Follow the Bouncing Ball

A combination of technologies, including a SmartBall acoustic leak detection device, provide condition assessments for a Maryland district

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Finding leaks in water transmission mains before they become safety hazards is an ongoing challenge for the Washington Suburban Sanitary Commission in Laurel, Md. In 2009, the district repaired 1,847 water main breaks and leaks in Prince George’s and Montgomery counties, the second-highest calendar year total in the past quarter-century.

 

But the real problem children in the agency’s 5,500 miles of water lines are 350 miles of 50- to 70-year-old prestressed concrete cylinder pipe (PCCP). The concrete core, steel cylinder, high-strength steel prestressing wires and mortar coating are highly sensitive to corrosion.

 

Consequently, the commission has inspected all 59 miles of 54- to 96-inch mains multiple times. Investigations have advanced from visual inspection and sounding to sonic/ultrasonic measures and electromagnetic assessments.

 

“If you depend on just visual to look for leaks, you’ll miss a lot,” says chief engineer Gary Gumm. “We learned that lesson after we knew we had a leak, but couldn’t locate it until we used electronic equipment. Then we had to really search for it visually to repair it. After that, we added electronic leak detection technology to our repertoire, and we use it before dewatering the pipe.”

 

However, some electronic leak detection equipment is tethered, often requiring excavation to create entry points. When Pure Technologies Ltd. in Columbia, Md., introduced the untethered SmartBall, an acoustic leak detection device, the commission embraced it and the P-Wave electromagnetic inspection system.

 

With the flow

SmartBall, introduced in 2007, is a sensor-filled aluminum core surrounded by a protective foam shell. Requiring only an insertion and extraction location, the device is deployed in 10-inch or larger pipes, then rolls with the flow. Various internal sensors pinpoint leaks as small as 0.1 gpm. The ball travels up to 15 hours before reaching memory capacity and depleting its rechargeable battery.

 

An accelerometer determines leak locations, as do acoustic pulses emitted by the ball. Receivers are attached to pipe appurtenances. The collected data is graphically represented in a position versus time profile.

Additional instrumentation calculates temperature and identifies valves, outlets and other metallic features. Once the ball is retrieved, the data is downloaded and analyzed for acoustic anomalies. The technology can be used with most pipe materials, including steel, ductile iron and PVC.

 

As part of a comprehensive PCCP inspection and management program, the commission contracts with Pure Technologies to conduct leak detection surveys. Before launching the ball, the agency closes sideline valves that could divert the device. “We try to turn the pipeline into a river and shoot for flows of 1.5 to 2 feet per second,” says Gumm. “Pure sets a net at the extraction port to catch the ball.”

 

Wave of the future

To inspect the prestressing wires, the pipe is dewatered, and a worker then pushes a cart with the P-Wave technology through the line, generating a magnetic signature of each pipe section. Broken wires alter the magnetic signal, creating an anomalous reading on the cart’s onboard computer. The readings are analyzed to pinpoint the breaks and estimate the total number of broken wires.

 

The commission takes transmission mains out of service in lengths of 6 to 18 miles. “That’s the size of our program for a year,” says Gumm. “We get away with this because these mains generally don’t have feeders to homes and businesses, and we have just enough redundancy. Locating and quantifying the extent of breaks gives us the pipe’s baseline condition. Monitoring it from that point on allows us to decide when to act.”

Thus far, Pure has inspected 27 miles of pipe 48 inches and larger, subsequently installing SoundPrint, an acoustic fiber optic cable that identifies active wire breaks in pipe sections and establishes a rate of deterioration.

 

A data acquisition unit transmits light through the cable, which is disturbed by the pressure wave created when a wire breaks. A second optical fiber determines the location of each event. Once data analysts at Pure confirm a wire break, they e-mail a PCCP management group within the commission.

 

“By June 30, 2013, Pure should have inspected and left cable in our entire 48- to 96-inch pipe, a distance of 77 miles,” says Gumm. “This year, we’re halfway through inspecting 13.3 miles of 54-, 60- and 66-inch pipe. A contractor will repair or replace the distressed areas.”

 

Contractors will either excavate and replace highly deteriorated sections of PCCP or repair them using external tendons (a wrap that squeezes and holds the concrete in place) or internal carbon-fiber-reinforced composite. “By using a combination of the latest technologies, it’s far more likely that we will identify the potential for big water main breaks in time to react before lines fail,” says Gumm.



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