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The secret to storage solutions

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Dry Bulk,

Roger Horner, ABDIE Solutions, Scotland, provides an insight into expert solutions for handling poor flowing products that are liable to bridge if stored in vessels or silos of an incorrect design, giving examples of challenging dry bulk materials and the best structures and systems to help resolve these issues.

Reliable and efficient dry bulk material storage and related handling systems are crucial for plant operations in many facilities. The key to designing an optimum storage and handling system is having a clear understanding of the dry bulk material it is being designed for, in order to predict and target the exact flow pattern. The density, friction, moisture, size, particle shape and strength of bulk products are all factors to be analysed prior to the design of a storage system. These material properties relate to the flow within the structure. Improper product flow can be detrimental to a manufacturer’s operations in a variety of ways, including product quality, cost and time efficiencies.

ABDIE Solutions has worked with a number of poor flowing products including malt grist, milled malt that is ready for distilling, which is light, dry and fine, and digestate (also known as draff), the waste product left after the distillation process. The density of digestate often varies depending on the process in which it is being handled, but is often very damp. Rolled oats and animal feeds have also proven to have very poor flow ability.

As these products are particularly challenging to flow, it is not beneficial to store the product in silos with a hopper enabling a feed to a conveyor, as the product will easily bridge if the hopper has sloping sides opposite to each other. Many products, particularly the light malt grist often faces this problem.

In the distilling industry, after the steeping of malted barley in hot water to extract soluble sugars, the residue which remains is distillers malt draff, which has particularly poor flow. Similarly, many textiles facilities have the challenge of handling dry filter cake (powder) in their processes. To target both challenging products, it is valuable to consider installing a chisel hopper with full length screw discharger and often also an agitator/breaker bar. This will direct and release over pressured products and avoiding the product packing tightly above the outlet, which can be a possibility with some damp and warm products being handled directly after a press plant.

It is also far more favourable to store the product in a four-sided silo with a hopper, with three vertical sides and one sloping side. This will enable the product to flow at different rates, faster down the vertical side, avoiding the chance of bridging within the hopper. The hopper will then be discharged using a full-length flood fed conveyor, ideally a screw feeder with variable pitch flighting. The width of the outlet and the conveyor will be determined by the product being stored. Often twin or multiple screws will be used to allow the outlet to be as wide as possible.

Many products, especially the malt grist, are stored in hopper silos with a single outlet. Most of these show signs of what is known as ‘hammer rash’, hammer rash is often encountered in silo operations, created in an attempt to remedy the build-up problem, as the side of the hopper, often the discharge point, is hammered with a mallet in order to dislodge a blockage and encourage the product to flow out of the hopper.

Over time, the hammering process takes its toll on the hopper and much like a skin rash, hammer rash presents a visual abnormal change in colour and texture to the metal sections. These ultimately have to be replaced, which is extremely costly and takes up valuable production time.

If the hopper is holding a known weight of product, which would be the case if a silo was holding the malt grist prior to being fed into a mash tun, then all the malt grist in the silo needs to be discharged for the mash to be successful. It is therefore imperative that the silo empties completely, avoiding the need to hammer. Having the correct design of hopper is crucial for this process to run smoothly. This type of structure worked particularly well at Grissan Energy UK Limited and William Grant & Sons.

In some cases, existing hoppers require modification in order to encourage products to flow out of the outlet. One example of this is installing a rotating agitator within the hopper.

This is a proven method of altering round bins that fail to discharge properly. In the client’s case, this was not due to poor design at the beginning of the project but due to a change in product going through the line.

All products handle differently depending on the stage of the process and climatic conditions, but in most cases it possible to design storage facilities to suit the application.

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