RFET (or RFT as it is commonly known) has been used to inspect diameters up to 72 inches and wall thicknesses up to 1.5 inch (not at the same time). The limitation on diameter and wall thickness is really imposed by the design of the instrument and probe carrier, not by the physics of RFT itself. For heavy wall such as cast iron pipe, frequencies around 20 Hz would be needed. And since this frequency is in the domain of probe vibration, a smooth ride is essential and the probe must be pulled very slowly. Larger diameter pipes will need multiple receiver coils for sensitivity to local defects.

For fin fan, use "near field" probes. The fins do not allow the field from the remote field probes to travel on the outside of the tube, so the signal is lost.

For heat exchanger tubes, clean the tubes to remove debris that may impede the passage of the probe or cause probe vibration, especially if the debris is magnetic.

Advantages

1. Can inspect ferromagnetic tubes up to 3.5 inches in diameter with 0.125 inches wall thickness.
2. Inspection speed (up to approx. 40 feet per minute).
3. Can detect large-area discontinuities such as steam erosion and baffle wear.
4. Amplitude changes in the signals sensed are not speed-sensitive.
5. Flexible probes can be used to inspect and travel through U-bend areas.
6. Permanent records can be obtained on test results.

Limitations

1. Some limitation to distinguishing ID from OD defects.
2. Evaluation of small flaws such as pits can be difficult.
3. Impingement erosion and wall loss detectability is limited to approximately 20% and greater.
4. Requires high inspection skills for data analysis and evaluation.
5. Instrumentation and test probes can be very expensive.
6. Tubes must be cleaned.
7. Inaccuracy in test results could occur if a discontinuity encountered differs in geometry from calibration discontinuities.