How Can You Use Eddy Current NDT for Tube Inspection?
Tubes may be inspected using eddy current (ECT) non destructive testing(NDT) from the outer diameter (OD), usually at the time of manufacture and from the inner diameter (ID), usually for in-service inspection, particularly for heat exchanger inspection.
ID Heat Exchanger Tube Testing
Heat exchangers used for petrochemical or power generation applications may have many thousands of tubes, each up to 20 m long. Using a differential Internal Diameter (ID or 'bobbin') probe, these tubes can be tested at high speed (up to 1 m/s with computerised data analysis) and by using phase analysis, defects such as pitting can be assessed to an accuracy of about 5% of tube wall thickness. This allows accurate estimation of the remaining life of the tube, allowing operators to decide on appropriate action such as tube plugging, tube replacement or replacement of the complete heat exchanger.
The operating frequency is determined by the tube material and wall thickness, ranging from a few kHz for thick-walled copper tube, up to around 600 kHz for thin-walled titanium. Tubes up to around 50 mm diameter are commonly inspected with this technique. Inspection of ferrous or magnetic stainless steel tubes is not possible using standard eddy current inspection equipment.
Dual or multiple frequency inspections are commonly used for tubing inspection, in particular for suppression of unwanted responses due to tube support plates. By subtracting the result of a lower frequency test (which gives a proportionately greater response from the support) a mixed signal is produced showing little or no support plate indication, thus allowing the assessment of small defects in this area. Further frequencies may be mixed to reduce noise from the internal surface.
In-line Inspection of Tubing
External eddy current encircling test coils are commonly used for inspecting high quality metal tubing of wall thicknesses less than 6 mm. When the tube is made of a magnetic material there are two main problems:
Because of the high permeability, there is little or no penetration of the eddy current field into the tube at practical test frequencies.
Variations in permeability (from many causes) cause eddy current responses which are orders of magnitude greater than those from defects.
These problems can be overcome by magnetising the tube using a strong DC field. This reduces the effective permeability to a low value, thus increasing the depth of penetration and masking the permeability variations, hence allowing effective testing.
Ferromagnetic tubing up to around 170 mm diameter are commonly tested using magnetic saturation and encircling coils. Testing may be in-line during manufacture or off line on cut length tube.
When tubes are welded (usually by the ERW method) the weld area is the usual site of defects and as the weld position is well controlled, it is more efficient to inspect the weld area only by means of a sector (or saddle) probe.
If you have a tube inspection application issue that ECT non destructive testing could help with, please get in touch at firstname.lastname@example.org.
Find out what ETher NDE equipment and probes would be perfect to use for Tube Inspection by using our Eddy Current NDT Applications table.
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