BLEVE and Heat Induced Tear
BLEVE (Boiling Liquid Expanding Vapor Explosion)
The following section presents important safety-related information on BLEVEs to consider in a situation involving Liquefied Petroleum Gases (LPG), UN1075.
LPGs include the following flammable gases:
- UN1011 — Butane
- UN1012 — Butylene
- UN1055 — Isobutylene
- UN1077 — Propylene
- UN1969 — Isobutane
- UN1978 — Propane
A BLEVE occurs when a fire-impinged or damaged tank car fails to contain its internal pressure and explodes with a sudden product release. This catastrophic failure is more likely to occur with damaged pressure tank cars, even in the absence of an active fire.
Main Hazards from an LPG BLEVE
- Fire: If the released substance is ignited, there is an immediate fireball.
- Thermal radiation: At a distance of about 4 times the radius of a fireball, the heat radiated from a fireball is enough to burn exposed skin in 2 seconds. Wearing protective clothing limits the thermal radiation dose.
- Blast: A concussive force caused by the sudden release of the pressurized substance. For a BLEVE occurring out in the open, the blast strength at a distance of 4 times the radius of a fireball can break window glass and may cause minor damage to buildings.
- Projectiles: Tank failure can throw metal fragments over large distances. These fragments can and have been deadly.
The danger decreases as you move away from the BLEVE centre. The furthest-reaching hazard is projectiles.
Heat Induced Tear (HIT)
A heat induced tear (HIT) is a rupture of a NON-PRESSURE tank car containing flammable liquids when exposed to the intense heat of a fire. The metal will soften and the pressure in the tank car will increase which can lead to containment failure. The tear generally occurs at the vapor space (upper side) of the container, venting large quantities of flammable liquid and vapors at high speed. A fireball and an intense heat wave will occur.
Compared to BLEVEs, HITs rarely result in the projection of tank car fragments. Heat induced tearing has occurred within 20 minutes of the derailment and as long as 10+ hours following the initial fire.
Responding to these types of incidents (BLEVE and HIT) requires specialized training, equipment and a tactical approach.
BLEVE — Safety Precautions (Interpreting the Distance Table)
Use with caution. The ERG BLEVE Safe Distances table (p. 359) summarizes tank properties, critical times, critical distances and cooling water flow rates for various tank sizes. The values below explain how to interpret that table.
- Tank dimensions are approximate and can vary depending on the tank design and application.
- Minimum time to failure is based on severe torch fire impingement on the vapor space of a tank in good condition, and is approximate. Tanks may fail earlier if they are damaged or corroded. Tanks may fail minutes or hours later than these minimum times depending on the conditions. It is assumed that the tanks are not equipped with thermal barriers or water spray cooling.
- Minimum time to empty is based on an engulfing fire with a properly sized pressure relief valve. If the tank is only partially engulfed, then time to empty will increase (e.g., 50% engulfed → twice as long). Assumes no thermal barrier or water spray.
- Thermal barriers or water spray cooling significantly increase the times to failure and the times to empty. A thermal barrier can reduce heat input by a factor of ten or more.
- Fireball radius and emergency response distance are based on mathematical equations and are approximate. They assume spherical fireballs, which is not always the case.
- Two safety distances for public evacuation: The minimum distance is based on tanks launched with a small elevation angle (a few degrees above horizontal, most common for horizontal cylinders). The preferred evacuation distance has more margin of safety, assuming a 45-degree launch angle — more appropriate for vertical cylinders.
- These distances are very large and may not be practical in a highly populated area. However, the risks increase rapidly the closer you are to a BLEVE. Furthest-reaching projectiles tend to come off in the zones 45 degrees on each side of the tank ends.
- Water flow rate is given in US gal/min needed to cool tank metal.
Warning: The data given are approximate and should only be used with extreme caution. Times for tank failure or tank emptying through the pressure relief valve are typical but can vary from situation to situation. Never risk life based on these times.