Pressure Relief Valve Sizing – Steam

The steam sizing equation presented in API RP 520, Part I (Section 4.4.1 of the 6th edition) should be used for sizing pressure relief valves for saturated or superheated steam flow. This equation is very similar to the vapor-sizing equation, with the physical properties of saturated steam built-in. Accordingly, the steam sizing equation yields results very similar to the vapor sizing equation, but requires less physical property data. Property differences between saturated and superheated steam are accounted for via a superheat correction factor (Ksh); RP 520 provides a table of Ksh values as a function of steam pressure and temperature. Deviations from ideal gas behavior at high pressures (> 1500 psi) enter the calculation via the Napier equation factor, KN, which is a simple function of the absolute pressure. Finally, the steam sizing equation also contains the discharge coefficient, Kd, in the same manner as the liquid and vapor sizing equations. When valve-specific data are not available for steam flow, API RP 520 recommends a value of 0.975, consistent with the behavior of other vapors.

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