Liquid Sizing.pdf

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LIQUID SIZING 0705

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LIQUID SIZING 0705

Introduction The company was formed in 1967 under the name of Introl Ltd. Its object was to provide a specialised control valve service for the rapidly expanding Energy Industries (Petroleum, Gas, Electricity) and for the ever-changing ever-changing Chemical Industry. The company very quickly achieved a reputation throughout these industries for high quality control valves of the conventional type and particularly for purpose designed high technology control valves. Introl have founded on a concept of in house design capability. Designs and prototypes have always been developed within the company and this remains an essential element of the company’s present day policy.  Accord  According ingly ly a larg large e devel develop opmen mentt and and desig design n depa departm rtmen entt is staff staffed ed by by quali qualifie fied d engin enginee eers rs who who are are avail availabl able e for customer consultation on problem applications. Kent Introl has always recognised the importance of maintaining high standards of quality and was the first control valve company to be awarded the British Standards approval of quality control systems — BS5750 Part 1 by the British Standards Institution in 1986. This was supplemented by approval of the systems to ISO 9001. Retention of these certifications requires continual maintenance of all Quality Assurance Systems to the satisfaction of the British Standards Audit Authority.

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LIQUID SIZING 0705

Scope of Manual The automatic control of modern processing plants relies heavily on the control valve as the final control element. These control valves may be required to operate continuously or intermittently to regulate process parameters such as flow rate, pressure level, temperature, etc. The introduction of computer technology within the industry and the demand for designs capable of handling a wider range of process requirements has necessitated a higher level of accuracy in the sizing and selection of these critical elements. The methods of control valve sizing and sound pressure level prediction for liquid and compressible fluids have previously been discussed in Introl Engineering Reports EN12 and EN9b respectively. This technical selection manual has been produced to provide a document incorporating all relevant aspects of valve sizing and selection, including revisions and additions e.g. multi-phase fluid sizing. In addition to sizing and sound pressure level calculation procedures, this manual provides information required during the specification of a control valve for a particular application including including selection guidelines, and material considerations.

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LIQUID SIZING 0705

Contents Contents ______  _________ ______ ______ ______ ______ ______ ______ ______ ______ ______ ______ ______ ______ ______ ______ ______ ______ ______ ______4 ___4

TS20 Control Valve Selection for Incompressible Incompressible Fluid Flows _____________________ _____________________4 4 TS20.1 - Nomenclature _________________________________________________________________ 5 TS20.2 - Liquid Flow Valve Sizing Procedure _______________________________________________ 6 TS20.3 - Process/Application data requirements ______________________________________________ 7

TS21 Liquid Sizing __________________________________ ___________________________________________________ ______________________ _____8 8 TS21.1 - Liquid Flows__________________________________________________________________ 8 TS21.1.1 TS21.1.1 - Introductio Introduction n ___________________ ____________________________ ___________________ ___________________ ___________________ ______________ ____ 8 TS21.1.2 - Flow Path Through a Control Valve __________________________________________ 9 TS21.1.3 - Flow Regimes - Normal, Semi-critical, critical _________________________________ 10 TS21.1.4 - Cavitation & Flashing ____________________________________________________ 11 TS21.1.6 - Viscous Flow___________________________________________________________ 13 TS21.1.7 - Pipework Configuration___________________________________________________ 14 TS21.2 - Valve Sizing Equations_________________________________________________________ 15 TS21.2.1 - Cavitation Index_________________________________________________________ 15 TS21.2.1 TS21.2.1 - Flashing Flashing Index ___________________ ____________________________ ___________________ ___________________ ___________________ ___________ _ 15 TS21.2.3 - Valve Flow Coefficient ___________________________________________________ 17 TS21.2.5 - Viscous Flow Correction _________________________________ _______________ ___________________________________ _________________ 19 TS21.2.7 - Pipework Correction Factor________________________________________________ 21 TS21.A TS21.A - Appendicie Appendiciess ___________________ ____________________________ ___________________ ____________________ ___________________ _________________ ________ 23 TS21.A.1 - Semi-critical Flow_______________________________________________________ 23 TS21.A.2 - Pressure Drop Considerations______________________________________________ 24 TS21.A2 - Contaminate Flow _______________________________________________________ 24

