Flow Characteristics

Sizing and SelectionIntroduction

The Valtek Sizing & Selection Manual is a comprehensive, easy-to-follow reference guide for determining the proper control valve for a particular application. The material follows the sequential order of the typical Valtek quote sheet (as shown in Figures 1-1 through 1-3).

This introduction discusses general information about control valve sizing. It also outlines the basic control valve parameters which must be known for correct sizing.

THE CONTROL VALVE

A control valve is a final control element used in instrumentation systems to regulate the flow, pressure or temperature of liquids and gases in process systems.

A control valve is different from other valves in that it has a power positioning actuator for moving the closure mechanism in response to an external signal. The actuator’s energy source is usually provided from an independent source.

The following reference table outlines the line of Valtek control valves, along with their applications, advantages and limitations:

TABLE 1-I: Globe Valves

Valve Application Advantages Limitations

Mark One Cryogenic to high temperature; vacuum to high-pressure

Higher DP than rotary; tight shutoff

Lower CV than rotary valve

Mark Two

High-pressure Fabricated, no casting delay Size limitations

Mark Eight

High flow rates High CV; self-draining in some applications

Angled top-works; maintenance

Three-way

Combining/diverting service 3-way capability Limited to 3-way service

Angle Erosive service; slurries Streamlined; self-draining Piping

TABLE 1-II: Rotary Valves


Valdisk High flow with low DP High CV; low cost; lightweight Low FL; low DP

ShearStream High flow; slurries; pulp stock High CV; high rangeability Low FL; low DP

TABLE 1-III: Severe Service


CavControl Minor cavitation control Minimizes cavitation damage; low cost alternative

Limited to minor cavitation

ChannelStream Anti-cavitation Eliminates cavitation damage Relative cost

MegaStream Gaseous noise control Wide range of attenuation Relative cost

Tiger-Tooth Anti-cavitation and gaseous noise control

Eliminates cavitation damage; wide range of attenuation

Relative cost

TABLE 1-IV: Special Service


Mark Four Check valve Non-slamming Limited application

Mark Six Cryogenics; cold box Top-entry; no flanges in cold box Limited application

Guardian Critical services Zero leakage; metal bellows stem seal Limited application

SELECTING THE RIGHT CONTROL VALVE

Control valves are available in a variety of styles and configurations, depending on the flow conditions, pressure and temperature requirements, material requirements, end connections, leakage classifications, cost, and customer preference. In general, the Valtek control valve selected will be one of four types: 1) globe, 2) rotary, 3) severe service or 4) special service.

Mark One globe-style control valves feature a standard cast carbon steel or stainless steel globe body, topentry trim with a characterized plug contour, high thrust double-acting piston actuator, four-way positioner, and fail-safe return spring. In addition to the carbon and stainless steel construction, Mark One bodies can be furnished in various grades of chrome-moly, alloy 20, Hastelloy “B” and “C”, Monel, Inconel, titanium, bronze, nickel, and other castable materials.

The Mark One is available in sizes ranging from 1/2 through 42-inch, and in ANSI Pressure Classes from 150 through 4500. Body styles include standard globe, globe with expanded outlets (Mark One-X), angle and three-way configurations. A variety of end connections is also available, including raised face, RTJ (ring-type joint), socketweld, buttweld, NPT (National Pipe Threads) and Grayloc. This versatility makes the globe design the most widely used of all control valve types.

Mark Two control valves are similar to Mark One globe style valves except that the body is fabricated from barstock. It is typically available in sizes from 1/2 through 2-inch and Classes from 150 through 2500. Special designs are also available for body sizes up to 6-inch with special applications to 15,000 psi.

Valdisk is a high-performance eccentric disk control valve with quarter-turn action. It utilizes an elastomeric or metal seat for tight shutoff. Valdisk features a flangeless wafer body that fits between raised-face line flanges.

One Valdisk body serves ANSI Classes 150 through 600 in sizes 2 through 8-inch, and Classes 150 through 300 in sizes 10 through 12-inch. The disc pressure class on Valdisk valves 12-inch and smaller is one pressure class less than the maximum body rating. For example, an ANSI Class 600, 8-inch Valdisk has a maximum shaft pressure drop of 720 psi. Sizes 14 through 30-inch use a Class 150 body as a standard. Valtek also has the capability to design and manufacture special high-pressure bodies and discs.

Valdisk is available in the same body materials as the Mark One. It also uses a piston actuator with a rotary transfer case in place of the linear yoke. Many parts are interchangeable between Valdisk and Mark One.

ShearStream is a high-performance ball valve designed to overcome the problems of harsh, particle entrained processes. Its high 300 to 1 rangeability, or turn down ratio, also makes ShearStream an ideal control valve for use in high temperature water and steam applications. It is available in sizes 1-inch through 12-inch and ANSI Pressure Classes 150, 300 and 600. Class IV and VI shutoff are achieved with Valtek’s unique Flex-LocTM metal seal and soft seal design, respectively.

ShearStream’s V-notch ball design reduces clogging and improves shearing action, making it an ideal control valve for particle entrained processes. The one-piece body design overcomes many long standing challenges faced by traditional ball valves: piping forces that unevenly load the seat, low rangeability due to limited orifice characterization, and unsatisfactory shutoff capabilities. Available body materials include 316 stainless steel and carbon steel. Ball materials available include 317 stainless steel with either hard chrome or Stellite facing.

MegaStream, ChannelStream and Tiger-Tooth control valves use special trim enclosed in the standard Mark One body to eliminate problems associated with high pressure drops. MegaStream minimizes the aerodynamic noise associated with gas services. Channel- Stream eliminates cavitation and vibration in liquid services. Tiger-Tooth is used in liquid services to eliminate cavitation and in gaseous applications when the required noise attenuation exceeds 20 dB. All three valves use the same materials of construction as the Mark One.

These special trims act to eliminate noise and damaging cavitation which would otherwise severely limit service life of the valve, cause premature mechanical failuref the component parts or prevent personnel from working near the valve.

Valtek also offers special service control valves which are designed for a specific application where standard control valves are not particularly suited or easily adapted.

Tek-Check check valves are often used with reciprocating and centrifugal pumps to provide directional flow control. The body and seat ring are both interchangeable with the Mark One. Tek-Check materials of construction are the same as the Mark One.

Mark Six cryogenic valves are specially designed for use in cryogenic air separation columns where all equipment is sealed and insulated in the cold box. Access to the trim is through the valve bonnet. Bodies are available in bronze and various grades of stainless steel.

Mark Eight Y-pattern valves offer a greater capacity per valve size than do standard globe valves, while at the same time retaining the advantages of the globe valve configuration. Materials of construction are identical to the Mark One.

SELECTION CRITERIA

To properly select the type of control valve best suited for a particular application, sizing calculations must be determined with accurate process conditions. Based on the estimated body size, consideration should then be given to the following selection criteria:

Flow Conditions

In general, applications for relatively low operating pressures and nominal operating temperatures may use any one of the “general service” control valves. Where pressure drops are small and flow rates are substantial, the Valdisk control valve, with its large capacity per size, may be chosen. The Mark One body style, on the other hand, should be chosen for those applications where operating pressures and pressure differentials are higher. Large pressure drops often require the use of Tiger-Tooth, MegaStream, CavControl or ChannelStream trim to reduce noise and damage to the valve and associated piping.

Pressure and Temperature Requirements

Regardless of the body style, selection of a control valve must be in accordance with established material properties. ANSI Standard B16.34 lists the allowable temperature and pressure combinations for a variety of body materials. Standard pressure classes include ANSI Classes 150, 300, 600, 900, 1500, 2500 and 4500. (Valtek also has the capability to work with higher ratings and special class valves.) For a given body material, each class denotes the maximum working pressure for a given operating temperature. Due to loss of material strength, the allowable working pressure decreases as the operating temperature rises. Lower temperature limits are also specified due to loss of ductility in some materials. Although a pressure class is usually indicated for an application, roper attention to selection of the pressure class is crucial to the safe operation of the control valve.

Standard Valdisk may be specified in services up to ANSI Class 600, 2 thru 8-inch, with reduced pressure classes in larger size valves. However, Valtek specializes in high-pressure, high temperature applications. For applications where the working pressures and temperatures are extremely high, special Mark One and Mark Two alves can be selected. These valves can be designed with pressure and temperature ratings thatexceed the arameters of ANSI Standard B16.34.

Material Requirements

Proper material selection is essential for reasonable valve life. Process fluid corrosion, high velocity rosion, entrained particles, cavitation or other problems can combine to destroy or consume improperly specified materials. Where carbon steel or stainless steel materials are unsuitable for a corrosive process, special alloys should be selected based on material compatibility with the process fluid. Where erosion exists, harder or more durable materials can be specified to prolong the valve life.

Since the control valve user is most familiar with the chemical composition and corrosive properties of the process fluid, the ultimate responsibility for material selection rests with the user.

Special materials of construction are available in all product lines. Alloy costs can be cut, however, by using Valdisk in applications where exotic alloys are required and where process conditions permit. Because of their size and weight, rotary valves tend to be less costly than comparable globe-style valves.

The Mark One minimizes costs with separable bonnet and end flanges. These flanges can be supplied in less expensive materials since they do not come in contact with the flowing medium. Carbon steel and stainless steel valves can be specified with carbon steel flanges. Exotic alloy valves such as Alloy 20, Hastelloy, Inconel and others are typically specified with 316 stainless steel flanges for significant cost savings. Using separable flanges – instead of integral flanges – can also result in significant cost savings.

End Connections

Most control valve specifications request raised face flange connections suitable for bolting to the process piping. In power piping and nuclear applications, buttweld ends suitable for welding to the process piping are typical. For limited applications, screwed end connections (NPT) and socketweld ends are specified for globe valves in sizes 2-inch and smaller. Some highpressure applications may require the use of RTJ (ringtype joint) flanges or patented Grayloc hub ends. Flangeless, wafer-style connections are specified in applications where rotary valves are suitable for the process conditions. These connections are usually raised face.

