火电厂锅炉再热器管道开裂原因分析

Cause Analysis of Cracking on Boiler Reheater Pipe in Thermal Power Plant

蒙西地区7台同型号亚临界参数燃煤火电锅炉相继发生再热器连接管道开裂泄漏失效事件,失效部位多发生于管道焊接接头内壁坡口倒角退刀槽处。对失效试样进行了宏观形貌、显微组织、扫描电镜、化学成分、力学性能、壁厚测量分析,并对7台锅炉设备的设计、运行数据进行调查。结果表明,锅炉燃烧煤种偏离设计煤种、电网AGC负荷自动调度、低氮燃烧改造等因素造成锅炉燃烧和运行工况发生变化,使得再热器减温水居于过量高投入水平,导致再热器微调减温器之后布置的直管和弯头长期承受交变热应力和非周期性振动,最终使管道焊接接头内壁坡口倒角退刀槽应力集中区萌生热疲劳裂纹,并扩展贯通。

Seven coal-fired thermal power boilers with sub-critical parameters of the same type in Western Inner Mongolia have successively suffered from cracking and leakage failures of the reheater connecting pipes,most of which occurred at the chamfered grooves of the inner wall groove of the pipe welding joints.In order to find out the cause of cracking,typical failure samples are intercepted and analyzed for macro-morphology,microstructure,SEM,chemical composition,mechanical properties,wall thickness measurement and design and operation data of seven boiler equipment are investigated.The results show that comprehensive factors such as the deviation of burning coal type from design of boiler,automatic dispatching of AGC load in power grid,low nitrogen combustion transformation caused changes in boiler combustion conditions and operation,which results in excessive high input level of reheater cooling water,therefore making long-term alternating thermal stress and non-periodic vibration of straight pipe and elbow arranged after fine-tuning of desuperheaterflow direction in reheater.As a result,thermal fatigue crack initiated in the stress concentration area of chamfered back tool groove of the inner wall of pipe weld joint and expanded.