Finding CUI Without Stripping the Whole Line

Why CUI Is the Most Expensive Damage Mechanism in US Refining

The published numbers are blunt. A study by ExxonMobil Chemical presented to the European Federation of Corrosion in September 2003 attributed between 40 and 60 percent of piping maintenance costs to corrosion under insulation. The same study found that 81 percent of piping leaks occurred in lines smaller than 4 in nominal pipe size, which is the small-bore, low-priority, rarely-inspected end of the system. DuPont has separately put its own direct CUI repair and replacement cost above 10 million dollars a year, excluding preventive maintenance and lost production.

Those figures explain the pattern American refiners keep seeing. CUI does not usually take out a main header that everyone watches. It takes out a 2 in drain, a dead leg, a vent line under a support, or a section of jacket where somebody cut an access panel and never resealed it. The failure is small, the consequence is not, and the location was almost always known to be susceptible if anyone had ranked it.

For scale on the underlying problem, the 2001 study Corrosion Costs and Preventive Strategies in the United States, produced by a research team enlisted through Congress, put total direct corrosion cost across the US economy at 276 billion dollars a year. CUI is one slice of that, concentrated in exactly the assets US operators are least willing to shut down to inspect.

The Inspection Paradox That Makes CUI Hard

Every other damage mechanism gives the inspector a surface to work from. External corrosion is visible. Internal corrosion is measurable through the wall with ultrasound. CUI is different: the insulation blocks direct visual access and blocks conventional contact ultrasound at the same time, because a UT probe needs to touch bare metal. Removing the jacket and insulation solves both problems and creates three new ones, namely cost, schedule, and the fact that badly reinstated insulation makes the next round of CUI worse than the last.

Which is why stripping everything is not a programme, even where budget exists. Wholesale removal on a live unit is disruptive, generates disposal volumes, requires scaffold on a scale that competes with turnaround work, and, because reinstatement quality varies, often reintroduces the water ingress paths it was meant to eliminate. The reinstatement is a corrosion control activity in its own right, and treating it as a clean-up task is how programmes create their own next cycle of damage.

The workable answer is a hierarchy: rank by susceptibility, screen without removal, and remove only where screening or visual evidence justifies it. That hierarchy is what a competent CUI inspection company is actually selling. The instruments are commodity items available to anyone; the sequencing and the ranking logic are not.

Susceptible Temperature Ranges: Where CUI Inspection Is Required

The temperature bands are the entry point for every CUI programme, because they decide which circuits are in scope at all. API 570, the piping inspection code, directs NDE for CUI at suspect locations on carbon and low-alloy steel operating between 10°F and 350°F (-12°C to 175°C). For austenitic and duplex stainless steels the band is 140°F to 400°F (60°C to 205°C), because the mechanism there is chloride stress corrosion cracking driven by chlorides concentrating as trapped water evaporates, not general wall loss. The edition checked is API 570, 4th edition, 2016, with Addendum 2 of 2018.

API 571, which describes the damage mechanism itself, gives the same carbon steel range of 10°F to 350°F. Within it the risk is not uniform. API 583, the recommended practice covering corrosion under insulation and fireproofing, identifies roughly 170°F to 230°F (77°C to 110°C) as the highest-risk band for carbon and low-alloy steel: hot enough to drive the reaction, not hot enough to keep the surface dry. Above about 350°F the surface dries and stays dry, and the mechanism largely stops.

Two exceptions catch people. Intermittent service: a line that normally runs above the band but sits idle, cools and cycles is in scope, because it spends real time wet and warm. And cold service: lines running below ambient dew point condense water on the outside of the steel continuously, and the insulation traps it. Insulated storage tanks sit in the same trap at shell courses and the roof-to-shell junction, which is why API 653 tank inspection intervals and external inspection intervals for atmospheric storage tanks need reading alongside the piping plan.

The Screening Hierarchy, Cheapest Signal First

The sequence that works runs from broadest and cheapest to narrowest and most expensive. Start with a desktop susceptibility ranking against the API temperature bands, service history, previous CUI findings and known design details that trap water. Then walk the line for external visual inspection of jacket condition, sealant, seams, penetrations, supports, low points, and rust bleeding through the cladding. Then apply non-intrusive NDE, meaning profile radiography, pulsed eddy current or guided wave, at the locations the first two steps flagged. Then strip, only where something has been found.

The discipline is in not skipping upward. Teams that go straight to an expensive technology screen thousands of feet of pipe at a uniform rate and find roughly the proportion of damage they would have found by ranking first, at several times the cost. The screening technologies are not there to survey everything. They are there to convert a large suspect population into a small confirmed one, and each step should shrink the population by an order of magnitude.

