P&Q University Lesson 10- Conveying & Material Handling

By |  September 25, 2015
Conveyors transport material to the various segments of a plant.

Conveyors transport material to the various
segments of a plant.

Today’s fully automated material-handling systems are cost-efficient and on-target when building stockpiles in any desired volume or configuration – when loading bins, silos and surge tunnels; when transferring material overland; or when streamlining loading, unloading and stockpiling at barge, railcar and marine distribution centers. Customized conveying and unloading equipment is designed to minimize truck and loader use and lower costs per ton, while ensuring product quality and meeting an operation’s exact parameters.

While haul trucks and wheel loaders had long been the standard for material handling and transport, rising fuel, maintenance and labor expenditures began adding significantly to costs per ton. Studies indicate that lifetime (8,000 to 12,000 hours) loader owning and operating costs are no less than 2.25 times higher than the unit’s initial purchase price – and that figure does not include labor and fuel costs. Using a loader is a very expensive way to stockpile, as loader component and tire wear accelerates when the machine is operated on an incline. It is best to limit haul truck and loader use to level applications, as opposed to stockpiling.

Beyond the significant short- and long-term savings, conveying and unloading systems tackle applications unreachable by trucks – such as inclined applications and dredging or waterfront applications. Overall, modern material-handling systems deliver greater efficiency based upon these three key benefits:

1. Improved product quality – Conveyors and truck unloaders eliminate the multiple material handling, while preventing the compaction and contamination typically caused by trucks, loaders or both.

2. Lower operating expense – Conveyors and truck unloaders cut labor and training costs. They are not reliant upon humans, require no breaks or shift changes, and will operate at maximum efficiency during every hour of operation. Plus, they have a lifespan of more than 20 years.

3. Limited inflationary effect – Rising fuel and energy prices have little effect on conveyor operating costs. Conveyors and truck unloaders are not sensitive to fuel shortages. Consider that electricity costs are fairly stable compared to diesel prices, and conveyors can move material during off-peak energy intervals.

Overland conveyors can take on inclines of up to 35 degrees.

Overland conveyors can take on inclines of
up to 35 degrees.

TELESCOPING STACKERS VS. CONVENTIONAL RADIAL

Automated telescoping radial stacking conveyors are one of the most important advancements in material handling over the past decade. Producers choose road-portable telescoping radial stacking conveyors over conventional radial stackers to achieve larger stockpile volumes, to solve unique stockpiling challenges and to hit precise loading targets.

Importantly, telescoping conveyors allow producers to overcome costly material segregation, which is the separation of material by particle size. Different applications of aggregate products require very specific and consistent material gradations. Segregation causes excessive variation in the product gradation. To those operations where segregation is an issue, automated telescoping stackers prevent deduct penalties and the costs associated with restoring previously in-spec material.

A standard radial stacker can help minimize stockpile segregation, but it cannot overcome it. An automated telescoping radial stacker is the only solution to creating a fully desegregated stockpile. It stockpiles in very thin lifts, or layers, with each layer consisting of a series of windrows of material. To accomplish this, the conveyor is in motion continuously – so automation is highly preferred over manual operation.

The telescoping conveyor consists of a conveyor (stinger conveyor) mounted inside an outer conveyor of similar length. The stinger conveyor can move linearly along the length of the outer conveyor, thereby varying the location of the discharge pulley. The height of the discharge pulley is variable as well as the radial position of the conveyor. The three-axis variation of the discharge pulley is essential in making the layered pile that overcomes segregation.

Depending upon conveyor size, automated telescoping conveyors can stockpile up to 30 percent more material than standard radial stackers. This is particularly important to the portable contractor who wants to build the largest stockpile possible before relocating, or to the producer who wishes to maximize stockpile volume on a site with limited space. Telescoping conveyors also can be programmed to create stockpiles of many different shapes, sizes and configurations. Perhaps an unusual site layout is more suited to a rectangular or pie-shaped pile, or a producer may want to stockpile inline over a reclaim tunnel for more live storage. This stockpiling flexibility results in higher production capacity at a lower cost.

Furthermore, the customized automation packages available to a telescoping radial stacker extend its capabilities far beyond stockpiling. For instance, a ready-mix producer has programmed a telescoping conveyor to feed each of his four aggregate storage silos. The conveyor automatically hits the exact mark on any one of the feed openings in the order desired. Consider that conventional radial stackers cannot build the linear piles needed when loading into a railcar or onto a barge. In the past, this would have required a stationary system with multiple conveyors. Now, the telescoping conveyor can be programmed to feed or load into bins, barges, rail cars or ships.

PORTABLE CONVEYING SYSTEMS

Within the portable arena where contract crushers, recyclers, excavators, and material suppliers must move from site to site, portable conveying systems streamline material unloading, transfer and stockpiling processes. The latest offerings in highly mobile material-handling systems play a major role in the successful and profitable portable operation. Here are four examples.

