When a fixed roof storage tank is surrounded by a pool fire, and when the vents are not correctly dimensioned, the inside pressure can raise until the mechanical limits of the structure. The aim of this study is to develop a tool that will allow to follow the variations of the pressure and temperature in the gas phase, as a function of the size of the vent. This will help the sizing of new tank's vents or evaluating the risk of already build structures. The model also gives the temperature's distribution in the liquid phase, at each time step. In case of undersized vents, this information will be convenient to describe the phenomenological aspect of the accident after the rupture of the tank i.e., to qualify and quantify the fire ball generated and its effect on persons and structures. Currently, if few analytical models exist to quantify the fire ball and its effects after the rupture, none are able to describe with accuracy the prerupture phenomena. The tool developed takes into account the two phases of the accident and allows to study complete and consistent scenarii. (C) 2010 American Institute of Chemical Engineers Process Saf Prog 30: 52-61, 2011