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  • Ali Oskuie

Corrosion Under Insulation and the Role of Monitoring Techniques in Improving Inspection Strategies

Corrosion Under Insulation (CUI) and Monitoring Techniques

Corrosion Under Insulation (CUI) is one the most insidious types of corrosion. Therefore, inspection guidelines consider it as one the major external corrosion mechanisms at plants that requires special care. Inspection guidelines try to answer the critical questions of “when to inspect”, “where to inspect”, and “how to inspect”. Due to the complexity of CUI mechanism, current approaches depend heavily on visual inspection of suspect locations at timings that are usually planned quite far from each other. This approach is not only costly but also cumbersome to the point of impossible. Additionally, the long history of CUI failures reveals the traditional approach’s lack of efficacy. Recent advances in monitoring techniques give insight to the onset of the CUI and provides the opportunity to surgically inspect and repair. In this post, the CUI methodologies proposed by API inspection codes and the improvements obtainable by monitoring techniques are discussed. The improvements to the traditional inspection approaches which epitomises in codes such as API 570 (the code for in-service piping inspections) and the risk-based approach (API 581) are considered.

Identifying CUI

The most basic and obvious way to inspect the CUI is to remove the insulation and the cladding to do the visual inspection. However, the indirect costs of creating the access to the asset by scaffolding, the man-power costs for removal and re-instating of the insulation and all the steps required in the process, brings headache for any inspection teams. However, the API inspection codes try to make it work by tightening the scope through pre-identifying the locations where CUI has a higher chance of occurrence. This is done by concentrating on locations on the assets where the conditions are suitable for the corrosion. This includes but not limited to the insulation rings on the vessels, any attachments, or appurtenances such as vents and drain piping, davit supports, and the bridles, etc. This procedural identifications of what is called Condition Monitoring Locations (CML) is well explained in API inspection code of API 570. The other step which tries to tighten the scope of inspection is to do regular site walk and do general visual inspection to find the parts of the system which show signs of the CUI, such as the red marks of corrosion on the cladding or any breaches in the insulation. Admittedly, CUI is full of surprises and occurs in places that you might not expect, based on the design conditions. Additionally, the visual inspection provides qualitative results only where you need data to plan forward. That is why non-destructive testing techniques are proposed by the codes to complement the visual inspection.

NDT Techniques for CUI

The non-destructive techniques (NDT) can provide invaluable information on the CUI for planning purposes. Some of the NDT techniques available are intrusive and only applicable after the removal of the insulation. There are other NDT techniques that can be used when the system is in-service condition. Among the intrusive inspection techniques, the ultrasonic thickness measurement (in spot or scanning mode) is abundantly used. Due to lower uncertainty in the measurement, API inspection codes require thickness measurements by spot UT or scanning on the corroded locations after the removal of the insulation and the visual inspection. The valuable data obtained by the thickness reading is crucial to determine the timing of the next round of inspection or any repairs on the assets.

It is worth noting that the in-service NDT techniques such as radiography and eddy current can also acquire quantitative data that gives them an edge over the other techniques when the asset surface is inaccessible. However, their uncertainty in detection of corrosion and costs are higher compared to intrusive techniques. Other methods for CUI testing exist which rely on detecting moisture underneath the cladding (infrared thermography). Interpreting the data for the latter should be done with care. As a General rule, API guidelines and codes give more credit to the intrusive inspection accompanied by the thickness measurement. If an asset was inspected intrusively and the data exists with no sign of corrosion, this asset can be left uninspected for a maximum of 10 years or its half-life.

Planning for CUI Inspection

Proper planning for the CUI inspection tries to balance the risk of the asset against the costs of inspection. The API inspection codes prioritise the assets for inspection based on the category of the fluid they contain. Assets with highly flammable and toxic process fluids are categorized as category 1 and should be thoroughly inspected every five years. Assets containing benign fluids, on the other hand, can be left uninspected for ten years. In API 581 for risk-based inspection the timing of the inspections is dependent on the value of the risk rather than the category of the fluid only. Highly flammable and toxic fluids can increase the consequence of failure and shorten the period for the next inspection. However, the probability of failure should be considered in the calculations, too.

