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Put it on plastic

Water supply

Refurbishment of the UK's ageing water supply system is a constant battle for water companies. But software developed by United Utilities is about to make main replacement a whole lot easier.

Roy West reports.

The Victorians left a great engineering legacy but today much of it is suffering from the ravages of time. As well as magnificent brick sewers and monumental treatment works, they laid down a superb network of cast iron mains often covering every road and street.

These mains still form a vital part of the supply network today, and refurbishment or replacement is a ceaseless challenge for the UK's water companies. Laid along roads now choked by traffic and submerged in urban sprawl, renewing them often spells trouble for water companies, customers and commuters alike.

Few would argue that open trench main laying needs to be minimised. The traditional approach to rehabilitation, which most UK water companies still use, involves scraping and on site relining of the old mains with concrete or epoxy coating.

But in the 1990s North West Water - the water utility now incorporated into United Utilities - started using new methods.

Instead of simply relining the old pipes and hoping they would hold up, engineers looked to high and medium density polyethylene and similar plastics, which offer a potential life of up to 50 years.

'People were already using plastic pipes for new installations, ' explains Paul Sage, modelling development manager of UU's Process Design Group.

'What we have done is to pioneer their potential for refurbishment projects.'

'Plastic offers an opportunity to improve no-dig technology, giving an even smoother surface flow than concrete, as well as avoiding many of the messy site and transport procedures needed.

'With concrete or epoxy there's still the risk of the main bursting, but plastic pipes simply use the old iron mains as a conduit and normally do not need to rely on its structural integrity, ' says Sage.

'There's also the problem of knowing whether a concrete lining is secure. An iron main relined with concrete can still be prone to leaks or the lining breaking off - especially when you tap in to make supply connections.'

The simplest and cheapest plastic solution - slip-lining - involves pulling a loose-fit new pipe through the old cast iron one. But matching the range of available plastic pipe sizes to the iron equivalents was not simple to find the optimum replacements to maintain the hydraulic characteristics of the system.

This led UU's engineers to devise other techniques such as 'swage' (or 'die-draw') lining, which is held under a joint licence with Advantica, and which gives a tight fit and minimises any reduction in diameter.

Where the existing pipe size needed matching or even increasing, they used a pipebursting or insertion technique while the more recently pioneered 'thin-wall' approach can be used where the host main is still structurally sound.

Armed with this range of plastic pipe techniques, it soon became clear that the water engineers needed a means of applying those materials and techniques in the most efficient way for a specific rehabilitation project.

Working closely with pipe testing and software specialists, UU Process Design Group gradually developed a software suite they named Poseidon. Little did the Greek God of the oceans imagine he was fated to become an acronym as Pipeline Optimisation System for the Engineering and Intelligent Design of Operational Networks.

Poseidon is designed to reference the various refurbishment techniques against the available pipe types, sizes, pressure ratings and costs.

Its ultimate aim is to identify the most cost effective option which will maintain - or even improve - the hydraulic performance of the supply network.

The engineers took as their starting point an all-mains hydraulic model of the network - AdvanticaStoner's SynerGEE - often used as standard in the industry. They then increased the demand - typically to around three times average domestic demand - and fed this into the Poseidon software.

A vital feature of the software is the ability to set a range of hydraulic constraints. As well as designing for three times the average demand, minimum pressures, maximum flow velocities and hydraulic gradients can be specified.

A unique feature is a safeguard known familiarly to UU engineers as their Plumbers' Rules. A standard hydraulic domestic demand is distributed across a district metering area (typically around 3,000 properties). This average demand may well differ dramatically from the actual demand at any given moment within a specific local area. The Plumbers' Rules were developed to take account of such a potential disparity.

'Basically it is an empirical formula that gives us a value based on the number and type of properties. This local flow can then be used to calculate the pipe sizes needed to feed the demand, ' explains Sage.

'Having identified a starting point that satisfies all the hydraulic limits, the software then embarks on a sophisticated process - known as cold annealing - to reach the smallest size of pipe practicable.'

A key advantage of the system is the potential for reducing the diameter of pipes. 'Poseidon often allows us to achieve better performance with smaller pipes, ' adds Sage.

'On top of the other benefits plastic offers over traditional relining materials, the interruption to the customer is kept to a minimum, with all supplies normally reinstated within the same working day.

In addition, the simpler connection procedure avoids any risk of debris getting into the water.'

Spurred by the success of Poseidon in mains rehabilitation projects, the UU team is now developing various other modules that can be used for operational purposes.

These include a leak detection facility which, like the main programme, has already been patented, and a programme to identify the optimum position for cross connections.

Also being refined is a module which automatically calibrates the hydraulic model to real pressures in the network and which should save many tedious hours of manual calibration.

Meanwhile it is the effectiveness of Poseidon's main role in coming up with the optimum specifications for rehabilitation projects that pleases UU.

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