E-Journal of Advanced Maintenance Vol.12 No.1 (2020) 1-6
Japan Society of Maintenology

Development of a Pipe-Wall Thinning Inspection Method
Based on a Wireless UT Sensor
Akinori TAMURA1,*, Masao ENDO1, Naoyuki KONO1, Shinobu OKIDO2,
Chenghuan ZHONG3, Erik FABRE3, Maria KOGIA3, Anthony J. CROXFORD4 andPaul D. WILCOX4
1Hitachi Ltd. R&D Group, 1-1 Omika-cho 7-chome, Hitachi, Ibaraki 319-1292, Japan
Hitachi-GE Nuclear Energy Ltd., 1-1 Saiwai-cho 3-chome, Hitachi, Ibaraki 317-0073, Japan3
Inductosense Ltd, Unit DX, St Philips Central, Albert Road, Bristol BS2 0XJ, UK.
4
University of Bristol, University Walk, Bristol BS8 1TR, UK.
2ABSTRACT
Aiming to reduce inspection time of pipe-wall thickness measurements in nuclear power plants, we have beendeveloping a new inspection method based on a wireless UT sensing technology which was originally proposedby University of Bristol. Utilizing the characteristics of the wireless UT sensor, the new inspection method
enables the pipe-wall measurement without removal of pipe insulation which is a time-consuming process ingeneral. One of the issues which needs to be addressed is existence of a metal jacket covering the pipe insulator.
Since the wireless UT sensor is based on electromagnetic induction between coils, a magnetic field between thecoils is interruped by the metal jacket and the wireless UT sensor is not applicable in this case. In this study, wedeveloped the intermediate coil system (PCT/JP2018/020043) as one of the options to overcome this issue. In
this system, two coils and cables are added into the original wireless UT sensor. Firstly, we developed anelectrical circuit model to optimize the system design, and confirmed that the developed electrical circuit model
has sufficient accuracy by the comparison with the experiment result. Then we performed the feasibility test of
the intermediate coil system in the pipe-wall thinning measurement by using the test apparatus which simulates
the pipe in the actual nuclear power plant. From these results, we have confirmed the feasibility of theintermediate coil system in the pipe-wall thinning measurement. Further development and evaluation will beconducted to apply this system into the actual plant inspection in the future study.
KEYWORDS

ARTICLE INFORMATION

Ultrasonic testing, wireless sensor, pipe-wall thinning, nuclear power plant,
inductive coupling

Article history:
Received 15 October 2018
Accepted 11 March 2020

1.

Introduction

Piping systems, such as within an oil and gas system, a chemical plant, a fossil-fuel plant, a
nuclear power plant and so on, are subject to corrosion and/or erosion which cause thinning of thepipe wall. Nuclear power plants have experienced some incidents due to the pipe-wall thinning.
Representative ones are at the Surry Unit 2 in 1986 [1] and at the Mihima Unit 3 in 2004 [2].
Following these incidents, a number of research programs were conducted [3-8], and then ultrasonic
testing (UT) is now commonly used for inspection of the pipe-wall thinning in the nuclear power
plants. In Japan, numerous ultrasonic thickness measurements are manually carried out during theoutage of nuclear power plants. Most of the pipes in the nuclear power plants are covered withinsulators, so the insulator must be removed before the ultrasonic measurement. Since thesupplemental works, such as the insulator removal, are known as time-consuming processes, theinspection technology which enables the pipe-wall thickness measurement without the insulator
removal is desirable to reduce the inspection time of the pipe-wall thickness measurement.
Zhong et al. from University of Bristol [9] proposed an innovative sensing technology based onelectromagnetic induction between coils. Since pulser/receiver and a transducer can be inductively
coupled in this technology, no cable is required between them. This means that the sensor has a*

Corresponding author, E-mail: akinori.tamura.mt@hitachi.com

ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

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A. Tamura, et al./ Development of a Pipe-Wall Thinning Inspection Method
Based on a Wireless UT Sensor

capability to perform the pipe-wall thickness measurement without removing the insulator if the coils
can be inductively connected through the pipe insulator. To confirm applicability of this technology(called a wireless UT sensor in this paper) to the actual plant inspection, a feasibility study wasperformed in the previous research [10]. Measurement accuracy, maximum offset between coils andeffect of the insulator on the inductive coupling were investigated. The sensor durability against
temperature cycles and irradiation was also confirmed [11]. One of the issues which needs to beaddressed is existence of a metal jacket around the pipe insulator. It is known that the electromagnetic
induction is strongly affected by the metal materials because magnetic field is converted into an eddy
current on the surface of the metal. In this study, we propose a new approach to overcome this issue.

