Divide and conquer

Dividing Cambridge Water's area of coverage into district meter areas is helping the utility overcome the problem of excessive leakage. Cambridge Water's design and standards engineer, John Brock, and consultant Mouchel's project engineer, Gareth Mallows, explain the part software plays in the solution

Cambridge Water's failure to meet Ofwat leakage targets in 2004-5 prompted the decision to investigate the introduction of district meter areas (DMAs).

DMAs were seen as a way of improving understanding of consumption in each zone, which would lead to maximum efficiency in tackling the leaks. A leakage review revealed that only 37% of supply was covered by DMAs, and they tended to be rural areas supplied directly from small towers.

The company supplies about 72,000 properties in the city and surrounding villages with water from chalk boreholes. Total demand is about 45,000m3/d, with an additional 20,000m3 being transferred in bulk for supply elsewhere.

The majority of the supply area was still covered by quarterly waste tests. There was only 5% DMA coverage in the city zone.

The utility had the view that district metering could be done fairly easily, if necessary, in the rural areas and the outer parts of the Cambridge zone, but was always deemed to be too difficult for the inner part, because it is a fully-interconnected network.

The age of Cambridge Water's 940km pipe network varies considerably, with some of it more than 150 years old. There had been a feeling that the network would not cope with the large number of valve closures required to create DMAs. However, the need to tackle leakage gave the impetus to look afresh at the possibilities.

Cambridge Water started by using its own resources to implement DMAs around the periphery of the network, where the introduction was relatively straightforward.
Consultant Mouchel was then invited, in April 2006, to explore whether district metering might be a viable option elsewhere. The outcome of that study was favourable and it was invited to carry out a detailed implementation study for district metering throughout.

Setting up the DMAs involved dividing the network into small areas, each ideally having a single-feed supply. Every area is metered individually so that consumption is identified for that part of the network. The sum of all those small parts provides a figure for total consumption.

An all-mains model had been built and calibrated in 2002 in Wallingford Software's InfoWorks, using data from the company's GIS system. The engineers were pleased to find the original digitisation was done well.

Much field data had to be collected to validate the model and Parkman - now Mouchel - was brought in to help. A total of 160 pressure loggers and 16 insertion meters were installed.

Consumption by 32 of the larger customers was also logged, representing about 10% of demand. In addition, data was collected from 48 points on the company's telemetry (SCADA) system.

The model was found to calibrate well. The engineers found the data-flagging tool especially useful, helping the team to keep track of the changes made to the original assumptions.

Updating of the model is carried out annually to incorporate new connections and any changes to operating practices. It is used extensively to evaluate enquiries from property developers, and to look at the potential impact of planned maintenance on customers.

The InfoWorks WS network model became the base model for the whole study and all subsequent field trials and pressure loggings have been based on it. The starting point was to undertake a desktop review of the system, working with Cambridge Water operational staff. The feasibility study had developed an initial split into 36 DMAs and the boundaries for these then had to be confirmed.

The open part of the system had been geographically split into eleven areas during the feasibility stage. Local geographical features such as the River Cam, the A14 and the rail network were used.

The majority of these eleven areas were then subdivided into three or four district meter areas to enable manageable field trials to take place. Only the city centre was designated as a unitary DMA.

Each of the areas had to be individually modelled in InfoWorks ahead of field trials. Network models were run for average and peak-day scenarios to flag up any apparent low pressures.

The modelling results were then reviewed to ensure customers within the proposed DMAs would not experience any issues in terms of level of service. Cambridge Water's GIS system was also used to identify any critical information within each DMA, such as major and sensitive customers. The preparations also confirmed which valves would need to be closed to isolate individual DMAs.

Modelling was an iterative process using two models - the original base version without DMAs and the working model, which was broken down into the constituent DMAs. The two models were continually compared, throughout the study, to ensure the proposed DMAs would not be detrimental to the system.

Controls were then set up for each individual area to look at the demand on both peak and average days. For each control, the boundary valves were closed for each DMA area. Modelling was then run with all the proposed meter location valves closed to ensure that the area was discrete. Pressure zero testing could then be undertaken to confirm the isolation from the rest of the network.

The pipe closed facility within InfoWorks was very useful for enabling rapid representation of a closed valve. The InfoWorks boundary trace tool was also used, enabling the engineers to check that the areas were discrete and that all the boundary valves and proposed meter-location valves had been identified.

This tool was used in conjunction with Cambridge Water's GIS system as a further check on the areas. Positive results from both pieces of software gave the team confidence that no boundary valves had been omitted from the network model.

The model was then used to simulate the scenario of the field trials in each DMA. It enabled the pinpointing of the nodes and areas with the lowest pressures - these would be critical monitoring points. Any pressure anomalies could also be identified for logging in the field.

The InfoWorks modelling was also used to create implementation packs for all eleven areas to facilitate the field trials. The packs were issued to the control room and key operational staff. To prove that each DMA was separate and discrete, each area was closed in and the pressure was checked. Field trials were carried out over 14-day periods to allow assessment of different scenarios. Pressure loggers were installed on the first day, so existing conditions could be monitored. Boundary valves were then closed on day six, with the pressure-zero test carried out on the seventh day.

A pressure-zero test involves closure of all inlet and outlet points to ensure that all feeds have been identified. The first step was to identify and close all boundary valves, generally 1am-5am. Customers with special needs were informed, as were large metered consumers.

A pressure gauge was set up at a standpipe at a convenient hydrant. The valve at the area's inlet was then closed to isolate the DMA, and the pressure on the standpipe gauge should then drop. However, pressures sometimes did not drop straight away, and might not drop to zero as some residual head may remain in the system.

If the pressure did not drop below 5m (the allowable figure deemed to represent residual head), then it was adjudged that not all feeds into the area had been identified.

On completion of the pressure-zero test, the supply valve was opened and the pressure gauge was used on a nearby hydrant to ensure pressure has been restored. After a pressure-zero test was undertaken, the boundary valves would generally be opened six days later and then the following day the pressure loggers were removed.

The overall correlation between field test data and modelled data was found to be excellent, and proved that Cambridge Water had a robust model.

On successful completion of all the pressure-zero tests, five areas indicated potential problems with the supply. Additional model runs were undertaken to identify solutions, such as mains reinforcement and rehabilitation.

Potential pressure problems were also predicted in ten areas at times of peak demand. Due to the nature of transfer across the system, these are furthest from the source of supply. A potential solution was identified, involving opening the boundary valves to adjoining DMAs to create super DMAs to maximise supply.

An information pack was produced for each DMA to facilitate the meter installation. Each pack contains an overview plan and schematic, mains information, a property listing, a list of sensitive and major customers, boundary valve and network modelling information, pressure-logging results and proposed meter locations.

Close correlation between model predictions and field-testing has given the engineers confidence in implementing the system. The meters, valves and instrumentation required to implement the scheme are being installed throughout 2008.

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