On-stream inspection can be carried out on pipes, valves, vessels, and distillation columns while in operation, in order to establish the degree of deterioration of the system either the projection or the tangential technique can be used. Since the introduction 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 reduced safety 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 correction factor, which is the true diameter (D) divided by the radiographic diameter Df . 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 Dinsulation and a source size of 3 mm, image quality requirement A of EN 1435 is met.
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-stream radiography it is important to know the direction of the product flow, so that a existence of localised wall thickness reduction can be better deduced. Films of 30 x 40 cm are generally 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 Dinsulation is chosen. The correction factor would then be: (2.5 x Dinsulation -0.5 x Dinsulation) / 2.5 x Dinsulation = 0.8. Selection of source, screens and filters The graph in figure 7-18 indicates which radioactive source is the most suitable, depending on pipe diameter and wall thickness. The quality of the radiograph can be optimised by applying filters and screens, see table 1-18.
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 Dinsulation 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 Dinsulation 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/m3 of content) / (specific density in kg/m3 of steel) x internal diameter = .... mm of steel Density of steel = 7.800 kg/m3 Density of content (oil and aqueous liquids) = 800 to 1.000 kg/m3
- 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 redu- ces the exposure time by a factor of up to 10, or if weaker sources can be applied a reduced safety area which is very attractive in cramped spaces e.g. offshore platforms, see the sections about Digital Radiography (e.g. this section)