You need to understand these practical experiences regarding the most important start-up and shutdown phases of turbine operation

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The most critical phases in the operation of a turbine are startup and shutdown – you need to be aware of these practical aspects. 2010-10-10 Turbine startup: 1. Why is startup considered the most important phase in the operation of turbine equipment? During the startup of a turbine, large temperature differences, thermal stresses, and thermal deformations occur between its various components. Most turbine accidents occur during startup. Accidents can be caused by incorrect warm-up conditions, improper operations by the staff on duty, and defects in certain structures of the equipment; even if no direct accident occurs at that time, the consequences resulting therefrom can still have negative effects on future production. Operating experience with modern steam turbines shows that cracks in cylinders, valve housings, and pipes, bending of the turbine rotor and cylinders, warping of the cylinder flange joints, loosening of tightly fitted components, changes in the condition of the metal structures, increased bearing wear, and other abnormalities that arise during the initial stages of operation are all direct consequences of poor startup quality. 2. What are the main changes related to the mechanical condition of the turbine during the turbine acceleration and load-bearing phases? The main changes related to the mechanical condition of the turbine during the acceleration and load-bearing phases are as follows: (1) Stress is generated in the pipes, cylinders, and valve housings due to internal pressures. ⑵ Centrifugal stresses are generated on the impeller, drum, moving blades, shaft sleeves, and other rotating components. ⑶ Bending stresses are generated in the diaphragm, impeller, static blades, and dynamic blades. ⑷ Due to the transfer of torque to the generator rotor, tangential stress is generated on the turbine shaft. ⑸ Vibration induces alternating stresses on the turbine blades, rotor, and other components. ⑹ An axial thrust acting on the thrust bearing appears. ⑺ Thermal expansion, thermal deformation, and thermal stress caused by the temperature rise of various components. 3. The start-up operation of a steam turbine can be divided into which three stages with different characteristics? The startup process of a turbine can be divided into the following three stages: (1) Startup preparation stage. ⑵ Ramp-up and acceleration to the rated speed stage. ⑶ Stages of generator grid connection and turbine load acceptance. 4. What are the different ways to start a steam turbine? The startup process of a turbine involves accelerating the rotor from a stationary or cranking state to its rated speed and bringing it into normal operation under load. Depending on the type of turbine unit and the specific circumstances, there are various methods for starting a turbine. Classified by the new steam parameters during the startup process: rated-parameter startup and sliding-parameter startup. Classified by the cylinder temperature level before startup: cold start and hot start. Classified by the steam inlet method during impulse startup: high and medium pressure cylinder steam inlet startup, and medium pressure cylinder steam inlet startup. Classified by the valve used for controlling the speed based on impulses: regulating steam valve start, automatic main steam valve start, electric main gate start, and main steam valve bypass valve start. 5. What are the necessary conditions for sliding-parameter startup of a steam turbine? The following necessary conditions must be met for the sliding-pressure start-up of a steam turbine: (1) For non-reheat units, there must be a condenser drain system; the diameter of the condenser drain pipes must be sufficiently large so that the steam generated by the boiler from ignition until the turbine reaches operating speed can be directly discharged into the condenser. ⑵ The diameter of the piping systems related to the cylinder and flange bolt heating system should be appropriately increased to meet the heating requirements of the flanges, bolts, and cylinder. ⑶ For units that use sliding-parameter startup, an auxiliary steam source must be provided for the shaft seal steam supply, the steam required for the ejector operation, and the heating steam for the deaerator. 6. What are the advantages and disadvantages of slip parameter starting? Sliding parameter startup has the following advantages and disadvantages: (1) Sliding parameter startup enables the turbine to start simultaneously with the boiler, thereby **reducing the startup time. ⑵ During slip parameter starting, the metal heating takes place under low parameters, and the impulse start and speed increase are carried out with steam supply throughout the entire circumference; as a result, the heating is more uniform, and it is also easier to control the rate of metal temperature rise. ⑶ Sliding-parameter startup can also reduce steam-water loss and heat energy loss. The drawbacks are: using changes in main steam parameters to control the heating of the turbine’s metal components; under manual control, the startup procedure is difficult to master, and if not handled properly, the rate of parameter change can be excessive. Based on a comprehensive comparison, the advantages of sliding-parameter startup outweigh its disadvantages; therefore, sliding-parameter startup is widely used in large-capacity units with a single cycle system at present. 