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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 tiny particles, fibers, chips, chunks, flakes or a small mixture of these. Types of dusts include, but are not limited to, metal dust, such as aluminum and magnesium; wood dust; plastic or rubber dust; biosolids; coal dust; organic dust, such as flour, sugar, paper, soap, and dried blood; and dusts from certain textiles.

The Dangers of Combustible Dust
Five elements are necessary to initiate a combustible dust explosion:
  1. combustible dust (fuel);
  2. ignition source (heat);
  3. oxygen in the air;
  4. dispersion of dust particles in sufficient quantity and concentration; and 
  5. confinement of the dust cloud.
If one of these five elements is missing, an explosion cannot occur. In order for the explosion to occur, there is an initial, or primary, dust explosion in processing equipment that may shake loose accumulated dust, or damage a containment system (such as a duct, vessel or collector). This causes the dust to become airborne; if ignited, this dust may cause one or more secondary explosions that can be far more destructive than a primary explosion due to the increased quantity and concentration of dispersed combustible dust and the larger ignition source.

The ease of ignition and the severity of a combustible dust explosion are typically influenced by particle size. Other factors that influence the explosiveness of dusts include moisture content, humidity, available oxygen, the shape of dust particles and the concentration of dust in the air. Different dusts of the same chemical material also can have different ignitability and explosibility characteristics, depending on physical characteristics such as particle size, shape and moisture content. These physical characteristics can change during manufacturing, use or while the material is being processed. Keep in mind that even weak explosions can cause significant damage, injury and death.

Methods of Protection

Facility Dust Hazard Assessments
Facilities should carefully identify the following in order to assess their potential for dust explosions:
  • materials that can be combustible when finely divided;
  • processes that use, consume or produce combustible dusts;
  • open areas where combustible dusts may build up;
  • hidden areas where combustible dusts may accumulate;
  • means by which dust may be dispersed in the air; and
  • potential ignition sources.
Dust Control
Following is a list of ways to help control dusts and prevent explosions:
  • minimize the escape of dust from process equipment or ventilation systems;
  • use dust collection systems and filters;
  • utilize 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;
  • clean dust residues at regular intervals;
  • use cleaning methods that do not generate dust clouds, if ignition sources are present;
  • only use vacuum cleaners approved for dust collection;
  • locate relief valves away from dust hazard areas; and
  • develop and implement a hazardous dust inspection, testing, housekeeping and control program (preferably in writing with established frequency and methods).
Ignition Control
The use of proper electrical equipment in hazardous locations is crucial to eliminating a common ignition source. Following is a list of recommended methods for ignition control:
  • use appropriate electrical equipment and wiring methods;
  • control static electricity, including bonding of equipment to ground;
  • control smoking, open flames and sparks;
  • control mechanical sparks and friction;
  • use separator devices to remove foreign materials capable of igniting combustibles from process materials;
  • separate heated surfaces from dusts;
  • separate heating systems from dusts;
  • properly use the right type of industrial trucks;
  • properly use cartridge activated tools; and 
  • adequately maintain all the above equipment.
Employee Training
While OSHA standards require training for certain employees, all employees should be trained in safe work practices applicable to their job tasks, as well as on the overall plant programs for dust control and ignition source control. They should be trained before they start work, periodically to refresh their knowledge, when reassigned, and when hazards or processes change.

Personal Protective Equipment
The importance of workers donning personnel protective equipment takes on an added dimension with the threat from combustible dust fires and explosions in the workplace. Many manufacturing facility managers, owners and employees are not aware of the fire hazard from combustible dust. In environments where workers can sustain life-threatening burns from vapor cloud explosions and flash fires, such as in the refinery sector, flame-resistant clothing is required.

A combustible dust explosion hazard may exist in the food, plastics, wood, rubber, furniture, textiles, pesticides, pharmaceuticals, dyes, coal, metals, fossil fuel power generation or one of many other industries that processes, handles, stores or transports materials or byproducts in fine, powdery form. If so, workers could be at risk, and the applicable federal, state and local laws and OSHA regulations must be identified and followed. A team of qualified managers or supervisors should be responsible for conducting the facility assessment (or for having one done by qualified third party) prior to the introduction of a hazard and for developing a prevention and protection scheme tailored to their operations. All personnel should be aware of and support the plant dust and ignition control programs.

Shane Stuller is director of site services for Indianapolis-based Safety Resources. For more information, visit www.safetyresources.com.

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