Acoustics leak detection: An introduction
Water distribution systems can be tested systematically by using acoustic equipment, which detects the noise generated by the leak from the pipe. Acoustic techniques have been widely used in the water industry and can produce the effective results . In the acoustic methods, the most common approach involves the cross-correlation in order to detect and locate the leaks. In general, the technique use acoustic sensors and is based on detecting the noise that occurs when a leak exists in the pipeline. The method works by placing sensor devices on both sides of the pipes where the leak is suspected. The sensors can be placed on the road surface or directly on the particular point such as fire hydrants as shown by Figure 1.
The location of the leak can be identified based on sound propagation velocity, time lag and distance between sensing points. It can be found by using the following equation :
where is the distance from the sensor to the distance of the leak, is a distance between two sensors, is a sound wave propagation velocity and indicates time difference between the arrivals of identical frequencies to each sensor. The performance of the leak detection is influenced by the distance between the sensors, . The shorter the distance between the sensors the better. Clearly all variables of this equation can be found easily from the experiment. Thus, this technique can give results with a high accuracy level. The efficiency and accuracy of the method are dependent on the skills of the operators. The cost of such a method depends only on the cost of the operators’ work. However, the method has several limitations. It has proven to work well in detecting and locating leaks in metal pipes. In contrast, when applied to soft materials, such as plastic, this method has been problematic . The reason is that plastic pipes are much more ‘elastic’, so the sound wave reduces to 300-600 m/sec. Due to their viscoelastic properties; plastic pipes also absorb sound energy, so the sound waves become weaker as they travel along pipes. The level of high frequency noise is also bigger, which makes analysis more complicated . On the other hand, the accuracy of the leakage detection can be affected by the presence of air in the pipe. This can reduce the bulk modulus and density of the liquid which will lead to a decrease in acoustic velocity. Additionally, the presence of the suspended solids in the liquid will make liquid more dense and hence decrease the acoustic velocity .
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