Analysis of PRV Stability In Relief Systems Part II

Screening. In part I 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 [3]), (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. The largest upstream pressure fluctuations are expected to occur during fast opening or closing of the PRV. The interaction of the pressure wave and valve opening/closing can cause instability. Note that during the opening of the PRV, a delay is typically observed in backpressure buildup because of the time needed to fill body-bowl and the discharge piping. Body-bowl choking and backpressure can influence the force balance on the disk and as a result can cause instability. In order to apply the simple screening model we need to establish the speed of sound in the inlet line, the PRV opening and closing times, and the acoustic pressure drop associated with PRV opening/closing.


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