What causes dust explosions?

Fire, Deflagrations and Dust Explosions

If sufficient concentrations of dust are mixed with oxygen, all it takes is an ignition source - a spark from moving machinery or static electricity, astray hot ember, or even a cigarette butt - and the dust will burst into flame. Three forms of combustible dust hazard are fire, deflagration/flash fire, and explosion.

Fire Triangle

A fire requires three elements: Fuel (dust), oxygen (air), and an ignition source. This is represented in the Fire Triangle. Fires can occur if dust settles on a hot surface and ignites, or if an ignition source reaches a build-up of dust. Hazards include flame, heat, smoke and toxic by-products.

Deflagration

A deflagration or flash fire requires the three elements of the Fire Triangle, and the dust must be dispersed in the air at the right concentration. The dust ignites and forms a fireball that expands rapidly, generating a flame front and a rapid pressure rise. Deflagration hazards include damage to buildings and equipment, with a risk to life as the flame front can engulf employees.

Dust Explosion      

A dust explosion requires all the four elements for deflagration plus confinement - the Combustible Dust Pentagon. With the added element of confinement, explosion hazards are particularly dangerous. Where an explosion risk has been identified, equipment should comply with explosion prevention and protection measures outlined in NFPA 68 and NFPA 69.

Anatomy of a dust explosion

A primary dust explosion or deflagration can disperse unburned dust through the air and form a dust cloud. Heat from the primary explosion can ignite this cloud and cause a secondary explosion. This secondary explosion can often be larger and more violent than the primary one. Secondary explosions can also occur when energy from the primary explosion reaches more dust through piping, ducts, ventilation systems or conveyors.