The development of a mathematical model for simulating the blowdown of pressurized hydrocarbon conveying pipelines under fire attack is described. The model is based on the resolution of the conservation equations using the method of characteristics. It accounts for real fluid behavior, pipeline mechanical strength, as well as phase and flow dependent transient heat transfer effects, and frictional pressure losses. Failure is assumed to occur when any one of the simulated triaxial thermal and pressure stresses in the pipeline wall exceed its ultimate tensile strength. The application of the model to a hypothetical example involving localized fire attack on an isolated natural gas pipeline reveals that the pipeline fails because of large thermal stresses on the outer surface in addition to the internal pressure stresses. The efficacy of emergency depressurization using different diameter relief valves as a means of protecting the pipeline mechanical integrity during fire attack is also quantitatively investigated. (C) 2007 American Institute of Chemical Engineers.