Magnetic Flux Leakage Testing
Magnetic Flux Leakage testing (MFL) is a technique used for the inspection of tubes made of ferrous materials. This technique will normally be applied as a fast screening technique if small diameter pitting is expected. Because of limitations to its sizing abilities the MFL technique is not often used as a stand-alone technique, and verification by other techniques is recommended. An advantage of the Magnetic Flux Leakage inspection is that it can also be used on airfin cooler tubes.
Magnetic Flux Leakage is sensitive to sharp type defects like pits and grooving. Internal and external pits can be detected. Depending on probe configuration MFL can distinguish between internal and external defects and can detect gradual wall-loss. For ID / OD discrimination the probe needs to be equipped with a second coil and to detect gradual defects and a Hall-effect sensor in the probe is needed.
The probe in this technique contains permanent magnets which are utilized to form a magnetic flux field in the tube wall. Defects will influence the path of the magnetic field and will cause some of the flux to leak out of the tube wall. This leakage field will be picked up by the coils and the Hall-effect sensors in the probe. Size of the leakage field is determined by pull speed of the probe and by the shape, the dimensions and the location of defects. Signals that represent the size of the leakage field and thus the condition of the tube are presented on a computer screen.
MFL technique has the following advantages and disadvantages:
- Sensitive to pits in carbon steel
- Can be used on fin tubes
- Sizing of defects is very limited
- Very poor for detecting axial cracks
No single inspection technique is suitable for all tube inspection and Magnetic Flux leakage is no exception. We would welcome your enquiry or questions so that we can help you to assess the best method for your tube inspection requirements.