Model And Application Of Ultrasonic Guided Wave Testing For Helical Tubes

Helical tubes with high heat exchange efficiency,such as finned tubes,internally-ribbed tubes,are widely applied in petrochemical and electric power industries.Due to a long-term operation in high-temperature,high-pressure,high-humidity and fluid-impact environment,helical tubes need a periodic inspection to prevent safety accidents.With advatanges of single-point excitation and long distance inspection,ultrasonic guided waves have the potential to become an effective testing method.However,the complex geometry and various types of helical tubes not only increase the difficulty of guided wave propagation characteristic analysis,but also bring new challenges to signal enhancement methods and transducer design.Therefore,the study in this dissertation is systematically developed around the model and application of ultrasonic guided wave testing for helical tubes.According to the spatial distribution of helical periodic components,a testing theory of guided waves in helical tubes is proposed based on the semi-analytical finite element method.With typical structure features,tube models of outside and inside arranged helical periodic components are developed,respectively,to reveal propagation characteristics of guided waves in helical tubes.The models are verified by experiments and the principle of the proper selection of excitation parameters is given to reduce the effect of helical periodic components.These reaserches extend the application scope of the semi-analytical finite element method,and also provide a solution process for propagation characteristics of guided waves in different types of helical tubes,which gives theoretical supports to the application of guided wave testing for helical tubes.Due to the limitations of the proposed semi-analytical finite element method when solving the problem of guided wave propagation containing defect echo signals,three-dimensional finite element simulation models of helical tubes are developed for guided wave testing.The characteristics of signal attenuation and defect response of the axisymmetric guided waves in helical periodic components are studied to reveal guided wave detection mechanism of helical tubes,which provides theoretical basis for the testing distance evaluation and defect quantitation.For the local structure characteristics of helical tubes,such as weld joints,clamped supports,encountered in guided wave testing,signal enhancement devices are utilized to simulate their effects on propagation characteristics of guided waves.Based on the theory of elastic wave reflection and signal waveform superposition,a signal enhancement method for the excitation,receiving and defect detection process of guide waves is proposed.The proper transducer arrangements in different types of local structure characteristics,signal enhancement conditions and the use of signal enhancement devices are also discussed,which gives scientific guidance for the field testing of helical tubes.To solve engineering problems of guided wave testing in the limited space,a longitudinal guided wave transducer based on a permanent magnet chain is developed for finned tubes,a typical type of helical tubes used in heating furnaces.A cross-sectional area loss of 1.2% wall thickness in finned tubes is detected by this transducer.By using the signal attenuation effect of weld joints and 180 degree elbows,the guided waves testing distance can be adjusted to remove the effect of the 180 degree elbow.Field application results show that the proposed transducer is able to inspect the finned tube in service.This dissertation provides a feasible method for propagation characteristics of guided waves in different types of helical tubes,which not only enriches the research of acoustic properties in periodic structures,but also helps to promote the application of guided wave testing for helical tubes.