Wood Dust Safety

Wood Dust in Sawmills - Compilation of Industry Best Practices  All field level hazard and risk assessments should be conducted by a qualified person before cleanup commences.

Two recent catastrophic fires and explosions occurring at BC sawmills have prompted a series of follow-up actions by industry, government and organized labour. One key step was to quickly poll operations and key organizations on industry practices specific to wood dust cleanup, control and associated fire prevention and protection measures.

This information has been compiled in the attached document and is available for your consideration. The materials cover a broad spectrum of fire prevention but do have a focus on combustible wood dust and specifically the cleanup and control of dust in sawmills.

When cleaning up combustible wood dust there is a potential to increase the risks associated with a variety of tasks and the potential for fire and explosion. The following introductory pages contain important considerations when addressing dust cleanup and control in your mill.

Of upmost importance is to:

• Minimize the use of compressed air for cleanup. Airborne dust can explode when in contact with potential ignition sources;
• Limit your ignition sources when cleanup activity is underway. No hotwork;
• Wetdown and misting is effective but be aware of impact and interface with electrical equipment;
• Lockout and de-energized equipment will decrease risks of energy contacts and ignition points;
• Dust accumulates in awkward locations – be aware of workers entering restricted, confined and workspaces at heights, and;
• Review the considerations following before you direct workers or engage contractors to cleanup.

To view other related wood dust documents please visit the SAFER website at www.safer.ca

Conditions For A Dust Explosion

When fine dust particles are mixed with air in the form of a dust cloud, the particles surface area is greatly increased and if exposed to ignition, will burn with enormous rapidity generating a huge amount of heat almost instantly.

A dust cloud of any flammable material will explode where: (1) the concentration of dust in air falls within the explosive limits, and (2) a source of ignition of the required energy for that dust cloud is present. Conversely, an explosion can be prevented if one, or preferably both, of these conditions are avoided.

Characteristics of Dust Explosions 
When a mass of solid flammable material is heated it burns away slowly owing to the limited surface area exposed to the oxygen of the air. The energy produced is liberated gradually and harmlessly because it is dissipated as quickly as it is released. The result is quite different if the same mass of material is ground to a fine powder and intimately mixed with air in the form of a dust cloud. In these conditions the surface area exposed to the air is very great and if ignition now occurs the whole of the material will burn with great rapidity; the energy, which in the case of the mass was liberated gradually and harmlessly, is now released suddenly with the evolution of large quantities of heat and, as a rule, gaseous reaction products.

Explosive Concentrations 
Although an intimate mixture of a flammable dust and air may burn with explosive violence, not all mixtures will do so. There is a range of concentrations of the dust and air within which the mixture can explode, but mixtures above or below this range cannot. The lowest concentration of dust capable of exploding is referred to as the lower explosive limit and the concentration above which an explosion will not take place as the upper explosive limit.

The lower explosive limits of many materials have been measured. They vary from 10 grams per cubic metre to about 500 grams per cubic metre. For most practical purposes it may be assumed that 30 grams per cubic metre is the lower explosive limit for most flammable dusts. Though this may seem to be a very low concentration, in appearance a cloud of dust of such a concentration would resemble a very dense fog. The upper explosive limits are not well defined and have poor repeatability under laboratory test conditions. Since the upper explosive limit is of little practical importance, data for this parameter is rarely available.

The most violent explosions are produced when the proportion of oxygen present is not far removed from that which will result in complete combustion. The range of the explosive concentrations of a dust cloud is not simply a function of the chemical composition of the dust; the limits vary with the size and shape of the particles in the dust cloud. Review various Ignition Sources.

Effects of a Dust Explosion 
The heat produced by the combustion of the dust particles in a dust explosion and any gases evolved will cause a rapid increase in pressure at the walls of the vessel containing the dust cloud. In factories it is the effect of this pressure wave on relatively weak items of plant and buildings which has caused the deaths and injuries to persons employed in handling materials giving rise to dust explosions.

Further, since the pressure wave produced by the explosion can cause further dust which may have accumulated in the plant or on internal surfaces of buildings to be thrown into suspension in air, additional fuel can be fed to the flame and a disastrous secondary explosion may follow.

Additional consequences following a dust explosion pressure wave are: the fires that may have been started by the dust flame; the implosion effect on the plant and buildings as the pressure within these rapidly returns to normal; the compromise of emergency exit routes and emergency lighting.

For more, visit the Dust Explosion awareness website.

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