Heat exchangers are used to heat and cool various fluids across a wide spectrum of industries. This is especially critical to the power generation and petrochemical industries where fixed equipment reliability is paramount. Tubes in heat exchangers made of such materials as austenitic stainless steel and Inconel are plagued by circumferential cracking, which conventional, single-coil eddy current testing (ECT) cannot reliably detect. This application note explores how an innovative probe technology is able to address this challenge. Non-ferromagnetic heat exchanger tubing is susceptible to circumferential cracking in the vicinity of the tubesheet. These cracks are induced by the localized stress created during the tube-to-tubesheet rolling process. Circumferential cracking is very difficult to detect and characterize with standard eddy current bobbin probes, currently the norm in the industry to inspect tubes, because: • bobbin probes are very sensitive to tube expansion and ferromagnetic materials, out of which tubesheets are usually made. When analyzing bobbin coil data, it is very difficult to isolate a small-volume crack from a complex signal comprised of a crack, a geometry change, and added material; • the orientation of standard bobbin coils is not well suited to circumferential cracks because eddy currents flow parallel to the indications.

1. Assessing Circumferential Cracking in
Non-Ferromagnetic Heat Exchanger
Tubes
Application Note

2. Heat Exchangers
Heat exchangers (HX) are used to
heat and cool various fluids across a
wide spectrum of industries.
• HX are especially critical to:
• Power generation
• Oil, gas, and petrochemical
• Tubes in HX are sometimes made from nonferromagnetic steels like:
• Inconel™
• Austenitic stainless steel
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3. The Challenge
Detecting, sizing, and positioning circumferential cracks in
the vicinity of the tubesheet of non-ferromagnetic HX tubes.
•
Circumferential cracking is very difficult to detect and characterize with
standard EC bobbin probes because:
•
They are very sensitive to tube expansion and ferromagnetic materials.
•
It is very difficult to isolate small-volume cracks from complex signals
comprised of cracks, geometry changes, and added material.
•
The orientation of standard bobbin coils is not well suited to
circumferential cracks because eddy currents flow parallel to
indications.
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4. The Challenge
Operators of such HX want to:
•
Identify leaking tubes affected by circumferential cracking.
•
Identify tubes showing signs of cracks that may grow to 100% through wall
while the heat exchanger is in service.
Sample scan data from calibration tube
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5. The Solution
Using a multiplexed eddy current array (ECA) tubing probe to
distinguish crack signals from tube sheet signals.
Three-part solution:
•
•
DefHi ® ECA tubing probe (patent pending)
•
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Ectane™ testing instrument
Magnifi ® acquisition and analysis software
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6. The Solution — Ectane
Eddyfi’s workhorse testing instrument.
•
Can be configured for almost any type of inspection technique, including
ECT and eddy current array (ECA).
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7. The Solution — DefHi
Leverages ECA technology to accurately detect, size, and
characterize circumferential cracking defects,
differentiating them from other unwanted signals, e.g., from
the tubesheet and roll transition.
•
•
Coils working in transmitter-receiver mode
•
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Multiplexed coils
Probe yields absolute signals with phase + amplitude information
© Eddyfi 2014

8. The Solution — DefHi
•
Because of the coil configuration in the probe, eddy currents flowing in the
inspected tube are perpendicular to circumferential defects, making
them easier to identify.
•
ECA technology know-how has also made it possible to optimize the
distance between the transmitter and the receiver coils so that the
expansion signal detected by the probe is flat compared to others,
making it extremely simple to isolate.
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9. The Solution — DefHi
The DefHi also offers the following benefits:
•
Benefit 1 — Better Sensitivity
The higher density of multiplexed coils in the DefHi enables you to estimate
the circumferential length and position of defects. The DefHi’s highdefinition sensitivity is uniform to detect defects oriented in any direction.
•
Benefit 2 — Better Speed
Scan tube ends in about an eighth the time it takes rotating probes.
•
Benefit 3 — Better Visualization
The Magnifi software uses the DefHi’s inspection data to produce 2D and
3D C-scans with color palettes to distinguish between defects making
defects easy to spot and characterize.
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10. The Solution — Magnifi
Eddyfi’s cutting-edge acquisition and analysis software for
graphical display (C-scan), record keeping, and reporting.
Actual scan data
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11. The Solution — Magnifi
Magnifi produces 2D and 3D C-scans.
•
Color patterns help distinguish between the tubesheet and the
circumferential cracking defect.
•
Tubesheets yield angular responses compared to defects, so they appear
clearly on the C-scans as having circumferential length and depth.
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12. The Benefits
The main benefits of the solution are:
•
Unlike ECT, the solution can discriminate between the various complex
geometries inside HX tubing to allow accurately detecting, sizing, and
characterizing small-volume circumferential cracks.
•
Rotating probe technology scans tube ends at approximately 40 seconds
per tube. Eddyfi’s solution can scan tube ends at 5 seconds per tube.
•
No need for a mechanical probe pushing device to accurately control the
axial speed of the probe.
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13. The Benefits
Additional benefits include:
•
Quantifying the circumferential extent of crack defects.
•
Accurate axial location and characterization of crack defects in the vicinity of
the roll transition.
•
Standard ECT bobbin coils of the DefHi can serve as a reference point for
typical ECT data signals.
•
The solution can be used for full-length tube examinations.
•
Personnel without extensive ECT data analysis experience can view the
solution’s 3D data and be confident about the inspection results.
•
Actual testing demonstrates detection and sizing of circumferential cracks at
the tubesheet as early as 50% wall loss and 30% in the free span area.
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© Eddyfi 2014

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