Article Archive
DEADLY DUST
Even small amounts of accumulated dust can result in fiery tragedy
Volume 23, No. 3

Back when Grandma cooked over an open flame it was common knowledge - never use flour to extinguish a grease fire. Use baking soda or salt. Better yet, put a lid over the pan. But never, never use flour. It might mean placing the kitchen and, quite possibly, Grandma at risk for a fiery dust explosion.

As a highly dispersed dust, flour can ignite. Flour is a starch which, like other carbohydrates such as sugar, burns very easily. About two ounces suspended in a cubic yard of air is enough to create an intense flash fire. If the dust cloud ignites within a confined space, the situation can become explosive.

Flour is not the only dust that poses an airborne threat. Most solid organic materials, as well as many metals and some nonmetallic inorganic materials, will burn or explode if finely divided and dispersed in sufficient concentrations. Combustible dusts can be intentionally manufactured powders, such as corn starch or aluminum powder coatings, or may be generated by handling and processing solid combustible materials such as wood and plastic pellets.

Polishing, grinding, transporting and shaping many of these materials can produce very small particles, which can easily become airborne and settle on surfaces, crevices, dust collectors and other equipment. Even seemingly small amounts of accumulated dust can cause catastrophic damage. The explosion that devastated a North Carolina plant in 2003 and killed six workers was caused by dust accumulations that, for the most part, were less than a quarter of an inch.

And yet, the respect that Grandma had for flammable dust in her kitchen is not shared by all corners of modern industry. The dust threat is most often identified with flour mills and grain elevators. Grain elevator explosions throughout the 1970s, including five elevator explosions in December 1977 that killed 59 people, led to much research and stricter Occupational Safety and Health Administration (OSHA) standards.

But the aforementioned plant that exploded in 2003 produced rubber syringe plungers and other pharmaceutical devices. Two other catastrophic dust explosions that same year - one at a Kentucky plant specializing in acoustic insulation for cars and the other at an Indiana foundry that made cast aluminum and aluminum alloy automobile wheels - killed an additional eight workers.

Angela Blair of the Chemical Safety and Hazard Investigation Board served as lead investigator for a report on combustible dust fires and explosions released by the CSB in 2006. She said investigators soon realized that the phrase "dust explosion" was not part of the common vernacular in industry.

"People do not comprehend that dust will explode," Blair said. "In those first investigations, we spent a lot of energy explaining and convincing people what dust was capable of doing."

Updating the statistics used in that report, there have been 348 dust explosion and fire incidents in U.S. industry since 1980. Those incidents resulted in 132 fatalities and 780 injuries. Including the Feb. 7 sugar refinery explosion in Port Wentworth, GA, at least eight of those incidents were defined as "catastrophic," involving multiple fatalities and significant community economic impact.

Citing the CSB dust explosion report, Rep. John Barrow, D-GA, and Rep. George Miller, D-CA, chairman of the House Education and Labor Committee, have announced plans to introduce a bill to force OSHA to issue new regulations governing industrial dust. A hearing on the issue was scheduled for March 12 in the wake of a sugar refinery explosion in Port Wentworth, GA, on February 7 that killed 12.

Ed Foulke Jr., head of OSHA, visited the Port Wentworth blast site on March 3 to announce that federal inspections would be carried out at hundreds of plants where combustible dust is a workplace hazard. OSHA mailed 30,000 companies with regard to combustible dust dangers.

According to the CSB report, the two most common fuel types involving dust explosions and fires were the food products industry with 24 percent and the lumber and wood products industry with 15 percent. However, chemical manufac-turing accounted for 12 percent of these fires and explosions. The rest were distributed among the primary metal industries, eight percent; rubber and plastic products, eight percent; electric services, eight percent; fabricated metal products, seven percent; equipment manufacturing, seven percent; furniture and fixtures, four percent; and others, seven percent.

The CSB's combustible dust hazard study serves as a primer of basic concepts about dusts and dust explosions. It states that a key factor in the three 2003 dust explosions studied by the CSB was that workers and managers were often unaware of dust explosion hazards or failed to recognize the serious nature of those hazards.

THE EVENTS

Each of the three 2003 explosions investigated by the CSB provide case studies of the different ways dust becomes a devastating hazard. In all three cases, the CSB determined that the companies involved failed to adhere to relevant NFPA standards.

In the North Carolina incident, the rubber compounding process involved freshly milled rubber strips dipped into a slurry of polyethylene, water and surfactant to cool the rubber and provide an anti-tack coating.

As the rubber dried, fine polyethylene powder drifted on air currents to the space above a suspended ceiling. While the visible production areas were kept extremely clean, the powder accumulated above the suspended ceiling, providing fuel for a devastating secondary explosion.

