E-Journal of Advanced Maintenance Vol.1 (2009) 52-62
Japan Society of Maintenology

Investigation of the On-line Monitoring Sensor for a Pipe Wall Thinningwith High Accuracy
Akihiro TAGAWA1,*, Kazunari FUJIKI1 and Takuya YAMASHITA1
1

Japan Atomic Energy Agency, 1 Shiraki, Tsuruga-shi, Fukui-ken, 919-1279, Japan(Received; August 22nd, 2008)
Abstract. In Japan, the safety of nuclear power plants (NPP) has been secured by performing a time-based
maintenance. However recently, NPPs were taking into consideration a new procedure using a condition-based
maintenance, aiming to improve both reactors safety and operation availability. Therefore, it is needed todevelop new sensors that can monitor, during reactor operation, the integrity of both piping system and pressure
vessel. Also, a proper measurement method has to be chosen in order to carry out an effective on-line
monitoring. In the present investigation, it was used the electromagnetic acoustic transducer (EMAT) because it
does not need a couplant in a high-temperature environment (up to 200 degrees C). Based on this method, twomeasurements techniques have been employed using a new developed sensor, depending on the required
purpose. The 1st technique is based on the Pulse Echo (PE) method. The paper analyses different methods forcarrying out plates thickness measurement using EMAT and supplying a vertical ultrasonic wave. The 2ndtechnique is based on the electromagnetic acoustic resonance (EMAR) that is usually used for material fatigue
assessment. The developed sensor measures, at 200 degrees C, a minimum 5mm±0.04mm plate thickness using
PE, and 2mm±0.01mm using EMAR, respectively. Moreover, it was found no attenuation of the sensor signal
even when the sensor performed a 200 hours continuously measurement.
KEYWORDS: Electromagnetic Acoustic Transducer, Pulse echo method, Electromagnetic Acoustic
Resonance, pipe wall thinning, monitoring

1.

Introduction

In Japan, the safety of nuclear power plants (NPP) has been secured based on a time-based
maintenance. But recently, NPPs were taking into consideration new procedures using a
condition-based maintenance in order to improve both reactors safety and reactors operation ratio. The
measuring method of a carbon steel pipe wall thinning received lately more attention, especially after
the occurrence of the pipe rupture accident in the secondary pipe system at Unit 3 of the Mihama NPP(Kansai Electric Power Co., Inc. in Japan) in August 9, 2004.
In the ruptured pipe (carbon steel) the thinning was developed after water passed the orifice of aflow-meter installed in the pipe and formed a vortex near tube wall, due to the combination of
mechanical erosion caused by internal water flow and corrosion caused by chemical reaction. It wasestimated that the pipe had gradually lost its thickness and that the loss of strength led to a rupture dueto the internal pressure in the tube (about 1 MPa). [1]One of the problems of the pipe wall thinning measurement is that the ultrasonic testing does nothave a sensor with the durability of a non-couplant sensor which can be used in a high-temperatureenvironment. Furthermore, due to the high cost, it is not practically feasible to attach many sensors formonitoring of wall thinning. In the past, it was shown that it is possible to have an ultrasonicinspection using a non-couplant Electromagnetic Acoustic Transducer (EMAT) [2-6]. The authors also
developed in the past an EMAT for a Fast Breeder Reactor (FBR) inspection. However, there is nosensor available to measure with a high precision a wall thickness in a high temperature environment.
In the present paper, it is investigated the possibility of on-line monitoring of a pipe wall thickness,using an ultrasonic sensor based of a non-couplant type. For this purpose, it was developed a low costsensor based on EMAT.
*

Corresponding author, E-mail: tagawa.akihiro@jaea.go.jp
ISSN – 1883 – 9894/09 © 2009 – JSM and the authors. All rights reserved.

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The sensor can be used 200 hours continuously at 200 degrees C. Even more, the sensor canmeasure a 10 mm carbon steel wall thickness with a ±0.1mm or more accuracy in a high temperatureenvironment.
2.

Targets

The main targets of the sensor are presented in Table 1. The targets of detection limit of thesensor were set based on the data found in the report [7]. The present study aims to obtain an accuracy
higher than the detection limit of (4±0.3mm) shown in the above report.

