Ultrasonic Examination – Remaining Thickness

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Figure 700-16 illustrates the use of longitudinal UT for determining the remaining thickness of a corroded shell. If the transducer is placed on a location of the shell component that is not corroded, as depicted by the position of Transducer A in Figure 700-16, a back reflection will be observed at a distance equal to the original thickness of the shell. On the other hand, if the transducer is placed on a location where internal corrosion has occurred, as depicted by the position of Transducer B in Figure 700-16, a back reflection will be observed that is less than the original thickness. The back reflection from a corroded surface of a vessel shell component can have an amplitude below the DAC, because irregularities in the corroded surface can scatter some of the reflected longitudinal wave.

Determination of Remaining Thickness of Corroded Pressure Vessel Shell with Longitudinal Wave UT Calibrated

The distance of the back reflection on the oscilloscope display is the remaining thickness of the shell component, and the size of the corroded area can be determined by moving the transducer along the surface until the back reflection returns to the original thickness.

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