District metering - at the front in fighting leaks

The ABB UK Flow Manager Tony Hoyle discusses the developing role of district metering in UK potable water distribution system leakage management and monitoring

Water scarcity is affecting people around the world as rain patterns alter in response to climate change.

In fact, the UK has got off pretty lightly so far compared with some less fortunate regions.

In China, for example, rainfall has dropped by 20% in the Northern Mountains, which are the source of the Great Yellow River.

This is threatening the water supply of 120 million people.

Supply problems in Britain may be somewhat less dramatic, but we are all aware of the drought situation in southern England in 2006 and the predictions for repeated instances in the near future.

Some water companies have already been forced to take drastic measures in response.

Thames Water, for instance, is to build a huge desalination plant to serve 900,000 people, while Folkstone and Dover issued compulsory metering notices in 2006 in an attempt to control demand.

Whatever other technical or regulatory solutions they may turn to, however, leakage management is one approach that no water company operating in Britain today can afford to ignore.

Great progress has already been made.

Overall the industry in England and Wales has cut leakage by 30% since 1995, according to Water UK.

For example, Thames Water is spending GBP190 million on finding and fixing leaks this year alone.

Yorkshire Water, meanwhile, is already reaping the benefits of its long-term, GBP10 million per year leak reduction programme.

The company has cut leakage by 234Megalitres per day in the decade from 94/95 to 04/05.

It is sobering to realise that without this reduction, the YWA reservoirs would have been empty in 2003.

District metering is a key weapon in the war against leaks.

The concept of District Metered Areas (DMAs) was first introduced to the UK at the start of the 1980s by the then UK Water Authorities Association.

A district is a defined area of the distribution system that can be isolated by valves and for which the quantities of water entering and leaving can be metered.

The subsequent analysis of flow and pressure, especially at night when a high proportion of users are inactive, enables leakage specialists to calculate the level of leaks in the district.

This can be used to determine not only whether work should be undertaken to reduce leakage, but also to compare levels of leakage in different districts and thereby target maintenance in those areas where it will have the greatest impact.

Leakage generally falls into two categories - background leaks and bursts or breaks.

Background leakage is the aggregation of losses from all the fittings on the network.

Such leaks are typically too small to detect individually.

Burst leakage occurs from holes or fractures in the network that can be located using a range of specialist equipment.

Even new distribution networks experience both types of leak, and the water industry in the UK must work with some of the oldest underground assets in the world.

So it is completely unrealistic to expect to reduce total losses to zero in Britain's mains network, which runs to over 300,000 km.

However, the role of DMAs is to divide the network into manageable sections that make it easier to determine where bursts are and to repair them.

While major bursts and gushes on the surface may be reported to water companies by the public, it is vital to keep on top of other, less obvious leaks.

While the most visible leaks may be losing water at a high rate, they are usually reported and rectified quickly.

Lesser leaks may not result in such spectacular losses per hour, but they can run undetected for far longer and often lead to higher overall losses.

District metering is now part of an established, active leak management programme among UK water companies.

Typical districts cover somewhere between 1,000 and 2,000 properties in urban areas.

However, coverage is by no means comprehensive, especially in more rural areas.

The role of the DMAs is changing as the leak management agenda progresses.

Initially, DMAs are used as a tool to drive down leakage in networks that had received little or no previous leak detection work, apart from dealing with reported problems.

At this stage, their role is to highlight those areas where companies should be concentrating their efforts, helping to get the biggest benefit for a given maintenance budget.

As work progresses and bursts are located and repaired, DMAs make the resulting successes easier to measure, since any improvements should be more noticeable when viewed locally, rather than by taking a snapshot of the distribution network as a whole.

Eventually the repair work reaches the point where the DMAs are being used to look out for fresh leaks as they spring up.

The metering accuracy required increases with each successive stage of the leak management programme.

In an ideal situation, it should be possible to estimate the level of leakage using a "top down" water balance.

This requires an assessment of total customer use, which can then be subtracted from the total flow into the system.

The difference equates to the leakage.

This approach is difficult to apply in most areas, however, largely because there are very few DMAs in which all the properties are metered.

This makes it impossible for water companies to get a sufficiently accurate picture of consumption.

The situation is changing slowly, especially in areas of particular water scarcity, where compulsory metering is either being mooted, such as the Thames Water request for compulsory metering in all homes sold in Bromley and Croydon from 2010, or made manadatory.

Most water companies therefore favour the option of a "bottom up" approach, which relies on metering the flows in and out of a district at night.

Best practice analysis of DMA flows requires the estimation of leakage when the flow is at its minimum, which is typically at night.