1. Truck-unloading systems – Road-portable truck-unloading systems can quickly transfer material from belly-dump or end-dump trucks onto a conveyor or into another truck. Consider truck-unloading systems (a height of 25 in. or less) that require only a minimal earth ramp, or none at all. On the other hand, conventional truck unloaders (at heights from 5 to 9 feet) typically demand a full day of ramp construction. That means eight to 10 hours of loader and operator time to move and configure more than 1,000 cu. yd. of material at an approximate cost of $2,000 each time a ramp is built.

2. Self-contained conveyors – Highly mobile, self-contained conveyors are diesel-over-hydraulic units that do not require electrical power. They deliver efficient stockpiling in remote locations. A complement to track-mounted crushing and screening plants, the self-contained conveyor delivers full-system portability. With a diesel engine mounted to its undercarriage and hydraulic operation of the head section and axle jacks, self-contained units are simple to move and are up and running in minutes. These units may be a good fit for certain in-pit portable setups, or for contractors tackling small, on-site processing contracts where base fill products are being generated right on the construction site.

3. Portable transfer conveyors – There are a number of innovative portable-transfer conveyor systems available. Stackable for cost-efficient transport, road-portable jump conveyors allow flexibility on jobsites as each unit can be removed from the line one at a time as the stockpile grows. Units with adjustable-height axle systems allow accurate feed into varying feed heights. There are also extendable-type conveyors, which eliminate the need for multiple transfer units. These extendable models feature a stinger that telescopes via a hydraulic cable winch system. Producers have the ability to automatically extend or retract just one adjustable unit to meet the required length of the stockpile – rather than having a loader operator continually shifting multiple units.

4. Low-profile telescoping radial stackers – Telescoping radial stacking conveyors are available in low-profile models, which maximize ease of setup, teardown and transport. They feature loading hoppers that fit directly under the crusher. This eliminates the transfer or discharge conveyor that is normally retrofitted to the portable plant. That means fewer loads to transport and fewer pieces of equipment to maintain on the site.

Idlers are rollers that transfer the conveyor motion while supporting the belt.

Idlers are rollers that transfer the conveyor motion while supporting the belt.

OVERLAND SYSTEMS

Overland conveyors offer cost-effective material transport and a wide spectrum of capacities that can vary from a trickle of material all the way up to 30,000 tons per hour. When used in place of truck transport, operations are significantly reducing fuel costs and expenses due to labor, workers’ compensation, Mine Safety and Health Administration training, emissions, maintenance and engine depreciation. Consider that a Texas-based mining operation reports that its 2-mile-long overland conveyor eliminates more than 140,000 truck trips and 570,000 vehicle miles on an annual basis.

Another key driver in an overland-conveying trend is ever-tightening environmental regulations. Haul trucks and loaders emit and stir pollution along the entire transfer path. As production sites expand and encroach upon suburban development, concerns arise over dust, noise and traffic. As such, quarry owners are often involved in decade-long negotiations over leasing and permitting. Community concerns could be eased by the fact that an overland conveyor offers quiet material transport, and may blend in with the environment.

Lastly, there are immovable issues such as the lay of the land. Haul trucks are not designed to tackle grades much above 6 percent. Operations are limiting truck transport to the area between the working face and the pit crusher as haul trucks are suitable for level applications alone. Current overland conveyor technology can take on inclines up to 35 degrees.

CONVEYOR BELTS STRETCH

There are many grades of rubber conveyor belt on the market with differing elasticity characteristics. Establishing and maintaining optimal belt tension can be one of the major challenges of a conveyor belt system. A conveyor system with inadequate belt tension may cause the drive pulley to slip inside the belt and possibly damage the belt or other drive components, and may also cause belt-tracking problems. Belt tension is typically maintained in one of two ways. A manual or screw take-up frame is used on the tail pulley bearings on conveyors that are shorter than 150 ft. long. The belt is tensioned by applying manual or hydraulic pressure and sliding the tail pulley back.

On longer systems, the belt is tensioned using a gravity take-up assembly. This system relies on gravity to tension the belt. An engineered amount of weight is hung below the take-up pulley. This keeps consistent downward pressure on the pulley, which keeps the conveyor belt tight throughout the system.

CONVEYOR BELT TRACKING

Poor belt tracking is one of the most common problems that a maintenance department will have with a belt conveyor. Common causes that lead to belt-tracking problems are:
■ Conveyor on uneven ground.
■ Crooked or bent structure.
■ Poor belt splice.
■ Uneven loading of material onto the belt.
■ Idlers and pulleys not square to the structure.
■ Material build-up on the return rollers.
■ Incorrect crown on pulleys (worn shell or lagging).

