CUI Inspection for Pressure Vessels Singapore — Corrosion Under Insulation Screening (PEC & Thermography)

What Is CUI Inspection on Pressure Vessels?

Corrosion Under Insulation, or CUI, is the corrosion of a vessel's steel shell that takes place underneath its thermal insulation, entirely out of sight. CUI inspection is the screening and assessment of that hidden corrosion on insulated pressure vessels, distillation columns and drums — detecting wall loss beneath the insulation and cladding without stripping it off. The whole point is to read the condition of the steel through the lagging, so that the vessel's integrity can be judged without the enormous cost of blanket insulation removal.

On a bare, uninsulated vessel an inspector can simply look at the shell and take ultrasonic thickness readings on the metal. An insulated vessel offers no such access: the carbon steel is wrapped in mineral wool or another insulant and sealed inside aluminium or galvanised-steel cladding. The corrosion that matters is therefore invisible, and the only way to check it directly is to cut the cladding open. CUI screening exists precisely to avoid doing that everywhere, using techniques that see through the insulation to find the areas that genuinely need to be opened up.

Our approach combines three complementary elements: Pulsed Eddy Current (PEC) to measure average wall thickness through the insulation, infrared thermography — delivered by drone on elevated surfaces — to flag wet insulation and moisture, and targeted ultrasonic testing at the specific locations screening highlights. This is a full-capability, screening-led programme, not a single-tool service, and it feeds directly into a plant's mechanical integrity programme.

Why Corrosion Under Insulation Is So Dangerous

CUI is dangerous for one simple reason: it does its damage where nobody can see it. Water enters through damaged cladding, failed sealant, insulation joints, or around penetrations such as nozzles, supports and instrument tappings. Once inside, it saturates the insulation and sits in contact with the warm steel, and corrosion advances steadily while the outside of the vessel looks perfectly normal. By the time evidence appears — a stain, a weep, or a full loss of containment — significant wall thickness may already be gone.

The risk is sharply temperature-dependent. CUI is most aggressive on surfaces operating roughly between 50°C and 175°C, where the metal is warm enough to drive corrosion but not hot enough to keep the trapped moisture boiled off. Vessels that cycle in and out of that band, or that have cold spots and dead-legs, are especially prone. Damaged cladding, poorly sealed penetrations, and areas beneath external attachments are the classic initiation points, because they are where water gets in and lingers.

Across the refining and petrochemical industry, CUI is consistently identified as one of the leading causes of unexpected loss of containment. What makes it a management problem as well as a safety one is cost: stripping every metre of insulation off a vessel just to confirm the steel is sound is hugely expensive and disruptive. Operators therefore need a screening method that tells them where to remove insulation, so that intrusive inspection is targeted rather than blanket.

The Methods We Use for CUI Screening

Effective CUI management on pressure vessels is layered. No single technique does everything, so we combine non-intrusive screening with confirmatory measurement, escalating from the least invasive method to the most.

Pulsed Eddy Current is the workhorse of the screening. A probe held over the cladding sends a magnetic pulse that induces eddy currents in the steel beneath; when the pulse stops, the currents decay at a rate set by how much steel is present, and the instrument converts that decay into an average wall thickness over the probe footprint. Because it works through non-magnetic insulation and standard cladding, PEC reads the wall without ever touching it — ideal for sweeping large vessel shells and heads.

Infrared thermography exploits the fact that wet insulation conducts heat differently from dry insulation. Moisture, water tracking and cladding breaches show up as thermal patterns on the surface, so a thermal survey quickly points to where CUI risk is concentrated. It is a risk-screening layer, not a thickness measurement, and it is most powerful when used to steer the PEC and removal work. Where screening indicates loss, targeted insulation removal and ultrasonic testing confirm the actual remaining wall for the integrity assessment.

Where Drones Add Value on Insulated Vessels

Pressure vessels and columns are often tall, and much of their insulated surface sits well above grade — shells, heads, skirts and the upper sections of columns. Reaching all of it with scaffolding or rope access purely to run a screening survey is slow and costly. This is where drone-delivered thermography changes the economics.

A radiometric thermal camera flown across the vessel captures the whole external surface from the air, mapping wet insulation and cladding breaches on shells, heads and skirts without a single scaffold pole. That thermal map becomes the plan for the intrusive work: PEC is then applied at accessible and roped locations, and insulation is removed only where thermography and PEC agree that moisture and wall loss are present. The scaffolding and access effort needed for screening collapses, because the aerial survey does the wide-area triage first.

This aerial-first workflow fits naturally alongside broader oil and gas drone inspection and refinery drone inspection scopes, where the same flight can also cover structures, flares and elevated pipework. It is the same discipline we apply to CUI screening on pipe racks, extended to the geometry of vessels.

What We Screen For

A CUI screening campaign on a vessel is built around the specific ways moisture and wall loss present themselves. Each is captured, located to a grid reference and reported for follow-up.

How CUI Screening Feeds Your Integrity Programme

CUI screening is not an end in itself — its value is in the decisions it enables. Complete, well-located screening data lets an inspection engineer do four things that directly protect the asset and control cost.

• Find CUI before it leaks — hidden wall loss is detected while there is still time to plan a repair, rather than discovered through a loss of containment.
• Target insulation removal — instead of stripping a whole vessel, removal is confined to the areas screening flags, cutting insulation, cladding and re-lagging cost dramatically.
• Feed API-aligned CUI management — the data supports the vessel's inspection plan and CUI-management programme, giving the inspector defensible evidence of coverage.
• Prioritise turnaround scope — screening done while the plant runs identifies the vessels and locations that truly need intrusive work in the next shutdown, so turnaround time is spent where it counts.

Because the screening can largely be performed with the vessel in service, it also helps de-risk the shutdown itself: much of the discovery work is done ahead of time, and the turnaround team arrives with a targeted, evidence-based scope rather than an open-ended one.

Standards That Frame CUI Inspection

Our CUI screening is planned around the recognised American Petroleum Institute standards that govern insulated static equipment. We cite these as the technical framework the work aligns to; each plant's own inspection strategy and its competent inspection body remain the authority for acceptance and fitness-for-service decisions.

Why Choose SG Drone Inspections

We are a specialist drone and NDT screening provider in Singapore, focused on giving plant integrity teams accurate, well-located CUI data with the least possible disturbance to the insulation and the least possible access cost.

Frequently Asked Questions