Zhuojin Pipe Fitting Co., LtdInternal Magnetic Array Eddy Current Detection for Small Diameter Steel Pipes
Literature Overview
This paper by Chen Tao and colleagues from Hubei University of Technology and Hubei Special Equipment Inspection and Testing Research Institute addresses a persistent challenge in the oil and gas industry: the reliable detection of internal and external wall cracks in small-diameter steel pipes during in-service inspection. The work, published in Sensors and Microsystems (2025, Vol. 44, No. 8), introduces an internally traversed magnetic array eddy current (MAEC) sensor design that overcomes the blind zone problem inherent in conventional eddy current inspection of small-bore piping. The research is funded by the National Key R&D Program (2022YFF0605600) and the National Natural Science Foundation of China (52105550), underscoring its significance in national pipeline integrity programs.
Core Technical Approach
The fundamental challenge in inspecting small-diameter pipes (in this case, 45# steel with 22 mm outer diameter and 18 mm inner diameter) is that conventional externally applied eddy current probes suffer from severe signal attenuation through the pipe wall, rendering internal wall defects virtually undetectable. The authors' solution is an internal traversed sensor that applies a magnetic array to enhance permeability modulation, combined with eddy current sensing for defect detection.
The methodology proceeds in three stages:
- COMSOL Multiphysics Simulation: A finite element model was established to simulate the coupled electromagnetic and magnetic field behavior of the magnetic array eddy current system. This allowed systematic variation of crack orientation angles and assessment of the resulting relative permeability changes and induced voltage signals.
- Signal Analysis for Crack Orientation: The relative permeability and voltage response were analyzed for internal and external wall cracks at various angles relative to the magnetic field direction. This parametric study revealed the directional sensitivity characteristics of the sensor.
- Physical Probe Fabrication and Experimental Validation: A working prototype was manufactured and tested on 45# steel pipe specimens with artificially introduced defects: internal wall cracks of 15 mm x 0.2 mm x 0.5 mm and external wall cracks of 15 mm x 1 mm x 1 mm, oriented at different angles.
Key Technical Findings
The following table summarizes the critical experimental parameters and outcomes:
| Parameter | Specification |
|---|---|
| Test material | 45# carbon steel pipe |
| Pipe OD/ID | 22 mm / 18 mm |
| Internal wall defect | 15 mm x 0.2 mm x 0.5 mm |
| External wall defect | 15 mm x 1 mm x 1 mm |
| Sensor configuration | 3 main channels in magnetic array |
| Simulation tool | COMSOL Multiphysics |
The three-channel magnetic array configuration proved effective in eliminating the blind zone for internal wall crack detection. By introducing a signal processing algorithm that synthesizes signals from multiple channels, the system achieved approximately uniform amplitude response for cracks at arbitrary angles. For external wall cracks, the sensor detected defects at all orientations, although the eddy current signal amplitude decreased as the angle between the crack orientation and the magnetic field direction diminished. This angular dependence is consistent with the fundamental principle that the induced eddy current disturbance is maximized when the crack interrupts the magnetic flux path most effectively.
Engineering Practice Implications
From a practical standpoint, this technology has significant implications for the integrity management of small-diameter piping systems, which are prevalent in:
- Heat exchanger tubes: Where internal and external wall corrosion and cracking occur simultaneously
- Instrumentation and control piping: Often too small for conventional inline inspection tools
- Petrochemical plant letdown valves and regulators: Where small-bore piping carries critical process fluids
- Hydrogen service lines: Where hydrogen-induced cracking in small diameter piping poses a safety hazard
The internal traversed approach eliminates the need for pipe removal, which is often impractical in operating facilities. However, engineers must consider the practical constraints of inserting a probe into a pipe with an 18 mm bore, including connector design, lubrication requirements, and the need for clean pipe interior surfaces free of scale or deposits that could impede probe travel.
Critical Reflections
The paper demonstrates a promising solution, but several questions remain for engineering application. First, the defect dimensions tested (0.2 mm depth for internal cracks) represent relatively small indications; the detection threshold for deeper, more realistic corrosion-induced cracks should be investigated. Second, the signal processing algorithm that achieves uniform amplitude response needs to be characterized in terms of false positive rates under realistic noise conditions. Third, the transition from laboratory pipe specimens to actual field conditions introduces additional variables such as pipe wall thickness variation, residual stress from prior welding, and the presence of multiple simultaneous defects.
The magnetic array concept draws upon the well-established principle of magnetic flux leakage combined with eddy current sensing, but the specific three-channel arrangement represents a novel engineering optimization. In my own experience with pipeline integrity programs, the most common failure mode in small-diameter piping is not through-wall cracking but rather wall thinning due to corrosion, which this method may not address directly. A comprehensive inspection strategy should therefore combine this MAEC approach with conventional ultrasonic thickness measurement.
Study Insights
This work exemplifies the convergence of computational modeling, sensor design, and experimental validation that characterizes modern non-destructive testing development. The use of COMSOL for pre-experimental optimization is now standard practice, but the value lies in how the simulation results directly informed the physical probe design. For engineers responsible for pipeline integrity programs, the key takeaway is that small-diameter piping inspection is no longer a dead-end problem; with appropriate sensor technology, reliable in-service detection of internal and external wall cracks is achievable. The next logical step would be to integrate this sensor into a full inline inspection tool capable of traversing pipe sections of several meters in length, which would transform this from a point-inspection method into a practical pipeline integrity management tool.