The truth about magnetic flux leakage as applied to tank floor inspections
The Magnetic Flux Leakage (MFL) is a Non-destructive testing (NDT) approach. It is used on ferromagnetic mild steel plates to identify and map material loss. MFL is a simple corrosion detection technology to use, quick, and reliable.
In 1889, MFL was proposed as a method for inspecting the magnetic penetration of ferrous materials. Until 1919, the MFL was used only for defect inspection, with a magnetizer generating magnetic flux into the specimen and induction coils collecting the leaking magnetic field created by any faults. Since then, scientists have focused their efforts on developing and improving magnetizing, sensing, and signal processing techniques. Meanwhile, a substantial study has been done on theoretical MFL distribution calculations, lift-off effects, and high-speed effects. New data processing algorithms are employed for defect recognition, categorization, and quantification, especially with the rapid growth of artificial intelligence.
In the petrochemical sector, magnetic flux leakage (MFL) is routinely used to inspect tank floors. Corrosion of the tank floor can result in product loss and environmental damage. As a result, tank floor inspections are critical to prevent such issues.
The approach magnetizes the tank floor with permanent or electromagnets, and the resulting magnetic field variations are recorded and analyzed. The magnetic field 'leaks' and 'leakage' are analyzed to establish the position and severity of the defect of the tank floor – both near and distant surface – if there is corrosion, pitting, or wall loss.
The capacity to inspect prominent regions quickly is a benefit of this technology over others. MFL systems detect material loss with diameters in the mm's over spans of hundreds of m2. Eddy currents are produced in the plate by the relative motion of the magnet assembly, even though this approach is known for its fast inspection speed. By distorting and resisting the induced magnetic fields, the plate has a negative impact on the signal response. Defects must be discovered and characterised to enable repair and keep storage tanks in use for as long as possible. The strength of MFL inspection is detection.
The level of induced magnetisation, defect orientation, sensor design, component magnetic characteristics, and other parameters such as scanning velocity influence MFL. The level of magnetic saturation reached within each plate in a permanent magnet-based system varies with thickness. This is due to the continuous flux generated by a permanent magnet assembly distributed over more significant amounts of material as the plate thickness grows. Saturation is regarded as the most critical aspect of an MFL inspection. When saturation is attained, more minor faults can be detected than in a material that is under-saturated. When modifications in material characteristics are noticed through possible repairs or discrepancies in material manufacturing, MFL inspection is difficult to characterise because saturation levels differ with material qualities. Non-homogeneous flux distribution is induced into the plate due to the nature of a permanent non-adjustable magnetic field. When velocity effects are combined with increasing plate thickness, the effect is magnified.
A material's magnetisation level is defined as the ratio of the applied magnetic field to the induced flux density within the material, which is traditionally represented by a BH curve, as shown in Figure 1. The BH curve shows the ability of a material to produce magnetic flux density (B) as a function of magnetic field intensity (H) applied to a material with no residual magnetisation. Figure 1 depicts the magnetisation levels of a typical ferromagnetic material, with four zones being considered as H increases.
Figure 1 - Depiction of a typical BH curve trend as a function of H, along with the associated magnetic domains of a ferromagnetic material at each level of the curve.
Stage (i) is the initial state of the material when it is not impacted by a magnetic field, which means that the magnetic domains are randomly aligned, resulting in a net magnetisation of zero. The applied H increases in stage (ii), causing the plate to get magnetised for the first time. The domains inside the plate begin to align as a result of this. Stage (iii) occurs when the applied H field is raised further, and the plate's domains are almost totally aligned in the direction of the applied magnetic field. Finally, stage (iv) is reached when the substance has reached saturation. When all magnetic domains are fully aligned in the direction of the applied H field, this occurs. After this point, any rise in H will not result in a meaningful change in B. This has a clear correlation with the increase in reluctance when the material domains align.
Principles of MFL
Magnetic Flux Leakage (MFL)
Magnetic lines of force (flux) are generated within a steel plate when a magnet is placed close to it. These flux lines prefer to pass through the plate rather than air. If the magnet is powerful enough, a near-saturated flux can be created in the plate.
Corrosion pits and wall thinning drive the material's magnetic flux "out", which can be monitored with a coil sensor or a Hall Effect sensor. Each of these sensors has its own set of benefits and drawbacks. A typical MFL setup is shown in Figure 2.
Figure 2 - Illustration of MFL principle
Sensors
Coil and Hall Effect sensors are the two types of sensors usually utilised with MFL technology. There are compelling justifications for using either sort of sensor.
Coil sensors
Coil sensors are passive and rely on Faraday's Law to work. An electric signal is induced within a coil passing through a magnetic field. The signal's amplitude can then be determined.
The resulting signal amplitude can be affected by the speed of scanning and/or the "lift-off" (height of the sensor/magnetic field from the plate under inspection), but there is less latitude than Hall Effect sensors. Coil sensors are less sensitive than Hall Effect sensors, but their sensitivity is more than enough, and they create fewer false calls and are less susceptible to inspection surface roughness.
Hall Effect sensors
A Hall Effect sensor is a solid-state device that creates a voltage signal based on flux density when placed into an appropriate electrical circuit.
