A numerical valve model has been validated to predict the discharge capacity in accordance to the requirements of valve sizing method EN ISO 4126-1 and the opening characteristic of high-pressure safety valves. The valve is modeled with CFD software ANSYS CFX and the model is extended with the Soave Redlich-Kwong real-gas equation of state to allow calculations at pressures up to 3600 bar. A unique test facility has been constructed to perform valve function andcapacity tests at operating pressures up to 600 bar with water and nitrogen. For gas flows the numerical results and the experimental data on mass flow rates agree within 3%, whereas deviations in flow force are 12% on average. The inclusion of fluid-structure interaction in the numerical method improves the results for the flow force well and also gives insight into the valve dynamics of an opening safety valve. In a comparison between the experimentally and numerically determined liquid mass flow rates a model extension accounting for cavitation reduces over-predictions by a factor of two to 20% for smaller disklifts and decreases the deviations in flow force from 35 to 7%. At higher disk lifts, the effect of cavitation is less and experimental and numerical mass flow rates agree within 4% and flow forces within 5%.