TS22 Liquid Velocity ________________________________ _________________________________________________ _____________________ ____25 25 TS22.1 - Factors Influencing Velocity Limitations _______________________________________ 25 TS22.2 - Velocity Calculation _________________________________ ________________ _________________________________ ______________________ ______ 26 TS22.4 - Flashing Flow _________________________________ _________________ _________________________________ ___________________________ __________ 28 TS22.5 - Procedure _______________________________________________________________ 29

TS23 Liquid Noise __________________________________ ____________________________________________________ _____________________ ___30 30 TS23.1 - Categories of Noise Vibration________________________________________________ 30 TS23.1 - Methods of Abating Liquid Liquid Generated Generated Noise __________________________________ _________________ ___________________ __ 31 TS23.3 - Liquid Noise Prediction ____________________________________________________ 32 TS23.4 - Procedure for Fixed Area Pressure Drop Stages __________________________________ _________________ _________________ 33 TS23.5 - Procedure for Tubotrol Valves _______________________________________________ 34

 Liquid  Liquid Sizing Sizing Exampl Examplee ______ _________ ______ ______ ______ ______ ______ ______ ______ ______ ______ ______ ______ ______ ______ ______ ____ _ 36

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LIQUID SIZING 0705

TS20 Control Valve Selection for Incompressible Fluid Flows

Selection of a control valve for an incompressible fluid (liquid) flow application involves a number of factors, which should be considered in a logical sequence. This section of the Technical Manual provides the information necessary to consider these factors, which include Cv calculation, fluid velocity and noise level prediction. It is important to note that omission of these aspects could lead to incorrect selection of a control valve for a particular application. The process and application information necessary to fully specify the size and type of valve required is detailed, together with a flow chart indicating the sequence of steps involved. The calculation includes consideration of the various flow regimes, together with the effects of processes such as cavitation or flashing. Additionally, where appropriate, techniques are detailed for evaluating the effects of both highly viscous fluids and pipework configuration on the calculated Cv value. To ensure correct selection of valve size and to maximise operational life, fluid velocity calculations and limitations are detailed for the various flow regimes.  Additi  Addition onall ally y in the the select selection ion of a contr control ol valve valve,, the pro proble blem m of envir environ onme menta ntall noise noise must must be take taken n into into accou account. nt. Therefore, a noise prediction technique forms part of the sizing and selection process.

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LIQUID SIZING 0705 Nomenclature Unit

Description

Imp

Metric

Cv  C V VISC 

U.S. units U.S. units U.S. units S.I. units p.s.i inches inches

U.S. units U.S. units U.S. units S.I. units Bar mm mm

-

-

-

-

-

-

-

-

 ∆Pvr limit

Valve Flow Coefficient Viscous Flow Coefficient Valve/Reducer Flow Coefficient Valve Flow Coefficient Cavitation Index Valve Pressure Recovery Factor Valve/Reducer Pressure Recovery Factor Valve bore size Pipe bore Size Pipe/Reducer Correction Factor Coefficient of Incipient Cavitation Inlet Head Loss Inlet Loss Coefficient Outlet Loss Coefficient Coefficient of Cavitation (1.1 K1) Valve/Trim Style Correction Factor Valve Reynolds number Number of Pressure Drop Stages Viscous Correction Factor Upstream Pressure Downstream Pressure Thermodynamic Thermodynamic Critical Pressure Vapour Pressure of Fluid (at flowing temperature) temperature) Supercooled Supercooled Vapour Pressure Pressure Drop Across Valve Sizing Pressure Drop Limiting Pressure Drop for Critical Flow Limiting Pressure Drop

T1 Q W G v

across Valve / Reducer Inlet Temperature Temperature Volume Flow Rate Mass Flow Rate Specific Gravity Fluid Velocity

SPL B HL T Z1  Ap

Sound pressure level Liquid noise efficiency term Liquid noise trim style correction Liquid noise valve opening reduction Liquid noise bulk flow factor Pipe attenuation

Cvr  Kv

CI Cf  Cfr  d D F Ki Kin K1 K2 KS NT NR n VK P1 P2

 ∆ P  ∆ Ps  ∆Plimit

-

p.s.i.a p.s.i.a p.s.i.a p.s.i.a

BarA BarA BarA BarA

p.s.i.a p.s.i. p.s.i. p.s.i.