Until recently, few monitoring technologies were available to monitor the CUI. That is why only recently the CUI integrity management guidelines started to include it in their portfolio. Like any other type of monitoring, CUI monitoring tracks the changes in the conditions which are either the signs of progressing corrosion (like corrosion coupons) or monitors the conditions (like the moisture) that, if persist, will lead to corrosion. In addition to proper inspection management systems and NDT techniques, CUI monitoring can make improvements in the CUI management planning and the scope of inspections.

Contrary to internal corrosion, no credits are given to online monitoring for the CUI damage factor in API 581. For internal corrosion, the internal thinning damage factor is credited with a reducing factor as much as 20. The reason for this generous reduction is obvious. The more you know about the assets, the less likely they will fail. Therefore, with the introduction of the CUI monitoring to external thinning in API 581, it is expected that the same credit will be given. The information received from the status of the asset under the insulation can make surgical remedial action possible and decrease the risk.

When using inspection codes such as API 510 and API 570, CUI monitoring provides obvious improvements. It simply gives the required justification to delag an asset or to leave it untouched. This is the case when the sites did inspections traditionally on a calendar basis without any bullet proof factual justifications. Regarding the extension of inspection on an asset, the CUI monitoring reduces the number of suspect areas to those from which we get the alarm.

Understandably, API’s RBI model puts much emphasis on the importance of inspections. The model is designed so that it multiplies the theoretical corrosion rate by a factor of four when inspections are missing. This conservatism wanes when adequate inspection data is obtained to predict the corrosion rate. Though justified, this can prompt the sites to inspect when it is too early. Having real-time data from the asset can reduce the uncertainty and with it the requirement for such a high safety factor. This makes future planning more realistic, corresponding to the real conditions of the asset. The site inspection would be satisfied too as they will justifiably have more green signs in front of their asset names on their RBI reports.

The other merit of installation of the CUI monitoring in the realm of API asset integrity methodologies is that it provides a framework for defining the main parameters and their thresholds affecting the susceptibility to the CUI. The parameters could be the number of days when the asset is wet, the corrosive conditions under the insulation, the real temperature underneath the insulation, etc. As in the case of API 584 for Integrity Operating Window, the threshold for these CUI parameters can be defined and traced. When these limits are passed, the procedure for updating the risk value should be started. As a result, instead of having a risk value which changes only with age and inspections, we can revisit our risk calculations and make any necessary corrections.

CorrosionRADAR’s CUI Monitoring Solution

One of the most successful and referenced technologies for CUI monitoring with the capability to integrate with the API integrity management systems is CorrosionRADAR (CR) technology. It is a technology developed and patented in response to the industrial need for remote CUI monitoring and to improve the inspection practices. Its technology is based on guided-wave electromagnetic principle and embedded sensor inside the insulation.

CR sensors are moisture or corrosion (corrosivity) sensors which are connected to a node that transmits the data to a cloud-based data repository. The sensors are designed in such a way to carry an electromagnetic wave unaffected by the field complexities (flanges, bends, pipe support, etc.). It operates using permanently installed flexible long-range sensors mounted along the outer surface of pipes (inside any insulation), eliminating the need for inspection scaffolding. The sensors consist of a moisture sensor and a sacrificial corrosion sensor (corrosivity). These sensors are connected to the channels on a node which send signals and receives them from the sensors. The nodes can accommodate either of the sensors or both of these together. Either of the sensors, if it is set-up alone, can cover a length of 100 meters on the asset. The data is then sent to cloud and is analyzed (Risk assessment or other similar assessments) on the system dashboard.

If you would like to find out more about CorrosionRADAR’s CUI Monitoring system or speak to one of our experts, please email

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