control valve sizing. bauman

7/30/2019 Control Valve Sizing. Bauman

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Control Valve Sizing

1991, ALL RIGHTS RESERVED

BULLETIN NO. SM-8


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TABLE OF CONTENTSValve Sizing ................................................. Page 2Valve Sizing Equations ................................. Page 3Viscosity Correction ..................................... Page 4Small Flow Valve Sizing .............................. Page 4Cv Table Series 24000 Lfttle Scatty ............. Page 5Cv Table Series 24000s Stainless Steel......Pag e 6Cv Table Series 21000 Elastomer Lined.. .. ..Pag e 7Cv Table Series 25000 LO-T Rotary.. ......... .Page 7Cv Series 26000 Corrosion Resistant .......... Page 7Cv Series 86000 Corrosion Resistant.. ........ Page 7CV Series 41000 Rotoglobe ......................... Page 8Incipient Cavitation ....................................... Page 8Damage Cavitation ................................... ..Pag e 8Avoidance of Cavitation ............................... Page 9Aerodynamic Noise Estrmate ..................... Page 10Graphical Method .................................... Page 11

UMANN ASSOC, LTD.35 Mirona Road

Portsmouth, NM. 03801Phone: (603) 436-2044

Fax: (603) 436-4901


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VALVE SIZINGSelecting the right size control valve is very important. For example, a valve that is too small mightwaste pump horsepower. On the other hand, a valve too large may not be able to handle reducedflows. %Most valve manufacturers publish a so-called rated C, Factor for each size and type of control valve.This simply indicates the maximum flow in gallons per minute of water which would pass through thevalve at a pressure drop of 1 psi. To select the proper size valve. however, requires full knowledgeof actual flowing conditions and method of calculating the required C,.Valve sizing equations as standardized by the ISA (Standard S75.01) have vastly improved inaccuracy primarily through the addition of a Pressure Recovery Factor F, and a Piping GeometryFactor F,. both of which were originated by Hans D. Baumann of our company in 1962 and 1968.respectively (formerly known as C, and R).The following equations have been adapted from the ISA S75.01 Standard to allow for practical every-day use without significant sacrifice in accuracy. For more comprehensive sizing use our computersoftware VS3.Proceed by calculating the required valve C, from given flow data, having prior determined whetherthe flow is critical or sub-critical.Next, select a valve size (or trim size) from the capacity tables shown on pages 5. 6. and 7. Note, tfa Lo-T valve is installed between reducers, multiply rated C, by F,. The other valve types, havingrelatively lower capacity, do not require this correction. However, if the required maximum C, is morethan 80% of the rated C,, or C, x F,, for a given valve size, then choose a next larger valve or trim.For example, for a given flow rate of 5670 Ibs/hr sat. steam at 250 psia inlet and 150 psia outlet, a C,of 13.5 is calculated. A l-1 14 Little Scatty with V-port trim has a maximum C, of 15. However, 13.5is more than 80% of 15. therefore, a l-l /2 size valve with a rated C, of 23 is recommended. Also.do not select a conventional globe valve or metal seated butterfly valve if the minimum required C, fallsat less than 5% of valve travel (see C, Tables).If no pressure drop is preselected. use 5 psi or 5% to 10% of maximum pump discharge pressure forsizing purposes.For additional information refer to Controol Valve Primer-4 Users Guide by Dr. Hans D. Baumann,a book published by the Instrument Society of America.VALVE CHARACTERISTICThe purpose of a valve flow characteristic is to linearize the installed relationship between actuatorsignal change and change of flow rate through the valve as much as possible. This ideally assuresa constant gain of the control valve.A Unear Characteristic does this only if the pressure drop across the valve is fairly constant,The Equal Percentage or Modified Equal Percentage Characteristic, on the other hand, doescompensate for varialrons in pressure drop with flow rate and should therefore be preferred if the APvanes morefhan 2:l between min. and max. Row. Another reason why this characteristic is preferred,is the fact that such a plug is stil l substantially off the seat with small flow rates. For example. a 2 V-port plug at only 1.7% of max. C, is stil l at 10% of rated travel.

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FORMULAS FOR SIZING CONTROL VALVESFOR LIQUID SERVICE

Subcritical Flow Critical Flowwhen AP is less than when AP is more thanF, CAP,) or equal to F, (AP,)Volumetric Flow

GI c,=Q -FL J APs

Flow by WeiqhtC = W c, = W5OOpp / 500FL,&hP,

FOR GAS AND STEAM SERVICESubcritical Flow Critical Flowwhen AP is less when AP is morethan F: (P42) than FL2 PJ2)

Volumetric Flow

G= &4X G= e

Flow by Weightc, = W * c,= W3.22,f&P(P,+p;IG, 2*0F,P,&

For Saturated SteamC = W c, = W2.1JdP(P14P,) 1.83F,P,

For Suuerheated SteamC" = W( 1 + 0.0007 T*J c,= w~1+o.ooo7T,h)2.1JAP(P,+P,) 1.83F,.P,

NOTE: When rotary valves are installed betweenreducers use C,F, instead of C, (in capacitytables).

