Research And Application Of Thermal Power Plant Steam-water System Safety Using Ultrasonic Guided Waves

The water level measurement of the key components of the thermal power plant steam-water system is an important indicator of the boiler operation monitoring and is related to the thermal power unit stable operation.Due to thermal power unit load changing,combustion conditions changing,and feedwater pressure changing,the water level of the key components fluctuates frequently,which has an important impact on the safe and economic operation of power plants.In addition,with the increase of the temperature of the main steam and reheat steam,the oxide film thickness of the high-temperature heating surface rapidly increases.Under the effect of thermal stress or growth stress,the oxide film peels off to cause pipes blockage and erosion of turbine blades,which greatly increases the unit forced shutdown rate,seriously affects the unit reliability and even jeopardizes the operation safety.In low-temperature heating surface,the pipes wall thickness gradually decreases due to the flow accelerated corrosion,eventually leading to failure of the equipment and pipelines.Therefore,the operation safety of the thermal power plant steam-water system is directly related to the safety of the entire power system,and is the most important issue for power generation companies.Compared with the traditional water level measurement and wall thickness monitoring technology,the ultrasonic guided waves have made up for the deficiency of the existing technologies for the water level measurement and wall thickness monitoring for the thermal power plant steam-water system.Based on the mature non-destructive testing technology of ultrasonic guided waves,the dissertation establishes the steam-water safety monitoring system of the thermal power plant,and analyzes the problems and challenges that the ultrasonic guided waves are facing in the steam-water safety monitoring system.In addition,the dissertation explores the application of ultrasonic guided waves to water level measurement and wall thickness monitoring of steam-water safety monitoring system.In the dissertation,the research focuses on the following:(1)Ultrasonic guided waves theoretical research:The dispersion curves and mode shapes of ultrasonic guided waves are investigated in free plates,large diameter-thickness ratio pipes,and water loaded plates.When the pipe radius is large enough,the longitudinal mode L(0,1)in pipes approaches to the A0 mode in plates,and the longitudinal mode L(0,2)in pipes approaches to the S0 mode in plates.A quasi-scholte mode exists in water loaded steel plate,which does not exist in free steel plate.Then,the propagation characteristics of ultrasound guided waves in free plates and water loaded plates are studied by numerical methods and experiments.In low-frequency region,A0 mode exists in free plates,and quasi-scholte mode only exists in water loaded steel plate.(2)Water level monitoring based on ultrasonic guided waves:Firstly,the frequency-wavenumber domain of signals received by Scanning Laser Doppler Vibrometer is analyzed for the propagation characteristics of the quasi-Scholte mode in the plates and pipes.Secondly,a water level characterization method based on the time of flight of target wave signal is designed.Thirdly,the optimization of excitation frequency is studied from two aspects of modal noise and detection waveform recognition for the water level measurement method by numerical method.The frequency thickness product selection range of ultrasonic guided wave excitation is 250 kHz mm-400 kHz mm.Fourthly,the applicability of traditional piezoelectric sensor to water level monitoring is studied.Fifthly,a electromagnetic acoustic transducer is designed for quasi-Scholte mode excitation in a single direction to water level measurement.The design optimization parameters of EMAT:the coil gap distance D is equal to the ultrasonic guided wave wavelength ?wave,the meander number of the coil is 5,the coil width is 0.9 mm,and the coil length is 30 mm.Finally,the time-frequency analysis method of the water level characteristic signal is studied based on the Wigner-Ville distribution,and a water level measurement system in closed container is developed based on ultrasonic guided waves.The linear fitting coefficient of the water level measurement data is very close to the theoretical prediction value,with an error of only 2.9%.(3)Wall thickness monitoring of high-temperature components based on waveguide:Firstly,this part studies the different modes in waveguide that produce different stress load distributions on the measured component surface.Secondly,the propagation characteristics of the SH0 mode in a rectangular cross-section waveguide is studied by the finite element method.For a 1 mm × 15 mm cross-section waveguide,the SH0 mode guided wave propagates in a rectangular cross-section waveguide at an excitation frequency of 2 MHz.The wave field is stable,and a high signal-to-noise ratio wave signal can be obtained.Thirdly,the distribution of temperature field in a rectangular cross-section waveguide is studied by means of the finite element method.The thermocouple temperature measurement experiment is used to verify the heat dissipation of the waveguide.Fourthly,the waveguide variants are studied and compared,and the coupling between the waveguide and the measured component is studied experimentally.Fifthly,using the designed waveguide dry coupling device,the thickness of the measurement components is monitored under normal temperature conditions.Finally,an experimental study was conducted on the application of thickness monitoring in high temperature environments.At room temperature(25“C),the difference between the wall thickness measurement and the actual thickness is 0.016 mm;when the wall thickness of the high temperature part under test is measured(25?-700?),the wall thickness calculated by the time difference of the reflection wave and the shear wave velocity is 9.6289 mm-10.1041 mm,the error range is 0.16%-3.7%,and the speed error between the velocity measurement and the fitting curve ranges from 0.1%to 2.5%.The dissertation research brings successfully guided waves to the water level monitoring and wall thickness monitoring of the steam-water system,and establishes the fundamentals of the steam-water safety monitoring system of the thermal power plant.We envision that one day the steam-water safety monitoring system of the thermal power plant will be established for power plants safety.