Description of problem: Corrosion Under Insulation (CUI) is external wall loss of carbon/low alloy steel (general, spotting and/or pitting) or External Chloride Stress Corrosion Cracking (ECSCC) of stainless steel due to moisture in the insulation. The main source of moisture is typically rain that penetrates through inadequate sealing of the weather protection jacket in the presence of a failed coating. Locations susceptible for CUI are those where the insulation is damaged, have penetrations through the cladding/covering, at or near improperly sealed joints, and at places where water can collect due to gravity such as support rings that hold the insulation. A large number of CUI defects are found at unpredictable locations, due to migration of water. The temperature range in which CUI of carbon/low alloy steel typically occurs is 5 to 175°C, while the corrosion rates are generally highest in the 50 to 110°C range. The corrosion rate is usually less than 1 mm/year, but due to temperature, the presence of chlorides and cyclical wetting/drying it can be much higher. External chloride stress corrosion cracking of stainless steel also occurs predominantly between 50 and 175 °C. The climate affects the extent of the CUI problem. Typically, at marine locations CUI is more severe.
Measures to reduce CUI are:
- Robust coating for corrosion protection (e.g., TSA)
- Avoiding water entry by proper construction and maintenance of jacket sealing
Three different types of corrosion problems, which lead to different inspection options, are being considered:
- External wall loss in C-steel
- Stress Corrosion Cracking (SCC) in stainless steel
- Very small pitting in stainless steel due to wet insulation
Inspection options – Carbon Steel: Before going into details of the inspection options, we want to stress the importance of application of a systematic RBI approach to identify the probability of occurrence of one of the corrosion mechanisms and the locations with highest risk. The preferred NDE technique for carbon/low alloy steels is visual inspection. The amount of visual inspection after delagging varies dependent upon the Inspection Strategy.
NDE techniques can sometimes be applied to inspect susceptible areas or to evaluate areas for the presence of moisture (i.e., where there is a higher likelihood of finding degradation).
Techniques available to detect wall loss are:
- Profile radiography
- Pulsed eddy current
- Long-range ultrasonics
- Guided wave ultrasonics
- Real-time radiography
- Visual (wall loss)
Techniques available to detect the presence of moisture are:
- Neutron backscatter
- Visual
Several of the inspection strategies allow partial delagging. It has to be kept in mind that with a rather unpredictable localisation of corrosion spots and a need for the high-risk systems to be subjected to considerable delagging, a 100 % delag is almost unavoidable.
Pitting in stainless steel caused by CUI
The techniques above applied after delagging (visual, etc.) are given in wall loss when access to corroded surface is possible. Note that scraping the corrosion products away can be dangerous when working on pressurized equipment as it may result in leakage. When delagging has taken place during operation and corrosion products have been found, it is often desirable to assess the remaining wall thickness through the corrosion products in order to determine whether or not it is possible and safely to remove them. For this purpose, use of a Pulsed Eddy Current (PEC) or radiographic profiling or similar technique is advisable, which is able to measure actual wall thickness or determine the surface profile of the remaining metal underneath the corrosion products (relative measurement). The option for screening without removal of the insulation is mainly meant to support decisions for further detailed inspections. The likelihood of occurrence of corrosion is related to insulation damage, which can be detected visually.
Inspection options and techniques – Stainless Steels: The preferred NDE technique for detecting ECSCC is eddy current.
For stainless steel, visual inspection has limited value, since cracks cannot always be visually identified and visual inspection of coating is unreliable. Several sites have detected ECSCC cracks by NDE, whilst the coating was visually in good order. Visual inspection should always precede any NDE techniques in order to determine surface condition, coating/wrapping condition, presence of salt crust and unfavourable construction details (e.g. insulation supports). The presence of pitting typically indicates the presence of chlorides and therefore a potential risk of ECSCC.
Eddy current is the preferred technique in lieu of dye penetrant because:
- Inspections can be readily done on-line;
- No removal of paint is required;
- Crack depth can be estimated with a reasonable accuracy;
- Cracking is often too “tight” for effective dye penetrant examination.
Although a less sensitive NDE technique, dye penetrant testing may be an acceptable detection technique provided that the correct surface preparation has taken place (e.g. paint removal, typically by abrasive flapper wheels).
