Boiling Liquid Expanding Vapor Explosions valve

At first it was thought that the spheres burst because their relief valves were too small. But later it was realized that the metal in the upper portions of tlie spheres was softened by the heat and lost its strength. Below the liquid level, the boiling liquid kept the metal cool. Incidents such as this one in which a vessel bursts because the metal gets too hot are known as Boiling Liquid Expanding Vapor Explosions or BLEVEs. 

Boiling Liquid Expanding Vapor Explosion (BLEVE) characterizes the failure mechanism of a liquid gas container where the major part of the contents are released by flash vaporization and explode while the remainder forms a pool which bums down. Three conditions need to be met (i) the liquid is supeiheated, (ii) a sudden pressure drop occurs, (iii) pressure and temperature conditions are such that flash vaporization is possible. This type of explosion is caused if a storage tank is exposed to a heat source from outside. A tank stmcture weakened by the high temperatures, a high inner pressure, and tensions in the container wall near the gas / liquid boundary layer eventually results in the catastrophic failure, however, merely causing pressure waves. An intact safety valve could delay, but possibly not prevent the vapor explosion [73]. 

If a flammable liquid is in a closed container, the vapor pressure will increase inside the container as the temperature of the liquid increases. This increase in temperature can come from many different sources. Increases in ambient temperature, radiant heat from the sun, or a nearby fire can increase the vapor pressure in a container. As the pressure increases in a container, it will reach the setting on the pressure-relief valve and the relief valve will function. If this pressure increase occurs in a container that does not have a relief valve, the container may rupture. Rupture may also occur in a container with a relief valve if the pressure rises too fast for the relief valve to vent the material into the air, or if the relief valve is not working properly. In either case, the rupture may be violent, with a fireball and flying pieces of tank that can travel over a mile from the blast site. This phenomenon is referred to as a boiling liquid expanding vapor explosion (BLEVE). 

BLEVEs (Boiling Liquid Expanding Vapor Explosions) A pressurized tank of VCM or associated piping exposed to an external fire may fail due to metallurgical weakening. Such failure may result in a catastrophic tank failure, a fireball and the potential for rocketing fragments. Relief valve overpressure protection will not prevent a BLEVE. 

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