The Mark One can be built with any one of the standard end connections, except the flangeless wafer style. The Mark One also uses face-to-face dimensions per ANSI Standard B16.10 for valve sizes 1/2 through 4-inch and ANSI Classes 150 through 600 (allowing the use of the same Class 600 face-to-face dimension for all three Pressure Classes with a given body size).

With separable flanges, the standard ANSI face-to-face dimension allows the use of a single body casting for ANSI Class 150, 300 and 600 for a given body size. This greatly reduces inventory costs and increases critical spare parts availability. For all valve sizes – except 6-inch and larger valves, Class 900 – the ISA face-toface dimension per ANSI/ISA Standard 75.03 is available, and is standard with all valves 6-inch and larger.

Leakage Connections

Control valve seat leakage can be an important factor in the selection of a control valve. Seat leakage is typically rated by leakage class per ANSI Standard B16.104. Most standard Valtek control valves shutoff tighter than Class IV, which corresponds to a leakage rate of 0.01% of the rated valve capacity. Class V or Class VI leakage can be obtained with proper actuator sizing or a soft seat selection.

With globe or rotary valves, a “bubble-tight” Class VI leakage can be specified using an elastomeric insert configuration (commonly referred to as a “soft seat”). However, there are some soft seat limitations to remember, such as temperature and pressure.

Pressure-balanced valves typically meet Class II and Class III leakage when used with a metal seat and metal or resilient seals. Pressure-balanced trim can achieve Class IV or Class leakage when used with a soft seat and exceptionally tight resilient seals.

Cost Considerations

The selection of a control valve type is generally based on the least expensive alternative which will meet the process control requirements. Rotary valves are very economical in applications where capacity requirements are high, and where process pressures and pressure drops are low. Globe valves are usually selected where flows are small and pressure drops are high, or where process fluids may be corrosive or erosive. Severe service control valves, which are typically modifications of a standard globe valve, should be considered for those applications where noise and cavitation must be controlled or eliminated.

Customer PreferenceThe accumulated experience of control valve users in a particular industry or process plant application may have a strong influence on control valve selection. Pulp and paper plants may use rotary valves almost xclusively due to the problems inherent in processing paper stock. Globe valves are generally selected in most standard applications due to their versatility. High performance rotary valves have gained in popularity as actuator capabilities and linkages have been improved to allow the valves to function as true control valves. Purchase price, performance, cost of maintenance and valve life also influence user preference.

Control Valve Sizing

INTRODUCTION

Valtek uses a systematic method for selecting body types, sizes, materials, pressure ratings and trim sizes based on flow characteristics. Valtek control valve flow capacity (Cv) is based upon the industry standard, ANSI/ISA S75.01. This standard and the corresponding measuring standards contain Equations used to predict the flow of compressible and incompressible fluids in control valves. Slightly different forms of the basic Equation are used for liquids and gases. Basic steps for sizing and selecting the correct valve include calculating the required Cv. Equations for calculating Cvfor both gases and liquids are found in this section.

Valtek has programmed the ANSI/ISA sizing Equations and procedures, making computer-aided sizing available on IBM-PC or compatible computers. These programs permit rapid control valve flow capacity calculations and valve selection with minimal effort. The programs also include exit velocity, noise prediction and actuator sizing calculations. See Section 22 for more details on computer-aided valve selection. These instructions are designed to expose the user to the different aspects of valve sizing. The step-by-step method outlined in this section is the most common method of sizing.

NOMENCLATURE

Flow Capacity

The valve sizing coefficient most commonly used as a measure of the capacity of the body and trim of a control valve is Cv. One Cv is defined as one U.S. gallon per minute of 60 degree Fahrenheit water that flows through a valve with a one psi pressure drop. The general Equation for Cv is as follows:

figure 3-1 Pressure Profile of Fluid Passing Through a Valve

When selecting a control valve for an application, the calculated Cv is used to determine the valve size and the trim size that will allow the valve to pass the desired flow rate and provide stable control of the process fluid.

Pressure Profile

Fluid flowing through a control valve obeys the basic laws of conservation of mass and energy, and the continuity Equation. The control valve acts as a restriction in the flow stream. As the fluid stream approaches this restriction, its velocity increases in order for the full flow to pass through the restriction. Energy for this increase in velocity comes from a corresponding decrease in pressure. Maximum velocity and minimum pressure occur immediately downstream from the throttling point at the narrowest constriction of the fluid stream, known as the vena contracta. 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 fluid pressure is shown in Figure 3-1. The slight pressure losses in the inlet and outlet passages are due to frictional effects. The major excursions of pressure are due to the velocity changes in the region of the vena contracta.

Figure 3–2: Choked Pressure Drop

Allowable Pressure Drop

The capacity curve shown in Figure 3-2 shows that, with constant upstream pressure, flow rate, q, is related to the square root of pressure drop through the proportionality constant Cv. The curve departs from a linear relationship at the onset of “choking” described using the Fi, factor. The flow rate reaches a maximum, qmax,at the fully choked condition due to effects of cavitation for liquids or sonic velocity for compressible fluids. The transition to choked flow may be gradual or abrupt, depending on valve design. ANSI/ISA liquid sizing Equations use a pressure recovery factor, FL, to calculate the <m>Delta</m>Pch, at which choked flow is assumed for sizing purposes. For compressible fluids, a terminal pressure drop ratio, XT, similarly describes the choked pressure drop for a specific valve. When sizing a control valve, the smaller of the actual pressure drop or the choked pressure drop is always used to determine the correct Cv. This pressure drop is known as the allowable pressure drop, <m>Delta</m>Pa,

Cavitation

In liquids, when the pressure anywhere in the liquid drops below the vapor pressure of the fluid, vapor bubbles begin to form in the fluid stream. As the fluid decelerates there is a resultant increase in pressure. If this pressure is higher than the vapor pressure, the bubbles collapse (or implode) as the vapor returns to the liquid phase. This two-step mechanism – called cavitation – produces noise, vibration, and causes erosion damage to the valve and downstream piping. The onset of cavitation – known as incipient cavitation – is the point when the bubbles first begin to form and collapse. Advanced cavitation can affect capacity and valve performance, which begins at a <m>Delta</m>P determined from the factor, Fi) . The point at which full or choked cavitation occurs (severe damage, vibration, and noise) can be determined from Equation 3.3. Under choked conditions, “allowable pressure drop,” is the choked pressure drop.

Liquid Pressure Recovery Factor, Fl

The liquid pressure recovery factor, FL, predicts the amount of pressure recovery that will occur between the vena contracta and the valve outlet. FL is an experimentally determined coefficient that accounts forthe influence of the valve’s internal geometry on the maximum capacity of the valve. It is determined from capacity test data like that shown in Figure 3-2.FL also varies according to the valve type. High recovery valves – such as butterfly and ball valves – have significantly lower pressures at the vena contracta and hence recover much farther for the same pressure drop than a globe valve. Thus they tend to choke (or cavitate) at smaller pressure drops 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 the maximum effective (choked) pressure drop across the valve.

Flashing

If the downstream pressure is equal to or less than the vapor pressure, the vapor bubbles created at the vena contracta do not collapse, resulting in a liquid-gas mixture downstream of the valve. This is commonly called flashing. When flashing of a liquid occurs, the inlet fluid is 100 percent liquid which experiences pressures in and downstream of the control valve which are at or below vapor pressure. The result is a two phase mixture (vapor and liquid) at the valve outlet and in the downstream piping. Velocity of this two phase flow is usually very high and results in the possibility for erosion of the valve and piping components.

Choked Flow

Choked flow occurs in gases and vapors when the fluid velocity reaches sonic values at any point in the valve body, trim, or pipe. As the pressure in the valve or pipe is lowered, the specific volume increases to the point where sonic velocity is reached. In liquids, vapor formed as the result of cavitation or flashing increases the specific volume of the fluid at a faster rate than the increase in flow due to pressure differential. Lowering the downstream pressure beyond this point in either case will not increase the flow rate for a constant upstream pressure. The velocity at any point in the valve or downstream piping is limited to sonic (Mach = 1). As a result, the flow rate will be limited to an amount which yields a sonic velocity in the valve trim or the pipe under the specified pressure conditions.

Reynolds Number Factor, FR

The Reynolds Number Factor, FR, is used to correct the calculated Cv for non-turbulent flow conditions due to high viscosity fluids, very low velocities, or very small valve Cv.

Piping Geometry Factor, FP

Valve sizing coefficients are determined from tests run with the valve mounted in a straight run of pipe which is the same diameter as the valve body. If the process piping configurations are different from the standard test manifold, the apparent valve capacity is changed. The effect of reducers and increasers can be approximated by the use of the piping geometry factor, FP.

Velocity

As a general rule, valve outlet velocities should be limited to the following maximum values:

Liquids50 feet per second

GasesApproaching Mach 1.0

Mixed Gases500 feet per second and Liquids

The above figures are guidelines for typical applications. In general, smaller sized valves handle slightly higher velocities and large valves handle lower velocities. Special applications have particular velocity requirements; a few of which are provided below.

Liquid applications – where the fluid temperature is close to the saturation point – should be limited to 30 feet per second to avoid reducing the fluid pressure below the vapor pressure. This is also an appropriate limit for applications designed to pass the full flow rate with a minimum pressure drop across the valve.

Valves in cavitating service should also be limited to 30 feet per second to minimize damage to the downstream piping. This will also localize the pressure recovery which causes cavitation immediately downstream from the vena contracta.

In flashing services, velocities become much higher due to the increase in volume resulting from vapor formation. For most applications, it is important to keep velocities below 500 feet per second. Expanded outlet style valves – such as the Mark One-X – help to control outlet velocities on such applications. Erosion damage can be limited by using chrome-moly body material and hardened trim. On smaller valve applications which remain closed for most of the time – such as heater drain valves – higher velocities of 800 to 1500 feet per second may be acceptable with appropriate materials.

Gas applications where special noise attenuation trim are used 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 substantial noise even in a straight pipe. The addition of a control valve to the line will increase the turbulence downstream, resulting in even higher noise levels.

Expansion Factor, Y

The expansion factor, Y, accounts for the variation of specific weight as the gas passes from the valve inlet to the vena contracta. It also accounts for the 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 the Equation to account for different behavior of gases other than air.

