Deferring Inspection on a Corroding Line
A refinery has a carbon-steel process line in wet sour service. Corrosion rates from the last three inspection intervals were 0.15, 0.20, and 0.28 mm/yr, trending upward. The current minimum measured thickness is 6.1 mm against a required minimum of 5.0 mm. The next scheduled turnaround is 18 months away, but the operations group wants to defer the line's inspection to the following turnaround, 30 months out, to reduce downtime. The API ICP inspector is asked to sign off on the deferral. The line has no injection-point monitoring and no corrosion coupons.
1. What should the inspector evaluate before agreeing to any deferral?
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The inspector should evaluate: (1) the remaining life at the current thickness and the credible corrosion rate - using the most recent rate (0.28 mm/yr) is the conservative choice given the upward trend; (2) the consequence of failure, which in wet sour service includes potential cracking and release, not just wall loss; (3) the adequacy of monitoring - there are no coupons and no injection-point monitoring, so the trend is based on limited data; (4) whether the upward trend reflects a process change that could accelerate further.
At the current rate, remaining life to the required minimum is roughly (6.1 - 5.0) / 0.28 ≈ 3.9 years, which on its face exceeds a 30-month deferral. But the trend is upward and monitoring is thin, so the rate could be higher than 0.28. The inspector should not treat a single-point calculation as sufficient.
2. What conditions, if any, would make a deferral defensible?
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A defensible deferral would require: (a) a documented remaining-life calculation using a conservative rate that accounts for the upward trend, with margin; (b) installation of monitoring - coupons or injection-point probes - before the deferral, so the trend can be tracked; (c) a defined re-inspection interval shorter than the full deferral, with a trigger to act if thickness falls below a set threshold; (d) confirmation that the line's other degradation mechanisms (sour-service cracking) are addressed by the appropriate inspection method, not just thickness measurement; (e) approval by the authorized inspector and the owner's engineering authority.
If those conditions cannot be met, the deferral should be refused and the line inspected at the scheduled turnaround. The inspector's sign-off should reflect the conditions, not a blanket approval.