ID eddy-current (EC) inspection is a form of nondestructive testing (NDT) employed to help detect pitting, cracking and other concerns such as erosion and corrosion. Often employed for nonferrous alloy heat exchanger tubing used in power generation, chemical processing and other applications, eddy-current testing also is used by fabricators and end-users to establish baseline data before putting a unit into service — to determine if any damage occurred during the fabrication and installation process. In addition, when power and processing industry operations have access to reliable baseline data — as well as the reference standards used — EC testing provides a valuable tool for monitoring corrosion and scheduling preemptive maintenance.
As a practical means for tube inspection, Eddy-current testing (ECT) has been available in the United States since the 1950s. According to the Heat Exchanger Institute, ECT is the standard NDT inspection technique for tubing, and it is included in the ASME Boiler & Pressure Vessel Code.
This article summarizes common eddy-current testing techniques. It provides examples, including a mini-case study, and data on C71500 copper nickel tubing — a material used in many heat exchanger applications. Most importantly, the article reviews challenges in eddy-current testing and discusses how to overcome potential limitations in the eddy-current testing process.
Eddy-Current Testing Techniques
Conventional ECT is based on measuring the impedance of a coil. The impedance changes as the electromagnetic field interacts with the material. The tested materials are limited to non-ferromagnetic materials such as austenitic stainless steel, admiralty brass, copper-nickel alloys and titanium.
Initially, a coil is placed in a tube and balanced (impedance matching). The test probe is pulled, and variations in coil impedance are recorded and analyzed. The impedance changes are related to the size, shape and location of imperfections.
Eddy-current testing offers certain advantages. Inside-diameter (ID) eddy-current testing inspection can be performed at speeds up to six feet per second, and tubing does not need to be as clean as it does with ultrasonic and other forms of testing.
Eddy-current inspection is available in two modes: differential and absolute. The differential mode is better suited to detect small defects such as pitting and cracking. The absolute mode detects gradual tube-wall changes. Combining differential and absolute modes in a single coil can detect defects under the support plates.
The reference standard used for both OD and ID eddy-current testing should contain artificial discontinuities. These include varying depths of a flat bottom drilled holes, a through-wall drilled hole, and ID- and OD-circumferential grooves that can be demonstrated to be comparable to the defects normally found during operation.
Challenges with Eddy-Current Testing
Understanding the differences between OD and ID eddy-current inspection of heat exchangers is critical for manufacturers, fabricators and operators of heat exchangers. There are often disputes as to whether the tubing’s inner diameter or outer diameter should be tested. It is suggested that prior to conducting ID eddy-current tests, the end user should first define the purpose of testing. Is it flaw detection, or is it baseline testing? Answering that question will drive the NDT process.
Baseline eddy-current testing was developed to monitor the tube condition throughout the life of the heat exchanger unit. Baseline tests typically are performed immediately after inserting tubes or installation of the unit. ID eddy-current testing at the tube manufacturer’s location may not be beneficial because the other components in the heat exchanger unit could influence EC testing.
There are other challenges in the eddy-current testing process. For example, test results can vary widely based on the company providing the NDT services as well as the interpretation skill level of the inspector. Obtaining reliable eddy-current testing requires hiring experienced inspectors with superior data analysis and evaluation skills. Operator evaluations can vary widely.
ID eddy-current analysis can be a more subjective process than OD eddy-current analysis. For the latter, inspectors often simply compare the measurements against the specifications and look for surface and subsurface flaws.
To further illustrate this point, one company hired two inspection services to conduct ID eddy-current tests study on C71500 seamless copper nickel 0.750” OD x 0.083” AW heat exchanger tubing. Both companies followed the same guidelines: ASME Section V, Article 8, Appendix VIII.
Company A performed testing, and the percent wall loss and amplitude in voltages of indications detected were documented, as well as borescope inspection images of the suspected indication form.
Company B performed a second ID eddy-current test using the same reference, standard tube and the same constant rate of withdrawing. The inspection data was recorded with the percentage of wall loss and amplitude in volts.
