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Chapter 4 PREVENTIVE MEASURES To forestall a dust explosion it is necessary to prevent the sequence of events that can lead to the simultaneous occurrence of the conditions for an explosion identified in Chapter 3 (see Figure 4~. Some of these conditions are always present and some occur from time to time and their frequency of occurrence can be reduced; however, none can be totally eliminated. The reason that there are not more explosions is that the simultaneous occurrence of these conditions is relatively rare in any single elevator or mill. {The 1980 data indicating that explosions occurred in 45 of-15,000 facilities can be interpreted to mean that the chances of simultaneous occurrence of the conditions is one in 333 per year.) Since none of these conditions can be totally eliminated there is no single, simple process for preventing explosions. On the other hand, applying what is known about the hazard can reduce the risk to a more tolerable level. AS stated many times in this report, dust will be present wherever grain is handled. me amount of dust can be reduced but never totally eliminated. Operating in an inert atmosphere has been proposed but the cost would be so great as to make it completely beyond reason. Totally eliminating suspended dust in enclosures may be extremely difficult if not impossible, especially in enclosures such as elevator legs, but the amount can be reduced. The possibility of dust suspension in concentrations above the lower explosive limit in open areas of an elevator cannot be ignored but can be reduced with relative ease. Layered dust in open work areas and hidden spaces can be reduced to a less hazardous amount through proper housekeeping. Ignition sources cannot be eliminated totally but the probability of their causing an explosion can be reduced. DUST CONTROL* Since grain dust is the fuel for an explosion, decreasing the amount of dust present at all points in a grain-handling structure is the most important ~mechanical. step to be taken and will produce the greatest results. m e installation of poorly designed dust control systems has - fostered a false sense of security and frequently has led to an inappropriate reduction in manual housekeeping. The disappointing performance of many improperly designed systems has generated skepticism concerning pneumatic dust collection in the grain-handling industry. *The term Dust collection" refers only to the mechanical collection system whereas "dust control" includes collection, housekeeping, and any other actions or equipment used to remove or prevent the generation of dust. 31

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32 Dust collection should begin at the point grain is received and should continue at every grain transfer point in the elevator. It is particularly important that suspended dust concentrations in enclosures be kept below the lower explosive limit and that layered dust, the main fuel for secondary explosions, not be allowed to accumulate at any location. AS stated in Chapter 3, the panel considers'the leg to be the most dangerous location in an elevator for an initial explosion for a number ' of reasons. First, buckets moving through grain in the boot generate and suspend more dust than any other process in the elevator. Second, most dust collection systems are designed solely to keep the interior of enclosures under negative pressure so as to prevent the escape of dust into the exterior working space; they presently do not reduce the dust concentrations in the leg below the lower explosive limit. Therefore, in many cases the presence of an operating dust collection system produces a false sense of security. The presence in the leg of moving equipment that may act as an ignition source makes the leg a doubly hazardous location. 'And, of course, the leg casing serves as a confining enclosure. A mechanical dust collection system must be supported by manual housekeeping to prevent the accumulation of dust 'in layers in open areas of the facility. Secondary explosions almost always have resulted from the ignition of the cloud created from layered dust. In addition, layered dust can lead directly and indirectly to primary explosions (e.g., in one explosion investigated by the panel a fire in a heavy layer of dust around the top of the leg enclosure was sufficiently hot to ignite the leg belt, which dropped and resulted in an explosion that destroyed the leg and damaged the elevator). Dust control should bean integral part of the design of' new - facilities. These designs should incorporate dust control systems and - should minimize the work required for manual housekeeping. In addition, the design should minimize grain movement that creates or releases dust. I GNITION SOURCES - - Reducing 'the number of ignition sources to a minimum is the second most important method of prevention. Like dust, sources of ignition always will exist in or be brought into an elevator. The use of electrical equipment complying with National Electrical Code standards should-be mandatory. Well designed electrical grounding should be used not only to prevent sparking due to isolation of electrical equipment but also to prevent the accumulation of static charges. It is probably helpful if conveyor belts and head pulley lagging are conductive though present knowledge of the contribution of these sources to explosions is very limited. 1

