Estimating the opening time of a direct spring operated pressure relief valve in the case of multiphase flow of fixed mass fraction in the absence of piping

Abstract

This paper addresses the effect of non-flashing multiphase flow on the dynamic behaviour of direct spring operated pressure relief valves (PRV). We employ DIER’s ω method to describe the mass flux through the valve for different xg gas mass fractions, which is combined with the one-degree-of-freedom mechanical model of the PRV and the vessel pressure dynamics, providing a simulation tool for predicting venting scenarios. We emphasise on the proper description of the fluid momentum forces on the valve disc. Three typical valve geometries are considered: a ’reaction-like’ geometry with large deflection angle reversing the flow, a simple disc valve with purely radial outlet and a conical geometry, for which the direction of the flow through the valve does not change significantly. It was found that by merely varying the mass fraction of the liquid and the gas, the valve opening time changes drastically, over a range of several orders of magnitude. By using a simplified mechanical analysis and assuming frozen mixture (i.e. fixed mass fraction), a formula was developed providing an order-of-magnitude estimation of the valve opening time.