Question 1

What is CUI (corrosion under insulation)?

Accepted Answer

CUI is external corrosion of pipe and vessel walls under wet insulation — consistently ranked among the costliest integrity problems in process industry. Mechanism: water enters through damaged jacketing or unsealed penetrations, the insulation holds it against hot steel (the 50–175 °C wall range is the classic worst zone — hot enough to concentrate, not hot enough to stay dry), and corrosion proceeds invisibly until inspection or failure. Defences, in order of effectiveness: keep water out (jacket integrity), use non-absorbent insulation where wetting is inevitable (cellular glass; hydrophobic aerogel), use inspectable systems — removable covers turn a destructive inspection into a two-minute open-and-look — and maintain coating systems under the insulation. Standards context: NACE SP0198 / EN 16961 cover CUI control practice.

Question 2

Does mineral wool recover after getting wet?

Accepted Answer

Partially at best. Water displaces air in the pores and multiplies conductivity (wet wool can conduct 5–20× more than dry); if it dries fully and the binder is intact, much of the performance returns — but in service it rarely dries fully (the jacket that let water in keeps letting it in), chloride residues stay behind on the steel (CUI fuel), and repeated cycles degrade the structure. Practice: treat soaked wool sections as replacement items, find and fix the water path, and on lines that get washed down or live outdoors consider non-absorbent materials or removable covers that can be taken off to dry.

Question 3

What is economic thickness of insulation?

Accepted Answer

The thickness at which the marginal cost of the next millimetre equals the marginal value of the energy it saves — beyond it you're buying insulation that never repays. It moved sharply upward in the 2020s: energy prices plus carbon pricing (EU ETS ~€77/t) raised the value of each saved kWh, so specifications written in the cheap-gas era under-insulate today. Modern practice: compute with current fuel + carbon price (the free calculator does both), check the personnel-protection constraint separately (it often governs above 250 °C), and remember the largest economic win is usually not thickness at all — it's covering the bare components that have NO insulation.

Question 4

R-value vs k-value (λ) — what's the difference?

Accepted Answer

k (λ, W/m·K) is the material property: how readily heat conducts through it, independent of thickness, but dependent on temperature. R (m²·K/W) is the assembly property: thickness ÷ k for a flat layer — what a given construction actually resists. Industrial practice quotes λ at mean temperature because the same product has very different λ at 10 °C and at 300 °C mean; building practice quotes R because constructions are standardized. For pipes neither tells the whole story: cylindrical geometry means each added millimetre adds less resistance and more surface — which is why pipe tables (like the ones on this site) are computed, not converted.

Question 5

How hot can an exposed surface be for worker safety?

Accepted Answer

Common industrial practice sets personnel-protection limits at 45–60 °C for metallic surfaces with incidental contact; ASTM C1055 provides the burn-threshold methodology (5-second contact producing first-degree burn ≈ 60 °C for metals), and many corporate specs adopt 45–50 °C for accessible equipment. Inzonex covers are engineered to ≤45 °C surfaces. Note the emissivity twist: a low-emissivity bright metal jacket runs HOTTER to the touch than a fabric surface at identical heat loss — high-emissivity skins make compliance easier at equal thickness.

Industrial insulation Q&A