Max Temp.
(℃)
200

Table 1 Development targets
Limit ofDurability
Accuracymeasurement（hours）
（mm）
(mm)
200
≦4
≦ ±0.3

Material
SS400

Regarding the sensor durability, the 200 hours was first consider as an initial target. During thistime, it can be understood the variations in the thickness measurements due to modifications of thesensing property, because of the changes in the initial demagnetization with temperature of thepermanent magnet. However, the durability test time is going to be extended to a two years operation
time in the future.
3.

Principle

3.1

Principle of ultrasonic generation
The ultrasonic wave is generated using an EMAT. The ultrasonic wave appears within anonmagnetic material due to the vibration of material surface subjected to the Lorentz force. This
force is generated on the surface of the material through the interaction of eddy currents generated bya coil and the external magnetic field produced by a permanent magnet [8-9].
However, for a magnetic material, the periodic distortion is mainly caused by themagnetostriction effect which then generates an ultrasonic wave. In the present investigation, the pipematerial was made of carbon steel which exhibits ferromagnetic properties. Therefore, EMAT uses
the magnetostriction effect to generate the ultrasonic wave.
3.2

Principle of magnetostriction effect type EMAT [10]When a ferromagnetic material is subjected to a magnetic field the material changes its shapedue to the reorientation of the magnetic domains, and this effect is called magnetostriction. If atime-varying magnetic field is produced by a high frequency current flowing in a coil, a ultrasonicwave will be produced on the surface of material due to the material elasticity. An electromotive force
will occur in a sensing coil through the interaction of ultrasonic wave and the additional magnetic
field. However, the magnetic permeability of material changes with the magnetic field intensity, andconsequently the magnitude of electromotive force will also depends on the magnetic field strength.
3.3

Principle of pipe wall thinning measurement
In the paper are investigated two measurement methods. In the first one, the pipe wall
thinning measurement using an ultrasonic wave is based on the Pulse Echo (PE) method. PE uses themultipath reflection of the ultrasonic wave from EMAT. The second thinning measurement uses
Electromagnetic Acoustic Resonance (EMAR) method which is based on the resonance frequency of
the ultrasonic oscillation.
3.3.1 Principle of the pipe wall thinning measurement using the PE method
The basic principle of the PE method is based on the arrival time of a reflected wave from the
opposite side of a pipe wall. Pipe wall thinning is calculated using differential-time Δt of thereflected wave using ultrasonic velocity of the material v. A multipath reflection occurs between theupper and lower surface. Therefore, wall thinning T can be calculated using Eq.(1) [10].

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A. Tagawa et al./ Investigation of the On-line Monitoring Sensor
for a Pipe Wall Thinning with High AccuracyTv (t n 1 t n )
2

v t
2

(1)

The main characteristic of the PE method is that direct calculation of the wall thickness can be
carried out using arrival time of the reflection wave. However, the PE method is not suitable formeasurement of a thin wall becauseΔt becomes too short to measure the wall thickness.
3.3.2 Principle of the pipe wall thinning measurement using EMAR method
The basic principle of the EMAR method is based on the resonance of ultrasonic wave
depending on wall thickness. The relation between nth resonance frequency and wall thickness isexpressed as following:

T

n
2

n

v
2 fn

(2)

where lambda λ is the wavelength. Therefore, wall thickness can be measured when ultrasonicvelocity v is known. The fn is the resonance frequency of the ultrasonic after using the Fast Fourier
Transform (FFT). In this paper it is carried out a FFT of the ultrasonic reception waveform, and aresonance frequency is specified by analyzing and interpreting the frequency spectrum.
4.

The trial production of a sensor

Selection of the Permanent Magnet
It is important to have a stronger magnetic flux density and a higher temperature
characteristic of the magnet used for EMAT.
Although the magnetic flux density of Neodymium magnet is generally strong at room
temperature, the magnetic flux density of samarium cobalt magnet has better characteristics at highR
temperatures, more than 200 degrees C. Therefore, it was selected the samarium cobalt magnet ( ○R R28HS at Shin-Etsu Chemical Co., Ltd.). Fig.1 presents the thermal characteristics of theR33H, ○
magnetic flux density of the samarium cobalt magnet. The graph shows the magnetic flux density up
to 240 degrees C, and confirms that this magnet can be used at higher temperature.

