RFT is an electromagnetic method of nondestructive testing whose main application is finding defects in the external walls of carbon steel or ferretic stainless steel tubing when the outside wall is not accessible, also used to detect wall
thinning. This technology offers good sensitivity when detecting and measuring volumetric defects resulting from erosion, corrosion, wear, and baffle cuts.
RFT probes use one or several transmitter coils (exciter coil) which sends a signal to the receiver coil (detector coil). The exciter coil emits a magnetic field. The magnetic field created by the transmitter coil travels through and out of the tube wall, radially and axially, towards the receiver. This is called through-transmission and is what defines RFT. The through-transmission allows external and internal defects to be detected with equal sensitivity. The detector is placed inside the pipe two to three pipe diameters away from the exciter and detects the magnetic field that has travelled back in from the outside of the pipe wall (for a total of two through-wall transits). Although RFT works in nonferromagnetic materials such as copper and brass, its sister technology eddy-current testing is preferred.
Benefits of remote-field testing over other electromagnetic testing techniques
- Suitable for ferromagnetic materials
- No need for direct contact with the pipe wall
- Less sensitive to probe wobble than conventional eddy current testing
- Equal sensitivity at the inner and outer surface
- Highly sensitive to variations in wall thickness
- Because the field travels on the outside of the pipe, RFT shows reduced accuracy and sensitivity at conductive and magnetic objects on or near the outside of the pipe, such as attachments or tube support plates
The main differences between RFT and conventional eddy-current testing (ECT) is in the coil-to-coil spacing. The RFT probe has widely spaced coils to pick up the through-transmission field. The typical ECT probe has coils or coil sets that create a field and measure the response within a small area, close to the object being tested.