Eddy current testing for heat exchanger tubes based on bias-magnetisation

The regular inspection of heat exchanger tubes in industrial pressure vessels is imperative, with the detection and characterisation of both inner diameter (ID) and outer diameter (OD) defects being of paramount importance. Owing to the limited diameter of heat exchanger tubes, conventional non-destructive testing methods, including saturated magnetic flux leakage (MFL) and eddy current testing (ECT), exhibit significant limitations in meeting the required detection criteria. Therefore, based on the eddy current method with biased magnetisation, the paper uses differential coils to detect permeability changes on the inner diameter surface caused by defects, enabling detection of both ID and OD defects. This paper analyzes the principle of magnetised eddy current testing, establishes a coil impedance model, constructs a simulation model of a permanent magnet biased AC electromagnetic field, and develops a multi-channel impedance detector. The findings from simulation and experiment demonstrate the method’s capability for concurrent detection of both ID and OD defects, and its ability to differentiate them via pronounced phase shifts in the signal response. The improved signal distinction between ID and OD defects achieved by the biased magnetisation method, compared to conventional eddy current testing, translates to significant potential for application in heat exchanger tube diagnostics.