Sizing for subcritical flow

[rogim]

When sizing for subcritical flow for vapour service according to API520 the isentropic expansion coefficient is needed. Assuming ideal gas the cofficient is set to the ratio between Cp and Cv.

However, how do I find the ratio as it is the ratio for the ideal gas that shall be used?

I can use the ratio for the actual gas, but that will be conservative(?)

 

[PaoloPemi]

if you know the composition you can calculate the ideal gas cp (and cv) as average of cp (and cv) of components in the mixture, in the same way you can calculate the real gas cp (and cv), a EOS or equivalent system is required for estimating real gas values from ideal gas values (supposing you have a library with temperature tabulated properties of pure fluids).
Things are more complex when the composition changes (for example during a blow down with liquid evaporating), in those cases I divide the whole process in a certain number of steps and I assume constant composition (and fluid properties) at each step, this can be done in Excel or similar tools.

 

[sheiko]

Read: "Using the ideal gas specific heat ratio for relief-valve sizing"; by Aubry Shackelford, Berwanger, Inc.; Chemical Engineering, November 2003 referred the latest API STD 520.

"We don't believe things because they are true, things are true because we believe them."

 

[rogim]

Thanks for your replies.

However, I found the ideal Cp/Cv ratio by depressurise the gas to atm pressure in the simulation model. This should be a very good approach as the compressibility factor is then close to 1.

Note, I have no change of phase during the depressurisation. Gas only.

 

[MortenA]

Many process simulators e.g. HYSYS can calculate the ideal Cp/Cv ration.

Best regards

Morten