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##### Liquid Control Valve Calculation

LIQUID CONTROL VALVE HELP

Liquid control valve calculations are used with the permission of ISA, the Instrumentation, Systems and Automation Society. The calculations are from ANSI/ISA-75.01-1985 (R1995), Flow Equations for Sizing Control Valves. © Copyright 1995, ISA - The Instrumentation, Systems, and Automation Society. For information, contact ISA Standards, www.isa.org

I used the calculations in sections 4 and 5 (pages 10 through 13) and Appendix E (pages 19 through 21). These calculations use different curves to estimate valve Cvs and to estimate flow rates when the control valve is in non-turbulent flow, a Reynold's number less than 40,000, in other words in the laminar and transient flow regimes. Note that the simplified calculations on page 23 of the standard use a third curve, the same curve used for prediction of valve pressure drop according to Appendix E. See the derivation of the equations in Appendix F, especially F1 (Reynolds number factor) on page 22 and compare with equations E2, E5 and E7 Reynolds number factors. These simplified calculations on page 23 will not match the calculations in Appendix E when in non-turbulent flow.

These calculations will allow preliminary sizing but in general the control valve vendor should take responsibility for sizing. Further Cv and Fd values should be taken from the vendor's catalogue. For preliminary sizing only the representative values below can be used.

REPRESENTATIVE VALUES OF VALVE CAPACITY FACTORS (Selected from Table D-1 of ANSI/ISA-75.01-1985)
 Valve Type Trim Type Flow Direction Xt Fl Fd Cv/dČ GLOBE Single Port Ported Plug Either 0.75 0.9 1.0 9.5 " Contoured Plug Open 0.72 0.9 1.0 11 " " Close 0.55 0.8 1.0 11 " Characterized Cage Open 0.75 0.9 1.0 14 " " Close 0.70 0.85 1.0 16 " Wing Guided Either 0.75 0.9 1.0 11 GLOBE Double Port Ported Plug Either 0.75 0.9 0.7 12.5 " Contoured Plug Either 0.70 0.85 0.7 13 " Wing Guided Either 0.75 0.9 0.7 14 GLOBE Rotary Eccentric Spherical Plug Open 0.61 0.85 1.0 12 " " Close 0.40 0.68 1.0 13.5 ANGLE Contoured Plug Open 0.72 0.9 1.0 17 " " Close 0.65 0.8 1.0 20 " Characterized Plug Open 0.65 0.85 1.0 12 " " Close 0.60 0.8 1.0 12 " Venturi Close 0.20 0.5 1.0 22 BALL Segmental Open 0.25 0.6 1.0 25 " Standard 80% Port Either 0.15 0.55 1.0 30 BUTTERFLY 60° aligned Either 0.38 0.68 0.7 17.5 " Fluted Vane Either 0.41 0.7 0.7 25 " 90° Offset Seat Either 0.35 0.60 0.7 29

Notes:

• Flow direction tends to open or close the valve, i.e. push the closure member away from or towards the seat.
• In general an Fd value of 1.0 can be used for valves with a single flow passage. An Fd value of 0.7 can be used for valves with two flow passages, such as double-ported valves and butterfly valves.
• In this table d may be taken as the nominal valve size, in inches.

References:

• "ANSI/ISA-75.01-1985 (R1995), Flow Equations for Sizing Control Valves," ISA - The Instrumentation, Systems, and Automation Society (1995).