Q96. For either a grain dust fire or explosion to occur and be sustained, which of the following elements must be present?
A dust explosion is the rapid combustion of fine particles suspended in the air, often but not always in an enclosed location. Dust explosions can occur where any dispersed powdered combustible material is present in high enough concentrations in the atmosphere or other oxidizing gaseous medium such as oxygen.
Dust explosions are a frequent hazard in underground coal mines, in grain elevators, and other industrial environments. On the other hand, they are also commonly used by special effects artists, filmmakers, and pyro technicians, given their spectacular appearance and ability to be safely contained under certain carefully controlled conditions.
Conditions required
There are four necessary conditions for a dust explosion
* A combustible dust
* The dust is suspended in the air at a sufficiently high concentration
* There is an oxidant (typically atmospheric oxygen)
* There is an ignition source
In some traditional analyses of dust explosions, confinement is considered to be a fifth requirement; this is not an essential condition, but can greatly aggravate the physical damage that results.
How do combustible dust explosions happen?
Any fire needs three elements. These elements are known as the “fire triangle”:
1. Fuel to burn
2. Oxygen
3. Ignition source (heat, spark, etc.)
A dust explosion needs two additional elements – known as the “dust pentagon”:
4. Dispersion of dust particles in the right concentration, and
5. Confinement of the dust cloud.
Dispersion means the dust particles are suspended in air. Confinement means the dust is in an enclosed or limited space. This restriction allows pressure to build up, increasing the likelihood of an explosion.
Figure shows the dust explosion pentagon. Figure from Occupational Safety and Health Administration (OSHA).
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