A boiling liquid expanding vapor explosion (BLEVE, /ˈblɛviː/ BLEV-ee) is an explosion caused by the rupture of a vessel containing a pressurized liquid that is or has reached a temperature sufficiently higher than its boiling point.^[1][2] 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 gas expanding rapidly. BLEVEs are manifestations of explosive boiling.

If the gas is flammable, as is the case with e.g., hydrocarbons and alcohols, further damage can be caused by the ensuing fire. However, BLEVEs do not necessarily involve fire.

Mechanism

There are three key elements in the formation of a BLEVE:^[3]

1. A material in liquid form at a temperature sufficiently above its normal atmospheric pressure boiling point.
2. A containment vessel maintaining the pressure that keeps the substance in liquid form.
3. A sudden loss of containment that rapidly drops the pressure.

Typically, a BLEVE starts with a vessel containing liquid held above its atmospheric-pressure boiling temperature. Many substances normally stored as liquids, such as carbon dioxide, propane, and other industrial gases have boiling temperatures below room temperature when at atmospheric pressure. In the case of water, a BLEVE could occur if a pressure vessel is heated beyond 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 ruptures, the pressure which prevents the liquid from boiling is lost. If the rupture is catastrophic, i.e., the vessel becomes suddenly no longer capable of holding any pressure, then the liquid will find itself at a temperature far above its boiling point. This causes a portion of the liquid to instantaneously vaporize with extremely rapid expansion. Depending on temperatures, pressures and the material involved, the 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 350 °C (662 °F) might be pressurized to 10 MPa (1,500 psi) above atmospheric (or 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:

• Atmospheric pressure
• Temperature of 350 °C (662 °F).

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 vapor explosively, and the 350 °C (662 °F) liquid water turned to gas would take up significantly more volume (≈ 1,600-fold) than it did as liquid, causing a vapor explosion. Such explosions can happen when the superheated water of a boiler escapes through a crack in a boiler, causing a boiler explosion.

For a BLEVE to occur, the boiling liquid must be sufficiently superheated upon loss of containment. For example, at a pressure of approximately 1 MPa (150 psi), water boils at approximately 177 °C (351 °F). Superheated water released from a closed container at these conditions will not generate a BLEVE. There is no consensus about the minimal temperature above which a BLEVE will occur. One proposed formula to predict it is to take it equal to 89.5% of the critical temperature of the fluid. The minimum BLEVE temperatures of some fluids, based on this formula, are as follows:^[4]

Physical BLEVEs

The term BLEVE is often associated to explosive fires from pressure vessels a flammable liquid. However, a BLEVE can occur even with a non-flammable substance such as water,^[5] liquid nitrogen, liquid helium or other refrigerants or cryogenics. Such materials can go through purely physical BLEVEs, not entailing flames or other chemical reactions. In the case of unignited BLEVEs of liquefied gases, rapid cooling due to the absorption of the enthalpy of vaporization is a hazard that can cause frostbite. Asphyxiation from the expanding vapors is also possible, if the vapor cloud is not rapidly dispersed, as can be the case inside a building, or in a trough in the case of heavier-than-air gasses. The vapors can also be toxic, in which case harm and possibly death can occur at relatively low concentrations and, therefore, even far from the source.

BLEVE–fireball

If a flammable substance, however, is subject to a BLEVE, it can ignite upon release, either due to friction, mechanical spark or other point sources, or from a pre-existing fire that had engulfed the pressure vessel and caused it to fail in the first place. In such a case, the burning vapors will further expand, adding to the force of the explosion. Furthermore, a very significant amount of the escaped fluid will burn in a matter of seconds in a raising fireball, which will generate extremely high levels of thermal radiation. While the blast effects can be devastating, a flammable substance BLEVE typically causes more damage due to the fireball thermal radiation than the blast overpressure.

Hazards of impinging fires

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 (such as near its top), it may rupture faster because the boiling liquid does not afford cooling in that area. Pressure vessels 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.^[1] 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. In an impinging fire scenario, it is virtually certain that the released vapors will ignite upon release, forming a fireball.

Safety measures

• Maintenance of pressure tanks to avoid damage or corrosion^[6]
• Emergency depressurization^[3]
• Pressure relief valves^[7]
• Passive fire protection^[7][6]
• Water spray cooling^[7][6]

Notable accidents

Notable BLEVE accidents include:

