Once again I turn to the professionals up North to provide us with some great information. Transport Canada has once again done an excellent job explaining the difference between a Boiling Liquid Expanding Vapor Explosion (BLEVE) and a Heat-Induced Tear (HIT) type explosion. So often when we see the large fireball, we will call the event a BLEVE; however, this is oftentimes an incorrect classification of the event (a BLEVE can occur with NO fireball).
For a BLEVE to occur we MUST have a “pressure vessel” involved; for a HIT to occur we can have an “atmospheric container” involved. Both events will result in a spectacular fireball (when the event involves a flammable material), but a HIT will most likely NOT propel its container as we see in a BLEVE event. Here is a gasoline TANK suffering a HIT. … It is officially a HIT and not a BLEVE!
In Transport Canada’s Transportation of Dangerous Goods latest newsletter, they explain the difference in these spectacular explosions…
During a BLEVE, a fire impinged or damaged tank car fails to contain its internal pressure and explodes with a sudden product release, propelling tank car fragments with great force and high speed over long distances. In addition, if the product (e.g. liquefied pressure gas such as propane) is flammable, there will also be a fireball and intense heat generated. This catastrophic failure is more likely to occur on damaged pressure tank cars, whether under fire conditions or not.
A HIT is a different type of tank car failure. Under the intense heat of a fire, non-pressure tank cars containing flammable
liquids will tear, generally at the top, causing the product to suddenly vent at high speed, generating a fireball and an intense heat wave. Unlike BLEVEs, HITs rarely result in the projection of tank car fragments. HITs have been known to occur within 20 minutes to several hours after a derailment of tank cars carrying flammable liquids such as crude oil or ethanol. Unlike a BLEVE, where all the product is quickly released or consumed, a tank car with a HIT may still contain product that can burn or leak for 8 hours or more.
For rail incidents involving tank cars containing flammable liquids under fire conditions, the risk of HITs makes the response very complex and dangerous. HITs can occur without warning and produce a fireball, generate extremely intense heat and release toxic gases. Anyone too close could suffer serious or fatal burns. Responding safely to a flammable liquids rail incident requires specialized training, specialized equipment and a tactical approach. First Responders must first focus on securing the scene; any other action could put both the responders and the public at risk.
CLICK HERE to see their entire article on this subject. It is a MUST READ for ALL Safety and Emergency Responders who deal with both FLAMMABLE MATERIALS and/or Liquefied Pressurized Gases.