Boiling Liquid Expanding Vapour Explosions: Dynamic Re-Pressurization And Two-Phase Discharge

A detailed re-analysis of the catastrophic failures, or BLEVEs, of four 4.5-tonne water capacity LPG vessels subjected to jet fire attack has indicated that the severity of the event and the intensity of the fireballs formed was possibly a function of the initiating mode of vessel failure and the thermo-hydraulic state of the contents. The mechanism of vessel failure appeared to be a two-step process; first the formation of an initiating over-pressure crack in the high temperature vapor wetted walls of the vessel which arrests. followed later by a final catastrophic 'unzipping' of the containment and the nearly instantaneous release of its contents. This behavior is similar to that observed earlier with very small vessels. Possible reasons for the final rapid failure of the vessel are crack instability, local thermal stresses caused by quenching at the crack tip by the two-phase discharge, or over-stress due to re-pressurization.In order to evaluate whether thermal stress and/or over-stress by repressurization could be responsible an experimental study of the venting process from a pressure liquified gas (PLG) vessel was undertaken. The release was suddenly initiated through a top slit orifice, simulating an accidentally formed crack. Measurements, over a range of fills. of the static and dynamic (top and bottom) pressure histories of refrigerant 22 (R22, chloro-difluoromethane) originally constrained at 20 degrees C (0.91 MPa) demonstrated that significant dynamic over-pressures on the head-space of the vessel can occur with volumetric fills greater than 50% and openings of less than 4% of the vessel cross section. These results, in addition to time synchronized video records of the interior fluid behavior and the vessel's two-phase discharge process, allow a preliminary assessment of their roles in the catastrophic failure of a vessel and the ensuing Boiling Liquid Expanding Vapour Explosions (BLEVE).