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When there is a temperature difference between the upper and lower cylinders of the turbine, the rotor eccentricity changes to a certain extent. When there is significant eccentricity, especially abnormal rebound, static and dynamic friction may occur inside the cylinder block. Heat is generated at these friction points, causing the temperature to rise; this in turn reduces the clearance between the moving and stationary parts, intensifying wear and tear and causing severe damage to the unit. The temperature difference between the upper and lower cylinders of a steam turbine is not uniform along the axis, with the maximum value typically occurring near the control stage. Tests have shown that for each 10°C increase in the temperature difference between the upper and lower cylinders of a high-pressure steam turbine, the radial clearance at the bottom of the control stage decreases by approximately 0.1 mm. The radial clearances of the steam seals on the partitions of high-pressure turbines are relatively small, usually ranging from 0.4 to 0.7 mm; therefore, the temperature difference between the upper and lower cylinders is specified to be no more than 35–50°C. If the temperature exceeds 50°C, it is possible for the radial clearance to disappear; if operation is initiated under such conditions, friction may occur in the radial steam seal, leading to friction between the moving and stationary parts. In severe cases, this can also cause wear on the rivets of the gasketing, resulting in more serious accidents. Especially when the rotor experiences thermal bending at the same time, the friction between the moving and stationary parts becomes more severe. When the turbine starts up or shuts down, the temperature of the upper half of the cylinder is higher than that of the lower half. This temperature difference causes deformation of the turbine cylinder, leading to an upward arching of the cylinder – a phenomenon known as thermal warping. In mild cases, this undermines the tightness of the joints between the components of the turbine, resulting in steam leakage; in severe cases, it reduces the clearance between the moving and stationary parts, causing friction between them. Additionally, the deformation of the cylinder alters the position of the bearing centers, leading to intense vibrations in the turbine unit. Generally, the operating procedures stipulate that the unit shall not be started when the temperature difference between the upper and lower cylinders exceeds 50°C before startup. If this temperature difference exceeds 50°C during the startup process, the unit must be shut down immediately. If such a temperature difference is observed during startup or after the unit is shut down while still hot, the unit will have to be stopped or its startup will be delayed. In particular, after a shutdown while the unit is still hot, the temperature difference between the cylinders usually has to drop to below 50°C once the cylinder temperatures return to normal levels, resulting in a longer delay in restarting the unit. Schematic diagram showing bending deformation caused by an excessive temperature difference between the upper and lower cylinders. Reasons for the temperature difference between the upper and lower cylinders: 1. The upper and lower cylinders of the turbine have different weights and heat dissipation areas; the lower cylinder is heavier than the upper one. The lower cylinder is equipped with steam extraction pipes, giving it a larger heat dissipation area. Under the same heating or cooling conditions, the lower cylinder dissipates heat more quickly and heats up more slowly, resulting in the upper cylinder having a higher temperature than the lower one ； 2. Inside the cylinder, steam rises while its condensed water flows downward, thereby changing the heating conditions in the lower part of the cylinder ； 3. In the surrounding space, the air temperature above the operating platform is higher than that below it; the air flow moves from bottom to top, resulting in different cooling conditions for the upper and lower cylinders, which causes the temperature of the upper cylinder to be higher than that of the lower cylinder ； 4. When the sequence in which the speed control valves are opened is incorrect, partial steam intake occurs, which also increases the temperature difference between the upper and lower cylinders ； 5. During startup, poor drainage of water from the cylinders occurs; after shutdown, cold steam or water returns to the cylinders through the extraction pipes, causing the temperature of the lower cylinders to drop ； 6. The lower cylinder is poorly insulated; since it is not as easy to ensure a tight seal around the lower cylinder as with the upper cylinder, this leads to air cooling of the lower cylinder ； 7. After shutdown, air convection occurs within the cylinders; the warmer air gathers in the upper cylinder while the air in the lower cylinder is cooler, resulting in different cooling conditions between the upper and lower cylinders. Technical measures to prevent large temperature differences between the upper and lower cylinders; Measures to prevent large temperature differences after shutdown: 1. Before shutting down the unit and applying the brake, all cooling water control valves and shut-off valves should be closed to ensure complete isolation of the cooling water. 