Advanced pressure management

More effective control of pressure means less waste of water. Advanced Pressure Management not only solves many of the problems associated with basic forms of pressure management, with its supporting systems, it provides a complete package to help companies bring down leakage and the rate of new bursts.

Many water companies have introduced basic pressure management, particularly in areas where the pressure is high. This is achieved by installing a fixed outlet PRV on the inlet to the DMA. The PRV drops the pressure down from the PRV inlet pressure (P1) to the PRV outlet pressure (P2). P2 is set manually after installation of the PRV. Because it cannot be varied easily, it must be set to a conservatively high level that will be safe under the worse case conditions. In addition, some contingency is usually added to take account of any future changes in the network such as the building of new houses or factories. Thus the pressure in the DMA, for much of the day and most of the night, is higher than is necessary to guarantee good service.

Figure 1 - A Typical DMA

The diagram above (figure 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 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 AZP point, where the average zone pressure (AZP) can be measured.

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).

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.  With low flow, 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.

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.

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 a good 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. It can be advantageous to have a higher P3ref value during the day than at night.