Bobbin Coil Research Articles

Modelling the Influence of the Tube Support Plate on the Eddy Current Testing of the Steam Generator: Sensitivity Study & Validation

Eddy current testing (ECT) of Steam Generator (SG) tubes is part of the maintenance program of Nuclear Power Plants. Tube Support Plates (TSP) are usually only considered as extraneous signals for tube inspection. However, for long-term operation of SGs, there is a safety concern related to the build-up of deposits in the TSPs flow holes. The clogging-up of the TSPs flow holes affect the safe thermal-hydraulic operation of SGs. The deposits come from the feedwater train and are mostly iron oxides (magnetite) with a weak electrical conductivity and magnetite permeability.The ability of ECT probes to detect and characterize TSPs deposits is therefore of great interest. The most common and industrial ECT technique, the bobbin coil, averages the surrounding electromagnetic field over 360°, whereas the geometry of tri-foiled and quadri-foiled TSPs, and thus of clogging, is rather complex and non- axisymmetric, hence motivating the evaluation of the performance of conventional ECT rotating probes used for SGs tubes inspection. Although simulation is a powerful tool to support such a study, it requires dedicated models to be made available to NDT experts, and CEA develops ECT physical models in CIVA for this purpose. In addition to standard fast CIVA modules based on 3D semi-analytical or 2D numerical calculations restricted to canonical or axisymmetric parts, respectively, CEA develops a module dedicated to the simulation of SG tube inspection based on a 3D numerical simulation. This module allows tube deformation such as ovalization, bending or tube expansion. It also allows the addition of external objects such as anti-vibration bars, different geometries of tube support plates and now their clogging by deposits. This model is used to study various influent parameters and to perform benchmarks in the framework of a scientific collaboration between the CEA and the IRSN. Here, the specificity of the TSPs and their clogging justifies qualifying the simulation model before evaluating the influence of material and geometry properties of the deposit as well as the performance of the inspection techniques. We present the main features of the simulation module in CIVA, its experimental validation, and the main trends in the distortion of the simulated signal as a function of variation in the clogging description.

Analytical model of eddy current bobbin coil probe responses at support plates in steam generator and heat exchanger tubes

The steam generators (SGs) of CANadian Deuterium Uranium nuclear reactors require periodic inspections to ensure their safe operation. Eddy current (EC) testing is the primary method by which the SG tubes are inspected. Many conditions in the SG tubes affect the EC response, such as fretting, pitting, cracking, as well as tube expansion, and the presence of tubesheet and support structures. When two or more of these parameters overlap, the EC signals from degradation induced wall losses are frequently difficult to interpret because these wall loss signals are often distorted due to interference with background signal variations. In this paper, a novel analytical model that describes the impedance of a bobbin coil with a plate encircling an arbitrary number of cylindrical conductors is developed. The model is validated against finite element method modelling and experiment. This model can be used to simulate the main probe response, while inspecting SG tubes near the tubesheet and support structures and thereby, provides the potential to separate out smaller flaw responses.

Modelling the Influence of the Tube Support Plate on the Eddy Current Testing of the Steam Generator: Numerical Tools and Experimental Validation

Eddy current testing (ECT) of Steam Generator (SG) tubes is part of the maintenance program of Nuclear Power Plants. Tube Support Plates (TSP) are usually only considered as extraneous signals for tube inspection. However, for long-term operation of SGs, there is a safety concern related to the build-up of deposits in the TSPs flow holes. The clogging-up of the TSPs flow holes affect the safe thermal-hydraulic operation of SGs. The deposits come from the feedwater train and are mostly iron oxides (magnetite) with a weak electrical conductivity and magnetic permeability. The ability of ECT probes to detect and characterize TSPs deposits is therefore of great interest. The most common and industrial ECT technique, the bobbin coil, averages the surrounding electromagnetic field over 360°, whereas the geometry of tri-foiled and quadri-foiled TSPs, and thus of clogging, is rather complex and non-axisymmetric, hence motivating the evaluation of the performance of conventional ECT rotating probes used for SGs tubes inspection. Although simulation is a powerful tool to support such a study, it requires dedicated models to be made available to NDT experts, and CEA develops ECT physical models in CIVA for this purpose. In addition to standard fast CIVA modules based on 3D semi-analytical or 2D numerical calculations restricted to canonical or axisymmetric parts, respectively, CEA develops a module dedicated to the simulation of SG tube inspection based on a 3D numerical simulation. This module allows tube deformation such as ovalization, bending or tube expansion. It also allows the addition of external objects such as anti-vibration bars, different geometries of tube support plates and now their clogging by deposits. This model is used to study various influent parameters and to perform benchmarks in the framework of a scientific collaboration between the CEA and the IRSN, related for example to the inspection of the U-bend tubes or of a friction wear under the anti-vibration bars. Here, the specificity of the TSPs and their clogging justifies further investigations between CEA and IRSN to qualify the simulation model before evaluating the influence of material and geometry properties of the deposit as well as the performance of the inspection techniques. We present the main characteristics of the simulation module in CIVA.

