ISA/IEC control valve specific heat ratio

[prq123]

All,
ISA 75.01 and IEC 60534 use the "specific heat ratio" for control valve sizing. Does this "specific heat ratio" mean the ideal gas specific heat ratio (evaluated by k = Cp (Cp-R) ) or does it mean the actual isentropic exponent (same isentropic exponent used in orifice sizing and relief valve sizing)? There does not seem to be any definitive answer in the the ISA or IEC specifications. Some control valve vendors use isentropic exponent while other vendors use ideal gas specific heat ratio. The values are very different when the gas is near the critical point or supercritical.

 

[hsbcn]

Hi prq123:

This Masoneilan control valve sizing handbook may be of use to you in researching the question:

http://www.dressermasoneilan.com/documents/.../OZ1000-0304-0509-web.pdf

Good luck!

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[prq123]

hsbcn,
Thanks for the link. Yes, I have a copy of this. Unfortunately it does not state how to calculate the "actual specific heat ratio". Note that the "actual" specific heat ratio mentioned in the link and the isentropic exponent are not the same value.

 

[ione]

The specific heat ratio for ideal gas is "incidentally" the same as that the isentropic exponent.
Glance at

http://webbook.nist.gov/chemistry/fluid/

to find fluid properties over a wide range of temperatures and pressures (even in the supercritical region). You’ll find there cp and cv values for specific given conditions and this will allow you to calculate the specific heat ratio required for the sizing procedure.

 

[prq123]

ione,
Thanks for the info. However, the issue here is not how to determine the specific heat ratio. Rather, what is the required "specific heat ratio" to be used for the ISA/IEC sizing equations (ie. is it ideal gas specific heat ratio, isentropic exponent, or even real gas specific heat ratio ?).

 

[ione]

You’ve to take into account the specific heat ratio, and more specifically the actual specific heat ratio. When ideal gas behaviour is not a reasonable assumption for your scenario (i.e. supercritical fluids) compute the specific heat ratio for the actual conditions.

Look better at page 16 of the link provided by hsbcn

http://www.dressermasoneilan.com/do...y/tech-docs/eng-data/OZ1000-0304-0509-web.pdf