Our study attempts to detect and evaluate outer piping corrosion
under insulation material. Such corrosion is accelerated by humidity and
correlates to temperature. To ensure that whole piping is corrosion-free
or within corrosion-allowable for safe operation, plant-owners currently
have to shutdown the operation periodically for precise inspection using
methods like ultrasonic thickness gage. This requires scaffolding to
elevated piping level (in a typical Japanese petrochemical plant, the
majority of piping runs at 7-8 m high), disassembling and reinstalling
insulation. To mitigate this work, our research focuses on utilizing AE
methods as an OSI tool and on enabling to screen/choose where among the
long distance of piping to be precisely inspected.
Another point of research is the utilization of fiber optic sensor.
Fiber optic sensor is explosion proof due to its non-electric principle.
Most location of a petrochemical plant requires explosion proof devices.
Short-time AE monitoring does not need to be explosion proof, but
assuming continual monitoring at a corrosion-critical or inflammable gas
areas, sensors need be explosion proof.
This paper reports on the outline of experiment, which used piping
mock-up. Corrosion was accelerated by sodium chloride (NaCl). Fiber
optic AE sensor was installed from 0.3 m to 3.9 m away from corroded
region and obtained AE signals.
Fiber Optic AE Sensor--Principle
When an object vibrates in elastic manner, the attached fiber optic
element on the object surface elongates and shortens simultaneously.
Light-wave frequency [f.sub.o] is modulated by such changes in length of
fiber optic element, because the number of light waves in that vibrating
region is constant at a moment. This is called "laser Doppler
effect", given as "[f.sub.o] - [f.sub.D]". The frequency
modulation [f.sub.D] is proportional to the changing velocity of fiber
optical length. Doppler effect is described as equation (1), with
[f.sub.D] as modulated frequency, [lambda] as light wavelength, and
dL/dt as velocity .
[f.sub.D] = -1/[lambda]dL/dt (1)
The frequency modulation [f.sub.D] is detected using
Mach-Zender/heterodyne interferometry as shown in Fig. 1. The laser with
frequency [f.sub.o] is emitted and divided with a half-mirror (HM) into
the sensing optical path and detecting optical path. At the detecting
optical path, frequency [f.sub.M] (80 MHz) is added by AOM
(acousto-optical modulator) to create the frequency "[f.sub.o] +
[f.sub.M]". This is combined with [f.sub.o] + [f.sub.D] from the
detecting optical path, again using an HM. The difference in frequency
given as "[f.sub.M] + [f.sub.d]" is converted into the voltage
output using a detector. Hereinafter, we call this sensor "fiber
optical Doppler" sensor or "FOD" sensor.
[FIGURE 1 OMITTED]
Fiber Optic AE Sensor--Calibration Data
For this experiment, a 65-m long, multi-layered FOD sensor was
employed as shown in Fig. 2 and Table 1. This sensor was originally
developed for field micro-seismic monitoring, and embedded into a
borehole near an excavated tunnel. The sensor was targeted for sensing
below 200-kJTz frequency. To reconfirm acceptance to corrosion monitor,
we have calibrated the frequency response of FOD sensor.
[FIGURE 2 OMITTED]
Calibration was in accordance with NDIS 2109-1991, particularly for
longitudinal wave detection. System and transmitting signal
specifications are shown in Fig. 3. Three PZT sensors were employed to
calibrate transmitting PZT sensor initially and then replaced a
receiving PZT sensor by an FOD sensor. Due to the 400-mm thickness of
steel cube, the elimination of reflected wave (80 us delayed at the
shortest distance) was carefully examined. Accordingly, frequency
response from 60 kHz to 300 kHz was calibrated at the same number of
transmitted waves (5 waves). Calibrated frequency response is shown in
Fig. 4 (Below 50 kHz, data is for reference only, as the number of
transmitted waves is only 3). Results for a 70-kHz-resonant PZT (40-dB
amplified) sensor was also shown. The 65-m long/multi-layered FOD sensor
has superior response to PZT sensor from 70 kHz to 140 kHz. This was
adequate for corrosion monitor .
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
Piping mock-up is shown in Fig. 5. It is made of a carbon steel,
5-m long, outer diameter 60.5 mm, thickness 3.9 mm (STPG-370-50A-sch.40)
and has inner flow of silicone oil for heating by a circulation pump.
Artificial corrosion area was located at 1 m from the right edge (Fig.
