Geometry Influence on Two-Phase Flow in Safety Valves

A careful design of pressure safety valves (PSV) is an essential requirement for safeguarding of industrial plants; reliable correlations are available for PSV design in the cases of liquid or gas discharge. Instead, if two-phase flow is possible, especially for low vapour quality (less than 10%), PSV design becomes very difficult owing to complex thermal hydraulic phenomena that happen between the two phases. Currently there are some calculation methods, based on different simplifying hypotheses, that try to predict the two-phase flow rate through a PSV knowing the inlet fluid conditions (pressure, quality or temperature) and the outlet pressure; however, none of them is acknowledged as being reliable for every situation and, therefore, there are not standards for PSV design under two-phase conditions. The PSV size is one of the most important parameters used for choosing between the two main prediction models, Homogeneous Equilibrium Model (HEM) and Homogeneous Non Equilibrium model (HNE). This paper shows the results of an experimental research carried out with steam-water two-phase flow through two PSVs having the same orifice diameter but with different discharge coefficients and inlet geometries. The experimental data are compared with the results of a calculation method based on the homogeneous model with non equilibrium hypotheses and an other one presented in API Recommended Practice 520, developed with equilibrium hypotheses