Bulk Cargo: Expert Advice for Solving Conveyor Carryback

Runoff can be an issue for many ports, making spillage from conveyors an added concern.  Belt conveyor operators in high-volume ports and terminals often discuss the efficiency “sweet spot,” which refers to the balance between the amount of spillage beneath the system and dust emissions above, relative to the volume of material being transported and the total uptime.  This balance is a myth.  There is no trade-off between maintaining a clean operation and ensuring productivity.  In fact, installing the correct material discharge and belt cleaning configuration can effectively address both concerns, along with a variety of other issues throughout the entire conveyor system.^[1] 

The quality and construction of belt cleaners are crucial for achieving optimal results. Two cleaners from different manufacturers, both made of polyurethane, may appear quite similar, but once installed and tensioned, operators will notice significant differences. Typically, a lower-quality cleaner (1) allows more material to carryback, (2) requires more maintenance labor, (3) needs to be replaced more frequently, (4) may not be suitable for the application’s volume or properties, and (5) could damage the belt, which is commonly the most expensive component of any conveyor system.

This article will discuss the various issues caused by inadequate belt cleaning and their solutions. Those problems are: [Fig. 1]

• Carryback
• Dust
• Equipment fouling
• Belt misalignment
• Maintenance labor
• Safety

Eliminating Carryback Improves Efficiency

Problem – Carryback refers to material that is not discharged, bypasses cleaners, and remains adhered to the conveyor belt beyond the head pulley. In addition to the loading zone, the discharge zone is often a main area where fugitive material accumulates beneath the conveyor system as “spillage.”  This is caused by carryback.  After passing through the cleaner(s), material drops at the snub pulley and/or the take-up pulley. It typically does not all come off at once. As the stuck material travels along the belt return toward the tail pulley, it falls off along the belt path, disrupted by return rollers.  Thus, cleaning the belt of as much carryback as possible is essential for workplace cleanliness and safety. [Fig. 2]

Solution – A primary cleaner blade endures the stress of heavy bulk material, so ensure it is properly graded for light, medium, heavy, or extra heavy duty. When selecting a primary belt cleaner, consider the pulley diameter, belt speed, production volume, and material properties. Wet, viscous, acidic/alkaline, and sharp/abrasive materials may require blades made from specialized polyurethane or material blends (tungsten tips, rubber, etc.). 

Invasive Dust Emissions

Problem – Dust from the return belt can be pervasive. Once the belt is loaded, the pressure of the cargo and the vibration of the belt compress dust into the trough of the belt, packing fine particles into the divots and cracks. The older the belt, the more imperfections it has. As the belt passes through the cleaners — especially the secondary and tertiary ones — unless the cleaners are properly tensioned, the fines can settle in the blemishes and dislodge along the belt path. This creates clouds of dust along the way, and the tiny gritty particles can infiltrate the smallest spaces, leading to equipment fouling.

Solution – To dislodge fines that drop like dust, a secondary or tertiary cleaner may be necessary.[Fig.3]  Positioned behind the head pulley, the secondary cleaner should be tensioned against a hold-down roll within a distance of 2 to 4 inches to ensure effective cleaning.  Contact with the belt should be in a negative-rake position and made from tungsten carbide or other abrasion-resistant materials for durability.  Tertiary cleaners often rely on the weight of the belt combined with a spring tensioner.  For extremely abrasive or sticky materials, consider using a washbox.  Mounted close to the discharge point, water is used to wash the belt, with slurry collected below and a squeegee blade clearing excess water.  When using a washbox, best practices include using a pre-cleaner to remove the majority of the material.

Dust is also minimized by enclosing the system at the discharge area with curtains and a return run seal.  When material is discharged, it can hit the back of the transfer chute, leading to dust formation.  As it falls down the chute onto the next conveyor, the impact creates turbulence that seeks an escape route.  The receiving belt below generates its own airflow to pull the dust along, but if the top of the transfer chute is open and there is wind, it could trigger the capillary effect and pull the dust back up through the chute.

