Article Archive
Grain Dust Peril
Latest Grain Elevator Blast Renews Safety Concerns
Volume 13 Issue 4

It is almost a joke to think that the main ingredient in something as wholesome and ordinary as bread can be a deadly explosive. But after the tragedy that struck Wichita, KS, on June 8, 1998, no one is laughing.

A grain dust explosion caused extensive damage to a half-mile-long grain elevator facility. It also resulted in the deaths of seven employees. Ten employees were injured, and three employees are still in critical condition.

Urban Search and Rescue Team (USAR) personnel, many of whom had also worked on the scene of the Oklahoma City federal courthouse bombing, were faced with the daunting task of locating survivors and recovering bodies in an unstable structure filled with more than six million bushels of grain, the dust from which could explode again at any time.

For fire and rescue personnel, the task was a difficult one. And for those in industry, it was a reminder about the ever-present threat of dust explosions.

"The timing couldn't have been worse," Larry Phillips, Safety and Health Director for the North Dakota Grain Dealers' Association, said. "The facility exploded a month or two before the regularly scheduled review of (current grain dust) regulations by the OSHA. But it is important to remember -- and OSHA has stressed this -- the timing (of the review) had been scheduled long before the explosion, as are reviews of all OSHA regulations. OSHA mandates all regulations be reviewed every 10 years, whether or not there have been problems ."

Even if the Wichita explosion does not have a significant impact on OSHA's review, it should encourage facilities to face the constant threat posed by grain dust.

BRIEF OVERVIEW

The first documented dust explosion occurred in a Turin, Italy, bakery in 1785. The explosion was caused by the ignition of flour dust by a lamp in a bakery storeroom. Fortunately, the explosion did not cause any fatalities. It did lead to the realization that grain dust is a highly explosive substance that must be handled carefully.

A famous modern-day grain dust explosion was the second one to occur at a large export grain silo plant in Corpus Christi, TX. The April 1981 explosion resulted in nine fatalities, 30 injuries, and more than $30 million in actual material damages. The suspected cause of the explosion was thought to be smoldering lumps of milo grain which ignited a dust cloud in a bucket elevator, though there has never been a consensus as to the cause.

In 1997, there were 16 grain dust explosions in the U.S., according to a Wichita Eagle article. According to a report released by Robert W. Schoeff, professor emeritus, Kansas State University, and Ralph Regan, safety director, U.S. Department of Agriculture's Federal Grain Inspection Service, 13 grain dust explosions were reported in the United States during 1996. During 1995, 14 grain dust explosions were reported, and the ten-year average for grain dust explosions was reported as 13 explosions.

In March 1988, the Occupational Safety and Health Administration (OSHA) passed new regulations regarding safety standards for the grain industry. The standard, OSHA 1910.272, set numerical level limits on the amount of dust that was allowable in certain areas of bucket elevators, which are simply conveying systems using an endless belt with a drive pulley at the top to bring the grain to the top of the structure. Since 1988, the number of grain dust explosions has decreased significantly, though it is impossible to say whether that is a direct effect of OSHA 1910.272.

THE SCIENCE OF A GRAIN DUST EXPLOSION

For a dust explosion to occur, several factors must come together, according to Dust Explosions in Process Industries by Rolf K. Eckhoff. First, there must be fuel, or grain dust. The critical parameter for grain particle size is 0.1 mm or smaller. As the size of the particle decreases, the risk of a deflagration or explosion increases.

The dust concentration contributes to the dust's flammability. In order for a dust explosion to take place, the concentration must be between 40 grams per cubic meter and 4000 grams per cubic meter. The actual limits may vary based upon particle size and composition.

However, the lower explosive limit is generally reached only in areas near grain handling and processing equipment. This limit is not usually reached in areas where employees actually work. Also, the dust must be in suspension in order for an explosion to take place; dust that is simply lying on the ground is not an imminent threat.

Another contributing factor is composition of the dust. Natural and synthetic organic materials, coal and peat, and metals can give rise to explosive dusts. Also, the moisture content of the dust is important. The drier the dust, the higher the risk of ignition.

The second major factor is the presence of oxygen, which is necessary for combustion.