20oC 100oC 150oC 200oC240oC
220oC
180oC

HcJ

HcB

R33H
20oC Br=Jr=1.163T
J-H
B-H

20oC

240oC
o
220
C
o
200 C
180oC
150oC
100oC

-30 -28-26-24 -22 -20 -18-16-14 -12 -10 -8 -6 -4 -2 0

12
10
8
6
4
2
0
-2
-4
-6
-8
-10
-12
-14
-16
-18
-20

Demagnetization Curves

B [X10-1 T] (kG)

Demagnetization Curvesoo100oC 150oC

20 C

200 C 240oC
o

180oC 220 C

HcJ

HcB

240oC
220oC
200oC
180 C
R28HS
o
150 C
o
o

o

100 C
o

20 C

20 C Br=Jr=1.12T
J-H
B-H

12
10
8
6
4
2
0
-2
-4
-6
-8
-10
-12
-14
-16
-18
-20
-22
-24
-26

B [X10-1 T] (kG)

4.1

-30 -28 -26 -24 -22 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0

6

H [X106/4 A/m] (KOe)H [X10 /4 A/m] (KOe)Fig. 1. The temperature characteristic of the magnetic flux density [6]4.2

Measuring system
The measurement system of the EMAT is shown in Fig.2. The system consist of an ultrasonicoscillator, a pre-amplifier, a diplexer, an oscilloscope, and a PC for data recording.

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A. Tagawa et al./ Investigation of the On-line Monitoring Sensor
for a Pipe Wall Thinning with High Accuracy
RAM-5000Mark Ⅳ
Mon ito r

Diplexer
RDX-6

EMAT

Receive r Rece iver
No.1
No.2

High P ower High Power
monitor
Out

Signal Out
Trigger Out

Signal In

Pre-Amplifier

High Power Out

Oscilloscope

Personal computer for data collection

CH 1

EXT

Fig. 2. Diagram of the EMAT measuring system

4.3

Manufacture of a trial sensor
Because the sensor design philosophy was to build a sensor with smaller size, higher
durability and cheaper cost, a simple sensor configuration was chosen. Fig.3 and Fig.4 show thedimension and the manufactured trial EMAT.

Samarium-cobalt magnet(W10×L2.4×T7）

9

10 2 10Coil
φ0.2mm×60turn

7

9

7

26

7

10

40

Fig. 3. Schematics of the trial EMAT structure

Fig.4 Photograph of trial EMAT
(left: upper side view-magnet, right: downside view -coil)

4.4

Characteristics of the EMAT transmission system
The ultrasonic wave transmitted by EMAT is a burst wave. The main characteristics of the
EMAT transmission system are shown in Table 2.
Frequency
Cycles per burst
Control
RF Level
Bias
P-P Volts

Table 2 Characteristics of EMAT transmission wave
2 MHz
Watts RMS
2 cycles
HP Filter
100
LP Filter3.14 V
Receiver Gain
4.99 V
Pre-Amplifier Gain
1600 V
Sampling rate

6400 W
1 MHz
5 MHz
34 dB
40 dB
25 MS/sec

Fig.5 shows the waveform of the transmitted burst wave with two waves. The transmitted

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A. Tagawa et al./ Investigation of the On-line Monitoring Sensor
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burst wave has the strongest reflection and the shortest dead zone. Several excitation frequencies weretried, ranging from 200kHz to 10MHz, and 2MHz was chosen because 2MHz is the strongest
reflective wave.
5
4
3

Amplitude(V)

2
1
0
-1
-2
-3
-4
-5
-6
0

0.5

1
Time (μsec)

1.5

2Fig. 5. Waveform of transmitted burst wave

5．

The Performance Test of The EMAT

5.1

Room temperature characteristic test
The following test was performed to confirm the performance of EMAT at room temperature.

