The development of a rigorous mathematical model for simulating the failure risk of pressurized spherical hydrocarbon containing vessels during blowdown under fire attack is described Accounting for the important thermodynamic and interfacial mass and heat transfer processes taking place during blowdown, the model simulates the thermal and pressure stresses in the vessel wall in both the radial and tangential directions. Failure is assumed to take place when the sum of these two stresses in either direction exceeds the vessel material of construction yield strength. Application of the model to the blowdown of a condensable hydrocarbon mixture indicates that failure occurs in the vapor space due to thermal weakening of the vessel wall. Finally, the risks associated with blowdown during fire attack for a cylindrical as opposed to a spherical vessel of the same volume and prevailing conditions are compared and contrasted.