This is a technique of electromagnetics method, which consists of inspection of all the length of tube using a probe built with two strong permanent magnets. These magnets generate a magnetic field which saturate the tube wall.
An absolute probe measures the time variation of magnetic field caused by thickness losses.
When a failure is found between two magnets, a perturbation in magnetic field takes place resulting in “leaking” of a small quantity of magnetic flow for the internal diameter of the tube.
This flow is detected by a differential bobbin, located between magnets. A bobbin put in the end of probe detects the residual magnetism and allows the discrimination between internal and external damages.
The main advantages in this technique are:
- Inspection of finned tubes
- It allows the inspection of tubes partially embedded
- It detects pits, circumferential crackings and thickness loss
- The presence of support plates and tubesheets is not very sensitive.
- It allows high speed of data acquisition
- The interpretation of results is relatively simple
The main disadvantages are:
- Sizing is precarious, it is recommended specially for detection of damages
- The fill factor required is similar to eddy-current
- It requires manufacturing of standard tubes to each type of examined tube