by Michael Capraro

Created on: November 30, 2009

BLEVE is an acronym for Boiling Liquid Expanding Vapor Explosion. Even if the term is unfamiliar to you, you might be familiar with the dramatic results of a BLEVE from news reports of chemical plant fires or train derailments. This is a brief introduction to BLEVEs with some additional notes for those interested.

BLEVEs occur when a vessel containing liquid above its normal (atmospheric pressure) boiling point fails catastrophically1,2. With the sudden drop in pressure, the liquid undergoes rapid transition to the vapor phase (i.e. boils) resulting in a pressure wave and flying debris propelled by the rapidly expanding vapor cloud3. If the vapor cloud ignites, you can get a spectacular fireball.

The vessels types that might be involved include tank trucks, rail cars, 55 to 85-gal. drums, high pressure cylinders, intermodal containers and fixed facility storage tanks.

Many people think you can have a BLEVE only with flammable liquids. This is not true. Perhaps the most common example would be your basement hot water heater. In a fire, these can and do BLEVE. Since the resultant cloud of water vapor is not flammable, you won't get a fireball, but you do get the pressure wave and shrapnel from the explosion.

BLEVE incidents are closely associated with fires. The fire both provides a heat source to raise the temperature and vapor pressure of the contained liquid, and may weaken the container metal causing vessel failure. However, you can have a BLEVE without a fire. Some materials like LPG (liquefied petroleum gas) have normal boiling points well below zero (about -42 C for LPG) and are stored and transported under considerable pressure. These could BLEVE in the absence of fire if some other factor led to sudden vessel failure4. These pressure vessels are very robust and designed to withstand considerable abuse (e.g., train derailments), so such a non-fire event would be very rare.

In a major incident, if there is a potential BLEVE hazard, on-scene emergency responders will want you to evacuate the area. A standard evacuation radius is one half-mile away from the incident5. They are not being overly conservative. To view an actual incident, go to http://www.youtube.com/watch?v=Xf3WKTwHpIU. In this case, you will see not only the huge explosion and fireball, but a large portion of railcar that was blown over 3600 feet away - significantly more than a half mile. You might want to pad your evacuation distance accordingly.

Before you high-tail it, however, if you

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