Screening for PRV Stability – Inlet Pressure Drop and the 3 Percent Rule

Chatter is most likely in vapor service, damage will most likely be due to large mechanical forces caused by the rapid valve closure and/or by PRV reduced flow capacity due to PRV damage. This is especially true for large valves and/or for valves in high pressure service. Chatter is least likely in flashing two-phase* flow service and/or in liquid service. Flutter will almost always occur in liquid service with fast opening or pop action valves. Cycling is most likely to occur in flashing two-phase flow service. Piping damage is most likely in liquid service due to the large magnitude of the water hammer pressure waves propagated upstream during rapid valve closure (full or partial), i.e. during chatter or during flutter. The 3 % inlet pressure is not sufficient to guarantee PRV stability. Stability may need to be assessed and confirmed for all credible scenarios and not necessarily just the one with the highest required flow rate. All models have limitations, some models are more useful than others.

This presentation was made by Georges A. Melhem, Ph.D. at the 78th API Refining and Equipment Standards Meeting in Las Vegas on April 20-27, 2013.

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