7. What are cold sliding parameter pressure start and vacuum start? ⑴ Start via pressure method. When starting by the pressure method, there should be a certain steam pressure in front of the electric main steam valve; the steam flow is controlled using control valves to spin the rotor and accelerate the machine for warming up. The temperature of the new steam is required to be 50–80°C higher than the metal temperature of the cylinder in the adjustment section; moreover, a superheat of 50°C must be ensured, in order to avoid excessive thermal stress as well as water hammer. ⑵ Start by vacuum method. During vacuum startup, before the boiler is ignited, all valves between the boiler drum and the turbine are opened, and vacuum extraction begins while the turbine is in its cranking mode. The new steam pipes of the turbine, the boiler drum, and the superheater are all placed under vacuum, after which the boiler is instructed to be ignited. The pressure and temperature in the boiler rise gradually; when the steam parameters are still low, the turbine rotor begins to rotate. Thereafter, the acceleration of the turbine and the increase in its load depend entirely on the gradual rise in boiler pressure and temperature. The disadvantage of starting by vacuum method is that, if the boiler is not controlled properly, water in the boiler superheater and drain water from the new steam pipes may enter the turbine, thereby damaging the equipment. Additionally, vacuum extraction is difficult and the turbine speed is hard to control; therefore, the vacuum method for slip-parameter starting is rarely used. 9. What are the main things to pay attention to when starting with sliding parameters? The following points should be noted when starting with sliding parameters: (1) During sliding-parameter startup, the period of intense metal heating generally occurs during the heating process at low load; at this time, it is necessary to strictly control the rate of pressure and temperature increase of the new steam. ⑵ During sliding parameter start-up, the metal temperature difference can be controlled according to the specifications for the start-up time at rated parameters. During startup, excessive thermal expansion may occur; in such cases, the boiler should be instructed to stop increasing the temperature and pressure of the new steam, so that the unit can remain in a state of warm-up at a stable speed or under a stable load. It is also possible to adjust the vacuum in the condenser, or to reduce the temperature difference in the metals by increasing the amount of steam supplied through the cylinder flanges. 10. Why is it necessary to maintain a certain oil temperature before starting the turbine? Before starting the unit, the oil system should be activated first, with the oil temperature maintained between 35 and 45°C; if the temperature is too low, the oil temperature can be raised in advance. Maintaining an appropriate oil temperature is primarily to establish a proper oil film in the bearing shells. If the oil temperature is too low, the increased viscosity of the oil will result in an overly thick oil film, which not only reduces the load-bearing capacity of the oil film but also leads to unstable operation. The oil temperature should not be too high either; otherwise, the viscosity of the oil becomes too low, making it difficult to form an oil film and thus losing its lubricating effect. 11. What precautions should be taken when supplying steam to the shaft seal before starting the turbine? The following points should be considered when supplying steam to the shaft seal: (1) Before supplying steam to the shaft seal, the steam supply pipeline should be warmed up first to ensure that all water is drained. ⑵ Steam must be supplied to the shaft seal while the shaft is rotating continuously; during hot start, steam should first be supplied to the shaft seal before vacuuming is carried out. ⑶ The timing of steam supply to the shaft seal must be appropriate; supplying steam to the shaft seal too early before startup can increase the temperature difference between the upper and lower cylinders, or increase the positive value of the expansion differential. ⑷ Pay attention to the matching between the temperature of the steam supplied to the shaft seal and the metal temperature. For hot start-up, it is best to use a backup steam source at an appropriate temperature, as this facilitates the control of the expansion difference. If the system permits, it is even better to adjust the temperature of the shaft seal steam to be higher than the temperature of the shaft seal itself. For cold start-up, it is preferable to use a low-temperature steam source for supplying steam to the shaft seals. ⑸ Care must be taken when switching the steam source for the high- and low-temperature shaft seals. Switching too quickly not only causes significant changes in thermal expansion, but may also lead to uneven thermal deformation at the shaft seals, resulting in friction, vibration, etc. 12. Why is it strictly prohibited to supply steam to the shaft seal when the rotor is at rest? Because steam is supplied to the shaft seal when the rotor is at rest, it not only causes uneven heating in the shaft seal section of the rotor. Bending deformation occurs, and steam leaking into the cylinder from the shaft seal area can also lead to uneven expansion of the cylinder, resulting in significant thermal stress and thermal deformation, which in turn causes the rotor to bend. Therefore, it is strictly prohibited to supply steam to the shaft seal when the rotor is at rest. 13. How to control