2. After shutting down the machine and applying the brake, close the following drain valves promptly: high-pressure and medium-pressure cylinder drain valves ； High and medium pressure cylinder steam inlet duct drain valve ； High and medium pressure main steam valves, control valves, drain valves ； Drainage valves before and after the non-return valves for extraction at each stage ； Drain valve in front of the high-pressure check valve. 3. After the rotating rotor has come to rest and the vacuum level has reached zero, stop the steam supply to the shaft seal; close all the control valves and isolation valves for the steam supply to the shaft seal, as well as the steam supply valves for the high and medium pressure cylinders. Then open the atmospheric drain valve on the shaft seal main pipe. 4. After shutting down the machine and applying the brake, it is necessary to check that the high and medium pressure main steam valves, control valves, high-pressure exhaust check valves, check valves for each stage of extraction steam, and electric valves for each stage of extraction steam are all properly closed and sealed. 5. Continuous turning of the turbine shaft should be initiated immediately after the unit stops; if it is not possible to start continuous turning, manual turning should be carried out regularly in accordance with the relevant regulations. 6. After shutdown, if the highest cylinder temperature exceeds 150°C, the turning operation must not be stopped arbitrarily; if stopping is necessary, appropriate measures must be taken. 7. After shutdown, it is necessary to regularly monitor the water levels in the high and low pressure feedwater heaters, shaft heaters, deaerator, and condenser, to ensure that the water levels in all tanks remain normal and to prevent cold water from flowing back into the steam extraction pipes. 8. After shutdown, regularly monitor the wall temperature of each extraction pipe to prevent water accumulation from returning into the cylinder. 9. After the unit is shut down, it must be ensured that the drain valves on the main steam pipe, reheat steam pipe, and reheat steam cooling section pipe are open. 10. Ensure that the insulation of the turbine unit and the extraction pipelines is in good condition; if any defects or cracks in the insulation are detected, contact the relevant parties for prompt repair. 11. During the winter period when the units are shut down, it is necessary to close the doors and windows of the plant at all times to reduce heat loss through natural convection. 12. After shutdown, close monitoring should be exercised on cylinder temperature and temperature differences; any abnormalities should be detected and addressed promptly. 13. If the temperature difference between the cylinders increases during any operation after the machine is shut down, such operation should be stopped immediately and the system restored to its original state; operations can resume only after the cause has been identified. Measures to prevent large temperature differences during startup and shutdown: 1. When supplying steam to the shaft seal of the turbine, it is necessary to ensure that the warming of the shaft seal system is thorough; the steam supply temperature should be 20–50°C higher than the temperature at the highest point of the cylinder. 2. When the temperature of the unit’s start-up cylinder is above 150°C, the vacuum level of the condenser should not be too high. 3. When the extraction steam temperature at the high-pressure bypass exceeds the maximum cylinder temperature by more than 50°C, it is permissible to use the cylinder heating device for pipe warming. During pipe warming, care must be taken to prevent steam from entering the cylinder. 4. For warming the pipes in the cylinder heating system, the drain valve at the manifold should be opened first; then, reverse flushing of the pipes from the manifold to the cylinder should be carried out. Finally, draining and warming of the steam inlet section pipes should be performed. 5. During startup and shutdown, the adjustment of the heating device should be made based on the temperature difference in the cylinders. 6. During shutdown, close attention should be paid to the superheat of the steam, ensuring that the superheat of the main steam remains at least 50°C to prevent wet steam from entering the cylinder. 7. Before starting the unit for a trial run, it is necessary to ensure that there is sufficient drainage time in the main and reheat steam pipelines, that the drainage valves are fully open, and to constantly monitor the drainage condition of the pipeline system. 8. During startup and shutdown, attention should be paid to draining water accumulated in the extraction steam pipes and the dead-zone steam pipes. Constant monitoring of the pipe wall temperature is necessary, and measures should be taken promptly to prevent cold steam from entering the cylinder. 9. During shutdown, the piping and the main body drain valves should be opened according to changes in steam temperature. 10. During the startup and shutdown of the unit, the pressure and temperature of the auxiliary steam main pipe should be monitored regularly to ensure that the steam superheat degree remains no less than 30°C. 11. Do not arbitrarily disable the water level protection mechanisms of high-pressure heaters, low-pressure heaters, and deaerators ； The protection of the extraction check valve must not be removed arbitrarily ； The high/low water level alarms for each heater must not be disabled arbitrarily.