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09/10/12 What Are Combustible Dusts?

Article- Sept 2012 By Larson

Explosion Proof 400 Watt Metal Halide

Larson Electronics Explosion Proof 400 Watt Metal Halide

Most of us who are familiar with the dangers of hazardous locations in the industrial workplace understand quite well the reasons for these dangers. Flammable and volatile liquids, gases, and vapors are often present as a byproduct, or an integral part of, the industrial work processes taking place. These substances represent a potent fuel source that in most instances requires only a source of ignition to release lethal energies in the form of fires and explosions. Thus, we understand the importance of protecting against introducing an inadvertent source of ignition. However, compounds with less obvious flammability and explosive potential can be present in less obvious forms, most notably combustible dusts. In order to help familiarize ourselves with what makes a dust combustible and why it can be so potentially dangerous, let’s look at some of the basics of these hazardous dusts.

What makes up combustible dusts?

In most cases, we equate flammability with obvious materials such as paper, sawdust, gasoline, solvents, wood, and similar materials. Few people would consider materials such as iron, aluminum, or even sugar as being very flammable if at all, yet these materials can be extremely flammable and even explosive when in the right FORM. If we were to consider for example a simple piece of iron, we would hardly consider it a fire or explosion threat. This is because in this large and solid form, it takes a great deal of heat for iron to reach a temperature which allows the metal to vaporize or burn. With the molecules of the iron bar so tightly compacted together in its large solid form, any heat applied is distributed throughout the piece  quite well, significantly slowing the rate at which the iron reaches a high enough temperature to allow combustion to take place. Compared to its mass, the piece of iron has a low surface area, so there is less surface area for heat to act upon and yet a lot of mass to allow absorption of the heat. Now, when we grind this same piece of iron down into a fine powder then some interesting things happen.

Why does dust explode?

Firstly, when we grind down the iron, we have in effect greatly increased the surface area of the iron while greatly reducing mass. Instead of one large and solid piece of iron, we now have millions of tiny piece of iron, each with a high surface area to mass ratio. The temperature at which the iron will ignite has not changed, however, there is so much less mass and so much more area, that when heat is applied the tiny piece of iron reaches ignition temperature very rapidly. The result is that the piece ignites and vaporizes very quickly, and when there are millions of these pieces in suspension in the air, the result can be a flash ignition of the all the particles, otherwise known as an explosion. This is true for almost any kind of material you can think of. Just about anything can burn if enough heat is applied, and when surface area is increased and mass decreased, the amount of heat energy needed is decreased as well.  

All Dust is not the same.

Another interesting thing to note with combustible dusts is that although not a linear occurrence, as a dust particle becomes smaller, its explosive potential increases. This is because as the particle gets smaller and easier to ignite, it also vaporizes faster, leading to a very quick burn rate once a particle is reduced to a certain size. If grain for instance were ground to what we’ll call moderately fine particles, dispersed into the air as a cloud of dust, then an ignition source applied, we could create a fairly intense flash of flame as the particles vaporize. However, if we divide the grain into extremely fine particles, that same dust cloud could ignite with much greater force as the particles will vaporize so much faster. It’s important to note that other factors affect burn rate, such as the composition of the material itself and its finite energy potential, but for most practical purposes, the more finely divided the material, the greater its explosive potential. Eventually a limit will be reached on explosive potential whereby making the particles any smaller will have little to no added effect on burn rate or energy release.

Still another factor affecting the combustibility and explosive potential of dust is its dispersal profile and concentration ratio. A very fine dust that is widely dispersed could be fairly easy to ignite, however, since its concentration is so low due to wide dispersal, its explosive potential will also be lower. Conversely, the same amount of dust, only with a tighter dispersal, could yield a much stronger explosion. There is a minimum concentration level that must be reached before dust can be considered explosive, and likewise there is a saturation limit at which the ratio of oxygen to dust particles inhibits burning. This is interesting because it touches on the problem of dust accumulation and explosions. For example, you may have a very small amount of dust actually suspended in the surrounding atmosphere, however, a thick layer of this same fine dust accumulated on top of equipment. The suspended dust can ignite and result in a fairly small explosion with limited destructive force, yet this force can be enough to dislodge the accumulated dust, thus placing it into atmospheric suspension where it to then ignites, resulting in a secondary explosion of catastrophic proportions.

As can be seen, there is much more to combustible dusts than simple accumulations or suspension of obviously flammable materials. Still more factors such as the electrical conductivity of dusts and moisture content add even greater scope to the problems associated with these dusts. First and foremost, in order to protect against the dangers of combustible dusts it is critical that good housekeeping rules be in place and practiced. Additionally, equipment used in locations where combustible dusts are encountered must be approved for use in such environments. Although dusts may be a more visibly obvious compared to flammable gases and vapors, the dangers although less obvious are no less serious.

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