Water Journal : Water Journal March 2011
refereed paper 102 MARCH 2011 water Abstract This paper explains the different techniques of leak detection on trunk mains and compares the results of a new non-invasive system with two established invasive technologies. Part 1 briefly reports simulation testing by Sydney Water on four commercial products. In Part 2, Gutermann equipment was subsequently tested by real-life comparisons in Manila, a busy Asian city with large levels of background noise, even in the middle of the night. Introduction Leak detection in large diameter water distribution mains or trunk mains is particularly challenging because the acoustic noise created by a leak must dissipate around the large circumference of the pipe before travelling along the pipe wall. Leak noise on large diameter pipes is much quieter and at a much lower frequency than the acoustic vibration used to find leaks in water reticulation networks. The leak noise is generated by the friction of water rubbing against the pipe wall as the water is escaping. A larger pressure differential between the inside and outside of the pipe forces the water to exit the pipe at a greater speed, creating a greater noise. Trunk mains are often designed with long distances between access points to the pipe, with an average distance of 500m between two points. The low frequency and quiet leak noise on trunk mains has made it almost impossible to detect leaks using leak noise correlators, as the leak noise was only detectable over a typical distance of 50m or less. In the last 15 years, innovators developed techniques for finding such leaks. Part 1: Technologies Invasive Technologies a) The Sahara System The UK Water Research Centre developed the Sahara system, which is now marketed by Pure Technologies. This is an invasive technology with an acoustic sensor inserted into the pipe, which travels along the inside of the pipe to detect the leak noise from close proximity. It has been very successful at finding leaks and has highlighted the fact that trunk mains need to be surveyed every one to two years to minimise leakage and prevent major incidents caused by main breaks on large pipes. The sensor is inserted into the pipeline through a device that allows the cable to pass through a seal. The hydrophone-transponder head is carried downstream by water flow acting on a drogue or small parachute attached to the head and aided by a hydraulic winch that regulates the rate at which the cable is fed from a cable drum (Figure 1). The size of the drogue used for each survey varies according to the diameter of the pipe and the velocity of the flow. The speed of the survey is also largely determined by the flow rate and is typically equal to about half of the flow velocity. As the survey progresses, signals from the hydrophone are transmitted to an operator on the surface via the umbilical cable. Any sounds, including leak noises picked up by the highly sensitive hydrophone, are monitored by the operator using headphones, and are also displayed in spectrogram form on a computer monitor according to amplitude and frequency. A second operator continually tracks the progression of the hydrophone- transponder head down the main on the surface. This is carried out using a specially designed receiver that amplifies a high-frequency signal emitted by the transponder. The two operators are in constant two-way radio communication, and when a leak is detected the transponder serves to accurately pinpoint its location. At the end of the survey run the cable, along with the attached head and drogue, is drawn back by the winch and fed onto its drum by a separate drum motor. Provided the drogue remains intact, previously detected leak sounds can also be verified as the head is retrieved and, if necessary, adjustments can be made to the magnitude and location of leaks. Survey distances of up to 2000m are possible from each insertion point. However, the length of a particular survey is primarily determined by the flow velocity and the number of bends and deflections in that section of pipeline. A minimum velocity of around 0.4m/sec is required, and as a rough guide, about 100m of survey distance can be achieved for every 0.1 m/sec increase in flow. The total change of direction that can be negotiated in the course of any one survey is about 180 degrees. The pressure in the pipeline also needs to be high enough to cause any leaks to generate a detectable noise, and ideally at least 20m head to make the sound of the leak clearly distinguishable above background noise levels. Very high pressures can make the initial insertion process more difficult, and can also make the cable feed somewhat harder due to the need to use tighter seals at the cable entry point. Nonetheless, the system asset management A Clark A comparison of three systems LEAK DETECTION IN TRUNK MAINS Figure 1: Operational diagram of Sahara (courtesy Pure Technologies).
Water Journal April 2011