Guidance for performing Ultrasonic Residual Thickness Evaluations

When performing an inspection of a storage tank (or other structure), ultrasonic thickness gauging has been the preferred method for determining residual thickness of the steel plates, pipes, seams, etc.  The thickness of the material can be determined from one side using a digital thickness gauge.

This equipment uses high frequency should (ultrasound), which travels at a know, regular velocity in a given material.  An electrical pulse from the thickness gauge is converted into ultrasound by the piezo electric crystal in the transducer assembly which  is in turn coupled into the test  piece.  The ultrasound travels through the test piece, reflects off of its back surface and returns to the transducer where it is converted back into electrical energy.  The interval between the transmitted and received signal or multiple secondary reflected signals, is referenced to the sound velocity in the material tested to produce a digital thickness measurement.

The standard inspection procedure has been to measure the residual thickness at least five locations per floor and shell plate.  Additional thickness measurements are obtained at suspect or locate areas of thinning.

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