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Boiling liquid expanding vapor explosion

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File:Philadelphia Energy Solutions Refinery BLEVE.png
A BLEVE–fireball at the Philadelphia Energy Solutions refinery, as rendered by the CSB

A boiling liquid expanding vapor explosion (BLEVE, Template:IPAc-en Template:Respell) is an explosion caused by the rupture of a vessel containing a pressurized liquid that has attained a temperature sufficiently higher than its boiling point at atmospheric pressure.<ref name="Kletz">Template:Cite book</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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 a portion of the liquid to boil and form a cloud of rapidly expanding vapor. BLEVEs are manifestations of explosive boiling.

If the vapor is flammable (as is the case with compounds such as hydrocarbons and alcohols) and comes in contact with an ignition source, further damage can be caused by the ensuing explosion and fireball. However, BLEVEs do not necessarily involve fire.

NameEdit

On 24 April 1957, a process reactor at a Factory Mutual (FM) facility underwent a powerful explosion as a consequence of a rapid depressurization. It contained formalin mixed with phenol. The burst damaged the plant. However, 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".<ref name=":22">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite magazine</ref> The expressions did not become of common use until the early 1970s, when the National Fire Protection Association's (NFPA) Fire Command and Fire Journal magazines started publishing articles using them.<ref>Template:Cite journal</ref>

MechanismEdit

There are three key elements in the formation of a BLEVE:<ref name=":0">Template:Cite book</ref>

  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 Template:Convert. That container, because the boiling water pressurizes it, must be capable of holding liquid water at very high temperatures.

File:Bleve explosion.svg
BLEVE mechanism

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 Template:Convert might be pressurized to Template:Convert 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:

At atmospheric pressure the boiling point of water is Template:Convert. Liquid water at atmospheric pressure does not exist at temperatures higher than Template:Convert. At that moment, the water would boil and turn to vapor explosively, and the Template:Convert 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.

The vaporization of liquid resulting in a BLEVE typically occurs within 1 millisecond after a catastrophic loss of containment.<ref name=":3" />

Superheat limit theoryEdit

File:Propane BLEVE conditions.png
In this diagram for propane, the orange curve represents its vapor pressure as a function of temperature. The minimum temperature above which a BLEVE can occur is the abscissa of the intersection between the atmospheric pressure horizontal line (blue) and a curve here called the superheat-limit locus. This is roughly a straight line with its upper limit at the gas critical conditions. A liquid expanding along AA' does not cross the superheat-limit locus and will not BLEVE. Conversely, for sufficiently high temperatures, as in the BB' expansion, the superheat-limit locus is crossed and a BLEVE will occur.<ref name=":4">Template:Cite journal</ref>

For a BLEVE to occur, the boiling liquid must be sufficiently superheated upon loss of containment. For example, at a pressure of approximately Template:Convert, water boils at Template:Convert. Superheated water released from a closed container at these conditions will not generate a BLEVE, as homogeneous nucleation of vapor bubbles is not possible.<ref name=":4" /> There is no consensus about the minimal temperature above which a BLEVE will occur. A formula proposed by Robert Reid to predict it is:

<math>T_{\text{min,BLEVE}}=0.895\ T_\text{C}</math>

where TC is the critical temperature of the fluid (expressed in kelvin). The minimum BLEVE temperatures of some fluids, based on this formula, are as follows:<ref name=":2">Template:Cite book</ref> Template:Table alignment

Substance Tmin,BLEVE
K °C °F
Water 579 306 583
n-Octane 509 236 457
n-Heptane 483 210 410
n-Hexane 454 181 358
n-Pentane 421 148 298
Ethyl eter 418 145 293
Phosgene 407 134 273
n-Butane 381 108 226
Chlorine 375 102 216
Ammonia 363 90 194
Propane 331 58 136
Propylene 327 54 129
Ethane 273 0 32
Carbon dioxide 272 –1 30
Ethylene 253 –20 –4
Methane 171 –102 –152

According to Reid, BLEVE will occur, more in general, if the expansion crosses a "superheat-limit locus". In Reid's model, this curve is essentially the fluid's spinodal curve as represented in a pressure–temperature diagram, and the BLEVE onset is a manifestation of explosive boiling, where the spinodal is crossed "from above", i.e., via sudden depressurization. However, direct correspondence between the superheat limit and the spinodal has not been proven experimentally. In practical BLEVEs, the way the pressure vessel fails may influence decisively the way the expansion takes place, for example causing pressure waves and non-uniformities. Additionally, there may be stratification in the liquid, due to local temperature variations. Because of this, it is possible for BLEVEs to occur at temperatures less than those predicted with Reid's formula.<ref>Template:Cite journal</ref>

Physical BLEVEsEdit

The term BLEVE is often associated to explosive fires from pressure vessels containing a flammable liquid. However, a BLEVE can occur even with a non-flammable substance such as water,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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–fireballEdit

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.

