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Categories of Chemical Explosions

In a physical explosion the high-pressure gas is produced by mechanical means, that is to say, even if chemicals are present in the container, they are not affected chemically by the explosion. In a chemical explosion the high-pressure gas is generated by the chemical reaction that takes place. The two major explosion types can be further divided into four sub types that result in the release of high pressure gas. The first type of explosion occurs as the result of the physical overpressurization of a container, causing the container to burst, such as the case of a children's balloon bursting when too much air is placed in the balloon.

Physical/Chemical Explosion

The second type of explosion occurs via physical or chemical means as in the case of a hot water heater or boiler explosion. The water inside the heater turns to steam when overheated, which results in a pressure increase inside the container, and the container fails at its weakest point. This is by far the most common type of accidental explosion that takes place.

Chemical Explosion

The third type of explosion involves a chemical reaction: the combustion of a gas mixture. Many of the explosive materials that are regulated and allowed to be offered for transportation by the DOT would involve this type of explosion should an accident occur. Therefore, most of our discussion of explosions and explosive materials in this hazard class will involve materials that explode by chemical means, with the high pressure being created by a chemical reaction. There are other types of explosions that produce high-pressure gases, some of which will be discussed in later hazard classes.

Explosives Tetrahedron

Just like the fire tetrahedron, there are several components (Figure 3.77) that must be present for a chemical explosive material like ammonium nitrate to explode. Since WWII has been the most common cause of

Explosive tetrahedron

Figure 3.77 Explosive tetrahedron.

firefighter and civilian deaths from explosives. An explosion involving a chemical explosive is really a fast-moving fire. In simple terms, an explosive that functions via chemical reaction creates a rapidly burning fire that is made possible by the presence of a chemical oxidizer. You must have a fuel, oxygen, but in the case of an explosive material oxygen from the air is not enough, so you must have a chemical oxidizer. You must have heat or something to initiate the explosion. Finally, there must be something to confine the explosive materials in order for an explosion to occur. Without confinement, the materials will just burn off. Confinement can be pretty much anything that will get the job done including the material itself. If you pile enough explosive material together the sheer weight of the material itself can provide the confinement for the explosion to take place.

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