The use of relief valves is crucial for the safety of power plants. Indeed, these valves, simple but robust in their design, provide the ultimate protection when all other safety systems are inadequate. This study is focused on valve opening characteristics which can be studied through the determination of flowforces applied on the valve disk. A spring-loaded safety relief valve (SRV) (1½ in. G 3 in.) and its transparent model are tested under static conditions. The spring is removed and the forces, exerted at the valve disk for different inlet pressures and lift positions, are measured in compressible, incompressible, and two-phase flows. Results indicate that even for relatively small qualities (i.e., 5–10%), two-phase mixtures approach compressible flow behavior (especially for the higher lifts) in terms of disk force. Additionally, an inverse flowforce of air and water is noticed above a certain value of valve lift. Numerical simulations with a commercial computation fluid dynamics (CFD) code are performed in a 2D axisymmetric model of the valve for validation purposes. The main motivation of these computations is to obtain the qualitative physical explanation of this phenomenon revealing the displacement of the sonic line which occurs in air flow simulations. Finally, the importance of precise adjustment of the valve ring (in the smallest valve opening) for its optimal use is stressed by quantitative analysis using CFD simulations.