Eddy Current Examination Research Articles

Eddy Current False Indications in Austenitic Steel and Titanium Alloys Heat Exchanger Tubes Activated by Stress

Eddy current non-destructive examination with the internal bobbin type probes is a mandatory test for heat exchanger tube manufacturing due to its high sensitivity to the different type discontinuities. In this paper, we want to highlight the problems related to the false indications in the nonmagnetic austenitic steel and titanium alloys due to magnetic inclusions. There are known origins for magnetic inclusions in stainless steel like chips or filings as a result of ferrous tool application, alloy depletion concerned with oxidation or corrosion, and magnetite deposits. The little-known origin of magnetic inclusions is concerned with stress induced martensitic transformation is shortly analyzed in this report as a possible mechanism for the creation of the magnetic inclusion in austenitic steel as in titanium alloy tubes. The phase transformation and as the result the presence of magnetic inclusions is generated by the activation of the spin magnetic moments of the free electrons under stress influence. These inclusions can radically influence the eddy current signal due to quite large difference in relative magnetic permeability but can be differentiated from real defects by re-inspection of the suspicious tubes at reduced operational frequency. During the eddy current examinations of stainless steel and titanium tubes by an internal probe, the false indications in tubes have been detected. The presence of these isolated indications is located always in the same length of the tested tube. The indications were detected in each tube and the signal amplitude was estimated in the range of 40-50% of the acceptance reference level. In stainless steels, the false indications generated are higher than in titanium alloys.

Sludge Deposit Mapping for Steam Generators

During plant operation, corrosion products from feedwater, drain and condensate system accumulate on the secondary side of steam generator in form of scaled deposits around the tubes and sludge piles on top of the tubesheet. These deposits increase potential for corrosion, affect fluid flow and reduce heat transfer efficiency of the steam generator. Using data obtained during periodical eddy current examination of steam generator tubing, information on sludge deposit location and thickness is extracted from low frequency absolute channels. Sludge deposits can be detected over the whole length of the inspected tube. Using automatic analysis deposit results are provided within 36 hours after the inspection. Results are imported in Sludge Mapping software and presented in 3D visualization of steam generator. Both PWR and VVER steam generator visualization is supported. Sludge mapping software provides visual information on sludge deposit thickness using color code and by calculation position of sludge indication, software draws the indication on the location within the steam generator where indication was found. Sludge mapping software provides information on heavy sludge loading areas of steam generator and can help with tracking of sludge build-up over time that can be used for optimizing steam generator maintenance. This paper presents INETEC’s Sludge Mapping solution, its functionality and features for visualization of sludge deposit location and thickness within the steam generator.

An autonomous robot for shell and tube heat exchanger inspection

AbstractShell and tube heat exchangers (STHEs) are critical to energy conversion efficiency of power plants. Eddy current examination is a way to evaluate working conditions of these tubes. However, the current testing apparatus requires human to manually insert an eddy current testing (ECT) probe into and extract it out of individual tubes, and meanwhile monitor measurement results for diagnosis. It is a time‐consuming and labor‐intensive procedure even for an experienced technician. To tackle this challenge, in this study, we developed a robot enabled ECT system for autonomous inspection of STHEs. The robotic platform employs Mecanum wheeled chassis for high mobility, machine vision to locate tube bundle and tube inlets, a rotational Cartesian mechanism to operate at planes with all possible inclinations, and a task‐specific mechanism for ECT probe delivery. Machine vision locates tube bundle and tube inlets by an April tag detection algorithm and a Circle Hough Transform algorithm, respectively. Assisted by a guiding cone, the ECT probe is continuously fed into the tubes with a fill factor of 0.819. During this process, the eddy current data are automatically collected and real‐time analyzed by convolutional neural networks, showing accuracy of nearly 100% for identifying defective and nondefective tubes and 85% for four types of defective tubes and nondefective tubes.

Model of Diagnosing and Searching for Incompatibilities in Aluminium Castings.

