Page 182 - CW E-Magazine (2-7-2024)

Special Report

specimen. The time taken for the sound
to traverse the thickness of a specimen
Receive socket Transmit socket
and return to the probe is usually dis-
LCD
played on an oscilloscope and refl ects
the distance the pulse has travelled and
thereby the thickness of the specimen.
Case
This technique is widely used for crack
detection and thickness monitoring of
Keypad Transducer in-service equipment that may suffer
from erosion-corrosion. Figure-16-A
shows the oscilloscope screen depict-
Sample block ing initial pulse, echo from the back
Battery compartment
(rear case) side of the specimen, and the echo from
the corrosion pit, using a normal probe.
Fig. 17: Direct reading ultrasonic thickness Fig. 18: Inspection Grid For UT Measurement Figure-16-B shows oscilloscope screen
gauge/meter In Pipe Bend
depicting echo from an internal crack in
the specimen, using an angle probe.

For monitoring thickness reduction
of equipment wall due to uniform
corrosion or erosion corrosion, an instru-
ment called ‘Direct reading ultrasonic
thickness meter’ is used (Figure-17).

Figure-18 shows an inspection grid
for thickness data recording for pipe
bend showing weld joints and data col-
The D-Meter probe transmits ultrasonic pulses into the sample being measured and
receives their echoes after they are refl ected from the other surface. The time elapsed is lection points. Figure-19 shows a real-
converted into thickness readings. life demonstration of thickness moni-
toring of tubes & pipes by ultrasonic
Fig. 19: Thickness monitoring of tubes & pipes by ultrasonic gauge (D-meter) thickness gauge (D-meter).
Radiographic technique
X or Y-rays This technique can be used to iden-
tify fl aws developed due to corrosion
in any asset. Generally, x-rays are sent
into the component and a picture is deve-
Not detectable crack
Detectable crack loped on a photographic film. Flaws
such as cracks due to stress corrosion
Pit Test Specimen cracking in welded joints, deposit for-
mation and/or corrosion pitting in tubes
can be located before a catastrophic
Photographic Film failure of the asset takes place. Figure-20
shows the principle of operation of
radiographic technique.

Developed Film
Hydrogen penetration monitoring
In acidic process environments,
hydrogen is a by-product of the corrosion
reaction. Hydrogen generated in such a
Fig. 20: Crack/Pit/Flaw due to corrosion detection by Radiography reaction can be absorbed by steel parti-

182 Chemical Weekly July 2, 2024

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