The methods below are the ones an inspection team actually reaches for when the question is "is there corrosion under this insulation?" Each one answers a slightly different question. Knowing which question you are asking is what keeps a CUI campaign from being either a stripping exercise or a false reassurance.
Two different questions
Inspection methods for CUI fall into two groups. Screening methods look through or around the insulation and indicate where follow-up may be warranted. Quantification methods measure the remaining wall at the locations and coverage actually examined so qualified personnel can use the results in an assessment. Most programs screen widely and quantify narrowly. The mistake to avoid is treating a screening indication as an exact local measurement—or treating a non-indication as proof that hidden metal is sound.
External visual inspection
The walk-down is the cheapest method and the one that finds the conditions that cause CUI rather than CUI itself: damaged or dented jacketing, open seams, missing sealant, wet or sagging insulation, staining, ice at cold spots, leaking flanges and steam traps above insulated lines. API 570 and API 510 require external inspections on a schedule, and for insulated equipment they include an evaluation of the insulation system.
What it misses: the steel. A dry, intact jacket says nothing about what happened under it during the last ten years. Visual findings are inputs to a ranking, not a verdict.
Insulation removal, visual and ultrasonic thickness
Stripping insulation exposes the selected surface for visual examination, coating assessment and local measurements such as straight-beam ultrasonic testing. What it establishes depends on the area exposed, surface preparation, grid or scan coverage, equipment and approved procedure; a sound spot reading does not rule out localized loss elsewhere. Whole-line stripping is rare, so programs often open inspection windows at selected supports, low points or penetrations and restore the weather seal afterwards. Some plants install removable plugs or ports at fixed points so the same spot can be re-inspected without cutting.
Trade-offs: it is slow, needs scaffolding on elevated lines, exposes hot surfaces, and every window is a new seam if it is not resealed well. The payoff is a real measurement and the chance to assess the coating and the insulation condition directly.
Profile and real-time radiography
Tangential radiography shoots through the insulation at a glancing angle so the pipe wall appears in profile; wall loss shows as a change in the edge. It works through insulation and jacketing, on small-bore piping and at supports, and digital detectors have made real-time radiography practical for screening long runs. Density comparisons can also reveal wet insulation.
Trade-offs: radiation permits and exclusion zones, limited applicability on large diameters, and sensitivity that depends on geometry and technique. It is a good choice for elbows, small-bore lines and supports where PEC is awkward.
Pulsed eddy current (PEC)
PEC estimates average remaining wall of carbon and low-alloy steel through insulation and jacketing without direct access to the pipe surface. A probe sends a magnetic pulse, and the decay of the eddy currents in the steel is converted to a wall-thickness estimate averaged over the probe footprint. Some systems can operate on hot equipment, but the probe, lift-off, jacketing, geometry, temperature rating and approved procedure all govern applicability.
Trade-offs: it averages, so a small deep pit under a wide footprint reads as modest loss; it does not work on non-magnetic stainless steel; and geometry near supports, nozzles and edges needs care. It is the workhorse screening method for insulated carbon-steel piping and vessels, and it is the technique demonstrated in the reading simulation. There is a full note on it in pulsed eddy current for CUI screening.
Guided wave ultrasonic testing
Guided wave, or long-range UT, sends low-frequency ultrasonic waves along a pipe from a collar clamped at one location and listens for reflections from changes in cross-section. It can screen tens of metres from one access point, which makes it attractive for insulated pipe racks, road crossings and sleeved lines.
Trade-offs: a short section of insulation must be removed for the collar; the method is sensitive to changes in cross-sectional area, not to thickness directly, so it flags general and localized loss above a threshold and misses shallow pitting; welds, supports and bends reduce the range and add reflections that need interpretation. Indications are followed up with local UT or PEC.
Finding the water instead of the corrosion
Two methods can help locate moisture conditions rather than measure metal loss. Infrared thermography can show wet insulation as a thermal anomaly under suitable thermal and environmental conditions. Neutron backscatter responds to hydrogen and can indicate moisture in an insulation system. Neither establishes steel condition; site access, geometry, calibration, radiation controls, equipment availability and cost determine whether either method is practical for targeting follow-up.
Side by side
| Method | Answers | Through insulation | Best at | Weak at |
|---|---|---|---|---|
| External visual | Is the insulation system compromised? | No | Jacketing damage, leaks, wet spots | The steel itself |
| Strip + UT | What is exposed and measured at this location? | No | Local direct measurement, coating check | Coverage, cost, access, new seams |
| Profile RT | Is there loss at this edge? | Yes | Small bore, elbows, supports | Large diameters, permits |
| Pulsed eddy current | What is the average wall under this footprint? | Yes | Fast screening of carbon steel | Small pits, stainless, complex geometry |
| Guided wave | Where along this run does the section change? | Mostly | Long inaccessible runs | Shallow pitting, feature-rich piping |
| IR / neutron backscatter | Where is the insulation wet? | Yes | Targeting other methods | Any statement about wall loss |
The order is the decision
Every method above costs crew time, and a campaign can cover a few hundred points. Which points come first is decided before any probe is switched on, and that decision is usually made by time since last inspection, by turnaround scope, or by whoever knows the unit best. All three are reasonable; none of them use the data the plant already holds about temperature, insulation type, coating age, environment and past findings.
A ranked list built from those attributes can make the proposed order explicit and testable. A properly separated retrospective backtest can show how quickly that frozen order reaches recorded findings in the covered historical dataset, while also exposing missed known positives and unknown outcomes. It cannot establish the hidden condition of uninspected locations or prove future performance. That is the idea explored in risk-based inspection for CUI and in our synthetic CUI Priority Workbench.
Move a weight, watch the order and the backtest change, and print the first-look plan for the top of the list. Then send one anonymized export and get the same treatment on your own unit.
Open the CUI Priority WorkbenchReferences
- API RP 583, Corrosion Under Insulation and Fireproofing, section on inspection methods and their applicability.
- API 570, Piping Inspection Code, external and CUI inspection requirements for insulated piping.
- EFC Publication 55, Corrosion Under Insulation (CUI) Guidelines, inspection strategy chapter.
- What is corrosion under insulation? for the mechanism and susceptibility ranges.