Modern conveyors are often complex systems with many interrelated components; failure of just one can lead to a belt shutdown and loss of production. Proper design, inspection and maintenance are critical.
MINES DEPEND ON CONVEYORS FOR A MULTITUDE OF MATERIALS HANDLING TASKS—AND THE NEWEST SYSTEMS ARE BEING ASKED TO PERFORM AT HIGHER WORKLOADS THAN EVER BEFORE.IS THE TECHNOLOGY UP TO THE TASK?
By Russell A. Carter, Managing Editor
Conveyor systems are a segment of mining technology that demands innovation—because the industry itself demands it. As mines become deeper, more remote and more expensive, end users are faced with a Hobson's choice of increased maintenance attention or higher downtime on conveyor systems that may, by necessity, be longer, more powerful and more complex than their predecessors—so the higher performance offered by sophisticated but expensive new conveyor technology must be accompanied by higher reliability to balance the cost-versus-benefit equation. At a conventional truck haulage operation, the loss of a truck or two may only superficially affect short-term production. Mines employing truly large in-pit or overland conveyor systems, for example, don't have that safety margin; a broken conveyor under those circumstances can bring production to a complete halt, perhaps for weeks at a time.
However, there is no doubt that conveyor systems and components are getting better, and with these advances comes higher end-user interest, wider project scope for new products and services, and increased investment in the technology. The Suppliers Report section of this issue, for example, contains news concerning Vale's S11D mine development—a massive project planned around the use of in-pit and overland conveyors, stackers, reclaimers and other bulk materials handling equipment that will be used from day one of the new mine's operation, instead of conventional truck haulage. At Codelco's El Teniente copper mine in Chile, ABB and Tenova TAKRAF have joined as technology development partners in a project to engineer and install an almost 12-km-long, 12,000-metric-ton-per-hour (mt/h), three-segment, regenerative system that will run uphill at an angle of 15° in some portions.
The push to overcome rolling resistance along the belt lines, reduce electrical energy consumption, and improve system reliability and longevity that drives this project is typical of the targets that conveyor manufacturing and engineering companies have set their sights on-generating recent innovations that apply to conveyor system conception, design, operation, maintenance and system health.
Gearless Drives for High-capacity Conveyors
In a recent white paper, TAKRAF explored the ways in which demands on belt conveyors in hard rock mining are shaping the development of conveyor components at the upper end of the performance spectrum. The performance requirements of belt conveyors are being redefined by the lower-grade ore and deeper deposits that characterize today's industry trends. This leads to new requirements for belt performance—and, similar new requirements with regard to conveyor geometries and mass flows make it necessary to think about alternative drive concepts. Gearless drives, for example, enable the use of larger drive units and are increasingly being looked at to power belt conveyors in place of conventional gear drives.
According to TAKRAF, motor efficiency curves in relation to motor torque for gearless drives show values that consistently surpass the characteristics of traditional conveyor drives. In addition, traditional maintenance of gear units—including oil change intervals—can be eliminated. This is of particular interest at operations where maintenance activities are difficult.
When awarded the contract for the conveyor system associated with the expansion of Codelco's El Teniente mine in 2012, TAKRAF turned to power technology specialist ABB with the aim of jointly developing a motor and drive concept that would meet client specifications and site conditions. They determined that certain designs might have merit as gearless drive motor candidates. These included:
Bearing-less motors: The rotor is directly connected to the drive pulley shaft by means of a flange connection. The stator is supported separately.
Bearing motors: The motor has two bearings. The stator is secured to the foundation or steel structure. A flexible coupling is used to facilitate the transfer of torque between the rotor and the drive pulley.
A closer look at bearing-less motors highlighted certain disadvantages, such as:
The need for a special drive pulley design to decrease shaft deflection.
The need to assemble a motor on site, because the rotor first has to be connected with the pulley shaft and the stator is then moved over the rotor. This means that the motor must be disassembled for shipment after factory assembly and completion of its test run.
In case of damage, the motor cannot be disconnected quickly.
These drawbacks prompted TAKRAF and ABB to focus on designing a motor with separate bearings. In this approach, the motor and drive pulley are connected by a flexible coupling. The overall aim is to reduce the amount of maintenance work in the field to an absolute minimum so that a faulty drive can be simply replaced and repaired in the shop. In order to accomplish this, the motor is mounted on a specially designed base frame.
The drive motors built for the El Teniente conveyor project will each have an output of 2,500 kW. Following extensive discussions with Codelco, the bearing motor concept described above was implemented for this contract. Commissioning of the first unit is slated for 2014.
