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CONTROL VALVE SIZING
(GAS AND LIQUID SERVICE)
Reference: GPSA Engineering Databook, 11th Edition
Gas Service (Volumetric Flow):
Qg= Cg* ((520 / G*T)0.5
)*P1*Sin [(3417 / C1)*((P / P1)0.5)]DEGAlternativelyCg= Qg/ ((520 / G*T)
0.5)*P1*Sin [(3417 / C1)*((P / P1)0.5)]DEG
Gas Service (Mass Flow):
Wg= 1.1*Cg*((1*P1)0.5)*Sin [(3417 / C1)*((P / P1)0.5)]DEGAlternatively
Cg= Wg/ 1.1*Cg*((1*P1)0.5)*Sin [(3417 / C1)*((P / P1)0.5)]DEGwhere
Qg= Volumetric Gas Flow Rate, ft / hr@ 14.7 psia, 60F (SCFH)Wg= Mass Flow Rate, lb/hr.
Cg= Gas Sizing Coefficient
G = specific gravity of gas (air = 1.0)
T = absolute temperature of gas at inlet, R
P1= absolute pressure of gas at inlet of valve, psia
C1 = Cg/Cv Ratio of Gas Sizing Coefficient to Liquid Sizing Coefficient.
DP = required pressure drop across control valve, psi
r1= density at valve inlet pressure & temperature, lb/ft
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CONTROL VALVE SIZING
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Typical C1& Cgvalues for valves in gas service:
C1 Cg1
C1 Cg1
1 32.7 270 31.9 528 Notes:
1 1/2 33.0 578 32.4 10202 33.2 851 32.0 2050
2 1/2 32.5 1600 32.0 2830
3 32.9 2150 31.5 3970
4 33.8 4100 33.2 6260
6 35.3 8200 34.4 13000
8 34.0 8300 36.6 18100
1 31.3 363 - -
1 1/2 28.9 1050 - -
2 30.3 1620 - -
3 27.9 3180 - -
4 28.1 5010 - -
6 28.4 8960 - -8 29.5 15100 - -
10 27.8 26900 - -
12 28.2 46300 - -
2 25.2 965 - -
2 1/2 25.2 1770 - -
3 25.2 3120 - -
4 25.2 6400 - -
5 25.2 10800 - -
6 24.5 17500 - -
8 24.5 31100 - -
10 24.5 48500 - -
12 24.5 75300 - -14 24.5 89000 - -
16 24.5 124000 - -
Linear Flow Characteristic:
Applications of Linear Flow Characteristic Control Valves:
Liquid level control & for certain flow control applications requiring constant gain.
Ball
Butterfly
1. At approximately 70% of valvetravel.
2. C1& Cgvalues vary with
percentage of valve travel.
3. For a more detailed analysis
of capacity of a given valve at
other percentages of travel
consult manufacturer's data.
The linear flow characteristic implies that the flow rate is directly proportional to the valve travel or opening.
This proportional relationship produces a characteristic with a constant slope so that with constant
pressure drop (delta P), the valve gain will be the same at all flows. Valve gain is the ratio of an
incremental change in flow rate to an incremental change in valve plug position. Gain is a function of
valve size & configuration, system operating conditions & valve plug characteristic).
Valve Style
Valve
Body
Size,
inches
Flow Characteristic
Equal Percentage Linear
Globe
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CONTROL VALVE SIZING
(GAS AND LIQUID SERVICE)
Selection Criteria for Linear Flow Characteristic Control Valve:
3. When 2 control valves are used in pairs as a 3-way valve.4. For compressor recycle (spill-back) applications.
5. For compressor anti-surge control valves.
6. For pump minimum safe continuous recirculation flow applications.
Equal-Percentage Flow Characteristic:
Applications of Equal-Percentage Flow Characteristic Control Valves:
Calculation steps for specifying gas service control valves:
1. Calculate the required Cgby using the sizing equation given above.
In this flow characteristic, equal increments of valve travel (opening) produce equal percentage changes in
the existing flow. The change in flow rate is always proportional to the flow rate just before the change in
position is made for a valve plug, disc or ball position. When the valve plug, disc or ball is near its seat &
the flow is small, the change in flow rate will be small; with a large flow, the change in flow rate will be
large.
