Eighteen force mains assessed, and the first one is now inspected twice
Introduction
Force mains are a pressurized component of gravity sewer systems that transport wastewater to a discharge point. Unfortunately, they are challenging to maintain, inspect, and repair due to the inability to take them offline for extended periods and the limited access points for inspection or maintenance.
Force main risks
A good understanding of the force main and its high-risk locations is an integral part of force main risk management. The vast majority of force main failures can be attributed to three main factors: internal corrosion, structure deterioration, and low scouring velocity.
- Internal Corrosion. Approximately 64 percent of force mains are composed of ferrous materials such as cast iron, ductile iron, and steel (EPA, 2010). The primary cause of failure in these force mains is internal corrosion caused by hydrogen sulfide (H2S) gas.
- Structure Deterioration. Force mains carry structures along their route, including air release valves (ARVs) at high points and blowoffs at low points. ARVs vent trapped gas and reduce the risk of sulfide corrosion. If an ARV fails, becomes submerged, or is not present, the resulting sulfide corrosion gradually deteriorates the metallic piping and valve. On a non-metallic force main these structures may be the only metallic components on the force main, and therefore the most likely points of failure.
- Low Scouring Velocity. The minimum self-cleaning velocity for a force main is two (2) feet per second. Below this velocity, deposits accumulate and restrict hydraulic capacity, and a moving deposit bed presents a risk of invert abrasion over time. Gas pocket formation is also more likely at lower velocities.
City of Naperville program description
In 2021 the City of Naperville, Illinois, inspected a single force main. The Cinnamon Creek force main is 6 inches, 2,261 feet of ductile iron installed in 1976, without a failure history. No failures were experienced on the City’s other force mains either, which left the City ranking its collection system by age and hoping age was a fair proxy for condition.
Four more inspections followed in 2022, and four more the year after that. Six annual programs later, eighteen (18) force mains have been inspected and the first reinspection has taken place five years after the initial inspection of that force main. The force mains reached from a 145 foot, 3.5 inch PVC force main to an 8,782 foot ductile iron force main. With these inspections, the City no longer ranks its force mains by age. It ranks them by measured condition, and sets each year’s capital budget against the ratings and cost estimates the program returns. The work has been delivered in partnership with RJN Group, Inc.
Inspection tool
Pipers®, INGU’s free-floating multi-sensor tools, provide an in-service condition assessment of force mains. In a single inspection, Pipers® identify gas pockets and leaks, determine the hydraulic grade line, and perform a magnetic inspection. The free-floating design allows Pipers® to navigate the force main’s flow without getting stuck in air release valves, tees, or deposits.
Running the program
Every inspection is carried out with the force main in service. The pumps are stopped briefly to accumulate volume in the wet well and to allow insertion of the Pipers® at the lift station, and then flow continuously for the duration of the inspection. No bypass is required and no force main is taken out of service, which is what keeps an annual cycle affordable.
Each force main comes back with a cleaning rating and a condition rating on a five point scale, combined into an overall rating that weighs condition more heavily, alongside the recommended action and a budgetary cost estimate for each option. Recommendations have ranged from installing an air release valve to replacing 200 linear feet of ductile iron pipe.
The second pass
Early 2026 the Cinnamon Creek force main was inspected for the second time, five years after the initial inspection. The City now has two inspection results for the same force main, recorded with the same tool under the same operating conditions, which shows how fast the force main is changing rather than only what condition it is in today.
Further reading
See RJN’s website for more information: Building a Prioritized Force Main Assessment Program