e et a nd f l o w s t h e f u l l l e n g t h

p a s t t h e b a f f l i n g t o t h e e x i t , which

s

l o c a t e d c l o s e t o t h e b ack o f t h e t u b e s h e et .

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Table 2-1

P R O P E R T I E S O F

SATURATED

WATER VAPOR

A N D L I Q U I D

Sat.

V o l u m e P e r P o u n d V o l u m e t r i c

Ratio

P r e s s u r e

Temp. Sat.

L i q

Sat. Vap.

s a t .

Vap.

psis)

OF)

~ t .,

~ t . Sa t . L i q

These

numbers

were

used

i n t h e ex am ple i n S e c t i o n 2 1 1

2 3

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Feed Out

Drains ooler

Shrouding

M i n i m Liquid Level Drains ooler

Drains

ut

Feed In

Sho rt Drai ns ooler Design

Feed Out

M i n i m Liquid Level

Drains

oo le r

l

Shrouding

rains

Out

Feed

In

Long

Drains ooler Design

st

Pass

r a b s

ooler

Shrouding

Drains ooler

Shrouding

Figure 2 1.

rains

ooler Shroud ing es igns

2 4

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In the case of the long drains cooler, the minimum operating level should be main-

tained above the flat plate of the shrouding. In the case of the short drains

cooler, the minimum operating level should be maintained at a point where the en-

trance into the shrouding (the snorkel area) is always covered, even throughout

plant transients. horizontal unit, of course, does have the capability of storing

relatively large quantities of water such that the level can be maintained within a

few inches in the vertical direction.

It is important to recognize that the level within a FWH is not necessarily the same

at all locations. The level can vary, depending on the pressure that exists at the

surface of the condensate, which is a function of the position of the steam inlet,

the design of the internals, and the flow through the unit. Discussions with ex-

perienced utility and vendor engineers indicate that these phenomena have been ob-

served under test conditions in the past. One thing that was established is that

the level can vary significantly from the back of the tubesheet to the other end of

the FWH. Under certain circumstances of operation, it could have a reverse slope;.

it could even have a two-way slope with a peak in the middle or vice versa.

Main-

taining a suitable level as the condensate approaches the drains cooling zone of the

WH

is essential. It is especially important to maintain the level above the en-

trance at all times. Therefore, the location of the liquid level control instru-

mentation should be as close as practical to that region.

mistake often made is

to provide liquid level control sensor points that are located physically a signif-

icant distance from the areas of concern. Some years ago, there was an experience

at a utility in Europe where

it

w s

determined that the water level at the U-bend

end was 18 inches higher than at the drains cooler entrance. An investigation re-

vealed that the FWH was very poorly designed. The heater was designed along the

principles governing water-to-water heat exchangers, which did not provide enough

clearance for steam flow. Therefore, large pressure drops occured, resulting in the

great level variations. Because both water and steam are present in a FWH each

zone must be considered individually during design. It is important that the shell

side of a

FWH

with a horizontal drains cooler be as free of obstruction as possible

and that it be properly sized so that there is good distribution of steam without

undue pressure drop.

The short drains cooler, while capable of performing satisfactorily if properly

designed and operated,

does offer more of a challenge than the long drains cooling

zone. It is important to remember that in this particular case, as the drains are

being condensed they have to travel the length of the shell before they reach the

inlet to the drains cooler. In so traveling, the drains could be impeded by

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b a f f l e s , s u p p o r t p l a t e s , e t c . w hich c r e a t e p r e s s u re d r o p s l i k e l y t o c a u se f l a s h i n g

and a r t i f i c i a l w at er l e v e l s . F or t h i s re as on ,

i t

may b e n e c e s s a r y t o m a i n t a i n a

l e v e l s uc h t h a t

t w o or

t h r e e r ows o f l ong U - t ube s

w i l l

b e submerged a t a l l t i m e s

p e r m i t t i n g p r e- c o ol i n g o f t h e d r a i n s . I f t h e d r a i n s a r e p r e- co o le d o n e o r

t w o

de-

g r e e s , t h e y w i l l n o t f l a s h .

I f

t h e y a r e n o t p r e -c o o le d a f ew d e g r e e s a nd i f t h e

p r op er l e v e l

i s

n o t ma in ta in ed a t t h e l o c a t i o n o f t h e s u c t i on i n l e t a t a l l times,

t h e y

w i l l

f l a s h upon e n t e r i n g t h e s n o r k e l or d r a i n s cooler pr ope r . When t he y f l a s h ,

the volume w i l l i n c r e as e d r a m a ti c a l l y a s i n d i ca t e d e a r l i e r . The v e l o c i t y a t t h e

s u c t i o n i n l e t t o t h e d r a i n s c o o l er

w i l l

n o l o n g e r r em ai n a t 2-3 f e e t p e r s ec on d , b u t

w i l l i n c r e as e

t o

a much h i g h e r v a l u e . T h i s w i l l g e n e r a t e more f l a s h i n g , d u e to

f u r t h e r l o s s i n p r e ss u re w i t hi n t h e d r a i n s

cooler ;

t h e r e fo r e , t h e f i r s t few b a f f l e s

w i l l a c t n o t a s a d r a i n s c o o l e r b u t a s a c on d en s in g r e g i o n . When t h i s phenomenon

i s

a s s o c i a t e d w i t h i mp ro pe r p o s i t i o n i n g o f t h e d r a i n s c o n t r o l v a l v e , w hi ch may b e o pe n

more t h a n

t

s h ou l d b e f o r t h e c o n d i t i o ns , f l a s h i n g w i l l b e in du ce d f a r t h e r i n t o t h e

d r a i n s c o ol e r . U n f or t u na t el y , many u t i l i t i e s do n o t f u l l y a p p r e c i a t e an d u n de rs ta n d

t h i s phenomenon. U n t i l a l l d e s i g n e r s a nd o p e r a t o r s pa y mo re a t t e n t i o n

t o

t h e d e s i g n

b a s i s o f t h e d r a i n s

cooler,

t h e s e p r ob le ms w i l l p e r s i s t .

