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Specialist article
01.01.2020  |  569x
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Tips to reduce high starting loads on feeders

It is essential for reliable performance that the outlet of a hopper is large enough to ensure the material to flow out so a design margin is necessary above the minimum needed. Slot outlets secure flow benefits and extra capacity but the width of these are often tapered to give progressive extraction by belt feeders. As the minimum size has to be greater than the ‘critical arching span’ to ensure reliable flow, the larger widths allow heavier loads to act on the feeder.

Excessive loads can arise on feeders with outlets that are significantly larger than the ‘critical arching span’ so it is important to base the design on measured properties of the material to be handled and avoid excessive overdesign. Even so, feeders can experience much higher starting loads than those of sustained running. The culprit is often not high compacting stresses, although those generated during filling a hopper may contribute to the state of packing and confinement, but often the result of high passive stresses that arise due to shear plane expansion of the confined product. This expansion is necessary to allow particles that had settled to a close packed, overlapping condition to move apart sufficient to allow them to pass each other in the shear plane, which is usually a few particles thick. The expansion needed clearly depends on the degree of granularity of the product.

Although the volume change that occurs in initial shear may be tiny, it must be made available by a compensating compaction within the bulk or by lifting the whole bed, which may be substantial. Hard granular solids in a confined condition can offer a massive resistance to compaction that is ultimately limited only by the strength of the constituent particles. Brute force is not the solution, as the powder needs may be many times that of continuous operation and would only surge for a short period. This could make both the capital and running cost of a drive based on starting load uneconomic for the duty.

Because this high loading only occurs following initial fill of a hopper, it can be avoided by running off a tiny amount of product at a very early stage of loading. This will initiate flow and develop a passive state of stresses in the outlet region, to form a ‘stressed arch’. Subsequent loading will not restore the original active stresses on the outlet. In the case of an empty feeder, the necessary extraction to achieve this condition can usually be achieved before the discharging material reaches the exit end of the feeder.

If this operating procedure is not practical, the best approach to relieve these shear expansion stresses is by providing local room for expansion. This may be done by way of flow inserts that offer local void space close to the outlet region, or by offsetting the discharge channels, so that space is available for the displaced bulk to move into.
Resistance to the initial flow of fine products may also be caused by the air demand of the expanding plane being opposed by the low porosity of the bulk. This gives rise to a compacting stress on the failure surface due to the pressure differential to ambient. Without attention this feature can give rise to the phenomenon of ‘Slurping’.

This behaviour occurs when the rate at which material separates from a flow surface to a vacant region of the flow channel is less than the potential exit rate. In such circumstances, loose material detaching from the failing surface increases progressively with a growing arch until, if the flow channel is wide enough, the size of the arch exceeds the ‘critical span’.

Beyond this point the arch collapses and fills the void to the outlet. If the collapsing material is dilated to the point of fluidity, the entire contents of the system may gush out in a ‘Flush’ or ‘Flood’ that can have serious consequences. Otherwise the void is filled and the process repeats in a cyclic ‘Slurping’ manner.

A solution in these cases is to provide the hopper outlet region with local access to ambient air or by supplying a small injection of air near the hopper outlet immediately prior to the start of discharging the system. In more difficult cases, a continuous bleed of dry air at a low, controlled rate may be supplied near the base of a freshly loaded fine product. The amount of air should be restricted to that required to replace settlement air escaping from the voids sufficient to prevent the bulk reaching the state of compaction where it becomes difficult to flow.

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