Effect of impinging firesEdit

BLEVEs are often 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.<ref name="Kletz" /> A pressure vessel is designed to withstand the set pressure of its relief valves, but only if its mechanical integrity is not weakened as it can be in the case of an impinging fire.Template:Sfnp In an impinging fire scenario, flammable vapors released in the BLEVE will ignite upon release, forming a fireball. The origin of the impinging fire may be from a release of flammable fluid from the vessel itself, or from an external source, including releases from nearby tanks and equipment. For example, rail tank cars have BLEVEd under the effect of a jet fire from the open relief valve of another derailed tank car.Template:Sfnp

HazardsEdit

The main damaging effects of a BLEVE are three: the blast wave from the explosion; the projection of fragments, or missiles, from the pressure vessel; and the thermal radiation from the fireball, where one occurs.Template:Sfnp

Horizontal cylindrical ("bullet") tanks tend to rupture longitudinally. This causes the failed tank and its fragments to get propelled like rockets and travel long distances.Template:Sfnp At Feyzin, three of the propelled fragments weighed in excess of 100 tons and were thrown Template:Convert from the source of the explosion. One bullet tank at San Juanico travelled Template:Convert in the air before landing, possibly the farthest ever for a BLEVE missile.Template:Sfnp Fragments can impact on other tanks or equipment, which may result in a domino effect propagation of the accidental sequence.<ref name=":3" />

Fireballs can rise to significant heights above ground.Template:Sfnp They are spheroidal when developed and rise from the ground in a mushroom shape.<ref name=":3" /> The diameter of fireballs at San Juanico was estimated at Template:Convert, with a duration of around 20 seconds. Such massive fires can injure people at distances of hundreds of meters (e.g., Template:Convert at Feyzin and Template:Convert at San Juanico).Template:Sfnp

An additional hazard from BLEVE-fireball events is the formation of secondary fires, by direct exposure to the fireball thermal radiation, as pool fires from fuel that does not get combusted in the fireball, or from the scattering of blazing tank fragments.Template:Sfnp<ref name=":3" /> Another secondary effect of importance is the dispersion of a toxic gas cloud, if the vapors involved are toxic and do not catch fire upon release.<ref name=":3" /> Chlorine, ammonia and phosgene are example of toxic gases that underwent BLEVE in past accidents and produced toxic clouds as a consequence.<ref name=":3" />

Safety measuresEdit

Template:See also

  • Maintenance of pressure tanks to avoid damage or corrosion<ref name="Transport-Canada-BLEVE">{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>

|CitationClass=web }}</ref>

  • Passive fire protection<ref name="CameoChemicals-NOAA" /><ref name="Transport-Canada-BLEVE" />
  • Water spray cooling<ref name="CameoChemicals-NOAA" /><ref name="Transport-Canada-BLEVE" />

Notable accidentsEdit

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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 expanding vapor explosion.<ref name=":1">Template:Cite journal</ref><ref>Template:Cite news</ref>
  • 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.<ref name=":1" />Template:Sfnp
  • 29 July 1943, Ludwigshafen, Germany – A chlorine tank car exploded at a BASF plant, killing 57.<ref name=":1" />Template:Sfnp
  • 28 July 1948, Ludwigshafen, Germany – Another tank car in the same plant, this time containing diethyl ether, exploded killing 209.<ref name=":1" /><ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>Template:Sfnp

|CitationClass=web }}</ref>

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  • 5 July 1973, Kingman, Arizona, U.S. – The Kingman explosion: An LPG-laden tank car BLEVEd, causing 13 fatalities.<ref name=":1" /><ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>

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See alsoEdit

ReferencesEdit

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SourcesEdit

Further readingEdit

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External linksEdit

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