Solving Bulk Handling Problems
While major problems will most likely require vendor assistance, most problems that occur in a poorly performing plant can be traced to the performance of individual units that comprise the system. Through a systematic analysis of the performance of each component's input and output values, the plant engineer can often ensure a high degree of on-time plant reliability without spending unnecessary time and money on obtaining outside advice.
System ConfigurationOperations in a typical in-plant bulk handling system include:
1. Receipt and storage of dry materials in bag or bulk
2. Debagging/drawing raw materials from storage to the plant process
3. Moving raw materials between storage and processing functions
4. Temporary day storage at a process location
Systems and equipment common to these operations include instrumentation, dust filtration and control logic.
Each of these operations requires equipment that is specifically designed for a particular function, the output of which should feed the next piece of equipment in the product flow path. In many cases, equipment performing each of the above listed actions is not manufactured by the same supplier, and process inputs and outputs must be carefully matched to provide reliable system operation. Individual equipment capacities also need to be matched both upstream and downstream.
For example, if a bag-slitting machine can cut open 500 50-kg bags per hour but is being loaded manually, the manual loading rates will not be able to match the machine rates. Hence, the requirement would be to have a suitable system upstream of the bag-slitting machine that is capable of feeding the machine to its capacity.
System ComponentsSome of the most common components of a typical bulk handling system are fans and blowers, rotary valves, feeders, screens, instrumentation, screw conveyors, bag slitting machines, bag lifters, diverters and deflectors, pipes and bends, belt conveyors and bucket elevators, buffer hoppers, intermediate storage (jumbo bags, etc.), bulk road and rail tankers, and size reduction equipment. Before embarking on troubleshooting, the plant engineer must have a thorough understanding of the basic principles of operation of each piece of equipment in the plant. This can be obtained by studying technical literature and data from suppliers, as well as by obtaining feedback from other companies using similar products.
Fans and BlowersBoth centrifugal fans and twin lobe roots-type positive displacement blowers are used widely for filtration and pneumatic conveying. For pneumatic conveying, it is best to depend on the positive displacement roots blower as it has a very narrow range of proportionality between pressure drop and gas flow. For a rise in pressure in the conveying line due to a material flow surge, for example, the gas (air) flow reduces only marginally, thus maintaining conveying velocities. The centrifugal fan, on the other hand, has a flatter pressure-volume curve and therefore a considerable reduction in fan gas flow occurs with a relatively small rise in pressure. This will lead to significant velocity drops in the conveying line, and material can drop out of suspension, leading to line chokes.
The positive displacement blower also aids in reentrainment of the material when a system is restarted after a power failure. On the other hand, a fan is preferred in a negative pressure system since product passing through the impeller in case of an upstream bag failure does not do much harm to the fan. With a roots blower, such an event could be disastrous. However, many cases exist where the roots blower has to be used in such negative pressure systems, and one has to ensure that no particles will carry over into the blower in case of filter bag failure. This can be done by installing primary and secondary filters for protection.
Rotary ValvesRotary valves are used to provide metered feeding into a pressurized pneumatic conveying line or gravity receptacle. When feeding into a pressurized pneumatic conveying line, problems such as blowback, low capacity, excessive noise and vibration, and equipment wear can occur. To avoid these problems, the equipment should be correctly specified for the application in which it is being used. All rotary valves will leak the air or conveying gas that is used in the downstream pneumatic conveying system. This leakage occurs due to a gap between the stator and the rotor, and the higher this gap, the greater the leak. Manufacturers have to compromise between having a large number of rotor vanes to minimize leakage (this simulates the labyrinth seal principle) and ensuring that material does not "wedge" into the vane pockets. The best compromise is usually a six- or eight-vaned rotor. If the pressure drop is high in the pneumatic conveying line, it may be worthwhile to consider a suction system instead of a pressure system, as this will ensure that no blowback occurs and that air will only be drawn into the rotor rather than forced out. This consideration should be discussed prior to placing the equipment order and will depend on the layout of the system.
Rotor speed is also an important consideration since peak performance is obtained only in a particular narrow speed band. However, high speeds do not necessarily mean high throughputs, since at high speeds the rotor pockets will not have sufficient time to collect material fully as they move across the inlet opening. Filling efficiency is best practically measured prior to placing an equipment order by sending some material to the equipment supplier for testing. An onsite check of filling efficiency is possible if adequate provision for this has been made while engineering the system.
