Sizing severe valve

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SELECTING AND SIZING

SEVERE SERVICECONTROL VALVES

BY JOHN WILSON

AS SEEN IN THE SUMMER 2004 ISSUE OF...


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42 | Valve M A G A Z I N E

Methods to select and size con-trol valves traditionally have

varied from one manufacturer tothe next, a situation that has givenrise to industry-accepted valveselection procedures. Two suchstandards are the calculation ofthe valve sizing coefficient, Cv, anda prediction technique for controlvalve noise. However, there hasbeen no accepted industry stan-dard developed to select a valve

trim for severe service applicationsthat experience cavitation, noise,or flashing.

Meanwhile, nearly all valvemanufacturers have developedtheir own experience-based guide-lines for selecting valve and trimcombinations for severe serviceuse, and some have published theirguidelines so that valve users canapply this methodology to theselection of valve trim, regardless

of the control valve supplier.One guideline is to place a limiton trim exit velocity or trim exitkinetic energy to protect againstdamaging conditions, be theynoise, cavitation, or flashing. Onthe surface, this velocity limitapproach appears simple and looksto be applicable to all types ofvalve trim. However, when oneexamines more closely the guide-lines underlying assumptions, it

SELECTING AND SIZING

SEVERE SERVICE

CONTROL VALVESCONSIDER USING THE GUIDELINES PROVIDED BY THEMANUFACTURER FOR THE VALVE BEING SPECIFIED RATHERTHAN A GENERAL, APPLIES-TO-ALL PROCEDURE.

BY JOHN WILSON

becomes apparent that this methodology may not produce accurate results andshould not necessarily be applied across the board.

According to the velocity limiting guideline, the trim exit kinetic energy shouldbe limited to a certain value. Equation 1 shows the calculation for kinetic energyexpressed as a function of density called kinetic energy density.

Equation 1 Vave2

KE 0____2gc

In Equation 1, KE is the trim exit kinetic energy, o is the density of the fluidat the trim exit, Vave is the average velocity at the trim exit and gc is agravitational constant.

There are several reasons why Equation 1 may not work on all types of controlvalves. First, this calculation presumably allows a single measure to handle bothincompressible and compressible flow by including density. In this case, density isusually calculated assuming there is subsonic flow at the trim exit, which is notlikely. The velocity is calculated assuming a constant velocity across the trim exitand by applying continuity and the known valve exit conditions. These twoassumptions are not entirely accurate since they yield the kinetic energy densityof the velocity averaged across an assumed ideal velocity profile.

A better measure of the exit flow energy is the average of the kinetic energy

density. This can be expressed as the actual kinetic energy density averagedacross the actual velocity profile. The trim exit density and velocity are notgenerally constant, which leads to an assumed value calculated from Equation 1.Equation 2 takes into account the true velocities and densities that can varyacross the trim outlet.

Equation 2 1 1KEave

_ ___ V2dSA s 2gc

2004 Valve Manufacturers Association. Reprinted with permission.


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By incorporating the surface inte-gral across the trim exit area, A, wecan take into account the differencesin velocity and density that can occur.If we assume that velocity and densityare constant, as is typically done in anintroductory physics course, we wouldsee that the kinetic energy calculationin Equation 2 equals that calculated inEquation 1. While it is easy to under-stand how this assumption is made, itis not what is actually occurring at thetrim exit.

Figure 1 shows a computerizedfluid dynamics (CFD) model of two dif-ferent types of noise-attenuating trims.In these models, the blue areas indi-cate areas of the lowest velocities,while red shows the areas of highestvelocities. Both CFD models have beenconducted with identical flow and inletand outlet pressures.

Figure 1 tells two different stories.The Trim Type A at left is a tortuouspath trim that relies on a series ofright-angle turns to accomplish thepressure reduction through the trim.This trim is normally selected using avelocity-based limit, but as the CFDshows, the trim exit velocities varydramatically across the trim exit area.This renders the assumptions made inEquation 1 inaccurate.

In contrast, Trim Type B at right isa multiple-path, multiple-stage trimthat has a much more uniform exitvelocity profile, which will yield com-parable results across the entire trimexit area. The exit profile of this trimbetter matches the assumption made inEquation 1.

To further illustrate the impact oftrue velocity on kinetic energy, we canexamine simple pipe flow models.These will be shown across one half ofa pipe diameter. In this case, we willcompare three different velocity pro-files with the same average velocity,equating to equal flow rates (Figure

Figure 2:Resultant velocity profiles across one-half pipe diameter

S u m m e r 2 0 0 4 | 43

Figure 1: CFD contours of Mach number for Trim Type A (left) and Trim Type B (right)

s Constant Velocity

s Laminar Velocity

s Turbulent Velocity

Velocity Profiles for Pipe Flow

Pipe Internal Radius, m

Velocity,m/s

25

20

15

10

5

0

0 0.05 0.1 0.15 0.2 0.25 0.3



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2). The types of velocity profiles thatwill be compared are shown in Equa-tions 3, 4 and 5.

Equation 3:

Constant Velocity Profile

V Vave

Equation 4:

Laminar Velocity Profile

V(r) 2Vave 1r

2

R

Equation 5:

Turbulent Velocity Profile

(1m)(2m)V(r) Vave 1

rm

2R

Again, note that the velocity profilesshown in Figure 2 equal the same aver-age velocity. However, we see a greatdeal of variance in the true velocitieswhen compared to an average velocity.This is because fully developed, incom-pressible pipe flow demonstrates theeffects of non-uniform velocity profiles.If these results are then carried over toEquation 1, we can expect to see avarying relationship to kinetic energy.Table 1 shows the results of the kineticenergy calculations for the differentvelocity profiles.

