Operating Manual

171
Useful life of plate and panel
CR plates (by handling), like DR detectors (by radiation) have a finite useful life which has
to be included in an economic evaluation. The working life of flat panel devices can range
up to millions of images, dependent on application-specific details, see paragraph 16.5.3.
Thus cost-per-image should be considered in any return-on-investment financial analysis.
CR plates in flexible cassettes can be used up to one thousand times.
If used in a rigid cassette their life can considerably be prolonged. In the field, care must
be taken that rigid cassettes are not too tightly strapped to a curved component (pipe)
to prevent permanent bending causing problems with automated readers as shown in
figure 4-16. Such readers refuse over-bended cassettes. This may result in undesirable
manual handling of the plates causing plate damage and excessive wear.
16.12 Work station
Hardware and software
A computer and extremely high-resolution display screen are recommended for digitised
films as well as for displaying and processing the images obtained with CR- and DR
techniques. The number of pixels of the display screen should at least match with the
digitisation spot- or pixel size of the applied CR plates or DR panels to achieve maximum
resolution. Radiographic images contain more information than the human eye can discern.
For this purpose workstations, as shown in figure 33-16, are used as an “image-processing
centre”. This workstation operates with powerful dedicated proprietary software (e.g.
“Rhythm” of GE Inspection Technologies) to manage, process and adjust images.
170
Automated/mechanised inspection
The choice of DR flat panel detec-
tors depends on the image quality
required and the number of parts
to be inspected to make it cost
effective (return on investment).
High performance DR detectors
are most suitable on stationary
locations, for example as part of a
production line where vast num-
bers of precision components are
checked at high speed with the
lowest possible radiation dose, or
in situations where mechanical
automation (robotising) can be
applied to achieve significant throughput improvements, see figure 31-16.
Girth weld inspection
Instant results as provided by DR
systems would also very attracti-
ve to replace film to inspect the
circumferential (girth) welds of
long distance pipelines under
construction, either on land or at
lay barges. Until recently DR
systems were more complex and
vulnerable than equipment for
film radiography, thus not suitable
for harsh field conditions. But this has changed.
The resolution of DR (although better than CR) is still (2008) too low compared to film to
meet the image quality requirements for the majority of such girth welds. Nevertheless, for
some type of girth welds, systems using CMOS line detectors with small pixels combined
with a high contrast resolution which rapidly orbit around the pipe, could provide a solution.
They can scan a weld in a few minutes with a reasonable image- and contrast resolution.
Figure 32-16 shows the scanner of such a system. The radiation source can be located insi-
de the pipe on a crawler (single wall panoramic image) or outside at a 180° shifted position
and rotates simultaneously with the line array (double wall image).
Similar scanners exist not using a band wrapped around the pipe but using magnetic wheels
instead. For such applications the CMOS line array must have a fast response- and erase
time in order to frequently (many times per second) refresh the information. CMOS detec-
tors are able do that; and are thus fast enough for girth weld inspection in the field.
Fig. 31-16. DR inspection of narrow beam weld and turbine blades
Fig.33-16. Work station
Fig. 32-16. CMOS weld scanner (courtesy Envision USA)
X-ray beam
Flat panel
Component
X-ray head