Incompressible or Compressible Fluid Flow 1

[jackboot]

A customer asked a very simple question that has turned into a massive project.

We make valves - all types. For the purposes here we have a safety valve or pressure relief valve.

The question: How much can I flow through your valve for a given pressure drop? (for water at 60 F)

So far:
First:
I have run the senerio in a CFD package with varying flow rates and pressures. From this I have obtained a Cv value which I used to plot flow rates verse pressure drop from 0 to 15,000 psi.
(Q=Cv*sqrt(pressure drop))

Second:
I experimentally tested the valve using a flow loop and I measured the flow rate and the corresponding pressure drops (using a manometer). However, I can not run high flow rates to obtain a high pressure drop. The maximum pressure drop I am able to obtain is 5.5 psi with my flow loop.

The Cv values were in close agreement (5%) compared to the experimental data to the computer simulation.

Everything is going good up to this point.

Now the problem:
I re-ran a few points from the flow curve that I plotted for a given flow rate to check that the computer agreed with the pressure drop - which it did. (Q=Cv*sqrt(pressure drop)) is the formula I used to produce my flow curves. However, my CFD package is also depicting areas of negative pressure (cavitation) at the high velocity areas and exit. The threshold where the "cavitation" develops looks to be approximately 100 psi pressure drop (with 14.7 psi (1atm) for the exit pressure).

What is truly happening? Are my flow curves valid for the pressure drops above 100 psi? If it does cavitate will the flow rate decrease?

jackboot

 

[davefitz]

I think it is probably cavitating.

One method to predict the flow of cavitating fluid is to use the equations from the "ISA handbook of control valves "
2nd edition, J. W. Hutchinson.

let Rc= critical pressure ratio= 0.96-0.28*SQRT(Pv/Pc)
Pv= vapor pressure (psia) at the inlet temperature
Pc= critical pressure = 3206 psia
W(lb/hr) = 63.4Cv*SQRT{Km(Pi-RcPv)/sv,i}
Cv= valve flow coef
Pi= inlet press, psia
sv,i= inlet spec vol, ft3/lbm
Km = valve cavitation index
Km=0.75 globe
=0.225 angle venturi
=0.55 angle seaat ring
=0.35 ball valve
etc.