FiltersMost plant problems in filter installations relate to high pressure drops across filters, "puffing" of filter bags, poor discharge from filters or dirty filters. The selection of the filter cloth and the air-to-cloth ratio is different for varying applications and materials. Many companies make the mistake of conveying different materials of similar bulk densities in the same pipelines using the same filter. If this is not planned beforehand with the system components selected accordingly, major problems will result due to incompatibility between the material being handled, the filter cloth and/or the cloth area available.
The filter must be sized for proper can velocities, filtration velocities and cloth area. Compromises in these areas may not result in any initial problems, but difficulties will surface as the plant grows older. Cleaning cycles in reverse pulse jet filters must be optimally set, and this can be done only after a period of daily operation once the plant is up and running.
Inlet designs should ensure that as much of the product as possible is separated prior to the air mass passing through the filter bank. This is done by specifying tangential entries/baffles, etc., within the entrance section of the filter unit. Initial daily checks of the pressure drop across the filter body and the top plenum chamber are essential to confirm that optimum filtration is taking place. Once the primary filter coat is formed and the cleaning cycles are optimized, the pressure differential between these two areas will generally remain constant over a long period of time.
It is preferable to have a transparent section on the filter casing to observe conditions of filter bags during cleaning and non-cleaning periods. Such viewing windows should have manual wipers, since fine powders can coat the inside of the transparent panel and impede visibility.
Broken bags will leave telltale deposits of powders on the plenum chamber where the filter element is fitted in. Removing the cover will show these areas. More expensive methods and instrumentation are available for online detection of broken bags, but these can be very expensive and should be installed only for very critical applications.
InstrumentationBy far the most common problems in bulk handling relate to instrumentation and control of the system operations.
Level Sensors. Level sensors are some of the most commonly used instrumentation, and these can lead to system control problems if they are specified or applied improperly. Level sensors generally come in four types: vibrating fork, capacitance, ultrasonic and electromechanical. Vibrating fork sensors work on the piezoelectric principle. The tuning forks of the instrument are brought to their resonance frequency by built-in piezoelectric crystals. When the bulk material contacts these forks, the vibrations are dampened, and suitable signals are generated that can be used for further control of the system.
Capacitance level sensors work on the principle of varying dielectric constant. The probe and the wall of the vessel make two plates of a capacitor. The probe insulation and surrounding air provide the dielectric constant. As the air insulation is displaced with the bulk material, the capacitance changes, which results in actuation of a relay to indicate material level.
Ultrasonic level sensors work on the principle of sound wave transmission and reception. Sound waves from the transmitter are sent to the depth of the silo level to be detected. These sound waves hit the material and are reflected back to the receiver. The time taken to hit the material and reach the receiver is proportional to the level of the material. This type of sensor is used where continuous level measurement is required.
Electromechanical (yo-yo) level sensors essentially consist of a weight fitted to a tape on a reeling drum. The weight is allowed to travel to the depth of the silo level to be detected. After reaching and settling on the material in the silo, the weight slackens the belt, which then retracts the weight to the top set point. The traveling speed of the weight is integrated with the timer so that the time taken to reach the material and retract to the home position indicates the silo level. This type of sensor is also used for continuous level monitoring applications.
Because of these differing principles of operation, it is crucial to specify the correct type of level instrument depending on the application and its importance with respect to system control. For critical applications, back-up sensing using either a second level sensor or a load cell system is advised.
The vibrating fork units perform well where the vibration amplitudes are effectively dampened by the bulk material in contact with the forks. Where light, fluffy and aeratable materials are involved, the vibrations do not get dampened enough for a signal to be generated, and sensing fails. These units are best suited to high bulk density cohesive powders. When sensing problems do occur, it is best to remove the instrument and check it separately by energizing and dipping the instrument in the material to gauge its behavior. Some units have a provision to alter the vibration characteristics, and this could also be evaluated.
Capacitance type units are generally reliable but again, the need for proper pre-ordering is paramount. Material characteristics influence performance to a great extent, and no material property is too insignificant to ignore. Typical problems relate to the inability of the unit to sense capacitance change due to moisture, or probe insensitivity due to coated powder deposits and/or inadequate probe surface. The set points of the units can often be adjusted after trial and error, and this procedure must be understood and applied by the plant engineer.
Ultrasonic units are generally robust and reliable but are more expensive than the other units. The mechanical "yo-yo" unit is simple and easy to install and use. Its most vulnerable feature is the tendency to get embedded in the powder mass, but this can be easily rectified by adjusting the belt length.
