Control Valve Capacity Testing
Control Valve Capacity Testing
(OP)
What differences can I expect in results from running the ISA S75.02 valve capacity tests (compressible - air) under vacuum (e.g. downstream “blowing engines”) versus pressurized (e.g. upstream air compressor)?
Thank you in advance for your input!
Thank you in advance for your input!





RE: Control Valve Capacity Testing
RE: Control Valve Capacity Testing
If in both cases (vacuum & pressurized) DP/P1 is .02 or less should I expect any difference in the test results and ultimately in the calculation of Cv?
I currently have this situation, different values for Cv on the same set of valves. The only obvious difference is the vacuum versus pressurized test condition. I am trying to determine the cause of the discrepancy.
Thanks again.
RE: Control Valve Capacity Testing
I wouldn't think so.
Can you retest the valves? I am thinking you have experimental scatter. How much do they differ? The Cv equation for gases should be calculated in absolute units. Make sure you are not using Gage units.
RE: Control Valve Capacity Testing
Cv tests run on air can be a bit tricky. As Jim mentioned, make sure you are using absolute pressures. Also, verify that your flow rate measurements are accurate enough. You won't have much flow at x=0.02 and less.
Your two setups should not matter if you have accurate flow rate and pressure measurements and you are using absolute pressures.
RE: Control Valve Capacity Testing
The value of Dp/Pi which causes choked flow is a function of Xt and also the gas's ratio of Cp/Cv.