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The first thing is to define the purpose of the model. A simple model will give simplistic information. I have used this for preliminary sizing and estimates. W = 63.2*Cv√(∆P)∕(density). W = pph, and 63.2 is a constant for the units used. Cv is the flow coefficient for the valve at the specific percentage open. ∆P is pressure difference in psi. Density is in pounds/cu ft.
In real life, Cv is a non-linear function of the valve stem position. This is found in the catalogs. Valve stem position is a function of controller signal and positioner accuracy. A complete model will include the estimated slip-stick motion of the valve stem. A more complete model will consider the possibility of choked flow or cavitation. If the flow fluid is compressible the appropriate equations consider those effects.
The standard definitions and equations are in the references below. Note that, in general, these equations are more reliable over “reasonable” ranges. Flow coefficients and even flow regimes can change widely at small valve openings. There is still debate over flow at low Reynolds numbers. Fluids at high viscosities are troublesome.
ANSI/ISA-75.01.01-2002 (60534-2-1 Mod), Flow Equations for Sizing Control Valves.
ISA-75.25.01-2000 (R2006), Formerly ANSI/ISA-75.25.01-2000, Test Procedure for Control Valve Response Measurement from Step Inputs
Cullen Langford, PE
A: There are two considerations in simulating valves: steady-state and dynamic. The steady-state model converts valve position into flow through the valve characteristic and pressure drop. The dynamic model converts controller output into valve position through deadband and velocity limit or the possible presence of a positioner, which closes the loop around the valve position through proportional or PID control.
Valve behavior is not a trivial consideration in fast loops like flow and pressure. Dead band can cause endless limit-cycling in level loops, and velocity limit can create expanding cycles on a large upset.
Simulation of all these properties is described in my book, Simulating Process Control Loops, ISA, 1990, pp. 125-131. Unfortunately, it is now out of print, but might be found in an engineering library.
Process Control Consultant
Q: My name is Matt Wilhite, Advanced Development Manager at Molex.
One of my colleagues has written a PhD thesis on the use of process control in electroplating. He and his sponsoring university (University of Limerick) are seeking a qualified external examiner to examine his thesis. Hopefully someone would see this as a potential opportunity to work with us in future or gain from our experience somehow. All travel and accommodation expenses for you or designate would be 100% paid.
I can be contacted by email and phone (mobile is easiest) listed below at any time.
Molex Ireland Ltd.
Shannon Industrial Estate
Shannon, Co. Clare, Ireland
int + 353 (0)61 702420 (office)
int + 353 (0)86 8157115 (mobile)
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