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)

 

[Justice100]

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

Ok, so they have derived their own equations for use of real Cp/Cv. Fair enough but most of the world uses the API 520 standard gas relief sizing equation and when doing so the ideal Cp/Cv should be used, as is stated in the standard - I am just writing this out for others.

 

[shvet]

most of the world uses the API 520 standard

Do you know this or believe in this?