Magnetic Flux Leakage Detection Of Defects In Long-distance Natural Gas Pipelines

Long-distance natural gas pipeline plays a key role in gas energy transportation and is called "energy lifeline".With the increase of service life,the pipeline safety problem becomes increasingly serious,leading to the high accident rate of pipelines in China.Regular inspection and maintenance of pipelines is the main means to ensure the safe and stable operation of pipelines.Among various pipeline detection technologies,magnetic flux leakage(MFL)detection technology is widely used in pipeline nondestructive testing due to its high degree of automation,low requirements for detection environment and reliable detection signal.Based on the theory of magnetic flux leakage(MFL)detection,the paper conducts simulation and experimental research.Through simulation analysis and experimental verification,the paper analyzes the corresponding relationship between the defects of natural gas pipelines and the magnetic leakage field,and develops a magnetic flux leakage detection system for natural gas pipelines.First of all,analyze the forming mechanism of natural gas pipeline defect leakage magnetic field,the magnetic dipole model are introduced to approximate solution of leakage magnetic field,in order to overcome the magnetic dipole model of error and accuracy is not deficiencies,such as the pipeline defect leakage magnetic field is solved by finite element method,established the mathematical model of leakage magnetic field finite element method to solve.Then,according to the actual situation of pipeline detection,the 3d finite element simulation model of flaw magnetic flux leakage detection in natural gas pipeline is established.The effects of length and depth changes of four defects with different shapes(rectangle,cylinder,trapezoid and asymmetrical triangle)on MFL signals are studied.The results show that the distribution of MFL signal is closely related to the shape of the defect.The minimum amplitude,the narrowest effective waveform width and the minimum signal strength of cylindrical defect magnetic flux leakage signal;The magnetic flux leakage signal of asymmetrical triangular defect is shifted obviously.Trapezoidal defect magnetic flux leakage signal is similar to rectangular defect,the signal strength is in the center.In addition,the interaction of the adjacent defect leakage magnetic field and the influence of the lifting value of the sensor on the magnetic flux leakage signal are studied,and the superposition phenomenon of the adjacent defect magnetic flux leakage signal is found,so the lifting value of the sensor should be reduced appropriately in the detection to achieve the best detection effect.Then,a magnetic flux leakage detection system for natural gas pipeline is designed.The whole signal acquisition circuit,with MCU as the control core,is composed of multi-channel sensor probe,multi-channel analog switch,A/D conversion module and data storage.Signal acquisition circuit requirements within 0.5ms sampling 120 hall sensors,multi-channel analog switch used in signal acquisition circuit of single channel 8 selected 1 MAX4638 EUE chip,the chip in the +5V single power input impedance is 3.5 ?,the total time for the gate 1 channel 25 ns,the time for completing the 120 hall sensor sampling theory for 3us,fully meet the design requirements.A/D conversion using ADS1256,is an 8-channel 24-bit high-speed low-power continuous approximation of A/D conversion chip,the sampling rate up to 30 KSPS,the accuracy up to 0.00001,in the conversion speed and precision to meet the circuit design requirements.The experimental prototype of pipeline was developed to realize the detection,display and historical data view of pipeline defects.The pipeline detection method is highly automated and has a good visual effect.Finally,the experimental platform was built and the pull experiment was carried out.The finite element simulation was verified by the experimental test results.The signal curves of the two have the same change trend,and the experimental results are consistent with the law of magnetic leakage obtained by the finite element analysis.The experimental results show that the accuracy of finite element simulation is verified by the experimental test results,and the corresponding relationship between different defect characteristics and MFL signal characteristics is obtained.The experimental prototype can realize the effective detection of natural gas pipeline defects.It provides reference for the accurate quantification of defects and the development of pipeline detection equipment.