POWER May 2023 - 30

OPERATIONS & MAINTENANCE
Understanding Ultrasonic
Examination in the Power Industry
Nondestructive testing (NDT) refers to the inspection of materials to ensure their
integrity and serviceability without damaging the components in the process. There
are various NDT methods used by inspectors, but one of the most frequently used
techniques in the energy industry is ultrasonic testing.
A.M. (Marty) Wenzig
ltrasonic testing (UT) uses the
same principle as naval sonar.
When sonar passes through water
and hits an object, a portion of the sound
reflects to the submarine and provides a
distance measurement to the object.
Sonar inspired early ultrasound inspectors
to explore ways to apply the concept
to other applications. By the 1940s, inspectors
developed ultrasonic techniques using
single element analog pulse-echo probes.
Analog Techniques
With the analog technique, sound is introduced
into a component part using
a single element ultrasonic transducer
(probe)
that
piezoelectrically converts
electrical impulses from the UT detector
machine into mechanical sound waves.
The transducer reconverts by turning
sound back into electric impulses that
can be displayed on a cathode ray tube.
A liquid or gel called couplant is used to
transmit sound into the part. The most
common types of sound waves used
in industrial inspections are longitudinal
(straight) beam wave and transverse
(angle) beam wave, with UT frequencies
between 1 MHz and 10 MHz.
Straight Beam. When inspectors use
straight beam (Figure 1) wave, longitudinal
waves are sent through the piece.
Because of distance, if the sound hits an
internal reflector, the sound transmits back
to the transducer faster than the sound returning
from the backwall of the part.
U
Angle Beam. Angle beam inspection
uses the same type of transducer but
mounted on an angled wedge to transmit
the sound beam into the part at a
known angle. An angle beam transducer
and wedge combination move back and
forth toward a weld, for example, so the
sound beam passes through it. As with
straight beam, reflectors aligned approximately
perpendicular to the sound beam
will send sound back to the transducer.
This is then displayed on the screen of
the UT detector machine.
Digital Techniques
The advancing nondestructive testing (NDT)
industry has observed the emerging trend
of more and more digital technologies.
Phased Array. Phased array ultrasonic
testing (PAUT, Figure 2), a common digital
technique, uses a transducer with multiple
elements that can be individually
activated allowing inspectors to steer the
sound beam. The resulting data form a
visual image through the inspected part.
With PAUT, the beam can map components
at appropriate angles. This process
simplifies inspections with complex geometry.
The transducer's small footprint
and ability to sweep the beam without
moving the probe aids the inspection of
components to which access is limited.
Sectorial scanning of this type is typical for
weld inspections. The ability to test welds
with multiple angles using a single probe
increases the probability of flaw detection.
Electronic focusing permits optimizratio
in
ing
beam shape and size at the expected
defect location. Focusing also improves
signal-to-noise
coarse-grained
material. C-scan images can then be
produced very rapidly. Although the
technique is more effective than regular
UT, phased array systems are high-cost
equipment and require additional training
and competency for UT personnel.
Full Matrix Capture (FMC). FMC is a
data acquisition strategy that allows capturing
every possible transmit-receive
combination for a PAUT transducer.
PAUT systems use probes with multiple
elements (typically from 16 to 128) that
are excited by a computer in a controlled
manner using a specific delay law. After
reception, the contribution of each element
is summed to produce a scan.
With FMC, each array element in a probe
is sequentially used as a single transmitter
while all array elements are used as receivers.
By capturing and storing A-scan signals
from every transmitter-receiver pair in
the array, it's possible to generate imaging
for any given focal beam and apply today's
latest post-processing algorithms.
Flaw Detection
Flaws may occur during the manufacturing,
fabrication, and/or in-service phases
of an operation. The types of flaws
anticipated during in-service are different
than those that occur during manufacturing
or fabrication.
Manufacturing and Fabrication Flaws.
Analog ultrasonics and radiography testing
(RT) are two main volumetric NDT
methods used in the manufacturing and
fabrication
industries. While detection
and sizing are possible with conventional
UT, indications and flaws observed using
these methods are in 2D. The acceptance
criteria in NDT requirements in
material and component specifications
usually appear in terms of
allowable
1. Analog straight beam technique. Courtesy: American Society for Nondestructive Testing
(ASNT)
30
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length or area size, such as no indication
greater than 1/8 inches or 3.18 millimeters.
There are four categories of material
processing defects:
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POWER May 2023

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