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Section 6 DUST CONTROL IN PROBLEM AREAS Effective dust control is especially important in certain problem areas in grain elevators. These areas involve both enclosed and open sources of dust. Examples of enclosed sources are bucket elevators (legs), horizontal conveyors (drag or screw), and distributors. Examples of open sources are trippers, conveyor belts and belt loaders, and loading/unloading facilities (truck, rail, marine). This section outlines the basic principles of pneumatic dust collection in problem areas and gives specific examples. It is extremely important to recognize that a dust-control program has two goals. One goal is to keep the levels of dust in the workplace and environment from creating inhalation health hazards. The other goal is to prevent explosions. Measures that attain one of these goals will not necessarily attain both. In other words, an elevator that easily complies with environmental and occupational regulations is not necessarily free of the hazard of grain-dust explosions. A program based on a pneumatic system can easily meet the respiratory standards on dust levels and still permit dust in enclosed spaces to reach explosible concentrations. On the other hand, a properly designed pneumatic system can eliminate explosible concentrations of dust in enclosed spaces, as well as satisfy the environmental and occupational requirements. Ideally, the materials-handling system in an elevator should be designed from the start with proper dust control in mind. Desirable features include very short free-falls, choke-fed spouts, and slow-running belts and elevators. Depending on the circumstances, such features minimize the generation of dust, the entrainment of air, or both. In a poorly designed elevator, proper dust control may be next to impossible. It will certainly not be economical. In such elevators it may be best to revise the materials-handling system to avoid the difficult dust-generation problems and then apply normal dust-control techniques. A pneumatic system must collect dust from every enclosed space in an elevator if it is to work effectively. The system also must collect dust at all grain-transfer points that are not enclosed. Enclosed Problem Areas The use of pneumatic dust control with enclosed equipment is relatively straightforward. As noted earlier, the equipment is on the intake side of the exhaust fan and is connected to the fan by ductwork. The fan creates a slight negative pressure inside the enclosed space and ductwork; the negative pressure creates a flow of dust-laden air through the system toward the fan. Since the system works by controlling the movement of air, enclosures around equipment should surround all possible dust emission points with adequate inflow of air. 49
50 A pneumatic system must maintain an airflow large enough to remove the dust that is generated. To do so, the system must continuously remove from enclosed equipment both the air displaced and the air entrained by the entering grain. A bushel of grain displaces 1.25 cubic feet (cu ft) of air. When grain is fed to enclosed equipment, therefore, the dust-collection airflow must be at least 1.25 cfm per bushel of grain fed per minute to remove displaced air. If the grain is choke-fed, it will entrain relatively little air. However, grain that is fed by a long free-fall, particulary one with free inflow of air at its source, can entrain 10 or more times the volume of air it displaces, and the dust-collection system must also remove this entrained air. It is clear that proper grain-feeding techniques help to minimize the size and cost of dust-collection ductwork and fans. The power consumed by the system is reduced correspondingly. Enclosed grain-handling equipment must be self-cleaning. It should be free of ledges or other surfaces where dust can build up. Conveyors should be designed to avoid dust buildup on the bottom surfaces of enclosures and there should be no internal structural members upon which dust can accumulate. Enclosing a poorly designed grain-handling device in sheet metal can make it more hazardous than it was when unenclosed. Bucket Elevators The most common vertical conveying device in the grain industry is the continuous bucket elevator. The buckets on the device scoop grain from the boot, at the bottom of the elevator, raise it to the head, discharge it, and return to the boot. The up-moving and down-moving sections of the elevator may be enclosed in separate, sheet-metal casing; in other designs, the