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Ferromagnetic heat exchanger tubing found in petrochemical plants cannot be inspected with conventional eddy current techniques. Common inspection methods for this material include IRIS, magnetic flux leakage (MFL), and remote-field testing (RFT). In general, RFT is preferred due to its defect sizing capabilities, decreased need for tube cleanliness, and high inspection speeds. This application note shows how innovative software features allow working around traditional difficulties encountered with RFT.
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State-of-the-Art RFT — Meetingthe Ferromagnetic Tube ChallengeApplication Note
Ferromagnetic HX Tubing
Found in petrochemical plants, it cannot be inspected with
conventional ECT. Common inspection methods:• Internal Rotary Inspection System (IRIS)
• Magnetic flux leakage (MFL)
• Remote-field testing (RFT)
RFT is usually preferred due to its defect sizing capabilities,
decreased need for tube cleanliness, and high inspection
speeds.
2 © Eddyfi 2014
The Challenge
Overcome the difficulties encountered when using RFT in
ferromagnetic heat exchanger tubes.• The RFT technique is widely accepted, but difficult to master
• Permeability of carbon steel tube material is unknown
• Can change considerably from one heat exchanger to the next
• At times, can vary significantly within the tubes of a single heat
exchanger
• Developing sets of calibration standards with different degrees of
permeability is neither functional or financially sound
3 © Eddyfi 2014
The Challenge
• The through-transmission nature of RFT makes it
sensitive to:• Nominal thickness variations
• Permeability changes
4 © Eddyfi 2014
The Challenge
The high sensitivity of RFT poses several challenges:1. Typically, the manufacturing thickness tolerance of carbon steel tubes
is approximately 10 %
• Such a variation in thickness can represent a rotation of about 60 degrees on your
channel, greatly affecting sizing interpretations
2. The magnetic permeability of carbon steel varies from grade to grade and
according to processing history
• Tubes processed at high temperatures do not have the same permeability as
those processed at low temperatures
• Variations in permeability have a considerable impact on RFT sensitivity and
sizing
3. Calibration tubes matching the properties of tubes to inspect are extremely
hard to come by. Differences in thickness and permeability almost always
make calibrations inadequate without the proper tools to compensate.
5 © Eddyfi 2014
The Solution
The shortcomings of RFT can be worked around with
software features, such as those found in Magnifi®, Eddyfi’s
acquisition and analysis software.
6 © Eddyfi 2014
The Solution
Magnifi uses a standard impedance plane and an absolute
voltage plane.• The impedance plane has proven efficient to:
• Detect and characterize small volumetric defects through standard
techniques.
• The absolute voltage plane:
• Enables more precise characterization of large volumetric defects.
Using both planes in concert allows improved detection, sizing, and
characterizing a wider range of defects.
7 © Eddyfi 2014
The Solution
8 © Eddyfi 2014
Magnifi is capable of saving the balanced, nominal operating
point.• This stored value creates a reference point that enables tube-to-tube
tracking of permeability variations.
The Solution
Normalizations are automatically applied to each channel in
an effort to improve accurate defect sizing.
9 © Eddyfi 2014
The Solution
• Magnifi issues warnings when the effects of magnetic
permeability variations are too large to normalize.• Users can then adjust the inspection frequency to compensate for magnetic
permeability differences between the calibration standard and the tubes in
the bundle.
• Magnifi can also detect landmarks automatically.• Allows accurately positioning defects without the need for an encoder.
10 © Eddyfi 2014
The Benefits
The value added by software-enhanced RFT is easy to see:• One-click normalization of all channels
• Automatically generated sizing curves
• Normalization tracking for easier, faster, and more precise characterization
of small and large volumetric defects
• The result: higher quality inspections that provide
much needed information to the asset owner
11 © Eddyfi 2014