A 90-inch water main runs parallel and is in close proximity to a Chicago Transit Authority (CTA) DC electric railroad. Prior testing indicated the presence of DC stray current interference on the 90 inch pipeline due to the effects emanating from the DC electric railroad. This ultimately led to more advanced testing and design of a bond using a reverse current drainage switch (RCDS) to mitigate the interference caused by the DC electric railroad. A reverse current drainage switch is a device that allows current to flow through a bond when the potential of the pipeline is more positive than the potential on the railroad. By utilizing this type of switch, the pipeline does not become more anodic as would be the case when DC stray current interference is present. Once the potential of the pipeline becomes more negative than the potential of the railroad the reverse current drainage switch opens to hinder the current flow in the bond. This paper discusses the final commissioning and testing performed on the reverse current drainage switch installed between the pipeline and the DC electric railroad.
The pipeline consists of approximately 8.97 miles (14.4 km) of 90-inch PCCP and is buried through out the majority of its length. There are approximately a 1,060-foot (323 m) and a 3,142-foot (958 m) long sections where the pipe is within tunnels. These locations coincide to locations where the main is closest to and crossing the DC electrical rails causing the interference. After previous testing on the 90-inch water transmission pipeline indicated the presence of dynamic stray current interference further testing and design work was performed to assess and mitigate the stray current interference issue. A reverse current drainage switch (RCDS) was selected to mitigate current from flowing from the pipeline to the DC electric railroad.
After the testing phase of the project, work began on designing a system that would both mitigate and monitor the DC interference current from the electric railroad. The final design was comprised of a RCDS and Coupon Test Stations.
The DC stray current mitigation system included installing a cable connection between the 90-inch water pipeline and the CTA rail line. During the testing phase the point of maximum current exposure was found to be within a portion of the pipeline that runs through an underground tunnel that had become inaccessible. Since further testing in the tunnel could not be performed the location for the bond cable was moved to a nearby test station. Moving the bond cable to the test station also allowed for the minimum length of directional boring required for the bond cable installation since the test station was closer than the calculated point of maximum exposure. The reduction in length of the bond cable minimized the electrical resistance of the bond cable and reduced the costs of the cabling and boring. The layout of the water transmission line and bond cable is show below in Figure 1.