5.1.1 Test conditions
The test took place under the following conditions. The temperature is room temperature (7degrees C). The EMAT working parameters have been described in Table 2. Specimen material is
SS400 (carbon steel). Specimen sizes are 70mm×100mm×2.01, 5.01, 10.01 and 15.00mm. Thickness
was measured with a micrometer (0.01mm accuracy).
5.1.2 Test result and consideration
Figures from Fig.6 to Fig.9 show waveform and results of FFT for each specimen in the
60-180 μs time interval. The waveform differential time Δt [μs] and the sound velocity v [m/s](SS400: 3230 [m/s]) wave are used to evaluate the wall thinning T[mm]. The wall thinning isestimated using Eq.(1) for the PE method. The wall thinning is estimated using Eq.(2) for the EMAR
method. PE method can measure 5mm of wall thinning with an accuracy smaller than ±0.1mm. In thecase of the EMAR method, 2mm of wall thinning can be measured with an accuracy smaller than
±0.01mm.
The wall thinning test result of both PE method and EMAR method and comparison with theactual measurement are shown in Table 3 and Table 4.

2

120

1.5

100
80

0.5
Power

Amplitude (V)

1

0
-0.5

60
40

-1
20

-1.5
-2

0
0

50

100

150

200

0

500

1000

1500

2000

Frequency (kHz)Time (μsec)

Fig.6. 15mm waveform (left: PE method, right: EMAR method)

56

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A. Tagawa et al./ Investigation of the On-line Monitoring Sensor
for a Pipe Wall Thinning with High Accuracy

2

180

1.5

160
140
120

0.5
Power

Amplitude(V)

1

0
-0.5

100
80
60

-1

40

-1.5

20

-2

0
0

50

100

150

200

0

500

1000

1500

2000

2500

3000

2500

3000

2500

3000

Frequency (kHz)Time (μsec)

Fig.7. 10mm waveform (left: PE method, right: EMAR method)
2

300

1.5

250
200

0.5
Power

Amplitude (V)

1

0
-0.5

150
100

-1
50

-1.5
-2

0
0

50

100

150

200

0

500

1000

1500

2000

Frequency (kHz)Time (μsec)

Fig.8. 5mm waveform (left: PE method, right: EMAR method)
2

350

1.5

300
250

0.5
Power

Amplitude (V)

1

0

200
150

-0.5
100

-1

50

-1.5
-2

0
0

50

100

150

200

0

500

1000

1500

2000

Frequency (kHz)Time (μsec)

Fig.9. 2mm waveform (left: PE method, right: EMAR method)
Table 3 Result of wall thinning measurement by PE method
Thinning A
ΔtMeasurement
Material
|A-B|
[mm]
[μs]
Thickness B [mm]
2.01
5.01
3.08
4.97
0.04
SS400
10.01
6.24
10.08
0.07
15.00
9.24
14.92
0.08
Table 4 Result of wall thinning measurement by EMAR method
Material
Thinning A
Δf [kHz]
Measurement
|A-B|
[mm]
Thickness B [mm]
2.01
809.7
2.00
0.01
5.01
323.6
5.00
0.01
SS400
10.01
161.6
10.00
0.01
15.00
109.3
14.90
0.10

When the ultrasonic velocity is constant the frequency difference is calculated using Eq(2):fn(n 1)v
2T

nv
2T

v
2T

(3)

Therefore, a resonance frequency only depends on wall thinning and sound velocity. The
theoretical value of frequency difference calculated using Eq.(3) is shown in Table3. It can be seenthat the theoretical value and the experimental value were well in agreement in Fig.10.
Therefore, it was confirmed that even 2mm of wall thinning is measurable with highaccuracy.

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for a Pipe Wall Thinning with High Accuracy

The frequency used for EMAR should be a multiple integer of the frequency in Table 5. In
the examination 2MHz was used as frequency of EMAR, as it is 6 times of the optimal frequency for
5mm (323kHz), as shown in Table 5.

Resonance frequency (Hz)3500
Experimental value
Theoretical value

3000
2500
2000
1500
1000
500
0
0

5

10
Thickness （mm）

1520

Fig.10. Relationship between the resonance frequency difference and wall thickness
Table 5 Relationship between resonance frequency and theoretical wall thickness
Thickness [mm]Thickness [mm]⊿f [kHz]⊿f [kHz]0.5
3230.010161.5
11615.0
15
107.7
2
807.5
20
80.8
5
323.0
-

