Bulk issues addressed

26-Feb-2008 - Last updated on 16-Mar-2017 at 15:03 GMT

Explosive materials; 'rat holing'; and hygienic design. Powder handling presents some thorny issues, says John Dunn

On Saturday January 26, fire broke out in a storage silo containing cereal flakes at the Weetabix factory at Burton Latimer, near Kettering. According to a BBC News report, an initial investigation found that dust from products made in the factory was inside an extractor on a hopper.

Three and a half years ago on Thursday August 12 2004, a BBC News report said that fire had broken out in an industrial oven and dozens of workers had been evacuated from ... the Weetabix factory at Burton Latimer.

As Oscar Wilde might have put it: "To have one fire may be regarded as a misfortune; to have two looks like carelessness."

But forget Oscar Wilde, because in the case of Weetabix we do not know and cannot guess at the reasons for this latest fire until the incident has been thoroughly investigated and the true cause of the fire established.

However, Paul Mayer, sales and marketing director of storage silo manufacturer Braby, is convinced that there are still a number of food companies, including some well-known names, that are not conforming to the ATEX legal requirements for controlling explosive atmospheres in factories. Yet this is 18 months after ATEX became mandatory for all existing plant and equipment.

ATEX forms part of the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR) 2002 and is policed by the Health and Safety Executive (HSE). In the food industry ATEX in effect applies to powders such as flour, sugar, starch and any other finely divided or dust-like dry organic ingredient. When these powders are mixed with air they can form explosive mixtures. And it doesn't require an awful lot of dust for it to happen.

According to Mayer, government statistics show that 37% of industry fires take place in bakeries. Yet three and a half years after ATEX was imposed, many food manufacturers have been slow to comply, he says.

ATEX compliance can be costly

The main problem, says Mayer, is that complying with ATEX can be costly. So in February Braby launched a low-cost, standard, off-the-shelf ATEX-compliant vacuum transfer and batch-weighing silo system for the food industry. The Vacatex system can be delivered and installed as a complete package for £85,000, significantly less than the typical £120,000 to £140,000 for a bespoke system. "It can be used by anyone handling flour, sugar, starch and so on," says Mayer.

The system will transfer up to 3t or 6t an hour and deliver pre-determined batches from 16kg to 196kg to within 200-300g accuracy over distances up to 40m.

But it is not just the expense of ATEX that has some companies dragging their feet, says Mayer. "There is still a degree of scepticism as to the validity and interpretation of ATEX, even among some very big blue-chip names in the bakery industry. But how many HSE inspectors are there to cover all the industries in the country who have the technical knowledge to be able to focus on something as specific as an explosion risk?" he asks.

"I will say, though, that every time I have sat down with someone who wants to listen, they have all come away with a desire to put it right. But there are some companies who won't even entertain it [ATEX] because as far as they are concerned it is unnecessary."

Part of the problem, says Mayer, is that fires and explosions don't always result in a notifiable injury, and thus the fire goes uninvestigated. "You could have a small fire in the corner of your bakery. It is extinguished. No one gets hurt. It is not notifiable."

But be warned, says Braby. Under the new Corporate Manslaughter Act due in April, there's a three months mandatory sentence for failure on ATEX, he says. "If you ignore ATEX and you injure or kill anyone, then someone's for breakfast with Her Majesty."

The Great Fire of London may have begun in a bakery in Pudding Lane. But fire and explosions are not the only things that bedevil dry, powder-like ingredients. There are 'rat holes', too.

Rat-holing problems

Ideally, dry ingredients should flow from a storage hopper under mass flow conditions - all the powder moves during discharge. But to achieve mass flow, hoppers and silos have to be properly designed to suit the flow characteristics of their particular ingredients.

Get the angles of the hopper walls wrong, and high friction materials stick to the insides. Only a rat hole in the central region of the hopper immediately above the outlet empties, leaving a stable stagnant zone around the rat hole. Result? Only a small proportion of the hopper's contents is readily retrievable.

And so, without the traditional clout to the hopper from a length of scaffold pole to loosen things up, ingredients from different batches can hang around inside, making a nonsense of any attempt at a batch traceability system.

Mark Waters, director of bulk handling equipment supplier Ajax Equipment says this 'residence time' can have all sorts of knock-on effects. "A food product might have a limited shelf-life, so if it is left in an enclosure for a length of time it could go off, even develop bacteria, leading to degradation of the quality of the product." And there's batch contamination. "You put in a new batch of ingredients and it gets contaminated by an earlier batch. In some poorly designed hoppers, stuff could be hanging around indefinitely," says Waters. "If you have a mass-flow designed hopper, then you have a geometry that will give you first in, first out discharge."

So if your silo is showing the tell-tale signs of the scaffold pole, how can you get true mass flow discharge without replacing your materials transfer system? One way is to use inserts, says Waters. "These alter the flow pattern inside the hopper. By changing the way the material discharges, inserts encourage complete discharge, simulating mass flow."

But inserts may not be the answer. "You can't always get access, and fixing the inserts has to be considered." Another option is to replace the discharge end of the hopper with one of the right geometry.

The mass flow characteristics of a hopper are directly related to the particular ingredient. So Waters' advice is for manufacturers to think ahead about what they might be making in the future and what sort of ingredients that might mean. "We suggest then that they get a hopper to discharge a range of materials. We would test the various materials and design the hopper for the most difficult. That would then give very good flow conditions for a range of materials."

So, you have stopped your ingredients from going bang. And you have got rid of the rat holes in your silos so that your ingredients all discharge perfectly. Super.

But hang on, why is it still costing us a lot of time and money to strip down the line to clean it. Can't the equipment supplier do something to make cleaning simpler, less costly?

Welcome to EHEDG Guideline 36: Hygienic Engineering of Transfer Systems for Dry Particulate Materials. Just launched, Guideline 36 is the latest document from the European Hygienic Engineering & Design Group (EHEDG) aimed at helping food equipment makers ensure that their equipment is designed to be cleaned as quickly, simply and thoroughly as possible.

Guideline 36 (available in the UK from Campden and Chorleywood Food Research Association - CCFRA) describes the hygienic design principles for powder transfer systems, including mechanical transfer systems such as bucket elevators and screw-feed systems, gravity-feed systems, belt conveyors, vibratory transfer systems, and pneumatic systems.

Karel Mager chairs the EHEDG working group on dry materials. He works in quality management at what was Quest International in The Netherlands, now part of Givaudan. The benefits of EHEDG-approved equipment mean the equipment is easier to clean, says Mager. "You have a sort of guarantee that you can more easily clean your equipment. It means you will not foul that equipment so quickly. And if you can clean it quicker, then you have less down-time on your process line."

Also, he says, the concern about allergens such as nuts and milk powder is an impetus to the better hygienic design of equipment. "The easier it is for you to clean it, the easier it is for you to get rid of allergens."

But Guideline 36 is not the end of EHEDG's work on dry materials, says Mager.

"You produce a powder and at the end of the line you have to pack it into 25kg bags or in to 'big bags'," says Mager.

"So we will write a guideline about that. Also, there are dust extraction systems, and simpler equipment like powder blenders and mixers." FM

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