In-Site Machining of Steam Turbine High-Pressure Casing-Case History

In-site machining of a high-pressure casing of a 75 MW steam turbine, which experienced severe deformation after a long period of service is fully described. The casing deformation was measured using an optical instrument (transit) which revealed a bottom casing concavity of about of 4.4 mm and top casing convexity about of 2.8 mm. This deformation caused rotor-stator rubbing, an increase in steam path radial clearances, turbine efficiency deterioration and maintenance problems. To restore the casing horizontal joint surface flatness in the top and bottom casing halves, a special portable milling machine tool was used, which was supported directly on the casing. Also due to its deformation, the casing central guide bores of the diaphragms carriers needed to be re-machined. This was made by using a special rotary arm supporting an electrical angle grinder which was centred individually for each guide bore. The casing restoration was made without any needs for welding. Due to the machining of the horizontal joint, the casing joint surface was displaced approximately 6 mm in the vertical direction without needing to change the turbine rotor axis position. After the re-machining, the casing was put back into service reaching the design operational conditions without any problems.