Hazardous substances such as petrol and lubricating oil, combine with the oxygen in the air when ignited, to generate large amounts of carbon dioxide, water vapour and heat.

The heat of combustion radiates, and is conducted back to the remaining liquid material, and serves to vaporise it and provide an expanding volume for the combustion reaction. This accelerating reaction is known as Boiling Liquid Expanding Volume Explosion.

The rate of expansion of the reaction is dependent on the heat of combustion of the material, and the latent heat of vaporisation. Materials such, as methanol which has a low heat of combustion and a high latent heat of vaporisation, do not present a major hazard in many situations.

Petrol, which consists of a mixture of hydrocarbons such as pentane, octane, and benzene, has a high heat of combustion, and a low latent heat of vaporisation, is always a major hazard. Thus there is a preference for use of alcohol fuels, in speed events at circuits such as Indianapolis, in America.

Diethyl ether, which has a very low latent heat of vaporisation, a low heat of combustion, and a very low flashpoint presents a special risk, it often serves to initiate incidents by facilitating the ignition of other flammable liquids.

The combination of flashpoint, latent heat of vaporisation, and heat of combustion, contribute to a property known explosive range, defined by Lower and Upper Explosive Levels. Within this range the vaporised material will ‘explode’, that is a shock wave will pass through the volume of vapor, with attendant immediate release of energy. This occurs as the combustible material vapour/ oxygen mixture approaches stochiometric proportions, and the reaction is chemically ‘balanced’.

b) Liquified Combustible Gases (Dangerous Goods Class 2.1).

Liquified Natural Gas, and Liquified Petroleum Gas, which consist of the hydrocarbons methane, propane and butane, present a special hazard, as they exist as vapour at normal temperatures and pressures. They combine readily with oxygen form theair to make an explosive mixture.

c) Flammable Solids (Dangerous Goods Class 4.1)

Flammable solids such as nitrocellulose contain their own chemically bonded oxygen, and will burn rapidly and often uncontrollably to form carbon dioxide, water vapour and heat. Nitrocellulose is often considered to be an explosive, particularly when finely divided.

d) Spontaneously Flammable Substances (Dangerous Goods Class 4.2)

Spontaneously flammable substances such as white phosphorus, can initiate fires if stored with other flammable materials. White phosphorus is usually stored under water, as it spontaneously combine with the oxygen in the air, if exposed, to generate heat and flame.

e) Substances Flammable If Wet (Dangerous Goods Class 4.3)

Substances which are flammable if wet, present a particular hazard. Calcium carbide releases acetylene when wet, and will provide extra fuel if inadvertently wetted, during firefighting.

f) Poison Gases (Dangerous Goods Class 2.3)

Gases such as chlorine, fluorine and bromine, are chemical oxidising agents. They will react with hydrocarbons, to generate heat and cause combustion. They should not be stored with Dangerous Goods Class 1, 2.1, 3 or 4.1, in particular.

g) Oxidising Substances (Dangerous Goods Class 5.1, 5.2)

Oxidising substances such as nitrates (potassium and ammonium nitrate), and organic peroxides (MEKP) are chemical oxidising agents. They will react with hydrocarbons, to generate heat and cause combustion. They should not be stored with Dangerous Goods Class 1, 2.1, 3 or 4.1, in particular.

h) Explosives (Dangerous Goods Class 1)

Explosives generally contain their own chemically bonded oxygen, and can react instantaneously to generate carbon dioxide, water vapour, nitrogen and heat. They do not need atmospheric oxygen to support combustion, however they are often capable of burning for some time, without detonating. They should not be stored with any other class of Dangerous Goods.

Alan Cotterell

19^th August 1999