BLEVE: Occurs when a liquefied gas is heated and evaporates from the vessel via the relief valve. Initially the liquid content inside the container will absorb this heat application, as the liquid acts as a heat absorber, however this will increase the pressure and result in further release of gas.  Then as the volume decreases due to relief valve operation, the surface area left exposed to vapour space increases. Ultimately the heat applied to this uncooled portion of the tank will reach breaking point and the cylinder will over stress and rupture. For this reason, flame impingement on the upper or vapour portion of the tank is most likely to cause BLEVE.

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This phenomenon is most usually associated with bulk LPG containers but can have the same affect on most liquefied gases (except cryogen’s).

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For each gas there is a ‘critical temperature’, above which it cannot be compressed by pressure alone. For example LPG (propane) has a critical temperature of 97 0C. Below this temperature it can be liquefied by increased pressure and it should be properly described as a vapour. Above this temperature it cannot be liquefied and is properly described as a gas, or to emphasis the fact that it is above its critical temperature, a “true gas”. The pressure required to liquefy a vapour at its critical temperature is called the ‘critical pressure’.

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Liquefied gases in cylinders do not obey the gas laws, since below critical temperature any change in temperature, pressure or volume will result in either liquefaction of gas or evaporation of liquid. Thus the pressure in a cylinder will remain constant as a gas is drawn off (provided the temperature remains constant) since more liquid will evaporate to make up for the gas drawn off until all the liquid has evaporated. 

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BLEVE should not be confused with ‘boil-over’, ‘slop-over’ and ‘froth-over’ which occur in liquid fuels.

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Boil-over:  The expulsion of crude oil (or certain other liquids) from a burning tank. The light fractions of the crude oil burning off producing a heat wave in the residue, which on reaching water strata may result in the expulsion of a portion of the contents of the tank, sometimes violently.

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Slop-over: Can occur when a water stream is applied to the hot surface of burning oil, provided the oil is viscous and above 1000C. It can also occur when the heat wave contacts a small amount of stratified water within the crude oil. It is also known to occur when the application of firefighting medium and tank contents exceeds the capacity of the tank, the burning oil layer overflows the tank.

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Froth-over: Not so relevant to firefighters but in general occurs when water already inside a tank comes into contact with a hot viscous oil which is being loaded, as in the loading of hot asphalt into a tank that contains some condensation.

 

Other interesting facts & figures relevant to LPG and of particular interest to firefighters are that it is made of various constituents, of which propane and butane are the most prevalent. It is colourless but may appear as a shimmer. LPG liquid is half the weight of water whereas LPG gas is twice the weight of air.  It ranges in flammability from 1.8 to 8.5% and is odourless. To overcome this distributors add methyl mercaptan as an odourant. It has an ignition temperature of around 4500C (petrol 3000C). It’s melting point is -1880C and it boils at -44.50C  (@ atmospheric pressure).

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Brendon Wood
GradDipEmergMgmt M.I.Fire.E FNZFBI 
Past President
New Zealand Fire Brigades Institute.