When Grain Dust Explodes
September 01, 1998
by Teresa Acklin
Increased awarenes of the elements that contribute to a grain dust explosion can save lives.
The deaths of seven persons in an explosion in June at one of the largest grain elevators in the United States once again thrust the volatile nature of grain dust into front-page news.
As of mid-August, the cause of the explosion at the DeBruce Grain, Inc. facility in Haysville, Kansas, was still being determined. The headhouse has been demolished and several teams have begun investigations. “We all hope to get some factual answers soon,” said Paul DeBruce, president and chief executive officer.
Mr. DeBruce said plans had been developed to repair the south end of the Haysville elevator initially. DeBruce hopes to begin receiving grain by mid-September. “We estimate that we will have the capacity to store 13 million bushels (or 354,000 tonnes) at that time,” Mr. DeBruce said. Before the accident, the facility had total grain storage capacity of about 570,000 tonnes.
Some changes will be made in functional operations during the rebuilding process, Mr. DeBruce said. The rebuilt structure will operate on a temporary basis using outside legs, “which resolves many dust control issues,” he said. A mineral oil application process will be installed as a redundant safety feature, he added, and the rebuilt facility also will have fully enclosed airbelts in the basement.
Depending on the structural integrity in the headhouse and north end, tentative plans call for the equipment in the temporary legs to be moved to permanent open legs in the current headhouse area, Mr. DeBruce said.
“We have discussed these changes with our employees, the majority of whom have agreed to stay on with us,” he said. “We are appreciative of the support our employees have expressed to us this summer. The tragedy of June is never far from our minds or hearts.”
The Haysville elevator, which is considered to be the largest elevator under one headhouse, was designed and operated with two dust collection systems. DeBruce Grain, which is based in North Kansas City, Missouri, acquired the elevator in 1996 from the former Garvey Grain, Inc., and spent about U.S.$100,000 to rework and rebuild those two dust collection systems, Mr. DeBruce said.
But even with state-of-the-art equipment and preventive housekeeping and sanitation procedures in place, there is always a threat of a grain dust explosion, experts say. Ron Noyes, an agricultural engineer at the cooperative extension program at Oklahoma State University, Stillwater, Oklahoma, U.S., said workers also should be aware of the elements that contribute to a grain dust explosion and how an explosion develops.
WHY GRAIN DUST EXPLODES.
Four basic physical elements must be present for a grain dust explosion to occur, Mr. Noyes said. They are a fuel source, such as very small particles of dry grain dust suspended in the air; oxygen; a confined space; and an ignition source, such as an overheated bearing, a belt, an electrical arc or static electricity.
There are two separate explosive phases primary and secondary during a major dust explosion, Mr. Noyes said. The primary explosion is caused by confinement of airborne dust and its contact with a heat source that ignites it.
The first explosion creates an air shock wave traveling at about 300 meters per second, he said, stirring up layered grain dust along gallery corridors, tunnels and vertical shafts in the elevator. A flame front traveling at about 3 meters per second follows the initial pressure wave, igniting airborne dust as it progresses through the structure.
Dust from the primary explosion may be carried along with the pressure wave, he said, providing even more fuel for secondary explosions. The secondary blasts send additional pressure waves through the structure.
Mr. Noyes said that grain industry research had measured pressures of 13.8 kilo pascals (or two pounds per square inch) during primary explosions and pressures of more than 550 kPa (or 80 pounds per square inch) during secondary explosions.
“If a dust explosion can be limited to a primary explosion because of good housekeeping and sanitation, far less damage can result than if secondary explosions occur,” he said.
Good housekeeping standards in the Fisherman Islands Grain Terminal in Brisbane, Queensland, Australia was believed to have prevented fire or secondary explosions during a grain dust explosion in January at the terminal. No one was injured in that blast and the damage was contained in one silo.
Mr. Noyes said primary and secondary explosions often occur only split seconds apart, “so they may be heard as one explosion or a series of explosions, like rolling thunder.”
Once the explosive process begins, he said, a continuous series of explosions may occur as long as adequate grain dust fuel and confined conditions exist. Thus, a chain reaction of secondary explosions can move with destructive force throughout any areas of a facility where grain dust levels exceed the “minimum explosive concentration” for particle size and energy nature of the particular grain product, he said.
“That's why empty silos can be blown out of an annex in some explosions, while full silos may not be directly affected,” Mr. Noyes said.
Dust explosions usually occur at grain transfer points, he said, such as bucket elevators or enclosed conveyors, where small dust particles become dislodged from kernels as fast-flowing grain hits the bucket elevator cups or changes direction in drag or belt conveyors. The turbulent movement creates high levels of suspended dust particles in the airspace, often close to a hot bearing in a leg boot section or a spark from tramp metal in a dump pit or drag conveyor.
Research has shown that there typically are from one to five kilograms of grain dust per tonne of stored grain, Mr. Noyes said. Thus, if a 325-tonne-per-hour leg handles wheat at full capacity, at the lower level of one kilogram of dust per tonne, about 325 kgs of grain dust are moving with the grain per hour. That can create levels of free grain dust recirculating in the air inside the leg that far exceed a minimum explosive concentration.
Mr. Noyes cited a National Grain and Feed Association book several years ago that stated, “Concentrations (of grain dust) in the bucket elevator almost always exceed the minimum limits and thus constitute an explosive condition.”
He added that when the minimum explosive concentration level was 50 grams per cubic meters of confined space, dust concentrations building inside an operating leg could exceed the safe limit very quickly. That can be the case, he said, even in some ventilated legs when excessively dusty grain is being transferred.
Some information for this article was provided by Bob Keating of Oklahoma State University.
Guidelines to reduce potential for grain dust explosion in elevator
|Procedures for reducing potential for a grain dust explosion in an elevator facility should be a standard part of good business management, says Ron Noyes, an agricultural engineer at Oklahoma State University's cooperative extension program.|
|He offered these guidelines to minimize the conditions for a grain dust explosion:|
|||Maintain a rigorous housekeeping and sanitation program inside the grain elevator structure.|
|||Implement a frequent bearing lubrication program based on the bearing manufacturer's specifications.|
|||Use a food-grade mineral oil spray system on grain during transfer and loadout.|
|||Install bearing temperature monitors on leg boot, head and knee pulley shafts; horizontal drag head and boot bearings; and belt conveyor drive and idler bearings.|
|||Install belt rub sensors inside bucket elevator leg casings to detect belt misalignment to prevent friction heating.|
|||Maintain a weekly bearing temperature monitoring program.|
|||Replace steel cups with plastic cups in elevator legs.|
|||Use anti-static belting material in legs and horizontal belt conveyors.|
|||Install quick-opening cleanout doors on leg boot side panels for grain and dust cleanout.|
|||Install dust aspiration systems at grain transfer points, and install ventilation systems in tunnels and galleries with open conveyors or in truck dump pits where dust accumulation is a problem.|
|||Install dust aspiration or suction ventilation systems on inside enclosed legs and conveyors to keep suspended dust below “minimum explosive concentration” levels.|
|||Clean out dust collectors and change filter bags at intervals recommended by the manufacturer.|
|||Clean out dust cyclone collector holding bins at scheduled intervals.|
|||Install dump pit baffles on truck dump pits to provide a major reduction in airborne dust during dumping operations.|