The plant's safety review process when the compounding system was designed and modified never addressed the dust explosion hazard. MSDS for the polyethylene slurry included no dust explosion warning. Workers had not been trained about the potential hazard.

Despite inspections by OSHA, firefighters, an insurance underwriter and an industrial hygienist, the dust hazard remained unidentified. Electrical equipment above the suspended ceiling was not rated for use around combustible dust as required by the National Electric Code.

"Even a seasoned expert in the control of combustible dust hazards would have been hard pressed to recognize that problem because the working area wasn't dusty," Blair said. "If that expert had spent two or three hours and seen those workers constantly wiping up, he might have started to think about what was in that dust."

No specific source of ignition for the explosion was determined.

"When we investigated in North Carolina, everyone we interviewed said the same thing - 'There is no way dust did this,'" Blair said. "Six months later when we made our public presentation, we took a half teaspoon of the powder that caused it and set off a tiny dust explosion on stage. That finally got it through to them."

In Kentucky, the manufacturing process began by impregnating a fiberglass mat with phenolic resin and then used air to draw the resin into the fiberglass webs, the CSB report states. On the day of the explosion, a curing oven left open because of a temperature control problem likely ignited the combustible resin dust stirred up by workers cleaning the area near the oven.

The CSB found that the building was not designed to prevent or minimize secondary dust explosions, such as using fire walls to separate production lines. Dust had accumulated in dangerous amounts throughout the production areas, in vent ducting and in dust collector housings. Workers routinely used compressed air and brooms to clean production lines, creating clouds of resin dust.

Again, an MSDS failed to communicate that the material posed a dust explosion hazard. Although the Kentucky Office of Occupational Safety and Health had inspected the facility, no citations regarding combustible dust had been issued. The facility had never been inspected by the state fire marshal. The Kentucky explosion killed seven workers and injured 37.

In Indiana, one worker died and several others were injured in an explosion involving equipment used to re-melt scrap aluminum. The scrap was chopped into small chips, pneumatically conveyed to the scrap processing area, dried and fed into a melt furnace. Transporting and drying the chips generated dust that was pulled into a dust collector.

The CSB determined that the explosion likely originated in the dust collector, which had not been adequately vented or cleaned, and was located too close to the aluminum scrap processing area. The collector was not designed or maintained to prevent dust explosions, nor to prevent spread through the ducting. When the dust accumulated, a large fireball emerged from the furnace. Dust had not been cleaned from overhead beams and other structures, leading to a secondary explosion.

"The dust collector was not properly located," Blair said. "It was too close to the building. The explosion wave propagated quickly back into the building where the people were."

Locating the collector further away might have meant more time for a sensor-controlled quick closing isolation valve, if one had been installed, to activate as the flame front moved through the system. Isolation valves direct an explosion to a benign location, minizing its effect.

Previous dust fires at the facility had gone uninvestigated. The Indiana Occupational Safety and Health Administration had not identified dust explosion hazards during previous facility inspections.

Strangely enough, the 2006 CSB report focusing on these three incidents and others was followed by an uncharacteristic lull in major dust explosions - until Port Wentworth.

"The reason it has been so long is likely not simply because industry is doing a better job," Blair said.

THE SCIENCE

The classic fire triangle consists of fuel, oxygen and ignition. A dust explosion requires two additional elements - dust suspension and confinement. Suspended dust burns more rapidly, and confinement allows for pressure buildup. Without suspension or confinement, an explosion is not possible. Further, the concentration of suspended dust must be within a particular range to explode, somewhat like the flammability range of vapors. The lowest amount of dust in air that will explode is referred to as the minimum explosible concentration.

NFPA defines a combustible dust as any finely divided solid material that is 420 microns or smaller in diameter. The particle size of table salt is around 100 microns. Finer particles are more explosive because they have large surface areas relative to their weight, allowing them to rapidly react with oxygen when dispersed. Other factors that influence explosiveness include moisture, ambient humidity, oxygen, dust particle shape and the dust concentration.

Many solid or bulk materials may not be explosive but may generate combustible dusts through handling or processing. Plastic pellets shipped from a polyethylene plant rarely pose an explosion hazard until they are handled and generate small particles as part of a different process. Likewise, aqueous solutions of a combustible material may try to produce a combustible dust.

Dust explosions can either be primary or secondary. A primary dust explosion occurs when a dust suspension within a container, room or piece of equipment is ignited and explodes. The secondary explosion occurs when dust accumulated on floors or other surfaces is lofted and ignited by the primary explosion. The blast wave from the secondary explosion can cause accumulated dust in other areas to become suspended in air, which may generate additional dust explosions.

