I have found specifications for valves (internal) where the Cv value of the valve is specified and the valve is almost exclusively used for gas application (dry Nitrogen).  I haven't worked on this stuff for a while, so I reviewed the derivation of the classic Cv equation.  This derivation assumes the fluid is incompressible.  Check out fisherregulators.com - one of their "technical monographs" goes over this in excellent detail.  For a new application, I know what my minimum flow requirements are and I know what my pressure range is.  Why not simply stay away from Cv and specify flow and pressure?  Is it appropriate to use Cv for gas application (obviously I think not)?  If not, is there an equivalent term used for gas application?

You still need some term to express how much gas the valve will flow for a given pressure and temperature AND pressure drop.

While Cv was originally developed for liquids, it's also used by a lot of valve vendors for gas services with appropriate correction factors to take account of any compressibility effects.

Fisher uses Cg for gas services which is Cv/C1.  The reason Fisher developed the use of Cg is that they state that they found valves with similar Cvs (eg, they would flow the same amount of liquid for the same dP) 'might' not flow the same amount of gas for the same dP.  Thus, they developed Cg and publish these for their valves/Forged Steel Valves and use them for gas sizing.  They also have Cs factors for use with steam, similar reasons.

Like TD2K said the standard Cv equation for incompressible flow was modified primarily to correct the units, i.e. SG to more specific gas law for density, USgpm to scfh, etc. The 'new' developed formula gave incorrect reading at dp/p ratios larger than something like 0.02 due to compressililty. Also the 'choked flow' range predicted was incorrect when reviewing different valve styles. In the end they discovered that the new gas formula didn't account for the difference in valve type, i.e. high recovery and low recovery valves. The basic liquid formula does by limiting the DP used in the equation to that of choke flow which is calcualted using the valve recovery coefficient Km (FL?). To account for this diff. in high and low recovery the C1 (recovery)factor was introduced together with the Cg factor. I've got a copy of the Fisher handbook which discribes this section a little more in detail. I can get you a scanned copy of this section if you leave yr e-mail. Next to the ISA formula described above which I believe Fisher have now also adopted you can also check the formula used by Mokveld being: US units Cv =(Q*SQRT(G*T))/(833*Fl*P1*(Y-0.148Y^3)) in which Y=1.63/Fl*SQRT(DP/P1) with a maximum of 1.5 Q=flow scfh G=specific gravity (air=1) T=temp in rankine P1 inlet pressure psia Dp=pressure drop psi In the end, using Cv for gas and liquids is a logical step as we can now check different valve type and sizing/selection from Vendors.

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