5) and was accelerated by NaCl solution and cyclic heating (maximum
~80[degrees]C). FOD sensors were located at 300, 2000, 3000 mm away on
the pipe, and 3900 mm away both on the pipe and on a welded flange. Each
FOD sensor was installed on the pipe via a U-shape bolt, while directly
attached on welded flange with a C-clamp, as shown in Fig. 6.
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
Corrosion AE Monitoring
AE monitoring was implemented 3 times with approximately 2-month
interval. First data was obtained 1 month after the start of continual
NaCl dripping. Until then, corrosion spread on the surface, but no
peeling was observed visually. At the third monitoring, corrosion
progressed aggressively, and peeling crack was confirmed even visually.
Figure 7 shows corrosion at the first and third monitoring.
[FIGURE 7 OMITTED]
Detected AE Waveform and Data Analysis
AE was detected by FOD sensors successfully even at 3.9 m away from
the corroded region, with enough SNR (signal-to-noise ratio) margin.
From those data, we made a sample analysis, 1) AE activity and corrosion
status, 2) AE difference by sensor location between on-pipe and
on-welded-flange, and 3) AE frequency-amplitude histogram.
Figure 8 shows a sample of AE waves and FFT data from the first
experiment. It was obtained by an FOD sensor 300 mm away from the
Figure 9 shows AE hits per 30 min during the second and third
monitoring at the same location (FOD at 3.9 m away). Corrosion was
clearly severe during the third monitoring, as confirmed by visual
observation. Figure 10 shows AE hits per 30 min, describing whether AE
data differs by sensor location between on-pipe and on-welded-flange,
both being at 3.9 m away from the corroded area. Data indicates the
welded flange location was slightly lower in detected AE hits than the
on-pipe sensor at the same distance. The attenuation of AE on the flange
location was not serious, giving us more flexibility to install an FOD
sensor on piping structures.
Figure 11 shows the correlation of AE peak frequency and peak
amplitude by sensor location. As monitoring duration is different from
each other, the density of AE hits is indicated qualitatively. However,
it is clear that 60-70 kHz peak frequency is more prominent at any
location, and larger amplitude events have lower frequency.
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
Following in-house experiment, we have conducted field test on an
insulated reactor vessel at the owner's plant (3.5-m outer
diameter, 25.5-m height). The vessel was partially corroded due to
ingress of rain water to the extent of 0.3 to 7 mm depth, as confirmed
by visual test. Four FOD sensors were installed at 90[degrees] interval
at the same height near corroded region. Figure 12 is a water-proof FOD
sensor, which was bonded to the vessel by epoxy resin. Figures 13 and 14
show AE hits per 30 min before and after de-rusting work. Some AE hits
still remained after the de-rusting work, which seemed to be from
internal liquid flow, because this vessel was being operated at the
normal condition. Separation of operation noise is under study,
installing the applicable equipments on site.
[FIGURE 10 OMITTED]
[FIGURE 11 OMITTED]
[FIGURE 12 OMITTED]
[FIGURE 13 OMITTED]
[FIGURE 14 OMITTED]
We have successfully detected and evaluated AE signals, caused by
corrosion progression using fiber optic AE sensor both in laboratory and
at plant. Assuming a pipe is roughly 10-m length, one sensor can cover
the pipe in order to screen CUI presence. Fiber optic AE sensor is
naturally explosion proof and this is especially advantageous in
 K. Kageyama, H. Murayama, K. Uzawa, I. Ohsawa, M. Kanai, Y.
Akematsu, K. Nagata and T. Ogawa: Doppler effect in flexible and
expandable light waveguide and development of new fiber-optic
vibration/acoustic sensor, J. of Lightwave Technology, 24, 2006
 High Pressure Institute of Japan: Recommended Practice for
Acoustic Emission of Corrosion Damage in Bottom Plate of Oil Storage
Tanks, HPIS G 110 TR 2005.
YUICHI MACHIJIMA (1), MASAHIRO AZEMOTO (1), TOYOKAZU TADA (2) and
HISAKAZU MORI (2)
(1) Lazoc Inc., Hongo 3-40-9, Bunkyo, Tokyo 113-0033, Japan; (2)
Process & Production Technology Center, Sumitomo Chemical Co. &
Ltd., Sobiraki 5-1, Niihama, Ehime, Japan
Table 1 FOD sensor specification.
Fiber optic Polyimide coated
Fiber length (m) 65
Height (mm) 6.00
Inner diameter (mm) 8.00
Outer diameter (mm) 22.00