Pipe failure in Prince George's County
The 54-inch water main is made of prestressed concrete cylinder pipe (PCCP), which often fails in a predictable way. The problem in this pipe was detected by fiber-optic sensors that heard the "pings" that occur when high-tension wires break. Read about the water shut-off.
Are you affected? Outage area map.
There are 350 miles of PCCP pipes in Montgomery and Prince George's counties.
Here's why they tend to fail.
How it is made
An outside coating of mortar protects the wires.
High-tension wires tightly wrap the pipe, providing much of the pipe's strength.
A layer of concrete surrounds the steel in one type of pipe. Another type does not have this layer.
A thin, steel pipe acts as a moisture barrier.
The innermost layer is a strong concrete core.
How it fails
Most commonly, the mortar coating cracks, either through mishandling, accident, poor manufacturing, erosion or just age.
Water seeps in and corrodes the wires. Wires break, creating a weak spot.
Internal water pressure overwhelms the concrete core. The steel gives way and the pipe bursts.
Three ways the sanitary commission looks for problems
Listening for wire breaks: Acoustic cables monitor the pipes, “listening” for wire breaks. Sensors in the Prince George’s main detected the first ping on July 12, then more followed. As of Monday, there had been 30 pings, enough to indicate a weak spot that would likely burst.
Locating leaks: A 7-inch, one-pound foam “SmartBall” filled with electronics floats through pipe, recording a soundtrack that will be analyzed for the distinctive sound that indicates an existing leak. It chirps as it travels so sensors along the route can pinpoint where it recorded the leak.
Finding damaged wires: Two types of technology detect broken wires electronically. A current is sent through pipes, either by a machine on a cart or a free-flowing robot, creating an electromagnetic field. Anomalies
indicate broken wires in that area.
Fixing water mains
Acoustic cables “heard” the first wire breaks on Thursday, then more followed. As of Monday, there had been 30 pings, enough to indicate a dangerously weak spot.
Crews will close valves to isolate the damaged pipe, dig down to expose the pipe and cut out a section with concrete saws. They will clear rocks and debris from the “bed” so the new pipe will lie on an even surface.
The new section, likely made of ductile iron, will be attached with concrete-and-steel adapters. After the pipe is inspected by WSSC, valves will be reopened and water will flow again.
More pipes, more breaks
In its 5,500-mile network of water pipes, the WSSC has more miles of the type of concrete pipe that has been prone to rupture than any of the 20 major jurisdictions surveyed by The Post, except Detroit.
Note: Some major breaks were caused by contractors striking pipes. Phoenix did not provide number of failures.
SOURCES: Mark Holley, president of Pure Technologies U.S. Inc.; Washington Suburban Sanitary Commission; EPA.- The Washington Post. Published June 21, 2013.