One lump or two?

London – A couple of years ago The Wolfson Centre received an enquiry from a food manufacturer asking for help with material characterisation and design analysis for a sugar-intake and storage silo.

Silo design involves the measurement of the flow properties of a bulk material, and selection of key design parameters for the silo to ensure the flow pattern is as desired and discharge is reliable.

After a few weeks, the prospective client decided against going ahead with the project, due to the cost - a few thousand pounds. The company, instead, opted to rely for the design on the manufacturer of the equipment who claimed to have provided “loads of sugar silos before and never had any problems”.

About six months later the same person telephoned to say that he had bought the silo and had one or two problems with it. The Wolfson Centre was asked to send its experts to the site to help troubleshoot the problems.

The issues were found to be lump formation, hardening of the inventory and rat-holing. On inspection, the cause was obvious: the silo was operating in a core-flow discharge pattern.

This core-flow pattern allows material to flow down through a live “core” in the middle of the stored material, with the material around the outside not moving until the level falls low enough to reclaim it.

Such a discharge pattern is common in silos and soften the “default” pattern if a device is not designed to work in any other special way.

Being the main sugar storage for the site, the owners naturally did not want the silo to run empty due to potential stoppage of the process. However, with a core flow pattern, the material in the bottom never moves until such time as all of the contents are emptied out: it is “first-in, last-out” discharge - the reverse of good stock rotation.

Sugar is sensitive to temperature, time and migration of moisture with changes in temperature, so the stagnant material began to form lumps and a large proportion became solid. The cost to the user was considerable. Arranging for a specialist silo-cleaning crew to come and undertake digging out of the hardened material under controlled confined-entry conditions was costing over £10,000.

Disposal of about seven tonnes of unusable sugar as waste had a landfill cost, in addition to the value of the sugar, of several thousand pounds at purchase prices. This was only four months after the installation had been got up and running, so it seemed that this expensive operation would require repetition two or three times a year.

Of course, the proper arrangement to have avoided this would have been to have had the silo designed to deliver the sugar in mass flow.
With a mass flow discharge pattern, all the material moves down when some is taken out of the bottom. There is no stagnant material in long term residence and it is delivered in a first-in, first-out pattern so stock is fully rotated.

This would greatly minimise the occurrence of lump formation and avoid the regular costs of digging out and sending tonnes of sugar for disposal.

To achieve mass flow the material flow properties must be measured and some calculations performed. This was the purpose of the material characterisation and silo design exercise that we had proposed, but that had been turned down “to save money” at the design stage.

Major modifications

Now the silo had been built, changing it from core flow to mass flow would mean major engineering modifications, so the user has decided to live with the problem and bear the cost of £20 to £40k every year for the regular clean-outs.

What of the manufacturer’s claim that they’d supplied sugar silos before, and had no reports of trouble? Were they lying? More likely the context was different - maybe their previous silos had gone into factories with a faster turn-over, and multiple silos that were emptied regularly, before lumps set in.

Or maybe they had been installed internally in buildings with stable temperatures. Either way, relying on that experience clearly led to trouble this time.

The lesson? Don’t cut out the small amount of time and money up-front on powder characterisation and design analysis dedicated to getting exactly the right equipment. This would have identified the problem and designed it out before metal was cut.

It may seem like money and time wasted if you think you can rely on the equipment manufacturer’s past experience, but this example - like so many others that are brought to us for troubleshooting - shows that past experience alone is no guide to future performance.

A few thousand pounds spent on a consideration of the operating context, the properties of the material and the design requirement to ensure smooth functioning, would have saved a regular cost of thousands in operation every year.

Sugar isn’t unique in needing careful design for reliable handling, and not every situation justifies the use of mass flow. To misquote a proverb, “design in haste, and you may repent at leisure.”