FIRE AND EXPLOSION
An explosion is characterized by a rapid release of high pressure gas in the form of a shock wave.
Physical explosions occur when a container or pressure vessel raptures, due to excessively high pressures developing internally. Physical explosions include boilers which explode include boilers which explode due to dry firing, or BLEVE's (Boiling Liquid Expanding Vapour Explosions) of flammable liquid drums or containers.
Chemical explosions involve reactions. These reactions may involve combustion, decomposition or many other processes. Dust clouds, vapour clouds and high explosives such as TNT are examples of chemical explosions.
Explosion damage structures and people, by releasing energy in the form of pressure waves and flame fronts.
High explosives such as TNT develop very high pressures in a localized area. A land mine and blow a victims leg off, but the victims eardrums located 4 ft. away will not be damaged. These types of explosions are known as detonations, because the pressure waves generated move at supersonic speeds.
Dispersed fuel mixtures such as flammable liquid vapours in air, or dust clouds in air can cause explosions. For these types of explosions to develop, the fuel/air mixture must be in the flammable range. An explosion occurs rather than a fire when ignition is delayed, allowing a quantity of fuel to accumulate. Upon ignition, the rapid combustion of the fuel mixture crates an explosion.
This type of explosion, known as a deflagration, is much slower developing than a detonation, and generates lower pressures. When a deflagration occurs in a building, the windows and doors will break and start to vent the overpressure. In some cases, this is the extent of the damage, but in other cases the volume of fuel causes the pressure to keep rising until the walls of the structure are blown out. Wall collapses are followed by roof collapses, and any personnel in the area who survive the explosions may be caught under the debris.
The pressures developed by vapour and dust explosions will not normally harm people, but the fire ball which travels with the pressure front, will burn any people it contacts. These burns can be fatal.
Determining the cause of an explosion involves the same steps as fire. The fuel source, oxygen source and heat source must be identified. The cause for the joining of the three must then be established. Cause may be accidental or intentional. Cause may be accidental or intentional. Explosions are often preceded by or lead to fires, so physical evidence is normally severely damaged.
Explosion investigation is a specialized area of fire investigation. It requires a careful scene examination, and recreation of events leading up to the explosion. Physical evidence is often dispersed over a large area. This type of investigation is best left to an expert.
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Walters Forensic Engineering | 277 Wellington
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