An essential element of any industry is castings, which is determined by the technical conditions for their reception. However, conducting production in the foundry technology is burdened with many difficulties associated with, for example, the inability to control all of the parameters that may affect the casting quality. Therefore, it is essential to undertake improvement actions in this area. Efforts are being made to use non-destructive testing (NDT) as a part of quality control, but these methods are rarely combined in a single diagnostic run. As a part of quality improvement, it is also essential to identify the root cause of the problem. For this reason, it is justified to develop a model of diagnosing and searching for non-conformities, which would combine NDT tests and quality management tools. The model included the visual, ultrasound, and eddy current examination in the diagnostic part, and the Pareto–Lorenz diagram correlated with ABC method, histogram, and 5WHY method (asking five questions why). The study’s originality is manifested in the combination of several NDT methods with quality management methods in one model. Using integrally configured methods in the proposed model, it was possible to: reduce diagnostic uncertainty, characterize the critical group of non-conformities, and identify the root causes of the quality problem. The model is a new and universal method that can be implied in any foundry company in order to ensure the stability of the production processes. The application of the model contributes to an increase in the detection speed and enables the reduction of non-conformities in aluminium castings, thus increasing the quality level of the offered products.

Simulation on the Effect of Frequency and Ferromagnetic Tubes Specifications by Far Field Eddy Current Examination

Tube leakage frequently occurred in on-service fossil power plants. Traditional NDT testing methods, including RT, UT, MT and PT are not applicable for the examination of large amount tubes. Fat eddy current examination is a new technique for testing ferromagnetic tubes defects, such as inner corrosion, thickness reduction and cracks. Far field eddy current simulation and verification for different specification tubes has been carried out. The result shows that the position of far field eddy current moves backwards with the increase of wall thickness and tube ID. To the defects with same size, examination sensitivity is higher in far field eddy current area. When the defect with same size locates in the inner and outer surface of the tube, the change of magnetic intensify differences is approximately the same, which is in accordance with the experimental result and is satisfied with far field eddy current examination theory. It has been concluded that defects in ferromagnetic tubes could be judged and recognized by far field eddy current examination technique, which shows prospective for the on-line examination and safety evaluation for in-service tubes in fossil power plants.

Model Construction for Far Field Eddy Current Examination Simulation on Ferromagnetic Tubes

Ferromagnetic tubes are widely applied in fossil power plants such as waterfall tubes, overheat tubes or reheating tubes. Outer surface corrosion, wearing, depth reduction, cracks are common defects during manufacturing and long term operation. Eddy current detection is a traditional method for the examination of surface and near surface defects. However the efficiency is restricted by lift-off effect and skim effect. Far field eddy current overcomes the above disadvantages with high efficiency and fast examination speed and high sensitivity. In this paper, numerical simulation has been carried out to study the far field eddy current Detection for defects in small diameter ferromagnetic tubes in fossil power plants.

Probabilistic structural integrity assessment for Inconel690 alloy steam generator tube with volume defect

Fit for service(FFS) assessment of Steam Generator Tube(SGT) plays a key role in safety operating of Pressurized Water Reactors. Engineering practices have emerged probabilistic method is a powerful tool in reducing redundant safety margin of FFS assessment strategy. The aim of this work is to develop a probabilistic structural integrity assessment method for typical Inconel690 alloy SGT with volume defect. The study consists of four parts: Firstly, an experiment program including tensile test, eddy current examination and SGT bursting test were introduced. Then, potential bursting pressure models were analyzed based on test dataset. Proper bursting pressure model was suggested. Thirdly, probabilistic bursting pressure prediction model was developed based on Monte Carlo Simulation. At last, parametric study was performed and an application instance on anti-vibration bar fretting wear was presented. The results indicated the proposed SGT structural integrity assessment method is less conservative than current repair criterions. This study represents a further step towards developing of smart FFS assessment of Nuclear Power Plant.

A New Repair Criterion for Inconel690 Alloy Steam Generator Tube with Fretting Wear

Structural integrity assessment of steam generator tubes(SGT) is important for the safety operation of pressurized water reactor. In this work, a repair criterion for Inconel690 SGT with fretting flaw was proposed. Firstly, experiments were conducted to address material strength, eddy current examination and bursting pressure of defected SGT. Then, a numerical method was proposed to predict bursting pressure. At last, all factors were concluded in a simplified statistical method of structural integrity assessment. For anti-vibration bar fretting, Condition Monitoring criterion, Operational Assessment criterion as well as the repair criterion for different inspection interval were proposed. The present criterion can provide reference for the inspection interval considering long-term temporary shutdown or standby.