A d d i t i o n a l l y , t h e t u r b i n e m a n u f a c t u r e r s may h a ve o v e r p l a y ed t h e i r

r o l e

i n i m p r e s s i n g

u pon t h e u t i l i t i e s an d t h e o p e r a t i n g p e rs o n ne l o f t h e power p l a n t t h a t t h e i n d u c t i o n

o f w at er i n t o t h e t u r b i n e m us t b e a v oi d ed a t a l l c o s t s . T h i s p o i n t h a s b ee n

s t r e s s e d i n many t e c h n i c a l p a p e r s a nd v e nd o r m an u al s to t h e e x t e n t t h a t t h e op er a-

t o r s i n t h e c o n t r o l room a r e s o w e l l awar e o f t h i s w arn ing t h a t t h e i r r a t i o n a l e f o r

pro pe r op e r a t io n of FWHs

i s

sometimes c l ou d ed . They f e a r t h a t t h e p o s s i b i l i t y o f

i n d u c i n g w a te r i n t o t h e t u r b i n e b y a l l o w i n g a h i g h w a t e r l e v e l i n t h e EWHs i s s o

g r e a t t h a t a p r im e co n c er n i s t o e ns u re t h a t t h e l e v e l s t a y s a s

l o w

a s p o s s i b l e.

T h i s l i n e o f r e as o n in g sometimes l e a d s t o t h e c o n cl u s io n t h a t a z e r o w a te r l e v e l i s

d e s i r a b l e b e c au s e

t

pr o v i d e s maximum t u r b i ne p r o t e c t i on .

T he r e ha ve be e n many i n s t a n c e s whe re t h i s ph i l o s ophy ha s be en a pp l i e d to t h e FWH

c o n t r o l s ys te m su ch t h a t d r a i n s o u t l e t c o n t r o l v a l v e s h av e been p u r po s el y l e f t f u l l y

opened. Some c a l l t h i s j a c k i n g o pe n o r s h o r t s t r o k i n g t h e v a l ve , e n su r i n g by

m ec h an i ca l m eans t h a t t h e v a l v e w i l l n e v er

close.

E xpe r i e nc e ha s shown t h a t w her e

t h e r e i s l a ck of knowledge of t h e $ l a s h i ng phenomenon,

t h i s p r a c t i c e

i s

q u i t e o f t e n

u s ed . T he o p e r a t i n g p e r s o n n e l r e a s o n t h a t t h e y a r e mak in g s u r e t h a t t h e s t ea m b lo ws

t h r ou g h , t h u s e n s u r i n g t h a t t h e l e v e l d o e s n o t i n c r e a s e . U n f o r t u n at e l y , when t h e s e

v a l v e s a r e l e f t o pe n, t h e s t e a m d o e s n o t s i mp l y b lo w t h ro u gh . I t s t a r t s t h ro ug h,

c o n d e n s e s , f l a s h e s , r e c o n d e n s e s , a n d so f o r t h . The d r a i n s

coolers

have been de-

s i g n e d b y t h e m a n u f a c t u r e r

t o

p a s s l i q u i d a t r e as on ab le v e l o c i t i e s

2-4

f e e t p e r

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s e co n d i n c u r r e n t d e s i g n s ) . When t h e vol um e i n c r e a s e s d ue t o t h e f l a s h i n g , t h e

v e l o c i t i e s a r e g oi ng

to

b e i n c r e a s e d s i g n i f i c a n t l y i n t h e d r a i n s c o o li n g z on es .

Under t h e s e c o n d i t i o n s , a g r e a t d e a l o f e n e r g y

i s

b e i n g r e l e a s e d

to

i m p i n g e t h e

s t ea m / wa t e r m i x t u r e , w i t h d am ag in g f o r c e , a g a i n s t t h e FWH s i n t e r n a l s .

t may be h e l p f u l t o t h i n k o f d r a i n s t h a t a r e a b o u t

t o

f l a s h l i k e a n e x p l o s i v e ,

which

i f

h a n d l e d p r o p e r l y , c a n b e b o t h m an ag ed an d t r a n s p o r t e d . H ow ev er , i f t h e y

a r e n o t h a n dl e d p r o p e r l y , t h e y w i l l e x p l o de w i t h t e r r i f i c f o r c e a nd c a u s e damage.

T he d e t o n a t o r m u s t b e r em ov ed f r o m t h e e x p l o s i v e . How c a n t h i s o b j e c t i v e b e accom-

p l i s h e d w i t h t h e f l a s h i n g ? The f l a s h i n g c o n d i t i o n s ho ul d b e e l i m i n at e d , s t a r t i n g

w i t h t h e s u c t i o n o f t h e d r a i n s c o o l e r a l l t h e way down

t o

t h e d r a i n s c o n t r o l v a l ve .

T h i s c a n b e a c c o mp l i sh e d

by

m a i nt a i ni n g t h e p ro p er l e v e l i n t h e d r a i n s c o o l e r.

N o t

o n l y s h o ul d t h e l i q u i d l e v e l c o n t r o l l e r b e l o c a t e d a t t h e p o i n t wh ere c on d en s at e

e n t e r s t h e d r a i n s c o o l e r , b u t a l o c a l ga ge g l a s s s ho ul d be i n s t a l l e d a t t h e same

l o c a t io n f o r v i s u a l l e v e l v e r i f i c a t i o n . t i s a l s o a go od i d e a f o r t h e l ow e r r a n ge

o f t h e c o n t r o l l e r t o b e no l ow er t h a n t h e bo t to m o f t h e s h e l l so t h a t t h e f u l l r an ge

is

e f f e c t i v e . A ny th in g b el ow t h e s h e l l b o tt om i s u s e l e s s , a nd o n e s h o ul d n o t p ro -

m ot e t h e i d e a t h a t a n o p e r a t o r wo uld a l l o w t h e l e v e l t o d r o p t h a t l ow.

