San Diego County Water Authority
Pure Technologies conducted a comprehensive Magnetic Flux Leakage (MFL) assessment on behalf of the San Diego County Water Authority (SDCWA) on their 72-inch and 75-inch mortar lined steel water transmission main, called the Second San Diego Aqueduct (SSDA). Pure inspected just over 5 miles of the SSDA, which was built in the 1950s and is an important link between the Metropolitan Water District and the San Diego County Water Authority. The SSDA inspection is the largest diameter MFL inspection to date.
|Magnetic Flux Leakage technology|
|November 2011 – July 2012|
|5 miles (8 km)|
|72 and 75-inch (1828mm -1905mm)|
pipe sections identified with areas of broad metal loss
MFL tool able to detect through 1/2-inch mortar thickness
After the Carlsbad desalination plant was approved for construction, SDCWA needed to verify that its major transmission main could handle the additional pressure created from the plant, which would eventually tie into the pipeline. A condition assessment was requested to decide whether the pipeline would need to be repaired to handle the additional strain. While the transmission main had been previously inspected using other technologies, SDCWA required a higher resolution assessment as well as a quantitative analysis of the remaining pipe wall thickness.
A custom, extra-high resolution MFL tool, owned by the San Francisco Public Utilities Commission (SFPUC), was loaned to SDCWA for the Aqueduct inspection. The tool was developed by SFPUC and Pure Technologies during a three year project from 2007 to 2011 that inspected SFPUC’s San Joaquin Pipelines.
MFL is non-destructive testing used to detect corrosion, pitting and wall loss in lined metallic pipelines. With MFL technology, permanent magnets are used to temporarily magnetize the pipe and the magnetic field changes are recorded and analyzed. The magnetic flux is uniform if there are no flaws in the wall of the pipe. If internal or external flaws are present, such as pitting, corrosion or other forms of damage, the magnetic flux is distorted beyond the wall of the pipe, and this distortion or ‘leakage’ is measured by Hall Effect sensors.
The condition assessment began with a geometric survey of the dewatered pipeline. The geometry tool mapped the inside of the pipeline for any distortions or structural anomalies that may have been caused by overburden or poor bedding and installation. The tool measures the internal diameter of the pipeline with fingers spaced 1-inch apart, with each finger recording the relative change in pipe diameter around the circumference at a rate of 500 samples per second.
Following the geometric survey, the MFL tool was used to identify and locate high-resolution metal loss in the pipe wall caused by corrosion, pitting, and external pressure. As the tool travels through the pipeline, sensors record variations of the magnetic field around defects. MFL technology has a magnetic field so strong that the pipe wall will carry no more magnetism if the pipe wall is intact. Where there is a defect, the remaining wall is too thin to carry the full magnetic field and the flux is diverted around the defects, or “leaks” out of the pipe.
Because of their size, both the Geometry and MFL tools were assembled inside large manholes and pulled by manned ATV’s. In order to move the tools up or down slopes, winches were set up above the ground to pull or lower the tool. Access points were located every 3,000 to 5,000 feet for the safety of the inspection staff pulling the tool and used to charge the ATV’s, as well as to set up the winch system when it was required.
The results of the SSDA inspection were positive; it demonstrated the effectiveness of the MFL tool in inspecting a pipeline with 1/2-inch mortar lining. Out of the approximately 892 pipe sections inspected, seven pipes exhibited areas of broad metal loss due to corrosion. Several of these pipes were verified by SDCWA as having metal loss and repaired.
“The MFL scan was successful in providing a comprehensive look at the condition of the pipe wall. We were able to ﬁeld verify the data and eﬀectively use it to make rehabilitation decisions.”
– Gary Eaton, Director of Operations and Maintenance San Diego County Water Authority