Acoustic Emission Testing – Applications and Limitations

AE can be used for the in-service inspection of pressure vessels to detect cracks that have resulted from any form of deterioration during service that can cause cracking. It can be a very sensitive NDE method for detecting cracks, but the reliability of the test results is highly dependent upon the qualifications of the vendor conducting the test. It makes no difference if the cracks are internal or surface, very fine or filled with corrosion scale, or are in vessel components with complex geometries. However, the deterioration of the vessel must reach the stage where cracks have developed for AE to detect the deterioration. It will not detect the initial stages of creep or hydrogen attack (see Sections 738 and 739) that could lead to the development of cracks shortly after a vessel is returned to service.

The greatest benefit of AE is that a single test will detect and locate cracks that have developed in any component or weld seam in a pressure vessel. Full coverage inservice inspection of a vessel cannot be obtained with any other NDE method in as short a time. However, it must be understood that AE will detect and locate cracks only. It provides no information that can be directly used for a fitness-for-service analysis, or to estimate the remaining life of the vessel. Other NDE methods, such as UT or RT, must be used to determine the size and orientation of cracks detected by AE. Nevertheless, AE is a valuable NDE method because it pinpoints the locations where these other examinations have to be made, and, therefore, can significantly reduce the total time required for in-service inspection.

The greatest disadvantage of AE is that the pressure vessel must be prepared for a hydrostatic pressure test. This should not be thought of as a deterrent to using AE, particularly if full coverage in-service inspection is desired and the results of the test can be further evaluated with other NDE methods at specific locations.

Alternative procedures have been used to develop a stress sufficiently high to cause acoustic emissions at flaws (such as thermal stresses during cool-down or increasing the operating pressure 10% to 20% during service), but they may not always provide reliable AE test data.

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