Belt tracking can be assisted by correctly installing belt-training idlers onto the trough side and the return side of the belt. Trough training assemblies typically have two urethane hourglass-shaped guide rollers that the belt contacts before the trough assembly, forcing the entire top assembly to pivot and steer the belt back straight. Return training assemblies with guide rollers or single-roll return training rollers should also be used on the return side of the belt.

Maintaining optimal belt tension is a major challenge of a conveyor belt system.

Maintaining optimal belt tension is a major challenge of a conveyor belt system.

IMPACT BEDS

A new and fairly common component on a conveyor is an impact bed used primarily in the load zones. The major advantage of an impact bed is that its stronger frame design will accept a lot more abuse from heavy load and impact. The other major advantage is the ability to skirt the load zone down to a flat surface (the impact bar) rather than individual rollers. This allows the load zone to be completely sealed. With impact idlers, there is always a gap in the skirting between the rollers where material can escape. With an impact bed and proper skirt board, monitoring spillage problems can be eliminated. An impact bed is designed using flat impact bars made of either urethane or rubber with an ultra-high-molecular-weight (UHMW) cap. The UHMW is a wear surface with a low coefficient of friction. The sliding surface and correct impact bed installation is important to prevent friction heat buildup that will result in premature impact bar wear.

CONVEYOR COMPONENTS

Well-specified conveyor components, such as idlers and pulleys, are integral to performance efficiency, maximum uptime, and minimal maintenance – as well as to the prevention of costly belt damage.

IDLERS

Basically, an idler is a roller that transfers the conveyor motion, while supporting the belt. There are two types of idlers: carrying and return idlers. However, within these categories there are a number of idler designs to manage the impact upon the belt and to meet a variety of application requirements. While conventional idler rolls are made of steel, recent developments have led to the use of a specially formulated high-density polyethylene material and a specially formulated polyurethane material. These types of rolls last up to three times longer than steel rolls, prevent material build-up and belt mistracking, and minimize noise during operation.

Troughing idlers are found on the carrying side, along the length of the conveyor. They comprise a center idler roll with wing idlers on either side. The wing idlers are set at an angle known as the troughing angle. This angle ensures that the load-bearing capacity of the belt is the same along the conveyor’s full length and that the material load will transfer without spillage.

With return or bottom idlers, the mass of the return belt is the only load that return idlers are required to support. As such, return idler sets are spaced at two to three times the pitch of their equivalent carrying-side idler sets.

A conveyor belt cleaner removes stuck-on material from the belt.

A conveyor belt cleaner removes stuck-on material from the belt.

To keep the belt properly aligned and to prevent costly belt damage, self-aligning idlers or return training idlers are able to detect belt misalignment and automatically re-align the belt. Self-aligning idlers are a lower-cost belt-training solution normally manufactured for both the carry and return side of the conveyor belt. When belting mistracks, it contacts the side guide rollers causing the self-aligner to pivot.

The automatic return training idler provides continuous alignment, centering the belt and reducing or eliminating any belt damage. Automatic return training idlers are particularly effective for operators of portable equipment or mobile track-mounted equipment. They are an ideal choice in a variety of tough applications, or in applications where self-aligning idlers do not fit. Return training idlers can be applied to either high- or low-end belts.

The market is shifting toward the use of replacement and retrofit rolls over the practice of discarding the entire idler. Typically, idler malfunction involves only one of three rolls – and if producers choose to replace the entire idler due to time and labor factors, they are wasting 75 percent of their investment. Everyone wants to minimize conveyor downtime and few want to move the belt out of the way to replace one roll. Consequently, many operations are choosing to replace rolls in the shop during maintenance hours. The task is far simpler than it used to be, made easier by new idler design innovations that streamline installation. Plus, growing replacement roll demand has resulted in the affordability of built-to-order retrofit rolls that fit any frame brand or type. The use of readily available, adaptable, and easily changeable replacement rolls will undoubtedly cut costs.

The heartbeat of an idler lies in the integrity of its seal and the protection of the bearings. Since an idler’s seal is typically its first fail point, it’s always best to closely scrutinize seal design. For example, current seal innovations may involve improved contact seals that result in less seal drag, triple-labyrinth seals that increase the distance that contaminants must travel, and grease fills that easily trap contaminants.

PULLEYS

A pulley is a rotating cylinder mounted on a central shaft that is used to drive, change direction of or maintain tension on a conveyor belt. There are many pulley designs with the main categories being drum and wing pulleys. The drum pulley has a uniform diameter from side to side. The wing pulley supports the belt on individual vanes that taper down from the inside to the outside to direct stray material out of the pulley and off to the sides.

In comparison to the standard wing pulley, the recently developed V-shaped wing pulley deflects fugitive material, prevents wing bending and belt damage, and provides a more smooth operation with less vibration, less noise, and less of an impact and load on the bearings. The V-shaped wing pulley has a round end view, which allows continuous belt contact. The belt is essentially rolling around a circle, which significantly reduces any beating action on the belt. On the other hand, the standard wing pulley is more prone to material buildup. It is also is shaped as a many-sided polygon. So the belt is always raising and lowering over one or two wings as the pulley is going around, which causes the belt to flap and results in more belt wear and much more vibration and noise.

