11th European Conference on Non-Destructive Testing (ECNDT 2014), October 6-10, 2014, Prague, Czech Republic Evaluation of conductive deposits volume using eddy current signal Valery P. Lunin, Vladimir V. Chegodaev, Andrew G. Zhdanov, Anton A. Stoliarov Department of Electrical Engineering & Introscopy, National research University Moscow Power Engineering Institute, Moscow Russia Phone: +7 916 18 777 81; e-mail: LuninVP@mpei.ru, zhdanov.andrew82@gmail.com Abstract Knowledge of the deposit condition within a steam generator is valuable for assessing the impact of the deposit on the potential for accelerated corrosion of the heating surfaces, as well as determining a degradation of signal detection performance. Any layer of conductive deposit might affect the secondary side of tubing. This additional layer around the tube wall decreases the heat transfer in the steam generator and thus reduces its operational efficiency. However, the deposit can be removed by chemical cleaning. Therefore it is useful to evaluate the total deposit volume in the steam generator to plan the chemical cleaning operation (basically the amount of reactants and the timetable). It is also useful to check afterwards the efficiency of the cleaning process. This paper presents method to evaluate the thickness (volume) of deposit on tubes based on eddy current inspection with a standard bobbin coil, routinely used for the inspection of tubing. The method comprises creating a calibration curve having at least three-four regions of deposit material with different thickness, subjecting its to an eddy current signal, wherein a parameter of the signal reflected from these regions is used to obtain the required curve. Keywords: Eddy current, conductive deposit, heat exchanging tubes 1. Introduction Deposits might affect the secondary side of steam generators (SG) tubing. This additional layer around the tube wall decreases the heat transfer in the SG and thus reduces its operational efficiency. However, the deposit can be removed by a chemical cleaning. In this event, it is useful to evaluate beforehand the total deposit mass in the SG to plan the chemical cleaning operation (the amount of reactants and the timetable). It is also useful to check afterwards the efficiency of the cleaning process with the same non destructive evaluation (NDE) method. We have developed, in the framework of R&D program, a method to evaluate the thickness of conductive deposit on tubes. This method is based on an eddy current (EC) testing with a standard bobbin coil, used routinely for the inspection of SG tubing. The EC signals were simulated with MagNum3D [1] for various configurations of tubes and deposits. The simulation results show that a bobbin coil used in absolute mode at lowest frequency (depending on system applied) is sensitive to a conductive deposit. The relationship between the EC signal and the thickness of the deposit layer is not linear and a look-up-table, based on simulation results, was build. A specific algorithm processes this base of pre-calculated signals to estimate the average thickness and volume of the conductive deposit. The aim of research was: to identify common statistical dependencies of EC diagnostic signal parameters from the conductive deposits on heat exchanging tubes of nuclear power plant (NNP) SGs; formation of the criteria that would be detected corresponding to the areas with deposits (for including in software PIRATE [2] for automatic extracting; to define parameters of EC signal, corresponding to the conductive deposits, which allow to estimate its geometrical parameters; development of algorithm for constructing a calibration curves for estimation of conductive deposits.
2. Investigation of characteristic properties of EC signal from conductive deposits To identify specific statistical properties of diagnostic signal from deposits and to define criteria that would be identified automatically for extracting the areas with deposits, corresponding signals at different frequencies in differential and absolute modes were investigated. Just about 16 thousand (15801) real signals from operational testing at various NPP were analyzed. Analysis of indications from deposits was carried out by comparing its with indications from structural elements support plates and anti-vibration lattices. Signals were analyzed using the software PIRATE. Consider typical absolute EC signal with excitation frequency 60kHz (figure 1). The signal contains various indications and blue highlighted is from the support plate. In the upper left part shown scaled plot, framed blue, and the right shows the hodograph of this signal. Fig.1. Indication from the support plate on the absolute signal at frequency 60kHz Figure 1 shows that an indication from support plate is on imaginary component of the signal after its normalization. These indications are of the same type, their hodograph is oriented vertically (has an angle close to 90 ) and does not depend on the type of the SG. Indication from the same support plate on the differential mode and at frequency 60kHz is shown in figure 2. Analysis of indications shows that the support plates better identified on absolute mode at low frequency. This is because the structural elements increases the thickness of the tube wall, accordingly, low-frequency excitation has a biggest depth of penetration and the best "feel" constructive elements. After normalization and calibration signal indications from structural elements appear only on the imaginary component of signal. Consider the typical indication from a defect, and as a defect take through wall defect at the calibration tube, which is used for signal calibration. Figure 3 shows this indication on absolute mode at frequency 60kHz.