Where:C, = Valve coefficientC,, = Rated Cv of Valve Selectedd = Valve diameter (inch)Fd = Valve Style ModifierF, = Pressure Recovery FactorF, = Reducer Correction FactorF, = Viscosity Correction FactorG = Gas specific gravity (air =l .O)G, = Specific gravity @ flowingtemperaturePI = Upstream pressure, psiaP* = Downstream pressure, psiaAP = Pressure drop P,-P2, psiPC = Pressure at thermodynamic criticapoint, psia (water = 3206 psia)P, = Vapor pressure of liquid at flowingtemperature, psiaAP, = Maximum AP for sizing

Use: P,-P, when outlet pressure ishigher than vapor pressure.I -\

use: P+.96-0.2E++"when outlet pressure is equal tolower than vapor pressure.

0 = Gas flow rate at 14.7 psia and OFscfh9 = Liquid flow rate, U.S. gpmT = Flowing temp., R.(460+F)T,h = Steam superheat, OFW = Flow rate, pounds per hourWater =1 @ 60F

OTHER METHODS: c, = & ; c, x -%; XT= .84F,* ; FL = m; FL = C,/.Qr-3-


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VISCOSITY CORRECTIONWhere highly viscous fluids (usually above 40 cSt) or small flow conditions are encountered, it will benecessary to correct the calculated C, turbulent, by dividing C, turbulent by F,, to ensure sufficientcapacity of the selected valve.

C~..,hrlsitc fkrOr,-d RFor small flow valve trim where C, r~ 0.1, this has to be done, regardless of viscosity, even if the flowmedium is a gas.FOR UQUIDS FOR GASES

Then determine F,. using the lower of F, or Fd:Laminar FlowGm > O-5) F, = 0.776 JRey/ (c,/dZ)Laminar Flow(C, s 0.5) Fn = 0.026&Transitional Flow(AlI GA F,' =Do not use this equation for Re, values below 10.Where Re, (Valve Reynolds Number):

Use Cv, at least 2 times the calculatedCv turbulent. This may have to be in-creased later if calculations show se-lected CvR is too small.Where n = 1 +(89Od%&*) + log (Re,)n = 2 + log (Re,) for small flowvalves (C,,, < 0.5). NOTE: Minimum n = 2

FOR LIQUIDS FOR GASESRev= 17300 Fd qVpT-Ci

v - Kinematic Viscosity (centistokes), if vfscosity is in Centipoise (u), divide u by G, to obtain v.For gases use Y at atmospheric conditions. For other definitions, see page 3.F,, (Valve Style Modifier) and F, (Pressure Recovery Factor) For Standard Valves:84000/86000 F, = 0.35 F, = .80 V-Port & Splined Trim Fd = 0.7 F, = .98Globe Valves Fd = 0.46 F, = .90 Mikroseal C, 2 0.3 Fd = 0.2 F, = .85Butterfty Valves, F, = 0.30 F, = .7070' (Open) Tapered P?gz Ffi. 102) 2 1 i:ic : f 1::See Cv Tables For OthersExample: Bunker C Oil @ 1 oOF, P, = 100 psig, P, = 75 psig,G, = 0.99. q = 70 gpm, v = 1100 centistokes

Re = 17300 x .45 x70 = 101" 1100 J.9 x28

So, salact l-112 Globe valveve, parabolic F~ = 0.776 ,fitii/(28/1.5=)=0.63pfug,rated C, = 28, FL =0.9, Fd = 0.46C = 13 = 22.1"a- 0.63

(O.K. 79% of Rated CJd-


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Cv TableSERIES 24000 BRONZE LllTLE SCOlTY CONTROL VALVE

10 30 40 j 800_-,www60 i 1


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Cv Table24000s STAINLESS STEEL CONTROL VALVE

ERCENTAGE OF PLUG_TRAVEL+- 5 1~ 10 ! 20 30 40 -70~609 70 1 80 1 00 j TM.,, ~66~~6 VALVEALVE MODEL ., PSIZE NO.IIN.)

