PCI11403

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http://slidepdf.com/reader/full/pci11403 1/77

Note: The source of the technical material in this volume is the Professional

Engineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for Saudi

Aramco and is intended for the exclusive use of Saudi Aramco’s

employees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,

or disclosed to third parties, or otherwise used in whole, or in part,

without the written permission of the Vice President, Engineering

Services, Saudi Aramco.

Chapter : Instrumentation For additional information on this subject, contact

File Reference: PCI11403 E.W. Reah on 875-0426

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Specifying Control Valves For Cavitating Flows

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Engineering Encyclopedia Instrumentation


CONTENTS PAGE

PREDICTING CAVITATION DAMAGE 1

Cavitation In Liquid Flow Streams 1

Pressure And Velocity Profiles 1

Fluid Vaporization 3

Cavitation Intensity 4

Dynamics Of And Damage Caused By Cavitating Fluids 5

Cavitation Damage 6

Methods To Predict Cavitation And Cavitation Damage 9

Application Ratio ( Ar ) 9

Control Valve Cavitation Damage Coefficient Kc 11

Predicting Cavitation Damage: Comparing Ar To Kc 13

DPcav 17

Selection Of Prediction Methods 18

Other Factors That Influence The Severity Of Cavitation Damage 18

System Design Considerations To Minimize Cavitation 19

Valve Placement 19

Sharing The Pressure Drop 20

SELECTING ANTI-CAVITATION TRIM 22

Anti-Cavitation Valve Trim: Role Of And Basis For Selection 22

Role 22

Basis For Selection 22

Common Anti-Cavitation Valve And Trim Design Strategies 23

Low Recovery Designs 23

Materials Of Construction 25Pressure Drop Staging 26

Expanding Flow Areas 29

Tight Shutoff 30

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CONTENTS PAGE

Commonly Available Trim Options 31

Single-Stage Anti-Cavitation Trim 31

Multi-Stage Cavitation Trim 32

Characterized Cages 34

Rotary Valve Anti-Cavitation Trim Options 35

Anti-Cavitation Trim Selection Procedures 38

Initial Trim Selection 38

Selection Of Appropriate Anti-Cavitation Trim 38

Sample Trim Selection Problem 40

SELECTING AND SIZING ANTI CAVITATION CONTROL VALVES 43

Manual Sizing Methods 43

Sizing Procedures 44

Using the Fisher Sizing Program 45

Sizing Equations Supported 45

Setting Options 45

Entering Sizing Data 46

Calculated Results 48Specification On The Saudi Aramco ISS 49

Entries On The Saudi Aramco ISS That Relate

To Cavitating Services 49

Importance Of Accurate Inputs 51

Obtaining Assistance 51

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CONTENTS PAGE

WORK AID 1: APPLICABLE PROCEDURES AND EQUATIONS AND

PUBLISHED KC VALUES THAT ARE USED TO PREDICTCAVITATION DAMAGE 52

Work Aid 1A: Procedures And Equations That Are Used To PredictCavitation Damage (By Comparing Ar To Kc) 52

Work Aid 1B: Kc Values That Are Used To Predict Cavitation Damage 54

Work Aid 1C: Procedures And Equations That Are Used To PredictCavitation Damage (By Comparing DPcav To DPflowing) 55

WORK AID 2: APPLICABLE PROCEDURES AND EQUATIONS, PUBLISHED

KC VALUES, AND VALVE SPECIFICATION BULLETINS THAT ARE USED TO SELECT ANTI-CAVITATION TRIM 57

Work Aid 2A: Procedures And Equations That Are Used To Select

Anti-Cavitation Trim 57

Work Aid 2B: Kc Values That Are Used To Select Anti-Cavitation Trim 59

WORK AID 3: APPLICABLE PROCEDURAL STEPS, COMPUTER-BASEDSIZING SOFTWARE, AND VALVE SPECIFICATION

BULLETINS THAT ARE USED TO SELECT AND SIZE ANTI-CAVITATION CONTROL VALVES 60

Work Aid 3A: Procedural Steps That Are Used To Select And Size Anti-Cavitation Control Valves 60

GLOSSARY 64

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LIST OF FIGURES PAGE

Figure 2 Liquid Flow Through A Restriction 1

Figure 3 Pressure And Velocity Profiles 2

Figure 4 Pvc, Pv and P2: Flashing And Cavitation 3

Figure 5 Pressure Drop And Incipient Cavitation 4

Figure 6 Bubble Formation, Collapse, And Rebound 5

Figure 7 Cavitation Damage 6

Figure 8 Conditions That Result In An Increase In The Value Of Ar 10

Figure 9 Typical Kc Values 11

Figure 10 Standard Valves: Determining The Kc 15

Figure 11 When Kc = Km 16

Figure 12 Reducing The Value Of Ar With Control Valve Elevation 19

Figure 13 Sharing The DP With Multiple Valves 20

Figure 14 Sharing The DP With A Fixed Restriction 21

Figure 15 Pressure Recovery 23

Figure 16 Kc As A Function Of Pressure Recovery (Km) 24

Figure 17 Kc As A Function Of Materials Of Construction 25

Figure 18 Pressure Drop Staging 26

Figure 19 Pressure Drop Staging With Drilled Hole Technology 27

Figure 20 Pressure Drop Staging With Stacked Disk Technology 28

Figure 21 Maintaining Pvc Above Pv With An Expanding Flow Area Trim Design 29

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Figure 22 Seat And Piston Ring Seals For ANSI Class V Shutoff 30

Figure 23 Single Stage, Drilled Hole Cage Design 31

Figure 24 Two-Stage, Drilled Hole Cage 32

Figure 25 Three-Stage, Drilled Hole Cage 33

Figure 26 Decreasing Pressure Drop Application And Cage Characterization 34

Figure 27 Anti-Cavitation Ball Valve Design 35

Figure 28 Cav V Tube Bundle 36

Figure 29 Rotary Attenuator Trim Option 37

Figure 30 Anti-Cavitation Valve And Trim Selection Procedures 39

Figure 31 Kc Values For Cavitrol III Anti-Cavitation Trims 41

Figure 32 Km Values That Are Published In Fisher Catalog 10 42

Figure 33 Review Of Valve Sizing Procedures 43

Figure 34 Options To Enable Cavitation Checks And Warnings 45

Figure 35 Required Inputs For Valve Sizing With Cavitation Checks 46

Figure 36 Typical Kc Table In An FSP Help Screen 47

Figure 37 Calculated Results 48

Figure 38 Entries On The Saudi Aramco ISS 50

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Saudi Aramco DeskTop Standards 1

Cavitation In Liquid Flow Streams

Cavitation is a phenomenon, or fluid behavior, that can occur in a liquid as the liquid flows through a flowrestricting device such as a control valve. Cavitating liquids are of special concern because cavitation canproduce unwanted noise, vibration, and equipment damage.

Pressure And Velocity Profiles

The phenomenon of cavitation is best understood when viewed in relationship to the pressure andvelocity flow profiles of a liquid that passes through a restriction.

Upstream Pressure And Velocity - Figure 2 illustrates a liquid that is flowing

through a fixed restriction. The upstream pressure is noted as P1 and the

upstream velocity of the liquid is noted as V1

.

Vena Contracta - The flow stream contracts as it passes through the

restriction, and it continues to contract even after the fluid has passed through

the physical restriction. The maximum amount of flow stream contraction occursat a point that is located downstream of the physical restriction. The point of maximum contraction (minimum cross-sectional area of the flow stream) is

referred to as the vena-contracta. The fluid pressure at the vena contract isnoted as Pvc, and the fluid velocity at the vena contracta is noted as Vvc.

Downstream Pressure And Velocity - Downstream of the vena-contracta,

the fluid expands and the downstream pressure, P2, recovers to a pressure that

is greater than Pvc but less than P1; similarly, the downstream fluid velocity, V2,

decreases.

PhysicalRestriction Vena Contracta

Flow A6363

P1

V1

PvcVvc

P2

V2

Liquid Flow Through A Restriction

Figure 2

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Effect Of Restriction - At the vena contracta, the velocity of the liquid flow

stream (Vvc) increases to a maximum value, as shown in Figure 3. According toBernoulli's equation, the increase in velocity is accompanied by a substantial

decrease in pressure and Pvc.

Downstream Conditions - Downstream of the vena contracta, the fluid stream

expands into a larger area. As the fluid expands, the downstream velocity V2becomes lower than Vvc, and downstream pressure P2 increases to a pressurethat is greater than Pvc but somewhat less than P1.

Pressure And Velocity Profiles

Figure 3

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Fluid Vaporization

Fluid Vapor Pressure P v - Every fluid has a vapor pressure (Pv). The fluid vapor

pressure is the pressure below which a liquid at a given temperature will begin to

vaporize. Fluid vaporization is similar to boiling a liquid, except that in thisinstance, the fluid is vaporized by reducing its pressure, rather than by increasing

its temperature.

Pvc Versus Fluid Vapor Pressure - If the local pressure at the vena contracta

falls to or below the fluid vapor pressure as shown in Figure 4, vapor bubbles

begin to form in the liquid stream. As the vena contracta pressure drops further

below the vapor pressure of the liquid, vapor bubbles form at a greatly increased

rate.

P2 Less Than Pv: Flashing - If P2 remains below Pv, the vapor bubbles remain

in the flow stream and the fluid is said to be flashing . Flashing has been discussedin Module PCI 103.02.

P2 Greater Than P v: Cavitation - If P2 rises above the value of Pv, the vapor

bubbles that formed at the vena contracta will collapse at a location that is

downstream of the vena contracta. The collapse of vapor bubbles is referred to as

cavitation.

Flow

P1 P2

Restriction

Vena Contracta

P2 (Cavitating)

Pv

P2 (Flashing)

Pvc

P1

Distance

P r e s s u r e

A6365

Pvc, Pv and P2: Flashing And Cavitation

Figure 4

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Cavitation Intensity

The onset and development of cavitation can be illustrated on the familiar choked flow curve that isshown in Figure 5.

Incipient Cavitation - The onset of cavitation (incipient cavitation) begins with

the formation of a few small bubbles as Pvc approaches Pv. Incipient cavitation

generally occurs at a pressure drop that is slightly less than the choked-flowpressure drop (the DPallow).

