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B.C. sawmill tragedy: How does dust explode?

The Lakeland Sawmill in Prince George April 25, 2012 after a fire and explosion at the mill.

John Lehmann/Globe and Mail

The deadly explosion at B.C.'s Lakeland Mills Ltd. earlier this week prompted sweeping new guidelines for inspections and safety across the province.

At issue is the risk of dust explosions, a "deadly serious" threat to sawmill workers, says Neil McManus, a consulting industrial hygienist at NorthWest Occupational Health & Safety.

The Globe asked Mr. McManus to explain why sawmills are particularly risky for dust explosions.

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Globe: What is combustible dust?

Mr. McManus: Combustible refers to the ability to burn. Dust particles of various sizes from the same material that is combustible will burn _ given the appropriate circumstances. Those circumstances do not necessarily include the ability to explode.

Globe: How does a dust explosion occur?

Mr. McManus: In order for a dust explosion to be able to occur the following conditions must apply:

* the particles must be the appropriate size and must be fine, compared to coarse. How fine is a question for testing. How fine is a matter of considerable interest and importance as applied to coal-fired generating stations. The station grinds the coal to the appropriate size in order for rapid combustion to occur in the boiler

* the particles are airborne in the appropriate concentration

* the particles are contained within the boundaries of a structure

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* there is a source of oxygen (the air)

* there is an energetic source of ignition

Globe: You once likened a dust explosion to a detonating bomb. How so?

Mr. McManus: Every explosion produces a pressure wave. The circular ripple that occurs when one drops a pebble into calm water is an example of a pressure wave. As the wave moves outward, it decreases in height.

There are two kinds of explosions: deflagrations and detonations. The pressure wave moves slower than the speed of sound during a deflagration explosion. The pressure wave moves faster than the speed of sound during a detonation. Dynamite explosions are detonations.

Globe: You have said dust explosions are engineering problem, not a safety issue. How so?

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Mr. McManus: Engineering and safety are intimately related. Safety -- being the condition such that nobody will suffer accident injury or death at work -- is an outcome from good engineering. Engineering responds to safety issues through redesign and installation of structures to prevent harm to people.

Globe: What are other countries doing to deter explosions?

Mr. McManus: My understanding is that Brazil requires all companies to assess the risk of their activities. If the risk in context of the number of workers exceeds some threshold, the company will employ one or more of this group -- a safety technician, a safety engineer, a nurse and/or an occupational health physician. Attainment of the status of safety engineer is a rigorous process involving a minimum of licensing as an engineer in Brazil (minimum of 5 years post graduation plus adherence to a strict code of ethical conduct) followed by 660 hours of training in health and safety. The designation of a safety engineer is a rigorous process. The curriculum is responsive to the need for training in specific areas. (To illustrate, I have taught about confined spaces and portable ventilation systems in Brazil.) This program is a specialization and not a masters degree. To my knowledge, nothing equivalent to this exists in North America.

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