2.

A Wireless Ultrasonic Sensor

Figure1 illustrates the wireless UT sensor proposed by Zhong et al [9]. It consists of two parts: asensor and an inspection rod. The sensor, attached on the surface of the inspection target, has a sensor
coil and a transducer which are connected with each other. The inspection rod contains a transmittercoil and a receiver coil. As shown in the left of Fig. 1, there is no cable between the inspection rod andthe sensor because they are inductively coupled by the coils. The right of Fig. 1 shows an applicationof the wireless UT sensor to the pipe-wall thinning measurement of the actual nuclear power plant.
Firstly, the sensors are bonded to the pipe surface with high temperature adhesive because thetemperature of the pipe becomes around 200 deg C. Then the pipe is covered with the insulator as
shown in the right of Fig. 1. Once installation of the sensor and the insulator is completed, theultrasonic measurement without removing the insulator can be available if there is no metal jacket. As
described above, the electromagnetic induction is strongly affected by the metal jacket, which isgenerally installed on the pipe insulator in the nuclear power plant. To resolve this issue, a simplest
way is to replace the metal jacket with non-metallic one. On the other hands, it is worth consideringother options to address this issue because it might be difficult to replace the metal jacket in somecases.
In this study, we propose the intermediate coil system (PCT/JP2018/020043) which enables thethickness measurement without removing the insulator even if the insulator is covered with themetallic jacket. In this concept, two coils and cables are added into the insulator with the metal jacket
as shown in the right of Fig. 1. Since the signal from the inspection rod can be transmitted to theunderneath of the metal jacket through the cable, the thickness measurement without removing theinsulator and the metal jacket may be achievable by the intermediate coil system. In the following
section, feasibility of the intermediate coil system is investigated.

Transmitter coil

Receiver coil

Inspection rod

Inspectionrod

Pulser/ReceiverThicknessmeasurementby UT

Inductive coupling

Metaljacket

Sensor coil
SensorTransducer

Inspection
targetIntermediate
coil system

Interruptedby the metal

10〜20mm30〜50mm

SensorInsulator
Carbon steel pipe

Fig. 1. Concept of a wireless ultrasonic sensor and its application to the pipe-wall thinning inspection

2A. Tamura, et al./ Development of a Pipe-Wall Thinning Inspection Method
Based on a Wireless UT Sensor

3.

Development of the Intermediate Coil System

To realized the intermediate coil system, we first developed the electrical circuit model. The
electrical circuit model of the original wireless UT sensor which consists of three coil circuits wasalready published by Zhong et al. [9]. By referring to this model, the five coil system was constructed
as shown in Fig. 2 to simulate the intermediate coil system. The subscripts of the symbols corresponds
to each coil. Each parameter in this electrical circuit model was calculated or measured by the same
method as the original wireless UT sensor’s one [9]. Because this model includes the impedance ofthe transducer attached on the inspection target, we can simulate the received signal (i.e. A-scan) withvarious coil designs. In order to validate the developed electrical circuit model, we manufactured theprototype of the intermediate coil system as shown in the right of Fig. 2. The transducer which cangenerate 3.5 MHz longitudinal wave was attached on the carbon steel plate with 20 mm thickness.

Transmitter coil

I1

M12L1C1Metal jacket

V1 R
1V31

I4L4V21
R2I3C3M45

I2
IC2C2L3V12V3 R3V32

IC4C4V24

L5R4R5V54

V45

V5C5RsdZpz

Sensor coil

30mmTransducer
Intermediatecoils

Sensor coil

V23
V13I5
L4Transmitter/receivercoils

M23

Receiver coilIntermediate coils

received signalFig. 2. An electrical circuit model for the intermediate coil system and the manufactured prototype.

received signal(a) The received signal from the original wireless UT sensor.

(b) The received signal from the prototype of the intermediate coil system.

Fig. 3. Validation of the electrical circuit model for the intermediate coil system (continues to next page).

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A. Tamura, et al./ Development of a Pipe-Wall Thinning Inspection Method
Based on a Wireless UT Sensor

received signal

100
500-50-100
00.5

1

1.5

2

2.5

3time [sec]3.5
10-5(c) The received signal from the simulation of the developed electrical circuit model.
Fig. 3. Validation of the electrical circuit model for the intermediate coil system.

