“Controlling grain dust is a valuable part of good grain storage management. Although grain dust emission control used for this is an expensive technic, this cannot be compared to the serious losses caused by a possible dust explosion.”
ABP/TDS Grain Storage Systems
Increasing food need along with increasing population necessarily make grains which is most important food product be produced with highest yields and protected. So, grain storage with proper conditions is becoming more important day by day. Possible dust explosions in silos used for grain storage can cause serious results like wounding, death and material damage. In this article, I would like to draw your attention to dust explosion that may occur during storage of grain.
GRAIN DUST EXPLOSIONS AND PREVENTION
Controlling grain dust is a valuable part of good grain storage management. Although grain dust emission control used for this is an expensive technic, this cannot be compared to the serious losses caused by a possible dust explosion.
GRAIN DUST EXPLOSION ELEMENTS
For a grain dust explosion to occur, four basic physical elements must be present:
Very small particles of dry grain dust from wheat, milo, oats, barley, wheat or oat flour, corn starch, etc.
Grain dust must be suspended in the air to create an explosion, but layers of dust in confined space provide explosive potential.
Adequate air supply with normal oxygen levels.
A vertical elevator leg casing or housing, an enclosed drag conveyor, a dust bin, a down spout, an aeration duct, a basement tunnel, a bin deck gallery, a bin, a silo, etc.
4. Ignition Source:
An overheated bearing in an elevator leg boot, head or conveyor; an elevator leg belt rubbing against leg sidewall casing; an electrical arc from a non-explosion proof electrical device; an electrical short; phosphine pellets or tablets exploding in a wet aeration duct; static electricity; a cigarette lighter or lit cigarette; a cutting torch; metal sparks from a grinder; metal to metal sparks; a dropped tool; lightning, etc.
Additionally, low relative humidity weather conditions are thought to be a major factor in explosions where static electricity may have been the ignition source and where the grain dust was very dry.
HOW DO DUST EXPLOSIONS DEVELOP?
During a major dust explosion, there are two separate explosive phases – primary and secondary explosions. Primary and secondary explosions are often so close together (a split second apart) that they may be heard as one explosion or a series of explosions, like rolling thunder.
The primary explosion is caused by confinement of airborne dust in contact with a heat source that ignites the dust. The first explosion sends an air shock wave, or a pressure front, at about 300 m per second along gallery corridors, tunnels, and vertical shafts in the elevator, which stirs up layered dust. A flame front traveling at about 3 m per second follows the pressure wave, igniting airborne dust as it progresses through a structure. Part of the dust from the primary explosion source may be carried along with the pressure wave, providing additional fuel for secondary explosions.
So, if a dust explosion is limited to a primary explosion because of good housekeeping and sanitation, far less damage is done than if secondary explosions occur. Once initiated, a continuous series of explosions occurs as long as adequate fuel and confinement are present. The result is a chain reaction of secondary explosions that move with destructive force throughout an elevator (wherever grain dust levels are above the MEC), causing major structural damage. This is why empty silos are blown out of an annex in some explosions, while full silos may not be directly affected.
WHERE DO DUST EXPLOSIONS OCCUR?
Dust explosions usually occur at grain transfer points, – in bucket elevators or enclosed conveyors – where small dust particles become dislodged from kernels due to tumbling, agitation, and kernel impacts, as fast-flowing grain hits bucket elevator cups or changes direction in drag or belt conveyors. This turbulent grain movement causes high levels of suspended dust particles (two to 20 microns in diameter) in the airspace, often close to a hot leg boot section bearing or a spark from tramp metal in a dump pit or drag conveyor.
GUIDELINES TO MINIMIZE GRAIN DUST EXPLOSION CONDITIONS
1. Maintain a rigorous housekeeping and sanitation program inside the grain elevator structure. Keep grain dust cleaned up in all working areas of the elevator.
2. Implement a weekly or bi-weekly (or as specified by the manufacturer) bearing lubrication program, based on the bearing manufacturer’s specifications.
3. Use a food-grade mineral oil spray system on grain during the transfer and unloading processes.
4. Install bearing temperature monitors on leg boot, head, and knee pulley shafts, on horizontal drag head and boot bearings, and on belt conveyor drive and idler bearings.
5. Install belt rub sensors inside bucket elevator leg casings to detect belt misalignment to prevent friction heating.
6. Maintain a periodic (weekly or bi-weekly) bearing temperature monitoring program. Document periodic bearing temperature readings and compare with previous readings. A substantial bearing temperature increase (10 to 20° F or more in a week or two) may indicate bearing failure and the need to replace the bearing.
7. Replace steel cups with plastic cups in elevator legs.
8. Use anti-static belting material in legs and horizontal belt conveyors.
9. Install quick-opening cleanout doors on leg boot side panels for grain and dust cleanout.
10. Install dust aspiration systems at grain transfer points or ventilation systems in tunnels and galleries with open conveyors, and truck dump pits where dust accumulation is a problem.
11. Install dust aspiration or suction ventilation systems on inside enclosed legs and conveyors to keep suspended dust below MEC levels.
12. Clean out dust collectors and change filter bags at intervals recommended by the manufacturer.
13. Clean out dust cyclone collector holding bins at scheduled intervals.
14. Install dump pit baffles on truck dump pits to provide a major reduction in airborne dust during dumping operation.