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Preventive measures to prevent water or air from entering the turbine: 1. Hazards and symptoms of water or air entering the turbine: The entry of water or cold air into the turbine can cause the cylinder to deform, eliminate the clearance between moving and stationary parts leading to friction, and cause the main shaft to bend. This results in damage and fracture of the blades, leakage at the joints between valves and the cylinder, friction between moving and stationary components, wear of the thrust bearings, permanent deformation of the turbine’s high-temperature metal parts, and metal cracks caused by thermal stress, all of which affect the equipment’s service life. According to the relevant regulations in accident investigation procedures, for large-scale turbines, any of the aforementioned consequences resulting from water or air entering the turbine constitute serious equipment failures. When steam enters the turbine or cold air gets in, there are usually noticeable signs, mainly as follows: 1. The temperature difference between the upper and lower cylinders of the high- and medium-pressure cylinders increases significantly, or this increase accelerates. During the startup, shutdown, and normal operation of the unit, the temperature difference between the upper and lower cylinders of the high and medium pressure cylinders remains generally within 50°C; the rate of change of this difference is usually within 5°C per minute. If this limit is exceeded, measures must be taken. 2. The main and reheat steam temperatures drop suddenly, resulting in a decrease in superheat. The main steam temperature should be 50 degrees higher than the highest metal temperature in the cylinders; the superheat of the steam must be at least 50 degrees. The rate of change of the main steam temperature should be within the range of 5°C per minute, and the main steam temperature on the turbine side must not be 10°C lower than that on the boiler side. 3. Turbine vibration increases. The entry of water or cold steam into the turbine causes the high-temperature metal components to cool down suddenly, leading to rapid contraction. This results in significant thermal stress and thermal deformation, changes in the expansion difference of the unit, intense vibration of the unit, and axial and radial rubbing between the moving and stationary parts. Therefore, during normal operation of the unit, the main cause of abnormally increased shaft vibration is the entry of water or cold air into the turbine. 4. Vibration occurs in the extraction pipeline. When water or cold air enters the regenerative exhaust pipeline, the steam hammer effect generated by saturated steam as well as the thermal deformation of metal materials can cause severe vibrations in the pipeline. This is similar to the pipeline vibrations that occur when a high-temperature superheater is brought online too quickly; such vibrations can damage equipment and may also pose a risk of injury to people. 5. The turning current increases or the turning device trips while in the turning mode. During the start-up and shutdown of the unit, the current associated with rotor turning usually varies by 2-3 A; friction between the rotor and the steam seal can easily lead to an abnormal increase in current or to a trip of the rotor turning system. II. Preventive measures for technical handling: (1) Regarding equipment technical requirements: 1. In the main steam system, apart from the drain pipes located in front of the electric main steam valves of the turbines, drain pipes with an inner diameter of no less than 25 mm should also be installed at all other pipe joints; additionally, inspection pipes for draining water into the ditch must be fitted. 2. The bypass system of the main steam pipeline and the condensate drain pipes serve not only to discharge steam but also to effectively remove water vapor; therefore, these bypass and condensate drain pipes should be arranged to start from the bottom of the lowest-level pipe in the steam pipeline and be as close as possible to the turbine. 3. The drain water from the hydrophobic expansion tank should be connected to the high-, medium-, and low-pressure drain headers according to their pressure levels. The drain water from the turbine itself should be connected separately to the expansion tank or header, and must not be connected to any other drains. The drain pipes are connected at a 45° angle, in order of decreasing pressure level; the drains with higher pressure are located farther away from the drain expansion vessel. The connection pipes that carry the drains from the vent pipes to the condenser should be large enough. 4. All extraction steam pipes must be equipped with sufficiently large drain pipes. The drain pipes located before and after each check valve and globe valve should not be connected together; instead, they should be separately connected to the drain manifold leading to the condenser. All drain connections on the condenser must be installed above the maximum water level in the hot well. The extraction steam check valves must be able to close automatically when the heater becomes full of water. 5. There should be two temperature measurement points on the extraction pipe: one located near the heater and the other near the extraction outlet, so that it is possible to determine whether the heater is operating properly based on these two temperatures. 6. The steam supply pipe for the steam seal should be kept as short as possible. Drain pipes should be installed before and after the steam seal regulator as well as on the steam supply header for the steam seal. A check valve must be installed on the shaft seal steam pipe connected to the low-pressure heater. 7. The regenerative heater and deaerator shall be equipped with reliable multiple protections to prevent rising water levels from returning to the turbine, as well as alarm devices to alert operators. 