2 . 1. 1 .2 V e r t i c a l C h a n n e l Down F e e dw a t er H e a t e r s . v e r t i c a l c h a n n e l down

FWH

c a n

b e, a s f a r a s t h e d r a i n s c o o l e r

i s

c o n c e r n e d , o f

t w o

types . One

is

t h e t h r e e - z o n e

WH w i t h a c o n de n s in g z o n e , a d r a i n s c o o l i n g z o ne , a nd a d e s u p e r h e a t i n g z on e t h a t

m u st b e l o n g e r t h a n t h e d r a i n s c o o l i n g zo ne . Th e o t h e r i s t h e two-zone FWH w i t h o u t

a d e s u p e r h e a t e r .

t i s

i m p o r ta n t t o a d d r e s s e a ch o ne o f them s e p a r a t e l y b e c au s e o f

t h e i r p e c u l i a r i t i e s .

I n a v e r t i c a l c h a n n e l down FWH t h e f i r s t t h i n g

t o

o b s e r v e is t h a t t h e c a p a c it a nc e *

i n a t h r e e - z on e FWH i s a p p r o x i m a t e l y h a l f t h a t i n a t wo -z on e FWH b e c a u s e , a s shown

i n F i gu r e 2-2 t h e s p a c e t h ro u g h w h ic h t h e d e s u p e r h e a t e r s h r o u d in g p e n e t r a t e s i s

e l i m i n a t e d i n c o mp u ti n g t h e c a p a c i t a n c e . I n a tw o-z on e FWH a l l o f t h e c ro ss -s ec -

t i o n a l a r e a a ro u nd b o t h p a s s e s o f t u b e s i s a v a i l a b l e f o r ma i nt a i ni n g t h e l e v e l .

T ho se t u b e s n o t i n c o r p o r a t e d w i t h i n t h e d r a i n s cooler a r e c o n si d er e d i n e f f e c t i v e

s u r f a c e b e c a u s e t h e y a r e su bm er ge d an d s e r v e n o r e a l h e a t t r a n s f e r p u r po s e. How-

e v e r , t h i s a r r an g em e nt i mp ro ve s t h e c a p a c i t a n c e f a c t o r a s co mp ar ed t o a t h re e -z o ne

d e s i g n .

* Ca pa c it an c e, a s r e l a t e d

t o

t h e a d eq u ac y o f d e s i g n i n g a c o n t r o l s s ys te m , is d e f i n e d

a s t h e s t o r a g e volume ( u s u a l l y g a l l o n s ) o f l i q u i d p r e s e n t p e r i n c h o f l e v e l c ha ng e

i n t h e l e v e l c o n t r o l r an ge .

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Steam In

rains

rains Outlet

T wo Z on e F e e d w a t e r H e a t e r

T h r e e Z o n e F e e d w a t e r H e a t e r

F i g u r e 2 2.

C o m p a r i s o n O f F e e d w a t e r H e a t e r C a p a c i t a n c e

2 8

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In a three-zone FWH, one must pay special attention to the distance between the top

of the drains cooling zone and the exit

of

the desuperheating zone because it is

important to maintain the drains water level between these two points. well-

designed

FWH

should have the exit of the desuperheating zone well above* the drains

cooling zone and a low liquid level that is maintained several inches** above the

drains cooling zone. High-level conditions that allow the overflow of water into

the desuperheating zone are detrimental to the life of that zone.

The vertical FWH by its nature, will usually have considerably less capacitance

than a horizontal FWH. Unless the diameter of the shell is such that adequate ca-

pacitance is provided, it is somewhat analogous to controlling the level in a straw:

the minute that one starts sucking, the level will disappear. It is imperative to

provide enough volume so that the load changes do not drive the level out of the

designed control range due to lack of capacitance. Table 2-2 shows the desired

reservoir capacity needed by most control systems in gallons per inch of depth,

depending upon the quantity of the drains that are being handled. Practical exper-

ience has shown that if these guidelines are followed, it is possible to control the

level within the 25-45 inch span typically recommended by experienced personnel.

However, it is noted that many FWHs now in existence have a smaller band, which may

demand superior performance from the level control system.

For proper level control, it may be necessary to increase the shell diameter for a

given tube bundle diameter so that the proper capacitance is available. Figure 2-3

displays a modification that was made on a FWH in just that manner. The lower por-

tion of the WH shell was enlarged to what is referred to as a belly band. This

modification not only provides greater capacitance, but also lowers the velocity of

the flashed steam (from inlet drains) as it exits from the lower portion of the FWH

Figure 2-4 shows internals of the same FWH that had its shell enlarged. portion

of the drains cooling zone shroud was cut away (as indicated by the white line) to

increase the difference in elevation between the drains cooling and desuperheating

zones from approximately 14-24 inches.

In

considering such a modification, it is

At

least 24 inches (and preferably more to accommodate fluctuations in level con-

trol).

**Some experts recommend 5 inches or more,

performance and capability depend upon the response of the system being used.

Typically, systems in use today are represented in Table

2-2.

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Table

2-2

DESIRED CAPACITANCE FOR TYPICAL*

FEEDWATER HE TER LEVEL CONTROL SYSTEMS

Fluid Flow-

Through Valve

Gallons/Minute)

Reservoir Capacity

In gallons/inch

o depth)

*This is

a

composite of typical capacitance data for many existing systems It is

shown for illustrative purposes only Similar data for a specific l v l control

system should be obtained from the vendor

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FSgwe 2 3 Belly Band

Xo-rBification o Verlicenl

aadwatar BcaB r S1 Lgbl [ e t w e e a

Xrrows

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necessary to determine whether it will lead to undesirable reductions in drains sub-

cooling. For this particular FWH, that was not a problem.

It has been stated that the lowest water level allowed should be at least several

inches above the top of the drains cooler. For reasons of economy, some may desire

smaller

FWHs, not allowing room for such a margin at the low level. It must be

remembered that if the water level should drop below'the shroud itself, there will

be steam but no drains going into the drains cooler, which is not the way to operate

the unit.

In a two-zone

FWH

by these same guidelines, the level is easier to maintain, with-

out increasing the size of the shell, inasmuch as the complete shell cross-section

minus the cross-section of all tubes is available for the required capacitance

levels.

