Holiday Detection Dispute on a Buried Pipeline
A 3 km buried steel pipeline has been coated with a fusion-bonded epoxy (FBE) and is being inspected before backfill. The specification requires holiday detection at a test voltage appropriate to the coating thickness, and a maximum of 1 holiday per 10 m. The inspector uses a high-voltage detector set to 15 kV on a 500 µm FBE coating. Several holidays are found, and the contractor argues the voltage is too high and is creating false holidays. The inspector must resolve the dispute before backfill proceeds.
1. How should the inspector evaluate whether the test voltage is appropriate, and what is the risk of using too high a voltage?
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Appropriate voltage: The test voltage should be selected per the applicable standard and the coating thickness. For FBE, the voltage is typically set according to a thickness-based formula or table (e.g., a common rule of approximately 5 V per µm, with a cap). At 500 µm, a voltage near 2.5 kV may be appropriate, depending on the standard used; 15 kV is far above that and is likely excessive.
Risk of excessive voltage: Too high a voltage can cause dielectric breakdown of sound coating, producing false holidays, and can damage the coating at defects, creating real weak points. It can also mask the true holiday count, making the acceptance criterion meaningless.
Action: Verify the voltage against the standard and the coating thickness, reset the detector, and re-test. Document the voltage setting and the basis for it.
2. The contractor claims the holidays are false. What evidence should the inspector gather to resolve the dispute?
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Evidence to gather:
- The detector's voltage setting, calibration/verification record, and the standard used to select it.
- The measured coating thickness at the holiday locations (to confirm the voltage basis).
- Visual examination of each indicated holiday: a true holiday shows bare steel or a pinhole; a false holiday from excessive voltage often shows a burn or a coating defect caused by the test.
- Re-test at the correct voltage to see whether the indications persist.
- Photographs and location records for each indication.
Resolution: If indications disappear at the correct voltage, they were likely voltage-induced. If they persist and show bare steel, they are true holidays and must be repaired per the specification. The inspector should document the findings and the basis for the final determination.
3. If true holidays exceed the acceptance criterion, what is the correct disposition and reporting?
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Disposition: The coating fails the acceptance criterion (more than 1 holiday per 10 m). The contractor must repair the holidays per the manufacturer's procedure (e.g., FBE repair patch or compatible repair coating), re-test the repaired areas, and demonstrate compliance before backfill.
Reporting: Record the holiday locations, count per 10 m section, test voltage and basis, repair method, and re-test results. Note the nonconformance and the corrective action taken. The report should allow a reviewer to reconstruct the decision: voltage used, thickness at each location, visual findings, and final acceptance.
Key point: The acceptance criterion is only meaningful if the test method and voltage are correct; the report must show both.