Bobbin Probe Research Articles

A radiographic calibration method for eddy current testing of heat exchanger tubes

The most technically available NDT method for in-service inspection of heat exchanger tubes is eddy current testing. Using a differential ID bobbin probe, tubes can be tested with high speed, but for accurate estimation of tube condition, precisely machined calibration blocks from the same material are necessary. The purpose of this work was to calibrate the eddy current system by radiography without the use of expensive calibration blocks. Wall thickness measurement and deterioration determination due to corrosion/erosion/pitting was evaluated in this study by using radiograph ie film density measurements. It was found that this method can determine the remaining wall thickness as well as pitting corrosion in the tubes using differential and absolute film density measurements. Results show that good precision can be obtained with this method of eddy current system calibration.

Principal Component Analysis of Eddy Current Signals Obtained from Steam Generator Tubes by Bobbin Probe

Principal Component Analysis of Eddy Current Signals Obtained from Steam Generator Tubes by Bobbin Probe

Prediction of Bobbin Probe Eddy Current Signals for Tubular Components with the Conductivity and Permeability Variation

Prediction of Bobbin Probe Eddy Current Signals for Tubular Components with the Conductivity and Permeability Variation

Model-based interpretation of experimental eddy current signals obtained from steam generator tubes by bobbin probe

Model-based interpretation tools for eddy current signals have been developed by the novel combination of neural networks and finite element modelling for quantitative flaw characterization in steam generator tubes. In the present work, interpretation of experimental eddy current signals was carried out in order to verify the developed inversion tools. A database was constructed using synthetic eddy current signals generated by the finite element models. The hybrid neural networks of a probabilistic neural network (PNN) classifier and back propagation neural network (BPNN) size estimators were trained using the synthetic signals. Experimental eddy current signals were then obtained from axisymmetric artificial flaws fabricated in steam generator tubes. Estimation of the flaw parameters was carried out by feeding experimental signals into the neural networks. The excellent performance obtained in the present work demonstrates the high potential of the developed inversion tools as a practical interpretation of experimental eddy current signals.

Dynamic thresholding for automated analysis of bobbin probe eddy current data

An automated algorithm is presented for the analysis and classification of eddy current bobbin probe data obtained from nuclear power plant steam generator tubes (SGT). This algorithm attempts to find a balance between the two seemingly conflicting requirements of SGT eddy current data analysis, namely, detecting and identifying as many of the actual defects as possible, whilst limiting the number of false alarms to a minimum. Initial results presented in this paper look very promising. Steam generator tubes (SGT) in nuclear power plants are routinely inspected using eddy current techniques. Although a variety of probes are employed for inspecting steam generator tubes, the most commonly employed method involves the use of multifrequency differential bobbin type probes. Such probes are very simple and rugged. The structural integrity of the tube is assessed by analyzing the impedance changes of the probe as it travels within the tube. The inspection is usually carried out using four different frequencies to extract additional information concerning the state of the tube. Unfortunately the probe is sensitive to the presence of a number of other structures within the steam generator such as tube support plates, conductive deposits, tube sheets, etc. in addition to flaws present in the tube. The multifrequency measurements allow selective suppression of signal artifacts introduced by such benign structures as well as garner additional information concerning the flaws (1). The inspection of steam generators involves the testing of several hundreds of kilometers of tubing. The data generated by the inspection procedure is very large. Since manual interpretation of the data is extremely cumbersome and time consuming, the industry has considerable interest in automating the analysis procedure. Although a number of automatic analysis systems have been proposed over the years, their performance have fallen short of levels obtained using manual procedures. Typical problems associated with automatic procedures include high false alarm rates and or poor detection/characterization performance. This paper describes anautomatic procedure for analyzing multifrequency eddy current probe data derived from the inspection of nuclear steam generator tubes. The procedure uses a multi-step method to accomplish the task. The first step involves a filtering process to minimize the noise contained in the signal. The second step involves a two-stage process for identifying segments of the signal stream that represent defects. The third stage involves the classification of signals. Initial results obtained using this approach indicate that it is possible to build an automated system capable of meeting the seemingly conflicting the need for high detection and low false alarm rate.

Modeling of eddy current probe response for steam generator tubes

Sample calculations were performed with a three-dimensional (3D) finite-element model to describe the response of an eddy current (EC) probe to defects in steam generator (SG) tubing. Such calculations could be very helpful in understanding and interpreting EC probe response to complex tube/defect geometries associated with the inservice inspection (ISI) of SG tubes. The governing field equations are in terms of coupled magnetic vector and electric scalar potentials in conducting media and of total or reduced scalar potentials in nonconducting regions. To establish the validity of the model, comparisons of the theoretical and experimental responses of an absolute bobbin probe are given for two types of calibration standard defects. Simulation results are also presented on the effect of ligament size in axial cracks on bobbin probe response.

Transmit-receive eddy current probes for circumferential cracks in heat exchanger tubes : Obrutsky, L.S.; Cecco, V.S.; Sullivan, S.P.; Humphrey, D. Materials Evaluation, Vol. 54, No. 1, pp. 93–98 (Jan. 1996)

Conventional eddy current bobbin probes are known to be ineffectual in detecting circumferential cracks in tubing. Multi-pancake and/or rotating pancake probes are required to detect circumferential cracks. It has recently been demonstrated in CANDU steam generator tubes, with deformation, ferromagnetic deposits, and copper deposits, that multi-channel probes with transmit-receive (T/R) coils are superior to those using surface impedance coils. Unlike rotating probes, the design of a new probe denoted as C3 permits fast, single-scan inspection of a full-length tube at inspection speeds comparable to conventional bobbin probes. Since 1992, the probe has been used routinely for steam generator in-service inspection at 4 CANDU plants. Defective tubes have been plugged and the units returned to service, and they continue to operate without leaks. This paper describes the basic features of T/R surface probes. Two-dimensional voltage diagrams showing computer-derived probe response to frequency, lift-off, carbon steel supports, magnetite deposits and copper deposits are presented and compared with corresponding signals from impedance coils. Theoretical and experimental results show that T/R probes are able to detect defects in the presence of variable lift-off (due to tube-wall deformation) with ten times the signal-to-noise ratio as that exhibited by comparable pancake-type impedance probes. In addition, T/R more » probes are less sensitive to magnetite deposits, and possess good phase discrimination to internal defects. « less