Load Cells. Most bulk handling operations require weighing as part of the system requirement. Load cells are commonly used weighing elements, and the selection of the proper type of load cell is an important consideration.
The principle of operation of the load cell depends on a wheatstone bridge configuration that averages weight readings, which are converted to milliamperes. Weight is sensed by a strain gauge-the strain proportional to the load applied is converted to current signals.
Plant engineers generally tend to over specify accuracy requirements of load cells in the belief that greater accuracy will help them make a better product; however, this is not always true. Accuracy requirements are best determined at a joint meeting with production and marketing staff. The market is a key evaluator of product specifications, and high accuracy, when not required, will only hinder sales by delivering more expensive (and thus un-buyable) products to the market.
The cost difference between a I0.5% accuracy requirement and a I0.1% accuracy requirement can be quite substantial. Weighing accuracies are always specified on the gross weight of the container that is holding the material and the weight of the material to be weighed. To obtain better accuracies in material weight for a given accuracy spread, it is advisable to make the container as light as possible by using different construction materials, such as fiberglass or aluminum. This can provide dramatic accuracy improvements for the same accuracy range, since the accuracy is transferred more to the material. The container weight should be restricted to less than 25 to 30% of the total weight of the system. The lower the container weight with regard to the total system weight, the higher the product weight accuracy.
To save money, engineers sometimes specify a single live load cell and two or three "dummy" load cells in a hopper or vessel weight application. Such specifications need to be drafted carefully since bulk powders lodge themselves toward vessel sides (depending on fill, discharge methods and openings), thereby leading to more weight on one side or more of a load cell set and thus inaccurate output signals.
Typical operational problems are incorrect weights, hunting weight indications and erratic behavior of the system electronics. Problems are generally not serious and often just need the electronics adjusted; however, this type of troubleshooting will probably require the services of a qualified electronic engineer. Again, pre-order selection of the type and number of load cells for the given application and accuracy requirements are very important to ensure proper on-site performance.
Flexible Spiral ConveyorsFlexible spiral conveyors are used prevalently in bulk handling plants to elevate powders over relatively short distances to feed reactors, vessels, hoppers and other machinery from ground level. They consist of a centerless spiral auger driven at one end and encased in a polymer-based tube. Product is fed directly into the tube from a feed hopper. The spirals run at anywhere from 100 to 500 rpm. Typical problems include breakage or unraveling of the spiral, or product compaction.
It is advisable to purchase a small length of extra spiral when ordering the equipment in case the spiral breaks. Mechanical clamps can be made to add on one bit of spiral to the main spiral without any adverse effects on performance. Most breakage is at or near the drive end and results from high inertial torque during startup on load and due to hard lumps in the material. Unraveled bits of spiral can also be cut off and a new piece added. In some cases, the rest of the spiral can be stretched to regain the original length. It is best to discuss these issues with the vendor prior to placing the order.
Aeromechanical ConveyorsAnother common element in most bulk handling facilities, aeromechanical conveyors are used to elevate powders over relatively short vertical distances to feed reactors, vessels, mixers and other equipment. A series of plastic discs are fitted at regular intervals on a wire rope assembly, and this endless rope-disc assembly runs inside a twin tube arrangement connected by sprockets at each end. Product is fed into the lower end through a feed hopper and is discharged at the opposite end. Problems in operation can include rope breakage and inadequate throughput.
Broken ropes can be mended by adding on a separate piece. Low throughput can be corrected by speed changes and by redesigning the feed hopper.
Plant LayoutMany problems can be avoided simply by walking around the plant and making changes to the layout as needed. Material flow, chutes, enclosures, pits, conveying lines, maintenance platforms and support structures, equipment wear and bends in pneumatic conveying lines should all be evaluated to ensure optimum equipment performance.
Material Flow. The best layout is one wherein the bulk material uses gravity to the maximum extent possible. This has to be considered based on the cost of a vertical layout against the cost of a horizontal layout, and depends on land cost in the area where the plant is located. Bulk solids are influenced greatly by their angle of repose, and to maintain this critical angle, large-diameter conical bottom hoppers or silos would require large heights. To save on height, a single large vessel could be split into two smaller ones, but this strategy reduces headroom. When filling silos or hoppers with bulk solids, it is advisable to fill in the center so that the product forms a uniform central cone within the hopper. For two hoppers side by side with a central fill point being fed from a single vessel above, the height requirement would be much greater than if the two vessels were fed at the periphery of the vessels on top. The choice is between height and vessel filling pattern. The latter could hinder plant performance because of flow problems due to non-uniform vessel fill.
