Controlling Carryback At Conveyor Transfer Points

Dan Marshall, Process Engineer, Martin Engineering

Dust and carryback from belt conveyor discharge points are an ongoing problem for bulk handling operations. Efficient discharge at a transfer point requires a holistic approach with several components working in tandem. This approach aims to extend equipment life, improve safety, and address the root causes of inefficiency and downtime that reduce production and raise the cost of operation.

Reducing Carryback

Primary cleaners or “scrapers” can fail for many reasons, but usually because of:

• Increased belt speed or material volume for which the blades are not specified.
• Changes in material type, size, or property for which the blades are not specified.
• Difficult access that prevents regular inspection and servicing.
• Policies or schedules that allow blades to go beyond their service life.
• Improper installation of the blade, tensioner, and/or assembly.
• Inferior design or materials for manufacturing the blade.

When material is allowed to cling to the belt as carryback, it spills along the belt’s return. Spillage can accumulate quickly and encapsulate the belt. Cleaning spillage while the system is still running is extremely hazardous. Every year, catastrophic injuries or fatalities occur when workers cleaning around a running conveyor come into minor contact with a fast-moving belt and are pulled in. If equipment, such as a front-loader, is used for cleaning and bumps the mainframe, serious system misalignment can result, leading to belt mistracking. These hazards can be avoided by reducing the need for cleanup labor, thereby lowering operational costs.

Mistracking can be caused by many factors, including a crosswind, off-center loading, or a frozen idler. A drifting belt can be a major cause of carryback at the discharge point if the blade is not centered on the head pulley. In this instance, adhered material bypasses the cleaner and becomes carryback.

Recommendation: Install a Tracker™ at a location equivalent to 3 to 4 times the belt width before the discharge zone to ensure the belt is centered on the head pulley. Consider installing a modern cleaner, such as a CleanScrape® Primary Belt Cleaner, which has 4x the life of traditional blades and requires no retensioning or additional service after installation. This reduces maintenance and significantly improves safety.[Fig.2]

Controlling Dust

Carryback is a major source of fugitive dust because much of what bypasses the primary cleaner blade is fine material hiding in cracks and divots of the belt. Excessive dust emissions reduce air quality and can result in workplace safety violations. It can also limit visibility, cover signage, and discourage access required to maintain system health and efficiency.

The dust and fines can also easily migrate into return rollers and takeup pulleys, fouling the bearings and causing them to seize. A frozen idler or roller increases belt friction, leading to mistracking as well as static and heat, which are major causes of belt fires. Frozen idlers and rollers must be replaced promptly, requiring unscheduled downtime, which reduces production and increases operating costs.

Recommendation: Install secondary and tertiary cleaners to ensure the belt is thoroughly clean on the return. To avoid buildup on the catch slope leading to the transfer chute, consider adding a Vibrating Dribble Chute. Install belt trackers or crown rollers along the lower belt path to ensure alignment on the return.

Conclusion

Production increases can change throughput volumes and belt speeds, which can result in carryback, spillage, and dust. Retroactively installing equipment that improves both safety and efficiency results in reduced labor costs for maintenance, fewer equipment replacements, greater compliance, and an overall lower cost of operation.