| Pipeline transportation is one of the four modes of transportation and plays an important role in the national economy industry.Pipe Elbow is an important part of piping system,many pipe elbows connect different pipes to different directions.The pipe elbows are in the main scour position of the fluid all the year round.During the service period,the pipe elbows are vulnerable to the influence of complex environmental changes,such as corrosion cracks and heavy leaks,which threaten the structural safety of the whole pipeline system.Thus endangering the structural safety of the entire piping system.Nondestructive testing is a key technique for safe operation and maintenance of pipelines,Compared with various nondestructive testing methods,Ultrasonic guided wave as a long distance and efficient detection method has a prominent advantage in detecting defects in curved pipes.In this paper,the following work has been carried out for the study of guided wave detection technology in curved pipelines.Firstly,This paper is based on the elastic wave theory in solids,the derivation process of different modal frequency dispersion curves of ultrasonic guided waves in hollow cylinders is given.Put DN50 Oil Pipeline as an object,reseshed the influence of the variation of pipe thickness and radius on the dispersion curve and the structural characteristics of each modal wave.At the same time,the influence of excitation frequency on different modal guided waves is analyzed.The longitudinal L(0,2)mode and the torsional T(0.1)mode are selected as the detection mode waves.By comparing the propagation characteristics of guided wave between the two modes,one can find that the transmissivity of the guided wave in the bending tube can be changed by comparing the propagation characteristics of the L(0,2)mode with that of the T(0,1)mode in a straight pipe,and it can be found that the transmissibility of the guided wave can be changed in the bend tube.Unlike guided waves propagating in straight tubes,modal transitions occur when the guided waves pass through the curved pipe area.Secondly,the three-dimensional finite element model of curved pipe is established for the object of study,and the mode of L(0,2)modal and T(0,1)is used as the modal wave.The relationship between the frequency and the curvature radius of the two modes and the transmission transmittance of the guided wave,the influence of the excitation period on the reflected amplitude and the width of the wave packet,the modal transition and the transmission ratio under different bending angles,and the mode transition of the 90° pipe are analyzed.The results of numerical analysis show that the bending pipe with different radius of curvature has different transmission coefficients,and the excitation frequency varies with different transmission coefficients,the optimum excitation cycle number should be 3-10 cycles.The L(0,2)mode will produce a new modal F(1,1)when it passes through the elbow,propagating a distance through the elbow and coupling into L(0,2).The T(0,1)mode propagates a distance(about 600mm)after the elbow and converts a new modal F(1,2).The bending pipe with different bending angles also has different modal transition relations and transmission rate.The L(0,2)modal wave is converted to F(1,2)at the bending angle 10°~80°,and the bending angle 90°~170° to F(1,1).The T(0,1)mode 10°~170° only to the F(1,2)mode at the bending angle.Then by introducing related defects and resersh the influence of crack defects and circular hole defects on their amplitude and emissivity in the two modes.The result is come out,For two modes,the detection effect of crack defects with different widths is almost equal.Both the depth of crack defect and the diameter of circular hole will increase the amplitude of reflection echo of the two modes.The T(0,1)modal guided wave will produce a new mode F(1,2)when it encounters the defect on the elbow.The interaction mechanism of detecting modal guided waves and defects is the basis of quantitative detection of guided waves.By set different defect parameters in finite element model,the influence of crack defects and hole defects on the amplitude and reflectivity of two modes is studied.The results show that for the two modes,the detection effect of the crack defects with different widths is similar.The depth of crack defect and the enlargement of diameter of hole defect will increase the amplitude of two modes reflected back and be linear.And a new modal F(1,2)is produced when the T(0,1)modal guide waves interact with defects on the elbow.Finally,based on the MsSR3030 R instrument to carry out the detection experiment research,design and optimize the L(0,2)modal guided wave detection sensor.The sensor is used to detect the defects on the elbow,and the feasibility of the sensor is verified.Using L(0,2)modal and T(0,1)modal guided wave sensor,the elbow with defect of defect-free pipe,crack and flat bottom hole is analyzed experimentally,and the experimental results are in agreement with the simulation conclusion.In order to develop the guided wave detection technique in service pipe,The testing experiment of water filled elbow and pipe with weld seam is carried out.,and the results show that the water filled pipeline affects L(0,2)modal detection effect,but T(0,1)modal detection is not affected.The weld on the elbow will affect the effect of L(0,2)modal guided wave detection,resulting in lower defect recognition rate. |
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