According to OSHA, combustible material can burn rapidly when in a finely divided form. If such a dust is suspended in air in the right concentration, under certain conditions, it can become explosive. Even materials that do not burn in larger pieces, given the proper conditions, can explode in dust form.
There is no OSHA standard on combustible dust, but under the Combustible Dust National Emphasis Program, OSHA can issue fines for violations under 18 different standards, including 1910.22, the main umbrella standard. You can also be cited under the General Duty Clause, which can apply to recognized hazards not specifically addressed in OSHA standards.
NFPA® introduced a standard to promote awareness of combustible dust hazards. The standard was created to promote and define hazard analysis, awareness, management and mitigation. It helps in identifying combustible dust hazards and what to do about them.
NFPA 652 requires all facilities that process, handle, convey, or manufacture potentially combustible dust to have their dust tested and keep the results on file. This is required, NFPA says, even if your type of dust has no history of causing an incident.
A wide variety of materials that can explode in dust form exist in many industries. Examples of these materials include: food, grain, tobacco, plastics, wood, paper, pulp, rubber, pesticides, pharmaceuticals, dyes, coal, metals (e.g., aluminum, chromium, iron, magnesium, and zinc). These materials are used in a wide range of industries and processes, such as agriculture, chemical manufacturing, pharmaceutical production, furniture, textiles, fossil fuel power generation, recycling operations, and metal-working and processing which includes additive manufacturing and 3D printing.
The vast majority of natural and synthetic organic materials, as well as some metals, can form combustible dust. NFPA’s “Industrial Fire Hazards Handbook” states that “any industrial process that reduces a combustible material and some normally noncombustible materials to a finely divided state presents a potential for a serious fire or explosion.”
The Combustible Dust National Emphasis Program issued by OSHA in 2008 instructs inspectors on how to identify combustible dust hazards.
The program contains policies and procedures for inspecting workplaces that create or handle combustible dusts. In some circumstances these dusts may cause fires or an explosion. OSHA says those dusts include, but are not limited to:
- Metal dust such as aluminum and magnesium
- Wood dust
- Coal and other carbon dusts.
- Plastic dust and additives
- Other organic dust such as sugar, flour, paper, soap, and dried blood
- Certain textile materials
Industries that handle combustible dusts include, but are not limited to:
- Food Products
- Forest and furniture products
- Metal processing
- Tire and rubber manufacturing plants
- Paper products
- Wastewater treatment
- Recycling operations (metal, paper, and plastic.)
- Coal dust in coal handling and processing facilities
OSHA says facilities should carefully identify the following in order to assess their potential for dust explosions:
- Materials that can be combustible when finely divided;
- Processes which 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.
The primary factor in an assessment of these hazards is whether the dust is in fact combustible. According to the NFPA, “material that will burn in air” in a solid form can be explosive when in a finely divided form. Combustible dust is defined by NFPA as: “Any finely divided solid material that is 420 microns or smaller in diameter (material passing a U.S. No. 40 Standard Sieve) and presents a fire or explosion hazard when dispersed and ignited in air.” In some cases, additional information such as test results will be available from chemical manufacturers.
Different dusts of the same chemical material will have different ignitability and explosive characteristics, depending upon many variables, such as particle size, shape, and moisture content. In some cases, dusts will be combustible even if the particle size is larger than that specified in the NFPA definition, especially if the material is fibrous, says OSHA.
Industrial settings may contain high-energy ignition sources such as welding torches.
Hazard analysis considerations
The amount of dust accumulation necessary to cause an explosive concentration can vary greatly, according to OSHA. This is because there are so many variables – the particle size of the dust, the method of dispersion, ventilation system modes, air currents, physical barriers, and the volume of the area in which the dust cloud exists or may exist. As a result, says OSHA, many basic rules of thumb regarding accumulation can be subjective and misleading. The hazard analysis should be tailored to the specific circumstances in each facility and the full range of variables affecting the hazard.
OSHA details the locations that need to be considered in an assessment. One place for a dust explosion to initiate is where dust is concentrated. In equipment such as dust collectors, a combustible mixture could be present whenever the equipment is operating. Other locations to consider are those where dust can settle, both in occupied areas and in hidden concealed spaces. A thorough analysis will consider all possible scenarios in which dust can be disbursed, both in the normal process and potential failure modes.
OSHA emphasizes that workers are the first line of defense in preventing and mitigating fires and explosions. If the people closest to the source of the hazard are trained to recognize and prevent hazards associated with combustible dust in the plant, they can be instrumental in recognizing unsafe conditions, taking preventative action, and/or alerting management, says OSHA.
Employers with hazardous chemicals (including combustible dusts) in their workplaces are required to comply with 29 CFR 1910.1200, the Hazard Communication standard.