Operating Manual

201200
Exposure time
Obviously different exposure times are required for gas filled or liquid filled pipelines.
Below are a few examples.
For gas filled pipelines:
Depending on diameter and wall thickness : Iridium192 or Cobalt60, see figure 7-18
Focus-to-film distance : minimum 3 x D
insulation
Irradiated thickness : 2 x nominal wall thickness
Film type : minimum C5 (EN584-1)
Film density : minimum 2.5 in the centre of the pipe projection
For liquid filled pipelines:
Depending on the diameter, wall thickness : Iridium192 or Cobalt60
Focus-to-film distance : minimum 3 x D
insulation
Irradiated thickness : 2 x nominal wall thickness plus steel
equivalent of the pipe content
Film type : minimum C5 (EN584-1)
Film density : minimum 2.5 in the centre of the pipe projection
The steel equivalent of the pipe content is determined as follows:
(specific density in kg/m
3
of content) / (specific density in kg/m
3
of steel) x internal diameter
= .... mm of steel
Density of steel = 7.800 kg/m
3
Density of content (oil and aqueous liquids) = 800 to 1.000 kg/m
3
Fig. 8-18. Preparations for on-stream radiography
Fig. 9-18. On-stream radiography of pipe with corrosion
Notes:
• In the most commonly used insulation materials absorption is negligible.
• The long exposure times cause over-irradiation at the edge of the pipe. As a result
the pipe wall shows up ‘thinner’.
Figure 8-18 shows preparations for on-stream radiography being made. The end piece for
the gamma-source is positioned above the pipe, while the flat film cassette is placed below.
Figure 9-18 shows an on-stream radiograph of a pipe with severe pitting corrosion. Since the
introduction of digital radiography the CR-method, using storage fosforplates, is rapidly
becoming an alternative for traditional film. The main advantage being that it reduces the
exposure time by a factor of up to 10, or if weaker sources can be applied a reduced con-
trolled area which is very attractive in cramped spaces e.g. offshore platforms, see chapter 16.
In industry this area is often called “safety area”, which is wrong, to the contrary, it is an
unsafe area with highest radiation close to the source.
18.7 Flash radiography
Flash radiography or pulse radiography can be carried out when information is required
about the condition of the outer surface of an insulated pipe, without having to remove the
insulation.
Figure 10-18 shows flash radiography in progress. Since only the aluminium cladding and
insulation need to be penetrated, relatively soft radiation is applied, while exposure time is
limited to only a fraction of a second. In that time sufficient “pulsed radiation” is generated.
to create an image on the superfast F8 + NDT 1200 (film+screen combination) see section
6.3. It is safe to make radiographs manually without the need for additional safety arrange-
ments. Systems up to 200 kV exist (Golden Inspector) suitable to penetrate 10 mm wall
thickness of steel.
Fig. 10-18. Flash radiography of an insulated pipe section