Research On Detection Technology Of Gasifier Bend Corrosion Damage Based On Ultrasonic Guided Wave

Pressure bend is a key component of material transportation in gasifier system.The accumulation of long-term corrosion effect in the production process will lead pipeline damage and leakage,which will seriously affect the operation of equipment,and bring serious consequences.Therefore,it is necessary to monitor the corrosion state of the bend.Ultrasonic guided wave technology has been widely used in testing chemical pressure vessel and pipeline because of its advantages of long detection distance,non-contact and on-line monitoring.Because there are still some issues which have not be solved about the theory and method of ultrasonic guided wave detection for bend detection,this research utilized numerical simulation and experimental analysis to investigate the propagation law of ultrasonic guided wave in the bend of an outlet bend of Texaco carburetor in a chemical plant.Series of algorithms from the enhancement and extraction of defect echo signal characteristics to the quantitative identification of defect location was proposed to improve performance of ultrasonic guided wave detection technology appling in the field of bend defect detection.Firstly,the propagation characteristics of ultrasonic guided waves in the bend were studied.Based on numerical simulation and experiments,the commonness and difference of ultrasonic guided wave propagation laws in straight pipe and bend were studied and compared.Compared with the propagation law and characteristics in straight pipe,the ultrasonic guided wave has similar characteristics when it propagates in the front and back sections of the bend.However,when the guided wave propagates near the elbow of the bend,most of the energy of the guided wave gathers outside the elbow,while the energy inside the elbow is very weak.Secondly,the enhancement method of defect echo features was studied due to the weak echo signal in the bend and the difficulty in identifying and extracting defect features.The results pointed out that the denoising effect of db8 wavelet with soft threshold denoising method has the best performance among several common signal denoising methods.The SNR of straight pipe and bend signal were increased by 1.11 and 1.05 times respectively in the experiment with db8 wavelet.In addition,in order to further improve the defect echo features in the guided wave signals,the enhancement method of defect echo features was studied.The smooth and pseudo Wigner-Ville distribution algorithm was proposed for time-frequency analysis of the signals,which can effectively highlight and strengthen the echo characteristics of the defect.On the other hand,the time reversal technique was proposed,which increased the reflection coefficient of straight pipe and bend pipe defect to 3.9 and 1.1 times respectively.Then,a method of modal extraction of bend pipe defect echo based on time-frequency ridge line was proposed to solve the problem that there are many modes of ultrasonic guided wave echo signal,which makes it difficult to extract the defect wave packet of the specified mode.Based on the Wigner-Ville distribution algorithm,the time-frequency characteristics differences of guided waves in different modes were analyzed and compared.Moreover,the modal identification and separation method suitable for bend defect echo signal was proposed so as to extract the characteristics of specific modal defect echo.Experimental results showed that L(0,1)and L(0,2)modal components could be identified and separated successfully by this method.Finally,in order to solve the problem of quantitative identification of bend damage location,damage features identification algorithms in axial,circumferential and radial directions were proposed.In the axial direction of the defect,Gabor matching pursuit algorithm and Hilbert time-domain envelope algorithm were used to locate the defect in the axial direction.The experimental results showed that the error of the axial location was less than 3.01%.In the circumferential direction of the defect,the circumferential reflection coefficient method was proposed to circumferential location of the defect.It was pointed out that the amplitude of the defect return wave increased linearly with the increase of the circumferential size of the defect.The experimental results showed that this conclusion was only applicable to the damage detection before the elbow.In the radial direction of the defect,a radial quantitative method based on bi-modal relative strength of flaw echo was proposed,and the relative change process of L(0,1)and L(0,2)components of defects during the transition from non-penetrating to penetrating was analyzed.Finally,the semi-quantitative evaluation of radial depth for pipeline defects was successfully achieved.This research can not only be used for the monitoring and identification of the bend corrosion state in the production process of gasifier,but also be extended to the pipeline corrosion monitoring in chemical industry park,municipal pipe network,electric power equipment,gas pipe network and other fields,which has important research and application value.