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Appendix C
EXPLOSIONS IN MILLS HANDLING GRAIN PRODUCTS
Mills of any type (feed, flour, soy, rice, etc.) are subject to the
same dust explosion hazards as grain elevators since their input is grain
and, before process ing, i t is handled in the same manner as in elevators
(e.g., movement in legs and on conveyor belts, drying, storage in silos).
Mills, however, are subject to additional dust explosion hazards because of
the actual processing of the grain. The hazards arise because, at Rome
point in the processing, the grain is ground into fine particles of
dust-like size, and flammable concentrations cannot be avoided. (Exceptions
to this processing will be discussed later.)
The first point of danger is the grinding operation, usually a hammer
or roller mill, where explosions can occur for two reasons. First,
ingestion of tramp metal or stones can produce sparks sufficient to ignite
the ground material. Second, the moving parts of the mill can break and
produce sparks. Even if such events do not cause an explosion in the mill,
they can ignite the ground material that then is transported to a point
where a primary explosion can occur.
Af ter the grinding operation the material usually is moved to the next
processing point by pneumatic conveyors similar to those used in dust
collection systems. Transporting the material by means of bucket conveyors
has all of the hazards attendant to an elevator leg without the possibility
of dust collection. Augers and drag conveyors are considerably less
hazardous .
further processing in most cases involves treatment with water in sane
form, either liquid or steam. The explosion hazard at this point is remote
except for one factor. Material that escapes from the processing apparatus
settles on floors, walls, beams, etc., and, if not removed, eventually dries
to form a dangerous layer of dust.
The product next is dried and moved within the mill in the same manner
and with the same hazards as grain in an elevator. Fortunately, the product
entering this portion of the mill system does not contain fine dust in the
same concentration as grain entering an elevator.
Operations such as flour production that do not involve a wet process
are extremely hazardous. In the early part of this century it was realized
that the explosio.~ potential of flour and corn starch was very great.
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Consequently, there is considerable emphasis on housekeeping, reinforced. by
the hygienic standard" required for processing material for human
consumption. . The one exception noted by the panel while visiting a mill
producing.flour was ion the bagging operation. The bagging machine was
located in a separate small building and a layer of flour, inches deep, was
on the floor in sane places.
. . .
Rice mills and pellet mills are somewhat unique and must be discussed
separately. The belief that rice mills will not explode is widespread;
however , rice mills have exploded in the past (e . g ., in Jonesboro, Arkansas ,
on May 28, 1964 ~ (U.S . Department of Agriculture 1979) . Rice dust and corn
dust of the same moisture content have similar explosive characteristics.
One of the rice mills visited by the panel operated in two ways that
contributed greatly to reducing the hazard. The mill shut dawn operations
just prior to the harvesting period to overhaul all of the equipment and to
clean the mill totally. The owner-operator also discarded all the dust
accumulated by the collection system.
Dust pellet mills are in a class by.themselves because their input is
the.expIosive material, grain-dust. In the one-mill visited by the panel it
was obvious that the major hazard existed in the receiving operation. Dust
from rail cars or trucks was dumped into a pit and pneumatically. conveyed to
silos. The introduction of an ignition source at this point would have
caused an immediate explosion either in the pneumatic system or in the .
silo. During the panel's. visit dust was being unloaded from a closed-body
truck by means of a fronted loader. Ignition sources on the loader were
readily apparent--no protection from ignition by the motor exhaust;
headlights (on) with only slight protection; standard, battery operated,
starting system; and no grounding (rubber tired wheels). Dust from the
Silos was fed to a hammer mill, wet processed, pelletized, dried, and stored
in bins. The operations following the hammer mill were not particularly
hazardous and there was little opportunity for dust to accumulate in layers.
A seed plant's material handling equipment operates at slow speeds so
as to handle the grain gently. -Dust cloud generation and the explosion
hazard is therefore considerably lower than in other mills.
Same mills contain explosion and fire hazards greater than. those.due
to dust (e.g., the use of hexane in oil extraction). However, the panel's
concern was limited to the hazard due to dust.
Re commendations for the safe operation of the elevator portion of
mills are, of course, the same as those discussed in the main text of this
report . me principal dif ference between recommendations applying to the
elevator and to the process ing sys tem is that dust collection cannot be
appl fed to the processing machinery; manual housekeeping assumes a greater
role due to "leaks. from the processing machinery. The one recommendation
specific to mills concerns conveying systems and hammer mills.
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All mill operators questioned by the pane] about what they believed
should be designed differently in new mills to improve safety indicated that
the hanoner mill should be outs ide the s tructure containing the general work
area. Al though the panel bel ieves that this response was influenced greatly
by the uncomfortable level of noise produced by a hammer mill, it agrees with
the suggestion on a safety basis. Its implementation not only would place a
dlangera~s process outside the general work area but also would increase the
employees' awareness of any sounds produced by other malfunctioning
equipment .
Because of the prevalent use of pneumatic conveyance in mills,
suppress ion devices assume an important role in explos ion prevention.
Systems involving pneumatic conveying of large amounts of explosive dusts in
high concentrations should always be protected by explosion suppression
devices. Although explosion suppression devices are not considered and
flour mill safety is emphasized, the Incorporated National Association of
British and Irish Millers (1973) general publication on protection against
dust explosions is recommended reading for all involved in the operation of
elevators and mills.
In summary, although feed mills, flour mills, and grain elevators
differ substantially from one another, they share, in varying degrees, the
same dust explosion hazards. The emphasis for hazard reduction in each type
of facility is therefore on dus t control, and the panel recommends that
hammer mills, other grinding equipment, and their dust collection systems be
isolated physically and pneumatically from the main facility. The efficacy,
feasibility, and efficiency of this recommendation were judged to have a
medium hazard control potential.
REFERENCES
Incorporated National Association of British and Irish Millers, Ltd. Dust
Explosions in Elour Mills and Bulk Flour Containers, Second (Revised)
Edition, London, 1973.
U. S. Department of Agriculture, Prevention of Dust Explosions in Grain
Elevator--An Achievable Goal, USDA, Washington, D.C., 1979.
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Representative terms from entire chapter:
dust explosion