and reduce thermal stress when starting a steam turbine under rated parameters? When starting the turbine using the rated parameters, the rotor is impulsed instantly; steam at a temperature close to the rated value enters the cylinder where the metal temperature is lower. Similar to the initial stage of warming up the steam pipes, the steam will cause intense condensation and heat release within the metal. It causes a rapid increase in the temperature of the inner wall of the cylinder and the outer surface of the rotor; such a fast rise in temperature can lead to significant thermal stresses. Therefore, when starting in a cold state under rated conditions, it is necessary to control the rate at which the metal heats up by methods such as limiting the flow rate of fresh steam and extending the time required for warming up and loading the system. Reduce excessive thermal stress and thermal deformation caused by uneven heating. 14. What are the principles for selecting steam parameters when performing pressure-based slip parameter starting? After cold sliding-parameter startup, the steam flow rate entering the cylinder is sufficient to enable the turbine to smoothly pass through the critical speed and reach full speed. To ensure uniform heating of all metal components, the inlet steam pressure should be set at a reasonably low level to increase the volumetric flow rate of the steam. The temperature should have sufficient superheat and match the metal temperature to prevent thermal shock. During hot sliding-parameter startup, the appropriate main steam temperature and reheat steam temperature should be selected based on the metal temperatures of the regulating stages of the high-pressure cylinder and the steam inlet chambers of the medium-pressure cylinder; that is, the temperature difference between these two should meet the requirements regarding thermal stress, thermal deformation, and expansion differences in the turbine. It is generally required that the steam temperature be 50–100°C higher than the metal temperature of the inner wall of the casing in the regulating stage, but it must not exceed the rated temperature. To prevent condensation heat release, it is required that the steam superheat be no less than 50°C, ensuring that after throttling through the control valve and expansion through the nozzle, the steam temperature remains above the metal temperature of the control stage. 15. What is negative temperature difference start-up? Why should starting under negative temperature difference be avoided as much as possible? A start-up in which the steam temperature is lower than the metal temperature at the hottest part of the turbine during rotation is known as a negative temperature difference start-up. Because during negative temperature difference start-up, the rotor and cylinder are first cooled and then heated, going through a thermal cycling process, which increases the wear and fatigue life of the unit. If the steam temperature is too low, excessive tensile stress will be generated on the rotor surface and the inner wall of the cylinder. Tensile stress is more likely to cause metal cracks than compressive stress, and it can also lead to cylinder deformation, altering the clearance between the moving and stationary parts. In severe cases, this can result in frictional failures between these parts. Additionally, when a thermal turbine starts with a negative temperature difference, the temperature of the turbine’s metal components drops, and the time required to increase the load must be extended accordingly; therefore, starting with a negative temperature difference is generally not used. 16. How should the temperature differences across various parts of the steam turbine be controlled during startup and shutdown? During the startup or shutdown of high-parameter, large-capacity units, it is inevitable for temperature differences to arise among various metal components due to different heat transfer conditions in those components. However, excessive temperature differences can cause excessive thermal stress and thermal deformation in the metal components, accelerating the wear and degradation of the unit and leading to dynamic and static friction-related accidents. This is not allowed. Therefore, the rate of steam temperature increase or decrease should be controlled in accordance with the specifications of the turbine manufacturer ； Rate of temperature rise and fall of metals ； Temperature difference between upper and lower cylinders ； The temperature difference between the inner and outer walls of the cylinder, the inner and outer walls of the flange, the temperature difference between the flange and the bolts, as well as the thermal expansion difference between the cylinder and the rotor. Controlling the rate of change of the metal temperature and the temperature differences between various parts is necessary to prevent excessive thermal stress and thermal deformation in the metal components. Strict monitoring of the rate of change in steam temperature is crucial; it is not allowed for this rate to exceed specified values, nor are large sudden increases or decreases permitted. 