Conceptual design of pressure relief systems is an important stage in the design of a safe process plant. The conceptual design stage consists of the following steps: deciding on the location of pressure relief devices, selecting the general type of pressure relief device for each identified location, i.e. safety valve and/or bursting disc (rupture disc), or other relief device, and selecting the special features for the chosen device type. Some regulations, codes and standards, and a decision tree for the selection of a relief device have been described in the literature. This paper presents four decision trees that have been developed for the different steps in the conceptual design stage. Only positive pressure in pressure vessels is considered here.