Shackelford, AE, Using the Ideal Gas Specific Heat Ratio for Relief-Valve Sizing, Chemical Engineering, 12, 110 (November, 2003), pp 54-59.
In API Recommended Practice 520, the basis for evaluating the ideal gas specific heat ratio has been modified from standard conditions (in the sixth edition) to relieving conditions (in the seventh edition). This provides the impetus for evaluating the use of the ideal gas specific heat ratio in the vapor-sizing equations as well as the validity of the ideal gas assumption to provide a good estimate of the mass flux through a nozzle. Presented here are the results of an evaluation for a few pure components, which indicates that the ideal gas specific heat ratio at the inlet temperature provides a very good estimate of the isentropic expansion coefficient under ideal gas conditions, although the temperature choice does not appear to have a significant effect on the mass flux calculations.
On the other hand, the use of the actual inlet density (typically via the inlet compressibility factor) can lead to an under-prediction of the required discharge area for a relief valve if used in conjunction with the ideal gas specific heat ratio under some conditions.