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Why this pig will fly

27 May 2011

EXCLUSIVE The inventor of 'ice pigging', Professor Joe Quarini of the Department of Mechanical Engineering at the University of Bristol, talks to FP Express about why this technique is important to the food sector

ABB has been working with Bristol University about a technique that uses ice to clean pipes. Ice pigging is said to overcome many drawbacks of conventional cleaning by pushing a plug of icy slurry under pressure through pipes to remove debris.

Ice pigging is a new buzzword in food processing. It’s important to the industry because of the impact it has on food safety and hygiene as well as in reducing downtime. Professor Joe Quarini is at the forefront of this groundbreaking initiative.

Invented at the University of Bristol, “ice pigging” will also lead to big cost reductions and environmental improvements in a range of cleaning operations.

There are several features within this Automation System that can’t be used by ABB systems but will become available within the next six months when the new PLCs are released. ABB supplied all the flow, pressure, conductivity and suspended solids instrumentation, as well as PLCs, drives and low-voltage electrical products to help turn the research project into a practical working system.

“We’ve got a good relationship with ABB and they’ve helped by providing best-in-class equipment,” says Professor Quarini. To define ‘’ice pigging’’, he explained how it all started.

‘’In the oil industry, the relatively warm crude oil that comes out of the oil well on the seabed has to be processed into saleable products like petrol, diesel, aviation flues etc. The processing is done in large efficient land-based petrochemical plants. To get the crude from the well to the processing plant it invariably is made to travel through steel pipes.

‘’These tend to be colder than the crude so as the crude cool, the vapours condense to liquids. The liquid thickens up, the viscosity goes up and some very long chain oil molecules turn into waxes and deposit on the pipe wall, the coldest part of the system.

‘’The pipe you started with, which had, say, an eight inch internal diameter, is slowly losing its ability to carry the design volume flow-rates as the effective flow diameter is getting smaller and smaller as it furs up with wax etc, just like arteries would fur up in the heart. Somebody has paid a lot of money to drill that oil well and build that pipe; their interest isn’t so much in clean pipes, they want pipes that give them the hydraulic performance they require to make the venture economically viable. If the pipe gets too small, money is lost.

Texan oil industry
‘Pigging’ has been going on in the hydrocarbon recovery industry for the past 100 years, Professor Quarini says. One of the interesting stories that provides an explanation for the genesis of the term ’pigging’ comes from Texan oil industry.

‘’Because of the need to clean the pipe in the oil industry, a metal object wrapped in a leather jacket was put in the pipe and a driving pressure was applied to one side of the object. The leather provided a reasonable hydraulic seal and the object experienced high pressure behind and low pressure in front of it, and behaved like a projectile (a blowpipe and dart, or cannon and cannon ball arrangement).

As the object travelled down the steel pipe, the leather wore until there was metal-on-metal sliding contact and it made a screeching sound, which led to the Americans comparing it to a pig’s squeal.

‘’Pigging is really putting an obstacle – a piston – in a pipe,’’ says Professor Quarini. ‘’The dimensions of the piston have to be such so that you can hold pressure, and you apply a pressure differential between the front-end and the back-end of the piston of the object, which then moves through the pipe and cleans it.

‘’Most of the pigging patents around belong to the oil companies or the people who service the oil companies. Pigging is an easy way of cleaning a long pipe with minimum fuss. The pig or piston has to be the right size: it has to be slightly smaller in diameter than the pipe or it gets stuck. If it’s much smaller, the pressure or fluids that are pushing it, go around it and don’t push it at all – it’s left at the bottom.

‘’This means conventional pigging is only really successful if the pipe is a fairly constant diameter and fairly straight. If you have an obstruction like a valve or a T, it doesn’t know what to do. For oil companies that’s fine because the pipelines on sea beds and elsewhere tend to be fairly straight and of constant diameter.’’

Which brings Professor Quarini to the subject of ‘’pigging’’ in the food industry. ‘’If you were making yogurt for instance, and you came to the end of the batch, and you needed to clean because it’s a new formulation, what you currently do is push water behind it. The water ‘drills’ a little hole through the centre of the yogurt and you get a lake of milky rubbish.

‘’As more water is pushed through the small hole progressively gets bigger and bigger, until the pipe is clear of yogurt. This process of slow removal of material starting from the centre of the pipe and gradually moving out towards the wall is known in the industry as ‘ratting’, which results in large volumes of valueless dilute product.

‘’You can’t do anything with it: the water boards will refuse to take it because of the BOD – biological oxygen demand – so you need a huge effluent treatment plant,’’ says Professor Quarini. That means it’s costing you to throw the waste away. It’s also costing you in the product you haven’t sold, as all the stuff in the line is now being wasted. And it’s costing you in downtime – you’re having to push water and the yogurt sticks to the wall.’’

Impossible topologies
Professor Quarini describes it as like trying to wash emulsion paint out of your brush, ‘’which goes on forever. It would be great if you could have a ‘plastic pig’, which pushed the yogurt through all the way to the delivery nozzles, and that way all the stuff (inventory) in the pipe gets sold to the punter for money. You probably still have to clean but at least now you’ve only got traces of the stuff as you’ve got rid of most of the bulk for money.’’

Because there are only traces left, the cleaning process is faster. In addition, fewer chemicals are used so it’s less caustic to dissolve things because you only need to clean away the remaining bits. The food industry likes the idea of pigs but doesn’t use conventional ones because food industry pipes have impossible topologies.’’

Real estate is expensive so in a food factory they try to squeeze in as much as they can, Professor Quarini explains. ‘’The pipes go backwards and forwards, there are many added elements such as valves, Ts, heat exchangers, pumps, stirrers, and the pig just couldn’t cope with that.

‘’So pigging would be advantageous in the food industry, it would give environmental benefits and for the food manufacturer it’s an added bonus because he’s not throwing away his waste – he’s selling it. He can reduce his effluent treatment and his downtime, so your factory’s operating in productive mode for longer periods.’’

This, says Professor Quarini, is where the ‘ice pig’, comes into its own. ‘’It’s an array of small crystals of ice, just water, in a liquor – water with a freezing point depressant, usually salt in it – and it’s a very thick ice slurry. If I put an ‘ice pig’ on a table it would pile up like a big pyramid or cone, it wouldn’t flow like a liquid. But if I prodded it with my finger, it would go through it.

Reduced effluent
‘’The ice pig is compliant, it doesn’t care what shape the pipes are, it simply fills them. Think of a glacier going through a valley – it will adopt the shape of the valley through which it is moving, scraping topsoil off, leaving the hard valley bedrock behind.

‘’The ‘ice pig’ scrapes and removes the yogurt, leaving the stainless-steel pipe untouched. In addition, it goes around things which are solidly anchored such as valves and instruments. But it moves like a plug so instead of flowing through like water, you’re pushing most of the product through to the punter. There is an area between the product and the pig where there is melting, so that has to be taken into account. But you’ve reduced your effluent load by about 90%, so you’re only going to throw away about 10%.’’

The cleaning time is now minimal and a clean-in-place (CIP) can rapidly be done because there’s got nothing (or certainly a lot less) to remove. Environmentally it’s superb because the reduced stuff you’re throwing away is now colder, and the problem with effluents is they’re warm. The water boards don’t like you placing warm stuff in their system because increased temperature reduces the dissolved oxygen content, and thereby reduces the ability of the water to support aquatic life.

‘’The only negative is you need electricity to get the refrigeration systems to work to make it but in terms of cash that isn’t significant.’’

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