Firstly, we experimentally compared the received signals between the original wireless UT
sensor and the prototype of the intermediate coil system. In the experiment, the distance between thecoils was kept small to have sufficient signal to noise ratio. As shown in Fig. 3, clear signals wereobtained in both cases. The thickness evaluated by the velocity and the time difference between thepeaks agreed well with the actual thickness (measured by a micrometer) in both cases. This result hasconfirmed that the intermediate coil system is realizable. Figure 3(c) shows the received signal by thesimulation of the developed electrical circuit model. Although the signal amplitude was slightly
different from that of the prototype, the time difference between the peaks agreed well. From thesecomparison, we have confirmed that the developed electrical circuit model has sufficient accuracy andcan be used for the system design.

3.

Feasibility Confirmation

To confirm feasibility of the intermediate coil system in the pipe-wall thinning measurement, weperformed the experiment with the pipe covered with the insulator and the metal jacket. The design ofthe intermediate coils was optimized by the developed electrical circuit model to increase the signal tonoise ratio in this test condition. A test apparatus is shown in Fig. 4. The transducers were attached on
the surface of the carbon steel pipe whose outer diameter is 114.3 mm. This is one of therepresentative sizes in the pipes of the nuclear power plants. The pipe was covered with the silicate
calcium insulator with 50 mm thickness. The Metal jacket with 0.4mm was also installed on theinsulator in the same way as the pipe in the actual nuclear power plant. The intermediate coil system
was installed into the insulator as shown in Fig. 2.
The photo of the manufactured test apparatus and the measurement results are shown in Fig. 5.
The red line in the graph shows the received signal from the original wireless UT sensor (i.e. does not

Intermediate coil system

Metal jacket (thickness is about 0.4mm)
Transducer and sensor coil

Silicate calcium
insulator

Carbon steel pipe
6mm

114.3mm

240mm

50mm

Fig. 4. A test apparatus for the feasibility test of the intermediate coil system

4A. Tamura, et al./ Development of a Pipe-Wall Thinning Inspection Method
Based on a Wireless UT Sensor

include the intermediate coil system). As the insulator is covered with the metal jacket, no signal wasobtained in this measurement. This means the electromagnetic induction between the coils wasinterrupted by the metal jacket. On the other hand, clear signal with sufficient signal to noise ratio wasobtained from the intermediate coil system. The evaluated thickness by the velocity and the timedifference between the peaks obtained with the intermediate coil system agreed well with the actual
pipe thickness. The maximum difference between them was approximately 0.1mm, which iscomparable to that of the manual UT measurement. The common wear rate at the downstream of anorifice would be approximately 0.1 to 0.3 mm per year. Thus, the developed sensor has capability tomeasure the wall thickness variation of the pipes. From these results, we have confirmed thefeasibility of the intermediate coil system in the pipe-wall thinning measurement.

Intermediatecoil
With Intermediate coil
Metal jacket(0.4mm)Insulation(50mm)
Carbon steel
pipe

Sensing part

Without Intermediate coil

114.3mmFig. 5. A manufactured test apparatus and a representative measurement result

4.

Conclusions

Aiming to reduce inspection time of the pipe-wall thinning measurement in nuclear power plants,
we have been developing the non-contact ultrasonic sensor which enables the thickness measurement
without removing the insulator covering the pipe. Measurement accuracy, maximum offset between
coils and effect of the insulator on the measurement performance were investigated in the previousstudy. The issue which needs to be address is existence of the metal jacket around the insulator
because the electromagnetic induction between the coils is interrupted by the metal jacket. In thisstudy, we proposed the intermediate coil system (PCT/JP2018/020043) as one of the options toovercome this issue. Firstly, we developed the electrical circuit model of the intermediate coil system
to optimize the design of the coils. Then we validated this model vie comparison of the received
signal with the experiment, and found that the developed eclectic circuit model has sufficient accuracy
and can be applicable to the system design. Finally, we performed the feasibility test by using the test
apparatus which simulates the pipe in the actual nuclear power plant. As a result, we have confirmed
the feasibility of the intermediate coil system in the pipe-wall thinning measurement. Further
development and evaluation including sensor durability against high-temperature will be conducted toapply this system into the actual plant inspection in the future study.

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A. Tamura, et al./ Development of a Pipe-Wall Thinning Inspection Method
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