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33 Systems to check heat, motion, and alignment should be used on leg belts, bearings, and other enclosed grain-handling equipment. Where practical, the systems should be incorporated into an interlock system thatwill halt the grain-handling machinery before these types of malfunctions cause ignition of suspended grain dust and an explosion. Mechanisms to prevent the introduction of foreign objects into leg boots and mills should be placed so as to screen all entering grain. The introduction of miscellaneous ignition sources such as cigarettes and other burning material and control of welding and cutting operations should, of ccurse, be prevented by rigidly enforced work rules. VENTING AD S~P~SION Explosion venting is a mixed blessing. The venting of a dust explosion results in an initial high-pressure discharge of hot gases and burning particles followed by a low-pressure discharge of burning particles lasting many times longer than the initial discharge. it is obvious that an explosion in a leg vented within a headhouse can lead to-a secondary explosion in a dirty headhouse as easily as an explosion in a nonvented leg, and the danger to employees in the headhouse is lessened only slightly. Some legs have a blow-out panel at the top to vent an explosion upwards, out of the headhouse; however, the panel has seen cases where the blow-out panel remained undisturbed after an explosion but the leg casing had blown open within the headhouse. The venting of other enclosed volumes in an elevator is subject to these same condition.. The proper venting of enclosures with geometries as complex as those found in elevators requires an understanding of explosion dynamics beyond that obtained from the study of simple structures such as cylinders and spheres. Indiscriminate venting should be avoided in order not to increase (instead of lessening) the hazard. Venting, of course, will not prevent a primary explosion but, properly applied, may reduce the possibility of a secondary explosion. Venting involving the use of numerous windows, corrugated metal siding, and other easily blown-out panels is preferred to the construction of solid concrete structures from which an explosion can distribute large pieces of solid masonary rubble over a wide area. -Venting, even via retrofit, is particularly desirable for totally enclosed sections of the leg that are constructed of reinforced concrete and cannot be cleaned readily . These spaces become bombs when ignition sou roes, including falling and burning belts, enter the wells, and the venting of the exterior walls of these wells will prevent destructive rupture of the enclosure. In addition, efforts to keep these spaces clean of dangerous quantities of layered dust should be made. Active explosion suppression devices (the event triggers the release of a massive quantity of a suppressant such as Halon 1301} also are a mixed blessing. Such devices represent a large capital investment; therefore, their protective action should be evaluated against the consequences of

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34 potential explosion {i.e., the amount at risk) and the probability of their success in preventing that explosion. They are subject to a false releases;. therefore, the probability of this action times the cost of recharging and lost operating time must be added to the initial capital costs. A Comparison of these costs versus use of these funds in other, possibly more beneficial activities should be made. Explosion suppression devices are more valuable when used in elevator legs and dust collection systems than in other areas of the elevator facility. PERSONNEL To this point, only mechanical prevention has been discussed, but in the great majority of explosions investigated by the panel, -the chain of events leading to the simultaneous occurrence of the conditions needed for an explosion was initiated by an action or a lack of action by one or more persons. (All explosions other than those caused by ran act of God. ultimately can be traced to a human cause.) As stated above, the reduction of dust in the grain-handling structure is the most important ~mechanical. method of prevention, and the proper actions of persons associated in any manner with grain elevators or mills is equally important. These actions can range from something as immediate as the timely lubrication of a bearing to something as far removed as the teaching of the correct method for fighting dust fires. The nature of the hazard of dust explosions intrinsic to grain elevators and mills should be made known to all who work within them or are in any way responsible for their operation. The proper and complete education of management in the grain industry on the hazard of dust explosions should be the first step of an explosion prevention program. Although by no means prevalent in the industry, there are still some who either do not believe grain dust explodes or that an inordinate amount of dust is required for an explosion. mis was evidenced in a few of the conversations panel members had with grain industry employees and management at symposia on grain dust explosions. Another fatalistic attitude detected by the panel was that work in an elevator or mill is a dangerous occupation and that nothing can be done about the danger. Employee education programs on the hazards of dust explosions should be part of standard operating procedures in all facilities. It is not . sufficient to have merely a short discussion and slide presentation on previous explosions. A demonstration of the explosion potential of dust should be made, preferably with dust gathered from within the facility elevator in the presence of the employees. .(T}~e demonstration should be \carefully planned and performed by a person knowledgeable about the explosion hazard. ~ One employee on each shift, assisted by a safety committee in larger installations, should be given the responsibility and authority for safety in the facility. He should report directly and only to the senior manager of

