Dust plus ignition can be deadly; cotton gins wrongly accused

By: Ron Smith, Southwest Farm Press

It’s not just the dust that’s trouble in grain bin explosions. Without an ignition source an explosion or “deflagration” is highly unlikely, says Dr. Calvin Parnell, Texas A&M Regents Professor and cotton chair in the biological and agricultural engineering department. He is also director of the Center for Agricultural Air Quality Engineering and Science (CAAQES) at College Station.

“Grain dust is not dangerous until it becomes entrained in the air at high enough concentrations and in the presence of an ignition source, a hot bearing, for instance,” Parnell said.

Grain bin explosions, he told participants in a recent Coastal Bend Grain Storage and Handlers Safety Conference in Sinton, Texas, are different from explosions caused by dynamite or ammonium nitrate. The latter two are detonations; grain bin dust explosions are deflagrations.

With an ignition source and a minimum explosible concentration (MEC) of dust particles in an enclosed space, such as a grain bin, an explosion is possible. More than one explosion usually occurs. “The primary explosion is followed by secondary explosions,” Parnell said. The first typically is relatively small, creating up to two pounds per square inch (psi) of pressure or less. “Secondary explosions are stronger, and pressure may exceed 150 psi. At 2 psi, the explosion can knock down a brick wall,” he said. “At CAAQES, we focus on primary explosions.”

Parnell said incidents such as a sugar plant explosion in Georgia “increased the Occupational Safety and Health Administration (OSHA) interest in testing dust for explosive potential. That led them to cotton gin dust and the finding that gin dust is explosible.

“About two years ago OSHA and the National Institute for Occupational Safety and Health asked CAAQES to study dust explosions in cotton gins,” he noted. “We found that cotton gin dust is not explosible. We were concerned that if cotton gin dust was determined to be explosible but was not they would have all kinds of (needless) expense.

Test procedure flawed

“We found the test procedure was not correct. (Other labs) were using pressure only to determine explosibility.”

If pressure increased in an enclosed container to more than 1 bar (about 14.5 psi) they considered the tested dust explosible. “We found that the ignition alone, using 10,000 joules, would create 1 bar of pressure,” Parnell said. “We could get the same reading with no dust in the chamber.

“We see flaws in the procedure (OHSA is using). So we developed a new one. We focus on the inerts in the dust. There is a lot of ash content, for instance, that prevents a deflagration in the chamber. There is not enough MEC, and we couldn’t get results at any concentration.

“We feel like we have a more correct procedure instead of relying on pressure only. Our tests show that 87 percent of the dust in a cotton gin does not burn. OSHA is using results from an incorrect testing system,” he said. “They will have gin dust classified as explosible when it’s not. They err on the side of safety but we can do better than that. It takes time to make changes (in regulations),” he said. “But we need to make changes.

“We also need to find ways to prevent dust explosions and keep people from getting hurt. We are more likely to see MEC in grain bins. We cannot control MEC, so we have to make certain we have no ignition sources.” Those may include faulty wiring, hot bearings or cigarettes or cigars. “Insist on ‘no smoking’ when loading or unloading grain,” Parnell said.

Primary explosions typically begin in the boot or leg of unloading equipment. “We must eliminate ignition sources, especially at grain transfer points where MEC are most likely. That’s where we find primary explosions followed by multiple secondary explosions that can move tons of concrete outside a building. That’s a very serious issue.”

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