Dust to Dust
For a dust explosion to occur, everything has to be lined up just right. Unlike a fire, which can be started by a trio of elements — fuel, an ignition source and oxygen — a dust explosion can occur only if a pentagon comprising two additional elements, dispersion and confinement, come together.
That destructive constellation is now being investigated as a possible cause behind the explosion of two sawmills in British Columbia that blew up months apart from each other. On January 20, 2012, an explosion and fire destroyed Babine Forest Products mill in Burns Lake, killing two workers and seriously injuring more than a dozen. On April 23, an explosion and the resulting fire razed Lakeland Mills in Prince George, claiming two employees and injuring 22.
The similarities between the two explosions are striking — both are sawmills, dust was present in both facilities and both are working with beetle-infested wood. A similarity so uncanny that it prompted a review, an industry-wide directive order and several investigations to find out just what went wrong, and how it could go so wrong.
The destruction of the two sawmills has triggered a search for answers that will be close to the heart of many industries that generate fine particles — from mills that process wood, grain and sugar; chemical manufacturing plants and facilities that fabricate rubber and plastic products; to coal-fired power plants.
On April 25, provincial labour minister Margaret MacDiarmid convened an urgent meeting involving representatives from the industry, United Steelworkers and WorkSafeBC to review mill safety and compile a document on industry best practices relating to combustible dust control in sawmills.
The following day, the provincial safety regulator issued a directive order to all sawmill operators to conduct a full hazard identification, risk assessment and safety review with particular focus on combustible dust, dust accumulation and potential ignition sources. “WorkSafeBC officers will be following up on these orders to confirm the ordered actions have been taken and sawmills are in compliance with the Workers Compensation Act and Occupational Health and Safety Regulation in regard to combustible dust and potential safety hazards,” a WorkSafeBC statement noted in April.
Until the investigation has concluded, Roberta Ellis, vice-president of corporate services with WorkSafeBC in Richmond, British Columbia, says it “cannot speculate, based on the similarities, as to the cause of these events.”
It did, however, reveal in a statement in May the ignition sources in both investigations appear to have been located at the conveyor belt level where electrical and/or mechanical equipment was in operation in areas contained by walls and equipment. These areas are located at the basement or lower level of both mills.
Officers inspecting all sawmills and associated site facilities under the directive order “will be paying particular, but not exclusive, attention to areas similar to those identified in the Babine Forest Products and Lakeland Mills investigations,” the statement notes.
The Council of Forest Industries in Vancouver has also established a task force comprising representatives and stakeholders from the wood products manufacturing industry to investigate combustion risks in mills. The task force, which reports to a CEO Action Committee, “is a significant collaborative effort to ensure that British Columbia’s wood products manufacturers are operating to the absolute highest standards,” Hank Ketcham, chief executive officer of West Fraser, says in a statement.
Don Kayne, chief executive officer of Canfor, adds that “we have taken every opportunity to increase our efforts in safety since the serious incidents at Lakeland and Babine, and this joint initiative is a next and very important step.”
Stephen Hunt, director of western Canada (district 3) with the United Steelworkers, went to Babine Forest Products mill with several union representatives following the incident. “Nobody from anywhere could recall a catastrophic explosion in a sawmill that literally blew the mill right off the face of the earth,” he says. When the mill in Burns Lake exploded, “I think it is safe to say that everybody is convinced that it was a one-off; something happened in that mill that was so unique that they could not be replicated,” Hunt continues. “Then three months later, we blow another one up.”
The two incidents have raised many questions on why dust particles are hazardous and what makes them combustible.
Combustible dust — also known as dry, deflagarable dust — is a wood particulate with an average diameter of 420 micrometers or smaller and has a moisture content of less than 25 percent, notes information from WorkSafeBC. “The drier the dust, the finer it can become and it is more of a fire explosion hazard,” says John Astad, director of the Combustible Dust Policy Institute in Santa Fe, Texas.
Combustible dusts are often organic or metal dusts that are finely ground into very small particles. Types of dusts include, but are not limited to metal dust (such as aluminum and magnesium); wood dust; plastic dust; biosolids; organic dust (like sugar, paper, soap and dried blood); and dusts from certain textiles, notes information from EMSL Canada, Inc. in Mississauga, Ontario. The United States-based firm is one of two companies identified by WorkSafeBC as being able to perform combustible dust testing.
