Chemical Engineering March 2019 - 36

Cover Story
* Multiple insulated components of various
diameters and with some thickness variations
can be viewed in one radiograph
CR requires imaging cassettes to be
placed in a laser reader, where they are
scanned to obtain a digital image. DDA images
develop directly on the detector and
need not be processed in a reader. They are
obtained in significantly less time than CR
(seconds versus minutes). They also require
less radiation than is needed for radiography
with traditional film and for CR. On the other
hand, durability of the cassettes and other
factors are more favorable for CR than for
DDA (Figure 7).
FIGURE 8. A tank with heavy
deposits is shown on the
top image. The lower image
shows a pulsed eddy-current
inspection that was performed
without removing the deposits.
The inspection identified
that the areas beneath ladder
rungs experienced the
greatest losses
of the wall thickness. Profile radiography
has been used to measure wall thickness
values since the 1950s [7]. Today, it
typically follows RTR for piping of NPS 8
and smaller.
With the advent of CR and DDA, profile
radiography is the technology of choice for
inspecting smaller-diameter piping (Figure
6). Though occasionally, film radiography
is used in CUI studies, CR and DDA have
largely displaced traditional profile radiography
with film. With these techniques:
* The piping is typically exposed to gamma
rays from an Iridium-192, Selenium-75
or Cobalt-60 gamma source. This point
is common with traditional radiography
with film
* The remaining thickness can be assessed
along pipe edges
* Shorter exposures are needed to obtain
images than for traditional radiography
with film
* Digital radiographs and images can be
stored and shared quickly
FIGURE 9. A guided-wave
testing (GWT) transducer ring
is placed on insulated piping
to screen for potential CUI
damage
Pulsed eddy-current (PEC) method. For
CUI examinations, PEC (Figure 8) is often
used in piping with NPS 8 or greater. This
method is also used when inspecting vessels
for CUI, because this allows for a decrease
in insulation removal. A PEC probe
scans the insulated equipment and identifies
general (averaged) losses, but isolated
pits can be difficult to detect. The detection
sensitivity of this electromagnetic inspection
technology depends on the
footprint of the probe, liftoff or insulation
thickness and on the steel thickness [2].
As with other electromagnetic techniques,
edges (for example, nozzles, flanges or
the end of a structure) pose a challenge
for inspection.
Guided wave testing (GWT). GWT, an ultrasonic
inspection method (Figures 9 and
10), can be used to screen for potential CUI
damage in piping. The inspection is performed
with minimal insulation removal, and
it has been used in piping with asbestos
insulation. A special tool (transducer ring)
is clamped around the pipe that transmits
guided waves in both directions along the
pipe. Reflected signals from defects and
pipe features, such as welds, are received
by the transducer ring and sent to the main
unit. The tested sections must be carefully
selected, since the length inspected in one
scan is limited by the number of elbows, fittings
or welded shoes. Losses due to corrosion
must be greater than 4 to 10% of the
pipe cross-section to be detectable [2].
Ultrasonic thickness measurement. In
CUI studies, direct ultrasonic thickness
measurements can be made on pressure
equipment with diameters larger than NPS
1 where the insulation has been removed
or in locations with insulation plugs. Since
removing insulation is expensive, these inspections
are not performed routinely during
CUI pre-screening.
36
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Chemical Engineering March 2019

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