Slide-Back Down Inclined Belts
The slip-back of material on inclined belt conveyors is invariably the result of the bulk solid being in a loose, weak condition due to dilation. If the effect is marginal, it may be that the material is disturbed by the repeated, minor changes in cross section as the product passes over the rollers. As long as there is no spillage, the conveyor is able to handle the capacity and the material is delivered in a condition suitable for the following operation, the situation should be tolerable. More serious problems arise when the condition of the powder is so loose that it will not travel up the slope, or it starts upwards and then cascades back down in an ever more fluid condition. This situation is usually initiated by the pre-handling of the feed to the conveyor by a system that either injects air into the bulk. This can be caused by collecting directly from pneumatic delivery and using such devices as air slides or fluidising pads to promote flow. The excess air content in the voids will result in the powder flowing in a loose condition. Air is also entrained in agitated, unconfined conditions, such as free fall, particularly from non-mass flow hoppers, or passing the material down steep chutes that involve a rapid change of direction. The pressure of excess air in the voids opposes the development of the particle-to-particle forces that resist their relative motion, and hence the mass will deform more readily. In extreme conditions the powder will behave like a fluid, but even with a lower air content there is a hazard that once the bulk is unstable and commences to move downhill it will become looser and avalanche down an inclined conveyor. The situation is more vulnerable in warm or hot conditions, because air is more viscous at elevated temperatures and therefore slower to leach out of a mass or bulk material.
It is not practical to rectify or avoid this problem by preventive maintenance. It must be addressed at the system design or review stage, from the viewpoint of powder ‘state control’. That is, to review the flow-route and design the system to cause the powder to flow in a condition that is under control. Provided the powder is reasonably settled in the supply stream, it should be transferred to the conveyor by a chute that is steep enough to guarantee flow, but not so steep that the material experiences a radical impact and change of direction. It is therefore beneficial for the chute to impart horizontal motion in line with the belt conveyor run, to affect as smooth a transition as possible to avoid disturbance of the bulk. The cost of such attention to detail is rarely significant compared with the costs that can arise from perennial handling difficulties. Chutes are relatively low cost items and, as such, often do not receive the degree of design attention that their essental function merits.
It is too late to do anything about it if the material is over-dilated when presented to the conveyor. Feeding a bulk material to an inclined belt conveyor in a fluid condition is inviting problems. Generally, a de-aeration feed system would be the best way to deal with such a situation. This would comprise a mass flow feed hopper with a de-aeration frame, an extraction slot that maximised the cross section of draw-down and an outlet gate controlled by a level probe. The purpose of this equipment is to maintain stock in the hopper for sufficient time for the powder to attain a stable condition when it is fed out.
Again, the cost should be faced before problems arise, as they are inherent in a sound system. Handling is not a value adding function and so there is a tendency to skimp on specifications. It is however an essential production function and, if does not work well, can subtract value from the bottom line intensively and fast. ■
Re: Retrograde Movement Of Materials At Belt Roller Points
Hello Thomas,
Mr. Lyn Bates has given detailed answer. I am adding few remedial hints / possibilities :
1) If possible, feed chute inclination (layout) should be reviewed to create more velocity of material in the direction of belt travel.
2) If the belt sag % is higher (i.e. comparatively loose belt), then the material agitation and tendency to roll / slide back at idlers will increase. Reduce the belt sag by increasing the tension or by reducing the idler pitch. 1% of sag is the best if you can go up to that level. However, this is a serious design issue and should only be done with the aid of conveyor designer, and after seeing its suitability without overloading belt, pulleys etc. If you have this problem with smaller conveyors, often the belt and conveyor components are of oversize and may have a room for extra forces
Meanwhile, please inform the material size, conveyor inclination, conveyor inclination at feed point, carrying idler pitch, belt speed for better understanding of the problem.
Regards,
Ishwar G Mulani.
Author of Book : Engineering Science and Application Design for Belt Conveyor.
Email : parimul@pn2.vsnl.net.in
Tel.: 0091 (0)20 5882916 ■
Re: Retrograde Movement Of Materials At Belt Roller Points
To Mr. Bates
Thanks for the boost in the proper design of chutes that can eliminate this type of flow behavior.
To Tominer,
I assume the product is a fine powder, similar to cement, and is aerated in is transfer.
I will offer a more informed opinion, if I can see the chute design and the product size distribution. This consultation will be at no charge.
We have worked on the transfer of cement powder to substantially reduce fluidization during its drop from bin or conveyor onto an incline belt. The trip is to not allow the material to increase in velocity during the transfer and to maintain consolidation of the flow stream. We analyze the flow field and proper chute geometry using Discrete Element Modeling (DEM). We have found a particular chute form that achieves this goal.
Lawrence Nordell
Conveyor Dynamics, Inc.
email: nordell@conveyor-dynamics.com
website: www.conveyor-dynamics.com ■
Re: Retrograde Movement Of Materials At Belt Roller Points
Dear Thomas,
You also have good suggestions from Mr. Nordell.
Coming to my earlier reply, I would like to add few more sentences.
If your problem is related to comparatively small length conveyors, equipped with screw take-up then tighten the take-up to permissible limit. This can give some relief in your problem but it cannot eliminate the problem all together, if the same is arising due to improper inclination, chute, etc.
Incidentally, your question does not mention fine powdery material, so I have presumed it to be average sized material.
Regards,
Ishwar G Mulani.
Author of Book : Engineering Science and Application Design for Belt Conveyor.
Email : parimul@pn2.vsnl.net.in
Tel.: 0091 (0)20 5882916 ■
Retrograde movement of materials at belt roller points
I have a question on what appears to be retrograde movement of dolomite materials at each roller station.
Has anybody done any research on the cost effect of this down slope movement of materials and its effective lengthing of travel distance from tail to head roller.
Is the annual cost of retrograde material movement significant?
What preventative maintainance proceedures can be integrated into a maintainance schedule on either a daily, weekly or monthly schedule that is cost efficient (if any)?
I've also noted that this process is magnified at the discharge hoppers due to the vertical movement of materials as compared to the horizontally inclined movement of the belt.
What benefits are attained using feeding chutes that discharge materials coliniar with the movement of the belt and are they cost effective and over what payback period?
Lots of questions from a new miner!
tominer ■