FAQs - MFL Tank Inspection

Q. What preparation is required before a magnetic flux leakage (MFL) tank inspection takes place?

A. Cleaning requirements for Magnetic Flux Leakage (MFL) Inspection
Some preparation is required in order for a MFL inspection to take place. Any product should be drained from the tank and the tank should be cleaned.

Cleaning techniques will vary depending on the type of product stored in the tank, the time since the tank was last cleaned and opinions held by the engineer in charge of the cleaning operation. Acceptable cleaning methods include shot blasting, grit blasting / sand blasting, high pressure water jetting, ultra high pressure water jetting etc.

The media used in the blast cleaning process should be removed from the floor before the inspection takes place. It is very difficult to completely remove all of the shot from the surface of the floor. Two problems can occur if the MFL scanner passes over any loose shot (or any other loose ferrous material).

The shot will become embedded in the scanner rollers and cause excessive vibration.

Spurious defect indications will be detected / recorded if any shot passes underneath the sensor head. The aim of the cleaning process is to remove any contaminants such as product residue, corrosion build, scale and loose ferrous material from the surface of the tank floor which can affect the quality of a MFL inspection.

Product residues can also give off toxic or flammable gasses. The floor should be clean enough for a good visual inspection (you should be looking at the surface of the steel, not a layer of product residue/scale etc - see examples below).

Surface oxidisation / rust will not present a problem for MFL inspections.

Tank floor conditions for MFL inspection click on thumbnail for a larger image
Acceptable conditions
Unacceptable conditions

Q Can I use Magnetic Flux Leakage (MFL) to inspect the lap welds as well as the floor plates ?
A No, Magnetic Flux Leakage is used to inspect the parent plate material for corrosion defects and cannot be used for the lap welds. The most common method of inspecting storage tank floor plate lap welds is by use of a Vacuum Box.


Q Is MFL capable of detecting bottom side corrosion as well as top side corrosion ?
A Yes, the MFL flux leakage signal is a bi-polar signal which can be detected at the top (near surface) of the tank floor even when the corrosion originates on the bottom (far surface) of the tank floor.


Q Can MFL be used to size corrosion ?
A Most MFL systems (like the MFL 2000) are simple detection devices and require another inspection method , commonly ultrasonics, to quantify the material loss. The FloormapVS2i system uses a combination of MFL signal amplitude and defect width to quantify the corrosion level and is currently achieving the same sizing accuracy as manual UT.

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