All too often, Corrosion Under Insulation (CUI) is an out of sight, out of mind problem lurking just beneath the surface. That is, until you finally spot a leak that disrupts your system, potentially causing a safety incident or near miss in the field.
The struggles of CUI are widespread and if you don’t yet have a history of CUI leaks, it’s only a matter of time until you encounter your first. So, why is CUI such a common problem and is there anything you can do to stop it? Should you be concerned about CUI at your operations?
Our highly experienced team of API-certified technicians in the Braun Intertec Nondestructive Examination (NDE) Group can shed some light on the widespread problem of CUI for system operators.
What is CUI and what causes it?
CUI damage, according to the definition in API 583, is characterized by either general metal wastage or pitting due to the localized breakdown of passivity. More simply, it’s a form of oxygen corrosion which occurs on carbon and low alloy steel when exposed to moisture and oxygen.
This corrosive damage occurs when water is absorbed by insulation or collects beneath it. Water is absorbed due to breaks in the insulation or pipe jacketing and cannot dissipate from the system. Sources of moisture range from rainwater, spillage from process operations or even condensation on a metal surface in a humid environment.
When considering your own system’s vulnerability to CUI, strictly speaking, the API 583 standard lists a temperature range of 32 F to 212 F when looking for moisture in a system. However, this range should be extended to 10 F to 350 F to account for fluctuations in the system’s operating temperature, ineffective insulation maintenance, temperature gradients within the equipment considered and various operating modes.
We find that this prescribed temperature range can be a tricky factor in gauging the risk and severity of CUI damage because of how users tend to report temperatures and whether they’re reporting the metal or process temperatures. But, generally speaking, the more severe damage takes place at higher temperature ranges between 170 F and 230 F.
Other factors to consider when it comes to CUI risk are contaminants in the insulation. Chemicals like chlorides and sulfides are sometimes present and can contribute to a corrosive environment.
Why do CUI inspections and maintenance programs matter and who should be concerned?
Effective CUI programs matter for all systems. However, these programs are especially important for older systems with aging piping or where insulation has not been well maintained. Where corrosion is present, it’s common to see anywhere from 5-15 millimeters of corrosion every year—but in the most extreme situations this rate of corrosion can be much more severe. The issue of unaddressed corrosion within your system isn’t merely a question of reliability. This yearly rate of corrosion represents a major safety risk for your personnel that should not go ignored.
Quite simply, every system operator should be concerned about CUI. Expectedly, systems in high humidity, high precipitation regions are prone to having the worst CUI issues. However, even in a dry climate moisture collecting under insulation is still possible due to nearby cooling tower plumes and steam tracing leaks.
Similarly, all operating conditions, whether it’s in active operation or not should still have a CUI program. In fact, systems that cycle in and out of the CUI temperature range during regeneration cycles or are frequently out of service at ambient conditions, can experience the most destructive corrosion damage even when normal operation of the system is outside the CUI temperature range.
What standards inform CUI programs?
API standards 570, 571, 574 and 583 all cover aspects of CUI from damage mechanisms, areas susceptible to corrosion as well as the design, maintenance and inspection practices for piping. API 570 is most useful when looking for the areas most commonly vulnerable to CUI. API 583 is the primary resource to reference for inspection and prevention techniques.
The National Association of Corrosion Engineers (NACE) also publishes a useful resource on the topic of CUI guidelines.
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