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, structural deterioration, and low sourcing velocity.
- Internal Corrosion. Over 60% of existing force mains are composed of ferrous materials like steel, cast iron, and ductile iron. The primary cause of failure in these force mains is internal corrosion due to hydrogen sulphide (H2S) gas.
- Structural Deterioration. Force mains have air release valves to prevent gas buildup at high points reducing the risk of sulphide corrosion. If an air release valve fails (or is not present), the resulting sulphide corrosion can gradually deteriorate metallic piping and valves, ultimately causing force main failure.
- Low Sourcing Velocity. Too low flow in force mains can lead to deposit build up which will restrict capacity and can be a risk for abrasion over time.
City of Crystal Lake project description
Pipers® inspection tool
Pipers®, INGU’s free-floating inspection tools, provide a comprehensive assessment of force mains. Pipers® effectively identify gas pockets (and leaks), determine the hydraulic grade line, and perform a magnetic inspection in a single inspection. The free-floating design allows Pipers® to navigate the force main’s flow without getting stuck in air release valves, tees, or deposits.
- Gas pocket detection. The Pipers® record sounds caused by gas pockets, pumps, and other noise sources in the pipeline. Gas pockets are usually louder than the background noise in the pipeline and can be confirmed by the specific spectral signature at its location.
- Hydraulic grade line inspection. The hydraulic grade line indicates the amount of frictional pressure loss throughout the force main, where regions of the hydraulic grade line with a steeper slope experience more flow friction suggesting increased internal surface roughness and/or diameter restrictions.
- Magnetic inspection. Pipers® measure the magnetic flux density inside pipelines. These measurements allow for the identification of material changes, pipeline features (e.g. air release valves and casings), and deterioration of metallic piping.
Operational description
For the deployment, the pumps were shut off, and the Pipers® were inserted through a check valve at the lift station. A bar screen was set up at the outlet of the discharge manhole, with a crew member on standby with an extended catch basket available to collect the Pipers® upon arrival.
In the end, two (2) Pipers® in total were deployed in the force main, and both Pipers® were retrieved successfully.
Project results
The Pipers identified 6 locations with a metal loss signature. These locations were assessed to be high risk as metal loss indicates at least 30% wall loss. Four of the identified locations were close together and are shown in the map below.
The City of Crystal Lake performed a validation of the identified metal loss locations and confirmed the identified metal loss by the Pipers®. The second image below shows photos of the cutouts that were performed as part of this assessment.
As visible in the photos, the corrosion seems to be external and the inside of the pipe seems in good condition. The Pipers® did not detect any air pockets in this segment of the force main and the external corrosion assessments revealed the presence of highly corrosive soils along the force main, so external corrosion was suspected as the primary cause of the wall loss and confirmed by the cutouts.
This confirmation by the City encouraged them to move forward with their next highest priority force main set to be inspected later this year.
Pipeline details
Pipeline Length | 1,100 feet |
Pipeline Diameter | 6 inches |
Pipeline Material | Cast Iron |
Content | Wastewater |
Location | United States |
INGU’s Pipers® have inspected over 100 force mains across the United States and Canada. These inspections encompassed a wide range of pipeline lengths, from 0.05 to 70 miles, and diameters, from 3 to 108 inches. The force mains inspected were composed of both metallic materials (such as ductile iron and cast iron) and non-metallic materials (such as PVC and HDPE), and were designed for various pressure ranges.