Steam turbine operation quiz bank

Thread Content

1. Under what circumstances should an emergency shutdown be performed? An emergency shutdown shall be initiated under the following circumstances: (1) The vibration of any bearing in the turbo-generator set reaches the emergency shutdown value. (2) There are obvious sounds of metal friction and impact inside the steam turbine generator set. (3) Water hammer occurs in the turbine, or the temperatures of the main and reheat steam drop by 50°C within 10 minutes. (4) Oil supply to any bearing of the steam turbine generator set is interrupted, smoking occurs, or the oil return temperature of the bearing rises suddenly to the emergency shutdown value. (5) Sparks are coming from the shaft seal. (6) A fire breaks out in the turbine oil system and cannot be extinguished quickly, posing a serious threat to the safe operation of the unit. (7) The generator or exciter smokes, catches fire, or the hydrogen system explodes. (8) The turbine speed rose to the trip speed of the emergency governor (3330 r/min), but the emergency governor did not activate. (9) The metal temperature of any bearing in the turbine rises to the emergency shutdown value. (10) The lubricating oil pressure dropped to the emergency shutdown level, and starting the AC and DC lubricating oil pumps was ineffective. (11) The oil level in the main oil tank of the steam turbine dropped abruptly to the emergency shutdown level; despite refilling with oil, it could not be restored. (12) The axial displacement of the turbine reaches the emergency shutdown value. (13) The turbine expansion difference has reached the emergency shutdown value. 2. Describe the phenomenon of water hammer in steam turbines and the principles for handling it during operation. Phenomenon: (1) The main steam or reheat steam temperature drops linearly. (2) There is a loud water impact sound or vibration in the steam pipes. (3) White steam appears or water droplets splash at the stem, flanges, and shaft seals of the main steam valve and control valve. (4) Load decreases, the unit emits abnormal noises, and vibration increases. (5) Axial displacement increases, the metal temperature of the thrust bearing rises, and the expansion difference decreases. (6) An increase in the temperature difference between the upper and lower cylinders of the steam turbine, or an alarm is triggered. Handling principle: (1) In the event of water hammer in the unit, it should be handled as an emergency shutdown due to vacuum loss. (2) Note that the drain valves for the turbine proper and related steam pipes should be open. (3) Pay attention to monitoring parameters such as axial displacement, expansion difference, metal temperature of thrust bearings, and vibration. (4) Carefully listen to the internal sounds of the turbo-generator and accurately record the coasting time. (5) If water enters the steam turbine due to a full water level in the heater or deaerator, the extraction valve for that unit should be closed immediately, the faulty heater should be isolated from the system, and water drainage should be increased. (6) If water enters the turbine, causing the temperature differences between the upper and lower metal surfaces in the high-pressure and medium-pressure cylinders to exceed the specified limits, the vacuum should be broken immediately to shut down the machine urgently. (7) After the turbine speed reaches zero, immediately initiate continuous barring. (8) When turning the rotor, pay special attention to whether the current for rotating it increases, and record the rotor’s eccentricity. When the rotor is severely deformed or there is friction between the internal moving and stationary parts, and the turning gear cannot rotate the rotor, it is strictly prohibited to force it to turn. (9) After an emergency shutdown due to water hammer in the unit, it is strictly prohibited to restart it within 24 hours ； Before restarting, the turbine should be rotated continuously for at least 6 hours; the temperatures of the upper and lower cylinders as well as the rotor’s eccentricity must meet the specified requirements. (10) Once the steam turbine meets the startup conditions, start it. During the startup process, it is necessary to monitor parameters such as rotor eccentricity, axial displacement, expansion difference, metal temperature of the thrust bearings, and vibration to ensure they are within the specified control limits, as well as to check the drainage condition of the turbine itself and the steam pipes ； (11) If abnormal noises or grinding sounds are detected when the steam turbine is restarted, the vacuum should be immediately broken to shut down the machine, and reports should be made at each level. (12) During the coasting process, if the axial displacement of the turbine, the expansion difference, vibration, the metal temperature of the thrust bearings, and the return oil temperature increase significantly, and the coasting time is markedly reduced, reports should be made at each stage. It is necessary to decide whether to remove the cylinder for inspection based on the condition of the thrust bearings; otherwise, startup is not permitted. (13) If abnormal noises inside the turbine and friction in its rotating parts are detected during shutdown, the casing should be opened for inspection. 3. What are the causes of water hammer? The reasons for water hammer include: (1) excessive or uneven evaporation in the boiler, as well as improper chemical water treatment, which leads to steam-water sloshing. (2) The boiler temperature and pressure reducing valve is leaking or improperly adjusted, resulting in improper steam pressure control. (3) An excessive rate of pressure increase during startup, or an excessive rate of temperature drop during controlled shutdown, reduces the superheat of the steam, even bringing it close to or to the saturation temperature, resulting in the steam containing water. (4) Improper operations by operators and issues with the feedwater regulator cause the boiler to overfill with water. (5) During the turbine startup process, the pipe warming time is insufficient, resulting in incomplete drainage of water. (6) When water spray is used for cooling the reheat steam cold section, improper operation or faulty valves can cause the cooling water to accumulate within the tubes of this section or to flow back into the high-pressure cylinder; when the unit starts up, this accumulated water is carried into the turbine by the steam. (7) The water level in the heaters of the steam turbine regenerative system was too high, and the water level protection device malfunctioned, causing water to flow back into the steam turbine through the extraction steam pipes, resulting in water hammer. (8) Overfilling of the deaerator causes water to enter the shaft seal system via the steam balance pipe. (9) At startup, if the shaft seal piping system is not properly warmed up and drained of water, water may accumulate inside the shaft seal. During shutdown, when switching to the standby shaft seal steam source, improper handling can result in water being carried along with the steam supplied to the shaft seal. 4. What are the hazards of water hammer in a steam turbine? The hazards of water hammer in steam turbines include: (1) friction and collision between moving and stationary parts. The introduction of cold water or cold steam into the turbine causes the metal components, which are at high temperature, to cool down suddenly and contract rapidly, resulting in significant thermal deformation and a sharp change in relative expansion, which in turn causes intense vibrations in the unit. Axial and radial friction and impact occur between the moving and stationary parts; large shaft bending is generated during radial friction and impact. (2) Damage and breakage of leaves. When a large amount of water enters the flow passage of the turbine, it causes damage and fracture to the blades, especially those that are longer. (3) The thrust bearing is burned out. The density of the water or cold steam entering the turbine is much greater than that of the steam; as a result, the same acceleration as that of the steam cannot be achieved within the nozzle. This causes the inlet angle of the relative velocity of this fluid to be much larger than that of the steam, preventing the flow from entering the flow channel in the correct direction and leading to impacts on the back arc at the inlet of the moving blades. In addition to generating a braking force on the moving blades, this also creates an axial thrust, thereby increasing the axial thrust of the turbine. In actual operation, the axial thrust of the turbine can increase to 10 times that during normal operation. Overloading the thrust bearing causes the tungsten alloy to burn out. (4) Steam leakage at the mating surfaces of valves or cylinders. If valves and cylinders are cooled rapidly, it can cause permanent deformation of the metal. It causes steam leakage at the joint surface of the valve or cylinder. (5) Causes metal cracks. During the start-up and shutdown of the unit, if cold water or cold steam enters frequently, the metal may develop cracks under the effect of repeated alternating thermal stresses. If the rotor surface at the steam seal is repeatedly cooled by water or cold steam from the steam seal supply system, cracks will form and continue to expand. 5. What are the phenomena of turbine blade fracture? What measures should be taken during operation to prevent blade damage? The phenomena of blade fracture include: (1) a sudden noise occurring inside the turbine or the condenser. (2) Sudden increase in vibration or jitter of the unit. (3) When the blades are severely damaged, to maintain a constant load, the steam flow rate must be increased, that is, the valve opening must be enlarged. (4) When broken blades fall into the condenser, they damage the cooling water pipes; as a result, the water level in the condenser rises, the conductivity of the condensate water increases, and the current drawn by the condensate pump increases. (5) Broken blades entering the extraction steam pipe may cause valve sticking. (6) In the coasting and barring modes, a metallic friction sound can be heard. (7) Operating intermediate pressure rises. To prevent blade damage, the following measures should be taken: (1) The power grid should operate at a normal frequency to avoid situations where excessively high or low frequencies cause certain stages of blades to enter the resonance zone. (2) During operation, maintain the steam parameters as well as the pressures and vacuums in various monitoring sections within normal ranges; if these values exceed the limits, operation should be carried out with reduced load. (3) Strengthen the chemical monitoring of steam and water. (4) During operation, strengthen the monitoring of vibrations to prevent the turbine from deforming due to uneven heating caused by cold water or steam ingress, or other factors, which may lead to a reduction in the dynamic and static clearances and resulting in localized rubbing. (5) During the major overhaul of the unit, a thorough and detailed inspection should be carried out on the damage to the flow-through components; records of any damage to the blades, shrouds, and struts should be kept, and adjustments to the blades should be made as necessary. 6. What are the main causes of shaft bending? What are the main measures to prevent the main shaft from bending? 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 shaft bending. (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 that the following valves are in their correct positions; (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 startup, the main steam pipes, reheat steam pipes, and various headers should be thoroughly warmed up. (5) During the start-up process, the vibration of each bearing in the unit must 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 rotation 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 smooth operation of the drainage system. (7) After installing the steam heating device, careful adjustments must be made to ensure that there are no cross-variations in temperature differences above and below, as well as left and right of the cylinder flange; all temperature differences should 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, close monitoring of the water levels in various water tanks and heaters should be carried out to prevent water or cold steam from entering 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) Desuperheating water shall not be used for hot start. 7. The main measures to prevent bearing damage are: (1) Strengthen the monitoring and adjustment of oil temperature and oil pressure; regularly calibrate the oil level gauge, oil pressure gauge, and oil temperature gauge. (2) The oil purification unit is operating properly; oil quality is tested regularly, and it should meet the standards. (3) Closely monitor the bearing white metal temperature; upon detecting any abnormalities, promptly identify the cause and eliminate it. (4) The oil system equipment is reliable in both automatic and standby modes, with strict regular testing conducted. (5) The start-up and shutdown of the operating oil pump or oil cooler should be carried out smoothly and carefully, with adequate air release to strictly prevent bearing damage due to oil shortage. (6) Pay attention to monitoring the vibration, shaft misalignment, and thrust difference of the unit. Prevent water ingress into the turbine, shaft bending, bearing vibration, and damage to the flow passages that could lead to wear of the bearing shells. (7) The turbine generator rotor shall be reliably grounded. (8) Before starting, the interlock setpoints for the AC and DC oil pumps should be carefully adjusted in accordance with the design requirements, and the wiring should be checked to ensure it is correct. (9) Valves in the oil system shall not be installed vertically, and the oil system should be cleaned after major repairs. (10) Regularly check the oil levels in the main oil tank, high-level oil tank, oil purification system, and seal oil tank, as well as the operation status of the oil filter during operation. If it is found that the oil level in the main fuel tank is dropping rapidly and refilling the oil does not work, the DC lubricating oil pump should be started immediately to shut down the machine. (11) The power fuse for the DC lubricating oil pump should have sufficient capacity and be reliable. 8. What are the main causes of turbine overspeed and what are the principles for handling it? The main reasons for turbine overspeed are: (1) the generator loses load to zero, and the turbine speed control system does not function properly. (2) The speed got out of control during the emergency stop device overspeed test. (3) After the generator is disconnected, the high and medium pressure main steam valves, control steam valves, extraction check valves, etc. become stuck or do not close properly. (4) Fault or malfunction of the turbine speed monitoring system. Principles for dealing with turbine overspeed: (1) Immediately break the vacuum to initiate an emergency shutdown, and confirm that the speed has decreased. (2) If it is detected that the rotational speed continues to increase, decisive isolation and pressure relief measures should be taken. (3) Identify the cause of the overspeed and eliminate the fault; only after a thorough inspection confirms that the turbine is operating normally can it be restarted. The critical safety device and all overspeed protection devices must function properly upon verification before the turbine can be connected to the grid under load. (4) During restart, close attention should be paid to and monitoring should be carried out on aspects such as turbine vibration, internal noises, bearing temperature, axial displacement, and thrust bearing temperature; if any abnormalities are detected, the startup process should be stopped. 9. What are the measures to prevent the turbine from overspeeding? Technical measures to prevent turbine overspeed include: (1) All overspeed protection devices must be in good condition, properly activated, and functioning correctly. (2) Under normal parameters, the control system should be able to maintain the turbine operating at its rated speed. (3) Under rated parameters, after the unit loses its rated load, the control system shall be able to maintain the unit’s speed below the speed at which the emergency shutdown device activates. (4) The speed variation rate of the regulating system shall not exceed 5%, and the hysteresis rate shall not exceed 0.2%. (5) The high and medium pressure automatic main steam valves and throttle valves shall be able to close quickly and tightly, without any sticking. (6) The regular testing device for regulating the security system should be in good condition and reliable. (7) It is necessary to conduct tests on the static characteristics of the control system; tests on the turbine’s control system should be carried out after major repairs to the turbine or after maintenance of the speed control system. (8) After the installation of new units or after technical modifications to a unit’s control system, tests on the dynamic characteristics of that control system should be conducted to ensure that the speed of the turbine does not rise beyond specified limits when the load is suddenly reduced. (9) An overspeed test should be conducted after a major overhaul or installation of the unit, after the emergency shutdown device is disassembled or adjusted, when the unit is restarted after being shut down for a month, and before load rejection tests are carried out on the unit. (10) An oil filling test for the emergency safety device should be conducted after the unit has operated for 2,000 hours; if the test is unsuccessful, an overspeed test must still be carried out. (11) When conducting an overspeed test, appropriate parameters should be selected; pressure and temperature must be kept within specified ranges. The bypass system should be activated, and only after the parameters have stabilized can the overspeed test be carried out. (12) During overspeed tests, the governing valve should be gradually opened in a smooth manner; correspondingly, the rotational speed should gradually rise to the trip speed of the overspeed protection device. If the governing valve is suddenly opened to its maximum position, the turbine must be immediately shut down to prevent any serious overspeed accidents. (13) Conduct periodic operational tests on the automatic main steam valve, control valves, and extraction check valves as stipulated. (14) When it is found during operation that the main steam valve or control valve is stuck, the sticking must be promptly resolved. Prior to doing so, measures to prevent overspeed must be taken. If the sticking of the main steam valve cannot be immediately resolved, the turbine must be shut down for handling. (15) Strengthen the supervision of oil quality by conducting regular analysis and testing to prevent water or impurities in the oil from causing sticking or corrosion in the control components. (16) Strengthen the supervision of steam quality to prevent scaling on the door rods due to salt in the steam, which could cause sticking. (17) Operators must be familiar with overspeed indicators and strictly adhere to emergency shutdown procedures. (18) When the unit is out of service for an extended period, it is necessary to take proper measures for shutdown protection to prevent steam, water, or other corrosive substances from entering or remaining inside the turbine and the oil supply control system, thereby avoiding corrosion of the valves or control components. (19) After a major overhaul of the unit, a valve tightness test should be conducted; the testing standards and methods shall follow those specified by the manufacturer. During operation, a valve tightness test should be carried out once a year. (20) During the operation of the steam turbine, pay attention to checking the relationship between the opening degree of the control valves and the load, as well as the pressure changes after the control valves. In case of any abnormalities, promptly investigate and determine the causes. (21) To prevent a large amount of water from entering the oil system, it is necessary to strengthen monitoring and adjust the steam seal pressure so that it does not become too high; similarly, the negative pressure in the front tank and bearing box should also not be too high. (22) For units that operate with sliding pressure, a margin should be left when adjusting the valves during the sliding-parameter startup process; they should not be opened to their maximum position to prevent overspeed due to load rejection. (23) During shutdown, the emergency safety device should be activated first, the main steam valve and control valves should be closed, and the generator should be tripped via reverse power. However, care must be taken to ensure that the time taken for the generator to be disconnected from the system is not too long, as this results in a state with no steam supply; an excessive duration in such a state can lead to an increase in the temperature of the exhaust cylinder and an increase in the expansion difference. 