Probabilistic evaluation of the area of coverage of a probe used for eddy current non-destructive inspections

This study proposes a method to probabilistically evaluate the area of coverage of nondestructive inspections to detect defects on a surface of a structure. For the specific problem, this study considers the effect of the distance between two neighboring scanning lines on the detectability of eddy current testing against near-side cracks. Thirty-eight type 316L stainless steel plates having a fatigue crack were prepared, and eddy current examinations were performed with a sufficiently fine scanning pitch. The full width at half maximum of the spatial distribution of the amplitude of the signals was approximated using a Gaussian function. A probability of detection model considering the distance between two neighboring scanning lines is proposed because in actual inspections a scanning line does not always run directly above a crack. The results demonstrated that the proposed model enables a reasonable probabilistic evaluation of the effect of the distance between two neighboring scanning lines.

Role and importance of NDE in nuclear power plant life extension

Nuclear power plants are ageing but there is a need for their further use. Ageing may lead to the gradual degradation of the safety margins of structures and components. Service life extension of nuclear power plants became a world tendency, the key condition of which is the assurance of components’ structural integrity until the end of the extended lifetime. Non-destructive evaluation is a solid pillar of structural integrity assessment, and in the case of nuclear power plants, the general objectives (social and commercial) of non-destructive evaluation are highly relevant.The paper will show examples on the substantial role and importance of non-destructive evaluation in ensuring nuclear component’s long-term integrity. The examples are selected from the technical justification of service life extension of Paks Nuclear Power Plant, Hungary. They will cover ultrasonic and eddy current examinations of critical areas and sub-components of the reactor pressure vessel, the reactor internals, the steam generator and some pipeline systems. The results contributed to completion of key time-limited ageing analyses which were absolutely necessary for getting the operating license beyond the design life.

A study on the inclusion sizing using immersion ultrasonic C-scan imaging

Inclusion sizing, especially for large inclusions greater than 30μm provides important reference for metallurgical process control and fatigue life assessment of steel. Ultrasonic non-destructive testing (NDT) shows great advantages in detecting infrequently occurred large inclusions than eddy current, magnetic particle, microscopic or macroscopic examination procedures. In this paper, the performance of inclusion sizing by immersion ultrasonic C-scan imaging is studied numerically. A two-dimensional model that consists of spherically focused transducer, water couplant and steel with embedded inclusion is established and solved numerically by the finite element method. The signal intensity distributions of inclusion with different sizes are acquired and the effects of inclusion type, shape, orientation on signal intensity distribution are analysed. The results show that the 6dB-drop threshold has the smallest relative error compared with the 12dB-drop threshold and the full-drop threshold, which is better for determining inclusion size larger than 100μm. Experiment is also performed to validate the simulated results.

Model Assisted Probability of Detection Curves: New Statistical Tools and Progressive Methodology

The probability of detection curve is a standard tool in several industries to evaluate the performance of non destructive testing (NDT) procedures for the detection of harmful defects for the inspected structure. Due to new capabilities of NDT process numerical simulation, model assisted probability of detection (MAPOD) approaches have also been recently developed. In this paper, a generic and progressive MAPOD methodology is proposed. Limits and assumptions of the classical methods are enlightened, while new metamodel-based methods are proposed. They allow to access to relevant information based on sensitivity analysis of MAPOD inputs. Applications are performed on eddy current non destructive examination numerical data.

The criterion for blanking-off heat-transfer tubes in the steam generators at VVER-based nuclear power plants based on the results of eddy-current examination

The problem of defining the criterion for blanking off heat-transfer tubes in the steam generators at nuclear power plants on the basis of signals obtained from the standard multifrequency eddy-current examination is considered. The decision about blanking off one or another tube is presently made with reference to one parameter of the relevant signal at the working frequency, namely, with reference to its phase, which directly depends on the depth of the flaw being detected, i.e., a crack in the tube. The crack depth equal to 60% of the tube wall thickness is regarded to be the critical one, at which a decision about withdrawing such a tube out from operation (blanking off) must be taken. However, since mechanical tensile rupture tests of heat-transfer tubes show the possibility of their further use with such flaws, the secondary parameter of the signal, namely, its amplitude, must be used for determining the blanking-off criterion. The signals produced by the standard flow-type transducers in response to flaws in the form of a longitudinal crack having the depth and length within the limits permitted by the relevant regulations were calculated using 3D finite-element modeling. Based on the obtained results, the values of the eddy-current signal amplitude were determined, which, together with the signal phase value, form a new amplitude-phase criterion for blanking off heat-transfer tubes. For confirming the effectiveness of this technique, the algorithm for revealing the signal indications satisfying the proposed amplitude-phase criterion was tested on real signals obtained from operational eddy-current examination of the state of steam generator heat-transfer tubes carried out within the framework of planned preventive repair.

