TECHNICALLY SPEAKING
The installation layout for Portsmouth Water's Emsworth DMA
Because of these uncertainties, it is usual to enter a very conservative
table of pressure values, limiting the water that can be saved. This can now be optimised, thanks to the new advanced pilot valve (APV) that is designed to provide an adjustable outlet P2 and is connected to the PRV in place of its conventional pilot valve. The APV smoothly adjusts P2 in response to instructions from the i2O controller. The server, in addition to receiving and storing data, also updates the
control algorithms on a daily basis and downloads these to the controller. Furthermore, it also handles alarms and provides reports both to the operator and the system provider. The main advantage of the new approach is to offer a managed service with a computer server that is able to receive and store data from the controller and sensors. This achieves a further reduction of losses over those from other forms of pressure management, giving a short payback period for the installation costs. The better information generated allows the company to manage assets more efficiently. In the long run, a more stable pressure regime results in additional financial benefits through fewer burst, as well as better customer service.
Case study 1: Portsmouth Water
This was one of the first of 12 prototype advanced pressure management systems implemented in the UK, installed in August 2008. The Emsworth DMA contains 2,689 mainly residential customers, with a mains length of 18.339km. The DMA inlet is fitted with a 100mm Cla-Val GE PRV and a 100mm Sensus WP Dynamic flow meter. The new system was fitted alongside the existing pilot rail and pressure sensors, allowing to duplicate flow and pressure logging and switch control to the original pilot rail during the trial.
The implementation process was carried out in five stages: 1. Automatic monitoring of flow and pressure data by the i2O system. 2. Automatic optimisation of the fixed outlet PRV pressure (P2). 3. Flow modulation to achieve the specified minimum critical point pressure (P3) of 20m. 4. Flow modulation with different day and night target pressures (20m and 18m).
5. In January 2009, the target minimum night pressure was reduced to 16.5m.
The main benefit observed was in a significant leakage reduction, quantified by a night flow reduction by 2.0l/s, from 6.8l/s to 4.8l/s, or almost 30%. Leakage reduction can then be calculated using the FAVAD equation (fixed and variable area discharges), which in this case amounts to 140m3/day.
Case study 2: United Utilities
One of the latest implementation trials is in the DMA of Lister Drive for United Utilities, installed in February 2009. This site was chosen because UU believed the pressure control was already very good and any further benefit would be a real bonus. The DMA contains 1009 domestic and 31 industrial connections and
has a total mains length of 8734m. The DMA inlet is fitted with a 75mm GE Cla-Val PRV and a 100mm Helix 3000 flow meter. Previously, the PRV outlet pressure had been set conservatively high to
take account of daily flow related head loss and anticipated variations in demand during the year. Under peak flow, the critical pressure P3 reached 25m, with 23m as a minimum under low flow conditions. As the system was started up on 14 April 2009, the pressure was
reduced by 1m intervals to start with. Two days later, the flow modulation stage started, with the system set to achieve 20m at the specified minimum critical point (P3).
The graph shows how the implementation process was successfully
completed – achieving and maintaining a target minimum pressure of 20m. This in turn allowed the valve outlet pressure to be lowered from 28m to an average of less than 25m. Leakage, expressed as minimum night flow, has reduced steadily
throughout the trial period, from a level of 315m3/d before the system was installed to 201m3/d for the period October 2009 to January 2010. This represents a substantial 36% reduction in minimum night flow in this DMA – 114m3/d.
Burst reduction
Reducing the maximum average zone pressure (AZP) in a DMA can have a very significant effect on the level of new bursts. In many cases, a reduction in maximum AZP of a given percentage
results in a higher percentage drop in the level of new bursts. Members of the IWA Water Loss Task Force recently reported data from
112 pressure management schemes around the world. They found that the average pressure reduction across all the schemes
was 37%, resulting in an average reduction in new bursts of 53%, or that the average percentage reduction in new burst frequency is 1.4 times the percentage reduction in maximum AZP. Because of the successful trials in the UK (including Southern Water,
Severn Trent and Thames Water), the first advanced pressure management system outside the UK was installed in February in Reggio Emilia, Italy, and in April the first of 50 systems was installed in Kuala Lumpur, Malaysia. ■■■
May 2010 Water & Wastewater Treatment 37
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