Far field test or RFET is one of the electromagnetic testing methods that is commonly used in the field of non-destructive testing. Other electromagnetic inspection methods include magnetic flux leakage, conventional eddy current, and alternating current field measurement testing. Far-field testing is related to eddy current testing, and the term “far-field electromagnetic testing” is commonly used when describing far-field testing. However, there are major differences between eddy current testing and far field testing, which will be discussed in this section.
The RFET is mainly used to inspect ferromagnetic tubes, as conventional eddy current methods have difficulty inspecting the full thickness of the tube wall due to strong skin effects in ferromagnetic materials.For example, using conventional eddy current coil probes to inspect a 10 mm thick steel tube (such as those that may be found in heat exchangers) requires frequencies around 30 Hz to achieve sufficient penetration from ID to OD through the tube wall. Using such a low frequency leads to very low sensitivity in detecting defects. In principle, the degree of penetration can be increased using partially saturated eddy current probes, biased magnetic probes, and pulsed saturation probes. However, due to the large volume of metal as well as potential changes in permeability within the product, these specific eddy current probes are still limited in their inspection capabilities.
The problems that arise in the testing of ferromagnetic tubes can be significantly reduced by using the far field test method.The RFET method has the advantage of providing almost identical sensitivities in identifying defects on the inner and outer surfaces of the ferromagnetic tube. This method is very sensitive to changes in wall thickness and is usually less sensitive to changes in the fill factor between the coil and the tube. The RFET can be used to inspect any conductive tubular product, but is typically less sensitive than conventional eddy current techniques when non-ferromagnetic materials are inspected.
It is a fast and reliable screening tool for assessing the condition of iron pipes (carbon steel, ferritic stainless steel and twin). This method typically operates at very low frequencies (100 Hz to 1 kHz) and can inspect 12 mm (1 / 2 inch) thick carbon steel tubing with very high amplified signals (>80 dB).Two channels are usually used for inspection: the “absolute” channel for wall thickness reduction and the “differential” channel for local defects. RFET is very good at measuring wall thickness reduction but less efficient at detecting indentations (compared to ECT). To improve inspection results, it is always recommended to clean the inside of pipes using a tool such as water jet lancing.
Advantages of pipe inspection using RFET
• Suitable for ferromagnetic tubes such as those found in heat exchangers and boilers.
• Identification of local corrosion, dents.
• Equal sensitivity to internal and external defects.
• Capable of inspecting pipes up to 75mm diameter with 5mm thickness.
• Relatively less sensitive to lifting the probe or shaking it.
• Typical draw speeds between 6 inches and 12 inches per second
