Carryback spillage in belt conveyors is a common issue that can lead to several operational challenges. It is the material that sticks to the conveyor belt after the material has been discharged. This residual material is carried back on the return side of the belt and can fall off at various points along the conveyor’s path, leading to spillage.
The major areas of concern due to Carryback spillage are the following:
- Increased Maintenance: Leads to more frequent cleanup and maintenance; thereby increasing the operational costs.
- Equipment Damage: Can cause premature wear and tear on conveyor components.
- Safety Hazards: Creates slip and trip hazards for workers around the conveyor.
- Material Loss: Represents a loss of product, which can be significant over time.
- Productivity: Unscheduled shutdowns will lead to lower productivity.
In order to control the carryback spillage, it is extremely important to install an effective belt scraper that can scrape the carryback material into the chute. Enough importance is always not given to install an efficient scraper & make that scraper effective. The scraper can only be effective when:
- Condition of the belt is reasonably good
- There is space to install the belt scraper & maintain it
- The position of the chute backplate is such that the scraped materials flows into the chute along with the main material.
The cost of the scraper not being effective is extremely high. It has a huge impact on the operation cost. Calculating the cost of carryback spillage involves considering several factors like:
- Cost of cleanup
- Impact on equipment maintenance
- Volume of material lost
Here’s a simplified way to estimate the cost of carryback spillage:
- Estimate the Volume of Carryback: Determine the thickness of the residual layer of material on the belt and the belt’s dimensions.
For example, if a 1200mm wide belt running at 3.5 m/sec has a residual layer of 1mm, the volume carried through in 1 hour would be 15.1m³. Generally, 75% of the belt carries the material & hence only 75% of the return side of the belt will have the residual layer. Therefore, the total volume of material carried through the return side of the belt in 1 hr. will be 11.3m³ - Calculate Annual Volume: Multiply the hourly volume by the number of operating hours per day and days per year. If the conveyor runs 16 hours a day for 250 days a year, the annual volume would be significant.
- Determine the Percentage of Spillage: Estimate what percentage of this carryback actually spills. Generally one third of the material that sticks to the belt falls on the floor; the spilled volume can be calculated accordingly.
- Assess Clean-up Costs: Include the man-hours for shovelling, equipment hours for loaders, and any delayed maintenance activities due to cleanup.
- Consider Material Loss: High-priced materials lost due to carryback can significantly raise operational costs. If the material is unrecoverable, factor in the production cost of the lost material.
- Account for Equipment Wear: Carryback can lead to premature wear of components like idlers and pulleys, increasing replacement costs and downtime1.
Here’s a basic formula to calculate the cost of carryback spillage:
Cost of Carryback Spillage = Clean-up Cost + Material Loss Cost + Additional Cost due to Low life of Equipment
Clean-up Cost = (Volume of Carryback × Operational Time × Spillage Percentage) × Cost of manpower/ equipment for cleaning 1 m3 of material
The numbers that will come out is quite staggering. It therefore calls for use of modern efficient belt scrapers.
Consider a plant with 40 conveyors handling coal ; if the difference in scraper efficiency is 2% , it will lead to 10876 tons of additional material on the floor in a year. The additional cost of cleaning in India will be 7mn Inr. The cost of labour in Europe is 40 times than that in India & therefore overall cost is much higher.
For all practical purpose, there is a difference of more than 10% of cleaning efficiency between a good & bad plant from the point of spillage management. A difference of 10% in the above example will mean more than 54000 tons of additional material on the floor. The additional cleaning cost along with the damage caused on the conveyor parts due to this additional spillage will pull down the profitability of the operation. We have observed that improvement of carryback spillage has led to following kind of improvement with users of belt conveyors:
- Reduction of roller consumption: from 5-6 nos a month in a particular conveyor to 1-2 nos a month in a steel plant
- Improvement of Belt Life: from 12-14 months in a coal washery to 16-18 months
- Reduction in belt sway: The belt sway reduced from 120mm to 40mm in an iron ore mine thereby eliminating the material spillage and the damage in the belt & stinger frame in the return side.
- Improvement of lagging life of pulley: The lagging life of a snub pulley improved from 7 months to 12 months in a cement plant.
The above examples are specific to a particular application. However, similar results are observed in the bulk material handling operations where the carryback spillage is not controlled.
The carryback spillage is not only affecting the increase in cost but it also affects the safety, efficiency & the overall performance of the conveyor. Prioritising proper spillage management in a must for operation of belt conveyors.