ORTDIA.IIN.)-

I11

LUG hECOVERY SlYLERAVEL RATED CV (FLOW TO OPEN) FACTOR MODIFIERIIN.) 1 (FL) (Fd

1246775: 375i--

-iG?x 375

1 pas j 813

I 2458% 150

iccc

0:

006 I009a 1301s08

1

046

0 080 12019uz-9 52

046

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SERIES 21000E~A~_T_OMER LINED BUlTERFLY VAtyE - _ ~-~~~~~~~~~~~,-~-~~~~~~:~~~::~. ;I0-~~~ . i 1 /3 . 05 + 2.5 1 8 : 19 39 64 ? 91 121 154 190 216 230 i .a65 79 025

4 12 6 16 28 56 97 132 180 232 292 I 354 j 435 .a5 77 0.71 6' - 25 T-125 35 69 139 229 327 441 566 707 1 836 975 85 77 j 0.2

-

6' i 15 32 a0 i 130 190 265 345____8 I 20 45 102 195 310 465 695

lo' T31 73 174 265 474 665 75012 i 45 105 750 6a0 960 1080-16' : 65 II 164 391

__!Xe t570 1060 1500._ _

I_ 16i5

(lbF~- Pressure recweqfactor(2\ KC - Coetf~cmt of inclptentcavdatm(3)Fp- Mult1plyva~erat~dC~byFp1pcompensatefor reducer losses

(D=lrnesire/d=valveslre)

--SE~I~S~26O~O~CORRO~lON RESlSTANTCONTROL-VALVEr~ PORT. j PLUG .____VALVE

,i%,MODEL ;TRAVEL;

PERCENTOFPLUGTRAVEL(flowloopen,NUMaER / (%I, j (INCH) j 5 10 20 30 40 50 60 70 a0 90 IOOFL F

i 26OOl I .312 : .50 ;.OOOG'XQLl~~ 00013~ooO24,ooO3650~0004 .OoOjS 0004 .OOOi ..0009

.00048~,00060; 0007 1 OOO8~lOO9I~l {226005 / 312 :; 26010~.312 ; 50 ;

XNl12 ;,0018.1.0025 ?.0033 ..004!00~;00007 .00015 mo4 .(X)08 m13 t ,002 : / .007!

iqoc&lo5 7- o09;mllm .'

i 76070 i 312 : SO I 0001 .0oa2~~06~~?008 .?017 0031 j .0048 Y ,007 1 ,010 j .013 j 02 1 1iQ~O50 I 312 50 j 0003 .0005 .OOl ,002 004 ; .008 1 ,012 ,017 : ,023 ! .033 ! 05

1' j 26100 j ,312 50 j 008

VALVE, MODEL PERCENTOF~UAT~RTRAVEL


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CAVITATION(Fig. 1 b) occurs with liquids only when high velocity reduces the static pressure inside a valvebelow the pressure level at which the liquid starts to boil and produce vapor bubbles. These vaporllapse whenever the downstream pressure is higher than the so-called vapor pressure (PJpressure waves. These implosions are responsible for a very high noise level and

great damage (under prolonged service) of the affected valve body or trim parts. Avoid the damagecavitation by not allowing the valve pressure drop to exceed K, x (P,-P,).

of the Lo-T rotary control valve is beneficial, since the teeth reduce the size of the vapor bubblesdeflect the implosions away from the vane. If the process permits, air injection can be applied withLo-T valve to greatly reduce cavitation noises.

process called flashing (Fig. 1~) will occur if the downstream pressure is equal to or less thanvapor pressure P,. This process is relatively harmless and there is no noise problem as long as highare avoided (connect outlet directly to flash tank or condenser) and as long as stainlessr chrome moly body and trim are employed.

INCIPIENT CAVITATIONin a valve is usually a gradual process beginning with only small portions of a plug or vaneng vapor bubbles. This so-called incipient cavitation, which is relatively harmless, will start

occur when the following pressure drop in a valve is exceeded:*pm - Kc O,-A

& = Coefficient of incipient cavitation vanes typically between 0.2 and 0.4.P, = Valve inlet pressure (psia)P, = Vapor Pressure of liquid at flowing temperature

(Water @ 70F = 0.38 psia)DAMAGE INDUCING CAVITATION

drops high enough to produce significant damage (and usually objectional noise levels) shouldavoided, parficularly over prolonged time periods. While there is no exact scientifically establishedit, the following rules may serve as a guide:AP damage = K, (P, - P,)

K,=K, x K,xFL3 P, = Inlet Pressure (psia)K, = (l/d).2s P, = Outlet Pressure (psia)6 = (loo/p,).~ P, = Vapor Pressure (psia)d = Valve Size, Inches


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MAX. INLET PRESSURE TO AVOID EXCEEDING 90 DBA CAVITATION NOISE OF LO-T VALVESMedium: Cold Water at atmosoheric downstream oressure.

