1. INTRODUCTION
1.1 Magnetic Flux Leakage (MFL) technology has been used in the monitoring of Underfloor or Far Side (FS) corrosion for about 11 years. As with all Non Destructive Testing methods, MFL has both advantages and disadvantages, as well as pitfalls for the unwary. This paper attempts to explain the underlying principles of the method and highlight the advantages, disadvantages and pitfalls.
2. PRINCIPLES OF OPERATION
2.1 GENERAL
MFL technology is similar to Magnetic Particle Inspection (MPI) - without the ink! In both cases the component is magnetised to a level at which the presence of a significant local reduction in material thickness causes sufficient distortion of the internal magnetic field to allow flux lines to break the test surface at the site of the discontinuity. In the case of traditional MPI, a ferromagnetic powder, in wet or dry form, is used to mark the spot so that it is readily visible by the inspector. With MFL, suitable sensors are used to give an electrical signal at the leakage site. This signal may operate an audible or visual alarm to alert the inspector, or may store the event for computer mapping of the area. Thus both techniques require two basic things, a method of magnetisation, and a method of detecting the leakage field.
2.2 In either technique magnetisation can be achieved using Electro-magnets or permanent magnets. Similarly, just as there are several types of ink that can be use in MPI, there are several types of sensor that can be used in MFL. These include Coils, Hall effect sensors, Magnetostrictive and similar devices.
2.3 MAGNETISATION
2.3.1 Because the MFL method responds to both far side (FS) and near side (NS) corrosion it is necessary to introduce a strong magnetic field into the component wall. The closer this field becomes to saturation for the component, the more sensitive and repeatable the method becomes. For typical steels used in Bulk Liquid Storage Tank construction this value is generally between 1.6 and 2 TESLA. In this range any residual magnetism from previous scans or operations will be eliminated during subsequent scans so that the resulting flux leakage signals remain relatively constant and repeatable. Working below the 1.6 T level will still detect pitting on the first scan, but residual magnetism tends to cause a progressive deterioration of signal amplitude during subsequent re-scanning unless alternate scans are made from opposite directions.
2.3.2 For a given magnet system the flux density achieved in the component will depend on the thickness and permeability of the material. For most storage tanks the steels used are in the mild steel range equivalent to the old EN 2 grade and these steels have similar permeability. So the factor controlling flux density becomes one of plate thickness. There will be an upper thickness limit for each given magnet system above which flux density will be too low to give adequate sensitivity to pitting. One advantage of using an Electro- magnet is that the magnetising current can be increased to cater for a wider range of plate thickness than can be achieved with a given permanent magnet. However, this will be at the expense of size, weight, and the use of an independent (battery) power supply.
2.4 SENSORS
2.4.1 Centred between the poles of the Magnet Bridge and stretching the full scanning width of the system is an array of Hall effect sensors. These are spaced at 7.5 mm between centres to give optimum resolution and coverage. The sensing range of each sensor is sufficient to allow overlap with its neighbour.
2.4.2 Hall effect sensors give a voltage signal proportional to the flux density of the field passing through the sensing element. Figs 5 & 6 show the field patterns for sound and pitted material. Because position the sensing elements parallel to the scanning surface, it follows that it is the Normal (Vertical) component of the magnetic flux leakage vector which will be measured. If the sensing elements were to be arranged perpendicular to the surface, then it would be the Tangential (Horizontal) vector that would be measured. There are advantages and disadvantages with both these alternatives and these will be discussed later.
2.4.3 The sensors are arranged to be about 2-3mm above the scanning surface to avoid wear and other mechanical damage during scanning. Since sensitivity is reduced as this distance is increased, lift off is normally only reduced to help compensate for thicker fibreglass coatings.
Application –
• To carry out flaw detection on a large tank floor area within an economy time frame.
Advantage –
• Very powerful in scanning a large floor area.
• Less dependent on operator technique.
• Efficient and easy operation.
• A reliable indication of corrosion on tank floor.
Limitations –
• Impossible to achieve 100% coverage due to limitation of physical access.
• Cannot differentiate between response from top side and bottom side indications.
• Qualitative but not quantative inspection tools.
• Due to environmental and physical restriction during inspection, no reliable quantative indication is possible.