World Grain - November 2023 - 46

GRAIN OPERATIONS
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 (2 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.
Many primary explosions originate in elevator legs.
Stored grain typically contains 2 to 10 pounds of grain dust
per ton. If a 12,000-bushel-per-hour leg handles wheat at
360 tons per hour, at the lower level of two pounds of dust
per ton, 720 pounds per hour of grain dust is moving with
the grain. If this leg is 130 feet high, the leg trunk casing
volume is about 500 cubic feet. At the MEC level of
0.05 ounces per cubic foot, only 25 ounces, or 1.56 pounds,
of free grain dust recirculating in the air inside the leg is
needed to reach the MEC.
An NGFA report on grain dust levels in bucket elevators
confined space with heat, a flame, or ignition spark does not
necessarily result in an explosion. However, all the physical
elements for an explosion are present in most grain handling
facilities. What's missing is that the dust must be stirred up to
the point that it is uniformly mixed or concentrated in the air
volume around the heat source for ignition to take place.
How dust explosions develop
During a major dust explosion, there are two separate explosive
phases - primary and secondary explosions. Primary
and secondary explosions often are 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 shockwave, or a pressure
front, at about 1,000 feet per second along gallery corridors,
tunnels, and vertical shafts in the elevator, which stirs up layered
dust. A flame front traveling at about 10 feet 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.
Secondary blasts send additional pressure waves throughout
the structure.
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 they occur?
Dust explosions usually occur at grain transfer points -
in bucket elevators or enclosed conveyors - where small
46
states that " Concentrations in the bucket elevator almost always
exceed the minimum limits and thus constitute an explosive
condition. " So, when only 0.05 ounces of dust per
cubic foot is needed to reach the MEC, as dust concentrations
build inside a leg, they can quickly exceed the MEC,
even in some aspirated or ventilated legs when excessively
dusty grain, like sorghum, is being transferred.
Belt speeds for a 12,000-bushel-per-hour leg typically
run between 600 and 800 feet per minute, or about 10 to 13
feet per second. The belt in a 130-foot leg makes one revolution
in about 20 seconds. Part of the airborne dust tends
to circulate continuously as the air is dragged along by the
cups in the leg casing. Even though only a portion of the
total dust is entrained in the air in the leg casing, much of
the dust in non-ventilated legs remains concentrated in the
air circulating in the leg housing during continuous operation,
usually exceeding NGFA's MEC value of 0.05 ounces
per cubic foot.
Explosion safeguards
Continuous housekeeping and sanitation (regular cleaning
of the elevator), and regularly scheduled bearing service
should be top priorities at all grain elevators and at flour
and feed mills.
Many insurance companies insist on strict housekeeping,
sanitation, and preventive maintenance at insured elevators.
Grain, broken kernels and grain dust accumulate in
the leg boots and should be cleaned out periodically. Some
elevators install easily removable doors on leg boot side
panels for quick, easy cleanout.
Carol Jones is an emeritus faculty member and former Buchanan Chair in
Biosystems and Agricultural Engineering at Oklahoma State University.
She may be reached at jcarol@okstate.edu
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November 2023 / World Grain / World-Grain.com
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World Grain - November 2023

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