Research On Damage Detection Method For Power Station Equipment Using Ultrasonic Guided Waves

With the increase of coal-fired generators steam parameters, the safety and residual life assessment of critical equipment is becoming increasingly important. The nondestructive testing technology using ultrasonic guided waves get rid of point-by-point scanning using by conventional ultrasonic body wave detection, which could significantly improve the detection efficiency, making the comprehensive detection of the critical equipment in a limited maintenance period possible. Therefore, the development of rapid non-destructive testing method by using ultrasonic guided waves is important to improve power plant operation safety, and promote the maintenance strategy change to condition-based maintenance.In this paper, the nondestructive testing methods of power station equipment using ultrasonic guided waves were studied. The main research works were given as follows.(1) As the large size and large diameter-thick ratio components,such as boiler deaerator and drum, the propagation of ultrasonic guided waves in effective detection range of the sensor is similar to the propagation in the plate, which means instead of the actual component the plate can be used to study the rapid damage detection method using ultrasonic guided waves under laboratory condition.Firstly, the propagation of ultrasonic guided wave in the plate is studied, the dispersion curves, the mode structures and wave field distribution at different frequencies are analyzed and calculated. On this basis, the guided wave mode excitation characteristic of piezoelectric wafer were simulated by finite element model coupling piezoelectric effect, and then the model was verified by experiment. The influence of the piezoelectric wafer radius, thickness, excitation frequency to the excited mode was studied to get the ultrasonic guided wave single-mode excitation conditions. Based on the above research, the Ao mode sensor array was built for component detection, and the damage in plate was imaging detected using triangular imaging algorithm, and the damage can be quickly and accurately identified by the damage image.(2) As the small thick-walled components, such as superheater tubes, reheater tubes and water wall tubes, the rapid NDT method for pipes was developed based on ultrasonic guided waves.Firstly, on the basis of model validation, the reflection and mode conversion of the pipe ultrasonic guided waves at the damage were studied using three dimensional finite element model in order to characterize pipe damage. The reflection of both the incident axisymmetric mode and converted modes including axisymmetric and flexural modes were studied, which provides a theoretical guidance for the development of the boiler pipe damage detection method. On this basis, the T(0,1) mode and L(0,2) mode transducers applicable to boiler pipe detection are developed based on the magnetostrictive effect. And excitation frequency, the uniformity of magnetic field, the magnetic field intensity of the designed transducer were optimized. And then the pipe damage detection experiments using the axisymmetric mode were carried out based on the damage detection system built in laboratory condition. The results show that the axial position of the damage in pipe can be quickly located based on axisymmetric mode.Finally, the T(0,1)-F(n,2) mode sensor array was designed, and the pipe damage imaging method based on virtual focusing is developed using the F(n,2) modes converted by incident T(0,1) at the damage. And then the pipes with multi-damage were detected using damage imaging detection method. The results show that this method can rapidly identify multiple damage in pipe and locate the damage axial and circumferential positions simultaneously, which provides a technical method for the rapid damage detection during boiler maintenance.