Sizing for subcritical flow
Sizing for subcritical flow
(OP)
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(?)
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(?)





RE: Sizing for subcritical flow
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.
RE: Sizing for subcritical flow
"We don't believe things because they are true, things are true because we believe them."
RE: Sizing for subcritical flow
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.
RE: Sizing for subcritical flow
Best regards
Morten