Eddy current (ECT) non-destructive testing (NDT) can be used for inspecting tubes from the outer diameter (OD) generally during manufacture and from the inner diameter (ID) generally during in-service inspection, specifically for heat exchanger inspection.
ID Heat Exchanger Tube Testing
Heat exchangers employed for power generation or petrochemical applications may have several thousands of tubes, each with a length of up to 20 m. These tubes can be inspected at a high speed (up to 1 m/s with computerized data analysis) with the help of a differential internal diameter (ID or “bobbin”) probe, and phase analysis can be used to evaluate flaws such as pitting to an accuracy of around 5% of tube wall thickness. This enables accurate estimation of the tube’s remaining life, thus enabling operators to select the suitable action, such as tube replacement, tube plugging, or replacement of the entire heat exchanger.
The material and the wall thickness of the tube govern the operating frequency is calculated by, extending from a few kHz in the case of a thick-walled copper tube, up to about 600 kHz in the case of thin-walled titanium. Generally, this technique is used for the inspection of tubes up to about 50 mm in diameter. It is impossible to inspect magnetic stainless steel or ferrous tubes with typical eddy current inspection equipment.
Generally, dual or multiple frequency inspections are employed for inspecting tubes, specifically for suppressing undesirable responses caused by tube support plates. When the result of a lower frequency test (which leads to a proportionately greater response from the support) is subtracted, a mixed signal is generated, which exhibits minimal or no support plate indication, thereby enabling the evaluation of minor defects in this area. Additional frequencies may be incorporated to decrease noise from the internal surface.
In-Line Inspection of Tubing
In general, external eddy current encircling test coils are employed for testing superior quality metal tubing of wall thicknesses under 6 mm. If the tube is made of a magnetic material, then there are two major challenges:
- Due to the high permeability, there is minimal or no penetration of the eddy current field into the tube at practical test frequencies
- Differences in permeability (from many causes) create eddy current responses which are in higher orders of magnitude than those from defects
These challenges can be overcome by magnetizing the tube with a strong DC field. This minimizes the effective permeability to a small value, thereby increasing the penetration depth and concealing the permeability differences, and hence enabling efficient inspection.
Generally, ferromagnetic tubing up to about 170 mm in diameter is inspected with magnetic saturation and encircling coils. Inspection may be in-line during production or offline on cut length tube.
While tubes are welded (generally by the ERW method), the weld region is the common spot of defects, and since the weld position is completely controllable, it is more effective to test the weld region only using a sector (or saddle) probe.
This information has been sourced, reviewed and adapted from materials provided by Ether NDE Ltd.
For more information on this source, please visit Ether NDE Ltd.