Increasing Cavitation Intensity - As the pressure drop increases to higher

values, the intensity of bubble formation increases and the bubbles becomelarger. The conditions under which cavitation becomes fully developed or "fullblown" are difficult to predict. The more important concern is the fact that

cavitation and cavitation damage can occur at any point in the flow curve

between the onset of incipient cavitation and the occurrence of flashing.

A6368

Q(GPM)

²P

Potential For CavitationDamage

IncipientCavitation

²P allow

Cavitating(P2 > Pv)

Flashing(P2 < Pv)

Pressure Drop And Incipient Cavitation

Figure 5

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Dynamics Of And Damage Caused By Cavitating Fluids

The collapse of vapor bubbles (cavitation) results in mechanical attack of the surfaces

that are exposed to the collapse.

Release Of Damaging Forces During Cavitation - Several current theories

attempt to explain the mechanism by which vapor bubbles cause damage. The

most prominent theory is that vapor bubbles that undergo non-symmetrical

collapse create a high-velocity microjet, as illustrated in Figure 6. The microjet's

velocity is estimated to vary from several hundred to several thousand feet per

second. The forces that are produced by the high velocity microjets are sufficient

to produce mechanical damage. Because the microjet must impinge directly on a

boundary surface to produce damage, the bubbles that collapse at a safe distance

from a pressure boundary surface will not produce damage. However, because

millions of bubbles are continuously forming and collapsing in a cavitating liquid, a

large number of bubbles will collapse near boundary surfaces.

Imploding vapor bubbles may go through a "rebound" process. During the rebound

process, bubbles may collapse and reform (rebound) into smaller bubbles.

Although each successive bubble regrowth becomes smaller and smaller, the

collapse pressure of the smaller bubbles is typically much higher than collapse

pressure of the original bubble. Because of their higher collapse pressures,

reformed bubbles have increased potential to produce cavitation damage.

1. Initial Vapor Bubble

2. Surface of BubbleStarts To Flatten

3. Fluid PenetratesBubble

4. Formation of Microjet

A6367

Microjet Small Bubble(Rebounding)

Bubble Formation, Collapse, And Rebound

Figure 6

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Cavitation Damage

Figure 7 illustrates the damage that is produced by cavitation. The illustration also underscores the factthat the bubbles must collapse near a boundary surface in order to produce damage.

Flow

Restriction Cavitation Damage

Vapor Bubble CollapseVapor Bubble Formation A6366

Cavitation Damage

Figure 7

Susceptible Components - The control valve components that are most

commonly damaged by cavitation include valve bodies, closure members

(plugs, disks, and balls), cages, and seat rings. Piping and fittings that arelocated downstream of the control valve are also susceptible to cavitation

damage.

Visual Appearance - Parts that have been damaged by cavitation have a

rough, pitted, cinder-like appearance. Cavitation damage is generally easy todistinguish from flashing erosion and corrosion damage because components

that have been damaged by flashing erosion and corrosion often display asmooth and polished appearance.

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Magnitude And Speed Of Cavitation Damage - While specifiers can

generally predict the occurrence of cavitation from service conditions, thedegree of cavitation damage depends on the complex interaction of a number of

variables. Some of these variables are explained below.

• Proximity Of Implosions To Boundary Surfaces - If the vapor cavities donot implode near a boundary surface, damage may not be a concern.

• Pressure Drop - Given a fixed inlet pressure (P1), a decrease inbackpressure (P2) will generally cause Pvc to fall further below Pv,causing more and more bubbles to form in the fluid.

• Material Properties - Ordinary materials may be quickly damaged by

cavitation, while tougher materials (such as Stellite, or Alloy 6) may resistcavitation damage for long periods of time.

• Cavitation And Corrosion - Cavitation attack can remove many of theprotective coatings (films, oxides, etc.) that are ordinarily present on amaterial surface, and, as a result, it will leave the base material more

vulnerable to chemical attack. Damage from both corrosion and cavitationis likely to occur at an accelerated pace in the absence of such protectivecoatings.

• Cavitation And Erosion - The mechanical attack of cavitation can also

remove hardfacing and plating that is designed to provide protection inerosive applications. If protective face materials are worn away, the base

material will be fully exposed to both erosion and continued cavitationattack.

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• Air And Gas Content - The presence of air and other gasses in the fluid

stream can have several effects on cavitation mechanics and cavitationdamage. Often, increased air or gas content can disrupt the mechanicalattack of cavitation by cushioning boundary surfaces from bubble

collapse. When the cushioning effect is present in a cavitating liquid, thecavitation is referred to as "soft" cavitation.

• Single Species Fluids Versus Mixtures - Most cavitation studies areperformed with pure water as the test fluid. Water is considered to be theworst case scenario for cavitation. Water produces a very damaging form

of cavitation that is commonly referred to as "hard" cavitation. Manyliquids, especially hydrocarbon liquids, are actually fluid mixtures. Eachcomponent of a fluid mixture has a different vapor pressure; therefore, the

fluid vaporizes in stages, and some components of the mixture may not

vaporize at all. The result is a "softer" form of cavitation.

• Exposure Time - The amount of time that a material is exposed tocavitation is also an important consideration. For example, some valves

may experience intense cavitation only during startup conditions whenpressure drops are large or during brief periods of emergency or surgeoperation. The use of more-or-less standard valves with appropriate

materials may provide adequate protection under these circumstances.On the other hand, if a standard valve is continuously subjected tointense cavitation, the time-to-failure may be very short.

• Summary Of Factors - While the factors that contribute to cavitation

damage are all interrelated, it is helpful to consider each factor asbelonging to one of three general categories:

- Cavitation attack mechanisms (the intensity and degree of cavitationas a function of the system conditions and the fluid properties).

- Material response characteristics (the materials of construction andother equipment features that inhibit or allow cavitation damage)

- Exposure time

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Methods To Predict Cavitation And Cavitation Damage

The occurrence of cavitation and cavitation damage in an initially selected valve can be

predicted with various equations and calculations. In this context, an initially selected

valve is one that has been selected and sized for a specific application. The popular methods for predicting the occurrence of cavitation and cavitation damage are based on

an evaluation of three parameters. These parameters are:

• Ar - The application ratio, or system cavitation index .

• Kc - The control valve cavitation damage coefficient .

• DPcav - The pressure drop at which cavitation damage will occur.

Application Ratio ( A r )

Index Of System Conditions - The application ratio (Ar ) is a calculated value

that expresses the relative tendency of a specific application, viewedindependently of the control valve, to cavitate. The application ratio is also

commonly referred to as the system cavitation index . The application ratio

describes the potential for the occurrence of cavitation in a particular system.

Viewed independently of other factors, Ar does not predict cavitation damage.

Calculation And Upper Limit - The equation that is used to calculate A r is as

follows:

=−

∆

where: Ar Application ratio (system cavitation index)

DP The pressure drop across the valve (P1-P2)

P1 The fluid pressure at the control valve inlet

Pv The fluid's vapor pressure

Note that the value of Ar will be different at different flow conditions (different inletand outlet pressures). For example, if the pressure drop (DP) is very high at theminimum flow condition, the value of Ar will be high at the minimum flow condition.

Higher values of Ar signal an increased tendency toward cavitation. The upper

limit, which signifies the maximum tendency to cavitate, is 0.99. If the value of A r

is 1.0, P2 will be equal to Pv and the fluid will be flashing i.e., the vapor cavitieswill remain in the flow stream because P 2 does not increase to a pressure that is

above Pv. If an application has an Ar of 1.0, cavitation is no longer the major

concern; instead, the specifier focuses on minimizing flashing damage, as

explained in Module PCI 103.02.

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Equation Interpretation - It was previously shown that cavitation can occur

whenever the pressure at the vena contracta dips below the vapor pressure of the liquid and P2 recovers to a pressure that is greater than Pv. Ar is a simple

term that accounts for the conditions that increase the likelihood that the Pvc will dip below the Pv. Figure 8 shows some of the conditions that will cause thevalue of Ar to increase. For purposes of illustration, a "typical" pressure recovery

curve (the pressure dip at the vena contracta" is shown.

Conditions That Result In An Increase In The Value Of Ar

Figure 8

Utility Of A r - The factor Ar is useful because it allows the specifier to quantifythe conditions that determine the relative potential for the occurrence of cavitation in a particular process system.

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Control Valve Cavitation Damage Coefficient K c

The control valve cavitation damage coefficient K c describes the relative tendency of a

specific control valve to experience cavitation damage. The tendency of one valve to

experience greater damage than another is determined by the design of the valve and thevalve trim. Valve and trim issues will be discussed in the next section of this Module.

Purpose Of K c - The maximum pressure drop that a control valve can experience

in a cavitating liquid stream is equal to P1 - Pv. A larger drop would result in a

flashing condition because P2 would be less than Pv. The coefficient Kc indicates

the portion (percentage) of the maximum drop (P1 - Pv) that can be taken across a

control valve before cavitation damage occurs. Low Kc values indicate a lower

control valve resistance to cavitation damage, and higher K c values (to a

maximum of 1.0) indicate a higher control valve resistance to cavitation damage.

Derivation And Location Of K c Values - Manufacturers determine and assignKc values for each specific valve type. These values are based on tests, analytical

assessments, and application experience. Kc values are published in

manufacturer's literature and they may be included in reference tables within a

sizing program. As an example, Figure 9 shows one of several tables of K c values

that are included in the Fisher Sizing Program.

Typical Kc Values

Figure 9

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Parameters For Identifying K c Values - Kc values are listed according to

several valve and application parameters. These parameters are shown inFigure 9, and they are discussed below.

• Valve Type - Kc values are unique for each different valve type. For

example, the table in Figure 9 lists Kc values for a Fisher V-Ball (V150,V200, or V300) control valve.

• Valve or Trim Style - The value of Kc also depends on the type of valvetrim that has been selected. For example, the table in Figure 9 lists Kcvalues for a V-Ball control valve with a rotary attenuator. Another chart

(not shown) lists Kc values for a V-ball control valve without anattenuator.

• Kc and Valve Size - The Kc value that is assigned to a specific control

valve may also be a function of valve size. Valve size is not related to thedamage that can result from the collapse of vapor cavities. Instead, thesize of the valve is significant because of the pipeline vibration thatgenerally accompanies cavitation in high flow rate applications. As shown

in Figure 9, the pressure drop limit (DPlimit) for each Kc coefficientdecreases as the valve size increases. Some manufacturers consider acceptable levels of vibration intensity as they assign Kc values, and

some manufacturers do not.

