Basics of Belt Conveyor Cleaning
Material transported on a conveyor is discharged at the head pulley and the return belt is usually subjected to some form of contact cleaning to remove residual material at the head pulley.
While carry-over may represent a very small percentage of the total quantity of conveyed product annually, this small percentage is nonetheless a significant direct and indirect cost to the overall conveyor systems' operating and maintenance bill.
With this in mind, belt cleaning methods and a number of different types and designs of belt cleaning equipment have become available over the last two decades, with improved cleaning results.
There is therefore an increasing need for designers and users of belt cleaning technology to have a clear understanding of the available equipment, the features of the different cleaning techniques as well as where the cleaners should be located for optimum results.
b) What causes material carry-over ?
Any conveyor type where the belt on the return strand is 'open' for example, a troughed belt conveyor, pocket belt conveyor, sandwich conveyor etc. presents an opportunity for residual material which is carried over the head pulley, to be deposited along the return strand of the conveyor.
Almost without exception, a small percentage of the conveyed material will not be discharged with the bulk of the load stream and will stick to the belt surface, resulting in the potential for spillage along the return strand of the conveyor.
The consistency of the material i.e. whether it is dry or wet; the characteristics of the material i.e. whether the product comprises large lumps or fine dust; and the mechanical design of the conveyor i.e. the belt speed and pulley diameter for example, have an effect on the amount of carryover on a conveyor.
In addition to the above, the quality of the carrying-side belt cover plays an important role in carryover as a rough surface will tend to form a stronger bond with the product in contact with the belt than would a smooth belt surface. In this case a far greater centrifugal force is required to eject the surface material at the head pulley.
c) Standard belt cleaning practice
Belt cleaning techniques are aimed at removing the greatest possible percentage of carryover at the source i.e. at the head / discharge pulley. Belt cleaners must not however cause damage to the belt in the course of operation.
It is standard practice to install scrapers on a belt conveyor. Examples of these cleaning devices and an indication of where they would be installed is provided below :-
- Primary belt scraper is installed inside the head chute at a point below the natural material trajectory where the belt is in contact with the pulley. Primary scrapers are usually contact scrapers.
- Secondary belt scraper is also usually installed within the head chute however this scraper is positioned to contact the belt once it leaves the head pulley. Secondary scrapers are usually contact scrapers and are either blade-type scrapers or rotating brushes although the latter is not common.
- Tertiary scrapers are sometimes installed although this is not common practice. In this case the scraper is outside the head chute and scrapings must be returned to the head chute by a mechanical chain conveyor or similar. Tertiary scrapers are usually contact scrapers.
- Belt washing systems are also used in cases where the material carried over is extremely difficult to remove from the belt and/or where the disposal or handling of the washing water plus spillage can be done easily or conveniently. Washing systems are usually considered to be non-contact cleaners in as much as water is used to clean the belt and squeeze rollers provide a drying function.
- Return belt ploughs are often used to remove spillage from the 'clean' side of the return belt, where spillage off of the carrying side has fallen onto the return belt. Product is discharged onto the ground or into a launder.
- Belt turnovers are sometimes used in which case the belt is turned through 180 degrees and the 'dirty' side of the belt is on the top of the return-side. In this way the spillage does not fall off of the belt along the length of the return-side but rather, is deposited at one point at the tail-end where the belt is again turned through 180 degrees ahead of the tail pulley.
d) How do belt cleaners work ?
Belt cleaning devices can be categorized as contact or non-contact cleaners. Both types of cleaners are used on bulk materials handling conveyors although contact cleaners are without question, more commonly employed.
A belt turnover is an alternative approach to dealing with spillage and is also discussed below.
i. Contact-type cleaners
These cleaners utilise spring-loaded, wear-resistant scraper blades pressed against the moving belt and in so doing, scrape the residual material off of the belt.
The design, shape and position of the scraper mechanisms differ from one supplier to another although the basic contact scraper locations are as indicated in the sketch above i.e. the primary and secondary scrapers.
Belt ploughs are also contact-type scrapers however these scrapers rest on top of the return belt, constrained only in the horizontal direction to prevent the ploughs coming adrift and being trapped in the tail pulley.
Ploughs use rubber-type blades and unlike their spring-loaded scrapers, ploughs are not forced to maintain contact with the belt.
The belt is however flattened either side of the plough by flat return idlers, to ensure consistent contact between belt and plough.
Another type of contact cleaner is the rotating brush cleaner.
The brush cleaner is located at the head-end and rotates against the belt travel, which produces a 'flicking' action to liberate the residual material on the conveyor belt.
This cleaner uses an auxiliary drive unit or the travel of the belt or is driven via the head pulley rotation.
This cleaner can only be effective in applications where the material is dry and fine-grained. This type of cleaner is not used frequently in bulk conveying applications.
ii. Non-contact-type cleaners
Belt washing systems are classified as non-contact cleaners. The use of high-pressure water sprayed onto the passing belt cleans residual material off of the belt.
A series of water jets are mounted onto a spray bar which is housed in a water trough enclosing a short section of the return belt. The spray nozzles are angled towards the passing belt and in so doing, the high pressure water peels product off the belt.
Having wet the return belt in the cleaning process, it is important that the excess water is removed from the belt surface and this is achieved successfully with squeeze rolls. Water left on the belt can in itself pose a problem if it is not removed within the water trough.
Belt washing systems are effective in a variety of applications however, the slurry which is generated at the washing points must be carefully considered and a suitable water supply and slurry removal system must be integrated into the plant design.
iii. Belt turnovers
In cases where cleanliness is particularly important and any spillage must be dealt with comprehensively, as well as cases where the conveyed material is extremely difficult to remove from the return belt, then belt turnovers can be considered.
Turnovers are used in addition to belt scrapers at the discharge point and their function is to turn the return belt over so that the dirty-side of the return belt is on top and the clean surface is in contact with the return idlers.
The adjacent diagram indicates how the return belt is rotated in the conveyor structure.
When a turnover is installed at the head-end of a conveyor, a second turnover must be used at the tail-end, to rotate the belt back such that product is always loaded onto the same surface of the belt.
Turnovers in themselves do not necessarily reduce spillage on a conveyor although the absence of contact with the return idlers does reduce the propensity for spillage.
As the return belt travels towards the tail-end, the residual material dries and loosens. When the belt is then turned over at the tail, the loose product tends to fall off the belt but is restricted to a concentrated, manageable area.
Belt turnovers are used in applications where conveyors are long and / or the conveyor is located in a remote area relative the rest of the Plant. In such cases spillage is often neglected and turnovers are a practical solution to limiting the clean-up to one location only.
The layout considerations require a reasonably long turnover station length and the height required to accommodate the turnover structure are considerations to be born in mind.
Example of a belt turn over station