Lessons Learned from a Comprehensive City of Phoenix 24-Inch Ductile Iron Force Main Condition Assessment

The City of Phoenix has recently launched an assessment program for its force main assets. The initial focus is a pilot study of the Ahwatukee lift station 40 force main, a triple barrel 24-inch ductile iron force main. Barrel 1 was originally constructed in 1974, with Barrels 2 and 3 installed in 1998. Each barrel is 3.1 miles long. The pipes traverse developed areas along the Interstate 10 corridor in Phoenix, AZ. The assessment pilot program developed inspected all three barrels using Pure Technologies’ (Pure) SmartBall platform for acoustic anomalies (AA) for gas pockets and leaks. Barrel 1, the older of the pipes which had experienced one leak, was also assessed using a combination of technologies including PICA’s See Snake remote field electromagnetic (RFEM) inspection, and Pure’s SmartBall platform with pipe wall assessment (PWA). RFEM and PWA were collected on the same 3.1 miles of 24-inch ductile iron force main constructed primarily in 1974. Permanent and temporary physical and operational modifications were required to complete the assessments. All obstructions along the alignment of Barrel 1 were removed, deployment fittings and valves were installed, and extensive pigging was performed on Barrel 1. The approximate cost of pipe preparation and support for the RFEM and PWA inspections were $24/foot and $2/foot, respectively. The disparity in costs between the RFEM and PWA inspection technology vendor costs was less significant, at $9/foot and $7/foot for RFEM and PWA, respectively. Analysis of these data allowed a direct comparison of the accuracy, precision and agreement between the two technologies. AA results identified no detectible leaks on any of the three barrels. One AA result consistent with a gas pocket was identified on Barrel 1. PWA results for Barrel 1 identified 41% of the pipe segments to have anomalies. RFEM results identified and detailed 50% of the pipe segments to have a total of 1,180 defects, including 20 locations where there was less than 10% remaining wall thickness on 11 pipe segments. The difference in total damage reported and locations of reported damage may be related to the difference in fundamental detection physics whereby RFEM directly measures loss of ferromagnetic material and PWA detects the change of the magnetic susceptibility of a material when subjected to a mechanical stress. An overall positive correlation rate of approximately 54% was observed. Approximately 21% of pipes were reported to have an anomaly based on PWA data, and correlated to a pipe with wall loss reported based RFEM data. Approximately 33% of pipes were reported to have no anomaly based on PWA data, and correlated to a pipe with no wall loss based on RFEM data. The results of these assessments are being used to develop rehabilitation plans for Barrel 1. Excavation and physical validation where defects were reported are in progress and are expected to determine the extent to which the emerging PWA technology can identify distressed pipes.