Research On The Outside Inspection Method For Defects At Carbon Steel Pipe Elbows Based On Remote Field Eddy Current

Metallic pipes are widely used in almost all industries,during the serving process,due to pipe material aging,harsh environment or poor working conditions and other reasons,cracks,corrosion,pits and other damage will be produced in the pipe wall.Especially the elbow of pipe part,the corrosion failure that caused by internal media erosion and impact is more likely to occur.This becomes weak lines of the whole pipelines.The accumulation of pipes' damages will eventually cause leakage or explosion accidents,which bring hidden dangers to safety production.Therefore,in order to find problems and maintenance in time,it is very necessary to having regular nondestructive testing for the pipes on active service.The remote field eddy current(RFEC)testing technique is very important in the nondestructive testing of metal pipelines.Not only it has the same sensitivity for both inside and outside wall defects,but also it is not sensitive for extraction of the sensor.However,in the practical application,the conventional RFEC testing sensor needs inside-passing condition,which means the equipment has to shut down then put the sensor into the pipe.To meet the inspection needs of testing for pressure pipeline on active service,this dissertation explains the new design of external RFEC sensors for inspecting the easily corrosion of pipelines elbow part.The main research work is as follow:1.Based on the finite element simulation,targeted for implementing the RFEC phenomenon by stimulating magnetic field twice through the wall of pipes,researched the affection factors of stimulating magnetic field penetration like placing the excitation mode,magnetic shielding and the number of the excitation elements,confirmed the structural style of external RFEC sensor by the double-excitation unit which made the rectangular coil as the excitation coil,added an external magnetic shield cover with an angle of 120 degrees from the coil,The simulation shows that this sensor can not only shorten the circumferential distance of the remote field,but also enhance the magnetic field signal of the remote field area.2.The elbow defect external RFEC testing simulation model established on the finite element software.The detection performance of the sensor for different orientation of the external wall defects and the quantitative relationship between the amplitude and phase of the sensor signal and the depth of the internal and externalwall defects were studied.The results show that the phase of the sensor signal changes linearly with the depth of the defect and can be used to quantify the depth of the elbow defect.However,due to the different sensitivity of the sensor to the inner and outer wall defects,it is not possible to quantify the depth of the inner and outer wall defects when inner and outer wall defects exist in the same place The response of the sensor caused by the circumferential defect is significantly weaker than the axial defect,but the difference of phase change caused by them is small.3.According to the simulation results,the external RFEC sensor is developed and the hardware platform of the RFEC detection system is built.The phase-locked amplifier is designed based on Lab VIEW to extract the phase and amplitude of the detection signal.The system was tested on Q235 pipe elbow,45 steel ladder tube and other specimens.The variation of the characteristic of the test signal is consistent with the simulation result,which verifies the validity and practicability of the designed probe.Compared with the conventional RFEC sensor,the external sensor proposed in this paper has the advantages of compact structure,No equipment downtime and so on.Therefore,the research results can provide new ideas and methods for the in-service testing of pressure pipelines.