14 April 2015
Gary Ryan, Design Engineer at BMT, discusses some of the key considerations that conveyor designers and operators must recognise when specifying both new build plant and upgrades to existing conveyor systems.
BMT has many years experience in the design of new conveyors, as well as modifications and upgrades of existing conveyors. For many of our projects the conveyor availability and reliability are critically important - for example, a conveyor breakdown may reduce the coal supply to a power station with obvious consequences.
Ideally, a conveyor should be designed so that it does not require maintenance. However in practice, for conveyors built to a price, regular maintenance is required while in service. The challenge for a conveyor designer is to minimise the total life cycle cost of the conveyor, balancing capital and all maintenance costs. A conveyor designed to operate with minimal planned maintenance will have greater availability - ensuring that the physical design allows the maintenance to be undertaken quickly and easily will increase availability further still. Furthermore, ensuring that the design allows easy access for maintenance will also increase the chances of the work being done at the correct intervals making the conveyor more reliable.
Every conveyor designer also needs to understand the priorities and the operating and maintenance practices of the site where a conveyor is to be used, so that their design is appropriate to the site. On one recent project BMT was asked not to use fibre optic communications despite their advantages with adjacent HV cables. The reason was very simple - the site maintenance workforce could not repair fibre optic cables easily so appropriate technology was deployed.
One way to reduce conveyor maintenance costs is to keep the design as simple as possible with additional components only added if there is justification. The simplest conveyor design would therefore have just two pulleys and a single drive. If the belt strength and cost becomes a concern, then multiple drives can be justified. If a shuttle head is required then more pulleys are justified. The BMT team often examine conveyors where excessive numbers of pulleys have been used without any rational justification. Additional components that do not add to the conveyors operational capability are simply mechanisms waiting to fail and cause unnecessary downtime.
For smaller conveyors the simplest take-up system often uses portable hydraulic jacks to tension the belt, and threaded screw rods with lock nuts to then hold the tension. The hydraulic jacks allow the tension to be set accurately. This take-up arrangement is common on bucket wheel excavators, travelling stackers and similar machines. Images 1 and 2 show a jack and screw take-up on one of six belt feeders designed by BMT on a transfer vessel. The pictures show the take-up as commissioned and more recently, approximately eight years after commissioning (image 3). We understand the take-up has not been touched since commissioning. The conveyor take-up system is another element where the design can be unnecessarily complex. Some conveyors, such as the underground main gate conveyors require particular take-up control strategies because of the specific characteristics of the system.
However, for many conveyors, the use of constant tension winches and live (gravity) take-ups is unnecessary and adds unwanted, continuously moving parts. The simplest take-up for many large conveyors is a fixed winch system, where the belt is tensioned, and then the winch is locked. BMT recently changed some 12,000 tonnes per hour conveyors to a fixed winch system after the site experienced a spate of pulley failures, which were largely attributable to the “smart’ take-up winch system not working correctly.
On a recent plant audit of a pocket belt conveyor, the take-up design relied on threaded rods to both apply and hold the tension. Maintenance personnel had been tightening the belt until it seemed right. This conveyor has experienced a number of pulley failures attributable to excessive belt tension – a simpler design using portable jacks would have made tensioning the belt easier, and would have eliminated the over tensioning problem and consequent plant failures.
Conveyors can also be made complicated with items that are secondary to the basic conveyor function. For example, the three Latrobe Valley mines in Victoria, Australia have gradually been eliminating turnovers and spill belts installed in existing conveyors. The justification is that the turnovers and spill belts require skilled maintenance in the mine, and increase the maintenance time for the whole conveyor. By comparison a piece of mobile plant used for clean up is maintained off site and is readily replaceable.
The selection of electrical field equipment such as light fittings, switches etc. should also be based on reliability and minimising the maintenance requirements, rather than the initial cost. For example, many new conveyors in Australia now use swivel type poles for light fittings, so that the light fitting can be lowered to walkway level for maintenance, rather than requiring a person to use an elevating platform or some other means to access the light fitting.
