Combustible dust, as defined by the Occupational Safety and Health Administration (OSHA), is “a solid material composed of distinct particles or pieces, regardless of size, shape or chemical composition, which can present a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations.” The presence of this dust, both in open and unseen areas, can present a grave hazard to employees, employers and facilities as explosions can be catastrophic in nature. An OSHA Fact Sheet titled, “Hazard Alert: Combustible Dust Explosions” explains how dust explosions can occur:
“In addition to the familiar fire triangle of oxygen, heat and fuel (the dust), dispersion of dust particles in sufficient quantity and concentration can cause rapid combustion known as deflagration. If the event is confined by an enclosure such as a building, room, vessel or process equipment, the resulting pressure rise may cause an explosion. These five factors (oxygen, heat, fuel, dispersion, and confinement) are known as the Dust Explosion Pentagon. If one element of the pentagon is missing, an explosion cannot occur.”
Combustible dust explosions typically occur in two waves. The first wave, also known as the primary explosion, starts with just the “right” concentration of airborne accumulated dust. This dust is held captive within a limited or enclosed space, such as inside the chamber of processing equipment. This captive dust is then subjected to a heat source which causes the dust to ignite. The ignited dust can burn very rapidly and release gases causing the pressure to rise within the enclosure and can result in an explosion.
Unfortunately, the first explosion is usually only the beginning. The primary explosion disturbs and shakes up dormant dust which has collected over time on a variety of surfaces within the area. Some examples of these surfaces can be the top of or underneath machinery, ledges, rafters, duct work, inside suspended ceilings, on top of support beams, etc. The second wave, or secondary explosion, occurs as this additional dust becomes suspended in the air and also ignites. Secondary explosions are often more destructive than primary ones because of the sheer volume and concentration of additional dust available to fuel them.
Many employers and employees are unaware of the potential threat of dust explosions or fail to recognize it as a serious hazard in their facility. In the Chemical Safety Board (CSB) video, “Combustible Dust: An Insidious Hazard”, Stephen Selk, a CSB Investigator says, “The big problem with combustible dust is that we underestimate its hazards. We become complacent and we fail to take the necessary precautions.” There may also be inadequate information available to help employers recognize a combustible dust hazard on their Safety Data Sheets (SDS). After reviewing the SDS of 140 substances known to create combustible dust, the CSB found they contained deficient information to assist the end-user in determining the hazard: 41% of the documents did not warn of the potential hazard at all, while the remaining 56% did not clearly or specifically describe the hazard in a way which was easy to identify (CSB, 2006).
There is a long list of industries vulnerable to the hazard of dust explosions including, but not limited to the following: agriculture, chemicals, food (such as sugar, candy, spice, starch, flour, feed), grain, fertilizer, tobacco, plastics, wood, forest, paper, pulp, rubber, furniture, textiles, pesticides, pharmaceuticals, tire and rubber manufacturers, dyes, coal, metal processing (such as aluminum, chromium, iron, magnesium, zinc), recycling operations and coal.
The key to the hazard assessment is correctly identifying whether or not the dust is indeed combustible. The National Fire Protection Agency (NFPA) Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing and Handling of Combustible Particulate Solids (NFPA #654) and NFPA #454, Standard for Combustible Metals, Metal Powders and Metal Dusts, both define combustible dust as “any finely divided solid material that is 420 microns or smaller in diameter and presents a fire or explosion hazard when dispersed and ignited in air.” Other variables to consider, in addition to particle size, are how the dust will be dispersed, what kind of ventilation is available, air currents, sources of ignition, the presence of physical barriers to either provide dust confinement or which provide separation of work processes one from another.