Measurement Principle

The TBIT conducts a metal loss survey by means of the high-resolution flux leakage method (MFL). The steel bottom plates are magnetized to saturation by high-performance permanent magnets. Under normal conditions (i.e. no flaws), the magnetic flux can travel through the plates undisturbed. In contrast, the flux ‘leaks’ out of the plates, if either internal or external metal loss is present. This leakage is then detected by one or more of the 72 hall-effect sensors installed on the TBIT.

The MFL amplitude used for determining metal loss geometry strongly depends on the length and width of a feature. Thus an automated evaluation has to incorporate into the sizing algorithm not only the signal amplitude, but also the signal width and length. In contrast to a simple amplitude trigger which tends to miss small but deep pitting defects, the sizing algorithm implemented in the TBIT system is the basis for a realistic online depth-threshold criterion. The TBIT is therefore capable of detecting small and deep pitting while rejecting large but uncritical features as irrelevant.

Figure  1
Flux line distribution of tank floor with internal flaw.

Figure  2
Flux line distribution of tank floor with external flaw.