Operating Manual

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18.2 Radiographs of objects of varying wall thickness
For radiographs of an object with limited differences in wall thickness, it is common to base
exposure time on the average thickness to obtain the required film density of at least 2. It
is possible that parts of the film are either under- or over-exposed if there are great diffe-
rences in wall thickness. This can be explained by the shape of the toe (lower part) of the
characteristic curve of the X-ray film used. The film gradient (contrast) is lower and, con-
sequently, so is the defect discernibility. In accordance with EN 1435, therefore, there is a
limit to the thickness range covered by one single exposure.
There are a number of practical ways to prevent over-exposure of thinner and under-expo-
sure of thicker sections. These can be divided in two groups: compensation by single film
or by two film techniques.
For exposures on one film, the following techniques can be applied:
• Reduce contrast by utilizing a filter on the X-ray tube to make the radiation harder.
• Reduce contrast by increasing the radiation energy using higher tube voltage or using
Iridium192 or Cobalt60 sources.
• Compensate the difference in wall thickness as the left sketch of figure 1-18 shows,
with material B of similar composition as object A.
• Instead of insertion of B in the previous method, use a special putty (filling material)
mainly consisting of metal powder.
When two films are used, the following techniques can be applied:
• Simultaneous use of two films of different sensitivity but similar screens, for a single
exposure. For example an Agfa D7 and D4 type film could be used. This is the least
complicated and most practical method (see figure 1-18 at right).
• Simultaneous exposure on Agfa D7 and D4 films with different screens (see figure 1-18).
• Make two exposures on film of the same sensitivity and screen type:
one with the exposure time based on the thinner and one on the thicker section.
• Make two exposures on film of the same sensitivity but different screen types.
Fig. 1-18. Compensating for differences in wall thickness
Single film technique
Material A - test object
Material B – material of comparable absorption
Double film technique
Section C. evaluate on D4
Section D. evaluate on D7
18.3 Radiography of welds in small diameter pipes
For pipe welds, the single wall-single image technique (SW-SI), or if this is not feasible, the
double wall-single image technique (DW-SI) is to be applied. For small diameter pipes this
alternative is not really practical, as a disproportionate number of double wall-single image
exposures needs to be made due to the limited effective film length (see section 12.2). In
such a case the double wall-double image technique should be used (DW-DI). Normally, the
DW-DI technique is only applied on diameters <75 mm and wall thickness of <8 mm. Both,
the weld on the source side and film side of the pipe are simultaneously interpreted.
Two more DW-DI techniques are suitable for small diameter pipes:
• the elliptical technique and
• the perpendicular technique
Elliptical technique
The elliptical technique, as illustrated in figure 2-18, is the preferred technique, but should
only be applied if the following conditions are met:
• external diameter (D
e
) is <100 mm (in practice 75 mm)
• wall thickness (t) is <8 mm
• weld width < D
e
/4
The number of exposures is determined by the relation between wall thickness (t) and
diameter (D
e
). If t / D
e
is < 0.12, two images - rotated 90° in relation to each other - are
sufficient for 100 % coverage. If t / D
e
is equal to or bigger than 0.12, three exposures -
rotated 60 or 120° in relation to each other (i.e. equally divided over the circumference) -
is considered to be a 100 % examination.
Fig. 2-18. Elliptical double wall–double image technique
Film Film