Research On The Key Technology Of Tip Clearance Measurement Of Last Stage Shrouded Blade Of Steam Turbine

Moving blade is steam turbine’s core work component.The last stage shrouded blade of steam turbine’s condition is closely related to its operation safety and efficiency.However,they are usually under challenges from harsh environment,for instance,steam,high temperature,oxidation,corrosion and water drops.Tip clearance measurement is the central content of turbine blade condition monitoring.However,the special structure of shrouded blade and harsh environment such as wet steam leads to the absence of ability to online measure turbine shrouded blade tip clearance effectively.Therefore,this paper proposes an on-line measurement of turbine shrouded blade tip clearance based on eddy current sensor that utilizes the sudden change in permeability and conductivity at the joint of shrouded blade to realize the modulation of gap measurement signal.Through simulation,the eddy current sensor with anti-harsh environment and wide bandwidth features was designed and developed.The construction of the measurement system and tests on the eddy current sensor were completed.The main research work is as following(1)We modelled and analyzed the principle of the on-line measurement method of tip clearance of free blade,and found that the key to measure tip clearance of free blade is to form spatial modulation in circumferfntial direction,which can effectively suppress the influence of distribution parameters and circuit driftt,and realize long-term on-line measurement of tip clearance.We also found that there is no spatial modulation in the shrouded blade geometrically,but there are sudden changes in conductivity and permeability at the tip joint.This is related to the propagation of electric vortex,leading to the development of a new method the measure the tip clearance of shrouded blade using spatial modulation.The key technology to realize the measurement is pointed out.(2)A method of measuring tip clearance of shrouded blade based on eddy current sensor was proposed.The influence of sudden change of permeability and conductivity on the coil impedance of eddy current sensor was analyzed.The simulation model was built using Ansoft Maxwell software to analyze the influence of joint width difference and tip clearance on coil impedance,thus,to verify feasibility of the proposed measurement method.(3)A design method of anti-harsh environment and wide-band eddy current sensor was developed.Simulation analysis was applied to determine the optimal geometric parameters of sensor coil and shell.Parallel connection of high stable capacitance and probe coil form LC resonance,which improves sensor’s response bandwidth.The results demonstrated that the eddy current sensor is reliable and it meets the requirements of field application.(4)The tip clearance measurement system of shrouded blade was built.A calibration device was designed using the unique structural characteristics of shrouded blade to realize the high-precision calibration of tip clearance measurement.The on-line measurement of the tip clearance of shrouded blade was realized by the measurement experiment on a steam turbine in the laboratory researching on highspeed dynamic balance.