Eddy current testing (ECT) is a commonly used technique to inspect non-ferromagnetic heat exchanger and condenser tubing. The typical bobbin probe configuration has proven to be efficient at detecting volumetric flaws such as pitting, fretting, erosion and general corrosion. However, this probe design faces a major limitation with respect to cracking defects especially along the circumferential axis of the tube. Motorized rotating pancake coil (MRPC) probes have been developed to overcome the limitation of circumferential cracking and to provide a high resolution mapping of the tube, sensitive to all types of flaws in any orientation. However, this inspection technique remains very slow and not appropriate for inspecting the entire length of the tube. Eddy current array (ECA) tubing probes were introduced in the late 1990s with success for the inspection of Steam Generators. However, these probes are very specialized, equipment and software-specific, and cost prohibitive outside the scope of SG applications. Interest has recently increased with regards to circumferential cracking resulting in high interest in ECA probes. Manufacturers are responding by proposing various “arrays” which can be categorized as non-multiplexed or multiplexed array probes. Non-multiplexed probes are generally limited to a combination of bobbins and coils equal to the number of physical channels of the source ECT tester (usually 4 or 8). This category also includes Air Conditioning (AC) probes. Multiplexed probes leverage the physical inputs of an ECT instrument via time slots and can physically accommodate up to 256 channels. This paper provides the laboratory and field results of bobbin probes, non-multiplexed array probes, and multiplexed array probes. The performance of the various techniques is compared and other inspection requirements such as C-Scan imaging are described in details.

1. 12th EPRI Balance-of-Plant Symposium
August 6th-8th, 2012
Inspection of stainless steel heat exchanger
tube with eddy current array probe

2. Introduction
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 Context
 Eddy Current Array + Bobbin probe design
 Overview
 Operating principle
 Result on various calibration defects
 Comparison with low resolution array probe
 Results on real tube
 Conclusions

3. Context
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 Non ferrous heat exchanger and condenser tubes may be affected
by several damage mechanisms:
 Pitting
 Erosion
 Fretting
 Cracking
 The bobbin probe inspection faces major limitations with respect
to:
 Detection and sizing circumferential cracking
 Evaluate circumferential extent of volumetric flaws
 Rotating probe can be use to improve the inspection but this
solution remains slow
 The Eddy Current Array + Bobbin probe technology is the solution
to perform high resolution inspection at the bobbin speed in a
single pass.

4.  One-Pass “Combination” Bobbin plus Array Probe
 Bobbin probe
 High resolution oval-coil array 1
 Centering devices
DefHiTM Probe1
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1 US Patent Pending - Eddyfi NDT Inc.

5. Operating mode
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 Oval-coils array + Bobbin
 12 transmitter
 12 receivers
 2 bobbins
 Excitation pattern
 2 poles
 2 skip coils
 6 time slots for the array
 1 time slot for the bobbin
Time slot 1Time slot 2Time slot 3Time slot 4Time slot 5Time slot 6Time slot 7
360 °
0 °

6. Results for ASME standard
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 Calibration on the ID
groove
 OD Pits detected on at
least 2 channels
 Sizing based on phase to
depth curve.

7. Circumferential Cracks
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 Detection of external EDM
notch (0.005“ width)
 80% x 50°
 60% x 50°
 40% x 50°
 40% x 25°
 20% x 50°
 Bobbin results for the
same defects

8. Taper Defects
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 Since the probe provides
an absolute signal, it is
possible to see a
representative profile of
the defect.

9. Shallow Small Defects
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 The high resolution of the probe allows a better detection of
small isolated pit
 20% ID round bottom hole, 0.125” diameter
 20% OD flat bottom hole, 0.125” diameter

10. Defects at Support Plate
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 Wear scars
 60%, extent: 270°
 20%, extent: 270°
 OD pits
 80%, diam.: 0.375”
 30%, diam.: 0.312”
 Circumferential crack
 60%, extent: 50°

11. Sensitivity vs Lift-Off
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 Defect phase angle is constant even with high lift-off
variation providing reliable depth sizing.
Probe against
the tube wall
1mm (0.040”)
lift-off

12. 8 channels array
Large amplitude variation
on the same defect
depending on its position
DefHi probe with 24 channels
Constant amplitude
whatever the position of
the defect
High Resolution Value
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 Reliable and repeatable detection
ASME hole scanned 3 times with 15° rotation

13. High Resolution Value
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 Improved defect representation in C-scan
ASME 40% FBH and 4 x 20% FBH
DefHi probe with 24 channels
Several coils see the defects
providing uniform response
8 channels array
Only one coil can see the defects
resulting in non uniform signature.

14. Data from real tube
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 Deep OD defect, large volume

15. Data from real tube
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 Shallow isolated OD pits

16. Data from real tube
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 OD general corrosion

17. Data from real tube
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 OD Pit close to the TSP not detected with the bobbin

18. Data from real tube
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 OD Isolated Pit / Corrosion – Field Tube

19. Conclusions
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The DefHi eddy current array probe provides several
benefits for non-ferrous tubing inspection:
 One-pass inspection providing bobbin and array probe
signal
 High definition which provide reliable and repeatable
detection of small defects whatever the position around the
circumference
 High sensitivity to pitting and circumferential cracking (ID
and OD circ. crack detected down to 20% depth and 25°
extension)
 More reliable characterization of flaws with C-scan imaging
 Improved detection of circumferential cracking under tube
sheets