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Control Valve Operation and Design Criteria for Beginners 2Today, control valve can be sized easily with software, but in the reality 83.43% ofthe control valve are sized incorrectly. 76.1 % are too big, 5.13% are too small, 2.2%are both (hahaha). The reason is lack of understanding about vapour pressure (whichmakes valve cavitate and flash), fluid property (if the service is slurry, the slurry canblock the trim and every body thinks that the valve is undersize) ,If the valve is sized incorrectly and the valve needs to be changed, everybody willknow it. And it is hard to change it insitu, not including how to perform the seat leaktest when the valve had been changed.Instrument Society of America's method (ISA S75.01 "Control Valve SizingEquations) can be used for calculating the sizing coefficientAs per the Flow chart, Control valve sized by Calculating the flow coefficient (Cv)For liquid service, ISA's standard (ISA S75.0] "Control Valve Sizing Equations) ISused.Liquid Service with Turbulent liquid flow (including flashing service)

Check the criticalFlow

RegionCv without Reducer

.6.Ps = PI - (0.96 - 0.28 ~(Pv/Pc) Pv = PI - FFPyWhere:.6.Ps = critical pressure drop for cavitation (psi)PI = upstream pressurePv =vapor pressure of liquid at flowing temperaturePc = critical pressureFF= liquid critical pressure ratioIf P < CF-2 ( Ps), flow is sub-critical. If not, flow is critical.If reducers are provided, check critical flow with (Fu>-I Fp) 2 (Ps)instead of FI? ( Ps).Where:FL (Cl' ) = Pressure recovery factor (valve - reducer critical flowfactor)Fu' (CFR) = Combined pressure recovery factor and pipinggeometric factor (valve - reducer critical flow factor)P = PI + P 2

Sub-critical Flow Critical FlowCv = (NI ql FL) .,[ (O F /.6.Ps)v = NI q .,[ (OF /.6.P)

WhereCv = valve flow coefficient(in2)q = liquid flow rate(gpm)

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OF = specific gravity at flowingtemperatureL'1P= actual pressure dropPl- P2Cv = Cv (without reducers)/R Cv = FL I Fu) Cv (withoutReducer Correction reducers)

Metric EnglishN J 1.17 1.00

Example:Liquid with Specific gravity of 0.5 is flowing through a control valve at the rate of 33m3/h, inlet pressure of 33.1 kg/cm2G and outlet pressure 31.5 kg/cm2G. Calculate theflow coefficient assign vapour pressure of 20.2 kg/cm2A and critical pressure of 43.3kg/cm2AAssume single seat globe valve with reducer; CrR "" 0.8 , R"" 1, CF R I R ""0.84, Kc(Coefficient of incipient cavitation) "" 0.58P = (33.1 + 1.03) - (31.5 + 1.03) = 1.6 kg/cm2Ps=P1 - (0.96 - 0.28 J(Pv/Pc) Pv = (33.1+1.03)-( 0.96 - 0.28 )20'%3.3 ) 20.2

= 18.60 kg/cm2Flow is sub critical, since P = 1.6> (FLP- / Fp) 2 ( Ps) = (0.94) 2 (18.6) = ] 6.4 kg/cm2Cv = (1.17)(33) ~0.5/1.6 "" 17.06 without reducer correctionHence, Cv ""(20.4)1(0.96) = 17.77 with reducer correction

Liquid Service with Laminar liquid flowIf viscosity of upstream is higher than 1OcP, viscosity compensation is necessaryReynold numbers factor ( need to be included on calculation.Cv=(I.17q/FR) j"(Gr /6P)Rev= 76000q/ (V j" Cv )F R can be find from the table below

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Representat ive Sizing Coeff icients for Single-Ported Globe Style Valve BodiesV alv e S iz e Port DIa, Rotod{inch } Volv. Plug Style Flow Characteristic (in.} Trovel C v Ft x , Fn[in.)

_~_"~"._~~"~"~ "".L ~~ "~-~~ ~Of,{)~""I-" 0.6112 Post Gui,jed E q ua t P e rc ,- e nt ag e 038 24 1 0.90 0.54-",V4 Po!;.tG~lided Equ,1f PerCnt~ge 0.56 0.50 5[,2 0.84 0.61 0.61Micro Forrn " EquJ~ Perc-entnge : 1 1 8 :V 4 3W 089 0.66 072

1 1 2 3 1 4 4.91 0.93 0.80 0.673/4 3 1 4 8.84 om 092 062

Cage Gwded bne~H 15116 3 1 4 20.6 0.84 0.64 034Equat Pel'c~l'Ilage 16il6 3 . 1 4 17.2 0.88 067 U3BMicro-Form '.. Equ J! P -e n: :e n \. ) ge 3J B :{f4 32 0 084 Q,m) 0.72\l2 3.'4 5.18 0.91 071 0.671 1 12 3! 4 31 4 102 on 080 0.62CiJ9P. (;ulde

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At Nor Flow Rate < 85%At min Flow Rate >10% 5%

About the authorM Iqba l Matondang, when not jogging on hills or riding motorcycle, is an instrumentengineer that had been working in several oil and gas projects. He obtain his RSc.From Bandung Institute of Technology (ITB). Indonesia in 2006. He lives in Bandungwith his family.