Tracking Conveyor Problems: Not So Simple
It's a universal axiom in conveyor maintenance that big trouble almost always begins as a small problem—a tiny rip, a slightly misaligned pulley, a dry bearing or something similar. But, as Honeywell's Process Solutions group pointed out, detecting problems and managing the health of belt conveyors is a complex activity that can include at least a dozen separate requirements, including:
Cover defect, edge damage, spice damage and belt rip detection;
Pulley and idler health detection;
Drive health monitoring;
Belt wear, steel cord damage and other internal defect detection;
Material offloading and chute blocking; and
Belt wander detection.
About a year ago, Honeywell introduced its Belt Asset Inspection System (BeltAIS), a product suite it is developing to handle belt cover defect, idler and belt wear monitoring. Recognizing that there are a number of monitoring solutions that address specific aspects of conveyor system components and operations, Honeywell maintains that these solutions may provide a high volume of measurement data but also may not lend themselves to the integrated solutions that many of its mining customers have come to prefer.
BeltAIS, said Honeywell, offers an alternative to multiple standalone solutions. Users can employ it to maintain a continuous, online view of conveyor operations and perform a wide range of monitoring and analysis functions. The system, according to the company, will provide an intuitive, user-friendly interface to the conveyor that can boost the effectiveness of maintenance and operational personnel.
To implement its initial BeltAIS product, the Cover Defect Monitoring solution, Honeywell turned to an existing camera-based solution developed for pulp and paper producers to check for defects in the papermaking process. For its mining application, Honeywell said the system offers highly localized analysis for monitoring defects along a conveyor belt.
In a similar vein, ContiTech announced in June that it had developed innovative electronic warning and inspection systems to detect conveyor problems at early stages, providing a way to protect conveyor systems from major damage. Its CONTI PROTECT splice elongation measurement can detect irregularities in belt splice length with the help of magnetic markers, while its belt rip detection system finds longitudinal rips in conveyor belts early on by means of vulcanized conductor loops. The belt warning system's improved technology is claimed to eliminate false warnings and thus unneeded system downtime. Online support is also available.
ContiTech also said that its CONTI INSPECT systems supply data that can be used to make reliable prognoses regarding remaining belt service life and estimate future investment costs more accurately. A mobile belt thickness measurement system measures the thickness of the conveyor belt across the entire length of the belt. Surface damage can be detected at an early stage with its continuous surface inspection system, which employs sophisticated scanning technology to inspect and compile a detailed image of belt surface quality. In addition, the company said its cord monitoring system can monitor steel cord within the carcass using a magnetic-inductive procedure.
Martin Engineering, a well-known supplier of bulk material handling equipment, offers a somewhat different approach to help conveyor users reduce operating costs and improve safety, with inspection and maintenance programs designed specifically for each individual system. Its "Walk the Belt" initiative provides regularly scheduled reviews of belts, cleaners, tracking, chutes, dust control and other components by experienced specialists. The program establishes an evolving record of each belt for current analysis and future reference, and provides immediate transfer of data and photos to facility managers. By taking responsibility for routine maintenance and identifying potential issues before components fail, technicians assist customers in maintaining system performance and extending service life, while minimizing fugitive material and unplanned shutdowns.
"Every conveyor is different, even with-in the same facility," observed Martin Engineering Global Market Development Manager Mark Stern. "So, we create a specific inspection plan based on the design, capacity, throughput requirements and the desired level of fugitive material abatement."
Stern said that while it's common for conveyor owners to perform service on their systems only when a component fails, it's actually less expensive in the long run to incorporate continuous maintenance into a plant's operational plan.
The Martin Engineering technicians do more than just walk along the length of the belt: one of their inspection techniques is standing stationary at a number of points along each conveyor and watching one or more complete revolutions of the belt, noting its condition, tracking, carryback and other observations. They take detailed notes, identifying trouble spots by component name and location as they are found, often logging data directly into a smart phone or tablet immediately at the site. Digital cameras and other devices allow the technicians to take photos, record video, make rough measurements and perform other tasks, then share the information with operations personnel. Maintenance and repair tasks can be included in the program and performed routinely, or scheduled upon request.
Safe Parking for Loaded Belts
A heavily laden conveyor can have an enormous amount of potential energy, and has to be treated with caution if accidents are to be avoided. Steve Powell, product manager for Twiflex Ltd., recently commented on the benefits of his company's "parked-off" conveyor brakes, and how they can improve safety during maintenance.
Powell explained that a parking brake is a key safety element in conveyor systems, used to lock the conveyor into one position when idle, while being serviced or during a power outage. This reduces the possibility of the conveyor starting to move under its own load, a situation that could become very dangerous, very quickly.
To account for the possibility of a total power failure, most parking brakes are spring applied. This means that when no external power is supplied to them, the pressure of the spring is applied to the pad and the brake clamps shut, thereby locking the conveyor in a fixed position.
