Marginal Cathodic Protection Before a Planned Excavation
A 20-inch natural gas transmission line has a 3,000-foot section where close-interval survey reads -0.72 to -0.78 V CSE. The rectifier serving the section was at 18 A and 24 V; last week its output fell to 12 A after a transformer tap change. Native potential is -0.55 V CSE, and instant-off readings average -0.68 V CSE. Soil resistivity is 2,500 ohm-cm. A parallel 138 kV power line crosses the route 800 feet away. Coating conductance has doubled since the last survey. The line is coated, but recent excavations found several coating holidays. The company uses the -0.85 V CSE criterion and the 100 mV polarization criterion. A planned excavation is scheduled in 10 days at milepost 112, inside the section. The corrosion technician proposes 'turning up the rectifier' and rechecking in six months.
1. Evaluate whether the section meets the company's cathodic protection criteria.
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The -0.85 V CSE criterion is not met: readings are -0.72 to -0.78 V, and even the instant-off average of -0.68 V is less negative than -0.85 V. The 100 mV polarization criterion may be met: native potential is -0.55 V and instant-off is -0.68 V, a difference of 130 mV, which exceeds 100 mV. But that conclusion depends on the instant-off readings being truly IR-free and on the native potential being representative. The close-interval readings include IR drop and cannot be used directly for the 100 mV criterion. The doubled coating conductance and reduced rectifier output make the section marginal. The correct judgment is: probably meets the 100 mV criterion on the average, but not proven; investigate before accepting. Do not treat the -0.85 criterion as met.
2. What should be done before the excavation at milepost 112?
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Do not wait six months. Verify protection at the excavation with IR-free readings, a coupon, or instant-off measurements, and confirm the native potential. Check the rectifier and groundbed, restore output if the cause is correctable, and consider temporary CP or a supplemental anode if protection is marginal. The excavation should have a corrosion technician on site, coating inspection, and a plan to repair holidays. If readings show active corrosion or potentials that fail the criterion, excavate and inspect at additional locations, not just the planned one. Document all readings and rectifier settings. Turning up the rectifier without checking the groundbed could overdrive the system, cause interference, or damage the transformer; it also may not raise current if resistance is high.
3. The rectifier output fell from 18 A to 12 A. What would you check, and why not simply increase the tap?
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Check AC supply voltage, fuse, circuit breaker, transformer tap, rectifier stack, DC output meter, anode-bed cable, groundbed resistance, and recent soil moisture or excavation near the groundbed. A tap change alone can explain the drop, but so can a failed anode, damaged cable, or drying groundbed. Increasing the tap raises voltage, not necessarily current; if resistance has risen, current may stay low while voltage and heating rise. Measure structure-to-soil potentials before and after any adjustment, and watch for stray-current interference from the 138 kV line. Restore current only within rectifier rating and design limits. If the groundbed is the problem, repair or replace it rather than overdrive the rectifier.