RFT is particularly suitable for the detection and quantification of wall thickness reductions. Internal and external defects can be detected but not distinguished. RFT testing is not suitable for testing finned pipes. With RFT, the wall thicknesses of up to 12 mm can be inspected and at a pulling speed of 0.1 m/s to 0.3 m/s. This method is highly sensitive to variations in wall thicknesses and tends to be less sensitive to probe lift-off, centering, wobbling, and fill-factor.

1. The principles of Remote field testing:

The basic probe is made of one exciter coil and two pickup coils. Two magnetic fields are present: the direct field, in the vicinity of the exciter coil, is rapidly attenuated with distance while the indirect field propagates along the tube axis and is rediffused back through the tube wall.

RFT probe is constituted from one exciter and two receiver coils set apart at a distance equal to twice the tube outer diameter or more. The zone where the receivers are located is called the remote field zone. Then, receiver coils are going to feel the indirect field from the exciter rather than the direct one, typically used with ECT.

Any defect located at the exciter and/or receivers location will have an impact on the magnetic field read by the coils.

RFT technique is equally sensitive to ID and OD defects, it cannot be used to determine if wall loss occurs inside or outside the tube wall.

Picture 1: The principles of Remote field testing (RFT)

Picture 2: Use Remote field testing to inspection the ferromagnetic pipes.

2. Advantages and Disadvantages of Remote field testing:

- Advantages:

• Suitable for Ferromagnetic Pipes and Tubes such as those found in boilers and Heat Exchangers
• Same Sensitivity is achieved for internal and External flaws
• Technique is relatively insensitive to probe lift off or wobble
• Can be used to examine very thick tube wall (up to 12mm)
• A low fill factor enables the inspection of partially scaled tubes and boiler tubes with swages and bends
• Centralization of the probe is not as critical as it is for ECT inspections

- Disadvantages:

• Some limitation to distinguishing ID from OD defects.
• Evaluation of small flaws such as pits can be difficult.
• Requires high inspection skills for data analysis and evaluation.
• Tubes must be cleaned.
• Inaccuracy in test results could occur if a discontinuity encountered differs in geometry from calibration discontinuities.
• Flaws under support plates and adjacent to end sheets can be difficult to detect.