GPSA Control Valve Sizing

CONTROL VALVE SIZING (GAS AND LIQUID SERVICE)

Page 1 of 6

Reference: GPSA Engineering Databook, 11th Edition

Alternatively

Alternatively

where

Gas Sizing Coefficient G = specific gravity of gas (air = 1.0)T =

Ratio of Gas Sizing Coefficient to Liquid Sizing Coefficient.

Gas Service (Volumetric Flow):

Qg = Cg * ((520 / G*T)0.5)*P1*Sin [(3417 / C1)*((DP / P1)0.5)]DEG

Cg = Qg / ((520 / G*T)0.5)*P1*Sin [(3417 / C1)*((DP / P1)0.5)]DEG

Gas Service (Mass Flow):

Wg = 1.1*Cg *((r1*P1)0.5)*Sin [(3417 / C1)*((DP / P1)0.5)]DEG

Cg = Wg / 1.1*Cg *((r1*P1)0.5)*Sin [(3417 / C1)*((DP / P1)0.5)]DEG

Qg = Volumetric Gas Flow Rate, ft3 / hr @ 14.7 psia, 60°F (SCFH)

Wg = Mass Flow Rate, lb/hr.

Cg =

absolute temperature of gas at inlet, °R

P1 = absolute pressure of gas at inlet of valve, psia

C1 = Cg/Cv DP = required pressure drop across control valve, psi

r1 = density at valve inlet pressure & temperature, lb/ft3


Page 2 of 6

Valve Style

Flow CharacteristicEqual Percentage Linear

Globe

1 32.7 270 31.9 528 Notes:1 1/2 33.0 578 32.4 1020

2 33.2 851 32.0 20502 1/2 32.5 1600 32.0 2830

3 32.9 2150 31.5 39704 33.8 4100 33.2 62606 35.3 8200 34.4 130008 34.0 8300 36.6 18100

Ball

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 - -

Butterfly

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 - -

Liquid level control & for certain flow control applications requiring constant gain.

Typical C1 & Cg values for valves in gas service:

Valve Body Size,

inches C1 Cg1 C1 Cg

1

1. At approximately 70% of valve travel.

2. C1 & Cg values 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.

Linear Flow Characteristic:

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).

Applications of Linear Flow Characteristic Control Valves:


Page 3 of 6

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.

Calculation steps for specifying gas service control valves:

Selection Criteria for Linear Flow Characteristic Control Valve:

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.

Equal-Percentage Flow Characteristic:

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.

Applications of Equal-Percentage Flow Characteristic Control Valves:

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.

1. Calculate the required Cg by using the sizing equation given above.

2. Select a valve from the manufacturers catalog (see table above). The valve selected should have a Cg which equals or exceeds the calculated value.. The assumed C1 value for the Cg calculation must match the C1 value for the valve selected from the catalog.


Page 4 of 6

Alternatively

where

G = specific gravity of liquid (water = 1.0 @60°F)

where Km = valve recovery coefficient (see table below for values)

Liquid Service (Volumetric Flow):

QL = Cv * (DP / G)0.5

Cv = QL*(G / DP)0.5

QL = Volumetric Liquid Flow Rate, gpm at inlet pressure & temperature

DP = less than the maximum allowable pressure drop (DPmax) across control valve for sizing or actual pressure drop, psi

Note: For flashing liquids the following equation should be used to determine the maximum allowable pressure drop that is effective in producing flow.

DPmax = Km ( P1 - rc*Pv)

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

If actual delta P is less than DPa, then the actual DP should be used in the equation above for calculating flow or coefficient..


Page 5 of 6

Valve Style

Flow Characteristic

Globe

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 19.92 1/2 0.71 49.2 0.88 32.6

3 0.68 66 0.89 40.44 0.68 125 0.8 73.56 0.73 239 0.82 100

Ball

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 69112 0.6 1640 0.71 1010

Butterfly

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 SrivastavaChemical Engineere-mail:

Typical Km & Cv values for valves in liquid service:

Valve Body Size,

inches

Equal Percentage- Standard

Linear- Anti-cavitation Trim

Km Cv1 Km Cv

1

1. At approximately 70% of valve travel.

1. Calculate the required Cv by using the sizing equation given above. The DP used in the equation must be the actual valve pressure drop or DPmax, whichever is smaller.

2. Select a valve from the manufacturer's catalog, with a Cv equal to or greater than the calculated value.

[email protected]

Disclaimer: The information and methods included within this spreadsheet are presented for "control valve sizing" calculations. It is intended to be used by technically skilled persons at their own discretion. I do not warrant the suitability or accuracy of these methods.

mailto:[email protected]


Page 6 of 6

: The information and methods included within this spreadsheet are presented for "control valve sizing" calculations. It is intended to be used by technically skilled persons at their own discretion. I do not warrant the

Documents

GPSA Control Valve Sizing