Digital Radiography in the Electric
Power Industry

by Stuart Kleven*

INTRODUCTION
The supply of electricity to our homes and businesses is something most of us take for granted since it is almost always available and is seldom interrupted. The supply of electricity is generated and transmitted by electric utility companies to distribution systems that eventually bring it to our location for use. The distribution yards are complex systems with numerous switching mechanisms and isolation equipment. The reliability of these items is of the utmost importance to maintain the constant flow of electricity. Some components used for protecting the line and preventing shorting are fabricated with metallic contacts embedded in thermoset polymers. The integrity of the components can be verified by various dielectric tests prior to use. These tests, however, do not test for small internal discontinuities and fractures that may have formed during the molding process. In some applications, these irregularities may increase the potential of reduced service life of the product. To help minimize this situation, a 100% or a random sampling of parts is radiographed to detect any discontinuities or fractures that may be present.

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The use of the digital radiographic system introduced new technology to our customer.

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RADIOGRAPHIC TESTING
The manufacturer of switching and isolation equipment typically performs radiography using an X-ray tube with radiographically sensitive paper for imaging. Recently, the paper products being used for this technique were discontinued. The manufacturer then subcontracted the radiographic work, with the results being obtained on radiographic film. This provided an excellent record of the test results and was significantly more sensitive than the paper imaging process. Discontinuities can take on many shapes and forms. The discontinuities may be long and slender and difficult to see along the polymer to conductor interface. They may be round or irregularly shaped and may be large or very small. Variation within the molding and curing processes for the polymer can result in isolated internal irregularities. The curvature of metallic components can hide discontinuities due to the change in density at the edge of the polymer bond. Exposure for uniform density at the edge is difficult to achieve and can add extra cost due to the multiple exposures needed to image the area.

 

Digital Radiography Using Photostimulable Luminescent Phosphor Plates
The need to identify all possible internal irregularities in the adjacent polymer prompted us to recommend the use of digital radiography in accordance with ASTM E2033 (ASTM International, 1999) in lieu of the film process. The use of the digital radiographic system introduced new technology to our customer. The image is formed using a conventional X-ray tube (300 kV, constant potential) with a 4 mm (0.2 in.) focal spot. The imaging plate is a replacement for the paper and film. The image is formed when the radiation passes through the part and interacts with the material, similar to standard radiography. The X-ray quanta energy is trapped in the phosphor layer of the imaging plate. An image is formed when a europium ion is changed from a +2 to a +3 ion. This releases an extra ion into the halogen ion empty lattice in the phosphor crystal. The imaging plate is then scanned by a laser and the trapped electrons are released. A photomultiplier tube picks up the energy that is released from the laser scan and the resultant released light energy is converted to a digital image. The digital image is then transferred to the work station computer and monitor (Figure 1).

Figure 1 - Discontinuities in molded components.

 

The reading of the plate erases the plate and makes it available for the next exposure. The plates can be used over and over without image degradation. Our company just retired an older system which was in use for seven years. After seven years, each plate had produced between 15 000 and 40 000 images on the existing plates, which are 356 by 432 mm (14 by 17 in.) and 254 by 305 mm (10 by 12 in.) in size.

The digital radiographic process allowed the exposure time to be reduced, with no downtime for film processing. The image was available on a high resolution monitor within 30 to 60 s. This sped up the test process and allowed the customer to view the images almost immediately, thereby reducing valuable time that wouldÕve been spent waiting to test components at our facility during the development of the radiographic techniques. Some components had round conductors that had some slight separation at the interface. The use of digital radiography allowed the image to be bracketed using the workstation computer. The edge was brought into proper viewing density by applying the brightness and contrast function. This allowed a narrow area to be viewed at optimal conditions. The discontinuity size can be small and narrow. Magnification of the object provided added capability for detection of the smaller discontinuities.

Customers required the size of each discontinuity to be recorded. The computer was again a useful tool, since measuring capabilities are part of the program. The discontinuities were sized using a linear measurement and by area.

 

SYSTEM VERiFICATION
The computed radiographic system is checked by use of a line pair gage. The scanning laser slit is the factor that controls the system sensitivity. A smaller slit produces finer lines of scanning; however, a larger digital file is created that must be displayed and manipulated. The line pair gage is imaged at 0, 45 and 90 degrees and is compared to the number of line pairs detected. This is recorded and is periodically rechecked to make sure that the digital radiographic system has not degraded. For steel items, a penetrameter is imaged with the part and the required sensitivity, normally 2-2T, must be displayed. The monitor is checked for brightness using a white light meter measured in candelas per square meter. A reading of 343 cd/m2 (100 fL) is obtained when the pixel intensity is approximately 530 to 540. The imaging plates are checked using density profile software. All plates are exposed at the same radiation level. The density histogram for each is added together and an average is obtained for all the plates. No individual plate should vary more than 3% from the average. Measured plates from our company were within 1%. The contrast sensitivity is checked with a series of step blocks with 1% changes in thickness. All the steps should be visible to demonstrate the ability to detect slight changes in density. ASTM International is currently in the process of developing a practice and hardware for the testing of digital systems. The use of this phantom could be used as a means of system control.

 

REFERENCES
ASTM International, ASTM E2033-99, Standard Practice for Computed Radiology (Photostimulable Luminescence Method), West Conshohocken, Pennsylvania, ASTM International, 1999.

 

* Alloyweld Inspection Company, 796 Maple Lane, Bensenville, IL 60106; (630) 595-2145; fax (630) 595-2128; e-mail <alloyweldinsp@sbcglobal.net>.

Copyright © 2004 by the American Society for Nondestructive Testing, Inc. All rights reserved.

 

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