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Seismic Activity Calls for Modified Radiographic Technique in Piping System Inspection
by Alex Bagarry and Alfred Sambranov*
Many things in this world are give and take. In setting up an acceptable radiographic procedure for a special job, the author had to balance unsharpness and contrast trade-offs. This is how they did it - using the "Basics" of course.
Frank A. Iddings
Tutorial Projects EditorThe underground gas distribution piping systems in our communities have a limited useful lifetime due to service effects, internal and external corrosion, and original fabrication quality.
We recently had the opportunity to participate in an engineering evaluation to assess the service condition of 300 mm (12 in.) iron piping that had been buried underground for approximately 40 years. Because of recent seismic activity in the area, several lengths of this well used piping were excavated and cut out for evaluation. The weld butt joints were cut into 900 mm (36 in.) pipe lengths and put aside for gamma radiographic evaluation.
The original welds were fabricated without any radiography. The relatively low service pressure and the fact that these systems were buried under the ground did not originally require radiographic inspection.
The existing code for this category of piping is now the American Petroleum Institute (API) Standard API-1104. We reviewed this standard carefully and determined that merely meeting the minimum requirements of API-1104 would not be adequate in this data collection and analysis effort.
A discussion among all parties involved concluded that it would be necessary to use a modified technique that would provide a higher level of sensitivity, contrast, and overall image quality.
Exact centering of the gamma source is the most important factor.
We settled on a single wall exposure technique that utilized a 550 mm (22 in.) long rigid guide tube. This was inserted into the inside diameter and offset to one side to provide the maximum source-to-film distance, using a source inside, film outside (single wall) technique. (See Figures 1 and 2.)
Figure 1 - Technical 1: single wall exposure, higher contrast.
Figure 2 - Ultrasonic inspection.
Exact centering of the gamma source is the most important factor if the most accurate evaluation and measurement of incomplete weld penetration or weld centerline cracking is to be achieved. The rigid tube was laid out with a ruler inscribed on the top of the tube. The weld center distances varied from pipe to pipe. Insertion into each length of pipe could be quickly made to the exact center of the weld for each exposure.
By using a single wall exposure, single wall view technique (Figure 1) instead of a double wall exposure, single wall view technique (Figure 2), we were able to maximize the contrast of the apparent radiographic image on the film, further improving the data for evaluation. Using this technique provided us with a calculated geometric unsharpness value of 0.175 mm (0.007 in.).
Please see the table for a comparison of these techniques.
The higher percentage of relative contrast (difference) evident on the radiographic film between the image quality indicator (IQI) and shim densities, even though with a slightly greater calculated geometric unsharpness value, combine for a better overall image quality and higher sensitivity when compared to the results achieved by technique 2.
This was very graphically demonstrated by comparing film of the same object (weld) using both techniques. The technique 1 film had superior visibility of fine tight cracks in the weld root area, and so provided us with a more accurate evaluation of the overall weld joint condition.
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