According to the U.S. Occupational Safety and Health Administration (OSHA) and the National Fire Protection Association, combustible dusts are any finely divided solid material – 420 microns or smaller in diameter – that present an explosion hazard when suspended in air in certain conditions. To get a sense of proportion, beach sand varies from 100 – 2000 microns; fine sand around 250 microns.
Besides organic materials such as wood, flour, sugar, plastic, rubber and coal, there are many other dusts including metals and chemicals that can also explode under the right conditions.
Conditions for dust explosion
A dust explosion can occur when the five basic conditions of the Dust Explosion Pentagon come together in a perfect storm.
NFPA Standard 664 defines a combustible dust hazard as:
- A layer of accumulated dust that exceeds one-eighth inch thickness over five per cent of the area or 1,000 ft2 whichever is smaller; and
- When dispersed in the air, it’s at a concentration in excess of 25% of the Minimum Explosive Concentration (MEC) – a dust cloud behind a dump truck driving down a dry, dusty, dirt road is a good representation.
The best explosion and fire mitigation strategy is to:
- Prevent combustible dust accumulation (i.e.: minimize fuel source and potential for dispersion); and
- Prevent ignition sources.
According to OSHA, an initial or primary explosion may dislodge more accumulated dust from rafters, false ceilings, duct work and other areas, into the air. This dislodged dust in the air may cause one or more secondary explosions. These can be far more destructive than the first explosion because of the increased quantity and concentration of dispersed combustible dust.