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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.
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12th EPRI Balance-of-Plant Symposium August 6th-8th, 2012
Inspection of stainless steel heat exchanger tube with eddy current array probe
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
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.
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.
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 1 Time slot 2 Time slot 3 Time slot 4 Time slot 5 Time slot 6 Time slot 7 360 °
0 °
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.
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
Taper Defects
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Since the probe provides an absolute signal, it is possible to see a representative profile of the defect.
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
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°
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
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
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.
Data from real tube
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Deep OD defect, large volume
Data from real tube
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Shallow isolated OD pits
Data from real tube
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OD general corrosion
Data from real tube
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OD Pit close to the TSP not detected with the bobbin
Data from real tube
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OD Isolated Pit / Corrosion – Field Tube
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
