Regarding the operating temperature on equipment, is there no corrosion under insulation (CUI) issue if it’s above 350°F (177°C)? If yes—and this is the most important part—what about an operating condition where the equipment is cyclic or in shutdown mode?
Absolutely. Corrosion under insulation (CUI) can be a serious issue for pipes and process equipment covered with thermal insulation. The corrosion range has kind of been established from 100 to 350°F (38 to 177°C) for mild steel, and 350°F is really kind of the upper limit where liquid water can exist before it vaporizes into its gaseous state.
I definitely would not say that only equipment that operates in between that temperature range is subject to CUI. Every piece of equipment in an industrial facility, in my opinion, is subject to CUI because of the high temperature ranges that are often cyclical, which will drop down into the CUI range. And then, also, during maintenance and outages you will see that equipment brought down out of service, and you can see significant amounts of time where it's in that CUI danger range. (Related reading: Third Party Long-Term Testing: CUI & Thermal Results.)
So, definitely, we want to help people understand what that bandwidth is really. Don't think that if you don’t have something operating in that operating condition, then you won't have to worry. CUI can occur at any operating temperature range because of those cyclical and outage conditions.
Figure 1. Pipes covered with thermal insulation. (Source: Kyryl Gorlov/Dreamstime.com)