Light Management Case Study

Chapter 5 DiscussionDiscussion DiscussionDiscussion

City of Cape Town Municipality Water Demand Management succeeded in the implementation of pressure management with installation of pressure reducing valves and flow controllers, with the scarce rainfall pattern, water has become an important source of supply to the community and with the population of the area increasing, water needs to be saved. During the first three months of this year, City of Cape Town Municipality came up with strategy to install PRV’s in Monte Vista due to high pressure points in the area as a result of old water infrastructure.

Due to the pressure reducing valves only been completed and running for one month, it can give an indication of what the pressure in the system,

The total flows taken from the logging results are shown in Figure 4.2.1 and summarised in Table 4.2.1.

Terms and concepts

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The minimum night flow (MNF) and average daily demand (ADD) flow rates are 14.4 l/sec (51.7m3/hr) and 53.1 l/sec (191.2 m3/hr) respectively. The ratio of the MNF to the ADD flow rate is

27.1% which suggests a low to medium leakage problem. The MNF as a percentage of ADD is much higher than the expected value in terms of international benchmarking (10-20%) and the night flow analysis. The current modelled peak flow during the summer is 232.1 l/sec (835.6 m3/h). This compares favourably with the actual summer peak of 216.6 l/sec (779.8 m3/h).

Flow Logging Summary, the average metered consumption is higher (77.7 l/sec) than the average logged consumption (53.1 l/sec). However, for the period Nov 2008 to Aug 2009, the average logged consumption is closed to the average metered consumption, being 77.1 l/sec. It is unlikely that there are other supplies into the area. A zero pressure test to confirm this has been undertaken to finalise the design of advanced pressure management for the area.

5.2.1 Flow logging

The approximate cost to repair a pipe burst in Cape Town was taken as R25 000 (based on estimate by Mr Ebrahim Amos). The value of reduced pipe bursts in Monte Vista can therefore be estimated as: 8 x 12 x R25 000 = R2.4 mil/yr.

If the water savings and reduction in pipe bursts are taken into account the payback period for the project can be calculated as 5 months.

5.4.1 Logging Results

A comparison of pressure logging results at the critical points before and after commissioning of the project are shown in Figure 4.5.1 and Figure 4.5.2. The critical point night time pressure reduced from 50m to 40m for the upper PRV zone and from 65m to 40mm for the lower PRV zone..

5.4.2 Logging Results before and after Pressure Management Installation at critical points

Figure 4.5.1 presented critical point at Glenville and D’almedia installation before and after installation of PRV, the pressure reduction was about 1bar and 2 bars, that means background leaks and pipe bursts were minimised, because pressure is directly proportional to the