Fig.2. Indication from support plate on the differential mode at frequency 60kHz The indication has real and imaginary parts of the signal. Hodograph has a complex shape and to define the defect parameters is almost impossible. Figure 4 shows the indication of the same defect on a differential mode at frequency 60kHz. On the differential mode at frequency 60kHz through wall defect is quite good readable, hodograph has the appearance that corresponds to typical representation of defect hodograph. Figure 5 shows a typical EC signal on the absolute mode at the same frequency 60kHz, where the blue part selects the conductive deposit. Fig.3 Indication from through wall defect on the calibration tube on absolute mode at frequency 60kHz
Fig.4. Indication from the defect through the calibration of the tube on the differential channel with a frequency 60 khz Fig.5. Typical EC signal on the absolute mode at frequency 60kHz, where the blue part selects the conductive deposit
These signals show that the indications of conductive deposits on absolute mode at low frequency quite good recognized and it can be defined parameters for automatic search. On the contrary, signals on differential mode from conducting deposits are not recognized: there is no possibility to get information about the parameters of the deposits. The angle of the hodograph from conductive deposits is the same as the angle of the hodograph from structural elements (approximately 90 ). Thus, in the analysis of indications from deposits it is necessary to take into account the angle of the hodograph. From the total number (15 801) viewed tubes 155 tubes with conductive deposits were identified. Figure 6 shows the distribution of the difference between deposit-hodograph angles and support-plate-hodograph angles. Mainly hodograph from conductive deposits, as seen in figure 6, is oriented in the same way as the hodograph from support plates (most of the variance is in the range of 15 ), but for automatic search is desirably to find all conductive deposits, so the tolerance was selected ±25. Thus, the analysis of indications from conductive deposits were allocated to the following criteria to automatically detect indications corresponded to conductive deposits: signal amplitude on absolute mode at lowest frequency, which is set for the regular EC testing; angle of the hodograph, that after normalization and calibration procedure, has approximately the same value as the angle of support plate hodograph with deviation ±25. Fig.6. The distribution of the difference between angles of deposit based hodograph and structural element based hodograph 3. Approach to the assessment of conductive deposits In addition to identifying conductive deposits and its location on the tube with automatic analysis of the signal we want to give some quantitative assessment identified deposits. Deposits are characterized by axial length along the tube and the cross-sectional area. The length along the tube is determined through step scan and the number of samples. To evaluate the cross-sectional area of conductive deposits we must define the signal parameter, which changes with changing this area. To determine what signal parameters correlated with the area, the following experiment was conducted. There were taken tube samples with plexiglass frame with different outside diameters, but exactly the same length. The frame was filled with a mixture of powder, corresponding in content and proportions to chemical
composition the real deposits. Next it was conducted eddy current testing of samples and analyzed signal. A characteristic appearance of test signals is presented in figures 7 and 8, which contain the plot calibration tube with standard defects and part of the sample with artificial deposits of different thickness. On figure 7 the thickness value is 0,2mm and the amplitude 0,1V. On figure 8 the thickness is 2,0mm and the amplitude 0,35V. Thus, the measured amplitude can be assessed cross-sectional area of deposits containing powders, or the deposit thickness, construction of features, representing the dependence of the amplitude of the signal from cross-sectional area (thickness) of deposits. Fig.7. Indication of the sample with artificial deposit (thickness 0,2mm) Fig.8. Indication of the sample with artificial deposit (thickness 2 mm)
4. The method for construction of calibration characteristic By analogy with the construction of the calibration characteristics to assess the defect depth, it is based calibration feature to estimate the thickness of deposits on sample on which there is a set of deposits for which reliably known thickness and length. In this case, when the EC probe scans two calibration tubes, one with defects and the other with deposits, and on indications, two calibration characteristics are built simultaneously, one for defects, other for deposits. Calibration tube with deposits represents a tube of steel, the size and material of the crosssection which should be identical to the tubes of the SG. For this tube plexiglass frame is attached. The frame along the axis has internal holes of different diameters, with the change of the diameter is increasing. The number of steps is determined by the number of points on which you can build a calibration curve, in our