One special circumstance that can develop in a three-zone vertical WH is where the

desuperheating zone surface is less than the drains cooling surface. In such a

case, it becomes necessary to artificially raise the top of the desuperheating zone

so as to meet the aforementioned level range criteria. This can be accomplished by

providing special baffling at the bottom of the desuperheating zone, thereby cre-

ating a dead zone from that point down to the tubesheet. When this happens, there

may be a tendency to skimp on realistic control ranges leading to operational prob-

lems. There is also a possibility that some novel design of drains cooler will be

attempted to cut its height. Over the years, there have been a number of variations

of two-pass shellside drains cooler (otherwise called double-shrouded or reverse

flow syphon-drains cooler ) designs that have rarely worked. These supposedly will

allow maintaining an operating level a few inches above the backside of the tube-

sheet.

Although some of these designs might work at stable load conditions and with

no imperfections in the manufacturing process, actual operating conditions make them

impractical. Flashing in the upflow pass invariably occurs, and the drains cooler

becomes ineffective.

2.1.1.3 Vertical Channel Up Feedwater Heaters. vertical channel up

FWH

that in-

cludes a drains cooler is subject to similar difficulties of flashing during varying

load conditions. In this design, the drains cooler shrouding encloses several rows

of tubes, and water level is maintained at the bottom of the shell (U-bend end).

These drains must

e

lifted to the top of the unit, and the potential for flashing

is exceptionally high.

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2.1.2 Examples Of Dra ins Coo ler Problems

F i g u r e 2-5 shows p i c t u r e s o f t y p i ca l damage fr om f l a s h i n g i n d r a i n s coo l i n g zones .

t

i s i mp o rt an t t o n o t e t h a t t h e f l a s h i n g t h a t t a k e s p l ac e i s s o v i o l e n t t h a t t h e

b a f f l e s and shroud s have been th inne d tub es des t roy ed and welds c rack ed . When

t h i s hap pe ns t h e a b i l i t y t o o p e r a t e t h e

WH

i n

a

normal manner

i s

l o s t a nd a d d i-

t i o n a l pr ob le ms de v e lo p t h a t mu st b e c o r r e c t e d ; i f t h ey a r e n o t t h e d e s t r u c t i o n o f

t h e FWH f o l l o w s r a p i d l y . fe w e x am p le s o f t y p i c a l p ro bl em s a r e c i t e d b el ow f o r

emphasis

Case : I n o ne u t i l i t y i n 1 970

it

was d is c o ve r ed t h a t a v e r t i c a l

channel down WH had cracked welds i n

i t s

d r a i n s c o o l i n g s h ro u d. Ex-

t e n s i v e r e p a i r s w er e made c o n s i s t i n g o f c o m pl et e r em ov al o f t h e d r a i n s

c o o l e r s h ro u d a nd r e p o s i t i o n i n g o f t h e l i q u i d l e v e l . The r e p a i r s w er e

s u c c e s s f u l a nd t h i s

EWH

h a s b ee n o p e r a t i o n a l s i n c e a l th o u gh a s l i g h t

s a c r i f i c e i n t o t a l p l a n t p e rf or ma nc e h ad t o b e a c ce p te d du e t o t h e e l i m -

i na t i o n o f d r a i n s coo l i ng . R epa i r o f t he c r acked sh r oud ing w as con -

s i d e r e d i m p r a c t i c a l

Case

2: A

ve r t i ca l channe l dow n

FWH

devel oped a c r acked we ld i n t he

p a r t i t i o n p l a t e t o t h e t u be s h ee t a r e a d ue t o v i o l e n t f l a s h i n g i n t h e

d r a i n s c o o l i n g z on e. T h i s p l a t e s e p a r a t e s t h e d r a i n s c o o l e r an d de-

supe r hea t e r zones . t was t e m p o r a r i l y r e p a i r e d by u s i n g a s e a l a n t t h a t

i s

e f f e c t i v e a t h i gh t em per a t u r e s . T he l eakage r a t e w hi ch was ove r 20

ga l l o ns pe r m inu t e w as c u t down t o 2 ga l l o ns pe r m i nut e . T he

FWH

was

a b l e t o o p e r a t e i n t h i s ma nn er f o r t h r e e more y e a r s w h i l e new

FWH

was

pur chased. I n a new des i g n o r

a

r ep lacement

FWH

a good way t o minimize

t h i s p ro bl em is by u s i n g s e p a r a t e s h ro u d w a l l s f o r t h e d e s u p e r h e a te r a nd

d r a i n s c o o l e r z o ne s r a t h e r t h an a common p l a t e which s ee s th e f u l l t em-

p e r a t u r e d i f f e r e n t i a l b et we en t h e t wo zo n es .

Case

:

A

w el d c r ac k e d i n a s h i e l d p l a t e i n a p o r t i o n of t h e d r a i n s

coo l e r sh r oud o f a ve r t i c a l channe l down FWH. The weld cr ac k s were de-

t e c t e d a f t e r ob se rv in g t h e i n a b i l i t y t o c o n t r o l t h e l e v e l . The

FWH

was

p r o p e r l y r e p a i r e d an d r e t u r n e d to s e r v i c e .