BarA BarA Bar Bar

p.s.i.

Bar

°F U.S. gall./min lb/hr

°C 3 m /hr kg/hr

-

-

ft/sec

m/sec

dBA

dBA

-

-

dB dB

dB dB

-

-

dB

dB

degrees

degrees

Greek Characters θ µ p ν 

* *

Pipe Reducer Angle Dynamic Viscosity Fluid Density Kinematic Viscosity

-3

centi-Poise (1x10 Ns/m )

3

lb/ft centi-Stokes (mm2/s)

* usually given in metric units

TS20.1

2

Page 6

kg/m3

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LIQUID SIZING 0705 Liquid Flow Valve Sizing Procedure The following flowchart details the overall sequence of steps used during the selection of a

control valve for a particular application. For individual consideration of liquid sizing, liquid velocity and liquid noise, reference should be made to Sections TS21, TS22 and TS23 respectively.

START

Select Trim Style*

Calculate Cavitation Index

N

Is cavitation Index Yes Determine the Valve Flow Coefficient

Select Design CV and Valve

Determine Cf Value at Valve Opening N

Re-calculate Cavitation Index and Valve Flow Coefficient

Calculate Pipework Correction

Is Design CV OK? Yes Calculate Flow Velocities

N Select Design CV & Valve Size

Is Velocity Yes Calculate Sound Pressure Level

N Is SPL OK

Select Different Trim Style

Yes END

TS20.2

* Pressure drop limit 50 Bar (725 psi) per stage

Page 7

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LIQUID SIZING 0705 Process/Application Data Requirements The information required to fully specify the size and type of valve for liquid service applications can be broken down into different categories. For valve sizing and selection, this information can be classified as essential, preferred or additional. The following chart categorises the information required into these three areas. The information presented here relates to valve selection only and for actuator selection refer to TS8O.

1

Quantity

2

Line Fluid

3

Flow Rate

Process Units

Flow Units -

Flow Condition

Max

4 5

Inlet Outlet

Pressures

6

P

7

Temp. at Inlet

8

Specific Gravity

9 10

Vapour Pressure

11

Critical Pressure

12

DP Actuator Sizing

13

Design Press./Temp.

14

Line Size In/Out/Sch.

15 16

Predicted SPL (dBA)

17

Calculated Cv

18

Viscosity

19

Valve Size

C.M.

20

Body Form

Design CV

21

Catalogue No.

22

End Conns. Style

Rating

23

Rated Press.

Temp.

24

Body Material

Tr i m

25

No of Seats

Design

26

Type

Rings

27

Char’s

Flow Dir 

28

Material

Trim

29

Type of Bonnet

30

Packing

31

Max. Leakage

32

Stem Dia

Lub. /Lub No Valve Duty

 Absolu  Absolute te minimu minimum m flow flow infor informat mation ion (essen (essentia tial) l) Information required for full analysis (preferred)  Additi  Additiona onall desi design gn inform informati ation on Full valve specification

TS20.3

Page 8

Temp Units Normal

Minimum

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LIQUID SIZING 0705

TS21

TS21.1

Liquid Sizing

Liquid Flows

This section covers the various factors to be considered when sizing and selecting a valve for liquid service applications. Procedures are detailed for determining the valve flow coefficient (Cv) along with the necessary corrections required to account for viscous effects and valve pipe reducer combinations. Additionally, calculation procedures are presented to ensure that cavitation and flow erosion are avoided.

