First, I had just found your post/question while doing a search online and saw my name. I'm floored by having me called out to help, now let's see if I can deliver. Hopefully being two months late is not an issue (btw my email is in my user profile).
To get Cv for valves using gases, the calculation is different than for liquids as you pointed out. Primarily has to do with the relative compressibility of gases as compared to liquids.
I'm going to offer two links to help you out:
1) Cv equation for gas at both low and high pressure drops: http://www.steam-toolbox.com/flow-coefficients-d_277.html
2) Swagelok's site has a nice PDF with the same equations and with more detail on how this is done.
Hey - thanks for responding - very good of you. I had looked at some of your blogs, including I think those 2 attachments that you've referenced before?
We've only recently gotten SW and it's all based on the FLOW so it's been hugely fustrating over the past 2-3 months.
Trying to work through the issues, we've had good local SW support and that's starting to necessarily extend out to SW-CFD experts in the US and France.
Basically, we were trying to set up simulations as a replication of a physical flow co-efficient valve test, and were getting the answers close to expectations on some products (old test results). But the results of other valves were miles off and that opened a whole can of worms. At the start, we hadn't even realised that SW was measuring volume flow rate rather than standardised conditions. So we were really not setting up correctly but we've certainly (since my post) made some progress regarding parameters, criterion setting, manual refinement, setting up 'dummy lids' as measurement sufaces and especially adaptive meshing.
I thought it would be simple ..... Set an inlet of 60 psig, Outlet of 59psig, Measure a pressure drop and Measure a flow rate and then equation out to SCFM and CV. But it didn't work out that way because of incorrect CFD set up, parameters, etc.
Still not there really -- for example I don't really understand some differences between static, total, dynamic pressure and why static is meant to be the appropriate one.
But trying some cross-checks now all we've learned against the other method of setting an inlet volume flow rate for air, and an outlet static pressure, taking measurements at surfaces 'set distances' from the test specimen (and a load of info goals just to see what's happening). It should work out at same CV as the set inlet and outlet method. If so, I just hope that SW support will 'rubber-stamp' the CFD set-up process in the context of the objective - CV test replication. Or, maybe there will be nuances that come out for very large flow, or convoluted flows, or low pressure drops, etc - that's the concern.
But at this stage, there can't be much else.
I might post up where this ends? - which hopefully will be in 2-3 weeks as I run some more simulations.
If there's anything esle specific to this gas-flow through valve type scenario that you think I should be paying particular attention to, a reply post would be great.
Thanks again for responding and the posts.