Suspended dust burns more rapidly while confinement allows for pressure to build up. The initial explosion can cause dust that has settled over the years to become airborne, resulting in a secondary explosion that propagates throughout the plant, often with disastrous results.
Apart from primary and secondary explosions, there is also a “multitude of non-consequential combustible-related fires,” Astad notes. While the majority of combustible dust-related incidents do not result in injuries, fatalities or property loss, “these can be precursors to catastrophic events.”
No amount of dust, it seems, is too little to be ignored. Any surface that has more than five per cent of dust accumulation on it has to be cleaned off. In fact, a thickness that exceeds an eighth of an inch is too much, says Craig Kennedy, owner of Kennedy Forest and Safety Consultants in Williams Lake, British Columbia. “That is the thickness of two nickels.”
Potentially explosive particles can accumulate wherever dust is generated, says Dr. Graeme Norval, PhD, associate chair and undergraduate coordinator with the department of chemical engineering and applied chemistry at the University of Toronto. For sawmills, this means places where the blades are located and where lumber gets transported. In sugar mills, vulnerable spaces include those where grinding operations, transportation of materials on conveyor belts and bagging of sugar and flour take place.
In other words, “anytime you are changing a particle size, that is when you have a vulnerability,” Astad notes.
Many new, automated mills are using more efficient, computerized technology to process wood, generating finer dust in greater amounts which, when suspended, presents an explosion risk, Kennedy explains. “They are pounding wood through really fast,” he adds, citing bandsaws, fast-moving table saws and circular saws as examples.
Hunt agrees. “We are finding that, like in many workplaces, the size and speed of the saw blades have changed; the nature of the dust coming off has changed.” These changes demands the accompaniment of safety controls, although “it appears that that may have been left behind,” Hunt suggests.
Transfer points like conveyor belts, bucket elevators, mixers and dryers are vulnerable to the risk of dust explosion. Friction from moving equipment — like a bucket elevator with its metal rubbing against a casing — and industrial-powered equipment, such as forklifts, can also cause fires or explosion when hot surfaces from motors, brakes and engine exhaust system meet suspended dust in sufficient amounts. “All that is missing is an ignition source,” Astad says, noting that explosion-proof forklifts should be used to ensure safe operation in environments where dust is being generated.
Any activity that involves heat, electricity and an open flame can serve as an ignition source. A worker doing hot work on the other side of a process equipment, Astad cites by way of example, can accumulate sufficient electrostatic charge to potentially ignite dust accumulation in that area. The possibility of an electric rotor or electrical panel arcing or sparking presents an additional risk.
“A lot of these ignition sources are inherent with the process equipment,” he notes. “It is not just one area we have to look; it is a holistic approach.”
Determining the minimum ignition temperature at which dust clouds can ignite is key to preventing dust explosions. There are two types of minimum ignition temperature: layered (for undisturbed dust) and suspension temperature (for suspended dust). The American Society for Testing and Materials provides guidelines for performing laboratory tests to evaluate the deflagration parameters of dusts in ASTM E1226: Standard Test Method for Explosibility of Dust Clouds.
These tests provide crucial information that forms the basis for the development of a dust accumulation control program. Once the minimum ignition temperature has been determined, “we can evaluate our facility and see if we have temperatures that are approaching that,” Astad says. From that knowledge springs the need to communicate the hazard to rank and file, implement engineering and administrative controls and evaluate the training required, he adds.
Characterizing Dust Samples
A combustible dust analysis comprises a series of tests to determine whether or not a dust is combustible and potentially explosive. At least one litre of dust must be collected and submitted for analysis, notes information from WorkSafeBC. Combustible dust is typically analyzed in two ways:
- Particle characterization determines particle size and moisture content. The most important part of this test is determining the percentage of the dust sample that is both combustible and small enough to pass through a 40-mesh sieve less than 420 micrometer in size.
- Class II test involves a number of parameters that determine how explosive the dust is and if the sampled dust is considered a Class II hazardous material.