• DPlimit - The value of Kc that is published for each valve type and size is

valid to a specific upper pressure drop limit (DPlimit). For example, Figure9 shows that a V200 valve with a rotary attenuator has a Kc that is equalto 1.0 up to a pressure drop limit of 220 psid. If the pressure drop is

greater than 220 psid and less than 250 psid, the value of Kc is equal tothe recovery coefficient (Km) of the control valve. Recall from Module 2 of

this Course that the recovery coefficient (Km) is a published valvespecification that enables the specifier to calculate the pressure drop atwhich choked flow will occur.

• Kc And Valve Travel - Kc values for standard globe valves are generallythe same at all percentages of valve travel. For rotary-shaft control

valves, the value of Kc

may decrease significantly as the ball or disk isrotated toward the open position.

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Predicting Cavitation Damage: Comparing A r To Kc

To summarize the previous discussion, Ar is a calculated value that expresses the relative tendency of asystem to cavitate. The control valve cavitation damage coefficient Kc is a published value that

expresses the relative tendency of a control valve to experiencecavitation damage. The utility of Ar andKc is that they allow the specifier to quickly predict the potential for cavitation damage according to thefollowing:

• Kc £ Ar - If the Kc of the selected valve is equal to or less than the value of Ar at

a specific flow condition, the selected control valve will experience cavitationdamage.

• Kc > Ar - If the Kc of the selected valve is greater than the value of Ar at aspecific flow condition, the selected control valve will not experience cavitationdamage.

Evaluation At All Pressure Drops (Flow Conditions) - When predictingcavitation and cavitation damage, the specifier must calculate the value of Ar atall flow conditions; i.e., at the minimum, normal, and maximum flow. The

greatest potential for cavitation damage often occurs at the minimum flowcondition because the flowing pressure drop (DPflowing) is often the highest atthis flow condition. However, the specifier must also consider the following two

circumstances.

• Shutoff Pressure Drop - During startup, many processes operate at or near the shutoff pressure drop until a normal, steady-state flow conditionis achieved. Extreme cavitation damage can occur during the time that is

required for the downstream pressure to increase to a normal operatingpressure. To determine the potential for cavitation damage at startup, thevalue of Ar is calculated with the pressure drop set to the shutoff pressure

drop.

• Rotary-Shaft Valves - Although the value of Kc for most globe style

valves is constant over the rated valve travel, the value of Kc for rotary-shaft valves often decreases as valve travel increases; therefore, thepotential for cavitation damage in a rotary-shaft control valve is often

greatest at the maximum flow condition.

While it is sometimes tempting to evaluate the potential for cavitation at thelargest flowing pressure drop only, the factors that are explained above illustratewhy the potential for cavitation damage should be evaluated at all flow

conditions.

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Sample Problem - To illustrate the utility of Ar and Kc, consider an application

that includes the valve specifications, fluid properties, and service conditionsthat are described below.

Initially Selected Valve

Fisher ED, ANSI Class 300, 4-inch

Standard trim, 316 stainless steel trim material

Equal percentage flow characteristic

Fluid Properties

Fluid Clean Water

Fluid Vapor Pressure 0.949 psia

Service Conditions

Flow Condition P1, psig P2, psig Flow Rate

Minimum 800 200 5 000 gpm

Normal 800 500 9 000 gpm

Maximum 800 600 10 000 gpm

The steps to predict cavitation damage are as follows:

1. Calculate The Value Of Ar At All Flow Conditions - The value of Ar iscalculated with the following equation:

=−

∆

To solve the equation, P1 and Pv must be expressed in the same units.

P1 will be converted to psia by adding 14.7 to the value of P1 that isgiven in psig. The values of Ar at the given flow conditions are calculatedas follows:

Minimum Flow Ar = 600 psid

814.7 psia − 0.949 psia= 0.74

Normal Flow

Ar =300 psid

814.7 psia − 0.949 psia= 0.36

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Maximum Flow

Ar =200 psid

814.7 psia − 0.949 psia= 0.25

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2. Determine The Valve Kc - The next step is to locate the value of K c for the

selected control valve. The first step is to determine the category of service:

category 1 or category 2. Table 1 in Figure 10 lists the needed information.

According to the information in the table, the sample application is defined

as a category 1 application.

Table 2 in Figure 10 shows that for a cage-characterized, Category 1, 4-

inch control valve with a pressure drop that is greater than 100 psid, the

value of Kc is equal to the value of Km (the control valve pressure recovery

coefficient). The tables that are shown in Figure 10 are examples of the

tables of the Kc values that are available in the Fisher Sizing Program.

Globe/Angle Valves Without Cavitrol Trim

TABLE 1

FluidWater Hydrocarbon*

Trim

Material

Clean Salt/Sour Boiler Feedwater Sweet Sour

316 SST Cat. 1 Cat. 1 Cat. 1 Cat. 1 Cat. 1

416 SST Cat. 2 ** Cat. 2 Cat. 2 **

440C Cat. 2 ** Cat. 2 Cat. 2 **

316/Alloy 6 Cat. 2 Cat. 2 ** Cat. 2 Cat. 2

*The hydrocarbon must not contain sand

**Do not use this material with this fluid

TABLE 2

Trim Size Kc=1.0 Kc=Km Kc=0.85Km

Plug Characterized

Cat. 1 & 2(e.g. M-form

M-fluteCE, GL, EZ

All <75 psid 75-100 psid >100 psid

Cage Characterized

Cat. 2

1"-2"

3-4"6"-12

<300 psid

<200 psid<100 psid

>300 psid

>200 psid>100 psid

N/A

N/AN/A

Cage CharacterizedCat. 1

1"-12" <100 psid >100 psid N/A

Standard Valves: Determining The Kc

Figure 10

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As shown in Figure 11, Page 1-73 of Fisher Catalog 10 lists the value of

Km for a 4-inch ED control valve with an equal-percentage flowcharacteristic as 0.68.

F L O W C O E F F I C I E N T S

Equal Percentage Equal Percentage

CharacteristicCoeffi-

cients

Body

Size,

Port

Diameter

,

Total

Travel,

Valve Opening - Percent of Total Travel Km*

and

In. In. In. 10 20 30 40 50 60 70 80 90 100 C11&1-1/4

1-1/2

2

2-1/2

3

1-5/16

1-7/8

2-5/16

2-7/8

3-7/16

3/4

3/4

1-1/8

1-1/2

1-1/2

.783

1.52

1.66

3.43

4.32

1.54

2.63

2.93

7.13

7.53

2.20

3.87

4.66

10.8

10.9

2.89

5.41

6.98

15.1

17.1

4.21

7.45

10.8

22.4

27.2

5.76

11.2

16.5

33.7

43.5

7.83

17.4

25.4

49.2

66.0

10.9

24.5

37.3

71.1

97.0

14.1

30.8

50.7

89.5

120

17.2

35.8

59.7

99.4

136

.77

.70

.72

.71

.68

Cv(Liquid)

4

68

8

4-3/8

78

8

2

22

3

5.85

12.918.5

27.0

11.6

25.838.0

58.1

18.3

43.358.4

105

30.2

67.486.7

188

49.7

104130

307

79.7

162189

478

125

239268

605

171

316371

695

205

368476

761

224

394567

818

.68

.73

.72

.74

1-1/2

2

2-1/2

3

4

1-5/16

1-5/16

1-7/8

2-5/16

2-7/8

3/4

3/4

3/4

1-1/8

1-1/2

1.12

.923

1.57

1.75

3.82

1.56

1.42

2.57

3.11

7.65

2.22

2.09

3.82

4.77

11.4

3.10

2.84

5.44

7.07

16.9

4.27

4.11

7.64

10.7

25.5

6.17

5.83

11.5

17.0

38.2

9.01

8.58

18.2

27.9

60.5

13.1

12.8

26.7

41.5

85.7

18.2

18.5

35.1

58.0

105

23.1

24.3

43.9

70.7

112

.83

.77

.79

.75

.79

When Kc = Km(Excerpt From Fisher Catalog 10, Page 1-73)

Figure 11

3. Compare Ar and Kc - Because the value of Ar (0.74) at the minimum flowcondition is greater than the published value of Kc (Kc = Km = 0.68), theselected valve will experience cavitation damage at the minimum flow

condition. Cavitation damage is not predicted at any other flow condition.

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DPcav

An alternate method of predicting cavitation damage makes use of thefactor DPcav. DPcav is a calculated value that predicts the pressure drop at which cavitation damage will

begin.

Calculating DP cav - DPcav is calculated as follows:

∆ = −

The equation is valid whenever the valve outlet pressure P2 is greater than the

fluid vapor pressure, Pv. If P2 is less than Pv, a flashing condition exists and theequation is not valid. The values of P1 and Pv must be expressed in the sameterms; typically, psia.

Utility Of DP cav - By comparing the actual pressure drop (DPflowing) at each

flow condition to the calculated value of DPcav, the specifier can quicklydetermine whether the selected valve will experience cavitation damage.

• DPflowing < DPcav - Whenever the actual pressure drop (DPflowing) at a

particular flow condition is less than the value of DPcav, cavitationdamage is not expected.

• DPflowing > DPcav - Whenever the actual pressure drop at a particular flow condition is greater than the value of DPcav, cavitation damage is

expected.

Sample Problem - Refer to the valve specifications, fluid properties, andservice conditions that were given for the previous sample problem (see page16). Also, recall that the value of Km was found to be 0.68. This information is

used to calculate the value of DPcav as follows:

∆ = −

Note that the values of P1 and Pv are constant at all flow conditions; therefore,the equation must only be solved once. If the value of P1 were different at eachflow condition, each value of P1 and possibly different values of Kc should be

used to solve the equation. After the substitution of values and the conversion of P1 to psia, the following result is obtained:

∆Pcav = 0.68(814. 7 psia − 0.949 psia) = 553 psid

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The value of DPcav (553 psid) is compared with the actual flowing pressure

drops, and the following results occur:

DPcav (553 psid) < minimum flow DP (600 psid) - cavitation damage is

predicted

DPcav (553 psid) >normal flow DP (300 psid) - cavitation damage is not

predicted

DPcav (553 psid) > maximum flow DP (200 psid) - cavitation damage is notpredicted

Selection Of Prediction Methods

Whether the specifier elects to predict cavitation damage by comparing Ar with Kc or by comparingDP

cavwith DP

flowingis a matter of personal preference. Both techniques yield the same results.

