January 2010

Plant Safety and Environment

Designing for pressure safety valves in supercritical service

Use this rigorous method to prevent over-sizing

Doane, R. C., S & B Engineers and Constructors Ltd.

Pressure safety valves (PSVs) on vessels containing liquid hydrocarbon that may be blocked in during a fire can relieve a supercritical fluid, if the relieving pressure is higher than the critical point. The conventional method for calculating an orifice size is presented in API RP-521.1 This method, however, treats all "un-wetted" vessels the same, whether the fluid contained is supercritical, a vapor or a gas. Caution is given in the API text and states that the given equations are based on the physical properties of air and the perfect gas laws with no change in fluid temperature. The reader is cautioned to review these assumptions to "ensure they are appropriate for any particular situat

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