# Boiling liquid expanding vapor explosion

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Flames subsequent to a flammable liquid BLEVE from a tanker. BLEVEs do not necessarily involve fire.

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

## Mechanism

There are three characteristics of liquids which are relevant to the discussion of a BLEVE:[citation needed]

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 because a steady-state equilibrium has been reached between the heat received from the heat source and the heat lost to the environment, or because it has had a heat source removed (in this latter case both the temperature and the pressure will fall if heat is allowed to leave the system).
2. The boiling temperature of a liquid is dependent on pressure - high pressures will yield high boiling temperatures, and low pressures will yield low boiling temperatures. 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 the pressure is increased beyond normal atmospheric pressures, the boiling of hot water could be suppressed far beyond normal temperatures. The cooling system of a modern internal combustion engine is a common example.
3. When a liquid boils it turns into a gas. The resulting gas takes up far more space than the liquid did.

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, 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 100 °C (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 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.

### Water example

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:

• atmospheric pressure
• a temperature of 204.4 °C (400 °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 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 vapor explosion. Such explosions can happen when the superheated water of a steam engine escapes through a crack in a boiler, causing a boiler explosion.

### BLEVEs without chemical reactions

A BLEVE need not 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 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,[2] liquid nitrogen,[3] liquid helium or other refrigerants or cryogens, 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 vaporization (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.

## 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 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.

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

The term "BLEVE" was coined by three researchers at Factory Mutual, in the analysis of an accident there in 1957 involving a chemical reactor vessel. Anthony Santos was one of the Factory Mutual chemical engineers who coined the term.[5]

On August 18, 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 liter) gasoline tank exploded during a fire on Southwest Boulevard, killing five firefighters. This was the first time the term BLEVE was used to describe the explosion of a burning fuel tank.[citation needed]

Later incidents have included:

## Safety measures

Some fire mitigation measures are listed under liquefied petroleum gas.