5.2 High temperature characteristic test
The following test was performed to confirm the performance of EMAT at high temperature.
5.2.1 Test condition
The test took place under the following conditions. Temperatures are room temp. (7 degreesC), 100, 150, 180, 200 and 250 degrees C. The EMAT working parameter has been described in Table
2. Specimen material is SS400 (carbon steel). Specimen size is 70mm×100mm×10.01mm. Thickness
was measured with a micrometer (0.01mm accuracy).
5.2.2 Test result and consideration
Figures from Fig.11 to Fig.14 show the result of the high temperature test. The measurement
was judged by the following two criteria. First, the signal to noise ratio is greater than two. Second,
the peak separation is very clear. The reflected wave is confirmed for PE method at 150 degrees C.
But it is impossible for PE method to use the reflected wave when temperature is above 180 degrees C,
because the peak separation is not clear. However, it was possible to observe a frequency peak in the
EMAR method, and to calculate the wall thickness at temperatures up to at least 200 degrees C. Since
the sound velocity in the material at high temperature was unknown, it was extrapolated andconsequently the wall thickness was calculated using data up to 150 degrees C. Fig.15 shows therelationship between reflection time and sound velocity at high temperature. This sound velocity wasused in the present investigation.
The result of measured reflection time and sound velocity for wall thickness at hightemperature up to 200 degrees C by PE method is shown in Table 6.The result of measured wall
thickness at high temperature up to 200 degrees C by EMAR method is presented in Table 7. Using
EMAR method, it was confirmed that wall thickness can be precisely measured with 0.1 mm accuracy
when the temperature is bellow 200 degrees C. Furthermore, the result also shows the rightness ofapproximation for ultrasonic velocity. Above 180 degrees C, it also appears a resonance frequency ofthe magnet itself and authors consider that further optimization of EMAT structure including themagnet configuration will be required in the future.

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0.5
Power

Amplitude （V）

1

0
-0.5
-1
0

50

100
Time （μsec）

150

180
160
140
120
100
80
60
40
20
0

200

0

500

1000
1500
2000
Frequency （kHz）

2500

3000

Fig.11. Room Temp. 10mm waveform (left: PE method, right: EMAR method)

2

140
120

1

100

0.5

80

Power

Amplitude（V）

1.5

0
-0.5

60

-1

40

-1.5

20

-2

0
0

50

100
Time （μsec）

150

200

0

500

1000
1500
2000
Frequency （kHz）

2500

3000

2500

3000

Fig.12. 100℃ 10mm waveform (left: PE method, right: EMAR method)

70

1

50
Power

Amplitude（V）

60
0.5
0

40
30
20

-0.5

10
-1

0
0

50

100
Time （μsec）

150

200

0

500

1000
1500
2000
Frequency （kHz）

Fig.13. 200℃ 10mm waveform (left: PE method, right: EMAR method)

1

80
60
Power

Amplitude（V）

70
0.5
0
-0.5

50
40
30
20
10

-1

0
0

50

100
Time （μsec）

150

200

0

500

1000
1500
2000
Frequency （kHz）

Fig.14. 250℃ 10mm waveform (left: PE method, right: EMAR method)

59

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3300

Velocity of ultrasonic （m/sec）◆：Experimental value−：Extrapolation

3250

3200

3150

3100

3050
0

50

100

150
Temperature （℃）

200

250

300

Fig. 15. Relationship between temperature and sound velocity

Table 6 Dependency of reflection time on sound velocity at high temperature
Material
SS400

Temp. [℃]
Room Temp.
100
150
200

Δt [μs]
6.19
6.26
6.31
-

Sound velocity [m/s]3225.83230.03168.73148.7

Table 7 Wall thickness test result using EMAR method at high temperature (A=10.01mm)
Material

SS400

Temp.
[℃]
Room
Temp.
100
150
200

Sonic
Velocity[m/s]
3225.8Peak
Frequency[kHz]
1614.43230.03168.73148.71599.4
1584.41569.4

Δf

|A-B|

[kHz]
809.7

Measurement
Thinning B
[mm]
10.00

323.6
161.6
156.9

9.98
10.00
10.03

0.03
0.01
0.02

0.01

Moreover, it was investigated the reason of why ultrasonic wave was impossible to be
detected by PE method above 150 degrees C. Since it was possible to detect the resonance frequency
even at this high temperature, EMAT sensing proved to be suitable for monitoring purposes which
requires long time monitoring at high temperatures.
5.3 Durability test at 200 degrees C
5.3.1 Test condition
The test took place under the following conditions. The temperature is 200 degrees C. The
test time is 200 hours. The EMAT working parameter has been described in Table 2. Specimenmaterial is SS400 (carbon steel). Specimen size is 70mm×100mm×10.01mm. Thickness wasmeasured with a micrometer (0.01mm accuracy).
5.3.2 Test result and consideration
Figures from Fig.16 to Fig.17 show the results of before and after durability test at 200
degrees C. Measurement of the wall thickness by PE method is possible at 200 degrees C like the hightemperature test which was mentioned in 5.2. On the other hand, wall thickness was also measured
using EMAR method. By analyzing the data, it turned out that the measurement accuracy does notchange at high temperature even after a continuously 200 hours operation.