TS21.1.1

Page 9

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LIQUID SIZING 0705 Flow Path Through a Control Valve The flow path through a control valve is highly complex, including regions of high turbulence, flow separation and impingement. To allow description of the behaviour of the fluid properties through a control valve, a greatly simplified sketch of the flow path through a control valve is presented in Figure 21.1. This figure presents the flow being directed under the plug and indicates areas within the valve, which are referenced in the subsequent discussion.  As the flow flow passes passes from from the the valve valve inlet to the the trim inlet inlet the static pressure reduces due to frictional and turning losses. Fluid approaching the trim contracts in a similar manner to that shown schematically in Figure 21.2. During this contraction the static pressure decreases and the fluid velocity increases as illustrated in Figure 21.3. Subsequently, as the flow passes through the minimum geometrical flow area the streamlines continue to contract, until at a point just downstream from the trim outlet the streamlines become parallel. This minimum flow area is referred to as the vena contracta. At contracta. At this this point point the minimu minimum m stat static ic pres pressur sure e and and maximum flow velocity occur. The pressure at the vena contracta in relation to the upstream pressure and the fluid vapour pressure is important in determining the flowrate through the valve. Downstream of the vena-contracta the flow area expands resulting in a reduction in flow velocity and an increase in static pressure. The amount of pressure recovery is a function of the valve trim style, and is quantified by the term the valve pressure recovery coefficient (Cf) where:-

C  f   =

Fig. 21.1 Idealised Flow Path Through a Control Valve

Fig. 21.2 Contraction of Streamlines

∆ p ∆ pvc

This factor is an important term in valve sizing particularly in reference to critical flow and the occurrence of cavitation or flashing, both of which are discussed later.

Fig. 21.3 Variation in Static Pressure and Velocity

TS21.1.3

Page 10

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LIQUID SIZING 0705  A liquid liquid flow can genera generally lly be treate treated d as being being incompressible if there is no vapour formation. However, vapour bubbles are produced if the local static pressure falls below the fluid vapour pressure. This occurrence is not uncommon in control valve flows and leads to changes in the behaviour of the flow. Different flow regimes, dependant upon the level of vapourisation, are used to describe the behaviour of the fluid as it passes through a control valve. Normal Flow

Critical Flow

Normal flow describes the case when the fluid is assumed to be incompressible (no vapour formation). Under this condition the volume flow rate is proportional to the square root of the pressure drop across the valve, see Figure 21.4.

Critical flow occurs when the pressure drop is increased beyond the semi-critical zone, refer to point 4 on Figure 21.4. At this stage the pressure at the vena contracta has reached its minimum value, referred to as the “supercooled” vapour pressure, see Figure 21.5. Beyond this point, as the downstream pressure is reduced no further change in flow rate occurs. Also any subsequent increase in pressure drop results only in greater levels of cavitation or flashing.

Semi-critical Flow  When the static pressure at the vena contracta (the minimum flow area) falls just below the fluid vapour pressure, bubbles form and the flow can no longr be assumed to be incompressible. This represents the start of semi-critical flow and corresponds to the break down in the relationship between flow rate and pressure drop shown in Figure 21.4. The onset of the semi-critical zone also coincides with the occurrence of incipient cavitation when the vena contracta static pressure is just lower than the fluid vapour pressure. In the semi-critical flow regime any subsequent reduction in downstream pressure leads to increased levels of cavitation and reduced rate of increase in flow rate as indicated by the curve between points 2 and 4 in Fig 21.4.

Fig. 21.4 Different Flow Regimes as a Function of Root of Pressure Drop

TS21.1.4

Page 11

Most valve sizing techniques omit the semi-critical flow regime and assume that normal flow occurs up to point 3 of Figure 21.4 and that critical flow occurs thereafter. This omission greatly simplifies the calculation procedure, and generally results in errors less than 2% in the Cv calculation. A calculation procedure for the semi-critical flow regime is presented presented in Appendix 21 .A. 1.

