48. Why is it necessary to warm up the turbine at low speed?

The purpose of warming up is to thoroughly preheat the metal components of the turbine, thereby reducing the temperature difference between the inner and outer walls of the cylinder flanges as well as between the flanges and bolts. This helps to minimize internal stresses in the metal, allowing the cylinder, flanges, and rotor to expand evenly. The pressure difference resulting from expansion remains within safe limits, ensuring that the dynamic and static clearances inside the turbine do not disappear and cause friction. At the same time, it increases the speed under load accordingly, reducing the time required to reach full load and thus achieving energy savings.

When starting a turbine, it is required to operate at low speed for a certain period of time. During cold start, low-speed warming is intended to allow all components of the unit to expand evenly as they are heated, thereby preventing deformation and loosening of components such as cylinders, diaphragms, nozzles, shafts, impellers, steam seals, and shaft seals. For turbines that have not been fully cooled, especially those without a barring device, it is also necessary to perform low-speed warm-up during startup. This is done to prevent the shaft from bending or deforming, thereby avoiding friction between the moving and stationary parts of the turbine. The speed during warm-up should not be too low. Because the rotation speed is too low, it is difficult to establish a bearing oil film, resulting in bearing wear. At the same time, the rotation speed is too low. Control is difficult; shutdowns tend to occur when steam temperature and pressure fluctuate. If the warm-up speed is too high, it will result in a rapid warm-up. A large amount of steam will enter the steam chamber. As hot air rises, the upper part of the cylinder and rotor becomes hotter than the lower part. The rotor will gradually bend upward. Therefore, when supplying steam to the uranium seal, special care must be taken to ensure that the steam pressure is not too high; it is sufficient for a slight amount of steam to emerge from the steam vent of the seal. Also, pay attention to cranking the machine to ensure even heating of the rotor.