Remote Field Eddy Current Test (RFET)
To identify corrosion or material thinning in heat exchangers and boilers used in power plants, remote field eddy current testing is frequently employed. For the aim of detecting and sizing corrosion and mechanical damage in tubes, EIWAA provides RFET.
Eddy current testing is based on the electromagnetic induction principle for both weld and tubes. The advantages of RFET include its non-contacting nature, ease to cover large inspection areas, and ability to detect corrosion and thinning as part of a preventative maintenance program. Eddy Current tube inspection examining “In Service” Non-ferrous heat exchanger tubes by using the non destructive RFET Equipment and the techniques designed to detect eddy-current signals originating from metal loss in non-ferrous tubing, by the measurement of amplitude and phase of eddy current responses to defects and comparing them to manufactured defects in standard reference tubes Mode of operation of tube inspection is Absolute and Differential and where Absolute means this is a single coil arrangement. Absolute coils are sensitive to gradual changes in tube dimensions such as gradual thinning. Differential means it is a twin coil arrangement. The coils are connected in opposition. Because of this differential, probes are not sensitive to long defects but are highly sensitive to sharp defects such as pitting corrosion. The results of the eddy current tube inspection scan are plotted by electronically, providing a permanent record of each tube. The severity of any damage is assessed by comparison with the signals from an agreed reference tube.
Eddy Current Tube Inspection Restrictions are,
Obstructed (OBS) – A tube that may have been partially inspected along 100% of the tube length due to the probe passage being obstructed, typically by silt scale or a dent. If a partially inspected tube is identified with a reportable flaw, that flaw must be noted.
Plugged(PLG):-A tube that has been plugged (PLG) is no longer in use. A plug will be included and inserted into the tube’s two ends.
Inaccessible (INA) – Any tube that cannot be temporarily inspected because of scaffolding, etc. is classified inaccessible (INA).
No Access(No ACC) – Any tube that cannot contain the probe.
Eddy Current Tube Inspection capabilities and limitations:
- Bends cannot be inspected by using standard probes (Separate inspection would be required using a specialised probe).
- Defects at tube sheet cannot be detected.
- Circumferential cracking cannot be detected using standard probes (Separate inspection would be required using a specialised probe).
- Magnetic inclusions/Deposits can cause spurious indications.
- The minimum diametrical defects detectable by conventional eddy current probes shall be approximately 1.5mm.