Fig. 21.5 Static Pressure Variation for Different Flow Regimes

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LIQUID SIZING 0705 used on flashing applications. To aid the engineer in selecting the correct trim and material combination, a flash index presented in Table 21.2.  As previous previously ly detailed, detailed, vapour vapour bubble bubbles s are generate generated d within the liquid if the local static pressure falls below the fluid vapour pressure. This subsequently results in the phenomena known as either cavitation or flashing. In addition to the effect these have on valve sizing, structural damage to the valve or adjacent pipework may occur. In order to accurately size the valve and minimize the effects of flashing and cavitation, consideration of these phenomena is essential. Flashing If the downstream static pressure remains below the fluid vapour pressure, see Figure 21.5, then these vapour bubbles will remain in the downstream flow, and the process is referred to as flashing. Incorrect selection of valve trim style d/or materials for these applications could result in serious erosion damage to valve trim and possibly to the valve body. The characteristic of flashing damage is that of a smooth polished appearance as shown in Figure 21.6.  As previo previousl usly y detaile detailed, d, correct correct selec selectio tion n of a valve/ valve/tri trim m style is fundamental for valves on flashing duty. High pressure recovery valves are generally considered to be more susceptible to flashing erosion damage than low pressure recovery designs. The cage guided design of trim is used extensively on high duty flashing applications whereby the flow is directed over the plug

Fig. 21.7 High Pressure Drop Trim Design used on Flashing Flow Cavitation In the event that the pressure recovery is sufficient to raise the static pressure above the vapour pressure, see Figures 21.5, then the vapour bubbles will collapse, this process being known as cavitation. The onset of this phenomena is referred to as incipient cavitation, and occurs when the pressure drop from the valve inlet to the vena contracta is equal to

C  f  2 K i ( P 1 − P v ) where Ki. is the coefficient of incipient cavitation. Collapsing vapour bubbles release extremely high levels of energy and noise. If these bubbles implode in close proximity to a solid surface then the energy released tears away the material leaving a rough pitted surface as shown in Figure 21.8.

Fig. 21.6 Trim Erosion Due to Flashing to dissipate the energy within the confines of the trim. Experience has shown that in the case of flashing flows it is good practice to use a single stage of pressure letdown and utilise trim materials with good erosion resistance. Figure21.7 illustrates a high duty trim design

TS21.1.4

Page 12

Fig. 21.8 Typical Cavitation Damage to a Valve Trim

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LIQUID SIZING 0705 Solution for Cavitating Flows With regards to valves on potentially cavitating duties correct selection of valve/trim style and material combination is a requisite. High pressure recovery valves are inherently more susceptible to cavitation damage than low pressure recovery designs. A comparison between the variation of the static pressure through a valve, for both high and low recovery valves, is shown in Figure 21.9. This figure reveals the greater potential for the flow to cavitate with a high recovery trim design. Thus, the low pressure recovery cage guided design of trim is used to remove cavitation potential, whereby the flow is normally routed over the plug so that any cavitation, jet impingement and/or highly turbulent zones occur within the confines of the trim. In the event of high potentials of cavitation, usually associated with high pressure drops, further consideration to the trim design is required. The principle is to utilise a low pressure recovery design and to apportion the pressure drop over a number of stages, so that the static pressure within the valve does not fall below the liquid vapour pressure. This principle can be achieved in a variety of ways including the use of variable area stages of pressure letdown within the trim (HFD, HFT designs), fixed area stages of pressure letdown either within the valve or downstream elements and/or a combination of both. The variable area solution provides a higher performance than fixed area units and inherently have a much wider rangeability.

Fig. 21.9 Comparison between Low and High Recovery Valve Designs

The cavitation index (CI) see TS21.2.1, is used to indicate the potential of the flow to cavitate. When selecting a valve for a set of operating conditions the cavitation index should be brought to within acceptable limits by using the principles discussed previously. Thus, the first resort is to utilise single stage cage guided trims such as the ported or HF design. If these fail to eliminate cavitation, then the two stage HFD or three stage HFT should be considered. If these should fail to meet the criteria, then additional stages of pressure letdown can be incorporated either in the form of the Turbotrol valve presented in Figure 21.10 or fixed area stages.