Case : Another c as e was a FWH t h a t d e ve lo pe d c r a c k s a t t h e b ac k of t h e

-

t u b e s he e t a nd t h e p a r t i t i o n w el d as w e l l a s t h e s i d e o f t h e p a r t i t i o n

p l a t e w e lds and t he sh r ouds p r ope r . The r e pa i r s we re made w i t h on l y

p a r t i a l s u cc es s due t o t h e i n a b i l i t y t o r ea ch c e r t a i n a r e a s of t h e

c r acked we l ds f o r r e pa i r w e l di ng. T he l eakage o f 10 ga l l o ns pe r m i nu te

was c u t down t o 1 / 3 o f a ga l l on p e r m i nu t e and t h e

FWH

was re t u r n e d t o

se rv ic e. Two more FWHs d e v el o pe d s i m i l a r c r a c k s a t t h e ba ck o f t h e t u b e -

s h e e t wh er e t h e d r a i n s c o o l e r s h r o u d is a t ta c h ed . I n t h i s p a r t i c u l a r

i n s t an ce no r ep a i r s w er e made s i n ce t he s e were ve r t i c a l channe l down

FWHs

w i t h on l y two zones and t h e l eakage w as no t t he r e f o r e i m por t an t

f o r t h e o p e r a t io n o f t h e FWHs . More of t h e same phenomenon was ex pe r-

i e n ce d i n t wo a d d i t i o n a l FWHs. The p o s s i b i l i t y o f d e t e c t i n g l e a k s i n t h e

i n t e r n a l sh r ouds w i t hou t r em ov ing t he

FWH

i s l im i te d t o t h e v e r t i c a l

chann el down des ig n . Hor izon ta l FWHs and v e r t i c a l chan nel up

WHs

d o n o t

o f f e r t h e c a p a b i l i t y o f wa t e r c o n ta i nm e n t w hi ch is n e c e s s a ry t o d e-

t e r m in e l e a k s e x t e r n a l l y . I n t h e s u r ve y e d u t i l i t y a p p ro x i ma t el y 1 00

WHs a r e v e r t i c a l channe l down he a t e r s and app r ox im a t e l y 60 o f t h e s e

h av e i n t e r n a l s h ro u d s. T he i n t e r n a l w eld f a i l u r e s w er e c a u s e d i n p a r t

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mkxt d@il~?~troy@dy

a

hhiqh-vebciky

rmpbknqmeat a b a ~ ~ l t

flashing

n

e d r a i n s

cwle r zane of a

har i zon t ak f@e&&Qar

eatan

Pabe

band @ mag@Prom Lrnslrkng

n

affle

hinnLng

nd t ube a~lsge n

th draans

ecmlar

Z O R ~

f d v e r t i c a l vertical Pee6water heatbar.

fee6Zbsater heatex,

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by inferior welds during the manufacturing process. Failure was ac-

celerated by improper operation of the FWHs for a number of years and, in

some cases, by an excessive temperature differential across the shrouds

themselves. Temperature differential problems can sometimes be present

if the design of a desuperheating zone is such that steam comes into

contact with the drains cooler shroud.

Case

5:

In another case involving a vertical head down installation, the

shell-to-bundle clearance was insufficient to provide adequate capaci-

tance, and the level controls were constantly hunting, even in stable

load operation. Rather than add a separate flash tank to accept drains

from the next higher WH space was not available), the utility cut

through the shell in the area of designed operating level and welded in a

section of 36-inch diameter pipe shown in Figure

2-6

that was capped on

the outside. This significantly improved level control, especially at

stable load operation.

Case 6: A mid-Atlantic utility operating a nuclear station with three

strings of FWHs found one of the first-stage

LP

EWHs in the condenser

neck having problems, while the other two were operating well. After

much searching, it was found that an improper setting of the level con-

troller was allowing levels below drains cooler entrance with subsequent

failure of the stainless steel tubing in the bottom rows due to high

velocities and vibration caused by the lower density steam flowing into

the drains cooler. Adjustment of the level control to the proper po-

sition stopped the problem, but damage was significant enough to justify

considering FWH replacement. An almost identical problem with a two-

string situation at another utility required extensive repair.

Figure

2 6. Vertical Channel Down Feedwater Heater With Welded Blister

2 16

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In summary, the experience of the experts who contributed to this guide clearly

points to flashing and improper operation of drains cooling zones as one of the

most important contributors to the failure of FWHs, particularly in horizontal

FWHs. This judgment is supported by the failure analysis data summarized in EPRI

Report CS-1776. The utility survey on which that report was based also showed that

the real cause of many

FWH

failures was never determined.

gain

experience sug-

gests that many of the unknowns could have belonged in the level control and

drains cooler zone category.

EPRI Report CS-1776 also contains a concise summary of drains subcooler problems as

discussed in the literature. Pages 4-1 through 4-14 are recommended reading for

operators to gain further understanding of the operation of the level control sys-

tem and the drains subcooler zone as well as the major problems to be avoided. A

good appreciation of the experiences outlined above and in CS-1776 will help plant

personnel avoid or minimize many of the problems that threaten the life of their

feedwater FWHs.

2.1.3 Symptoms Of Level Control And

r ins

Cooler Zone Problems

Indications of drains cooling zone problems can be obtained by observing the liquid

level swing or the absence of level in a

FWK

An excessively high water level

could be an indication of drains cooler problems, although ruptured tubes in other

zones would result in the same symptom. Some units have liquid level indicators

located in the control room to facilitate monitoring the operating levels. Another

quick observation is the position of the drains control valve between each

FWH

If

the valve is always fully open, it is probably not doing its job or is consistently

overloaded.

An unusual noise that is often heard (like marbles in a bottle) is the sound of

flashing. It is not unusual

to

detect this type of sound downstream of the drains

control valve. However, when a similar noise is heard upstream of the valve, then

it

is an indication of potentially damaging flashing. Today's technology has pro-

duced equipment that is capable of differentiating between noises to pinpoint a

certain type. Acoustic monitors are available on the market that give the user an

opportunity to first establish that a flashing noise exists and then to quickly

determine whether this condition exists upstream of the drains control valve.

For comprehensive, accurate performance tests, the

ANSI/ASME Performance Test Codes

PTC 12.1, 1978) should be used as a detailed guide. However, for routine trouble-

shooting and quick checks of WH performance, the simple test outlined below can be

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made without a complete heat balance, special instrumentation, or elaborate compu-

tations. The test form shown in Figure 2-7 can be used to determine uncorrected

terminal temperature difference (TTD) and the approach for comparison with design

figures for these parameters, Note that the uncorrected

TTD

developed in line

4

of

Figure 2-7 for the actual column is different (for simplicity) from the corrected

TTD* developed in PTC 12.1 as indicated below:

TT (saturated temperature for actual steam pressure)

(actual feedwater outlet temperature)

TTD* (saturated temperature for designed steam pressure)

minus

(feedwater outlet temperature corrected to design conditions)

The TTD is, in effect, a measure of the heat transfer capability of the FWH. The

higher the terminal temperature difference above design (if the value is positive),

the poorer the performance of the FWH. The actual measurements can only be com-

pared to the values obtained during performance tests to see whether any serious

change is taking place. If for the same conditions the TT is substantially higher,

then the FWH has problems.

