Applications and Limitations of MT
MT is preferred to PT for detecting surface cracks because of its greater sensitivity, especially when wet particles are used. Very fine cracks can be detected by MT that escape detection by PT. This makes MT especially useful for vessels that have been exposed to process environments that contain H2S under conditions that can cause stress-corrosion cracking.
Although MT can detect subsurface cracks that are not too far below the surface, do not depend on MT alone to detect internal cracks (see Section 730).
Other methods more suitable for detecting internal flaws, such as UT and RT, should be employed to supplement MT. This is especially true for relatively thick (approximately 1.5-inch thick and greater) vessel shell components. It is more likely that cracks may originate internally in thick shells, and it will normally take longer for them to propagate to the surface where they can be detected by MT.
Similar to PT, MT does not give a reliable indication of the depth of a surface crack, which is the primary characteristic that affects the integrity and reliability of a pressure vessel. Therefore, the depth of cracks detected by MT should be determined by other NDE procedures (such as UT) to evaluate their significance, unless the MT indications are removed by grinding.
MT procedures can vary widely in reliability. They should be qualified for an inservice inspection of a pressure vessel by demonstrating their performance with sample flaws representative of the actual flaws that may have occurred in the vessel.
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