Theodosius Dobzhansky

The Gaz liquefied under Pression present a big risk in the event of rupture of the tank: the boiling-explosion, or in English BLEVE ( boiling liquid expanding vapor explosion ). When a gas is compressed, starting from a certain pressure, it is transformed into liquid; this property makes it possible to store great quantities of gas in tanks: Butane, Propane, LPG (Petroleum gas liquefied)… There is above the liquid a “gas sky” (most of the product is liquid, a small portion is gas and occupies volume remaining). When gas of the tank is tapped, the liquid boils (with room temperature) and the vapor thus produced comes to compensate for withdrawn volume; it is the normal functioning.

fig. 1 - In the tank, the liquid gas is surmounted by a gas sky under pressure

So now the tank breaks (following a shock, because of corrosion, or by embrittlement by a Feu), it occurs a catastrophic explosion then:

the pressure of the gas sky releases and creates outside a wave of overpressure, a detonation; the pressure drops abruptly in the tank;
the liquid is put at to boil violently to compensate for this decrease in pressure, the quantity of gas released “reinflates” the tank (the gas does not have time to escape by the escape); the tank explodes then literally, causing a second wave of overpressure (much more important than the first) as well as the projection of metallic lusters;
the released gas mixes with the air; if it is about a flammable gas, he can form a true fireball.

fig. 2 - the explosion takes place in three times: 1 - decompression due to a rupture of the envelope - 2 - violent boiling and recompression of the tank - 3 - catastrophic rupture of the tank and formation of the gas cloud

To limit the accident risk, the tanks are in general provided with valves of safety which make it possible to slowly evacuate the pressure with a boiling controlled before the rupture of the wall.

This accident can occur cold (with room temperature), for example in the case of Carbon dioxide or of Dioxygène liquid, the rupture of the tank which can be due to a shock or the Corrosion; in this case, the valve cannot play its part. But most of the time, it occurs hot, in consequence of a Incendie. In this case, the pressure increases with heat; when the valve starts, the liquid part decreases with the profit of the gas sky, but, the gas less better transmits heat, this one thus concentrates in the top of the tank what causes its embrittlement, then its rupture. Moreover, if the gas is flammable, the opening of the valve can create a long flame (“blowtorch effect”)

This accident is very dreaded in industries storing of gases liquid under pressure, like by the firemen in the fires of vehicles (trucks or tank cars, tanks of LPG).

External bonds

the BLEVE: phenomenology and modeling of the heating effects , INERIS, direction of the accidental risks, 2002, downloadable here
fires of cars to the LPG
Illustration of a bleve: Refinery of Feyzin 1966

Random links: