High-pressure safety valves with set pressures of more than 200 bar are required in industry, e.g. for polyethylene and synthesis gas applications. They are presently sized according to ISO 4126-1. Only equations for ideal gases are presented there, and there are no indications as to how the real gas factor and the adiabatic exponent for real gases are to be calculated. For this reason, an equation for the critical mass flow rate of a real gas through a nozzle was derived and compared with the model according to EN-ISO 4126-1 and with experimental data. It is recommended that the current ISO-standard be supplemented by the nozzle flow model for real gases. The first numerical calculations (ANSYS-CFX) show that the discharge coefficient for a high pressure safety valve measured at moderate pressures can be extrapolated to very high pressures if it is used in conjunction with the nozzle flow model for real gases. However, this numerical result must yet be validated for further valve types by experiment. For this purpose, BASF has set up a high pressure valve test facility in Ludwigshafen.