Heat exchangers are essential to the operation of many systems in various industries, processing plants and electric power generation plants, for example. A unique type of heat exchanger (HX) was introduced several years ago: the Twisted Tube® HX. They offer the highest levels of efficiency, but have always been a challenge for periodic inspections. Until now. A twisted tube is composed of two straight, circular sections near the tubesheets, separated by a length of helical, oval tube. This helix creates a small, central circular passage along the length of the tube where traditional bobbin probes can be pushed and pulled. However, standard ECT probes deliver suboptimal performances — detecting, characterizing, and sizing flaws are problematic because of poor signal-to-noise ratios. Flaws on the wall furthest away from the tube’s center (crest) produce much smaller, distorted signals than same-size flaws in the narrow section, closest to the tube’s center (dip). This is because of the liftoff created by the tube’s geometry between its walls and the ECT probe. This amount of liftoff translates into a lower probability of detection and less accurate sizing.

1. Twisted Tube® Heat Exchanger
Inspection with Eddy Currents
Application Note

2. Twisted Tube® Heat Exchangers
Heat exchangers are essential in many industries. A unique
type, the Twisted Tube® heat exchanger (HX), offers:
• Highest efficiency among HX
• Challenge for periodic inspections because of tube profile
2 © Eddyfi NDT Inc. 2009–2015

3. The Challenge
A twisted tube is composed of:
• 2 straight, circular sections near tubesheets
• 1 length of helical, oval tube
The helix creates a small, circular central passage along
the length of the tube where
traditional bobbin probes can
be pushed and pulled.
3 © Eddyfi NDT Inc. 2009–2015

4. The Challenge
Poor signal-to-noise ratios lead to suboptimal
performances with standard ECT probes.
• Because of liftoff created by tube’s geometry, similar-sized
flaws:
• Close to crests = small distorted
signals
• Close to dips = large
distorted signals
• Results: Lower PoD, less
accurate sizing
4 © Eddyfi NDT Inc. 2009–2015

5. The Challenge
• Common sizing method to determine flaw depth are
inaccurate because:
• Reduced sensitivity
• Large tube signal variations from flaw locations (crest/dip)
• IRIS also affected by non-linear geometry
• Reflecting echoes not perpendicular to inspected wall. Not an alternative.
• Tubes in Twisted Tube® HX come in variety of:
• Pitches
• Diameters (typical: 19.1–25.4 mm [0.75–1 in])
• Non-ferromagnetic and ferromagnetic materials
5 © Eddyfi NDT Inc. 2009–2015

6. The Solution
A probe that:
• Fits most Twisted Tube® HX
• Dynamically adjusts to changing geometry to minimize liftoff and optimize
inspection results
Result: Patent-pending Twistec™ probe
Designed with:
• Rotating head
• Equipped with two “feelers” that press down the coils
they contain against tube’s inner surface.
• Pair of oval coils matching helix pitch
of twisted tube.
6 © Eddyfi NDT Inc. 2009–2015

7. The Solution
Twistec probe remains close to surface under test for
more precise inspection data
• Feelers compress to fit through circular section at the tubesheet
Pulling speed:
• Approximately 0.3 m/s (12 in/s)
Life:
• 1500 tubes (60 to 80 tubes/h)
Results allow defects to be clearly identified. Twistec
probe signals are less noisy than those from a conventional
bobbin probe.
7 © Eddyfi NDT Inc. 2009–2015

8. The Solution
8 © Eddyfi NDT Inc. 2009–2015
Bobbin Probe Results Twistec Probe Results
Crest detection

9. The Benefits
• Minimized liftoff
• Minimized noise
• Optimized results
• Inner/outer-diameter defect distinction
• Determine whether defects are in crests or dips
• Easier-to-analyze scan results
• More accurate scan results
• Longer asset life
9 © Eddyfi NDT Inc. 2009–2015

10. www.eddyfi.com