When a pressure vessel is exposed to an engulfing fire, the vessel can fail due to a combination of high internal pressure and material degradation due to high wall temperatures. In fire-engulfed vessels containing pressurized liquids in equilibrium with their vapors, high wall temperatures will occur in the upper regions of the vessel where the wall is in contact with vapor. Several methods exist for thermal protection of vessels, including thermal insulation and external water spray cooling. Recent tests have shown internal spray cooling to be potentially effective as thermal protection. The present paper presents a theoretical evaluation of a new concept in thermal protection which involves cooling the upper regions of vessel during fire engulfment by directing 2-phase fluid along the upper regions of the vessel wall during pressure relief. Based on observations of the internal cooling device from small-scale tests, a model was formulated and integrated with an existing vessel-in-fire computer model. This model was then used to study the effectiveness of internal spray cooling on an example system consisting of a rail tank-car carrying propane. Simulation results indicate that internal wall cooling could be effective at reducing the risk of thermal ruptures of vessels. The simulation results for internally cooled vessels are compared with previously validated simulations of an uninsulated tank-car filled with propane exposed to engulfing fires.