Dust testing, collection systems can prevent an explosion
A large fireball burst through the roof at the northeast side of the mill. The explosion then travelled through the operating and basement levels. Fire spread throughout the premises, completely destroying the mill.
This is how WorkSafeBC described its investigation into an explosion at Babine Forest Products sawmill in Burns Lake, B.C., in January 2012. The investigation determined the cause of the explosion, which killed two workers and injured 20, was accumulation of wood dust. Three months later, a wood dust explosion destroyed the Lakeland Mills sawmill in Prince George, B.C., killing two workers and injuring 22 others.
“When I look at combustible dust in a workplace, I see sticks of dynamite waiting for their fuses to be lit,” says Raymond Roch, director at the Fire Inspection and Prevention Initiative (FIPI) in Richmond, B.C.
The hazard of combustible dust affects many industries, he says. Most combustible dust incidents occur in wood product companies, but they also happen in industries such as food (egg whites, powdered milk, cornstarch, sugar, flour, grain), metals (iron, aluminum, zinc, magnesium), chemicals (sulphur, pesticides), pharmaceutical, pulp and paper, minerals (coal), utilities, plastics, rubber, printing and textiles.
Dust is combustible when it is flammable or explosive. The fast-burning fire produced in a dust cloud is often called a deflagration. For an explosion to happen, five elements must be present:
• fuel (combustible dust)
• ignition source (a flame, hot surfaces, sparks from electrical equipment or static electricity)
• dispersion (or cloud) of dust particles
• containment of the dust cloud.
Sometimes, there is a series of explosions. The pressure wave created by the first explosion knocks dust off rafters and beams, which then becomes airborne. The fireball from the explosion then lights that dust and causes a secondary explosion, which can dislodge more secondary (or fine) dust and cause another explosion.
“So you get this domino effect,” says Roch. “That’s why you get entire facilities blown up: It wasn’t one explosion.”
If a company is producing dust, the first thing to do is determine whether the dust is combustible. Michelle Murphy, a partner at ioKinetic in Salem, N.H., says every company with dust should get it tested.
“There are some things we know for sure, but they are few. (We know) sand and salt are not combustible. But for the majority of materials, it’s very difficult to tell,” she says. “So, if you believe it’s not combustible, a quick screening test is a good way to confirm or deny that.”
If a material is combustible, Murphy recommends conducting Kst (explosion severity) and MIE (minimum ignition energy) tests.
The most effective way to prevent an explosion is to control the accumulation of combustible dust, Roch says. One of the most common engineering controls is a dust collection system. It has a capture hood that sucks in dust where it is produced and conveys it pneumatically through ductwork to the dust collector.
According to FIPI, a dust collection system is the most effective way to prevent explosions because it removes from the facility two of the five necessary elements for explosions: dust accumulation and airborne dispersion.
Another engineering control, often used at machine centres and on conveyance systems, is passive containment. To prevent dust produced by equipment or machinery from migrating, it is enclosed and then drops to an area where it is manually picked up.
However, both dust collection and passive containment systems can be hazardous. While they remove the risks of accumulation and dispersion from the general workplace, the systems themselves contain four of the five explosion elements: dust, oxygen, dispersion and containment. It’s essential to eliminate these hazards by providing the system with explosion prevention equipment, proper design to prevent spark generation and regular cleaning of the ducts.
Another control is wet dust suppression where dust is moistened as it is generated or a mist is sprayed into the air where the dust cloud forms to drop it out of the air. At Lantic, a Montreal-based sugar refinery, water is sprayed on the raw sugar.
“When raw sugar is dumped from a height onto a conveyer belt, it can release a dust cloud. A fine spray of water damps it down,” says Bob Copeland, vice-president of operations.
Building features can help lessen dust accumulation and make surfaces easier to clean. In B.C., Roch says, many companies are removing flat surfaces and making vertical and horizontal surfaces as slippery as possible by using glossy, enamel paint, for example. They are also sealing cracks to prevent dust from seeping in.
There are four key administrative controls: manual housekeeping, safe hot-work procedures, equipment maintenance (to check for loose or vibrating parts and ensure parts are not overheating) and training of supervisors and workers.
A good housekeeping program includes regular cleaning of out-of-the-way places, a fact confirmed by a recent study conducted by the Manufacturers’ Advisory Group (MAG), a safety group whose members include 55 of B.C.’s 140 sawmills.
The study, called the Sawmill Dust Initiative — which was launched in 2012 after the two mill explosions — analyzed 30 sawdust samples from a variety of trees. Ken Higginbotham, project manager of the study, says the results showed wood particles, whether large or small, are more combustible when dried. But even sawdust from recently harvested trees can be combustible if dry enough. The study also found small particles are more combustible and explosive than larger particles or particles of mixed size.
“Smaller materials tend to be more able to float in the air, and the smaller particles are more likely to end up on the beams at the top of the mill or flow down into the basement,” he says. “The study suggested that cleanup efforts need to be carried out everywhere but that particular attention should be made in some of those areas sometimes referred to as secondary, like the beams and the basements. You might not think of those areas as part of your daily cleanup but it’s an important thing to find out.”
Like many organizations in Canada, MAG uses the dust accumulation standard of 3 mm developed by the National Fire Prevention Association (NFPA) in the United States.
“Wherever the dust is, if it’s more than an eighth of an inch (3 mm), it should be cleaned up,” Higginbotham says.
Training should aim to ensure that every employee in the facility — including supervisors, operators, maintenance and even housekeeping staff — has a solid understanding of what a combustible dust is, how the hazard may present itself, how to protect themselves and what to do to avoid the hazards, says Murphy.
At Lantic, training on combustible dust is mandatory for new employees and existing employees receive regular refresher training. Instruction covers situations such as dust in enclosed spaces, raw sugar unloading, icing sugar packaging areas and areas near heat or spark sources.
Workers who are properly educated in dust hazards will also know they can go to management and tell them when a problem needs to be addressed, says Walt Beattie, president of Pottsdam, Pa.-based Beattie Fire Protection and Risk Consulting. Dust explosions usually occur as the result of an upset in the system, and often, as a result of several changes.
“By having an employee step into that chain of events that is setting up for a potential dust explosion and bringing attention to that area so that the overall system can be repaired or modified, that’s going to help them prevent that explosion from happening,” he says.
Ultimately, he adds, all employees should feel empowered to do what is needed to prevent something going wrong, including, if necessary, stopping a process.
With combustible dust, personal protective equipment (PPE) is not very effective, Roch says. If workers must be near a dust cloud, flame retardant clothing and face shields should be worn.
“But PPE is not something we want to see people focus on,” he says. “There isn’t any PPE that’s going to do you any good if there’s an explosion.”
The 2012 explosions affected major changes in the wood industry, Higginbotham says. Companies have invested several million dollars to install misters around machine centres where sawdust is produced. A lot of money has also been spent on collection equipment and on improved ventilation systems to minimize air movement around machines producing dust.
“One thing I’ve learned in 20 years in the industry in B.C. is there’s a very strong and real honesty about wanting to look after employees,” he says. “The explosions were a blow to the industry. But I think there’s a lot to be said for the response of everybody.”
Linda Johnson is a freelance writer based in Toronto. She can be reached at firstname.lastname@example.org.