Combustible dusts are fine particles that present an explosion hazard when suspended in air in certain conditions. A dust explosion can be catastrophic and cause destruction of entire buildings. Deflagration is the term used to describe when a dust cloud causes rapid combustion—after coming into contact with all five of the following elements: oxygen, heat, fuel, dispersion and confinement. These five factors are known as the “Dust Explosion Pentagon.” If one element of the pentagon is missing, an explosion cannot occur.
Secondary Explosions
An initial explosion in processing equipment or in an area where dust has accumulated may dislodge more accumulated dust into the air, or damage a containment system (such as a duct, vessel, or collector). As a result, if ignited, the additional dust dispersed into the air may cause one or more secondary explosions. These can be far more destructive than a primary explosion due to the increased quantity and concentration of dispersed combustible dust. According to OSHA statistics, many deaths in past incidents, as well as other damage, have been caused by secondary explosions.
The explosive level of a dust cloud nearly impossible to measure during an incident. Meters are available to measure the explosive levels of flammable gases and vapors, but none are currently available for combustible dusts. One rule of thumb: if the dust cloud totally obscures a light source at a distance of 6 to 9 feet, treat it as if it is in the explosive range, and consider evacuating the area.
OSHA recommends the following dust control measures:
• Implement a hazardous dust inspection, testing, housekeeping, and control program;
• Use proper dust collection systems and filters;
• Minimize the escape of dust from process equipment or ventilation systems;
• Use surfaces that minimize dust accumulation and facilitate cleaning;
• Provide access to all hidden areas to permit inspection;
• Inspect for dust residues in open and hidden areas at regular intervals;
• If ignition sources are present, use cleaning methods that do not generate dust clouds;
• Use only vacuum cleaners approved for dust collection; and
• Locate relief valves away from dust deposits. Ignition Control Recommendations
• Control static electricity, including bonding of equipment to ground;
• Control mechanical sparks and friction;
• Use separator devices to remove foreign materials capable of igniting combustibles from process materials and;
• Separate heated surfaces from dusts.
For these and other safety tips, contact our offices at Diversified Safety Services.
