How to Calc Cv with gas

[jackboot]

Question:
How does one calculate the Cv value for a piece of equipment?

I don't know if there is a standard for doing this - it would seem that the value would the meaningless if the flow changed from incompressible to compressible.

Further, I can only run the simulation with a CFD package to get the flowrates. So I don't have emperical data to back my number.

Does anyone have a clue what I should do?

jackboot

 

[davefitz]

If you can test both the choked and unchoked case , it is simple.

for choked,
W=63.6*0.667*Cv* SQRT( Pi*Xt/sv,i)

for incompressible approximation;
W=63.4 *Cv *SQRT{ (Pi-Po)/ sv,i}
Pi= inlet press, psia
Po= outlet press
sv,i = inlet spec vol, ft3/lbm

2 equations, 2 unknowns.Xt and Cv

 

[jackboot]

OK - But forgive me - I am slightly clueless on this subject.

What does the Xt variable represent?

Additionally - wouldn't there be two different Cv values -one for compressible and one for incompressible?

jackboot

 

[Tremolo]

jackboot,

Typically you can use the reported Cv value (presumably measured for incompressible flow) in combination with choked flow limitations. This is the approach suggested in the Crane Technical Paper (p 1-9).

Also, Crane does not mention any dependence of the loss coefficient information (which is equivalent to Cv; see Crane equation 2-6) on the compressibility of the fluid.

Finally, we have performed in house experiments with two-phase steam-water flow through globe valves. I have shown a good correlation between the data and the all-liquid Cv values for the globe valve if an appropriate choked flow model is applied. So, in my experience, it is ok to apply the incompressible Cv for compressible flow if you also use a choked flow model.

Therefore, if your CFD code calculates a total flow and pressure drop, then you should be able to apply Crane equation 1-11 on page 1-9 to determine a Cv value for your piece of equipment.

I am not clear on the equations provided by davefitz. It's probably just a units coversion thing, but where does the 63.6 (or 63.4) come from and what are the units for W?

 

[jackboot]

Tremolo,

Thanks for the information.

I have a question - if a customer requests a Cv value of a piece of equipment. And that equipment will flow liquid, gases, two phase, solids, and supersonic compressible flows - is there truly an answer for a Cv value?

I notice that there is a calculation for each and every aspect of flow - meaning laminar flow, transitional flow, choked flow and any other flow. I could see where a Cv would be useful where clean consistant fluid streams are seen day to day as in process or instrumentation systems. However, for a piece of equipment that will flow anything, would the Cv be a useful measure.

jackboot

 

[metalonis]

The term Cv is defined as the flow rate in gallons per minute of water through a valve with a pressure drop of 1 psi.

 

[Tremolo]

jackboot,

A single Cv value should be good for liquid and gas from laminar flow up to choked flow. I don't know about solids or supersonic flow.

It seems that the information that is really required is the hydraulic resistance through the piece of equipment. The Cv is one way to characterize the hydraulic resistance. Other ways to characterize the hydraulic resistance are to simply to calculate the effective flow area or discharge coefficient. This approach may make more sense for your wide range of fluid conditions.

TREMOLO.