In the case of two-phase vapour-liquid flow, especially for low vapour quality (< 10%), pressure safety valves (PSV) design becomes very difficult due to complex thermal-hydraulic phenomena occurring between the two phases. Currently, there are some calculation methods, based on different simplifying hypotheses, trying to predict the two-phase flow rate through a PSV knowing inlet fluid conditions (pressure, quality or temperature) and the outlet pressure. However, none of them is acknowledged as being reliable for any situation and, therefore, there is still a lacking of 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 (10 mm), but different discharge coefficients and inlet geometry. The experimental results are compared with the predictions obtained using a calculation method based on a homogeneous model with non-equilibrium hypotheses and another method proposed in API Recommended Practice 520, developed with equilibrium hypotheses. The results show that the PSV geometry and the discharge conditions are important factors for choosing the more suitable model for the sizing of a little PSV. (c) 2007 Elsevier Ltd. All rights reserved.