API 583 is explicit about the shape of this: non-intrusive methods are the primary screening approach where access and insulation condition allow, with removal reserved for targeted confirmation of anomalies that screening identified. Programme design at this level is a risk-based exercise and belongs alongside the rest of the mechanical integrity plan, which is why it connects to RBI programme design and OSHA PSM mechanical integrity and NDT rather than sitting as a standalone inspection campaign.

Pulsed Eddy Current: What It Resolves and What It Averages Away

Pulsed eddy current is the workhorse of CUI screening because it does not care what sits between the probe and the steel. It measures through insulation, aluminium or stainless weather jacketing, fireproofing and coatings, with no surface preparation and no removal, on live lines at operating temperature. For an inspector working at height on an insulated pipe rack, that combination is why PEC exists. Published guidance puts the practical envelope at a combined wall-plus-insulation thickness of about 6 in (152 mm), with wall measurement in the region of 0.125 in to 2.5 in.

The limitation is structural and every buyer needs to understand it. PEC returns an average remaining wall over the probe footprint, and the footprint is on the order of the insulation thickness. On a line with 3 in of insulation, the instrument is averaging over a patch several inches across. Broad general wall loss shows up clearly. An isolated deep pit, which is precisely the CUI morphology that causes leaks, can be averaged into a reading that looks acceptable.

So a PEC result is a ranking input, not a thickness for the record. The correct use is to flag areas where the average has dropped, then confirm those areas with insulation removal and contact ultrasound before any number enters a remaining-life calculation. A programme that files PEC averages as thickness data has built a false record, and it will look worse in an audit than having no data at all.

Profile Radiography and the Small-Bore Problem

Profile or tangential radiography shoots through the insulated line at a tangent so the pipe wall appears in silhouette on the image. Nothing is removed, and on small-bore pipe, elbows, tees and supports it does something PEC cannot: it shows the wall profile directly, including external metal loss and the remaining ligament at the point imaged. Given that 81 percent of piping leaks in the ExxonMobil study occurred below 4 in nominal size, efficient small-bore inspection is not a niche capability.

Its limitation is morphology-dependent. Tangential radiography sizes the remaining ligament accurately where the corrosion presents cleanly in profile, and can significantly undersize it where the attack is irregular, offset from the imaged tangent, or spread around the circumference. On pipe, that argues for multiple exposures around the line rather than a single shot. It also brings radiation control back into the picture, with exclusion zones, permits and the scheduling constraints PEC was chosen to avoid.

Digital and real-time radiography have taken most of this work off film in US plants, which shortens the shot-to-image cycle enough to make 360-degree coverage of a suspect elbow practical rather than theoretical. The technique choice is a scope decision worth making explicitly at tender rather than leaving to whatever the crew happened to bring in the van.

How a CUI Programme Prioritises Which Circuits Get Inspected

Prioritisation starts with three questions asked circuit by circuit. Is the metal temperature in a susceptible band, including the intermittent and cold-service cases. Is water getting in, judged from jacket condition, design details, drainage, steam tracing, personnel protection wrapping and proximity to cooling towers or wash-down areas. And what happens if it leaks, judged on fluid, pressure, location and consequence. High on all three goes first, and the ranking is a document, not an opinion.

Design details do most of the discriminating work. Supports and shoes where the jacket is interrupted, penetrations for instruments and vents, nozzles, flanges, dead legs, low points, terminations, damaged or missing cladding, and any location where an access panel was cut and resealed badly. Every experienced CUI programme in the United States finds the same list, and the list is more predictive of where damage sits than the pipe specification is.

The output is a location list with a method assigned to each. It should be explicit about which locations are being screened, which are going straight to removal because visual evidence already justifies it, and which are deliberately not being inspected this cycle and why. That last category is the one auditors ask about, and a programme that cannot explain its exclusions has not prioritised, it has just run out of budget.

Targeted Stripping and What the CUI Record Has to Show

Every screening step exists to justify this one. At a confirmed anomaly the insulation comes off, the surface is cleaned, the condition is documented photographically, and the wall is measured with contact ultrasound or a pit gauge. That measurement is the one that belongs in the inspection record and in the remaining-life calculation. The stripping scope should be specified as a location list with an extent rule: how far either side of the indication, and what triggers extending it, agreed before the crew mobilises.

A defensible CUI record shows the ranking logic and its inputs, the screening method and coverage per circuit, the raw screening results including locations that were clear, the removal locations and the reason each was chosen, confirmed thickness data with instrument and calibration, photographs of the as-found condition, the disposition of each finding, and the insulation reinstatement detail. That last item is not administrative. How the jacket goes back on determines whether the next cycle finds new damage in the same place.

Where records arrive from a contractor without screening coverage or the reasoning behind removal choices, they cannot support an interval decision, and an auditor will say so. NDT report validation is how that gap gets found internally rather than during an audit. For asset-wide history, registering findings to geometry through laser scanning during turnaround and digital twins keeps every strip point locatable years later instead of losing it in a spreadsheet. To scope a CUI programme, start at contact.

Request a consultation