Modelling of wear defects under Anti Vibration Bars in U-bend for the Eddy Current inspection of Steam Generator tube

The purpose of the anti-vibration bars (AVBs) positioned on both sides of the U-bend part of the steam generator tubes is to limit the vibratory phenomena. These tubes are prone to localized rubbing against the AVBs, resulting in friction-induced wear. To address this issue, eddy current (EC) monitoring and analysis tools like simulation are developed [1]. The objective of the joint study conducted by CEA LIST and IRSN, is to simulate the wear signal under the AVB and to assess the influence of the main geometric parameters on this signal. Within this context, new advanced geometric tools, such as the vibratory movement of an object have been developed to enhance the CIVA model developed at CEA LIST, [2][3]. This 3D numerical model has been experimentally validated on simple configurations involving a machined tube with rectangular-bottomed flats and an AVB, inspected by two axial bobbin coils. The obtained results demonstrate that the presence of AVBs significantly influences the signature of defects near them, especially for thin defect.

Development of Electromagnetic Cylinder-Type Probe for Inspection of Heat Exchanger Tubes

This article presents a cylinder-type magnetic camera (CMC) system in nondestructive testing (NDT) of a heat exchanger (HX) tube. The CMC system is developed for the inspection of corrosion on HX tubes of nuclear power plants. The CMC probe works based on the principle of eddy current testing, in which a single-large bobbin coil is used to supply eddy current on the tube, and a matrix array of InSb Hall sensor is used to measure the electromagnetic field around the corrosion. The CMC system can measure both the real and imaginary components of the magnetic field. The CMC system can operate in two modes: single-line scan mode by selecting a single-line Hall sensor or area scan mode by the electrical scan. The limitations and advantages of each operation mode will be presented. The CMC system has the potential for further applications such as evaluating corrosion size, shape, and locations.

Simulation of eddy current testing of steam generator tubes with wear and secondary side magnetite deposits

The steam generator (SG) tubes of pressurised water reactors are prone to degradation and are periodically inspected using eddy current testing (ECT) techniques. SGs provide a safety barrier between the radioactive primary side and the non-radioactive secondary side. Historically, a major degradation issue was stress corrosion cracking of tubes made of alloy 600. In modern SGs, better resistance to stress corrosion cracking is observed with the replacement of alloy 600 by alloy 690. However, this alloy does not prevent tubes from wear at support locations or wear resulting from fretting with loose parts. Long-term operation of SGs also leads to non-volatile species accumulation in the secondary side of the SGs, mainly magnetite. Magnetite deposits may build up as a dense and solid material on top of the tubesheet (hard sludge) and also in the flow slots of tube support plates, therefore modifying the general flow pattern and velocity of the secondary fluid. This has detrimental effects on the safe operation of SGs. The clogging-up ratio of the flow slots therefore needs to be assessed. The development of an ECT simulation model to address tube wear and magnetite deposits in SGs is presented. This model is based on the CIVA NDT platform. A bobbin coil signal is simulated and its performance for tube wear and magnetite deposit evaluation is discussed.

