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Revision as of 09:53, 5 April 2007 by Atyldesley (talk | contribs) (New page, previously uploaded as a jpg form!)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)How can dusts explode?
Most people are familiar with the idea that a large log will burn more quickly if you chop it up, and wood in the form of little twigs burns more quickly still. The same increase in the speed of burning continues if you turn the wood into sawdust, or even finer sanding dust, just so long as enough air can get to the individual particles. So if you shake a large heap of sanding dust up into a cloud, and provide an ignition source, in the right conditions, flame will pass rapidly through the cloud , generating much heat in a short space of time. But if you put a flame to a heap of sawdust on the floor, it can only burn slowly, because there is not enough air within the heap to allow this rapid combustion. So to create an explosion you need the right fuel, with a large enough surface area; the right concentration to allow the combustion to proceed, a source of ignition, and some degree of confinement.
It is not only wood dust that can burn as cloud like this. Many very common materials will do the same, such as coal, flour, sugar, powdered milk as well as lots of chemicals, and a few metals, like aluminium.
If this rapid combustion takes place within process equipment, the result is an explosion, because the heat from the burning is released so fast, it causes the remaining air to expands. It can then burst its way out causing tremendous damage.
Dust explosions like this have caused many industrial accidents. Dust explosions in coal mines have caused many fatalities around the globe. In industry the most common plants to be affected are wood processing (like chipboard manufacture) food processing, including storage of wheat and maize, and bulk plastics manufacture.
Common ignition sources for explosions are faulty machinery, which overheats, electrostatic sparks, products which self heat if held warm in large quantities, and plant maintenance such as welding or grinding that produces sparks or hot surfaces.
Much research has been carried out in Europe and elsewhere to understand how to control these dangers, but explosions still occur . The alternatives for making processes and plants depend on the industry. In the coal mining industry, stone dust is spread along mine roadways, literally to dilute the coal to the point where it does not burn. Some industries exclude air from the process, known as inerting. Typically this uses nitrogen or carbon dioxide, and if this is done properly nothing can burn. Other industries provide a deliberate area of weakness in their plant, to allow the hot gases to escape, this is known as explosion venting. The last common technique is called explosion suppression, in which the equivalent of an extremely fast acting fire extinguisher, linked to a special pressure sensor snuffs out the flames, before dangerous pressures are reached. For stories about incidents in France and the USA see
http://www.hse.gov.uk/foi/internalops/hid/din/529.pdf and
various completed investigations at http://www.csb.gov/