Carryback
Definition: Fugitive material that clings to the belt after the belt has discharged its cargo. Characterized by pile of fine, wet material or dried flakes under the return idlers and gravity take-up; material buildup on bend pulleys and other components. Carryback is a possible source of dust.
Level C1: Dirty ____ points
- 101 to 250 grams per square meter of carryback on the belt surface (Level I Cleaning) (For reference : 1.0 g/m2 = 0.003 oz/ft2.)
- Characterized by a layer of material 0.5 to 1 millimeter (0.02 to 0.04 in.) thick on the belt
- Material accumulates under the rollers
- Clean up required at least once a week
- Suitable for open cast mining operations where mechanized cleaning up is used Can be achieved by a single or dual belt cleaner system
Level C2: Clean ____ points
- 11 to 100 grams per square meter of carryback on the belt surface (Level II Cleaning)
- Seen as a film or streaks of carryback on the belt slightly discoloring the surface
- Small amount of accumulation under the return rollers—may be in the form of flakes
- Manual clean up required 2 to 4 times a month
- Suitable for most bulk-materials conveying applications
- Can be achieved with a dual or triple belt-cleaning system
Level C3: Very Clean ____ points
- 0 to 10 grams per square meter of carryback on the belt surface (Level III Cleaning)
- Characterized by a mostly slightly damp belt with few to no streaks of carryback
- Manual clean up required less than once a month
- Usually requires use of water sprays and a wash box to achieve this level consistently
The plant should determine the maximum possible points for any given category after reviewing their goals for control of fugitive material and enter the point values in the form.
| Scoring Form for Swinderman Scale | ||||||
|---|---|---|---|---|---|---|
| Weighting System for Scoring Area on Fugitive Material Emissions | ||||||
| Level | Description | |||||
| Rating | ||||||
| Point Scale | Baseline Date: ____ |
30 Days Date: ____ |
60 Days Date: ____ |
90 Days Date: ____ |
||
| D1 | Extremely Dusty | |||||
| D2 | Dusty | |||||
| D3 | Dust Free and Under Statutory Limit | |||||
| S1 | Extreme and Continuous Spillage | |||||
| S2 | Frequent Spillage | |||||
| S3 | Spillage Free or Occasional Spillage | |||||
| C1 | Dirty | |||||
| C2 | Clean | |||||
| C3 | Very Clean | |||||
| TOTAL SCORE | 100 | |||||
Sample of the Swinderman Scale Procedure: Happy Company
Procedure
The goal is to reduce the amount of cleanup required and to meet the statutory requirement for dust. Management selects an area for performance monitoring. Management and a service provider meet to agree upon the weighting of the three elements: dust, spillage, and carryback.
In this example, supervisors from production and maintenance at the Happy Company get together to develop the definitions and performance levels for their operation based on the problems and the desired outcomes of their operation. Once a month they walk the area as a team and grade the environmental conditions to assign scores that reflect the materials-handling system’s performance. In the sample scoring system, carryback is more heavily weighted, with the value of up to 60 points for a clean operation. Consequently, improvements in control of carryback will be shown as the greatest improvement in overall score.
Baseline Survey
Management and a service provider tour the area and take representative photos that provide a visual comparison scale from Acceptable to Not Acceptable. A baseline rating is established and a plan for improvement is implemented. In the survey conducted prior to any system improvement, Happy Company’s material-handling operations scored a 15. This score reflects a dusty plant with significant spillage and carryback problems.
30-Day Survey
After 30 days, Happy Company has improved performance to a score of 45. The installation of belt-cleaning systems improved cleaning performance, increasing the rating for carryback from Dirty to Clean. The plant is still evaluated as Dusty with Extreme Spillage.
60-Day Survey
After 60 days, Happy Company has improved performance to a score of 60. Improvements in the materials-handling systems have improved the level of spillage form Extreme Spillage to Frequent Spillage. Dust and carryback remain where they were at the previous survey.
90-Day Survey
After 90 days, Happy Company has improved performance to a score of 70. Dust and spillage have been virtually eliminated, and carryback has been reduced to acceptable levels with the installation of multiple cleaning systems.
Results from Happy Company
By comparing the baseline score to interim results, the analyst can deduce that first the carryback problem was addressed, then the spillage, and finally dust. From the final score, it appears that the original goal was met, but further improvement remains possible. Photos documenting the new level of performance are taken and submitted with the original standards as proof of performance.
See Also: Measuring Fugitive Materials from Belt Conveyors Part 1 | Part 2
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