Conveyor pulleys are often covered with some form of rubber, fabric, urethane, ceramic or other material. Lagging increases the traction between the belt and the pulley, which in turn reduces load and wear on the drive, belt, pulleys, bearings and take-up. The Conveyor Equipment Manufacturers Association recommends that lagging be used for all drive pulleys, particularly those in high-tension applications. Non-driving pulleys – especially on the carrying side of the belt – should be lagged in abrasive environments.

A common misconception is that oversized equipment lasts longer and requires less maintenance. In truth, systems should be sized to the expected load (maximum lump size), while allowing for a small capacity increase. For example, an oversized belt will result in additional belt and idler rolling resistance and less energy efficiency. The pulley shaft must be large enough to function without failing, yet not overkill. Obviously, larger shafts require larger hubs, bushings and bearings, so costs increase significantly with size.

Some producers will oversize a motor, thinking it will require less maintenance. However, the energy savings realized from a properly sized motor will outperform any maintenance savings derived from a larger model. Motor size should be matched to the horsepower requirements of the load. Always work closely with your selected manufacturer to properly size and specify conveyor components.

Overland conveyors offer cost-effective material transport.

Overland conveyors offer cost-effective material transport.

MAXIMIZING CONVEYOR UPTIME

When processing sand, gravel, minerals or other bulk material, a belt conveyor is a critical piece of equipment. When it’s down for planned or unplanned maintenance, a facility has only a short period of time before production is halted. Therefore, keeping a conveyor running for long predictable periods of time is essential to high productivity. Here is some insight on keeping belt conveyors running efficiently.

A belt conveyor consists of a structural framework (most are a lattice design), an endless belt guided on idlers, a motor and gearbox, and the tail, idler, return idler, tension, drive and head pulleys. The primary point of failure is often the pulleys. And when one of them has a bearing problem, the belt stops moving – even though the bearings are often not the root cause of the failure. Proper diagnosis of the problem can help increase bearing life and extend a conveyor’s overall uptime.

The main loads on a pulley bearing come from the belt pull, the weight of the pulley, the belt and the payload. Payloads have been increasing in recent years due to improvements in belt design and the use of stronger materials. There may also be a small thrust load originating from the belt, riding against the lateral belt guide rollers. In the case of a belt track problem, the common solution is to mount fixed bearings on both ends of the tail pulley. This results in additional thrust loads on both bearings. Thermal expansion of the shaft further increases the thrust loads.

As a rule, the bearing journal diameter of conveyor pulleys is predetermined by rigidity requirements set by the manufacturer. With drive pulleys, the journal diameter also depends on the driving torque. The rigidity of the journal must be such that it prevents the seals from fouling the housing covers. In addition, the journal diameter determines the bore diameter of the bearing.

Tolerances for the journal diameter are also set by the bearing manufacturer and should be followed closely. Mounting a bearing on an undersized journal will allow the bearing and adapter sleeve to creep. This generates heat and rapid wear of the journal and bearing components. Taking the time to replace a pulley with proper journals will ultimately result in an increase in uptime and productivity.

SAF pillow blocks are mounted predominantly on drive pulleys, but are also often used in head and tail pulleys. To accommodate long tensioning distances, they are sometimes used on take-up pulleys, in which case the pulley and housings are located on a carriage. Look for pillow blocks with labyrinth seals – a one-piece design made of machined aluminum or steel. The labyrinth gaps allow for shaft deflection of just over 1 degree from the true center position without the labyrinths fouling the pillow block or the shaft. Also available is a wiper/flinger seal that acts in conjunction with labyrinth seals to further prevent contamination. Taconite seals are another option for severely contaminated environments.

Certain housing features also help resist wear and improve pillow block performance. Examples include two-piece housing bodies made of cast iron with a reinforced base. Small housings have two mounting bolt slots, or four bolt slots as an option. The slotted-hole design allows for easier alignment of the pulley. Additional features include an easy-to-read temperature strip mounted to the pillow blocks to alert users of potential failure due to overheating. Blocks also contain strategically located predrilled and tapped holes to allow for remote monitoring.

Split pillow block housings should be able to accommodate straight or tapered bore spherical roller bearings. Tapered bores are almost always secured on the journal by means of a tapered sleeve. Adapter and withdrawal sleeves facilitate easier mounting and dismounting of the bearings and can allow the bearings to be changed out on-site.

