E-Journal of Advanced Maintenance Vol.5-1(2013) 25-33
Japan Society of Maintenology

Implementation of electromagnetic acoustic resonance in pipe inspection
Ryoichi URAYAMA1 Toshiyuki TAKAGI1,*, Tetsuya UCHIMOTO1, Shigeru
KANEMOTO2, Taku OHIRA3 and Takayoshi KIKUCHI3
1

Institute of Fluid Science, Tohoku University, 2-1-1Katahira, Aoba-ku, Sendai 980-8577, Japan
School of Computer Science and Engineering, The University of Aizu, Tsuruga, Ikki-machi,Aizu-Wakamatsu, Fukushima, 965-0826, Japan
3Plant Management Department, The Japan Atomic Power Company, 1-1, Kanda-Mitoshiro-cho,
Chiyoda-ku, Tokyo 101-0053, Japan2ABSTRACTElectromagnetic acoustic resonance (EMAR) provides accurate and stable evaluation. Its capability has been
demonstrated through online monitoring using a large-scale corrosion test loop operating at high temperature.
This study uses EMAR to evaluate the thickness of pipes in a nuclear power plant during its shutdown throughsignal processing based on superposition of nth compression. Sections of piping evaluated with EMAR include
those in long-term service, where thinning may produce scale-like surfaces, and those having complicatedgeometry. Moreover, we compare measurement results obtained with EMAR and with ultrasonic testing (UT).
The accuracy of EMAR depends on the pipe geometry, such as the pipe diameter and whether the pipe isstraight or an elbow, the presence of welding, and complicated wall thinning. We consider the causes of thedifference in thickness values between EMAR measurements and UT. Finally, we discuss how to implement
EMAR in pipe inspection.
KEYWORDS
nondestructive testing, electromagnetic acoustic resonance, pipe wallthinning, thickness measurement, signal processing.

1.

ARTICLE INFORMATION
Article history:Received 12 November2012
Accepted 13 February 2013

Introduction

The management of pipe wall thinning is a key issue for nuclear power plants. Currently, anultrasonic thickness gauge is used to inspect pipe wall thickness. Although the gauge provides
accurate evaluation of thickness, it is sensitive to scale-like surfaces and complicated geometry, andrequires highly skilled workers to operate. Electromagnetic acoustic resonance (EMAR), which has
been developed for online monitoring, has excellent accuracy and stability of evaluation [1][2].
Experimentation has confirmed that the measurement error of EMAR is lower than that ofconventional inspection techniques [3]. Furthermore, results obtained through field tests using alarge-scale corrosion test loop at high temperature show that EMAR provides measurements with highaccuracy and reproducibility [4]. In this study, we combine EMAR with superposition of nthcompression (SNC) for data processing to measure the thickness of pipes in a nuclear power plant
during its shutdown. Moreover, we compare the results obtained with EMAR with those obtained inan ultrasonic test (UT), and study their deviations in view of pipe geometry (e.g., pipe diameter andstraight pipe versus elbow pipe), the presence of welding, and wall thinning. We consider the causesof the difference between EMAR measurements and UT. Finally, we discuss how to implement
EMAR in pipe inspection.

2.

MethodAn electromagnetic acoustic transducer (EMAT) consists of permanent magnets and transmitterand receiver coils. The time-varying current in the coil induces an eddy current, the static magnetic
field of the magnet generates an Ampere force, and a shear wave propagates through the target [5].
*Corresponding Author, E-mail: takagi@ifs.tohoku.ac.jp

ISSN-1883-9894/10 © 2012 – JSM and the authors. All rights reserved.

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R. Urayama, T. Takagi, T. Uchimoto, S. Kanemoto, T. Ohira and T. Kikuchi
Implementation applicability of piping inspection using electromagnetic acoustic
resonance

EMAR is based on the through-thickness resonances of bulk waves [6]. Resonance is then observed
when the wavelength () satisfies the resonance condition n  = 2 d, where d is the thickness and n is
an integer. The resonance frequency of nth order is given by fn = n v / 2 d, where v is the sound velocity.
If the frequency axis of the original spectrum is compressed to the nth-order resonance frequency, thenth peak moves to the fundamental resonance frequency. Summing up the spectra of the nthcompression, the largest peak should appear around the fundamental resonance frequency f1, given by  f f1  argmaxx  ,f n  n (1)where x(f) is the spectrum intensity of SNC, and argmax (argument of the maximum) is the frequency
at which the SNC spectrum intensity is maximum. The peak value of the fundamental resonancefrequency is the average value of resonance peaks that are included in the calculation frequency range.
This value will be called the SNC peak value.
Finally, the thickness d is evaluated as
d = v/2f1.

3.
3.1.

