A robust numerical simulation is presented for predicting the risk of rupture during emergency depressurization or blowdown of cylindrical vessels containing high-pressure two-phase hydrocarbon mixtures under fire attack. Accounting for nonequilibrium effects between the constituent fluid phases, the model simulates the triaxial transient thermal and pressure stress profiles generated in both the wetted and unwetted wall sections of the vessel. A comparison of this information with the vessel material of construction temperature/yield stress data allows the precise evaluation of the risk of failure and, if applicable, the rupture mode during depressurization. Finally, based on the application of the model to a real system, the subtle differences between the failure risks associated with blowdown under fire as compared to those tinder ambient conditions are presented and discussed in detail.