Thread Content
The main reasons for shaft bending during operation are: (1) Local overheating of the rotor due to static and dynamic friction, which generates compressive stresses and leads to plastic deformation. After the rotor is cooled, residual tensile stress causes the main shaft to bend. (2) When cold steam and cold water are introduced into the steam turbine, tensile stress is generated in the cooled parts of the rotor, leading to plastic deformation and thus bending of the main shaft. (3) A failure in the shaft seal system allows cold air to enter the cylinder, causing the rotor to cool down rapidly; this eliminates the clearance between the rotating and stationary parts, resulting in friction that causes the main shaft to bend. (4) Wear of the bearing shells or thrust bearings causes the axis of the shafting to be misaligned, resulting in dynamic and static friction that leads to bending accidents. Measures to prevent bending of the main shaft: (1) Before starting, carefully check the following valves to ensure they are in the correct position; (2) The isolation valves and control valves for the high-pressure bypass cooling water must be closed tightly ； All steam pipes of the turbines, as well as the body drain valves, must be fully opened ； The desuperheating water valves leading to the boiler, as well as the intermediate tap valves of the feed water pump, should be kept tightly closed and opened only when the boiler requires it ； After filling each water seal with water, the water injection valve should be closed to prevent water from flowing back from the shaft seal heater to the steam seal. (3) Before starting the turbine unit, it is necessary to perform continuous rotor turning for more than 2 hours; for hot-starting, this process must continue for more than 4 hours without interruption, and the rotor bending value must be no more than 0.02 mm of the original value. (4) Before starting up, the main steam pipes, reheat steam pipes, and all headers should be thoroughly warmed up. (5) During the impulse start process, the vibration of each bearing in the unit should be closely monitored. When the rotational speed is below 1300 r/min, the bearing vibration must not exceed 0.03 mm; once the critical speed is reached, the bearing vibration must not exceed 0.1 mm. Otherwise, the machine should be stopped immediately. After shutdown, the bending of the main shaft should be measured, and the shaft should be rotated continuously for more than 4 hours before the machine can be restarted. If there is an interruption, the barring must be timed again. (6) After reaching 3000 r/min, the electric main steam valve and the drain valve should be closed to prevent excessive drainage volume from affecting the proper flow of drainage within the unit. (7) After installing the steam heating device, careful adjustment is required; it is not allowed for there to be varying temperature differences above and below, as well as left and right, on the cylinder flange, with all such temperature differences having to remain within acceptable limits. (8) When the boiler combustion is unstable, strict monitoring of the changes in main steam and reheat steam temperatures is required; if the temperature of the main steam or reheat steam drops by 50°C within 10 minutes, the plant should be shut down. (9) During shutdown, monitoring of the water levels in various water tanks and heaters should be intensified to prevent water or cold steam from flowing into the cylinders. (10) At low load, the outlet pressure of the condensate pump should be adjusted so as not to exceed the specified value, in order to prevent the rupture of the steel tubes in the low-pressure heater. (11) Before connecting the high-pressure heater, all protection tests must be carried out to ensure that the protections of the high-pressure heater function properly; otherwise, the high-pressure heater shall not be put into operation. (12) Coolant water shall not be used for hot start.