The inspection technique options for external corrosion in carbon steel are:
Radiography (film and real time)
There are various systems of radiography: some film and others real time. The principles are the same, the actual capabilities have to be evaluated in realistic field trials and can vary widely with the competency of the technician performing the inspection. Film can hardly be regarded as a full screening technique, but rather as a follow-up technique at selected spots.
Application: By the development of very sensitive detectors, which can be applied as single detectors or in arrays, real time radiography systems are now on the market that can rather rapidly scan pipes, either diametrically or tangentially. This can be done manually or by mechanical or robotic scanners. By using sensitive detectors, exposure times are significantly reduced and real time scanning can be obtained for pipes with diameters equal to or less than 24″ and 25 mm wall thickness (double wall exposure). Most systems are equipped with data recollection systems.
Operational constraints: There must be sufficient space between the pipes for the source and detectors. The actual required space depends on the particular system used.
Reliability and accuracy: High, provided that sufficient scans are made to obtain full coverage. A warning must be made for the presence of corrosion products: defects may be shaded by the radiation absorption of the scale. In that case a tangential scan may be more reliable than a diametrical scan.
Economics: Single scan speeds in the order of 2 to 3 m/min are obtainable. To obtain full coverage 4 to 8 pipe scans are required, depending on pipe size and bundle widths.
Guided waves
Application: Long stretches of straight pipe can be inspected. Access is required to parts of the pipe that should be delagged. Thick painting must sometimes be removed at the position of the sensor ring. Order of magnitude of coverable range: 10 to 50 m, depending on pipe roughness. Limited range can be indicative for general corrosion. Although the technique is widely applied, it is still developing further to increase its capabilities and can vary widely with the competency of the technician performing the inspection.
Operational constraints: Bends, flanges and branches form significant limitations.
Reliability and accuracy: At present the detectable size is determined by the cross sectional area (circumferential extent times the defect depth). Only defects with an area larger than 9% of the pipe circumferential area (cross sectional wall) can reliably be detected. Distinction between small deep defects and large shallow defects is very difficult and strongly dependent on operator experience. No information is obtained on the axial extent of the defect. Defects near pipe features (like welds, bends, etc.) cannot reliably be detected. Dense coatings (e.g., Bitumen, Polyurethane bonded insulation systems) can attenuate the waves strongly and reduces the inspection range to a couple of meters.
Pulsed Eddy current (PET and PEC)
Application: The main benefit of Pulsed Eddy Current is the ability to inspect without surface preparation and without removing coatings and insulation materials. The weather sheeting must be non-magnetic and no chicken wire should be present. PEC measures the average wall thickness over a “footprint area”. With the best available method the size of this area is roughly 50 mm in diameter for a distance between probe and steel surface of 50 mm. As a result, PEC can be used for assessing general wall loss, but not for detecting isolated small pitting corrosion.
There are two important applications relevant for refineries and chemical plants: detection of corrosion under insulation and detection of corrosion under passive fireproofing. For detection of corrosion under insulation, the PEC probe is placed on the metal jacket which covers the insulation material and the thickness of the steel underneath is measured (no distinction between internal or external corrosion). In this way, there is no need for removing insulation material. The second application, measurement through passive fire proofing (concrete and bricks), is important for supporting legs of LPG storage spheres and column skirts. The PEC probe is placed against the fire proofing to measure the thickness of the steel underneath. In this way, there is no need to remove the fireproofing.
Operational constraints: Galvanised steel claddings and metal grids (chicken wire) form limitations. Smaller isolated pits and localized corrosion cannot be detected or are averaged out and can lead to an inaccurate wall thickness measurement.
Reliability and accuracy: The detectable defects depend on lift-off and footprint area. Defects with a surface area equal to the footprint and more than 30% wall loss should be detected with a probability of detection of near 100%. Below 30% volumetric wall loss, PEC can give a non-conservative measurement of wall thickness. There have been cases of localized corrosion from CUI where pitting was leaking, yet the PEC-measured wall thickness was adequate. It is not advised to employ PEC for CUI detection because of the averaging of localized corrosion in which is the most prominent wall loss mode for CUI.
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Tangshan Aojie Petroleum Machinery Equipment Make Co., Ltd (Short for: OGEM Solids Control)
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