Magnetic Flux Leakage Testing ( (MFL Testing) is an important testing method that has seen significant development in pipeline inspection since the 1960s, becoming one of the most commonly used pipeline inspection methods in the oil and gas industry, such as Magnetic Pipeline Inspection Gadgets (PIGs)
What is Magnetic Flux Leakage Testing?
Magnetic Flux Leakage Testing is a non-destructive testing technique, which can not only find surface and near-surface crack defects but also detect internal defects like corrosion pits from the exterior.
The principle of MFL detection is based on magnetizing the ferromagnetic material being tested. Suppose the material’s internal composition is continuous and uniform. In that case, the magnetic induction lines are confined within the material, with magnetic flux parallel to the material surface, resulting in almost no magnetic field on the surface.
However, if the magnetized material has defects, its low magnetic permeability and high magnetic resistance cause distortions in the magnetic circuit. The induction lines change, with some magnetic flux passing directly through the defect or bypassing it from within the material, while some leaks into the space above the material surface, forming a leakage magnetic field at the defect location.
Magnetic induction sensors capture these leakage field signals, which are then processed by a computer. Analyzing the magnetic flux density of the leakage field can provide further information about defect characteristics such as width and depth.
Magnetic Flux Leakage Testing Equipment in Pipeline Inspection
The main equipment and materials for Magnetic Flux Leakage Testing include the Magnetic Flux Leakage Testing system, test specimens, and shims.
MFL detection equipment can be classified into 5 types based on magnetization methods: AC magnetization, DC magnetization, permanent magnet magnetization, composite magnetization, and comprehensive magnetization. Based on magnetization direction, it can be divided into circumferential magnetization and axial magnetization.
An MFL testing system should include a power supply, magnetization device, magnetic field sensors, scanning device, signal acquisition unit, signal display (or alarm) unit, and signal storage unit.
When necessary, it should also have demagnetization and position recording devices. The MFL detection system can also analyze and display the location and depth of detected defects.
The Process of Magnetic Flux Leakage Testing
| MFL Testing and Signal Acquisition: The internal detector is pulled into the pipeline, magnetizing the pipe wall and collecting magnetic flux leakage signals. The collected signals are stored in the data acquisition room, after which the detector is pulled out of the pipeline.
| Signal Pre-processing and Visualization: The detector parameters are first calibrated before the inspection. Then, defect signals are extracted using methods such as wavelet decomposition. Finally, visualization software presents the defect signals in image form.
| Anomaly Signal Identification: Anomalous signals are identified through signal feature analysis and classified, distinguishing between pipeline damage and non-damage states.
| Defect Quantitative Analysis: Based on the defect signals, the location, size, and depth of defects are inversely calculated.
| Equipment Safety Assessment and Prediction: Combining the defect inversion results with factors such as pipeline environment, a safety assessment and prediction of the pipeline’s operational state is conducted.
Advantages of Magnetic Flux Leakage Testing
Compared to methods like magnetic particle testing, MFL testing has the following advantages:
| No pre-treatment of the inspection object is required, making it easy to obtain detection signals with a high degree of automation.
| The introduction of computers has improved the accuracy of defect judgment based on detected signals.
| It can detect various types of defects such as pores, shrinkage cavities, and corrosion, allowing for preliminary quantification of defects.
| In pipeline inspection, it can simultaneously detect internal and external wall defects within a wall thickness range of up to 30mm.
| High efficiency, non-polluting, and automated inspection can achieve very high detection efficiency.
Limitations of Magnetic Flux Leakage Testing
| Only applicable to ferromagnetic materials.
| Low detection sensitivity; complex inspection conditions require processing large amounts of magnetic field signals, making it difficult to determine defects accurately.
| Rough quantification of defects; the presence of impurities in the material significantly affects detection signals.
| Due to the use of sensors to detect magnetic flux leakage, the method is not suitable for inspecting specimens with complex shapes.
| MFL testing is not suitable for very narrow cracks, especially closed cracks. Experiments have shown that very narrow fatigue cracks do not produce detectable signals in MFL testing.
Application Range of Magnetic Flux Leakage Testing
Petrochemical Industry: Oil and natural gas are often transported through long-distance pipelines; these pipelines are mostly buried underground and exposed to high humidity or high ground pressure environments, making them susceptible to corrosion and deformation. MFL testing technology has been widely applied in oil and gas pipelines to solve this problem.
With technological advancements, MFL testing technology has made significant progress in signal processing capabilities, processing speed, and defect judgment accuracy, demonstrating its important value in the oil and gas fields.
Image Source: Epcmholdings & Pipecaregroup & Researchgate & Rosen-group & Sciencedirect
