Corossion under insulation

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Dec 2015

Case Study“Corrosion Under Insulation”

(CUI)



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Introduction

Corrosion under insulation (CUI) is real threat to the onstream reliability of many of today's

plants. This type of corrosion can cause failures in areas that are not normally of a primary

concern to an inspection program. The failures are often the result of localized corrosion andnot general wasting over a large area. These failures can be catastrophic in nature or at

least have adverse economic effect in terms of downtime and repairs. The American

Petroleum Institute code, API 570, Inspection, Repair, Alteration and Rerating of In-Service

Piping Systems, identifies CUI as a special concern. CUI is difficult to find because of the

insulation cover that masks the corrosion problem until it is too late. It is expensive to

remove the insulation.

• Profile Radiography

• Ultrasonic Thickness Measurement

• Insulation Removal

• Gamma Ray

• Real-Time Radiography

• Digital Radiography Testing

Inspection Methods

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Ultrasonic Thickness Measurement

This is an effective method, but limited to a small area. It is expensive to cut the insulationholes and cover the holes with caps or covers. It is not practical to cut enough holes to get

a reliable result. The inspection holes cut in the insulation may compromise the integrity of

the insulation and add to the corrosion under insulation problem, if they are not recovered

carefully. This technique will not detect corrosion induced stress corrosion cracking

(CISCC) in stainless steels.



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Insulation Removal

The most effective method is to remove the insulation, check the surface condition of the

pipe, and replace the insulation. This approach will detect CISCC in stainless steels; may

require eddy current or liquid dye penetrant inspection. This is also the most expensive

method in terms of cost and time lost. The logistics on insulation removal will probably

involve asbestos and its attendant complications. Process related problems may occur, if

the insulation is removed while the piping is in service.

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Inspection Methods

Gamma Ray (RT)

Industrial radiography is used for a variety of applications. It is performed using two different

sources of radiation, X-Ray and Gamma ray sources. The choice of radiation sources and

their strength depends on a variety of factors including size of the component and the

material thickness. Portable X-Ray cameras used for field weld applications and thin wall

material inspection. Gamma sources vary from very low level fluoroscopic units to perform

real time corrosion under insulation surveys, to Iridium (Ir192) and Selenium (Se 75) sources

used for a variety of weld inspections, to Cobalt (Co 60) inspections for thick componenttesting



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When Does Corrosion Under Insulation Occur?

The problem occurs on carbon steels and 300 series stainless steels. On the carbon steels it manifests as

generalized or localized wall loss. With he stainless pipes it is often pitting and corrosion induced stresscorrosion cracking. Though failure can occur in a broad band of temperatures, corrosion becomes a

significant concern in steel at temperatures between 32 F (0 C) and 300 F (149 C). Corrosion under insulation

is caused by the ingress of water into the insulation, which traps the water like a sponge in contact with the

metal surface. The water can come from rain water, leakage, wash water, etc.

API 570 specifies to the following areas as susceptible to CUI:

• Areas exposed to mist overspray from cooling water towers.

• Areas exposed to steam vents

•Carbon steel piping systems, including those insulated for personnel protection operating between 25 F and

250 F. CUI is particularly aggressive where operating temperatures cause frequent condensation and re-

evaporation of atmospheric moisture.

•Carbon steel piping systems that normally operate in-service above 250 F (120 C) but are in intermittent

service.

•Dead legs and attachments that protrude from insulated piping and operate at a temperature different thanthe active line.

•Piping systems with deteriorated coatings and/or wrappings.

•Locations were insulation plugs have been removed to permit thickness measurements on insulated piping

should receive particular attention, etc.



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CUI Case

Tag: DA-1102 (Debutanizer Tower)

Visual Inspection Results: CUI

Operational Temperature: 147°F (not relevant)

Operational Pressure: 15.11 kg/cm² (not relevant)

Fluid: Not Relevant

Material: 515-70

Environment: saline environment by its

proximity to the sea

Figure 1 Figure 2 Figure 3


Figure 1: General View of the skirt in good conditions.

Figure 2: Manhold Access view in good conditions.

Figure 3: Nozzles not inspected because insulation.Figure 4: Vent of the skirt in good conditions.

Figure 5: The skirt presents corrosión on Ring A, Plate VI

Figure 6: Wear evidence material thickness loss .300,

.350” because high corrosión on Ring C.




Figure 10: deep undermining with lost of material because high corrosion on the 3rd ring, Plate VII, original

thickness 0.740” and remanent thickness of 0.282”,0.272”.

Figure 11: zones with lost of material because high corrosión on 3rd ring, plate VII.

Figure 12: remove support ring of insulation, which is destroyed because high corrosión installed on the 3rd

ring of the skirt.





• Apply additional material to recover the nominal thickness and restablish the mechanical properties of

the plate that conforms the skirt.

• Before and after they recover thicknesses on plates listed, in the previous sections , shall do non-destructive testing (ut , vt , pt , mt ) to determine if there is any other problem areas and thereafter to

corroborate thickness after the application of welding , finally recommends put a ring plate 13 mm (

0.500 " ) covering the entire thinning area with the purpose of give strength to the mechanical

structure of the skirt. Under welding procedure; ASME code section VIII and section IX , should be

checking temperatures during the contribution of material prior preheating . In order to not impair the

strength of the plate and not unleash the weakening of the same with consequences due to weight for

tower supporting the skirt .

Recomendations

Corossion under insulation

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