What Is MFL Tube Inspection?
Magnetic Flux Leakage (MFL) tube inspection uses a strong permanent magnet to saturate the
tube wall with magnetic flux. Where the tube wall is locally thinned — by pitting, corrosion,
erosion, or mechanical damage — the magnetic flux leaks out of the tube wall. Hall effect sensors
within the MFL probe detect this flux leakage and produce signals that indicate the presence,
location, and relative severity of the wall loss.
MFL is the preferred technique for high-speed inspection of ferrous (carbon steel and low-alloy
steel) heat exchanger tubes and boiler tubes — the tube populations for which ECT sensitivity is
significantly reduced due to the ferromagnetic properties of the material, and for which IRIS may
be too slow for full-bundle inspection within available outage windows.
MFL tube inspection detects:
- General wall thinning and pitting — inner diameter (ID) and outer diameter (OD) defects
- Baffle cut wear at support plate locations
- Erosion at inlet and outlet zones
- Through-wall corrosion and perforation
- Mechanical damage and denting
MFL cannot reliably detect:
- Axially oriented cracking — circumferential flux orientation limits sensitivity to axial
cracks
- Very shallow surface defects — below the method's minimum defect size threshold
- Defects in non-ferrous (austenitic) tubes — use ECT for non-ferrous tube inspection