OPERATORS EMBRACE INLINE TECHNOLOGIES TO ASSESS THEIR METALLIC FORCE MAINS
Historically, it has proven difficult to assess the condition of pressurized mains that carry sewage, especially those made with ferrous material. Metallic force mains have special operational challenges that don't apply to gravity sewer systems, and due to the presence of solids in the flow, force mains represent a far more abrasive environment than potable water systems.
According to the EPA, approximately 63 percent of material used in large diameter force mains is ductile iron, followed by cast iron and steel, and to a lesser degree, concrete pressure pipe. These materials are susceptible to internal corrosion from the sewage flow and from external forces in which the pipe is buried.
In a 2009 report published by to the Water Environment Research Foundation (WERF), internal corrosion is rated as the major cause of failure in metallic force mains 26 percent of the time, ahead of all other causes. External corrosion (19.2 percent) and third-party damage (19.4 percent) round out the next most common sources of failures in metallic force mains.
Force mains tend to operate with limited redundancy
Redundancy is not common in force mains, as most run constantly and cannot be taken out of service except for brief periods.
With redundancy limited, utilities have been challenged to deploy in-depth inspections. It has been more common to react to a failure rather than proactively assess the force main while in operation.
That reactive mindset is changing, especially with improvements in condition assessment practices and technologies. As well, many utilities have begun programs for inspecting their force mains as part of their overall assessment strategy, often as a result of consent decrees. The change is playing a role in asset management decisions, and the data is helping utilities make defensible decisions as when to repair, rehabilitate or replace their assets.
Pairing new detection technologies for condition assessment
Defects and deterioration of force mains can be wide ranging and vary from one pipe material to another. Pure Technologies (Pure) has found it prudent to use multiple techniques and technologies to address different operational challenges encountered on pressurized sewer mains. This can involve using inline leak and gas pocket detection tools paired with pipe wall assessment (PWA) technology to indicate locations where the pipe wall is under stress. The presence of stress can represent wall loss or damage, and is often a precursor to failure.
In the case of metallic force mains, gas pockets are of particular concern as hydrogen sulphide gas within the wastewater may be converted into sulphuric acid by bacteria on the pipe wall, which may cause corrosion and eventual breakdown of the pipe's exposed surface.
Pure's SmartBall® tool can accurately locate leaks and gas pockets in force mains of all materials, and when paired with pipe wall assessment sensors, provide a comprehensive screening solution. The areas where gas pockets overlap with stress anomalies represent the largest areas of concern of force main owners, as it represents a high likelihood of corrosion.
Metropolitan St Louis District inspects its ductile iron force main with success
In 2011, the Metropolitan St Louis District (MSD) responded proactively to a consent decree requiring the implementation of a force main management program that included all 114 miles of their force mains. Partnering with Pure, MSD developed a force main evaluation program to provide a risk-based analysis district's assets, and used PWA technologies and SmartBall leak and gas pocket inspections as a means of screening the force mains to prioritize pipeline repair or replacement.
MSD was an early adopter of SmartBall Pipe Wall Assessment, Pure's screening sensor technology that assesses the condition of metallic pipelines by identifying increased levels of stress. The SmartBall tool was also deployed using its traditional leak and gas pocket detection technology, pinpointing acoustic anomalies that represent a leak or pocket of trapped gas.
Analysis identifies 77 locations of stress and 120 gas pockets
The PWA analysis on 21 miles of high and medium force mains identified 77 locations showing stress anomalies. In addition, 120 gas pockets were discovered on just under 31 miles of force mains.
Proving their status as a proactive utility, St Louis continues to perform PWA assessments on the remaining ferrous force mains in their system to help determine where higher resolution inspections will occur.