How it works
Insulation is not waterproof, and cladding is not watertight for long. Rain, deluge testing, wash-down water, condensation and steam tracing leaks enter through damaged jacketing, unsealed seams and penetrations. The wet insulation then acts as a sponge: it holds water against warm metal, often with chlorides or other contaminants leached from the insulation or the environment. Temperature matters in two directions — heat accelerates corrosion while the surface is wet, and cyclic or intermittent service repeatedly wets and dries the surface, concentrating salts.
Carbon and low-alloy steels suffer general and localized wall loss. Austenitic stainless steels face a different threat: chlorides concentrated under insulation can cause external stress corrosion cracking, which grows from the outside surface with little visible metal loss.
Where it shows up in plants
CUI concentrates where water enters or collects:
- penetrations for nozzles, supports, instrument connections and vents;
- insulation terminations, damaged or missing jacketing and cladding seams facing upward;
- low points, vessel bottom heads and the lower side of horizontal lines;
- vertical-to-horizontal transitions, insulated dead legs and equipment in intermittent or cycling service;
- areas downwind of cooling towers or exposed to steam discharges.
The AMPP (formerly NACE) standard practice SP0198 treats CUI as a systems problem covering design, materials, protective coatings, insulation, jacketing and inspection. Owners typically combine it with the risk-based inspection logic of API 570 and API 510, since the temperature range and service history of each circuit determine its susceptibility.
How it is found
Visual inspection of the jacketing flags where water can enter, but confirming damage generally requires removing insulation at selected locations or using screening methods such as profile radiography, pulsed eddy current, guided-wave ultrasonics, or techniques that detect wet insulation. Screening narrows the search; insulation removal and direct thickness measurement confirm it, with the results feeding condition monitoring locations and a remaining-life assessment.
What to do about it
The durable defence is a coating on the metal itself, selected for wet service at the operating temperature and for cycling, so that water that does get in finds a barrier. Around it: maintained and sealed jacketing, insulation materials suited to the service, removal of insulation that no longer serves a process or personnel-protection purpose, and a documented inspection plan for the susceptible circuits. Where wall loss is found, the remaining strength is assessed before choosing between continued service, repair and replacement.