What is combustible dust? Dust particles that can catch fire and/or cause an explosion. OSHA defines combustible dust in their current ‘National Emphasis Program on Combustible Dust’ as:
“A combustible particulate solid that presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations, regardless of particle size or shape.“
How can dust combust and create an explosion? Before a dust explosion can occur, five elements must be present:
- Fuel (the dust)
- Oxygen from the surrounding air or other sources
- An ignition source, such as static electricity, electric arc, a glowing ember, a hot surface, welding slag, frictional heat, or flame
- Dispersion of the combustible dust particles into a dust cloud suspended in the air
- Confinement of the dust cloud, such as in a vessel, room, building, enclosure, or ductwork
Those five elements make up what is called the Dust Fire and Explosion Pentagon (see image below). If all five are present, dust can combust and create an explosion. If any one of the five items is absent, there will be no explosion.
What is the combustible dust ‘fire and explosion pentagon’?
The fire explosion pentagon describes the elements that must be present for a combustible dust fire or explosion to occur that are listed above. The best way to reduce the risk of a combustible dust fire or explosion is to ensure that all five elements of the fire and explosion pentagon are not present together at any one time. The two elements that are easiest to control are the fuel source and ignition source. The size of the dust particles, the chemical properties of the dust, the moisture content, and the dispersion of the dust cloud all contribute to the risk of an explosion. Installing ventilation, using pressure-relief venting on equipment, using spark/ember detection and extinguishing systems, and using explosion protection systems, sprinkler systems, and other specialized suppression systems can all reduce the impact of an explosion should one occur. No explosion relieving construction is necessary as an explosive environment does not exist by design. All areas subject to the accumulation of dust shall be designed with minimum transport velocities to satisfy NFPA requirements and prevent dust fall out during transportation.
Combustible Dust Management
The approach to combustible dust management for material handling systems should follow a path of passive to active. All passive means shall be considered, applied and/or dismissed before the application of active dust control. By applying all passive means before active control, the following can be accomplished:
- Minimize the volume of combustible dust that is available to be collected (unless collected specifically for process compliance)
- Minimize wear in material handling ducts, especially in elbows
- Minimize dust filter bag wear and degradation
- Minimize rotary screw and airlock wear
- Minimize dust filter bag change-out frequency
- Minimize the volume of water and chemicals introduced for suppression