The Remote Field Eddy Current (RFEC) inspection technique is a nondestructive method which uses low frequency AC and through wall transmission to inspect pipes and tubes from the inside. The through wall nature of the technique allows external and internal defects to be detected with approximately equal sensitivity.
The RFEC tool uses a relatively large internal solenoidal exciter coil which is driven with low frequency AC. A detector, or circumferential array of detector coils, is placed near the inside of the pipe wall, but axially displaced from the exciter by about two pipe diameters. Two distinct coupling paths exist between the exciter and the detector coils. The direct path, inside the tube, is attenuated rapidly by circumferential eddy currents induced in the tube's wall. The indirect coupling path originates in the exciter fields which diffuse radially outward through the wall. At the outer wall, the field spreads rapidly along the tube with little further attenuation. These fields re-diffuse back through the pipe wall and are the dominant field inside the tube at remote field spacing. Anomalies anywhere in the indirect path cause changes in the magnitude and phase of the received signal, and can therefore be used to detect defects.
Skin effect consideration limits conventional eddy current inspection techniques to inspection of only the surface nearest to the probe. The remote field technique is capable of inspecting the entire wall thickness without the need to use ultra low frequency. Like conventional eddy current techniques, RFEC probes respond well to slits because these interact strongly with eddy currents and produce little perturbation of magnetic flux. Although RFEC probes have been used for well casing inspection for many years, it is a rather complex phenomenon. The interaction with defects is now well understood, thanks to the anomalous source defect models and computer animations that we have developed.
There is great interest in the detection of stress corrosion cracking (SCC) in pipelines, but tests of ultrasonic tools in gas pipelines have not been entirely satisfactory. There is therefore continued interest in the potential application of RFEC techniques. RFEC probes are already used for commercial inspection of heat exchanger and pressure tubes. There are several other important potential applications for RFEC techniques including the inspection of water and gas distribution lines. These have elbows and tees which are difficult for any other tool to negotiate. We are collaborating on this application as also on the potential use of RFEC for inspection of water supply pipes and lined gas lines.
Early in 1988, we started a three year collaborative program with Pipetronix in order to develop advanced magnetic inspection tools for pipelines, and further investigate the potential application of the remote field eddy current technique to pipeline monitoring. Our research was supported by an NSERC/Pipetronix $210,000 p.a. U.I. grant, and a $70,000 p.a. Ontario URIF award for equipment.
Our work on the remote field eddy current inspection technique has also progressed very well. This progress can be partly atrributed to the fact that we are the only group using combined analytic, finite element and experimental investigations. Our group can lay good claim to being amongst the world leaders and was awarded the 1990 American Society for Nondestructive Testing's Achievement Award for some of this work.
In 1991 we were awarded a new 3-year NSERC Strategic Grant worth $61,800 p.a. for research on the use of remote field eddy current inspection for heat exchanger tubes. This enabled us to expand collaboration with Russell Technologies, Edmonton, who are the world leaders in the use of remote field techniques for inspecting small diameter tubing. Additional support for this work was received in 1993 from an NRC IRAP award to Russell Technologies.
IN 1992 we also received a 3-year contract for US$458,817 from Gas Research Institute, Chicago for an investigation of remote field slit defect interactions using a special "phantom exciter coil" simulation.
In 1993 we received a second 3-year contract for US$536,760 from Gas Research Institute, Chicago for research on the effects of line pressure stress, magnetic properties and test conditions on magnetic flux leakage signals.
The result of this support has not only been to help establish our group as being among the world experts in the five fields of our R&D (ferromagnetic hysteresis theory, the interaction between magnetic and stress effects, magnetic Barkhausen noise, the remote field eddy current technique and magnetic inspection techniques for pipelines), and to produce a large number of publications, but to also generate many techniques with promising industrial importance.
All of this has led to very close industrial collaboration, particularly with Pipetronix and also with AECL Research and, more recently, with Russell Technologies. All of these organizations now employ graduates from my Applied Magnetics Group. We have written more than 200 reports for these groups, have exchanged test samples, equipment and software as well as personnel for training. We put considerable effort into technology transfer.
Papers on the Remote Field Through Wall Inspection Technique
Introduction to Remote Field Log Interpretation
Download the RFEC Tutorial presented at the Fourth International Conference on RFEC Technique, 1997.
Revised August 2002
