Is there a maximum value used for spec ht coef, C?

[Bill3752]

I am performing some RV calcs for a NH3 plant, where very high pressures/temps are the norm. Often, k (specific ht ratio) is very high - 4 or more; this yields a very high C. Would you exercise some engineering judgement, and use a max C of say 420 in these cases? Anyone know of any documentation on this subject?

 

[EGT01]

Bill,

Take a look at API RP-520, Part 1, 7th edition, January 2000. Appendix B has a discussion about the flow equations and how to address non-ideal/real gas situations. Here's an excerpt but be sure to review the complete section...

B.1.6 Even though many vapors encountered in refinery
service do not follow the ideal gas laws, in most cases a pressure
relief valve (PRV) is adequately sized based on this
assumption. However, there may be unusual situations where
deviations from ideal behavior are significant. In those cases,
an isentropic expansion coefficient is used to characterize the
actual pressure-volume relationship that exists in the PRV
nozzle. Since this coefficient is used in the same way as the
ideal specific heat ratio, the form of the vapor sizing equation
is identical. The coefficient C is calculated for a real gas using
the isentropic expansion coefficient n instead of the specific
heat ratio k.

B.1.7 Determining the isentropic expansion coefficient for
a real gas is somewhat complicated because it is a function of
both pressure and temperature and it will vary throughout the
expansion process (for an ideal gas the isentropic expansion
coefficient will remain constant). The coefficient can generally
be obtained from an equation of state that describes the
pressure-volume relationship along any thermodynamic path
but is restricted to an isentropic expansion path.


Also, here's a link to a previous thread that may help...
Using 'real' Cp/Cv Ratios
thread1203-157455

I'm attaching a copy of the old Technical Inquiry that I refered to in the referenced thread...

http://files.engineering.com/getfil...4f-4af455cdb6c6&file=520_Part_I_Inquiries.htm

Regarding the old Technical Inquiry, here's a summary of the policy API has regarding the Technical Interpretations...
> Our policy is that we only provide interpretations on the most recent
> editions of our standards. As the standards are revised over the course of
> time some of the older interpretations may no longer be valid. For those
> that still have validity, I have chosen to leave them on the website as a
> courtesy and because there are not very many of them. Also, some of the
> older ones offered to much consulting and "how to" information, which is
> not permitted per our policy. I did not want to give people the impression
> that API would continue to respond to questions that did not comply with
> the appropriate format. The website is now the only place where we publish
> interpretations.

 

[Bill3752]

Thanks EGT, I found your post and the referenced threads very useful. I am running into many critical point situations; your feedback has shown that I ought to either default to more conservative specific heat ratio info (std conditions) or perform the needed calcs to estimate an isentropic coeficient.