Terminal Pressure Drop Ratio, xT

The terminal pressure drop ratio for gases, xT, is used to predict the choking point where additional pressure drop (by lowering the downstream pressure) will not produce additional flow due to the sonic velocity limitation across the vena contracta. This factor is a function of the valve geometry and varies similarly to FL, depending on the valve type.

Compressibility Factor, Z

The compressibility factor, Z, is a function of the temperature and the pressure of a gas. It is used to determine the density of a gas in relationship to its actual temperature and pressure conditions.

CALCULATING CV FOR LIQUIDS

Introduction

The Equation for the flow coefficient (Cv)in non-laminar liquid flow is:

IMAGE 3.1

Where:Cv = Valving sizing coefficientFp = Piping geometry factorq = Flow rate, gpm< m>Delta</m>Pa = Allowable pressure drop across the valve for sizing, psiGf = Specific gravity at flowing temperature

The following steps should be used to compute the correct Cv body size and trim number:

Step 1: Calculate Actual Pressure Drop

The allowable pressure drop, <m>Delta</m>Pa, across the valve for calculating Cv, is the smaller of the actual <m>Delta</m>P from Equation 3.2 and choked <m>Delta</m>Pch from Equation 3.3.

3.2

DP = P1 - P2 (3.2)

Step 2: Check for Choked Flow, Cavitation and Flashing

Use Equation 3.3 to check for choked flow: DPch = FL2 (P1 - FFPV) (3.3)

Where:FL = Liquid pressure recovery factor FF = Liquid critical pressure ratio factorPV = Vapor pressure of the liquid at inlet temperature, psiaP1 = Upstream pressure, psia

See Table 3-I for FL factors for both full-open and partstroke values.

FF can be estimated by the following relationship:

PV FF = 0.96 - 0.28 (3.4) PC

FF = Liquid critical pressure ratioPV = Vapor pressure of the liquid, psiaPC = Critical pressure of the liquid, psia(see Table 3-II)

If <m>Delta</m>Pch (Equation 3.3) is less than the actual <m>Delta</m>P (Equation 3.2), use <m>Delta</m>Pch for <m>Delta</m>Pa in Equation 3.1.

PV = Vapor pressurePc = Critical pressure

Figure 3-3: Liquid Critical Pressure Ratio Factor Curve

Table 3-II: Critical Pressures

LiquidCritical Press (psia)LiquidCritical Press (psia)

Ammonia 1636.1Hydrogen Chloride1205.4

Argon 707.0 Isobutane 529.2

Benzene 710.0 Isobutylene 529.2

Butane 551.2 Kerosene 350.0

Carbon Dioxide 1070.2 Methane 667.3

Carbon Monoxide 507.1 Nitrous Oxide 1051.1

Chlorine 1117.2 Oxygen732.0

Dowtherm A 547.0 Phosgene 823.2

Ethane 708.5 Propane615.9

Ethylene 730.5 Propylene 670.3

Fuel Oil 330.0 Refrigerant 11 639.4

Fluorine757.0 Refrigerant12 598.2

Gasoline 410.0Refrigerant 22 749.7

Helium 32.9Sea Water3200.0

Hydrogen 188.1Water 3208.2

Nitrogen 492.4

It may also be useful to determine the point at which substantial cavitation begins. The following Equation defines the pressure drop at which substantial cavitation begins:

DP (cavitation) = Fi2 (P1 - PV) (3.5)

In high pressure applications, alternate analysis may be required; verify analysis with factory if <m>Delta</m>P > <m>Delta</m>P (cavitation) > 300 psi (globe valves) or 100 psi (rotary valves).

Where:Fi = Liquid cavition factor(Typical values for Fi</sub are given in Table 3-I)

P<sub>1 = Upstream pressure, psiaPV = Vapor pressure of the liquid, psia

The required CV for flashing applications is determined by using the appropriate <m>Delta</m>P allowable [<m>Delta</m>Pch calculated from Equation 3.3].

Step 3: Determine Specific Gravity

Specific gravity is generally available for the flowing fluid at the operating temperature. The appendix provides fluid property data for 268 chemical compounds, from which the specific gravity, Gf can be calculated.

Step 4: Calculate Approximate Cv

Generally the effects of nonturbulent flow can be ignored, provided the valve is not operating in a laminar or transitional flow region due to high viscosity, very low velocity, or small CV. In the event there is some question, calculate the CV, from Equation 3.1, assuming FP=1, and then proceed to steps 5-7. If the Reynolds number calculated in Equation 3.6a is greater than 40,000, FR can be ignored (proceed to step 8 after step 5.)

Step 5: Select Approximate Body Size

Based on CV From the Cv tables in section 4, select the smallest body size that will handle the calculated CV.

Step 6: Calculate Valve Reynolds Number Rev and Reynolds Number Factor FR

Use Equation 3.6a to calculate valve Reynolds Number Factor:

(3.6a)

Use Equation 3.6b to calculate valve Reynolds Number Factor FR if Rev < 40,000, otherwise FR = 1.0:

(3.6b) Where: CVS = Laminar flow Cv

(3.6c) CVT = Turbulent flow Cv (Equation 3.1) Fs = streamline flow factor

Step 7: Recalculate Cv Using Reynolds Number Factor

If the calculated value of FR is less than 0.48, the flow is considered laminar; and the CV is equal to Cvs calculated from Equation 3.6c. If FR is greater than 0.98, turbulent flow can be assumed (FR = 1.0); and CV is calculated from Equation 3.1. Do not use the piping geometry factor FP if FR is less than 0.98. For values of FR between 0.48 and 0.98, the flow is considered transitional; and the CV is calculated from Equation 3.6e:

For laminar and transitional flow, note the <m>Delta</m>P is always taken as P1 - P2.

Step 8: Calculate Piping Geometry factor

If the pipe size is not given, use the approximate body size (from step 5) to choose the corresponding pipe size. The pipe diameter is used to calculate the piping geometry factor, FP3, which can be determined by Tables 3-III and 3-IV. If the pipe diameter is the same as the valve size, F1 is 1 and does not affect CV.

Step 9: Calculate the final Cv

Using the value of Fp, calculate the required Cv from Equation 3.1.

Step 10: Calculate Valve Exit Velocity

The following Equation is used to calculat entrance or exit velocities for liquids:

Where:V = Velocity, ft/secQ = Liquid flow rate, gpmAV = Application flow area, in2 of body port (Table 3-VIII)

After calculating the exit velocity, compare that number to the acceptable velocity for that application. It may be necessary to go to a large valve size.

Step 11: Recalculate C<sub>V</sub> if Body Size Changed

Recalculate CV if the FP has been changed due to selection of a large body size.

Step 12: Select Trim Number

First identity if the valve will be used for on/off or throttling service. Using the CV tables in Section 4 select the appropriate trim number for the calculated CV and the body size selected.When cavitation is indicated, refer to Section 14 to evaluate special trims for cavitation protection.

LIQUID SIZING EXAMPLES

Example One

Given: Liquid …………………………………………………. WaterCritical Pressure (PC) ……………………. 3206.2 psiaTemperature………………………………………… 250° FUpstream Pressure (P1) …………………….. 314.7 psiaDownstream Pressure (P2P) ……………………104.7 psiaSpecific Gravity …………………………………….. 0.94Valve Action ……………………………………… Flow-to-openLine Size …………………………………….. 4-inch (Class600)Flow Rate …………………………………………….. 500 gpmVapor Pressure (PV) …………………………… 30 psiaKinematic Viscosity (V) ………………………… 0.014 centistokesFlow Characteristic ………………………………Equal Percentage

After calculating the exit velocity, compare that number to the acceptable velocity for that application. It may be necessary to go to a larger valve size.

ANSI Classes & Body Material Selection

ANSI CLASSES

It is important that the valve specifier understands ANSI Class ratings and pressure ratings; and they must clearly designate the appropriate standard class, special class, or intermediate class rating when specifying valves.

When Valtek designates a valve pressure class according to ANSI B16.34, the following standard designations are generally used: Class 150, 300, 600, 900, 1500 or 2500. These designations apply to valves with flanged, buttweld, “Grayloc”, socketweld, or NPT threaded end connections.

In addition to the above mentioned classes and end configurations, ANSI B16.34 designates a standard Class 4500 for buttwelding end valves. Buttwelding end valves may also be upgraded to Special Class 150, 300, 600, 900, 1500, 2500, and 4500. Special Class ratings are available when certain non-destructive examination requirements are met for buttwelding end valves. Properly applied Grayloc-style end connections are also suitable for many high pressure applications. Therefore Valtek offers Special Class and standard Class 4500 valves with Grayloc and buttweld end connections.

ANSI B16.34 also permits standard and Special Class 400 ratings with appropriate end connections. Although available by special request, Valtek does not offer the Class 400 rating as a standard option.

ANSI B16.34 also permits the use of intermediate ratings for buttweld end valves. For example: A customer requires a WCB body material for service at 300OF and 6500 psi. ANSI B16.34 provides a method for determining an intermediate rating for this valve (a buttweld end valve) so that it would not be necessary to require a Class 4500 valve rating (with greater size, weight and cost). This method is described in detail in Annex F of ANSI B16.34-1981. Using this example, the method in Annex F would allow the valve specifier to use an intermediate rating of Class 3300. Although designing to intermediate ratings increases engineering time and adds pattern time when castings are required, it minimizes weight, size and material costs for some applications.

The intermediate rating method can also be used to establish pressure classes greater that Class 4500. However, the valves specifier must be careful not to interpret a 6600 psi pressure rating as a Class 6600 which has a maximum working pressure of 13,200 psi.

BODY MATERIALS

The control valve user normally specifies the body material, which is often the same material as the pipe. The most common choices of body material are carbon steel, chrome-molybdenum steel and stainless steel.

Carbon steel is the most commonly used material for bodies. It handles most non-corrosive liquids and gases up to 800 degrees Fahrenheit for continuous service, or to 1000 degrees Fahrenheit for occasional service. (See ANSI B16.34 for specific temperatures and pressures.) Carbon steel can be used for most condensate and steam services.

