5.1 The purpose of RFT is to evaluate the condition of the tubing. The evaluation results may be used to assess the likelihood of tube failure during service, a task which is not covered by this practice.
5.2 Principle of Probe Operation—In a basic RFT probe, the electromagnetic field emitted by an exciter travels outwards through the tube wall, axially along the outside of tube, and back through the tube wall to a detector7 (Fig. 2a).
Note 1: Arrows indicate flow of electromagnetic energy from exciter to detector. Energy flow is perpendicular to lines of magnetic flux.
5.2.1 Flaw indications are created when (1) in thin-walled areas, the field arrives at the detector with less attenuation and less time delay, (2) discontinuities interrupt the lines of magnetic flux, which are aligned mainly axially, or (3) discontinuities interrupt the eddy currents, which flow mainly circumferentially. A discontinuity at any point on the through-transmission path can create a perturbation; thus RFT has approximately equal sensitivity to flaws on the inner and outer walls of the tube.7
5.3 Warning Against Errors in Interpretation.Characterizing flaws by RFT may involve measuring changes from nominal (or baseline), especially for absolute coil data. The choice of a nominal value is important and often requires judgment. Practitioners should exercise care to use for nominal reference a section of tube that is free of damage (see definition of “nominal tube” in 3.2.3). In particular, bends used as nominal reference must be free of damage, and tube support plates used as nominal reference should be free of metal loss in the plate and in adjacent tube material. If necessary, a complementary technique (as described in 11.12) may be used to verify the condition of areas used as nominal reference.
5.4 Probe Configuration—The detector is typically placed two to three tube diameters from the exciter, in a location where the remote field dominates the direct-coupling field.7 Other probe configurations or designs may be used to optimize flaw detection, as described in 9.3.
5.5 Comparison with Conventional Eddy-Current Testing—Conventional eddy-current test coils are typically configured to sense the field from the tube wall in the immediate vicinity of the emitting element, whereas RFT probes are typically designed to detect changes in the remote......