In Part 1 of this series we reviewed the principle of magnetic flux; and in Part 2 we show how this natural phenomenon is applied to detecting defects in pipes. The key is to remember that Gauss’ Law states that for an enclosed volume in a magnetic field, the total flux in must equal the total flux out.
The literature seems to indicate that when a pipe is uniform, the magnetic field is also uniform. Thus, a uniform signal is sent to the sensor within the smart pig as it travels down the pipe. However, as shown in the Figure above, when there is a defect in the pipe, the field becomes nonuniform. The flux goes around the defect and this is called magnetic flux leakage.
When the magnetic field lines go around the corrosion or erosion, the sensor picks up an increase in the magnetic field (remember the flux is still a constant) as is shown in the above Figure. This magnetic field intensity deviates from the baseline number that the sensor has experienced so far along the healthy part of the pipe. The change in B is the signal indicating an anomaly in the pipe.
Come back for Part 3 of this series where we shall explore how the sensor actually measures the flux signal.