Repair and reconstruction of engineering structures - Assessment and Refurbishment of Steel Structures - page 258

Figure 6.96 Corrosion damage in the
tank in Figure 6.95 from the inside
Therefore corrosion on the bottom should be analysed as a process characterized by
variable intensity, but occurring on both sides. This combined estimate of corrosion
losses is of greater significance around the peripheral of the bottom zone.
Assessment of corrossive damage may be carried out when the tank is full, if acoustic
emission methods are used. This involves registering by a set of sensors the sound waves
generated by the leakage of liquid through the tank bottom or the corroded plates.
However, using this method is difficult and so is not often chosen.
Recently developed methods of determining bottom damage caused by corrosion are
based on measurement of steel plate thickness by ultrasonic techniques and the use of an
appropriate measuring grid. With this method the tank has to be emptied. The method
must be regarded as not very reliable, even if a dense measuring grid is used, because the
point measurements do not guarantee that the areas indicated are the ones most exposed
to perforation hazards.
Reliability in this respect is secured by measurements performed over all of the
bottom. The measurements can be made by means of testers, which have been used for
several years. These techniques can detect local plate thickness variations (corrosion
pits), by the variable intensity in the magnetic flux generated by the tester. The tester is
mounted on a trolley and looks like a lawn mower. Its relatively small size and weight of
50 kg enable it to fit easily through a side manhole (Figure 6.97). Power is supplied either
by a cable from a generator placed outside the tank or a battery inside the tester. The
tester is equipped with a dozen sensors to monitor the magnetic flux variations caused by
differences in the bottom plate thickness. Thus, by moving the tester over the bottom
surface it is possible to register the plate thickness at every place on the bottom surface.
Sensors can be freely mounted on the trolley. As a result of such mounting, the areas
adjacent to the overlapping steel plate edges and other irregular surfaces can be inspected
as well. Prior to measuring, the tester is calibrated by means of master templates whose