BLEVEs are most commonly associated with liquefied petroleum gas (LPG) vessels, but any liquefied pressurised gas container may cause a BLEVE when heated or involved in a fire.
When pressurised containers are heated, without adequate cooling or venting, a BLEVE could occur. This is usually a result of a vessel containing a liquefied pressurised gas being heated by an external fire. The contents of the vessel are heated above their boiling point and the pressure in the vessel increases.
A BLEVE can occur at any time and depends on:
- Whether the vessel is full, half full or empty
- The condition of the vessel
- The functioning of the pressure relief valve
- The duration of flame impingement above the liquid level
The main hazards from a BLEVE involving a flammable gas are:
- Fire
- Thermal radiation from the fire
- Blast
- Projectiles
The danger from these hazards decreases further away from the BLEVE centre. Projectiles are the hazards with the greatest reach. The size of the BLEVE depends on the size and weight of the container, along with the amount of liquid that remains inside the container at the moment of the BLEVE. Generally speaking, the bigger the container, the bigger the BLEVE. Most flame-induced liquefied gas BLEVEs occur when half to three-quarters of the liquid remains in the container.
For further details on a BLEVE, its causes and effects, refer to Foundation for Hazardous Materials. Guidance on BLEVE hazard areas, evacuation distances, tank properties, critical times and cooling water flow rates for various tank sizes can be found in the Emergency Response Guide (ERG). The information provided is for guidance and should only be used where there is no specialist advice or site-specific information. Where times are given for tank failure or tank emptying through the pressure relief valve, the times provided in the ERG are typical but they can vary from situation to situation. Regarding cordons, the distances provided in ERG are very large and may not be practical in a highly populated area. However, it should also be understood that the risks increase rapidly the closer a person is to a BLEVE. Personnel should keep in mind that the furthest-reaching projectiles tend to come off in the 45-degree zones on each side of the tank ends.
Liquefied gas tanks/containers need significant heating (usually this means by direct flame) to reach BLEVE conditions but there is no ‘safe period’ when a pressurised container is subjected to significant direct flame contact. Therefore, if adequate cooling is not available a BLEVE should be expected at any time.
Any significant flame contact on the surface area of liquefied gas tanks or containers above the level of the liquid (i.e. the ‘dry walled’ areas at the top of the cylinder) is most dangerous as the internal gas will not conduct heat away as quickly as internal liquid.
The pressure relief valve operating will temporarily reduce overpressure, leading to a torching flame from the valve. This reduction in tank contents may hasten the onset of a BLEVE as the dry wall area increases as the tank is emptied.
In the event of a BLEVE a fireball can engulf exposed personnel; using heavy, coarse sprays should be considered to provide a measure of protection for personnel and equipment against fire effects.
refer to Control measure – Cordon control: Cylinders and pressurised gas and Cool pressurised gas containers.