Get a handle on PRV stability. There is general agreement that the 3% inlet pressure loss rule (IPL3) is not sufficient to guarantee PRV stability and does not work all the time. This is confirmed by recent findings from actual PRV stability measurements and dynamic modeling. IPL3 only considers irrecoverable pressure loss. IPL3 assumes that the fluid dynamic pressure is ultimately recovered at the disk surface as the PRV is closing. This recovery of fluid dynamic pressure can keep the PRV open, even at reduced lift. But this is only possible if the inlet line length is less than the ”critical length”. In other words, the returning pressure wave can keep the PRV open before the PRV reaches full closure only if it can get there before the PRV closes. One might even argue that as long as the ”total” wave/dynamic pressure drop in the inlet line is less then PRV blowdown, the PRV can operate in a stable manner, even at reduced lift. The pressure wave travel time depends on the speed of sound of the fluid/pipe system and the presence of any acoustic barriers.
Watch the white paper video on PRV stability by Georges A. Melhem, Ph.D., FAIChE on Process Safety tv®. Sign up for FREE or log in to watch all the white paper videos.
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