However, there is a downside that needs to be considered: although a spring-applied brake offers protection in the event of a power failure, it also makes maintenance a far harder task. When a spring-applied brake is powered down ready for maintenance work to be carried out, the brake is tight against the disc; the springs need to be compressed to access the pads, which leaves a lot of potential energy in the brake.
To address this issue, Twiflex has incorporated a unique feature into its conveyor brake products which addresses the risk of brakes clamping shut unexpectedly during maintenance, an occurrence that can pose a hazard to personnel. The "parked off" feature can quickly be applied while the brake is in the field and allows for maintenance and pad removal without the risk of the brake clamping shut.
Conventional industrial brake design typically employs a mechanical lock-out concept in which a nut and center bolt arrangement is used to hold the spring force and prevent the disc from closing during maintenance. However, this lock-out arrangement only works as long as the nut can hold its integrity. If the nut fails because its thread shears, the brake will close. Unexpected closing of the brake could cause injury to maintenance workers-or anyone within the vicinity of the conveyor system.
Twiflex said its "parked off" feature is different from other solutions in that it actively removes the spring force from the brake while it is powered off, so that there is no forceacting on the pad and therefore no potential of an unexpected closure when correctly applied. With hydraulic pressure applied, maintenance workers can unwind an adjusting spindle, which releases the spring pack. When the hydraulic pressure is removed, the spring has the freedom to extend without acting on the brake. At this point, the spring load and hydraulic pressure are both zero and the brake has no stored energy.
The "parked off" feature has a number of benefits, according to Powell. First, basic maintenance such as brake pad replacement can be carried out quickly and with a reduced risk of harm from unexpected closure when compared to conventional brake designs. Second, the brake pressure can easily be adjusted by setting the brake to "parked off" and adjusting the number of shims from the end cover. Finally, installation is made easier and the brake can be installed without the need for hydraulic pressure.
It's not difficult to find examples of remarkable conveyor engineering throughout the global mining industry; systems extending 50 km or more are in place at a number of mine and quarry operations, and the world's longest system—the 100-km-long Bu Craa phosphate conveyor in Africa's Western Sahara—has been operating for decades. Even so, long, undulating conveyor routes continue to pose challenges to system designers as demands for better belt capacity, longer flight length and steeper vertical reach steadily increase.
RBL-REI, a privately owned French company, has installed a number of conveyor systems employing Curvoduc technology, a trademarked system that is well-suited for conveyor projects that often must negotiate long, twisting, uphill, and downhill routes to deliver ore, coal and other bulk commodities. According to RBL-REI's Vincent Guillemot, Curvoduc conveyors can extend up to 17 km per flight and can be built with tight curve radii of 250 m, offering flow capacities up to 20,000 mt/h.
One of the company's more noteworthy projects, said Guillemot, includes two parallel belts running between a quarry and what is regarded as the largest cement plant in the world, owned by China Resources, at Fengkai, China. Initially, this project involved two parallel belts stretching a length of 40 km. Each line comprises three flights: one that is 10 km long, a second of 13 km long and a final section that is 17 km long.
The system can transport 2,500 mt/h of limestone and is driven by 28.5 MW of power. Its three flights include 13 horizontal and 83 vertical curves. The first half of the original project was commissioned in 2010, 18 months after engineering began; the second line, six months later. However, according to the company, the project hasn't ended; in the coming months, it will extend the overall distance covered by the system by adding an additional 13-km flight running from a new quarry to the the initial start point of the existing system-making the entire conveyor network 53 km long.
In 2012, RBL-REI commissioned an 11-km-long system at the Koniambo nickel mine in New Caledonia. Koniambo Nickel SAS is a joint venture owned by Société Minière du Sud Pacifique SMSP (51%) and Glencore Xstrata (49%).
The conveyor system links the mine, situated in a mountainous environment, with the process plant located on the shore of the island. It consists of two overland conveyors that negotiate 11 horizontal and 43 vertical curves over the 11-km route. Because much of the route is downhill, the system was built to provide regenerative power-up to 500 Kw-to the mine's power grid.
The company is currently in the engineering phase of a large project for potash producer Uralkali at underground operations that extract sylvinite ore in the Perm region of Russia. The system will connect a new mine to an existing plant, and will also return tailings from the processing plant to the mine for backfill. RBL-REI's two-way system will transport ore at a rate of 2,500 mt/h and tailings on the return side at 900 mt/h-again, over a route that includes a number of horizontal and vertical curves. This isn't the first time that RBL-REI has designed that type of setup; it installed a similar system in Australia—where it has been running for more than 20 years-and another in China.