Guidelines for Providing Process Conditions for RBI - Part 2: CUI and How it Relates to Risk Based Inspection

In Part 2 of our series on guidelines for providing process conditions for RBI, we move on to discuss Corrosion Under Insulation (CUI) and how it relates to Risk Based Inspection. CUI is essentially a type of corrosion that can occur when there is moisture present on the surface of insulated equipment. In addition to being very difficult to predict, CUI can be even more challenging to locate without removing insulation. This article provides some tips on predicting and locating CUI from a Risk Based Inspection perspective.

Corrosion Under Insulation is a concern at many facilities and one that costs the process industry millions of dollars a year to manage. Risk Based Inspection (RBI) is a tool that is currently being used to help understand the consequence of a failure from CUI and to develop inspection and maintenance plans to find potential CUI failures before they become a problem. Collecting data and information related to potential CUI failures, and using the data and information to determine a risk ranking and associated inspection and maintenance plan will go a long way to helping ensure a safer and more reliable facility.

Data Collection:

During the data collection phase of your RBI implementation, a helpful guideline is to take the following into account when considering damage mechanisms for a particular piece of equipment.

• Most RBI CUI risk calculators use an operating temperature range of 0 to 350°F to calculate an expected external corrosion rate for insulated carbon steel assets. Based on industry experience, the highest probability for CUI occurs between 170 and 230°F. Due to the random nature of this damage mechanism, it makes sense to stay on the conservative side for CUI. If a vessel operates in a range of temperature and sometimes that range is between 0 and 350°F (especially 170°F to 230°F), assign the operating temperature to fit in the CUI temperature range.
• Area Humidity is a key factor depending on the geographical location of the facility. The higher the humidity, the more likelihood of CUI. Many RBI software tools require the user to assign a humidity factor of Low, Medium or High depending on geographic location. High typically makes sense along the Gulf Coast, Low for many parts of Canada.
• Insulation jacketing condition, insulation type and coating factor can all play an additional role in determining probability of failure for CUI.
• Based on industry experience, CUI may also be found in the temperature transition section of towers or other fixed assets where the temperature increases or decreases from within the CUI temperature range to something above or below that temperature.
• Another potential physical factor to take into consideration is whether the insulated carbon steel assets undergo external wetting on a regular basis due to proximity to cooling water towers or condensation dripping down from above.

Potential Inspection Locations

Once the risk priority for CUI has been determined, inspecting for CUI will be enhanced by using the following guidelines:

• Areas to focus CUI inspection activity for fixed assets include:
  • Barrier penetrations larger than 10" diameter and smaller penetrations that aren't properly sealed, termination of insulation, damaged insulation and insulation support rings.
• Piping components may experience accelerated CUI near the following locations:
  • Bottom of vertical runs, low points of horizontal runs as well as Barrier penetrations larger than 10" diameter and smaller penetrations that aren't properly sealed, termination of insulation, damaged insulation
  • A Circuit Complexity factor taking into account the number of insulation penetrations and terminations, number of vertical runs, and total piping circuit length can also aid in prioritizing piping CUI inspection activity

Remember, the point is that you want to get as accurate as you can when gathering/providing process information as it might significantly impact the risk. Keep in mind that all RBI team members need to work closely to ensure a good understanding of when and where to look for CUI and the level of risk that it may present. Using the aforementioned guidelines for CUI can help ensure that it is considered when appropriate.

Stay tuned for the next entry in this eight-part series covering guidelines on assigning process conditions for RBI efforts:

1. Guidelines for Providing Process Conditions for Risk Based Inspection (RBI) Implementation and Revalidation (Introduction)
2. Corrosion Under Insulation (CUI) and How it Relates to Risk Based Inspection (this article)
3. Process Fluids and Consequence Models
4. High Temperature Damage Mechanisms
5. Low Temperature Damage Mechanisms
6. High Temperature Hydrogen Attack
7. Environmental Cracking Damage Mechanisms
8. Concluding Remarks

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