Shear Wave UT – Applications and Limitations

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UT is a very efficient NDE method. A large amount of data for evaluating the integrity and reliability of a vessel can be obtained in a relatively short period of time, without requiring extensive preparation of the vessel or interfering with other work in the area.

Longitudinal wave UT is applicable for determining the remaining wall thickness of a corroded pressure vessel under almost any circumstances, and most UT technicians have the skill and experience to perform these examinations. Longitudinal wave UT will also detect and locate hydrogen blisters, or similar internal flaws. Shear wave UT is very useful for detecting cracks and provides essentially the only method for determining the size (depth) of cracks with sufficient accuracy for making a fitness-for-service analysis to evaluate the integrity and reliability of a vessel.

The greatest limitation on the use of UT for in-service inspections is that the accuracy of the data obtained is highly dependent on the skill and expertise of the technicians performing the examinations. This is especially true for detecting and sizing cracks by shear wave UT. Qualification of procedures and certification of technicians is not sufficient to guarantee acceptable results. Only those vendors should be used that can demonstrate that they have the required skill and expertise to provide accurate data. Samples of various types of cracks that have developed in pressure vessels during service should be saved for future use as “standards” for the qualification of UT procedures and technicians for in-service inspections.

Another limitation on the use of UT for in-service inspection is that it was not used during the construction of most of the vessels that are now operating. Consequently, many indications of flaws detected by UT are very difficult to classify as either innocuous fabrication flaws or more serious indications of deterioration occurring during service. UT procedures are available that can usually distinguish between fabrication flaws and those that have developed during service, but they require highly skilled UT examiners to properly apply. The increasing use of UT during the construction of vessels, as provided for in PVM-MS-4750 and PVM-MS-4749, included in this manual, may eventually alleviate this difficulty.

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