Research On Microwave Detection Method Of Inner Surface Of Metal Pipeline

Metal pipelines are widely used in laying nuclear power plant pipelines,boiler pipelines and long-distance oil and gas pipelines.However,some small-diameter metal pipelines play an important role in them.Defects on the surface of nuclear power plant pipelines may cause pipeline leakage.Once leakage occurs,radioactive substances destroy chemical bonds of human tissues,posing a great threat to the environment and people's lives.For the inspection of inner surface defects of small-diameter metal pipes,there are some limitations when using magnetic flux leakage inspection method,eddy current inspection method and ultrasonic inspection method due to the influence of material properties and pipe size.However,microwave detection method does not need coupling agent,has high power and low propagation loss,and is suitable for detection in small-diameter metal pipelines.Based on microwave transmission theory and reflection theory,this paper proposes a method for detecting the inner surface of small-diameter metal pipeline based on microwave reflection method.The metal pipe is regarded as a metal circular waveguide,and the microwave is fed from one end of the metal pipe and receives the reflected signal.The defects on the inner surface of the metal pipe are detected by measuring the reflection coefficient and return loss of the microwave in the circular waveguide.Based on the basic theory of microwave,this paper analyzes the influence of defects in metal pipeline on the transmission characteristic parameters of microwave in pipeline and the intensity of electric and magnetic fields in pipeline.The mechanism of defect detection on the inner surface of metal pipeline based on microwave is analyzed.Using HFSS finite element simulation software,the simulation model of inner surface defect detection of small diameter metal pipeline is constructed,and the detection ability of microwave to inner surface defect of metal pipeline is studied by analyzing the frequency domain waveform of S parameter.The influence of circumferential length,axial length and depth of circumferential crack on return loss parameters of metal pipe inner surface is studied by controlling variables.The influence of the depth and axial length of the thinning area of the pipe wall on the return loss is analyzed.The results show that when there are defects on the inner surface of metal pipeline,the change of electromagnetic field distribution in pipeline caused by the change of local inner diameter of pipeline affects the transmission of microwave energy in pipeline.There will be a large return loss at one or more frequency points in the frequency domain.The change of depth,width and length of circumferential crack will cause the change of amplitude and phase of return loss in frequency domain,and the energy loss of microwave transmission in pipeline will increase with the increase of width,depth and length.The change of depth and width of tube wall thinning will cause the change of return loss.Moreover,the influence of the depth of tube wall thinning on return loss is greater than that of the width.