When radiographing curved objects, for example a circumferential weld in a pipe, as figure 3-12 shows, the resulting image will be distorted. Variations in density will also occur. As a result of the curvature of the pipe with a wall thickness t, the material thickness to be penetrated increases to T, so film density is lower at the ends of the film than in the middle.
Moreover, if defects are projected nearer the ends of a film, distortion of the defect image will become greater. The film length suitable for defect interpretation is therefore limited. This so-called ”useful film length” is, depending on the nature of the work, defined in codes e.g. in EN 1435.
It is not always practicable to apply the single-wall technique as shown in figure 3-12.
In order to still achieve 100 % examination, the double-wall / single-image technique (DW-SI) is applied. (In NDT jargon the abbreviations DW-SI and DW-DI are frequently used for Double Wall–Single Image and Double Wall-Double Image respectively.)
In that case several radiographs are made, spaced equally around the circumference of the item under examination. The number of radiographs to be made depends on the standard or code to be complied with.
In codes, useful film length is determined by the percentage of extra wall thickness which may be penetrated in relation to the nominal wall thickness (t) of the pipe. Percentages of 10, 20 and 30 are commonly applied. For general use, 20 % is a practical value whereby the lightest section of the film shall have a density of at least 2.
The number of radiographs necessary for 100 % examination of a circumferential weld can, through calculation, also be obtained from the codes. When large numbers of similar welds are involved, this is an important figure, because too many radiographs would be uneconomical and too few would lead to insufficient quality of the examination.
The minimum number of radiographs required for various pipe diameters and wall thicknesses at varying source positions can be derived from the graph in figure 4-12. The graph is applicable to single wall and double wall technique, whereby the maximum increase in thickness to be penetrated is 20 %, in accordance with EN 1435 A.
An X-ray tube with an outside diameter of 300 mm is used to examine a circumferential weld in a pipe of a diameter De of 200 mm and a wall thickness t of 10 mm. The distance between the focal spot and the outside of the X-ray tube is 300/2 = 150 mm.
F = half the X-ray tube diameter + De = 150 + 200 = 350 mm.
t/De = 10/200 = 0.05 and
De/F = 200/350 = 0.57
The intersection of the two co-ordinates (0.05 and 0.57) is in the range where n = 5, so the number of radiographs must be at least 5.
When using a source placed against the pipe wall, t/De = 10/200 = 0.05 and De/F = 200/(200+10) = 200/210 = 0.95.
The intersection of the two coordinates now lies in the area where n = 4. So, by using a radioactive source which is located closer to the pipe surface, one less exposure would still ensure compliance with EN 1435A. Initially, the code would however have to allow the use of an isotope instead of an X-ray tube.