What You Need to Know About Dust Explosions

According to recent research conducted by the U.S. Chemical Safety and Hazard Investigation Board (CSB), over 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 over 700 injuries as well as untold millions of dollars in damages.¹ It is likely there were 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 (ANPR) in the Federal Register as an initial step in 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 acting Assistant Secretary of Labor for OSHA Jordan Barab when announcing the proposed rulemaking.

DUST RISKS

What is most amazing is how little those in the chemical process industry (CPI) understand about the risks posed by dusts. While most people understood that certain dusts are an explosion risk — for example, coal dust, grain dust and fertilizer dust — very few understood the broad range of powders and dusts that actually posed a fire and/or explosion danger in the 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 the OSHA inspectors and chemical industry workers were asking. A careful examination of the nature of finely ground materials, and how fires work, 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. But how do these relate to our examination?

When we understand that fire is a chemical process — an oxidation reaction — we more easily can 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 moves 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 pressure relief 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 elimination 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. It is a fact that most catastrophic explosions — those that create the greatest devastation and loss of life within the plant — 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 rapidly can expand and cause millions of dollars in damages and the risk of severe injury or death.

The National Fire Protection Association (NFPA) has determined that dust accumulations of as little as 1/32^nd 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 is the focus of OSHA's inspections.⁴

There are three essential components to the reduction of hazardous dust accumulations. These are identification of problem areas, identification of problem dusts and elimination of dust hazards. Within all industrial facilities where powdered materials either are handled or generated, some dust escapes from the 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 the equipment itself, on stairs, railings, support steel, light fixtures, etc.

Over time, the accumulation of dust may become extensive. Those surfaces that are highly visible often are cleaned on a periodic basis and may present a very minimal hazard. However, “hidden” surfaces generally are overlooked. These include support steel, roof support members and trusses, equipment surfaces above eye level, light fixtures and elevated ductwork, piping or cable trays.

These problem areas are the areas that will draw the attention of OSHA inspectors and are the areas that safety professionals should concentrate on cleaning.⁴ The watchword in reducing the risk of dust explosions is housekeeping: cleaning up dust accumulation in all areas, visible or hidden.

The second aspect of reducing the dust explosion risk is to identify the hazardous nature of the accumulated dusts. Not all dusts present a fire or explosion risk. 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 publications^2,5,6,7 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 the problems involves the general housekeeping chores discussed above, but goes beyond that 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 to eliminate sources of heat, sparks, combustion, etc. wherever possible. To facilitate this process, OSHA and the Chemical Safety Board rely on the NFPA and their collection of codes, standards and guidelines, which will be discussed in Part 2 of this article.

References:

U.S Chemical Safety and Hazard Investigation Board, July 29, 2008.
NFPA 68-2007, Standard on Explosion Protection by Deflagration Venting, National Fire Protection Association, 2006.
NFPA 654-2006, Prevention of Fire and Dust Explosions from the Manufacturing, Processing and Handling of Combustible Particulate Solids, National Fire Protection Association, 2006.
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.
NFPA 484-2009, Combustible Metals, National Fire Protection Association, 2009.
NFPA 495-2006, Explosive Materials Code, National Fire Protection Association, 2006.
NFPA 499-2008, Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas, National Fire Protection Association, 2008.

Michael Maxwell is vice president of engineering and technology and manager of applications for Griffin Filters LLC. He received a B.S. in Chemical Engineering from Iowa State University and an MBA from Lewis University. He has over 36 years of experience in dust collection design, application and installation, and has been a member of the National Fire Protection Association since 1994.