Industry standards that are derived from NACE and API identify the piping systems that operate between 25 ˚F to 250 ˚F as having the greatest risk for developing corrosion under insulation (CUI). CUI can be broken into two categories, the first being corrosion of carbon steel due to contact with aerated water and forming corrosion cells. Carbon steel piping operating at temperatures greater than 250 ˚F is warm enough that the piping surface stays free of moisture. When operating below 25 ˚F any water that is present at the surface is frozen and does not provide a wet environment for a corrosion cell to develop.
The second main category deals with stainless steels and their susceptibility to external stress corrosion cracking and pitting; however this paper will discuss experiences with the corrosion of carbon steel piping. Currently all piping that is identified as operating from 0 ˚F to 300 ˚F and insulated are part of a CUI inspection strategy that involves identifying breaches in insulation and removing insulation in suspect areas. Systems that have the greatest potential for issues with CUI have been the steam utility stations, low pressure steam and condensate piping, and regeneration piping that are insulated for heat conservation and personal protection purposes. Since the external corrosion rates are relatively the same for small bore and large bore piping, the smaller nominal wall thicknesses of line sizes 1-1/2 inch and smaller are more prone to developing through wall failures.
Steam utility stations have shown the highest potential for CUI. These piping systems are constructed of carbon steel, insulated for heat conservation, and for the most part are dead legs that stay at ambient conditions, as these sections of piping are not equipped with a steam trap and are usually 20 feet from the header. The typical orientation of the piping is a vertical leg that drops down to grade level from a main header. The piping is insulated to help prevent heat lose and condensate formation. The typical design of the steam utility station is to put a u-bolt support and valve near grade level, however, the main issue with this orientation is that it leaves a high potential for water ingression due to the number of insulation penetrations. Although properly sealed at installation, which is caulking, over time the caulk has the potential to break the seal and allow moisture ingression and eventual coating failure and corrosion cells to form. Here the areas that pose the greatest potential to develop a leak have been at the u-bolt to pipe interface and on the topside of the valve.
Inspection for CUI starts with a visual inspection of the insulation for defects that could allow moisture to enter the system. Based on the orientation of the piping, areas of insulation are selected for removal to examine the base metal. The above case of the ethylene guard drier moisture analyzer was caught on a CUI inspection. The initial insulation inspection showed that there were several breaches in the insulation and that the insulation appeared to holding water. After removing the insulation around the valve assemble and support, a very small leak was found in the form of bubbles coming from underneath the corrosion scale at the u-bolt support location, Figure 10. The section of u-bolt in direct contact with the pipe was completely corroded away. The insulation was stripped further back to the main header to where the frost rings were present. This was approximately an 8 foot deadleg section of piping. 
The section of piping that was still frozen near the header showed no significant signs of corrosion and the coating was still in good condition. During a maintenance unit outage, an operator was in the process of purging the process gas dryers, when the valve he was attempting to open snapped off at the 3/4 inch nipple between the valve and header. The valve was located on the dryer effluent filter bypass line. The system operates at 60˚F and is insulated. After investigating the process gas dryer system, a majority of the bleed valves were subject to CUI. The valves and nipples were replaced with stainless steel to prevent future instances of CUI at these locations. This is an example of piping that continuously operates at or near ambient conditions and is insulated to minimize ambient temperature swings, Figures 12, 13 and 14. 
Source: http://www.allriskengineering.com/library_files/AIChe_conferences/AIChe_2008/data/papers/P108061.pdf
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