Pressure relief of near critical processes working at two phase region is simulated dynamically. The design procedure for pressure relief devices is demonstrated by an example. The example shows that the required relieving area of the safety valve depends on time and scenario. This is the basis for the selection of the type and the size of safety valve. When the process relieves through the selected valve the outflow and the pressure of the protected vessel varies dynamically. This phenomenon cannot be discovered by the static approach that is normally used in the design of pressure relieving devices. The traditional sizing equations, API 520 and DIERS, are also difficult to apply in near critical region and they include coefficients that are sometimes difficult to obtain. The sensitivity may be so large that a different size of the relieving device is selected depending on the model. Usage of two common models, API 520 and DIERS, in the region of two phases is discussed.