Looking at the laminar profileresults, we see that the Equation 1under-predicts the average kineticenergy density (Equation 2) by halfand the maximum kinetic energy by

four times. Again, this is due toassuming an average exit velocity anddensity profile at the trim exit.

It is starting to become clear thatthe simplified equations for calculatingkinetic energy density do not yieldaccurate results across the entire trimoutlet. As Figure 1 shows, applyingthis to all types of valve trim is not anentirely accurate representation of alltrims available in the market.

As mentioned earlier in this article,in order for Equation 1 to be applied,velocity had to be constant across theentire trim exit area and exit densityhad to be subsonic. For the velocity tobe subsonic, the density at the outlet ofthe trim has to equal that in the down-stream pipe. Figure 3 shows the Machnumber profiles for the trim typesdenoted in Figure 1.

Figure 3 explicitly shows that the

flow is supersonic at the trim outlet forTrim Type A. Trim Type B has muchlower exit velocities, but still has some


S E V E R E S E R V I C E C O N T R O L V A L V E S

Figure 3: Contours of trim exit Mach number for Trim Type A (left) and Trim Type B (right)

Table 1: Kinetic energy calculations for different velocity profiles

Profile Type Kinetic Energy Density of Average Kinetic Energy Maximum Kinetic Energy

the Average Velocity, kPa Density, kPa Density, kPa

(Equation 1) (Equation 2)

Laminar 50 100 200

Turbulent 50 53 75Constant 50 50 50



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S u m m e r 2 0 0 4 | 45

Figure 4:Velocity profile across trim exit for Trim Type A (left) and Trim Type B (right)

Figure 5: Density profile across trim exit for Trim Type A (left) and Trim Type B (right)

Figure 6: Kinetic energy density profile across trim exit for Trim Type A (left) and Trim Type B (right)

600

500

400

300

200

100

0

VelocityMagnit

ude(m/s)

Position (m)

0.0015 0.0020 0.0025 0.0030 0.0035 0.0040 0.0045 0.0050 0.0055 0.0060 0.0065

6.00e+02

5.00e+02

4.00e+02

3.00e+02

2.00e+02

1.00e+02

0.00e+02

VelocityMagnitude(m/s)

Position (m)

0.0730 0.0732 0.0734 0.0736 0.0738 0.0740 0.0742 0.0744

1.15e+00

1.10e+00

1.05e+00

1.00e+00

0.950e+00

0.900e+00

0.850e+00

0.800e+00

0.750e+00

0.700e+00

0.650e+00

Density(kg/m3)

Position (m)

0.0730 0.0732 0.0734 0.0736 0.0738 0.0740 0.0742 0.0744

1.20e+05

1.00e+05

8.00e+04

6.00e+04

4.00e+04

2.00e+04

0.0e+00

ke-density

Position (m)

0.0730 0.0732 0.0734 0.0736 0.0738 0.0740 0.0742 0.0744

1.150

1.100

1.050

1.000

0.950

0.900

0.850

0.800

0.750

0.700

0.650

Density(kg/m3)

Position (m)

0.0015 0.0020 0.0025 0.0030 0.0035 0.0040 0.0045 0.0050 0.0055 0.0060 0.0065

120000

100000

80000

60000

40000

20000

0

ke-density

Position (m)

0.0015 0.0020 0.0025 0.0030 0.0035 0.0040 0.0045 0.0050 0.0055 0.0060 0.0065



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level of supersonic flow. Since the flowis supersonic at the outlet, the otherassumption for Equation 1 has beenrendered inaccurate.

We have looked at a number of dif-ferent ways to express the outlet veloc-ity and density and the subsequentkinetic energy density. It is now impor-tant to break the models shown in Fig-ures 1 and 3 into hard data to showthe difference in the subsequent veloc-ity, density, and kinetic energy calcula-tions across the entire trim exit area.

Comparing the profiles in Figure 4and Figure 5 shows that the velocityand density profiles for Trim Type A

vary dramatically across the trim exit.These types of trim most commonlyuse exit velocity and exit kinetic

energy density as a criteria in valvetrim selection.

As noted, these calculations assumethat both velocity and density are con-stant across the trim exit. In actuality,Trim Type B maintains better uniformproperties at the trim exit. Similarresults are seen in Figure 6 when theseresults are combined to determine thekinetic energy density profile acrossthe trim exit.

As Table 2 shows, the kinetic energydensity of the average velocity is gen-erally not similar to the average of thekinetic energy density, and the latter isa better representation of the true

kinetic energy at the trim outlet.When selecting valve trim for a

severe service application, utilize the

guidelines provided by the valve manu-facturer for the valve being specifiedas opposed to using a general, applies-to-all procedure. The very fact thatstandards committees have not agreedupon a single selection method forsevere service trim validates the prac-tice of relying upon the manufacturersexperience and technologies. VM

JOHNWILSON is the Severe Service Business Man-

ager at Fisher Controls International,LLC

(www.fisher.com) in Marshalltown, IA. He

received his BS in Chemical Engineering from the

University of Nebraska-Lincoln. For the past five

years,he has worked extensively on severe service

applications focused on the power and hydrocar-

bon industries. Reach him at 641.754.2554 [email protected].


S E V E R E S E R V I C E C O N T R O L V A L V E S

Average Average Kinetic Maximum Average Kinetic

Exit Exit Energy Kinetic Energy Density,

Density, Velocity, Density, kPa, Energy Density, kPa,

kg/m3 m/s Equation 1-1 kPa Equation 1-2

Trim Type A 0.845 314 42 120 55

Trim Type B 0.813 427 74 83 74

Table 2: Kinetic energy density of average velocity


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