Explaining dust explosions

BC’s recent sawmill blowups draw attention to the ways explosions start and how they can be prevented in the future.

BY SIMON FRIDLYAND

The worry over plant and mill explosions has increased this year because of at least two recent sawmill blowups in Canada. A wood dust explosion occurs when a fine dust in suspension in the air is ignited, causing a very rapid burning and then a release of gaseous products. This leads to a subsequent pressure rise of explosive force that will damage a plant, a property and people. This is exactly what has happen recently in two British Columbia sawmills.
Dust explosions can be categorized into two types -- primary and secondary. A primary explosion takes place in a enclosed atmosphere, such as a dust collector or specific part of a manufacturing plant. A post-mortem investigation can probably identify exactly where an explosion began. The resulting shock wave will damage and often rupture the plant, allowing the products of the explosion (burning dust and gases) to be expelled into the surrounding area, disturbing any settled dust and initiating a larger secondary explosion.
The secondary explosion can cause severe damage to the surrounding plant and buildings. All large-scale dust explosions, including the ones at the BC sawmills, result from chain reactions of this type.
There is always a concentration of dust inside dust collectors, cyclones or other wood dust handling machinery, as well as in confined enclosures. So a source of ignition could come from overheated bearings, sparks, electric arcs and such. In principle, these are all the components needed for the primary explosion to take place.
If ignition of a combustible dust cloud takes place in an enclosed area (e.g., machinery or the plant) there will be a rapid build-up of pressure, with resulting damage to the plant and building. Means must be provided for the early release of this excess pressure so that damage to the plant can be minimized.
The most convenient way of providing explosion protection is to install explosion reliefs such as vents, bursting panels or explosion doors.
The relief must be capable of operating almost instantaneously, as usually there is only a very small safety time factor between the operation of the vent and the bursting pressure of the plant. When the vent opens, the products of the explosion are discharged from the plant or mill, keeping the explosion pressure at a lower level than the design strength of the building and protecting it from the worst effects of the explosion.
Care should be taken in the placement of such explosion reliefs, and the products of the explosion must be vented to a safe place in the open air.
The equipment must also be designed to prevent the transfer of energy to upstream and downstream areas. This is usually achieved by installing isolation devices, such as chemical isolation, high-speed gate valves, abort dampers, and so on.
There are also explosion detection and suppression systems designed to identify the explosion at an early stage and suppress it before it gets out of control.
Since implementation of all of these systems is not a simple matter; it must be handled by a professional engineer specializing in this field. In Ontario, dust collection systems, as well as potentially explosive processes, must be covered by Pre-Start Health and Safety Reviews (PSRs). A PSR is a report signed and sealed by a professional engineer and states the status of the plant and equipment related to the dust collection system and potentially explosive processes.
The primary explosion usually has enough energy to bring into suspension the dust that has accumulated on flat surfaces such as ducts, beams, equipment, etc., and ignite the dust-air mixture. The time required for a dust explosion to develop fully is only about 50 milliseconds. There may be several secondary explosions going on, one after another, resulting in the complete destruction of building complexes. The explosions of facilities such a sawmills are also followed by fires.
The strategy for mitigating secondary explosions lays with the removal of dust from the facilities, including dust accumulated on beams and other hard to reach areas.
A layer of accumulated fugitive dust exceeding 1/8th of an inch over 5% of the area or 2,000 sq ft, whichever is smaller, is enough to cause a secondary explosion. Therefore, housekeeping becomes an important measure in the explosion mitigation process.
WorkSafeBC recently issued the following directive to sawmill employers:
“In the past three months, explosions and fires at two sawmills in British Columbia have resulted in fatalities and serious injuries to workers. This employer is ordered to, without undue delay, at its sawmill and any related facility:
- undertake a comprehensive risk assessment with respect to hazards created by combustible dusts, which assessment must include a thorough inspection of the employer’s facility, and
- develop and implement an effective combustible dust control program based on the risk assessment.”
WorkSafeBC said a prevention officer would be conducting a follow-up inspection on site no later than May 9, 2012, to evaluate the employer’s actions taken immediately to fully comply with the order in the Inspection Report. “Failure to comply with this order without undue delay will result in further action, including possible sanctions and stop work orders. A WorkSafeBC officer will follow up on this order to evaluate compliance.”
In my opinion, in order to investigate the potential primary causes of an explosion in a sawmill, specialized knowledge of applicable codes and standards, as well as experience in this area, are required. With all due respect to the people who run sawmills, the explosion mitigation issues of plant equipment may not be their cup of tea. This type of work should be done by a professional engineer specializing in this field. However, the housekeeping aspect of a combustible dust control program must be developed immediately by the sawmills' employers.
Simon Fridlyand, P.Eng., of SAFE Engineering Inc., specializes in industrial health and safety concerns and PSR compliance. For more information, visit www.safeengineering.ca.

[SIDEBAR STORY]

'There was guys with their skin hanging off', says worker at exploded BC sawmill
By The Canadian Press
Prince George, BC -- Evening shift workers at a Prince George, BC, sawmill ran for their lives after an earth-shaking explosion and massive fire sent walls crumbling down on top of them, killing two people and critically injuring at least seven more.
The explosion occurred at the Lakeland Mills site at approximately 9:45 p.m. on April 23, 2012. The explosion resulted in a fire that completely destroyed the sawmill, operated by Sinclar Group Forest Products Ltd. of Prince George.
''It was quite gruesome,'' said Brian Croy, first vice-president of the United Steelworkers Local 1-424, in an interview from his home. ''When you walked out, there was guys with their skin hanging off their arms and stuff from being burned.''
Croy said he was among six people inside the mill's lunchroom talking about training when the explosion happened. ''That thing came up so fast, so quick. I don't know where it came from, but it was almost like a cannon going off. It blew through there. It ended just that quick,'' he said.
''It's almost like you were coming out of the war zone. Everything was levelled. I met that one fellow. I think his fingers were blown off, and his clothing, a lot of it was gone. It was burned off and his hair (was too).''
In all, Sinclar Group Forest Products said 24 people were in the sawmill when the blast occurred, a further 16 were in the planer mill next door and four were working in the yard.
''I can't tell what caused the explosion,'' said Croy. ''All I know is that there was no warning, no nothing, because we'd walked through the mill and stuff to come up to that lunchroom and didn't smell no gas, no nothing.
The mill's primary products are premier-grade, kiln-dried studs. The mill also supplies fibre for two bioenergy systems, including the district energy plant.
In January, an explosion and fire killed two workers at Babine Forest Products in Burns Lake, about 230 km northwest of Prince George. WorkSafe BC is still investigating the cause, but the mill was flattened and 250 people were thrown out of work.
BC Labour Minister Margaret MacDiarmid said she would “send an order out to all the sawmills in the province asking them, telling them, instructing them to inspect from top to bottom their mills, to make sure all steps are being taken to address current safety policy.
''There's a common factor here and we're all aware of it and it's sawdust. So although we don't know what caused the initial fires or explosions, we know that sawdust may be a factor.''
This article is an edited compilation of stories by The Canadian Press. For more information, see our guide to understanding, preventing and controlling dust explosions, Part 1, ‘Dust, Dangerous Dust’, which was published in Machinery & Equipment MRO’s September 2011 issue. Part 2, ‘Great Balls of Fire’, appeared in the November 2011 issue. Both are available online www.mromagazine.com, as are the original news stories about the BC mill explosion.

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