Multi-frequency and Multichannel Technique_ – ASME Article 8
Exam of tubes of heat exchangers of non-magnetic materials such as stainless steel, copper, aluminum and titanium alloys. The tubes are examined by internal side and have the major definition and precision between non-destructive methods available. It is possible to distinguish accurately the depth of pits and cracks in internal and external surfaces of tubes up to 95% of thickness.
This is a technique of electromagnetics method which consists of scanning of all the length of pipe using one or more probes which generate magnets fields which induce Eddy currents in examined tubes.
The presence of discontinuities causes a disturbance in impedance in the testing system which is correlated to artificial defects with known dimensions, induced in standard tubes, similar to the tested ones. This correlation is represented by “ impedance plan”.
Among all techniques, this is one which presents greater sensitivity and versatility, once it is capable of detecting and sizing (with some limitations), all types of damages normally found in tubes , from losses of localized and external thickness to circumferential, longitudinal and branched cracking through equipment use and updated software which allow for the operation with multiple frequency, absolute and differential multiple channels and various types of probes (flexible, pancake, rotating, weld scan, bobbin, etc – See picture 20).
The main problem observed in the inspections by eddy-current, arrives when tubesheets or support plates are made with materials similar to the tubes . This situation can cause “distortions” in signals which limit the detection and the sizing of damages.