Magnetic flux leakage testing, explained

Magnetic flux leakage testing pushes a strong magnetic field into a ferromagnetic pipe wall until the steel is near saturation. Intact wall carries the flux quietly. Where the wall has thinned — corrosion, mechanical damage, metal loss — the steel can no longer carry its share, and flux leaks out of the surface. Sensors riding the wall read that leakage field, and software reconstructs the defect's length, width, depth and clock position.

What an MFL run reports

The tool fleet in this line references each feature to the nearest circumferential weld within +/-0.1 m and to circumferential direction within +/-5 degrees — the coordinates a dig crew needs. The service side identifies, across 8–56 inch lines: internal and external wall corrosion and mechanical damage (metal loss); cracks and abnormal weld defects; pipeline features (girth welds, bends, lengths); and installations (valves, tees). Its multi-sensor fusion and adaptive filtering give a signal recognition accuracy of +/-5 cm.

The method's honest limits

The product literature for the tapping detection machine states the limits plainly, and they apply across MFL: cracks running parallel to the magnetic field, pinholes, and defects at circumferential welds are harder to detect and size, because of field distribution, sensor layout density and permeability distortion in heat-affected zones. Good programmes therefore pair methods — the MFL vs eddy current comparison shows the pairing logic — and validate with digs.

Operating envelope (this line)

• Pressure 0–10 MPa; temperature -10 to 80 degrees C.
• Bends at 1.5D radius or greater; deformation through straight sections of 15–20% OD depending on tool.
• Continuous runs of 50–60+ hours; diameters 159–1,422 mm across 20+ tool specifications.

For programme structure rather than physics, read the pipeline inspection guide.