Risk-Based Inspection of a Crude Oil Import/Export Line: The Corrosion Engineer’s Role

This paper uses the case study of a line used to import and export stabilized crude oil to illustrate the steps, the benefits and the role of a corrosion engineer for a typical semi-quantitative Risk-Based Inspection (RBI) process. The line was originally designed for dry service, but currently receives a significant amount of produced water which accumulates in deadlegs with resultant deterioration. Historically, this line also experienced a number of high profile failures attributed to microbial activity, preferential weld decay and corrosion under insulation. Consequently the HSE issued an improvement notice which included a requirement for an RBI study to mitigate these threats and to provide operational integrity assurance. The scope of the RBI review considered in this paper covers only the corrosion circuit, which is from the tank isolation valve to the storage tanks. On the basis of the identified threats and the associated risks, the following inspection, monitoring, and repair (IMR) actions were recommended by the RBI team: (1) an intermediate, non-invasive, rolling inspection at a 12-monthly frequency. This includes a 100% ultrasonic testing (UT) or saturated low frequency eddy current (SLOFEC) scan of all susceptible deadlegs and a USFD (using time of flight diffraction, ToFD) of all welds on the deadlegs; (2) a thorough, non-invasive inspection at a 36-monthly frequency. This includes a full line walk and general visual inspection; a 100% UT scan or SLOFEC on all deadlegs; inspection of small bore branches for fatigue cracks with magnetic particle inspection (MPI) or eddy current (EC) Inspection and visual examination of all accessible supports; (3) a survey to identify corrosion monitoring locations for probes, coupons and sampling points; and (4) Identification and deletion of all unnecessary deadlegs during the next turn around (TAR) maintenance. These actions have since been implemented and provide the largest single contribution to the significant reduction in the number of failures/anomalies experienced on this line. The primary role of the corrosion engineer in this process is to ensure that all the credible threats for this circuit (generic and from inspection history) are identified, risk assessed, and appropriate inspection/monitoring techniques and frequencies identified, and assigned in order to mitigate the high risk threats.

Surface profiling with high density eddy current non-destructive examination data

The pressure tubes (PT) in CANDU (CANada Deuterium Uranium) reactors undergo creep induced deformation due to operating pressure, temperature and radiation conditions. While global deformation of the tube in the form of elongation and diametral creep is well characterized and monitored by station inspection systems, local PT deformation and the presence of inner surface artifacts due to wear are not as directly monitored, but can still provide additional information of fuel channel condition. A surface profiling technique for monitoring local deformation and identification of surface wear using an eddy current probe mounted in a small (50mm×25mm) planar probe body is presented. The sensitivity of the eddy current probe to small lift-off variations combined with high density C-Scan information is used to extract information on smoothly varying local deformation as well as monitor more significant wear on the inner surface of pressure tubes. Vector separation of components permits independent identification of axial and circumferential surface features. Analysis of this data can be used to characterize local PT deformation due to constrictions at fuel bundle ends and loaded garter spring spacers, as well as identify areas where shallow mechanical wear has occurred. Examples of the features that may be identified are presented.

Evaluation of the electromagnetic characteristics of type 316L stainless steel welds from the viewpoint of eddy current inspections

This study evaluated the electromagnetic characteristics of austenitic stainless steel welds from the viewpoint of eddy current testing. Seven welded plate specimens, which were welded using JIS Z3221:2010 YS316L welding metals, were prepared. Two welding metals and several welding conditions were adopted to discuss the generality. The results of eddy current examination of the specimens using three different types of eddy current probes, that is, an absolute pancake probe, a differential plus-point probe, and a uniform eddy current probe, confirmed that the welds cause a large noise especially when the absolute pancake probe was used. The analysis of the signals through finite element simulations revealed that the magnetic property of the welds is not negligible from the viewpoint of eddy current testing. Complementary experiments were conducted using a vibrating sample magnetometer to validate the results.