10. What aspects should be given special attention to prevent rotor bending during hot start-up? In addition to taking measures to prevent rotor bending before starting up in a hot state, the following tasks should also be carried out: (1) Before initiating a hot start, the team responsible for the operation must understand the circumstances of the previous shutdown, identify any abnormalities, note the issues that require attention, and explain these points to each operator so that everyone is aware of them. (2) Before starting the machine in hot condition, the rotor must be rotated continuously for more than 4 hours, with the rotor’s vibration measured to be no more than 0.02 mm of the original value. (3) The shaft seal steam must be supplied first before evacuating the vacuum. (4) All pipes and manifolds should be more fully insulated with heating pipes and boxes. (5) Strict requirements are placed on the startup parameters and the opening degree of the bypass (the bypass should be opened only when the condenser has a certain level of vacuum); the main steam temperature must be 80–100°C higher than the temperature of the high-pressure inner cylinder, with a superheat of over 50°C. During warm-up and load acceptance, monitoring of the main steam and reheat steam temperatures should also be intensified; the steam temperatures must not fluctuate repeatedly. (6) Strengthen the monitoring of vibrations. During hot start-up, vibrations can occur easily due to temperature differences among various components, and stricter monitoring is required at this time. If the vibration exceeds the specified value, the machine should be stopped immediately by shutting off the power. (7) During startup, hydrophobicity of all components should be enhanced. (8) Extreme hot-start conditions should be avoided as much as possible. (9) Before hot startup, the control system should be degassed, as air remaining in the control system may cause the control valves to move significantly during startup, leading to unstable boiler parameters and steam containing water. (10) An overspeed test cannot be conducted during hot-state startup. (11) During hot start, the load should be increased as quickly as possible to the level corresponding to the cylinder temperature. 11. Under what circumstances is it generally prohibited to start or operate a steam turbine? Generally, it is prohibited to operate or start the turbine under the following conditions: (1) Abnormal operation of the emergency safety device. (2) The automatic main steam valve, throttle valve, and extraction check valve are sticky and cannot close tightly; the tightness tests for the automatic main steam valve and throttle valve fail. (3) The speed control system is unable to maintain the turbine operating at no load (or it cannot keep the speed within the range that triggers the emergency shutdown device after the unit loses load). (4) The bending value of the turbine rotor exceeds the specified limit. (5) The temperature difference between the upper and lower sections of the high-pressure inner cylinder is greater than 35°C, while the temperature difference between the upper and lower sections of the high- and medium-pressure outer cylinders is greater than 50°C. (6) When obvious friction sounds are heard inside the unit during shaft rotation. (7) When any oil pump or turning gear fails. (8) The oil pressure is not within the specified range or the oil temperature is below the required value. (9) When the oil level in the tank is below the specified value after the oil system has been filled. (10) There are severe leaks in various systems of the turbine. (11) When the insulation equipment is substandard or incomplete. (12) In the event of failure of the protection devices (low oil pressure, low vacuum, axial displacement protection, etc.) and failure of the main electric valves (such as the electric main steam valve, high-pressure heater feed valve, water inlet valve, etc.). (13) Failure of the main instruments, including the tachometer, deflection meter, vibration meter, thermal expansion meter, differential expansion meter, axial displacement meter, speed control and lubricating oil pressure gauges, seal oil pressure gauge, temperature gauges for thrust and seal bearings, hydrogen-oil pressure difference gauge, hydrogen pressure gauge, cooling water pressure gauge, main steam or reheat steam pressure and temperature gauges, cylinder metal temperature gauge, vacuum gauge, etc. 12. What are the main technical measures to prevent damage to turbine bearing shells? The main technical measures to prevent damage to the turbine bearing shells are: (1) All valves in the oil system should be equipped with label plates, and the switching of operations in the oil system must be carried out in accordance with the regulations. (2) The valves in the lubricating oil system are of the exposed rod type or equipped with scales. (3) Regular testing of high and low pressure oil supply equipment. (4) The lubricating oil pressure shall be based on the bearing shell farthest from the cooler along the turbine centerline. (5) A higher rating fuse should be selected for the power supply of the DC oil pump. (6) When the oil pump is stopped after the turbine reaches a constant speed, attention should be paid to changes in oil pressure. (7) The fuel tank level should meet the specified requirements. (8) The lubricating oil pressure should meet the design value. (9) Before shutting down, the lubrication oil pump should be tested to ensure it is functioning properly before proceeding with shutdown. (10) Strictly control the oil temperature. (11) The turbine should be shut down urgently if smoke is generated due to a loss of oil supply to any bearing, or if the temperature of the oil returning to the bearing rises suddenly to the emergency shutdown value. 13. Under what circumstances will a turbine stop operating due to a vacuum loss fault without being damaged? (1) The vacuum has been reduced to the specified value, but the load still not being reduced to zero. (2) At the rated steam pressure, the main steam temperature rises to the maximum allowable value. (3) The main and reheat steam temperatures are too low. (4) The main steam pressure rises to the maximum allowable value. (5) The water supply to the generator exceeds the specified value, and the water shortage protection fails to activate. (6) All plant power is lost. (7) When the main oil pump fails and cannot maintain normal operation. (8) There is a large hydrogen leak in the hydrogen cooling system, and the hydrogen pressure inside the generator cannot be maintained. (9) The condensate pipe ruptured, preventing the deaerator water level from being maintained. (10) Leakage in the condenser cooling water pipes, with circulating water leaking into the steam side. 14. What issues should be considered in operational procedures to prevent static and dynamic friction in steam turbines? Pay attention to the following points: (1) Before each start-up, it is necessary to carefully check the deflection of the main shaft; only when the deflection is within the allowable range can the start-up be carried out. (2) The temperature difference between the upper and lower cylinders must be within the specified range. If the temperature difference between the upper and lower cylinders is too large, it will cause significant thermal bending of the cylinders. Practice has shown that a large temperature difference between the upper and lower cylinders is often the initial cause of shaft bending. (3) During the hot start of the unit, the state changes are relatively complex; operators should pay special attention to controlling and monitoring issues such as the steam inlet temperature and the steam supply for shaft seals. Most past accidents involving shaft bending occurred during hot starts. (4) Strengthen the monitoring of unit vibration. When static and dynamic friction occurs below the first critical speed, it poses the greatest threat of shaft bending; therefore, when the bearing vibration of the turbine reaches 0.03 mm at speeds below medium speed, the machine must be shut down immediately. It is absolutely forbidden to reduce the speed to warm up the machine while the vibration is increasing. When the machine is shut down due to an abnormal condition, it is necessary to check the rotor’s coasting time; if a significant change compared to normal conditions is observed, efforts should be made to determine the cause. (5) After the turbine is shut down, make sure to cut off all water sources connected to the utility systems to prevent water from entering the cylinders. To enhance the monitoring of equipment after it is shut down, it is necessary to continue with the regular routine inspection system; any abnormal conditions detected should be analyzed and addressed immediately. 15. Describe the general steps for diagnosing vibration faults in steam turbine generator sets. The steps for diagnosing vibration faults in steam turbine generator sets are as follows: (1) Measure the vibration frequency to determine the nature of the vibration. If the vibration frequency does not match the rotor speed, it indicates that self-excited vibration has occurred, allowing for the identification of the specific cause of such vibration. If the vibration frequency matches the rotational speed, it indicates that forced vibration has occurred. (2) Determine whether the bearing housing that has experienced severe vibration has good stability; if not, it should be reinforced. If the stability of the bearing housing is not the main cause, it can be concluded that the excessive vibration is due to excessive excitation force. (3) Determine the nature of the excitation force. (4) Identify the source of the excitation force, that is, the specific component where the vibration defect occurs and its nature. In vibration fault diagnosis, the location where the vibration is greatest usually indicates the site of the defect. But sometimes, especially in shafting systems with multiple rotors connected together (particularly flexible rotors), the vibration caused by a defect in one rotor bearing can result in greater vibrations in other rotor bearings. This involves both issues related to bearing stiffness and the vibration modes of multiple shafts connected together; this factor must be taken into account during detailed analysis. 16. What damages can vibration in a running turbine cause? Vibration of an operating steam turbine can cause the following hazards: (1) Wear of certain shaft seals at the low-pressure end, resulting in a loss of sealing effectiveness; air then leaks into the low-pressure casing, affecting the vacuum level ； The shaft seals on the high-pressure side are worn, resulting in an increased amount of steam leaking out from the high-pressure cylinder. This causes localized heating of the rotor, leading to bending, and the steam enters the bearing oil, causing it to emulsify. (2) Severe wear of the diaphragm steam seal will increase the leakage of steam between stages; in addition to affecting efficiency, this also increases the axial thrust, leading to the melting of the tungsten alloy in the thrust bearings. (3) When the slider wear is severe, it affects the normal thermal expansion of the unit, thereby causing other accidents. (4) Breakage of the tungsten alloy in the bearing bush, and loosening or breaking of the securing screws. (5) A decrease in the fatigue strength of the rotating parts will cause damage to blades, discs, etc. (6) Loose or damaged components of the generator and exciter. (7) The speed control system is unstable. 17. What are the main reasons for the increase in axial displacement of steam turbines? The main reasons for the increase in the turbine’s axial displacement are: (1) a decrease in steam temperature and pressure, overload of the flow-through sections, and the shutdown of the regenerative heaters. (2) The steam leakage due to wear increases the gap in the diaphragm shaft seal. (3) Poor steam quality causes scaling in the flow passages. (4) Water hammer occurs. (5) Overloading of the turbine: Generally, the axial thrust of a condensing turbine increases as the load increases ； For extraction or back-pressure turbines, the maximum axial thrust may occur at an intermediate load. (6) Damage to the thrust bearing. 18. What measures should be taken in operation and maintenance to prevent low-temperature brittle fracture accidents? To prevent low-temperature brittle fracture accidents, the following measures should be taken: (1) Avoid or reduce thermal shock damage. During startup, the main steam temperature should be controlled with at least a 50°C superheat. When starting the unit, the warming-up procedure and duration must be followed as specified in the regulations, in order to adjust the temperature of the rotor’s inner cavity to match the internal stresses and prevent the material from being subjected to excessive stress. Therefore, the rotor needs to be thoroughly preheated, with care taken to control the rate of temperature rise of the metal as well as the temperature difference between the inside and outside of the cylinders. (2) During normal operation, the primary and secondary steam temperatures must be strictly controlled to avoid exceeding limits or experiencing significant fluctuations. (3) The overspeed test should only be conducted after 3–4 hours of warm-up at 25% low load. (4) During medium-speed warm-up, full speed shall only be reached once the temperature of the lower wall of the high and medium pressure inner cylinders exceeds 250°C, to ensure that the temperature of the rotor’s central hole is above the temperature at which brittle failure occurs. (5) Adopting a sliding pressure operation mode to adjust the load during normal operation can reduce the magnitude of thermal stress variations. In particular, using a sliding-parameter shutdown helps to reduce thermal stress. 19 What are the common causes of changes in the oil level in the main engine’s oil tank? Reasons for the increased oil level in the main engine tank: (1) Excessively high pressure in the pressure equalization tank or excessive steam volume at the end shaft seals. (2) Malfunction of the shaft exhaust pump results in poor steam discharge from the shaft seal, causing water to enter the oil. (3) The copper tubes of the oil cooler are leaking, and the water pressure is higher than the oil pressure. (4) The oil level gauge gets stuck, resulting in an incorrect oil level reading. (5) During startup, the bearing cooling water of the high-pressure oil pump and the lubricating oil pump leaks into the oil. (6) When the oil temperature at the outlet of the cold oil cooler increases and its viscosity decreases, the oil level will also rise. (7) An excessively low oil level in the sealed oil tank causes a high oil level in the main oil tank. The reasons for the decrease in the oil level in the main engine tank are as follows: (1) Leakage or accidental activation of the tank emergency drain valve and other components of the oil system. (2) The filter pump of the oil purifier fails to automatically start and pump oil into the main oil tank when the oil level reaches its upper limit. (3) Leakage in the copper tubes of the oil cooler. (4) The oil temperature at the outlet of the oil cooler is low, and the oil level has also decreased. (5) Oil leakage from the bearing oil seal. (6) The fuel tank has just had water drained from it. (7) The oil level gauge is stuck. (8) An excessively high level of seal oil in the tank causes a low level of oil in the main tank. (9) Oil supply to the generator during shutdown. 20. What are the causes and consequences of an increase in the temperature of the turbine startup exhaust cylinder? (1) When the turbine starts up, steam passes through a throttle valve and then through nozzles to drive the impellers of the speed-regulating stage; after throttling, the entropy of the steam increases while its enthalpy drop decreases, resulting in a higher temperature of the exhaust steam after work is done. During the entire startup process prior to grid connection, very little steam is consumed; at this stage, the work is primarily done by the regulating stages. The waste steam, as it flows toward the exhaust cylinder, has a low flow rate and low velocity, as well as a large cross-sectional area, which results in a significant draft effect. The exhaust temperature rises due to significant blowdown losses. As the rotor rotates, friction occurs between the blades (especially the longer blades in the latter stages) and the steam, which is also one of the factors that raise the exhaust steam temperature. When the turbine starts up, the vacuum level is low, and accordingly the saturation temperature rises, which means that the exhaust temperature increases. An excessively long startup time of the steam turbine may also cause the exhaust casing temperature to become too high. (2) When the load is increased during grid-connected power generation, the main steam flow increases as the load rises; the turbine gradually enters its normal operating condition, and the proportion of power consumed by friction and aerodynamic losses becomes smaller and smaller. As the vacuum in the turbine exhaust cylinder gradually increases, the exhaust temperature also decreases gradually. (3) An increase in the exhaust cylinder temperature leads to greater thermal deformation of the low-pressure cylinder shaft seal, which can cause the center of the turbine’s pit to shift, resulting in increased vibration and greater friction between the moving and stationary parts; in severe cases, this can damage the low-pressure cylinder shaft seal. (4) When the temperature of the exhaust casing reaches above 80°C, the water spraying system for the exhaust casing automatically activates to cool it down; the temperature of the exhaust casing must not exceed 120°C. 21. What are the reasons for bearing oil loss in a steam turbine? The reasons for bearing oil loss in a turbine include: (1) misoperation during the oil system switch-over while the turbine is in operation. (2) If the main oil pump loses pressure and the lubricating oil pump does not start operating, this will result in a loss of oil supply; it can also cause a loss of oil supply at the moment just before the lubricating oil pump starts operating. (3) If a large amount of air remains in the oil system and is not removed in time, it can cause an instantaneous loss of oil to the bearing shells, leading to their damage. Failing to remove air in advance as required when switching between the oil filter and the oil cooler can allow a large amount of air to enter the oil supply pipeline, resulting in an immediate loss of oil supply to the bearing shells. (4) The lubricating oil pump does not supply oil during startup and shutdown. (5) The oil level in the main fuel tank is too low, air enters the filler, causing the main fuel pump to lose fuel supply. (6) Loss of oil to the bearing shells occurs when the DC oil pump cannot be activated in time due to a disruption in plant power supply. (7) The fuel supply pipeline broke, resulting in a large amount of oil leakage and a disruption in fuel supply. (8) Residual fibers and other debris in the oil system during installation or maintenance can cause blockages in the oil inlet. (9) The bearing shells shift during operation; for example, if the bearing shells rotate, it can cause the oil inlet to become blocked. 22. What are the differences between the reasons for the increased temperature of individual bearings and the increased temperature of bearings in general? Reasons for the increased temperature of individual bearings: (1) Increased load, uneven distribution of stress on the bearings, and higher load on certain bearings. (2) Poor oil inflow or poor oil return. (3) Debris enters the bearing, and the black metal shell comes off. (4) The shaft seal steam on the bearing side is excessive or there is significant steam leakage. (5) Gas is present in the bearing, and the oil flow is poor. (6) Vibration causes damage to the oil film and poor lubrication. Bearing temperatures have generally increased: (1) The oil outlet temperature of the cooler has risen for certain reasons. (2) Deterioration of oil quality. (3) Excessively high negative pressure in the bearing housing or main oil tank for oil return, as well as poor oil return flow. (4) The speed of the turbine unit increases. 23. What are the reasons for vibration during the operation of the feed water pump? (1) Excessive traffic, operating under overload. (2) At low flow rates, periodic turbulence occurs in the fluid within the pipeline, causing unstable operation of the pump. (3) Vaporization of the feedwater pump. (4) The bearing is loose or damaged. (5) Loose impeller. (6) Shaft bending. (7) Unbalance in the rotating part. (8) The coupling centers are not aligned. (9) The foundation screws of the pump body are loose. (10) Severe wear of the balance disk. (11) Foreign objects enter the impeller. 24. What are the signs of corrosion in the copper tubes of a condenser? The phenomena of corrosion in the condenser copper tubes include the following: (1) Electrochemical corrosion. When the condenser is in operation, since the cooling water flowing through the copper tubes is not purified chemical water but often contains electrolytes such as salts and alkalies, the cooling water becomes conductive, thereby causing electrochemical corrosion. (2) Impact corrosion. This is a major form of damage to the copper tubes in condensers. It occurs mostly at the inlet end of the copper tube. The high and uneven water flow here causes impact corrosion. Furthermore, when the cooling water contains a high amount of sand, mechanical friction can also cause wear and corrosion of the condenser copper tubes. (3) Dezincification corrosion. This is the result of electrochemical reactions. The inner surface of the copper pipe is covered with an oxide layer that serves to protect it from electrochemical corrosion. However, during operation, factors such as sediment erosion, friction from debris, and water flow can cause this protective layer on the inner surface of the copper pipe to wear off. Electrolysis occurs when steel and zinc are in water, causing the zinc in the copper pipe to be dissolved and carried away by the water. Copper pipes that have lost zinc become porous, their texture turns brittle, and their mechanical strength **decreases**. 