Radiographic Examination – Applications and Limitations
It is sometimes thought that RT is the best NDE method that can be used for detecting flaws, because it is mandated by the ASME Code for certifying the quality of newly constructed pressure vessels. This is not true. RT has attained its preeminence in the ASME Code by virtue of the evolution of weld joint efficiencies and permitted design details around the types of fabrication defects that can be readily detected by RT.
The reliance of the ASME Code upon RT should not be construed to imply that it is the optimum NDE method to use for the in-service inspection of vessels. In fact, most of the flaws that can develop as a consequence of the deterioration of a vessel during service can be better detected by other NDE methods.
One circumstance where RT can be used to considerable advantage is when a direct comparison is desired between the present condition of a vessel and its condition when new, and other NDE methods were not used during construction to provide baseline data.
A significant limitation upon the use of RT for in-service inspection is that it will only detect cracks that are essentially parallel to the direction of the incident radiation, and have a sufficient width to be visible in the radiograph as limited by the grain of the film. The detectability of cracks diminishes greatly as they deviate further from this orientation. Taking several radiographs with different angles of incident radiation can overcome this shortcoming, but this would considerably increase the time and cost of the inspection.
RT does not give a reliable indication of the depth of a flaw through the shell of a pressure vessel. Therefore, the depth of a crack detected by RT must be measured by another NDE method, such as UT, to evaluate the effect of the crack upon the integrity and reliability of the vessel.
RT is also severely limited for the in-service inspection of nozzle openings and welds, which tend to be locations of relatively high stress where deterioration during service is likely to occur. Nozzles are usually fabricated from plate and forging or pipe materials with different thicknesses. Satisfactory radiographs of nozzle openings can rarely be obtained, because the workpiece must have an essentially uniform thickness for the variation in density of the radiograph to be within an interpretable range.
RT is the most time-consuming and expensive of all NDE procedures. The set-up time for the equipment is usually much greater than the time required for the exposure, and this must be followed by development of the film in a darkroom and interpretation of the resulting radiograph. Several man-hours can be required for each exposure. Additional time and cost penalties are incurred indirectly by restricting and delaying other work in the area due to the serious radiation hazard associated with RT. Therefore, the suitability of other NDE methods for detecting the forms of deterioration that might have occurred during service should be investigated before RT is employed.
Despite all of the disadvantages associated with the use of RT for the in-service inspection of pressure vessels, there can be no dispute that a radiograph can provide very valuable data concerning the integrity and reliability of a vessel. The radiographs provide permanent records that can be compared to the results of future inspections or reinterpreted in the light of new information concerning the deterioration that can occur during service.
Categories: In-Service Inspection | Tags: Radiographic | Leave a comment