17. What precautions should be taken during startup? Starting a steam turbine is one of the critical operations carried out by operators. Adequate preparation is necessary before starting, thorough inspections must be conducted, and tests should be performed prior to startup. During the startup process, it is important to: (1) strictly follow the established procedures and regulations; forcing the turbine to start when it does not meet the required conditions is not allowed. ⑵ During the startup process, it is necessary to control parameters such as steam and metal temperature rise rates, as well as temperature differences between the upper and lower cylinders, the inner and outer walls of the cylinders, flanges and bolts, and expansion differences, in accordance with the manufacturer’s specifications. In particular, the rate of steam temperature rise must be strictly controlled; the temperature increase rate is not allowed to exceed the specified value, nor are any significant spikes or drops permitted. ⑶ At startup, the steam entering the turbine must be free of water, the parameters must match the metal temperature of the cylinder, and thorough drainage and warming of the pipes are required. ⑷ Strictly control the vibration levels during the startup process. ⑸ During the slip-parameter startup of high-pressure steam turbines, the phases with intense metal heating are the low-load periods after start-up and after synchronization; it is in these phases that significant thermal expansion differences and temperature differences in the metal occur. Adjustments can be made by regulating the vacuum, using steam injection cylinders, employing heating devices for flanges and bolts, and adjusting the steam temperature used for the shaft seal. ⑹ During startup, control the changes in steam parameters according to the specified curve to maintain an adequate steam superheat. ⑺ The process of removing air from the control system must be repeated until all the air is eliminated. After expelling the air, keep the high-pressure oil pump running continuously until the unit reaches full speed before stopping it, to prevent air from entering the control system again. ⑻ Under any circumstances, if the steam temperature drops or rises by 50°C within 10 minutes, the machine should be shut down. ⑼ At the moment of startup, it is essential to control the rotational speed and prevent it from increasing too rapidly. After connecting to the grid, the valves should be opened in stages; it is strictly prohibited to open them fully all at once after connection to the grid. ⑽ After connecting to the grid, attention should be paid to the temperatures of air, oil, water, and hydrogen; these temperatures should be adjusted to ensure that the hydrogen temperature in the generator remains above 35°C. 18. What are the characteristics of high-pressure steam turbine startup? High-pressure steam turbines are structurally complex with small clearances between moving and stationary parts. They have the following characteristics: (1) The axial clearance in high-pressure steam turbines is quite small; if the heating during startup is uneven, the differential expansion value may exceed the specified limits, which could lead to axial friction between moving and stationary parts. Therefore, controlling differential expansion is very important. ⑵ The radial clearance in high-pressure units is also very small; therefore, it is extremely important to control the temperature difference between the upper and lower cylinders as well as the rotor bending. The unit must not be started if the temperature difference between the cylinders or the rotor bending exceeds the specified values, and measures must be taken to restore them to normal levels. ⑶ The cylinder walls and flanges of high-pressure units are quite thick, so cylinder flange heating devices are generally used. It is important to note that the temperature of the heating steam must be higher than the temperature of the cylinder flange. During heating, the flange temperature should be lower than the cylinder temperature. Flange bolts are relatively thick, so they expand slowly when heated; attention should be paid to the temperature difference between the flange and the bolts. To reduce the temperature difference between the upper and lower cylinders, it is necessary to drain the drain water from the lower cylinder as much as possible during startup, make proper use of the steam heating device, and enhance the insulation of the lower cylinder. To eliminate thermal bending of the rotor, continuous barring must be engaged after shutdown and before startup. ⑷ When starting a high-pressure unit, special attention should be paid to the vibration of the unit. If the vibration exceeds the specified level, the machine should be stopped immediately and an barring gear should be used; reducing the speed to warm up the machine should not be employed to eliminate the vibration. 19. Why should the stable speed during turbine startup for warming up be kept away from the critical speed of 150–200 r/min? This is because during startup, the parameters of the main steam and the vacuum level fluctuate, and the critical speed values provided by the manufacturer may vary to some extent in actual operation. If a certain speed is not avoided, the turbine speed might fall into the resonance range when operating conditions change, resulting in greater vibrations. Therefore, it is specified that the stable speed during warm-up should be 150–200 r/min away from the critical speed. 20. Why is a certain level of vacuum specified as a condition for starting a steam turbine? A certain level of vacuum is required before starting the turbine; it is generally around 0.06 MPa. 