Other Factors That Influence The Severity Of Cavitation Damage

If a specific application includes factors and conditions that would tend to diminish or increase theintensity of the cavitation damage that is expected, these factors should be considered in a subjectiveassessment that is based on experience and professional engineering judgment. A list of relevant factorsand conditions was given previously. A review of these factors and conditions follows:

• Proximity of boundary surfaces - If there are no pressure boundary surfaces that

can be damaged by cavitation, cavitation damage will not occur.

• Cavitation and corrosion - If corrosion and cavitation occur simultaneously,

cavitation damage may occur at an accelerated rate.

• Cavitation and erosion - If erosion and cavitation occur simultaneously,

cavitation damage may occur at an accelerated rate.

• Air and gas content - If the fluid includes air or gas, the degree of cavitation

damage may be reduced because of cushioning effects.

• Single-species versus fluid mixtures - If the fluid is a mixture, the severity of thecavitation and cavitation damage is often less than would be experienced with a

single-species fluid such as water or a refined product.

• Exposure time - If the maximum operating pressure drop exceeds DPcav

for

short periods of time only, the selected valve may not experience significantcavitation damage.

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System Design Considerations To Minimize Cavitation

The potential of cavitation in a given application may, in some cases, be minimized or

eliminated by the system design. Although this Module focuses on cavitation as it relates

to control valve selection and sizing issues, specifiers should be aware of the systemdesign strategies that can be implemented for the purpose of minimizing cavitation and

cavitation damage.

Valve Placement

While minimizing cavitation and cavitation damage through valve placement is not always

possible, relocating a valve to minimize cavitation and cavitation damage may be the

most cost-effective solution for some cavitation problems.

Lower Elevations - Any technique that increases the backpressure on the valve

will help to minimize the potential for cavitation; i.e., reduce the calculated value of

Ar . One way to increase backpressure is to install the control valve at a low

elevation as shown in Figure 12.

Installation A in Figure 12 increases the static head downstream of the control

valve. Depending on conditions, this technique may or may not result in a

significant reduction in the value of Ar .

Installation B in Figure 12 results in an increase in both P1 and P2. Installation B is

more likely to reduce the value of Ar than is installation A.

Reducing The Value Of Ar With Control Valve Elevation

Figure 12

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Sharing The Pressure Drop

Multiple Valves - Cavitation may be prevented by sharing the overall pressure

drop across two valves in series as shown in Figure 13. This strategy increases

the backpressure (P2) on the first control valve and it presents a lower inletpressure to the second valve. The overall effect is a reduction in the pressuredrop across each control valve. To reduce the cost and the control systemcomplexity that may be introduced with two control valves, a control valve may

be used in combination with a downstream block valve.

P1

P2 P1

P2

A6377

Sharing The DP With Multiple Valves

Figure 13

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Downstream Restrictions - Another way to lower the pressure drop across

the control valve is through the use of a "breakdown" orifice plate (fixedrestriction) that is located in the pipeline downstream of the control valve, as

shown in Figure 14. The use of an orifice plate or other restriction has thefollowing significant disadvantages.

• Because the pressure drop across the valve is reduced, a larger valvemay be required to pass the needed flow rate.

• When the control valve is throttled at low flow rates, the velocity of thefluid through the orifice plate may be so low that the majority of the

pressure drop is still taken across the control valve. If the low-flowpressure drop across the valve is large enough, the control valve maycavitate.

When the valve is opened to produce a higher flow rate, the restrictionthat is imposed by the orifice plate may limit flow.

Often, the result of these two conditions is a limited effective flow range.

• While control valve cavitation may be prevented by the orifice plate, theorifice plate itself may cavitate. Consequently, piping and components

that are downstream of the orifice plate may sustain cavitation damage.

• If the backpressure device becomes worn, the backpressure on thecontrol valve (P2) will decrease and cavitation may occur in the valve.

P1

P2 P1

P2

FixedRestriction A6378

Sharing The DP With A Fixed Restriction

Figure 14

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Anti-Cavitation Valve Trim: Role Of And Basis For Selection

Role

When it has been determined that an initially selected valvewill experience cavitation and cavitationdamage, the specifier may select special anti-cavitation trims. The role of these special trims is tominimize or eliminate cavitation within the control valve and to minimize cavitation damage. In someinstances, it may not be practical or economically feasible to totally eliminate cavitation. In suchinstances, the role of the special trim may be only to prevent cavitation damage.

Basis For Selection

Anti-cavitation trim is selected on the basis of either of the following conditions:

• The selection of a trim for which the published Kc value is greater than the

calculated value of Ar at each flow condition.

• The selection of a trim for which the calculated value of DPcav is less than theactual flowing drop at each flow condition.

The following section will provide an overview of the strategies that are commonly used to increase theKc of a control valve.

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Common Anti-Cavitation Valve And Trim Design Strategies

Many different types of special anti-cavitation control valves and anti-cavitation trim options are availablefrom control valve manufacturers. Regardless of manufacturer, most anti-cavitation valve and trim

designs employ similar strategies to eliminate or reduce cavitation and cavitation damage. Thesestrategies and the principles that they are based on are discussed below.

Low Recovery Designs

Recall from Module PCI 103.02 that a low recovery control valve produces a smaller pressure dip at thevena contracta than does a high recovery valve. Refer to Figure 15. Low recovery valves are, therefore,less likely to cause fluid vaporization and cavitation. Accordingly, most anti-cavitation valves and valvetrim options are based on low recovery valve and trim designs.

Pressure Recovery

Figure 15

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Kc And Pressure Recovery - Figure 16A illustrates a high recovery valve (Km= 0.5) in a high pressure drop application. Note that P2 approaches Pv.Because the valve is a high recovery design, the pressure at the vena contracta

falls below Pv and vapor bubbles form in the flowstream. Because P2 is greater than Pv, the bubbles will collapse and cavitation will occur.

Figure 16B illustrates a low recovery valve (Km = 0.95) in the same applicationas the valve that is illustrated in Figure 16A. Note that P2 again approaches Pv,

but, because the valve is a low recovery design, the pressure dip at the venacontract is quite small. The fluid pressure remains significantly above Pv, andthe potential for cavitation is minimized. The small pressure dip at the vena

contracta means that P2 can approach Pv without causing cavitation.

The plots that are shown below illustrate why low recovery valves (such as

sliding-stem, globe style valves) tend to have much higher Kc

coefficients thando high recovery valves (such as ball and butterfly valves).

Kc As A Function Of Pressure Recovery (Km)

Figure 16

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Materials Of Construction

Although the recovery characteristics of a valve or trim may determine the pressure conditions under which cavitation occurs, the recovery characteristic of a device does not necessarily predict the

occurrence of cavitationdamage. Cavitation damage is determined, in part, by ability of the selected trimmaterials to resist cavitation damage.

Material Toughness - The material property that provides the greatest

resistance to cavitation damage is toughness. As a general guideline, materialsthat provide resistance to cavitation damage include - in order of increasingresistance to damage - 316 stainless steel, 440C stainless steel, 17-4 stainless

steel, tungsten carbide, and Stellite (Alloy 6).

Kc Values And Material Properties - A valve trim that is made of cavitation-

resistant materials may experience cavitation without sustaining cavitationdamage. For example, if a high recovery valve (Km = 0.5) is manufactured from

standard materials, the value of the Kc coefficient may be equal to or less thanthe value of Km, as shown in Figure 17A. The value of Kc is constrained to the

value of Km because the standard materials may sustain damage from anyoccurrence of cavitation.

Figure 17B illustrates a valve that is made of cavitation-resistant materials. Thevalue of the Kc coefficient is greater than the pressure recovery coefficient Kmbecause the valve can tolerate some degree of cavitation without sustaining

cavitation damage.

Kc As A Function Of Materials Of Construction

Figure 17

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Pressure Drop Staging

Sharing The Pressure Drop - Through the use of a pressure drop staging

strategy, the pressure recovery characteristics of a valve trim can be enhanced

to prevent Pvc from reaching Pv. Pressure drop staging refers to a design inwhich fluid flow is directed through a series of several small restrictions, or stages, as opposed to a single large restriction. Each successive restrictiondissipates a certain amount of the available energy and presents a lower inlet

pressure to the next stage. As shown in Figure 18, the pressure dip that occursat each restriction is much smaller than the pressure dip that would result from asingle, large restriction; therefore, pressure drop staging may maintain Pvc at a

pressure that is greater than Pv. Even if Pvc falls below Pv, as shown in Figure18, the intensity of the cavitation will be far less than the intensity of thecavitation that would occur with a standard trim. When the cavitation intensity is

dramatically reduced, cavitation damage may be prevented by materials that areresistant to cavitation damage.

Pressure Drop Staging

Figure 18

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Drilled Hole Technology - The method by which a pressure drop staging

strategy is incorporated into a trim design varies according to the control valvemanufacturer. As shown in Figure 19, many manufacturers base their anti-

cavitation products on a drilled-hole technology. Note that the flow direction isflow-down, which is typical for a drilled-hole, anti-cavitation cage. The speciallydesigned holes:

• Minimize pressure recovery while maintaining flow capacity.

• Redirect the flowstream toward the center of the cage to reduce the riskof cavitation damage to critical trim and body parts.

• Change the flow field to reduce noise and vibration.

Valve Plug

Drilled-Hole Cage

FlowFlow

Seat Ring

A6380

Pressure Drop Staging With Drilled Hole Technology

Figure 19

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Stacked Disk Technology - Some manufacturers employ a "stacked-disk"

anti-cavitation trim design. In this design, several metal disks are stacked toform a complete cage assembly, as shown in Figure 20. A flow pattern of

successive, right-angle furrows is cut into each disk. A stacked-disk cagedesign is often referred to as a "tortuous path" design.

Tortuous Flow Path

Disk

A6381

Flow

Pressure Drop Staging With Stacked Disk Technology

Figure 20

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Expanding Flow Areas

The expanding flow area concept of damage control is closely related to the pressure drop stagingconcept. Figure 21 shows a pressure versus distance curve that compares the pressure conditions within

two control valves. One valve includes a trim that is designed to take equal pressure drops across eachstage. The other valve trim is based on a design in which the flow area of each restriction is larger thanthe flow area of the previous restriction. Accordingly, the majority of the pressure drop is taken across thefirst restriction and the pressure drop at each successive restriction becomes smaller and smaller.Because cavitation is most likely to occur in the final restriction, the trim is designed so that the pressuredrop across the last stage is a small percentage of the total drop. In a well-designed device, the valve willbe able to take a large pressure differential yet maintain the vena contracta pressure above the vapor pressure of the liquid, thus preventing the liquid from cavitating.