In view that the entire Class II test is expensive, costing more than $4,000, the most practical way is to perform an explosive severity test, which is an initial screening to find out if the dust has explosive properties before continuing with the remaining tests. ‘If the dust is not found to be an explosive threat, the analysis can be aborted to avoid unnecessary fees,” WorkSafeBC information notes.For combustible dust sampling and analysis, the safety regulator recommends following the Occupational Safety and Health Administration (OSHA) ID-201SG sampling method guideline. The OSHA Combustible Dust Emphasis program (CPL 03-00-008) also provides information on sample collection.
Undertaking a risk assessment to identify high-hazard areas and developing a preventive maintenance program is a good starting point. However, housekeeping can only mitigate the hazard so much if the interaction between the operational environment and engineering controls do not work in sync to keep the dust out.
Kennedy cites a wood processing facility he inspected that has two large openings in one part of the mill. “The wind is blowing through one side and out through the other. It is like a big wind tunnel out there,” he describes. “The dusts that were cleaned off were blown back onto the girders and the beam by the wind.”
Seasonal change also needs to be factored into consideration when it comes to designing engineering controls. Water misters, used to dampen down airborne dust, are not suited for use during winter when the temperature drops below freezing. Roof fans, which keep small amounts of fine dust out of a building, introduce hot air into a facility in summer “and in winter, it sucks the cold air in,” Kennedy says.
“Many facilities think housekeeping is just blowing it down with compressed air,” Astad says. On the contrary, that can put dust in suspension and heighten the risk of an ignition, he cautions.
In fact, equipment designed to collect dust can sometimes pose a hazard itself. About 40 per cent of combustible dust explosions reported in the United States and Europe over the last 25 years have involved dust collectors, notes information from Designing Your Dust Collection System to Meet NFPA Standards, published in 2008. Down south, dust collection systems have now become a primary focus of inspection required by the Occupational Safety and Health Administration’s National Emphasis Program on the safe handling of combustible dust since 2007.
One of the key factors in preventing explosions in the dust collection system is to maintain a reasonable conveying air velocity in every part of the duct. Low velocity can cause dust to drop out of the air and build up inside the duct. Too high a velocity, on the other hand, results in energy wastage. The characteristic of the dust is also an influencing factor. Coarse dust can cause erosion to the duct while moist and sticky dust can smear the duct wall, the paper notes.
“The most important thing that should be considered is the efficiency of the collector,” says Bes Blentic, sales manager with Camfil Farr Air Pollution Control in Toronto. Finer dust requires a lower velocity while heavier dust needs air to be churned at a higher speed to prevent them from settling in the duct, Blentic adds.
Personnel who maintain dust collector systems should be provided with training to ensure that they understand the fire and explosion hazards associated with combustible dust, Astad notes. He is, however, quick to point out that training on preventing dust explosion hazards should involve not only workers and stakeholders, but also all relevant regulatory agencies, such as the Fire Commissioner. “This is not solely a WorkSafeBC issue,” Astad says, noting the provincial safety regulator primarily looks at the operational environment of a plant from an occupational safety perspective.
On the other hand, the built environment involves looking at the physical structure of the plant and its fire safety and prevention, which falls under the purview of the Office of the Fire Commissioner. This includes administering the Fire Services Act, enforcing fire safety legislation, inspecting and investigating fire incidents, and responding to major fire emergencies.
“We need to distinguish what is an operational environment and what is a built environment,” Astad notes. “Workplace fire and explosion hazard is an issue where guidance can be acquired from both local assistants to the Fire Commission and WorkSafeBC fire prevention officers,” he adds. “Solely relying on one agency and ignoring the other does not comprehensively address the issue.”
ON ALL FRONTS
A multi-faceted dust accumulation control program involves conducting regular risk assessments; dust control through containment, engineering systems and housekeeping; ignition control and emergency procedures in the event of a dust explosion, notes information from the Forest Industry Task Force on Mill Safety.
For risk assessments, workspaces to consider include sawing and debarking operations, planers, chipper enclosures and chip screening areas. Particular attention needs to be paid to less conspicuous areas, such as conduit, pipe racks, cable trays and rafters. Areas that produce fugitive dust should be identified and ways to enclose or contain it in that location need to be developed. All dust control systems must be inspected, cleaned and maintained in good working condition, the information adds.
Clean-up should also be scheduled in relation to the extent of possible dust accumulation. Apart from horizontal surfaces, overhead and vertical areas — such as beams and walls — need to be covered. “You have to be conditioned to notice that there is dust collecting and deal with it,” Dr. Norval says.
