Inspection Frequency and Acceptance Criteria for a Pipeline Coating
An inspector is assigned to inspect the external coating of a 10-km pipeline. The coating is a fusion-bonded epoxy (FBE) applied at a mill. The specification requires that the coating thickness be 350-450 μm, with no holidays (pinholes) and no defects larger than 5 mm. The inspector has a holiday detector and a DFT gauge. The pipeline is delivered in 12-m lengths. The inspector plans to inspect 10% of the pipe lengths for DFT and 100% for holidays. However, due to time constraints, the inspector decides to inspect only 5% for DFT and 50% for holidays. The inspector finds that the DFT readings on the inspected pipes are mostly within range, but a few pipes have DFT as low as 320 μm. The holiday detector finds several holidays on 20% of the inspected pipes. The inspector is unsure whether to accept the shipment.
1. Evaluate the inspector's sampling plan and the implications of the reduced inspection.
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The inspector's decision to reduce inspection from the specified 10% DFT and 100% holiday inspection to 5% and 50% is a deviation from the specification. This increases the risk of accepting non-conforming pipes. The specification likely requires 100% holiday inspection because holidays can lead to corrosion. Reducing to 50% means many holidays may go undetected. For DFT, 5% sampling may not be statistically representative; a few low readings (320 μm) indicate that some pipes are below the minimum 350 μm. The inspector should follow the specified sampling plan unless approved by the client. If time constraints are real, the inspector should seek approval for a reduced plan with justification, possibly using statistical sampling. However, the current findings already show non-conformances (low DFT and holidays), so the inspector should reject the affected pipes and may need to inspect more to determine the extent.
2. What actions should the inspector take regarding the non-conforming pipes?
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The inspector should:
- Document all non-conformances: Record the DFT readings below 350 μm and the locations of holidays.
- Reject the non-conforming pipes: Pipes with DFT below 350 μm and those with holidays should be marked and rejected. The specification allows no holidays, so any holiday is a defect.
- Increase inspection: Since defects were found, the inspector should increase sampling to the specified levels (10% DFT, 100% holidays) to determine the extent of the problem. If the mill's quality control is poor, 100% DFT inspection may be warranted.
- Notify the manufacturer and request corrective action (e.g., rework or replacement of non-conforming pipes).
- Consider the cause: Low DFT may indicate application issues; holidays may indicate surface contamination or application problems. The inspector should review the mill's quality records.
- Accept only conforming pipes: After rework, re-inspect the pipes before acceptance.
The inspector should not accept the shipment as is; the non-conformances must be addressed.