Testing and Modeling of an Acoustic Instability in Pilot-Operated Pressure Relief Valves

Abstract

Pressure relief valves (PRVs) are included as an essential element of many compressor piping systems in order to prevent overpressurization and also to minimize the loss of process gas during relief events. Failure of the valve to operate properly can result in excessive quantities of vented gas and/or catastrophic failure of the piping system. Several mechanisms for chatter and instability have been previously identified for spring-loaded relief valves, but pilot-operated relief valves are widely considered to be stable. In this paper, pilot-operated PRVs are shown to be susceptible to a dynamic instability under certain conditions where valve dynamics couple with upstream piping acoustics. This self-exciting instability can cause severe oscillations of the valve piston, damaging the valve seat, preventing resealing, and possibly causing damage to attached piping. Two case studies are presented, which show damaging unstable oscillations in a field installation and a blowdown rig, and a methodology is presented for modeling the instability by coupling a valve dynamic model with a 1D transient fluid dynamics simulation code. Modeling results are compared with measured stable and unstable operation in a blowdown rig to show that the modeling approach accurately predicts the observed behaviors.

Dynamic behaviour of direct spring loaded pressure relief valves connected to inlet piping: IV review and recommendations

“…Typically, one encounters the first, quarter-wave, harmonic of the pipe -that is a standing wave of wavelength of four times the pipe length. Examples of such vibrations can be found in Allison and Brun (2015), where the authors found that the quarter-wave frequency to dominate valve oscillation. Similarly, in Misra et al (2002) the quarter-wave eigenfrequency of the downstream piping was found to match with the observed vibration frequency.…”

“…As described in Singh (1983); Urata (1969); Song et al (2013); Darby (2012); Bazsó and Hős (2012) or Allison and Brun (2015), the forces acting on the valve body can be characterized into three forms: force due to pressure distribution, force due to shear stress and impulse forces. In most cases, the shear stress can be neglected because of the small geometry associated with the pipe and hence the relative low Reynolds numbers involved.…”

“…There are many reports of valves being unstable at small openings, e.g. Allison and Brun (2015); Hős and Champneys (2012); Bazsó and Hős (2013).…”

“…In fact, Misra et al (2002) analysed an air-actuated control valve both numerically and with in-situ measurements and found also the same quarter-wave instability. Another documented example of the quarter-wave instability is given in Allison and Brun (2015), were the authors studied a PRV both experimentally and with numerical simulation. They also found that instability was most likely to occur with small opennings of the valve and that the instability could potentially be removed by closing the PRV quickly enough.…”