Safety Analysis On Low-pressure Cylinder Blade In Stream Turbine Under Small Volume Flow Rate Conditions

As more and more clean energy sources are connected to the grid for power generation,thermal power units participate in grid peak regulation for a long time,so the low-pressure cylinder of the steam turbine often needs to operate under the condition of small volume flow.At this time,the flow field in the low-pressure cylinder is extremely complex,and phenomena such as windage and eddy currents will appear in the flow passage,causing the blades to work in a very harsh environment.At the same time,the water erosion range and water erosion intensity of the blades are quite different from the design conditions under the condition of small volume flow rate.As the working environment changes,the stress,deformation,and modal properties of the blade also change.These problems will affect the safe and stable operation of the low-pressure cylinder,so it is of great significance to study the safety of the low-pressure cylinder blades of steam turbines under small volume flow rate condition.First of all,this paper takes the low-pressure cylinder flow passage of a 300 MW steam turbine unit as the research object and establishes a six-stage blade model of the low-pressure cylinder with an additional guide ring.The flow field of the low-pressure cylinder under the flow condition is numerically simulated.The pressure characteristics,temperature characteristics,eddy current characteristics,and water erosion characteristics of the low-pressure cylinder of the steam turbine under a small volume flow are analyzed.The research results show that: under the condition of small volume flow,the final stage first enters the blasting condition,and the outlet pressure of the final stage blade is higher than the inlet pressure at 15% LPTHA.As the load decreases,the temperature of the runner begins to rise.The high temperature region first appears at the bottom of the guide ring and moves continuously to the blade region.There are two high temperature regions on the blade surface.At the steam outlet side of the suction surface of the vane tip,the exhaust gas temperature has exceeded the intake air temperature at 5% LPTHA.Under the condition of small volume flow,the steam flow in the flow passage of the low-pressure cylinder is very complicated,and there are vortices such as separation vortex,backflow vortex and clearance vortex For the water erosion characteristics of the low-pressure cylinder,the humidity is positively correlated with the blade water erosion,and the pressure is inversely proportional to the blade water erosion.Compared with the design condition,under the small volume flow rate,the water erosion range of the last stage inlet edge,and the water erosion also occurs at the outlet edge under the influence of backflow.Secondly,the structural model of the last two-stage moving blades is established,the strength characteristics of the last two-stage moving blades are studied by using the method of fluidstructure coupling,and the modal analysis of the last stage moving blades is carried out.The stress,deformation,mode shape,frequency and other strength parameters of the stage moving blade are determined,and the minimum safe load of the low-pressure cylinder is determined.The research results show that the maximum equivalent stress of the blade is inconsistent with the maximum deformation region.As the load decreases,the maximum equivalent stress and maximum deformation of the last two stage buckets first decrease and then increase,and the sub-last stage buckets are not dangerous under small volume flow,while the maximum equivalent stress of the final blade at 5% LPTHA is 943.77 MPa,which exceeds the yield limit of the material and is prone to plastic deformation.The mode shape of the last stage blade has nothing to do with the load change,and the centrifugal force is the most important factor affecting the modal frequency.At10% LPTHA,the effect of temperature on blade modal frequency exceeds that of steam power.Considering the equivalent stress,deformation and modal safety,the minimum safe load of the low-pressure cylinder is 10% LPTHA.