Adjustable current waveform via altering the damping coefficient: A new way to reduce Joule heating in electromagnetic forming coils

The temperature rise of the working coil due to Joule heating is a very important issue in electromagnetic forming processes. A technically simple and cost-effective method was proposed to address this problem, which only needs to introduce an additional resistor in the discharge circuit. The core of this method is to alter the current waveform during the forming process by adjusting the damping coefficient of the discharge circuit. The effectiveness of the method was numerically analyzed in detail and verified by the experimental investigations. It has been demonstrated that the Joule heating in the coil can be significantly reduced when a resistor was applied, while ensuring the forming depth of workpiece unchanged. As an example, more than 50 % of the Joule heating in a bobbin coil can be decreased when a resistor of 400 mΩ is applied. Meanwhile, the versatility of the developed method was further verified through the analysis of an electromagnetic forming system with a spiral coil and a high-frequency discharge. Furthermore, this work found that the line resistance of discharge circuit should not be as small as possible. It has been proved that when the line resistance is already small, further reducing it has little effect on the improvement of forming efficiency, but it will greatly increase the Joule heating in the coil. The developed method of achieving adjustable current waveform and its application in reducing the Joule heating of coil could be of great significance to the development of electromagnetic forming.

Cylinder-type magnetic cameras and their coverage performance evaluation for inspection of heat exchanger tubes

Eddy current array (ECA) probes are recently used to inspect heat exchanger tubes in a nuclear power plant. The ECA probes are usually produced by arraying multiple exciting/sensing coils with a transmitting-receiving scan approach. This paper introduces an alternative solution with cylinder-type magnetic cameras, which use a single exciting bobbin coil and a multiple magnetic sensor matrix. Two cylinder-type magnetic probes with different diameters (13.3 mm and 17.2 mm) and numbers of Hall sensors (11 × 12 and 16 × 23) were fabricated to inspect heat exchanger tubes. We propose a procedure to investigate both the circumferential and axial coverage performances of the cylinder-type magnetic probe. The coverage performance in the circumferential direction shows a better result than that in the axial direction (e.g., 72% with 55.17%) even they have the same spatial resolution. And, the coverage performances of the two probes have averages value over 89%.

Reading depth of the magnetic Barkhausen noise. I. One-phase semi-hard ribbons

The aim of this two-part work is to determine the effective depth from which the magnetic Barkhausen noise is detected. To achieve this, we have tested different materials for which the effect of reading depth is critical. This first part presents results of the measurements of a set of martensitic steel ribbons. The ribbons of varying thicknesses (from 10 μm up to 1 mm) have been measured in different magnetizing configurations (solenoid and yokes) ensuring homogeneous quasi-static magnetization. The Barkhausen noise signal obtained has a standard one-peak profile. The rms intensity of the Barkhausen noise signal rises up to a ribbon thickness of ~200μm when a sample-wrapping search coil is used for detection. However, the practical industrial method of the surface-mounted bobbin coil gives a lower rise of the rms intensity in a smaller thickness range up to ~50μm.

Flaw Classification Algorithm for Heat Exchanger Tubes Using a Bobbin-Type Magnetic Camera

This paper presents an algorithm that estimates the presence, location, shape, and depth of flaws using a bobbin-type magnetic camera consisting of bobbin probes and a bobbin-type integrated giant magnetoresistance (GMR) sensor array (BIGiS). The presence of the flaws is determined by the lobe path of the Lissajous curves obtained from bobbin coil with respect to the applied frequency. The location of the flaw, i.e., whether it is an inner diameter (ID) or outer diameter (OD) flaw, can be determined from the rotational direction of the lobe with respect to the frequency change. The shape of the flaw is then determined from the area of the lobe and the BIGiS image. At this stage, multi-site damage can be determined from the BIGiS image. The effectiveness of the flaw classification algorithm was evaluated using various types of artificial flaws introduced into small-bore tube test specimens made of austenitic stainless steel.