Chrome-moly steel is used for higher temperatures and pressures than carbon steel, including such services as high pressure steam or flashing condensate which requires corrosion and errosion resistance. Chrome-moly is stronger than carbon steel and, in some cases, is as strong as stainless steel. It costs less than stainless steel, but is not as corrosion resistant.

Stainless steel is specified for high temperature services (1000 degrees Fahrenheit and up) or in corrosive applications. It is more corrosion resistant than either carbon steel or chrome-moly.

Special alloys—such as Hastelloy B and C, Monel, nickel and titanium—are also available. Consideration should be given to the types of material that have been used successfully in the past for similar applications.

Valtek is cautious about choosing materials for the user. Except in the most basic applications, the control valve user should specify the body materials.

Because of the lack of actual correct fluid data, Valtek does not recommend body material or guarantee corrosive resistance or fluid compatibility. Valtek has no control over fluid composition.

Once the body material has been chosen, check the pressure/temperature tables in ANSI B16.34 (found at the end of this section) to determine the application limits for the selected material.

The valve body can be cast, forged, wrought or fabricated. Castings are usually the first choice in standard sizes and ratings. Forgings are used for smaller sized Mark Two valve bodies. (Generally these valves have high pressure ratings—1500, 2500 and 4500 for those special materials not available in castings.) Barstock bodies are recommended when delivery is critical and a casting or forging is not available. Fabricated bodies are a convenient way to manufacture large angle valves.

Bonnets are generally manufactured from barstock of the same material as the body. The exception is smaller (6-inch or less) low pressure chrome-moly valves, where a stainless steel bonnet is normally standard. The basic rule is to specify the general type of bonnet material only, not the ASTM (American Society of Testing Materials) specification number, because Valtek’s standard material varies according to valve size and pressure rating.

Material specification is based upon the ASTM specifications listed in ANSI B16.34 for standard service valves, and ASME (American Society of Mechanical Engineers) specifications. ASME codes are preceded by the letter “S,” such as SA216-WCB.

The tables in this section are from ANSI B16.34 and B16.24 (bronze). They determine the valve body’s pressure rating, such as Class 150, 300, 600, etc. The design or maximum pressure and temperatures (as given by the control valve user) should be used. Pressure and temperature values may be extrapolated between the lines.

The “special class” tables allow higher pressure and temperature values for weld-end valves with some nondestructive examination of critical areas of the body and bonnet.

Table 5-II provides the applicable ASTM codes for Valtek’s common body materials.

Tables

Table 5-I: Valve Body Material Temperature Limits (OF)

Material Lower UpperCast Iron -20410Ductile Iron -20 650*Carbon Steel (Grade WCB) -20 1000Carbon Steel (Grade LCB) -50 650Carbon Moly (Grade WC1) -20 8501-1/4 Cr - 1/2 Mo (Grade WC6) -20 10002-1/4 Cr - 1 Mo (Grade WC9) -20 10505 Cr - 1/2 Mo (Grade C5) -20 11009 Cr - 1 Mo (Grade C12) -20 1100Type 304 (Grade CF 8) -425 1500Type 347 (Grade CF8C) -425 1500Type 316 (Grade CF8M) -425 15003-1/2 Ni (Grade LC3) -150 650Aluminum -325 400Bronze -325 550Inconel 600 -325 1200Monel 400 -325 900Hastelloy B -325 700Hastelloy C -325 1000Titanium 600Nickel -325 500Alloy 20 -50 300

* The carbon phase of carbon steel may be converted to graphite upon long exposure to temperatures above 775° F. (Check applicable codes for maximum temperature rating of various materials.)

TABLE 5-II: Material Standards

Table 5-III: Valve Body (Pressure Containment) Materials

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* Valtek standard for cast bodies ** Valtek standard for Mark Two bodies

Body Forms

In-line

In-line style bodies feature smooth, streamlined, constant internal area passages with no pockets, permitting high capacity with minimum turbulence. Cast bodies are designed with nearly constant wall thickness, to eliminate weight and reduce cost. This is particularly important when manufactured in expensive stainless or alloy steels. If necessary, this body form can also be manufactured from barstock.

Figure 6-1: In-line Forms

Angle

The angle-style body form is completely interchangable with the globe-style except for the body – all other valve parts remain the same. If required for additional protection of the body, a special Venturi seat ring, which extends to the outlet flange, is available for handling erosive fluids. The angle valve incorporates a selfdraining design. The design also allows for smaller space requirements than a globe valve.

Figure 6-2: Angle Forms

Three-way

Three-way bodies are used for either combining or diverting services. Due to Valtek’s excellent parts interchangeability, a standard globe valve easily converts to three-way service with the addition of a three-way adaptor, upper seat ring, two gaskets, a three-way plug, and bonnet flange bolting.

Figure 6-3: Three-way Forms

OffsetWhen inlet and outlet piping can be offset, this design is the simplest, least expensive barstock style. A Mark Eleven works best in self cleaning applications. Other than the body, the offset design is completely interchangeable with the standard Valtek globe valve.

Figure 6-4: Offset FormsExpanded Outlet The expanded outlet valve, such as a Mark One-X, permits the installation of a small valve in a larger line without using line reducers or expanders. The valve is a standard in-line globe valve, except for the body which incorporates expanded outlets. Therefore, all parts – except the body – are interchangeable with the Mark One. Because line expanders and reducers are not used, field installation expenses are reduced. The expanded outlet valve is less costly than a full-size Mark One with the same size end connections.

Figure 6-5: Expanded Outlet Form

Steam Jacketed

Steam jackets are used to heat the fluid passing through the control valve. The steam jacketed valve body uses a standard globe-style body with oversized, blind flanges for a full jacket or standard flanges for a partial jacket. The jacket usually is rated for 150 psi and comes equipped with 3/4-inch NPT supply and drain connection.

Figure 6-6: Steam Jacketed Form

End Connections

Standard globe valve bodies have a raised face hub for either separable or integral flanges. Separable flanges are highly recommended because less expensive carbon steel separable flanges can be specified for use on alloy valves as a cost-saving measure. (Stainless steel flanges may be required with a high temperature/pressure service. See factory for specific

limitations.) Separable flange valves are also easier to install with the mating piping because the flanges can be rotated to fit the line flange hole pattern. To achieve better sealing with the mating piping, the flange face is machined with groove serrations.

Integral flanges can also be provided with a flat face, RTJ (ring-type joint), or tongue and groove connections – depending upon the user’s requirements.

NPT (National Pipe Threads) connections are provided for small valves (2-inch and smaller). They are designed with a female NPT to mate with piping using male NPT threads, and are usually used in pressures less than ANSI Class 600. Because of the threaded connection, these connections are limited to non-corrosive services. See Figure 6-9.

Socketweld connections are usually used in high pressure, high temperature fluids in sizes 2-inch and smaller. The connection uses a bore in the body end which mates with the corresponding piping. A weld is then applied between the body face and the pipe. See Figure 6-10.

For high pressure, high temperature services above 2- inch, buttweld connections are used. Buttwelds are common to steam and water services in power plants. Usually, the buttweld angle machined into the body matches the angle machined into the piping. A full penetration weld is then applied to the butt joint. The material in the body and piping should be compatible to ensure proper welding. See Figure 6-11.

Table 6-I provides specification information concerning each of the end connections:

Table 6-I: Typical End Connections

(a) ANSI B16.10, Class 600 Globe Valves (b) ANSI / ISA S75.03© Valtek Standard (d) ANSI B16.10, 1986 (e) Normally valves 2-inch and less are socketweld, not buttweld (f) Integral flange available as an option in sizes 1/2 through 4-inch ISA or ANSI face-to-face (g) ANSI / ISA S75.03 covers valve sizes 1/2 through 16-inch. Larger sizes are Valtek standard

Figure 6-9: Threaded End Connection (NPT)

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Figure 6-10: Socketweld End Connection

Figure 6-11: Buttweld End Connection

Separable Flange Material

Interchangeable separable flanges are standard for Mark One valve bodies through 4 inch in Class 150, 300 and 600 ANSI ratings, and for 6 and 8 inch bodies in Class 300 and 600. With separable end flanges, a Class 600 body can be adapted for Class 150, 300, or 600 service by simply changing to the proper end flanges.

Separable flanges are usually furnished in carbon steel for maximum cost savings, although other alloys can be specified if the atmospheric conditions or the temperature warrants it.

NOTE: Carbon steel bodies, carbon steel end and bonnet flanges should not be used when temperatures are 800 degrees Fahrenheit or greater.

Table 6-II: End Flange Material



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(a) Carbon steel end flanges with zinc plated steel half rings can be used in most all corrosive applications since the flanges are not wetted by the fluid. (b) Stainless steel flanges and half-rings are usually only necessary when atmospheric conditions or temperature limitations require stainless steel. Carbon steel will usually suffice.© Optional alloys are also available.

Trim

Unbalanced and Pressure-balanced Trim

Unbalanced Trim Types

Valtek offers three unbalanced trim designs: 1) standard full area trim which provides maximum Cv with a removable seat ring; 2) reduced trim which provides a lower Cv in a wide variety of sizes or when larger bodies are required; and 3) integral seat trim which utilizes the seat machined into the body and an oversized plug to provide additional Cv beyond the capabilities of standard full area trim.

Mark One, Mark Two and Mark Eight valves can be converted from one unbalanced trim type to another since all seat rings and plugs within a given size and pressure class are completely interchangeable. Integral seat trim is available by changing the plug and removing the seat ring. Table 8-III lists the values for unbalanced, full trims.

NOTE: All Mark One, Mark Two, Mark Eight, and Mark Eleven bodies are machined with integral seats, except ANSI Classes 900, 1500 and 2500.

Figure 8-1: Unbalanced Trim Design

Pressure-balanced Trim

Some high pressure drop applications or valves with large seat diameters may require pressure-balanced trim. Pressure-balanced trim reduces the actuator thrust requirement by reducing the trim’s effective offbalance area. However, with high-thrust cylinder actuators, pressure-balanced trim may not be required. Often, an over-sized cylinder actuator may be the most economical choice.