• 13 December 1926, Saint-Auban, France – A 25 ton chlorine BLEVE killed 19 in the first accident recognized as a boiling liquid vapor explosion.^[8][9]
• 24 December 1939, Zărnești, Romania – A rail tank car containing a butadiene (80%) and butene (20%) mixture BLEVEd and caused a fireball. It killed 60 people.^[8][10]
• 29 July 1943, Ludwigshafen, Germany – A chlorine tank car exploded at a BASF plant, killing 57.^[8][11]
• 28 July 1948, Ludwigshafen, Germany – Another tank car in the same plant, this time containing diethyl ether, exploded killing 209.^[8][12][13]
• 7 July 1951, Newark, New Jersey, U.S. – Seventy tanks of liquefied petroleum gas (LPG), for a total of 2600 tons, exploded at Port Newark. There were no fatalities.^[8][14]
• 24 April 1957, United States – At a Factory Mutual (FM) facility, a process reactor containing formalin mixed with phenol BLEVEd as a consequence of a rapid depressurization. The burst damaged the plant. No fire developed, as the mixture was not flammable. In the wake of the accident, researchers James B. Smith, William S. Marsh, and Wilbur L. Walls, who were employed with FM, came up with the terms "boiling liquid expanding vapor explosion" and its acronym "BLEVE".^[15][16]
• 8 January 1957, Montreal, Quebec, Canada – At a Shell refinery, 5100 tons of LPG stored in several tanks exploded, causing one fatality.^[17][18][19]
• 28 June 1959, Meldrim, Georgia, U.S. – The Meldrim Trestle Disaster: An LPG rail tank car disaster killed 23.^[8]
• 28 March 1960, Glasgow, Scotland, United Kingdom – The Cheapside Street whisky bond fire: A warehouse storing 3900 tons of whisky experienced a fire in which several casks underwent BLEVEs. There were 19 fatalities.^[20]
• 4 January 1966, Feyzin, France – The Feyzin disaster: 1000 tons of LPG exploded at an Elf refinery, causing 18 fatalities.^[8]
• 30 March 1972, Rio de Janeiro, Brazil – 1972 Reduc explosion [pt]: 1000 tons of LPG exploded at a refinery, killing 28.^[8][21]
• 5 July 1973, Kingman, Arizona, U.S. – The Kingman explosion: An LPG-laden tank car BLEVEd, causing 13 fatalities.^[8][22]
• 8 December 1977, Cartagena, Colombia – A process reactor burst in an ammonia BLEVE that killed 30.^[8][23]
• 23 February 1978, Waverly, Tennessee, U.S. – The Waverly tank car explosion: An LPG tank car exploded, killing 16.
• 11 July 1978, Alcanar, Spain – The Los Alfaques disaster: In the worst ever tank truck BLEVE and road accident involving hazardous materials, a vehicle laden with 25 tons of propylene suffered a mechanical failure next to a crowded camping site. The BLEVE killed 216 people.
• 30 May 1978, Texas City, Texas, U.S. – Multiple storage tanks holding 1500 tons of LPG exploded, killing seven people.^[8][24]
• 30 August 1972, Good Hope, Louisiana, U.S. – A BLEVE developed after a ship collision between MV Inca Tupac Yupanqui and butane-laden barge TB Panama City. There were 12 fatalities.^[8][25]
• 1 August 1981, Cerritos, San Luis Potosí, Mexico – A rail tank car carrying chlorine exploded, causing 29 fatalities.^[8][26]
• 23 July 1984, Romeoville, Illinois, U.S. – The Romeoville petroleum refinery disaster killed 15 people.^[17]
• 19 November 1984, San Juan Ixhuatepec, Mexico – The San Juanico disaster: In the worst ever BLEVE accident, as well as one the deadliest industrial disasters ever occurred, more than 500 people died when a series of BLEVEs hit a Pemex LPG storage terminal.^[8][27]
• 28 January 1986, above Merritt Island, Florida, U.S. – The Space Shuttle Challenger disaster: The disintegration of the spacecraft was caused by the BLEVE of the liquid hydrogen and oxygen external tank compartments.^[8][28]
• 23 December 1988, Memphis, Tennessee, U.S. – The Memphis LPG tank truck disaster killed nine people.^[29]
• 19 April 1993, Waco, Texas, U.S. – A contributor to the aftermath of the Waco siege was the BLEVE of an LPG tank caused by the intervention of the military.^[30]
• 4 March 1994, Weyauwega, Wisconsin, U.S. – The Weyauwega derailment, no fatalities.
• 22 June 2002, Tivissa, Spain – In the first ever reported liquefied natural gas (LNG) BLEVE, a tank truck exploded. Only the driver perished.^[31]
• 10 August 2008, Toronto, Ontario, Canada – The Toronto propane explosion at the Sunrise Propane Industrial Gases killed two people.^[32]
• 27 August 2012, Chala, India – The Chala LPG tank truck disaster: An LPG tank truck road accident in the state of Kerala caused 20 fatalities among the bystanders.
• 21 July 2014, Lice, Turkey – An LPG tank truck explosion caused the death of 34 people.^[33]
• 6 August 2018, Bologna, Italy – Two people perish in the Borgo Panigale explosion, an LPG tank truck blast caused by a road accident.^[34]
• 21 June 2019, Philadelphia, Pennsylvania, U.S. – The 2019 Philadelphia refinery explosion: Following a fire, a process vessel containing a butene, isobutane and n-butane suffers a massive BLEVE.
• 24 December 2022, Boksburg, South Africa – The Boksburg explosion, an LPG tank truck BLEVE that killed 41.