Ultrasonic Time Of Flight Diffraction (TOFD) Detection In Spherical Tank Butt Weld And Defect Analysis

Spherical tank is a kind of steel pressure vessel,which is widely used in oil,liquefied natural gas and other materials storage.Its body is composed of many spherical shell plates welded,and the welding quality directly affects the service life of spherical tank.As a new nondestructive testing technology,ultrasonic TOFD detection has the advantages of fast detection speed and high detection rate in spherical tank butt weld.It has been widely used in weld detection of pressure vessel,but ultrasonic TOFD detection technology still has limitations.In this paper,there is a near surface blind area in ultrasonic TOFD detection technology,because the redundant information produced near the defect endpoint can also be received by the probe,the transverse resolution of the image is low.The limitation of defect feature recognition is studied in three aspects.Therefore,this article mainly aims at the near surface blind area,and the acoustic beam diffusion angle of the probe makes the receiving probe receive redundant information produced near the defect end point,which results in low lateral image resolution,and the limitation of defect feature recognition is studied in the ultrasonic TOFD detection technology for the butt weld of spherical tank.The main contents of the study are as follows:(1)Compare the effects of different frequency,wafer size and refraction angle,scanning times and probe center spacing on the results of ultrasonic TOFD.(2)Research on ultrasonic TOFD detection blind area and the principle of ultrasonic phased array detection,and used ultrasonic TOFD and phased array inspection technology to scan the custom block in the weld surface crack of artificial design block at the same time.The results show that ultrasonic phased array can detect the cracks near the surface of the weld seam,The ultrasonic phased array can detect the near surface crack of the tailored weld.The quantitative error of the phased array is less than 3.00 mm,while the quantitative error of the TOFD detection is less than 1.00 mm,and the quantitative error of the phased array is larger,compared with The quantitative error of TOFD detection for defects is less than 1.00 mm.Using ultrasonic TOFD and phased array detection to detect the weld simultaneously,whichcan not only reduce the work load,but also realize the accurate localization and quantification of defects.(3)Research on The principle of synthetic aperture focusing,and based on the principle,established the mathematical model of ultrasonic TOFD detection D scan image,and the ultrasonic TOFD detector was used for non-parallel scan of the customized T-B test weld,and acquired the D-scan image,and applyed the mathematical model of SAFT image processing,and the defect length measured in the image processed by SAFT is compared with the parabola pointer and the end point 6dB length measurement.The results show that the D scan image processed by SAFT has higher lateral resolution,the absolute error of the defect length is less than0.5mm and less quantitative error to defect length.(4)Research on edge detection,the Canny operator which is more suitable for TOFD image defect edge feature extraction is selected.,and the traditional Canny operator is improved and the improved Canny operator is used to extract the defect feature.The results show that the improved Canny operator can effectively extract the defect feature information and avoid the loss of the edge feature information due to the change of gray level in the image and the error of the defect location is less than 1.2mm.