entire bucket elevator may be enclosed in a single, concrete housing. The combination of bucket elevator and housing usually is called the "leg." The leg is the most dangerous component of a grain elevator as the preponderance of grain dust explosions originate here. As the buckets scoop grain from the boot they stir up more dust than any other operation in the elevator. Furthermore, most pneumatic dust-collection systems maintain only enough negative pressure in the leg to prevent dust from escaping. Such systems do not prevent buildup of explosible concentrations of dust in the housing. The hazard is reinforced by the bucket elevator's potential for producing sparks and hot spots. Tests have shown that a properly designed pneumatic system can avoid explosible concentrations of dust in a leg (see Appendix C). The airflow needed will vary with the design and other factors. However, there is a rule of thumb for calculating the necessary airflow (Figure 6-1). The calculation involves the cross-sectional area of the leg casing, the speed of the elevator belt, and a factor (f) ranging from 1.25 to 1.50, the f factor to be selected according to the dust load. The factor f remains the same regardless of the cross-sectional area and belt speed. The equation is; WxDxBxf= Airflow
51 In this equation, W and D are the width and depth of the leg casing in feet, B is the belt speed in feet per minute, and the airflow is in cubic feet per minute. In the example of Figure 6-1, the leg casing is 2.0 ft wide and 2.5 ft deep, and the belt speed is 700 fpm. The volume flow required in the leg is; 2.0 x 2.5 x 700 x 1.25 = 4375 cfm That is, a volume flow of 4375 cfm should be sufficient to reduce internal dust suspensions adequately, provided the aspiration pattern is properly designed. A volume flow of more than 4000 cfm is much higher than is usually used in elevator legs. It has been commonly believed that a volume flow of about 750 cfm was sufficient to accommodate a typical 20,000 bph leg. We now know, however, that 750 cfm will not avoid explosible concentrations of airborne dust; such a volume flow in some cases may not even handle the air displaced by the entering grain. In the tests reported in Appendix C, numerous samples were taken by a vacuum method from various points in legs with no aspiration and in legs with conventional aspiration (i.e., suction on top of boot housing). It was shown that adequate volume flows applied properly could reduce the levels of airborne dust in legs below the minimimum explosible concentration. The use of adequate volumes of air, combined with proper aerodynamics, has been applied at only a few elevators, but in all cases it substantially reduced the levels of airborne dust. It should be remembered that the calculation given here is a rule of thumb. The volume flow actually required in the leg will vary not only with belt speed, but also with the way grain is fed to the leg, the suction in associated equipment, and possibly with other factors. Furthermore, as indicated in Figure 6-1, the calculation uses only the cross section of the up leg. The suction hoods in this kind of system should be on the sides of the up leg above the boot. Hoods must be designed and positioned with care so that air movement does not deflect the belt. Such deflection can cause the buckets, or cups, to strike or rub against the casing. One way to avoid belt deflection is to apply suction equally to both sides of the leg casing. Air should move from the leg into the hood at a face velocityâthe velocity at the entrance to the hood--of no more than 1000 fpm; a lower face velocity is desirable if possible. This limit on face velocity, produced by a properly designed transition, is necessary to avoid scalping grain and deflecting the belt. Reducing the speed of the belt can sharply reduce the generation of dust in a leg. The airflow needed in the leg is reduced correspondingly, as shown by the airflow equation above. On the other hand, if the belt runs too slowly, the buckets will not discharge properly. If the diameter of the head pulley is 48 inches, the minimum belt speed for proper discharge is about 480 fpm. However, belt speed in U.S. elevators typically is about 800 fpm. Thus operators should have some leeway in selecting the belt speed that is the best compromise between dust generation and grain-handling capacity.
52 wrww \ WWCNWW ftnAMtMA ) T rfl Horizontal Cross ^ yy Ml ^ ' Section of Leg , 1 1 ,^D^ iMMWWW Boot - Feed FIGURE 6-1 Rule of thumb for calculating airflow in elevator leg.
