Pinpoint Accuracy

A prototype of a large-diameter pipe leak correlator enables an Illinois utility to locate an elusive leak.

Water entering the basement of a school in downtown Chicago, Ill., was the first clue the Chicago Water Department had that something was amiss. However, city crews were unable to locate the source of the leak.

The department called its service provider, Pitometer Group of ADS Environmental Services. ADS, the city’s authorized service provider of the Echologics large pipe LeakFinderRT technology, saw an opportunity to test the new software in the field.

All previous achievements had involved induced known leaks. On the first attempt, and in a matter of hours, the leak correlator pinpointed the leak in a large-diameter main without an intensive setup or intrusive techniques.

Multiple choices

The street where the school is located has two 8-inch iron water mains and one 36-inch prestressed concrete cylinder pipe. Working at night, Bill Doyle and Nicole Avalos, ADS leak detectors, correlated the 8-inch mains using a standard Echologics correlator. Finding no leaks, they then investigated the 36-inch main using LeakFinder specialized software and hardware.

The software uses the computer’s soundcard and other multimedia components to record and play back leak noise, and to perform the correlation and associated signal conditioning. The enhanced correlation function improves the definition of correlation peaks for narrow-band leak noise in plastic and large-diameter pipes, multiple-leak situations, and where leak sensors are closely spaced. It also is more effective than the traditional function for small leaks or high background noise.

Basic hardware includes two accelerometers, two hydrophones, wireless transmitters, and a two-channel wireless receiver with headphones.

Doyle and Avalos installed hydrophones on a 1-inch pitot tap on the pipe’s south end (Blue Station). At the north end, they chose a fully charged hydrant on a 12-inch leg (White Station). An out-of-bracket noise (probably a pump running) at first obscured any sounds. However, filtering the noise from the correlation enabled Doyle and Avalos to hear a leak in the main. They measured the distance between the two hydro-phones, including the hydrant leg, using a measuring wheel. The length was 829 feet.

X marks the spot

To measure the velocity of sound on the 36-inch pipe, Doyle and Avalos created a leak outside the bracketed area by opening a hydrant, then recorded the time delay and velocity of the sound. The velocity was 3,542 feet per second. Entry of the pipe length and sound velocity into the calculation pinpointed a leak 408 feet north of the Blue Station.

“We marked the measurement on the street with blue paint, but ground microphoning of the area was inconclusive,” says Doyle. “The measurement fell directly over a known concrete-to-ductile-iron adapter in a repaired section of the concrete pipe.” They reported the leak location measurement to the water department, which scheduled repairs for early one Saturday.

When a crew dug down to the adapter, they found a torn gasket exactly 408 feet from the Blue Station. The Chicago Water Department theorizes that it probably suffered a small tear during construction that gradually worsened over time.


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