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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. 123
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124 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. 1
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125 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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