WATER & WASTEWATER
Leak detection has been identified by the Water Conservation and Water Demand Management (WC&WDM) Strategy of the City of Cape Town (CoCT) as one of the best methods to minimise water losses in its reticulation network. By Kirsten Kelly
Leak detection in the MOTHER CITY
T
he City concentrates on detecting both visible and non-visible leaks, supporting reticulation depots to pinpoint the location of existing leaks, and improving the leak repair response time to a maximum of 48 hours,” says Unathi Noludwe, senior technician: Water Demand Management Division, CoCT.
Non-visible leaks Different types of equipment are used by the CoCT to detect non-visible leaks: • Mechanical listening stick – used at all contact points (meters, valves, fire hydrants). • Noise loggers – deployed on pipe fittings and programmed over a period of time (recordings are usually taken at 02:00). • Leak noise correlators – this needs accurate information around the pipe material and diameter. Then the exact location of certain noise profiles (such as hissing leak sound) on the pipe are determined by correlating the noises that reach both sensors and Damp areas on the ground or very green grass patches surrounded by dry grass are visual indications of a leak
TABLE The CoCT’s leak detection results over the past three years
Year 2018/19 2019/20 2020/21
Network coverage (km) Number of leaks detected 5 471 323 4 640 568 6 662 262
measuring the difference it takes to travel on the pipe from the leak location to each sensor. • Data loggers – measures the flow and pressure of water in order to analyse the minimum night flow. • Ground-penetrating radar – locate the pipe direction and depth.
The CoCT uses four leak detection methods: • Visual leak detection – drive or walk on streets to inspect reticulation networks for obvious ground leaks. There is a 3 km target per team per day. • Localise – conduct leak noise surveys by using a listening stick and noise loggers. • Locate – identify a leak location by use a leak noise correlator. • Pinpoint and confirm – find the exact leak location by using a ground microphone. “We also use step testing as a leak detection method. Step testing is the process of localising leakage into specific pipe segments of the distribution system for subsequent replacement or repair. It is conducted at night when there is low demand, as we often shut off certain valves. A leak is usually found in an area with a disproportionate drop in flow,” adds Noludwe.
Leak detection in non-discrete areas can be challenging, as it is difficult to isolate the leak
IMIESA October 2021
Total savings (Mℓ) 4 4.4 5.89
due to feedwater from other areas. “The CoCT has therefore developed a different approach where a score card is used,” explains Noludwe. Parameters like type of pipe material, age of infrastructure, average pressure, pipe burst record, soil type, period of supply, leak detection record and type of land use are included on the scorecard. Leak detection will be prioritised in areas with a high score.
Methods
Non-discrete zones
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One of the leak detection methods used by the CoCT is a mechanical listening stick
Discrete zones Discrete zones are monitored by controllers like i20, Zednet, MyCity and MNF/AF. The minimum night flow (MNF) is compared to the average flow, and leak detection is considered if the MNF is more than 30% of the average flow. “We also use the SanFlow model (night flow analysis) that helps in identifying key problem zones. It uses the MNF, average pressure, number of connections, number of properties and the residential population as part of its calculation. Unavoidable annual real losses formulae are also used,” states Noludwe.
Results “Over the past three years, we have covered the entire water distribution network of Cape Town by using non-visual methods. Most of the leaks occurred by the water meters, followed by service connections,” adds Noludwe. Going forward, the CoCT aims to cover 100% of its network on an annual basis by 2031. The metro also wants to pilot and explore new leak detection technologies.