Fig. 21.10 Seven Stage Turbotrol Valve

TS21.1.5

Page 13

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LIQUID SIZING 0705 The flow rate of a fluid through a valve is proportional to the square root of the pressure drop within the normal flow regime, assuming the flow to be turbulent. The factor that determines the turbulence level within the fluid is related to its viscosity and its effect requires consideration during the sizing of a control valve for liquid service. In considering the flow of a fluid through a valve there are two distinct groups of forces affecting the motion of the fluid particles. These are viscous forces, which are proportional to the fluid velocity (∝V), and inertia forces, which are proportional to the square 2 of the velocity (∝ V ), see Figure 21.11. The predominance of one of these forces over the other leads to two different types of flow. If the viscous forces dominate, the flow is termed laminar (or viscous), if the inertia forces dominate the flow is termed turbulent. The influence of these two flow types on control valve sizing should not be overlooked. If the viscous effects are ignored then gross undersizing of a control valve can occur. Fig 21.12 Diagramatic Representation of Laminar & Turbulent Flows.

Turbulent Flow

Fig. 21.11 Deviation from Normal Flow Relationship Due to Viscous Flow

Reynolds Number The occurrence of laminar or turbulent flow is indicated by the value of the ratio of inertia to viscous forces;

Laminar Flow (Viscous) Laminar flow generally occurs with fluids having high viscosities and/or low flow velocities. Under such conditions the movement of individual particles are along clearly defined lines (streamlines) , see Figure 21.12. There is no movement transverse to the streamlines, although particles in different streamlines may have different velocities. In the case of laminar flow in a pipe, the fluid layer in contact with the wall is at rest and the velocity of the various streamlines increase progressively as the pipe centre in reached. The resistance to the flow is due to viscous shear forces between adjacent layers of fluid.

TS21.2.1

Turbulent flow occurs at relatively high velocities and with fluids having low viscosities. It is characterised by the mixing of fluid particles between adjacent layers or streamlines, the particles gaining or losing momentum in the process. The particles thus have velocity components transverse to the streamlines as well as along the streamlines. The inertia forces are too large for the viscous forces to restrain the particles motion.

Page 14

Re =

 Inertia Viscousfor ces

=

 ρν d  d  µ 

This ratio is referred to as the Reynolds number, after Osbourne Reynolds who first demonstrated this effect.This relationship has been applied to the valve flow problem, and a modified form of the Reynolds number, applicable to control valve terminology, is used as the basis of determining whether the flow through a valve is laminar or turbulent. Thus, the basic valve flow sizing equations apply to turbulent flow and a correction factor is then introduced if viscous forces are dominant. The procedure used for this calculation is presented in TS 21.2.4.

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LIQUID SIZING 0705  A contro controll valve valve presents presents a single single componen componentt within within a piping system. In determining the function of the valve, the overall piping arrangement should be considered. The pressure loss across the different piping components such as adjacent isolating valves, elbows and tees can usually be determined for different flow conditions by utilising head loss coefficients. However, certain components not only introduce pressure loss but will also effect the capacity of any adjacent valve due to changes in the velocity pressure head (dynamic pressure). This will tend to have a greater affect on the performance of the valve under choked flow conditions. The piping components most likely to cause this are reducers and expanders, see Figures 21.13 and 21.14.

Fig. 21.13 Line Size Valve

 Althou  Although gh it would would be possib possible le to test test valves valves and adjacent fittings, to determine the correction factors, it is more practical to estimate them. A calculation of the correction can be made by assuming that the reducer and expander result in a sudden contraction and sudden enlargement in series. The pressure drop across these can then be determined by using the following expression. 2

 ∆p=k   ρv  p=k 

(where k is the head loss coefficient.)

The head loss can be incorporated into the valve sizing formula by means of a pipework correction factor. Under critical flow conditions the effect of the contraction on the pressure recovery coefficient also becomes important.The pressure recovery will be modified by the reducer and expander, and consequently, a combined pressure recovery coefficient is required. This is used in the calculation of the limiting pressure drop which is used to determine whether choked flow occurs due to vapourisation of the fluid. A procedure for determining the effect of the pipework is detailed in TS 21.2.7.

TS21.2.1

Page 15

Fig. 21.14 Valve fitted with Pipe Line Reducers

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LIQUID SIZING 0705 TS21.2

Valve Sizing Equations

The sizing procedures detailed in the following sections are dependant on the valve trim style. In these procedures an engineer would generally start with the standard trim design (contoured) and depending upon pressure drop, cavitation, flashing, or sound pressure level a higher duty trim design may be selected. The starting point for the trim selection is the determination of the cavitation index or flashing index, both of which are detailed here. In the case of high pressure drop applications refer to Appendix 21 .A.2 for a guide to the trim design and material selection.

