Boiling liquid expanding vapor explosion

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File:BLEVE.jpg
A BLEVE erupting from a tanker

A boiling liquid expanding vapor explosion (BLEVE, 11px /ˈblɛviː/ BLEV-ee) is an explosion caused by the rupture of a vessel containing a pressurized liquid above its boiling point.[1]

Mechanism

There are three of characteristics of liquids which are relevant to the discussion of a BLEVE:

  1. If a liquid in a sealed container is boiled, the pressure inside the container increases. As the liquid changes to a gas it expands - this expansion in a vented container would cause the gas and liquid to take up more space. In a sealed container the gas and liquid are not able to take up more space and so the pressure rises. Pressurized vessels containing liquids can reach an equilibrium where the liquid stops boiling and the pressure stops rising. This occurs when no more heat is being added to the system (either because it has reached ambient temperature or has had a heat source removed).
  2. The boiling temperature of a liquid is dependent on pressure - high pressures will yield high boiling temperatures, and low pressures will yield low pressures. A common simple experiment is to place a cup of water in a vacuum chamber, and then reduce the pressure in the chamber until the water boils. By reducing the pressure the water will boil even at room temperature. This works both ways - if you increase the pressure beyond normal atmospheric pressures you could suppress the boiling of hot water far beyond normal temperatures.
  3. When a liquid boils it turns into a gas. The resulting gas takes up far more space than the liquid did.

A BLEVE typically 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, oxygen, and other similar industrial gasses have boiling temperatures, at atmospheric pressure, far below room temperature. In the case of water, a BLEVE could occur if a pressurized chamber of water is heated far beyond the standard 212°F. That container, because the boiling water pressurizes it, is 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 incapable of holding any pressure at all, then there suddenly exists a large mass of liquid which is at very high temperature and very low pressure. This causes the entire volume of liquid to instantaneously boil, which in turn causes an extremely rapid expansion. That expansion is so rapid that it can be classified as an explosion; fully capable of inflicting severe damage on its surroundings.

Imagine, for example, a tank of pressurized liquid water held at 400°F. This vessel would normally be pressurized to 250PSI above atmospheric ("guage" pressure). Were the tank containing the water to split open, there would momentarily exist a volume of liquid water which is

  • at atmospheric pressure, and
  • Four hundred degrees.

At atmospheric pressure the boiling point of water is 212°F - liquid water at atmospheric pressure cannot exist at temperatures higher than 212°F. It is obvious, then, that 400°F liquid water at atmospheric pressure must immediately and instantaneously flash to gas causing an explosion.

It is important to note that a BLEVE need not be a chemical explosion - there does not 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 cause the vessel to rupture in the first place or from an internal source of localized heating such as skin friction. This heating can cause the substance to ignite, adding a secondary explosion caused by the primary BLEVE. While any BLEVE can be extremely devastating, a flammable substance such as propane can add significantly to the danger.

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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,[2] liquid nitrogen[3] liquid helium or other refrigerants or cryogens, and therefore is not usually considered a type of chemical explosion.

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, it may rupture faster without the liquid to absorb 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 at the correct rate.[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.


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, also termed a vapor cloud explosion (VCE). If the materials are toxic, a large area will be contaminated.[4]

Incidents

In August 1959 the Kansas City Fire Department was hit with their largest loss of life in the line of duty deaths to date, when a 25,000 gallon gas tank exploded during a fire on Southwest Boulevard killing five firefighters. This was the first time BLEVE was used to describe a burning fuel tank. Significant industrial BLEVEs include accidents at Sunray, Texas in 1956, Glasgow, Scotland in 1960, Feyzin, France in 1966; Crescent City, Illinois in 1970; Kingman, Arizona in 1973; Texas City, Texas in 1978; Murdock, Illinois in 1983; and San Juan Ixhuatepec, Mexico City in 1984.[5] In july 11th 1978, a BLEVE occurred with an ethylene tank truck in the Los Alfaques Disaster in Spain. In 2010 Bialystok in Poland. In the Columbine High School massacre, if the two propane tank bombs planted by Eric Harris and Dylan Klebold had successfully detonated, the result would have been a BLEVE explosion.

Safety measures

Some fire mitigation measures are listed under liquefied petroleum gas.

See also

References

  1. ↑ 1.0 1.1 Script error
  2. ↑ "Temperature Pressure Relief Valves on Water Heaters: test, inspect, replace, repair guide". Inspect-ny.com. http://www.inspect-ny.com/plumbing/Water_Heater_Relief_Valves.htm. Retrieved 2011-07-12.
  3. ↑ "Nitrogen Dewar Explosion at Texas A&M University". http://www.thebatt.com/2.8485/chemistry-lab-explosion-costly-1.1195774.
  4. ↑ "Chemical Process Safety" (PDF). http://chemelab.ucsd.edu/processdesign/safety/safety-notes.pdf. Retrieved 2011-07-12.
  5. ↑ "PEMEX LPG Terminal, Mexico City, Mexico. 19th November 1984: Accident summary". www.hse.gov.uk. 1984-11-19. http://www.hse.gov.uk/comah/sragtech/casepemex84.htm. Retrieved 2011-07-12.

External links

40x40px Look up boiling liquid expanding vapor explosion in Wiktionary, the free dictionary.

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