These are generally used for pressure control applications. They are also used where a large percentage
of the total system pressure drop is normally absorbed by the system itself, with only a relatively small
percentage by the control valve. These are also recommended for applications where highly varying
pressure conditions can be expected.
2. Select a valve from the manufacturers catalog (see table above). The valve selected should have a Cgwhich equals or exceeds the calculated value.. The assumed C1value for the Cgcalculation must match
the C1value for the valve selected from the catalog.
1. When the ratio of differential pressure across the control valve at minimum flow over the differential
pressure across the control valve at maximum flow is equal to or less than 1.5.
2. When the differential pressure conditions across the control valve under all specified flow conditions
are more than two-third (2/3) of the differential pressure across the control valve in the closed condition.
7. For pressure reducing service where pressure drop is constant within +/- 15% over all specified flow
conditions.
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CONTROL VALVE SIZING
(GAS AND LIQUID SERVICE)
Liquid Service (Volumetric Flow):
QL= Cv* (P / G)0.5Alternatively
Cv= QL*(G / P)0.5
whereQL= Volumetric Liquid Flow Rate, gpmat inlet pressure & temperature
G = specific gravity of liquid (water = 1.0 @60F)
DP =
DPmax= Km( P1- rc*Pv)
where
Km = valve recovery coefficient (see table below for values)
rc= Critical pressure ratio = 0.96 - (0.28*(Pv/ Pc)0.5
)
Pv= Vapor Pressure of liquid at valve inlet temperature,psia
Pc= Absolute thermodynamic critical pressure, psia
Note: For flashing liquids the following equation should be used to determine the maximum allowable
pressure drop that is effective in producing flow.
If actual delta P is less than DPa,then the actual DP should be used in the equation above for calculating
flow or coefficient..
less than the maximum allowable pressure drop (DPmax)across control valve for sizing or
actual pressure drop, psi
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CONTROL VALVE SIZING
(GAS AND LIQUID SERVICE)
Typical Km& Cvvalues for valves in liquid service:
Km Cv1
Km Cv1
1 0.77 7.83 0.87 8.31 Notes:1 1/2 0.7 17.4 0.82 11.7
2 0.72 25.4 0.81 19.9
2 1/2 0.71 49.2 0.88 32.6
3 0.68 66 0.89 40.4
4 0.68 125 0.8 73.5
6 0.73 239 0.82 100
1 0.74 11.6 - -
1 1/2 0.72 36.3 - -
2 0.6 53.4 - -
3 0.67 114 - -
4 0.68 178 - -
6 0.68 316 0.73 2488 0.61 512 0.72 407
10 0.66 967 0.72 691
12 0.6 1640 0.71 1010
2 0.55 38.3 - -
2 1/2 0.55 70.4 - -
3 0.55 124 - -
4 0.55 254 - -
5 0.55 428 - -
6 0.55 713 - -
8 0.55 1270 - -
10 0.55 1980 - -
12 0.55 3070 - -
14 0.55 3630 - -
16 0.55 5070 - -
Calculation steps for specifying liquid service control valves:
Prepared by: Ankur Srivastava
Chemical Engineer
e-mail: [email protected]
Linear- Anti-
cavitation Trim
Globe
1. At approximately 70% of valve
travel.
2. Select a valve from the manufacturer's catalog, with a Cvequal to or greater than the calculated value.
1. Calculate the required Cvby using the sizing equation given above. The DP used in the equation must
be the actual valve pressure drop or DPmax, whichever is smaller.
Ball
Butterfly
Disclaimer : The information and methods included within this spreadsheet are presented for "cont
calculations. It is intended to be used by technically skilled persons at their own discretion. I do no
suitability or accuracy of these methods.
Valve Style
Valve
Body
Size,
inches
Flow CharacteristicEqual Percentage-
Standard
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CONTROL VALVE SIZING
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CONTROL VALVE SIZING
(GAS AND LIQUID SERVICE)
ol valve sizing"
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