Posted on Thursday, October 29, 2020 by Sune Dupont
How Acoustic Measurements are used to Locate Water Leaks
We have all heard it, and the UN Sustainable Development Goal No. 6 highlights it — clean (drinking) water is a scarce source that we need to protect. But how are leaks in pipe networks actually discovered if the leak does not manifest itself as a new water fountain in Mrs Smith’s backyard?
The Physics of Leaks
When a fluid exits a pressurized pipe, acoustic noise is generated. This fact has been known for more than a hundred years and been used extensively for detecting leaks in fluid systems containing pressurized medias such as oil, gas and water. We now know, that the source of the noise is mostly due to turbulence and cavitation bobbles generated, where the fluid exits.
Basic Leak Detection using Acoustics
In many cases, a simple listening device placed on a curb stop (valve at beginning of service line) close to the leak is enough to hear and recognize the fizzling sound of a leak. Historically, a simple stick was actually used, much like the stethoscope used by a midwife. Present day instruments are more advanced and often implement either accelerometers or hydrophones (underwater microphones). Furthermore, many systems exist, where the listening devices are permanently installed in the grid and offer constant monitoring. This makes it possible to discover evolving leaks before they become severe. Here a good coupling between the pipe and the sensor is required. Therefore, the sensor is often either equipped with a strong magnet (accelerometers) or integrated directly into the pipe (hydrophones).
Getting More Out of Data
To get most out of data, some systems also introduce advanced data treatment like frequency analysis and cross-correlation techniques. The former enables filtering away unwanted (known) ambient noise sources by comparing spectral components. Crosscorrelation, in principle, facilitates leak pinpointing, i.e. finding the exact location of the leak. This requires two sensors, where the leak must be located between the sensors (See figure 2). Performing a cross-correlation of simultaneously captured measurements will give the time delay of the leak noise between the sensors. If the speed of sound in the pipe is known, this delay can be converted into a distance, i.e. the location of the leak. Especially noise correlation has proven its worth during the last two decades. However, several pitfalls exist using this kind of analysis.
A Simple Approach to a Complicated Problem
Integrating an acoustic noise sensor into a water meter is one way of solving this task. Meters are often placed at the end of every service connection which facilitates a very good coverage of the grid. The sensor will get an inherent optimal coupling to the grid, since a water meter is an integrated part of the pipe. Furthermore, modern smart meters all have a wireless transmission of data, which the acoustic sensor can piggyback on. However, to succeed with this tactic, the acoustic sensor must be cheap such that the combined price of the water meter is acceptable to the customers. Furthermore, the acoustic sensor should have a low current consumption since water meters often need to run on a battery for more than 15 years. Our newest water meter developed here in Denmark by Kamstrup actually introduced this novel technique without compromising any of the existing features of the meter. To reduce data transmission, a single measure of the acoustic noise level at the meter is returned once a day to either a central or local system.
Testing in Real Life
Figure 3 shows a map, where every dot marks an acoustic sensor integrated into a water meter. Within a radius of 250 meters, this amounts to more than 250 sensors monitoring the grid for leaks. During a test trial period (half a year) six previously unknown leaks were found in the shown area. These were all on the service lines and none could be heard on the curb stops, where you normally would listen for leaks, i.e. the leaks were only found owing to the acoustic sensor integrated into the water meter. Meter-based noise measurements also create new challenges. The main one being that meters are often installed inside houses, where ambient noise sources like pumps and district or central heating are more likely to cause acoustic interference. This is, fortunately, a local challenge and as seen in the top panel in figure 4, the noise pattern from a pump is often very different from a leak, making it possible to distinguish most leaks from pumps based on post data analysis.
Our initial test showed that many leaks can only be detected by the nearest meter. However, in some cases, multiple meters detect the leak. This is often seen as very similar noise graphs (Figure 4 bottom panel). The similarity rises as the sensors register the same noise source and are mostly seen in metal-based pipes, where the noise can travel far, or for leaks on the mainline where the noise source is close to multiple sensors.
To sum up, acoustic measurements are a great and simple tool for leak detection. Several technologies exist and have been used with success for the last decades. The newest step forward is water meters equipped with acoustic noise sensors. This is a simple approach to a complicated problem and even though simple, it is striking how far you get simply by the power of many.
