POWER July 2020 - 17

FOCUS ON O&M
5. There are several disadvantages of manual
ultrasonic thickness gaging, including missing
flaws that lie between the grid points.
Courtesy: Olympus
ment of the condition is difficult if you're
taking single-point measurements with
a transducer as it may only provide partial
information (Figure 5).
Limitations of Manual Thickness
Gaging with Dual-Element
Transducers
For ultrasonic thickness gaging applications
such as corrosion monitoring, the
transducer uses longitudinal waves (or
compression waves), and the measurement
is performed from one side of the
part. A dual-element transducer is often
used to isolate the transducer's pulser
from the receiver, helping to minimize
the interface echo to improve the detection
near the surface.
The range of frequencies typically
used with this dual-element pitch-catch
inspection (Figure 6) is between 0.5
MHz to 10 MHz, and the operator spot
checks the component, point by point
inside a grid, to find the minimum thickness.
The results of this method are
7. An automated scanner, used with an ultrasonic
testing probe or transducer, can take
measurements in a raster pattern to improve
the data. Courtesy: Olympus
highly dependent on the operator's abilities
and are difficult to replicate from
one operator to another or even from
day to day.
The disadvantages of manual ultrasonic
thickness gaging typically include:
■ Long and demanding process
■ Results are less reliable because a
flaw can be missed if it lies between
the grid points
■ Operator dependent, permanent recording
is not integrated into the process
■ Limited use of the data for mechanical
engineers
■ Inconsistent probability of detection
Upsides of Automation-and the
Downsides
For inspectors, the main issues with
thickness gaging using conventional UT
are that it's time-consuming, has a low
data density, and it cannot be effectively
used to monitor corrosion because the
8. Phased array probes contain multiple
piezoelectric elements. This illustration shows
a phased array testing coverage area. Courtesy:
Olympus
probe or transducer, it can take measurements
in a raster pattern to improve the
data density (Figure 7).
However, automated UT inspection
has limitations of its own, such as low
productivity due to the transducer's limited
coverage. Each line of the scan inspects
only a few millimeters at a time.
To completely inspect the delimited area,
multiple back-and-forth movements are
necessary. Also, the scanner's fast mechanical
movements contribute to additional
issues, including:
■ Security concerns for operators
■ Prone to scanner failures
■ Coupling becomes difficult at high speed,
leading to unreadable data points
Advancing Automated Corrosion
Inspection Through Phased Array
When incorporated into an automated
scanner configuration, phased array
technology can further improve the
effectiveness,
reliability,
and
safety
Technology that facilitates corrosion inspection
already exists, but there is a new tool
that can offer key improvements by combining
advantages of two common techniques:
conventional pitch-catch ultrasound and
phased array.
6. The range of frequencies typically used
in a pitch-catch inspection is between 0.5
MHz and 10 MHz, and the operator must
check the component point by point inside a
grid. Courtesy: Olympus
July 2020 | POWER
data is captured periodically and cannot
be directly compared with previous
results. Automating the inspection can
partly resolve these problems. When an
automated scanner is used with a UT
www.powermag.com
of the inspection. Phased array (PA)
probes (Figure 8) are composed of
multiple piezoelectric elements. Pulsing
and receiving of the elements are
electronically
controlled
to
generate
17
http://www.powermag.com
POWER July 2020

Table of Contents for the Digital Edition of POWER July 2020