One common oversight is dust accumulation at heights, such as on top of girders or overhead pipes. “One day, the pipe shakes and now you have a large accumulation of dust in the air,” Dr. Norval says. “Just because you don’t see dust on the floor doesn’t mean there’s no dust problem. People look at the floor, but they don’t look at on top of things.”
When performing clean-up operations — methods of which include vacuuming, water wash, brooms and compressed air — personal protective equipment and proper safe work procedures should be provided.
A comprehensive combustible dust control program, however, is not complete until that hazard has been identified and communicated to all workers. Until then, “we can talk about engineering and administrative controls all day, but that’s putting the cart before the horse,” Astad suggests.
In the meantime, the search for answers to the two devastating explosions continues. The United Steelworkers is engaged in the investigation conducted by WorkSafeBC, as well as conducting its own investigation, which involves interviewing workers at Lakeland Mills, Hunt says. “The key for us is to ensure the timing is right for the investigation,” he notes. “If you go to people too quickly, there is still quite a bit of anger and frustration.”
The Council of Forest Industries task force is looking at establishing an auditable dust control standard that can be implemented through a third party to serve as a “powerful means of ensuring a consistently high standard across the industry,” says Corinne Stavness, manager of public affairs with the council.
“We are making good progress,” she adds, noting that the two incidents have brought together the forest products industry in British Columbia in an unprecedented fashion. “It is our aim that this will be a positive legacy of these horrible tragedies — that we have really set a new standard in coordination and openness between companies, sharing more information on dust and all other safety hazards and increasing the pace at which we learn from each other and implement safety improvements.”
Since May 28, Lakeland Mills had restarted its planer mill for about six weeks to process inventory that remained following the explosion and fire. A company statement says there is enough lumber to keep the mill running for approximately 30 shifts, or one shift per day for six weeks. No decision has yet been made on the longer-term future of the mill’s operations.
“What we learned from this is don’t take any dust for granted,” Hunt says. “It can all explode if they have enough quantity.”
Jean Lian is editor of ohs canada.
On May 2, WorkSafeBC released an investigation update on the explosion at Babine Forest Products that leveled the mill on January 20, 2012. More than 80 witness interviews have been conducted, with as many as 20 WorkSafeBC personnel working on the investigation. The footprint of the incident site required in excess of 3,000 lineal feet fencing, placing it amongst the largest scene investigations in organizational history.
Possible factors contributing to the risk of explosion that continue to be examined include production level records (both recent and historic); type of wood being milled prior to the incident; exhaust and ventilation systems and schedules; effect of cold weather in the days preceding the incident date (-41 degrees Celsius); effect of cold on water pipes and misters; and sawdust accumulations.
There is no evidence to suggest the explosion was caused by arson, lightning strike, hot oil, hydraulic oil, gear oil, oxygen and acetylene.Natural gas, propane and sawdust continue to be examined as possible fuel sources, while hot surfaces or friction, electric arcs from motors and switches, and several other electrical components are being looked at as possible ignition sources.
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Hello, This post is given to OSH Canada in order to assist the scrutiny to focus on a possible ignition source that could be generated from the described physical evidences in the two explosion events. I am a physicist and electrical engineer. For more than 30 years I am working in the field of static electricity risk analysis in industry, including investigation of dust materials fire & explosion. The very cold and dry weather has a strong effect on the build up of electrostatic charge due to any kind of friction between materials/parts in the mill. The charge can accumulate on electrically conductive parts as well as isolative materials. The electrostatic mechanism of dust ignition involves some parameters such as discharge type (spark, brush, PBD, cone) and charge density/energy, dust particle (distribution size, concentration in space, moisture content). Dust explosions created by such ignition source are generally characterized by two explosions following each other, the first is weak (sometime even not seen or heard) and the second is much stronger that is yielding mass destruction. The investigation already eliminated some common ignition source and now some electrical components are being looked. As static electricity is known to be a hidden ignition source with very unique characteristics, I strongly recommend OSH Canada to include in the investigation and to look for the possibility for electrostatic ignition. Feel free to contact the undersigned for any further assistance if needed. Sincerely, Eyal Zadok E-mail: firstname.lastname@example.org
Posted July 18, 2012 04:26 AM