Falope, Gboyega Bishop Oyewale; (2002) Modelling the transient thermal response of pressurised vessels during blowdown under fire attack. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis presents the development of a number of mathematical models for simulating the transient response of pressurised vessels containing condensable hydrocarbon mixtures during vapour space blowdown under fire attack. This is followed by the quantitative evaluation of the consequent failure risks associated with such operations. Accounting for non-equilibrium effects between the constituent fluid phases, the models simulate the multi-dimensional transient thermal and pressure stress profiles generated in both the wetted and unwetted wall sections of different geometry vessels.A comparison of this information with the vessel material of construction yield and ultimate tensile stress data at the prevailing conditions, allows an evaluation of the risk of failure and, if applicable, the rupture mode during depressurisation.The study considers cylindrical as well as spherical vessels, as their different spatial 3-D structures result in different stress containment capabilities. Two types of fire scenarios involving total engulfment by a pool fire as well as high heat intensity localised jet fire attack are modelled.A major part of the study involves the application of the above models to hypothetical failure scenarios involving blowdown of condensable multi-component hydrocarbon mixtures. For example the blowdown under fire attack of a cylindrical and a spherical vessel with the same volume, initial pressure and equivalent orifice diameter of 3.02 m3, 116 bara and 10mm reveals that in both cases failure (plastic deformation) occurs at approximately the same time. In each case failure occurs in the vapour space due to the mechanical weakening of the vessel wall combined with the total thermal and pressure stresses. This is in contrast to the blowdown of the same vessels under ambient conditions where rupture due to low temperature induced ductile/brittle transition may occur in the wetted wall section.In the case of localised jet fire attack on the other hand the effect of the jet flux heating is to expose the vessel to severe thermal stresses, which far exceed the accompanying pressure stresses. Failure in this case is signified by the total stresses being in excess of the ultimate tensile strength of the vessel wall material. Finally, the importance of accounting for real fluid behaviour at the discharge orifice when modelling the characteristic dimensions and heat intensity of jet fires are highlighted. This is considered to be important since the most likely fate of the released inventory during blowdown is its instantaneous ignition thereby resulting in a jet fire. Application of a real fluid model based on homogenous equilibrium flow to the Chamberlain's [1987] empirical jet fire correlations produces good agreement with the published field data. The salient manifestations of real fluid behaviour such as two-phase flow on jet fire characteristics on the other hand are demonstrated by simulating a hypothetical jet fire formed during the blowdown of the 116 bara spherical vessel under fire attack.

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