Because the pressure-balanced plug fits closely to the sleeve (or retainer), the trim should be used in relatively clean services.

As a standard, pressure-balanced trim is designed to ANSI Class II shutoff for valve sizes 1⁄2 to 3-inch and Class III shutoff for valve sizes 4-inch and larger shutoff. These specifications call for a maximum seat leakage of 0.5 and 0.1 percent (respectively) of rated valve capacity.

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Flow direction is normally under the plug for failclosed applications and over the plug for fail-open applications. The sleeve area minus the stem area is designed slightly larger than the seat area, creating an off-balanced area that assists closing with flow under for fail-closed applications.

Figure 8-2: Typical Pressure-balanced Design

Allowing the fluid pressure to act on both sides of the plug results in a net force equal to the pressure times the off-balance area. This balancing force is made possible by transfer holes in the plug. Leakage past the plug is prevented by a seal around the top of the plug head. Figure 8-2 shows a typical pressure-balanced design. The following list provides information about the types of plug seals available for pressure-balanced service:

Metal Seals

1. NiResist Rings for all sizes and pressures, and temperatures up to 800 degrees Fahrenheit. For temperatures over 600 degrees Fahrenheit, the surface of the pressure-balanced sleeve bore must be hardened. Pressure-balanced sleeves for temperatures up to 800 degrees Fahrenheit are constructed from the following materials;

- Carbon steel with electroless nickel plating (up to 600 degrees Fahrenheit)

- 410-416 high temperature stainless steel (up to 800 degrees Fahrenheit)

- 316 stainless steel with Stellite overlay or(up to 1600 degrees Fahrenheit)

2. Muskegon Multi-sealTM Rings for temperatures from 300 to 1600 degrees Fahrenheit. When Muskegon rings are used, the bore surface of pressure-balanced sleeves must be hardened with one of the following materials:

- 410-416 high temperature stainless steel (up to 800 degrees Fahrenheit)

- 316 stainless steel with Stellite overlay (up to 1600 degrees Fahrenheit)


Figure 8-3: Pressure/Temperature Limitation for Teflon Seals

O-ring Seals with Backup Rings

Buna-N O-rings are standard for those applications with temperatures between -60 to 250 degrees Fahrenheit and pressures to 6000 psi. Viton O-rings can be used for higher temperatures – between -40 to 435 degrees Fahrenheit – and for pressures up to 6000 psi. Back-up rings are used in conjunction with O-rings. Special materials are available in O-rings and back-up rings for temperatures up to 500 degrees Fahrenheit.

Teflon (TFE) Seals

The maximum pressure/temperature usage for Teflon seals in pressure-balanced applications is shown in Figure 8-3:

Leakage Requirements

Table 8-I provides the class shutoff available with pressure-balanced trim and Table 8-II provides the seat loading ratings which are necessary to obtain the specified leakage rate. ANSI standard B16.104 discusses shut-off classes in section 5.

Table 8-I: Shutoff Capabilities of Pressure-balanced Trim


Table 8-II: Seat Loading Rates to Obtain Specified Leakage Class

Flow Direction

unbalanced, or standard, full trim

Unbalanced trim design generally requires that the direction of flow should assist the motion of failure, for example, flow-over for fail-closed and flow-under for failopen. The force required to fail-open or closed is a function of the off-balanced area. This area is equal to the seat area in fail-open applications and the seat area minus the stem area in fail-closed applications.

Flow Direction

full area, first and second reduction

Pressure-balanced trim design generally requires that the direction of flow be opposite to standard (unbalanced) trim to oppose the motion of failure–for example, flow-over for fail-open and flow-under for fail-closed. The force required to fail-open or closed is a function of the off-balanced area. This area is equal to the sleeve area minus the stem and seat area in fail-closed applications. When flowing under the seat, the net offbalanced area pushes downward

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to assist closure. For fail-open applications, the off-balanced area is equal to the sleeve area minus the seat area, thus there is an upward force with flow over the seat to assist failing open.

Trim Materials

Valtek uses 316 stainless steel as standard plug and seat ring material except in the case of special alloy bodies where trims are usually furnished in the same material as the body. It is difficult to assign specific limitations to the use of stainless steel because of the insufficient information about the actual condition of the flowing stream. However, Valtek applies one general rule: Hardened trim is considered for all choked flow conditions or for temperatures above 600 degrees Fahrenheit.

Hardened Trim Selection

Hardened trim is used in control valves to protect the trim against erosion and/or corrosion. As shown in Figure 8-4, hardened surfaces may include the seat surface of the plug and seat ring, the full seat ring bore, the full contour of the plug or the lower guide area of the plug stem.

Valtek stocks No. 6 Stellite for many valve trim parts which require hardened trim. Stellite offers a good combination of relative hardness and corrosion resistance. For corrosion resistance, special alloys—such as Alloy 20, Hastelloy C and Monel—are also available.

A major problem with material selection is deciding when to apply a hard face to protect the control valve trim. Scientific studies have not adequately predicted when hard facing should be used. Therefore, opinions and conclusions based on experience must be used to set practical guidelines.

Aside from corrosion, the main factors that cause wear in valve trim are the conditions of the process fluid:

Gas versus liquid Velocity and pressure differential Temperature Flashing Presence of abrasive solids.

All these factors must be examined when considering hardened trim.

Figure 8-4: Hard Facing Variations With Seat and Plug

Gases Versus Liquids

Clean gases are not usually a source of trim erosion, even at high velocities. However, entrained solids or liquid droplets in high velocity gas can wear the trim rapidly. Depending on the fluid’s composition, liquids at high velocity can produce accelerated erosion. For example, at high velocities water causes more damage than lubricating oil.

With liquids, another harmful effect is cavitation which can erode most trim material, even hardened trim. Liquid application valves require the use of hardened trim more often than gas application valves.

Velocity and Pressure Differential

Erosion caused by flowing fluid is a function of the velocity of the fluid. Velocity is dependent on flow rate and area. In order to have a large flow rate through a relatively small area, large differential pressures are required. Therefore, hardened trim selection becomes a function of differential pressure. Pressure differential values which are anticipated to require the use of hardened trim are shown above. The following should be considered when using the differentials from Table 8-7:

1. When operating differentials are 50 percent more than the values in Table 8-7 (factor above figures by 1.5), the use of full bore and full contour hardened trim is recommended. 2. The factory should be consulted on any choked liquid application where the differential pressure exceeds 500 psi. 3. Use hardened full bore, full contour and lower guide area whenever the temperature exceeds 600 degrees Fahrenheit. 4. Liquid applications with high pressure drops should also consider the valve Sigma. (Refer to Section 13 for a discussion about Sigma values.)

Table 8-7: Pressure Differential (psi) Requiring Hardened Trim

Temperature

As temperatures increase, many trim materials become susceptible to erosion because of the general deterioration of their mechanical properties. Therefore, the selection of the hardened trim must be compatible with high temperature conditions. For example, hardened 440C would not be recommended for service above 800 degrees Fahrenheit, whereas Stellite can be used up to 1500 degrees Fahrenheit. At the other end of the temperature scale–such as cryogenic service–most available hardened materials become excessively brittle and 316

stainless steel becomes relatively hard. Whenever the temperature exceeds 550 degrees Fahrenheit, seating surfaces should be hardened. The plug stem and bushings should be hardened above 600 degrees Fahrenheit regardless of the pressure differential.

Corrosion

The erosion and abrasion of valve and trim is aggravated by the corrosive effect of the process fluid. In some cases, this may be the deciding factor in selection of the hardened trim. In other cases, it may dictate the use of a trim which is resistant to corrosion by the fluid, but which cannot be hardened.

Types of Hardened Trim

The term “hardened trim” may cover such materials as: 1) stainless steel hard-faced with Stellite; 2) flamesprayedwith tungsten carbide or aluminum oxides; 3) hard materials such as wrought Stellite 6B or the various sintered metal carbides and oxides; and 4) materials which are hardened by heat treatment, such as 416, 17-4 PH, 440C, or 329 stainless steels, or K Monel K-500.

Hard-Facing

The most common material used by Valtek for hardfacing is Stellite No. 6, a product of Haynes Stellite Co. It is a cobalt-based alloy. Stellite No. 6 is the most common of all hard-facing materials used in the control valve industry. It has one of the best combinations of corrosion, abrasion and impact resistance. In addition, it has a low coefficient of friction with itself and an even lower one with Stellite No. 12.

Heat Treatable Materials

Type 17-4 PH (product of Armco Steel) combines high hardness with good corrosion resistance.

17-4 PH is a precipitation hardened stainless steel. Its corrosion resistance is comparable to that of 304 stainless steel. Maximum tensile strength, hardness and corrosion resistance are obtained by the hardened condition H900. In this condition, the upper operating temperature limit for steel is 800 degrees Fahrenheit and the lower limit is -10 degrees Fahrenheit. A minimum temperature of -320 degrees Fahrenheit can be expected if the heat treatment is changed to condition H1150M; this condition is also the most ductile at any temperature.

Type 440C stainless steel is a high carbon, 17 percent chromium steel and is available (for trim sizes 3.50 and smaller) in bar and cast form. It can be hardened to Rockwell C56 to 60. It has fair corrosion resistance which deteriorates rapidly above 800 degrees Fahrenheit. However, it is dimensionally unstable during heat treatment and is susceptible to cracking after heat treating. For these reasons, caution must be taken when using 440C for seat rings. However, 300 series stainless with Stellite seat surfaces is an alternative. Monel

K-500 is a copper-nickel alloy, with a chemical composition that differs from Monel 400 by the presence of 3 percent aluminum. Whenever the specifications call for Monel trim, the

bushings will normally be supplied in Monel K-500. This ensures a differential hardness between the plug stem and the guide bushings.

The corrosion resistance of Monel K-500 and Monel 400 are comparable. The major limitation of Monel K-500 as a bushing or plug stem is its availability only in bar or wrought form. Monel K-500 is hardened by a single thermal treatment consisting of aging at the required temperature followed by controlled cooling.