Hall Effect sensors have a higher sensitivity than coil sensors, although they are more susceptible to induced eddy currents. To eliminate eddy current effects caused by starting/stopping and operation, appropriate filtering and signal rectification and some cross-referencing are required.
Hall Effect sensors enable more lift-off of the inspection head and magnetic field from the inspection surface, resulting in less instrument wear. Scanning over rougher inspection surfaces, such as around a weld spatter zone, is also possible.
Magnets
The saturated field can be generated by a variety of magnets. Electromagnets, as well as powerful rare earth magnets, are acceptable. The latter approach does not require an external power supply and is typically lower in weight, but it cannot be switched off.
Defect Types
Many different defects might occur in a storage tank. These include general corrosion on the top and bottom sides, significant underbelly lake corrosion, minor corrosion pits on both sides, and sulphur-reducing bacteria corrosion on the topside. As shown in Figure 3, they are divided into three categories, each with its own set of features.
- Dish-shaped corrosion with a sloping edge, indicative of general corrosion
- Corrosion pits with a conical shape
- Corrosion-resistant pipes
Because the strength of the MFL signal is affected by defect volume, isolated thin but deep defect pits or pipes may be ignored.
Because of the sloping edges, dish-shaped corrosion can be more challenging to detect. The MFL apparatus will detect a change in plate thickness. As a result, once the MFL inspection head enters a large area of corrosion, the system can only identify more plate thickness loss. It may be feasible to discern the borders of such corrosion and determine an area of general thinning due to widespread corrosion with follow-up ultrasonic thickness inspections.
Figure 3 - Illustration of generic defect types and locations
Equipment Design
MFL Scanner
Several properties are shared by all MFL inspection equipment used for floor scanning. The magnetic bridge and inspection head are placed together on a wheeled carriage. An operator can grasp and guide the instrument like a lawnmower using a handle attached to the carrier that rises to just over waist height. This handle is frequently equipped with a controlling computer. Some instruments are propelled by hand, while others are driven by a small motor that may be turned off for manual use.
The inspection head is typically up to 36 sensor devices, resulting in an inspection width of 250mm to 300mm. Each sensor's signal can be evaluated independently, which improves resolution. Typically, the instrument's display informs the operator which sensors have observed flux leakage and, as a result, which areas are likely to have experienced a loss in floor thickness. When a signal exceeding a threshold level is detected, the motorised instruments usually have an engaged automatic stopping feature. When the instrument comes to a complete halt, signs on the back of the instrument indicate where the operator should undertake any more UT inspections.
Some instruments can be dismantled for use in tight spaces or on smaller surfaces. There is various specific "hand scanning" equipment available for such needs.
MFL scanning can be done automatically, with information from each scan run being saved. This can be analysed right away or saved to a computer to create an MFL image of the entire tank floor. Following that, the data can be analysed outside of the inspection environment. Areas needing follow-up UT inspections can be determined based on this data.
Signal Processing
To obtain a clear signal from noisy data, signal processing of the input from each sensor may be required. There is no requirement that the operator comprehends all aspects of the processing. Despite this, the operator will be needed to set a signal threshold value to distinguish between a reportable defect indication and lower-level spurious and false indicators.
It may be feasible to change the threshold value retrospectively using automated inspection data, allowing the operator to convey more information about the tank's condition without rescanning at greater sensitivity. This type of data analysis should be done by a more senior operator who has worked with automated inspections before.
Certain users support using thresholding, while others argue that because signal amplitude is affected by various circumstances, using a threshold number is undesirable because some problems may be ignored.
Reference Plate
Setting the sensitivity of the MFL equipment should be done on an appropriate reference plate composed of material with similar magnetic characteristics and thickness as the floor sections to be inspected.
Figure 4 shows an example of a conventional reference plate design. The dotted circles depict dish-shaped pits drilled through the plate thickness by 20 per cent, 40 per cent, 60 per cent, and 80 per cent through the wall with a 22mm ball end drill.
Figure 4 - Standard reference plate design
Magnetisation of the reference plate
To avoid the production of an induced magnetic field, operators should avoid extended contact between the MFL magnetic bridge and the reference plate. Regularly demagnetize reference plates and test for the presence of any residual induced magnetic field. A gauss metre or similar device should be used for testing. If the reference plate still has an induced magnetic field measurable by MFL equipment, it must be changed. Before doing any inspection, the induced magnetic field must be checked.
Defect Notification
The operator is usually alerted to the presence of an MFL indication by the illumination of one or more LEDs on the instrument display. If the instrument is powered, it will come to a halt directly behind the instrument carriage, in the indicated area. A row of LEDs may be present at the instrument's base to locally emphasize a suspected point(s).
If the instrument is manually operated, the operator may need to run it back and forth over the suspected location to identify it correctly. It is recommended that the operator move the instrument over the area in various directions to avoid the magnetic saturation of the questionable area.
Within any inspection operating procedure, the defect identification mechanism for each given instrument should be clearly described. Different approaches may be required if a service company uses more than one type of instrument.
Final Words
MFL is sometimes paired with another technology since MFL cannot distinguish whether the corrosion is on the tank floor's bottom or top side. There is currently limited training available for MFL equipment users in this application. Appropriately trained and qualified individuals must perform ultrasonic testing. This is not just a "measurement of thickness" but an evaluation of corrosion, and the technician must fully comprehend the technique to be used.