The cavitation index (CI) indicates the potential of the flow to cavitate under a certain set of operating conditions. CI should be used to select select a trim style style that will eliminate any potential of the flow to cavitate. A full explanation of cavitation, the cavitation index and methods to remove/ reduce cavitation has been given above.

For a single stage of pressure letdown (i.e. microspline, contoured, ported, or HF trim) use the equation below to calculate the cavitation index. Read the values of Cf and K1 for the trim style selected from Table 21.2.

C  I  = ∆ p − C  f  2 K i ( P 1 − P v )

If the cavitation index is still positive when specifying an HFT trim then either fixed area restrictions or a turbotrol valve should be considered. When determining the cavitation performance of either a turbotrol or fixed area devices then each pressure letdown stage should be evaluated separately using equation 21-01. Table 21-1 Allowable Levels of Cavitation Material Cavitation Index (CI) Single stage Multi-stage psi bar psi bar 316L 5 0.3 3 0.2 17.4 PH 8 0.5 5 0.3 Full stellite grade 6 20 1.4 10 0.7 Full stellite grade 12 26 1.8 12 0.8 Monel 8 0.5 5 0.3 Ferralium 10 0.7 8 0.5

In the selection of a valve for a flashing application the trim style and material should be chosen to eliminate! reduce erosion potential. In determining the correct solution the influence of both the valve pressure drop and percentage flash should be accounted for. A guide to this selection can be obtained by using the Flash Index presented in Figure 21.15a/b. The Flash Index combines the effects of pressure drop and % flash and indicates the appropriate trim design, trim material and overlay.

21.01

Note: - when the valve opening has been determined the corrected value of Cf (see Figure 21.16) should be used in the above formula.  A negati negative ve value value of of CI indicates no cavitation, whereas, a positive value indicates possible cavitation damage; the higher the value the greater the potential of cavitation damage. The allowable level of the cavitation index is a function of the trim design and material. For single stage trims CI values up to the levels shown in Table 21.1 the valve should operate satisfactorily without any significant wear. If the value of CI is higher than these limits then a higher class of anti-cavitation trim such as the HFD or HFT should be used. When selecting multi-stage trims it is good practice to eliminate all cavitation. The equation below is used for these trim styles (where n is the number of pressure letdown stages in the trim).

C  I  =

∆ P  n

− C  f  2 K i ( P 2 +

∆ p n

− P v )  

Fig. 21.15a Flash Index. Contoured Trim

21.02 Fig. 21.15b Flash Index. HF Trim

TS21.2.1

Page 16

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LIQUID SIZING 0705 Table 21.2 Valve Pressure Recovery and Incipient Cavitation Coefficients Valve Type

Series 10

Trim Style

Trim Size

Flow Direction

Cf 

K1

Microspline

All sizes

Over

0.95

0.95

Full

Under Over

0.9 0.85

0.8 0.81

Reduced

Under Over

0.9 0.8

0.8 0.82

Ported

All sizes

Over or under

0.93

0.9

HF, HFD, HFT

All sizes

Over or under

1

0.95

Under Over Under Over

0.9 0.85 0.9 0.8

0.8 0.81 0.8 0.82

Contoured

Full Contoured Reduced

Series 14 Ported

All sizes

Over or under

0.93

0.9

HF

All sizes

Over or under

1

0.95

Ported

Full

Over or under

0.92

0.9

HF

All sizes

Over or under

0.97

0.95

XHF

All sizes

Over or under

0.98

0.95

HFD

All sizes

Over or under

0.99

0.95

XHFD,HFT,XHFT

All sizes

Over or under

0.97

0.95

Contoured

Full Reduced

Over and under

0.9 0.8

0.87 0.84

HF, HFD, HFT

All sizes

Over and under

1

0.95

‘V’ Port

All sizes

Mixing and diverting

0.91

0.9

4 Stage

All sizes

Over

1*

0.95*

7 Stage

All sizes

Over

1*

0.95*

Vane