For these installations, the bearings are pushed up the tapered outside diameter of the sleeves until the proper amount of interference fit secures the bearing and sleeve to the journal. To determine if the bearing has an adequate amount of interference, measure the radial clearance reduction as the bearing is being mounted. Small bearings can be mounted using specialized spanner wrenches. For medium and large bearings, use hydraulic nuts. Medium and large sleeves are also available with hydraulic removal features.

When mounting bearings, keep these seven things in mind.

Vibratory feeders handle applications that require controlled feeding of difficult materials.

Vibratory feeders handle applications that require controlled
feeding of difficult materials.

1. Do not unpackage the bearings until you are prepared to mount. Make sure your work area, your hands and your tools are as clean as possible – have clean gloves, rags (shop cloths) and hand cleaner available to avoid getting any contaminants in the bearing.

2. Before mounting, inspect the pulley journals and measure their diameters. Measure the journal dimensions at four points circumferentially and three points axially. The circumferential measurements will determine roundness, while comparing axial measurements along the same plane will determine taper. Averaging all of the measurements together will determine size. As a rule, the tolerance for roundness and taper is half the journal tolerance.

If any of these dimensions are out of spec, the journal (pulley) should be replaced. The journals at the seal position likewise need to be in good condition. Check for wear or damage that may prevent adequate contact or create too much clearance for a labyrinth seal. These circumstances also require pulley replacement. If the journals on both ends of the pulley pass inspection, record this information on a journal inspection chart.

As well, it’s always best to use a new sleeve for each installation. When reusing housings, inspect them thoroughly and look for wear or damage on the bearing seat and seal surfaces. Also check the base and around the block for cracks, breaks or wear. If any such problems are found, replace the housing. The mounting surface on the housing frame should also be flat and free of damage to provide proper support, and should be repaired if necessary.

3. Straight-bore bearings require an interference fit to the journal. The safest and easiest method of mounting this type of bearing is to use an induction heater or, as an alternative, an oil bath heater. Make sure the oil is free of sludge and contaminants. Never heat the bearing more than 250 degrees. Never use a torch to heat the bearing.

4. Avoid pushing against the bearing cage or the rollers during installation.

5. Fully saturate packing-type seals with oil before fitting. Lubricate the lips of oil seals to prevent damage and make them easier to install.

6. Pack the labyrinth grooves with grease and make sure they are in the proper housing grooves.

7. Take measurements to ensure the bearing’s correct location on the journal. This will prevent an abnormal thrust load on the bearings.

CONVEYOR BEARING LUBRICATION

Conveyor bearings are lubricated with grease or oil. Oil should have antioxidants and defoaming agents, and extreme pressure additives. Its viscosity at operating temperature should be 17 to 25 cSt. Check the oil level and condition periodically to determine a suitable change interval – it generally varies from one to six months depending on operating conditions. Use oil analysis to check for contaminants, oxidation and overall quality.

If grease is used, select a lithium-complex grease with an oil that is a VG220 grade or higher. The NLGI grade depends on the ambient temperature. If the conveyor is outdoors and in a northern climate, consider using different grades for winter and summer.

It’s better to set the re-lube intervals to a smaller quantity more often than to re-grease with a large quantity. Excess grease can lead to overheating, which accelerates bearing deterioration. It’s important to note that in most applications, conveyor bearings do not run hot unless there is a problem with the bearing installation, operating conditions or both. See the conveyor manufacturer’s instruction manual for recommended grease fill amounts and replenishment intervals.

The use of oil and grease commonly requires extensive maintenance, so consider alternative solutions. NSK’s Molded Oil is available in normal-operating and high-temperature versions. They minimize and can eliminate the need for lubrication, and they offer bearings added protection against solid and liquid contaminants.

The market is shifting toward using replacement or retrofit rolls versus discarding entire idlers.

The market is shifting toward using replacement or retrofit rolls
versus discarding entire idlers.

Slurry pumping

There are many factors that must be considered for a given pumping application, especially when it comes to pumping slurries. The intent of this information is to give you a basic understanding of slurry pumping and to help you select the pump that is best for your application. We will discuss what a slurry pump is and delve into the fundamental principles of centrifugal slurry pumps, selection and sump design.

First off, what is a slurry pump and what makes it different from a water pump? A slurry pump is designed to handle large solids within a slurry and the slurry is often at high concentrations. In a grinding circuit it is normal for slurry pumps to pump at concentrations approaching 65 percent solids by weight. In order to do this, slurry pumps are much larger and heavier. They have large internal passages and are constructed of special abrasion-resistant materials that are much thicker than those of water pumps.

There are few major wet end components to any centrifugal pump. They are the casing, impeller, side liners and gland seal.

There are three types of casings: volute, semi-volute and concentric. Each casing type has its own advantage and there are tradeoffs between each. The volute casing is the most efficient casing design. It is a spiral design. As the distance between the impeller perimeter and casing increases, the slurry velocity decreases and the pressure increases. This is the most common design used today. It is also the most efficient design. In a concentric casing design, the distance between the impeller and casing is the same throughout the pump. This design is best for high-pressure pumps because the pressure between the impeller and internal walls of the casing is kept constant, which reduces shaft deflection. This reduces problems around the gland area, which are common in high-pressure applications.

