DC Transit Stray Current on a Parallel Pipeline
A 10-in. coated gas pipeline parallels a DC light-rail line for 3,000 ft at 30-ft separation. At a test station near the midpoint, pipe-to-soil potential is stable at -1.02 V CSE when no train is nearby. When a train accelerates, the potential shifts to -0.68 V; 20 seconds after the train passes, it swings to -1.19 V before recovering. A 10-A shunt in a bond to a nearby grounded structure shows current flowing from the pipe during train acceleration and into the pipe afterward. Soil resistivity is 3,000 ohm-cm.
1. Identify whether the pipe is picking up or discharging stray current during each phase, and state whether corrosion risk exists.
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During train acceleration, the pipe potential shifts positive from -1.02 V to -0.68 V. A positive shift means the pipe is discharging current to the environment; this is the anodic, corrosion-risk phase.
After the train passes, the potential swings to -1.19 V, more negative than the static value. That indicates current pickup, which is cathodic and not directly corrosive at that location.
The risk is therefore dynamic and localized: the pipe discharges while the train is accelerating and picks up afterward. The bond shunt confirms current leaves the pipe during acceleration. A static -1.02 V reading alone would miss the interference.
2. Recommend a mitigation approach and the tests needed to size it. Include a bond resistance estimate if 4 A of drainage is desired at a 0.34 V driving potential.
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Mitigation should provide a controlled drainage path from the pipe to the rail or grounded structure, such as a resistor bond, diode bond, or forced-drainage system. The bond must not drain the pipeline's own cathodic protection when no train is present.
For 4 A at 0.34 V, R = V/I = 0.34/4 = 0.085 ohm. That is a very low resistance bond and would need current control or a diode to prevent continuous CP loss.
Tests needed: time-synchronized pipe-to-soil potentials during train cycles, bond current logging, rail-to-earth voltage, soil resistivity, and potential mapping along the parallelism. Confirm whether the grounded structure is part of the stray-current path before bonding to it.
3. Explain why simply increasing the pipeline rectifier output is not an adequate remedy for this interference.
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Increasing CP output raises the pipe's cathodic polarization, but it does not remove the positive shift caused by train acceleration. The pipe can still discharge stray current during the anodic phase even while the rectifier is operating. Higher output can also increase current pickup after the train passes, worsen interference on nearby structures, and create overprotection.
The correct approach is to control the stray-current path, not merely raise the rectifier. Confirm the effectiveness of any bond by repeating the time-synchronized potential and current surveys.