The approach

in

line 7 is the difference between the drains outlet temperature

and the feedwater inlet temperature.

In addition to the general data recorded at the top of this form, critical readings

to be gathered are:

Line 1: Steam-side pressure (saturation pressure) as measured in the FWH

shell;

Lines 3 6: Temperature reading

of

the feedwater flow at the

WH

outlet

and inlet connect ons

Line 5: Temperature of the drains as they leave the FWH shell; and

Line

8:

Measurement of actual feedwater flow.

Lines 2, 4, and do not involve data gathering in the plant but are obtained from

the steam tables (for Line

2)

or simple arithmetic (Lines

4

and 7 .

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G E N E R A T I N G S T A T I O N U N I T N O.

TE S T NO. HEATER NO.

ACTUAL W LOAD W L O A D O F

DESIGN

DAT E TIME

OBSERVED LEVEL C OM M E NT S)

A C T U A L D E S I G N

C O N D I T I O N C O N D I T I O N

1. Shel l - s ide s team opera t ing

pres su re psia ps ia

2.

Shel l - s ide steam corresponding

sa tu ra ted tempera ture F F

F

3

Feedwater o u t l e t temperature F

4.

TTD

( terminal temperature

d i f f e r enc e uncor r ec t ed )

Item 2 I t e m 3 Item

5 Drains ou t l e t t empera ture

6

Feedwater in le t t empera ture

7.

Approach

I t e m

5

t em

6

Item

7

8 Feedwater flow

Figure 2 7. Feedwater Heater

e s t

Sheet. ( to determine ckains cooler

behavior

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F or p r op er a n a l y s i s o f t h i s d a t a , i t w ou ld b e d e s i r a b l e t o show t h e l o c a t i o n o f t h e

w a t e r l e v e l w i t h i n t h e s h e l l a nd t h e m ea ns by w hi ch t was de te rm in ed . Assuming

d r a w i n gs o f t h e FWH i n q u e s t i o n a r e a v a i l a b l e , a s t u d y o f t h e p ro bl em c a n t h en be

made.

The nu mb er s i n t h e a c t u a l c ol um n a r e c om pa re d t o t h o s e f o r t h e 1 0 0 l o a d d e s i g n

c o n d i t i o n s . F i g u r e 2-8 i s a v e r y s i m p l e e xa m pl e o f a f a m i l y o f c u r v e s sh ow in g u n i t

m ega watt l o a d p er c e n ta g e of f u l l l o ad v e rs u s s h e l l o p e r a t i n g p r e s s u r e i n p s i a . The

t o p c u r v e i s f o r t h e h i gh e s t s t a g e FWH ( i . e . , h i g h e s t t e m p e ra t u r e a nd p r e s s u r e ) ,

T he se a r e s t r a i g h t l i n e s f ro m 1 00 l o a d t o 0 l o ad . U si ng F i g u re 2-8, o n e c a n

q u i c k l y d e te r m in e w ha t t h e o p e r a t i n g p r e s s u r e s h ou l d be a t a ny p a r t i a l l o a d co n-

d i t i o n . I t s t a n d s t o r ea so n t h a t w i t h a p a r t i a l l o a d , t h e TTD a nd t h e a p p r o ac h

v a l u es s ho ul d b e b e t t e r ( l e s s ) t h a n a t f u l l lo ad . Many u t i l i t i e s h ave f a m i l i e s o f

cu rves showing TT a nd a p pr o ac h a t v a r i o u s l o a d s t h a t h a ve b e en p r e p ar e d b y t h e

m a n u f a c t u r e r a nd s u b m i t t e d t o th em f o r t h e p u r p o se o f p e r fo r m a n c e a n a l y s i s . Op-

e r a t o r s sh o ul d r e f e r t o t h e s e c u r ve s , i f t h ey wa nt more p r e c i s e i n f o rm a t io n ; b u t

f o r t h e pu rp os e o f t h i s t e s t , t h e d a t a i n F i g u r e 2-7 s h ou ld be s u f f i c i e n t .

B y

l o o k i ng a t t h e a p p r o ac h t e m p e r a t ur e , t c a n b e de t er m i ne d i f t h e r e

is

f l a s h i n g i n

t h e d r a i n s c o o l e r a nd f l a s h i n g i n t h e s y st em ( as su mi ng e a r l i e r t e s t s have shown

t h a t t h e

FWH i s

n o t d e f i c i e n t f ro m t h e t h e r ma l d e s i g n s t a n d p o i n t ) . Most F W H s i n

u s e t o d a y a r e d e s i g n e d f o r a n a p p r o ac h o f a p p r o x i m a t e l y

l o 0 .

I f

t h e

FWH

i s o p e r -

a t i n g a t 75 l o a d i n s t e a d o f l o o , t h e n t h e a p p r o a c h m ig h t b e

OF

o r OF. I t h e

t e s t sho ws t h a t t h e t e m p e ra t ur e o f t h e a pp r oa ch i s 2 O 0 ~ - 3 o0 ~ , h e r e is s t r o n g r e a -

s on t o s u s pe c t t h a t t h e

FWH

i s

e x p e r i e n c i n g p ro b le m s i n t h e d r a i n s c o o l i n g z on e

a nd /o r t h e o p e r a t i o n of t h e d r a i n s c o n t r o l v a lv e .