Electromagnetic Testing of Rod Cluster Control Assemblies in Pressured-Water Reactor Power Plants

This paper presents an electromagnetic testing system for rod cluster control assemblies used in pressurized-water reactors. The system uses several encircling-type magnetic cameras equivalent to a number of the control rods; each sensor probe composes of an encircling Hall sensor array (EHaS) and a bobbin coil. The EHaS has 16 Hall sensor elements that measure the electromagnetic field distribution in the radial direction of the control rod induced by the bobbin coil for defects. Experiments are performed on artificial defects on the cladding tube of real control rods to simulate short-circumferential grooves (SCGs), sliding wears (SWs), and circumferential cracks (CCs). The system can inspect the artificial SCGs, SWs, and CCs with depths up to 20%, 30%, and 40% of the cladding tube thickness (0.47 mm), respectively. Furthermore, the shape and depth of the defects could be estimated. The standard deviations of depth estimation are 18%, 5.8%, and 6.0% for CCs, SCGs, and SWs. The SCGs and SWs have a small and similar estimation error, but the CCs have the highest error of estimation, and have a small width of 0.2 mm.

Nondestructive testing and evaluation of high pressure feed water heat exchanger tubes using differential-type bobbin coils and solid-state integrated GMR sensor array

Nondestructive testing and evaluation of high pressure feed water heat exchanger tubes using differential-type bobbin coils and solid-state integrated GMR sensor array

Development of a high sensitive magnetic flux leakage instrument for imaging of localised flaws in small diameter ferromagnetic steel tubes

The authors report the development of a high sensitive, 16 channel magnetic flux leakage (MFL) instrument for imaging of localised flaws in small diameter ferromagnetic steel tubes. The instrument consists of a magnetising unit of two bobbin coils wound on a ferrite core, a sensing unit of flexible giant magneto-resistive (GMR) sensor array and an image visualisation unit. 3D-non-linear finite-element modelling has been carried out to optimise the inter-coil spacing of the bobbin coils and to identify the number of GMR elements and their locations between the bobbin coils. A series of experiments has been carried out to evaluate the flaw detection performance of the MFL instrument. Studies reveal that the instrument can image as small as 1.1 mm diameter hole and 0.46 mm deep shallow outer diameter circumferential notch in 17.2 mm diameter Mod. 9Cr-1Mo steel tubes used as steam generator material in prototype fast breeder reactor, with a signal-to-noise ratio better than 8 dB. Studies also indicate that intensity of MFL signal measured by the instrument increases with increase in hole diameter and notch depth. The instrument is portable and easy to use for field inspection of ferromagnetic steel components.

Bobbin Coil Probe With Sensor Arrays for Imaging and Evaluation of Longitudinal Cracks Inside Aluminum Tubes

Imaging and evaluation of inner-wall cracks are significant for the integrity management of aluminum tubes. In this paper, a novel bobbin coil probe with sensor arrays is presented for the imaging and evaluation of longitudinal cracks inside aluminum tubes. The finite element method model is developed to analyze the characteristic signals of the crack. The bobbin coil probe with inside tunnel magneto resistance (TMR) sensor arrays is developed for imaging of inner-wall longitudinal cracks. The depth and length of longitudinal cracks are evaluated by the characteristic signals. The results show that the detection sensitivity of the longitudinal crack can be improved greatly when the lift-off of the bobbin coil decreases with inside TMR sensors. The inner-wall cracks inside the aluminum tube can be imaged visually using the bobbin coil with inside TMR sensor arrays. The crack depth and length can be evaluated accurately by the detection sensitivity of the Bx in and the distance between the peaks of the Bz in respectively.

SKIN EFFECT IN EDDY CURRENT TESTING WITH BOBBIN COIL AND ENCIRCLING COIL

SKIN EFFECT IN EDDY CURRENT TESTING WITH BOBBIN COIL AND ENCIRCLING COIL

Development of magnetic flux leakage measuring system for detection of defect in small diameter steam generator tube

Abstract Magnetic flux leakage based non destructive technique for detection of defect in small diameter steam generator tube is evaluated. The detect-ability of a flaw depends on the magnetic flux density passing through a specimen to be inspected. The process of magnetizing local region of the small diameter tube is complex, when the magnetizer arrangement is placed inside the tube. Bobbin coil is considered to magnetize the local region of small diameter ferromagnetic tube and a hall sensor is placed at the center of the magnetizer to sense the leakage field. ANSYS Maxwell EM V16 based Finite Element Method (FEM) and Analytical model is used to validate the proposed magnetizer arrangement. Experimental results were found to be in good agreement with both numerical and analytical results. The obtained results suggest that the performance of the proposed bobbin coil is good to inspect the small diameter tube.

