The problem with basic pressure management, ie. using a Pressure Reducing Valve (PRV) with a fixed outlet, is that for much of the day and at night, the pressure in the DMA is higher than is necessary to guarantee a good service to the customer. Advanced Pressure Management was developed to solve this problem and thereby bring down leakage and the rate of new bursts.
Fig 1 shows a typical DMA. A PRV has been installed at the inlet to the DMA. The critical point is that point in the DMA. The critical point is that point in the DMA which is either the furthest or at the highest elevation or both in relation to the DMA inlet. It is the point that will normally see the lowest pressure. There is a further point shown, the Average Zone Point (AZP), where the average zone pressure (P4) ca be measured.
Figure 1 - A Typical DMA
Many water companies have introduced basic pressure management,particularly in areas where the pressure is high. This is achieved byinstalling a fixed outlet PRV on the inlet to the DMA. The PRV dropsthe pressure down from the PRV inlet pressure (P1) to the PRV outletpressure (P2). P2 is set manually after installation of the PRV.Because it cannot be varied easily, it must be set to a conservativelyhigh level that will be safe under the worst case conditions. Inaddition, some contingency is usually added to take account of seasonal fluctuations and possible changes in the network such as the building of new houses orthe building or change of use of factories.
The limitations of basic pressure management are illustrated by the following diagrams. Fig 2 shows the system at a time of maximum demand during the daytime. The high flow rates in the pipes create a large head loss between the inlet to the DMA and the critical point. If the PRV has been set up correctly, P2 will be set high enough to provide adequate critical point pressure (P3).
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| Figure 2 - Pressures in a DMA under high demand/flow rate conditions i.e. during the day |
Figure 2 shows the system at a time of maximum demand during the daytime. The high flow rates in the pipes create a large head loss between the inlet to the DMA and the critical point. If the PRV has been set up correctly, P2 will be set high enough to provide adequate critical point pressure (P3).
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| Figure 3 - Pressures in a DMA under low demand/flow rate conditions i.e. during the night |
Fig 3 shows the same system at night when the inflow is lowest. The head loss between the inlet to the DMA and the critical point is minimal and the pressure rises across the whole DMA until it is close to P2.
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| Figure 4 - Varying critical point and PRV outlet pressures with basic pressure management |
This can be seen graphically in Fig 4. P2 remains stable while P3 varies considerably as the head loss across the DMA varies with changing demand.
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| Figure 5 - Varying critical point and PRV outlet pressures with Advanced Pressure Management |
With Advanced Pressure Management, P2 is continuously adjusted so that the pressure is always kept at the minimum level necessary to guarantee the required level of service to all the water company's customers. This is achieved by varying P2 in order to achieve a steady level of P3 just above the reference level (Fig 5). The P3 reference level (P3ref) is set by the water company and is usually a value between 15m and 20m.