According to a 2004 Water Environment Research Foundation (WERF) survey, 7.5% of wastewater collection systems are force mains. Most are located in highly critical areas in communities where the consequence of failure can be severe. Even more troublesome for utilities are the operational and technological limitations associated with pressurized pipes.
With advancements in technology and a willingness to develop proactive pipeline integrity programs, utilities can successfully reduce failures, mitigate risk, reduce capital expenditures, and increase confidence in the overall operation of their force mains.
Utilizing inline inspection technologies, a utility can gain a comprehensive understanding of the condition of their force mains and pinpoint areas which need immediate attention. Sections of the pipe with many years of remaining useful life can be identified, saving the cost and effort of a larger-scale replacement.
The importance of managing pressurized sewer pipes
The failure of most force mains equates to a high cost financially, operationally, environmentally and socially for utilities. Despite this, many utilities have traditionally opted for a reactive approach to managing these assets, often due to the difficulty of the inspection process.
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Utilities who have implemented proactive programs to manage their force mains, along with their entire linear asset inventory, have been able to:
Reduce failures – force main failures can cost a utility from $500K to well over $1M, as well as negative public sentiment and adverse media exposure.
Reduce capital expenditures – condition assessment programs can be implemented for roughly 5% – 15% of the cost of full-scale replacement programs.
Mitigate risk – through a comprehensive condition assessment program, a thorough understanding of risk can inform optimized repair, rehabilitation and replacement strategies.
Increase confidence – in their overall force main operations as well as with their customer base and communities.
Optimize operational expenditures – by implementing a force main management strategy, utilities can move from reactive operation and maintenance to proactive planning, thus optimizing budget allocation.
Force main pipe types and modes of failure
According to a survey by WERF and the National Association of Clean Water Agencies (NACWA), the primary cause of force main failures is internal corrosion.
The majority of force mains are metallic pipe, which includes cast iron, ductile iron and steel pipes. Almost half of metallic force main failures are due to external or internal corrosion, with an additional quarter of failures due to surge pressure and joint leakage. This means that nearly 75% of metallic force main failures can be prevented by implementing a proactive pipeline management program.
Force main failure with metallic pipes
While non-ferrous pipes such as prestressed concrete cylinder pipes (PCCP), reinforced concrete cylinder pipes (RCCP) and bar wire wrapped pipes (BWP) make up a smaller percentage of force mains, they dominate in diameters above 0.91 m. What is important to note is that failures on these non-ferrous pipes tend to be more catastrophic.
Nearly 55% of failures of non-ferrous pipes are from corrosion and structural defects. Another 10% from surge pressure and joint leakages means that nearly 65% of non-ferrous force main failures are also preventable.
Developing a proactive risk-based condition assessment program
Implementing a strategy that focuses on gathering data through condition assessment is crucial to ensure the safe operation of wastewater infrastructure, and to optimize capital expenditures.
How to assess force mains
Inspecting force mains is significantly more challenging than inspecting gravity mains. These challenges include: lack of redundancy; lack of or limited access points; cost of inspection; environmental concerns related to the nature of sewer systems; and technology limitations.
New standards of best practice for force main management involve a variety of methods and technologies to provide data and information with which to make decisions. Utilities can now often perform a detailed condition assessment while the force main remains in service.
There is no “one-size-fits-all” way of assessing force mains. Any approach should be tailored to risk tolerance, material, diameter and past failure history. Utility managers are turning to programs that reduce damage to assets, prioritize investment to minimize community impact of asset failure, and reduce the consequence of failure by enabling system control.
General assessment approaches should include maintenance and failure history; checking design for today’s loading conditions; monitoring for transient pressures; assessing critical control valves; performing inline, accurate lead and gas pocket assessment, and inline pipe wall assessment.
Research conducted as part of the WERF: 2010 Guidelines for the Inspection of Wastewater Force Mains, shows that the most common failure mode for force mains is internal hydrogen sulfide corrosion which starts as a gas pocket forming in a pipeline. So, monitoring for hydrogen sulfide is extremely important.
Faced with deteriorating buried sewer infrastructure and challenges associated with the complexity of force mains, proactive utilities are taking the important step to perform condition assessments on these assets. Information gained allows utilities to take action needed on critical assets and to better understand the condition of their pipelines.
This approach enables a powerful, cost-effective strategy for asset management with significant operational, financial and community benefits.
This article appears in ES&E Magazine’s June 2019 issue.