There are two main impeller designs: open and closed. An open impeller is an inefficient design that is rarely used. It is an inefficient design because it is not capable of stopping suction side recirculation, due to the fact there is no front shroud. A closed impeller is the most common style impeller used today. The purpose of the impeller is to impart the energy from the motor through the shaft onto the slurry. The impeller must be balanced and fixed to the shaft. For most slurry pump applications statically balancing the impeller is sufficient.

Telescoping radial stackers can stockpile more material than standard radial stackers.

Telescoping radial stackers can stockpile more material than
standard radial stackers.

The diameter of the impeller affects the head produced. Head is basically the same as pressure, but head takes into account the slurry’s specific gravity. The larger the diameter for a given speed the greater the head produced, whereas the depth of the impeller affects the flow rate.

Therefore the deeper the impeller for a given speed the greater the flow rate. As an example, a large diameter impeller running at a slow speed could produce the same head as a small diameter impeller running much faster. Impellers also vary by the number of vanes in the impeller. In most slurry pumps there are generally three, four or five vane impellers. The more vanes an impeller has, the more efficient the impeller is and the more head it will produce at a given flow rate.

The tradeoff is that with more vanes in the impeller, it cannot pass as large of a solid due to the reduced distance between vanes. A three-vane impeller is capable of passing a larger solid than a four vane, but it will produce less head for a given flow rate. The depth of the impeller is the other limiting factor to the maximum particle size the impeller can pass.

There are three types of gland seals that are used to keep the slurry from leaking out of the pump at the shaft: water flush, centrifugal and mechanical. A water flush seal consists of a stuffing box with a lantern ring, packing and a gland follower.

In this arrangement clean fresh water is pumped into the stuffing box at a pressure 5 to 10 psi greater than the slurry pump’s discharge pressure. The volume of water will vary with the pump size; it can vary from ~5-90 gpm. Consult your pump manufacturer for your specific water requirements. The packing needs to be adjusted periodically. It should be tightened, using the gland follower, until there is a constant thin steady stream of water coming out of the packing and it should be approximately the diameter of a pencil. Also the water coming out of the packing should not be hot. If it is hot, the packing is too tight and the gland follower should be loosened. When there is no adjustment left on the gland follower the packing must be replaced.

With a centrifugal seal configuration, there is an expeller behind the back side liner, which spins at the same speed as the impeller. The expeller is basically a thin open “impeller” with numerous straight vanes. As the impeller spins it creates a zone of pressure at the periphery of the expeller keeping the slurry from entering the gland.

Aggregate operations use pumps to tranport water and slurry.

Aggregate operations use pumps to tranport water and slurry.

A centrifugal seal can only be used in applications where the expeller generates more head (pressure) than there is suction head (pressure). A centrifugal seal can generate pressures that range from 5 to 20 percent of the discharge head depending on the expeller to impeller ratio, manufacturer and where the pump is operating on the curve.

A centrifugal seal will seal against the most suction head when the pump is operating at the BEP (Best Efficiency Point). Packing is also used in a centrifugal seal. The purpose of the packing is to keep the pump from leaking when it is not operating. The pump is capable of not leaking without the packing installed while running, but will start leaking once the pump is shut down. One of the major advantages of a centrifugal seal over water flush is that gland water is not required. The disadvantage is if there is too much suction head it won’t seal and if there is not enough suction head then the expeller can suck air into the pump. A centrifugal seal will also cost more than a water flush seal.

A mechanical seal consists of two hard pieces, generally tungsten carbide, pressed together by a spring. One piece is fixed to the pump casing and the other piece spins with the shaft. The seal is formed by the two pieces of tungsten carbide rubbing against each other. One of the common causes of seal face failure is trapped air around the face.

When this happens, the air pocket keeps the slurry away from the seal face and prevents the slurry from cooling the seal. The seal then overheats and cracks. Another common cause of seal failure is shaft deflection. Shaft deflection pries open the seal faces and allows solids to penetrate, which causes the seals to fail. A third cause of seal failure is high surface speed due to oversized seals that have to fit over the existing shaft sleeve. Quench water may be required to cool the seal depending on the pump speed. This water provides no sealing action and needs to be about the same pressure as your garden hose. Mechanical seals cost the most.

There are various materials of construction used in slurry pumps, but they can be classified into two main types of materials: metals and elastomers. Elastomers, such as rubber, are best applied in applications where there is a large concentration of ultrafine material (~200 mesh/ 75 micron) or there is a chemical in the slurry that would react with the metal. The problem caused by the ultrafine material is that the fine particles eat away at the micro grain structure (the bond) between the hard material, such as chrome, and the iron causing the hard, abrasion resistant material to “fall out” of the metal. Elastomers, however, should not be used in applications where the particles have sharp edges, there is tramp material in the slurry, there are oils or other hydrocarbons in the slurry and when the slurry is at a high temperature. For all other applications metal slurry pumps are generally recommended, with high chrome iron being the most common material.

