What is Corrosion Under Insulation (CUI)?
Corrosion Under Insulation (CUI) is a hidden threat that - obviously -occurs under insulation. This is why detection is a challenge. The industry (e.g. chemical/petrochemical industry) invests a lot of money in inspections to detect any CUI corrosion problems as early as possible. For some larger chemical companies, this inspection cost runs up to several hundred thousand euros per year per plant.
The inspections are supplemented by preventive maintenance involving the dismantling of high-risk pipelines to check the condition of the installation and, if the estimated risks of condensation and corrosion become too high, the provision of completely new insulation. In doing so, a Risk-Based Approach is followed based on:
- The age of the pipe and insulation;
- The type of insulation, type of piping and coating applied:
- shape of the installation (straight pipe, bends, reactor vessel, etc.);
- the effect of any leakage due to corrosion at that location (safety, economic, ecological).
This is mostly based on experience but is conducted without data on the effective corrosion condition of the plant. However, a good estimation of the effective condition and the risks is necessary and must ensure a minimum cost by not replacing the insulation or pipes too soon, but also not too late in order to avoid leakages and unforeseen stoppages. This is because the economic losses associated with shutdowns quickly mount up (>100 000 €/h for a chemical plant).
Visual inspection and alternatives for detection of Corrosion Under Insulation (CUI)
Even today, visual inspection is still the most reliable technique for detecting CUI corrosion. This involves removing the insulation (partially) and visually inspecting the pipe/equipment. In addition to visual inspection, there are other techniques in use that help detect moisture intrusion or corrosion under insulation, such as:
- X-ray radiography (real-time radiography (RTR) / computed radiography (CR) / digital radiography (DR)):
Through interaction of X-rays with the structure, a profile is obtained of the outside diameter of the pipe. If a change in profile is noticeable, it often means that corrosion products are present. However, it is not possible to estimate a wall thickness reduction based on this.
- Neutron Backscatter:
This technique uses a radioactive source and detects damp insulation on pipes and vessels. It is a relatively fast technique.
- Infrared Imaging:
Since there is often a surface temperature difference between areas of dry and damp insulation, an infrared camera can be used - under the right conditions - to detect damp insulation.
In addition, there are sensors that detect moisture intrusion. These allow you to react quickly and give a warning even before coating degradation or material loss from the steel has occurred.
Measurement of material loss
However, it is often more useful to measure whether or not there has been a significant loss of material underneath the insulation. If there is direct access to the pipe (by removing the insulation or through certain measuring holes), the residual wall thickness at that location can be measured using UT (ultrasonic testing). Nevertheless, there are also techniques that do not require the insulation to be removed beforehand. In this area, the main techniques used are Guided Waves (GW) and Pulsed Eddy Current (PEC):
- Guided Waves (GW): Transducers that are placed on the pipe surface (after local removal of the insulation) send ultrasonic surface waves into the pipe. These waves interact with the pipe material and based on the information from the received waves, a possible wall thickness decrease is estimated. Depending on the conditions, up to > 100m of pipe can be tested with 1 measurement. GW is mainly used for measuring long stretches of straight pipe. For curves and more complex geometries, however, signal loss occurs and the technique becomes less interesting.
- Pulsed Eddy Current (PEC): PEC is a semi-quantitative electromagnetic inspection technique for measuring wall thickness loss on ferromagnetic structures such as carbon steels. With this technique, a magnetic field is first applied and then abruptly removed. This causes eddy currents in the steel pipe, which dissipate at a speed related to the thickness of the pipe. The probes are placed on the weather jacket. The measurement is rather local. PEC is used for both pipelines and more complex geometries such as tanks, supports, and other structures. For long stretches of pipe, this technique is less efficient than guided waves.
What is the minimum detectable defect for PEC?
Corrosion spots with a diameter equal to the footprint of the probe will be detected very well. For deeper corrosion spots, the diameter can be smaller but the wall thickness decrease shown by the PEC device will then be under evaluated, as PEC averages the measurement over the footprint zone of the probe.
The larger the diameter of the corrosion spots, the better it will be evaluated. The footprint of the probe to be used depends on the thickness/properties of the insulation (in general, thicker insulation requires a larger probe, which increases the minimum detectable defect). Contact us if you have a specific question about this.
When is it recommended to remove insulation?
As soon as the PEC technique indicates >15% wall thickness reduction, it is recommended to dismantle the insulation and perform a visual inspection.
What to expect of PEC?
- Detection of corrosion at the bottom of different types of insulation/concrete/silicate
- Weather jacket/insulation/coating removal not required
- You get an idea where the wall thickness decrease occurs
- The sensors can be placed on surfaces up to 120°C
Our colleagues at TÜV AUSTRIA Belgium have the latest PEC technologies available, which are performed by level 2 and level 3 eddy current experts. Do not hesitate to contact us if you have further questions on PEC or other NDT technologies.
More questions about Corrosion Under Insulation (CUI)?
Are you interested in keeping CUI corrosion at bay? Or do you want your other corrosion testing needs met? Contact us for more information!
