Services · Non-Destructive Testing
Tank Floor Inspection by MFL Scanning per API 653
A floor that looks sound from inside the tank can be losing metal from underneath. Magnetic flux leakage reads both faces of every plate in one pass, and tells you which face is corroding.
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TCR inspects above-ground storage tank floors by magnetic flux leakage (MFL) during an out-of-service API 653 inspection, mapping product-side and soil-side corrosion plate by plate, proving indications up by ultrasonic thickness measurement and covering the weld margins by UT. In Saudi Arabia the service is delivered by TCR Arabia, on the SABIC Approved Contractor List for advanced NDT including MFL.
What MFL Finds on a Tank Floor
A tank floor corrodes from two faces: the product side, where water and sludge settle, and the soil side, where the foundation, drainage and cathodic protection decide the rate. Spot ultrasonic readings sample only a few points of the floor. MFL scans the whole plate area and finds the loss wherever it sits.
- What it detects: pitting, general metal loss and erosion in carbon-steel floor and annular plates.
- Which face: the scanner discriminates product-side from soil-side loss, so a floor losing metal underneath is not mistaken for one that needs a new lining.
- How it works: magnets saturate the plate; where metal is missing, the field leaks out of the steel and the sensors between the poles read the leak.
- What it does not give on its own: a certified remaining thickness. Every significant indication is proved up by ultrasonic thickness measurement before it is reported as a number.
What a Floor Scan Establishes
MFL is an out-of-service method: the tank is emptied, cleaned and made safe for entry, and the scan is only as good as the floor it is run on. Magnetic debris and weld spatter give false indications, so the cleaning standard is agreed before the team mobilises, not argued on the day.
- Every indication is located against a numbered plate and a position on the floor map, not a photograph.
- The whole floor area is scanned, with the instrument verified on a reference plate for the plate thickness and the coating.
- Indications above the reporting threshold are proved up by ultrasonic thickness gauging, which sets the number that is reported.
- The margins beside lap welds and the shell-to-annular weld, and a small square at each plate corner, are examined by ultrasonic testing and recorded as such.
- The floor map, the proved-up thicknesses and the register of untested areas go to the API 653 inspection of the tank.
What the Scanner Covers, and What It Does Not
Every floor method has limits, and a report that hides them is worse than a smaller scan. The limits below are stated on every TCR MFL report, together with the areas left to ultrasonic testing, so the owner can see exactly which part of the floor the conclusion rests on.
| Aspect | Position |
|---|---|
| Plate thickness | Confirmed per tank against the scanner's stated range before the team mobilises. Sensitivity falls as the plate gets thicker |
| Coated floors | Inspected through the coating; the coating thickness that still allows a satisfactory scan falls as the plate gets thicker, and is agreed per tank |
| Annular plates and weld margins | The steerable scanner reaches the annular plates and close to the plate welds; the strip it cannot reach is examined by UT |
| Plate corners | A small square at each corner of a floor plate cannot be scanned and is examined by UT |
| Surface condition | Magnetic debris and weld spatter produce false indications; cleaning is part of the scope |
| Tank status | Out of service, cleaned and gas-free. For a tank that must stay in service, see the methods below |
What the Report Contains
The MFL report is written for the person who has to decide which plates to repair, patch or replace, and for the inspector who signs the API 653 bottom evaluation. It separates what was measured from what was screened, and names every area that was not examined by either method.
- A floor map, plate by plate, with every indication located and classified as product-side or soil-side.
- The ultrasonic prove-up value against each significant indication, with the instrument identified.
- The minimum remaining thickness found on each plate and on the annular ring.
- A register of untested areas: weld margins, plate corners and any plate the scan could not cover, with the method used there instead.
- The data the API 653 bottom evaluation needs: remaining thickness, the side the loss is on, and its distribution across the floor.
- Where the floor is borderline, the inputs for a fitness-for-service assessment by TCR Advanced Engineering rather than a replacement recommendation made on the scan alone.
Where MFL Sits Among the Tank Methods
MFL is the method for the whole floor when the tank is open. It is one of several non-destructive testing methods an API 653 inspection draws on, and each answers a different question. The full inspection, with every test and how the results are put together, is described on storage tank inspection and audit per API 653.
| Method | Tank status | What it answers |
|---|---|---|
| Magnetic flux leakage floor scanning (this page) | Out of service | Where the floor is losing metal, on which face, across the whole floor |
| Acoustic emission testing | In service | Whether the floor is actively corroding or leaking now, graded I to IV, and when to open the tank |
| Robotic in-service tank inspection | In service | Ultrasonic thickness of the floor without draining or entry |
| Advanced ultrasonics | Either | Shell corrosion mapping and weld examination by PAUT |
| Conventional NDT | Out of service | Vacuum box testing of floor welds, MT and PT of shell-to-floor and nozzle welds, UT thickness of the shell courses |
Who Delivers It
TCR Arabia, the group's 60/40 joint venture in Dammam, runs MFL tank floor scanning in Saudi Arabia with the Silverwing FloormapVS2i computerised floor scanner, and is on the SABIC Approved Contractor List for advanced NDT including MFL, confirmed on 20 August 2019. In India, TCR Engineering scans tank floors with MFL floor scanners as part of its API 653 tank inspection.
- Instrument: Silverwing FloormapVS2i, a computerised MFL tank bottom scanner with automatic feature sizing and product-side and soil-side discrimination.
- Prove-up: ultrasonic thickness gauging, and phased array ultrasonics where an indication needs to be characterised.
- Inside Saudi Arabia: delivered by TCR Arabia Company Limited.
- Elsewhere: scoped per enquiry, with the instrument, the team and the reporting standard agreed before mobilisation.
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Frequently asked questions
Does the tank have to be emptied for MFL floor scanning?
Yes. MFL is run from inside the tank, so the tank is taken out of service, emptied, cleaned and made safe for entry. For a tank that must stay in service, acoustic emission testing screens the floor for active corrosion and leaks, and robotic in-service inspection measures floor thickness without draining.
Can MFL tell product-side corrosion from soil-side corrosion?
Yes. The FloormapVS2i discriminates between loss on the top face and loss on the underside of each plate, so the report separates internal corrosion from soil-side corrosion instead of giving one combined figure.
What floor thickness can be scanned?
The plate thickness range is confirmed for each tank against the scanner's stated capability. Sensitivity falls as the plate gets thicker, and the coating thickness that still allows a satisfactory scan falls with it, so both are confirmed before the team mobilises.
What happens to the areas the scanner cannot reach?
The strip beside lap welds and the shell-to-annular weld, and a small square at each plate corner, are examined by ultrasonic testing. The report lists every such area and the method used there, so no part of the floor is reported as scanned when it was not.
Does MFL give the remaining plate thickness?
MFL locates and sizes metal loss across the floor. Every significant indication is then proved up by ultrasonic thickness measurement, and the reported remaining thickness is the ultrasonic value, with the instrument identified.
How does an MFL scan fit into an API 653 inspection?
It supplies the floor data for the bottom evaluation: remaining thickness, which face the loss is on and how it is distributed. The shell, roof, welds, foundation and appurtenances are covered by the other tests in the API 653 inspection, described on the storage tank inspection and audit page.