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A Dust Explosion refers to the rapid combustion of fine powders of a solid flammable material in suspension in the air. Due to its very rapid nature and the subsequently violent expansion of combustion gases, it's considered an explosion.
Although most people consider the powders of many flammable materials as harmless and inert in a normal atmosphere (such as grain, coal, flour, sugar, powdered milk as well as lots of chemicals, and a few metals, like aluminium), the right concentration of these materials in air can create an air to fuel ratio lean enough to create the aforementioned combustion.
A good way to understand this phenomenon is to analyze the familiar situation of starting a camp fire. 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. The conclusion then is that to create an explosion you need an ignition source, the right fuel and, more importantly, a large enough surface area in contact with the oxidant (the oxygen present in the air being the most common source) to provide the right air to fuel ratio to allow the combustion to proceed.
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.
External links
For stories about incidents in France and the USA see
- http://www.hse.gov.uk/foi/internalops/hid/din/529.pdf
- csb.gov various completed investigations