Prioritizing Pipe Repairs
More than $1 million: That is the potential cost of emergency repairs, cleanup, disruption and possible regulatory fines associated with the failure of a typical wastewater force main today. Every wastewater utility desperately wants to avoid the cost and calamity of a force main failure, but wholesale replacement of a force main system can cost upward of $5 million.
In 2008, the Grand Forks, N.D., Wastewater Collection Utility (GFWCU) was facing a similar situation. But using the latest acoustic inspection technology to inspect its aging wastewater force main system enabled the utility to identify, further assess and plan for the repair of only the damaged pipe sections rather than expensively replacing the entire pipeline. The estimated savings totaled a whopping $2.5 million.
Force mains are most often used where the low topography of an area is insufficient for the application of traditional gravity sewers to deliver wastewater to the desired downstream discharge location. Like the aging water pipe infrastructure, many wastewater force mains have been in service for decades and inevitably are becoming prone to more frequent failures. According to Water Environment Research Foundation guidelines, these failures primarily are caused by internal and/or external corrosion.
Ductile iron, polyvinyl chloride (PVC), cast iron or prestressed concrete cylinder pipe (PCCP) are the most frequently used materials for wastewater force mains. Ductile iron pipe and fittings are often installed with both an internal and external noncorrosive coating to avoid premature deterioration or failures. PVC is generally not affected by the corrosive environment present in wastewater force mains, but it is prone to other failure mechanisms sometimes predicated by leaks. Cast iron force mains are generally older than other materials and lack any corrosion-resistant coatings or linings. More recently installed PCCP force mains are often lined with special liners to resist corrosion, but many older PCCPs are not lined and also lack external mortar coating protection.
Rather than continuing the “run-to-failure” practice with their critical wastewater infrastructure, many utilities are implementing proven asset management principles and proactively performing condition assessments of their high-risk force mains. Considering that most force mains are simply water pipes employed in wastewater applications, it is no surprise that utilities are turning to the latest tools and advanced technologies that have proven successful in detecting the defects that often lead to pressurized water main failures. There are, however, two major problems with performing condition assessments on wastewater force mains.
“The general absence of wastewater collection system redundancy and the lack of accessibility into the pipelines are significant challenges when dealing with wastewater force mains,” said Travis B. Wagner, P.E., wastewater specialist with Pure Technologies Inc. Wastewater infrastructure simply does not have the built-in redundancy enjoyed by many water distribution system counterparts, according to Wagner. Water transmission mains are usually part of an interconnected system where a utility can shut down a major pipeline yet still deliver water to its customers through another pipeline. Only a single force main, on the other hand, is typically available to transport wastewater from one point to another.
“If you have to take the force main out of service to perform a condition assessment, you are often forced to implement an expensive bypass pumping operation,” Wagner said. “And bypass pumping costs can actually be higher than inspecting and/or replacing the force main itself.”
Gas Pockets in Grand Forks
Constructing an expensive bypass sewer pipeline and embarking on cumbersome internal cleaning and inspections was precisely the scenario the GFWCU wanted to avoid. The wastewater utility initiated a project in 2008 to conduct a condition assessment of an 8.8-mile wastewater force main that traverses directly through the heart of the city. The force main consisted of both 24- and 30-in. PVC and PCCP constructed between 1960 and 1980, and the utility was concerned about the condition of several sections of the PCCP portion.
“We wanted to look for leaks, but we also wanted to detect the presence of hydrogen sulfide (H2S) gas pockets inside the pipe,” said Shawn Gaddie, operations manager of Advanced Engineering and Environmental Services, a consulting engineering firm working with the city of Grand Forks.
When oxygen depletes from wastewater and gas pockets form within the pipe, H2S gas released from the wastewater can be converted to sulfuric acid by bacteria in the slime layer on the pipe wall, and can cause corrosion and eventual breakdown of the pipe’s exposed surface.
“If you’ve got H2S gas formations in those air pockets, the gas corrosion is just going to eat away at this prestressed concrete cylinder pipe,” Gaddie said. “Once the reinforcing wires are exposed, it’s just a matter of time.”
The Grand Forks wastewater force main had key areas with no available redundancy, so isolating the force main and inspecting it with conventional internal robotic CCTV equipment was not an option. The utility began exploring alternative technologies enabling leak detection and condition assessment of in-service wastewater force mains, including the acoustic-anomaly detection solutions SmartBall and Sahara from Pure Technologies.
Originally developed by PPIC (since acquired by Pure Technologies), the Sahara inspection tool uses an acoustic sensor tethered to a cable and is pulled through the force main by the flow of wastewater. The cable enables the sensor to be stopped, retrieved and repositioned during the inspection to locate any detected anomalies.
The SmartBall used an untethered acoustic-anomaly detection sensor, a “free-swimming” solution that is more appropriate in situations in which few pipeline access points are available. With only two insertion and extraction points along the entire 8-mile alignment, the GFWCU decided that Pure’s untethered acoustic detection solution was more appropriate for its situation.
The SmartBall consists of a 7-in. foam ball with a 2.5-in. aluminum core that contains a data acquisition system. The device is released into the live force main and is propelled through the pipeline by the flow of the wastewater stream. Moving untethered along the bottom of the pipe at approximately 90% of the flow velocity, it records the acoustic activity within the pipe and accurate positional information is downloaded when the device is recaptured downstream. The acoustic data recorded is downloaded, cross-referenced with time-stamped positional data and analyzed to determine the presence, approximate size and location of leaks and gas pockets along the pipeline.
The two-day Grand Forks force main survey was performed while the pipe remained in service and required two separate SmartBall insertions and retrievals. The initial survey data analysis detected no actual leaks, but several acoustic anomalies within the pipeline were identified as possible H2S gas formations ranging from 2 to 18 ft in length. Later pipeline repairs at one of those locations gave engineers a chance to validate the gas pocket data by inserting a CCTV camera to view the pipe interior. The evidence of the gas pockets identified was there, and corrosion damage was clearly visible.
“We observed corrosion deteriorating the interior mortar and the metallic reinforcing structure at the crown of the pipe where the gas pocket had occurred,” Gaddie said, noting that SmartBall’s gas-pocket detection had identified a point of potential catastrophic failure. “There had been a pipe failure very near this spot about 15 years earlier, and another failure again at this location would mean shutting down one of the city’s major transportation corridors for a very expensive emergency repair of the pipe.”
Proactive Approach, Lower Capital Costs
The GFWCU was able to save a significant amount of budget it had earmarked for wastewater force main rehabilitation and replacement. The utility’s master plan estimated that nearly $4 million would be needed to clean, inspect and repair significant portions of the PCCP force main, but the SmartBall assessment results enabled the utility to prioritize locations for repair rather than expensively replacing entire sections, reducing that budget number to an estimated $1.5 million—a $2.5-million savings.
The SmartBall inspection effort has convinced the GFWCU that proactive condition assessment is vital when it comes to managing critical infrastructure. Proactively identifying defects within this critical wastewater force main proved to be significantly more cost effective than reactive repair and maintenance of pipelines under failure conditions—not to mention the immense environmental impact and public reaction to a major sewer main failure in an urban area.
“A proactive approach to understanding your problem far outweighs reactive measures when it comes to prudently spending capital dollars,” Gaddie said. “Using this SmartBall technology gave us direction we needed. It took us from applying blanket approaches to the problems to at least being able to bring it down to a level where we could prioritize and understand what pipeline locations we really needed to examine and focus on."