25. What should be done if the turning gear stops operating for some reason when the cylinder temperature is high? In cases where the cylinder temperature is high, if there is a problem with the turning gear, it can be handled according to the following principles: (1) When the turning gear cannot function, manually turn the shaft. Meanwhile, maintain the continuous operation of the oil system. During this period, monitoring of the bearing temperature should be strengthened. (2) If the rotor cannot be rotated due to reasons such as collisions between the moving and stationary parts inside the turbine caused by thermal shock and the resulting deformation, a cylinder sealing procedure should be carried out, and an attempt to rotate the rotor can be made again after 1 hour. Under no circumstances should one attempt to start the machine by feeding steam to the crew or use a crane to force it to rotate. (3) When the barring motor experiences overcurrent, the temperature difference between the upper and lower parts of the cylinder exceeds the specified value, or an obvious sound of metal friction is heard, continuous barring should be stopped and regular barring should be used instead. Rotate the shaft 180° regularly as required. Closely monitor the data in the TSI eccentricity table, and carefully record the eccentricity values, as well as the time and number of disk rotations. (4) If two out of the three jacking oil pumps fail, one jacking oil pump should be started. As long as the jacking oil pressure is normal, shaft turning can be carried out; however, the DC lubricating oil pump should be started to increase the amount of lubricating oil. If all three top shaft oil pumps fail to operate, the cylinder should be sealed, and maintenance personnel should be contacted to carry out repairs as soon as possible. After repair, the rotor should be rotated 180° to straighten it before resuming continuous turning. (5) When resuming continuous barring after an interruption, the rotor eccentricity should be monitored, and attention should be paid to any frictional sounds between the stationary and rotating parts of the unit. (6) Cylinder stuffing method: Close all drain valves on the cylinder and steam extraction pipes to isolate all sources of steam entering the turbine and condenser. Once the temperature difference between the upper and lower cylinders is less than 50°C, straighten the rotor using the 180° rotation by gravity method. After the rotor’s vibration level returns to normal, resume continuous rotor turning. 33. What are the reasons for high temperatures in the thrust bearings during turbine operation? How to adjust? During operation, high thrust pad temperatures can be caused by: (1) high oil temperature at the outlet of the cooler. (2) Lubricating oil pressure is low. (3) Insufficient oil in the thrust bearing. (4) Thrust bearing wear. (5) High axial thrust. (6) Water impact occurs. (7) Sudden load changes, vacuum variations, and changes in steam pressure and temperature. Adjustment measures: (1) When it is detected that the metal temperature at any point of the thrust bearing rises by 5°C or continues to rise, the cause of this increase must be identified, and the situation should be reported to the shift supervisor and the plant manager. Check the outlet temperature of the cold oil cooler and adjust it to the normal level. Check whether the lubricating oil pressure and the oil flow in the thrust bearings are normal. (2) If the metal temperature of the thrust bearing is abnormal, listen for any unusual noises inside the unit, and check the load, steam temperature, steam pressure, vacuum, axial displacement, and vibration levels. If any abnormalities are detected, adjust them to normal values. (3) When the metal temperature of the thrust bearing or the return oil temperature of the thrust bearing reaches the alarm value, it should be reported to the shift supervisor, the load should be reduced, and close monitoring should be carried out. (4) When the metal temperature of the thrust bearing or the bearing return oil temperature reaches the shutdown value, a vacuum-based emergency shutdown should be initiated. 26. Describe the principles for dealing with a drop in condenser vacuum. (1) If a drop in vacuum is detected, it should be confirmed by comparing with the exhaust steam temperature. The cause must be identified promptly, appropriate countermeasures taken immediately, and the situation reported to higher-level management. (2) When the vacuum level drops, the backup vacuum pump should be started. If the vacuum continues to decline even after reaching the load-reduction value, the load should be reduced by an amount corresponding to the degree of vacuum drop until it reaches zero. (3) If the treatment is ineffective and the unit load is reduced to zero yet the vacuum cannot be restored, the machine should be shut down by tripping the circuit breaker. (4) When the vacuum level drops, attention should be paid to the operation of the steam pump; if necessary, switch to electric pump operation. (5) As the vacuum decreases, attention should be paid to changes in the exhaust temperature. (6) If the vacuum drops rapidly and reaches the shutdown value during operation, the protection will trigger a trip of the unit; otherwise, manual shutdown should be carried out. (7) During shutdowns due to low vacuum, the high and low pressure bypasses should be promptly disconnected and closed, as well as the main and reheat steam pipes leading to the condenser drain; it is prohibited to activate the secondary bypass from the boiler to the condenser. (8) Strengthen the monitoring of the temperatures and vibrations of various bearings in the unit. 27. What should be done if the circulation water is interrupted during unit operation? (1) That is, manually shut down the steam turbine generator set while keeping the condensate system and vacuum pump operating. (2) Promptly cut off and close the bypass system, shut down the main and reheat steam pipes leading to the condenser’s drain, and prohibit the activation of the 5% startup bypass from the boiler to the condenser. (3) Pay attention to the temperature changes of various users in the closed water system. (4) Strengthen the monitoring of lubricant temperature, bearing metal temperature, and bearing return oil temperature. If the bearing metal temperature or return oil temperature rises to near the limit, a vacuum-based emergency shutdown should be initiated. (5) Close the inlet and outlet valves for the condenser’s circulating water; wait until the exhaust steam temperature drops below the specified value before restoring the circulation of water in the condenser. (6) Check that the safety diaphragm of the low-pressure cylinder is intact; otherwise, notify maintenance personnel to replace it promptly. 28. How to deal with oil leakage in the turbine oil system lubrication system? Once the duty officer notices a drop in the oil level in the lubricating oil tank, they should first check the oil gauge to confirm that the level has indeed dropped, and then investigate the cause. (1) Check whether the accident drain valve is airtight. Perform a water drainage check on the cold oil cooler; if there is a leak, isolate the leaking cold oil cooler. (2) Check the oil system pipes for leaks, and take strict precautions to prevent oil from leaking onto high-temperature pipes and equipment. (3) When the fuel tank level drops to the low-level alarm threshold, fuel should be added. (4) In the event of a large-scale oil leak in the oil system, immediate measures should be taken to seal the leak in order to reduce the amount of oil leaking or to change its direction, preventing it from reaching high-temperature pipes and equipment. At the same time, the oil tank should be refilled promptly and any defects fixed. (5) If the oil level in the tank drops rapidly due to severe oil leakage, reaching the shutdown threshold, or if the lubricating oil pressure falls to 0.06 Mpa without the protection system activating, immediately break the vacuum to initiate an emergency shutdown. (6) In the event of a fire caused by oil leakage into high-temperature pipes or components, use dry powder fire extinguishers or foam fire extinguishers; water should not be used to extinguish the fire. The fire alarm code “119” should be activated immediately to alert the fire department, and the situation should be reported to the shift supervisor and relevant supervisors. 29. What is the importance of the pressures at various monitoring points of the turbine? (1) The pressures at each monitoring section of the turbine are the extraction pressures at those sections; since, except for the last and second-to-last stages, the extraction pressure at each section is proportional to the main steam flow rate. Based on this relationship, during operation, the normal functioning of the flow path can be effectively monitored by observing the pressures at each monitoring and regulating stage as well as the extraction steam pressures in each section. Each unit has corresponding extraction steam pressures at its rated load, and after the unit’s installation or major overhaul, it is necessary to determine, through testing under normal operating conditions, the relationship between the load, main steam flow rate, and the monitoring pressures at various stages, so as to use this as a standard for routine operation monitoring. (2) During normal operation and at a certain load, if the pressure in the monitored section rises, it indicates that scaling may have occurred in the sections downstream of that section, or that other metal components have fallen off and caused blockages ； Of course, if the pressure at the governing stage and in the high-pressure cylinder rises simultaneously, it may be due to restricted opening of the medium-pressure governing valve or the shutdown of steam extraction at a certain stage of the medium-pressure cylinder. (3) When monitoring the pressure in each section, it is necessary to check not only whether the absolute value of the pressure exceeds the specified limit, but also whether the pressure difference between sections exceeds the specified limit. If the pressure difference across a certain stage is too high, it may cause damage to equipment such as blades. 30. Describe the causes of abnormal pressure in the turbine regulation stage and the methods for handling it. Under normal operation, the pressure at the regulating stage is roughly proportional to the main steam flow rate. The reasons for abnormal pressure at the regulating stage include: (1) inaccurate readings due to issues with the instrumentation. (2) Salt deposits accumulate in the flow passage of the turbine, reducing the flow area. (3) Due to the fragmentation of metal parts or mechanical debris blocking the flow path, or damage and deformation of the blades. (4) With the main engine load remaining constant, if the main steam flow deviates from its designed value for various reasons – such as the removal of multiple heaters, severe leakage in the boiler reheater, serious internal leakage in the main engine’s low-pressure bypass, or sudden changes in vacuum – then the main steam pressure and temperature will change significantly, resulting in abnormal main steam flow, which is reflected in abnormal changes in the pressure of the control stages. (5) The host is operating under overload. Handling of abnormal regulating stage pressure: (1) After the unit’s major overhaul, under certain operating conditions, there should be original records of the corresponding regulating stage pressure, so as to enable comparison during daily operation. When the pressure at the main turbine regulation stage is abnormal, it is necessary to first conduct a detailed analysis to identify the cause, and enhance the monitoring of relevant parameters such as main steam pressure, temperature, and vacuum, as well as the main turbine’s vibration, expansion difference, shaft displacement, and whether there are any abnormalities in the extraction steam pressures at various stages. (2) In the event that the pressure in the control stage becomes abnormal due to a fault in the thermal measurement points, since the main steam flow rate may also become abnormal at this time, it is necessary to enhance monitoring of the coordination control system and the automatic drum level control; manual adjustments should be made if required, and the main steam flow rate should be monitored more closely through indirect methods. Contact the instrument control personnel as soon as possible to handle it. (3) The reduction in the area of the flow passage due to salt deposition there occurs gradually; after the unit has been operating for a certain period, the pressure at the regulating stage should be compared with its original value. Once salt deposition is detected, the unit should be shut down as soon as possible. Meanwhile, during regular operation, it is necessary to strengthen the management of the quality of steam and water, in order to prevent scaling on the blades caused by substandard steam quality. (4) When there are abnormal changes in the pressure at the regulating stage, accompanied by increased vibration of the main unit, significant changes in shaft displacement, or rises in parameters such as condensate water hardness and conductivity, or when the heaters become filled with water, it is indicated that the blades of the main unit are damaged. In such cases, the load should be reduced or the unit should be shut down in strict accordance with the procedures to prevent the accident from escalating. (5) When the unit is under high load, the main steam parameters should operate as close as possible to their rated values. At loads corresponding to significantly increased main steam flow rates, in addition to checking the various parameters of the main steam, it is also necessary to verify whether there are any leaks in the steam system downstream of the main steam valve that could be causing the increased flow rate. When removing heaters, enhanced monitoring of the pressure at the regulating stage is required (especially when multiple heaters are removed simultaneously). (6) When the pressure at the regulating stage rises to the specified value, the unit should request load reduction. 31. Why can adopting variable-pressure operation for steam turbines yield economic benefits? The main reasons why operating a turbine at variable pressure (sliding pressure operation) can yield economic benefits are as follows: (1) Under normal conditions of operation at constant pressure at low loads, it is difficult for large boilers to maintain the temperatures of the main steam and reheat steam from dropping; whereas with variable pressure operation, it is easier for the boiler to maintain the rated temperatures of the main steam and reheat steam. During variable-pressure operation, as the main steam pressure decreases while the temperature remains constant, although the superheated enthalpy of the steam drops with falling pressure, the enthalpy of saturated steam increases by more, resulting in a significant rise in the total enthalpy. This is an important reason why variable-pressure operation offers economic benefits. (2) When operating at a reduced steam pressure with constant steam temperature, the volumetric flow rate and flow velocity at each stage of the turbine remain approximately unchanged, allowing the efficiency within the turbine to be maintained without decline at low loads. (3) Under variable-pressure operation, the temperatures of each stage of the high-pressure cylinder, including the exhaust temperature of the high-pressure cylinder, will increase. This ensures the temperature of the reheat steam and helps improve the efficiency of the thermal cycle. (4) During variable-pressure operation, the feedwater pressure may be reduced accordingly; the use of electric variable-speed feedwater pumps can significantly reduce the electricity consumption of the feedwater pumps. Furthermore, operating the feed water pump at a reduced speed helps to reduce the erosion of the equipment by water flow and prolongs its service life. 32. What aspects should be considered to improve the operational efficiency of power units? To improve the operational economy of the unit, attention should be paid to the following aspects: (1) Maintaining the rated initial steam parameters. (2) Maintain the rated reheat steam parameters. (3) Maintain the most favorable vacuum. (4) Maintain the minimum condensate subcooling. (5) Make full use of heating equipment to increase the feedwater temperature. (6) Pay attention to reducing the plant power consumption rate. (7) Reduce the pressure loss of new steam. (8) Maintain the optimal efficiency of the turbine. (9) Determine a reasonable operating mode. (10) Pay attention to the economic distribution of the turbine load. 33. What are the main losses within the stages of a steam turbine? What is the cause of the loss? Within a turbine stage, the main losses include nozzle loss, blade kinetic energy loss, residual velocity loss, blade height loss, fan loss, partial steam injection loss, frictional blowing loss, steam leakage loss, and wet steam loss. (1) Nozzle losses and blade losses are caused by the mutual friction between the steam streams as they pass through the nozzles and blades, as well as the friction between the steam streams and the surface of the blades. (2) Residual velocity loss refers to the fact that steam still possesses a certain velocity when it leaves the moving blades; this portion of kinetic energy is not utilized in this stage, and thus it constitutes a loss for that stage. However, when the vapor stream flows into the next stage, its kinetic energy can be partially utilized by that stage. (3) Blade height loss refers to the loss caused by vortices formed by the steam flow at the base and tip of the nozzle and rotor blades. (4) Fan loss refers to the energy loss caused by the flow hitting the blades, as the blades are arranged in a circular pattern along the rim, resulting in a fan-shaped cross-section of the flow channel. As a result, the pitch, circumferential velocity, and inlet angle vary throughout the height of the blade, which leads to such energy loss. Additionally, the steam flow generates radial movements that further consume its energy. (5) Some of the steam inlet losses are due to \"blowing\" losses that occur when the moving blades pass through the arc sections without nozzles, as well as steam rejection losses that occur when the moving blades move from the non-operating arc sections into the operating arc sections with nozzles. (6) Frictional blowing loss refers to the fact that the high-speed rotating impeller comes into friction with the steam surrounding it, causing this steam to rotate as well; this process consumes a portion of the impeller’s useful work. The swirl formed by the steam flow between the diaphragm and the nozzle under the action of centrifugal force also consumes the useful work of the impeller. (7) Steam leakage loss refers to the loss that occurs in a steam turbine due to pressure differences; as a result, part of the steam escapes through various gaps between moving and stationary components, rather than passing through the nozzles and blade passages, and thus does not contribute to power generation, leading to such losses. (8) Moisture loss refers to the situation in the low-pressure section of the turbine where the steam is in a moist state. The water present in this moist steam not only fails to expand and generate work, but it also consumes the kinetic energy of the steam flow; it further exerts a braking effect on the movement of the blades, thereby wasting useful work, and it also causes erosion of the blades. 34. What is the effect of changes in main steam pressure on the operation of a turbine when the main steam temperature remains constant? With the main steam temperature remaining constant, the increase in main steam pressure has the following effects on the turbine: (1) The overall enthalpy drop increases, improving operational efficiency. However, when the main steam pressure exceeds the limit, it poses a threat to the safety of the unit. (2) The regulating-stage blades are prone to overload. (3) The steam humidity increases in the last stages of the turbine unit ； (4) It causes an increase in internal stress in components such as the main steam pipes, main steam valves and throttle valves, cylinders, and flanges, reducing their service life and leading to damage. With the main steam temperature remaining constant, a decrease in main steam pressure has the following effects on the turbine: (1) The available enthalpy drop of the turbine decreases, steam consumption increases, efficiency drops, and output is insufficient. (2) The flow passage part of the steam turbine is prone to overload. (3) For the small steam turbine driving the feed water pump and the deaerator, which are supplied with steam extraction, the excessively low main steam pressure results in a corresponding decrease in the extraction steam pressure; this prevents the small steam turbine and the deaerator from operating normally. 