Bridging the 3% Inlet Pressure Loss Rule Gap

The potential for pressure relief valve instability (chattering) is a known hazard. Because of this, 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 (the 3% rule).

It is now widely known and recognized that the 3% rule is not sufficient to guarantee pressure relief valve stability. Through measurement, incidents, and modeling it has been shown that some installations with an irrecoverable inlet pressure loss of less than 3% can be unstable, while some installations with irrecoverable inlet pressure loss greater than 3% can be stable. Published research by Chiyoda, Pentair, and ioMosaic show 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 and the Petroleum Environmental Research Forum have co-sponsored two major studies on PRV stability. The results of those studies have been incorporated into recent editions of the API 520 standard. 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 ioMosaic recommends it be used in conjunction with an estimate of the critical line length. 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, ioMosaic recommends 1D dynamics to demonstrate expected stable performance.

This publication, presented by Dr. Melhem, shows how the 3% rule came about, its shortcomings, and how to ensure that new installations and modifications to existing installations can be designed and evaluated. He will demonstrate this by showing how to bridge the 3% rule with new methods including force balance and critical line length.