The process of moving and handling grain at grain facilities generates significant quantities of combustible dust. As seeds or kernels are handled, fugitive grain dust is produced and can become suspended in air or accumulate on floors, ledges, beams and equipment. And when conditions are right, suspended grain dust can ignite and explode, causing catastrophic damage and huge financial and human loss.
Elements needed for a fire:
- Fuel — combustible dust.
- Oxygen — air supply with normal oxygen levels.
- Ignition — an overheated bearing, an elevator leg belt rubbing against leg sidewall casing, a cutting torch, an electrical short, a lit cigarette, etc.
Additional elements needed for an explosion:
- Confinement — a vertical bucket elevator casing, housing or enclosed conveyor.
- Suspension — combustible dust particles concentrated in the air.
During a major dust explosion, there are two separate explosion phases — primary and secondary explosions. The initial (primary) explosion — typically the less powerful of the two — usually occurs within the confinement of the bucket elevator or conveyor when an airborne combustible dust mixture comes in contact with a heat source and ignites. The situation quickly turns from bad to worse when the blast wave from the initial explosion dislodges fugitive combustible grain dust, which accumulated due to inadequate housekeeping, into the air where it ignites causing one or more catastrophic secondary explosions.
The development and implementation of a comprehensive combustible dust and ignition control program is vital to preventing primary explosions and mitigating additional damage caused by excessive fugitive combustible dust accumulations.
Occupational Safety & Health Administration (OSHA) and National Fire Protection Association (NFPA) have strict standards on the management of fugitive combustible grain dust and ignition sources to prevent explosions — below are recommendations based on these industry standards.
OSHA 1910.272 Grain Handling Facilities
NFPA 61 Standard for the Prevention of Fires & Dust Explosions in Agricultural & Food Processing Facilities
- Establish a preventive maintenance program. Without preventive and/or predictive maintenance, bearings, belts, buckets, pulleys and milling machinery become potential ignition sources. Periodic inspection, cleaning, lubrication, alignment and tightening are critical to keeping equipment functioning properly and safely.
- Install and maintain motion and alignment monitoring devices to detect slippages, misalignments or bearing failures. Operators should be trained in the operation and workings of motion and alignment monitors/sensors, as well as familiar with required protocols in the event of an alarm. Monitors/sensors and alarms should be inspected and tested regularly in accordance with the manufacturer’s guidelines.
- Remove foreign objects and ferrous (iron) material from the incoming grain stream by installing and maintaining magnets at receiving pits. These metal contaminant materials can cause sparks and damage equipment as they move through augers, legs and milling equipment.
- Use appropriate electrical equipment and wiring methods. Use of inappropriate electrical equipment, faulty installations and lack of electrical maintenance are fire ignition hazards and should be avoided.
- De-energize or remove all machinery that presents an ignition source before using compressed air to clean. Regulations require the entire elevator to be de-energized before blowdown can occur. Nationwide recommends not applying compressed air for cleaning. Sweeping and vacuuming are safer alternatives.
- Control open flames and sparks. Require the issuance of a hot work permit for all work involving electric or gas welding, cutting and brazing or similar flame or spark producing operations.
- Strictly enforce a no-smoking policy. “NO SMOKING” signs should be posted in key locations.
- Develop a Dust Hazard Analysis is accordance with NFPA 61. Determine where fugitive grain dust accumulations occur and minimize or eliminate them. Pay special attention to areas that allow extra fugitive grain dust to accumulate, such as gaps in spouting; casings of bucket elevators; pneumatic conveying pipes; screw augers and drag conveyer casings. Areas in need of repair should be corrected immediately.
- Consider food grade mineral oil. Spraying edible oil on or into a moving stream can help control fugitive grain dust emissions. See Control Grain Dust Emissions With Oil.
- Develop and implement a written housekeeping program with established methods, frequency and inspection. Follow a written clean-up schedule to help ensure all areas of your facility are inspected and cleaned daily — more frequently during peak periods of operation.
- Inspect for combustible dust residues in open and enclosed areas at regular intervals. No area is off limits. Remember to check pipes, ledges, beams and equipment.
- Control the use of compressed air to blow dust from ledges, walls and other areas. Regulations require the use of an air blowdown permit to help ensure all necessary precautions are taken before compressed air is used; however, Nationwide does not recommend the use of compressed air for cleaning. Sweeping and vacuuming are safer alternatives.
- Minimize the free fall of grain. Use choke feeding techniques and dust-tight enclosures at all transfer points.
- Use a properly designed pneumatic dust collection system. Follow maintenance, testing and inspection guidelines recommended by the manufacturer. Collectors must be equipped with a monitoring device (capable of being read at an accessible location and checked frequently) to indicate when maintenance, such as a filter change, is required.
- Install dust aspiration systems. With proper aspiration, dust concentrations in legs and conveyers can be kept below the lower explosive limits.
- Minimize the escape of fugitive grain dust from process equipment or ventilation systems. Consider enclosing the conveying system, pressurizing the general work area and providing a lower pressure inside the enclosed conveying system.
- Put a contingency plan in place. This helps you deal with fugitive combustible grain dust that accumulates rapidly following an equipment or system failure.
Employee training on how to recognize and prevent the fire and explosion hazards is a major component of a successful combustible grain dust and ignition control program. Employees are your first line of defense so they should be trained thoroughly and often and empowered to take preventive/corrective actions or alert management at the first sign of a combustible grain dust control problem. The OSHA Grain Handling Standard 1910.272 contains additional details and minimum guidelines for employee training.