AN OVERVIEW: GRAIN DUST EXPLOSIONS
FIRE PROTECTION
photo by the author.
The locations change—Kansas City, for instance, or Galveston or Westwego—but the results are usually the same when dust in a grain elevator explodes. The fire department is called. There is death and injury to several people, hundreds of thousands of dollars in property loss, unemployment and economic loss to the community for months to come.
An elevator is a facility where grains are dried, weighed and reshipped. Most commonly the grain is delivered to the elevator by truck or barge, stored until a large quantity is on hand and then loaded into railroad cars or ocean-going freighters.
The potential for grain elevator dust fires and explosions exists in rural villages and major cities, along country railroad sidings and along the shores of the Great Lakes and the Gulf of Mexico—anyplace where grain crops are moved to consumer markets. Few fire departments can proclaim with certainty that they have no grain storage and handling risks in their areas.
The magnitude of the grain dust problem is monumental. Consider these facts: 2900 grain elevator fires occur each and every year, and this figure does not include explosions. There are 15,000 grain facilities in the United States. Statistically, therefore, almost one in every five will experience a fire in any given year. The statistics for grain dust explosions are not much better. At the present rate, one of every eight or nine elevators operating in the United States will explode within the next 50 years.
In the past decade, 149 people have died in this country from grain facility explosions and fires. Twelve died and 72 were seriously injured in 1981 alone. And 1982 is off to an even worse start: nine deaths through June.
The table summarizes data for the 11 states with the worst records since 1958. Nebraska leads the list in total number of incidents, but Texas has recorded the greatest number of deaths in these fires and explosions, more than three times higher than Iowa’s second highest toll.
The simple fact is that adequate safety standards for grain elevators do not exist in the United States. Australia, on the other hand, has stringent requirements for dust removal and only one explosion has occurred in that nation in the last 60 years.
Could have been different
The U.S. Department of Agriculture recently noted that the “United States has possessed necessary knowledge and many of the prevention techniques (to prevent grain dust explosions) for at least 60 years. It is obvious that they have not been implemented. Otherwise these problems would not exist.”
The problem today remains one of a lack of effective legislation, and inadequate enforcement of and compliance with the regulations that do apply.
Dust from corn breakage is eight times more explosive than coal dust and ignites at a temperature of 400 degrees lower than the coal dust. Corn dust contains more explosive energy per unit weight than does an equal amount of TNT! And other grain dusts are just as bad.
The potential for a grain elevator explosion is going to remain with America’s fire fighters for many years to come, it seems; so it behooves us to learn as much as possible about these events.
Many of the most hazardous grain and other dusts are products familiar to everyone, a fact that well might breed the carelessness that leads to disaster. Dust from food products like flour and sugar are particularly hazardous.
The reason that dusts burn with explosive quickness lies in their small particle size and the resulting very large surface-tovolume ratio. Therefore, there is an extremely large surface area exposed to air (specifically, to oxygen) when the dust particles are suspended in the air. The materials are thus easily ignitable in suspended dust form, whereas they ignite with difficulty, if at all, in the form of larger pieces. Indeed, the dusts of some products usually considered nonflammable are themselves highly dangerous.
The terms dust and powder are both correct and they can be used interchangeably, although there are preferences within various industries. Either term refers to particles that have a diameter of less than 1000 microns, or 0.04 inch.
Dust suspensions have lower and upper explosive limits (LEL and UEL) just as flammable vapors do. The meaning of the terms is also the same when applied to dusts. LEL is the minimum concentration of suspended dust necessary to sustain flame propagation. Above the UEL, the dust concentration in air is too great for sustained combustion to occur.
It is common to speak about dust “explosions” but this is not always accurate. Some dusts deflagrate rather than detonate. A deflagration occurs when the flame speed through the gaseous combustion products is less than the speed of sound (1088 feet per second). A detonation, with its accompanying shock wave, occurs when the flame speed equals the velocity of sound. Brisance is a property of the shock wave from a detonation and is a measure (psi) of its shattering effect.
Lincoln foumal photo by Harald Dreimains
A dust explosion can occur only when the dust is suspended in air; a dust fire occurs in deposits of dust on flat surfaces. Don’t leap to the false conclusion that a dust fire is of less concern than an explosion, though. The transition from a fire to an explosion can take place virtually instantaneously. The only requirement is that the dust deposit be disturbed suddenly by a heavy hose stream, for example.
Fire can also follow rather than precede a dust explosion when the force of the explosion scatters burning particles, igniting dust accumulations throughout the room or building.
Another possible sequence is that of a primary, or initial explosion, followed by secondary explosions. The first explosion, usually relatively minor, will cause the accumulated dust from various surfaces in the area to become suspended. Burning particles from the primary explosion then ignite the newly suspended dust and a second, and usually more severe, explosion occurs. The series of explosions can continue through a third and more, each progressively more severe.
One of the greatest dangers from a dust explosion is the expansion effect which leads to a great pressure increase in a restricted area. Grain dust explosions can generate pressures of 150 psi and higher. When you consider that many buildings are designed to withstand no more than 2 psi of extra lateral pressure, the total structural collapse that often accompanies a grain elevator explosion is readily understood.
