Size and geometry influence in pressure safety valve design in two phase flow. Comparison among three prediction methods

In the case of two- phase flow, especially for low vapour quality (less than 10%), pressure safety valves (PSV) design becomes very difficult owing to complex thermal hydraulic phenomena that occur 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 flow 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 (10 mm) but different discharge coefficients and inlet geometries. The experimental data are compared with the results of three calculation methods: the first one is based on equilibrium hypotheses and is the reference of API Recommended Practice 520; the second one is a calculation method based on the HNE hypotheses; the last one is a recent method, called HNE-DS, introducing some correction parameters in the HEM hypotheses to consider the boiling delay and is under the consideration of a working group ISO on sizing of PSV. The results show that the PSV geometry and the discharge conditions are important factors in choosing the more suitable model for the sizing of small PSVs