Analysis of PRV Stability In Relief Systems Part II

In part I [1] of this paper we established a detailed dynamics methodology for the modeling of PRV stability. We demonstrated that (a) the irrecoverable inlet pressure loss due to friction has essentially no impact on PRV stability (also see [2]), (b) PRV instability is caused by the coupling of PRV disk motion with the pressure wave caused by excessive acoustic pressure drop (1/4 wave) during PRV opening/closing, (c) the instability does not amplify, and (d) liquid systems are the most likely to cause damage to piping and piping components. In this paper we provide a simplified model for the assessment of PRV stability where the inlet line geometry is simple and/or where the inlet line acoustic length can be established. This simplified model has also been proposed in the 3rd ballot of API-520 part II.

PRV stability is heavily influenced by the inlet and discharge piping configuration. Excessive inlet pressure loss or backpressure can cause PRV chatter and/or flutter. As the PRV starts to open, the pressure upstream of the PRV starts to decrease due to sudden expansion. This gives rise to an expansion wave that will travel upstream. As the expansion wave reaches the pressure source (Vessel) upstream, it reflects and travels back towards the PRV as a compression wave.


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