Bridging the 3% Inlet Pressure Loss Rule Gap

Because the potential hazard of pressure relief valve instability (chattering) is already recognized, relief systems design basis documentation must demonstrate expected stable pressure relief valve (PRV) operation and performance for a multitude of credible scenarios. Historically, expected stable pressure relief valve performance has been demonstrated by showing that the irrecoverable inlet line pressure loss is less than or equal to 3% of the pressure relief valve set pressure (3% rule). 

It is now widely known and recognized that the 3% rule is not sufficient to guarantee pressure relief valve stability. It has been shown (through measurement, incidents, and modeling) that some installations with irrecoverable inlet pressure loss less than 3% can be unstable while some installations with irrecoverable inlet pressure loss greater than 3% can be stable. Numerous publications including research by Chiyoda [1], Pentair [2] and ioMosaic [3] showed that pressure relief valve instability leading to flutter and/or chatter is due to the coupling of the PRV disk motion with the quarter wave pipe/fluid mode frequency without resonance. 

The American Petroleum Institute (API) and the Petroleum Environmental Research Forum (PERF) have cosponsored two major studies on PRV stability. The results of those studies have been incorporated into recent editions of the API 520 standard [4]. Recent editions of API 520 allow the user to perform an engineering analysis to demonstrate expected stable PRV performance for installations where the irrecoverable inlet pressure loss exceeds 3%. One engineering analysis method described by API 520 is the force balance recommended by ioMosaic. The force balance is a simple method and should be used in conjunction with an estimate of the critical line length as also recommended by ioMosaic. This simple method is most applicable to simple piping geometries. Where complex piping geometries are encountered and/or inlet line lengths exceed the critical line length criterion, 1D dynamics is recommended by ioMosaic to demonstrate expected stable performance.

Read this paper for an overview of the OSHA PSM Standard compliance requirements, analysis of the current 3% inlet line pressure loss rule, and recommendations for bridging the 3% gap.

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