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)
Copyrighted 2001, 2003