The Ignition-Control Strategy
Industry's inclination, according to a former USDA investigator, is to concentrate on ignition sources. Ignition occurs most frequently in the bucket elevator—a continuous conveyor belt with equally spaced buckets (often metal) that elevates the grain and discharges it into a spout (see figure 1). The top section of a bucket elevator, where the drive is
located, is referred to as the "head." The bottom section, where grain enters the elevator, is known as the "boot." The "leg" connects the head and the boot.
Ignition sources are varied and often notoriously difficult to pinpoint. In a study of fourteen explosions, the USDA identified ten different "probable sources," the largest group (four) being "unknown." Most studies agree that welding or cutting (also known as "hot work") is the largest known ignition source, accounting for perhaps 10 to 20 percent of all explosions. Other common ignition sources include electrical failure, overheated bearings, foreign metal objects sparking inside the leg, and friction in choked legs. "Jogging the leg"—trying to free a jammed bucket conveyor by repeatedly stopping and starting the driving motor—is a primary cause of friction-induced explosions.
Ignition-source control can take several forms. One is mechanical. Mechanisms including electromagnets and special grates can minimize the problem of metal objects entering the grain stream. Belt speed, alignment, and heat monitors can be used to detect hazardous conditions and shut down the equipment before suspended dust is ignited. The effectiveness of these devices varies, but quality has improved since their introduction to the grain-handling industry ten or fifteen years ago. Another approach to ignition control is behavioral. Employees are instructed not to jog the legs. Rules against smoking are strictly enforced. Permit procedures are instituted to ensure that hot work is done safely, and preventative maintenance schedules are instituted and implemented.
Whatever the combination of mechanical and behavioral requirements, ignition-source control has two limitations. First, there are countless potential ignition sources. The National Academy of Sciences (NAS) has reported the results of two surveys in which the ignition source remained unknown in over half the cases. Virtually every piece of equipment, as well as every grain transfer point, is a potential ignition source. Eliminating or controlling them all seems impossible. Second, as an NAS panelist put it, "ignition control is fine if you have perfect people; but people will make mistakes and then the equipment fails." A common example is jogging jammed conveyors instead of inspecting and digging out the elevator boot. This problem has been recognized for years, yet it persists largely unabated. According to an insurance representative, operators of some country elevators encourage this time-saving but risky practice. In short, everyone agrees that good operating procedures are a good idea. They are also inherently
difficult to enforce. Since operational breakdowns are inevitable, critics consider this loss control method inadequate by itself.