The Transition from Conventional Radiography to Digital Radiography

First published in Materials Evaluations Magazine on May 2009

 

Digital radiography (DR) is earning its place in the NDT market, as specialists realize this technology surpasses film and film replacement technologies.

 

Portable digital Radiography provides many benefits to users. Fast images upon request for immediate analysis mean there is no compromise on image quality and no repositioning. DR systems enable reduction of working time and costs while enlarging the profits of NDT service providers. The article will focus on Non Destructive Testing with a portable DR system.

 

The article will show inspections conducted in a laboratory and in a refinery with portable amorphous silicon flat panel combined with iridium (Ir-192) source and a pulsed 270kV X-ray source. The examples will show that exposure time has been shortened ten fold and inspector safety is increased. Plant shut down is reduced and the inspections are conducted in minimum time. Various software tools for immediate analysis on site will be demonstrated with examples from petrochemical and pipes industry, aerospace testing and in the service of art.

 

Vidisco's portable DR flat panel systems offer mobility, high quality images and efficient inspection anywhere. DR is the ideal combination between near to real-time results and sophisticated analysis software tools, enabling NDI personnel to work in the laboratory and on site with the same equipment, achieving results quickly and efficiently and, being able to increase the level of analysis.

 

 

1. DIGITAL RADIOGRAPHY IN A LABORATORY

 

In 2008 a vast project was conducted in order to test the capability of digital radiography systems in the field of pipe NDT. The system used in the test was a Vidisco Ltd. foX-Rayzor portable X-ray inspection system, which contains the flat amorphous silicon 14 bit (16,384 gray levels) panel, with Golden XRS-3 (270kV pulsed X-ray source). Several pipe welding samples with intentional defects such as slag, undercut, corrosion, porosity and cracks were tested. Criteria for the success of the test were the time taken to achieve an image and the visibility of the defects and the IQI wires.

 

 

 Set up and test X-ray of carbon steel 60.3mm pipe

FIGURE 1: Set up and test X-ray of carbon steel 60.3mm pipe

 

 

Figure 1 shows Carbon steel 5355 pipe with 60.3 mm outer diameter and 2.9 mm wall thickness (~6 mm total wall thickness), welded in a V shape. The external setting and the X-ray shot parameters are recorded, as are the conditions and time of X-ray. Measurements of the pipe diameter and wall thickness are also recorded as (removable) annotations on the image. Exposure time was 3.54 seconds (53 pulses). Focal detector distance (FDD) was 50 cm and the “shooting” angle 90 degrees. Wire type IQI EN13FE was used to verify 2% sensitivity. A steel block was placed for this test as a measurement reference. The defects in the weld are clearly visible.

 

 

Set up and test X-ray of weld ellipse in pipe

FIGURE 2: Set up and test X-ray of weld ellipse in pipe

 

 

In Figure 2 lack of root penetration is clearly seen on the top of the weld. On the bottom left of the weld one can discern porosity and on the lower right slag and undercut are visible. The pipe is made of Carbon Steel 5355 and has 88.9 mm outer diameter with 3.2 mm wall thickness (total wall thickness 6.4 mm).

 

Figure 3 shows a T shaped Carbon Steel 5355 pipe with 60mm outer diameter and 2.9 mm wall thickness (total wall thickness to penetrate in X-ray, ~6 mm). The exposure was just 4.3 seconds and the distance between the detector and the source 50 cm. The angle was 90 degrees and the IQI used was EN10FE, to verify 2% sensitivity. Slag and lack of root penetration are clearly visible in the top left welding, as are the IQI wires in the center.

 

 

Set up and test X-ray of carbon steel 60mm pipe

FIGURE 3: Set up and test X-ray of carbon steel 60mm pipe

 

 

2. DIGITAL RADIOGRAPHY IN THE FIELD

 

 A test of the efficiency of digital radiography was recently conducted in one of the TOTAL refineries in France. The test set out to understand the advantages of working with a digital radiography inspection system using a gamma ray source. The system used in the test was the foX-Rayzor portable inspection system with the 13 mm thin amorphous silicon flat panel. Criteria for the success of the test were, time to set up the detector and source on site, time to take a good image, the quality of images in comparison with known images of the tested object, and analysis tools available on site.

 The source used was an Ir192, 16Ci gamma source. The distance between the detector and the source was 50 cm – the same distance used when conducting inspections with film. Exposure time ranged between 8 to 16 seconds. The images appeared on the screen in real time, without the need for development or scanning. If an image was not good enough, it was immediately repeated and a new image was once again available in seconds. There was no need to compromise on image quality as good images were achieved on site.

 The system is portable and carried in one ruggedized case to any location in the refinery. The Vidisco DR system offers a combination of the highest quality images, shortest time to image and superior FlatfoX software for analysis of results on site. The software is user friendly and enables the use of state of the art image enhancement tools to facilitate the highest level of analysis. The software tools most commonly used for on site analysis are: Window leveling, Sharpening, Emboss, Overlay, Wall Thickness Measurement and Averaging. With these tools the level of analysis the operator can conduct on site is significantly increased. This gives the operator the tools to make a correct and immediate decision regarding the quality of the image and the information obtained.

 

2.1 Window Leveling

The Window Leveling Tool is a software feature that allows the NDT operator to make the most of the information created by the DR system. The Portable Flat imager offers a 14 bit dynamic range image, which means 16,384 grey levels of information. A computer screen normally shows only 256 Grey levels. The Window Leveling Tool allows the operator to lighten up or darken an image, examining a particular spectrum of grey levels each time, allowing visibility of more specific information on an 8 bit screen.

