Water leak detection - precise pin-pointing of faults

Feb 02, 2006

Different from electro-acoustic principles to locate water leaks, which are using different kinds of microphones to analyse loudness of leak noises for pin-pointing, correlation is based on the run-time difference of acoustic signals created by the water discharging from the leak.

The noise emitted from the point of fault spreads equally into both directions along the pipe. On it’s way it passes by various points of contact to the tube, such as main valves, gate valve or fire hydrants. If two microphones are now attached to these points in a way that the leak is somewhere inside a measurement section, the distance between one accelerometer to the leak is usually bigger than the other. Therefore the sound reaches the first microphone earlier than the second – a measurable time delay is caused. At first the correlator mathematically evaluates this run-time difference (Δt). The ensuing calculation of the distance to the sound source (d) is shown in Figure 1.
Besides the time delay the (Δt) variables L and v are contained in this equation and for that reason have significant influence on the resulting calculation of the position of the leak. L in this case is the pipe length between the measurement points that is filled with water, whereas v means the sound velocity at which the noises are spreading out in the pipe system.
For the application of a correlator in the every-day operation of water leak detection for these reasons not only knowledge about the exact length of the measurement section is required, but even information about the pipe material and diameter is essential since the sound velocity directly depends on these two data. The accuracy of a measurement does not only depend on high mathematical precision of the instruments used, but even more depends on the availability of all data required. Whereas the pipe length can be measured quite easily outdoors at a significant precision, inaccurate pipe data do falsify the measurement results enormously.
To avoid such sources of mistakes the measurement section should be varied in such manner that the time delay becomes as small as possible – this means that the position of the leak would be in the middle of the section (or at least as close to the centre as possible). For the practical use of a correlator no first result should immediately be trusted. A simple "moving" of the microphones and a second measurement often leads to increasing accuracy.
If a different second measuring configuration cannot be set up, because there are no more points of contact that allow the unit to recognize the noises measurably (e. g. at dead-end pipe sections), a sound velocity measurement is highly recommendable. For this purpose an "artificial" leak could be created besides the real on. This additional sound sources may be open fire hydrants or not fully closed valves, which create a significant noise as well. In an ideal case such "artificial" leak is created out of the real measurement length. By utilizing a certain firmware feature the correlator is enabled to calculate the real sound velocity inside the pipe based on the "created leakage".
As already known from the application of electro-acoustic principles to locate leaks in water supply systems usually non-metallic pipe material means a special challenge. This is valid as well for correlation. Audibility of leak sounds in this case is not necessarily required in order to correlate successfully, but the signal must be recognisable at least at two points of contact. The sound insulation capacity inside of non-metallic pipe material often leads to the fact that in bigger distances from the leak only using accessible contact points do not deliver a suitable result of correlation. A solution to this kind of problem is the application of so called hydrophones. This special kind of microphones are directly contacted to the water inside the pipe. Inside the medium the leak noise usually spreads out much better and can be recognised in much larger distances compared to the pipe material. For coupling of such hydrophones directly into the water column fire hydrants can be used as well as building them in by dismantling water meters at the customers.
In addition to the classical possibilities of correlating leaks, instruments of the latest generations offer the option to connect microphones directly to the main unit. These receivers can in such case be used as normal electro-acoustic water leak detectors. For this reason correlators today can not only be used for exact pin-pointing of leaks, but as well for pre-location by using test-rods and additionally to confirm correlated positions acoustically by using ground microphones. Besides portable instruments in various configurations like shown in Figure 2, PC-based technology is available to be mounted on specialised water leak detection vehicle solutions, either as a notebook or a desktop computer system.

 
[Source: 3R INTERNATIONAL: Journal for Piping, Engineering, Practice
 Special Edition 13/2005]

This Article is published on UNITRACC by courtesy of 3R INTERNATIONAL

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