The Effect of Elevated Temperatures on the Failure of Unfired Pressure Vessels

Abstract

The effect of elevated temperatures on the failure of pressure vessels constructed of mild steel was studied both analytically and numerically. This structural response issue is relevant to Homeland Security and tanks that are subjected to elevated temperatures from fires that result from blast or impact loading. Elevated temperature conditions may also be encountered during accidental fires, for example rural LPG tanks exposed to forest fires. For the analysis conducted, the material was assumed to behave in an elastic-plastic manner with linear hardening. The thermal loading for the results presented was based upon information relevant to a LPG (propane) vessel exposed to an external fire. Two-phase fluid conditions exist inside the vessel and, due to vapor/liquid stratification, the liquid occupies the lower region of the vessel whereas the vapor occupies the upper region of the tank. Due to the comparatively large thermal mass associated with the liquid phase, there can be a significant temperature gradient in the fluid and the wall between the lower regions of the tank and the upper regions occupied by the vapor. Results from the analyses indicate, as expected, that the pressure required to reach a fully plastic condition (tank failure) throughout the wall decreased almost linearly with increasing temperature. It was, however, somewhat surprising to find that the temperature gradient from the outside (hot) wall to the inside (cold) wall had little effect on the pressure required to reach the tank failure. Also considered in this study was the effect of tank wall thickness. A recent decision by ASME to decrease the design margin on its unfired pressure vessels from 4 to 1 to 3.5 to 1 allows the wall thickness on these vessels to be reduced by 12%. As discussed herein, results from analytical models indicate that this reduction in wall thickness will reduce the pressure required for tank failure by 0.6 MPa (∼90 psi). Results from the numerical model indicate that, when a two-dimensional temperature gradient is imposed on the wall adjacent to the liquid/vapor interface, a large thermal stress is induced that may increase the possibility for a BLEVE (Boiling Liquid Expanding Vapor Explosion), however more research is needed to confirm this possibilit. Results also indicate that wall thickness, in this case, has little effect on the wall reaching a fully plastic condition (assuming the internal tank vessel pressure to be at relief valve set pressure).