Combustible Dust: What You Need to Know

Jan. 1, 2010
Prepare for OSHA’s new combustible dust standard by learning about dust explosions and how to prevent them.

This two-part series examines the causes of dust explosions, their devastating impact and the measures suggested by OSHA, the National Fire Protection Agency and the Chemical Safety and Hazard Investigation Board to eliminate them. Part 1 examines the risks of dust and how to reduce the hazards.—Ed.

According to recent research conducted by the U.S. Chemical Safety and Hazard Investigation Board (CSB), more than 280 explosions and fires have resulted from the ignition of dusts or dust clouds in U.S. industrial plants over the last 25 years. These incidents have caused 119 fatalities and more than 700 injuries as well as untold millions of dollars in damages.1 There were likely at least 80 additional incidents that were not documented in the CSB study.

OSHA launched a combustible dust national emphasis program (NEP; OSHA Directive CPL-03-00-008) in October 2007, providing procedures for inspecting for dust hazards, policies for industrial plants to implement designed to reduce dust explosion risks and guidelines on implementing National Fire Protection Association (NFPA) standards and codes.

The NEP resulted in an unusually high number of general duty clause violations, indicating a strong need for a combustible dust standard. The general duty clause is not as effective as a comprehensive combustible dust standard would be at protecting workers, so on Oct. 21, OSHA published an advance notice of proposed rulemaking in the Federal Register as an initial step in the development of a standard to address the hazards of combustible dust.

“Since 1980, more than 130 workers have been killed and more than 780 injured in combustible dust explosions,” said Jordan Barab, acting assistant secretary of labor for OSHA, when announcing the proposed rulemaking.

Dust Risks
Most amazing is how little those in the chemical process industry understand about the risks posed by dust. While most people understand that certain dusts, such as coal, grain and fertilizer dust, present an explosion risk, few understand the broad range of powders and dusts that actually pose fire and/or explosion danger in industrial plants. This fact was brought home by the explosion at Imperial Sugar.

How can a powder—a solid, typically non-hazardous material like sugar or plastic—become a fire and explosion (or, more appropriately, a deflagration) risk? This was the question OSHA inspectors and chemical industry workers were asking. A careful examination of the nature of finely ground materials and the nature of fire is necessary to understand why this risk exists.

We all know that it takes three components to make a fire: a fuel the fire will feed on, a source of oxygen to sustain the fire and an ignition source, such as a spark, flame or heat.

When we understand that fire is a chemical process—an oxidation reaction—we can more easily understand how a seemingly non-hazardous material can become a great fire or explosion risk. In fact, most materials can oxidize. A prime example is iron that rusts, or chemically reacts, to form iron oxide. When we place a bar of steel where it is exposed to air, the surface will slowly rust. This process typically takes days, weeks or longer.

However, if we grind that iron bar up into very fine particles, there is significantly more surface area that becomes exposed, and this surface will react with the available oxygen at a much more rapid rate. The finer the particles are, the quicker the reaction will proceed. This reaction also releases heat, so if the reaction proceeds at a rapid rate, it will generate heat at a quicker rate, and this heat will cause the gas around the particles to expand.

This rapid heat generation and oxidation of the fine particulate creates a flame front. If that flame front moves at less than the speed of sound, it generally is considered to be a deflagration.2 When a deflagration occurs in an enclosed space, an increase in pressure results when the expansion of the internal gases caused by the heat generated is restricted by the enclosure walls. This creates an explosion, where the expanding pressure wave can cause damage to the enclosure.

If the flame and pressure wave move faster than the speed of sound, the explosion is classified as a detonation.2 Detonations generally are associated with high explosives and, in general, cannot be controlled with pressurerelief vents of any type.

This same mechanism will occur with any material that can oxidize with the release of heat (called an exothermic reaction). Therefore, most organic chemicals, plastics, foods, metals, carbon compounds, pharmaceuticals and chemical intermediates can present a risk when they are in the form of very fine powders. The finer the powder, the greater the risk. In contrast, materials that require the addition of heat to oxidize (endothermic reactions) will not spontaneously ignite or explode.

