Baltimore County Assesses Force Mains with Advanced Non-Destructive Technologies

Aging large-diameter buried water and wastewater infrastructure is a massive obstacle facing utilities across North America. Currently, a huge funding gap exists between what is needed for assessment, repairs and replacement and what is possible for utilities to spend on programs that address this problem.

Wastewater force mains pose a unique challenge for utilities, as they are typically more challenging to assess than water transmission mains because they lack redundancy, and therefore the ability to shut down for comprehensive condition assessments.

However, a failure to a wastewater force main can deliver more catastrophic results than a water transmission main since the structural damage caused by a failure is exacerbated by the environmental impact of having wastewater released into the environment and surrounding community.

“Wastewater force main inspection requires a different approach than to use common gravity sewer technologies in order to provide an assessment of the pipe’s current condition,” says Travis Wagner, engineering manager at Pure Technologies.

“Typically, electromagnetic (EM) inspections have required either the dewatering or depressurization of Prestressed Concrete Cylinder Pipe (PCCP) force mains in order to allow manned or robotic entry for inspection. However, the evolvement of EM assessment techniques in recent years for in-service PCCP water mains now allows for these inspections to be conducted under full operation of a PCCP force main.”

Despite the challenge facing many utilities, Pure Technologies has collected data from more than 8,000 miles of pressure pipe condition assessment and has found that only a small percentage of these pipes — less than 5 percent — are in need of repair and therefore have a significant remaining useful life; meaning it is often feasible for a utility to selectively rehabilitate its pipeline in favor of replacing it entirely.

Baltimore County, Md., and the Baltimore County Department of Public Works (DPW) have committed to ongoing projects that evaluate the condition of their wastewater pipeline assets and replacing areas that are at risk of failure.

In recent years, regular inspection and condition assessment of force mains has become heavily regulated in the United States — meaning inspections are now required by law. This is in part due to the problem of aging infrastructure around the country being highlighted by industry reports and studies, but also a stronger political commitment to preserving environmental integrity.

In September 2005, Baltimore County entered into a consent decree brought forth by the U.S. Department of Justice, the Maryland Department of the Environment (MDE) and the Environmental Protection Agency (EPA). The consent decree stipulated that Baltimore County inspect all force mains in its collection system with one or more methodologies appropriate to the specific characteristics of each force main. The consent decree also specifically stated several force mains be inspected and evaluated within six years of the effective date of the consent decree.

Although the consent decree stipulates that the force mains be inspected, it allowed Baltimore County the flexibility to specify the method or technology at the time the inspections are performed. The Baltimore County DPW has taken this opportunity to go beyond a minimalist approach, choosing to inspect its force main inventory with advanced non-destructive condition assessment technologies, reaffirming their ongoing commitment to providing reliable service and preventing pipeline failures.

To meet its consent decree obligation, DPW contracted two engineering firms, PHR+A/Jason Consultants, a joint venture, and RJN Group to inspect its wastewater force mains. Both firms subsequently contracted Pure Technologies for leak and gas pocket detection along with condition assessment of the force mains.
October was a busy month for Baltimore County DPW with 11 force mains scheduled for inspections using SmartBall, PipeDiver and PureRobotics.

As part of the condition assessment of PCCP force mains, leak and gas pocket detection is crucial since their presence is often a preliminary indicator of a potential failure location. Gas pockets in force mains are of significant concern as hydrogen sulfide gas within the wastewater may be converted to sulfuric acid by bacteria in the slime layer on the pipe wall, which may cause corrosion and eventual breakdown of the pipe’s exposed surface.

SmartBall is a free-swimming tool that measures the acoustic activity associated with leaks and gas pockets in pressurized pipelines. When acoustic anomalies are present, the data is analyzed to determine if it is a leak, gas pocket or just an external sound.

For structural condition assessment of the force mains, Pure Technologies used its PipeDiver and PureRobotics technologies.

