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.

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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

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The Challenge

• The through-transmission nature of RFT makes it

sensitive to:• Nominal thickness variations

• Permeability changes

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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.

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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.

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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.

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The Solution

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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.

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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.

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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

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