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 a significant increase in the pressure on the steam side of the condenser, which may result in positive pressure on that 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 level not only prolongs the time required to establish vacuum, but also results in a lower heat release rate due to the reduced amount of steam passing through the turbine; this slows down the heating of the turbine and makes it difficult to stabilize its speed, thereby extending the start-up time. 21. Why does the vacuum in the condenser drop during the turbine startup? During the cold start of a steam turbine, the vacuum level is generally still relatively low; some air remains in the cylinder and pipelines and is not completely evacuated. This air is carried toward the condenser by the steam flow during the start-up process. During startup, steam does not immediately exchange heat with the condenser tubes and thus does not condense right away; therefore, the vacuum in the condenser always decreases during startup. When the steam entering the condenser after start-up begins to condense, and the exhaust pump continues to remove air, the vacuum can be restored to its original value quite quickly. 22. Why is the exhaust temperature higher during turbine startup and acceleration as well as at no-load conditions, compared to normal operation? What measures can be taken to reduce the exhaust temperature? During the turbine’s acceleration process and at no-load conditions, due to the low amount of steam supplied, the steam expands and does work mainly in the high-pressure section once it enters the cylinder. By the time it reaches the low-pressure section, the pressure has dropped to a level close to that of the exhaust steam; as a result, the blades in the low-pressure section do little or no work, which leads to significant aerodynamic friction losses. This in turn heats the exhaust steam, raising its temperature. Furthermore, at this time the valve opening is adjusted to a very small value, causing the fresh steam with its rated parameters to undergo significant throttling, which also raises the exhaust steam temperature. At this time, the vacuum level of the condenser and the exhaust steam temperature are often not consistent with each other; that is, the exhaust steam temperature is higher than the saturation temperature corresponding to that vacuum level. Large units typically have water injection cooling systems installed in the exhaust cylinders; when the exhaust temperature is high, condensed water is injected to lower it. For units without a rear cylinder water injection system, the no-load operating time should be minimized as much as possible. When the turbo-generator is connected in parallel and operating at partial load, the exhaust temperature drops to its normal value. 23. Why is it necessary to monitor the vibration of the unit during the turbine’s speed-up and load-increasing process? When large-scale units are started, vibration occurs mostly during medium-speed warm-up and the acceleration phases before and after that, especially during the process of passing through the critical speed, when the vibration of the unit increases significantly. At this stage, if the vibration is high, it is most likely to cause friction between the moving and stationary parts, wear of the steam seal, and rotor bending. Once the rotor bends, the vibration increases, and the greater the vibration, the more severe the friction becomes. Such a vicious cycle can easily cause permanent deformation and bending of the rotor, leading to severe damage to the equipment. Therefore, during the warming-up or speed-up process, if significant vibration occurs, the machine should be stopped immediately by shutting off the power; the shaft should be aligned manually, and once the cause of the vibration has been eliminated, the engine set can be started again. After the unit is connected to the grid at full speed, for every 10,000 increase in load, the steam flow changes significantly, and the temperature rise within the metal increases rapidly. If the main steam temperature is not properly controlled, a large temperature difference can occur between the inner and outer walls of the metal, leading to vibrations in the unit. Therefore, an additional warm-up period is required for each increase in load, to allow the unit to heat up gradually and evenly. In summary, during the turbine acceleration and operation under load, it is necessary to regularly monitor the vibration of the turbine. 24. Why should axial displacement protection be activated before startup? During startup, the steam flow is high momentarily; the steam must first pass through the high-pressure cylinder, while almost no steam reaches the medium and low-pressure cylinders. The axial thrust is substantial and is entirely balanced by the thrust disc. If the axial displacement exceeds the allowable limit at this time, it will also cause sliding friction between moving and stationary parts; therefore, the axial displacement protection should be activated before startup. 25. Why are temperature rise and temperature drop rates required to remain within certain ranges during startup and shutdown? During startup and shutdown of the turbine, its cylinders and rotor undergo a process of heating and cooling. During startup and shutdown, a certain temperature difference is inevitably present 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, stress formation occurs in the opposite direction. When the metal stress in the cylinder exceeds the material’s yield stress limit, the cylinder may undergo plastic deformation or develop cracks. The magnitude of this stress is proportional to the temperature difference between the inner and outer cylinders, which in turn is proportional to the rate of change of the metal’s temperature. Since there is no monitoring mechanism for metal stress during startup and shutdown, an indirect indicator is used, namely the rate of increase and decrease in the metal’s temperature, as a means to control thermal stress. 