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 employee deaths, injuries, and destruction of entire buildings. In many combustible dust accidents, employers and employees were unaware that a hazard even existed. It is important that employers determine if their operations have this hazard, and if so, that they take action now to prevent tragic consequences.

A wide range of industries possess combustible dust hazards and a wide variety of materials cause combustible dust incidents. (Combustible dusts can be intentionally manufactured powders, such as corn starch or aluminum powder coatings, or may be generated by handling and processing solid combustible materials such as wood and plastic pellets. For example, polishing, grinding, transporting, and shaping many of these materials can produce very small particles.)

Action steps

The following are general action steps that should be taken to identify and control dust hazards. For detailed guidance specific to your operations, consult with a qualified professional, as well as industry standards, as applicable (for example, those published by the National Fire Protection Association).

• Assess your facility for processes that generate dust. Have dusts analyzed for combustibility hazards. (There are several factors that determine how explosive a dust can be, as well as some key measurements for determining dust explosibility. These include particle size, moisture content, ambient humidity, oxygen available for combustion, the shape of the dust particle, and the concentration of dust in the air, i.e., minimum explosive concentration or MEC.)
• Understand the five elements needed for a combustible dust explosion: Combustible dust (fuel); ignition source (heat); oxygen in air (oxidizer); dispersion of dust particles in sufficient quantity and concentration; and confinement of the dust cloud.
• Understand that there can be a primary explosion or event that shakes loose hidden dust (for example, from piping or rafters) which can cause a devastating secondary explosion.
• Ensure operations have proper engineering design and controls and inspect/maintain those controls. Seek outside assistance/expertise as needed to ensure proper dust control procedures/methods are in place.
• Develop an ignition control program, limiting and controlling potential ignition sources in dust accumulation areas.
• Train employees on the explosion hazards of combustible dusts.
• Implement a housekeeping program with regular cleaning frequencies for floors and horizontal surfaces, such as ducts, pipes, hoods, ledges, and beams, to minimize dust accumulations within operating areas of the facility. (Even seemingly small amounts of accumulated dust can cause catastrophic damage. The CSB estimated, for example, that the explosion that devastated a pharmaceutical plant in 2003 and killed six employees was caused by dust accumulations mainly under 0.25 inches deep. The CSB study references National Fire Protection Administration (NFPA) warnings that a catastrophic explosion can occur from as little as 1/32 of an inch of accumulated dust — around the thickness of a dime — covering just 5 percent of a room’s surface.)
• Ensure working surfaces are designed in a manner to minimize dust accumulation and facilitate cleaning.
• Ensure electrical equipment and electrically-powered cleaning devices such as vacuum cleaners are approved for the hazard classification for Class II locations.
• Select and use industrial trucks that are approved for the combustible dust locations.
• Prohibit the use of cleaning methods that can disperse combustible dust into the air (i.e., compressed air and dry sweeping).
• Ensure the ready availability of SDSs for the chemicals which could become combustible dust under normal operations. Note: The Chemical Safety Board has found that many SDSs do not provide sufficient data on combustible dust hazards. However, OSHA revised the HazCom rule in 2012. In the revised rule, combustible dust hazards must be addressed on labels and SDSs. Label elements are provided for combustible dust in the final HCS and include the signal word “warning” and the hazard statement “May form combustible dust concentrations in the air.”
• Implement an emergency action plan and maintain exit routes properly.

For chemicals in a solid form that do not present a combustible dust hazard, but may form combustible dusts while being processed in normal downstream uses, OSHA allows the chemical manufacturer some flexibility in labeling requirements. The manufacturer or importer may transmit the label to the customer at the time of the initial shipment, but the label does not need to be included with subsequent shipments unless it changes. This provides the needed information to the downstream users on the potential hazards in the workplace, while acknowledging that the solid metal or other materials do not present the same hazards that are produced when these materials are processed under normal conditions of use.

Training action plan

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. 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.