Costly Equipment Fouling

Problem – Carryback and dust are not only violations of workplace safety but also lead to costly consequences throughout the entire system. This results in reduced equipment lifespan and expensive replacements, which sometimes require unscheduled downtime.

• Belting – Carryback material accumulating at the discharge zone can encapsulate the tail pulley, rollers, and belt. In this situation, the belt (typically the most costly component of any conveyor) will rest on the abrasive material and erode, reducing the equipment’s lifespan.
• Pulleys and motors – Often, the head pulley acts as the drive pulley, meaning any fugitive dust from the discharge gets drawn into the air intake, clogs the working parts, and leads to a breakdown.
• Rollers – Fine particulates can foul bearings and cause the rollers to seize over time, leading to roller wear, belt damage, and frictional heat. Heat, combined with combustible dust and oxygen, equals a potential disaster. Furthermore, roller faces can become fouled with caked material, resulting in mistracking. [Fig. 4]

Solution – The resolution includes enclosing the system and installing a belt cleaning setup that eliminates carryback and dust. Seal exposed bearings and axles for rolling components. Install a filter in the drive pulley’s mechanical air intake and monitor it. Replace equipment immediately to prevent frictional heat and damage. Running equipment to failure is a recipe for disaster. The preferred option is to inspect the system regularly or use a monitoring system, such as a position indicator that shows the status of cleaners and provides data on when they need servicing.

Mistracking Away From Efficiency

Problem – Many conveyors come with a “tracking” system that is intended (ineffectively in many cases) to keep the belt from coming into contact with the stringer and shredding the edges. Some operators who experience mistracking on the return install a “crown” roller design, which is thicker in the middle. If the belt drifts slightly, this design uses its weight to realign it back to the center. When these return rollers are fouled by carryback, they no longer realign the belt. [Fig.5] If the belt is not centered on the tail pulley, the cargo is likely to be loaded off center. This can result in the belt drifting further off center, spilling cargo from a full belt and potentially damaging the entire system.

Solution – Modern belt alignment systems like the Martin® Tracker™ HD can remedy misalignment more effectively than merely installing crown rollers.  Designed for either the return or carrying side of the belt, these devices use sensing arms to detect slight variations in the belt path and immediately correct them by turning a roller or idler in the opposite direction.

Trackers are recommended for all systems because many factors can cause mistracking, not just carryback and broken equipment.  However, mistracking results from carryback, so the underlying cause should be addressed first.  Belt tracking on the return is also advised, regardless of issues stemming from carryback.  With longer gaps between rollers on the return side than the upper carrying side has between idlers, the return side is susceptible to wind, belt camber, and other elements that can lead to drift. [Fig.6]

Maintenance Labor Raises The Cost of Operation

Problem – Running a system to failure due to an overstretched workforce will lead to more expensive and prolonged downtime.  Inspection and maintenance of a conveyor system should be routine and performed regularly to prevent unscheduled downtime.  It is easier said than done since belt cleaner tensioning is a continuous challenge for many operators.  Under-tensioning leads to carryback and expedites the blade wear.  In contrast, over-tensioning results in higher power costs, increased heat from friction, runs a higher risk of pull-through, and also accelerates blade wear. After the lock-out/tag-out procedures, changing cleaners can be an ergonomically awkward task performed beneath a conveyor or inside the drop chute, often requiring two or more people and/or a confined space entry certification.

