Hazard-Specific Issues - Dust Explosibility

• Dust Explosibility - Overview
• Terminology
• Factors and Substances Implicated in Explosions
• WHMIS Classification and Hazard Communication
• References

Dust Explosibility - Overview

Three separate dust explosions in 2003 that killed 14 people and injured 81 prompted the United States Chemical Safety Board (CSB) to examine this hazard. The CSB's lead investigator identified commonalities amongst these accidents suggesting that the material safety data sheets (MSDSs) for the materials implicated in these explosions inadequately describe dust hazards and cited a general lack of awareness of dust hazards amongst company workforces.

In Canada, the MSDS is a requirement of the Workplace Hazardous Materials Information System (WHMIS).

Terminology

The term "explosible" is used to describe a dust which is able to cause a dust explosion. "Explosible" is preferred over the term "explosive" to avoid confusion with high explosives, whose explosion properties are different from those of dusts dispersed as a cloud and ignited. A dust explosion propagates by the combustion of particles with surrounding gas, whereas explosives do not require a surrounding atmosphere.

Although the term "combustible" is often used synonymously with the term "explosible", it should be noted that not all materials that will burn in air cause dust explosions even if finely divided and dry; i.e., not all combustible dusts are explosible, but all explosible dusts must be combustible.

Factors and Substances Implicated in Explosions

Like all fires, a dust fire occurs when fuel (the combustible dust) is exposed to a source of ignition in the presence of oxygen. Removing any one of these elements of the fire "triangle" eliminates the possibility of a fire.

For dust explosions, two additional conditions are required: dispersion and confinement. Suspended dust burns more rapidly and confinement allows for pressure build-up. The initial explosion can cause dust that has settled over a period of years to become airborne resulting in a secondary explosion that propagates throughout the plant often with catastrophic results.

Industries at risk of dust explosions include: metal processing; wood product manufacturing; chemical manufacturing; food and pharmaceutical production; grain storage, fabrication of rubber and plastic products; and coal-fired power plants. Combustible materials which have been implicated in dust explosions include coal; chemicals; wood dust; rubber; grain dust; sugar; flour; and a number of metals such as, for example, aluminum. OSHA's Combustible Dust poster provides examples of products or materials from which combustible dust explosions could occur if they are processed in powdered form.

For more information, please refer to the CSB's "Investigation Report - Combustible Dust Hazard Study, Report No. 2006-H-1".

WHMIS Classification and Hazard Communication

Although the Controlled Products Regulations (CPR) do not set out criteria for the explosibility of dusts, this does not preclude a supplier's obligations to disclose information relating to this hazard as required by the existing requirements of the CPR:

12(11) A material safety data sheet shall disclose, in addition to the information required to be disclosed by subsection (2), any other hazard information with respect to the controlled product of which the supplier is aware or ought reasonably to be aware.

As such, where applicable, MSDSs are to identify this hazard and disclose information on the appropriate engineering controls to prevent these explosions through, for example, information provided in pertinent guidelines such as those recommended in the standards issued by the National Fire Protection Association.

While it is appreciated that suppliers cannot predict every possible use of their products, the processes and substances implicated in dust explosions are well documented as are preventive measures; i.e., such information is "hazard information... of which a supplier ought reasonably to be aware". This information is subject to disclosure on the MSDS. Examples of appropriate wording are provided in OSHA 3371-08, "Hazard Communication Guidance for Combustible Dusts".

References

National Fire Protection Association (NFPA) Standards:

• NFPA 61, Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities
• NFPA 68, Standard on Explosion Protection by Deflagration Venting
• NFPA 69, Standard on Explosion Prevention Systems
• NFPA 484, Standard for Combustible Metals
• NFPA 499, Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas
• NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids
• NFPA 655, Standard for the Prevention of Sulfur Fires and Explosions
• NFPA 664, Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities

United States Chemical Safety Board:

• "Combustible Dust: An Insideous Hazard" (video)
• Investigation Report - Combustible Dust Hazard Study, Report No. 2006-H-1, November 2006

 United States Occupational Health and Safety Administration (OSHA):

• Combustible Dust National Emphasis Program Instruction, OSHA Directive CPL 03-00-008, (2008)
• Fact Sheet, (March 2008), Hazard Alert: Combustible Dust Explosions
• Hazard Communication Guidance for Combustible Dusts, OSHA 3371-08, 2009
• National News Release, 09-475-NAT, U.S. Department of Labor's OSHA announces rulemaking on combustible dust hazards
• Poster: "Combustible Dust"
• Safety and Health Topics - Combustible Dust
• Safety and Health Information Bulletin (SHIB) (07-31-2005) Combustible Dust in Industry: Preventing and Mitigating the Effects of Fire and Explosions.