The methods for calculating heat flux to a vessel exposed to fire are well documented. API RP 520, 521, Standard 2000 and NFPA 30 are the basic industry guidelines providing correlations to calculate this heat flux. Other guidelines are available for less general cases. Given the heat flux, a relieving rate can be calculated to prevent vessel damage caused by over pressure. Considering the height and geometry variables, heat flux is calculated based on the exposed vessel wall area that is wetted by internal liquids. Some add a contribution owing to heat flux through unwetted surfaces. The above references provide guidance for the basic cause of calculating external fire heat flux to a typical vessel for the purpose of vent sizing. The following paper provides guidance for the three special external fire cases. The authors believe that these three cases are not addressed in the Codes or elsewhere, thus justifying their presentation here. These papers were presented in summary at the 2000 DIERS Users Group meetings.