There is a lot of literature and information to prove that corrosion under insulation (CUI) is a very common but very complicated phenomenon. It is very difficult to find and it takes a lot of time and economic cost.
Corrosion under the insulation layer refers to the corrosion of pipes or equipment covered by the insulation layer due to the ingress of moisture and corrosive substances. This type of corrosion is generally difficult to find because it is usually hidden under the insulation layer. The traditional detection method is:
1. Remove the pipe surface covering;
2. Check the pipeline;
3. Restore the original appearance of the surface covering.
This process involves a lot of cost and logistics problems. Even cutting only the cover layer is a very time consuming and labor intensive process, and can complicate the situation if the surface covering material contains asbestos and requires safe removal.
Nondestructive testing technology
It has played a huge role in the detection and prevention of corrosion under insulation. This technology covers a variety of different methods, each with its own advantages and disadvantages.
Non-destructive testing technology for corrosion under insulation
There are many non-destructive testing techniques suitable for corrosion under insulation, but most of these techniques are usually only detected effectively in the event of significant fluid leakage. Of course, the industry needs more technology to detect without removing the insulation cover. Usually, these detection methods are not used alone, but they are combined with each other to take advantage of their respective advantages and make up for their respective deficiencies to achieve the best detection results.
Here are four common nondestructive testing techniques that are most suitable for corrosion under insulation:
Visual inspection is the simplest and most obvious method. It involves removing the cover (partial or all), visually inspecting the surface for signs of corrosion or damage, and then deciding whether or not to replace the insulation. This method is very effective, but the disadvantage is that the cost is high. The engineer can only check the outer area of the pipe and cannot check the inner space of the pipe. The removed insulation may also need to be replaced, which adds additional economic costs. Moreover, the entire inspection process must be done with care and appropriate, otherwise the integrity of the piping structure may be compromised and other additional risks may be compromised.
Radiography is another common method of corrosion detection under insulation. This X-ray technology can also be subdivided into a variety of different technologies, such as real-time radiography (RTR), computer radiography (CR), and digital detector array (DDA) technology. Each method has different strengths and weaknesses, and its applicability depends on the particular application. For example, a computer radiography camera is only suitable for detecting both sides of a member, while a digital detector array device has higher precision and less image distortion, but its performance generally decreases as the diameter of the tube increases due to structural limitations.
Each method is based on the X-ray principle, highlighting some areas of variation or uneven thickness through the insulation layer (which may indicate the accumulation of corrosion), which in turn provides a view of the pipe diameter profile. Advanced software and digital imaging technology enable images to be viewed in real time and enhanced to provide operators with the best view. The advantages of radiography also include portability, fast analysis, traceability, and the ability to perform digital archiving.
Ultrasonic thickness measurement:
Ultrasonic thickness measurement is an efficient, high-precision, multi-purpose remote visual inspection technology that is widely used in many industries. It works on the principle of ultrasonic propagation and measures wall thickness through various linings and coatings. Another advantage of this technology is that it only needs to measure one side of the component under test. However, this technology also has disadvantages: inspection is limited to a small range, and calibration of each different material is required.
Pulsed Eddy Current Testing (PEC)
Pulse eddy current technology mainly uses electromagnetic wave technology to determine the wall thickness of any conductive material. This method does not require the probe to be in contact with the test surface, but still has high accuracy. However, since this technique only provides an average wall thickness, it is usually only used as a screening technique and is not used to detect isolated corrosion areas. In addition, pulsed eddy current technology can only be used for the detection of carbon steel and low alloy steel structures, which limits its application range.
The above is a very extensive overview of various testing methods for corrosion under insulation. For specific applications, more careful judgment and analysis is required to select the most appropriate technology or combination of technologies. For example, the first thing to consider is safety. However, it is the best location and portability of the testing equipment, as well as minimizing the number of equipment and manpower requirements. In the final analysis, quality is the most critical consideration. However, there is no doubt that non-destructive testing technology has played a huge role in detecting and preventing corrosion under insulation, and each method has its own merits, and users can choose it flexibly.