March 18, 2014Across the United States, water and wastewater infrastructure is aging and becoming increasingly difficult for utilities to manage. The most publicized result of this is an increase in large-diameter pipeline failures, which can be expensive to rectify and damaging to communities and public confidence in the utility.
Industry reports also offer a bleak outlook about infrastructure in the United States. The American Society of Civil Engineers 2013 Report Card on America’s Infrastructure gave water and wastewater infrastructure a ‘poor’ rating and estimated that the cost to renew these systems would range from $200 billion to $1 trillion over the next 25 years.
While most of the discussion surrounding American water infrastructure involves pipe failures and the fiscal impact of renewal, water loss from leaking pipes is a major problem for utilities that often goes unnoticed. The U.S. Environmental Protection Agency (EPA) estimates that on average, between 15 and 20 percent of water never reaches the consumer, but could be as high as 60 percent in some municipalities.
This loss accounts for a huge financial cost for operators in terms of billing and wasted energy used to pump and treat the water, but also represents the waste of a critical natural resource.
In places like Dallas, managing water loss is an important matter for utilities, especially in the summer months when users are affected by severe droughts and forced to restrict consumption. The droughts also bring extreme heat and dryness, which dries out the soil and causes pipes to shift. This can lead to the accelerated development of leaks.
To mitigate this problem, Dallas Water Utilities (DWU) has completed an annual summer leak detection program since 2004 on its large-diameter water transmission mains that range in size from 12 to 84 in. The inspection program focuses on a variety of piping materials including Prestressed Concrete Cylinder Pipe (PCCP), Cast Iron Pipe and Ductile Iron Pipe.
To date, DWU has inspected 100 miles of pipe in the program, locating 120 leaks. This has allowed for a major reduction in water loss and helped ensure service reliability.
Water systems in large metropolitan areas are made up of thousands of miles of pipe varying in size; the distribution system, which delivers water directly to taps, is quite large and features small-diameter pipe; transmission mains, which transport high volumes of water throughout an area, are made up of a smaller amount of large-diameter pipe. Because so many areas depend on these pipelines for supply and their high consequence of failure, maintaining transmission mains effectively is a high priority for operators. For DWU, the criticality of these pipelines was a major factor in adopting a leak detection program that focused on its large-diameter pipe.
According to a study completed by the American Water Works Association (AWWA), leaks on large-diameter pipelines account for only 5 percent of the total leaks, but more than 50 percent of the total water lost from leakage. The discrepancy is created because transmission mains have a much higher capacity and operating pressure than distribution mains, meaning small leaks are actually leaking at a very high rate.
By focusing leak detection programs on large-diameter pipes, operators can achieve a much larger reduction in water loss by identifying and repairing even one leak.
There are several methods of locating leaks on large-diameter pipelines. Non-invasive methods, such correlators or listening sticks, work quite well on small-diameter distribution mains but often lack the accuracy needed to address large pipes as the sound of a leak does not travel as well as pipe diameter increases.
Conversely, inline leak detection methods aren’t well suited for distribution mains due to pipe size and complexity, but are effective in accurately locating leaks on large-diameter transmission mains because they bring the leak detection sensor directly to the source of the leak, unlike non-invasive systems.
For inspection of its transmission mains, DWU uses Sahara leak detection — a tethered platform that combines acoustic leak detection and inline CCTV — offered by Pure Technologies Ltd. The tool is non-destructive and is pulled by the flow of water by a small drag chute while the line remains in service. When the sensor is inserted into a tap, it remains tethered to the surface to allow for immediate confirmation of suspected leaks and gas pockets, internal pipe wall conditions and pipeline features by winching the sensor back and forth from the surface. The sensor is also tracked at ground level by a staff member, allowing for precise spot markings for excavations.
“Since introduction to Dallas’ program in 2004, Sahara technology has been a reliable tool for locating and eliminating leaks on larger diameter pipelines,” DWU assistant director Randy Payton says. “The program allows the Department to plan and prepare the repair in lieu of responding to a failure.”
The tool is capable of locating very small leaks due the sensitivity of the acoustic sensor. In terms of reducing water loss, small leaks may actually represent the best opportunity for long-term improvement. Leaks on large-diameter pipelines typically form and mature over a period of decades. They tend to grow larger over time, up until a point where the pipe fails or the leak surfaces.
Locating and repairing a large leak prevents it from leaking for the “tail end” of its life, and from failing catastrophically. Catching a leak while it is very small does this as well, but also prevents the decades of sustained water loss that would occur as it grows into a large leak. Using technologies that can locate small ‘pinhole’ size leaks can identify small leaks early on before they grow into larger leaks or lead to pipeline failure.
In the annual pre-planning stage, DWU identifies the ideal access points needed the inspections based on their knowledge of platform from previous years — there are usually about 30 insertions through 2-in. access points each year. Inspections are usually done during the summer months when most of the leaks are developing, and higher volumes in the pipelines allow longer distances to be inspected. DWU also controls the water flow closely during inspections to optimize the inspection distance. After many years of inspection, DWU staff has become adept at identifying the best insertion points and controlling the flow rate to maximize the tool’s capabilities.
During tethered inspections, there is significant traffic control required when the transmission mains runs beneath busy streets, since the tool is controlled and tracked above ground by staff members. To avoid major commuter disruptions, the City will reroute traffic and thoroughly plan the insertions to avoid high traffic times — for example, inspections frequently start in the mid-morning when traffic slows as opposed to during morning rush hour. Beyond traffic control, staff from DWU and Pure will often work on weekends when downtown Dallas is less busy.
There are also unavoidable environmental challenges that require adjustments. Sometimes the water main will run under a busy highway or an environmental obstacle like a river, making it impossible for the staff member on the ground to track the tool and mark exact leak locations. In this case, the operator needs to review potential leaks more closely by winching the tool back and forth to determine the exact location.
DWU’s leak detection program has been extremely successful, locating 120 leaks in the 100 inspected miles. The estimated water savings from these leaks is about 7.2 million gallons per day.
DWU has also seen a 17 percent reduction in catastrophic water main failures, likely as a result of the proactive approach in fixing leaks. Leaks, particularly in metallic pipe materials, are often a preliminary indicator of a failure location as it is a preliminary sign of distress. The reduction in failures has reduced property loss claims and service interruptions, as well as reduced treatment and delivery costs.
“Several factors affect the success of leak detection,” adds Payton. “After nine years, the utility continues to be impressed with its accuracy within the varied environments and piping materials.”
Through continued commitment to leak detection on its transmission mains, DWU is improving service reliability and saving significant amounts of water. DWU also completes regular structural assessment of its transmission mains to identify distress that could lead to pipe failure.
Adam McKnight, P.E., a project manager and Andy Dettmer, Ph.D., P.E., is a senior project manager at Pure Technologies, based in Dallas.