Keeping the Lid on Dust

January/Febraury 2009

By Diana Mota Morgan

Passing the white-glove test could be more important than you think. Under certain conditions, a seemingly harmless, nearly invisible layer of dust could be a combustible time bomb. Dust buildup in industrial work sites has fueled fires and explosions, causing employee deaths, injuries, and the destruction of buildings and equipment. Combustible materials—even materials that typically are not combustible—can burn rapidly in a finely powdered or divided form, and this material, when suspended in the air in the right concentrations, can become explosive.

Dust explodes in an instant, says John Gilstrap, ReMA director of safety, because a fire will spread across the surface area of any combustible material. When that material is dust, he explains, each dust particle has a huge surface area relative to its size, so it ignites quickly. "The fire propagates very, very quickly from particle to particle in a suspended cloud—so quickly that it produces a pressure wave that we call an explosion."

The United States experienced more than 280 dust explosions from 1980 to 2005, resulting in 119 deaths and 718 injuries, according to a 2006 report by the U.S. Chemical Safety and Hazard Investigation Board (Washington, D.C.). At least 13 more deaths and 40 injuries have occurred since 2005, according to news reports. Though none of the reported incidents occurred at scrap processing facilities, the potential for a combustible dust explosion at a scrapyard exists, say Gilstrap and Guy Colonna, an engineer with the industrial and chemical engineering division of the National Fire Protection Association (Quincy, Mass.). "Because of the processing, the potential is there," Colonna says.

About six months ago, OSHA added scrap recycling to its list of industries at risk for combustible dust explosions. "I think it is a concern," says Jerry Sjogren, safety director for E.L. Harvey & Sons (Westborough, Mass.). "A lot of scrap companies [work] with metals that could burn, and dust from those metals might be a concern; obviously paper dust is a concern," he says. "We need to make sure that our housekeeping standards are up." Further, scrap companies should investigate whether the combination of materials they process, the processing techniques, and the environment put their facilities at risk for combustible dust explosions.

How Dust Gets Fired Up
Most solid organic materials, many metals, and some nonmetallic inorganic materials will burn or explode if finely divided and dispersed in sufficient concentrations. Materials that ignite in their solid form can become even more dangerous as a dust, Colonna says. "If you take a piece of rubber, you might be able to burn it or melt it, but it won't be able to explode," he says. "As a dust, it could become [explosive]. Just because [a material] doesn't burn in its normal form, [that] doesn't mean it can't if you pulverize it, chip it, or abrade it."

In general, smaller dust particles raise the risk of a dust explosion, Gilstrap says. "The finer the particle size, the greater the hazard because you increase the surface area." Dust particles of 420 microns or less can serve as fuel in a dust explosion, according to an article in the November/December 2008 NFPA Journal. That's roughly the size of a grain of white sugar. Dusts are often a mixture, or distribution, of sizes, and only 2 percent needs to be 420 microns or smaller to be potentially explosive.

Even the same materials will have different ignitability and explosibility characteristics depending upon variables such as particle size, shape, and moisture content, according to an OSHA safety and health information bulletin. Those variables can change as the material passes through processing equipment. "In some cases, dusts will be combustible even if the particle size is larger than that specified in the NFPA definition [420 microns or smaller], especially if the material is fibrous," the bulletin states. Scrapyards face the additional complication of material that can vary from day to day or batch to batch.

A recipe for a combustible dust explosion includes the fire triangle of fuel (in this case, dust), an ignition source, and an oxidizer such as oxygen—plus two more factors: the suspension of a sufficient quantity and concentration of dust particles in the air and confinement of the dust cloud. If one element of the potentially deadly and destructive combination, known as the explosion pentagon, is missing, an explosion cannot occur. If those five factors exist, however, a dust cloud that ignites within a confined or semi-confined space can burn rapidly and may explode, resulting in fires, more explosions, flying debris, and collapsing building parts.

Even if combustible dust does not ignite and cause an initial explosion, it still can pose a risk. The pulse of an initial explosion from some other cause can shake loose dust that's accumulated within the facility or damage a containment system such as a duct, container, or dust collector and release dust into the air. If ignited, the fugitive dust could cause one or more secondary explosions, which Colonna and Gilstrap say are often more destructive than the initial blast. "Sometimes the secondary explosion is the most catastrophic because it moves through the facility and can take out an entire plant," Colonna says, as one dust explosion ignites the next dust cloud. "If you don't pay attention to the housekeeping," Gilstrap says, "[if] you don't clean the dust off of surfaces, and you have a small ignition, that small ignition becomes the big ignition, and it's that chain of events that OSHA's concerned about."

