Pressure Relief Valve Inlet Piping Hydraulic Considerations

The principal requirements on the hydraulic performance of a pressure relief valve inlet line are specified in ASME Boiler and Pressure Vessel Code, Section VIII. At paragraph UG-135 (“Installation”), Section VIII states,

The opening through all pipe, fittings, and nonreclosing pressure relief devices (if installed) between a pressure vessel and its pressure relief valve shall have at least the area of the pressure relief valve inlet. The characteristics of this upstream system shall be such that the pressure drop will not reduce the relieving capacity below that required or adversely affect the proper operation of the pressure relief valve.

More specific guidance concerning acceptable inlet pressure drop is provided in Non-mandatory Appendix M of Section VIII, which specifies that for valves in compressible fluid service, “the cumulative total of all nonrecoverable inlet losses shall not exceed 3% of the valve set pressure. The inlet pressure losses will be based on the valve nameplate capacity corrected for the characteristics of the flowing fluid.” This guidance of a maximum of 3% of the set pressure for the frictional inlet flow losses is reiterated in API Recommended Practice 520 Part II, and is the generally accepted practice. The adverse effect that this is intended to avoid is the repeated opening and closing of the valve known as chatter. Chattering typically causes a reduction in relief capacity and can result in premature valve failure. See Guidelines for Pressure Relief and Effluent Handling Systems, Section, for additional discussion of chatter.

Recall (Section 1233) that a pilot-operated pressure relief valve can be installed with its pressure sensing line connected directly to the protected equipment, rendering the opening and closing of the valve independent of the flowing inlet pressure drop.

The evaluation of the non-recoverable (i.e., frictional only) inlet line pressure loss is performed using standard equations for fluid flow through pipe and fittings. Each pipe segment or fitting is represented in the calculation by a flow resistance coefficient or an equivalent length of pipe. As specified in the excerpt from Section VIII Appendix M quoted above, the flow rate is assumed to be the full nameplate (i.e., rated) capacity of the valve. This inlet pressure loss is evaluated for all causes of overpressure in which the valve is expected to relieve. “Nonrecoverable losses” refers only to the frictional losses, neglecting the effects of potential and kinetic energy.

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