"A primary dust explosion typically occurs inside a piece of equipment," Blair said. "Secondary explosions are often caused by poor housekeeping, accumulations that are not cleaned up."

Depending on the extent of the dust deposits, a weak primary explosion may cause very powerful secondary explosions. However, the initiating event for a secondary dust explosion might not be a dust explosion at all, the CSB report states.

The best way to prevent secondary dust explosions is to eliminate the dust. Good housekeeping, designing and maintaining equipment to prevent dust leaks, using dust collectors, eliminating flat surfaces where dust can accumulate and sealing hard-to-clean areas can effectively prevent secondary dust explosions.

The CSB warns that proper equipment and techniques to clean combustible dust accumulations must be used. Care must be taken to minimize dust clouds, and use only vacuum cleaners approved for combustible dust locations.

Several of the incidents studied by the CSB involved dust explosions that spread through pipes or vent ducts, from one piece of equipment to other equipment or other areas of the facility. Pressure can increase as the explosion moves from one location to the next, increasing the damage.

"One way to eliminate dust accumulation is to have a dust collector," Blair said. "Suction hoods over the equipment serve as point collectors, sucking up the dust from various locations in the plant, transporting it through duct work to some kind of device separating the dust from the air, either a filter unit, a cycling separator or a bag house filter."

In moving the dust, the collection system concentrates it in various locations. This can become fuel for a potential explosion. Constant maintenance is required to prevent the collection system from becoming a source of danger instead of safety. If the worst happens, the system must be designed to deal with it.

"It has to have explosion venting," Blair said. "It must be vented properly and vented to a safe location. Putting it most simply, the way you vent a dust collector is to put a big blowout panel on it that flies open when an explosion happens."

NFPA standards for dust collectors consider the risk of propagation, with recommendations to provide isolation valves or distance to minimize chances of a dust explosion spreading.

NFPA's two principal voluntary consensus standards to prevent and control dust explosion risks are NFPA 654 and NFPA 484. NFPA 654 details the hazards of combustible dusts, specifies building construction requirements and the type of equipment to use in dust-handling operations. It addresses selection and design of protective systems by referencing other NFPA standards. NFPA 654 recommends analyzing processes for hazards, controlling dust and ignition sources, constructing the building to address dust hazards and training employees.

NFPA 484 applies to fine particles of metals, including aluminum. It is distinct from NFPA 654 because the nature of metallic dusts makes them exceptionally vulnerable to ignition. Once ignited, metal dusts release a large amount of energy, making some of the protective systems required by NFPA 654 inappropriate. NFPA 484 details equipment design and explosion protection systems. It also requires that management systems address combustible dust hazards.

Other NFPA standards related to combustible dust explosion hazards include NFPA 61 (fires and dust explosions in agricultural and food processing facilities), NFPA 68 (deflagration venting), NFPA 69 (explosion prevention systems), NFPA 70 (National Electric Code), NFPA 499 (classifying dust processing locations for electrical equipment installation), NFPA 655 (prevention of sulfur fires and explosions) and NFPA 664 (wood processing and woodworking facilities). In regard to NFPA 499, some categories of dust are electrically conductive. Current can pass through a layer of such dust causing short circuits or arcs.

"Our report did not recommend any changes in the NFPA standards," Blair said. "The explosions we investigated were potentially preventable had the NFPA standards been implemented the way they were written."

The only comprehensive OSHA standard that addresses combustible dust hazards is the 1987 Grain Handling Facilities Standard that has effectively reduced the risk of dust explosions in grain handling. OSHA lacks a comprehensive standard to require general industry to implement the dust explosion prevention and mitigation measures embodied in NFPA fire standards.

"Although OSHA has cited employers for dust explosion hazards, most OSHA enforcement activities related to combustible dust hazards have been in response to incidents, rather than focusing on prevention," the CSB report states.

Most states adopt one of two national fire codes - the IFC or UFC - which incorporate, through NFPA consensus standards, principles and practices that can help prevent and mitigate combustible dust explosions. While the technical guidance in the NFPA standards is widely considered to be effective, the U.S. fire code system allows states to adopt only parts of it and local jurisdictions to adopt different codes from the states.

Implementing comprehensive changes or improvements to effectively tackle the problem of dust explosions on a national scale is difficult, the CSB report states. Code enforcement often varies across states and smaller jurisdictions. Training programs for fire inspectors do not generally cover combustible dust hazards.

CONCLUSION

It does not come marked with a hazmat placard. You might be able to write your name in it, but otherwise it seems invisible. Dust is common and inevitable in many work environments. The best way to eliminate the danger of a combustible dust explosion is through engineering design and by enforcing work practices and guidelines.

See Also: BitterSweet and Dust Storm

 
 

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