(2)

Experiment
Measurement condition

The EMAT probe used in this study consists of two Sm-Co-based permanent magnets, anexciting coil and a pickup coil. The transducer transmits shear waves normal to the target surface.
Each of the rectangular-shaped magnets has a width of 10 mm, length of 20 mm, and height of 20 mm,and its surface inductive flux is 459 mT. The diameter and number of turns of the coils are 10 mm and40 for the exciting coil, and 20 mm and 80 for the pickup coil. Figure 1 shows the measurement
component for EMAR. We use a high-power pulser/receiver (RITEC, RPR-4000), a wide-range
decade filter (NF Corporation, FV-628B) for the detection frequency, a high-impedance preamplifier(RITEC, PASJ-0.1-20) for amplifying the detection signal, and a PC for data collection. The
transducer is driven by burst signals with an applied voltage of 1000 Vp-p at 2 MHz and a period of100 s. The driving frequency is swept from 1.5 to 3.5 MHz at intervals of 10 kHz. The signal
amplitude of each frequency, which is the 200-s period after the end of the exciting signal, iscomputed with super-heterodyne processing at intervals of 1 kHz. The sampling rate is 50 MS/s.
3.2.

Outline of measurements

In this study, we use EMAR to measure pipes of the secondary cooling system of Tsuruga
Nuclear Power Plant Unit 2 (Japan Atomic Power Company). The power plant is a pressurized-water
reactor that started commercial operations in February 1987. Measurements are done during thereactor outage for periodic inspection. We extract eight test sections from the plant as targets for
EMAR measurements: four straight pipes and four elbows. The purpose of including both elbows andstraight sections of piping is to assess the effect of pipe geometry on measurements. In addition, thetested sections include those in long-term service to evaluate effects of advanced thinning. There are atotal of 195 measurement points.
The straight pipes are made of STPT38 (carbon steel for high temperature) and have outer
diameters of 48.6, 114.3 and 165.2 mm and nominal wall thicknesses of 5.0, 8.6 and 7.1 mm. Figure 2
shows a straight pipe of target section for EMAR. The elbows are made of STPT38 and SB410(carbon steel for pressure vessels and a boiler) and have outer diameters of 89.1 and 558.8 mm andnominal wall thicknesses of 5.5 and 10.0 mm. Figure 3 shows an elbow pipe of a target section forEMAR. When the EMAT is inclined by the weld padding, the EMAR is measured about 15 mm from26

E-Journal of Advanced Maintenance Vol.5-1 (2013) 25-33
Japan Society of Maintenology

the ends of the welds. Therefore, the measuring points of the EMAR in the vicinity of welds are about
10 mm from the points of the UT. Figure 4 shows the setting of the EMAT probe near the weld. The
white circles on the pipe are the UT measurement positions. If there is a welding line, the starting
position can be measured at a distance of about 20 mm around the downstream side from the welding
line [7]. The measurement position of the EMAR is near this position.
UT is performed by a qualified person before the EMAR measurements. We compare the results
obtained with EMAR with those obtained in UT.
3.3.

Normalize SNC peak value

Figure 5 shows the SNC signal of the calibration specimen, which is a STPT38 carbon steel platewith thickness of 5.01 mm. We use the peak value of the fundamental resonance frequency of thecalibration specimen to normalize the value of the SNC peaks. This value is hereafter called thenormalized SNC peak value.

4. Results and Discussion
Figure 6 compares the thicknesses obtained in the UT and through EMAR with SNC. Closed andopen circles indicate the results at base pipes and welded pipes, respectively. The results did notdepend on pipe diameters or whether the pipe was straight or an elbow, and effect of the thickness waslower. However, there are differences between the EMAR and UT results near the welding. Figure 7
shows the relationship between the normalized SNC peak values and the difference between theEMAR and UT results. The result shows that when the normalized SNC peak values are more than0.15, 0.1, and 0.05, the root mean squares (RMSs) of the differences are 0.18, 0.21, and 0.36,
respectively. When the normalized SNC peak value is more than 0.15, the thicknesses obtained withEMAR and UT agree well and are highly reliable. However, if the normalized SNC peak value is lessthan 0.05, the difference between the UT and EMAR results becomes large and reaches 3 mm. There
are discrepancies between EMAR and UT measurements when the probes are close to the welding. As
mentioned previously, because of the configuration of the reducer and size of the EMAT, thetransducer was put 10 mm away from the measurement points of the UT thickness gauge in thevicinity of welds. The differences in measurement results indicate the difference in measurement
position and the influence of the weld. Because the heat near the weld changes the sonic velocity atthe entrance, it is possible that UT and EMAR measured the change in sonic velocity.
When the normalized SNC peak value was attenuated to less than 0.05, the results of UT andEMAR differed at several measurement points. When pipe wall thinning from flow-accelerated
corrosion (FAC) occurs, an inner surface appears with a scale-like shape. We created a specimen witha scale-like shape on its back surface to evaluate the effect on measured thickness and the normalized
SNC peak value. Figure 8 shows the schematic of the specimen. The material is SS400 (carbon steel)with a width of 100 mm, length of 100 mm, and thickness of 9.9 mm. To machine the specimen with2-mm pitches and a 0.4-mm depth, we used a ball mill with a diameter of 3 mm because it is a typical
FAC shape. Figure 9 shows the resultant SNC signal. The SNC peak value is attenuated slightly, but
its shape has a sharp peak. The thinning depth, which was calculated from the fundamental resonancefrequency, was 0.3 mm. The scale-like thinning shape does not significantly affect the measurement ofthickness using EMAR with SNC.
We made another test piece, one with an inclined bottom, as shown in Figure 10. The specimen is100 × 120 mm in size with a maximum thickness of 12 mm and inclination of 5°. Figure 11 shows theresultant SNC signal. The SNC peak value is significantly attenuated, and its shape is obscure.
Because the ultrasound is scattered by the slope of the bottom, the SNC signal is attenuated. Inaddition, the SNC signal shows information on the inclination thickness. Several SNC peaks due tothe thickness variation of the inclination emerge around the fundamental resonance frequency. Thebottom incline significantly influences the measurement of thickness using EMAR with SNC.
We also prepared a test specimen in which two carbon steel plates were welded. The specimen
represents as-welding in which the weld overlaying is not removed. Figure 12 shows the SNC signalof the weld overlaying, and Figure 13 shows the SNC signals near the weld. These signals areattenuated, but both of the SNC peaks are sharp and clear. On the weld overlaying, the SNC

