Remote Field Testing or “RFET”

Remote Field Testing, abbreviated as “RFET,” is one of several electromagnetic testing methods commonly used in non-destructive testing. Magnetic flux leakage, conventional eddy current, and alternating current field measurement testing are some other electromagnetic inspection methods. Eddy current testing is associated with remote field testing, and the term “Remote Field Electromagnetic Testing” is frequently used to describe remote field testing. There are, however, several significant differences between eddy current testing and remote field testing that will be discussed in this section.

Because conventional eddy current techniques have difficulty inspecting the full thickness of the tube wall due to the strong skin effect in ferromagnetic materials, RFET is primarily used to inspect ferromagnetic tubing.

For example, using conventional eddy current bobbin probes to inspect a steel pipe 10 mm thick (such as that found in heat exchangers) would necessitate frequencies in the 30 Hz range to achieve adequate I.D. to O.D. penetration through the tube wall. The use of such a low frequency results in extremely low flaw detection sensitivity. In theory, the degree of penetration can be increased by using partial saturation eddy current probes, magnetically biassed probes, and pulsed saturation probes. However, due to the large volume of the metal present as well as potential permeability variations within the product, the inspection capabilities of these specialized eddy current probes are still limited.

The use of the remote field-testing method can greatly reduce the difficulties encountered when testing ferromagnetic tubes. The RFET method has the advantage of providing nearly equal detection sensitivity at both the inner and outer surfaces of a ferromagnetic tube. The method is highly sensitive to variations in wall thickness and less sensitive to changes in fill-factor between the coil and tube. When inspecting non-ferromagnetic materials, RFET is less sensitive than conventional eddy current techniques.

This is a fast reliable screening tool for the condition assessment of ferrous tubing (carbon steel, ferritic stainless steel and Duplex). Works typically at very low frequencies (100 Hz to 1 kHz) and very high signal gains (> 80 dB) can inspect up to 12 mm (1/2 in.) of carbon steel pipe. Typically, two channels are used for examination, Absolute for wall thinning and Differential for localized defects RFT is very good at wall thinning measurement but less performing to pitting (as compared to ECT). To improve inspection results it is always advised to perform cleaning of the internal bore of tubes with some form of water jet lancing tool.

RFET - Aqualis

Advantages

  • Suitable for ferromagnetic tubes such as those found in heat exchangers and boilers.
  • Localized corrosion, Pittings.
  • Equally sensitive to internal and external flaws.
  • Can inspect tubes up to 75mm OD with 5mm thickness.
  • Relatively insensitive to probe liftoff or wobble.
  • Typical pull speed between 6” and 12” per second.