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Purging with an icy pig

Engineers in the Somerset town of Bridgwater are using food industry techniques to help it clean up drinking water supplies. Paul Thompson reports on ice pig technology use.

You would be forgiven for thinking that nothing much happens in Bridgwater. For most of the many thousands of visitors who drive through the town on their way to Exmoor or the Butlin’s holiday camp at Minehead it barely warrants a shrug.

But the town is set to become the showcase for an innovative pipe cleaning technique thanks to engineers at water and sewerage network operator Wessex Water.

Under its Operation Clean and Clear, the company is investing some £11.5M in upgrading almost 30km of water pipelines around the Somerset town. Engineers are in the latter stages of an 18 month project to reline, replace and clean out the existing network of water mains which serve Bridgwater in a one-hit scheme which will introduce a new pipe-cleaning technique developed alongside research partners at the University of Bristol and Bristol Water.
Parts of the network date back to the early 1900s and with these cast iron and ductile iron pipes starting to cause concern, the decision was made to carry out the work after a network appraisal in 2008.

“There had been a couple of major incidents of discoloured water which had been picked up by the Drinking Water Inspectorate. We want to maintain purity of supply to our clients and were able to identify a number of ageing cast iron and ductile iron pipes that were causing problems to the network,” says Wessex Water project manager Davin Eversett.

Who’s who

Client Wessex Water Services
Main contractors: Wessex Engineering and Construction Services Lewis Civil Engineering

 

Those old pipes are unlined and have corroded, leaving iron deposits that can affect water quality when disturbed.

“We have to maintain flow rate through the pipe so that solids remain settled and do not purge through the system if that rate fluctuates. At the moment the network is in a settled state and we are trying to make sure it stays that way as we work. The whole of the scheme’s programme of works is based around that single criterion of minimising impact on water quality to our customers as much as possible,” says Eversett.

Potable water for most of Bridgwater is drawn from the Danesborough reservoir to the west of the town. It feeds through the network via two link mains - the northern and the southern. Branches from these mains carry water throughout the town before the two meet again to the west and feed into the Bath Road pumping station where water is collected into a holding tank and sent on to serve Polden East, Polden Central and West Huntspill.

“We have found that the supply returns to normal much quicker than it might have done under normal circumstances”

Davin Eversett, Wessex Water

 

Under the first phase of the two-part scheme Wessex Water and its south Wales-based AMP5 partner Lewis Civil Engineering will install, reconnect clean and flush pipes predominantly on the northern link. This section feeds one of Bridgwater’s largest employers, fruit juice producing giant Gerber. Around a fifth of the network’s maximum instantaneous flow of 250l/second can be directly apportioned to demands from its plant. This flow rate is required throughout the working day. The Bath Road pumping station also has to maintain flows of 150l/second to feed Bridgwater’s outlying districts during peak demand periods.

First phase

The first phase of the work will allow the southern area to be supplied via the northern link once improvement work has been carried out. Currently demands on the northern link are low at around the 70l/second mark but this is set to change during the second phase of the project and on completion. Once finished, water flows will be shifted between the northern to southern links on a regular basis, making the network more efficient and ensuring that both links can accommodate the maximum flow rates. Currently the network is in a settled state where all the main solids have dropped out of suspension, but any increase in flow could stir these solids up to affect the quality of the drinking water.

So the project team is cleaning out the existing retained network using an innovative method that will clear settled material while lessening impact of the work on Bridgwater’s population.

Normally pipes are cleaned using systems where a foam swab or “pig” is dragged through the network, clearing debris as it progresses. But in Somerset, Eversett’s team is using a pig formed from ice (see box). It is, he claims, more effective, less intrusive and more efficient than standard pigging methods.

“Normal methods can be fairly intrusive. You have to cut the pipe to get the swab in and they can easily get stuck.

Ice pigging

The supply is off for longer and the swabs can damage the pipe. With the ice pigging system the ice is pushed through an existing hydrant and expands to fit the bore, it is then pushed along the pipe run by the head of water,” explains Eversett.

Some 9m3 of ice is introduced through the valve to form the plug, which picks up loose deposits from the pipe invert as it travels through the bore in a similar way to a glacier picking up moraine as it travels down a mountain valley.

And because the ice slush plug doesn’t create a bow wave of sediment as it passes through the bore, the impact the work has on water quality is minimised. Should the ice pig ever get stuck there are no costly excavations needed to rescue the swab, just a wait while it melts.

“It’s pointless to say there is no impact because it’s not true. It is very much reduced though. We monitor turbidity and have found that the supply returns to normal much quicker than it might have done under normal circumstances,” says Eversett.

The pass at Bridgwater takes around 1.5 hours to progress through the 1km of 450mm diameter pipe.

It’s not an issue for Eversett though. He is happy with the performance of the new method and is confident it will go on to become another tool in the belt for water engineers.

“Because it is a Drinking Water Inspectorate undertaking, the scheme has to be complete by July 2011. There shouldn’t be any problem hitting that,” he says.

It is a date the 45,000 Wessex Water customers in the area will be looking forward to with relish.

Where ice pigs come from

Developed by Professor Joe Quarini at the University of Bristol the ice pigging method was initially intended to help clean pipes in the food industry.

Liquids used to clear pipes were inefficient but Professor Quarini hit upon the idea of stiffening those liquids so that they could displace material in pipes and clean more efficiently.
Rather than a single block of ice, the pig itself is a mass of crushed ice that is introduced into the pipeline and pumped through it, displacing material as it travels.
For the Bridgwater project, the pig is produced at Bristol Water’s depot in Kingswood using commercial ice makers to form a slurry like a giant Slush Puppy. The ice contains chemicals, normally salt, to help depress the freezing point and stop it becoming a solid plug. Following production the ice slurry is transported to site in a tanker where it is injected into the pipeline.
Thanks to its self-lubricating properties the ice mass is easily pushed through the pipe and can accommodate changes in diameter and direction as it encounters them, unlike traditional cleaning methods.

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