Research On In-Service Inspection Technique For Pressure Piping Based On Pulsed Remote Field Eddy Currents

Wall thinning,pits and other defects caused by corrosion or erosion is most threatening to the reliability and safety of pressure pipes which are commonly used in petroleum,chemical and thermal industries.The traditional remote field eddy current(RFEC)method for ferromagnetic pipeline has the same sensitivity to inner diameter(ID)and outer diameter(OD)defects,which makes it difficult to meet the online detection requirements and identify the defect's specific location.It is necessary to simultaneously obtain the size and location of defect in a single testing in order to ascertain richer and more accurately states information of pipes before pipeline maintenance.The study in this thesis focuses on quantitative evaluation and identifying location of defects for pressure pipeline on active service,and an outside inspection method based on pulsed remote field eddy currents(PRFEC)is presented.First,based on our preliminary work,the magnetic flux distributions caused by ID,OD and hybrid defects were respectively analyzed and their signal responses acquired by an outside RFEC probe were studied by using finite element analysis.Signal characteristics which can be used to infer the defect location were then extracted from the time-domain waveform,and they verified the feasibility of the outside inspection method of PRFEC applied for identification defect's location.Second,the significant size parameters of exciting coils and magnetic shield portion were simulated and optimized through single factor test and orthogonal design method.The simulation results showed that the excitation energy of sensor was increased by 25.7% and the peak value of testing signal was increased by 1.17 times after parameters optimization.Afterwards,based on the characteristic of magnetic field propagation,the principles of inside and outside RFEC inspection were compared and analyzed.Then,the repetitive frequency,duty cycle,voltage of the excitation pulse was optimized.On this basis,the magnetic perturbations caused by ID and OD defects and their response signals were studied,and meanwhile the correlations of signal peak and time of zero-crossing(TZC)with the defect depth and location were revealed.The experimental results show that: i)the probe signal peak value increases almost linearly with the increase of ID or OD defect depth,which can be used to quantify defect depth;ii)the TZC of ID or OD defect is in different periods,which can be used to identify defect location.Finally,according to the simulation results of parameters optimization for an outside sensor,the external RFEC sensor was developed and a PRFEC test jig was set up.The detecting prefabricated ID and OD defects experiment on a Q235 elbow was conducted.The carbon steel plate within hole defects was tested.The experimental regularities are consistent with the simulation results,which verify the validity and the feasibility of the method.Compared with conventional internal detection methods,an outside inspection method based on PRFEC is presented in this thesis.On one hand,the method overcomes the technique drawbacks of RFEC,which is difficult to identify the location of defects due to the same sensitivity to both ID and OD defects.On the other hand,the method solves the problems as equipment shut down caused by the sensor inside of the pipe.The research results will provide certain referenced value for the in-service testing of industrial pipelines.