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Section 1 INTRODUCTION Grain elevators in the United States are designed and operated in such a way that at least 30 of them will suffer a dust explosion in any given year (1). Some of these explosions may cause only minor damage, but people will be injured in roughly half of the incidents, and people will be killed in about one fifth of them. What is more, elevator operators may sustain significant loss of equipment and stock. There can be no question that dust explosions in grain elevators are a serious problem in this country. Running a grain elevator, like any business, involves certain risks. The key to success lies in managing those risks effectively. Dust fires and explosions are a large part of the total risk in operating grain elevators. Means of preventing fires and explosions, therefore, must play a correspondingly large role in the overall management and operating programs at these facilities. All elevator operators do a certain amount of risk management. They must keep handling and processing machinery in working order to meet delivery deadlines. They must do preventive maintenanceâoiling, repair, replacement, and regular inspectionâto avoid costly breakdowns. These activities make good common sense. They also happen to be effective risk management. Employee training and good communications can help prevent mistakes. These activities, too, are good sense and effective risk management. The elevator manager who recognizes and deals effectively with the risks involved in his business generally has a well-run operation and stands a better chance of showing a profit at the end of the year. Effective risk management, in other words, is effective management. The two cannot be separated. The first step in managing the risk of dust fires and explosions in grain elevators is to recognize the problem. Many operators say they may have had a few smoldering fires in their elevators, but never an explosion. They seem to think that because they have not yet had an explosion they will never have one. Such operators are relying more on luck than on effective risk management. The only sure way to prevent dust explosions is to eliminate or control their causes. (1) Prevention of Dust Explosions in Grain ElevatorsâAn Achievable Goal, U.S. Department of Agriculture, Washington, D.C., 1980.
Causes of Explosions Grain dust explodes when three conditions occur at the same time. There must be an explosible concentration of dust suspended in air; the dust must be suspended in an enclosed space; and there must be a source of ignition, such as a spark or a hot surface.* To avoid this situation, the elevator operator must enforce sound programs of maintenance, employee training, safety, and physical security and above all he must enforce an efficient housekeeping program to keep his elevator clean. Housekeeping programs should be based on the fact that the root of the explosion problem is dust. Dust is the material that explodes, and modern, high-speed, grain-handling methods generate large amounts of it. In many explosions the exact source of ignition has been unknown. But the one factor common to nearly all explosions in grain elevators is excess dust. Therefore, although a housekeeping program should prevent the accumulation of trash, spilled grain, discarded tools, and similiar debris, the program should be geared primarily toward dust control. How well must dust be controlled? Many elevator operators think they need only avoid explosible concentrations of airborne dust in their facilities. That degree of cleanliness is necessary, but it is not good enough. Careful study has shown that layers of dust on floors, ceilings, walls, ledges, and equipment are also a serious hazard. Unusual currents of air, for example, can pick up layered dust, creating an explosible concentration where none existed before. More importantly, an initial explosion can violently disperse layered dust into air, leading to a series of secondary explosions. We now know that the largest part of the damage from grain-dust explosions results from devastating secondary explosions fueled by layered dust (see NMAB report 367-2). Estimates of the amount of layered dust that is hazardous vary widely for a number of reasons. However, a layer of fine, dry dust only l/64th of an inch thick inside an elevator can be an explosion hazard. In general terms, if accumulations of dust are visible, the elevator has a potential explosion problem. The thicker the layer of dust, the greater are both the probability of an explosion and the severity of the resulting damage. Methods of Dust Control The danger of dust explosions in grain elevators can be reduced or eliminated only by an effective dust-control program. Such a program has three parts; mechanical housekeeping, manual housekeeping, and various measures that minimize the creation of dust. Each of these parts is essential to the program. For example, the operator does not have a choice between mechanical and manual housekeepingâboth must be used. * For details on explosibility parameters refer to the previous report in this series, NMAB 367-2, "Prevention of Grain Elevator and Mill Explosions."
It should be noted that "dust control" and "dust collection" have different meanings in this manual. Dust control includes all means of combating the grain-dust problem. Dust collection includes only means of collecting dust. Thus a dust-control program, as noted above, includes mechanical and manual housekeeping and measures for minimizing the generation of dust. Mechanical and manual housekeeping, in turn, include various means of collecting grain dust. The only mechanical housekeeping system that is known to be effective is the pneumatic type. Over the past 20 years, many elevator operators have installed pneumatic dust-collection systems, primarily systems using bag (fabric) filters. However, many of these pneumatic systems do not control dust well enough to reduce either the risk of explosions or the severity of those that occur. They are inadequate even when combined with effective manual housekeeping and dust-minimization measures. Such systems generally have not done the job for several major reasons; â¢ There are no realistic design standards for pneumatic dust-collection systems for grain elevators. â¢ Systems are fabricated and installed incorrectly. â¢ Buyers select the lowest bid, with little regard for the contractor's expertise or reliability. â¢ Operators do not know how the system should be expected to perform. â¢ Systems are operated and maintained improperly. This manual is designed to help correct these shortcomings. It gives detailed information on all aspects of pneumatic dust-collection systems for grain elevators. Abbreviations and definitions of terms used in dust-control work in the grain industry appear in Appendix A. Although the manual is for elevator operators, it contains guidelines for designers, installers, and contractors involved with pneumatic dust-collection systems. The manual also contains much information that should be useful to grain-elevator management.