GPSA Control Valve Sizing

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)]DEG Alternatively

Cg = 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)]DEG Alternatively

Cg = Wg / 1.1*Cg *(( 1*P1)0.5)*Sin [(3417 / C1)*((P / P1)0.5)]DEG where Qg =

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

Wg =

Mass Flow Rate, lb/hr.

Cg = G= T= P1 =

Gas Sizing Coefficient specific gravity of gas (air = 1.0) absolute temperature of gas at inlet, °R

C1 = Cg/Cv P = 1 =

Ratio of Gas Sizing Coefficient to Liquid Sizing Coefficient. required pressure drop across control valve, psi

absolute pressure of gas at inlet of valve, psia

density at valve inlet pressure & temperature, lb/ft3

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CONTROL VALVE SIZING (GAS AND LIQUID SERVICE) Typical C1 & Cg values for valves in gas service:

Valve Style

Globe

Ball

Butterfly

Valve Body Size, inches 1 1 1/2 2 2 1/2 3 4 6 8 1 1 1/2 2 3 4 6 8 10 12 2 2 1/2 3 4 5 6 8 10 12 14 16

Flow Characteristic Equal Percentage Linear C1

Cg1

C1

Cg1

32.7 33.0 33.2 32.5 32.9 33.8 35.3 34.0 31.3 28.9 30.3 27.9 28.1 28.4 29.5 27.8 28.2 25.2 25.2 25.2 25.2 25.2 24.5 24.5 24.5 24.5 24.5 24.5

270 578 851 1600 2150 4100 8200 8300 363 1050 1620 3180 5010 8960 15100 26900 46300 965 1770 3120 6400 10800 17500 31100 48500 75300 89000 124000

31.9 32.4 32.0 32.0 31.5 33.2 34.4 36.6 -

528 1020 2050 2830 3970 6260 13000 18100 -

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

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CONTROL VALVE SIZING (GAS AND LIQUID SERVICE) 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. 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. 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. Calculation steps for specifying gas service control valves: 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 C g which equals or exceeds the calculated value.. The assumed C 1 value for the Cg calculation must match the C1 value for the valve selected from the catalog.

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Liquid Service (Volumetric Flow): QL = Cv * (P / G)0.5 Alternatively

Cv = QL*(G / P)0.5 where QL = G= P =

Volumetric Liquid Flow Rate, gpm at inlet pressure & temperature specific gravity of liquid (water = 1.0 @60°F) less than the maximum allowable pressure drop (Pmax 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.

Pmax =

Km ( P1 - rc*Pv)

where Km = rc =

valve recovery coefficient (see table below for values) Critical pressure ratio = 0.96 - (0.28*(P v / 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 Pa, then the actual P should be used in the equation above for calculating flow or coefficient..

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CONTROL VALVE SIZING (GAS AND LIQUID SERVICE) Typical Km & Cv values for valves in liquid service:

Valve Style

Globe

Ball

Butterfly

Valve Body Size, inches 1 1 1/2 2 2 1/2 3 4 6 1 1 1/2 2 3 4 6 8 10 12 2 2 1/2 3 4 5 6 8 10 12 14 16

Flow Characteristic Equal PercentageLinear- AntiStandard cavitation Trim Km 0.77 0.7 0.72 0.71 0.68 0.68 0.73 0.74 0.72 0.6 0.67 0.68 0.68 0.61 0.66 0.6 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.55

Cv1 7.83 17.4 25.4 49.2 66 125 239 11.6 36.3 53.4 114 178 316 512 967 1640 38.3 70.4 124 254 428 713 1270 1980 3070 3630 5070

Km 0.87 0.82 0.81 0.88 0.89 0.8 0.82 0.73 0.72 0.72 0.71 -

Cv1 8.31 11.7 19.9 32.6 40.4 73.5 100 248 407 691 1010 -

Notes: 1. At approximately 70% of valve travel.

Calculation steps for specifying liquid service control valves: 1. Calculate the required Cv by using the sizing equation given above. The P used in the equation must be the actual valve pressure drop or Pmax, whichever is smaller. 2. Select a valve from the manufacturer's catalog, with a C v equal to or greater than the calculated value.

Prepared by: Ankur Srivastava Chemical Engineer e-mail: [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.

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ed for "control valve sizing" tion. I do not warrant the

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GPSA Control Valve Sizing

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