Additional Information

Table 8-8 provides general guidelines concerning Valtek’s trim materials. Table 8-9 shows the wear and galling resistance of various material combinations. And, Table 8-10 indicates the temperature limits of various trim materials.

Table 8-8: Trim Material Characteristics

Table 8-9: Wear & Galling Resistance of Material Combinations

Soft and Metal Seats

Soft Seats

A soft seat is used in applications requiring ANSI Class VI “bubble-tight” shutoff (ANSI B16.104, 1976-FCI 70-2). Its design consists of an elastomer insert sandwiched between a metal seat ring and retainer (see Figure 8-5). The assembled soft seat is completely interchangeable with the hard seat for a given size and pressure rating. Figure 8-6 shows the differential pressure and temperature limitations for Teflon, a common soft seat insert material.

Figure 8-5 Soft Seat Design

Figure 8-6: Teflon Soft Seat Pressure and Temperature Limits

Metal Seats

Class IV shutoff (ANSI B16.104, 1976-FCI 70-2) is the industry standard for metal seated valves. This class calls for maximum permissible seat leakage of 0.01% of rated valve capacity. All Valtek valves are seat leak tested after assembly and exhibit substantially lower leakage rates than called for by this class. This exceptional seat tightness is obtained by 1) a self-aligning seat ring (aligns with the plug during assembly) and 2) by increasing the seat loading through increased actuator thrust. See Table 8-11 for available shutoff classes.

Table 8-11: Unbalanced Seat Leakage Classes per ANSI B16.104

NOTE: The required seat load (in pounds per inch of linear seating force) are included in parenthesis.


* See explanation on page 8-12 or contact factory.

* * Requires Teflon gaskets.

Note: Slightly higher pressures and temperatures (approximately 15 percent) can be achieved with filled TFE materials. Kel-F inserts can be used at cryogenic temperatures, at or below -200 degrees Fahrenheit with the same pressure limits as TFE above.

Figure 8-7: Class V and VI Leakage Versus Orifice Diameter

Class V Versus Class VI

Due to the common belief that Class VI shutoff is more stringent than Class V under all circumstances, the following should be noted:

Class V allowable leakage is defined as .0005 cc per minute per inch of orifice diameter per psi differential. Therefore, allowable leakage increases with orifice diameter and differential pressure.

Class VI allowable leakage is independent of pressure differential and is only a function of orifice diameter. For large orifice diameters or small pressure drops, Class VI shutoff can be less stringent than Class V. For example, refer to Figure 8-7. For an orifice diameter of 4 inches and a 450 psi pressure differential, Class VI allows nearly twice the leakage as Class V.

This above information is important because it shows that Class VI shutoff can be obtained with metal seats. It is not true that Class VI shutoff cannot be obtained in high temperature service simply because a soft seat cannot be used. Keep in mind that seat loading must be increased to 250 to 400 lbs. per linear inch of seating force to obtain the Class VI shutoff.

Gaskets

Figure 8-8: Bonnet, Seat Ring and Sleeve Gaskets, Unbalanced and Balanced Designs

Gaskets are used in control valves to prevent leakage around the seat ring, bonnet or pressure-balanced sleeve. Refer to Figure 8-8.

Valtek® globe valves are designed with the bonnet and seat ring gaskets fully retained. Since the bonnet bottoms metal-to-metal in the body, bonnet gasket compression is determined by the machined depth of the gasket step on the bonnet. This compression is equivalent to the gasket manufacturer’s requirement.

When the bonnet is fully installed, force is transmitted through the seat retainer to secure the seat ring in position. The body, seat retainer and seat ring are all machined to close tolerances to provide the proper seat ring gasket compression. Unlike the bonnet, the seat ring does not bottom in the body. Thus allowing the small clearance remaining to compensate for manufacturing tolerances and thermal expansion.

Table 8-12 provides general information for selecting the proper gasket material with respect to temperature and pressure ratings.

Table 8-12: Gasket Specifications

*Asbestos-free gasket

Teflon Gaskets

Flat Teflon gaskets are an economical first choice and should be used whenever possible within the pressure/ temperature limits, which are listed in Figure 8-9:

Figure 8-9: Teflon Gasket Allowable Pressure Versus Temperature

Kel-F Gaskets

Flat Kel-F gaskets are used primarily for cryogenic services. Whenever the customer specifies Kel-F or the temperature falls below the limits of Teflon (-200 to 350 degrees Fahrenheit), Kel-F is generally used. The temperature range for Kel-F is -423 to 350 degrees Fahrenheit. The pressure should be limited as shown in Figure 8-9.



Spiral Wound Gaskets

Spiral wound gaskets consist of alternate layers of metal and nonmetalic materials wound together. Because spiral wound gaskets are crushed during assembly, they can never be reused. With the exception of some CavControl, ChannelStream and Tiger-Tooth designs, spiral wound gaskets should not be used in valves with soft seat designs. The force needed to compress a spiral seat gasket is partially transmitted through the soft seat insert, which is more compressible than a spiral gasket. Hence, the soft seat is likely to extrude before the spiral gasket is fully compressed and may damage the seat ring or cause the seat to leak. Valtek’s most commonly used spiral wound gaskets are discussed below:

AFG is a non-asbestos filler material for standard, spiral-wound gaskets and may be directly substituted for asbestos material in most applications. It has been tested in steam service up to 1000 degrees Fahrenheit and in air at 1500 degrees Fahrenheit. Its sealability is virtually equal to that of graphite gaskets.

The temperature range for 304 stainless steel/asbestos gaskets is -20 to 750 degrees Fahrenheit with the maximum pressure rating of ANSI class 2500. They are used in valves through 8-inch, in carbon steel and chrome moly.

316 stainless steel/asbestos gaskets have a temperature range from -20 to 1000 degrees Fahrenheit with a maximum ANSI class rating of 2500. They are usually used in stainless steel valves, and in carbon and chrome moly valves sizes 10-inch and above.

316 stainless steel/Grafoil gaskets have a temperature range from -423 to 1000 degrees Fahrenheit for a full pressure rating (ANSI class 2500). They are commonly used for high pressure, high temperature, severe service applications up to 1000 degrees – especially severe service valves.

Inconel/Grafoil gaskets have a temperature range from -20 to 1500 degrees Fahrenheit at a full pressure rating. They are commonly used for high temperature applications (above 1000 degrees Fahrenheit), or where Inconel is preferred over 316 stainless steel for that particular fluid.

Valtek also offers other types of custom spiral wound gaskets. A listing of the metallic and non-metallic windings are included in Table 8-13, along with the appropriate color code:

Table 8-13: Spiral Wound Gasket Temperature Range and Standard Color Codes

* Industry standard for metal and filler material as adopted by the Metallic Gasket Division of the Fluid Sealing Association. * * Asbestos-free gasket

Metal O-Rings

Inconel X-750 gaskets have a temperature range from -20 degrees to 1500 degrees Fahrenheit at full pressure rating. This gasket material is used where the customer specifies metal O-rings, very high temperatures, or where Grafoil cannot be used (oxidizing service above 800 degrees Fahrenheit). Other special materials are available for high temperature or corrosive environments.

Bonnets, Metal Bellows Seals, Bonnet Flanges, Bolting, Packing and Guides

Introduction

The proper selection of bonnet, flanges, bolting, packing and guides is important to the operation of any control valve. In addition, many applications require that a low leakage packing system be installed in the valve to prevent the fugitive emissions of process fluids. This section has two parts. The first part describes Valtek’s globe body valves. The second part (beginning on page 12-12) describes Valtek’s rotary valves.

Globe ValvesS

Bonnet Materials

Bonnets are normally manufactured from the same material as the body. Table 12-1 provides the standard bonnet material for a given body material:

Table 12-I:Standard Globe Body/Bonnet Materials

Bonnet Types

Standard Bonnet

Valtek’s standard bonnet is usually constructed of the same material as the body. It handles temperatures from -20 degrees to 750 degrees Fahrenheit, depending on the packing used. In some cases, class 900 - 2500 standard bonnets can be rated as high as 800 degrees Fahrenheit, depending on the packing used.

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Figure 12-1: Standard Bonnet

Extended Bonnet

The extended bonnet protects the packing and actuator soft goods from excessive heat or cold which may inhibit packing or actuator performance. It is constructed from carbon steel for temperatures from -20 to 800 degrees Fahrenheit, and from 304 or 316 stainless steel for temperatures from -150 to 1500 degrees Fahrenheit. For continuous cryogenic applications, a cryogenic extended bonnet should be used.

Figure 12-2: Extended Bonnet

Cryogenic Extended Bonnet

The cryogenic extended bonnet permits stagnant, moderate temperature gas to form in the bonnet, which acts as an insulator to minimize heat transfer. This design also protects the packing from the extremely low temperature of the service fluid. It is usually manufactured from 304 or 316 stainless steel and handles temperatures down to -423 degrees Fahrenheit. Standard construction consists of stainless steel bonnet flange and bolting.

Figure 12-3: Cryogenic Extended Bonnet

Metal Bellows seals

A metal bellows seal can be encased either in the body or in an extended bonnet for those services where fluid leakage to atmosphere needs to be totally eliminated.

Guardian Metal Bellows

The Guardian standard metal bellows seal – which is encased in the body – uses a standard bonnet, as shown in Figure 12-4. Four sizes and two pressure ratings are available for temperatures up to 650 degrees Fahrenheit.

The metal bellows are rated either by pressure, temperature or cycle life. Pressure ratings can be increased by reducing the calculated cycle life. Conversely, cycle life can be lengthened by reducing the operating pressures.

The Guardian bellows is in a relaxed state at the valve’s closed position. The stroke length has been reduced from that of a standard Mark One to increase the life of the bellows. External pressure is used to balance the pressure load on the bellows. An anti-rotation pin prevents accidental rotation of the seal. A “tell-tale” tap in the standard bonnet provides early indication of bellows failure, should the bellows rupture.

The standard metal bellows is constructed from Inconel 625. It is also available in Hastelloy and other weldable materials.

