Operating Manual

18.5 Determination the depth position and diameter
of reinforcement steel in concrete
Similar to the method for determination of the depth position of a defect in metals is the
determination of the depth position (cover) of reinforcement steel in concrete.
Subsequently, the true diameter of the reinforcing bar (D) can be calculated.
Correction factor = d / (H-d).
The dimension of the radiographic image (D
f
) on the film is multiplied by this correction
factor. The true diameter of the reinforcing steel is therefore D = D
f
.
d / (H-d).
18.6 On-stream inspection - profiling technique
On-stream inspection can be carried out on
pipes, valves, vessels, and distillation
columns while in operation, in order to esta-
blish the degree of deterioration of the
system either the projection or the tangential
technique can be used. Since the introducti-
on of digital radiography, the CR-method
using storage phosphorplates, is increasingly
becoming an alternative for traditional film
in case of on-stream exposures, see chapter
16. The main advantage being that it reduces
the exposure time by a factor of 5 to 10, or if
lower energies (Iridium192 instead of
Cobalt60) can be applied it results in a redu-
ced controlled area, which is very attractive
in cramped spaces and personnel nearby e.g.
on offshore platforms.
Projection technique
The projection technique is most commonly
used. With this technique the two walls are
projected on film simultaneously, as shown in
figure 5-18. The image projected is larger than the actual object dimensions. It is important
to know the degree of magnification so as to be able to determine the true wall thickness.
If both walls of the pipe are projected on the film, it is straight forward to establish the cor-
rection factor, which is the true diameter (D) divided by the radiographic diameter D
f
.
This method should be used as much as possible.
With the projection technique, the source is placed at a certain distance from the pipe.
At a film-to-focus distance of 3 x D
insulation
and a source size of 3 mm, image quality require-
ment A of EN 1435 is met .
Fig. 5-18. Projection technique for on-stream radiography
correction factor =
filter
film
D
insulation
D
pipe
= source-to-insulation
distance
or
(source)
The actual pipe wall thickness (t) is equal to the
image on film (tf ) multiplied by the correction
factor (see fig. 5-18).
Most common is on-stream radiography of
insulated pipes, for which half the insulated
diameter determines the sharpness. In on-stre-
am radiography it is important to know the
direction of the product flow, so that a existen-
ce of localised wall thickness reduction can be
better deduced. Films of 30 x 40 cm are gene-
rally used for pipe diameters up to 250 mm.
Larger diameters require more films.
Tangential technique
In the pipe diameter range of 250 to 400 mm
the tangential technique, as shown in figure 6-
18 is sometimes applied. Only one wall is pro-
jected. The perpendicular projection produces
a sharper image. This allows a shorter focus-to-
film distance, and consequently a shorter expo-
sure time. Generally, a focus-to-film distance
of 2.5 x D
insulation
is chosen.
The correction factor would then be:
(2.5 x D
insulation
-0.5 x D
insulation
) / 2.5 x D
insulation
= 0.8.
Selection of source, screens and filters
The graph in figure 7-18 indicates which radio-
active source is the most suitable, depending
on pipe diameter and wall thickness. The qua-
lity of the radiograph can be optimised by
applying filters and screens, see table 1-18.
Fig. 6-18. Tangential technique for on-stream radiography
Table 1-18. Selection of source, screen and filter for the various areas in figure 7-18.
filter
film
Fig. 7-18. Areas of application for selection of source,
screen and filter in on-stream radiography
This graph appears enlarged in theappendix.
diameter in mm
wall thickness
Zone Source type Source size Screens Filter
front and back
I Iridium192 2 mm 0.027 mm Pb 1 mm Pb
II Iridium192 2 mm 0.027 mm Pb 2 mm Pb
III Cobalt60 3 mm 0.5 mm Cu of RVS 1 mm Pb
IV Cobalt60 3 mm 0.5 mm Cu of RVS 2 mm Pb
V Cobalt60 4 mm 0.5 mm Cu of RVS 4 mm Pb
Correction factor =
(2.5 x D
insulation
-0.5 x D
insulation
) / 2.5 x D
insulation
= 0.8
D
insulation
FFD
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