Conveyor systems are the lifelines of any bulk material handling plant.
A critical component in the conveyor system is the transfer point where the bulk material is transferred from or to a belt.
If the transfer point is not designed and maintained properly, bulk material may fall off the conveyor system, resulting in spillage.
The moment the material becomes fugitive, it instantly becomes a hazard, adding a burden to the operating costs, productivity, and efficiency of the plant. Spillage also significantly reduces equipment service life and poses great risk to safety of the employees.
Understanding and controlling spillage
Spillage from conveyor may be the result to various problems, so it is important to find out the root cause and address it at its source. A comprehensive spillage control audit helps in identifying that cause and list out practical solutions.
Common causes of spillage include off-centre loading of material, belt sway, improper sealing of the skirt zone, inadequate belt cleaning, overloading of belt, poorly designed transfer point, and more.
With implementation of proper control measures, however, efficiency and productivity of the plant can be improved, and operating costs and safety risks reduced.
The major steps to be followed to achieve a spill-free transfer point are load zone containment, proper belt support, carry back control and control dust generation.
Belt alignment
This is one of the key factors to be considered when implementing any spillage-control measures. Perfect alignment of a belt while in operation – without any sway – must be ensured to prevent spillage.
There are many things that can cause the belt to sway. The most common are carry-back of material, off-centre loading, improper belt tension, mis-aligned pulleys and idlers, and faulty splice. Heavy cross winds can also cause belt sway on open conveyors.
Improvements like the provision of proper belt cleaning, replacing worn-out pulley lagging, and ensuring pulley alignment helps to control the belt sway within acceptable limits.
While it is ideal to address the root cause of the sway and correct it, using a belt tracker is a quick workaround and is commonly practiced.
A belt tracker consists of the tracking roller and a sway-sensing mechanism (usually a roller). On sensing sway of the belt, the tracking idler changes its direction to steer the belt back to its true line.
Multiple belt trackers are typically installed on a conveyor depending upon the length of the belt and severity of the problem. Belt-tracking rollers are rubberised to increase the traction between the roller and the belt and improve tracking performance.
Dynamic belt tracking systems use advanced technology to provide precision-sensing and positive, multi-plane movement to track the belt continuously and ensure its true running.
Load-zone containment
Well-engineered modular transfer chutes are particularly important in order to contain materials prone to becoming fugitive.
Engineered transfer points reduce the impact force, reduce dust generation, and ensure centre-loading and smooth transfer of material to the receiving conveyor, thus eliminating causes of spillage.
A properly designed skirting system combined with skirt seals and wear liners creates an effective sealing system that contains the material well.
Selecting the right type of skirt sealing is critical and must be done with the objectives and application conditions in mind.
Sufficient attention must also be given to the skirt board design. Too much gap between the skirt board and the belt results in ineffective sealing, regardless of the skirt sealing choice.
Skirt board enclosure should have adequate volume to contain induced air. Dust curtains may be provided inside the skirt board to create dead zones that promote settlement of airborne dust.
Adequate belt support
The success of load-zone containment depends hugely on the close tolerances that can be maintained for the skirt seal. The impact of material often leads to sagging of the belt and gaping of the seals at the point of impact, thereby allowing material to become fugitive.
To overcome this issue, proper support should be provided beneath the belt at the impact zones. Impact cradles should be used below the load zone, creating a low-friction bed for the conveyor belt and preventing sagging.
By providing a firm support to the belt, the skirt sealing forms a close seal with the belt and prevents spillage and leakage of air-borne dust. A well-designed impact bed also dampens the force of material on the belt, resulting in extended service life.
New innovations such as dynamic impact beds with garland profile offer three-dimensional relief and are highly recommended for load zones with heavy impact, spillage, and mis-tracking issues.
Carry-back control
Carry-back is among the most common causes of material spillage in any plant. It often leads to a host of operational issues such as belt mis-tracking, idler seizure, uneven belt wear, and accelerated belt cover wear.
Spilt material also poses safety risk to the workers. Belt cleaners are the most effective way to combat material spillage due to carry back.
A well-engineered belt cleaning system effectively eliminates carry back, thus reducing material loss and clean-up costs. It also improves plant productivity and the service life of belt components.
A multiple-cleaner approach, varying upwards in the efficiency of cleaning, is often recommended to solve the carry-back problem. This essentially consists of at least a primary cleaner installed on the head pulley, clear off the material stream, and a secondary cleaner installed at the point where the belt leaves the discharge pulley.
Depending upon the severity of carry-back, additional cleaners may be installed. It is always recommended to refer to a belt cleaner selection chart provided by the manufacturer to select the right type of cleaner for the given application.
A vast range of belt cleaners is available in the market; however, proper selection of the one to suit the application condition and operating parameters is key to meeting the objective.
It is equally important to properly install and maintain the belt cleaners to get the optimum results. Service and maintenance-friendly features like quick-release tensioning systems and track-mount blade cartridges make the life easier for the technicians and can cut downtime.
Sustainable solution
Properly setting up the equipment plays a major role in achieving the objectives in a sustainable manner. Due importance must also be given to regular maintenance and performance monitoring of the installed equipment.
Partnering with a manufacturer offering all the spillage-control accessories, as well as maintenance of the equipment, is the best way.
Manufacturers with well-equipped research and development facilities can make quick changes to their products to meet site requirements, which is better than using a standard product that might not be suited to the given application.
These companies provide a holistic solution, including spare management, helping to eliminate major challenges.
While plants should be wary of manufacturers that offer only standard products that cannot be customised, they should avoid switching between suppliers without acknowledging the importance of maintenance. This would result in dumping the plant with a variety of equipment from several suppliers, none of which deliver the desired results.
This feature appeared in the October issue of Australian Mining.