2.1.4

O p e r a t i o n a l P r a c t i c e s T o void Or M i t i g a t e P r o bl e m s

I t

s h o u l d be e mp ha s iz e d t h a t mo s t o f t h e f o r e g o i n g d i s c u s s i o n s as su me a c o n d i t i o n

w here a l l F W Hs a r e o p e r a t i o n a l . I f some a r e o u t o f s e r v i c e , t h e r em ai nd er o f t h e

FWHS, w hi ch a r e i n t h e o ve r l o a d c o n d i t i o n , c a n n o t be e x p e c te d t o m a i n t a i n t h e a p -

p r oa c h t h a t was p r e v i o u s l y d i s c u s s e d . F o r e xa mp le , i n a s i n g l e s t r i n g o f e i g h t FWHs

( a s c en d i n g n u m e r i ca l l y w i t h p r e s s u r e w i t h t h e e i g h t h b e i ng t h e h i g h - pr e s s u re F W H ,

i f FWNs 4 an d 5 a r e o u t , t h e t e m p e r a tu r e o f t h e w at er g o i n g i n t o FWH 6 w i l l be much

l ow er t h a n n or m al . T h a t p a r t i c u l a r FWH t h e r e f o r e , i s g oi ng t o

be

oper a te d under a

g r e a t o ve r l o a d c o n d i t i o n . A t o n e u t i l i t y h a v in g mor e t h a n 300 FWHs maximum poten-

t i a l o v e r l o a d c o n d i t i o n s o n do wn st re am

F W H s

range f rom 120 -304 wi t h one ad j ace n t

ups t ream WH c u t o u t . U nde r s uc h c o n d i t i o n s , o n e sh o ul d e xp e c t t h e a pp r oa c h t o b e

much hig he r t ha n normal. I t i s p a r t i c u l a r l y i m po rt an t i n t h e s e c ir c um s ta n ce s t o

make s u r e t h a t f l a s h i n g i s e l i m i n a t e d b o t h i n t h e d r a i n s c o o l e r a n d u p s tr e am of t h e

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Unit MW oad

2 )

F i g u r e

2-8.

U n i t

W

Load

Vs.

Feedwater

eater hell

O p e r a ti n g P r e s s u r e

Ge neric Example) .

These curves may be developed f rom th e s ta te d designed

p r e s s u r e s , r t h e y may be approximated

by

measu r ing one o r more po in t s a t

s t e a d y - s t a t e p l a n t c o n d i t i o n s.

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d r a i n s c o n t r o l v a l v e , b e ca u se f l a s h i n g u n de r t h e s e c o n d i t i o n s

i s

o f s u c h g r e a t i n -

t e n s i t y a nd f o r c e t h a t d e s t r u c t i o n o f a h e a t e r

i s

u s u a l l y r e a ch e d w i t h i n a s h o r t

pe r i o d o f t i me . check w i t h t h e FWW s u p p l i e r s h o u l d b e made b e f o r e o p e r a t i n g a t

a ny o v e r l o a d c o n d i t i o n . t would b e an e x c e l l e n t p r a c t i c e f o r p l a n t o p e r a t o r s t o

know t h e v e r s a t i l i t y o f a n y

FWH

t h a t m i gh t b e r e q u i r e d t o o p e r a t e u n de r a b no rm al

c o n d i t i o n s . ( S e c t io n a d d r e s s e s t h i s a r e a i n more d e t a i l . )

The a b i l i t y o f t h e f e e dw a t er s y st em t o h a nd l e ab no rm al c o n d i t i o n s i s ve r y i m por t an t .

I f t h e sy s t em does no t p r ov i de m eans o f bypass i ng t he f eedw a t e r s i d e o r bypass i ng

t h e d r a i n s , s a f e a bn or ma l o p e r a t i o n c a n b e s e v e r e l y l i m i t e d . f e e dw a t er s y s te m

t h a t d o e s n o t h av e some f l e x i b i l i t y of o p e r a t i o n

is

poor l y des i gned and

w i l l

deve l op

p ro bl em s. I n p e a ki n g p l a n t s , t h e r e w i l l be a demand fo r power t h a t

w i l l

c r e a t e

c y c l i c a l c o n d i t i o n s . The l a c k o f p ro p e r a d j us t m en t o f c o n t r o l s w i l l i n c r e a s e t h e

r a t e o f d e t e r i o r a t i o n an d d e s t r u c t i o n o f t h e EWHs. s t r i n g of FWHs t h a t can op-

e r a t e i n a s t e a dy

s t a t e c o n d i t i o n w i t h o u t p ro bl em s may e x h i b i t s i g n i f i c a n t f l u c t u -

a t i o n o f w a te r l e v e l when c y c l i c a l c o n d i t i o n s a r e e x p e r ie n c ed .

This point empha-

s i z e s t h e n ee d f o r t h e o p e r a t o r s

to

r ecogn i ze t ha t bl owi ng s team t h r ough a d r a i n s

c o o l e r i s neve r

a

p r o pe r o p e r a t i n g p r a c t i c e . One s h o u l d b e l o o k i n g f o r a l e v e l t h a t

i s

r e l a t i ve l y s t eady . W hil e unde rgo i ng l oad changes , t h e r e shou l d be some l e ve l

f l u c t u a t i o n ; b u t upon r e t u r n t o st e a d y s t a t e , e ve n a f t e r g o in g fro m a 10 0 o a d t o a

25 l oa d , t h e l e v e l s ho ul d s t a b i l i z e . I f t d o e s n o t , t h e s wi ng in g o f t h e l e v e l i s

a n i n d i c a t i o n o f p r ob l em s i n t h e s y s te m ( p r o b a b l y c o n t r o l s ) u nd er s t e a d y o p e r a t i o n

t h a t s h o u ld b e i n v e s t i g a t e d . The t e s t t h a t was p r e v i o u sl y d e sc r i b e d i s a gu id e t o

a c c o mp l is h t h i s e nd .

