Two-phase flow phenomena with depressurization—consequences for the design and layout of safety and pressure relief valves

Abstract Depressurization through safety and pressure relief valves has gained increasing interest in safety strategies for chemical plants. A best-estimate design of these valves needs reliable information on various thermohydraulic phenomena. Flashing and phase separation in saturated liquids and vapour-liquid mixtures are very complicated thermodynamic and fluid-dynamic processes, as discussed in this paper. The critical flow through the valve is a function of the inlet conditions, mainly pressure and void fraction, and of the thermodynamic properties. There is still some disagreement in the literature whether and to what extent slip between the phases influences the critical mass flow rate. Finally, two-phase flow friction in the pipes and in the valve itself can control the mass flow rate during depressurization considerably. In pipes of very large diameter reliable measurements on two-phase friction are still lacking.