Valve Size: 2 3 4 6 8 ,O, 12 16Max. P, (Psig) 150 105 65 45 42 32 25 20

NOTE: The above figures are based on the valve being approximately 2/3 open.NO CAVITATION CAVITATION FLASHING

1 pt I Pl I P%

t------ --

- B -B* - -t -----y ------I -t ----- \ ,_, ; -

1 I IFmJRF 18 FlGUAE lb FIGURE Ic

FULL CAVITATIONFull cavitation or choked flow will be reached if a critical pressure drop (AP,) is exceeded.

AP, = F, (P, - P,)Where: F, is the Pressure Recovery Factor.

For globe valves, full area trim:V-Port plug = 0.98Parabolic plug = 0.90Lo-T = 0.7 @ 100% travel (see page 7)

AVOIDANCE OF CAVITATIONIf possible, cavitation should be avoided by using valves having a higher F, factor, by reducing thepressure drop across the valve or by increasing the outlet pressure (elevation, etc.). Resistanceplates are sometimes applied to absorb some or most of the valve pressure drop when placeddownstream of the control valve. Another method is the application of two valves in series (separatedby at least six pipe diameters). The overall Kc or FL of such a combination is the square root of eachindividual F, factor.The F, required to avoid damage producing cavitation must be more than:

For example:One 8 Lo-T valve near wide open has an F, factor of 0.72. Two valves in series have a combined F,of m= .84. thereby increasing the allowable AP by about 58%! Note: It is important that both valvesbe identical. Assuming an inlet pressure of 115 psia. the two valve K,, = (118)


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AERODYNAMIC VALVE NOISE PREDICTIONThe calculation of the aerodynamic noise level for control valves is based on methods establishedby Hans D. Baumann which are concurrent with ISA Standard 75.17 (1989). They are furthermodified to suit the particular low noise characteristics of the Lo-T line. The method has been kverified by air and steam tests on selective sizes and it is believed that the calculations yield typicalaccuracies of within + 3 dBA. However, the somewhat simplified equations shown below areaccurate for globe valves only if the actual C, is less than 65% of maximum rated C, or less than9 C, per valve diameter squared, and at valve outlet velocities of less than 0.1 Mach. For higherMach numbers see correction below.A pocket calculator solvable calculation method is shown below. A simplified graphical methodfor field use and quick estimating purposes for the Lo-T valve is shown on page 11.SPL = 48 + 10 log(C, F, P, P,) + 5 log(D) - 20 log&,) - 20 log(D/d) + n,,, + 20 log(F,) + F. + L, indBA @ 1 m from outer pipe wall.Where: C, = Flow coefficient under actual conditions

FL = Pressure Recovery CoefficientP, = Inlet pressure, psiaP2 = Outlet pressure, psiaD = External pipe diameter (inch)d = Valve Size (inch)t = Pipe wall thickness (inch)Lo = Pipe wall thickness Schedule 40 pipeF, = Valve style modifier (see Page 4)

For Lo-T Fd = 0.30 (2), 0.25 (3). 0.24 (4) 0.22 (6), 0.18 (8 - 16)For globe valves F, = 0.33 @ 65% of Rated C,.Otherwise F,, = [(e/2.4) + 6 I 1001; 4 = % of max. rated C,.

% = For globe valves: 25 log (P,/P,-1) below PJP, = 2.5. andI .75 + 15 log(P,/P,-1) above 2.5.%l = For Lo-T rotary valves: 12 log(P,/P, - 1.05)F, = Gas property correction factor = 10 log [(C,) x (G,)T - 122.

(C, = Speed of sound in gas (f/s). G, =Specific gravity of gas)Saturated Steam -2Superheated SteamNatural Gas 1;Air 0Helium +1

Mach Number Correction: L, = 26 log O.O23P, C,F, 1D2 P2 )I8aumann, H.D. 1987, A Method for PredictinJet Noise Theories ASME paper 87-WA/NC hAerodynamic Valve Noise Based on Modified Free-7.

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Lo-l AERODYNAMlC NOISE PREDICTION (air.gas.steam)PRESSURE RATIO P,/P2 [PSIA!

100

95

70

105: 1.2 1.5 2 3 4 5 6 7 a910

6 /

/ / P,: 115 PSIA I

PRESSURE RATIO P,/P2 IPSIAI

Calculate ratio PI/P,, follow P,/P2 line to valve size and read SPL at intersectionwith valve size.Above values based on 80% rated valve capacity (rated G). For flow at 40%,rated G. subtract 3 dBA.SPL based on Sched. 40 pipe. For Sched. 80, subtract 5 d0A.Note: If inlet pressure is half as shown, subtract 6 dBA.If inlet pressure is twice as shown, add 6 dBA.

110

a5

control valve sizing. bauman

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