The third major factor is the ignition source. Sparks from welding and cutting equipment, or cigarettes, can ignite dust. If a bolt, or some other piece of metal, located on moving equipment rubs or scrapes against another metal surface, sparks again could result, leading to an explosion. A choked bucket elevator, a conveyor belt slipping, an electrical malfunction, or lightning could also start a fire leading to an explosion.

Finally, in order for an explosion to occur, the ignition must take place within an confined space. An explosion is defined as the tremendous pressures built up and released instantaneously, an explosion can not take place in an open area. However, a dangerous flash fire could result.

It is also important to recognize that there are two major types of dust explosions: the primary explosion and the secondary explosion. The primary dust explosion is initiated by an ignition source. Secondary explosions occur when the blast wave from a primary explosion propagates and causes layers of dust in other areas to become suspended in air. Dust suspension by the primary explosion is extremely flammable. It can be ignited by the primary dust flame within microseconds of each other.

PREVENTION

When it comes to grain dust explosions, prevention needs to receive the utmost attention. According to Phillips, the best way to prevent a dust explosion is to eliminate the dust, period.

"A dust explosion can't take place without dust," he said. "The best method of prevention is good old housekeeping. As you see dust beginning to build up and it shouldn't get at it right away.

"Housekeeping is just as important, if not more important, as all of the (job) responsibilities of the workers. Managers have to realize that in order to keep these places safe, they have got to take care of the dust."

Robert Schoeff, a dust expert who works at Kansas State University, said if any one of the four elements necessary for an explosion is removed an explosion cannot occur.

"It's not practical in agribusiness to eliminate the oxygen (with inerting gases)," Schoeff said. "Focus should be on controlling the dust, keeping it out of suspension, or controlling the concentration."

Schoeff suggested using pneumatic dust collectors or spraying edible mineral oil on grain in order to prevent the generation of dust. Machinery should also be checked regularly for points where grain might spill out and accumulate.

The best way to prevent dust explosions is to eliminate ignition sources, he said. Dust controlling measures will help prevent a secondary explosion, but not a primary explosion.

Phillips said that grain elevator managers need to be aware of all possible ignition sources. There can be many.

"Explosions are caused by improper welding and cutting operations, cutting, or repairs in the elevator without taking the proper precautions, employees smoking (in the elevator area), overheated bearings, and malfunctioning equipment, among other factors as well," he said.

Adherence to standards regarding hot work permits, the use of the proper types of repair tools, mandates against employees smoking in elevator areas, and routine preventive maintenance of machinery can help alleviate the possibility of ignition. Some elevators have gone so far as to not permit any smoking whatsoever on the grounds of the elevator.

Schoeff also stressed the role of proper maintenance in preventing secondary explosions. Dust should not escape if machinery is in correct working order. He suggests using of explosion suppression systems that can sense pressure change associated with the combustion that can lead to an explosion.

IN CASE OF EXPLOSION

"Handle a dust explosion with kid gloves," Phillips said. "As long as there is an ignition source (such as a fire) remaining, there is the possibility of another explosion."

Both Phillips and Schoeff cautioned fire personnel about using water fog nozzles in a straight stream setting. Water sprayed by a straight stream pattern on to smoldering grain could cause dust to become suspended, leading to another explosion.

"If there is a residual fire, use a fogging nozzle," Schoeff said. "Don't hit it the grain with a straight spray. It is best to adopt a defensive strategy."

Phillips advised using as much water as necessary, but keeping pressure flow low.

"Use volume, not pressure. You can't just bust in and treat it like a regular structure fire."

Phillips also stressed that rescue services should keep their own safety in mind.

"Rescue services are there to preserve life and property -- life first, property second," Phillips said. "Don't risk your life to save property. Property can be replaced. You can't put a price on life."

Rescue personnel need to be aware of the unique dangers associated with grain dust explosions, such as shifting grain, fatigued structures, and hidden spotfires. They also need to deal with the structural instability of the elevator in the proper manner. When grain dust explosions occur, the overpressures can be high enough to cause reinforced concrete to fail or wooden cribbing, and, in some cases, total failure of the structure.

"You need to be very careful," Phillips said. "There needs to be some pretty fancy and well-thought out shoring to go in and get the victims out."

"Take each explosion on a case by case basis," he said. "Each one is different."

It is important to realize that in the 200 years since the Turin explosion,the depth of knowledge about grain dust explosions has grown considerably.

Now, it is simply a matter of applying that knowledge in order to safely put bread on our tables.

 
 

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