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R. Urayama, T. Takagi, T. Uchimoto, S. Kanemoto, T. Ohira and T. Kikuchi
Implementation applicability of piping inspection using electromagnetic acoustic
resonancemeasurement obtains a thickness of 9.71 mm from results shown in Fig. 12 and Eq. (2), but thethickness obtained by the micrometer is 10.1 mm. The SNC thickness is smaller than the real value.
The shape of the weld overlaying seems to influence the SNC measurement. Conversely, the side of
the weld is evaluated to be 8.42 mm using Eq. (2) and the EMAR results shown in Fig. 13, and it
agrees well with the true value (8.45 mm).

5. Conclusion
This study used EMAR combined with SNC signal processing to measure the thickness of pipes
in a nuclear power plant. Moreover, EMAR and UT measurements were compared.
The results show that when the normalized SNC peak value is greater than 0.15, the difference
between UT and EMAR measurements is very small. In addition, the signal-to-noise ratio of EMAR
is large. At such a location, EMAR obtains the same thickness as the UT.
Conversely, at some measurement points near the weld, the SNC signals are reduced, and there
are differences between thicknesses measured with EMAR and UT.
We created three test specimens that each had a feature representing a possible cause of thediscrepancies, and considered thickness dispersion and signal attenuation as possible causes. We used
the three test pieces to simulate a scale-like wall thinning, an inclined bottom, and a weld.
The scale-like shape of wall thinning does not significantly affect the measurement of thickness
using EMAR with SNC. The thickness on the weld is found to be thinner than the maximum thicknessof the weld overlaying under the influence of padding welding. The slope of the bottom appears to bea major cause of signal attenuation.
A sloped bottom is expected to cause thickness dispersion and signal attenuation when evaluating
the thickness of pipes in a nuclear power plant. The normalized SNC peak value can be used as aparameter to represent the decrease in reliability of the measurement due to a sloped backside.

Fig. 1 Experimental apparatus and measurement operation.

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E-Journal of Advanced Maintenance Vol.5-1 (2013) 25-33
Japan Society of Maintenology

Fig. 2 Straight pipe of target section

Fig. 3 Elbow pipe of target section.

Fig. 4 Setting of EMAT probe near the weld.

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R. Urayama, T. Takagi, T. Uchimoto, S. Kanemoto, T. Ohira and T. Kikuchi
Implementation applicability of piping inspection using electromagnetic acoustic
resonance

Fig. 5. SNC signal of the calibration specimen.

Fig. 6. Comparison of evaluated thickness between EMAR and UT thickness gauge.

Fig. 7. Relationship between the SNC peak values and the difference in thicknesses obtained with EMAR
and UT.

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E-Journal of Advanced Maintenance Vol.5-1 (2013) 25-33
Japan Society of Maintenology

Fig. 8. Schematic of the specimen simulating scale-like shape.

Fig. 9. SNC signal of the simulated scale-like shape.

Fig. 10. Cross section of the test piece with an inclined bottom.

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R. Urayama, T. Takagi, T. Uchimoto, S. Kanemoto, T. Ohira and T. Kikuchi
Implementation applicability of piping inspection using electromagnetic acoustic
resonance

Fig. 11. SNC signal of the test piece with an inclined bottom.

Fig. 12. SNC signal of the center of weld overlaying.

Fig. 13. SNC signal near the weld.

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E-Journal of Advanced Maintenance Vol.5-1 (2013) 25-33
Japan Society of Maintenology

Acknowledgment
Part of this study was supported by the Global COE Program of Tohoku University, “WorldCenter of Education and Research for Transdisciplinary Flow Dynamics”.

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[5] R. B. Thompson, “Physical Acoustics Vol. XIX”, Academic Press, New York, pp.157–200 (1990).
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