Authors: Dr R Raman, Arriscar Pty Ltd, S Medonos, Petrellus Engineering Ltd. Proceedings of the Hazards Australasia, November 26-27, 2013, Perth, Australia.
The use of liquefied natural gas (LNG) is becoming popular in recent years as a transport fuel, due to its environmental advantages. However, LNG vapour being flammable the perception is that an LNG vessel, if subject to an external fire, may result in a boiling liquid expanding vapour explosion (BLEVE), similar to what could occur with liquefied petroleum gas (LPG) vessels.
Using a vessel heat-up model, the fire impact characteristics, heat transfer and thermodynamic characteristics, and structural impact of a LNG fuel vessel on a transport truck is explored. The main temperature variables are the external wall, the wetted and not-wetted side of the internal wall of the double-walled vessel, and the liquid and vapour temperatures which maintain the vapour-liquid equilibrium in the vessel. The set of constitutive differential equations were integrated using finite difference marching technique. Small time steps were used to avoid numerical instability. At each time step, the physical, thermodynamic and heat transfer properties of LNG, vessel material and the insulation material were calculated at the prevailing temperatures. The opening of the vessel pressure relief valve as the contents heat-up, and the chattering of the relief valve as the liquid/vapour maintain their thermodynamic equilibrium were also included.
The results are presented as wall and product temperature profiles, vessel pressure profile and the allowable tensile strength of vessel wall, and the applied stress (hoop and axial) in the vessel. It was found that for transport vessels of small inventories, inventory depletion would occur in a short time and that even for jet fires, the applied stress never exceeds the allowable tensile stress, indicating that a BLEVE of a LNG fuel tank in a transport truck would not occur when subject to an external fire.
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