Monica Chauviere spent 25 years advising refineries how to mitigate CUI in oil refineries on the U.S. Gulf Coast and around the world. In her current role as a CUI consultant, she is a vocal advocate of best practices and a debunker of methods and products that are counterproductive to protecting equipment and piping that operate in the CUI temperature range.
The following is the first in a series of articles based on our recent conversation. Future topics will include a 40-year CUI case history, the purpose of insulation and its role in CUI, CUI mechanisms and myths, as well as promising new coating and insulation technologies that can save facility owners time and money.
Root Causes of Corrosion Under Insulation (CUI) Misunderstood
Although corrosion under insulation (CUI) is a fairly straightforward problem, there is a lot of confusion and misinformation in the oil and gas industry. These days, people focus mostly on CUI coatings. If something is corroding, you want to put a barrier between the corroding steel and the water that is causing it. Yes, there are a number of immersion-grade coatings that are resistant to hot and boiling water, which are suitable for use in the CUI temperature range. (For a discussion on this topic, see The Detrimental Effects of Wet Insulation in the CUI Range.)
However, such coatings will not permanently prevent CUI because they don’t address its cause. A bandage is not a cure. CUI coatings may postpone corrosion under insulation, but because coatings are applied by humans, they are never perfect. Additionally, if wet conditions exist in the presence of coating imperfections, CUI will occur at a rate that is a function of temperature, duration and frequency of wetness, as well as contaminants in the water.
The root cause of CUI is the prolonged wetness of unprotected steel. This condition is most often the result of the use of water-absorbent insulation. When wet insulation is in contact with unprotected carbon steel at an elevated temperature for an extended duration of days, weeks or months, the resulting CUI can be severe to potentially catastrophic.
Figure 1. Severe CUI under water-absorbent insulation. Photo: Monica Chauviere.
Another way that prolonged wetness can occur (regardless of water-absorbent insulation) is when there are equipment design flaws—places that trap and hold water like a bowl or on a shelf. When water cannot escape or drain away, such as at an insulation support ring that is welded continuously to a vertical vessel or vertical piping, the hot or boiling water (behind or inside the rigid insulation) can remain trapped and in contact with the steel for weeks, depending on the operating temperature.
What Happens Inside Hot, Rigid Insulation Systems
There are people in the industry who speak of water existing "behind the insulation", but this occurs only in cases where water-repellent rigid insulation is installed on equipment of improper design, or where caulking has been improperly applied. Design flaws can prevent drainage, and caulk applied at bottom terminations may also trap water.
When rigid insulation is a water-absorbent type, the water is not behind the insulation; it is held within the insulation, in intimate contact with the steel wherever it rests on the steel (due to gravity).
Almost all refinery equipment is outdoors. Insulated equipment and piping move with thermal expansion and contraction. If rigid insulation is installed, it often suffers mechanical damage. Because all of this outdoor insulation is exposed to humidity, rainstorms, wind and even hurricanes and typhoons—and because there is no such thing as a perfect insulation system—it is impossible to keep water from making its way into a hot insulation system.
Now let's talk about hot insulation. When water gets into rigid, water-absorbent insulation, it will readily be absorbed by, or wicked into the insulation. Whether the insulation becomes saturated and holds water against the steel depends on the duration of the rain event, of course. But regardless of the volume of water absorbed, the thermal conductivity is drastically impacted, compromising the ability of the insulation to do its job.
However, if the insulation is a rigid, non-absorbent type, and if the insulation support rings are properly designed in such way that there are no water traps, any water that enters the insulation will freely flow out. (Related reading: Understanding Insulation Chemistry Proven to Inhibit Corrosion Under Insulation (CUI) – Whitepaper.)
The water flows behind the insulation along the interstitial space (that always exists between rigid insulation and the steel). The water follows the path of least resistance and gravity until it comes to the exit point, which is the first joint between two sections of the non-absorbent rigid insulation. In this case, any water that enters behind the insulation drains out within minutes after a rainstorm ends.
In my opinion, industry standards do not adequately address the role of insulation regarding CUI. Fortunately, our industry has come to understand that addressing CUI involves multiple factors, and we even have an industry standard that calls for a "systems approach."
But this and other industry standards and documents still do not recognize that it is the insulation that is the primary factor that influences the risk of CUI. The documents warn that if the insulation is a water-absorbent type, this can exacerbate the corrosion problem; however, there is no advisement against the use of water-absorbent insulation, or recognition that the original reason the insulation was specified is negated by even very small amounts of water absorption.
- The root causes of CUI are often misunderstood.
- A CUI coating is a bandage and does not remove the root cause.
- Long-term water contact is the cause of CUI.
- There are two ways that water is held against the steel long-term (days or weeks):
- Water-absorbent-type insulation, which has absorbed water and holds it in contact with the steel substrate
- Water holdup due to equipment and installation design flaws, which prevents ready drainage/escape
- Water can readily drain out of a rigid, non-absorbent insulation system if the equipment design and insulation support details are properly designed and the insulation is properly installed.
- Industry standards and documents do not adequately dissuade the use of water-absorbent type insulation.