53 Horizontal Conveyors Enclosed horizontal conveyors are usually screw or drag conveyors. Common methods of airflow control for these devices are shown in Figure 6-2. It is extremely important that more air be exhausted from the conveyor housing than is displaced by incoming grain, and that adequate makeup air inlets where necessary be provided at the proper locations. It is also important that the face air velocity not exceed 800 fpm. This limitation avoids air-conveying velocities near the moving grain, permitting only the fine dust to enter the dust-collection system. Scales and Garners Hopper scales are among the most troublesome dust-emission points in conventional elevators. The emissions are usually intermittent. They tend to be high during the loading or discharge cycle or during both cycles. The capture of dust by pneumatic methods is complicated, in that pressure differentials between the inside and outside of the scale hopper can affect the accuracy of the scale significantly. Scale installations vary considerably, but the typical arrangement is an upper garner, a scale hopper, and a lower garner. This bin-scale-bin sequence can be used to illustrate workable dust-collection methods. The upper garner usually is filled from the elevator head. The incoming grain and the air entrained with it build pressure and release dust in the bin. Dust-laden air is thus forced from any openings in the bin or the spouting leading to it. A dust-collection take-off at the top of the upper garner can create negative pressure in the bin, thus eliminating dust emissions. The pressures inside and outside the scale hopper must be equal during the weighing cycle to obtain accurate weight. Therefore, the hopper cannot be under negative pressure, or suction, during the weighing cycle, nor can the scale room be under significant negative or positive pressure. If the scale hopper is under negative pressure or the scale room under positive pressure, the hopper is buoyed by the higher pressure in the room. Additional grain is required to overcome this flotation effect, so the scale will weigh heavy. If the scale room is under negative pressure during the weighing cycle, the scale will weigh light. Two general methods can be used to prevent dust emissions from hopper scales. One method assumes an airtight bin-scale-bin sequence. The other method allows for leaks, which usually exist. In the first method (Figure 6-3), pressures generated in the bin-scale- bin sequence are vented to a "Chinese hat" (see Figure 6-3). Aspiration by the dust-collection system creates an*area of negative pressure at the Chinese hat, where displaced dust is collected. However, the arrangement does not create negative pressure in the scale hopper, so accurate weight is obtained.
54 I 5 c o a .s>Â«- -Â£SÂ§ Q m CD 4-l H ai * -H Â£ -P 0) 03 h M 0 -H w a to i-H (t) (0 4J H C O O >. N Q) -H > ^ c O O K O (N I vD D O H Cn
55 Floor Floor Scale Hopper Floor Lower Garner Aspiration (Chinese Hat) Canvas or Flexible Connection in Duct Gate NOTE: System must be airtight. FIGURE 6-3 ScaleâGarner intervent system.
56 The second method (Figure 6-4) places the entire bin-scale-bin sequence under negative pressure, except during the weighing cycle. Only the upper garner is under suction during the weighing cycle. Cycling of the slide gates and suction damper is shown in Figure 6-4. Distributors The term "distributor" is applied to many significantly different devices that have the same function; distribution of grain in variable directions. Distributors are sometimes very troublesome emitters of dust. They are not troublesome because they generate dust or pressure themselves. The problem is that distributors are often the points of release of high pressure and large amounts of dust created during long free-falls of incoming grain. The most common conventional type of distributor, particularly in older elevators, is the Mayo spout. Other conventional distributors are the turnhead and the distributor box. The Mayo spout is connected to one source of grain, is angled downward, and is rotatable, usually through 360 degrees (see Figure 6-5) . Thus the spout can deliver grain to any porthole around its lower circle of rotation. The turnhead is a rotating enclosure that receives grain from one spout and directs it to any of several others. The distributor box differs from the turnhead in that it is stationary and uses internal valves or gates to divert the grain (see Figure 6-6). It is the rule in dust control that use of free-falls and long, steeply inclined, unrestricted spouts should be minimized if not eliminated. The lower the speed of the grain entering a distributor, the easier it is to maintain negative pressure in the device and capture airborne dust. The closest feasible approach to choke-feeding should be used. Even then, however, suction on the distributor will be required to avoid dust emissions. Installation of Mayo distributors is not recommended because of the difficulty of controlling the associated dust emissions. The main problem is providing adequate suction at the many escape points without interfering with the rotation of the spout. Flexible suction ducts can be used, but generally require too much maintenance. The pressure generated in a steeply angled Mayo spout can sometimes be handled by taking suction from the bins fed by the spout (Figure 6-5). In such cases each bin must be kept under negative pressure while it is being fed to avoid blowback through the porthole. It is also good practice to fit the end of the spout with a flared skirt or circular bristle brush that encloses the opening between spout and porthole. The diameter of the porthole should not exceed the diameter of the spout fitting by more than 20 percent. The turnhead distributor presents dust-collection problems similar to those of the Mayo spout. Again, these problems can be solved by placing suction on the bins that the turnhead is feeding. Also, the turnhead distributor can be tightly enclosed with sheet metal (Figure 6-6) so that it can be treated as a box distributor.
