Pneumatic Dust Control in Grain Elevators: Guidelines for Design Operation and Maintenance (1982)

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Section 2 DUST-CONTROL TECHNIQUES AND EQUIPMENT An efficient dust-control program in a grain elevator, as pointed out in Section 1, has three parts; mechanical housekeeping, manual housekeeping, and methods for minimizing the generation of dust. Mechanical housekeeping using pneumatic systems is the primary subject of this manual. However, the other aspects of dust-control programs deserve brief treatment at this point. The ultimate goal of a dust-control program is perfection—no dust, either suspended in air or accumulated on surfaces inside the elevator. A sound program must be designed to approach perfection as closely as possible; at least to the point where explosible suspensions and dust layers do not exist. Guidelines aimed at peak performance in dust-control programs appear in Table 2-1. The human element is extremely important in dust control, as in other areas of elevator operation. Well-trained and vigilant employees are vital to a successful program. Equally vital is well-organized and effective management. In this vein, one person on each shift in an elevator should be assigned the responsibility and authority for implementing the dust-control program. This individual should report directly and only to the senior manager of the elevator. Minimizing Generation of Dust Normal elevator operations will always produce dust, but there are ways to minimize the effect. The more the grain is handled and agitated, the more dust is produced. More efficient handling of stock, therefore, will reduce the generation of dust. Deeper layers of grain on conveyor belts are helpful. It is also helpful to avoid long free-falls and steep-angle descents of grain in spouts and to choke those spouts where these problems cannot be avoided. Besides reducing the generation and dispersal of dust, avoidance of long free- falls reduces the amount of air entrained by falling grain and carried into enclosed sections of the elevator. Reducing entrained air, in turn, reduces the load on the mechanical dust-collection system. Manual Housekeeping Manual housekeeping is always necessary in a grain elevator. The extent of the need depends on the effectiveness of the other parts of the dust-control program. That is, the need for manual housekeeping depends on how well the generation and dispersal of dust are controlled and on how well the mechanical system keeps dust from escaping from enclosed sections of the facility. The key to a good manual program is to anticipate the need for cleanup and schedule it accordingly. Removing dust after it has accumulated—if only to a depth at which footprints first become visible—is not good housekeeping. It is playing catch-up, and it is hazardous.

TABLE 2-1 Guidelines for Dust Control in Grain Elevators 1. Techniques for preventing or reducing the generation of dust shall be used at all possible locations. 2. Pneumatic or other mechanical dust-collection techniques shall be used at all dust-producing locations in or adjacent to a facility. 3. Pneumatic dust-collection systems shall be designed and operated to capture virtually all dust emitted from the stock stream and so prevent the occurrence of explosible concentrations of airborne dust and subsequent layering of dust. 4. Manual cleanup shall be scheduled and performed so as to prevent the accumulation of settled dust at any location in quantities which, if dispersed into the air, could support propagation of an explosion at that location. 5. Manual cleanup should be applied to all surfaces in a facility as needed. 6. Vacuuming should be the preferred technique for manual cleanup inside facilities. 7. Aspiration of legs and other components of stock-handling systems shall be applied to minimize the airborne dust therein. 8. Interior parts of stock-handling systems should be designed and aspirated to minimize suspensions and formations of layers. 9. Blow-down of settled dust with compressed air is a hazardous cleanup method because it can create explosible concentrations of airborne dust. If blow-down is used for major accumulations of dust on surfaces not accessible to other cleaning techniques, all stock-handling equipment should be shut down and remain out of operation until the dust has substantially settled and been removed.

The manual-housekeeping schedule needed to control dust effectively in an elevator can be determined by experience. However, the manager can estimate his housekeeping needs by means of certain measurements. Dust inside an elevator can be removed manually by several methods; broom and shovel, blowing down, washing down, and vacuuming. One problem with broom and shovel is that surfaces covered with hazardous but very thin layers of dust--as little as 1/64 inch thick—may look clean to a sweeper, who may thus ignore them. Blowing down with compressed air may create an explosible cloud of dust and, at best, simply redistributes most of the layered dust. Washing down with a hose is rare in this country, but also can initially create explosible clouds of dust. Vacuuming (with a cleaner approved for use in Class II, Group G locations) is the preferred method for manual cleanup. Vacuuming can collect small amounts of dust extremely well. Also, it does not disperse dust into the air, so that normal operations need not be shut down during cleanup by vacuuming. Mechanical Housekeeping Mechanical housekeeping systems in grain elevators have been designed traditionally to keep dust from escaping from enclosed sections of these facilities. However, mechanical systems of the pneumatic type evidently can do more than prevent emissions of dust. Recent tests in operating grain-handling facilities suggest that properly designed pneumatic systems can prevent airborne dust in enclosed spaces—such as elevator legs—from reaching concentrations high enough to explode. These very promising results have not yet been fully confirmed and further testing is under way. Pneumatic dust collection involves only a few common-sense principles. Grain dust is heavier than air and so becomes airborne only when disturbed in some way. Once airborne, dust settles by gravity if left undisturbed. Apart from the effect of gravity, airborne dust moves only when carried by the air it is suspended in. The air, in turn, moves in response to differences in pressure. Furthermore, air moves from areas of higher pressure to areas of lower pressure. Grain dust in an enclosed space, therefore, can be controlled by controlling the air flow associated with the system. Dust-collection systems using these principles have five basic components, exhaust fans, capture enclosures, ductwork, dust-separation devices, and storage bins. These components are described in general terms in the remainder of this section. Their application to dust-control problems is described in detail in subsequent sections of this manual. Exhaust Fans An exhaust fan is a centrifugal or axial device that maintains a difference between the air pressures on its intake and exhaust sides. The pressure is lower than atmospheric on the intake side of the fan and higher than atmospheric on the exhaust side. In a dust-collection system, dust is captured on the low-pressure, or intake, side of the fan and discharged on the high-pressure, or exhaust, side if the fan precedes the filter in the system. If the order is the reverse, the dust is removed by the filter, and the cleaned air is exhausted on the high-pressure side of the fan.

