Cathodic Protection Interference on a Pipeline Crossing
A 12-inch coated natural gas pipeline (Pipeline A) crosses a 16-inch water main (Pipeline B) at a perpendicular distance of 10 feet. Pipeline A has an impressed current cathodic protection (ICCP) system with a remote anode bed 500 feet away, output 30 A, 50 V. Pipeline B is bare steel and has its own sacrificial anode system. Recent close-interval surveys on Pipeline B show a potential of -0.75 V vs. CSE at the crossing, while the native potential is -0.55 V. Pipeline A's pipe-to-soil potential at the crossing is -1.10 V. A coating holiday on Pipeline A near the crossing is suspected. The operator wants to ensure both pipelines remain protected without causing interference.
1. Determine if interference is occurring and identify the type. Calculate the driving voltage and recommend a mitigation strategy.
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Interference is likely occurring. Pipeline B's potential shifted from -0.55 V (native) to -0.75 V, a 200 mV cathodic shift, indicating it is picking up current from Pipeline A's ICCP. This is cathodic interference (beneficial but potentially problematic if current discharges elsewhere). The driving voltage for interference is the potential difference between the two pipelines: Pipeline A at -1.10 V and Pipeline B at -0.75 V, so 350 mV. However, the real concern is if Pipeline B has a coating holiday where current discharges, causing anodic interference and accelerated corrosion. Since Pipeline B is bare, current pickup is distributed; but at the crossing, current may discharge on the far side. To assess, perform a current span survey or use a Pearson probe. Mitigation: (1) install a bond between Pipeline A and B with a resistor to drain current in a controlled manner, (2) adjust ICCP output or anode bed location to reduce interference, (3) improve coating on Pipeline A to reduce current demand, (4) install additional anodes on Pipeline B near the crossing. The bond resistance should be sized to allow sufficient current to keep Pipeline B polarized to at least -0.85 V, while not overloading the ICCP. Calculate required bond current: if Pipeline B needs 100 mA to maintain protection, and voltage difference is 350 mV, R = V/I = 0.35/0.1 = 3.5 ohms. Use a variable resistor to fine-tune.
2. What safety considerations are critical when performing this interference mitigation work?
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Safety considerations: (1) Lockout/tagout of the ICCP system before making bonds or adjustments to prevent electrical shock. (2) Use of personal protective equipment (PPE) including insulating gloves and tools. (3) Verify that the bond does not create a hazardous voltage on Pipeline B; install overvoltage protection (e.g., spark gap or diode) to protect against fault currents. (4) Ensure proper grounding and bonding to avoid stray currents. (5) Test for explosive atmospheres before hot work; use gas detectors. (6) Follow confined space entry procedures if accessing test stations. (7) Coordinate with both pipeline operators. (8) After installation, monitor potentials to ensure no overprotection (e.g., Pipeline B potential should not go more negative than -1.2 V to avoid coating disbondment).