Sjogren says he trains his employees about the dangers of poor housekeeping. Say that "you've got a building that's allowed to accumulate a lot of dust and a lot of loose paper or other combustible material," he explains, and then "you've got an electric motor that's overheating, and it causes a fire. If you can keep that fire to the electric motor, you're only [replacing] a motor. But if you've got a lot of dust accumulated around it and you've got a lot of paper and debris around it, it's going to allow that fire to travel from that point to something else. Before you know it, your whole building is engulfed in flames."

Assessing the Risk
When assessing a facility's combustible dust risk, scrap recyclers need to review their entire process, not just their final product, to identify anything that might generate dust, Colonna says. Handling, conveying, or manipulating combustible components of any shape or size can create combustible dusts.

OSHA's safety bulletin recommends that facilities identify "materials that can be combustible when finely divided; processes that use, consume, or produce combustible dusts; open areas where combustible dusts may build up; hidden areas where combustible dusts may accumulate; means by which dust may be dispersed in the air; and potential ignition sources." Further, the analysis should identify areas that require a special electrical equipment classification because of the presence of—or the potential for—combustible dust.

Look for dust in obvious places where it becomes concentrated, such as dust collection equipment. But Colonna and Gilstrap caution processors not to overlook less obvious places, too. "Dust that's hidden behind stuff burns just as well as the stuff that's out in front," Gilstrap says. "Check conspicuous areas and hidden areas, such as beams and rafters," Colonna says.

Roof beams in a building can have a surface area equal to about 5 percent of the floor area, he notes—a significant surface upon which dust can collect.

At E.L. Harvey, "One of the things we learned was our electrical panels and our motor starters and all of our motor equipment … the dust can [get] inside of those," Sjogren says. "Even if you've got really tight boxes, the dust gets in there and builds up inside there. That, combined with a resistance in an electrical connection, causes friction and causes heat, which in turn could cause a fire and could lead to an explosion. So you need to be really careful about maintaining those things. We've actually had companies come in and do infrared surveys and check the temperature on [motors] and tighten up lose connections."

How much dust does it take to cause an explosion? It varies, Colonna says. "For some dusts, 1/32 of an inch, spread out, could be too much." From his perspective, "If you can write your name in it or leave your footprints, that's too much." There are no simple rules of thumb when it comes to safe levels of dust accumulation, according to OSHA's safety bulletin, which advises tailoring the hazard analysis to each facility's specific circumstances.

Less important than the volume, according to OSHA, is the dust's combustibility. "All dusts are unique," Colonna says, and so are the environmental factors around them. The only way to find out the potential combustibility, he says, is to have a laboratory test it.

Don't let one test give you a false sense of security, though. "Every batch is different," he says. "It's a case-by-case scenario if the input is not consistent." Changes to the processing method, materials, and equipment used "may be the magic change—that's the bad one" that increases the risk, he says.

Minimizing the Risk
"There are things you can do to minimize the generation of dust," Colonna says. Or as Sjogren puts it, "Try and catch it at the source and prevent it from being a problem." A facility might be able to process or move high-risk materials more slowly, for instance.

In processes that generate dust, Colonna says, "don't let your dust escape. Use capture devices—and maintain them properly or they can become ignition devices." He suggests eliminating as many flat surfaces as possible—especially in high places—where dust can collect as well as finding out where the dust is coming from.

Both men reiterate the importance of good housekeeping, though Sjogren cautions that some housekeeping processes can become part of the problem. He suggests sweeping or vacuuming dust instead of blowing it with an air hose, which could create a dust cloud that might fuel a combustible dust explosion.

The NFPA Journal article warns that "puffs," or small explosions, could be precursors to larger, potentially deadly explosions. The association recommends training employees to recognize conditions that signal the development of a serious situation. It has produced a voluntary standard and set of procedures for the prevention of combustible dust explosions, NFPA 654. OSHA's Web page, www.osha.gov/dsg/combustibledust/index.html, also contains information and links on the issue.

An Ounce of Prevention
Even though no one has reported a combustible dust explosion in the scrap recycling industry, such explosions most likely have happened and can happen again, Gilstrap and Colonna warn. It would be an "unusual occurrence," Colonna says, "because the necessary conditions have to come together at the right time," but even so, he's concerned that companies aren't taking the issue seriously. Gilstrap agrees: "The risk exists." •

Diana Mota Morgan is associate editor of Scrap.