Numerical Simulation On MFL Inspection For Heat Exchanger Pipes

Magnetic flux leakage(MFL) is one of the most commonly used Non-Destructive Testing (NDT) methods. Because of the obvious advantages like high efficiency, high sensitivity and simple devices, it is widely used in inspection of gas pipes and railway tracks. Whereas, research on the application of MFL to inspection of heat exchanger pipes with retaining plate is not carried out so often. Heat exchanger, as an general equipment in many industrial sectors like chemistry, petroleum and power generating,is likely to fail due to shock, cracks, corrosion and deformation. Statistics suggest that 90% of defects in ferromagnetic heat exchanger pipe occur under the retaining plate or nearby, making it a key monitoring area. By utilizing Ansoft finite element software, this paper conducts analog simulation to analyze factors that will exert influences on magnetic flux leakage signal nearby the retaining plate.Numerical simulation on low-speed magnetic flux leakage inspection in heat exchanger pipes is described. Statistics suggest that 90% of defects in ferromagnetic heat exchanger pipe occur under the retaining plate or nearby, making it a key monitoring area. This paper illustrates influence of different velocities, defect depth and position on the characteristic of flux leakage signals. Result of the numerical simulation provides theoretical principle for application of Magnetic flux leakage (MFL) technique to inspection of heat exchanger pipes. It possesses great significance to enhance the possibility for defect inspection as well. By using finite element software Ansoft, this paper describes numerical simulation on low-speed MFL inspection in heat exchanger pipes. Conclusions of the simulation are listed as following.(1). Influence of the retaining plate is illustrated. Compared with the situation where no retaining plate exists, the magnitude of the output signal is much smaller. With a constant speed and depth, magnitude of the two peak values increases with the distance between the defect center and X-axis is increased. When the distance is more than 20mm, influence of the retaining plate is no longer that obvious;(2). With a constant speed and position, the magnitude of the MFL signal has same ratio to the depth of surface defect, which indicates that the deeper the depth the easier for it to be inspected; (3). Because the speed is very low, also with the existence of the retaining plate, current effect caused by moving magnetic field is not that conspicuous. But still magnetic field strength decreases when the probe speed is increased.Result of the numerical simulation provides theoretical principle for application of the MFL technique to inspection of heat exchanger pipes. It possesses great significance to enhance the possibility for defect inspection as well. Further work needs to be done to establish a 3-D model to simulate the condition more practically.