A fan reduces the pressure on its intake side and increases the pressure on its exhaust side. Air and entrained dust are forced through the system solely in response to the differences between these pressures and atmospheric pressure. Ductwork The intake ductwork in a pneumatic system connects the low-pressure side of the exhaust fan to all points in the system where the internal pressure must be lower than atmospheric. When a pneumatic system is operating, atmospheric pressure forces air and entrained dust into the system. The intake ductwork channels the air and dust to the intake side of the fan. Here the air and dust are transported to the exhaust side of the fan, where the higher-than-atmospheric pressure forces them through the discharge duct to the filter or other dust collector. The pressure in a duct always has two components; static pressure (SP) and velocity pressure (VP). Static pressure at any point in a duct is the pressure exerted on the wall of the duct. Velocity pressure in a duct is the pressure that a moving stream of air would exert on a flat plate at right angles to the direction of movement of the air. The sum of the two pressures is the total pressure (TP), i.e., TP = SP + VP. If the pressure on the wall of the duct is lower inside than outside, the static pressure is said to be "negative." If the reverse is true, the static pressure is "positive." The velocity pressure, on the other hand, is always positive. Dust-Collection Devices Industry in general uses four basic types of devices to remove dusts and other particles from air. The four types are wet scrubbers, electrostatic precipitators, mechanical collectors or cyclones, and fabric filters. Cyclones and fabric filters are in common use in the grain industry, although the use of cyclones is declining. Following are the characteristics of the four types of devices. Wet Scrubbers. Wet scrubbers remove particles from air by scrubbing it with water (e.g., sprays, water spinners, wetted bed collectors). The devices are effective on a wide range of particle types and sizes. As particle size decreases to the fine range, however, the energy consumption of a wet scrubber rises sharply. Also, ordinary water cannot be used in wet scrubbers mounted out-of-doors in cold climates. For these reasons and others, the devices find virtually no use in the grain industry. Electrostatic Precipitators. The electrostatic precipitator uses high voltage to attract electrically charged particles to the collecting surfaces. Normal operation of the devices involves a certain amount of sparking, so they cannot be used with combustible and explosive grain dust. Such a device has been field-tested by a grain company, but the results are not yet available.

Mechanical Collectors. A mechanical collector or cyclone induces a spinning action in a stream of air in a cylindrical vessel with a conical bottom. Particles are thrown out of the airstream and collected. The cyclone is limited by its inability to separate fine or very light particles. Thus the use of the device in the grain industry is declining as the Environmental Protection Agency grows more concerned over the discharge of fine, inhalable particles to the atmosphere. Fabric Filters. Fabric filters or baghouse units filter particles from air with high efficiency. Moreover, they are efficient for particles of widely varying size, including very fine and very light particles. The initial cost of a fabric filter is several times the cost of a cyclone. Also, the filters must be carefully maintained and operated to retain their design performance. Nevertheless, the air discharged from a baghouse is extremely clean, and the devices are the best available for dust collection in the grain industry. Dust-Control Terminology Abbreviations and definitions of terms used in dust-control work, as noted earlier, are given in Appendix A. However, explanations of three basic terms--pressure, volume flow, and velocity—are in order here. Pressures in pneumatic dust-control systems usually are stated in inches of water. A pressure of 1 inch of water, for example, is the pressure exerted by a column of water 1 inch high, or 0.0361 pounds per square inch (psi). A pressure of 1 inch of water is also the velocity pressure exerted by air moving through a duct at approximately 4000 feet per minute (fpm). Atmospheric pressure (14.7 psi) is 407.5 inch of water. Volume flow is the volume of air moving through a duct. Usually it is given in cubic feet per minute (cfm) . In a branch duct, the volume flow is the same at all points. In a main duct, the volume is the sum of all the volume flows in the branch ducts. Velocity is the speed of the air moving through a duct. Usually it is given in feet per minute. Velocity is equal to the volume flow divided by the cross-sectional area of the duct in square feet (sq ft). For a given volume flow, velocity may be increased by making the duct smaller and decreased by making it larger. This also introduces pressure drops in the duct. Use of Pneumatic Systems The practical use of pneumatic systems to collect grain dust in elevators is explained in detail in subsequent sections of this manual. The treatment starts with points of emission of dust from enclosed and open sources. It then proceeds in order through dust-collection hoods and transitions, ductwork, filter collectors, and fans. Finally, installation and acceptance of pneumatic systems, as well as training of plant personnel in the operation and maintenance of pneumatic systems are presented.