Combustible Dusts: Explosive Potential|
According to OSHA, 30,000 U.S. facilities are at risk of fires and explosions caused by the presence of combustible dusts. OSHA also notes that between 1980 and 2005 118 workers were killed and another 718 injured in incidents involving a combustible dust explosion. In one of the most deadly recorded incidents in thirty years, a 2008 combustible dust explosion at a Georgia sugar refinery killed 14 people and injured more than 40. On December 7, 2009 three men were severely burned when grain dust at the Simmons Feed Mill in Fairland, Oklahoma ignited and caused a dust explosion while they were using cutting torches inside of a grain silo. These incidents occur all too often in industrial operations all over the United Stated although fortunately many have resulted in only minor injuries and damages. What they underscore however is the continuing need for improvements in all aspects of hazardous workplace safety because although combustible dust conflagrations on a large scale are relatively few, the potential cost in lives and damages makes allowing any incident to occur unacceptable.
Industrial professionals have known for over one hundred years the dangers posed by combustible dust. Just about any material when divided finely enough acquires the potential to fuel catastrophic explosions and fires. Aluminum, steel, coal, grain, plastics, resins, flour, wood and a whole list of others both organic and non organic can become extremely flammable when ground, shaved or in some other way processed into fine particulates. This increased flammability is the result of increased surface area relative to the mass and density of the material. Oxidation, otherwise known as burning, occurs at the surface of a substance when it reacts with oxygen. The speed and ease with which this reaction takes place increases as the surface area of a material is increased.
For example, a chunk of coal has a moderate density and a relatively low surface area, thus it will take a significant amount of energy to begin combustion and it will oxidize very slowly once it is ignited. This is because there is less surface area available to react with oxygen and the high density to surface area ratio means that more heat energy will be dissipated within the mass of the coal. This same piece of coal, however, when ground into a fine powder becomes extremely easy to ignite as its surface area has been greatly increased and its density decreased although its mass remains the same. Each particulate within this coal dust then requires very little energy to ignite as there is less density for heat energy to be lost to through conduction. This means that something as small as a spark could be sufficient to begin combustion of such a dust cloud. Compounding the problem, the increased surface area of the coal dust also speeds the rate of combustion which leads to violent increases in pressure and temperature, hence the resulting explosion when dust is ignited.
Many of the more severe dust explosions that have occurred happened as secondary explosions. This means that a small and relatively minor explosion or fire took place which was responsible for igniting the dust and causing a secondary and more significant explosion. This is because although large amounts of dust may be created during industrial processes, most safety measures are effective in keeping the amount of dust actually suspended in the work atmosphere below the threshold of ignition. Although effective, these measures are not 100% efficient. The result is still some amount of dust being created, and all of this dust eventually has to settle somewhere. This leads to dust accumulation and buildup that can become significant over time and eventually turn into the equivalent of a ticking time bomb. This dust accumulates inside ventilation ducts, on machinery surfaces and interiors and on top of lighting fixtures where cleanup is difficult and often overlooked. When this dust is disturbed by a small scale explosion it becomes once again dispersed and highly flammable, resulting in a secondary explosion of much larger proportions.
Since even with the best safety measures and practices it is almost impossible to completely remove the dust that serves as the fuel for fires and explosions, it then becomes necessary to add further protection by removing potential sources of ignition. In almost all dust explosion incidents inadvertent ignition is the initial causal factor. This can be an errant spark, a hot piece of machinery, static electricity, or even the tiny arc created by a light switch. Preventing inadvertent ignition is an industry unto itself with the development of electrical equipment designed specifically to address this problem. Adding to this are the federal and local equipment regulations created to provide guidelines for its design, production and incorporation into the industrial workplace. At its most basic, electrical equipment designed to protect against inadvertent ignition is generally categorized as explosion proof or intrinsically safe and falls within several ratings levels intended to address the level of danger presented in a specific type of hazardous environment.
Hazardous environments are rated by the types of materials present, their concentration and presence frequency and their volatility and or flammability. In the case of locations designated as hazardous due to the presence of combustible dusts, equipment generally must carry an explosion proof rating of Class II, Div 1 and or Class II, Div 2 depending upon the how the environment has been classified. These ratings insure that the equipment has been tested and proven effective in preventing ignition in environments where dust is or may be present on a regular or occasional basis. This protection is achieved by limiting the amount of heat the electrical equipment is capable of producing, designing the equipment to contain ignitions within itself and or ensuring the equipment does not produce sufficient energy to create sparks or flame. A Larson Electronics EPL-48-2L-LED Explosion Proof LED light for example is rated Class II, Division 1 - 2, Groups E,F and G, which means that this light is safe for use in environments where combustible dusts may be present. This is achieved through specially designed lamps and housings that produce low heat levels and are capable of containing ignitions of materials that may occur within the units housing. In this way, protection is achieved even though combustible dusts may be present in sufficient quantities to pose a significant threat.
Industrial operations very often by their nature make it impossible to completely remove the threat of fires or explosions. Flammable materials and by-products of operations are a normal part of the industrial process and there is simply no way to make these materials 100% inert and safe. Because of this, workers must be trained in safety and prevention, the proper equipment must be used, and all the mandated rules and regulations observed. Anyone operating in a hazardous location or managing such operations must keep themselves updated on the latest developments with OSHA and other regulatory bodies as well as remain vigilant against the dangers initially innocuous appearing materials such as dust present. It is only through a combination of safety measures such as these that the threat of fires and explosions can truly be reduced to the lowest extent possible.