For a fire to occur, there must be an adequate supply of oxygen and fuel, along with a sufficient source of energy in the form of heat. These three conditions are colloquially known as the fire triangle. A dust explosion is merely a specific type of fire where two of the three elements in the fire triangle are subject to certain conditions. The heat element is referred to as the ignition source.
For dust to explode, there must be at least 30 grams per cubic metre of combustible material where the particles are smaller than 200 micrometres and the moisture content is less then 30%. Once these conditions exist and there is sufficient ignition energy, the explosion sustains itself through the auto ignition of adjacent dust particles.
Although aluminum dust and wood dust explosions are the most common, other combustible dusts pose serious safety hazards such as aspirin, coffee, corn flour, milk, peat, garlic, rice and tobacco.
Once the dust ignites, it forms a fireball that expands outwards at a speed of approximately 10 metres per second. This fireball creates a steadily increasing pressure wave in front that travels at the speed of sound. This pressure wave usually causes most of the damage by knocking out walls, thereby ensuring a supply of oxygen for the fireball. The fireball causes further damage by igniting other combustibles exposed by the force of the pressure wave.
Although it is possible to create a dust explosion using a power saw in your basement, most dust explosions occur in large industries. Some of the most common processes that are affected by dust explosions are mills and grinders, spray dryers, silos and hoppers, cyclones and filters.
There are five possible methods of protection. These are prevention, suppression, venting, containment and isolation. Prevention would constitute the removal of any one of the elements of the fire triangle. If prevention is impossible, then you must accept the potential risk of a dust explosion.
The three most common suppressants used in suppression protection are water, halon gas and powder (usually sodium bicarbonate). The different kinds of dust explosion will dictate which kind of suppressant to use. The release of the suppressant is commonly triggered by a diaphragm sensitive to the pressure wave.
By using venting, containment or isolation as a method of protection, one is merely trying to keep the extent of the damage to a minimum. Containment and isolation are self-explanatory, while venting usually takes the form of a breakaway panel, door or wall.
In 1977, 54 people died in a grain elevator explosion in the United States. Dust explosions are a very real hazard, but with modern day technology, there is no excuse for industries not having a method of prevention.
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