Inlet

Pressure

PvVena Contracta Pressure

P2

InletPressureTo ThirdStage

Conventional"Equal Drop''

Three-StageCage

Cavitrol IIIThree-Stage Cage

1 32

Fluid Travel Through The Valve Stages

P r e s s u r e

A6382

Maintaining Pvc Above Pv With An Expanding Flow Area Trim Design

Figure 21

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Tight Shutoff

High pressure drop, cavitating applications require tight shutoff. Because high velocity leakage across theseat would result in rapid damage to critical sealing surfaces, seat leakage and seal ring leakage are not

tolerable. Accordingly, anti-cavitation trims are typically available only in valve constructions that providea minimum of ANSI Class V shutoff. Globe style valves that are typically capable of tight shutoff includesingle-seated valves with unbalanced plugs (Fisher ES series) and balanced valve constructions withspring-loaded piston seals and lapped metal seats (Fisher ET series, illustrated in Figure 22).

View ARetainer Ring

Anti-ExtrusionRing

Backup Ring

Spring-LoadedPTFE Seal

ValvePlug

Lapping For ANSI Class VShutoff

View B

Cage

SeatRing

Cage Wall

ValvePlug

A

B

A6541

Seat And Piston Ring Seals For ANSI Class V Shutoff

Figure 22

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Commonly Available Trim Options

Anti-cavitation products are available for a broad range of service conditions. The following section willdiscuss commercially available trim options, the design strategies that each trim option is based upon,

and the relative degree of cavitation protection (the Kc) that each trim option provides.

Single-Stage Anti-Cavitation Trim

Single-stage anti-cavitation trims are generally specified for applications with moderate pressure drops.Depending on the valve size, the Cavitrol III, 1-stage cage that is shown in Figure 23 can provideprotection against cavitation damage at pressure drops up to 600 psid.

Single Stage, Drilled Hole Cage Design

Figure 23

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Multi-Stage Cavitation Trim

Cavitrol III - Drilled Hole, 2 Stage - Increased cavitation protection is

achieved through the use of additional pressure-reducing stages. Figure 24

illustrates a two-stage cage. The objective of multi-stage trims is to further stagethe pressure drop to maintain Pvc above Pv. In addition, the DPlimit is increasedsubstantially.

Two-Stage, Drilled Hole Cage

Figure 24

Other Approaches - Stacked disk trims provide additional stages of protection

by increasing the number of right-angle turns in each disk.

Cavitation Protection Versus Capacity - Additional stages of cavitation

protection generally result in a reduction in capacity. To compensate for the loss

in capacity, plug travel can be increased through the use of an extended cage,as shown in Figure 24.

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Cavitrol III - Drilled Hole, 3 Stage - Further increases in cavitation protection(increased values of Kc) are achieved through the use of three-stage cages, asshown in Figure 25. The pressure drop limits (DPlimit) are also increased.

Additional stages of pressure reduction (e.g., a 4-stage trim, which is not shown

in the figure) result in additional increases in cavitation protection.

Three-Stage, Drilled Hole Cage

Figure 25

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Characterized Cages

Multiple-stage trim designs limit flow capacity, and they often lead to the requirement for a larger valve. Inapplications where the pressure drop is constant, there is no alternative to the selection of a large valve;however, in applications where the pressure drop decreases as the flow rate increases (as is typical for many applications) characterized cages provide an alternative to the selection of larger, more costlycontrol valves. Figure 26 illustrates the basic concept of a characterized, anti-cavitation cage. The holesnearest the seat are two-stage, Cavitrol holes. The two-stage holes stage the pressure drop and providemaximum cavitation protection when the plug is near the seat; i.e., when the pressure drop is high andthe Ar (the needed Kc) is high.

As the plug moves away from the seat and the pressure drop decreases, the requirement for cavitationprotection decreases. The moderate cavitation protection that is needed and an increase in the capacityof the trim are provided by rows of single-stage holes that are located above the two-stage holes. As thevalve plug approaches the wide-open position, the pressure drop decreases and the value of Ar (therequired Kc) is greatly reduced. Because there is no need for cavitation protection at high flow rates,several rows of straight-through holes are located above the single-stage holes. The straight-through-holes provide increased capacity but no cavitation protection. The result of this characterized cage is

maximum cavitation protection when the plug is near the seat and maximum capacity at the maximumflow conditions. Selection of characterized, anti-cavitation trim requires the assistance of the vendor.

Percent Of Rated Flow Capacity

∆P

Significant CavitationProtection Is Required

Moderate CavitationProtection Is Required

No CavitationProtection IsRequired

0 100

Minimum

Maximum

A6545

2-Stage Holes1-Stage Hole

Straight Through Holes

Decreasing Pressure Drop Application And Cage Characterization

Figure 26

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Rotary Valve Anti-Cavitation Trim Options

While rotary-shaft valve are generally considered to be high-recovery devices, and therefore prone tocavitation, a few anti-cavitation trim options are available. Generally speaking, the degree of cavitation

protection that is provided by rotary valve trim options is not as great as the protection that can beachieved with sliding-stem, globe style valves. Accordingly, anti-cavitation rotary-shaft valve options aregenerally viewed as "low-tier", severe service solutions. In other words, rotary-shaft, anti-cavitationcontrol valves may be most appropriate when cavitation occurs only occasionally; e.g., during startup or shutdown, or for brief periods of normal operation.

Neles Q Ball Valves - Anti-cavitation trim for ball valves typically includes a

baffle or diffuser in the ball bore. The ball valve trim design that is shown inFigure 27 includes parallel, perforated plates that are located inside the ballbore. The purpose of the perforated plates is to create backpressure on the

valve when the ball is throttling and to reduce the pressure recoverycharacteristic of the valve. Only a moderate reduction in pressure recovery is

possible.

Throttling Position

Fully Open Position A6344-1

Perforated Baffles

Ball

Flow

Anti-Cavitation Ball Valve Design

Figure 27

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Tube Bundles - Some cavitation-prone applications have requirements for

control valve rangeability that cannot be satisfied with a globe style valve.Extremely wide rangeability is a requirement that is often best satisfied with the

selection of a a V-notch ball valve. However, V-notch valves are inherently high-recovery designs, which makes them particularly vulnerable to cavitationdamage. To allow the use of a V-notch ball in a moderate pressure drop (to 500

psid) application, a tube bundle (referred to as Cav V) can be attached to theoutlet of the valve, as shown in Figure 28.

The tube bundle serves the following three distinct functions:

• It creates a back pressure in the valve, thereby dividing the overallpressure drop between the tube bundle and the ball seal.

• It prevents formation of a severely restricted vena contracta, thus

eliminating high flow velocity and the associated drop in static pressure.

• If cavitation does occur, the tube bundle restricts the size of vapor

bubbles to the diameter of the individual flow tubes. Tests with softaluminum rods have shown that the damage that occurs downstream of the Cav V trim is insignificant when it is compared with the damage that

occurs without the flow tubes.

Flow

Ball Segment

Tube Bundle A6392

Cav V Tube Bundle

Figure 28

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Fisher Rotary Attenuator - A Rotary Attenuator (see Figure 29) is a trim

option that is available for most Fisher V-notch ball valves. The attenuator isdesigned to reduce the pressure recovery of the valve. The attenuator is

recommended for applications in which the major objective is to reduce lowfrequency vibration that is caused by pressure transients, and to provideprotection against moderate, intermittent cavitation. To provide protection

against sustained, intense cavitation, sliding-stem, globe style valves with anti-cavitation trim options are recommended.

Vee-Notch Ball Segment

Attenuation

Element

A6546

Rotary Attenuator Trim Option

Figure 29

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Anti-Cavitation Trim Selection Procedures

Initial Trim Selection

Trim selection begins with the selection of a particular trim, according to the criteria that are important tothe selection of any control valve; e.g., the ANSI Class pressure-temperature rating, the pressure droprating, shutoff requirements, materials of construction, balanced versus unbalanced plugs, and so forth.Because cavitation often occurs in high pressure applications, high pressure valves are often required.Tight shutoff (ANSI Class V or better) is also required to prevent damage to seating and sealing surfacesthat could result from high-velocity leakage.

Selection Of Appropriate Anti-Cavitation Trim

Several different approaches may be taken to select a specific anti-cavitation trim. The followingdescribes one methodical approach. The approach is also illustrated in Figure 30.

Step 1: Calculate A r - The first step in the selection of anti-cavitation trim is to

calculate the value of Ar at each flow condition. In addition, if the valve is to beoperated at or near the shutoff pressure drop for sustained periods of time, thevalue of Ar should also be calculated at the shutoff pressure drop (DPshutoff ).

Step 2: Check For Flashing - If the value of Ar at any operating condition is

1.0 or greater, the process fluid is flashing , not cavitating. Because flashingerosion would tend to quickly damage the small passages of an anti-cavitation

trim, anti-cavitation trim should not be selected.

Step 3: Select Valve/Trim - The calculated value of Ar is the minimum value

of the control valve Kc that is required to prevent cavitation damage; therefore,

the next step is to select a particular valve and trim for which the publishedvalue of Kc is greater than the value of Ar at each operating condition. The

DPlimit for which the Kc is valid must be greater than the pressure drops thatare used to calculate Ar . As is the case for any selection process, the valve andtrim must have adequate ratings in terms of pressure, temperature, and

pressure drop limits.

If the selected valve or valve trim meets all the requirements, the specifier

continues with the sizing process. If the selected valve or valve trim does notmeet all of the requirements, the specifier must return to the initial valveselection procedure and select a different valve or trim style.

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Anti-Cavitation Valve And Trim Selection Procedures

Figure 30

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Sample Trim Selection Problem

For the basis of a sample trim selection problem, consider the valve specifications, fluid properties, andservice conditions that are listed below.

