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Types of major chemical/industrial hazards - Boiling Liquid Expanding Vapor Explosion (BLEVE)

The BLEVE occurs when the catastrophic failure of a vessel containing liquid flammable material in pressurized condition takes place. The visual effects of the BLEVE is shown in the photo.

 

For a BLEVE situation following four conditions must be present:-

  1. There must be a substance in liquid form. Most of the destructive BELEV's that have occurred have involved flammable liquids and liquefied flammable gases. BLEVE can occur with any liquid, even water. The only difference is that with non¬flammable liquids there is no fireball. However, there will still be damaging effect including the propagating of creaks in the structure of the container together with possibility of subsequent failure and propulsion.
  2. The liquid must be in a container like sphere, bullet, and road/rail tanker.
  3. The contained liquid must be at a temperature above its normal boiling point at atmospheric pressure at the time container allows the pressure inside to build up above atmospheric pressure, the fluid, in the container is able to remain in the liquid state, even through its temperature is above its normal boiling point. This increase in pressure raises the Boiling point of the contained liquid above its boiling point.
  4. There must be a failure of the container in order to have BLEVE. This container failure can be due to following courses:
  • Flame impingement.
  • Internal structural weakness of the container
  • Failure of improperly designed SRV
  • Impact from a mechanical cause such a road accident, tanker derailment allowing flammable liquid to flow out.

The physical force that causes the BLEVE is because of the large liquid to vapour expansion of the liquid in the container. LPG will expand 250 times its volume when changing from liquid to vapour and water will expand 1700 times its original volume. It is this expansion process that provides the energy for propulsion of the container and the rapid mixing of vapour from the container with air, resulting in the characteristic fireball when flammable liquids are involved. In most BLEVEs caused by exposure to fire, the container failure originates in the metal area of the vapour space because it is extremely difficult to heat the container metal significantly where it is in contact with liquid. The liquid conducts the heat away from the metal and acts as a heat absorber. Therefore, the metal around the vapour space can be heated to the point of failure.

The Hazards of BLEVE

A BLEVE poses the following hazards:

  • Fireball with thermal radiation with some rainout forming pool fires.
  • Missiles and Major fragmentation
  • Rocketing vessel parts
  • Overpressure from minor shock waves

The past experience has shown that whenever there is BLEVE; it is only the heat radiation of the fireball and the over pressure, which do the off-site damages. The effects of both can be well understood by the zones of the heat and over pressure generated by the BLEVE. BLEVE is causing damage due to heat and pressure both so, it is most dangerous.

 

 
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