By Carlos Molina
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CORROSION UNDER INSULATION
Corrosion Under Insulation (CUI) is the corrosion of piping, pressure vessels and structural components resulting from water trapped under insulation or fireproofing.
Probability of failure in Corrosion Under Insulation is drove at least by 10 critical factors, making CUI one of the most common types of damage mechanisms. It attacks more agressivelly pipes of 4″ diameter or less. That is why in some cases, if possible, designers will choose stainless steel over carbon steel when there is probability of CUI in piping. Besides, the older the piping or equipment and insulation system, CUI is more likely to appear.
CUI is a potentially harmful form of corrosion, difficult to predict with certainty and difficult to find without 100% insulation removal. So the best idea is to have a good insulation from the start. As a designer, you should ake sure your tank and piping has the best insulation design, given that maintenance of insulation is also costly. A good summary of strategies to prevent Corrosion Under Insulation can be found here
As a tank inspector, you should know that the defense against CUI is using appropriate coatings under well maintained insulation barriers. It can be present at the lower portions of tank shells, and in protrusions of the isolation barrier, as many other locations. As for the API 653 exam, I don´t remember any specific questions about CUI in my exam back then in 2012, but you should study it, because it is in the BOK.
Now you have downloaded a new pdf with 38 more questions taken from parts described in the BOK for the March 2015 API 653 exam. With this, we reached 240 questions in the API 653 questions series.
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