Asset Integrity
Tank Floor MFL Scanning: Inspecting What You Can’t See
In brief Magnetic flux leakage (MFL) scanning magnetises a storage tank floor plate and detects the flux disturbance caused by metal loss — screening the full floor rapidly for corrosion on both surfaces, including the hidden underside. Within a tank integrity programme based on recognised methodologies such as EEMUA 159, MFL screening plus follow-up ultrasonic verification turns an out-of-service inspection into a quantified floor condition map that supports repair and re-inspection interval decisions.
A storage tank floor is the least visible pressure boundary on any site: a few millimetres of steel plate lying on a foundation, corroding — if it is corroding — from the side nobody can ever look at. Floors fail from underneath. Magnetic flux leakage scanning exists for exactly this problem, and it has become the centrepiece of the modern out-of-service tank inspection.
The underside problem
Product-side corrosion can at least be seen during an internal inspection. Soil-side corrosion cannot: moisture, contaminated foundations and stray current attack the plate from below, commonly as isolated pitting that punctures a floor while the visible surface looks respectable. Historically the answer was spot ultrasonic thickness readings on a grid — statistically thin coverage that can and does miss isolated pits between grid points.
Floor screening needed a technique that covers all the plate, both surfaces, at walking pace. That is MFL.
How magnetic flux leakage works
An MFL scanner saturates the floor plate with a strong magnetic field between the poles of a magnet bridge. Where the plate has lost metal — a pit, a lake of general wall loss, top side or bottom side — the field “leaks” out of the plate, and sensors between the poles detect the disturbance as the scanner passes over it.
The practical characteristics that make it the floor-screening workhorse:
- Full coverage, fast. Sweep by sweep, the scanner screens entire plates rather than sampling points — a complete floor in a fraction of the time a UT grid would take.
- Both surfaces. Flux leakage responds to metal loss wherever it sits — including the invisible underside.
- Screening, then verification. MFL flags and locates; follow-up ultrasonic testing at each indication verifies and sizes the actual remaining thickness. The pairing — MFL coverage plus UT accuracy — is the method.
Its constraints are equally practical: the tank must be out of service, cleaned and gas-free; coatings must be within scannable thickness; plate edges, annular zones and obstructed areas need supplementary techniques; and results depend on calibration against reference plates and on operator competence — another place where accredited inspection earns its keep.
Where MFL sits in a tank integrity programme
A floor scan is one instrument in a larger discipline. Tank integrity programmes built on recognised methodologies — EEMUA 159 being the widely used European reference for above-ground storage tank inspection, maintenance and repair — tie the whole outage together:
- Pre-outage review — history, previous findings, corrosion allowances, settlement records.
- Internal inspection — shell, roof structure, welds, nozzles, coatings.
- Floor survey — MFL screening of plates, supplementary examination of annular plates and critical zones, follow-up UT sizing of indications.
- Assessment — findings against acceptance criteria; repair scope (patch plates, weld repairs) defined and executed under proper procedures.
- Close-out — a floor condition map, repair records, and a re-inspection interval justified by evidence rather than habit.
That last output is where surveys pay for themselves: an interval argued from quantified corrosion rates is defensible to insurers and regulators, and often materially different from the default that was being applied out of caution — in either direction.
What a good survey report contains
Insist on: a plate-by-plate floor map with located indications; verified minimum thicknesses from follow-up UT; segregation of top-side vs suspected underside loss; annular and critical-zone findings; repair recommendations with acceptance criteria referenced; and the calibration and personnel records that make the data stand up. A tidy PDF with “floor generally satisfactory” is not a survey — it is an opinion with a cover page.
Metlab’s tank integrity capability
Tank work draws on most of what Metlab does in one engagement: MFL floor scanning and UT verification by certified NDT technicians, EEMUA-based tank integrity surveys, corrosion assessment and mapping, statutory vessel examinations where applicable, and weld repair support — procedures and welder approvals — when findings demand intervention. Delivered nationwide from Cork and Dublin, at night and inside shutdown windows, under INAB-accredited inspection and testing scopes.
A tank outage is expensive — the data it produces shouldn’t be thin. Talk to us before the tank is opened and we’ll scope the survey to earn the outage.
Frequently asked questions
What does MFL tank floor scanning detect?
Metal loss in the floor plate — pitting and general corrosion — on both the product side and the soil side. Underside corrosion is the critical case: it is invisible to visual inspection and is the classic cause of tank floor leaks. MFL is a screening technique; indications are typically verified and sized by follow-up ultrasonic testing.
Does the tank have to be taken out of service?
Yes — floor scanning requires an empty, cleaned, gas-freed tank. That is why MFL is scheduled within planned out-of-service inspections, and why making the most of the outage — full floor coverage, annular plates, follow-up UT, and the rest of the internal inspection scope — matters so much.
What is EEMUA 159?
EEMUA Publication 159 is the widely used industry guide for the inspection, maintenance and repair of above-ground vertical cylindrical storage tanks. Tank integrity programmes built on it use inspection findings — including floor condition from MFL surveys — to justify repairs and set re-inspection intervals on an engineering basis.