Knowing the maximum capacity of existing conveyors is crucial when owners and operators wish to increase the overall throughput of transporting systems. Aspec Engineering’s Sara Vance and Dr Paul Munzenberger provide insight on the processes behind determining the maximum capacity.
Belt conveyors have attained a dominant position in transporting bulk materials as they are reliable, safe and have low maintenance and energy requirements. Conveyors are commonly used to transfer bulk materials between key operations, such as from a stockpile to a process plant or from a process plant to a loadout system.
Because of their position in the industry, the capacity of a conveyors is crucial for increasing the overall throughput of their operations. We often get asked “How much more can we put through our system?”
If a belt conveyor has sufficiently sized loading and unloading equipment, then the maximum capacity depends on the limitations of its weakest components. The first step is to determine the capacity of the main components:
• the conveyor drive,
• volumetric capacity (how much can physically fit on conveyor),
• idlers (load rating & bearing life),
• belt pulleys,
• belt,
• take-up (belt tension),
• brakes and flywheels and
• support structure.
For existing conveyors, the determination of the above parameters is more than just carrying out standard calculations. For example, the conveyor drive performance can be validated against the power draw data from the Supervisory Control and Data Acquisition (SCADA) systems. In some cases, the existing conveyor has been conservatively designed and consumes less power than it was installed with. This excess capacity can be used to convey additional material. The volumetric utilisation of the conveyor, at the existing speed, can also be determined through laser scanning or other measurements to determine if any additional material can be loaded on the belt to fill it completely.
Once the capacity of each component is determined, the maximum possible capacity of the conveyor can be identified. If the maximum capacity, using the existing components, is not sufficient to meet the owner’s requirements, then the components that fail to meet the requirements can be upgraded.
Taking the reasonable assumption that increasing the width of the conveyor belt is not cost effective, the main way to increase the conveyor’s capacity is to increase the belt speed or to fully utilise the volumetric capacity at the conveyor’s current belt speed. In the first case, there will be an increase in the velocity dependant resistances, and in the latter, there will be in increase in the load dependant resistances. Either option, or a combination of both, will result in the need for more drive power, or a reduction in the conveyor’s resistances. If the conveyor is required to lift the conveyed material by any amount, there will be an additional requirement to further increase the conveyor’s drive power or to further reduce the conveyor’s resistances so that a greater amount of material can be lifted in a given time.
The main consequence of increasing the drive power requirements of a conveyor is that the conveyor belt tensions will increase due to the additional resistances. This will occur regardless of whether the velocity of the conveyor is increased, or its volumetric capacity is better utilised without a speed increase. Not only will the increased drive size impose more tension on the conveyor belt, the increased take-up force required to generate the additional pulley traction needed to transfer the power into the conveyor belt will further increase conveyor belt tensions.
Tension requirements can reach the point that a stronger conveyor belt would be required. Increased strength requirements of pulleys and supporting structures may also be needed, and transition lengths and the reaction forces generated in horizontal and vertical curves will also be affected. If the conveyor is long enough, the starting and stopping dynamics should also be recalculated.
When upgrading a conveyor, it is not ideal to change the rating of the conveyor belt. As such, it may be necessary to employ a range of resistance reduction techniques to allow the existing conveyor belt capacity to be used. The methods chosen will likely depend on what components of the existing conveyor are due to be replaced during the upgrade and the type of conveyor that is being upgraded.
Options for resistance reduction include: using larger diameter idlers which lower rolling resistance, lighter idlers which reduce the acceleration tensions by reducing conveyor inertia, idlers with lower rim drag or operating the conveyor at even higher speeds than intended to reduce the total mass of material on the conveyor and thus the weight dependant drag forces.
If additional resistance mitigation is required it may become necessary to replace the existing belt with a modern, low rolling resistance, conveyor belt that will generate less resistance at each idler station, thereby reducing the drive force and thus lowering the belt tensions.
The issue with using these resistance reduction strategies is that they require specific, and sometimes more expensive, components which carry the risk that in the future someone may want to save money by specifying cheaper replacement components without knowledge of why the specific components were originally used.
As an example, for an asset owner that wishes to improve the capacity of their conveyors, upgrades can be implemented to increase the capacity of the governing components. Given the typical data shown in Figure 1, it may be recommended to replace the drive unit and increase the take-up weight. The new drive unit would need a higher ratio gearbox to speed up the conveyor to increase its throughput and larger drive to provide the additional power. Additional take-up mass is also recommended to transfer the additional power to the conveyor belt. In many cases, the capacity of existing conveyors can be increased with relatively low cost and minimum interruption to its operation.