API 570 EXAM STUDY GUIDE [MODULE 2 : PART 7] CORROSION UNDER INSULATION - PART 1

CORROSION UNDER INSULATION - PART 1

(API 570 EXAM STUDY GUIDE : MODULE 2 : PART 8)

API 571 : 4.3.3 Corrosion Under Insulation (CUI)

CUI is the most severe headache of all industries. Insulation cannot be avoided, but it causes corrosion.

Description Of Damage

- CUI happens to Piping, Pressure vessels and Structural Components
- CUI happens due to Water/moisture trapped under insulation or fireproofing.
Note: Fireproofing is also included in this.

Affected Materials.

- Insulation affects to those materials which are insulated.
- Carbon Steel, Low Alloy Steel, 300 Series SS and Duplex Stainless Steel

Critical Factors

There are four critical factors
a. Design of insulation systems
b. Insulation type
c. Temperature
d. Environment

a. Design of insulation systems

Poor design and /or installation will cause water to become trapped inside the insulation.

b. Insulation type

- Insulation materials that hold moisture (Wick) will cause corrosion.
- Damage is aggravated by contaminants that may be leached out of the insulation, such as chlorides.

c. Temperature

- Cyclic thermal operation or intermittent service can cause corrosion
- Equipment's that operates below dew point will condensate water on the metal surface and it will increase corrosion
- Corrosion rates increase with increasing metal temperature up to the point where the water evaporates quickly. Corrosion will be severe for operating temperatures between 100Deg C and 121 Deg C
- For CS, 300 and Duplex SS operating temperature range that can cause corrosion is in the table given below.

d. Environment

- Plants located in areas with high annual rainfall or warmer, marine locations are more prone to CUI.
- Environments that provide airborne contaminants such as chlorides or SO2 can accelerate corrosion
- Chloride can be from cooling tower drift or marine environments
- SO2 emissions can be from the stack.
Note: CUI affects externally only.

Affected Units or Equipment

- All insulated piping and equipment are susceptible to CUI.
- Damaged insulation and insulation related items such as vapor barrier, weatherproofing or mastic may cause corrosion.
- Leaking steam tracing may cause corrosion to the insulated lines.
- Termination points of insulation like flanges, valves, vertical lines etc.
- Irregular insulation surfaces such as valves and fittings
- Locations where lines enter or exit point in insulation like steam tracing lines in an insulated line or protrusions through insulation like ladder and platform clip, lifting lugs, nozzles and stiffener rings
- The first few feet of a horizontal pipe run adjacent to the bottom of a vertical run

Appearance or Morphology of Damage

Carbon and low alloy steels are subject to localized pitting corrosion and or localized loss of thickness. After the insulation is removed from these lines, CUI damage often appears as loose flaky scale covering the corroded component.

300 series SS and duplex are subject to pitting and localized corrosion.

For 300 series SS specifically in older calcium silicate insulation (Chloride contain will be there), localized pitting and chloride stress corrosion cracking. can occur. The duplex is less susceptible to chloride stress corrosion cracking. 

In some localized cases, usually found under a failed paint/coating system, corrosion can appear to be carbuncle type pitting.

Telltale signs of insulation and paint/coating damage often accompany CUI.

Prevention / Mitigation

Mitigation is best achieved by 

1. using appropriate paints/coatings

2. Maintaining the insulation/sealing/vapor barriers to prevent moisture ingress.

Note: In atmospheric corrosion also prevention is best achieved by proper surface preparation and painting.

Paints/Coatings

- Flame sprayed aluminium coatings on CS, coating corrodes by galvanic action and protects the steel.

- High-quality non-metallic coatings, properly applied to the surfaces to be insulated can provide long term protection

Insulation

- Careful selection of insulation materials is important.

- Thin aluminium foil wrapped on SS piping and equipment has been used on stainless steels as an effective barrier under insulation.

- Closed-cell foam glass materials will hold less than mineral wool

- Use Low chloride insulation for 300 series SS

- If possible avoid insulation. This decision can be taken by reviewing the balance between heat conservation and cost due to corrosion

Inspection and Monitoring

1. Develop an inspection plan

2. If the external coverings are in good condition and there is no reason to suspect damage behind them, it may not be necessary to remove them for inspection of the vessel.

3. Inspect common areas of concern in process unit.

4. If CUI is found the inspector should inspect other susceptible areas on the vessel.

5. Multiple inspection techniques to be utilized to produce a cost effective approach.

Inspection techniques need insulation stripping

• Full or partial insulation stripping : Visual inspection, UT for thickness verification
• Partial insulation stripping: Guided wave UT
• Without insulation stripping: Real-time profile X-ray, Deep penetrating eddy current inspection.

The above methods are used to asses the thickness also.

• For detecting wet insulation, without insulation stripping: Neutron backscatter techniques, IR thermography. Thickness loss cannot be detected by this method.

Missing insulation can be detected by IR thermography.

Related Mechanisms

- Atmospheric Corrosion

- Oxidation

- Chloride SCC

Study of CUI will not be complete, if we do not cover the CUI portion in API 570. So in the next blog, we will discuss CUI which is in API 570 also. This will help us to have a complete understanding of CUI for the API 570 inspector exam.

Below are some links which will help you to have an understanding of CUI. These links are for making a general understanding of CUI and its application in industry. Going through the links you can also understand the importance of CUI in practical.

Presentations on CUI

1. Link 1
2. Link 2 
3. Link 3

Inspection of CUI

1. www.eddyfi.com
2. www.qsa-global.com

This series will continue...

Click Here for Further Reading on this series

This blog is not a replacement for the training provided by authorized partners/experienced institutions nor this blog guarantee success in API exams. This blog is the outcome of the approach followed by the author for the preparation of API exams.

Disclaimer: Photos are taken from different websites. If this violates any copyright rules this will be removed. Please inform me if any of the photos or contents violate copyright rules. (through 13hareesh13@gmail.com). This blog is meant for educational purposes only. If it is used for any other means without the written consent from the author, the author will not be responsible. The author does not guarantee the accuracy of the data used in this blog. All the information, data, photo/image, calculations presented in this blog should be used at the sole risk of the user. The blogger is not liable or responsible in any way for any damage, losses or costs arising from the use of the blog for any purpose whatsoever or whosoever it may be. The blog is to be used at your sole risk and responsibility.