35. What are the various ways of variable-pressure operation for steam turbines? There are several modes of variable-pressure operation for steam turbines: (1) pure variable-pressure operation. That is, within the entire range of load variations, the throttle valve remains fully open, with the load changes being controlled entirely by the boiler pressure. (2) Throttling voltage regulation operation. To overcome the drawback of slow load adjustment during full voltage transformation operation, under normal conditions the speed control valve is not fully opened, thereby maintaining a certain degree of throttling on the main steam pressure. When the load increases suddenly, the throttle valve, which was not yet fully open, quickly opens to its full extent in order to meet the demands of this sudden increase in load. Thereafter, as the steam pressure in the boiler increased, the control valve was closed again until it reached the opening degree corresponding to the original sliding pressure operation. (3) Composite voltage transformation operation. This is an operating mode that combines variable-pressure operation with constant-pressure operation, and there are specifically the following three methods. ① It operates at variable voltage under low load and at constant voltage under high load. At low loads, the last one or two control valves are closed while all the other valves remain open. As the load increases gradually and the steam pressure reaches the rated value, the main steam pressure is kept constant; instead, the last one or two control valves are opened further to continue increasing the load. In this mode, the unit exhibits variable-speed operation at low loads, while at high loads it contributes a certain capacity to frequency regulation; this is a relatively ideal operating approach. ② Operate at variable voltage under high load, and at constant voltage under low load. Large-capacity units employ variable-speed feedwater pumps. Although their speed can vary over a wide range, there is still a minimum speed limit. Additionally, at low pressures and high temperatures, the heat absorption ratio of the boiler changes significantly, making it somewhat difficult to maintain the main steam temperature. Consequently, the minimum operating pressure of the boiler is also limited. This approach meets the above requirements and features variable voltage operation under high load conditions. ③ Operate at constant pressure under high and low loads, and operate with variable voltage in the intermediate load range: in the high-load area, adjust the load using control valves to maintain constant pressure operation ； In the intermediate load range, one or two control valves are closed, resulting in a sliding pressure operation mode ； In the low-load region, it operates at a constant pressure at a lower level. This operating mode is also known as the fixed-slip-fixed operating mode, as it combines the advantages of the above two modes. 36. Describe the factors that affect changes in positive and negative expansion differences. The main factors causing the expansion difference to increase in the positive direction are briefly described as follows: (1) During startup, the warm-up time is too short, and the speed or load is increased too rapidly. (2) The heating temperature of the steam in the cylinder interlayer and flange heating device is too low or the flow rate is too low, resulting in a weak heating effect. (3) The sliding performance of the slider system or bearing plate is poor, and the slider system has become stuck. (4) Excessively high shaft seal steam temperature or excessive supply of steam to the shaft seal causes excessive elongation of the shaft journal. (5) During unit startup, parameters such as steam inlet pressure, temperature, and flow rate are too high. The temperature rise of the main and reheat steam is too rapid during startup. (6) Wear of the thrust bearing leads to an increase in axial displacement. (7) The insulation effect of the cylinder’s insulation layer is poor, or the insulation layer has fallen off. During the cold seasons, the temperature in the turbine room is too low or there is draft wind. (8) Cold steam (or cold water) is introduced into the interlayer of the double-cylinder. (9) An inaccurate zero point of the drift indicator or worn contacts cause numerical deviations. (10) Multi-rotor units, the mutual influence caused by changes in the expansion difference between adjacent rotors. (11) Effect of vacuum changes. (12) The impact of changes in the extraction volume at various levels: if the primary extraction is shut down, the effect on the high-pressure expansion difference is significant. (13) The bearing oil temperature is too high. (14) During the coasting process after the unit is shut down, due to the effect of the Poisson effect. The main factors that cause the expansion difference to increase toward a negative value are summarized as follows: (1) A rapid drop in load or sudden load shedding. (2) A sudden drop in main steam temperature, or the inlet steam temperature during startup being lower than the metal temperature. (3) Water impact. (4) Excessive heating in the cylinder interlayer and flange heating device. (5) The shaft seal steam temperature is too low. (6) Axial displacement change. (7) The bearing oil temperature is too low. (8) A sudden increase in rotational speed during startup occurs, as the axial size of the rotor decreases under the effect of centrifugal force, with particularly significant changes in the low-pressure difference. (9) High-temperature steam flows into the cylinder interlayer; it may originate from a steam heating device, or from leaks in the steam inlet casing or shaft seal. During startup, a steam heating device is generally used to control the expansion amount of the cylinder, while the rotor’s expansion is primarily controlled by the inlet steam temperature and flow rate of the turbine, as well as the temperature and flow rate of the shaft seal steam. During startup, the expansion difference generally moves in the positive direction. When a turbine is shut down, as the load and speed decrease, the rotor cools faster than the cylinder, so the expansion difference generally moves in a negative direction. This is particularly severe during sliding parameter shutdown; a steam heating device must be used to supply cooling steam to the cylinder interlayer and flanges in order to prevent the expansion differential protection from activating. After the turbine rotor stops rotating, negative expansion differential may worsen; therefore, during shutdown, shaft seal steam at a certain temperature should be maintained to avoid adverse consequences. 37. Analyze the locations and times at which the maximum thermal stress occurs during the turbine startup process. The time at which the maximum thermal stress occurs in the turbine cylinder and rotor should be at the moment when the temperature difference between the inner and outer walls of the metal is greatest under unstable operating conditions. At a certain steam temperature rise rate, the turbine enters a quasi-steady state; the temperature difference between the rotor surface and the central hole, as well as between the inner and outer walls of the cylinder, approaches the maximum value corresponding to that temperature rise rate. Therefore, thermal stress also reaches its maximum value when the turbine enters this quasi-steady state. During start-up/shutdown and changes in operating conditions, the areas in the turbine where the maximum stresses occur are usually the control stages of the high-pressure cylinder, the steam inlet area of the medium-pressure cylinder in reheat units, the steam seals around the control stages of the high-pressure rotor, and the front steam seal of the medium-pressure rotor. These areas experience high operating temperatures, and significant temperature changes occur during startup, shutdown, and changes in operating conditions, resulting in large temperature differences and thus high thermal stress. Furthermore, heat stress concentration occurs at areas where there are changes in the component structure, such as at the root of the impeller, the transition radii at the shaft shoulders, and the shaft seal grooves; the heat stress in these areas is 2 to 4 times that of smooth surfaces. 38. How to check for leaks in one half of the condenser? (1) Contact the shift supervisor to reduce the unit load to about 70% of the rated load (when the condensate hardness is too high, the load must be reduced further, and the second set of extraction systems should be activated). (2) Appropriately increase the steam supply pressure for the shaft seal. (3) Slightly increase the inlet valve of the circulating water for the condenser on the side that will not be inspected for leaks. (4) Close the air valve from the condenser on the leak-check side to the extractor. (5) Close the circulating water inlet valve and the connecting valve on the side used for leak detection, adjust the circulating water air valve; after these valves are closed, the switch handle must be moved to the manual position. (6) After confirming that the unit is operating normally, open the drain valve of the condenser on the side that is not in use. (7) The condenser vacuum must not be lower than 85 kPa, and the exhaust steam temperature should not exceed 70°C. (8) Open the manway of the condenser on one side to turn it off and enter to check for leaks. (9) After checking for any omissions, the team leader shall verify that there is no one present and no tools left behind, and then close the condenser manway and drain valve. (10) Open or close the circulating water inlet valve on one side of the condenser, adjust the circulating water air valve and the circulating water connection valve, and adjust the circulating water inlet valve on the other side accordingly. (11) Deactivate the condenser on one side until the air valve of the extraction pump is opened. (12) Check for leaks in the other side condenser using the same method. (13) During the leak detection process, the vacuum level of the condenser should be no less than 87 KPa, and the trend should remain stable; otherwise, the cleaning of one half of the condenser should be stopped, and operation of the condenser on the side being cleaned should be resumed as soon as possible. 39. What precautions should be taken when filling the condenser with water to check for leaks? (1) A dedicated person should monitor the condenser for leaks, ensuring that the inlet and outlet valves of the condenser’s circulating water are closed and that the power is cut off with the equipment locked. (2) Before filling the condenser with water, it is necessary to check and ensure that the jacks at the bottom of the condenser are in place and firmly supported, and that all water on the water side of the condenser’s circulating water system has been drained. (3) When checking for leaks in the condenser cooling water pipes, the metal temperature of both the high-pressure and medium-pressure cylinders should be below 300°C. To detect leaks in the condenser cooling water pipes, water should be added until the pipes are completely submerged, with the manholes on both the steam side and the water side opened. (4) During inspection, if pressurization is required, the pressure shall not exceed 50 Kpa; the maintenance personnel must seal the shaft seal at the turbine end, and the low-pressure cylinder’s atmospheric safety valve must be secured properly. When checking for air leaks on the steam side of the condenser, it should be noted that this can only be done when the metal temperature of the high and low pressure cylinders is below 200°C. (5) After water is introduced, close monitoring of the temperature difference between the upper and lower cylinders should be intensified. Once water leaks out from the steam-side manhole, open the steam-side monitoring manhole and the top vent valve, and close the steam-side manhole. After irrigation, operate in conjunction with maintenance personnel to check for leaks. After checking for leaks, fill it with water; once it is confirmed that no one or tools are left behind, close the manhole on the water side as well as the drain valve. (6) After a thorough inspection, place the equipment in standby mode. 40. What is the speed variation rate of a speed control system? What are the requirements for it from the perspective of ensuring optimal operation of the turbine? When the turbine operates alone, the percentage of the difference between the stable speed n2 corresponding to no load and the stable speed n1 corresponding to full load, relative to the rated speed n0, is called the speed variation rate of the speed control system, denoted by the symbol δ. The speed variation rate indicates the extent of change in the turbine’s speed from no load to full load. The rate of speed variation should not be too high or too low; the general range is 3% to 6%. Smaller values are used for units used for peak shaving, while larger values are used for units that provide base load. When the rate of speed change is too low, even small variations in the grid frequency can result in significant changes in the load on the units. This leads to large fluctuations in load during normal operation, affecting the safe operation of the units and reducing the dynamic stability of the speed control system. The speed variation rate is excessively high. Although the speed control system exhibits good dynamic stability during operation, when the unit sheds load, the dynamic overspeed increases, making overspeed prone to occur. On the other hand, to ensure that the turbine can be easily connected to the grid during startup and to prevent overload at full load, a large rate of speed change is required in these two sections of the static characteristic curve. At the same time, it is required to ensure that the overall rate of speed change does not become too large, so the values in the intermediate section are smaller. To ensure the stable operation of the unit across the entire range, the rate of change of speed must cause the static characteristic curve of the speed control system to slope smoothly and continuously in the direction of increasing power; there should be no upward or horizontal segments in this curve. 41. Describe the working principle of the deaerator? What are the reasons for the increased oxygen content at the deaerator outlet? The deaerator consists of a deaeration tower and a water storage tank at the bottom. The deaeration tower is equipped with a sieve-like, porous water distribution tray. The condensate from the condensate pumps and the steam trap discharge water from the high-pressure heaters enter the deaeration tower through upper pipes, where they are dispersed into fine droplets by the sieve-like, porous water distribution tray before falling downward. The steam extracted from the turbine enters the lower part of the deaerator, flowing from bottom to top where it comes into contact with the falling tiny water droplets to exchange heat; this process heats the water to its saturated temperature. The gases present in the water are continuously separated and removed through the exhaust pipe at the top of the deaerator, while the condensed water flows into the storage tank located at the bottom. The vapor-gas mixture discharged from the deaerator passes through an excess steam cooler, where the useful substances and some of the heat contained in the steam are recovered before the mixture is released into the atmosphere. The main reasons for the increased oxygen content at the outlet of the deaerator include a sudden increase in the unit’s load, an increase in pressure inside the deaerator, a decrease in the temperature of the water entering the deaerator or an excessive amount of water entering it, high oxygen content in the condensate, low steam supply to the deaerator, and a small opening degree of the deaerator’s exhaust valve. 42. Why does the temperature of the exhaust cylinder rise when the turbine starts? (1) During the startup process of the turbine, the control valves are opened to allow steam to flow in at full capacity. After stages such as impulse starting, speed increase, and medium-speed warming up (at 1200 r/min) over a period of about 1.5 hours to reach 2800 r/min, as well as valve switching, the turbine is gradually brought online at full speed with the load being increased accordingly. During turbine startup, steam is throttled and then passes through nozzles to drive the impellers of the speed-regulating stage; after throttling, the entropy of the steam increases while its enthalpy drop decreases, resulting in a higher temperature of the exhaust steam after work is done. (2) During the entire startup process prior to grid connection for power generation, very little steam is consumed; at this stage, work is primarily done by the regulation stages. The waste steam, as it flows toward the exhaust cylinder, has a low flow rate and low velocity, as well as a large cross-sectional area, which results in a significant draft effect. The exhaust temperature rises due to significant blowdown losses. As the rotor rotates, the blades (especially the longer blades in the latter stages) come into friction with the steam, which is also one of the factors that cause the exhaust temperature to rise. During the startup of the turbine, the vacuum level is low, and as a result the saturation temperature increases, meaning that the exhaust temperature rises as well. An increase in the temperature of the exhaust casing leads to greater thermal deformation of the low-pressure cylinder’s shaft seal. This can cause a shift in the turbine’s centerline, resulting in increased vibration and greater friction between moving and stationary parts; in severe cases, the low-pressure cylinder’s shaft seal may be damaged. (3) When the load is increased during grid-connected power generation, the main steam flow increases as the load rises; the turbine gradually enters its normal operating condition, and the proportion of power consumed by friction and aerodynamic losses becomes smaller and smaller. As the vacuum in the turbine exhaust cylinder gradually increases, the exhaust temperature also decreases gradually. An excessively long startup time of the steam turbine may also cause the exhaust casing temperature to become too high. We should follow the procedural requirements and complete the startup process based on the procedure card, so that the temperature of the exhaust cylinder will remain within limits. When the temperature of the exhaust cylinder exceeds 80°C, water injection into the exhaust cylinder will automatically start to cool it down, and it is not allowed for the temperature of the exhaust cylinder to rise above 120°C. 43. Describe the composition and working principle of the self-sealing shaft seal system. The self-sealing shaft seal system consists of the shaft end steam seal, the shaft seal steam supply main, the shaft seal steam supply pressure adjustment mechanism, the shaft seal cooler, the shaft exhaust fan, the desuperheater, and related pipelines. Generally, the shaft seal at the exhaust end of the high-pressure cylinder is divided into 4 sections and 3 chambers; the pressure decreases from high to low, and the 3 chambers are connected respectively to the deaerator, the shaft seal steam supply main, and the shaft seal steam extraction pipeline leading to the shaft seal cooler ； The shaft seals at the exhaust end of the intermediate-pressure cylinder and both ends of the low-pressure cylinder are divided into 3 sections with 2 chambers each; these two chambers are connected respectively to the main steam supply pipe for shaft sealing and to the extraction steam pipe leading to the shaft seal cooler. During normal operation of the unit, steam leaking from the shaft seals at both ends of the high and medium pressure cylinders enters the shaft seal steam supply main, which is then used to supply steam to the shaft seals at both ends of the low pressure cylinder, thereby achieving self-sealing. During unit startup, no-load, and low-load conditions, it is possible to choose to supply steam to the shaft seal feed pipe using auxiliary steam, main steam, or cold reheat steam, via three pressure control valves respectively, in order to prevent air from entering the cylinder. After the shaft seal system achieves self-sealing, excess steam is discharged through the relief valve to adjust the pressure in the shaft seal steam supply main. To prevent high-temperature steam from entering the steam seals at both ends of the low-pressure cylinder and causing thermal deformation of the seal elements and bearing housings, condensate water is sprayed into the steam supply pipe for the shaft seals via a temperature reducer, thereby maintaining the temperature of the seal steam between 121 and 177°C. 44. What is a sliding parameter shutdown? What are its advantages and disadvantages? Sliding parameter shutdown refers to the process of gradually reducing the load by lowering the steam parameters through the boiler, while keeping the throttle valve fully open; as a result, the metal temperature of the turbine also decreases accordingly, until the load reaches zero. After the generator is disconnected, the steam parameters can be further reduced to lower the speed of the turbine until the rotor comes to a stop. Advantages of sliding-parameter shutdown: Since sliding-parameter shutdown utilizes steam at low parameters and high flow rates to ensure uniform cooling of all the heated components of the turbine, and it allows the metal temperature to be reduced to lower levels, **the cooling time of the cylinder is shortened. Additionally, waste heat from the boiler can be used to generate electricity, and steam with low parameters and high flow rate can be utilized to clean the flow passages of the turbine. Where conditions permit, both the high-pressure and low-pressure heaters as well as the deaerator can undergo random sliding shutdowns, thereby improving thermal efficiency and reducing steam and water losses. Disadvantages of sliding-parameter shutdown: During the shutdown process, it is more likely to experience a large negative expansion difference compared to a shutdown at rated parameters. It imposes very strict requirements on boiler operation, and it is difficult to control the uniform decrease in steam temperature. In steam turbines, operations and adjustments are frequent; if monitoring is not strict, water hammer and undercooling of the heated components can occur, leading to equipment damage. 