When a pressure vessel containing a pressure liquefied gas is subjected to fire heat input the heat transfer process in the liquid and vapour space is significantly affected by the pressure relief valve (PRV) action. Experiments have shown that liquid temperature stratification builds until PRV action causes boiling induced mixing which dissipates the stratification. This stratification and destratification process can have a significant effect on the thermal energy stored in the vessel. Studies have shown that the hazards associated with loss of containment of pressure liquefied gases and BLEVEs (Boiling Liquid Expanding Vapour Explosions) are strongly affected by the energy stored in the liquid phase in the pressure vessel at the time of failure. It is desirable to minimize the stored energy in a vessel to reduce hazards if a failure were to occur. A study was initiated by Transport Canada at Queen's University to quantify the effect of various PRV parameters on the thermal energy storage in a pressure vessel exposed to fire. It is believed that different types of PRV action (i.e. continuous flow, cycling with small blowdown, cycling with large blowdown) can have a significant effect on the energy stored in a vessel exposed to external heat input. This paper describes results obtained from experiments carried out using a test pressure vessel equipped to study 2-phase energy storage and transient blowdown. The experiments were designed to see how different PRV cycling patterns affected energy storage in the vessel. In the tests conducted, steam entered the vessel vapour space and heat was driven down into a cool liquid while a PRV operated. The test objective was to see if different PRV cycling patterns changed the rate of heat transfer to the liquid.
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