26. Why is it necessary to slightly close the drain valve on the main steam pipeline after startup? During the period from pipe warming to startup of the main steam pipeline, the pipe warming process is essentially complete; at this point the temperature of the main steam pipeline is nearly equal to that of the main steam, so little condensate is generated. Furthermore, after startup, hydrostatic pressure must be established inside the cylinder. If the main steam valve remains fully open at this time, positive pressure will be generated in the steam trap, which will push back the hydrostatic pressure inside the cylinder and prevent it from being discharged – this is very dangerous. The water storage pipe at the lower part of the hydrophobic expansion tank is connected to the hot well of the condenser. When the main steam pipe’s drain valve is opened fully, an excessive amount of steam is released, resulting in a mixture of steam and water flowing in the pipe. This leads to water hammer, which can damage the pipes and affect the vacuum level of the condenser ； Additionally, with the drain valves fully open, heat loss is high; therefore, after startup, all drain valves on the main steam pipeline should be closed. 27. Why is a superheat value for steam specified when starting and stopping a turbine? If the superheat of the steam is low, during startup, due to a significant drop in temperature in the earlier stages, the temperature in the later stages may drop to the saturation temperature at that pressure, causing the steam to become wet steam. Steam containing water poses a severe threat to the blades; therefore, it is safer to maintain the superheat of steam at 50–100°C during startup and shutdown processes. 28. What issues should be considered during hot start? The following points should be noted during hot start: (1) Before starting in a hot condition, it is necessary to ensure that the shaft is rotated continuously; the bending degree of the main shaft must not exceed its original value. If this condition is not met, starting is not allowed. The shaft must be rotated continuously until it is in proper alignment. Continuous turning should last for more than 4 hours without interruption. In case of any interruption, continuous barring operation should be performed for a duration 10 times that of the interruption period. ⑵ Supply steam to the shaft seal first, then evacuate. The high-pressure steam valve for the shaft seal must be closed tightly; high-temperature steam should be used for the shaft seal (adequate water drainage is required before supplying steam to the shaft seal). When the vacuum level reaches 39.997 kPa, notify the boiler to start up. ⑶ It is necessary to enhance the hydrophobicity of the body and pipes to prevent cold water and cold steam from flowing into the cylinders or pipes, thereby causing water hammer vibrations. ⑷ At low speed, a thorough inspection of the unit should be carried out; once it is confirmed that there are no abnormalities, the unit should be brought up to full speed and operated in parallel with an appropriate load. During the acceleration process, it is necessary to prevent the speed from rising too quickly and then dropping. ⑸ At low speeds, the vibration of the unit must be closely monitored; once the bearing vibration becomes excessive, the machine should be stopped immediately, the barring gear should be engaged, and the shaft bending degree should be measured. (If, for some reason, the barring gear cannot be engaged, do not force it. The cause must be identified and measures taken before attempting to engage the barring gear again.) ⑹ The cylinder flange heating device should be activated at the appropriate time. 29. Why is shaft sealing steam supplied first and then a vacuum created during hot start-up? 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 seal; since cold air flows downward to the lower cylinder, its temperature drops sharply, resulting in an increased 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 can prevent the turning gear from functioning properly and cause the main shaft to bend. Therefore, during hot start, shaft seal steam should be supplied first, followed by the creation of vacuum. 30. Why cannot the vacuum level be too high during low-speed warm-up? During low-speed warm-up, if the vacuum level is too high and the steam flow for warming up is too low, the unit does not heat up sufficiently, which in turn prolongs the warm-up time. Furthermore, when passing the critical speed, it is necessary to exceed it as quickly as possible, and the methods for this include: ① increasing the steam flow rate ； ②Increase the vacuum. If the vacuum is raised too high upon startup, the time it takes to exceed the critical speed increases. It is unsafe and not allowed for the unit to operate in the vicinity of the critical speed for an extended period of time. 31. What is cylinder swelling? How does cylinder expansion change during the startup and shutdown of the unit? The absolute expansion of a cylinder is called cylinder swelling. The startup process is the heating process of the turbine cylinder, rotor, and each component. During startup, cylinder expansion gradually increases ； During shutdown, the metal temperatures of various parts of the turbine decrease, the cylinder gradually contracts, and the cylinder expansion reduces. 