The reported imperfections along the length of the tube were cut out for metallographic examination. Following are some examples of actual imperfections compared to the wall-loss percentage reported by companies A and B (table 1).
Limitations of ID Eddy-Current Testing
ID eddy-current inspection can be beneficial if testing and data interpretation are reliable. Understanding the strengths and limitations of this testing technique is critical.
Because testing is conducted from the inside of the tube, it is difficult to verify the nature and size of discontinuities. Depth only can be estimated using phase angles/amplitudes of the signals developed from a reliable reference standard.
If the calibration tube does not contain proper artificial imperfections, the results can be misleading, and interpretation of the data becomes very difficult. The ability to accurately interpret the signal and correctly identify potential failure mechanisms is essential for ID eddy-current testing.
If ID eddy-current testing is performed prior to fabrication, defects developed during the shipping and installation process might be missed or ignored.
Other than customers’ specific requirements, there is no single standard accept/reject criteria for ID eddy-current testing of heat exchanger tubing. This makes wide-ranging practical experience in EC inspection and analysis all the more important.
Other limitations include natural discontinuities, which typically are not identical in shape or size, compared to the reference standard used to calibrate the tester. In addition, verification by a borescope is limited to evaluating the actual type and size of the indication to help make a determination for acceptance.
All of these potential issues make ID eddy-current testing a challenge and allow subjectivity to creep into the decision-making process for new tubing. As discussed below, this test method may be more appropriate for baseline testing with a dependable reference standard to help identify corrosion problems in older equipment.
Before anything else, the first step is determining why you are conducting eddy-current testing. Is it to establish a baseline for corrosion testing sometime in the future, or is it for flaw detection and acceptance or rejection of tubing prior to installation? If the goal is flaw detection to ensure the tubing meets size and quality specifications on the shell side, OD testing is the most appropriate. If the goal is to establish a baseline for future corrosion/erosion testing on the tube side, then ID testing is more appropriate. Depending on the reason for inspection, some end users require more than one test, including hydrostatic testing, ultrasonic and other forms of NDT.
Although eddy-current testing is one of the fastest methods for testing tube, it pays to spend ample time preparing for a new heat exchanger project. Best practices start with developing a good specification and sharing drawings with all parties, including third-party inspectors. It also is helpful to decide what quality of tubing is actually needed before launching a heat exchanger project. Not all projects require flawless tubing, and that is good: Some tubes may contain minor imperfections that do not impact performance. Determine the acceptance and rejection criteria ahead of time and review all of the specification line items with manufacturers and fabricators, so that everyone is clear about expectations. Decide how to handle rejected tubing prior to starting the project. In some cases, a second test might reveal that tubing is acceptable, saving significant time and money.
If the purpose of the testing is to detect tube-side flaws such as corrosion detection inside the tube, the artificial or natural ID imperfections may need to be inside, which can better simulate the desired imperfections than outside flat bottom holes or notches (table 2).
However, in order to improve the reliability of the ID eddy-current testing for detecting service-related flaws such as vibration wear or tube OD corrosion, an OD flat-bottom hole (FBH) or OD wear-scar type reference standard may need to be considered.
Interpretation of the signal is not difficult when compared with a good reference standard. However, the artificial discontinuities need to be in the same location, on the tube side or the shell side, and need to be as close as possible to what they want to monitor for. Some end users keep old heat exchanger tubing — which provides the actual condition of the tubing after service — as a reference. This means that if the application is the same, using the same design of heat exchangers, you already know what the corrosion shape and characteristics look like.
Eddy-current testing can be a useful tool when it is planned for a specific purpose, and the manufacturer, fabricator and owner-operator communicate effectively. Understanding the goals (baseline testing or acceptance/rejection of tubing) of eddy-current testing effort prior to starting a heat exchanger project is critical. Being clear about what the testing will be used for also is critical. Rejecting material because of misinterpretation of ECT results may lead to a failure to identify the true problems and lead to costly schedule delays.