Control Valve Capacity Testing

[GainControl]

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!

 

[JimCasey]

It depends how much vacuum you have downstream. If the absolute pressure ratio is about 2 or more, it exceeds critical drop. Velocity becomes sonic, there is a standing shock wave in the vena contracta, and downstream pressure becomes independent of flowrate. Starting at atmospheric pressure, for a fixed valve position if you drop the downstream pressure, flow will increase. Progressively lower downstream pressure increases the flow, until you hit the critical pressure drop. This is also known as choked flow. I don't like the term "choked" because it is misunderstood. If you open the valve more, you still get more flow, and if you increase upstream pressure you also will get more flow. "Choked" is merely a thermodynamic expression that to us means it's time to use a different Cv equation.

 

[GainControl]

Thank you, JimCasey, for your response.

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.

 

[JimCasey]

>>DP/P1 is .02 or less should I expect any difference <<

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.

 

[FlowBee]

I agree with Jim that you probably have some type of experimental scatter. Did the tests have data at more than one pressure drop ratio? If the ratios are well spaced and the Cv is relatively constant between them, the test should be good. Of what order is the discrepancy?

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.

 

[davefitz]

For a DP/Pi of 0.02, you could determine Cv, but you would need to have a much larger DP/Pi to also calculate the other valve characteristic that determines its choked flow behavior with compressible fluids. Using the ISA nomenclature, the parameter Xt will define how the valve will flow under choked flow conditions.

The value of Dp/Pi which causes choked flow is a function of Xt and also the gas's ratio of Cp/Cv.