Grain Dust Handling part I: Explosion Hazards


Dust Explosions in General

A dust explosion occurs when fine particles suspended in the air ignite and burn rapidly, causing a violent increase in pressure. In order to cause an explosion, the combustible mixture of air and dust must be contained in some type of vessel. Grains and other agricultural products are a common fuel for dust explosions due to the nature of their handling and storage. Any time that grain is handled or moved, potentially explosive dust is generated.

Grain is usually stored in large upright silos. The grain is elevated to an entrance in the top of a silo and allowed to fall down and gradually fill the silo. As it falls, dust separates out and becomes suspended in the airspace of the silo, creating an explosion hazard.

Many factors have an effect on the violence of a dust explosion. First and most obvious is the dust material itself. Some materials that are not even considered combustible in bulk form, such as aluminum, are capable of high-pressure explosions when they are in dust form. Most grain dusts are combustible and can cause an explosion, but some types are much more dangerous than others, especially dust associated with corn or sugar.

The size of the dust particles also plays an important role in determining the severity of an explosion. A solid fuel only burns at its surface, where it is exposed to air. A cloud of very fine dust particles has a much greater surface area than a cloud of coarser particles. In addition, fine particles weigh less and tend to stay suspended in air longer. Generally speaking, particles smaller than 840 microns (0.033 inches) can be an explosion hazard.

Dust particles must reach some minimum concentration in the air before they can support combustion. This concentration varies with the material in question. In addition, dust that has settled on walls or surfaces may be stirred up by a primary explosion, possibly causing secondary explosions.

In a dust cloud containing .020 ounces per cubic foot of dust., a human being would not be able to see beyond about three feet. A concentration of .020 ounces per cubic foot is below the minimum explosive limit for most grain dusts. Obviously, concentrations of dust above the minimum explosive limit usually do not occur in occupied areas. However, these concentrations frequently exist in bucket elevators, conveyor housings, bins, silos, and other such structures where grain is moved around.

Different types of grain dust have different combustibility and explosive characteristics. The United States Bureau of Mines has devised a scale to relate the explosion severity of one type of dust to another. This is shown in the following table.

Do not be misled by the table above. The maximum explosion pressure for any one of these grain dusts is rarely reached. In order for grain dust to reach its maximum possible pressure, two things must happen. First, the dust must mix with air in much higher proportions than the minimum explosive limit. For maximum explosive pressure there must be enough dust to consume all available oxygen without any leftover dust. This proportion is around one ounce per cubic foot for most grains. Second, the explosive mixture must be contained in a vessel strong enough to withstand the maximum pressure. If the vessel breaks, all of the pressure is vented immediately and the maximum pressure is not achieved.

Perhaps the most damaging property of grain dust explosions is the cascade effect. Grain dust that has settled on floors or walls can be thrown into the air by a dust explosion, thus providing fuel for secondary explosions. Often, these secondary explosions cause more damage than the first. In this way, a dust explosion can jump from room to room or from silo to silo. This is a common phenomenon in grain dust explosions. For example, one of the most dangerous areas for grain dust explosions is in the bucket elevator area of a silo. The grain is always in motion, so dust is constantly generated. In addition, possible sources of ignition such as static discharge and friction heated bearings are always present. Frequently, a primary explosion in this area cascades into the silo itself, causing a much larger and more damaging secondary explosion. The importance of good housekeeping in preventing cascade explosions cannot be overemphasized. It is necessary to remove dust buildup on interior surfaces regularly.

Often, the ignition source and location of the primary explosion are unknown, as shown in the following tables. This is due in part to the fact that not all of these explosions are investigated.

Reference:
National Fire Protection Association, Fire Protection Handbook 11-56, Storage Practices and Hazards, Sixteenth Edition 1986

Kennedy, Patrick M., and John Kennedy, Explosion Investigation and Analysis, 1990.

National Grain and Feed Association, Dust Control for Grain Elevators, 1981

National Fire Protection Association, NFPA 61A -Standards for the Prevention of Fire and Dust Explosions in Facilities Manufacturing and Handling Starch, 1989

National Fire Protection Association, NFPA 61B -Standard for the Prevention of Fires and Explosions in Grain Elevators and Facilities Handling Bulk Raw Agricultural Commodities, 1989.

U.S. National Materials Advisory Board, Prevention of Grain Elevator and Mill Explosions, 1982.

National Fire Protection Association, Industrial Fire Hazards Handbook, chapter 8, 1990