Dust Hazard Reduction
Once we understand how and why dusts can become fire and explosion risks, we can determine how best to bring industrial facilities into compliance with OSHA’s NEP and the eventual combustible dust standard. While it may not be possible to eliminate all dust, the primary goal must be the prevention of major, or catastrophic, explosions inside the facility.

To accomplish this, the first task is to reduce or eliminate accumulations of combustible dusts from exposed and hidden surfaces inside the plant. Most catastrophic explosions—those that create the greatest devastation and loss of life within a facility—are secondary explosions caused when the shock or pressure wave from a smaller, primary explosion causes accumulated dust on horizontal surfaces to become airborne, where it is ignited by the primary ignition source. As this accumulated dust can extend for great distances away from the initial ignition source, the small fire or explosion can expand rapidly and cause millions of dollars in damage and even severe injury or death.

The NFPA has determined that dust accumulations of as little as 1/32nd of an inch (approximately the same thickness as an average paper clip) are sufficient to create a dust deflagration when dispersed and exposed to an ignition source.3 The removal of accumulated hazardous materials is the primary emphasis of OSHA’s NEP plan and the focus of OSHA inspections.4

There are three essential components to the reduction of hazardous dust accumulations. These include: identification of problem areas, identification of problem dusts and elimination of dust hazards.

Within all industrial facilities where powdered materials are handled or generated, some dust escapes from processing and conveying equipment. This dust naturally settles on horizontal surfaces in close proximity to the dust source. These surfaces may be on top of equipment or on stairs, railings, support steel, light fixtures, etc.

Over time, the accumulation of dust may become extensive. Highly visible surfaces are often cleaned on a periodic basis and may present a minimal hazard. However, hidden surfaces— such as support steel, roof support members and trusses, equipment surfaces above eye level, light fixtures and elevated ductwork, piping or cable trays—are generally overlooked.

These problem areas will draw the attention of OSHA inspectors and are the areas that facility managers should concentrate on cleaning.4

The watchword for reducing the risk of dust explosions is “housekeeping”—cleaning up dust accumulation in all areas, visible and hidden. The second aspect of reducing dust explosion risk is identifying the hazardous nature of accumulated dusts. Not all dusts present fire or explosion risks. While it is preferred that all accumulated dusts be cleaned up for safety reasons, the emphasis here is to concentrate on those dusts that present a real fire or explosion risk.

Conduct an inventory of all the powdered materials within the facility and review material safety data sheets (MSDS) to determine if a risk already has been identified. In the event the MSDS is incomplete, or if no MSDS exists for the powder under review, physical hazard testing may need to be conducted. Several NFPA publications2,5,6,7 provide additional information on explosive and combustible materials. Of course, a hazard may be assumed to exist for all unidentified materials and a cleanup undertaken anyway.

Eliminating problems involves the general housekeeping chores discussed above but goes beyond to address the sources of the primary dust and ignition sources. This involves a review of the process and process equipment to minimize any openings where dust can escape and eliminate sources of heat, sparks, combustion, etc., wherever possible. To facilitate this process, OSHA and the CSB rely on the NFPA and its collection of codes, standards and guidelines, which will be discussed in Part 2 of this article.

Michael A. Maxwell is vice president of engineering and technology and manager of applications for Griffin Filters. He earned a bachelor’s degree in chemical engineering from Iowa State University and a master’s degree in business administration from Lewis University. He has more than 36 years of experience in dust collection design, application and installation and has been a member of the NFPA since 1994. This article originally appeared in MHM’s sister publication, EHS Today

1. U.S Chemical Safety and Hazard Investigation Board, July 29, 2008.
2. NFPA 68-2007, Standard on Explosion Protection by Deflagration Venting, National Fire Protection Association, 2006.
3. NFPA 654-2006, Prevention of Fire and Dust Explosions from the Manufacturing, Processing and Handling of Combustible Particulate Solids, National Fire Protection Association, 2006.
4. CPL 03-00-008 Combustible Dust National Emphasis Program, 03/11/2008, U.S. Department of Labor, Occupational Safety and Health Administration, Washington, D.C.
5. NFPA 484-2009, Combustible Metals, National Fire Protection Association, 2009.
6. NFPA 495-2006, Explosive Materials Code, National Fire Protection Association, 2006.
7. NFPA 499-2008, Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas, National Fire Protection Association, 2008.

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