Chutes. Chutes or pipe ducts are generally used to connect unit equipment for product flow. Where the layout does not permit vertical gravity feed, the chutes have to be inclined to enable smooth material flow between units. The choice here is again between gravity chutes and chutes with suitable flow aid devices such as electric vibration, fluidized porous media or other energy inputs. Product characteristics are an important consideration in selecting chute type and design.
When equipment from different suppliers is used, different dimensional openings must be matched, and the interconnection between such openings can often create a flow problem. In many cases, equipment suppliers can make minor changes in the outlet configuration and dimensions of their equipment to suit downstream units if such adjustments are specified at the outset of the equipment order. Such preliminary specification can provide major cost savings compared to matching equipment interfaces after the plant is ordered and under installation. Once the plant is in operation, a periodic check of all interfaces between equipment will provide telltale signs when flow or throughput problems arise.
Enclosures. Typical bulk handling plants have a considerable amount of unseen ambient dust in practical conditions, and instruments and other enclosures should generally meet IP65 or higher standards. Here again, the decision should be carefully made between the environment and the cost. If the handling system generally deals with large pellets or granules with little dust entrainment, enclosure specifications can normally be to IP55. If the system is handling fine powder of around 50 microns and below, the enclosure specifications should be reviewed carefully. Dust is all-pervasive and omnipotent. It is therefore desirable to ensure that no dust can enter the instrumentation or panel box internals.
Pits and Pitfalls. Pits are well-known and popular devices in the layout of bulk handling plants. Once it is known that no headroom is available for the layout of the bulk handling plant after it is built, the first solution is often to put equipment in a pit. However, this will immediately increase costs because of the excavation, sealing, waterproofing and drainage required, in addition to the need to have a suitable cleaning system to remove dust and spillage accumulation. It is best to avoid pits, and this can only be done if the layout is planned in advance of placing equipment purchase orders.
Conveying Lines. The best pneumatic conveying system is one that has no bends, since bends contribute to a major portion of the energy consumption and are also the cause of most choking and pressure drop problems. Deciding on and modifying plant layouts well in advance of equipment ordering can enable an elegantly routed pneumatic conveying system that does not have to veer and skirt around already installed equipment, columns, pipe racks, cable trays, steam, oil and water lines and a host of other hardware typical of a production plant. If the material handling system layout is handled as part of the overall utility planning, most problems will sort themselves out on the drawing board. Arbitrarily making plant routing changes to pneumatic conveying lines to circumvent obstacles can lead to inadequate system performance since the conveying capacity, pressure drop, etc., of pneumatic conveying systems depends on the conveying pipe routing and number of bends.
Running pneumatic conveying pipes next to hot lines, such as steam or hot oil lines, is likely to lead to problems. The heating and cooling of the conveying air can cause product settlement or caking in the lines due to moisture absorption.
Maintenance Platforms and Support Structures. Because of the variety of equipment from different suppliers typically used in a bulk handling plant, the maintenance areas and access to these areas for each piece of equipment should be part of the design. In some cases, access platform and support structures may need to be custom-built to allow maintenance personnel to approach the equipment for maintenance. Adequate preplanning can also solve many maintenance problems. For example, long filter elements need to have almost one and a half times the unit height above them for filter removal, but this can often pose problems due to the unavailability of headroom. If this lack of headroom is communicated to the filter supplier, optional side-removal units can often be supplied.
Wear and Abrasion. Bulk handling plants are extremely susceptible to performance problems due to the inherent nature of the products being handled. Not only should the plant be designed to perform the tasks it is meant to perform, but it should also be built around the properties of the material it will be handling. Materials like fly ash and silica sand have excellent flow properties, but their abrasive nature can create major wear problems. Even the specification of the bags containing the bulk material will play a role in determining plant performance. When cutting paper bags in a bag slitting machine, for example, the abrasive nature of the paper can wear down the machine's knife hub and lead to problems.
Bends. Bends in pneumatic conveying lines are the most likely candidates for rapid wear, especially when the conveying is in dilute phase suspension flow mode. Various methods of preventing and minimizing bend erosion are available and need to be built into the plant at the engineering stage. Vertical bends should also have a drop leg on the vertical riser to enable cleaning of choked lines.
For long horizontal pneumatic conveying lines, it is advisable to have small internally threaded sockets welded at an angle at 2 to 3 meter intervals. These will be capped under normal circumstances and can allow jetting of compressed air into the conveying line when the line has choked. This avoids the need to dismantle large sections of lines to clear chokes.