45. What precautions should the turbine take during the controlled shutdown and load reduction process of the unit? (1) Strengthen the monitoring of main steam parameters, especially ensuring that the superheat is above 50°C. (2) Ensure that the temperature difference on both sides of the high-pressure and medium-pressure main steam valves is less than the specified value. (3) During the sliding parameter shutdown process, the rate of decline in the temperature of the reheat steam should try to keep up with the rate of decline in the temperature of the main steam, and the temperature difference between the main steam and the reheat steam should remain within the limits specified by the regulations. (4) Closely monitor the changes in the unit’s sound, vibration, axial displacement, thermal expansion difference, as well as the metal temperatures of the support bearings and thrust bearings; these should remain within normal ranges. (5) Pay close attention to ensure that there is no water hammer in the turbine as well as in the main and reheat steam pipes; check that all drain valves are functioning properly, and open the manual drain valves in a timely manner. (6) Regularly check that the cylinder metal temperature, the temperature difference between the upper and lower cylinders, and the stress levels of the high and low pressure rotors are within normal ranges. (7) During the sliding parameter shutdown process, no tests that may affect the opening degree of the high and medium pressure main steam valves as well as the control steam valves are permitted, and turbine overspeed tests are strictly prohibited. (8) Notify the chemistry department to strengthen supervision of the quality of condensate water; it is prohibited to feed water into the deaerator if its quality does not meet the standards. 46. How to keep the oil system clean, free of water, and with proper oil quality? The following tasks must be carried out: (1) After the major overhaul of the unit, the fuel tank and fuel pipelines must be thoroughly cleaned. Before starting the unit, the fuel system should be flushed through oil circulation, and only once the quality of the fuel is satisfactory can it be fed into the control system. (2) The shaft seal comb teeth should be replaced during each major overhaul, and the gap between the comb teeth should meet the requirements. (3) The fuel tank exhaust fan must be operating properly. (4) Adjust the steam supply to the shaft seal in a timely manner according to load changes, to prevent excessive shaft seal steam pressure from leaking into the oil system. (5) Ensure the cold oil cooler operates properly; the cooling water pressure must be lower than the oil pressure. After shutdown, it is especially important to prevent the water pressure from being higher than the oil pressure. (6) Strengthen the chemical monitoring of turbine oil, regularly check the quality of the turbine oil, and periodically drain water. (7) Ensure the oil purification unit is operating properly. 47. What issues should be considered when supplying steam to the shaft seal before starting the unit? (1) Before supplying steam to the shaft seal, the steam supply pipe should be warmed up first to drain any condensate. (2) Steam must be supplied to the shaft seal under continuous turning of the shaft. For hot start-up, steam for the shaft seal should be supplied first, followed by vacuuming. (3) The timing of steam supply to the shaft seal must be appropriate; supplying steam to the shaft seal too early before turning the turbine on can increase the temperature difference between the upper and lower casings or widen the expansion differential. (4) Attention should be paid to the matching between the temperature of the steam supplied to the shaft seal and the metal temperature. For hot start-up, using an appropriate backup steam source is beneficial for controlling the expansion difference; it is even better if the system allows the temperature of the steam supplied to the shaft seal to be adjusted to be higher than the temperature of the shaft seal itself. For cold start-up, a low-temperature steam source should be used. (5) Care must be taken when switching the steam source for the high and low temperature shaft seals; a too rapid switch not only causes significant changes in the thermal expansion difference but may also lead to uneven thermal deformation at the shaft seal sites, thereby resulting in friction and vibration. 48. Can an overspeed test be conducted when using a sliding parameter shutdown? Why? When shutting down using the sliding parameter method, it is strictly prohibited to conduct a turbine overspeed test. Because by the time of shutdown via a sliding parameter method and the generator is disconnected from the grid, the steam parameters upstream of the main steam valve have already dropped significantly. Moreover, during the sliding shutdown process, in order to ensure that the steam provides optimal and uniform cooling for the turbine metal, the superheat of the main steam is generally kept at a value close to the minimum allowable level, while keeping the throttle valve fully open. Furthermore, if an overspeed test is to be conducted, the unit’s speed must be controlled using throttle valves, which can very well lead to an increase in main steam pressure and a decrease in superheat; in some cases, the steam temperature may even drop below the saturation temperature corresponding to that pressure. Conducting an overspeed test under such conditions can result in a water hammer accident in the turbine. On the other hand, since the valve bodies and spools of the turbine’s main steam valve and control valve may not operate smoothly or may get stuck due to asynchronous cooling, especially after the turbine itself has been cooled during a reduced-parameter shutdown process, where there are significant changes in its thermal expansion and axial displacement, it is not permissible to conduct an overspeed test. 49. What does a thermal test of a steam turbine generally include? What preparations should be made before the test? Thermal tests of steam turbines mainly include: (1) test items and test objectives. (2) Thermal system and operating mode during testing. (3) Layout of measurement points, measurement methods, and testing equipment used. (4) Selection of the test load point and measures to maintain load stability. (5) Conditions required of the equipment during testing, and measures to be taken to meet these conditions. (6) Determine the calculation method according to the test requirements. (7) Organization and division of labor in the experiment. The following tasks should be carried out before the test: (1) Gain a comprehensive understanding of the main and auxiliary equipment as well as the thermal system. (2) Conduct a comprehensive inspection of the unit’s thermal system to eliminate various leaks and equipment defects. (3) Install the measurement points and instruments required for the test and calibrate them. (4) Prepare the test outline. 50. What are the requirements for the regenerative system in turbine thermal tests? What measurement points are generally installed for thermal characteristic tests? Requirements for the regenerative system in thermal tests: (1) The heater’s tube bundle must be clean, with no leaks either in the bundle itself or at the flanges of the tube sheet. (2) The stop valves on the steam extraction pipes are airtight. (3) The bypass valve of the heater is airtight. (4) The drain trap can maintain a normal drainage level. Thermal characteristic tests generally incorporate the following measurement points: (1) Main steam pressure and temperature before the main steam valve. (2) Flow rates of main steam, condensate, and feedwater. (3) Pressure behind each throttle valve. (4) Pressure and temperature after the regulation stage. (5) Pressure and temperature of each extraction chamber. (6) Inlet and outlet water temperatures of each heater. (7) Steam inlet pressure and temperature for each heater. (8) Steam leakage pressure and temperature at each shaft seal. (9) The steam trap temperature of each heater. (10) Exhaust pressure. (11) Heat section pressure and temperature. (12) Cold section pressure and temperature. (13) Reheater desuperheating water flow rate, make-up water flow rate, stem leakage steam flow rate. 51. How to conduct a mechanical overspeed test? What are the qualification criteria? Mechanical overspeed protection test method: (1) Coordinate with the relevant departments to prepare for the test. (2) Confirm that the unit’s manual tripping and electrical overspeed protection tests were successful. (3) Before the test, it should be confirmed that the oil injection test, as well as the tests for the tightness of the main steam valve and control valves, have been successful. (4) Confirm that the main AC/DC lubricating oil pumps and standby pumps have started up properly, with interlocks activated. (5) Confirm that the unit has been warmed up at 25% load for 4 hours. (6) Confirm that the generator has been disconnected, the unit maintains a speed of 3000 r/min, and the main steam parameters meet the requirements. (7) The electrical overspeed channel connected to the thermal control interlock ETS. (8) Bring up the “Overspeed Test” control panel on the DEH screen. (9) In the “Overspeed Test” control panel, click the “Allow Test” button – the light will turn on ； (In some units, it is necessary to use manual operation to set the key to the “Test” position; once the “Test Allowed” light on the “Overspeed Test” control panel comes on, the next steps can be carried out.) (10) In the “Overspeed Test” control panel, click the “Mechanical Overspeed” button, and the light will turn on. (11) On the DEH “Control Setpoint” sub-screen, set the target speed to 3330 r/min and the increase rate to 100 r/min; press the “Proceed” button, and the light will turn on. (12) When the rotational speed approaches 3300 r/min, the mechanical overspeed protection activates, causing the main valve, the speed control valve, and the exhaust check valve to close rapidly. (13) Check that the AC lubricating oil pump of the main unit starts up properly in conjunction with it. (14) Record the speed at which the emergency safety device activates, and press the “Clear” button on the “Overspeed Test” control panel to clear the maximum speed. (15) If the mechanical overspeed protection does not activate at a speed of 3330 r/min, the machine should be stopped manually by tripping it. (16) After the test is normal, click the “Mechanical Overspeed” button on the “Overspeed Test” control panel, and the light will go out. (17) In the “Over-speed Test” control panel, click the “Allow Test” button to exit the test. (18) During the reduction in speed, pay attention to checking the reset condition of the impactor. (19) When the unit speed is less than 2900 r/min and the resonance zone is avoided, reconnect the brake and start the machine up again to maintain a unit speed of 3000 r/min. (20) If the unit speed drops below 2850 r/min, the high-pressure starting oil pump should be started. (21) After checking that everything is normal, connect them in parallel and bring the system online under load following the normal startup procedure. The qualifying standard is: for the mechanical overspeed protection test, it must be conducted twice in a row under the same operating conditions, and the speed at which the protection mechanism activates in both tests should not exceed 18 r/min. 52. Under what conditions can a turbine undergo a load-shedding test? What are the qualification criteria? The turbine may undergo a load-shedding test only after the following tasks have been completed: (1) The load-shedding test should be carried out only after it has been confirmed that the speed control system has passed its no-load tests, load tests, and overspeed tests. (2) The tests shall be conducted under normal conditions of the boiler and electrical equipment, with reliable operation of all types of safety doors. (3) Test measures shall be comprehensive and may only be carried out upon approval by the dispatcher or the responsible engineer. (4) The full-load test can be conducted only after the tests at 1/2 and 3/4 of the rated load have been successful. Additionally, personnel assignments should be made prior to the test. Passing criteria for the turbine load rejection test: After the rated load is removed, the speed of the unit increases; if this does not trigger the operation of the emergency safety device, then it is considered a pass. It is considered good if the rotational speed does not exceed 8%–9% of the rated speed. 53. What are the main components of a turbine control system? A complete turbine control system includes the following functional systems: (1) Monitoring system. A monitoring system is an essential device to ensure the safe operation of a turbine, as it can continuously monitor changes in various parameters of the turbine. Turbine parameter monitoring is typically carried out by a DAS system; the measurement results are sent to the control system as constraints, to the protection system as protection conditions, and to the sequence control system as control parameters. (2) Protection system. The function of the protection system is to enable the protective devices to act promptly based on the actual situation in case of a fault in the power grid or the turbine itself, thereby stopping the turbine from operating or taking certain measures to prevent the accident from escalating or causing damage to the equipment. The protection functions of large-capacity steam turbines include overspeed protection, low oil pressure protection, axial displacement protection, expansion difference protection, low vacuum protection, vibration protection, etc. (3) Control system. The closed-loop control system of a steam turbine includes a speed control system, a power control system, a pressure control system, etc. (4) Online thermal stress monitoring system. Thermal stress cannot be measured directly; it is usually calculated indirectly using modeling methods by measuring the temperature values at certain specific points in the turbine. In addition to being used for monitoring, the results of thermal stress calculations can also be used to correct the turbine acceleration rate and load change rate. (5) Steam turbine automatic start-stop control system. The turbine automatic start-stop control system is capable of carrying out the entire process, including turning the turbine, evacuating the vacuum, accelerating it for grid connection, operating under load, operating at full load, as well as reducing the load and shutting down the turbine. The prerequisite for enabling the automatic start and stop of the turbine is that all necessary control systems are available and can operate properly. These systems include automatic control systems, monitoring systems, thermal stress calculation systems, and bypass control systems, among others. (6) Hydraulic servo system. The hydraulic servo system consists of a turbine oil supply system and a hydraulic actuator. The oil supply system provides pressurized oil to the hydraulic actuator. The hydraulic actuator consists of components such as electro-hydraulic converters, hydraulic motors, and position sensors, and its function is to control the operation of the corresponding valves in accordance with the instructions from the electronic control system. 54. What are the advantages of using a electro-hydraulic control system DEH? The use of an electro-hydraulic control system has the following advantages: (1) The use of electrical components improves the precision of the control system, reduces the lag rate, enables rapid reduction of power output in the event of load rejection, and enhances the dynamic overspeed characteristics. (2) Achieve full-range regulation of rotational speed to control the steady acceleration of the turbine. (3) It can participate in primary frequency regulation of the power grid based on the selected static characteristics (the slope of which can be easily improved, as well as the maximum amplitude of frequency modulation), in order to meet the requirements of the turbines, boilers, power grid, and other elements. (4) A power system is employed, which helps to resist internal disturbances and improve the frequency regulation dynamics, thereby enhancing the unit’s adaptability to loads. (5) It can be easily integrated with machines, furnaces, and main control devices to achieve automatic control of machinery, electricity, and furnaces. 55. Describe the function and working principle of the unloading valve. The unloading valve is installed on the hydraulic block of the hydraulic actuator. Its main function is to allow the pressure oil in the lower chamber of the actuator’s piston to be released rapidly through the unloading valve, when an emergency shutdown is required due to a malfunction in the unit and the emergency trip device causes a loss of pressure in the AST oil; the valve then closes under the action of spring force. Working principle: The quick-unload valve contains a cup-shaped spool; the lower part of this spool is connected to the high-pressure oil circuit beneath the piston of the hydraulic actuator. High-pressure oil enters the upper part of the spool through the throttle orifice at the input port, via the emergency shut-off oil circuit. Since the needle of the regulating needle valve completely blocks the passage there, the oil pressure at the upper part of the spool becomes equal to the emergency shutdown oil pressure. Therefore, the force of the oil pressure acting on the upper part of the spool, combined with the spring force, is greater than the force of the high-pressure oil acting on the lower part of the spool; as a result, the spool is pressed against the base, and the path for high-pressure oil to reach the return inlet is closed. When the emergency shut-off device activates and causes a loss of pressure in the AST oil, the oil pressure above the spool is almost zero. Given that the spring’s stiffness is not high, the high-pressure oil at the bottom of the spool overcomes the force of the spring and pushes the spool aside. This allows the high-pressure oil circuit to connect to the return line and flow back to the tank. As a result, the pressure of the oil beneath the piston of the hydraulic actuator drops rapidly, thereby quickly closing the steam inlet valve. The regulating needle valve can be used for manual unloading. 56. What is a rotor coastdown curve? What is the purpose of drawing it? The time period from the moment the automatic main steam valve and control valves close after the generator is disconnected from the grid, until the rotor comes to a complete stop, is known as the rotor coastdown time. The curve that shows the relationship between the rotor coastdown time and the rate of speed reduction is called the rotor coastdown curve. After the new unit has been in operation for a while and all its components are functioning properly, it is possible to measure the rotor’s coasting curve during shutdown periods. This curve can then be used as the standard coasting curve for that unit. When drawing this curve, it is necessary to control the vacuum level in the condenser, ensuring that it decreases at a constant rate. Subsequent shutdowns should be recorded under the same conditions, so that the coasting curve can be drawn again, facilitating comparative analysis of any issues that may arise. If the coasting time decreases sharply, it may be due to bearing wear or friction between the moving and stationary parts of the turbine ； If the coasting time increases significantly, it indicates that the valves in the new steam or reheat steam pipelines, or the extraction check valves, are not properly sealed, allowing pressurized steam to leak into the cylinder. When the top shaft oil pump starts too early and the condenser vacuum is high, the coasting time also increases. 57. Why is it necessary to wait until the vacuum level reaches zero before stopping the steam supply to the shaft seal? If the steam supply to the shaft seal is stopped before the vacuum level reaches zero, cold air will enter the cylinder from the shaft end, causing localized cooling of the rotor and the cylinder. In severe cases, this can lead to friction at the shaft seal or deformation of the cylinder; therefore, it is required that the vacuum level reach zero before stopping the steam supply to the shaft seal. 58. Why is it specified that the vacuum level should be reduced after shutting down the turbine, so that it reaches zero when the rotor comes to rest? During the coasting process after a turbine stops, the best way to maintain the vacuum level is to gradually reduce it, aiming to bring the vacuum level to zero once the rotor has come to rest. This is because: (1) the shutdown coasting time is related to the vacuum retention time; the vacuum is reduced at a certain rate with each shutdown, which facilitates the comparison of the coasting curves. (2) If the vacuum drops too slowly during coasting, the unit will remain at the critical speed for a longer period of time, which is detrimental to the safety of the unit. (3) If the vacuum drops too rapidly during the pre-idle phase, and reaches zero even when there is still some rotational speed, the high heat generated by the aerodynamic losses of the long blades in the subsequent stages can cause the exhaust temperature to rise. This also hinders the removal of water accumulated inside the cylinders, increasing the risk of corrosion of the turbine metal after shutdown. (4) If the rotor has already stopped but there is still a high vacuum, and the steam supply to the shaft seal cannot be stopped either, this will also lead to an increased temperature difference between the upper and lower cylinders, as well as uneven deformation of the rotor resulting in thermal bending. In summary, it is best to bring the speed to zero and the vacuum level to zero during shutdown; in actual operation, a vacuum break valve is used for control and adjustment. 