 

 

Window levelling tool screen

FIGURE 4:Window levelling tool screen

 

 

Figure 4 shows the Window Leveling Tool in action. An X-ray image of an aircraft wing rib (made of Aluminum casting) is viewed in the range that brings to light both the crack on its thick end on the right and the defects around the bolt hole on the thin end on the left.

 

 2.2 Sharpening

The Sharpening Tool is a sophisticated algorithm, which sharpens the image with minimum added noise. The Sharpening tool comes in handy especially when using the Ir-192 source, which shoots its rays in every direction and therefore has a large focal spot that causes unsharpness in the images. This feature can help bring out details of defects in an image.

 

 2.3 Emboss

The Emboss Algorithm translates the gray-level scale into depth, creating a 3-D effect and making the image appear as if stamped in metal. This makes it easier to detect defects such as corrosion and porosity in pipes.

 

 

Emboss effect compared to original X-ray

FIGURE5: Emboss effect compared to original X-ray

 

 

Figure 5 illustrates how the Emboss effect brings out the sunken materials in a 3.5” outer diameter Carbon Steel pipe and makes it much easier to see. With a click of a button the image is transformed on the screen immediately for convenient analysis.

 

2.4 Overlay

Overlay Mode combines two images one on top of the other. This mode is most useful when different parts of the X-rayed object require different exposure times. This allows viewing different thicknesses and materials in one image.

 In Figure 6 the differences between the materials of the pipe and its insulation make it difficult to see all the details with just one exposure. The first exposure is 10 seconds long and the second just 3 seconds. Then the images are laid automatically exactly on top of each other using the Overlay Mode. Now all the details can be viewed in one image for clear analysis.

 

 

Insulated pipe in overlay mode

FIGURE 6: Insulated pipe in overlay mode

 

 

2.5 Averaging

The Averaging Function combines two or more images by averaging their values at every pixel. The Automatic Averaging Function averages a number of images (as set by the user) by taking consecutive images one after another and then displaying one image that is the result of averaging the consecutive images. The result is a cleaner image due to reduction of noise, which will show finer details.

 

2.6 A combined Test

A cut of a pipe was brought to the laboratory from the refinery for a comparison between the pulsed X-ray source and the gamma ray source.

Figure 7 shows that the pulsed X-ray source was able to achieve sharper results in minimal exposure time. Just one exposure with XRS-3 270kV source (c) gave a high quality image that surpassed even the image created by averaging 6 images with Ir192 (b) and was most certainly better that a single exposure with Ir-192 (a).

 The Emboss effect shows that new defects can be detected in the image created with the pulsed X-ray source (c). The image on the left (a) was created with 9 seconds exposure with Ir-192, the middle image (b) is an averaging of 6 such images (9X6=54 seconds exposure with Ir-192) and the image on the left (c) is an image taken with XRS-3 pulsed X-ray source, with 2.03 seconds of exposure only. Not only is X-ray less dangerous that the radioactive Iridium, its rays are more focused and directional and the exposure time is shortened.

 

 

Soutirage Catalyseur FCC Unit, comparison with XRS-3 - Emboss tool

FIGURE 7: Soutirage Catalyseur FCC Unit, comparison with XRS-3 - Emboss tool

 

 

3. EXPOSURE TIME COMPARISON

 

DR portable inspection systems have been available to NDT operators for some years. To complete the picture of the capabilities of pipe NDT with portable DR, here are results of exposure times as obtained in daily field work of a large NDT provider in Finland.

 Table 1 details the time comparison of NDT of pipes in several sizes that are made of various materials. The table compares exposure times required when using film with the exposure times when using a portable DR system. All tests were conducted using Ir-192 source.

 

Table 1: Exposure Time Comparison

Item Inspected

 Pipe Diameter

Material

Wall Thickness

Liquid
Content

Exposure time with Ir-192 and foX-Rayzor
Time to Image

Exposure time with Ir-192 and Film 
No Development**

Fire water hose

208mm

st 35

7.2mm

None

30 seconds

3 minutes

Glass fiber profile

700mm

Glass fiber

approx. 25mm

None

70 pulses*(about 4.3 seconds)

30 seconds

Process water pipe

150mm

ss2343

Total one wall 6mm

Water

20 seconds

15 minutes

Steam cooler

250mm + insulation

10CrMo

Total one wall 40mm

None

50 seconds

approx.1 hour

Low pressure steam pipe

400mm + insulation

st 35

12mm

None

30 seconds

approx. 20 minutes

Fuel Lye pipe

100 / 80mm

ss 2343

6mm

Lye

15 seconds

10 minutes

 

* Test conducted with pulsed XRS-3 source

** Exposure time only, not including film development

 

CONCLUSION

 

The portable amorphous Silicon DR system has proven itself to be efficient because it offers practical solutions to both laboratory uses of NDT and also to conducting inspections on site. In the refinery it took just 3 hours for  the operators of the portable DR system to conduct inspection in 10 locations. A similar inspection when using film or film replacement technologies that require scanning and development can take more than one day (including film development but excluding analysis, which adds more time to end result of the test).

 

Not only was the inspection time shortened, there was no need to shut down the plant and the quality of the images was known immediately. There was no need to return to conduct further inspections because if a location mistake had been made, the operator could tell at once (because images are seen immediately on screen) and correct it immediately – thus always achieving good images in an on site inspection.

 

In laboratories in museums and in aerospace facilities portable X-ray makes analysis easier due to sophisticated enhancement software. Results are immediate and in the highest quality.

 

To conclude: DR systems offer the NDT provider an opportunity to achieve good results in a shorter time and to increase the quality of analysis. The service provided can therefore be improved and its cost reduced. Profitability and operator safety are increased significantly.
 

For more information, please read the article Cutting Edge Digital Radiography Technology in Daily NDT Use.

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