EDDY CURRENT TUBE INSPECTION
Eddy Current Testing is ideally suited to the inspection of non-ferromagnetic tubing. The principle of this inspection is based on electromagnetic induction.
An alternating electric current can induce a magnetic field and a magnetic field can induce an electric current.
A probe with a thin copper wire wound around an electrically non-conductive probe is energized within the bore of a conductive tube.
The current traveling through the windings generates a magnetic field which in turn induces a small electric current in the tube wall (eddy currents).
Eddy Current inspection is a very fast technique with data acquisition rates of up to 2m/sec being feasible, making it an ideal technique for the inspection of equipment with large numbers of tubes as the case with turbine condensers and distillers, for instance.
The use of multi frequency equipment allows data capture at a number of frequencies and modes (absolute and differential) simultaneously.
The generalized wall thickness can be measured down to an accuracy of ±0.05mm while localized defects as small as 1.0mm in diameter can be measured to an accuracy of ±10% of wall thickness.
System calibration is carried out on a calibration tube of the same material and dimensions as the tube being inspected into which artificial defects are machined in accordance with ASME V Article 8.
Data analysis is complex and in this regards Steel Test has a number of personnel with more than 10 years experience and have analysed hundreds of thousands of tubes during their careers.
In the majority of inspections the standard bobbin probe is used. For special applications such as the detection of defects inside the area of the tubesheet specialized probes such as rotary pancake probes can be utilised.
Tubes are classified in accordance with the deepest defect detected along the tube length. (Different reporting formats are available on request from the client). Using specialized software the analysed data is imported into tubesheet drawings in which the tube status is shown by colour coding.
For the best results the tube should be as clean as possible as the closer the probe diameter is to the bore of the tube the higher the sensitivity that can be achieved.