The rate of pressure rise is also important as far as structural damage and personal injury potential are concerned. A brisance exceeding 15,000 psi per second is frequently reported, a level that is likely to be fatal to any human in the vicinity.
A properly designed grain handling facility will have pressure-alleviating vents built into it. Experience indicates that there are numerous grain elevators throughout the United States that lack these safeguards, however.
The temperature of the flame front that carries through a room or building as a result of a grain dust explosion is important for at least two reasons. First many of the serious injuries and deaths are due to bums. Second, fire is quickly spread throughout the facility by a high-temperature shock wave, igniting all combustibles in its path.
Don’t overlook the possible presence of toxic gases and combustion products after a dust explosion. This is a particularly serious problem for employees who are likely to be injured and rendered unconscious for at least a few moments after an explosion. Being unable to evacuate themselves from the area, these people can very quickly inhale lethal combustion gases. No fire fighter should ever enter any building where an explosion has occurred without SCBA.
The challenge of dust fires is actually two-fold: to extinguish the fire with minimal damage, and to prevent an explosion.
Fires in dust deposits can be either smoldering or flaming fires. The smoldering fire will often be found deep within a pile of dust and may have been burning for a long time before it was discovered.
Greatest danger
The greatest danger to fire fighters attempting to extinguish a dust fire lies in the rapidity with which the fire can be transformed into a dust explosion. The dust must absolutely not be disturbed in any way that would cause it to become suspended in the air. Remember, the ignition source is already present. The development of the large surface-to-volume ratio is all that is needed to generate an explosion.
The safest course of action could be no action at all, just let the fire burn itself out. The circumstances in which this is practical are obviously limited, however, and a constant watch will have to be maintained.
More often, the fire will have to be attacked and extinguished. The type of dust involved will dictate the extinguishing agent that is used. Water can be used on all grain dusts and, in fact, on just about any dust except those of combustible metals (class D fires).
Remembering the critical importance of not suspending any dust in the air, use fog lines only. And, even with a fog stream, operate it at the lowest practical pressure. Never use a straight stream or high-pressure fog on any dust fire. To do so will only invite an airborne dust cloud and explosion.
Sprinklers may be effective on a flaming dust fire but, like fog streams, the water probably will not penetrate far enough into a dust pile to extinguish a deep-seated smoldering fire. Furthermore, deepseated fires will not likely produce enough heat to activate the system.
Wet water offers the greatest hope for success in these types of fires due to its lowered surface tension and greater penetration into the dust pile. Foams are ineffective. They won’t penetrate the surface layers and it isn’t probable that the blanket can be maintained long enough to extinguish the fire by excluding oxygen.
Lincoln Journal photo
The Raymond, Neb., Grain Elevator Explosion
Nebraska leads the country in grain elevator explosions but ranks only seventh in the number of off-farm storage sites, according to the U. S. Department of Agriculture figures. Last November’s explosion in Raymond, Neb. (population, 200), killed four and seriously injured three.
In the Raymond incident, officials suspected that a propane leak might have been involved, but one report by a federal researcher indicated that about 80 percent of the nation’s 15,000 grain elevators contain enough dust accumulations to cause an explosion with any spark, even static electricity. That report came after a Council Bluffs, Iowa, explosion killed five last June 9. Other major explosions last year occurred in Missoisi, Texas, Louisiana. Illinois arid Oregon.
The 4:30 p.m. explosion in Raymond came alter reports of trouble with a grain dryer, according to the Lincoln Journal Several people said they had smelled propane. After the explosion, fire consumed the 10O-foot structure, and only windless conditions and a determined group of mutual-aid fire fighters kept the fire from the nearby town.
Firefighters controlled the blaze and removed the last body at 2 a.m.
Lincoln Journal photo
Some dust fires have been fought successfully by injecting water deep into the pile. This method is very slow, however, often requiring days or even weeks to complete extinguishment.
Perhaps you have read that some dust fires have been extinguished by covering the pile with soil. This is not recommended unless other methods have failed, though. It is hazardous and involves moving very large quantities of soil. The pile will have to be covered to a depth of about 3 feet if all oxygen is to be excluded.
The method of absolute last resort is digging the fire out of the pile. This is very hazardous because there is the everpresent risk of sudden flaming combustion when the fire is exposed to air, as well as that of raising a dust cloud which could explode. Try this technique only if extinguishment is essential in the shortest possible time.
Don’t walk
Another caution should be mentioned. Do not walk on a pile of burning dust. Your body weight could cause collapse of surface layers covering hollow pockets that have burned underneath. You could suddenly fall through into the burning dust.
Many of the techniques for handling dust fires and explosions in general, and specifically those involving grain dusts, lie in the realm of prevention. Nothing can be done once a facility explodes except to remove the injured persons and extinguish the fire. Dust fires are also best prevented. Cleanliness in the grain handling facility can avoid dust accumulations that may subsequently ignite or explode. Education of management personnel and employees is a major factor in preventing these disasters.
