Problems in hoppers? Go with the flow

Eddie McGee, technical manager of Ajax Equipment, explains how flow problems in hoppers can be eliminated by using novel geometry and inserts.

Flow problems in hoppers and silos are a perennial problem for chemical engineers. All too often the expected mass flow fails to materialise in practice. Common performance problems are caused by 'rat-holing' and arching, leaving raw materials trapped in the hopper.At best this can result in a reduction in the hoppers' working capacity and poor flow performance, while at worst the static material can degrade or cake against the hopper walls. Production suffers from unreliable flow, and the hopper may need taking offline for repair and cleaning more frequently - a disruptive and costly exercise.

The ideal is a mass flow type hopper, where the material slips on the wall contact surfaces and there are no 'dead' regions of storage, thus providing the optimum flow condition of the bulk material. Mass flow hoppers have the additional advantage of securing reliable flow through the outlets when the material is not slipping on the container walls.

particular problems

The ability of the material to slip at the hopper wall is governed principally by the bulk solid's friction coefficient against the wall material and the shape of the hopper. Wall friction can be measured using the simple apparatus shown in figure 1, and the required wall angles for mass flow can easily be calculated.

Conical hoppers pose a particular problem for chemical engineers. This hopper shape may be easy to make, but it does not create optimum flow, as the bulk solid has to converge simultaneously in two planes as it approaches the outlet. This requires a lot of work, yet the potential energy of the bulk solid available for this work is limited.

If reliable flow is to be achieved then conical hoppers require very steep wall angles. However increasing the height of the hopper headroom to achieve steeper wall angles and thus mass flow is often not possible due to physical plant constraints.

As a consequence, arching and rat-holing problems are commonplace, and operators reach for the scaffolding poles to initiate flow, meaning that the outside surface of the hopper becomes beaten with hammer rash.

The central region of the hopper immediately above the outlet empties well, but there is a stagnant zone that will form a stable rat-hole. This zone is where the bulk of the hopper storage capacity is tied up, and only a small proportion of the hopper's contents are readily retrievable. Titanium dioxide has very high friction, even against polished stainless steel, and it is rarely practical to make a conical hopper wall sufficiently steep to generate mass flow. However, other geometries can be exploited that offer intrinsically better flow shapes, such as using a wedge or v-shaped hopper. This will improve the flow shape because the bulk material will have to deform in only one plane as it approaches the outlet.

relaxing stresses

In turn, this means that the wall angle can be relaxed, making it possible to squeeze the required capacity within the available headroom without compromising flow angles. It is possible to combine two successive stages of single plane reduction so that the hopper walls approach a final circular outlet that sits neatly with downstream feeder requirements.

Another approach is to create hoppers where the side-walls converge steeply while the end walls diverge. This will relax the confining stress in one plane - coined Sigma2 Relief- and provides a narrow, yet effective, slot outlet to ensure flow.

'Caking' in storage hoppers is, of course, best avoided by using mass flow type hoppers to ensure that no material remains undisturbed in storage for an indefinite period.

In particular, fine powders have awkward characteristics: tending to 'flush' when in a loose condition, attaining a poor flow nature when settled or compacted, and becoming virtually impossible to handle if moisture absorbs, resulting in caking. Inserts are used to prevent this, generating mass flow in hoppers at wall angles much less than that required by conventional mass flow hopper design.

Inserts work by modifying the conventional converging flow pattern in a conical silo to an extraction pattern that provides circumferential relief for the hoop stresses, resisting convergence of the bulk.

Made of mild and stainless steel suitable for either conical or wedge-shaped hoppers, inserts can either be incorporated into new equipment as an integral construction, or supplied loose for retrofit installation within existing equipment.

A recent installation at a manufacturer involved replacing two battered non-mass flow hoppers with new units incorporating inverted v-shaped flow inserts. This created a hopper that gave better drawdown pattern - thereby preventing material build-up and maximising live storage capacity, while avoiding variable residence periods of storage.

correcting deviations

Another example of the difference inserts can make concerns a sack-filling problem for fine aerated powders. A new fully automated sack-filling and stitching line for detergent powder would work at only 40% capacity because of significant variations in powder condition. Erratic density variations compromise the weight control and verification system.

Instead of systematically checking the weight and correcting minor deviations every fourth or sixth bag, as is suitable for bulk materials in a stable flow condition, it was necessary to check the weight of virtually every bag, dramatically reducing the filling rate.

Even so, the bags would sometimes be unable to hold the required weight due to the low density of dilated product. Instability of the sacks on the stitching line required constant manual attention to prevent slumping and mis-stitching, while the amount of mess and spillage severely compromised good housekeeping.

Here, inserts were used to prevent 'through-flow' of fluid material in the hopper by establishing a mass flow pattern of movement in the supply hopper. A secondary benefit was that they assisted in the de-aeration of the powder to a more stable and controllable flow condition. Following installation of the inserts, the plant immediately achieved full production with a cleaner working environment.

Some hoppers are so far from meeting the required design criteria for mass flow that it is not practical to undertake a simple conversion. In all cases it is necessary to test the powder's flow characteristics.

Fortunately the chemical engineer can now turn to a whole framework of techniques designed to optimise recalcitrant bulk solids storage and flow problems in hoppers.