Warning "Ping" from PCCP Pipe Averts Danger
It promised to be like any other Washington area 4th of July weekend - hot and humid -with pool parties and fireworks; lots of fun in the sun and water.
By I.J. Hudson
It promised to be like any other Washington area 4th of July weekend - hot and humid -with pool parties and fireworks; lots of fun in the sun and water. But just a few days before the official holiday this year, a "ping" from below ground in a DC suburb registered on a computer in Calgary, Alberta. That ping was "heard" by the Acoustic Fiber Optics (AFO) monitoring system, installed in almost 28 miles of pipeline in the Washington Suburban Sanitary Commission's (WSSC) distribution system.
In pipes monitoring by the AFO system, sound waves caused by small fissures or other structural changes affect laser light projected through a fiber optic cable inside the pipe. And because the entire length of the cable acts as a sensor, a sensor is never more than a pipe diameter from the event. That helps pinpoint the area of concern.
In this case, the ping was created by re-enforcing wire snapping in a 96-inch diameter pre-stressed concrete cylinder pipe (PCCP) in a Potomac, MD, neighborhood.
One ping is a "heads up." But the AFO monitoring system recorded eight pings. That strongly suggested a 16-foot section of the massive water transmission main was losing its integrity. How much time before it would fail? There was no way to know.
WSSC General Manager/CEO Jerry N. Johnson called an "all hands" meeting for his key team chiefs and field managers at 8 p.m. Wednesday. The guidelines: cost was not an issue, public safety was paramount. Which options made sense for this critical situation? How much time did they have? They emerged about six hours later with a plan. They needed to dewater this pipe and replace the suspect section. That meant Johnson would have to order mandatory water use restrictions for the 1.8 million residents of two of Maryland's largest counties…on a holiday weekend.
|An Acoustic Fiber Optics monitoring system helped the WSSC prevent the failure of this section of damaged 96 inch pipe. Photo courtesy of Ronald Williams, WSSC Photographer.|
Work could start quickly because WSSC had recently established a new on-call contract to repair its major pipelines. WSSC immediately isolated the failing pipe leaving a parallel 48-inch water main to provide continuous service to its customers. That meant the water would be going through a pipe with one-quarter of the capacity of the original main. A failure of this 48-inch pipeline would put at risk the system's ability to supply the entire network. The mandatory restrictions were necessary to reduce forces on the smaller pipe, and to reduce demand in an attempt to ensure water was available should the need for fire fighting arise.
Local media gave the repair work and water restrictions wide coverage, while also airing additional public service reminders and visual captions or "crawls" on television broadcasts about the restrictions. They also covered WSSC police officers enforcing the restrictions by handing out hundreds of warnings for first time offenders, and $500 dollar tickets for second offenses. Maryland's State Highway Administration posted the alert on its electronic digital signs along major highways.
Work continued around the clock. The WSSC emergency contractor uncovered the suspect pipe section allowing technicians to use a tool to create another "ping" on that section of pipe allowing the monitoring site in Canada to confirm this was the bad section. When the pipe was removed, a preliminary inspection showed an area of major corrosion, broken wires and a crack. Failure was imminent.
The monitoring system had worked, and the management response using information provided with new technology was the correct one.
The water-use restrictions remained in place until July 6th. At the peak, water usage was reduced by 17 percent, not as much as had been hoped. But it was enough to keep the system intact while the pipe was fixed and the transmission line returned to service.
Infrastructure repair or replacement of broken water mains is an ongoing and expensive operation for a large water utility.
"AFO has given us a cost effective way to proactively manage major elements of our buried infrastructure," said Gary Gumm, WSSC Chief Engineer. "We've been in the situation where we have to react to a failed pipe and it's quite a different and more difficult challenge."
That point was brought home in December 2008, when a 66-inch main burst two days before Christmas and created a virtual river along River Road in Montgomery County. Air and water rescues of motorists trapped in their cars were nationally televised. The 66-inch main did not have an AFO monitoring system. As is already the case with all of the 96-inch transmission line, it will this year.
WSSC has 77 miles of water mains 48 inches and larger. Current plans are to have all of those large mains inspected and equipped with fiber optic cable by the end of fiscal year 2013.
AFO installation costs about $225,000 per mile of PCCP; that includes internal inspection of the pipe and the cost of monitoring equipment. Continuously monitoring each mile adds $13,000 a year to the cost.
The expense may seem high, but a break in a transmission main of this size would have caused major property damage and posed a serious risk to public safety. Instead, because WSSC is using a cutting-edge approach to system inspection and preventive maintenance on this pipe, the repair to this deteriorating section of an eight-foot diameter transmission line was merely an inconvenience over a holiday weekend. WW
About the Author: I.J. Hudson is Contract Project Manager, Public Affairs & Community Relations, at the Washington Suburban Sanitary Commission.