Implementation Status of Performance Demonstration Program for Steam Generator Tubing Analysts in Korea

Some essential components in nuclear power plants are periodically inspected using non-destructive examinations, for example ultrasonic, eddy current and radiographic examinations, in order to determine their integrity. These components include nuclear power plant items such as vessels, containments, piping systems, pumps, valves, tubes and core support structure. Steam generator tubes have an important safety role because they constitute one of the primary barriers between the radioactive and non-radioactive sides of the nuclear power plant. There is potential that if a tube bursts while a plant is operating, radioactivity from the primary coolant system could escape directly to the atmosphere. Therefore, in-service inspections are critical in maintaining steam generator tube integrity. In general, the eddy current testing is widely used for the inspection of steam generator tubes due to its high inspection speed and flaw detectability on non-magnetic tubes. However, it is not easy to analyze correctly eddy current signals because they are influenced by many factors. Therefore, the performance of eddy current data analysts for steam generator tubing should be demonstrated comprehensively. In Korea, the performance of steam generator tubing analysts has been demonstrated using the Qualified Data Analyst program. This paper describes the performance demonstration program for steam generator tubing analysts and its implementation results in Korea. The pass rate of domestic analysts for this program was 71.4%.

Manufacturing Stress Corrosion-Cracking Tube Specimens for Eddy Current Technique Evaluation

To detect degradation in steam generator (SG) tubes, periodic inspection using nondestructive examination techniques, such as an eddy current testing, is a common practice. Therefore, it is critical to evaluate and validate the reliability of the eddy current technique for ensuring the structural integrity of the SG tubes. The eddy current technique could be evaluated by comparing the data estimated by the eddy current with the destructive examination data of field cracks, which would be both costly and labor intensive. A viable alternative to pulled tube data is to manufacture crack specimens that closely represent actual field cracks in laboratory environments. A crack manufacturing method that can be conducted at room temperature and atmospheric pressure conditions is proposed. The method was applied to manufacture different types of stress corrosion cracking (SCC) specimens: axial outer-diameter (OD) SCC for straight tubes, circumferential ODSCC and primary water SCC (PWSCC) at hydraulic expansion transition regions, and axial PWSCC at the apex and tangential regions of U-bend tubes. To help the growth of SCC into the tube, corrosive chemicals (sodium tetrathionate) and tensile stress were applied. Eddy current and destructive examination data for SCC specimens were compared with the available field crack data to determine whether those SCC specimens are representative. It was determined that the proposed method could manufacture the representative crack specimens.

원전 증기발생기 전열관 와전류검사용 보빈코일의 권선 수 변화에 대한 전기적 특성 연구

Two kinds of eddy current probes are mainly used to perform the steam generator tube integrity assesment in NPPs. The first one is the bobbin probe using for inspection of volumetric defect like a fretting wear. The second one is the rotating probe using for inspection of non-volumetric defect like a crack. The eddy current probe is one of the essential components which consist of the whole eddy current examination system, and provides a decisive data for the tube integrity in accordance with acceptance criteria described in specific procedures. The design of ECT probe is especially important to improve examination results because the quality of acquired ECT data is depended on the probe design characteristics, such as coil geometry, electrical properties, operation frequency. In this study, it is analyzed that the coil winding number of differential bobbin probe affects the electrical properties of the probe. Eddy current bobbin probes for the steam generator tubes in NPPs are designed and fabricated according to the results. Experiment shows that the change in coil winding number has much effects on the optimum inspection frequency determined by the tube geometry and material. Therefore, the coil winding number in bobbin probe is very important in the probe design. In this study, a basis of the coil winding number for the eddy current bobbin probe design for steam generator tubes in NPPs is established.

An accurately controllable imitative stress corrosion cracking for electromagnetic nondestructive testing and evaluations

Abstract This study proposes a simple and cost-effective approach to fabricate an artificial flaw that is identical to stress corrosion cracking especially from the viewpoint of electromagnetic nondestructive evaluations. The key idea of the approach is to embed a partially-bonded region inside a material by bonding together surfaces that have grooves. The region is regarded as an area of uniform non-zero conductivity from an electromagnetic nondestructive point of view, and thus simulates the characteristics of stress corrosion cracking. Since the grooves are introduced using electro-discharge machining, one can control the profile of the imitative stress corrosion cracking accurately. After numerical simulation to evaluate the spatial resolution of conventional eddy current testing, six specimens made of type 316L austenitic stainless steel were fabricated on the basis of the results of the simulations. Visual and eddy current examinations were carried out to demonstrate that the artificial flaws well simulated the characteristics of actual stress corrosion cracking. Subsequent destructive test confirmed that the bonding did not change the depth profiles of the artificial flaw.