Initially Selected Valve

Fisher ET

4-inch

Linear flow characteristic

Fluid Properties

Fluid Clean Water

Fluid Vapor Pressure 0.949 psia

Service Conditions

Flow Condition P1, psig P2, psig Flow Rate

Minimum 1 200 100 5 000 gpm

Normal 1 100 200 9 000 gpm

Maximum 1 000 200 10 000 gpm

Step 1: Calculate A r - The first step in the selection of anti-cavitation trim is to

calculate the value of Ar at all flow conditions; i.e., the value of Ar must be

calculated with the use of the upstream and downstream pressures at theminimum, the normal, and the maximum flow conditions. The equation to

calculate Ar is as follows:

=−

∆

Because the vapor pressure is given in psia and P1 is given in psig, P1 will be

converted to psia by adding 14.7 to the pressure that is given in psig. Thevalues of Ar at the given flow conditions are calculated as follows:

At minimum flow Ar = 1 100 psid

1 214.7 psia − 0.949psia= 0.91

At normal flow Ar =

900 psid

1 114.7 psia − 0.949psia= 0.81

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At maximum flow Ar =

800 psid

1 014.7 psia − 0.949psia= 0.78

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Step 2: Check For Flashing - All of the calculated values of Ar are less than 1.0;

therefore, the fluid is not flashing.

Step 3: Select Trim - The next step is to select a particular valve trim for which:

• the published value of Kc is greater than the value of Ar at all flow

conditions

• the published DPlimit is greater than the actual pressure drop at any of the

flow conditions

• the published pressure, temperature, and pressure drop limits are adequate

for the service conditions

For purposes of achieving economy, a common selection strategy is to first

investigate the least expensive (least complex) trim option. To begin, refer to thetable that is shown in Figure 31. This table is one of several tables of K c values

that are included in the Fisher Sizing Program. This table lists K c values for all

Cavitrol III trims.

The cost and complexity of the trims increase as the number of stages increases.

For economy, a Cavitrol III 1-stage trim will be evaluated. Analysis of Cavitrol III 1-

stage trim reveals that a 4-inch valve with a pressure drop that is greater than

500 psid has a Kc rating that is equal to its Km.

Cavitrol III Trims

Trim Size Kc=1.0 Kc=Km

Cavitrol III1 stage

1"-2"3"-6"

8"-12"

<600 psid<500 psid<400 psid

600-1440 psid500-1440 psid400-1440 psid

Cavitrol III2 stage

1"-2"3"-6"

8"-12"


N/A1800-2160 psid1200-2160 psid

Cavitrol III

3 stage

1"-12" <3000 psid N/A

Cavitrol III4 stage

1"-12" <3000 psid 3000-4000 psid(Kc = 0.99)

Kc Values For Cavitrol III Anti-Cavitation Trims

Figure 31

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The Km for a 4-inch ET valve with 1-stage Cavitrol trim is published on page 1-

91 of Fisher Catalog 10. As shown in Figure 32, the Km is 0.81. The value of Km is less than the value of Ar (0.91) at the minimum flow condition and it isequal to the value of A

r (0.81) at the normal flow condition; therefore, the

Cavitrol III1-stage trim will not provide the needed protection against cavitation damage.

Liquid Flow Coefficients (C v)

Class 600 Linear

CharacteristicBody Port Maximu

m

Minimum

Trim Size, Diameter Travel, Throttling Valve Opening, Percent of Maximum Travel Km*

Stage In. In. In. Cv(2) 10 20 30 40 50 60 70 80 90 100

OneStage

1

1-1/2

2

2-1/23

1-5/16

1-7/8

2-5/16

2-7/83-7/16

1(1)

7/8(1)

1-1/8

1-1/21-5/8

(1)

1.9

2.5

3.9

4.24.6

.25

.59

.84

.841.65

.48

.72

1.49

6.8310.8

2.36

2.54

6.68

16.222.3

5.04

6.03

12.3

25.034.3

7.36

9.32

17.3

33.045.3

9.47

12.8

22.1

41.255.5

11.2

15.6

26.7

48.864.7

13.1

18.2

30.9

55.572.7

14.6

20.8

34.4

61.780.0

15.5

22.5

36.1

64.486.7

.81

.86

.87

.82

.79

4

6

8

4-3/8

7

8

2-1/8(1)

2-1/4(1)

3-3/8(1)

5.2

10

15

3.47

4.6

16.2

22.7

30.0

70.2

43.3

65.3

124

63.4

99.7

176

81.8

134

227

100

165

276

116

195

324

131

219

370

144

241

412

151

259

439

.81

.83

.89

Two

Stage

1

1-1/2

2

2-1/2

3

1

1-5/16

1-7/8

2-5/16

2-7/8

1

1-1/2

2

2-1/2

3

0.28

0.44

0.92

1.10

1.20

0.11

0.22

0.80

1.75

3.14

0.41

1.20

3.05

5.25

8.23

1.08

2.23

5.29

8.71

13.3

1.75

3.26

7.56

12.2

18.5

2.43

4.29

9.83

15.6

23.5

3.10

5.31

12.1

19.1

28.7

3.78

6.35

14.3

22.6

33.8

4.45

7.37

16.5

26.1

38.9

5.12

8.40

18.8

29.6

44.0

5.80

9.40

21.0

33.0

49.0

.96

.96

.96

.96

.96

4

6

8

2-7/8

5-3/8

7

4

4

6

1.90

3.00

7.00

2.83

6.05

18.4

11.2

22.5

47.2

19.4

38.0

74.5

27.4

53.7

101

35.5

69.4

129

43.2

85.2

156

50.5

100

184

57.1

115

211

63.2

130

238

69.0

144

265

.96

.96

.96

Km Values That Are Published In Fisher Catalog 10(Excerpt From Fisher Catalog 10, Page 1-91)

Figure 32

Evaluation of a Cavitrol III 2-stage trim in a 4-inch valve (refer to Figure 31 onthe previous page) reveals that the value of Kc is 1.0 up to a pressure drop limitof 1 800 psid. Accordingly, a Cavitrol III 2-stage trim will provide the needed

protection. It should be noted that when this valve is sized, the required valvesize may be larger than 4-inch because of the reduction in flow capacity of the

anti-cavitation trim.

The pressure, pressure drop, and temperature ratings that are listed in theproduct specification bulletin (Fisher Bulletin 51.1:ET) indicate that the selected

valve and trim are compatible with the service conditions; therefore, theselection process is complete.

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Manual Sizing Methods

The procedures for sizing a control valve for a cavitating application are the same as theprocedures for sizing a control valve for any liquid flow. The procedures are reviewed

below and they are shown in Figure 33.

Done

Valve Selection Complete

Basic Selection Criteria

Cavitation Selection Criteria

Select Valve Size Based

On Cv

Pactual

< Pallow

?

Check For Choked Flow At

All Flow ConditionsCalculate ∆ P

allowat each flow

condition

∆ Pallow

=Km

(P1-r

cP

v)

Psizing

= Pallow

Psizing = Pactual

Yes

Calculate Required Cv

At All Flow Conditions

Cv

= Q G/ P

Size Available?

Yes

No

Verify Sizing

Calculations Actual K m =Estimated K m?

No

Yes

No

Review Of Valve Sizing Procedures

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Figure 33

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Sizing Procedures

Step 1: Check For Choked Flow - As with all liquid sizing, the allowable

pressure drop, or choked flow pressure drop, is calculated to determine if

choked flow exists. To calculate the DPallow, an estimated value of Km is used.The estimated value is one that is representative of the published Km's for thetype and the size of the valve that has been initially selected. The equation tocalculate the allowable pressure drop is as follows:

∆ = −

Where:

DPallow the choked flow pressure drop (refer to PCI 103.02)

Km control valve recovery coefficient

P1 upstream pressure

r c critical pressure ratio (refer to PCI 103.02)

Pv vapor pressure of the fluid

Step 2: Determine DP sizing - For the purpose of calculating the required Cv,

the pressure drop is set to the lesser of the actual drop or the DPallow at eachflow condition.

Step 3: Calculate The Required C v - The required control valve Cv is

calculated for each flow condition according to the following basic equation:

=∆

Step 4: Select Valve Size Based On C v - Using the calculated Cvrequirements, the specifier refers to manufacturers' sizing catalogs to select aspecific valve size. If the needed valve size is not available in the type of valve

that has been initially selected, the specifier must repeat the valve selection andsizing procedures.

Step 5: Verify Sizing Calculations - If an appropriate valve size is available,

the specifier should compare the actual Km of the selected valve to theestimated value of Km that was used to perform the initial sizing calculations. If there is a significant difference between the estimated Km and the actual Km,

the calculations to determine DPchoked and the required Cv are repeated. Theprocess is repeated until a suitable valve size is identified.

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Using the Fisher Sizing Program

While valve sizing can be performed manually, most specifiers rely on the speed, accuracy, andconvenience of computer-based sizing software to calculate control valve Cv requirements. In addition tosimplifying control valve sizing, sizing software may also include features that assist in the selection of anti-cavitation valves and trim. For purposes of illustration, the Fisher Sizing Program will be used as abasis for explaining the operation of a typical sizing software program. Other control valve manufacturersalso offer sizing software.

Sizing Equations Supported

The functionality of the Fisher Sizing Program varies according to sizing method that is selected. Recallfrom Module PCI 103.02 that the specifier may calculate Cv requirements with the use of the ISA sizingequations or with the use of Fisher Controls' sizing equations.

ISA Equations - The sizing equations that are included in ISA Standard S75.01

do not include terms or equations for calculating Ar or DPcav. Accordingly,

many of the cavitation checks and sizing aids are not available if the ISA Liquidsizing method is selected.

Fisher Equations - If the specifier selects either the Fisher Liquid or the Fisher

Water sizing method, the software automatically calculates the values of Ar and

DPcav, compares Kc to Ar , and displays a warning if Ar > Kc.

Setting Options

In order to take advantage of the built in cavitation checks, the specifier must press the F3 key and selectthe options Cavitation Check On and Warnings On , as shown in Figure 34.

Options To Enable Cavitation Checks And Warnings

Figure 34

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Entering Sizing Data

Basic Liquid Sizing Procedure - Information is entered according to the

procedures for liquid sizing that were discussed in Module PCI 103.02. The only

difference is that the value of Kc must be entered in the Valve Specificationssection of the calculation screen, as shown in Figure 35.