t i s e s s e n t i a l t h a t t h e o p e r a t o r s un d er st an d t h e i mp or ta nc e o f t h e d r a i n s o u t l e t

c o n t r o l v a lv e w i t h r e s p e c t t o c o n t r o l l i n g t h e w at er l e v e l w i t h i n t h e power p l a n t WH

s y st e m, Many c o n t r o l v a l v e s a r e n o t p r o p e r l y s i z e d a nd o p e r a t e d f o r t h e s e r v i c e

in ten ded . The t endency seems to be t o o v e r s i ze t h e c o n t r o l v a l v e s an d, i n a d d i t io n ,

t o o p e r a t e w i t h t h e v a l v e s o pe ne d w i de r t h a n n e c e s s a r y . b a s i c m i s t a k e i s n o t

r e a l i z i n g t h a t when t h e v a l v e i s opened w i de r t han neces sa r y , t he r e cou l d be

f l a s h i n g u p s tr e am o f t h e v a l v e , w h er e as t h e f l a s h i n g s h o u l d t a k e p l a c e o n l y d own-

s t r e a m o f t h e v a l v e s e a t . A ga in ,

t

i s

i m por t an t t o remember t h a t t h e pas s i ng o f

f l u i d t h ro ug h a c e r t a i n a r e a

i s

a f u n c t i o n n o t j u s t o f t h e p r e s s u r e a n d o pe n s p a ce ,

b u t a l s o o f t h e d e n s i t y a n d v olu me of t h e f l u i d an d t h e ge om et ry o f t h e a r e a . When

f l a s h i n g

is

a l lo w e d up s tr ea m o f t h e v a l v e , t h e f l u i d volume a n d v e l o c i t y i n c r e a s e

d r a m a t i c a l l y , a nd a v a l v e s i z e d on t h e b a s i s o f p a s s i n g o n l y w a t er c o u l d be t o o

sm a l l t o pa s s t h e m i x t u r e o f f l a sh ed s t eam and w a t e r ( tw o- phased f l ow ) .

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In one set of tests, identical drain quantities were passed by the liquid level

control valve when it was adjusted manually to only 60% open, as compared to what it

was passing when wide open under automatic control. When the valve was only 60

open, the pressure drop through it was high enough that the upstream pressure re-

mained sufficient to preclude flashing; when the valve was wide open, the pressure

condition at the point of inlet was such that flashing was occurring ahead of the

valve. Therefore, with the limited valve opening, only hot water was passing

through, whereas a combination of hot water and steam was flowing across the valve

seat when the valve was wide open. Even though the same quantity of drains flowed

in each instance, the 100% open valve condition was most likely damaging the valve

trim (i.e., the valve plug and the seat).

Similarly, another utility developed problems with a LP FWH drains valve to the

condenser. Upon investigation, the operating personnel were asked to override the

controls of this valve, which was not passing the required quantity of drains while

it was fully open. When the valve was manually set to a position

50

open, an in-

creasing level was experienced for a short period of time, followed by a decrease to

the design level. Prior to that adjustment, it was decided to bypass the drains

from the upstream FWH to the condenser, rather than to cascade to the lowest pres-

sure

FWH,

thereby minimizing flow through that troubled valve. When the real prob-

lem was recognized and solved, the operators returned to the cascading mode without

further difficulties.

It is also possible to encounter problems

if

the drains control valve is undersized

rather than oversized. If it is undersized, the alternative drains valve or dump

valve to the condenser will

be called upon to operate for long periods partially

open.

This situation may go undetected, resulting in poor plant performance as well

as deterioration of the dump valve. This also degrades the turbine water induction

protection, which relies upon proper dump valve operation during high-water-level

emergencies.

Failure to recognize such operating considerations is costing the electric utility

industry millions of dollars

in both poor performance and equipment destruction.

Both the approach and the TTD are affected.

FWHs are designed for an optimum TTD

and an optimum approach,

which can exist at any given operating condition, provided

an operating level is maintained that allows complete liquid phase flow to the

drains cooler without flashing. If it is necessary to submerge one or two rows of

tubes to accomplish this (in a horizontal

FWH ,

the TTD will still be superior to

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what w i l l e x i s t i f a low w a te r l e v e l i s p e r mi t te d and f l a s h i n g t a k e s p l a c e a t t h e

d r a i n s c o o l e r e n t r a n c e .

P er fo rm a nc e o f l i q u i d l e v e l c o n t r o l s y st e m i s d e p e n d e n t up on p r o p e r m o n i t o r i n g

a s

w e l l

a s t h e a b i l i t y o f a l l c om po ne nts t o d o t h e i r j o b. T h e re f o re ,

t

i s

n e c e s s a r y

t o

know wh er e t h e v a r i o u s l e v e l s s h o u l d be i n a FWH F o r e a c h

FWH

t h e r e s h o u ld b e a

s i m pl e s k e t c h o f t h e i n t e r n a l s s i m i l a r t o t h a t s hown i n F i g u r e 2-9, w hi ch i n d i c a t e s

w h e r e t h e

low

t u b e i s how many tu be s s hou ld be submerged, an d what happens when th e

l e v e l move s f r om h i g h to l ow . T he o p e r a t o r s i n

t h e

p ow er p l a n t s h o u l d h a v e a s u f -

f i c i e n t u n d e r st a n d i n g o f w h a t a l ow o r h i g h l e v e l m ea ns a nd how l e v e l s s h o u l d f l u c -

t u a t e d u ri n g t r a n s i e n t s .

F i g u r e s 2-10 a n d 2-1 1 d e p i c t b o t h h i g h a n d l ow s h e l l

l e v -

e l s f o r a h o r i z o n t a l s i t u a t i o n . F i g ur e 2-11 c l e a r l y i l l u s t r a t e s w he re a f l a s h i n g

c o n d i t i o n d e v e l o p s when a l e v e l i s m a i n t a i n e d too l o w

T he l a c k o f u n d e r s t a n d i n g c o n c e r n i n g t h e l e v e l s o n t h e FWHs c a n b e c o r r e c t e d by

r e qu e s ti n g a d d i t i o n a l i n f or m at io n s o t h a t t h e u t i l i t i e s h av e p ro p er d r a w in g s a nd

i n f o r m a t i o n .

One u t i l i t y p o s t s t h e s e l e v e l s i n t h e c o n t r o l room on d ia gr am s s i m i l a r

t o F i g u r e 2 -12 .

The

e xa m pl e s ho ws a v e r t i c a l c h a n n e l down u n i t , i n d i c a t i n g t h e

l