Corrosion Under Insulation on a Coastally Located Hot Line

The prior inspection was a spot survey, not a CUI assessment. Five uniform 12 in. windows on a 12-year-old insulated coastal line with damaged jacketing is very thin coverage, and the fact that all five read the same 0.34 in. suggests they were placed in convenient straight runs. CUI is a localized mechanism driven by where water enters and where it cannot drain, so the only question that matters is whether the windows happened to land in the susceptible areas. Nothing in the record suggests they did.

Metal temperature does not exempt the line. A 340 F process temperature under insulation can still produce metal temperatures in the CUI range, particularly where the tracer is idle, at low flow, and on the shaded underside. Do not screen the line out on process temperature alone.

Order insulation removal (permit-controlled, line in service):

• all three damaged/open jacket locations on top of the line, extending at least 1 ft beyond the damage in every direction;
• the low point where the caulking has failed, since water travels down and pools there;
• where the tracer tubing enters and exits the insulation and at any tracer dip or support;
• around pipe supports, guides and hangers, and at any penetration (vents, drains, instrument connections);
• every insulation termination, and any section where the jacketing is not sloped or sealed.

Record the jacket condition itself as a finding: the insulation system is part of the corrosion control, and it is currently failing.

• Insulation removal + visual + ultrasonic thickness grid (primary method). Most reliable, and the grid matters: over a stripped area take readings on a regular grid (e.g., 3-4 in. spacing) rather than four spots, because CUI produces pits that a coarse grid steps over.
• Profile radiography. Useful where access is poor and can reveal localized loss, but the insulation, tracer and support geometry complicate interpretation and it should not replace direct UT in a stripped area. It is a screening and confirmation tool.
• Guided-wave UT. Screens long runs from a single access point and is good at finding outliers, but it does not size remaining wall reliably and cannot give you a corrosion rate. Follow any anomaly with direct UT.
• Neutron backscatter / real-time radiography for wet insulation. These detect water in the insulation, not metal loss. Genuinely useful for deciding where to strip insulation, and that is the only claim you should make for them.
• Infrared thermography. Limited by the aluminum jacketing and the insulation; can occasionally show wet insulation but is not a CUI detection method here.
• UT through intact jacketing/insulation. Not a valid thickness measurement path on this line. The proposed plan (readings from a ladder at the same five locations) is essentially a repeat of the inspection that missed the problem.

State the method in the plan, and state the coverage: how many square feet stripped, where, and why those locations.

Arithmetic. Using the minimum and the six-year baseline at that region:

• rate = (0.340 - 0.220) / 6 yr = 0.0200 in./yr
• remaining life = (0.220 - 0.200) / 0.0200 = 1.0 year
• half of that = 6 months, which is the maximum interval before the next inspection.

Judgment.

• The scatter is the finding. 0.37 down to 0.22 in a 1 ft square is pitting, not general loss. Using the minimum with the conservative rate is correct, but the interpretation is 'localized attack under failed jacketing', not 'the line is thinning'.
• Cross-check the baseline. The insulation is 12 years old: 0.375 - 0.220 = 0.155 in. over 12 years = 0.013 in./yr. The six-year rate is higher, so the higher rate is the one to plan on. Both numbers belong in the record, since the difference tells you whether the attack is recent.
• Six months of margin is not enough to justify a watch-and-wait. Only 0.020 in. of wall remains above t_min, which is one interval's worth of metal at the measured rate. The realistic actions are repair or replace the pitted span (sleeve, patch or spool), or rerate with engineering calculations if the plant wants it to keep running.
• Fix the cause. Re-insulate properly: sloped, sealed jacketing, correct caulking at the low point, and the tracer re-routed or supported so it does not create a water path. Otherwise the repaired area re-corrodes.
• Carry it forward. Add these locations to the circuit CML list as permanent CUI-suspect points so the next survey opens the right places without re-discovering them.