Objectives & challenges
Under WSN’s dynamic leak detection programme, field crew is dispatched to survey water mains for leaks. They rely on visual cues, listening sticks and leak noise localisers to identify leaks. With an extensive network of around 6000 km of mains, each survey cycle can take up to a year to complete. However, with increasing public expectations, there is a need to find methods to further reduce response time towards leak incidents. WSN aims to deploy more advanced leak detection equipment/systems to continuously monitor and analyse leaks, thereby improving response time and minimizing impact to customers. With more leak monitoring systems installed island-wide, there can be significant reduction in the manpower which is normally required to perform the routine site inspection.
Review of current technology & tools in the market
Acoustic sensors based on accelerometers
Leak noise localisers and leak noise correlators are listening devices which use accelerometers to sense leak-induced sounds or vibrations caused by water escaping from a pressurized pipe. These devices are suited for the network in Singapore as metal pipes easily transmit water leak sounds (Hamilton & Charalambous 2013). Leak noise correlators are more sensitive than localisers, as they are able to automatically pinpoint leak locations based on parameters like pipe size, distance and pipe material. However, due to the sound propagation theory, these methods are only accurate for small diameter water mains; preferably 300 mm mains and below. In addition, the range of the sensors is limited to 250 m.
Acoustic sensors based on hydrophone sensors
It is also possible to determine leak positions by analysing data from hydrophone sensors that capture sound waves in water. Hydrophones are more sensitive than accelerometers and are effective in detecting leaks in larger pipes. However, such systems are known to be expensive and a detailed site study has to be conducted as the sensors are best installed in 100 mm or 150 mm air valves located not more than 750 m apart (Hamilton & Charalambous 2013).
High rate pressure sensors
High Rate Sampling Pressure Sensors are used to detect patterns of pressure transients which occur during a pipe burst. These sensors have a much wider detection range of 1.5 km, but produce lower resolution results in a range of a few hundred meters, compared to tens of metres for acoustic devices. Moreover, there is a need to distinguish pressure transients caused by leaks against those caused by network operations or regular draw-offs.
Virtual district metering areas
Virtual District Metering Areas utilize flow meters to monitor inflows and outflows to sub-sections of the water network. Combined with automated meter readings from customers premises, mass balance calculations can be used to derive the actual water loss in an area. Operators can then focus their leak survey efforts on areas where high water losses are observed.
Statistical analysis of pressure and flow
Statistical analysis of flows and pressures can be carried out to reveal deviations that may indicate a new leak. However, water flow fluctuations can be attributed to a variety of reasons such as seasonal variations and public holidays amongst other causes. Thus, statistical analysis also needs to take other datasets into account to improve its accuracy.
PUB experience with technology & gaps identified
Leak noise localisers and correlators have already been incorporated within the Standard Operating Procedures of WSN’s Leak Detection Unit. High rate pressure sensors have been installed in the network. The dedicated WaterWise software system developed by PUB and Visenti (http://www.visenti.com/) comprises 300 multi-parameter probes to detect both leaks and water quality issues in real-time (see Fig. 1a and b) (Allen et al. 2011; Whittle et al. 2011). Advanced computer algorithms are used to identify the pressure drop signatures associated with pipe bursts. After identifying a credible signature, the system attempts to identify the stretches of water mains which may contain the leak spot. Upon receiving the electronic alerts, officers will proceed to survey these mains for leaks. The survey efforts are therefore more focused compared with the traditional leak detection programme.
WSN has also installed a separate system of short range sensors (acoustic range: 250 m) at leak-prone areas in the network. These sensors provide a higher resolution for leak localisation, but are only semi-real-time as they are switched on in the early morning when ambient noise levels are lower.
Technology roadmap
WSN plans to expand the sensor coverage in the network to 90 % by 2018 with 400 additional multi-parameter sensors. The focus is now to distinguish the sharp pressure transients of leaks from the transients created by high customer consumption. WSN will also be testing the use of hydrophones to detect leaks in large transmission mains. As more trials are conducted, WSN can work with vendors to develop advanced software filters to better identify leaks.