October 2010, Vol. 22, No.10


'Pinging' avoids fireworks

As the Fourth of July holiday weekend began to ramp up in the nation’s capital, the Washington (D.C.) Suburban Sanitary Commission (WSSC) was more interested in tiny noises from the ground than fireworks in the sky.

Those tiny noises were the “pings” of wires breaking within a 2400-mm-diameter (96-in.-diameter) water supply main. Those wires provide strength for the prestressed concrete cylinder pipe — the largest in WSSC’s distribution system. When they begin breaking, it’s a signal that the pipe is weakening.

WSSC was alerted to the pings by an acoustic fiber-optic monitoring system installed 3 years ago in the 41-year-old pipe, according to Gary Gumm, WSSC chief engineer. “One wire breaking, in and of itself, is not a concern,” Gumm said. “But if you get a slew of these things, then you need to worry.”

Following a series of pings over a few days, WSSC decided to take the line out of service and replace that section of pipe.

A different situation

“This technology, an acoustic fiber-optic monitoring system, detected the problem, allowing us to take preventive measures now,” said Jerry N. Johnson, WSSC general manager, in a July 1 press release. “The system helped us prevent what could have been a much more serious situation.”

In December 2008, WSSC faced such a situation when a 1700-mm (66-in.) pipe ruptured, sending a torrent of water rushing down a major road in Bethesda, Md. That pipe carried 568,000 L/min (150,000 gal/min) of water. The rushing water instantly turned the roadway into a river and trapped several drivers in their cars until they were rescued by county fire and rescue services. The road was closed for 8 days while repairs were conducted.

This summer, however, the 2400-mm (96-in.) line was dewatered and replaced before a break, so there was no damage to the surrounding roads and homes, and no customers lost service, Gumm said.

WSSC was required to implement mandatory water restrictions while the pipe was out of service. WSSC prohibited watering lawns, washing cars, and topping off swimming pools and asked residents to take shorter showers, turn off faucets while washing hands, limit toilet flushing, and limit using washing machines and dishwashers.

The pipe is the largest of three lines that transmit water produced at WSSC’s 833,000-m3/d (220-mgd) water treatment plant to the bulk of the service area’s distribution system. The other two pipes are 1200 and 1700 mm (48 and 66 in.) in diameter, Gumm said.


Preventive maintenance

The fiber-optic sensors were installed 3 years ago following a preventive maintenance inspection of the pipe, Gumm said. WSSC dewatered the line and inspected it from the inside using three methods: A visual inspection checked for signs of deterioration, a sonic technology “listened” for voids and echoes within the pipe walls, and an inductive technique measured the integrity of the wire coils that are buried within the concrete, Gumm said.

“The strength of the pipe comes from prestressed wires — sort of like a Slinky® stretched,” Gumm said. The wires manage the tensile strength of the pipe. The concrete can easily handle the compressive forces of the water on the inside of the pipe, but the wires are needed to counteract the tendency of the outside of the pipe to bow from the pressure inside, he explained.

Following the inspection, WSSC established a baseline condition for that section of pipe and installed fiber-optic sensors that send a signal to WSSC when one of those wires break. The sound wave from the wire breaking propagates through the wires and concrete of the pipe and disturbs the light beam traveling through the fiber-optic sensor. The sensors also are coordinated with a global positioning system to pinpoint exactly where the break has happened, Gumm said.

Breaks in the middle of the pipe are more tolerable than breaks nearer the joints, Gumm said. The breaks heard in this 2400-mm (96-in.) main were near the end of a segment, he said.


Weakened wires

Corrosion from the outside of the pipe is the main cause of the wires breaking, Gumm said. The fluctuating pressure from inside the pipe contributes somewhat, but “the pipes are designed and built to withstand pressures well beyond what we have in the pipe,” Gumm said. The freeze–thaw cycle, as well as time, leads to cracks in the exterior that allow water to reach the reinforcing wires and corrode them. Each wire is about half the thickness of a man’s little finger, Gumm said.

“On this particular pipe that we took out of the ground, the areas where the wires were breaking was a rusty-looking mess,” Gumm said. But at the ends of that segment, where the pipe was cut out of the line to be kept for experimentation, the wires looked brand new.

So was that corrosion an indication that the pipe was about to fail?

It is impossible to tell with any accuracy how long the 2400-mm (96-in.) pipe would have lasted had WSSC not made the decision to replace it, Gumm said. That pipe could have lasted for as long as a year, but “we didn’t like what we were seeing, in terms of frequency and the timeframe [of the wires breaking].”

“You literally don’t know how much more time you have,” Gumm said. “Because of its location, and because of its size, and because we had some warning, we felt it prudent to go in.”

“When we got in there — while we’re not able to predict exactly when the pipe was going to break — it was very clear we made the right decision to go in and replace that pipe,” Gumm said.


Continuing work

WSSC has 124 km (77 mi) of 1200-mm (48-in.) and larger prestressed concrete cylinder pipe, Gumm said. The commission plans to have these fiber-optic sensors in all of those pipes by July 2013. Then, the program will start to deal with smaller lines as well. After that, WSSC’s goal is to perform inspections on each pipe segment every 6 to 7 years, Gumm said.

In the meantime, having the sensors in this pipe might have avoided some unwanted Fourth of July fireworks. Gumm said, “We were able to watch it as intended, and we were able to react to it as we intended.”

— Steve Spicer, WE&T


©2010 Water Environment Federation. All rights reserved.