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Real Or Ideal Cp/cv For Control Valve Calculations

control valve specific heat ratio

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#1 sumwunrandom

sumwunrandom

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Posted Yesterday, 05:53 PM

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 ShackelfordBerwanger, 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?



#2 breizh

breizh

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Posted Yesterday, 08:08 PM

Hi,

I believe it's based on ideal. Consider the document attached and annexes.

Probably good to ring valve suppliers.

EDIT: More convenient to use ideal when you don't have access to a simulator.

 

Note: Make sure you compare apple to apple,

Compare Xt calculated using the ratio K gas /K air at atmospheric conditions for ideal and at process condition for real.

 

Good luck. 

Breizh 

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