57 Weigh Cycle: A, B, C Closed Scale Dump: B & C Open A Closed Scale Fill: A & B Open C Closed Aspiration Upper Garner Scale Hopper Lower Garner FIGURE 6-4 ScaleâGarner intervent system.
58 Intervent Intervent FIGURE 6-5 Rotating or Mayo spout dust control.
59 Grain Aspiration Tapered Fitting FIGURE 6-6 Enclosed distributor dust control system.
60 The box distributor is immovable and relatively tight and can be kept under negative pressure rather easily. The device has few leaks. Also, it has a large top-surface area from which suction can be taken, using a proper transition fitting, without scalping grain. Unenclosed Problem Areas It is essential that a pneumatic system collect the dust generated at all unenclosed grain-transfer points in an elevator. There is little danger that an explosion will be initiated in unenclosed spaces. However, if airborne dust is not collected at grain-transfer points, most of it will settle on surfaces in the working space. This dust can then fuel secondary explosions in tunnels, galleries, and headhouses. Furthermore, layers of dust around working equipment are ideal sites for hidden, smoldering fires. Such fires can be the source of ignition for an explosion. GalleriesâTrippers and Bins Dust in the gallery of an elevator results mainly from the operation of trippers, "blowback" from tanks being filled, loaders, and excessive conveyor belt speeds. This dust can be collacted pneumatically. The type of installation depends on whether the bins are filled by traveling belt trippers or by stationary trippers and distributors. Trippers. The traveling tripper should have a well-fitted suction hood above and enclosing the ends of the tripper pulley. Suction hoods below the tripper pulley usually plug with grain and should be avoided. There should also be a hood at the head pulley of the tripper belt. In addition, hoods should be fitted at the spouts. Stationary trippers should have hoods at the discharge of each tripper. Hoods should also be fitted at the belt reloaders and distributors. Belt re loaders should not be treated like typical belt loaders because they require substantially more air. Figure 6-7 shows two typical dust-collection arrangements for manual or automated traveling trippers. Table 6-1 shows the airflows required for typical belt widths and capacities. These air volumes are for the tripper only. They do not include air for the hoods at the head pulley or belt loader. Both whole grain and dust must be handled at the belt discharge of a traveling tripper. The best approach usually is to return spilled grain to the topside of the belt with a miniature loop conveyor. Whole grain must also be handled along with dust at the head pulley of the tripper belt. It is not possible mechanically to return this grain to the grain stream. The grain can be captured by the dust hood at the head pulley by using high air velocity for pickup and relatively high velocity in the associated duct. Air velocity in the duct should approach that used for pneumatic conveyingâ4,500 to 5,500 fpm. Otherwise, grain will not remain airborne in the duct. The head-pulley duct should never be connected to the end of the tripper duct. This arrangement would make it impossible to provide enough suction at the head pulley at all times. Instead, the head-pulley duct should be tied into the main duct ahead of the tripper duct.
61 Top Hood Shown Plan View Only Spout Hood Style A Blast Gate Spout Hood Style B ELEVATION FIGURE 6-7 Zipper suction for tripper.