case, taking into account dependence of amplitude of the signal from the cross-sectional area of conductive deposits, rather three-four. The space between the tube and nozzle is filled with a mixture of powders, relevant content and proportions of chemical composition the real deposits in SG. Figure 9 demonstrates such calibration tube. Fig.9. Calibration tube for deposit assessment The disadvantage of this method of calibration curve construction is the need to know the chemical composition of deposits. It is known that the chemical composition of deposits is different for one SG from another even for the same NPP, not to mention the fact that for each NPP deposits differ in chemical composition too. Therefore there is no possibility to offer one calibration tube for all NPPs. But if for each of the SG to make their calibration tube, then this method of calibration is quite admissible. Unfortunately, testing this calibration tube for assessment of deposit parameters showed that the application of the method of powder filling does not allow to construct reliable calibration characteristics due to the absence of conductive component in used powders [3]. 5. Selection of the material filling the calibration sample Further research was limited to the selection of material filling the calibration sample, having electromagnetic parameters similar to the electromagnetic parameters of the real deposits, and thus providing the same EC signal. The samples represent a thin plate with a small length of different metals. These samples were imposed on the tube by length 200mm, made of the same material as the tubes of SG. Tube sample was jointed with the defect calibration tube that is used to test SG system MIZ-70.
Next, we need to consider the impact of deposit volume on signal magnitude. The volume of deposits is defined by the product of the cross-sectional area at its axial length. The crosssectional area is determined by the product of the thickness of the deposits on the length of the tube circumference. It is possible to assume, that the length of the deposit in azimuth direction can vary from zero to half the length of the outer tube circumference. We used stainless steel foil with the thickness 0,2mm with electromagnetic parameters close to electromagnetic parameters of constructive elements. The size of plates made of foil, one had 25mm length along the perimeter and 42mm axial length, other one had 25mm length along the perimeter and 110mm axial length. First, we studied the dependence of the amplitude indication on the thickness of deposits (plates overlap each other, increasing the thickness of 0,2mm). Figures 10 and 11 show the signals, where as a model simulating conductive deposit, a different number plates of stainless steel (from one to seven, respectively) were used. Fig.10. The signal from the sample, imitating conducting deposit in the form of one layer plate stainless steel (volume is 210mm³) The result of this experiment is construction the dependence of the signal amplitude on the thickness of deposit (figure 12).
Р Fig.11. The signal from the sample, imitating conductive deposit in the form of five layers of stainless steel plates (volume is 1050mm³) Амплитудная зависимость 2 1,8 1,6 Амплитуда сигнала, В 1,4 1,2 1 0,8 0,6 0,4 0,2 0 0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 Толщина, мм Fig.12. The dependence of the signal amplitude on the thickness of the sample, imitating conductive deposit in the form of plates of stainless steel
Conclusion Evaluation of conductive deposit is based on the existence of a quantitative relationship between the energy parameters of low frequency signal in absolute mode and the volume of conductive deposit layer. This evaluation is carried out simultaneously with conducting typical automated eddy current inspection procedure by multi-frequency method. The received signal contains information as about defects in tube metal, and about conductive deposit with fixing the coordinates of location and distances along the axis. To determine the geometric parameters of conductive deposits, first of all the program PIRATE in automatic mode according to certain criteria finds the location coordinates of conductive deposit. Then the program determines the length along the axis of this region through the scanning step and the number of samples. The effective value of low-frequency signal in absolute mode is calculated as a root of the sums of the squares of real and imaginary components of the signal, related to the amount of samples. Thickness and volume of conductive deposit layer is processed through calibration curve often approximated by a straight line. References 1 V.P.Lunin, A.G.Zhdanov Certificate of official registration of the computer program "Program finite element modeling MagNum3D" 2007611345 from 28.03.07 2 V.P.Lunin, A.G.Zhdanov, E.G.Schukis, D.Y.Lazutkin Certificate of official registration of the computer program "Program for data analysis of eddy current testing PIRATE" 2007611344 from 28.03.07 3 A.A.Stoliarov, V.V.Chegodaev, V.P.Lunin, A.G.Zhdanov, A.S.Lavrentiev Evaluation of volume of conductive deposit layer on outer surface of steam generator tubes, International conference "Information means and technologies". М. 2013, v.3, pp.53-60 (in Russian)