A boiling liquid expanding vapor explosion (BLEVE, // BLEV-ee) is an explosion caused by the rupture of a vessel containing a pressurized liquid that has reached a temperature above its boiling point. Because the boiling point of a liquid rises with pressure, the contents of the pressurized vessel can remain a liquid as long as the vessel is intact. If the vessel's integrity is compromised, the loss of pressure drops the boiling point, which can cause the liquid to convert to a gas expanding rapidly. If the gas is combustible, as in the case with hydrocarbons and alcohols, further damage can be caused by the ensuing fire.
There are three key elements that cause a BLEVE.
Typically, a BLEVE starts with a container of liquid which is held above its normal, atmospheric pressure boiling temperature. Many substances normally stored as liquids, such as CO2, propane, and other similar industrial gases have boiling temperatures far below room temperature when at atmospheric pressure. In the case of water, a BLEVE could occur if a pressurized chamber of water is heated far beyond the standard 100 °C (212 °F). That container, because the boiling water pressurizes it, must be capable of holding liquid water at very high temperatures.
If the pressurized vessel, containing liquid at high temperature (which may be room temperature, depending on the substance) ruptures, the pressure which prevents the liquid from boiling is lost. If the rupture is catastrophic, where the vessel is immediately no longer capable of holding any pressure, then there suddenly exists a large mass of liquid which is at a very high temperature and very low pressure. This causes a portion of the liquid to "instantaneously" boil, which in turn causes an extremely rapid expansion. Depending on temperatures, pressures and the substance involved, that expansion may be so rapid that it can be classified as an explosion, fully capable of inflicting severe damage on its surroundings.
For example, a tank of pressurized liquid water held at 204.4 °C (400 °F) might be pressurized to 1.7 MPa (250 psi) above atmospheric ("gauge") pressure. If the tank containing the water were to rupture, there would for a brief moment exist a volume of liquid water which would be at:
At atmospheric pressure the boiling point of water is 100 °C (212 °F) - liquid water at atmospheric pressure does not exist at temperatures higher than 100 °C (212 °F). At that moment, the water would boil and turn to vapour explosively, and the 204.4 °C (400 °F) liquid water turned to gas would take up significantly more volume (≈1,600-fold) than it did as liquid, causing a vapour explosion. Such explosions can happen when the superheated water of a boiler escapes through a crack in a boiler, causing a boiler explosion.
A BLEVE does not have to be a chemical explosion, nor does there need to be a fire: however, if a flammable substance is subject to a BLEVE, it may also be subject to intense heating, either from an external source of heat which may have caused the vessel to rupture in the first place, or from an internal source of localized heating such as skin friction. This heating can cause a flammable substance to ignite, adding a secondary explosion caused by the primary BLEVE. While the blast effects of any BLEVE can be devastating, a flammable substance such as propane can add significantly to the danger.
While the term BLEVE is most often used to describe the results of a container of flammable liquid rupturing due to fire, a BLEVE can occur even with a non-flammable substance such as water, liquid nitrogen, liquid helium or other refrigerants or cryogenics, and therefore is not usually considered a type of chemical explosion. Note that in the case of liquefied gasses, BLEVEs can also be hazardous because of rapid cooling due to the absorption of the enthalpy of vapourization (e.g. frostbites), or because of possible asphyxiation if a large volume of gas is produced and not rapidly dispersed (e.g. inside a building, or in a trough in the case of heavier-than-air gasses), or because of the toxicity of the gasses produced.
BLEVEs can be caused by an external fire near the storage vessel causing heating of the contents and pressure build-up. While tanks are often designed to withstand great pressure, constant heating can cause the metal to weaken and eventually fail. If the tank is being heated in an area where there is no liquid, it may rupture faster without the liquid absorbing the heat. Gas containers are usually equipped with relief valves that vent off excess pressure, but the tank can still fail if the pressure is not released quickly enough. Relief valves are sized to release pressure fast enough to prevent the pressure from increasing beyond the strength of the vessel, but not so fast as to be the cause of an explosion. An appropriately sized relief valve will allow the liquid inside to boil slowly, maintaining a constant pressure in the vessel until all the liquid has boiled and the vessel empties.
If the substance involved is flammable, it is likely that the resulting cloud of the substance will ignite after the BLEVE has occurred, forming a fireball and possibly a fuel-air explosion. If the materials are toxic, a large area will be contaminated.
The term "BLEVE" was coined by three researchers at the Factory Mutual insurance company, in the analysis of an accident at one of their research facilities in 1957 involving a chemical reactor vessel.
On 18 August 1959, the Kansas City Fire Department suffered its second largest loss of life in the line of duty, when a 25,000 gallon (95,000 liters) gasoline tank exploded during a fire on Southwest Boulevard, killing 5 firefighters.
Examples of other BLEVE incidents have included:
Some fire mitigation measures are listed under liquefied petroleum gas.
Transport Canada published a training video for emergency response personnel to respond to and prevent BLEVEs. They also advise that expert advice can be obtained from Transport Canada's Canadian Transport Emergency Centre, CANUTEC.