RADIOGRAPHY

Radiography is a nondestructive test method that utilizes electromagnetic energy (radiation) from X-rays or gamma rays; it detects both surface and internal discontinuities. Radiographs are two-dimensional negative images (projections) of objects on radiation sensitive films.

X-ray generators or "tubes" transform electrical energy to produce x-rays. They stop emitting when the power is switched off. In contrast, decaying radioactive "isotopes" emit gamma rays continuously. Cobalt 60 and Iridium 192 are the more commonly used industrial gamma ray sources. They are stored and transported in shielding devices known as exposure devices.

Radiography can be performed on most metallic and non-metallic materials, such as weld and weld overlays, castings, forgings, valves and their components, machined parts, pressure vessels, and structural steel. Radiography is commonly performed in power plants, gas plants, pulp mills, refineries, and on pipelines. Radiography shadow shots can be used to determine the extent of internal and/or external corrosion/erosion in process piping, pressure vessels and valves. Radiographic film provides an easily viewed and permanent record of the component that has been tested.

Radiographs show discontinuities in two dimensions only. To determine the depth of a discontinuity additional tests, such as ultrasonic inspection may be required. For radiography to be performed access to both sides of the subject material is required, as the film must be on one side with the source on the opposite side. The exposed film is processed similarly to normal photographic film; it is viewed with a high intensity white light viewer.

The part thickness determines the radiation source to be used. The practical limit for carbon steel gamma radiography is approximately 2.5" for Iridium 192 and 9" for Cobalt 60. With x-rays, the x-ray generator energy determines the maximum thickness penetrated. Commonly used portable X-ray tubes can readily penetrate up to 1.5" of carbon steel. Thicker materials require much larger fixed facility X-ray generators.

Safety is a fundamental consideration when using either type of radiation. X-ray and gamma radiation are known as ionizing radiation that penetrates solid matter significantly. Both require clearance of large areas adjacent to the component from persons or items that may be affected by radiation. A significant radiation dose can cause physical changes in the human body. Strict Federal and Provincial regulations, coupled with highly trained and qualified personnel ensure the safety of everyone.

Test results are interpreted in accordance with specifications or customer requirements. The customer receives the final radiographic film, a report and drawings (if required) identifying the location of discontinuities.