Combustible Dust: What You Need to Know

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.