Get a big load off your mind

07-Mar-2005 - Last updated on 12-Apr-2010 at 09:45 GMT

Handling 'neurotic' or 'sadistic' ingredients in bulk can present more problems than you originally expected, as John Dunn discovers

Imagine you have a 9m high, 3m diameter stainless steel silo standing outside in your goods inwards yard. And let's say that when it's empty it weighs 1,000kg, and when it's full of flour or cereal for your next batch of buns or biscuits, it weighs 8,000kg. How do you know how much is in it?

You will know because, like most weighing applications in the food industry, the silo relies on load cells to weigh its contents. Sitting on an array of loads cells, the silo will automatically record each change in weight as ingredients are pumped into or discharged from it during deliveries and processing.

But what happens if a 40-metres-per-second gust of wind blows into your yard? Well, the silo could actually appear to weigh three times as much. The wind load on the sides of the silo would upset the reading from load cells, making the silo and its contents appear up to three times heavier than they actually are.

As it happens, it would take a pretty freak gust of wind in your yard to do that -- 40m/s is 90mph, and anything over 75mph is a hurricane! Nevertheless, the figures illustrate one of the ways in which inaccuracies can creep into batch processing when weighing bulk food ingredients.

And with the new European food traceability regulations that came into force on the January 1 this year, there is now even more pressure on food manufacturers to ensure the accuracy and reliability of all their bulk handling equipment, not just weighing systems.

Under the new European Union food law, EC Regulation 178/2002, it has been mandatory from the start of this year for all food businesses to be able to identify where they have obtained a food ingredient or food product. And in the case of companies not selling to the final consumer, they must now also be able to identify where they have sold their products. This information has to be made available to the authorities on demand. The small firms lobby group, the Forum of Private Business, has estimated that for small food producers the new rules could add 6% to their wages bill, costing them £50m a year.

So far the legislation doesn't include a requirement to track how batches are split and combined on the process line to create particular products. And it is not yet mandatory to identify and record the bulking up of ingredients from a number of suppliers. However, there are provisions for the development of further rules in the future, the Food Standards Agency has warned.

So, thumping the side of a silo with a piece of pipe to discharge the last traces of ingredients stuck to the walls hasn't yet been banned by Brussels. But manufacturers are increasingly being pressurised into recording the source of all ingredients in each batch. And so inevitably, left-over ingredients from one batch that are stuck to the hopper walls will not be allowed to get into the next batch unrecorded, if at all. The writing is on the wall for 'hopper rash'.

Yet despite advances in the understanding of bulk ingredients and how they should be handled and stored, hopper rash is still a common complaint. According to Mark Waters, director of bulk handling specialist Ajax Equipment, the design of storage and handling equipment frequently ignores best practice.

It's too often a question of lowest cost, he says. The pressure from customers such as supermarkets to reduce costs means that food manufacturers are often driven to reduce their capital spending budget by going for the cheapest solution. "Many buy equipment which doesn't take into consideration the characteristics of the material that's being handled," says Waters.

And if hoppers do get clogged and feeders break down as a result, then it's a different budget that's saddled with the bill for the costs of cleaning and maintenance, as well as the cost of lost production from machinery downtime.

Ingredient personalities

Bulk ingredients have personalities, rather like human beings, suggests Lyn Bates, md of Ajax. He characterises them into seven personalities from normal, through neurotic and introvert, to sadistic. 'Normal' bulk materials are uniform, of consistent behaviour, and are unaffected by surrounding conditions. On the other hand, 'neurotic' materials are awkward to handle, unstable, prone to erratic changes, and sensitive to their surrounding conditions, he says. And 'introvert' materials "slyly conceal their nature and are prone to spring unpleasant surprises". They need to be thoroughly investigated to establish their potential behaviour, says Bates. And 'sadistic' materials are downright nasty and to be avoided, he warns.

The rule is that any variation in the composition or condition of a bulk ingredient essentially represents a different material. So even if bulk handling equipment is properly designed for an ingredient, there can still be problems caused by the way it will be handled and stored, adds Waters.

"When, for instance, an ingredient is discharged into a rigid intermediate bulk container (IBC) or flexible IBC (FIBC or 'big bag') it will be in a nice loose condition. But if it is then stored, perhaps with other big bags on top, the material at the bottom becomes quite compact." Also, says Waters, settling can occur when transferring ingredients in a big bag or rigid IBC half a mile across the factory. "The problem is getting it out and ensuring complete discharge."

As an example of where cheapest isn't always best, Waters refers to the case of a 4m3cocoa hopper and inclined screw elevator installed at the Singapore factory of an international confectionery company. Ajax was called in because the hopper was discharging from one side only, leaving half its contents behind to absorb moisture and stick to the hopper walls. The result was that the working volume of the hopper was reduced by 50%; down-stream processes were starved of cocoa; and there was potential for cross-contamination between batches.

"We think the equipment was bought on price, with not much thought given to the characteristics of the actual product, cocoa," says Waters. The Ajax solution was to install a second screw feeder in the hopper to ensure even and complete discharge to the screw elevator.

Another way to improve traceability and avoid any cross contamination from ingredients left behind in silos and hoppers is to use two silos side by side, suggests Phil Plummer, using one for production while the other is emptied ready for the next delivery. Plummer is sales director of Ingredients Handling Solutions which represents the German ingredients handling and automation company AZO.

"Rather than, say, having one 50t silo, you could split deliveries into two 25t silos -- that's still large enough to take a normal 23-24t tanker load. So you start with one full silo, but while you're using that on production, the other is being filled ready to bring on line when the other silo is totally empty."

That would avoid overlaps between batches and improve traceability, says Plummer. And if you had one silo dedicated to supplier A and the other to supplier B, that would then enable you to track the ingredients in any batch back to a particular supplier, he says.

But food manufacturing, and the issue of traceability, are still highly dependent on weighing ingredients at key stages during the process, says Plummer, particularly after they have been discharged into IBCs and big bags. And here the onus falls squarely on the load cell to get it right.

But the readings from load cells can be affected by all sorts of unexpected things, not just hurricanes, warns Peter Zecchin, head of process weighing at Vishay Nobel Weighing Systems. Speaking recently at a food sensors meeting at the Institute of Physics, Zecchin explained that temperature changes, humidity, impact and vibration, structural deflection, and pipework connections, as well as wind loads, could all adversely affect the accuracy of load cells under hoppers, silos and storage tanks.

As a vessel expands on a hot day, it shifts its weight on the load cells. Damp can get in to load cells -- they can in effect 'breathe' moisture as they operate, says Zecchin. Impacts and vibrations can come from less obvious sources such as lumps of ingredients falling into the vessel, and mixing machinery. And the forces generated can lead to uncertainty in the readings and possible damage to the weighing system.

Unreliable results

Zecchin is also chairman of the Institute of Measurement and Control's weighing panel which has produced a number of guides to industrial weighing systems (see http://www.npl.co.uk/instmc_weighing_panel). The panel is currently compiling a review of dynamic weighing systems such as batch weighers, belt weighers, and loss-in-weight feeders.FM