Introduction
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.
Pipers® comprehensive assessment of force mains
- 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.
Case study (RJN Group / City of Naperville)
The City of Naperville is taking a proactive approach when it comes to their force mains and has been using Pipers® as part of their force main inspections. Since 2021, 14 force mains have been inspected in the city and 4 more are scheduled for this year.
RJN Group, our partner for the City of Naperville, has created a case study describing the results of five of the Naperville inspections.
In all five force mains, the Pipers® inspection produced actionable data for the City of Naperville. In total six acoustic anomalies were identified and two force mains were found to be running below full capacity. Partial replacement was recommended for one of the force mains at road crossings with large magnetic flux density anomalies. An air release valve was located at this location, but was inoperable for an unknown number of years before 2006. The large magnetic anomalies potentially represent wall loss due to an old gas pocket. Due to the smaller 6-inch diameter and advanced age, 200 LF of 6-inch DIP was recommended to be removed and replaced.
For another force main a low scouring velocity was determined and potential tuberculation buildup restricting the diameter by up to 32.5 percent in some sections of pipe. Two acoustic anomalies and numerous magnetic flux anomalies were identified along the pipe.
Details of all force main results can be read in the case study with an overall conclusion is that the inspections were successful and allowed for targeted recommendations for follow-up studies, rehabilitation recommendations, or that the results/conditions did not warrant further action.