Two phase pressure drop influence on pressure safety valve performance

A reliable design of pressure safety valves (PSV) is essential for the safety of industrial equipment. However when two-phase flow is possible, especially for low vapour quality (less than 10%), the design becomes very difficult owing to complex thermal hydraulic phenomena that occur between the two phases. Furthermore, when a circuit for the recovery of the discharged flow is also present, the built-up back pressure can be rather high thus decreasing the predicted flow rate. As a consequence, for a correct PSV design, accurate methods for the calculation of the pressure drop in two-phase flow are of the utmost importance. At present, various methods, based on different hypotheses, are available. In particular, some methods provide more equations, specifically taking into account the local fluid flow pattern; others are based on general hypotheses which are assumed valid for every flow pattern. This paper shows the results of an experimental research carried out on a two-phase steam-water system. The experimental data are compared with the results of three different pressure drop calculation methods, and the performance of the models is evaluated as a function of the main process parameters