Table of Content
- Applications of MFL Technology
- MFL in the Petrochemical Sector
- Principles of Magnetic Flux Leakage
- Sensors utilised with MFL Technology
- Magnet Types and Corrosion Challenges in MFL
- Equipment Design for MFL
- Challenges and Innovations
- Key Takeaways
- FAQs
Magnetic Flux Leakage (MFL) is a Non-destructive testing approach used on ferromagnetic mild steel plates to identify and map material loss. MFL inspection is a simple, quick, and reliable corrosion detection technology to use. In 1889, MFL was proposed as a method for inspecting the magnetic penetration of ferrous materials.
Until 1919, the magnetic flux leakage equipment was used only for defect inspection, with a magnetiser generating magnetic flux into the specimen and induction coils collecting the leaking magnetic field created by any faults.
Since then, scientists have focused on developing and improving magnetising, sensing, and signal-processing techniques, including those used in MFL technology. Meanwhile, a substantial study has been done on theoretical MFL distribution calculations, lift-off, and high-speed effects. New data processing algorithms are employed for defect recognition, categorisation, and quantification, especially with the rapid growth of artificial intelligence.
Applications of MFL Technology
Image Credit: MFEIS
MFL is a versatile technique, like many NDT Methods. Its efficient defect detection makes it a preferred choice for many applications, some of which include:
1. Crack Inspection Flux Leakage Testing:
This can effectively identify surface-breaking and subsurface cracks in ferromagnetic materials, ensuring structural reliability.
2. Handscan Tank Bottom Corrosion Screening:
A portable solution for localised inspections, this provides rapid assessments of tank floor corrosion without disrupting operations.
3. Magnetic Flux Leakage Inline Inspection:
Used for Inspecting Pipelines and tubular structures, this ensures the integrity of transport systems in oil and gas, water distribution, and industrial facilities.
4. Tube Testing Constant Field Magnetic Flux Leakage:
It is essential for detecting wall thinning, pitting, and cracks in heat exchangers and boiler tubes.
5. MFL Tank Floor Scanner for Comprehensive Scans:
This provides full-coverage scans of tank floors to detect and map corrosion or material loss, critical for storage tank maintenance and safety.
6. Magnetic Flux Leakage Technology in Rail Inspection:
Applied in rail track testing to identify flaws such as cracks and wear, it enhances the safety of transport infrastructure.
7. Inspection of Pressure Vessels and Spheres:
Ensures defect detection in curved surfaces and Spherical Storage Units, where manual inspection may be challenging.
8. Detection of Stress Corrosion Cracking:
This is used for identifying stress-related flaws in industrial equipment prone to harsh operational environments.
9. Offshore Platform Integrity Monitoring:
It is ideal for inspecting corrosion and damage in offshore structures, where accessibility and environmental factors pose unique challenges.
10. Magnetic Flux Leakage Method for Weld Testing:
This provides precise weld assessments in pipelines and tank constructions, ensuring the durability of critical joints.
These applications demonstrate the indispensable role of magnetic flux leakage technology in ensuring operational safety and structural health.
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MFL in the Petrochemical Sector
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Magnetic flux leakage (MFL) is routinely used for tank floor inspections in the petrochemical sector. Corrosion of the tank floor can result in product loss and environmental damage. As a result, tank floor inspections using magnetic flux leakage technology are critical to prevent such issues. Advanced has become a cornerstone of industrial inspections, providing rapid and reliable results over large areas.
Tank Floor Inspection using MFL
The magnetic flux leakage principle is based on magnetising a ferromagnetic material and detecting distortions in the magnetic field caused by defects.
- In MFL tank floor inspection, the Magnetic Flux Leakage Equipment magnetises the tank floor with permanent or electromagnets, and the resulting magnetic field variations are recorded and analysed.
- The magnetic field 'leaks' and 'leakage' is analysed to establish the position and severity of the defect of the tank floor – both near and distant surfaces – if there is corrosion, pitting, or wall loss.
- Tank floor scanning with an MFL tank floor scanner can help inspect prominent regions quickly, which 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 of testing area.
- 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 harms the signal response.
Defects must be discovered and characterised to enable repair and keep storage tanks in use for as long as possible. The magnetic flux leakage method offers significant advantages for tank floor inspections, including high-speed scanning, reduced downtime, and cost-effectiveness. Modern MFL technology leverages AI and Advanced Signal Processing to enhance defect detection and classification.
Magnetic Saturation in MFL
The level of induced magnetisation, defect orientation, sensor design, component magnetic characteristics, and other parameters such as scanning velocity influence MFL. 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 an under-saturated material.
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 materials 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.
What are the Principles of Magnetic Flux Leakage?
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. A near-saturated flux can be created in the plate if the magnet is powerful enough.
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.
Sensors utilised with MFL Technology
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.
1. 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.
2. 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, 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.
Understanding Magnet Types and Corrosion Challenges in MFL
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. Dish-shaped corrosion can be more challenging to detect because of the sloping edges.
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 the general thinning area caused due to widespread corrosion with follow-up ultrasonic thickness inspections.
Equipment Design for the MFL
1. MFL Scanner:
All MFL inspection equipment share several properties 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, improving 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. Signs on the back of the instrument indicate where the operator should undertake any more UT Inspections when the instrument comes to a complete halt.
Some instruments can be dismantled for use in tight spaces or on smaller surfaces. There are 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.
2. 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.
Certain users support using thresholding, while others argue that because signal amplitude is affected by various circumstances, using a threshold number is undesirable as certain problems may be ignored.
3. 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 %, 40 %, 60 %, and 80 % through the wall with a 22mm ball-end drill.
4. 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 demagnetise reference plates and test for 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.
5. 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 emphasise 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 NDT 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.
Challenges and Innovations in Magnetic Flux Leakage Technology
While MFL is effective, it has limitations, such as difficulty in detecting very small defects or differentiating between types of flaws. Innovations like constant field magnetic flux leakage sensors and machine learning are addressing these challenges, improving magnetic flux leakage accuracy on Tank Floor Inspections.
Adhering to a robust magnetic flux leakage testing procedure is crucial. This involves calibration using reference standards, proper setup of the MFL inspection system, and skilled interpretation of results. The magnetic flux leakage testing market continues to evolve, driven by innovations in hardware and software, such as the development of lightweight MFL Tank Floor Scanners.
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 do it justice.
Key Takeaways
- MFL is a proven, efficient non-destructive testing method for detecting and characterising material loss, such as corrosion and pitting, in ferromagnetic steel tank floors. It provides rapid inspection over large areas, making it an essential tool for maintaining the structural integrity of storage tanks in industries like petrochemicals.
- Achieving proper magnetic saturation is crucial for accurate defect detection. The choice of sensors (coil or Hall Effect) significantly affects inspection results, with each offering distinct advantages in sensitivity, reliability, and handling of surface irregularities.
- Modern advancements in MFL technology, including artificial intelligence for data processing and signal analysis, have enhanced defect detection and classification. These innovations ensure more reliable and efficient inspections, reducing the risk of missed defects.
FAQs
1. What defects can Magnetic Flux Leakage (MFL) detect in tank floors?
A: MFL can detect various defects, including general corrosion, corrosion pits, and wall thinning caused by environmental or operational factors. It is particularly effective in identifying volumetric losses in material, though follow-up inspections, like ultrasonic testing, may be required for detailed characterisation.
2. What factors influence the accuracy of MFL inspections?
A: Several factors, such as the level of magnetic saturation, defect orientation, scanning velocity, sensor type, and material properties, impact the accuracy of MFL inspections. Proper calibration using reference plates and skilled interpretation of data is essential for achieving precise results.