59. Why is it necessary to engage the oil pump interlock switch during the turbine barring process? Although the turbine barring device is equipped with interlock protection, which causes it to shut down when the lubricating oil pressure drops to a certain level in order to protect the bearings of the unit, this protection system can sometimes fail. If the lubricating oil pump fails to supply oil or malfunctions, it can lead to friction against the turbine bearings, causing damage to them. Once the oil pump interlock is activated, if the AC oil pump fails, the DC oil pump can be started automatically to prevent shaft bearing damage. 60. What issues should be noted during the barring process? (1) Monitor whether the current of the barring motor is normal, and whether the ammeter reading fluctuates. (2) Regularly check whether there are any changes in the rotor bending indication value. (3) Regularly listen for any friction sounds inside the cylinder and at the high and low pressure seals. (4) Regularly check the operation of the lubricating oil pump. 61. Why must the lubricating oil pump continue to run for a while after the turning of the shaft is completed following a shutdown? The main purpose of keeping the lubricating oil pump running is to cool the shaft journals and bearing shells; even after shutdown, the temperature of the rotor’s metal remains high, and heat is transferred through the bearings along the direction of the shaft journals. If there is not enough lubricating oil to cool the rotor journal, the temperature of the bearing shells will rise; in severe cases, this can cause the bearing material to melt and lead to damage to the bearings ； Excessively high bearing temperatures can also cause the remaining oil in the bearings to oxidize rapidly, leading to smoking and even fire. During the operation of the low-pressure oil pump, the cold oil cooler must also remain in operation to keep the temperature of the lubricating oil below 40°C. After the high-pressure steam turbine is shut down, the lubricating oil pump should operate for at least 8 hours. Of course, each unit should be determined specifically according to the circumstances. 62. What maintenance tasks should be carried out after shutdown? Maintenance work after shutdown is very important; in addition to monitoring the operation of the cranking device, the following tasks also need to be performed: (1) Strictly cut off the sources of steam and water connected to the cylinder, to prevent steam and water from entering the cylinder, which could lead to an increase in the temperature difference between the upper and lower parts of the cylinder, and even cause damage to the equipment. (2) Closely monitor the exhaust temperature of the low-pressure cylinder and the water level in the condenser as well as that in the heaters; full water levels are strictly prohibited. (3) Pay attention to the cooling water of the generator rotor water-inlet seal support to prevent a disruption in the cooling water, which could damage the packing. (4) After the boiler is depressurized, all the drain valves and vent valves of the unit should be opened ； During winter, take proper frost protection measures to ensure that no water accumulates in any equipment or pipes. 63. Where is the maximum bending of the rotor after the turbine is shut down? When is it most dangerous to start the turbine? After the turbine is stopped, if the barring gear cannot be activated for some reason, the rotor will gradually bend due to temperature differences between the upper and lower parts of the cylinder or other factors. The area with the greatest bending usually lies near the regulating stage, and the maximum degree of bending occurs within 2 to 10 hours after shutdown; therefore, starting the turbine during this period is the most dangerous. 64. Why hasn’t the load been reduced to zero, so that the generator can be disconnected? If the load cannot be reduced to zero during shutdown, it is generally due to loose or stuck control valves, a malfunctioning extraction check valve that fails to close properly, or an influx of large amounts of steam from the heating system. If the generator is disconnected at this time, an overspeed accident will occur. Therefore, it is necessary to first eliminate the fault by using methods such as closing the automatic main steam valve and the electric isolation valve to reduce the load to zero, before disconnecting and shutting down the generator. 65. Why is it better to first reduce the steam temperature before reducing the steam pressure during a controlled shutdown? During normal operation of the turbine, the superheat of the main steam is relatively high; therefore, during a controlled shutdown, it is better to keep the steam pressure constant while appropriately reducing the steam temperature, thereby reducing the superheat of the main steam. This helps with the cooling of the cylinder, results in a lower temperature of the cylinder after shutdown, and shortens the time required for rotating the cylinder. 66. How to reduce the temperature difference between the upper and lower cylinders? ①Improve the drainage conditions in the cylinder by selecting an appropriate drain pipe diameter to prevent water accumulation at the bottom. ②During unit startup and shutdown, operators must correctly and promptly operate all drain valves. ③Improve the wind shields of the lower and middle pressure cylinders, enhance the insulation of these cylinders, and reduce cold air convection. ④Operate the steam heating device properly; when it is detected that the temperature difference between the upper and lower cylinders exceeds the specified value, use the steam heating device to heat these cylinders. 67. From which system do the cold water and cold steam that can enter the steam turbine usually come from? ①Boilers and main steam system. ②Superheater desuperheating water system. ③After the heater leaks and fills with water, it enters the turbine from the steam extraction system. ④The condenser is full of water. ⑤The steam turbine’s own drain system is imperfect and poorly arranged. ⑥The utility systems of the unit. 68. Under what circumstances should a turbine undergo an overspeed test? ①After the major overhaul of the unit ； ②After disassembly and repair of the emergency safety device ； ③During normal operation, the emergency safety device triggers erroneously ； ④Restart after being shut down for maintenance for a month ； ⑤Before the load rejection test ； ⑥After 2000 hours of operation of the unit ； It is not possible to conduct the emergency safety device oiling test, or the oiling test fails. 69. What are the reasons for the increase in the outlet temperature of the condenser circulating water? ①As the inlet water temperature rises, the outlet water temperature rises accordingly. ②The turbine load increases. ③The condenser copper tubes are dirty. ④The circulating water volume decreases. ⑤The secondary filter screen of the circulating water is clogged. ⑥The displacement has increased. ⑦Vacuum drops. 70. What are the reasons for the increase in exhaust temperature during the startup and acceleration of a steam turbine? Answer: ① The vacuum inside the condenser decreases; air is not completely removed, and steam mixes with air. Since air has poor thermal conductivity, this leads to an increase in the exhaust pressure as well as a higher saturation temperature. ②A large amount of drain water from the main steam and reheat pipes, as well as from the cylinder itself, is directed to the expansion tank; the steam that emerges from the expansion tank is then sent to the throat section of the condenser. The temperature of this drain water and steam is 4-5 times higher than the saturated temperature inside the condenser. ③During the warm-up process, the steam flow decreases and the slower flow rate prevents the frictional heating generated by the blades from being removed in a timely manner. 71. What are the factors that affect convective heat transfer? ① The driving force of fluid flow. ② Whether there is a phase change in the fluid. ③ The flow regime of the fluid. ④ Influence of geometric factors. ⑤ Physical properties of the fluid. 72. What are the technical measures to prevent bending of the turbine shaft? ①The cylinder should have good insulation conditions. ②The main steam pipes, bypasses, as well as the steam pipes and cylinder drains must meet the requirements. ③The thermometers for various parts of the cylinder are complete and reliable. ④The shaft wobble must be measured before startup; if it exceeds the specified value, startup is prohibited. ⑤Check the temperature difference between the upper and lower cylinders before starting. ⑥During hot start, the steam inlet temperature and shaft seal temperature are strictly controlled. ⑦Enhance vibration monitoring. ⑧After the turbine stops, take strict precautions to prevent water from entering the cylinder. 73. What are the conditions for an emergency shutdown of general pumps? Answer: ① When continued operation poses a clear threat to the safety of equipment and personnel. ②The pump or motor experiences severe vibration, or metal knocking or grinding sounds can be clearly heard. ③Any bearing or shaft seal smoking, or a sudden increase in temperature exceeding the specified values. ④When measures taken are ineffective to prevent water from vaporizing inside the pump. ⑤The pump’s casing is broken. ⑥The motor switch is smoking or on fire. ⑦Motor failure. 74. What are the reasons for the increase in the outlet temperature of the condenser circulating water? Answer: ① As the inlet water temperature rises, the outlet water temperature rises accordingly. ②The turbine load increases. ③The condenser copper tubes are dirty. ④The circulating water volume decreases. ⑤The secondary filter screen of the circulating water is clogged. ⑥The displacement has increased. ⑦Vacuum drops. 75. What are the reasons for the increase in exhaust temperature during the startup and speed-up of a steam turbine? ①The vacuum inside the condenser decreases; air is not completely removed, and steam mixes with air. Since air has poor thermal conductivity, this leads to an increase in the exhaust pressure as well as a higher saturation temperature. ②A large amount of drain water from the main steam and reheat pipes, as well as from the cylinder itself, is directed to the expansion tank; the steam that emerges from the expansion tank is then sent to the throat section of the condenser. The temperature of this drain water and steam is 4-5 times higher than the saturated temperature inside the condenser. ③During the warm-up process, the steam flow decreases and the slower flow rate prevents the frictional heating generated by the blades from being removed in a timely manner. 76. What are the hazards of a drop in turbine vacuum? ①An increase in exhaust pressure can be compensated by a decrease in enthalpy drop, which is uneconomical and also results in a reduction in the unit’s output. ②The exhaust cylinder and bearing housings expand due to heat, which may cause a change in centering and lead to vibration. ③Excessively high exhaust temperature may cause the steam copper tubes in the condenser to loosen, compromising their tightness. ④It increases the axial thrust of the turbine. ⑤The decrease in vacuum leads to a reduction in the volumetric flow rate of exhaust gas, which is detrimental to the operation of the last stages of the blades. Flow separation and swirling occur in these final stages, and significant excitation forces are generated on certain parts of the blades, which may damage them and lead to accidents. 77. What are the reasons for vibration in the deaerator? Answer: ① During insertion into the deaerator, improper heating leads to uneven expansion, or an uneven distribution of steam and water load. ②An excessive amount of water flowing into the various pipes of the deaerator causes vibration in those pipes, which in turn leads to vibration in the deaerator itself. ③It stopped operating due to the detachment of internal components. ④Suddenly introducing cold water during operation causes uneven temperatures in the water tank, resulting in shocks and oscillations. ⑤The deaerator is leaking. ⑥The deaerator pressure dropped too rapidly, resulting in steam-water bumping. 78. What is the impact of an excessively high delay rate in the control system on the operation of a turbine? Answer: ① When the turbine is operating at no load, an excessively high delay rate can cause instability in the turbine’s speed, thereby making it difficult to connect the turbine to the power grid. ② After the turbine is connected to the grid, an excessive delay rate can lead to fluctuations in the load. ③ When the load on the turbine suddenly drops to zero, the high delay rate prevents the control valves from closing immediately, resulting in a sharp increase in speed; this may trigger the activation of the emergency shutdown device. If the emergency shutdown device does not function, it can lead to a serious accident of over-speed operation. 79. Why is an emergency shutdown required when the vacuum level drops to a certain value? Answer: ① The reduced vacuum leads to excessive axial displacement, causing the thrust bearings to become overloaded and wear out. ②The reduced vacuum causes the blades to become overloaded due to the increased steam flow. ③The decrease in vacuum raises the temperature of the exhaust cylinder, and changes in the cylinder’s centerline lead to increased vibration of the unit. ④To prevent the low-pressure cylinder safety valve from activating and to ensure the safety of the equipment, an emergency shutdown should be initiated when the vacuum level drops to a certain value. 80. What is the effect of an increase in main steam pressure, with the main steam temperature remaining constant, on the operation of the turbine? Answer: ① The overall enthalpy drop increases, improving operational efficiency. However, when the main steam pressure exceeds the limit, it poses a threat to the safety of the unit. ②Overload of the regulating stage blades. ③The steam temperature in the last stages of the turbine increases. ④It causes an increase in internal stress in pressure-changing components such as the main steam pipes, main steam valves, control valves, cylinders, and flanges, reducing their service life and leading to damage. 81. What are the signs of water hammer in a steam turbine? ①The main and reheat steam temperatures drop by 50°C or more within 10 minutes. ②Main steam valve flange, cylinder joint surface, control valve stem; white vapor appears or water droplets splash at the shaft seal. ③The steam pipeline has water hammer noise and intense vibration. ④As the load decreases, the sound of the turbine becomes deeper and the vibration of the unit increases. ⑤As the axial displacement increases, the temperature of the thrust pads rises, and the differential expansion decreases or becomes negative. 82. What are the reasons why the cylinder cannot expand during the startup of a steam turbine? Answer: ① Improper selection of main steam parameters and condenser vacuum. ②Improper use or incorrect operation of the cylinder flange bolt heating device. ③The load increase speed is fast, and the warm-up is insufficient. ④The drain valves at the body and related extraction piping are not open. ⑤The slide pin system is stuck. 83. What are the reasons for the increase in axial displacement? Answer: ① The main steam parameters are not up to standard, causing overload in the flow-through section of the turbine. ②The stationary blades are severely fouled. ③Water is present in the steam supplied to the turbine. ④The condenser vacuum decreases. ⑤The thrust bearing is damaged. ⑥The system frequency decreases. ⑦Generator shaft alignment. 84. What is the significance of using a feedwater reheating cycle? Answer: By using feedwater reheating, a portion of steam is extracted from the middle section of the turbine to heat the feedwater, thereby increasing the temperature of the boiler feedwater. This prevents the extracted steam from releasing heat in the condenser, reducing losses due to cold sources. On top of that, an increased feedwater temperature reduces the amount of heat absorbed by the feedwater in the boiler. As a result, with identical initial and final steam parameters, the feedwater reheating cycle offers higher thermal efficiency compared to the Rankine cycle. 85. Why must the axial displacement protection be activated before starting up? Answer: During startup, the steam flow is high momentarily; the steam first passes through the high-pressure cylinder, while almost no steam reaches the medium and low-pressure cylinders. The axial thrust is significant and must be balanced entirely by the thrust disc. If the axial displacement exceeds the allowable limit at this time, it can also cause sliding friction between moving and stationary parts. Therefore, the axial displacement protection should be activated before startup. 86. Why should a slight amount of steam be allowed to escape from the deaerator’s oxygen discharge valve? Answer: The working principle of a deaerator is to use steam to heat water to its saturated temperature at that pressure, thereby separating the dissolved gases in the condensate water (including oxygen), which are then expelled through the oxygen discharge valve. If this valve does not open, the separated oxygen will dissolve back into the water, rendering the deaeration process ineffective. If the valve is opened too wide, although deaeration is achieved, a large amount of steam is lost along with the oxygen, resulting in losses of heat and steam-water mixture. Therefore, while ensuring an effective deoxygenation effect, the oxygen discharge valve should be kept as closed as possible to allow only a slight amount of steam to escape, thereby reducing steam and water losses. 87. Why is a vacuum tightness test conducted? Answer: For turbines, the level of vacuum has a direct impact on their operational efficiency. A higher vacuum means a lower exhaust pressure, resulting in a greater effective enthalpy drop; this reduces the amount of heat carried away by the circulating water, thereby increasing the thermal efficiency of the unit. When air enters the condenser, the vacuum level decreases, the effective enthalpy drop is reduced, and more heat is carried away by the circulating water. Based on the results of the condenser vacuum tightness test, it is possible to assess the performance of the condenser, so as to take measures to eliminate leakage points. 88. What are the hazards of excessively high or low wind temperature in generators? Answer: Excessively high wind temperature in the generator causes the temperature of the stator coils, the core, and the rotor to rise as well. This leads to weakening of the insulation and a reduction in mechanical strength, thereby **shortening the generator’s lifespan. In severe cases, it can cause damage to the generator’s insulation, resulting in breakdowns and accidents ； If the wind temperature is too low, dew formation is likely to occur; water droplets accumulate on the generator coils, reducing their insulation capacity and threatening the safe operation of the generator. 89. Why is it best to start high- and low-pressure heaters randomly? Answer: The random start of the high and low pressure heaters ensures even heating of these heaters, which helps prevent leakage at the joints of the steel (copper) pipes. It also helps prevent deformation of the flanges due to high thermal stresses. Since the steam extraction pipes connecting the heaters originate from the lower cylinder, random starting of the heaters effectively increases the number of drain points in the cylinder, thereby reducing the temperature difference between the upper and lower cylinders. Additionally, this simplifies the operations after the units are connected in parallel. 90. Why is a certain level of vacuum required when starting a turbine under impulse conditions? Answer: A certain level of vacuum is required before starting the turbine; generally, this is a vacuum of -60 kPa. If the vacuum level is too low, more fresh steam is needed to drive the rotor. The excessive waste steam being discharged into the condenser suddenly causes the exhaust pressure in the condenser to rise significantly, which may result in a positive pressure on the condenser side. This can damage the safety film used for venting air, and it also causes significant thermal shock to the cylinder and the rotor. When using an impulse rotor, the vacuum level should not be too high. A high vacuum not only prolongs the time required to establish vacuum, but also reduces the amount of steam passing through the turbine; as a result, the heat release rate is low, which slows down the heating of the turbine and thus extends the startup time. 91. Why is it necessary to specify that the temperature rise and temperature drop rates must remain within certain limits during startup and shutdown? Answer: During the start-up and shutdown of a turbine, its cylinders and rotor undergo a process of heating and cooling. During start-up and shutdown, a certain temperature difference must exist between the inner and outer cylinders. During startup, the inner cylinder expands more rapidly and is subjected to thermal compressive stress, while the outer cylinder expands more slowly and is subjected to thermal tensile stress. During shutdown, the nature of these stresses is reversed. When the stress on the cylinder metal exceeds the material’s yield stress limit, plastic deformation or cracking may occur in the cylinder. The magnitude of the stress is proportional to the temperature difference between the inner and outer cylinders, and this temperature difference is in turn proportional to the rate of change of the metal’s temperature. Therefore, during startup and shutdown, the rate of increase and decrease in metal temperature is used as indicators to control thermal stress. 