62 Whole grain at the head pulley of the tripper belt can also be caught in a hopper under the pulley. The hopper would discharge through an opening into the bin. However, this alternative may not be practical where grains cannot be comingled. TABLE 6-1 Air Volume for Traveling Trippers3 Belt Width Approximate Capacity Air Volume 24 inches 10,000 bph 3,500 cfm 30 inches 15,000 bph 3,800 cfm 42 inches 30,000 bph 5,500 cfm 48 inches 40,000 bph 6,000 cfm 60 inches 60,000 bph 6,800 cfm a The figures above assume belt speeds of 500 fpm or less. Speeds in excess of 500 fpm should be avoided because of a) excessive dust emission, b) maintenance problems, and c) grain breakage. Bins. Grain entering a bin from the gallery forces air out of the bin. Normally this displaced air contains dust and should be captured by the pneumatic system. The required bin suction usually is applied at the end of the tripper spout. This is the least costly way to vent the bin, but it may not be the best way. Often the spout is almost as big as the opening to the bin; the entering grain keeps displaced air from rising freely through the opening and into the dust hood. When this is the case, dust-laden air sometimes forces its way from the bin through a manhole or gravity vent. It would be better to provide a separate bin opening and hood to capture displaced air and dust that cannot leave the bin at the spout. The hood could be connected to the tripper duct or to an external bin-vent manifold. Dust collection from bins can be simplified if the bins are interconnected. One dust-collection take-off can effectively maintain negative pressure in a group of interconnected bins. Various codes have different requirements for interconnections, however, and some codes prohibit interconnection of bins in new construction on the theory that intervents can propagate explosions from bin to bin. However, numerous investigations show that as a result of high pressure the explosion will propagate to other spaces regardless of openings. In many cases interventing may be the only practical way to provide for a clean gallery.
63 When bins are not interconnected, each must have its own dust take-off and exhaust fan. This approach requires no interconnecting ductwork. Also, a fan need run only when its bin is being filled, so less power is consumed than by a large, centralized system with a high-horsepower fan. Even so, the system may be complex and expensive. Alternatively, a manifold could be used to connect many bins into a central system. Blast gates, or valves, could be used to isolate suction to the proper bin. Grain-dust explosions, sufficiently confined, may generate pressures of up to 120 psia. At pressures below 10 psia bin walls, tops, and discharge spouts through failure would propagate the explosion. Any bin-venting system, regardless of its design, should provide for free intake of air by the bin. Free intake of air is required to prevent the bin from collapsing when it discharges grain. Belts The head pulley on most belts presents two problems; carry-over of whole grain and carry-over of dust. It is best, where practical, to treat these problems separately. Whole grain that carries over the head pulley usually is released directly under the pulley. This grain is best handled by a mechanical pickup, such as a minature loop conveyor or screw conveyor. The reclaim conveyor can be used to return the collected grain to the grain stream on the top side of the belt. A short screw conveyor can be used to carry the grain to a discharge point where it can be picked up pneumatically or reelevated to the grain stream on the belt. Dust that carries over the head pulley most often will become airborne at the first return idler pulley. It should be collected at that point by a suction hood. Sometimes, however, dust will become airborne at several return idlers. This can be handled by a high velocity air sweep at the first return idler. In seme cases it is not possible to return the collected grain to the grain stream. An example is the head pulley on a tripper belt (see above, under Trippers). There are two ways to solve this problem. One approach is to handle the grain with a suction hood at the point of carry-over, using high air velocities for pickup and transport to the primary separator. Here a primary separator should be used to remove the grain from the dust stream before it reaches the filter. The other approach is to provide an opening in the bin directly under the head pulley. Grain that carries over the pulley can fall directly into the bin. Cleaners and Small Scales Other sources of grain dust in an elevator are cleaners and small open hopper scales. These devices should be enclosed insofar as possible. Ledges and other flat surfaces that can collect dust should be avoided. Aspiration