Figure12-4: Guardian Metal Bellows Seal

Guardian II Metal Bellows Seal

The Guardian II Metal Bellows Seal uses a formed bellows design with minimal welded joints. A full-cycle life of up to 5 million cycles can be expected while operating in process temperatures from -320° to 1000° Farenheit and pressures to 1100 psi. Inconel 625 is the standard material for the bellows assembly with Hastelloy C-22 as optional.

The bellows assembly is encased in a shroud, which acts as a pressure boundary in the service. This design allows a single, pressurized gasket seal and prevents fluid contact with the bellows housing during normal operation. External pressurization of the bellows increases cycle life and the maximum allowable operating pressure. At the same time “bellows squirm” is eliminated. The replaceable plug head allows trim changes without changing the bellows assembly. The Guardian II design includes an anti-rotation pin to prevent the plug and

bellows assembly from rotating, and a tell-tale tap that indicates bellows leakage. Additional monitoring ports in the bellows assembly housing are available.

Figure12-5: Guardian II Metal Bellows Seal

Formed Metal Bellows Seal

Valtek also offers a formed (or rolled) bellows that is encased in an extended bonnet, as shown in Figure 12-6. It is rated for operation at 150 psi at 100 degrees Fahrenheit or 90 psi at 600 degrees Fahrenheit. Special designs are available for pressures to 2900 psi at 100 degrees Fahrenheit and temperatures to 1100 degrees Fahrenheit at 150 psi.

Since bellows seals are designed for special service conditions and not to a particular design class, it is necessary to include complete and accurate service conditions when specifying.

Figure 12-6: Formed Metal Bellows Seal Extended Bonnet

Bonnet Flange and Bolting Material

Table 12-II provides Valtek’s standard bonnet flanges and bolting materials for Mark One, Mark Two, Tek-Check and Mark Eight globe valves. Table 12-III lists specific temperature limitations and material specifications for bonnet flange bolting.

Table 12-II: Globe Valve Bonnet Flange and Bolting Materials

(1) Alloy flange & bolting material required only when pressure or temperature limits of the standard carbon steel or B7, 2H materials are insufficient.(2) Temperature limit of -20 to 800 degrees Fahrenheit, depending on body limitation.(3) Temperature limit of -425 to 1500 degrees Fahrenheit, depending on body limitation.(4) Other alloys depending on design criteria.

Table 12-III: Bolting Temperature Limitations

* Alloy steel bolting, A193 Gr B7 bolts and A194 Gr 2H nuts can be used at moderate temperatures depending on the permissible differential expansion.

Packing and Packing Box

Standard Valtek packing boxes are deeper than most conventional types. The spacing between the wiper set and the main upper packing set prevents contamination of the upper packing. The upper set is positioned far enough away from the wiper set to avoid contact with any part of the plug stem, which has been exposed to the flowing medium. The wiper set is designed to minimize the amount of fluid on the plug stem.

The deep packing box is designed to permit a wide variety of packing configurations, including twin seal packing, without changing bonnets. Figure 12-7 shows the common packing box configurations.

Two widely-spaced stem guides – used with a large plug stem diameter – provide exceptional guiding. The upper stem guide also acts as a packing follower. The lower guide is situated close to the plug head for additional guiding support, ensuring accurate alignment of the seat ring and plug.

Packing configurations can include twin seals where equal amounts of packing are used at both ends of the packing box. This configuration is usually specified when a vacuum seal or lubrication is required. However, twin packing does not improve the sealing capability of the packing, and in some cases may hamper its performance. Lantern rings are provided between the packing sets if stem lubrication is required.

Valtek’s standard packing is the Teflon V-ring. This design provides a tight seal at the feather edge of each ring with a minimum amount of stem friction. To achieve a tight seal with Teflon, the packing box bolting is tightened to just over finger-tight. The major limitations of Teflon are its low service temperature limit of approximately 500 degrees Fahrenheit with a standard bonnet and its “creep” characteristics, which decrease its sealability under load.

Other available packing materials include PTFE braided, glass-filled Teflon, graphite/asbestos-free packing (AFP), (Inconel reinforced AFP) and Grafoil. Graphite/ AFP, and Grafoil are usually used in high temperature applications.

Table 12-IV shows packing temperature limitations for both standard and extended bonnets. Table 12-V, provides the minimum temperature limitations for packing used with cryogenic extended bonnets. Figures 12- 8, 12-9 and 12-10 provide pressure/temperature curves for Teflon TFE, glass-filled Teflon and carbonfilled Teflon asbestos. Table 12-VI provides the stem packing friction forces.

Vacuum Service Packing

When the process fluid is at a vacuum pressure (below atmospheric pressure), special consideration must be given to the packing configuration. Normally V-rings are used since they seal the best. Although Teflon is the most often used material, other packing materials may be used depending on the process temperature and pressure. Valtek has three available packing options.

1. If the process is always under a vacuum, a standard V-ring packing may be used with the top set of V-rings being inverted (chevron facing away from the plug head.)2. If the process pressure is both vacuum and positive at various times, the packing needs to

seal both directions. Twin V-ring packing is best used, inverting the top set (chevrons facing away from the plug head) with the bottom set being installed normal.3. When a vacuum seal is needed on the bonnet independent of the process pressure, a purge connection with a lantern ring spacer is available with either of the above configurations. This configuration allows the application to be monitored.

Fugitive Emissions Packing

When special packing is required to keep emissions through packing at a low level, SafeGuard and SureGuard packing sets are used. SafeGuard and SureGuard packing sets are available for new valves or may be retrofitted into existing Valtek bonnets to provide exeptional leakage control, reliability and longevity. Both systems are available in standard and twin configurations. See Figure 12-7.

SafeGuard is a Teflon based, V-ring packing set that utilizes the sealing ability of virgin Teflon while minimizing the effects of creep. This is accomplished by backing up the virgin V-rings with carbon-filled Teflon (to prevent extrusion and cold flow) and live loading the entire set to compensate for any Teflon creep due to thermal gradients or wear. Figure 12-8 illustrates the pressure/temperature curve for SafeGuard.

SureGuard is a Kalrez based, V-ring packing set that utilizes the exceptional sealing ability of a perfluorelastomer while maintaining the inertness equivalent of Teflon. Like SafeGuard, the sealing rings are backed up by carbon-filled Teflon to prevent extrusion and increase the life of the packing set. SureGuard XT is a similar packing set with PEEK backup rings, which has the capability to endure temperatures up to 550 degrees Fahrenheit in a standard bonnet, and up to 800 degrees Fahrenheit in a high-pressure, extended bonnet. Figures 12-8, 12-9 and 12-10 illustrates the pressure/temperature curve for SureGuard and SureGuard XT.

Figure 12-7: Typical Linear Valve Packing Configurations

Table 12-IV: Packing Temperature Limitations, Standard and Extended Bonnets

(1) ANSI B16.34 specifies acceptable pressure/temperature limits for pressure retaining materials. Consult factory for additional information.(2) If the appropriate body and bonnet materials are used.(3) PTFE is rated to -423 degrees Fahrenheit.(4) 8 to 12-inch, Class 150 - 600; and 3 to 12-inch, Class 900 - 2500 can be used to 850 degrees Fahrenheit. (5) Asbestos-free, high temperature packing.(6) Do not use Grafoil above 800 degree Fahrenheit in oxidizing service such as air.* Fluid temperatures; see Figures 12-8, 12-9 and 12-10.

Table 12-V: Minimum Globe Valve Packing Temperatures, Cryogenic Extended Bonnets

(1) ANSI B16.34 specifies acceptable pressure/temperature limits for pressure retaining materials. Consult factory for additional information.

Maximum Fluid Temperature/Pressue Curves for Valve Packing:

Figure 12-8: Standard Packing

Figure 12-9: Extended Bonnet / Extension, Class 150 - 600

Figure 12-10: Extended Bonnet, Class 900 - 2500

Table 12-VI: Typical Globe Valve, Stem Packing Friction ForcesNOTE: All numbers are in pounds-force

* See Table 12-VII for plug stem diameter versus valve size.

Table 12-VII: Plug Stem Sizes for Given Valve Sizes (inches)

Guides

Standard Guides

When selecting guide materials, the prioritized list below should be used: a. Grafoil-lined stainless steel*b. Glass-filled Teflon lined stainless steel*c. Solid bronze guided. Solid stellite guide

* The standard guide retainer material is stainless steel. In alloy valves the guide retainer is constructed of the same material as the body.

Grafoil Guides

Versatile Grafoil guides can be used in many applications. The following applies to Grafoil guides.

• 1/2 – 2-inch Valve SizePressures to 1000 psigTemperatures to 1500O FPressure drops to 250 psidExceptions:Maximum temperature 800O F on oxydizing or air serviceDo NOT use in cavitating conditions, or pressure drops over 250 PSID, or oxygen enriched services

• 3 – 4-inch Valve SizePressures to 600 psigTemperatures to 1500O FPressure drops to 200 psidExceptions:Maximum temperature 800O F on oxydizing or air serviceDo NOT use in cavitating conditions or pressure drops over 200 PSID, or oxygen enriched services

• 6-inch Valve Size & LargerPressures to 500 psigTemperatures to 1500O FPressure drops to 100 psidExceptions:Do NOT use in cavitating conditions or pressure drops over 100 PSID, or oxygen enriched services

Glass-Filled Teflon Guides

Glass-filled Teflon guides consist of a glass-filled Teflon liner and a metal retainer.

These guides are suitable for most chemical applications if the temperature and pressure fall within the limits shown in Figure 12-11.

Figure 12-11: Pressure vs. Temperature Limitations, Glass-filled Teflon Guides

Solid Bronze Guides

Bronze guides may be used in valve applications up through class 2500. Temperature of the fluid should not exceed 500 degrees Fahrenheit for the lower guide. Temperature of the fluid should not exceed 900 degrees Fahrenheit for the upper guide.

These guides are principally used in water service and in valves which require a high degree of cleanliness such as oxygen or hydrogen service valves. Bronze guides should not be used: 1) where corrosion exists; 2) in cavitating service where the pressure drop exceeds 400 psi for valves 1/2 through 4-inch and 250 psi for valves 6-inch and larger, or; 3) in valves requiring N.A.C.E certification.

