Water Journal : Water Journal July 2011
asset management technical features 104 JULY 2011 water Data Interpretation Data analysis is carried out immediately after the measurement and an indication of defect type and location is given. The data is presented in the form of a spectrogram (see Figures 4 and 5). The colour intensity in the spectrogram corresponds to the intensity of sound reflected by blockages and pipe defects. It is plotted as a function of the frequency of sound and distance from the pipe inlet. The acoustic technology is able to detect and locate lateral connections, pipe wall perforation, missing pipe wall sections, defective joints, major cracks and blockages and pipe ends. The technology uses a pattern recognition technique to define those defects for which the signatures exist in a specially developed conditions database. The acoustic technology calculates the distance to the defect using the speed of sound in air, which is adjusted by the thermocouple reading. With this method, the distance to the leading edge of the defect can be estimated with an accuracy of 10mm. In addition, the acoustic technology can estimate the hydraulic pipe roughness, energy losses and the overall hydraulic capacity of the sewer. Validation The current technology was validated in the laboratory and real sewer conditions. Field trials took place in the UK, Austria, Luxembourg and Holland under different weather and sewer conditions. Sewers of PVC, clay, concrete and brick were inspected, with diameters ranging from 150mm--650mm. In total, more than 3km of pipeline were surveyed and analysed. About 1km of sewers were surveyed both acoustically and with CCTV to benchmark the performance of the acoustic method against that by CCTV. The results have revealed that the acoustic technology proved to be at least 79% effective, which means that it was able to identify 79% of those defects that were detected by CCTV. Some other defects were neglected from the statistical analysis. Also, it should be noted, only those defects that had a variance in location less than +/- 0.3m were considered to represent the same defect in both methods. Limitations The acoustic technology is generally limited to pipes with dry weather flow conditions. The longest sewer pipe examined successfully in the field was 130m long between manholes. Considering the attenuation of the signal in this pipe it is estimated that sewers up to 2km long and ranging 150mm--650mm in diameter could be inspected with this acoustic technology. Due to acoustical reasons the first few metres of the pipe (distance equivalent to around five sewer diameters from the sensor location) are disregarded by the analysis as they appear to be in a 'blind zone'. Figure 3. The acoustic sensor software screenshot. Figure 4. The acoustic software representation of a clean pipe with an indication of the next manhole chamber at 14m. Figure 5. The acoustic software representation of a pipe with blockage at 7m and an indication of the next manhole chamber at 14m.
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