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2
1
0.5
Power

Amplitude（V）

1.5

0
-0.5
-1
-1.5
-2
0

50

100
Time （μsec）

150

200

200
180
160
140
120
100
80
60
40
20
0
0

500

1000
1500
2000
Frequency （kHz）

2500

3000

Fig.16. 10mm waveform before the increase of temperature (left: PE method, right: EMAR method)
2
1
0.5

Power

Amplitude（V）

1.5

0
-0.5
-1
-1.5
-2
0

50

100
Time （μsec）

150

100
90
80
70
60
50
40
30
20
10
0

200

0

500

1000
1500
2000
Frequency （kHz）

2500

3000

Fig.17. 10mm waveform after 197 hours temperature up (left: PE method, right: EMAR method)

The measurement wall thickness was 10.01 mm at room temperature (time=0 sec) while athigh temperature it changed to a 10.03 mm.(Fig.18) By taking into account the coefficient of linear
thermal expansion of iron (11.7x10-6 [mm/℃]) the wall thickness increases from 10.01 mm at room
temperature to 10.03 at 200 degrees C. The measurement accuracy at room temperature is 10.00±0.01
mm. Because the wall thickness measurement at 200 degrees C was also stable (10.03 mm) it can be
deducted that the measurement accuracy, considering iron thermal expansion, is about ±0.01 mm(using the high temperature velocity).

Measurement thickness （mm）

10.1

10.05

10

9.95

9.9
0

50

100

150

200

Time passage （hour）

Fig.18. The wall thickness test result of continuous 200 hours operation

6. Conclusion
In the paper it was investigated a wall thickness measuring method by checking themeasurement accuracy at both room and high temperatures (up to 200 degrees C). Two measurements
methods were employed: the conventional PE and the EMAR method. For wall thickness up to 5mm
it was checked that measurement accuracy is ±0.1 mm or less for both methods. However, a 2mm
wall thickness could be measured only by EMAR. This indicated that it is possible to measure thewall thickness using two types of measuring methods but employing only one EMAT. Furthermore, athick wall (over 5 mm) can be measured using two methods and using the same EMAT signal.

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The EMAR method could be used reliable for temperatures up to 200 degrees C and it was
confirmed in a 200 hours durability test.
However, in the future, will be also investigated the magnet configuration, in order to confirm
whether or not after the stabilization of magnet mechanical vibrations the EMAT (for PE method)
method could also be adopted for wall measurements at high temperatures.
Based on this investigation, it was also acquired the prospect of on-line monitoring of wall
thickness using EMAT.

References[1] Nuclear and Industrial Safety Agency (NISA), “Summary of the Interim Report on the Secondary
System Pipe Rupture at Unit 3, Mihama Nuclear Power Plant (NPP), Kansai Electric Power Co., Inc.
(KEPCO) ”, http://www.nisa.meti.go.jp/text/kokusai/041022.pdf, pp.1-6, 2004
[2] R.B.Thompson, “A Model for the Electromagnetic Generation and Detection of Rayleigh andLamb Waves”, IEEE Trans. On Sonics and Ultrasonics, SU-20, pp.340-346, 1973.
[3] B.W.Maxfield, A.Kuramoto and J.K.Hulbert, “Evaluating EMAT Designs for Selected
Applications”, Materials Evaluation, Vol.45, pp.1166-1184, 1987.
[4] R.B.Thompson, “Physical Principles of Measurements with EMAT Transducers”, in PhysicalAcoustics, Vol.XIX, Edited by R.N.Thurston and A.D.Pierce, New York, Academic, pp.157-181,
1990.
[5] Koorosh Mirkhani, Optimal design of EMAT transmitters, NDT&E International 37 (2004)
181-193.
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