Novel Transceiver Rotating Field Nondestructive Inspection Probe

Metallic tube inspection techniques using eddy current probes have evolved over the years from those employing a single bobbin coil to rotating coils and arrays, in an attempt to improve the speed and reliability of inspection. This paper presents a novel eddy current transceiver probe design that uses a rotating electromagnetic field. The transceiver coils consist of three identical windings located 120° apart on the same physical axis. A three-phase sinusoidal current source is used for exciting the coils. The phase voltages are identical in amplitude, but 120° apart in phase. The rotating magnetic field generated by the three-phase current is sinusoidal in space and time and so are the induced eddy currents in the tube wall. The sensor achieves mechanical rotating probe functionality by electronic means and eliminates the need for mechanical rotation. The terminal voltages of the three-phase windings can be measured during the scan. The defect’s axial and angular position can be estimated by analyzing the amplitude and phase of the sum of the three terminal voltage signals. The probe is sensitive to defects of all orientations and is as effective as conventional rotating pancake coil probes while offering the advantages of high inspection speed and greater reliability, since the probe does not rotate mechanically. A 3-D finite-element model based on reduced magnetic vector potential Ar, $ {V}$ -Ar formulation was developed to simulate and predict the response of the probe to a variety of defects. A prototype unit consisting of a probe connected to a three-phase constant current source and data acquisition system was developed and tested. Experimental results validating the simulation model and demonstrating the feasibility concept are presented.

Development of magnetic flux leakage technique for examination of steam generator tubes of prototype fast breeder reactor

For non-destructive examination of small diameter (outer diameter, OD 17.2mm) and thick walled (wall thickness, 2.3mm) ferromagnetic Modified 9Cr–1Mo steel steam generator (SG) tubes of Prototype Fast Breeder Reactor (PFBR), this paper proposes magnetic flux leakage (MFL) technique. Three dimensional finite element (3D-FE) modeling has been performed to optimize the magnetizing unit and inter-coil spacing of bobbin coils used for axial magnetization of the tube. The performance of the technique has been evaluated experimentally by measuring the axial (Ba) component of the leakage fields from localized machined defects in SG tubes. The MFL technique has shown capability to detect and image tube outside defects with a signal-to-noise ratio (SNR) better than 6dB. Study reveals that Inconel support plates surrounding the SG tubes do not influence the MFL signals. As the MFL technique can detect localized defects in the presence of support plates as well as sodium and the remote field eddy current technique is sensitive to distributed wall thinning, their combined use will ensure comprehensive inspection of the SG tubes.

Stabilization of Barkhausen noise readings by controlling a surface field waveform

Insufficient reproducibility and measured results that are difficult to interpret are the main obstacles to widespread practical implementation of the magnetic Barkhausen noise technique as a useful non-destructive tool. This work gives a clear experimental proof that precise control of the surface magnetic field stabilizes the subsurface Barkhausen noise readout with respect to the magnetizing yoke lift-off. The surface magnetic fields above the sample were measured at two different distances by Hall sensors; the real sample field was obtained by linear extrapolation of these surface field waveforms to the sample face. This extrapolated field waveform was then adjusted to the standard triangular/sinusoidal shapes with a multi-parameter digital feedback procedure. The Barkhausen noise signal was measured locally with a bobbin coil placed on the same surface area where the magnetic field was measured.

Rotating field eddy current probe with bobbin pickup coil for steam generator tubes inspection

Eddy current probes are used extensively to evaluate the integrity of steam generator tubes in nuclear power plants and to detect cracks in tube walls. The probe design has evolved from simple bobbin probes to mechanically rotating multi-coil probes providing high resolution images of tube integrity. This paper presents a novel rotating field eddy current probe with bobbin pickup coil that generates a rotating magnetic field, thereby avoiding mechanical rotation of probe. Three balanced identical rectangular windings carrying excitation currents with 120° phase difference generate an electrically rotating magnetic field that is sensitive to defects of all orientations. A bobbin coil located in the center is used as a pick up sensor. The phase of induced voltage in the bobbin coil is interpreted in terms of the circumferential location of the defect. The properties of the probe are investigated using a three dimensional finite element model, and validated experimentally using measurements.