Radiographic testing (RT)
Radiographic testing is a nondestructive examination (NDE) method of examining materials for hidden flaws by using the ability of short wavelength electromagnetic radiation (high energy photons) to penetrate various materials.
The specimen to be inspected is placed between the source of radiation and the detecting device, usually the film in a light tight holder or cassette, and the radiation is allowed to penetrate the part for the required length of time to be adequately recorded.
The result is a two-dimensional projection of the part onto the film, producing a latent image of varying densities according to the amount of radiation reaching each area. It is known as a radiograph, as distinct from a photograph produced by light. Because film is cumulative in its response (the exposure increasing as it absorbs more radiation), relatively weak radiation can be detected by prolonging the exposure until the film can record an image that will be visible after development. The radiograph is examined as a negative, without printing as a positive as in photography. This is because, in printing, some of the detail is always lost and no useful purpose is served.
In the oil and gas industry, radiographic testing is typically used to inspect weld repairs, pressure vessels and valves.
Advantages of Using Radiographic Testing
Radiography has several advantages over other NDE techniques. It is highly replicable, can be employed across a wide-range of materials, and the data gathered can be stored for future analysis. Radiographic testing requires minimal surface preparation and many radiographic systems are portable and can be deployed in the field and used at elevated positions.
Additionally, radiographic testing can provide trained personnel with a permanent record of weld quality.
Type of Radiography
Computed radiography (CR) employs a phosphor imaging plate instead of film that is used in conventional radiography techniques.
Real-time radiography (RTR) is performed in real-time, and works by emitting radiation directly through an object
Direct Radiography (DR) is similar to computed radiography. The primary difference is in how the image is captured. In this case, a flat panel detector is used for image capture with the resulting image displayed on a computer monitor.
Computed tomography (CT) takes multiple two-dimensional radiography scans (in the range of hundreds to thousands) and superimposes these to generate a 3D radiographic image.
This technique can provide 3D cross-sections and volumetric images of an object