Assessing Circumferential Cracking in

Non-Ferromagnetic Heat Exchanger

Tubes

Application Note

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 non-

ferromagnetic steels like:

• Inconel™

• Austenitic stainless steel

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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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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.

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Sample scan data from calibration tube

The Solution

Using a multiplexed eddy current array (ECA) tubing probe to

distinguish crack signals from tube sheet signals.

Three-part solution:

• Ectane™ testing instrument

• DefHi ® ECA tubing probe (patent pending)

• Magnifi ® acquisition and analysis software

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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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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.

• Multiplexed coils

• Coils working in transmitter-receiver mode

• Probe yields absolute signals with phase + amplitude information

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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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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 high-

definition 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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The Solution — Magnifi

Eddyfi’s cutting-edge acquisition and analysis software for

graphical display (C-scan), record keeping, and reporting.

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Actual scan data

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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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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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.

13 © Eddyfi 2014

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