Real or ideal cp/cv for orifice sizing

[Justice100]

Hello all,

I am using Crane for gas orifice sizing but am wondering if the real or ideal k (cp/cv) should be used? I know for relief valves it should be the ideal but unsure for a standard orifice plate.

This is a blowdown orifice so flow is choked.

 

[georgeverghese]

In all cases, it should be real gas isentropic exponent. There are pressure and temp ranges when real values approach ideal - see your thermo textbooks. Semi ideal and ideal values may be used as an approximation for low pressures, ambient temp ranges typically.

 

[pierreick]

Hi,
Yes, must be real exponent k=Cp/Cv. You may want to use an EOS like Peng Robinson to calculate them.
Pierre

 

[Justice100]

Thanks for your responses.

Can anyone point to a reference that explicitly states the use of real cp/cv?

 

[pierreick]

Consider the document attached in particular slide 7.
Your statement about safety valve and use of ideal values of Cp,Cv is incorrect.
Pierre

 

[apetri]

std. API formulation for PSV has been derived with ideal cp/cv ,

but for a blowdown design you can calculate the critical flow based on real density , cp/cv (for example from a EOS),

note that there are models (for example HEM) suitable to calculate critical flow also for two-phase (vapor + liquid)