Rev 1.41 NRM Fisher Liquid Valve Sizing

Fluid & Service Conditions Valve Specifications

-------------------------------------- -------------------------------------

Liquid ISOBUTANE Km -

Kc -

SG 0.900

P1 600.000 psig

dP 500.000 psid

Q 50000.000 gpm(US)

T 100.000 deg F Calculated Results

-------------------------------------

Intermediate Results Cv -

-------------------------------------- dP Allowable - psid

Pv - psia dP Cavitation - psid

Pc 528.800 psia Ar -

Rc -

Notes:

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Valve Recovery Coefficient.

F1-HELP F2-Calc F3-Option F5-Clear F9-Table F10-Exit

Required Inputs For Valve Sizing With Cavitation Checks

Figure 35

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Locating K c Values - Tables of Kc values for Fisher control valves (see Figure

36) are included in the Information Screens. The Information Screens areaccessed by pressing the F1 key twice to display an index, pressing the K key

to navigate quickly to the screens that display information on topics that beginwith the letter K, and selecting the entry Kc Tables from the index. The tables

that list Kc values for different valve types are on several screens. The specifier may view different screens by pressing the PAGE UP and PAGE DOWN keys,

or by pressing the F3 key (page up) and the F4 key (page down).

Typical Kc Table In An FSP Help Screen

Figure 36

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Calculated Results

After all the pertinent information has been entered, the specifier presses the F2 key to perform thecalculations.

Basic Sizing Information - The Calculated Results section of the screen

displays all of the basic sizing information: the required Cv, DPallow, Rc, and a

warning if choked flow exists.

Cavitation Information - In addition to the basic sizing information, the

Calculated Results section also displays, as shown in Figure 37, the

calculated values of DPcav and Ar , and a warning if Ar >Kc.

Rev 1.41 NRM Fisher Liquid Valve Sizing

Fluid & Service Conditions Valve Specifications

-------------------------------------- --------------------------------------

Liquid ISOBUTANE Km 0.750

Kc 0.750

SG 0.900

P1 600.000 psig

dP 500.000 psid

Q 50000.000 gpm(US)

T 100.000 deg F Calculated Results

--------------------------------------

Intermediate Results Cv 2329.002

-------------------------------------- dP Allowable 414.804 psid

Pv 71.906 psia dP Cavitation 407.093 psid

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Pc 528.800 psia Ar 0.921

Rc 0.857

Notes: CHOKED

Ar > Kc

PRESS [F7]

Liquid name (optional). Press [F4] for a list of liquids.

F1-HELP F2-Calc F3-Option F4-Choice F5-Clear F9-Table F10-Exit

Calculated Results

Figure 37

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Specification On The Saudi Aramco ISS

When specifying a valve for a cavitating application, the specifier must enter the data that is pertinent tothe specification of the particular valve, as presented in Modules PCI 103.01 and PCI 103.02.

Entries On The Saudi Aramco ISS That Relate To Cavitating Services

In addition to the basic valve selection and sizing information that is entered on lines 5 and 6, lines 49through 67, and lines 72 and 73 of the Saudi Aramco ISS, the specifier must also enter information thatrelates specifically to a cavitating application. The lines on the ISS that relate specifically to thespecification of an anti-cavitation control valve are discussed below. Refer to Figure 38.

Line 6: Valve Model/Type Number - The valve description that is entered on

this line should include a description of any special trim that is specified; e.g.,Cavitrol III 2-stage.

Line 32: DP Cavitation Worst Case - This value is the smallest calculatedvalue of DPcav. The lowest value of DPcav is the "worst case" because it is thelowest pressure drop at which caviation damage is likely to occur. The smallestvalue of DPcav occurs at the flow condition where the flowing pressure drop isthe highest which is generally, but not always, at the minimum flow condition.This value can be obtained from the FSP, either directly from a calculationscreen or from a table of values.

Line 33: Cavitation Service - This line provides a general alert to indicatewhether or not there is a potential for cavitation (as opposed to cavitationdamage). While the potential for cavitation damage or piping vibration problemscan be predicted by comparing the values of Ar and Kc, there is no absolute

guideline to precisely determine or quantify the severity level of cavitation;however, to determine whether or not an application has the potential tocavitate, Saudi Aramco engineers often use the following criteria:

• If Ar > 0.9 Kc, there is significant potential for cavitation and theapplication should be closely evaluated.

• If Ar < 0.9 Kc, the potential for cavitation is minimal.

To complete line 33, the specifier circles the appropriate selection: YES if theapplication has the potential for cavitation, and NO if the application does nothave the potential for cavitation.

Line 71: Cavitation Resistant Trim Required - On this line, the specifier

circles the appropriate selection: YES if an anti-cavitation trim is selected; andNO if a standard trim is adequate.

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Entries On The Saudi Aramco ISS

Figure 38

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Importance Of Accurate Inputs

Many severe-service valve failures can be traced to an incomplete or inaccurate description of fluidproperties or service conditions; therefore, every effort must be made to ensure that the inputs to the

sizing process reflect the actual operating conditions.

Vapor Pressure - For example, the fluid vapor pressure is a critical input to the

equations. Fluid vapor pressure can change significantly with temperature andwith fluid composition; therefore, the specifier must ensure that the vapor

pressure that is used in the sizing procedures is the vapor pressure at theoperating temperature.

Operating Conditions that differ from design conditions can negate the

validity of the most rigorous selection and sizing procedures. For example, if thevalve sizing information that is made available to the specifier indicates an

operating temperature of 150 degrees, but conditions change to 200 degrees,the vapor pressure may increase significantly. An increase in vapor pressurewill result in the onset of cavitation at a much lower pressure drop. Changes in

temperature may also cause a change in fluid density, which can affect the Cvcalculations and the selected valve size.

Obtaining Assistance

To ensure that the sizing inputs are accurate and to verify the suitability of a particular valve selection,specifiers may seek support and assistance from several sources.

Process Engineers - To ensure that the fluid properties are accurate,

specifiers may consult with Saudi Aramco process engineers.

Operations Personnel - To verify the operating conditions such as

temperature, flow rate, pressure, and pressure drop, specifiers may obtain the

most accurate information from operations personnel.

Manufacturers - The published information that relates sizing techniques, anti-

cavitation valve design strategies and options, and other cavitation-related

information represents only a fraction of the knowledge and experience that isheld by leading manufacturers. Whenever unusual conditions exist, or if aspecific construction cannot be identified that will meet all requirements,

specifiers may consult directly with vendors and manufacturers to identify themost appropriate solutions.

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Work Aid 1A: Procedures And Equation s That Are Used To Predict Cavitation Damage(By Comparing A r To Kc)

1. Calculate DPflowing

For each flow condition that is listed, calculate the flowing pressuredrop(DPflowing) as follows:

DPflowing = P1 - P2

2. Locate Kc Values

For the initially selected valve, locate the value of Kc at all three flow conditionsby performing the procedures that follow.

a. Refer to the Help Screens in the Fisher Sizing Program or to Table 1 in

Work Aid 1C and determine the category of service.

b. Refer to the appropriate table for the selected valve and the service

category and determine the value of Kc.

c. If Kc = Km, refer to Fisher Catalog 10 to obtain the Km value for theselected valve type and size.

3. Calculate Ar

Calculate the value of Ar at each flow condition. The appropriate equation is:

=−

∆

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Recall that, if it is given in psig, P1 should be converted to psia by adding 14.7

to the value of P1. The value of Ar at each flow condition may be calculated byinserting the appropriate values in the following equations and solving for Ar .

At minimum flow=

−=

At normal flow=

−=

At maximum flow=

−=

4. Predict The Occurrence Of Cavitation Damage

To determine whether or not cavitation damage is predicted, compare the valueof Kc to the value of Ar at each flow condition.

• If Ar > Kc at any flow condition, cavitation damage is predicted.

• If Ar < Kc at any flow condition, cavitation damage is not predicted.

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Work Aid 1B: Kc Values That Are Used To Predict Cavitation Damage

Manufacturers publish Kc values for their valves in various publications, specification bulletins, and other documentation. The tables that are shown below are taken from the Help Screens that are included in the

Fisher Sizing Program.

TABLE 1

Fluid

Water Hydrocarbon*

TrimMaterial

Clean Salt/Sour Boiler Feedwater Sweet Sour

316 SST Cat. 1 Cat. 1 Cat. 1 Cat. 1 Cat. 1

416 SST Cat. 2 ** Cat. 2 Cat. 2 **

440C Cat. 2 ** Cat. 2 Cat. 2 **

316/Alloy 6 Cat. 2 Cat. 2 ** Cat. 2 Cat. 2

*The hydrocarbon must not contain sand

**Do not use this material with this fluid

TABLE 2

Trim Size Kc=1.0 Kc=Km Kc=0.85Km

Plug Characterized

Cat. 1 & 2(e.g. M-form

M-fluteCE, GL, EZ

All <75 psid 75-100 psid >100 psid


1"-2"3" 4"

6"-12"


>300 psid200 psid

<100 psid

N/AN/AN/A


1"-12" <100 psid >100 psid N/A

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Work Aid 1C: Procedures And Equations That Are Used To Predict Cavitation Damage

(By Comparing DP cav To DPflowing)

1. Locate Kc Values

For the initially selected valve, locate the value of Kc at all three flow conditions

by completing the following procedures:

a. Refer to the appropriate Help Screen in The Fisher Sizing Program or to

Table 1 in Work Aid 1B and determine the category of service.

b. Refer to the appropriate Help Screen in The Fisher Sizing Program or toTable 2 in Work Aid 1B and determine the Kc of the selected valve.

c. If Kc = Km, refer to Fisher Catalog 10 to determine the value of Km for

the initially selected valve.


For each flow condition that is listed, calculate the flowing pressure drop(ÆPflowing) with the use of the following equation.

DPflowing = P1 - P2

3. Calculate DPcav

For all three flow conditions, calculate the value of DPcav. The equation to

calculate the value of DPcav is:

∆ = −

To solve the equation, P1 and Pv must be expressed in the same units (psia).The value of ÆPcav at each flow condition may be calculated by inserting theappropriate values in the following equations and solving for ÆPcav.

Minimum flow: ∆ = − = − =

Normal flow: ∆ = − = − =

Maximum flow: ∆ = − = − =

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4. Predict Cavitation Damage

To determine whether or not cavitation damage is predicted, compare the valueof ÆPflowing to the value of ÆPcav at each flow condition. Predict the

occurrence of cavitation damage as follows:

• If DPflowing > DPcav at any flow condition, cavitation damage is

predicted.

