Water two-phase flow through pressure safety valve with variable backpressure: check of calculation methods to estimate mass flow-rate and critical flow conditions

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; moreover, in this situation, the prediction of the critical flow onset (which occurs when the fluid velocity becomes equal to the sound velocity in the fluid) is very important for a correct design because it involves a choked flow situation. 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. This paper shows the results of an experimental research carried out through a PSV with steam-water two-phase flow. The experimental data are compared with the results of a calculation method based on the homogeneous model with non equilibrium hypotheses. The performance of the model is evaluated as a function of inlet and outlet parameters; an analysis of model critical flow prediction capability is presented too, with some uncertainties caused by the test procedures (here the mass flow-rate is imposed)