Operating Manual

109
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 simi-
lar 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.
Example 1:
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.
Example 2:
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 co-
ordinates now lies in the area
where n = 4.
So, by using a radioactive sour-
ce which is located closer to the
pipe surface, one less exposure
would still ensure compliance
with EN 1435A.
Initially, the code would howe-
ver have to allow the use of an
isotope instead of an X-ray tube.
108
12.2 Useful film length
When radiographing cur-
ved objects, for example a
circumferential weld in a
pipe, as figure 3-12 shows,
the resulting image will be
distorted. Variations in den-
sity will also occur. As a
result of the curvature of
the pipe with a wall thick-
ness t, the material thick-
ness to be penetrated incre-
ases to T, so film density is
lower at the ends of the film
than in the middle.
Moreover, if defects are pro-
jected nearer the ends of a
film, distortion of the defect
image will become greater.
The film length suitable for
defect interpretation is the-
refore limited. This so-cal-
led ”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 stan-
dard 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.
source
crack
distorted
image of
crack
film
t
T>t
T
Fig. 3-12. Image distortion caused by the curved shape of the object
Fig. 4-12. Graph for the minimum number of exposures in accordance
with EN 1435 A at maximum thickness increase of 20 %.
This graph appears enlarged in the appendix.
Operating range
not being used.