某进口350MW汽轮机低压第2级叶片和叶轮开裂故障原因分析

Cause Analysis of Cracking Failure of Low-pressure Stage 2 Blade and Impeller of an Imported 350MW Steam Turbine

某厂3号汽轮机为GE设计生产的350-16.7/538/538型凝汽式汽轮机,在2021年10月份检修过程中发现汽轮机侧和电机侧低压第2级叶片多个叶根及多处叶轮存在裂纹。通过叶片故障调查,叶片材料和断口理化检验分析,叶片数值计算分析评估,结果显示:第2级叶片和叶轮的裂纹失效模式均为微动磨损引起的疲劳开裂,源区位于第1齿叶根和叶轮接触的磨损部位;叶根和叶轮开裂的主要原因是运行工况下叶根3对齿受力不均匀致使叶根与叶轮第1齿局部接触应力偏低,从而造成叶片组的一阶动态振动落入k=20共振区域,叶片组振动下传至叶根和叶轮的接触处,使叶根与叶轮第1齿接触面处出现微动滑移并造成疲劳损伤,接触处逐渐萌生微裂纹并扩展,直至断裂;机组负荷频繁波动和深度调峰运行加速叶根和叶轮的微动磨损及微裂纹萌生,促进了叶根和叶轮裂纹萌生与扩展。

A 350-16.7/538/538 type 350MW steam turbine is designed and produced by GE for the No.3 steam turbine of a factory.During the maintenance in October 2021,cracks were found on several roots and multiple impellers of the low-pressure stage 2 blades at the turbine side and motor side.Through blade failure investigation,physical and chemical inspection and analysis of blade material and fracture surface,and numerical calculation analysis and evaluation of blade,the results show that the crack failure modes of the second stage blade and impeller are fatigue cracks caused by fretting wear,and the source area is located at the wear part where the first tooth blade root and impeller contact;The main reason for the crack of blade root and impeller is that the uneven stress on the three pairs of teeth of blade root causes the local contact stress between the blade root and the first tooth of the blade wheel to be low under the operating condition,which causes the first dynamic vibration of the blade group to fall into the k=20 resonance region,and the vibration of the blade group is transmitted to the contact between the blade root and the impeller,causing the fretting slip and fatigue damage at the contact surface of the blade root and the first tooth of the impeller,and the microcrack gradually sprouts and expands at the contact until it breaks;The frequent fluctuation of unit load and deep peak-shaving operation accelerate the fretting wear and microcrack initiation of blade root and impeller,and promote the crack initiation and propagation of blade root and impeller.