• Define combustible dusts. Combustible dusts are fine particles, fibers, chips, chunks, or flakes that could be an explosion hazard when they’re suspended in air under certain conditions. Many products can be fuel for a dust explosion when they’re used in powdered form. Here are some examples:
  • Flour, sugar, powdered milk, or other food-related products;
  • Wood;
  • Paper;
  • Textiles;
  • Charcoal and other carbon-rich materials;
  • Sulfur and other chemicals;
  • Aluminum, zinc, and other metals; and
  • Epoxy resin and other plastics.
• Introduce the hazards. A dust explosion can cause deaths and serious injuries. An explosion can destroy an entire building. In many combustible dust incidents, employers and employees didn’t know that a hazard even existed. Here’s an example of what happens during a combustible dust explosion.
  • In February 1999, a deadly fire and explosion occurred in a foundry in Massachusetts. An investigation found that a fire started in a molding machine and then spread to ventilation ducts. The small initial explosion in the ductwork disturbed resin dust that had settled on the outside of the ducts, creating a dust cloud. This dust provided fuel for a secondary explosion that was powerful enough to lift the roof and cause walls to collapse.
• Identify the types of industries that may be at risk. Combustible dust explosion hazards can exist in a variety of industries, including: agriculture, chemicals, food, grain, fertilizer, tobacco, plastics, wood, paper, tire and rubber, furniture, textiles, pesticides, pharmaceuticals, coal, metal processing, recycling operations, and others.
• Explain how dust explosions can happen. The familiar fire triangle is made up of: oxygen, an ignition source (heat), and fuel. All three conditions must be present for a fire to start. Because oxygen is normally always present in the air, fires are usually prevented by keeping the fuel separate from the ignition source. In addition to the fire triangle elements, the “dust explosion pentagon” adds two more conditions: dust particles and the confinement of the dust cloud. Similar to the fire triangle, if one part of the pentagon is missing, a combustible dust explosion can’t occur. If a dust cloud is ignited in an enclosed area such as a room, vessel, ductwork, or process equipment, it burns very fast and may explode. Even if the initial explosion is small, it could shake up accumulated dust in the area to form another dust cloud. Often these secondary explosions are far more destructive.
• Outline how to look for combustible dust hazards. To identify the factors that could contribute to an explosion, employers should evaluate the hazards of all: materials, operations, spaces (including hidden ones), and potential ignition sources. Some likely areas where dust can build up include:
  • Parts of the building’s structure,
  • Conduit and pipe racks,
  • Electrical cable trays,
  • Floors,
  • Areas above suspended ceilings, and
  • On and around equipment (including leaks around dust collectors and ductwork).
• Discuss what can be done to control dust build-up. If the dust isn’t building up, it lowers the risk of a dust explosion. There are many ways to keep dust under control. Some of these methods are to:
  • Conduct regularly scheduled inspections,
  • Clean up dust on a regular basis,
  • Use proper dust collection systems and filters,
  • Keep dust from escaping from equipment or ventilation systems,
  • Use surfaces that are easy to clean,
  • Provide easy access to inspect and clean hidden areas where dust might build up,
  • Use cleaning methods that don’t stir up dust clouds, and
  • Use vacuum cleaners approved for dust collection.
• Describe methods to prevent dust from igniting. Keeping ignition sources under control is an important part of any fire safety program. Some actions employers can take include the following:
  • Use proper wiring methods;
  • Make sure electrically powered equipment (including forklifts) is approved for the location;
  • Control static electricity (bond equipment to ground);
  • Keep smoking areas, open flames, sparks, and heated surfaces separate from dusts;
  • Follow a hot work permit program; and
  • Follow a preventive maintenance program.
• Discuss some methods to minimize injuries and damage from a combustible dust explosion. Some of the steps employers can take to minimize injuries and damage from a combustible dust explosion could include:
  • Developing an emergency action plan,
  • Maintaining emergency exit routes,
  • Setting up operations in areas where the hazards are away from workers and equipment,
  • Setting up isolation barriers around hazardous operations,
  • Making sure dust handling equipment has pressure relief venting that’s directed away from workers and equipment, and
  • Installing specialized spark detection and fire suppression systems in dust handling equipment.

Tips

The amount of dust accumulation necessary to cause an explosive concentration can vary greatly. 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, simple rules of thumb regarding accumulation (such as writing in the dust or visibility in a dust cloud) 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.

Many locations need to be considered in an assessment. One obvious 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.The facility analysis must identify areas requiring special electrical equipment classification due to the presence (or potential presence) of combustible dust. Various OSHA standards as well as industry (e.g., NFPA) can be consulted.

Checklist

Follow these checklists to prevent combustible dust hazards.

Hazard assessment

• Are there processes that accumulate dust? If so, have the dusts been analyzed for combustibility?
• Are periodic assessments conducted to determine dust explosion possibilities?
• Are employees and supervisors involved in identifying dust explosion hazards, for example through job hazard analyses?
• Is particular attention given during the hazard assessment to dust collection systems and areas not in plain view?

Prevention

• Have proper engineering design and controls been implemented for operations involving combustible dust; for example, dust control in process equipment and fugitive dust control; grounding and bonding; and separator devices?
• Is a comprehensive housekeeping program implemented?
• Are potential ignition sources limited and controlled in dust accumulation areas?
• Is it prohibited to use compressed air and similar cleaning methods that could disperse combustible dusts into the air?
• Are electrical services appropriate for Class II location? Consult applicable codes, for example the National Electrical Code.
• Are industry standards, such as NFPA 654, followed as applicable?
• Is a preventive maintenance program in place to minimize release of fugitive dust?

Note: Minor, but steady leaks, can cause large amounts of dust to accumulate.

Training

• Have employees received training on the hazards of dust?
• Have employees received training on specific characteristics of the dust that they work with?
• Have employees been trained to recognize potential ignition sources?
• Do employees know which processes/locations accumulate dust (for example, cable trays, pipe racks, floors, above ceilings, and structural members)?
• Are employees instructed to report plant fires and near misses?