PipeDiver is a free-swimming tool comprised of three parts — a battery module, electromagnetic (EM) module and a tracking module. It has EM sensors on each fin that take readings as it flows throughout the pipe with the wastewater. The EM sensors collect a magnetic signature reading for each pipe section to identify anomalies that are produced by wire breaks in the PCCP. PipeDiver is able to quantify the amount of wire break damage and is the best method of determining a baseline condition of a PCCP pipeline that cannot be removed from service.

PureRobotics is an EM inspection platform used when a pipeline either cannot be dewatered or the diameter is too small for a manned inspection. The robotic platform contains a CCTV camera, SONAR and EM sensors; the tool takes a magnetic signature reading and quantifies the amount of wire breaks using the same method as PipeDiver. Pure’s robotic assessment requires the line to be shut down and depressurized for inspection, although not dewatered, which many robotic assessment technologies require. All robotic inspections were completed at night to ensure the force main was shut down during a low usage time.

DPW and Pure Technologies completed just more than 15 miles of SmartBall inspections on all 11 force mains, almost 11 miles of PipeDiver inspections on seven different mains, and about three miles of robotic inspections on three different mains. The pipe diameters will vary for each main, ranging from 16- to 42-in. PCCP.

The busy inspection schedule posed the most significant challenge for the condition assessment project in Baltimore County. Over the month, 23 total inspections took place on 11 different force main sections, requiring extensive planning and coordination in order to complete the project on schedule.

While the SmartBall and PureRobotics inspections had standard insertion points, the PipeDiver insertions used bypass pumping stations in a variety of ways to insert the tool. Pure used bypass connections as pig launchers, as well as removing spool pieces in the bypass stations to deploy the tool.

In July 2011, Pure Technologies completed the initial phase of the project with DPW with an EM condition assessment using PipeDiver and a leak and gas pocket detection inspection using SmartBall on the Patapsco Force Main in Baltimore County.

The Patapsco Force Main carries sewage from the Patapsco Pumping Station, which receives flow from the Patapsco Sewershed in Baltimore County and Howard County. The force main discharges to the Baltimore City owned Southwest Diversion Sewer which carries flow to the Patapsco Wastewater Treatment Plant located in southwestern Baltimore City.

The Patapsco Force Main is made of 54-in. PCCP and was 31 years old at the time of inspection. The inspection covered the entire 1.5-mile alignment. The EM inspection was Pure’s first sewer force main EM inspection using PipeDiver, and the first in-service assessment of a PCCP force main in the world.

The PipeDiver tool allowed for a non-destructive EM assessment while the force main remained in service, which was required due to the lack of redundancy and ability to shut down on the Patapsco Force Main.
Of the 423 pipes inspected in the Patapsco Force Main, 24 pipes showed clear indications of distress for just fewer than 6 percent of the total pipeline. The results from the Patapsco SmartBall survey showed no anomalies associated with leaks or gas pockets.

Based on the PipeDiver inspection and subsequent Finite Element Analysis (FEA) of the Patapsco Force Main, a number of pipe sections exhibited prestressing wire breaks that suggested the pipeline had several sections with deteriorating conditions.

PHR+A/Jason Consultants recommended that the Baltimore County DPW start a high priority program for repair or replacement of several pipe sections based on the results of the FEA to reduce the risk of pipe failure. It was also suggested that the Patapsco Force Main should be re-inspected within five years to reassess the risk of failure and develop a deterioration model for the pipeline.

During Hurricane Irene in August 2011, a power failure occurred at the Patapsco Pumping Station causing a pressure transient in the force main. This pressure spike caused the failure of a pipe section that was determined to have exceeded the yield limit based on the FEA and the quantified number of wire breaks. Baltimore County seized the opportunity to replace other pipe sections that exhibited wire break damage beyond the yield limit while repairs were being completed on the failed section.

Pure Technologies and Baltimore County DPW will continue to complete force main condition assessment projects to ensure quality service delivery and prevention of pipeline failures.

Brian Ball is a project manager at Pure Technologies. 
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