What is corrosion under insulation (CUI)? Corrosion on the outer surface of insulated pipes, vessels and tanks, caused by water that gets through the insulation and is held against the metal. Here is a whole unit at once: switch the layers to see where the water comes from, where it gets in and gathers, and where to look first.
1 Water sources Water rain · deluge · cooling tower 2 Ways in Ways in where the cladding fails 3 Where it gathers Gathers low points · 6 o’clock 4 Temperature Heat illustrative °C vs two ranges 5 Inspect first Inspect where the conditions coincide Illustrative process unit, isometric, not to scale An illustrative process unit in isometric view, not to scale: a pipe rack with four insulated lines (L1 to L4), a column C-1, a drum D-1 under a deluge system, a storage tank T-1 with a sprinkler ring, a valve in a valve box, a cooling tower and a control room. Layer 1: Where the water comes from. N UNIT 100 · NOT TO SCALE C-1 D-1 T-1 CT CR L1 L4 L2 L3 1 Rainover the whole unit 2 2 Deluge systemover the drum 3 3 Sprinkler systemring on the tank roof 4 4 Cooling tower 5 5 Condensation 1 1 Failed joint sealantat a nozzle 2 2 Damaged claddingmechanical damage 3 3 Cladding not refittedafter maintenance 4 4 Around a valve box 5 5 Badly fitted claddingat a nozzle 1 1 Undersides of horizontal runs 2 2 2 2 Elbows 3 3 3 3 Vessel bases 4 4 4 Pipe supports 5 5 5 Nozzle connections 6 6 Drain legs 7 7 6 o’clockseen on the cut ends 15 °C L4 40 °C L3 60 °C T-1 85 °C D-1 95 °C L2 140 °C C-1 200 °C L1 1 1 Inspection windowonly as good as where it is cut 2 2 Guided wavescreens long runs Equipment on the unit Layer 1 · Water sourcesIsometric 30° · not to scale
Hover or tab to any equipment for what applies to it.
Unit 100 · illustrative Drawing CUI-ISO-01 Scale Not to scale Temps Illustrative
Layer 1 / 5 · Water sources
Where the water comes from Steel corrodes when it meets water with a supply of oxygen, and insulation usually leaves a space against the metal where water can sit with air. On a plant the water can come from rain, cooling towers, sprinkler and deluge systems, or condensation.
1 Rainover the whole unit 2 Deluge systemover the drum 3 Sprinkler systemring on the tank roof 4 Cooling tower 5 Condensation Sources HSE 2011 Cao et al. 2022
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