“An explosion is a sudden and rapid release of gases from a confined space,
usually accompanied by loud noise, a violent shock, and high heat.”
– NFPA 921.
There are four major types of explosions: mechanical, chemical, electrical and nuclear. Knowing which is the cause of your next fire loss could determine whether the event was a covered loss.
A mechanical explosion is when the high-pressure gas within a container expands beyond that container’s structural integrity. That point, referred to as the tensile strength, is the maximum stress the container can withstand before failure. In most fire investigations, this is the failure of aerosol cans, propane tanks and air compressors. There is no chemical change, meaning the compressed gas has not undergone any reaction or alteration.
Typically, mechanical explosions do not combust, meaning there is no flame or heat, however, they can result in subsequent combustion, acting as the initial cause of a fire or additional explosions.
The most interesting sub-type of a mechanical explosion is called Boiling Liquid Expanding Vapor Explosions (BLEVEs). BLEVEs occur under very specific circumstances. A flammable liquid must be stored under pressure and at a temperature above its atmospheric boiling point and is then heated above its boiling point. The container ruptures and the liquid rapidly vaporizes before being ignited. The most common items susceptible to BLEVEs are aerosol cans but they can also occur with butane lighters, above-ground storage tanks, or other similar containers.
A chemical explosion occurs when an uncontrolled chemical reaction releases energy. The two major subtypes are reactions occurring inside vessels that create gas as a product of that reaction, sometimes referred to as pop bottle bombs, and exothermic reactions that causes enough heat to increase the pressure of liquid or gas within a confined area, or a combustion explosion.
These explosions are commonly depicted in slow-motion Hollywood action scenes where a flame is shown traveling through the air. What Hollywood gets wrong is that the energy must be contained to create this effect and, therefore, these explosions could not happen in large rooms, outside, or in structures with many open doors or windows.
Combustion explosions are further classified as either deflagration or detonation. To put it simply, did the flame front travel slower or faster than the speed of sound?
An electrical explosion is caused by an arched flash, which occurs when stored electricity is discharged to a nearby conductor via the air.
Compared to mechanical and chemical explosions, where temperatures are in the hundreds of degrees, electrical explosions are typically thousands of degrees, discharging not only heat, light and force, but also plasma.
Rarely seen outside of commercial settings, electrical explosions tend to be the most visually spectacular, but also the most fatal.
There are two types of nuclear explosions: fusion, which involves the joining of atoms, or fission, the splitting of atoms, which releases massive amounts of heat. This heat creates a high-pressure wave, which is responsible for the initial damage.
Chicken and the Egg
In a fire investigation, explosions present an interesting chicken and egg problem. Did the fire cause an explosion? Or did the explosion cause the fire?
Fire pattern analysis, witness statements and ventilation analysis by a qualified investigator can all assist to reach a conclusion. Only after determining the sequence of events can an investigator turn to the cause of the explosion. Getting the right answer to those questions can be the difference between a covered fire loss in which the furnace exploded due to the fire and an excluded event caused by a neglected or improperly maintained furnace that caused an explosion.
While each policy’s exclusionary language is different, determining the answer to this chicken or egg fire question will result in better results.
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