64 should be applied to the enclosure of each cleaner and scale with sufficient makeup air to provide a sweep of the emission area. Enough suction should be applied to handle the air displaced by entering grain and still create an indraft velocity of 200 fpm at each opening in the enclosure. Truck and Rail Unloading Critical areas for controlling grain dust outside elevators are the receiving pits, where trucks and railroad cars are unloaded. Still, there are basic methods for solving the problem. When used properly, they will help to control dust emissions around truck and rail dumps. Truck Dumps. Control of grain dust at truck dumps is illustrated by Figure 6-8. In the figure, the plan view of the pit at grate level is divided into Areas B, C, and D. The truck crosses the pit from right to leftâentering via Area B and leaving via Area C. Options X and Y in the figure are possible arrangements of the dust-collection system. Areas C and D of the pit should be protected by a three-sided building. The back wall of the building should be in Area B, parallel to and 2 or 3 inches from the line dividing Areas B and D. This back wall is an automatic door. It should be closed when grain is being unloaded to keep wind from interfering with the dust-collection system. The door also prevents grain from being spilled on most of Area B, which is covered, thus eliminating a housekeeping problem. The two parallel walls of the building should extend at least 6 ft beyond the front of the pit (the left edge of Area C) to help prevent dust from escaping in that area. Area B is covered to keep grain from entering the dust-collection system. The cover also creates a space below grate level where airborne dust can be trapped long enough for the pneumatic system to collect it. If the collection system is connected to the pit at only one point (Option Y), a shield should be installed around the pit under the grate to provide an unobstructed air passage to the take-off. The shield prevents grain from filling the pit and restricting the airflow to areas where dust emissions could be a problem. Relatively little grain enters the pit through Area D. That area should be closed off with baffles that will swing to one side to let grain enter the pit. The baffles are designed to block the escape of dust-laden air from the pit to above floor level. By doing so, they sharply reduce the power consumed by the dust-collection system. The bulk of the grain falls into the pit through Area C, which should be left open except for the grating. Baffles in Area C would only reduce the unloading rate. A very important aspect of dust control at a truck dump is to provide the proper airflow through the pit area. The method of computing volume flow is shown in Figure 6-8. The major concern is to supply adequate airflow in all critical areas when the pit is full of grain.
65 Vertical View Option "X" Vertical View Option "Y" Area "C" Area "D" Area "B" Area "C" Area "D" Plan View Option "X" Plan View Option "Y" Angle "A1 Area "Bâ¢ Area Area II S^l II II i-.ll 15 degrees from low point of air outlet to top of grate, determines Area "B". Cover this area with solid steel plate to keep grain from entering dust system. 4' x 8' open area for easy entry of grain to pit. Baffles to be installed under grain grateâbaffles to cover 2/3 of open area. cfm Computation of air required to control dust 1. Air displacement caused by grain flow Example: 500 bushels goes into pit in 10 seconds. 500 bushels/10 seconds = 50 bps x 60 seconds = 3000 bpm x 1.25 cu ft/bushel = 3750 cu ft/minute 2. Square feet of Area "C" x 100 cfm = cfm 3. Square feet of Area "D" x 33 cfm = cfm Total 1, 2, 3 cfm FIGURE 6-8 Suggestions for control of grain dust at truck dumps.
66 It should be noted here that truck and car dumps have been persistent problems with respect to dust emissions. The difficulty has been an inability to concentrate sufficient inlet air in the center of the pit face from where most emissions escape. The treatment first described represents the most successful approach to date, but experimentation is currently underway with a new concept; one which is intended to serve the dual purposes of controlling emissions and cleaning incoming grain, all with less horsepower than is now typically applied. Rail Dumps. The methods of dust control at truck dumps also apply to railroad hopper cars. Most hopper-car pits are long and narrow and require less airflow than truck dumps. However, the length of the train makes it impossible to close in the ends of the building. Without this control on wind, somewhat higher air velocity than is used in truck dumps may be needed to control cross-drafts. Truck and Rail Loading There are several ways to control dust when loading trucks and railroad cars with free-pouring grain. The methods for open-top trucks, hopper cars, and boxcars are generally similar. However, trucks vary much more than railroad cars in height, length, and configuration. Flex Hoses and Hoods Dust from loading railroad cars can be captured relatively inexpensively with two flex hoses. Their free ends can be suspended by ropes. With hopper cars, the end of one hose is suspended near the spout at the top of the grain pile. The other hose extends into the hopper and collects most of the dust there before it escapes. With boxcars, a hose is placed on each side of the grain spout. The hoses can be held in place extended into the boxcar. The design airflow for this system, for both hopper cars and boxcars, should be 4,200 cfm for each hose or a total of 8,400 cfm. This airflow is based on an air velocity of 4,000 fpm. A hood over a hopper car