92. What are the regulations regarding steam superheat during the start-up and shutdown of turbines? Answer: If the superheat of the steam is low, during startup the temperature in the first few stages drops significantly, and the temperature in the subsequent stages may fall to the saturation temperature at that pressure level, resulting in wet steam. Steam containing water poses a serious threat to the blades; therefore, it is safer to keep the superheat of steam at 50–100°C during startup and shutdown processes. 93. Why is it necessary to wait until the temperature of the condenser shell drops below 50 ℃ before starting the circulating water pump to supply water in the event of a long-term interruption of the circulating water supply? Answer: If the circulation water is interrupted and the circulation pump cannot be restarted immediately due to equipment issues, the exhaust steam temperature will rise. The struts of the condenser, as well as the low-pressure cylinder and copper tubes, expand horizontally. When circulation water is reintroduced at this point, the copper tubes cool down first, while the struts of the condenser and the low-pressure cylinder do not cool down. As a result, the copper tubes contract while the struts remain unchanged, generating significant tensile stress. This stress can loosen the joints in the copper tubes, leading to leaks in them. Therefore, to ensure the safety of the condenser equipment, circulating water can be introduced only after the temperature of the condenser shell drops below 50°C. 94. Why hasn’t the load been reduced to zero, so that the generator can be disconnected? Answer: If the load cannot be reduced to zero during shutdown, it is generally caused by improper or stuck control valves, or by a malfunctioning or inadequately closed extraction check valve, which allows a large amount of steam to flow back from the steam supply system. At this point, the generator is disconnected, and an overspeed incident is likely to occur; therefore, it is necessary to first resolve the fault by methods such as closing the automatic main steam valve and the electric main steam valve in order to reduce the load to zero, before disconnecting the generator and shutting it down. 95. Why does the turbine rotor bend after friction occurs? Answer: Due to the temperature differences in the metal of the cylinder flanges, the cylinder deforms, causing the radial static and dynamic clearances to disappear. As a result, when the rotor rotates, radial friction occurs at the shaft seals at the ends of the unit as well as at the diaphragm seals, generating significant heat. The heat generated causes the temperature difference between the two ends of the shaft to increase rapidly. As this temperature difference grows, the rotor bends. This process repeats itself: the greater the temperature difference on either side of the shaft, the more the rotor bends. 96. Why is an exhaust fan installed in the turbine oil tank? Answer: The purpose of installing an exhaust fan in the oil tank is to remove gases and water vapor from within it. This prevents water vapor from condensing inside the tank, and it also ensures that the pressure inside the tank does not exceed atmospheric pressure, thereby allowing the oil returning to the bearings to flow smoothly into the tank. Additionally, the exhaust fan helps to remove harmful gases from the tank, thus preventing the quality of the oil from deteriorating. 97. What is the critical speed? What factors is it related to? Answer: During the start-up and shutdown of the unit, when the speed rises or falls to a certain value, the vibration of the unit increases suddenly; once the speed continues to rise or fall, the vibration decreases again. This speed at which vibration increases suddenly is known as the critical speed. The magnitude of the critical speed depends on factors such as the rotor’s thickness, weight, geometric shape, spindle span, stiffness, type of coupling, bearing stiffness, and elasticity. 98. Why is a water spraying cooling device installed in the exhaust cylinder? Answer 48: During the startup of the turbine, as well as when it is operating at no load or low load, the steam flow rate is very low; this amount of steam is not sufficient to remove the heat generated by friction between the steam and the impeller. As a result, the exhaust temperature rises, along with the temperature of the exhaust cylinder. Excessively high exhaust temperatures can cause significant deformation of the exhaust cylinder, disrupting the alignment of the rotating and stationary parts of the turbine. In severe cases, this can lead to vibration in the unit or other accidents. Therefore, high-power turbines are equipped with water injection systems in their exhaust cylinders to reduce temperatures. 99. What are the hazards of water in the shaft seal steam supply to the unit, and how should it be dealt with? Answer: Water in the steam supplied to the shaft seal can cause damage to the shaft-end seal during unit operation; in severe cases, it can lead to water hammer, posing a threat to the safe operation of the unit. When dealing with accidents involving water in the shaft seal, corresponding measures are taken based on the underlying causes. If it is observed that the sound of the unit becomes deeper, vibration increases, axial displacement rises, or there is a decrease in the expansion difference or even a negative expansion difference, the vacuum should be broken immediately to shut down the unit. The shaft seal steam supply system and the drain valves of the unit should be opened; once all the water has been drained, and all parameters meet the requirements for restarting, the unit can be restarted. 100. What functions does the DEH control system have? Answer: (1) Speed control, (2) Automatic synchronous grid connection, (3) Power control, (4) Valve position control, (5) Pressure-controlled mode, (6) Valve management, (7) CCS mode, (8) Primary frequency regulation, (9) Limiting and protection functions, (10) Valve tightness test, (11) Valve mobility test, (12) Operation parameter monitoring. 101. Procedure for bringing a condensate pump back into standby mode after maintenance is completed during turbine operation. Answer: ① The inspection and maintenance work have been completed, the work order has been retrieved, and the safety measures have been removed ； ②Contact electrical and thermal engineers to energize the motor and electric valves ； ③Slightly open the seal water valve ； ④Slightly open the vacuum release valve, and pay attention to monitoring changes in the unit’s vacuum level as well as ensuring that the operating pump runs steadily ； ⑤Slowly open the entrance door ； ⑥After checking that everything is normal, open the outlet door; make sure the pump does not rotate in reverse ； ⑦After a comprehensive inspection confirms normal operation, the interlock standby can be activated as needed. 102. How should a clogged water inlet filter in the generator air cooler be dealt with? Answer: ① Immediately open the bypass valve for the inlet filter screen, close the manual valves at its front and back, and contact the maintenance team to clean it as soon as possible ； ②Based on the wind temperature around the generator and the temperature rise in various parts of the generator, the unit operates at a reduced load ； ③Open the generator air cooler vent valve to release all the air ； ④If the temperatures of various components of the generator continue to rise, and reducing the load does not work, the generator should be shut down ； ⑤During the maintenance process, close monitoring of the generator should be carried out, and the filter screen should be put into operation as soon as possible after the work is completed. 103. Briefly describe the precautions during the speed-up process of a steam turbine unit Answer: (1) Listening to the turbine, the rotating parts of the generator are normal. ⑵Checks show that the vibration of each bearing is within normal limits; the vibration of the bearing cover at 1500 r/min should be less than 0.03 mm ； At speeds above 1500 r/min, the value should be less than 0.05 mm; if it exceeds 0.08 mm during speed increase, the machine should be stopped. At speeds beyond the critical speed, the value should not exceed 0.1 mm, and the machine must be stopped if this limit is surpassed. The machine shall shut down in case the shaft vibration exceeds 0.254 mm. (After the rotor has come to rest, start continuous barring; check for shaft bending, listen for abnormal noises, and determine the cause. If the coasting time is significantly reduced, continuous barring is prohibited, and intermittent barring should be used instead.) It is strictly forbidden to operate at speeds near the critical speed. ⑶Adjusting the shaft seal in a timely manner ensures normal operation. ⑷Ensure that the water levels in the condenser, various heaters, and deaerator are within normal ranges, and make adjustments promptly. ⑸Check that the oil level in the tank and the oil pressure are normal, as well as that the oil flow to all bearings is proper; maintain the lubricating oil temperature at 40–45°C. ⑹Pay attention to axial displacement; cylinder expansion and differential expansion are normal. ⑺Pay attention to the steam temperature rise rate; the temperature rise rates in various parts of the cylinder and the temperature differences are normal. 104. Preventive measures against fires in the turbine oil system. Answer: ① Cast iron valves are prohibited from being used in the oil system, and plastic gaskets, rubber gaskets, or asbestos paper gaskets are also not allowed ； ②The two emergency drain valves of the main fuel tank are located more than 5 meters away from the tank; they cannot be locked, and must be fitted with clear “Do Not Operate” signs ； ③The oil filter screen should be replaced and cleaned as specified ； ④Welding work on oil pipelines is prohibited ； ⑤When applying for a hot work permit, there must be no open flames around the oil system; if open flame work is necessary, effective safety measures must be taken ； ⑥Strictly implement the routine inspection system to promptly eliminate oil leaks ； ⑦After the maintenance work on the oil system is completed, the pipe insulation should be restored promptly, and it must be covered with iron sheeting. 105. What are the precautions for hot-starting a steam turbine? Answer: A. The temperature of the new steam entering the turbine should be more than 50°C higher than the temperature of the cylinder walls; it is necessary to ensure that the steam has a superheat of 80°C ； B. The temperature difference between the upper and lower sections at the regulating stage of the high-pressure cylinder shall not exceed 50℃ ； C. The rotor should be in a continuous turning state 2 hours before startup ； D. A vacuum of over -80 Kpa must be maintained ； E. Install the shaft seal first, then evacuate to prevent cold air from entering the turbine ； F. The automatic main steam valve and throttle valve cool down rapidly after shutdown; during hot start, ensure that the temperature difference between the inner and outer walls of the main steam valve is ≤50°C, and avoid overheating it too quickly ； G. Before starting up, thoroughly warm the steam leakage pipeline of the door rod, drain all the drain water, and enable steam leakage from the door rod. H. During hot start, the speed of acceleration should be increased appropriately to prevent a drop in cylinder temperature. 106. What equipment is mainly included in the turbine oil supply system? Answer: The oil supply system mainly includes the main oil pump, injector I, injector II, main oil pump start drain valve, high-pressure oil pump, AC/DC lubricating oil pump, jacking oil pump, oil cooler, oil filter, and overpressure valve, etc. 107. How is the terminal difference of surface heaters generated? How to reduce end difference? Answer: Surface heaters are used; the steam extracted from the turbine condenses in these heaters, releasing heat. This heat is transferred through the metal walls of the heat transfer surface to the main condensate water or boiler feedwater inside the tubes. In conventional surface heaters, due to heat transfer resistance, it is not possible to heat the feedwater to its saturation temperature at steam pressure; therefore, a temperature difference exists, namely the difference between the saturation temperature of the steam used for heating and the temperature of the feedwater after heating. Ways to reduce the temperature difference at the heater end: one is to increase the heating surface area of the heater, and the other is to make use of the superheat of the steam as much as possible in terms of design; both approaches will result in higher costs for the heater. At present, the terminal temperature difference of heaters manufactured in our country is generally 3–5°C. 108. What are the advantages and disadvantages of using high-pressure deaerators? Answer: Advantage ① The deaerator is a mixed-type heater; when used in a thermal system, the high-pressure deaerator helps reduce the number of high-pressure heaters, thereby saving materials ； ②If the high-pressure heater stops operating due to some issue during operation, the high-pressure deaerator can take over, so it has little impact on the normal operation of the boiler ； ③High-pressure deaerators can prevent \"spontaneous boiling\". Disadvantage: The feed water pump operates at high temperatures, making vaporization likely; therefore, it is necessary to raise the installation height of the deaerator or install a pre-pump at the inlet of the feed water pump. 109. What are the reasons for a rapid drop in water level in the deaerator during operation? Answer: ① Leaks in the tubes of components such as the boiler economizer, water wall, and superheater, or in the feedwater pipelines ； ②Excessive boiler wastewater discharge ； ③Insufficient make-up water volume for demineralized water ； ④Leakage due to the operation of the boiler safety valve or deaerator overflow valve ； ⑤Leakage was caused by an operator’s mistake. 110. What basic requirements must be met by a deaerator during operation? ①The deoxygenated water must be heated to the saturation temperature corresponding to the steam pressure in the deaerator; this is a necessary condition for separating gases from the water ； ②The gas separated from the water must be discharged promptly ； ③Take measures to reduce soda loss, keep the make-up water rate as close as possible within the specified range, and maintain stability ； ④The load distribution among the deaerators operating in parallel should be even to prevent overloading of the deaerators ； ⑤The water level in the water tank should be maintained within the specified normal range of variation. 111. Steps for starting up the low-temperature reheater. Answer: ① Verify that the low-pressure heater is in good condition, check that all gauges show accurate readings, and ensure that the water control interlock device is active. ②Open the low-pressure heater feed valve, verify that there is no leakage, open the low-pressure heater discharge valve, and close the feed side bypass valve. ③Check that the direct discharge door of the low-temperature heater is closed; open the drain outlet door and the bypass door, and close the drain inlet door. ④Turn the feed valve of the low-temperature heater 3–4 times to warm it up; slightly open the air extraction valve of the low-temperature heater as well, and pay attention to changes in vacuum level. After warming the low-temperature heater for 10 to 15 minutes, open the steam inlet valve fully and close the bypass valve for the low-temperature heater’s drain water. Once the water level in the low-temperature heater rises to the normal level, open the inlet valve for the drain water and use it to adjust the water level to the normal value. Slowly fully open the drain valve from the low-pressure heater to the condenser. 112. Indicators that the generator has been disconnected from the grid and all loads have been shed, as well as procedures for handling such situations when the emergency cut-off device does not activate. Answer: Symbol: The load indication is zero; the extraction check valves in each section close automatically. After the control valves close, they reopen. The turbine speed first increases and then decreases, staying within the speed range at which the emergency shutdown device activates. Treatment: A. Control the rotation speed at 3000 r/min ； B. Promptly disconnect the heat load of the heating units and close the isolation valves on the vacuum main ； C. Adjust the shaft seal steam pressure to within the specified range, and promptly activate the rear-cylinder water spray for temperature reduction ； D. Open the condensate recirculation valve, close the condensate valve leading to the deaerator, and maintain the level in the hot well ； E. Isolate the third extraction line for deaeration steam supply and valve stem leakage steam ； F. Stop the make-up water pumps; connect the drains from the high and low pressure heaters in series and direct them to the condenser ； G. Monitor changes in steam parameters, ensure that everything is functioning properly, send signals of \"Caution\" and \"Ready for paralleling\" to the electrical department, inform the shift supervisor, and get ready to take over under load. 113. Procedure for introducing rubber balls on the Condenser-1 side. Answer: ① Check that the condenser on side -1 is in operation; there are no maintenance tasks, and the rubber ball cleaning system is ready for use as usual ； ②Check that the rubber ball cleaning pump is shut down; the electric valve at the outlet of the rubber ball pump is closed, and the electric valve at the outlet of the ball loading chamber is also closed ； ③Open the vent valve at the upper part and the drain valve at the lower part of the ball charging chamber to drain all water and relieve pressure ； Open the upper cover of the ball chamber ； ④After loading an appropriate amount of rubber balls, close the lid of the ball-loading chamber and shut off the lower water discharge valve ； ⑤Move the ball-chamber cleaning handle to the cleaning position ； ⑥Slowly open the electric door at the outlet of the ball chamber, and once water flows out from the upper venting valve, close that venting valve ； ⑦Open the manual door at the inlet of the rubber ball pump and the manual door at the outlet of the ball loading chamber ； ⑧Start the rubber ball pump and open the electric valve at the pump’s outlet ； ⑨The system is running normally. 114. How to resume operation of the condenser after maintenance on one half of it is completed during operation? Answer: ① Check that the equipment after maintenance has been restored, and that the end caps and manholes on the maintenance side have been closed ； ②Close the drain valve on the maintenance side of the condenser, slightly open the circulating water outlet valve on that same side, and exhaust air from the water returning to the condenser on its maintenance side. When water appears in the exhaust pipe, close the exhaust valve and fully open the outlet valve for the condenser circulating water ； ③Open the circulating water inlet valve on the maintenance side of the condenser; pay attention to changes in the oil temperature at the outlet of the cold oil cooler and the air temperature at the inlet of the generator, and make adjustments as necessary ； ④Pay attention to changes in the quality of the condensate water; once the quality deteriorates, operation should be stopped immediately, and after the system is taken out of service, it should be inspected again ； ⑤Notify the monitoring personnel to pay attention to changes in the condenser vacuum, and slowly open the air extraction valve on the maintenance side of the condenser. Once the condenser vacuum returns to normal, load increase operations can be carried out ； ⑥Decide whether to start additional circulation water pumps based on changes in the circulation water pressure and vacuum. 115. Steps for activating the turning gear. Answer: (1) A comprehensive inspection confirms that the startup conditions are met ； (2) Start the smoke exhaust fan; (3) Start the AC lubricating oil pump or high-pressure electric oil pump – ensure that the lubricating oil pressure is normal and oil return is unobstructed, then activate the interlock for the DC lubricating oil pump ； (4) Open the barring lubrication oil valve ； (5) Remove the pin and rotate the handwheel counterclockwise; push the lever toward the working position (in the direction of the generator) until it reaches the working position (at an angle of approximately 45° to the horizontal) ； (6) Press the crank startup button to start the cranking ； (7) After checking that the barring motor current is normal, activate the barring motor interlock. 