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Results of the first industrial applications of the new generation of imaging platesAlexis BLETTNER -- Daniel CHAUVEAU -- Françoise GRESSET
Institut de Soudure - ZI les jonquières - 57365 Ennery - France
Non-film radiography techniques such as fluoroscopy, computed radiography and real time
radiography has been in medical and industrial use for a long time.
Unfortunately, these have generally been large, immobile systems not practical for field inspections. As a result, radiography technique pipe inspections in the field have been limited to conventional film techniques. The technology based on new generation storage phosphor sensor is now available to provide truly practical digital, non-film radiography for field inspections of pipe.
3.1 - PRINCIPLE OF IMAGING PLATES
Technology of Imaging Plates is build on the ability of certain phosphors to capture a latent image. This image is composed of crystals in which electrons become trapped in a higher energy level after being hit by an X- or gamma-ray photon. This state of entrapment can be resolved through stimulation by a laser beam. The return of the electrons to their original energy level is accompanied by emission of energy in the form of visible light. This process is often referred to as PSL (Photo Stimulated Luminescence). The more radiation reaching the phosphor crystals, the more entrapment is caused, the more PSL-centers are build and the more visible light is subsequently emitted during laser scans. This fully proportional relationship between irradiated dose, entrapment and amount of emitted light makes it possible to determine the originally received dose by measuring the amount of emitted light. Therefore the emission of the visible light is measured by a photomultiplier and digitized to build a raw image. Through the proper processing this raw image is transformed into a viewable image. After readout the image plate can be erased and is ready for re-use.
3.2 - DESCRIPTION OF THE EQUIPMENT
The equipment consists out of a cassette with an imaging plate, a scanner, a PC-based image processing station and image processing software. If needed images can be printed on a hard copy film. Through the development of customized software algorithms (e.g. automated wall thickness measurement for corrosion assessment), a considerable value is added making this system more than just a product producing digital images. The Digital Phosphor System is conceived to offer a solution to the specific needs of the NDT customer by optimizing the overall process of conducting NDT examinations.
3.3 - ADVANTAGE OF IMAGING PLATES
They are significantly faster than conventional film. Exposure times for photostimulable phosphor imaging plates can be 2 to 10 times faster than conventional film systems.
Imaging plates have shorter exposure times than most NDT film/screen systems. However image quality will be less than that of the typical NDT film/screen product.
Imaging plates have the greatest potential where speed is critical, where retakes are difficult or even impossible, where great variation of thickness are met and where the highest image quality is not necessary.
The complete process of an imaging plate is carried out without the use of any kind of chemicals. Thousands of exposures are possible in one plate.
With its extremely wide dynamic range and exposure latitude, the phosphors on the imaging plate provide a tremendous imaging capability. Additionally, with image processing algorithms it is possible to achieve enhance detail contrast. Together, this gives technicians more flexibility regarding dosage and minimizes the need for retakes.
Of course working with a digitised equipment brings all the advantages proper to the use of computerised imaging tools, such as digital storage, easy annotations, networking options, fast communication, integrated reporting, etc...
Finally, but not without importance, the imaging plate is not sensitive to visible light. However, since intense light will erase the image information, the handling of exposed imaging plates needs to be done in dimmed light conditions that means that there is no need of a darkroom. The scanner and all the other hardware will operate in daylight conditions.
3.4 - PROCESSING STATION
Combined with advanced imaging processing hardware and by using specific algorithm it is possible to improve the detail contrast, exposure field recognition and latitude reduction.
A good example of such an algorithm is the wall thickness assessment tool for on-stream applications. This software module will automatically calculate the wall thickness on a radiograph of a tube.
Interactive image processing joins the most powerful post-processing hardware. A powerful tool for advanced and interactive post-processing, the processing station allows the user to take full advantage of the system.
Using a mouse, the post-processing results are displayed instantaneously on the high-resolution monitor.
The main features of the software are given here:
4.1 - INTRODUCTION
All the images present in this paper are perform with the Digital Phosphor System (DPS) developed by AGFA. This system uses the new generation of imaging plates. This system has now been employed in the field for nearly three monthes, and diverse range of applications have been found and are summarised below.
Institut de Soudure have been performed approximately two thousand images on site. During this tests, the system is implanted in a vehicle and a certified operator scan and interprets the images near the exposure site.
In this configuration, it's possible to perform testing on site. After scanning of the imaging plates (two minutes) the operator can evaluate the quality of image and deliver a diagnostic. One other possibility is to set the whole system in a local near the site.
Figure 1 is a phosphor radiograph on an insulated valve on an oil refinery. The image shows the valve position, a chemical or sedimentary deposit is visible on the back of the valve.
|Fig||Fig 1: Phosphor radiograph on an insulated valve with deposit|
The second associated image is the image after image processing that has been used for improving the perception of the deposit. With this image processing, all the details are visible on the image. Deposit
Figure 2 is a phosphor radiograph on a part of a double-pipe heat exchanger used for a high pressure for water-gas exchanges.
|| Fig 2: phosphor radiograph on a part of a double-pipe heat
Normally with conventional radiographic film technique, it's possible to perform only seven
shots in comparison with the phosphor system that enables 27.
Theses benefits are obtained due to the exposure times reduced by a factor 10, the wide range of object densities captured on a single exposure and an optimum contrast display by processing. Figure 3 is a phosphor radiograph on a part of a welded tube with a corrosion crack.
|Fig 3: Image on a welded tube with a corrosion crack.|
The image has been performed with a conventional X-ray source.
The image quality allows to detect the crack with a very short exposure time (only 30 seconds with 1 mA current).
The quality image is reduced by the printer. Normally, this defect is clearly visible on the screen computer.
On the second associated image, a specific imaging processing was used who. The crack is now better detected.
Figure 4 is a phosphor radiograph on a part of a welded tube with an external clamp and resin used for stop leaking.
|Fig 4: Image on a pipe with an external clamp and resin.|
The image has been performed with a conventional X-ray source.
The image quality allows to detect the important corrosion on the left tube with a very short exposure time
The results obtained with an X-ray source should allow in the future to inspect some kind of welded components.
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