Listening for Defects

Acoustic technology provides improved condition assessment of asbestos cement water mains.

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Asbestos cement-lined water mains degrade when subjected to long-term exposure to sulfates and byproducts of biofilms that form on their interior walls. AC pipes also lose structural integrity as a result of "leaching," a chemical process in which calcium contained in the cement is absorbed by soil surrounding the pipe, resulting in reduced structural wall thickness. Water utility operators see the result of such effects in the form of "soft" AC pipe.

Condition assessment — or measuring the remaining wall thickness — of AC pipe has been problematic for many utilities, as most traditional methods are based on excavating sections of pipe and extracting samples for phenolphthalein dye testing. In many cases, samples cannot be obtained without taking the pipes out of service, and due to the costs and low sample sizes associated with such methods, few utilities have a true understanding of the condition of water distribution assets that are comprised of AC.

Fortunately, recent developments in non-invasive acoustics can help utilities to accurately and non-invasively determine the remaining wall thickness of AC transmission and distribution mains in their water systems. These developments have been tested and confirmed by a number of utilities, including the Las Vegas Valley Water District and the District of Maple Ridge, British Columbia. This new method involves measuring the velocity of acoustic waves in a pipe, using a leak-noise correlator and two acoustic sensors that are either placed on available appurtenances or on the outside walls of the pipe. The measured velocity is then used to calculate the average wall thickness of the pipe between the two sensors.

Background theory

The acoustic wave velocity of a fundamental vibro-acoustical wave (a pressure-induced wave) of the pipe is a function of the Young's Modulus of the pipe, and the Bulk Modulus of the fluid. An acoustic wave is induced in the pipe by lightly "tapping" on the pipe, or by running water through a hydrant. The propagation velocity of the wave is then measured based on the sensor spacing and the measured time delay between the two sensor locations. The average wall thickness of the pipe section between the acoustic sensors is then reverse-calculated from a theoretical model. As the pipe wall thickness decreases over time, the acoustical wave velocity decreases.

The acoustic propagation wave (the water hammer mode) propagates as a compression wave in the fluid, and a dilatational wave in the pipe. The pipe will breathe on a microscopic level, and therefore the pipe will go into stress. The implication of this is that only the structural part of the pipe that can carry load will contribute to the structural stiffness of the pipe. Therefore, deposits on the pipe wall such as tuberculation or graphite are not the included in the average wall thickness measurement. The measured wall thickness represents an average between the two sensors. Typically, the length of the pipe section over which the acoustic velocity is measured is 100 to 300 meters; however, if an anomalous measurement that could represent a degraded pipe is found, this distance can be decreased to anywhere between 30 to 100 meters. Closer measurements can also be taken by using existing fittings on the pipe that are more closely spaced.

The use of this method has been very accurate at assessing the condition of AC pipe that is in good, moderate or bad condition.

Las Vegas Valley Water District

Las Vegas Valley Water District turned to a condition assessment specialist to acoustically measure the remaining wall thickness of a major pipeline in its water system in order to more efficiently prioritize what it anticipated to be a large repair and replacement project. However, the district's engineers wanted to test the technology's accuracy beforehand on a section of 6-inch AC pipe that was scheduled to be abandoned — without breaking ground.

Following the assessment, samples of the pipe were obtained and sent to a consultant, whose lab specializes in AC pipe testing in accordance with ASTM standards. When the results came back, they were compared to the specialist's condition assessment report and found to be nearly identical.

The acoustic method employed found the remaining wall thickness of the pipe to be .74 inches. The result of .74-inch wall thickness was not expected, as the standard wall thickness of a pressure class 150 AC pipe is .66 inches. Phenolphthalein dye tests conducted by the consultant found the samples to be in excellent condition, with a remaining wall thickness of .75 inches. The consultant also conducted chemical analysis of the samples, which indicated that the pipe's chemical composition was within normal limits.

Based on its analysis, the consultant assigned a remaining service life of 48 years to the samples. The results of the analysis essentially confirmed the results predicted by the specialist and the acoustic-based technology.

Based on the reliability of these and other acoustic measurements, the water district has since cut back its sampling program on AC pipe, and uses acoustic technology as its primary condition assessment tool.

The District of Maple Ridge, British Columbia

The District of Maple Ridge, British Columbia, was experiencing leaks and main breaks in sec­tions of its water system, which is comprised mainly of ductile iron pipe. However, approximately 18 percent of its water system is comprised of asbestos cement and cast iron, and most of the leaks and breaks it experienced were occurring in these areas.

Normally, Maple Ridge would analyze the break histories of parts of its water system to prioritize sections for replacement. However, it turned to a specialist that leveraged acoustic-based methods in order to more accurately prioritize replacement projects — without breaking ground or disrupting service.

Maple Ridge's superintendent of water works wanted to gauge the accuracy of the acoustic condition assessment method prior to its full-scale deployment, so he had a specialist assess two sections of 6-inch AC pipe in the system, which the district already knew were badly degraded.

Once both sections of pipe were acoustically surveyed, Maple Ridge compared physical samples of the pipes to the acoustic condition as­sessment report and found the results to be nearly identical. The acoustic-based findings indicated that both pipes had remaining wall thicknesses of 7.7 mm, which meant that the pipes were significantly degraded, as they had lost more than half (54.2 percent) of their original wall thickness. The results closely correlated with the condition of the physical samples of the pipe. Impressed with the accuracy of the results, Maple Ridge now uses acoustic-based pipe condition assessments along with breakage histories to prioritize water system repairs and replacement.

These case studies are just a few of the many examples that help validate the accuracy and effectiveness of acoustic-based technology that leverages propagation velocity when it comes to determining the remaining wall thickness of water mains comprised of AC. The condition assessment capabilities of this type of survey-level technology are important for utilities in that they can help them to efficiently prioritize water system repairs and replacement and continue providing customers with safe, clean drinking water.

About the Authors

Marc Bracken, M.A.Sc., P.Eng., is vice president and general manager of Toronto-based Echologics and has more than 18 years of experience in acoustical and vibration engineering.

Dave Johnston, B.Eng., is manager of field service and technology for Echologics. He has played key roles in developing non-invasive leak detection and condition assessment solutions for many different pipe types and materials.



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