Comparison among Three Prediction Methods for Safety Valves Design in Two-Phase Flow in the case of a Small Valve

In the case of two- phase flow discharge, pressure safety valves (PSV) design becomes difficult due to complex thermal-fluid dynamic phenomena that occur between the two phases. Currently, there are two main types of prediction models, the Homogeneous Equilibrium Model (HEM) and the Homogeneous Non Equilibrium model (HNE), used for developing methods to calculate the theoretical mass flux G(t); this value has to be corrected by an experimental coefficient k(d), termed as two-phase "discharge coefficient", for obtaining the actual two phase mass flow-rate value W(r). Generally, for each method a different way of calculating k(d) is proposed, but various studies are looking for a general correlation that considers all the two-phase flow aspects. This paper will compare some experimental results obtained with a steam-water system and a small PSV (office diameter phi(or)=6 mm) with the predictions of three methods, an HEM, an HNE and a more recent method, called HNE-DS, proposed by the ISO working group on PSV sizing.