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Application of Eddy Current Testing in Equipment Corrosion Inspection
Release Date:
2022/12/29
In equipment corrosion inspection, the inspection technique is critically important and directly affects the quality of the inspection; the use of eddy current testing helps to improve inspection efficiency.
I. Principle of Pulsed Eddy Current Scanning Technology
Eddy current testing is a nondestructive evaluation technique that leverages the principles of electromagnetic induction. During implementation, the eddy currents generated by electromagnetic induction are utilized to conduct a practical analysis of the magnetic impedance within the equipment, thereby providing insights into its operational performance.
The application of eddy current testing in current equipment inspection is characterized by high speed, safety, and ease of implementation, making it highly significant for technical inspection applications. By combining pulsed eddy current with ultrasonic techniques and employing specialized sensors to perform comprehensive scans of equipment and process pipelines, this integrated corrosion monitoring technology can rapidly and accurately determine the wall-thickness distribution, identify potential defect locations, and assess their severity, thereby providing data-driven support for the maintenance and repair of equipment and process pipelines.
II. Characteristics of Pulsed Eddy Current Testing
The sensor has a small diameter (typically less than 2 cm) to ensure measurement accuracy; in most cases, it cannot perform non-contact measurements through insulation (provided the insulation layer thickness is no more than 4 cm); scanning is relatively fast, with continuous area scanning; and it must be used in conjunction with ultrasonic technology.
The essence of precise pulsed eddy current inspection lies in increasing the volume of inspection data for the same pipe component by a factor of dozens to hundreds, all at a lower cost. In recent years, as societal attention to safety issues has grown, petrochemical refineries have become increasingly conscious of safety in equipment operation and management, and government safety oversight agencies have intensified their regulatory efforts, leading to an ever-more urgent demand within the petrochemical industry for more advanced nondestructive testing technologies.
III. Advantages of Pulsed Eddy Current Testing
Compared with conventional techniques such as ultrasonic and radiographic testing, which are currently widely used, pulsed eddy current scanning technology offers the following advantages:
3.1 No surface treatment required
Traditional ultrasonic testing techniques are limited by the principles of ultrasonic wave propagation, requiring the pipe surface to be meticulously polished to a highly smooth finish prior to inspection and imposing stringent requirements on surface flatness.
However, pulse eddy current technology, based on the principle of electromagnetic induction, only induces currents in metallic conductors; therefore, surface coatings, paint layers, rust layers on pipe fittings, as well as non-metallic linings inside the pipes, have no impact on the inspection process or results. Moreover, this method does not require coupling agents or other auxiliary materials, making it highly convenient to use.
3.2 Broad detection range
Due to the “temperature drift” of ultrasonic wave velocity and the constraints imposed by the couplant’s melting and boiling points, temperature has a significant impact on ultrasonic testing technology.
In contrast, pulse eddy current technology based on the principle of electromagnetic induction is not subject to any temperature limitations and has already been successfully applied across a temperature range of –15°C to 500°C.
3.3 High-efficiency detection with no omissions
Compared with conventional ultrasonic “point inspection,” pulsed eddy current technology employs “area scanning,” resulting in higher inspection efficiency. In addition, because ultrasonic testing covers only a small area at each point, it is difficult to achieve comprehensive coverage of pipe components, making missed defects more likely.
In contrast, pulsed eddy current testing, since it detects surface areas, only requires dividing the pipe under inspection into several scanning surfaces based on the pipe diameter and probe size, thereby achieving “full coverage with zero missed defects.”
3.4 The test results are more intuitive.
Traditional ultrasonic testing yields wall-thickness measurements at individual scan points, which are relatively isolated from one another and exhibit poor continuity. In contrast, pulsed eddy-current testing employs real-time wall-thickness imaging, enabling the continuous variation of wall-thickness distribution along the pipe to be displayed during the inspection sweep, thus providing more continuous and intuitive inspection results.
3.5 Harmless to human health
The pulsed eddy current testing equipment is powered by a built-in 12V lithium battery, operates at low test voltages, and relies on the principle of electromagnetic induction, posing no harm to humans or the surrounding environment, allowing inspection personnel to use it with complete peace of mind.
IV. Limitations of Pulsed Eddy Current Scanning Technology
Compared with various conventional nondestructive testing techniques, pulsed eddy current testing, while offering numerous advantages, also has certain limitations, primarily in the high level of expertise required of inspection personnel.
First, the instrument is relatively complex to operate. As it is remotely controlled via a tablet, operators must be proficient in using the software. When inspecting pipe fittings of different materials, temperatures, and diameters, they must rely on extensive inspection experience to adjust the instrument’s various parameter settings, thereby achieving accurate inspection results.
Meanwhile, pulse eddy current testing is subject to numerous factors—such as the relative permeability of different materials and whether the magnetic field in the pipe is saturated—due to its underlying detection principle. Therefore, inspectors must, based on their on-site experience, switch between different probes and adjust multiple parameters to mitigate corresponding interference.
At the same time, for inspection personnel to accurately distinguish in inspection images which signals indicate corrosion, which indicate defects, and which are caused by environmental interference, years of training and accumulated experience are essential. As for single-point measurements, in theory each pipe component requires on-site calibration, making it impossible to truly perform inspections without removing the insulation.
Insulation systems on the production floor—particularly those that are eccentric or irregular—can pose significant challenges to pulsed eddy current inspection, making it difficult to ensure detection accuracy.
A sensor diameter that is too large can easily lead to false positives and missed detections.
Thermal insulation inspection places extremely high demands on the inspectors’ experience and theoretical knowledge; otherwise, it is difficult to ensure inspection quality.
Reverification of the accuracy of insulated pulsed eddy current testing is challenging, and it is difficult to assess the inspection results. Consequently, identifying areas on the production floor prone to severe corrosion is often problematic, which in turn limits the applicability of insulated pulsed eddy current testing.
Penetrating relatively thick insulation layers often results in very low inspection efficiency. In light of the aforementioned drawbacks, when employing pulsed eddy current testing, it is essential to comprehensively consider multiple factors to ensure the accuracy of the final inspection.
V. Practical Applications of Pulsed Eddy Current Testing
Pulsed eddy current inspection was conducted on the inlet and outlet pipelines of the atmospheric top circulation pumps P125/1 and P125/2 in the Atmospheric Distillation Unit No. 1, the inlet pipeline of the initial overhead pump P102, the outlet pipeline of the vacuum bottom pump P115/2, the inlet and outlet pipelines of the atmospheric bottom pumps P111/1 and P111/2, the shell-side outlet pipelines of the atmospheric top circulation heat exchangers 103/1 and 103/2, the bottom sludge oil withdrawal pipeline of the vacuum furnace 104, and the outlet pipeline of the atmospheric furnace 101.
Pulsed eddy current inspection was conducted on the shell of overhead reflux drum D405, the outlet pipeline, the condensate drain line, the pipeline associated with the pressure-control valve PV-403 for sour gas, and the level gauge nozzle of the catalytic cracking unit in Refinery No. 1.
Based on the inspection results, one area of wall-thickness corrosion-induced thinning was identified, located at the first elbow at the shell-side outlet of the atmospheric-top circulation heat exchanger 103/1 and 2 in the atmospheric–vacuum distillation unit (see the schematic diagram of the inspection location, Position 1 in Figure 1).
Conclusion
The development of pulsed eddy current testing technology has provided substantial assistance to equipment corrosion inspection, offering advantages such as high sensitivity, broad detection range, and intuitive results. However, during operation, it is essential to take into account the specific site and equipment conditions and to assign experienced inspectors to ensure the accuracy of the inspection outcomes.
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