Hello fellow Chemical Engineers,
If one is determining a gas breakthrough scenario through an existing control valve with a high dP (such that the flow will be choked), is it more appropriate to use the 'real' Cp/Cv (e.g. calculated by HYSYS) or the 'ideal' Cp/Cv?
I have compared results for both and found that using the 'real' ratio gives a 12 % higher flowrate. For the purpose of my calculation (determining amount of gas breakthrough), the 'real' ratio is more 'conservative'.
I would compare this to PSV sizing in which API 520 notes that 'ideal' ratio should be used as it *typically* results in a more conservative design, and that if you are not careful, using 'real' ratio can result in undersized orifices. I have trawled through Annex B of API520, posts in this forum and ENGTIPS (incl https://www.cheresou...2-psv-sizing/),where caveats to the API 520 statement are mentioned (such as that 'ideal' ratios are not always appropriate for a select few mixtures, or when gas is near critical point). I also note ' Using the ideal gas specific heat ratio for relief-valve sizing"; by Aubry Shackelford, Berwanger, Inc.; Chemical Engineering, November 2003.'
However, I can't seem to find any information anywhere on CONTROL VALVE SIZING specifically. This includes reading through control valve supplier 'technical bulletins' and the such.
Any thoughts?