Slurry pumps work under Bernoulli’s principle, which basically says for a fluid in a closed system a decrease in velocity results in an increase of pressure. So what does this mean? Once the pump is primed, meaning that the casing is full of liquid, the impeller spins, throwing the liquid outward towards the casing.

A variety of pulleys are available for various applications.

A variety of pulleys are available for various applications.

Since the liquid has no place to go, it must slow down, thus increasing in pressure according to Bernoulli’s principle. Since the liquid that was at the eye of the impeller has been thrown outward there is now a void or area of low pressure at the eye. This area of low pressure causes the new liquid to be sucked into the impeller and then thrown out. This process continues to repeat itself. The impeller imparts the energy from the drive (motor) into the liquid.

Now we know the lowest point of pressure in a pump is at the eye of the impeller and the highest point of pressure is along the casing, we need to discuss suction side recirculation. In an ideal application all the slurry that passes through the impeller would immediately discharge out of the pump, but in reality some of the slurry recirculates back through the eye of the impeller.

How does this happen? There is a direct path in slurry pumps from the point of high pressure to the point of low pressure at the eye of the impeller between the suction side and the impeller. Since liquids, like electricity, take the path of least resistance and flow from high to low pressure, some of the liquid will flow down the suction side of the pump back into the eye of the impeller. The larger the clearance between the impeller and suction side, the greater the recirculation. It is possible to have as much as 20 percent of your flow recirculate in this manner. As the slurry recirculates it takes the highest concentration (University of Kentucky Hydraulics Laboratory) of slurry down the suction side, which gets crushed consuming power and increasing wear. As the pump wears, suction-side recirculation increases causing the pump to produce less flow and head for the same speed and further accelerates wear.

This is a vicious cycle. In order to reduce suction-side recirculation some pump manufactures will put pump out vanes on the suction side of the impeller. As the impeller spins these vanes create a zone of pressure, reducing the pressure differential between the eye of the impeller and the suction side of the casing. This reduction in differential pressure reduces the amount of suction-side recirculation.

Automated telescoping radial stacking conveyors are a key advancement in material handling.

Automated telescoping radial stacking conveyors are a key
advancement in material handling.

Suction-side recirculation can be minimized through proper maintenance. This is done by making adjustments to the pump to minimize the clearance between the impeller and the suction side of the casing.

There are various ways in which this is done and it varies by pump manufacturer. Some methods require the entire pump to be disassembled and others can be adjusted in a couple of minutes while the pump is running.

The most time consuming method requires you to measure the clearance between the impeller and suction side, then disassemble the pump and remove the impeller. A spacer a little thinner than the distance measured is then placed on the shaft, the impeller replaced and the pump is reassembled. Other manufactures have you move the whole bearing assembly towards the suction side until the impeller touches. Then you back up the bearing assembly until the impeller spins freely and lock the bearing assembly back down. Then your drive needs to be realigned.

Yet another manufacturer has a special wear ring on the suction side that adjusts to the impeller. This is the easiest method and the adjustment only takes a couple of minutes while the pump is running. The other methods require some downtime. Regardless of how the pump is adjusted it is crucial to keep it adjusted to maintain the proper flow, head and to minimize wear.


Some idlers are made of impact-resistant material.

Some idlers are made of impact-resistant material.

Maintenance and material handling

The U.S. Mine Safety and Health Administration estimated that 85 percent of conveyor maintenance is a result of fugitive material spillage at transfer points. A transfer point is any point on the conveyor where material is loaded onto or unloaded from the conveyor.

And the cost reductions from eliminating spillage at just a single transfer point are significant. Material spillage is a costly proposition in tons of lost material per year, additional labor for cleanup, significant safety risks, and excessive wear and damage to conveyor components caused by any resulting belt mistracking. Operations should note these four key tips for eliminating spillage.

1. Reduce impact – Loading-zone impact causes wear and damage to the conveyor belt, weakening the belt carcass. To reduce impact at transfer points, position impact cradles under the conveyor. Or, use impact idlers at a transfer point. These are troughing idlers that have rubber-cushioned rollers to absorb impact.

2. Load in the center – Ideally, each transfer point should be designed to load the belt in the center and at a uniform rate. Off-center loading can be corrected by using systems such as deflectors, liners, baffles, screens, grizzly bars, or a curved loading chute – all of which are designed to consistently direct material flow onto the center of the belt.