• If DPflowing < DPcav at any flow condition, cavitation damage is not

predicted.

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Work Aid 2A: Procedures And Equations That Are Used To Select Anti-Cavitation Trim


Calculate the flowing pressure drop (DPflowing) at each flow condition. The

appropriate equation is:

DPflowing = P1 - P2

2. Calculate Ar

Calculate the value of Ar at each flow condition. The appropriate equation is:

=−

∆

Recall that P1 must be converted to psia by adding 14.7 to the value of P1. The

value of Ar at each flow condition may be calculated by inserting the appropriatevalues in the following equations:

At minimum flow=

−=

At normal flow = − =

At maximum flow=

−=

3. Determine Kc Values For The Initially Selected Valve

a. Refer to Table 1 in Work Aid 1B and determine the category of service.

b. Refer to Table 2 in Work Aid 1B and determine the Kc for the initiallyselected valve.

c. If Kc = Km, refer to Fisher Catalog 10 to determine the value of Km for the initially selected valve type and size.

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4. Predict Cavitation Damage

For each flow condition, compare the values of Kc and Ar to predict cavitationdamage.

• If the valve Kc < Ar at a specific flow condition, cavitation damageis predicted to occur at that flow condition.

• If the valve Kc > Ar at a specific flow condition, cavitation damage

is not predicted to occur at that flow condition.

5. Final Trim Selection

To identify the least complex trim that will provide the needed protection fromcavitation damage, first evaluate a 1-stage Cavitrol trim, as follows:

a. Refer to the Kc table that is included in Work Aid 2B and determine the

Kc for the selected trim, valve size, and pressure drop limit.

b. If the valve Kc > Ar at all flow conditions, the trim that is being evaluated

will provide adequate protection. Proceed to step 7. of this procedure.

c. If the valve Kc < Ar at any flow condition, cavitation damage will occur atthat flow condition.

d. If necessary, proceed to evaluate increasingly complex trims (according

to steps a, b, and c, directly above), in this order:

Cavitrol III 2-stage trim



6. Trim Specification

Refer to the appropriate table in the specification bulletin for the selected valveand determine the trim number for the selected trim.

Refer to the appropriate valve specification bulletin and ensure that the valvepressure drop ratings and temperature ratings are adequate for the application.

If the selected valve and trim meets all of the requirements, enter the trimspecification in the appropriate location.

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Work Aid 2B: Kc Values That Are Used To Select Anti-Cavitation Trim

Manufacturers publish Kc values for their valves in various publications, specification bulletins, and other documentation. The table that is shown below is taken from a Help Screen that is included in the Fisher

Sizing Program.

Trim Size Kc=1.0 Kc=Km

Cavitrol III1 stage

1"-2"3"-6"

8"-12"


600-1440 psid500-1440 psid400-1440 psid

Cavitrol III2 stage

1"-2"3"-6"

8"-12"


N/A1800-2160 psid1200-2160 psid

Cavitrol III3 stage

1"-12" <3000 psid N/A

Cavitrol III4 stage

1"-12" <3000 psid 3000-4000 psid(Kc = 0.99)

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Work Aid 3A: Procedural Steps That Are Used To Select And Size Anti-CavitationControl Valves

1. Initial Calculations

a. Select the Fisher Liquid sizing method.

b. On the screen that appears, select the Valve Sizing Option and the

minimum flow condition (MIN).

c. Press the F3 key, and select the following options: Cavitation Check ON,Warnings On, and Input Pv.

d. For the minimum flow condition, enter the fluid properties, the serviceconditions, and the valve specifications.

To determine the value of Km for the initially selected valve, refer toFisher Catalog 10.

To determine the value of Kc for the initially selected trim, refer to theHelp Screens by performing the following procedures:

• Press the F1 key twice to view an index of Help Screens.

• Press the "K" key to navigate to the topics that begin with the letter 'K'.

• Select "Kc Table" from the index of topics.

• Press the PAGE DOWN key until the Help Screens for standardvalves (valves without Cavitrol trim) are located.

• Determine the service category and the value of Kc.

e. After entering the values of Kc and Km, press the F2 key to calculate andto display the valve sizing information.

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f. Repeat steps d and e above for the normal flow condition and for the

maximum flow condition. Recall that sizing information can be copiedfrom one flow condition to another flow condition through the performanceof the following:

• Press the ESCAPE key.

• With the use of the left arrow key or the right arrow key, move thecursor to the condition to which values are to be copied.

• Press and hold the ALT key, and, then, press the C key.

• Enter the number of the flow condition that is to be copied to theselected flow condition.

• To copy the information to the new condition and to view thecalculation screen, press the ENTER key.

• Change the sizing inputs that are different for this flow condition (P1,dP, and Q).

• Press the F2 key to calculate the sizing information.

g. After the sizing information has been calculated for all three flowconditions, press the F9 key to display a table of sizing information.

h. Record the values of the following parameters that have been calculatedat each flow condition.

• Cv

• DPflowing (dP on the computer screen)

• Ar

• Kc

• Pcav

i. Determine whether or not cavitation damage is predicted.

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2. Trim Selection

Evaluate a Cavitrol III 1-stage trim by performing the following procedures:

a. Locate the Kc values for Cavitrol III trim by referring to the Help Screens.

• Press the F1 key twice to view an index of Help Screens.

• Press the "K" key to navigate to the topics that begin with the letter

'K'.

• Select "Kc Table" from the index of topics.

• Press the PAGE DOWN key until the appropriate Help Screen isdisplayed.

b. Review the Kc values for the various Cavitrol trims, and select the least

complex trim (the trim with the fewest number of stages) that will satisfythe following conditions:

• Kc > Ar at each flow condition

• DPlimit >DPflowing at each flow condition

c. To evaluate the trim that is selected in the step immediately above, the

valve sizing information must be recalculated with the use of theappropriate values of Kc and Km.

• The value of Kc was determined in the step immediately above.

• Because the exact valve size is not yet known, an estimated valueof Km is used to perform the sizing calculations for the initiallyselected Cavitrol III trim. To estimate the value of Km, refer to the

page in Fisher Catalog 10 that lists data for the Cavitrol III trim that isbeing evaluated, and note the value of Km for the size of the initiallyselected valve.

d. After entering the new values for Kc and Km, press the F2 key tocalculate and to display the valve sizing information.

e. After recalculating the sizing information for each flow condition, press the

F9 key to display a table of sizing information.

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3. Valve Size Selection

a. Refer to the minimum, normal, and maximum Cv's that are displayed onthe screen. Then, refer to the appropriate page of Fisher Catalog 10, and

select a valve size that will satisfy the Cv requirements. Note the actualvalue of Km for the valve size that is selected.

b. Final calculations. For each flow condition, change the value of Km to theactual value of Km that was determined in the step immediately above,and recalculate the valve sizing information. Press the F9 key to display a

table of values and note the calculated Cv requirements. Refer to theappropriate page of Fisher Catalog 10, and verify that the selected valve

will provide the needed capacity.

c. Refer to the appropriate specification bulletin, and verify that the pressure

drop and temperature ratings of the selected trim are suitable for theservice conditions.

Specify The Selected Valve On The Saudi Aramco ISS

Referring to the sizing information on the computer screen and to the valve specifications that are listedin the appropriate valve specification bulletin, enter the specifications of the selected valve and trim onthe Saudi Aramco ISS.

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GLOSSARY

DPcav The pressure drop at which a particular valve will become susceptible to

cavitation damage.

application ratio (A r ) The ratio of the system pressure drop to the pressure differentialbetween P1 and Pv that is used to provide an index of the susceptibilityof a system to cavitate.

Ar See application ratio.

aspiration The intentional injection of air or gas into a fluid stream for the purposeof minimizing cavitation damage.

backpressure The fluid pressure that exists downstream of a control valve.

cavitation In liquid service, the noisy and potentially damaging phenomenon thataccompanies vapor bubble formation and collapse in the flowstream.Cavitation is most commonly encountered in high pressure and highpressure drop services.

Cavitrol A registered trademark of Fisher Controls that applies to anti-cavitationtrims.

erosion The damaging effects of flashing or abrasive media impinging oncomponent surfaces. Erosion may be forestalled with hardenedmaterials or with valve designs that separate the flowstream from criticalvalve components.

flashing Phenomenon observed in liquid service when the pressure of the fluidfalls below its vapor pressure and when it does not recover to a pressureabove the vapor pressure.

high-recovery valve A valve design that, due to streamlined internal contours and minimalflow turbulence, dissipates relatively little flow-stream energy.

hydrodynamic noise The noise that is associated with cavitation. It sounds like gravel flowingthrough the valve and associated piping.

incipient cavitation The onset of cavitation, observed when the first vapor cavities begin toform in the liquid stream.

KcControl valve damage index that is used to describe a control valve'srelative susceptibility (due to its pressure recovery characteristics and itsmaterials of construction) to cavitation damage.

Km The pressure recovery coefficient for a control valve. Km is determined

by valve manufacturers and published in sizing catalogs. Km is used to

calculate the DPallow (choked flow pressure drop) for valve sizing

purposes. The value of Km may also be used to predict cavitation

damage.

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low-recovery valve A valve design that dissipates, due to the turbulence that is created bythe contours of the flow path, a considerable amount of flowstreamenergy.

microjets Microscopic, high velocity fluid streams produced as a result of vapor bubble collapse in cavitating liquids.

Pv The vapor pressure of a fluid.

Pvc Pressure at the vena contracta.

rebound The successive collapse, regrowth, and collapse of vapor bubbles in acavitating liquid.

recovery A relative term that describes the difference in pressure between thevalve vena contracta and the downstream system.

trim, anti-cavitation Trim that is specifically designed to eliminate or reduce cavitation andcavitation damage in a control valve. Common designs stage the totalpressure drop across one or several specially designed restrictions.

vapor pressure (P v) The pressure at which a given liquid begins to vaporize, given a constanttemperature.

vaporization The process by which a fluid changes state from a liquid to a vapor.Vaporization may be caused by increasing the temperature of the fluid or by reducing the pressure of the fluid.

vena contracta The location where the cross-sectional area of the flowstream is at its

minimum size, where fluid velocity is at its maximum value, and wherelocal fluid pressure is at its lowest value. The vena contracta normallyoccurs downstream of the actual physical restriction in a control valve.

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