Prevention of Grain Elevator and Mill Explosions (1982) / Chapter Skim
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Chapter 6: Basis for Recommendations
Chapter 6: Basis for Recommendations
Pages 43-62
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From page 43... ...
influences their attitude in taking action to prevent explosions. If resources were unlimited, the panel believes that the dust explosion hazard could be reduced to a negligible level in every type of facility.
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From page 44... ...
FIGURE 5 Facets of a systems approach to grain elevator explosions.
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From page 45... ...
Apply state-of-the-art H L M te chniques to reduce the concentration of airborne dust in and emanating from e levator legs . Control dust generation M M fir and airborne dust at a 11 gra in transf er and discharge points.
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From page 46... ...
The removal of the airborne dust can be accomplished using the same methods as in the leg. The dust problem in silos and the generation of additional dust can be lessened somewhat if dead boxes, grain ladders, and filling spouts that entrain a large portion of the dust in the grain stream are used.
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From page 47... ...
Every elevator having interior legs should utilize an adequate dust collection system in the leg because of the extremely hazardous condition resulting from suspended dust in proximity to potential ignition sources. The other dust control recommendations will contribute to a reduction of the hazard and are, to some degree, interdependent.
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From page 48... ...
Ef f icacy By Ef f iciency H H H H ~ H H ~ H M H H L H - H Next to the control of dust, the control of ignition sources is the most effective means for reducing the explosion hazard. Since the data on ignition in actual explosions are poor, it is not possible to give a meaningful ranking to ignition sources; therefore, the panel arbitrarily divided the sources into the eight categories shown in Table 2 and then assessed the probability of their occur rence and the ease of their elimination.
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From page 49... ...
1 TABLE 2 Ignition Sou roes 49 Probability Ease of Source of Occurrence Elimination Spontaneous ignition Low Easy Arcing from electrical apparatus 1 · Normal operation - Low Easy · fault Me ration Low . Easy Sparks from foreign materials Elevators, ferrous metals Low Easy Elevators, nonferrous metal Low Easy Elevators, other Low Easy Mills High Moderate Static electricity · Moving belts High Moderate · Moving grain/dust Low Difficult Hot Surfaces o Lad Low Easy · Bearings High Moderate · Radiators/Pipes Low Moderate Friction · Rubbing head pulleys Moderate Easy · Slipping.
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From page 50... ...
The potential for tramp metal to~cause a primary explosion does exist in a hammer mill. The energy released when a small piece of metal is struck by hammers in a mill is more than sufficient to ignite grain dust.
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From page 51... ...
me ignition temperature of grain dust layers exceeds 200°C. Hot lump surfaces can serve as ignition sources if they do not meet NEC requirements for use in dusty locations.
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She bursting strength of existing structures is small compared to the maximum pressures generated by most well-fueled grain dust explosions.
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They are very effective in suppressing explosions in enclosures, especially legs and dust collection systems, but they have two drawbacks: they are relatively expensive for small facilities and they are not 100 percent safe against false actuation, which adds to their operating cost because recharging is expensive. Research-and development being conducted by the manufacturers of these devices should be followed closely to determine if they are becoming more cost-e ffective for small facilities.
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From page 54... ...
In addition, the inerting of large volumes is dangerous because workers- can be asphyxiated. Education Recommendations Ef ficacy Establish an information center to distribute actively all available information on elevator and mill dust explosions and the ir causes and prevention.
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From page 55... ...
Ignorance of the dust explosion hazard can be alleviated by collecting available information in a central repository, a relatively simple but laboricus task and by distributing the available information. Numerous organizations now are engaged in disseminating information {e.g., the National Grain and Feed Association, the Grain Elevator and Processing Society, the Department of Agriculture, NIOSH, OSHA, the trade and union press, and various university and private research organizations)
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From page 56... ...
It was said that the pits were cleaned out regularly every few months' The importance and degree of housekeeping should be directly proportional to the degree of activity of the facility, not inversely proportional.- ' 'Maintenance is related to ignition sources in the same fashion as housekeeping is related to dust control. It is assumed that any normal plant operation should include preventive maintenance; however, in facilities where flammable dust is a problem, maintenance to prevent ignition sources assumes greater importance.
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From page 57... ...
Several aspects of the economic problem were considered by the panel, ranging from total cleaning of the grain to re-introduction of the collected dust into the grain stream. A properly designed and operated dust collection system collects only those dust granules that become airborne.
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If the economic value of dust can be increased, the costs of dust collection will become much more acceptable and the tendency to return dust to the grain will decrease. Information on the present disposition of collected dust, the amounts collected by elevators of various sizes and locations, and the cost of transporting dust is needed.
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From page 59... ...
The results are too preliminary for the panel to make any recommendation on the subject. Ignition Sources Re commendation Investigate the e ffect of electrostatics and humidity on the explosion hazard, including an examination of conveyor belt conductivity and the 'charging of ungrounded conductive structures.
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of the facility if, for- sane reason, there is a primary explosion. Thus, design criteria should consider' the isolation of sites where primary explosions may occur from those that may produce secondary explosions-.
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From page 61... ...
Although large grain corporations will disagree vigorously with this point, there are literally thousands of elevators and mills operating as independent entities whose understanding of the hazard is at best limited to a knowledge that elevator explosions are fueled by grain dust. The federal, state, and local occupational safety enforcement agencies are in no better position to decrease the dust explosion hazard for a 'number of reasons.
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From page 62... ...
Safety Consulting Engineers, Inc., Rosemont, Illinois, Electrostatic Characterization of-Grain Products, prepared for the National Grain and feed Association, Washington, D.C., October 1, 1980. Palmer, R.N., Dust Explosions and Fires, Chapman and Hall Ltd., London, 19 73.
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