Solution – Carrying out maintenance during a scheduled shutdown, when staff can focus on specific tasks in a controlled environment, is the optimal situation.  Many innovations to minimise maintenance time have been introduced in recent years.  For example, Martin Engineering’s N2® Position Indicators (PI) help determine scheduled downtime by providing data on the wear status of belt cleaners.  Delivered via mobile app, the N2® PI not only alerts users if an incident occurs, but the data it provides also helps operators understand wear times.  This informs managers and aids them in coordinating maintenance schedules for greater efficiency.[Fig.7]

Modern belt cleaner designs such as the CleanScrape® Primary Cleaner are tensioned during installation, tested for performance, and then almost never need to be retensioned.  With typically four times the lifespan of the average cleaner, the CleanScrape® features tungsten carbide tips for enhanced equipment longevity in rugged, demanding conditions.  It applies minimal pressure to the belt to keep it clean while ensuring mechanical splices can easily pass through.

Safety

Problem – Carryback increases safety risk as well as raising operational costs.^[2] Although it is strongly discouraged, operators continue to assign workers to clear spillage from around the system while the belt is running, shoveling the spillage back onto the moving conveyor, or into a skip or barrow – all of which drives up labor costs.  If a worker comes into contact with the belt, the risk of an injury or fatality are high.  In the bulk handling sector, working on or near moving conveyors remains a leading cause of workplace injuries and deaths.  Besides the devastating price paid by a worker’s family and colleagues in such circumstances, these incidents always result in costly fines, unscheduled downtime, and increased insurance premiums.  [Fig.8]

Solution – Avoid safety issues and labor costs associated with cleaning by eliminating the cause, carryback. Always follow the lock-out, tag-out (LOTO) procedure and never allow any worker to perform maintenance on or around a working conveyor. One innovation that enhances servicing and safety is the Safe to Service Martin® QC1+™ Primary Cleaner or Safe to Service SQC2S™ Secondary Cleaner. Instead of confined space entry or crawling under the system, these designs enable equipment to be pulled away from the mainframe and serviced quickly and ergonomically by a single worker from outside the chute. 

Anatomy of a Discharge Zone

According to the Conveyor Equipment Manufacturers Association (CEMA), “Conveyor belt cleaners are used to remove fugitive material, otherwise known as carryback, from the return side of the conveyor belt after the bulk material has been discharged.  Ideally, this will be accomplished from within the chute works so that the removed carryback will pass onto the next system element.”^[3]

By enclosing the discharge zone and extending the entry hood, operators can control airflow with dust curtains and return run seals.  When retrofitting a new enclosure, installing a replaceable wear surface plate made of smooth reinforced metal on the impact wall of the transfer chute will reduce buildup and be easier to replace than the entire structure later.  This design eliminates dust emissions from the capillary effect.  As part of the enclosure, accessible inspection doors that offer a clear view of the equipment should be installed.

However, dust remains within the enclosure, so it is advisable to use shaft seals for bearings to prevent fouling.  Once the appropriate primary, secondary, and tertiary cleaners are installed and carryback is significantly reduced, operators will notice an increase in material flow through the transfer chute.  It is recommended to install a dribble chute to ensure all captured material is handled.  The dribble chute should include a vibrator or air cannon to ensure that dust and fines are reintegrated into the cargo stream and do not accumulate inside the enclosure.  [Fig.9]

Inspect and Expect

Inspecting a discharge zone can be time-consuming and requires all your senses. Listen for squeaking rollers and idlers, debris hitting the sides of the enclosure or the inspection door, and the scraping of accumulated materials on the pulley against the mainframe or enclosure.  Smell for any metallic friction, as well as burning polyurethane or rubber.  Feel for excessive vibration in the system, which may indicate mistracking or a change in the discharge stream.  Lastly, always wear safety glasses and observe the operation of each component.  Inspect the belt during a full rotation and note how the splice passes through the discharge process.

Conclusion

In the punishing environment of ports and terminals, many aspects of workplace safety and operational efficiency are directly tied to how clean the conveyor belt is.  Since incidental contact with the high-speed belt has become the leading cause of conveyor injuries and fatalities, carryback is increasingly being scrutinized by inspectors from government safety organizations.  Eliminating the safety hazards associated with belt cleaning also brings benefits such as reduced unscheduled downtime, fewer equipment failures, and lower operating costs.