The Safety Corner

By: Lars G. Larsson

Dust Explosions

Following the excellent presentation on dust control at our March meeting, an explanation of how a dust explosion can occur may be in order.  Let’s first establish that, technically, wood and similar organic materials do not burn.  It is the vapours that are given off when these materials are heated that burn.  For ordinary purposes this is not really important, but it helps to explain why a dust explosion can occur.

If you take a piece of wood and hold a match to it, it will take a few seconds before the wood will start to burn.  The smaller the piece, the faster it will catch fire.  Fine dusts, such as wood flour, will have a relatively large surface in relation to its mass and will thus easily be heated to the point where vapours are formed.  It also should be pointed out that, while we call it a dust explosion, it is not a chemical explosion such as we would experience with dynamite.  Rather, it is a very rapid combustion.  If there is dry dust in an area, such as on overhead pipes, this dust could be disturbed and form a dust cloud.  Any spark, flame, or other source of ignition could set off a dust explosion.

Let’s assume that we have dust and that the particles are dry such as they would be in a heated basement workshop.  If we introduce a spark or other source of ignition into this environment and at the same time disturb the dust so that a minor cloud is formed, e.g., when cleaning up, then the conditions are right for a dust explosion.  The spark would ignite this small cloud that would burn with explosive force.  This small explosion would disturb more dust so that a larger cloud was formed.  This rapid combustion could have enough force to blow out windows, walls, or whatever has the least resistance. 

Dust explosions are easily prevented.  As with all fire prevention we are looking at the three main elements of a fire - FUEL, HEAT and OXYGEN.  Removing any one of these will prevent a fire from occurring.  Obviously we can’t remove the Oxygen (air) in this case.  That leaves us with the other two.  Heat means any source of ignition, from sparks and open flames to cigarettes and similar.  Electric motors such as we have on saws etc. can produce sparks.  If there is a furnace or other type of heater in the room, we can have open flames or very hot elements.  The fuel in this case is the dust, so the obvious starting point is to ensure that the shop is clean.  If we use proper dust extraction systems, most of the problems are solved.  Still, there will be fine dust collecting in a woodworking shop.  This dust must be cleaned off regularly and when this is done we must ensure that all sources of heat are controlled to avoid a potential disaster.