Leakage teams close boundary valves around the DMA and take very accurate readings at around 3:00 or 4:00am.

Customer demand is typically at a minimum at night and the percentage of the flow made up of leaks is therefore at its highest.

However, certain urban areas are seeing an increase in activity at night, with nightclubs, late-night takeaways and the shift to 24-hour licensing.

All this can lead to increased 'leakage' reporting when, in fact, increased flow rates could be the result of genuine rises in consumption at night.

Under-registration of flows is a significant challenge.

Many water companies are still relying on mechanical DMA flow meters, which can't cope with the low flows they need to measure at night.

For example, typical mixed commercial and domestic urban DMA will have two or three inward feeds and two or three pipes exiting the area.

Each feed line might expect typical flows of the order of 60 m3/h in the daytime, and this is could peak at 1.6 to 2 times as much.

The corresponding flow at night could drop as low as 2m3/h with low leakage.

With a typical turndown ratio that in the very best meters achieves 40:1, mechanical meters sized to deal with the peak demands simply cannot provide the necessary accuracy.

This problem is exacerbated in areas where heavy industry has given way to commercial developments and housing.

The overall level of consumption in such areas often falls to a fraction of the historical industrial demand, leaving mains pipes and flow meters hopelessly oversized for the job.

So-called "right sizing" programmes are an attempt by water companies to overcome this problem.

These are just some of the reasons why many water companies are upgrading their district meters to electromagnetic technology.

Electromagnetic meters offer improved accuracy over a far superior range of flows.

For example, the ABB AquaMaster meters offer +/-0.5% uncertainty and a dynamic turndown range of 1000:1.

In fact, the results available today are such that these meters could even detect a toilet flushing.

As well as offering a lower accuracy than electromagnetic meters when new, mechanical meters also present an issue of long-term reliability, since mechanical wear causes a progressive deterioration in performance.

A DMA-based leak reduction programme can only be successful if the data is reliable, so this is another strong driver for water companies to switch.

Of course, reputable instrumentation manufacturers put a great deal of effort into ensuring that meters of all types are accurate as they leave the factory.

But meters must be peridocially verified or calibrated to maintain confidence in their accuracy in the longer term.

For example, installation damage on an electromagnetic meter may not be spotted immediately.

Once the meter is in the ground, no one will know if the magnetic circuit has been distorted, if poor earth bonding is a problem, or if the meter is struggling against EMC interference from noisy pump motor cables or other sources of interference.

The ultimate check would be to remove the meter and send it away for recalibration using an accredited UKAS calibration rig.

However, this is an expensive option and may be completely impractical.

In-situ testing can be carried out, for example, using an insertion probe.

By taking a series of readings across the pipe, a skilled engineer can gauge the overall flowrate but it is a demanding job.

Clamp-on ultrasonic meters can also be used to check meters.

But the problem with both of these approaches is that neither is intrinsically as accurate as the meters they're trying to check.

Software-based verification tools can help solve the problem.

The ABB Checkmaster field validation and Calmaster II IRIS verification tools, for example, rely on ABB installing an electronic "fingerprint" in its electromagnetic meters during calibration.

This fingerprint stores information about the magnetic circuit associated with the individual meter.

The original fingerprint can then be checked against the meter current performance for signs of deterioration.

Water Industry Regulators like the Environment Agency recognised the benefits of verification tools several years ago for abstraction metering.

It encouraged robust metering regimes and required regular checking and verification certificates.

New technology which extends the reach of verification tools to mains and battery-powered flowmeters could allow water companies and OFWAT to implement periodic verification of strategic DMA meters.

This would provide an extra level of confidence in DMA leakage data.

Whichever technology is being used, correct installation is vital.

For example, meters need sufficient runs of straight pipework up and downstream to deliver accurate results.

Installing and accessing DMA meters can be difficult, especially in busy urban areas where the ground is already crowded with an array of underground assets, or where a pipeline runs under a major road.

Conversely, meters in remote areas may be nowhere near a potential power supply.

Thankfully, the battery technology today means that meters such as the AquaMaster can be sited pretty much anywhere, without having to worry about the availability of power supplies or the need for frequent access.

Accessing the data is the next challenge, but the latest technology can help here too.

For example, the ABB AquaMaster Explorer transmitters have an integral data logger with possibilities of 1 minute logging and GSM text messaging, so that leakage managers can collect all the data from the night lines from the comfort of the office.

Water companies face significant challenges in setting up and applying DMAs successfully, but there is now a growing body of experience in dealing with all the issues.

WRc is a great source of further information.

It has carried out many meter technology comparison studies and developed a comprehensive set of Best Practice guides.

These should be the first port of call for anyone involved in leakage management and meter replacement strategies.

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