ISA control valves Kv vs Pneumatic Kv

[MGZmechanical]

Hi, I'm checking some formulas for Kv and I've seen different formulas.

I use Norgren and ASCO formulas for pneumatic Kv value:
Kv = (Q/514) · sqrt(T1/(Dp·p2))

Now the ISA control valve handbook is:
Kv = (Q/(482·Y))·srqt(T1·Z/(Dp·p1))

So, if Y and Z are approximately 1 for air this is:
Kv = (Q/(482))·srqt(T1/(Dp·p1))

How is it that one is sqrt(T1/(Dp·p1)) and the other sqrt(T1/(Dp·p2))
Maybe I shouldn't use ISA formulas for pneumatic valves, but should't they be the same?

Regards,

 

[EnergyMix]

If the lower one is for liquids (and there is nothing to indicate that it isn't), then of course it would be different than the one for pneumatics.

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[MGZmechanical]

Hi, the ISA formula is for compressible fluids in non-choked turbulent flows.
ISA-75.01.01-2007 (IEC 60534-2-1 Mod)

Regards,

 

[EnergyMix]

Ok. I did some searching. I found the ISA reference first and confirmed that the formulas there do reference the inlet pressure. However, none of them set Y equal to 1 and none give the numerical constant that you state. With a little more searching I found a paper from ASCO where, provided that the outlet pressure is greater than or equal to half the inlet pressure, it gives the formula you state. I have to presume that case fits your situation. I was unable to track down the Norgren paper, but I'll take your word that it states the same as the ASCO paper.

My two cents is that any of the formulas would work if they were applied properly. Go with which ever you want to use. However, if you're designing valves, I recommend that you consider testing them to confirm that your actual K_v meets your stated K_v.

Want to know the do's and don'ts of Eng-Tips? Read FAQ731-376.
English not your native language? Looking for some help in getting your question across to others or understanding their answers? Go to forum1529.