Combustible Dust Safety
What is combustible dust?
- Combustible dust is defined as a solid material composed of distinct particles or pieces, regardless of size, shape, or chemical composition, which presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations. Combustible dusts are often either organic or metal dusts that are finely ground into very small particles, fibers, fines, chips, chunks, flakes, or a small mixture of these. However, larger particles can still pose a deflagration hazard. In addition, particles can stick together due to electrostatic charges accumulated through handling, causing them to become explosive when dispersed.
- Types of dusts include, but are not limited to: metal dusts, such as aluminum and magnesium; wood dusts; plastic or rubber dusts; biosolids; coal dust; organic dusts, such as flour, sugar, paper, soap, and dried blood; and dusts from certain textiles.
How do dust explosions 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 a 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.
Secondary Explosion Risks
An initial explosion in processing equipment or a deflagration in an area where fugitive dust has accumulated may dislodge more accumulated dust into the air, or damage a containment system. As a result, if successfully ignited, the additional dust dispersed into the air may cause one or more secondary explosions. These secondary explosions can be far more destructive than a primary event due to the increased quantity and concentration of dispersed combustible dust. Many deaths in past accidents, as well as other damage, have been caused by secondary explosions.
Which industries are at risk?
Combustible dust explosion hazards exist in a variety of industries, including: agriculture, chemicals, food (e.g., candy, sugar, spice, starch, flour, feed), grain, fertilizer, tobacco, plastics, wood, forest, paper, pulp, rubber, furniture, textiles, pesticides, pharmaceuticals, tire and rubber manufacturing, dyes, coal, metal processing (e.g., aluminum, chromium, Iron, magnesium, and zinc), recycling operations, and fossil fuel power generation (coal).
Identifying the Potential for Dust Explosions
Assessing the potential for dust explosions must include an assessment of all physical and health hazards. To identify factors that may contribute to an explosion, a thorough hazard assessment should be conducted of:
- All materials handled
- All operations conducted, including byproducts
- All spaces (including hidden ones)
- All potential ignition sources
Anticipated types of operations, uses or downstream material processing that generate dusts should be considered normal conditions of use of a substance. These include operations and uses such as abrasive blasting, cutting, grinding, polishing or crushing of materials; conveying, mixing, sifting or screening dry materials; and the buildup of dried residue from processing wet materials.
Recommendations for Controlling Dust Explosion Risks
- 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.
- Design and use surfaces that minimize dust accumulation and facilitate cleaning.
- 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.
- Locate relief valves away from dust deposits.
Ignition Source Control
- Use appropriate electrical equipment and wiring methods. Class 2 / Div. 1 or 2
- Control static electricity, including bonding of equipment to ground.
- Control smoking, open flames, and sparks.
- Control mechanical friction.
- Use separation devices to remove foreign materials capable of igniting combustibles from process materials
- Separate heated surfaces from dusts
- Separate heating systems from dusts
- Select and use industrial trucks properly
- Use an equipment preventive maintenance program.