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“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 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 (Figure 1). If one element of the pentagon is missing, an explosion cannot occur.”
Combustible dust explosions typically occur in two waves. The first wave, also known as the primary explosion, starts with just the “right” concentration of airborne accumulated dust. This dust is held captive within a limited or enclosed space, such as inside the chamber of processing equipment. This captive dust then is subjected to a heat source, which causes the dust to ignite. The ignited dust can burn very rapidly and release gases, causing the pressure to rise within the enclosure, and can result in an explosion.
Unfortunately, the first explosion usually is only the beginning. The primary explosion disturbs and shakes up dormant dust, which has collected over time on a variety of surfaces within the area. Some examples of these surfaces can be on top of or underneath machinery, ledges, rafters, duct work, inside suspended ceilings, on top of support beams, etc. The second wave, or secondary explosion, occurs as this additional dust becomes suspended in the air and also ignites. Secondary explosions often are more destructive than primary ones because of the sheer volume and concentration of additional dust available to fuel them.
Many employers and employees are unaware of the potential threat of dust explosions or fail to recognize it as a serious hazard in their facility. In the Chemical Safety Board (CSB) video, “Combustible Dust: An Insidious Hazard,” Stephen Selk, a CSB investigator says, “The big problem with combustible dust is that we underestimate its hazards. We become complacent and we fail to take the necessary precautions.” There also may be inadequate information available to help employers recognize a combustible dust hazard on their Safety Data Sheets (SDS). After reviewing the SDS of 140 substances known to create combustible dust, the CSB found they contained deficient information to assist the end user in determining the hazard: 41 percent of the documents did not warn of the potential hazard at all, while the remaining 56 percent did not clearly or specifically describe the hazard in a way which was easy to identify (CSB, 2006).
There is a long list of industries vulnerable to the hazard of dust explosions, including, but not limited to, the following: agriculture, chemicals, food (such as sugar, candy, spice, starch, flour, feed), grain, fertilizer, tobacco, plastics, wood, forest, paper, pulp, rubber, furniture, textiles, pesticides, pharmaceuticals, tire and rubber manufacturers, dyes, coal, metal processing (such as aluminum, chromium, iron, magnesium, zinc), recycling operations and coal.
OSHA suggests completing an in-depth dust hazard assessment analyzing the following areas:
- Materials that may be combustible
- Processes that utilize any combustible dust
- Open and especially hidden areas where dust may collect
- Opportunities that may cause dust to become airborne
- Any source of ignition
The key to the hazard assessment is correctly identifying whether or not the dust is indeed combustible. The National Fire Protection Agency (NFPA) Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing and Handling of Combustible Particulate Solids (NFPA #654) and NFPA #454, Standard for Combustible Metals, Metal Powders and Metal Dusts, both define combustible dust as “any finely divided solid material that is 420 microns or smaller in diameter and presents a fire or explosion hazard when dispersed and ignited in air.” Other variables to consider, in addition to particle size, are how the dust will be dispersed, what kind of ventilation is available, air currents, sources of ignition and the presence of physical barriers to either provide dust confinement or which provide separation of work processes one from another.
In addition to the hazard assessment, controlling and eliminating combustible dust also is critical. Per OSHA, the following are ways to mitigate combustible dust:
- Create a program to inspect for and test for the presence of dust; implement housekeeping and control measures.
- Use only appropriate dust collection systems and filters.
- Recognize and eliminate fugitive dust, which may escape from process equipment or ventilation systems.
- Reduce the amount of horizontal surfaces which may collect dust and require cleaning.
- Regularly inspect for dust residue in hidden and open areas.
- Provide access to hidden spaces to facilitate inspection.
- Identify specific housekeeping practices close to ignition sources so dust clouds are not created.
- Use only explosion-proof vacuum cleaners.
- Do not place relief valves in close proximity to dust deposits.
OSHA also provides several suggestions for controlling ignition sources, including the following:
- Only utilize approved electrical and wiring equipment and methods.
- Bond and ground equipment to the ground and control static electricity.
- Control open flames, sparks, smoking and friction.
- Segregate materials to prevent combustible material from work processes.
- Put distance between heated surfaces/systems and dust exposure.
- Follow proper operating instructions when using cartridge-activated tools.
- Implement a preventative equipment maintenance program.
The 2006 CSB investigative study concluded engineering controls and adequate safety practices exist in general industry to control combustible dust, but no comprehensive federal standard was in place to require adherence to these practices. The CSB advocated for OSHA to form a new combined standard based on the existing five NFPA standards (NFPA Standards 654, 664, 61, 484 and 655). In 2013, CSB voted to put this issue on its “Most Wanted Safety Improvements” program. The CSB has recommended the following codes be included in the proposed combined standard:
- Hazard assessment
- Engineering controls
- Building design
- Explosion protection
- Operating procedures
- Worker training.
Grainger has the products, services and resources to help keep employees safe and healthy while operating safer facilities. It also provides a network of safety resources that help companies stay in compliance and protect employees from hazardous situations. Count on Grainger for lockout-tagout, fall protection equipment, confined space products, safety signs, personal protective equipment (PPE), emergency response and so much more. For more information, visit www.grainger.com.
This publication is not a substitute for review of the applicable government regulations and standards, and it should not be construed as legal advice or opinion. Readers with specific compliance questions should refer to the cited regulation or consult with an attorney.
Sources for information
- Combustible Dust: An Insidious Hazard
- OSHA Fact Sheet, Hazard Alert: Combustible Dust Explosions
- Hazard Communication Guidance for Combustible Dusts
- Combustible Dust
- Combustible Dust in Industry: Preventing and Mitigating the Effects of Fire and Explosions
- NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids
- NFPA 664, Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities
- NFPA 61, Standard for the Prevention of Fires and Explosions in Agricultural and Food Processing Facilities
- NFPA 484, Standard for Combustible Metals
- NFPA 655, Standard for Prevention of Sulfur Fires and Explosions (Rev. 5/2014)