Stellite Guides

Stellite guides are suitable for all valves through class 2500 and to temperatures through 1500 degrees Fahrenheit.

A Stellite lower guide is normally supplied for valves such as ChannelStream, Tiger-Tooth, CavControl and some pressure-balance applications. A Stellite lower guide is used in choked flow at differential pressures greater than that allowed with Grafoil, Teflon or bronze guides.

The lower guide area on the plug stem that comes in contact with the Stellite guide must be Stellited if the base trim material is a 300 series stainless steel.

Rotary Valves

Rotary Valve Bodies

All Valtek rotary valve body designs include a bonnet. Valtek rotary valve bodies normally handle temperatures from -20 degrees to 750 degrees Fahrenheit, depending on the packing used. In some cases, class 900 – 2500 bodies can be rated as high as 800 degrees Fahrenheit, depending on the packing used.

Table 12-VIII: Rotary Body/Extension Materials

Extension Materials

For high or cryogenic temperature applications, standard extensions or cryogenic extensions are used. Extensions are normally manufactured from the same material as the body. Table 12-VIII describes the standard extension material for a given body material:

Extension Types

Standard Extension

The standard extension protects the packing and actuator soft goods from excessive heat or cold which may inhibit packing or actuator performance. It is constructed from carbon steel for temperatures from -20 to 800 degrees Fahrenheit, and from 304 or 316 stainless steel for temperatures from -425 to 1500 degrees Fahrenheit. For continuous cryogenic applications, a cryogenic extended bonnet should be used.


Figure 12-12: Standard Extension

Cryogenic Extension

The cryogenic extension permits stagnant, moderate temperature gas to form in the bonnet, which acts as an insulator to minimize heat transfer. This design also protects the packing from the extremely low temperature of the service fluid. It is usually manufactured from 304 or 316 stainless steel and handles temperatures down to -423 degrees Fahrenheit. Unlike the bolt-on design of the standard extension, the cryogenic extension is a welded extension integral with the valve body.

Figure 12-13: Cryogenic Extension

Extension Bolting Material

Table 12-III lists specific temperature limitations and material specifications for standard and extension bolting for ShearStream and Valdisk rotary valves.

Packing and Packing Box

Standard Valtek packing boxes are designed to permit a wide variety of packing configurations, including twin seal packing. Figure 12-14 shows common Valtek rotary packing box configurations.

Packing configurations can include twin seals where equal amounts of packing are used at both ends of the packing box. This configuration is usually specified when a vacuum seal or lubrication is required. However, twin packing does not improve the sealing capability of the

http://www.flowserveperformance.com/performhelp/_detail/sizing_selection:valtek:12:fig12.gif?id=sizing_selection%3Avaltek%3Abonnets_bellow_bolting_packing_guides

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packing, and in some cases may hamper its performance. Lantern rings are provided between the packing sets if stem lubrication is required.

Valtek’s standard packing is the Teflon V-ring. This design provides a tight seal at the feather edge of each ring with a minimum amount of stem friction. To achieve a tight seal with Teflon, the packing box bolting is tightened to just over finger-tight. The major limitations of Teflon are its low service temperature limit of approximately 500 degrees Fahrenheit with a standard body and its creep characteristics, which decrease its sealability under load.

Other available packing materials include braided PTFE, glass-filled Teflon, graphite/asbestos-free packing (AFP–Inconel reinforced), and Grafoil. Graphite/AFP, and Grafoil are usually used in high temperature applications. Table 12-IX shows packing temperature limitations for bodies, standard extensions, and the minimum temperature limitations for packing used with cryogenic extensions. Figures 12-8 shows pressure/temperature curves for Teflon packing in standard and cryogenic extensions. Table 12-8 provides the shaft packing friction forces.

Table 12-IX: Rotary Packing Temperature Limitations, Standard Extensions

(1) ANSI B16.34 specifies acceptable pressure/temperature limits for pressure retaining materials. Consult factory for additional information.(2) If the appropriate body and bonnet materials are used.(3) PTFE is rated to -423 degrees Fahrenheit.(4) 8 to 12-inch, Class 150 - 600; and 3 to 12-inch, Class 900 - 2500 can be used to 850 degrees Fahrenheit. (5) Asbestos-free, high temperature packing.(6) Do not use Grafoil above 800 degrees Fahrenheit in oxidizing service such as air or oxygen.* Fluid temperatures; see Figures 12-8 thru 12-9.

Fugitive Emissions Packing

When special packing is required to keep emissions through packing at a low level, SafeGuard and SureGuard packing sets are used. SafeGuard and SureGuard packing sets are available for new valves or may be retro-fitted into existing Valtek bonnets to provide exceptional leakage control, reliability and longevity. Both systems are available in standard or twin configurations. A fire-safe version is available for rotary valves. See Figure 12-14.

SafeGuard is a Teflon based, V-ring packing set that utilizes the sealing ability of virgin Teflon while minimizing the effects of creep. This is accomplished by backing up the virgin V-rings with carbon-filled Teflon (to prevent extrusion and cold flow) and live loading the entire set to compensate for any Teflon creep due to thermal gradients or wear. Figures 12-8 and 12-9 illustrate the pressure/temperature curve for SafeGuard.

SureGuard is a Kalrez based, V-ring packing set that utilizes the exceptional sealing ability of a perfluorelastomer while maintaining the inertness equivalent of Teflon. Like SafeGuard, the sealing rings are backed up by carbonfilled Teflon to prevent extrusion and increase the life of the packing set. SureGuard XT is a similar packing set with PEEK backup rings, which has the capability to endure temperatures up to 550 degrees Fahrenheit in a standard rotary valve, and up to 700 degrees Fahrenheit with an extension. Figures 12-8 and 12-9 illustrate the pressure/temperature curve for SureGuard and SureGuard XT.

Figure 12-14: Typical Rotary Valve Packing Configurations

Table 12-X: Typical Rotary Valve, Stem Packing Friction Forces*NOTE: All numbers are in inch-pounds

*Packing friction force is a small factor in determining actuator size. Refer to Section 16 to determine correct actuator size for application.

Note: See Table 12-XI for shaft size versus valve size.

Table 12-XI: Shaft Diameter at Packing for Rotary Valve Sizes

Note: Contact factory for Valdisk sizes over 12 inches.


Flow Characteristics Examples

Figure 1 shows the ideal characteristic curve for the three most common types of flow characteristics; quick opening, equal percentage and linear. These characteristics can be approximated by contouring the plug. However, inasmuch as there are body effects and other uncontrollable factors, plus the need for maximizing the flow capacity for a particular valve, the real curves often deviate considerably from these ideals. When a constant pressure drop is maintained across the valve, the characteristic of the valve alone controls the flow; this characteristic is referred to as the “inherent flow characteristic.” “Installed characteristics” include both the valve and pipeline effects. The difference can best be understood by examining an entire system.

Figure 1 Inherent Flow Curves for Various Valve Plugs

Example A

A centrifugal pump supplies water to a system in which a control valve is used to maintain the downstream pressure at 80 psig. The pump characteristics are shown in figure 2 and the schematic of the flow system is shown in Figure 3.

Figure 3: Flow Schematic without Piping Losses

The maximum flow required is 200 gpm. at which the pump discharge pressure (P1) is 100 psig. Piping losses are negligible. Using the ISA liquid sizing formula, the flow coefficient, or CV, can be determined:

The piping geometry factor, FP, and the Reynolds number factor, FR, are both assumed to have a value of one. A 2-inch Valtek Mark One control valve would handle the above application.

To determine the plug characteristic which should be specified, let us analyze the installed flow characteristic of “equal percentage” and “linear” trim in this 2-inch valve.

Based on the typical pump characteristic in Figure 2, Table 1 shows several values of flow, the required valve CV, and the percent of the maximum CV which the valve must have to control the flow.

Table 1: Valve CV and Pressure as a Function of Flow Rate, Without Line Losses

The percentage of total valve lift for equal percentage and linear plugs can be determined using Figure 1.The “installed characteristics” plotted as valve lift vs. flow in gpm, are shown in Figure 4.

Note that the installed linear characteristic is “pulled” toward the inherent quick open curve and the installed equal percentage curve is “pulled” toward the inherent linear curve. A study of Figure 4 shows that either installed characteristic will provide good control for this situation.

Figure 4: Installed Characteristics without Piping Losses

Example B

The previous example was idealized in that the downstream pressure was held constant and the pressure drop variation was due to the pump characteristic alone. Now consider a more realistic installation where the delivered pressure must be held constant after passing through the valve and with some line restriction, R, in series with the valve. Schematically it would appear as illustrated in Figure 5.

Figure 5: Flow Schematic with Piping Losses

To find the installed characteristics of equal percent and linear trim in a suitably sized valve, a pressure drop distribution must be chosen. A suitable choice would be 4 psi across the valve at a flow of 200 gpm. The control valve can then be sized for the maximum required CV:

A 3-inch Valtek Mark One 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 the square root law ( Q = R√DP ) the available pressure drop across the valve at various flowing quantities can be determined, keeping in mind the pump characteristic. This is shown in Table 2. As before, the percent of maximum CV which the valve must have to control the flow is calculated and the “installed characteristic” is plotted as Figure 6.

Table 2: Valve CV Pressure as a Function of Flow Rate with Line Losses

*CV 100 is assumed to be the maximum CV.

Note the inherent equal percentage trim exhibits a nearly linear installed characteristic, while the inherent linear trim appears to be almost quick opening installed. Let us examine these

curves from the standpoint of proportional band, considering the operating region from 50 to 150 gpm at low flows. It can be seen that for a given flow change, a very small change in lift is required for the linear trim as compared with the equal percentage trim. Thus, the sensitivity of the system is high.

Figure 6: Installed Characteristics with Piping Losses

Operating in the higher flow region, the opposite is true. That is, a larger change in lift (or instrument air output) is required for the same change in flow as the linear trim. Consequently, overall sensitivity will be decreased. The equal percentage trim would exhibit an almost constant sensitivity over the entire operating range. Therefore, one proportional band setting in the controller would be adequate for the equal percentage trim, whereas several would be necessary for linear trim.

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Flow Characteristics