116. What is the function of the main oil tank in a steam turbine? Answer: The main oil tank of the turbine is of great significance for purifying the turbine oil. The oil that flows back to the oil tank from the turbine contains air and mechanical impurities. These impurities may be the result of wear on the bearing shells, speed control valves, and turning gear devices, or they could be welding slag, metal oxides, and other residues left in the bearing housings and oil system during manufacturing, installation, or maintenance. Sometimes steam leaks from the shaft ends, and some moisture also mixes into the oil. All these impurities can be filtered, settled, and separated in the oil tank to prevent them from being sent back into the oil system. 117. What should be given special attention to when starting a turbine? Answer: Check the oil flow and vibration of each bearing ； The speed control and lubricating oil pressure should be normal ； Pay attention to maintaining the water level in the condenser hot well, and use recirculation for adjustment ； Pay close attention to the sounds coming from the front and rear steam seals as well as inside the cylinder ； Pay attention to changes in the condenser vacuum ； Control the steam supply to maintain a stable turbine speed during warming up ； Pay attention to monitoring the changes in cylinder wall temperature. 118. Operating procedures for switching the turbine lubricating oil cooler from -1 to -2. (-2 The cooling oil cooler has not yet been restored to normal operation after maintenance)? Answer: ① First, check that the maintenance work order for the -2 cold oil cooler has been completed, and that the stop-work sign on site has been removed ； ②Open the air release valve on the oil side of the -2 cold oil cooler, slightly open the outlet valve on the oil side of the -2 cold oil cooler to fill the oil side of this cooler with oil. Once oil starts to flow out from the air release valve, close it, and then fully open the outlet valve on the oil side ； ③Open the air release valve on the water side, slightly open the inlet water valve to fill the water side with water. Once water starts flowing out of the air release valve, check that there are no oil droplets present; then close the air release valve on the water side ； ④Open the water inlet valve on the water side fully ； ⑤Gradually open the oil side inlet throttle, while slowly opening the water side outlet valve, to maintain a stable lubricating oil pressure; the oil temperature at the outlets of the two oil coolers should not exceed 2℃ ； ⑥Gradually increase the throttle at the inlet of the cold oil cooler, and adjust the outlet valve on the water side to maintain stable oil temperature and pressure ； ⑦After confirming that the -2 cold oil cooler is operating properly, first close the inlet valve of the -1 cold oil cooler, then close its outlet valve; be careful to maintain stable oil temperature and pressure ； ⑧At -1, all the inlet and outlet valves of the cold oil cooler are closed. -1 The cold oil cooler remains in standby mode. 119. How should a fire in the turbine oil system be handled? Answer: ① When a fire breaks out in the oil system, immediate action must be taken to extinguish it, and the fire department should be notified along with reporting to management. ②Make every effort to control the fire so that it does not spread to the rotating parts and cables. ③If it cannot be extinguished and poses a threat to the safety of the crew, the vacuum emergency shutdown should be activated. ④As appropriate, activate the emergency oil drain valve to maintain the lowest oil level until the rotor has come to rest. ⑤During an emergency shutdown due to a fire in the oil system, it is prohibited to start the high-pressure oil pump. 120. Causes of abnormal vibration during turbine operation and troubleshooting steps. Answer: Reason: Abnormal oil temperature, causing oscillation of the oil film ； Insufficient oil supply to the bearing shells, interruption in the oil supply, or damage to the oil film due to foreign particles ； Saturated steam parameters, sudden changes in unit load ； The opening degrees of the main steam valves and control valves on both sides are not consistent, resulting in a large deviation in steam flow ； Uneven expansion on both sides of the cylinder ； Sliding pin system jammed ； Large temperature differences in the cylinder metal cause thermal deformation or large shaft bending ； Damage to the shaft seal or cooling of the shaft end causes the main shaft to bend ； Leaf detachment and septum deformation ； Loose rotor components or rotor imbalance ； Damage to the thrust bearing pads, increased axial displacement, or improper bearing clearance ； Moving parts in the front bearing housing have fallen off ； When the turbine generator is not in normal condition or during startup, the rotor bending value is high, exceeding the specified limit ； Low condenser vacuum ； Vibration caused by the generator exciter ； The set screws of the shaft bearings in the steam turbine generator set are loose ； Impurities in the oil cause wear of the shaft bearing tungsten alloy, or water enters the oil leading to oil emulsification. Handling: ① If it is observed that the bearing vibration is increasing gradually and the measured axis vibration exceeds the alarm value, it is necessary to report this to the shift supervisor; efforts should be made to eliminate the vibration. If the axis vibration exceeds the limit value, the machine should be stopped immediately ； ② If, during operation, a sudden impact sound is heard inside the unit, or if the conductivity of the condensate water increases suddenly, if the pressure in the monitoring section rises under the same load, and if vibration increases significantly, an immediate emergency shutdown must be carried out ； ③When the vibration of the bearing shells changes by ±0.015 mm or the shaft vibration changes by ±0.05 mm, the cause must be identified and eliminated. If the vibration of the bearing shells suddenly increases to 0.08 mm, the machine should be stopped immediately by shutting down the brakes ； ④If it is caused by load variations, the load should be reduced until the vibration disappears ； ⑤If the cause of the vibration cannot be identified directly, measures should be taken to reduce the load. If the vibration or abnormal noises still do not disappear, it should be reported to the shift supervisor and relevant supervisors for joint consideration and resolution. 121. When a steam turbine operates in winter, air often accumulates on the outlet side of the condenser circulating water. What are the reasons for this and what are the consequences? Answer: When a turbine operates in winter, with the unit under light load and a low temperature at the cooling water inlet, the amount of water circulating through the condenser is small, resulting in a low flow rate. This causes the temperature of the cooling water inside the condenser to rise, and the gases dissolved in the circulating water get separated and accumulate at the highest point of the water chamber at the outlet of the condenser, forming air bubbles. As the amount of accumulated air increases, the air pocket expands, hindering the flow of cooling water and causing the condenser vacuum to drop. During low-load operation in winter, the water side of the condenser should be vented regularly. 122. How should an increase in the axial displacement of a turbine be handled? Answer: ① An alarm is triggered due to large axial displacement; it is necessary to check the temperature of the thrust bearings, the temperature of the return oil, differential expansion, and vibration levels ； Listen carefully to the sounds inside the turbine ； ②When the axial displacement increases to +0.8mm/-1.2mm, the unit should operate at a reduced load until the alarm disappears, after which the cause should be investigated ； ③When the axial displacement increases to +1.2mm/-1.6mm, the turbine should trip automatically; otherwise, it should be shut down manually ； ④In the event of sudden changes in load or steam parameters, the load should be stabilized promptly and the steam parameters adjusted ； ⑤If the axial displacement of the unit increases, accompanied by abnormal noises and severe vibrations, it should be treated as an emergency shutdown ； ⑥If the temperature at any point of the turbine thrust bearing rises to 110°C, the vacuum should be broken immediately to initiate an emergency shutdown. 123. Into which three roughly distinct stages is the coasting curve of a turbine rotor divided? Answer: ① The first stage is the period immediately after braking; the rotational speed drops rapidly because at that moment the rotor of the steam turbine generator still has a high inertial rotation speed, resulting in significant aerodynamic friction losses. ②In the second stage, the energy loss of the rotor is primarily used to overcome the frictional resistance from the governor, main oil pump, bearings, etc. This amount is much smaller than the frictional losses associated with blower operation, and such frictional resistance decreases as the speed drops; therefore, the speed declines more slowly during this period, resulting in a longer duration. ③The third stage is the phase when the rotor is about to come to a stop. Since the oil film is no longer intact at this stage, the resistance at the bearings increases rapidly, causing the rotor’s speed to drop quickly until it finally stops. 124. How to reduce pressure loss in pipelines? Answer: ① Select an appropriate working fluid flow rate. ②Minimize the number of pipe fittings as much as possible, reduce local resistance losses in the pipes, and eliminate unnecessary pipe accessories such as valves, elbows, tees, reducers, and throttle orifices. ③Choose a pipe village with relatively low absolute roughness. ④The total length of the pipes should be shortened as much as possible. ⑤Choose accessories with a lower coefficient of local resistance loss. 125. What measures should be taken to prevent cavitation at the inlet of a centrifugal pump? Answer: ① Improve the geometric shape of the centrifugal pump impeller ； ②Reduce the pressure loss in the suction pipeline of the centrifugal pump ； ③Use a reasonable suction height ； ④A booster pump or impeller wheel is installed in front of the impeller inlet to improve cavitation resistance ； ⑤Cavitation-resistant materials are used. 126. Describe the steps for performing an emergency shutdown. Answer: 1. Press the emergency stop button or the manual shutdown button, close the isolation valve of the vacuum main pipe (quick-close valve). Check that the automatic main steam valve, control valves, low-pressure hydraulic actuators, all extraction check valves, and the quick-close valve for the third extraction are closed. Verify that the speed of the turbine is decreasing ； 2 Send “Attention” and “Turbine Danger” signals to the electrical department ； 3 Start the high-pressure electric oil pump ； 4 Stop the vacuum pump and open the vacuum break valve ； 5 Close the electric main steam valve, and retract the start valve handwheel ； 6. Stop the rotor; reduce the vacuum to zero, cease steam supply to the shaft seal, record the coasting time, and engage continuous shaft rotation ； 7 Complete other shutdown operations. 127. What are the steps to start operating the oil cooler? Answer: ① Slightly open the inlet throttle, open the air release valve on the oil side; after all air has been released, close the air release valve, then slowly open the inlet throttle fully. ②Slightly open the inlet water valve; open the air release valve on the hot water side. After all the air has been released, close the air release valve, then slowly open the outlet water valve fully. (If oil spots are found in the water, it indicates that the cold oil cooler is leaking and cannot be put into use.) ③Gradually increase the throttle at the outlet; adjust the water valve at the inlet of the cold oil cooler based on the oil temperature at the outlet of the cold oil cooler. The oil temperature should be 40±5°C, with a difference of no more than 2°C compared to the oil temperature at the outlet of the operating cold oil cooler. 128. What are the causes of thermal shock in steam turbines? Answer: ① The steam temperature does not match the metal temperature at startup. ②Thermal shock caused by starting in an extremely hot state. ③Thermal shock caused by large fluctuations in load. ④Thermal shock caused by water ingress into the cylinders and shaft seals. 129. Why is it specified that the vacuum level should be reduced after shutting down the machine, so that it reaches zero when the rotor comes to rest? Answer: ① The shutdown coasting time is related to the vacuum retention time; the vacuum is reduced at a certain rate with each shutdown, which facilitates the comparison of coasting curves. ②If the vacuum drops too slowly during idle operation, the unit’s speed will remain at the critical speed for a longer period of time, which is detrimental to the safety of the unit. ③If the vacuum drops too rapidly during the idle phase, with the engine still running at a certain speed, the vacuum can drop to zero. The high frictional losses generated by the long blades in the subsequent stages produce a lot of heat, which can lead to an increase in the exhaust temperature. This also hinders the removal of water accumulated inside the cylinders, increasing the risk of metal corrosion. ④If the rotor has stopped but there is still a high vacuum, and the steam supply to the shaft seal cannot be stopped either, this will lead to an increased temperature difference between the upper and lower cylinders as well as uneven deformation of the rotor, causing it to bend. In summary, it is best to bring the speed to zero and the vacuum level to zero during shutdown; in practice, a vacuum break valve is used for control. 130. What issues should be considered when both the main steam pressure and temperature decrease? Answer: ① When both the main steam pressure and temperature drop, contact the boiler operator to request that normal conditions be restored, and inform the shift supervisor to reduce the load. ②During the process of decline in steam temperature and pressure, attention should be paid to parameters such as the expansion difference of the high-pressure cylinder, axial displacement, bearing vibration, and the temperature of the thrust bearings. It is also necessary to closely monitor whether white steam appears or water droplets splash from the main steam valve, shaft seals, and cylinder joints; the machine should be stopped immediately if water impact is detected. ③When both the main steam pressure and temperature drop, and although there is a superheat of 150°C, if the main steam temperature is more than 50°C lower than the temperature in most of the control steam chamber, report to the shift supervisor and request an emergency shutdown. 131. What is the monitoring section pressure? Answer: The pressure in the control steam chamber and the extraction pressures at various sections are collectively referred to as the monitoring section pressure. In condensing steam turbines, except for the last stage or two, the pressure in the control steam chamber and the extraction pressures are approximately proportional to the steam flow rate. By monitoring these pressures during operation, it is possible to determine changes in the fresh steam flow rate, the level of load, as well as whether there is scaling, damage, or blockages in the flow pathways. 132. What is oil film oscillation? What measures can be taken to prevent oil film oscillation? Answer: Oil film oscillation is a self-excited vibration phenomenon in which, when the journal drives the lubricating oil to flow at high speed, the high-speed oil flow in turn excites the journal, causing it to vibrate intensely. Measures: ① Increasing the specific pressure of the bearing allows for an increase in the bearing’s load, a reduction in the length of the bearing bushes, and adjustment of the center of the bearing bushes. ②Control the lubricant temperature to reduce its viscosity. ③Reduce the top clearance of the bearing shell to be equal to or slightly less than the sum of the clearances on both sides. ④Check valves are installed on each jacking oil branch pipe. 133. Why is the shaft seal installed first and then vacuuming performed during hot start-up? Answer: During hot start, the temperatures of the rotor and cylinder metals are high. If vacuum is drawn first, cold air will enter the cylinder through the shaft seals; since cold air flows downward to the lower cylinder, its temperature drops sharply, increasing the temperature difference between the upper and lower cylinders. This causes the cylinders to deform, leading to friction between the moving and stationary parts. In severe cases, this prevents the turning gear from functioning properly, resulting in bending of the main shaft. 134. What is oil film oscillation? What measures can be taken to prevent oil film oscillation? Answer: Oil film oscillation is a self-excited vibration phenomenon in which, when the journal drives the lubricating oil to flow at high speed, the high-speed oil flow in turn excites the journal, causing it to vibrate intensely. Measures: ① Increasing the specific pressure of the bearing allows for an increase in the bearing’s load, a reduction in the length of the bearing bushes, and adjustment of the center of the bearing bushes. ②Control the lubricant temperature to reduce its viscosity. ③Reduce the top clearance of the bearing shell to be equal to or slightly less than the sum of the clearances on both sides. ④Check valves are installed on each jacking oil branch pipe. 135. What should be noted during the water filling test of vacuum systems? Answer: Before filling the vacuum system with water, it is necessary to ensure that all maintenance work inside the condenser has been completed, and that all vacuum gauges located below the water level have been removed. Temporary supports must be added to the bottom support springs of the condenser to prevent deformation under stress, before water can be filled in. After draining water upon completion of the test, the temporary supports should be removed. 136. What is the function of the automatic main steam valve? Answer: The function of the automatic main steam valve is to quickly cut off the steam supply and stop the operation of the turbine once the turbine protection system is activated; therefore, it serves as the actuating element of the protection system. 137. What are the causes of shaft bending? Answer: ① Friction between the moving and stationary parts, improper assembly clearance, a large temperature difference between the upper and lower cylinders during startup, thermal deformation of the cylinders, and thermal bending of the main shaft during hot startup can all cause localized overheating of the rotor, leading to its bending. ②In a unit that is in a hot state, cold steam and cold water enter the cylinders, resulting in an excessive temperature difference between the upper and lower parts of the rotor. The thermal stress on the rotor exceeds the yield limit of the material, causing the main shaft to bend. ③The rotor raw materials contain excessive internal stress; after operating at high temperatures for a period of time, this internal stress is gradually released, resulting in bending of the main shaft. ④The components on the set rotor are skewed, stuck, and experience relative displacement. Sometimes, when the blades break off, the rotor experiences excessive bending moments and severe vibrations, which can also cause displacement of the assembled components and the main shaft, leading to bending of the latter. ⑤The operation management is not strict; for example, the system is started when the conditions for startup are not met, vibrations occur and abnormal situations are not handled properly, and water enters the cylinders after shutdown, all of which lead to bending of the main shaft. 138. Where does the economic benefit of operating the unit in slip mode come from? Answer: 1) At low unit load, reducing the steam pressure facilitates maintaining a stable steam temperature. Although the superheated enthalpy of the steam decreases due to the drop in pressure, the enthalpy of saturated steam increases significantly, resulting in a notable rise in the total enthalpy – this constitutes the main source of economic efficiency in sliding-pressure operation ； 2) The feedwater pressure decreases accordingly, the speed of the feedwater pump drops, which reduces the energy consumption and wear and tear of the feedwater pump ； 3) When the steam pressure decreases while the steam temperature remains constant, the volumetric flow rate and flow velocity at each stage of the turbine remain approximately unchanged, thus preventing a decline in internal efficiency ； 4) The temperatures at each stage of the high-pressure cylinder as well as the exhaust temperature of the high-pressure cylinder have increased, which helps to maintain the temperature of the reheat steam and thereby improves cycle efficiency. 139. What is the function of condensate recirculation? Answer: 1) Protect the condensate pump to prevent vaporization due to too low flow rate ； 2) It provides protection for the low-pressure heater and steam seal coolers (heaters). 140. What are the main functions of the turbine bypass system? Answer: 1) Before starting the turbine, activate the bypass to ensure a minimum evaporation rate in the boiler ； 2) Activating the bypass during unit startup/shutdown or load shedding can protect the boiler reheater (preventing dry burning) ； 3) Accelerate the improvement of steam parameters to reduce startup time ； 4) Recover the working fluid and some heat, and prevent noise generated by exhaust steam ；