Volumetric & Advanced NDT · MFL
Magnetic Flux Leakage Scanning
The tank floor’s underside, screened at walking pace.
Magnetic flux leakage (MFL) scanning screens storage tank floor plates for corrosion on both surfaces — including the invisible, soil-side underside. A magnet bridge saturates the plate with magnetic flux; where metal loss thins the plate, flux is forced out of the steel, and sensors passing over the leakage flag the defect’s position for ultrasonic verification.
The explainer
Watch how it works.
Schematic · loops automatically
1Saturate the plate
The scanner’s magnet bridge drives the floor plate to near magnetic saturation between its poles.
2Scan at walking pace
The bridge travels the plate in overlapping lanes — full coverage, both surfaces interrogated at once.
3Metal loss leaks flux
Where corrosion has thinned the section — top side or soil side — surplus flux bulges out of the plate.
4Sensors flag the spot
The sensor row intercepts the leakage; the system marks position and severity on the floor map.
5UT verifies, EEMUA decides
Ultrasonic follow-up measures true remaining thickness at each flag; findings feed repairs and the re-inspection interval.
The physics
A tank floor plate is a magnetic conductor. The MFL scanner’s powerful magnets drive the plate to near saturation between their poles — the steel carries as much flux as it can. Lose metal anywhere in the section, top or bottom, and the remaining steel cannot carry the same flux: the surplus bulges out of the plate surfaces as a local leakage field.
A row of sensors between the poles rides just above the plate and intercepts that leakage as the scanner travels. The response scales with the volume of metal missing — pits, lakes of general loss and grooving all disturb the flux — and crucially it does not care which surface the loss is on: soil-side corrosion, the failure mode that visual inspection can never see, signals just as clearly as top-side damage.
MFL is a screening technique: fast, full-coverage, sensitive — but semi-quantitative. Practice pairs it with ultrasonic follow-up: MFL sweeps the floor at walking pace and drops markers; UT then measures the exact remaining thickness at each flagged location. Within a tank integrity programme run to recognised methodology such as EEMUA 159, that pairing turns an opened tank into a quantified floor-condition map and a defensible repair and re-inspection decision.
Where it earns its keep
Typical applications
- Storage tank floor surveys during out-of-service inspection
- EEMUA 159-based tank integrity programmes
- Pre-repair condition mapping and repair scoping
- Re-inspection interval justification with quantified floor data
Standards & schemes
| Designation | Covers |
|---|---|
| EEMUA 159 context | Above-ground storage tank inspection, maintenance and repair methodology |
| Qualified procedures + UT verification | MFL screening with EN ISO 16809-based ultrasonic thickness confirmation |
MFL floor scanning is performed by certified technicians within Metlab’s tank integrity teams, with UT verification and reporting designed for repair scoping and interval-setting.
Why can’t we just UT the floor on a grid?
Because underside pitting is spatially random and a practical UT grid samples a fraction of one percent of the plate. MFL screens one hundred percent of the swept area at walking pace and tells the UT exactly where to measure — coverage first, precision second.
Does MFL find defects on the underside specifically?
It detects metal loss anywhere in the plate section — the flux does not distinguish surface. Discriminating top from bottom uses the visual condition of the top surface plus UT at the flagged point; unexplained loss with a clean top side is, by elimination, the soil side.
What preparation does the tank need?
Emptied, cleaned to bare accessible plate, gas-freed and vented for entry. Scan quality follows floor cleanliness — product residue and scale cost sensitivity — so cleaning standard is agreed as part of the outage plan.
Testing · Inspection · Certification
Need MFL on your asset?
Describe the component, the material and the question — a specialist confirms the technique, the standard and the turnaround.