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[Brightonnk]

Hi all,

 

So I'm getting some testing done to find the flow coefficient (cv) value of a range of valves. The valves I have that use water as their medium. The test house use air as their medium. After speaking to the man in charge, he has advised me that they can assume flow conditions as incomrpessible if the pressure differential ratio from inlet to outlet is with 0.02.  (this is what is stated in the standard of conformance)

 

Now this makes sense because if the inlet pressure and outlet pressure are only 2% out then that assumes that the medium (air) they are using is not compressed, resulting in a cv value that would be the same if I was to use water (as water is incomrpessible). However- the tests i am conducting are to validate some theoretical calculations I have done. So, If my pressure drop across the valve is based on water, would I have to change my calculations to mimic the test procedure which uses air. The reason I think this is because my pressure drops relate to diameter, length, friction factor and density and obviously the density of water and air are completely different.

 

I guess I am a bit confused as to how the cv value can be the same if im using 2 different mediums, The only thing i can think of that makes sense is that the pressure drop across the valve will be different due to different densitys and viscositys but the overall cv value will still be the same.

 

any advice on this would be appreciated

Thanks

Neera