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35 the elevator or mill. His duties should include the development and use of a safety checklist (an example of such a list appears in U.S. Department of Agriculture 19801. m e safety officer should be cognizant of the proper method of fighting dust fires and should assure himself that the local fire service is proficient in this matter. Elevator and mill managers should treat housekeeping and maintenance as major functions of elevator operation. There is, however, a great tendency to postpone these functions during those periods when the elevator is operating at maximum capacity. However, this is the period of-greatest risk, and a prearranged schedule of periodic preventive maintenance and housekeeping should be adhered to regardless of the press of business. INDUSTRY I NFORMATTON in the grain-handling industry, large corporations tend to be more knowledgeable about the technical details of grain dust explosions than small companies or the operators of individual country elevators. With respect to elevators, the amount at risk, including elevator personnel, is much greater . . . in larger elevators, but this is offset by the much larger number of smaller elevators, approximately 14,000 versus 1,000. Although efforts to prevent explosions in large elevators should not be neglected, a reduction of the hazard in small elevators would yield a greater result (i.e., the cumulative number of injuries and fatalities in small elevators is greater than that in large elevators). Unfortunately, protection of small elevators is the most difficult to accomplish because less capital is available. Employee awareness of actual grain dust explosions is important, and this can be accomplished by circulating information on actual explosions and their investigation to all elevators and mills, down to the-.grass roots. level, through media such as the Department of Agriculture, state agriculture schools, m e Cooperative Extension Service, and trade publications. The details of each explosion plus a report on its causer should be supplemented by recommendations concerning prevention. Unfortunately, however, obtaining and distributing such information is difficult because of legal constraints. Cooperation among industry, government, educational institutions, and trade and professional organizations in generating and disseminating such information is needed to dispel some of the explosion myths revealed to the panel during its visits to various facilities. ~ In its investigations of explosions, the panel encountered some resistance from elevator managers in its attempts to determine the cause of explosions. Even in those cases where reluctance to discuss the circumstances surrounding the explosion was not experienced, further investigation often was hindered by legal actions instituted by representatives of injured employees. In a few instances elevator managers and others, who could have provided detailed information, associated the panel's intentions with those of government investigators who were seeking

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36 information on which to base personal or corporate blame. Requests to the panel for depositions or actual courtroom testimony placed the investigators in a compromising position with respect to their main concern, which was to determine the sequence of events leading to an explosion. The ultimate . purpose was to determine the cause of the explosion as background information for.this report and to inform others of means for preventing similar explosions in the future. . . Seminar=, symposia, and other open meetings on grain elevator safety sponsored by government, industry, educational institutions, and trade and professional organizations have been held in the recent past with increasing frequency, and some have been devoted solely to explosion prevention while. Others have devoted only a few sessions to the subject. One annual meeting. sponsored by a consortium of organizations and devoted solely to grain dust explosions probably would be more effective and efficient than a number of smaller meetings or larger meetings with only a session or two devoted to the problem. It is hoped that the need for meetings dedicated solely to grain. dust explosions will not continue indefinitely. RESEARCH AND DEVELOPMENT - Research concerning dust.collection in elevators and mills is needed. A variety of commercial equipment is available but its effectiveness and correct use is not well understood. (The panel has prepared a report on this subject, NMAB Report 367-3.) This research can borrow heavily from the -information and experience gained in other industries handling explosive dusts (e . g ., coal and flow r) . Also needed is an industry~wide study of the grain-handling system from . farm to ultimate consumer with emphasis on reducing dust generation. The handling methods now used evolved over a long period during which little if any attention was given to the dust explosion hazard. Miss research should be aimed at identifying general modifications that can be made in the present system to increase safety. Included in this study should be the design of mills and new elevators in a complete range of sizes from roughly 100,000 to 10,000,000 bushels. As mentioned before, questions concerning electrostatic phenomena and relative humidity are part of dust explosion lore. mere is a need for research on these topics that will relate theory to practice. Considerable information is available on the effect of relative humidity on the build-up of electrostatic charges and the ignition of dust by electrostatic discharges (Palmer 1973), but this information is rarely applied to determine The degree \of hazard in an actual elevator or mill environment. For example, the manufacturers of conveyor belts claim that conductive belts reduce the possibility of static charge generation and storage on belts conveying nonconductive materials such as coal and grain. Research is needed to relate the electrical characteristics of conveyor belts to their contribution to the explosion hazard in real situations. The relationship between the conductivity of a belt and its ability to cause ignition is not well understood. 1

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37 The generation of electrostatic charges during the pneumatic conveying of grain dust under conditions of low absolute humidity also has received little if any attention. Typical elevators and.mille should be studied to determine if there are points where hazardous electrostatic conditions can. Occur due either to basic design or malfunction. If such points are found, the elimination of the hazard will not be difficult; the problem at the moment is recognition. Research to discover new and more valuable uses for grain duct. would encourage the installation of dust collection systems. The pelletizing of dust for use as animal feed is one step in this direction. At present, pelletizing operations use only a small percentage of the available grain dust, but the competition for dust among a few plants relatively close to one another has raised the price at times to almost one-half that of grain. An important factor in the development of a large pelletizing industry is the transportation costs after collecting the large amount of dust dispersed over a wide geographic area. A complete assessment study of the cost~effectivenese. Of converting grain dust to pellets for animal feed, including the safety benefits resulting from increased dust collection, is needed and would be- a suitable project for the U.S. Department of Agriculture or an industry trade organization. The search for additional uses for grain dust also should be increased. REFERENCES PaLner, K.N., Dust Explosions and Fire=, Chapman and Hall Ltd., London, 197 3. .. U.S. Department of Agriculture, Safety Operations Plans, A Self-Evaluation Checklist for Operators of Grain Elevators , U.S.D.A., Washington, D.C., 1980. \

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