Research On High Power Pulsed Eddy Current Test Technique For Residual Wall Thickness Of Pipeline Under Insulation Layer

Pipeline with insulation layer is widely used in petroleum,chemical industry and electric power industry.The wall thickness thinning caused by corrosion or erosion seriously threatens the reliability and safety of its operation.Therefore,it is very important to carry out regular nondestructive testing.The pulse square wave signal contains rich frequency components,and the detected signal of the pulsed eddy current technique contains more information about the integrity of the device under test.Because the pulse eddy current technique does not need to contact the probe and the device under test,and also does not need to pretreat the device under test and peel off the external insulation layer,its detection has the advantages of wide range and fast speed,and can realize non-stop online detection of the defects of the pipeline wall thickness thinning under the insulation layer.In this thesis,the theory and approach of pulse eddy current technique for the pipeline thickness under the insulation layer are studied systematically and deeply.Specific research contents are as follows:(1)The pulsed eddy current detection probe for residual wall thickness of pipeline under insulation layer is designed and optimized.The four-layer flat plate model was used to approximate the pipeline model with insulation layer.Based on that,the mapping model of residual wall thickness and the signal of the pulsed eddy current technique for the pipeline thickness under insulation layer was established.And the pulse eddy current time-domain voltage difference signal was obtained by numerical simulation.It was found that 0.2s～0.5s was the stable stage of pipeline wall thickness detection,and the pipeline wall thickness evaluation approach based on differential peak value was proposed.Followed that the structural parameters of the probe were optimized by considering the signal strength,linearity and sensitivity.Finally,the coverage area of the designed probe was analyzed.(2)The pulse eddy current testing system for residual wall thickness of pipeline under insulation layer is built up.The system includes three parts: PECT probe,hardware circuit and upper computer software.The hardware circuit adopts the modular design idea,and the power supply,probe excitation,data collection,master control modules are integrated in the hardware circuit box.The upper computer software is developed with the test hardware circuit design,which can realize the real-time receiving,processing and visualization of the multi-channel detection probe data through the serial port.Finally,the system experiment platform was built to complete the overall debugging of the detection system.(3)The experiments of the residual wall thickness detection system of pipeline under insulation layer are carried out and the results are analyzed.A novel voltage extraction method based on random sampling consistency algorithm is proposed,which can effectively improve the accuracy and stability of detection and reduce accidental errors.The induced voltage signal of different wall thickness at different lift-offs were detected in the experiment,and the calibration model of wall thickness was obtained.The absolute thickness measurement error was all within ±0.5mm.A multi-probe detection method based on time-sharing excitation is proposed to reduce the coupling of excitation magnetic fields between array probes.The repeatability and stability of the system were verified by the system repeatability experiment and the change of the characteristic voltage with time at room temperature.