can control dust effectively during loading. If the hood extends on both sides of the spout, the car can move in either direction during loading. The hood should be about 16 ft long and 20 inches wide and should have a flexible skirt. Suction should be ducted into each end of the hood. The device should be able to move at least 4 ft vertically, preferably by means of a powered hoist. A telescoping grain spout must be used, and the suction ducts must be telescoping or flexible. The closer the hood fits to the hopper car, the better it will work. Deadbox Spouts and Hoods The deadbox spout can be used effectively with both trucks and railroad cars (Figure 6-9). Grain falls freely down the spout, is stopped momentarily in the deadbox, and then drops about 1 ft to the grain pile. The short fall from the deadbox minimizes the generation of dust. Entrained air and dust
67 liberated in the deadbox are aspirated back up the spout by the dust-collection system. On a grain spout 14 inches in diameter, the cross-sectional area of the air conduit will be about 3.2 sq ft. Air velocity should be 1,500 fpm, so the design airflow should be about 4,800 cfm (1,500 fpm x 3.2 sq ft). The deadbox spout requires vertical clearance and must be retractable and power operated. The nose of the spout must be kept close to the grain pile, but not buried, which can be a problem for the operator. At flow rates lower than about 25 percent of design capacity, grain falls uninterrupted through a deadbox, generating considerable dust on impact with the grain pile. A deadbox should have plug-relief doors to prevent back-up and choking. A very effective dust-collection system is a combination deadbox and hood. Suction is taken from the deadbox itself and from a hood on each side of the deadbox (see Figure 6-10). Proprietary Loading Devices Various proprietary devices are available for controlling grain dust during loading operations. These devices employ the deadbox (and other choke feeding), venturi, centrifugal force, or reverse airsweep principles. They operate most efficiently when held close to the grain pile. Spout Design, Wind Spout design is an important aspect of dust control during loading. The best design is a short spout with no air inlet at the top (Figure 6-11). The worst design is a spout that permits air to enter at the top and has a long free-fall. The volume of air entrained with such a spout can be more than 30 times the volume of the entering grain. This condition can create a severe dust problem at the discharge point. Wind can sharply reduce the effectiveness of dust-control measures during loading. Wind currents are best controlled by enclosing one end of the loading building with roll-up or bi-fold doors. This approach works well when loading trucks, but is less convenient with railroad cars. Marine Loading Controlling dust when loading vessels with free-pouring grain involves methods much like those used with trucks and railroad cars. Ships vary much more than barges in hatch size and in height above the waterline at light and full draft. At export elevators radical changes in water level have to be considered when designing dust control for loading spouts. The most flexible means of controlling grain dust when loading vessels is the hatch tent. Other devices generally have two disadvantages when used with existing loading installations. The first is that the added weight usually cannot be supported without extensive structural changes. The second drawback is that most existing installations were not designed to hold dust-control devices near the grain pile in all loading conditions.
68 Air Suction Air Conduit Must be Flex Hose or Swivel Joint Telescoping Grain Spout Outer Sleeve Must Be Telescoping or Collapsible Must Maintain 12" Clearance to Be Most Effective Dead Box FIGURE 6-9 Retractable spout deadbox for truck load-out.
69 T) 8 I S 3 to X o 0) 'o c o -H -p id -H O CJ s D O
70 . Storage Tank or Garner Free Air Intake Slide Gate Control No Volume of Entrained Air to Deal With At Discharge Entrained Air Can Be 30 Times Vol. of Grain on Long Free-Falls GOOD DESIGN Easiest to Control Dust POOR DESIGN Most Difficult to Control Dust FIGURE 6-11 Load-out spout design,
71 Hatch Tents The hatch tent, or covering, is the basis of a number of dust-control systems for marine loading. With the tent in place, negative pressure can be maintained on the hatch using flexible hoses that collect the dust. Little if any structural change is needed to install a hatch-tent system. The tent may be hung from the ship's rigging or simply draped across the hatch. The latter approach is not effective during topping-off procedures. Other Loading Devices The deadbox principle described under Truck and Rail Loading (Figure 6-9) is also used with vessels. The same proprietary devices mentioned on page are also used for marine loading operations. Such systems require additional height to accommodate the devices. As noted earlier, the installation must be strong enough to support the extra hardware. A few marine dust-control systems have gravity spouts whose discharge end can reach the grain pile at all times. The spout is partially buried in the grain pile and kept under negative pressure.