Liquid temperature stratification and its effect on BLEVEs and their hazards

Recent fire tests involving over forty, 4001 automotive fuel tanks filled with commercial propane have shown that the likelihood of a BLEVE and the severity of its hazards are significantly affected by the detailed thermodynamic condition of the lading at the time of failure. When the liquid in a tank is heated by fire impingement on the tank external shell, the liquid near the heated wall will tend to rise because of buoyancy effects. This leads to the development of temperature stratification where the liquid near the top of the tank will be at a higher temperature than liquid lower down. The pressure in the tank is dictated by the warmest liquid. This means that when the liquid is stratified the pressure in the tank is higher than the pressure one would calculate from the average liquid temperature. If the tank fails at the pressure relief valve (PRV) set pressure the resulting release will be less powerful if the liquid is stratified. When the PRV is activated on a tank it usually vents vapour to the surroundings. This vapor flow results in boiling action in the liquid and this boiling causes heat transfer and mixing and these cause destratification of the liquid. The time for destratification increases with the scale of the system. Eventually, the PRV may eliminate the stratification and the liquid will consist of a near isothermal liquid mass. If the tank fails when it is full of liquid, and the liquid is uniformly at the saturation temperature for the PRV set pressure, then the resulting BLEVE and hazards will be maximized for the given tank. Based on recent fire test data, this paper discusses how temperature stratification and destratification is affected by the fire type and the PRV action. The paper also discusses how this temperature stratification effects the likelihood of a BLEVE, and the severity of the associated hazards including fireball heat flux, blast overpressure and projectiles.