A simulation tool for the preliminary design of a pressure relieving device for processes at near critical conditions is described. The selection of a pressure relieving device can be divided into two stages. The first stage is to determine the required emergency outflow as a function of time, which provides information to select a pressure relief valve or a rupture disk. The second stage is to simulate the process with the pressure relieving device selected at the first stage to fmd out the behaviour of the process as a function of time. A comparison between the proposed and the API 520 method is shown with the aid of an example.