Magnetic Flux Leakage
Magnetic flux leakage (MFL) uses a powerful magnet to magnetize the conductive material under test (usually steel). Where there are defects — corrosion or material loss — the magnetic field “leaks” from the steel. This ET method can be used in two very different situations: tubing (e.g. fin-fan air coolers) or surface (e.g. tank floors).
During inspection, a magnetic circuit is formed between the part and the probe. The magnetic field induced in the part saturates it until it can no longer hold any more flux. Eventually, the flux overflows and leaks out of the part wall. Strategically placed sensors can accurately measure the leakage field.
MFL is generally considered a fast inspection technique, suitable for measuring wall loss and defects such as pits, grooves, and circumferential cracks, most commonly in tubes, pipelines and storage tanks. MFL can be used on aluminum-finned carbon steel tubing, because the magnetic field is almost completely unaffected by the presence of such fins. Improved MFL tubing probes now offer a reliable absolute signal and do not require periodic manual adjustments while in use.
The primary advantage of the MFL is the ability to locate and estimate the size of defects over large areas in a quick and efficient manner. Continuous research and development has resulted in the most advanced Magnetic Flux Leakage MFL tank bottom inspection allowing for top & bottom defect discrimination with high scanning resolutions and full mapping capabilities, this enables tank engineers to determine the optimum repair strategy and improve the outcome of Fitness For Service (FFS) assessments, Remaining Life Assessment (RLA), and Risk Based Inspection (RBI) programs.
Benefits of MFL
- One of the few methods used to inspect finned tubes, (NFT is an acceptable alternative)
- Can be used on all ferromagnetic material
- Good sensitivity to pitting
- High speed inspection