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(a)2
1.5Time difference

Burst wave generator
RAM-5000Mark IVOven

Signal Out

Voltage (V)1Trigger Out

EMAT

Diplexer
RDX-6

Specimen

PreAmplifier

0.50-0.5-1

Thickness=-1.5Signal In-2
High Power Out

050

Sound Velocity×Time Difference
2100

150

200

Time (μsec)

Personal computerfor data collection

Oscilloscope

FFT(b)
120
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Resonance
Frequency

Thickness=Resonance number×Sound Velocity
2×Resonance frequency

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In nuclear power plants in Japan a periodical inspection
has been carried out once every 13 months. However,maintenance activities and inspections are concentrated
during plant shut down periods and tend to be fragmented.
Also, there were indications that the required inspection
interval was too long in some cases. To deal with theseproblems, a new inspection code (maintenance program) wascarried out in 2009, in which the checking intervals have ascientific rationale.
Many of the past studies had focused on the conditionmonitoring technique of active components such as a motor.
However, there is also a high interest in the condition
monitoring of passive components, such as a piping. For theirmonitoring, it is necessary to install a sensor with hightemperature durability under the thermal insulator. Also, thesensor should be able to be manufactured with a low costprice.
In this present study an electromagnetic acoustic
transducer (EMAT) composed of a permanent magnet and acoil were used. The sensor was specifically developed for apiping monitor. It excels in durability because EMAT has asimple structure, and also thus has a low price. Moreover, it

0500

1000

1500

2000

2500

3000

Frequency (kHz）                                   is possible to attach it to the pipe using a permanent magnet.
The pipe wall thinning monitoring sensor was developed tomeet the targets described in Table 1-3. The data received
from EMAT are taken to a computer as shown in Fig.1-13.
The sensor is detects multiple ultrasonic wave reflections
in a wall, and thickness is calculated from the intervals
between them (Fig.1-14(a)). The frequency composition of
the reception signal can be analyzed to find resonancefrequencies, which are used together with sound speed tocalculate thickness (Fig.1-14(b)).
It was difficult to measure the thickness of thinner wall by
the reflection time difference. It was also difficult to measure
the thickness of a thick wall using the resonance frequency.
This study was able to properly measure the wall thickness
using different techniques applied to the same received data.
By this method a 2mm board thickness was measured
with±0.1mm accuracy, which is much better then thedevelopment target of 4mm±0.3mm. The sensor wasconfirmed to be usable at high temperature (200 ℃). In thefuture, it will be possible to enhance the safety of piping byinstalling these sensors on them.

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
Tagawa, A. et al., Investigation of the On-line Monitoring Sensor for a Pipe Wall Thinning with High Accuracy, E-Journal of Advanced Maintenance vol.1,no.1, 2009, p.52-62.
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