8. A drain tank shall be installed for the reheat cold-section piping, along with automatic drain devices for high and low water levels. In addition to the control valve, a power-operated globe valve shall also be fitted for the reheat steam attemperation water. When the steam flow within the reheater ceases, both the control valve and the globe valve must be able to close rapidly. Upon a load rejection in the turbine, the attemperation water valve must be capable of closing automatically. (II) Operation monitoring: 1. Strengthen operational supervision to prevent water hammer phenomena. Should any signs of water hammer in the steam turbine occur, emergency shutdown measures must be taken promptly. 2. Before starting the unit, the drain valves on the main steam pipes, cold reheat steam pipes, and hot reheat steam pipes should be fully opened. Especially before starting the unit while it is still hot, the main steam and reheat steam pipes need to be properly warmed up to ensure smooth drainage. 3. When the unit is started, before the vacuum has been properly reduced, the turbine operator activates the shaft seal. On the DCS “Shaft Seal System”, the mode is set to “Automatic”, with the temperature of the shaft seal supply line adjusted to 120°C, and this temperature is maintained between 120°C and 200°C. The pressure of the shaft seal supply line is also set to “Automatic” with a value of 50 mbar. 4. Pay attention to monitoring the changes in the metal temperature of the cylinders, as well as the water levels in the high and low heaters and the condenser. Monitoring of water levels must not be neglected even after the machine is shut down. When a risk of water ingress into the cylinders is detected, the cause should be identified promptly, and efforts should be made to cut off any sources that could lead to water entering the cylinders. 5. During the sliding-parameter shutdown and startup of the steam turbine, the steam temperature and pressure must be controlled in strict accordance with the operating procedures to ensure the necessary steam superheat. 6. The water level adjustment and protection devices of the high-pressure heater should be inspected and tested regularly to ensure that their performance meets the design requirements. If the protection mechanisms of the high-pressure heater do not meet the operational requirements, it is prohibited to put the high-pressure heater into operation. 7. After the boiler is shut down and the steam parameters cannot be guaranteed reliably, steam is generally not supplied to the turbine; if such supply is required for special reasons, necessary technical measures must be established in advance. 8. Regularly inspect the heater tube bundle; once a leak is detected, the water supply should be cut off immediately to isolate the steam turbine, and repairs should be carried out promptly. 9. Strengthen the monitoring of the deaerator water level, regularly inspect the water level control devices, and prevent water-overflow accidents. 10. The steam seal system shall be able to meet the steam supply requirements for starting the unit under various operating conditions, and it is necessary to check that the continuous drainage in all areas is functioning properly during normal operation. 11. Maintain the necessary superheat of the steam during shutdown, and pay attention to the switching of auxiliary steam sources: switch between the auxiliary steam header source and the deaerator steam source in a timely manner. After shutdown, it is necessary to keep accurate records of the shutdown process, analyzing and documenting changes in parameters such as cylinder metal temperature, cranking current, eccentricity, absolute cylinder expansion, and expansion difference. All auxiliary steam systems, drain systems, as well as valves in the pipelines that could supply cold water to the cylinders must be closed tightly to prevent situations where water from the deaerator, condenser, or heaters enters the cylinders. 12. During shutdown, it is best for both vacuum and rotational speed to be zero at the same time. This prevents cold air from entering. Check that the drain valves on the turbine itself are opened in a timely manner. 13. Operators should be aware that allowing water to enter the turbine at low speeds poses a much greater threat to the equipment compared to when it is operating at its rated speed or under load. 14. Before activating the turbine shaft seal, it is necessary to start the shaft turning device first; before introducing steam, the pipes must be properly drained and warmed to ensure that the steam supplied to the shaft seal has a superheat of 14°C. When reducing the load, pay attention to adjusting the steam supply to the shaft seal as needed. After the vacuum is lost, the supply of steam to the shaft seal as well as the operation of the shaft seal fan should only be stopped once the vacuum level drops to zero. III. Organizational measures for pre-control during disposal: 1. Before startup, ensure that the switching of high and medium pressure drain valves, as well as the interlock tests for the water levels of high and low heaters, are successful, and that the interlocks function properly automatically. 2. The drain valves of the turbine itself, the main steam and reheat steam supply pipes, as well as various extraction pipe systems, shall be opened or closed automatically in a timely manner in accordance with the regulations. IV. Pre-control measures for safe disposal: 1. After the unit is shut down, maintenance personnel must carefully check the operation of the turbine protection systems to prevent any failure in their function. 2. If cold air or water is detected during the turbine’s cranking process, it is necessary to continue operating the cranking mechanism until the temperature difference between the upper and lower cylinders of the turbine returns to normal. At the same time, strengthen the monitoring of internal sounds in the turbine, rotor eccentricity, turning current, etc. 3. When the unit is starting up or after it has stopped, if cold air or cold water enters the turbine, emergency shutdown measures must be taken: the vacuum system should be shut down, the turbine’s speed should be reduced to below 2700 rpm to break the vacuum, and once the vacuum level reaches zero, the shaft seal system should also be shut down while ensuring that the turning gear continues to function properly.