3. Maintain proper belt scraper tension – For effective cleaning, belt scrapers should be installed at the appropriate locations and at the proper angles. Since multiple belt-cleaning systems are often factory-installed, operators need only concern themselves with maintaining accurate belt scraper tension and replacing worn scrapers. The majority of blade-to-belt cleaning systems feature some form of tensioning device that should typically be checked or adjusted on a weekly basis. An improperly adjusted belt scraper will lead to material carry back, premature wear to components, and eventual spillage and belt mistracking headaches.

When re-tensioning is required, the cleaner and tensioning unit should allow for easy access and maintenance, without the need for special tools or multiple service technicians. Tensioning instructions are often located on the side of the belt scraper manufacturer’s bracket.

4. Select and maintain proper skirtboards – Skirtboards are key to preventing material spillage in and around the loading zone – from the moment that material leaves the loading chute and until it reaches belt speed. So skirting usually extends from the loading chute and along some distance of the conveyor. Skirting should make slight contact with the belt, and should be mounted close enough so that the gap can be sealed with flexible rubber or urethane strips. Multiple-layer edge seals (a primary seal against the chute wall and a secondary seal that lies on the belt surface outside the chute) are best as they can contain any escaping fines.

Note that seal wear life is dependent upon minimizing belt sag, which can allow material entrapment. Wear liners should be installed inside the chute to protect the sealing strips from the forces of the load. To prevent spillage and maintain skirting life, hoppers should be lined, checked for wear areas or both.

Also, it’s important that skirtboards and loading and discharge chutes be selected and installed to match the characteristics of the material as well as the conveyor.


Eliminating tramp metal handling

Removing damaging tramp metal from aggregate processing is one of the most important procedures to enhance product purity and to protect expensive operating equipment. While there are multiple ways to search for and remove both ferrous and nonferrous tramp metals from the product stream, one of the most efficient methods is the use of industrial metal detectors. These metal detectors fit within a range of belt widths and can be field-adjusted to fit most conveyor configurations. Manufacturers also offer a variety of options that help personnel quickly locate and remove the tramp metal once the offending material passes through the detector.

How does tramp metal enter the production stream? Any place where heavy machinery is involved is a potential source for wayward metal. Examples are loading equipment where pieces of bucket wear liners or cheek plates break off and fall into the product stream. Front-end loader bucket teeth, rock bolts and even tools can find their way into the aggregate before entering transfer points, chutes and crushers.

Metal detectors act as the last line of defense before the aggregate enters a critical stage of operation within the processing plant. Prior to this stage, however, most mining and quarry operations use some type of suspended magnet positioned above the primary conveyor belt to remove any large pieces of magnetic tramp metal embedded in the aggregate. These workhorse magnets remove large amounts of ferrous materials conveyed in heavy burden depths on almost any type of conveyor or chute. They are ideal for separation applications on fine or coarse materials, large or small tonnages. The suspended magnet is usually positioned right before a transfer point so the belt – and subsequent downstream equipment – does not suffer damage caused by larger tramp metal.

The metal detector is placed after the suspended magnet and before any screens, cone or jaw crushers, transfer points or other processing equipment. Should any ferrous, nonferrous or stainless steel metal pass through, the metal detector detects it and signals the conveyor belt to stop, even when the material is conveyed on steel corded belts. This is particularly important if the suspended magnet cannot remove metal that is embedded in the rock or piece of concrete. The metal detector can locate tramp metal trapped in deep burden depths and signal operators before the metal can damage other equipment.

Most metal detectors can be set for higher detection sensitivity, ensuring that even the smallest problematic tramp metal can be detected; thus providing better protection for crushers, screeners and conveyor belts. This ability to locate metal particulates even on high-speed belts saves operators time and money because of less equipment downtime.

Metal detectors also can be used effectively on highly conductive products such as gold, copper and iron ore, giving operators far more flexibility and protection.

Once installed, these metal-detection systems operate reliably for long periods of time, frequently in adverse environments, with little attention or maintenance. The power required to operate both the metal detector and reject device is minimal; a standard 115-volt or 230-volt line is sufficient.


Sources:

Contributors to this chapter include the following, in alphabetical order:

Steve Cook
Sales Manager
Luff Industries Ltd.

Eriez Manufacturing Co.

Jarrod Felton
Vice President of Sales, Marketing & Engineering
Superior Industries Inc.

FLSmidth Krebs

NSK Americas


Lesson 10 Quiz

1. According to recent studies, how much more expensive are the owning and operating costs of a haul truck or wheel loader than its purchase price?

2. What do you call the rollers that transfer the conveyor motion while supporting the belt?

3. What are the two categories of idlers?

4. Name some common causes of belt-tracking problems.

5. What is a common misconception about oversized equipment?

6. Which can stockpile more material: standard radial stackers or telescoping radial stackers?

7. What is used in aggregate operations to transport water and slurry?

8. How does tramp metal enter the production stream?

 

Click here for the quiz answers.

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