Pressure relief systems are used to protect pressure vessels and related equipment against situations of excess pressure. In an emergency situation, they should vent sufficient mass to reduce the pressure to a safe level. The effective discharge area of a pressure relief valve is calculated with the theory of flow through convergent-divergent nozzles taking into account a correction factor. The international standard ISO 4121 and that of the American Petroleum Institute API 520 are recommended for sizing. In order to examine the validity of these methods for sizing valves for use at very high pressures, relief experiments should be performed over a wide range of pressures and temperatures. Because of their industrial importance, high pressures of 150-300 MPa, and large reactor capacities of up to 300,000 t/a, the polymerization of ethylene was selected as an example reaction.