Table of Contents ECNDT '98
Session: Chemical, Petrochemical
On-Line Radiographic Wallthickness-Measurement of Insulated Piping in the Chemical and Petrochemical IndustryA. Hecht* - BASF. R. Bauer - Giessen Uni. F. Lindemeier - AMI. a.o. Germany.
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Radiographic wall-thickness measurements had been carried out since the 50´s, but there is only very little literature about this special radiographic technique. Fig. 1 shows an early example of a real industrial application:
Fig. 1: Wall-thickness measurement on a high-pressure tube during hot-service using Mesothorium as -ray source (BASF-slide from 1954)
A major disadvantage of this radiographic method is, that the results are obtained off-line after film processing. There is a great industrial need for making radiographic wall-thickness measurements really on-line, which requires the availability of a filmless -ray imager. This paper deals with first results obtained with a new flat and lightweighted Radiation Image Detector (RID).
Fig. 2: Setup for tangential imaging (general) /1/
Radiation source is typically Ir192. For thicker pipes or pipes with outer diameters (OD) greater than 250 mm Co60 is often used. Tangential imaging is normally not carried out with X-rays or Se75. This technique requires harder radiation, because the tangential arrangement leeds to great materials-thicknesses (and also great changes in materials-thicknesses) to be penetrated by the rays, see fig. 2.
Wall-thickness measurements when using -rays offer two methods of evaluating the image:
By use of fast films and the arrangement in fig. 2, both methods of film-evaluation are possible with only one radiographic shot.
Because of the method´s disadvantage of being off-line, there was always the industrial need for a filmless method. The relatively easy to handle "Lixi-Scopes" had been the first solution for a really on-line monitoring of corrosion under isolation (CUI) at the pipes outside /4/. With this system the rays cannot penetrate through the wall because of the used weak -ray source. It is therefore not suitable for method 1 or method 2.
In /5/ a crawler-controlled line-camera is described for obtaining radiographic wall-thickness images and in /6/ an (heavy-weighted) image-intensifier mounted on an external pipeline-crawler. A recent development is an external pipeline-crawler for radiographic corrosion monitoring, where an Ir192 source in combination with a fluoroscopic imager is used /7/. Having only a limited resolution in intensity (optical density) this device is not suitable for quantitative wall-thickness measurements (method 1).
|Fig. 3: Setup for on-line wall-thickness measurement and corrosion monitoring at insulated pipes|
The RID`s size is 330 mm x 320 mm x 46 mm and its weigth is 8 kg. The active imaging area is 204.8 mm x 204.8 mm and consists of 512 x 512 pixel-elements, each 0.4 mm x 0.4 mm. Each pixel consists of a szintillator, which transforms the -rays into visible light and a light-sensitive semiconductor directly coupled to a TFT-transistor. The semiconductors can be considered as small capacitors, which are charged before the measurement and discharged by the emitted light from the szintillator. All pixels of the array are read out every 200 ms which leads to an image-refresh-rate of 5 Hz.
The electrical discharge-current, which is directly proportional to the incoming dose-rate of the X- or -rays, is digitized by the RID´s ADC with a resolution of 16 bit, equivalent to 65536 single intensity (or gray-scale) values! By this great dynamic range an over- or underexposition of the radiographic image is nearly impossible. Additionally, the RID is then sensitive both for high-dose X-rays and low-dose -rays.
With an exposure time of only 200 ms one complete -ray image is available on the PC-screen. By temporal averaging, the image´s noise can be reduced. Averaging of 25 images within 5 seconds leeds to a sufficient signal-to-noise ratio. The image is stored on the PC´s hard disk and can be evaluated on-line or off-line by software.
Fig. 4: Digital radiographic image of a pipe with 60 mm diameter and 2.6 mm wall-thickness, obtained with 800 Gbq Ir-192 and new Radiation Image Detector
At present a first prototype for industrial wall-thickness measurements at insulated pipes is under construction. A commercial system for industrial needs will be available in the near future. By use of this prototype, we will also check the limitations of this on-line technique like:
Fig. 5: Line plot from fig. 4, showing optical density (digital values) across the tubes diameter (see also schematic curve from fig. 1). t´ can be measured between a and b.
The pixel-resolution of 0.4 mm x 0.4 mm is sufficient for radiographic wall-thickness measurements. For weld-inspections a pixel-resolution of 50 µm x 50 µm is at least required.
A RID-based radiography-system can be a future part of the basic equipment of ambulance cars. Real-time radioscopic examinations and diagnosis during transportation can help to preparing surgical measures in the hospital. Persons injured at the spinal column have to be layed or transported properly, which also can be checked by RID-based radiography.
A racehorse can be examined during training or before a race to check wether it is allowed to start at a competition or not. An injured horse can be radiographed directly at the riding-stable or at the horse show, and therapeutical measures can be initiated immediately. If the horse had moved during exposure, the examination can be repeated on-site, because the veterenary surgeon can observe the blur on the radiographic image directly after exposing.