Operating Manual

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This way an image is obtained of the pipe “horizon” with possible presence of
corrosion (swelling or pitting). The image is presented real-time on a portable
monitor. The battery-powered equipment uses soft radiation of low intensity, so that it
can manually be moved along the pipe. The system can also be used to locate welds
under insulation, providing the weld crown has not been removed.
17.3 Computer Tomography (CT)
Unique features
For medical diagnostic purposes, techniques have been developed to obtain a radiographic
quasi 3D picture, a so called CT-image with a high resolution of a few tenths of a mm.
Powerful computers are used to transform a large number of absorption variations that
occur when irradiating a human body with a moving source around the stationary patient,
and their coordinates into a comprehensive 3D (volumetric) image. This technique is now
also used in industry, e.g. for checking the integrity of components with complex geome-
tries, high quality castings, miniature electronic circuits as built into mobile telephones and
even for 3D metrology: a method to measure even internal (inaccessible) dimensions of
components that otherwise cannot be measured at all. CT systems with a resolution of only
a few microns for a wide variation of tasks have already been successfully applied. To inter-
pret the results, CT images can be freely rotated and virtually sliced in all directions for dif-
ferent views of a defect or other anomaly; a unique and very useful feature.
Computing capacity and scanning time
In NDT contrary to medical applications, it is usually the object that rotates between the
source and the detector as shown in figure 11-17. This can be done continuously or stepwise
to obtain a great number of 2D images that ultimately are reconstructed into a 3D CT image.
The object is scanned section by section with increments of say 1° over 360° with a very
narrow beam of radiation (small focus X-ray or collimated gamma-ray). The more incre-
ments, the better the CT quality. The receiver in this illustration is a flat panel detector.
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• CCD-camera as a substitute for the relatively slow conversion screen
• Photo array detector, minimal size per diode (pixel) approx. 100 microns to
inspect slowly moving objects (airport luggage checks)
• Flat panel detector consisting of millions of light-sensitive pixels.
Although the image intensifier is still most commonly used, the flat panel detector is
becoming more and more attractive. Flat panel detectors provide various pixel sizes with
extensive image dynamics (a very wide density range, far greater than is possible with film).
Since the signals received by the computer are digital, the screen image can be optimised for
interpretation (contrast, brightness, sharpness, magnification, filtering, noise suppression)
and subsequently stored.
These advanced systems also offer the possibility of comparing the image obtained with a
reference image and of automatic defect interpretation, see chapter 16.
Selection of the most suitable (expensive) system is made even more difficult because of the
rapid development in sensor- and electronic technology.
Fluoroscopy, image intensifiers and-magnifiers are more elaborately described in the
booklets: “Die Röntgenprüfung” and its translation “The X-ray Inspection” [3].
Portable real-time equipment
A portable version of real-time equipment is used to detect external corrosion under
thermal insulation. It is generally very difficult to detect corrosion on piping with
insulation still in place whereas removing and re-installing the insulation is a costly
and time consuming operation. Sometimes the likely presence of corrosion is
indicated by moisture/water detected in the insulation, see section 17.4.
External corrosion in a low-alloy steel pipe becomes apparent by local swelling of the
pipe surface as a result of volume increase of the corrosion layer.
Figure 10-17 illustrates a system by which the swelling and even severe pitting can be detected.
On one side a strongly collimated source or X-ray tube is located that must be aligned in a
way as to direct a narrow beam of radiation along the tangent of the pipe towards a small
flat panel detector behind.
1. pipe 6. tangent ( pipe horizon)
2. sheet metal (Al) cladding 7. portable monitor
3. insulation material 8. adjustable bracket
4. collimated radiation source 9. sliders/rollers
5. miniature flat panel detector 10. corrosion spot
Fig. 10-17. Schematic arrangement of a portable real-time radiographic corrosion detector
1
X-ray
source
Focus-object
distance
Focus- detector
distance
Rotating object
Focus
Detector
Fig. 11-17. Rotating component to create a 3D CT image