Electrical field equipment such as switches and sensors are often subject to mechanical damage, from spill, maintenance activities and clean-up work. For conveyors on mine sites, the essential design requirements for these electrical devices typically include that the items be robust and waterproof, as they will be subject to high pressure water hosing, mechanical damage from spill, and clean-up work etc.
When a cost can be placed on lost production time, it becomes relatively easy to justify choosing equipment based on low maintenance requirements and high reliability. Typically, the additional cost of using the correctly selected equipment is very small compared to the production losses.
A maintenance focused design arranges the conveyor structure to facilitate maintenance. Image 4 shows the structure arrangement at the take-up pulley removal position on an 8500 t/h conveyor recently designed by BMT. The main structure is arranged to maximise the space for insertion of a C frame and has anchor lugs for pulling the belt out of the way. The cabling and fire service pipes are placed clear of the pulley removal position. The walkways are placed to be useful for the maintenance work. The aim is to have a quick pulley changeover procedure with minimum work. BMT's over-arching design approach is to ensure the structures suit the maintenance work from the start of the design, rather than trying to work out the maintenance procedures after the structure has been designed.
On sites with multiple conveyors, further maintenance savings can be achieved by ensuring the conveyor designs maximise the standardisation of components and work procedures. As an example of component standardisation, conveyors at Loy Yang Mine in Victoria, Australia have one drive pulley design and one driven pulley design in use throughout their 2000 mm wide movable conveyors. The manufacturing details of the pulley shells have changed over the years, but the respective pulleys shells, shafts and bearings all remain interchangeable. The benefits of standardisation of components include the reduced spares costs and greater flexibility in allocating the available spares.
On a recent project, a contractor proposed 14 different pulleys designs for three conveyors, all with the same belt width and installed power. If the design had been focused on maintenance from the start, six pulley designs would have been sufficient with a consequent saving in what has to be maintained and stocked as spares.
The benefits of the routine condition monitoring and appropriate servicing are well known. From a design viewpoint, the plant layout has to make the routine work as simple and quick as practical. Typical design details include provision of adequate walkways, space for oil discharge from gear reducers, inspection openings etc. The walkways should be appropriately placed to facilitate the maintenance, not obstruct it. If the first step in the maintenance procedure to change a pulley or drive in a conveyor is “remove the walkway” then it is probable that the designer did not consider maintenance sufficiently.
A final consideration with conveyor maintenance is the function of the control and protection system. Our preference for large conveyors is to have all the conveyor status information available in the programmable logic controller (PLC), so that appropriate logic can be applied to monitor and manage the conveyor. At first this may seem to complicate the conveyor; however there are significant benefits in having the conveyor information and settings readily accessible. For example, if one drive fails on a multi drive conveyor, it is common to adjust the belt tension settings, starting times and/or torque limits to continue plant operation, perhaps at reduced load, or with stacker or excavator operating position limited to one end of the conveyor.
The control and protection logic should be carefully worked out, clearly documented and made as simple as possible. We have analysed conveyors where overly complicated starting speed-time ramps have caused more severe dynamic events in a conveyor than a simple linear ramp with some smoothing at the start and end. If the mechanical and structural designers do not understand the control logic then it is possible the control system will be inconsistent with the mechanical and structural design, resulting in plant damage or failure. Furthermore, if the maintenance personnel cannot readily understand the control logic then it is likely that after a few years something will change and there will again be plant damage and/or delays as a result.
With all the conveyor status information available in the PLC it is practical to provide warnings of impending faults rather than simply stop the conveyor when a fault occurs. A warning may give the plant operators time to respond. The history stored within the system can also assist in identifying the circumstances of a failure.
If the optimum design of a conveyor is that which achieves the minimum whole of life cost, then the maintenance costs become significant, and the conveyor designer must consider minimising the maintenance costs and the time required for maintenance, not just the initial cost.
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