32. What is differential expansion? What do the positive and negative values of differential expansion indicate? When a turbine is started or stopped, both the cylinder and the rotor expand when heated and contract when cooled. Due to the difference in mass between the cylinder and the rotor, as well as the different heating conditions, the rotor expands and contracts more rapidly than the cylinder. The difference in axial expansion between the rotor and the cylinder is known as differential expansion. When the differential expansion is positive, it indicates that the axial expansion of the rotor is greater than that of the cylinder ； When the differential expansion is negative, it indicates that the axial expansion of the rotor is less than that of the cylinder. When the turbine starts up, the rotor heats up rapidly, and the value is generally positive ； When the steam turbine shuts down or sheds load, differential expansion tends to become negative. 33. What factors are related to the magnitude of differential expansion? During the startup, shutdown, and operation of a steam turbine, the magnitude of differential expansion is related to the following factors: (1) During unit startup, the heating devices for the cylinder and flanges are not utilized properly, resulting in either excessive or insufficient amount of heating steam. ⑵ During warm-up, the rise rate is too fast or the warm-up time is too short. ⑶ During normal shutdown or slip-parameter shutdown, the steam temperature drops too rapidly. ⑷ The load increase rate is too fast. ⑸ After load shedding, operation at no load or low load lasts for too long. ⑹ Water hammer occurred in the turbine. ⑺ During normal operation, the steam parameters change too rapidly. 34. What is the relationship between axial displacement and differential expansion? The zero points for both axial displacement and differential expansion are at the thrust pads, and the zero-point positioning method is the same. When the axial displacement changes, although the magnitude is small, the total displacement of the major axis changes. When the axial displacement is positive, there is a displacement of the large axial generator in that direction; the differential expansion changes in the negative direction ； When the axial displacement changes in the negative direction, the turbine rotor displaces toward the turbine head, and the differential expansion value increases in the positive direction. If the unit parameters remain unchanged and the load is stable, the differential expansion and axial displacement do not change. During the startup and shutdown of the turbine and when steam parameters change, differential expansion varies, while axial displacement remains unchanged. Changes in axial displacement during operation inevitably lead to changes in differential expansion. 35. Under what conditions does differential expansion take on a negative value? Due to the difference in steel materials used for the cylinder and the rotor, the linear expansion coefficient of the rotor is generally greater than that of the cylinder. Additionally, given that the rotor has a small mass but a large heating surface, the differential expansion remains positive during normal operation of the unit. When the load decreases or is suddenly reduced, the main steam temperature and the reheat steam temperature drop, causing water hammer in the turbine ； Improper use of the steam heating device during unit startup and shutdown can both result in negative differential expansion. 36. How to handle excessive differential expansion during unit startup? During unit startup, if the differential expansion is excessive, the operator should take the following actions: (1) Check whether the main steam temperature is too high; contact the boiler operators to appropriately lower the main steam temperature. ⑵ Warm up the unit at a stable speed and under a stable load. ⑶ Appropriately increase the condenser vacuum to reduce the steam flow rate. ⑷ Increase the heating steam supply to the cylinder and flange to cause the cylinder to expand rapidly. 37. How to control differential expansion when starting a steam turbine? The following measures can be taken depending on the unit conditions: (1) Select appropriate startup parameters. ⑵ Establish appropriate heating and pressurization curves. ⑶ Activate the cylinder and flange heating devices in a timely manner to keep the temperature difference of the metals in various components within the specified range. ⑷ Control the acceleration speed and the time for warming up at a constant speed; after loading, adjust the load increase speed based on the cylinder temperature. ⑸ Adjust the vacuum in a timely manner when starting up the machine for warming up. ⑹ The steam supply for the shaft seal should be used appropriately, with adjustments made in a timely manner. 38. What are the hazards of an excessive temperature difference between the upper and lower cylinders of a steam turbine? During the startup and shutdown of high-pressure steam turbines, it is easy for a temperature difference to arise between the upper and lower cylinders. Sometimes, after the unit is shut down, the removal of the cylinder insulation can also lead to a large temperature difference between the upper and lower cylinders; in severe cases, this difference can reach around 130°C. Usually, the temperature of the upper cylinder is higher than that of the lower cylinder. The upper cylinder has a higher temperature, resulting in greater thermal expansion, while the lower cylinder has a lower temperature, leading to less thermal expansion. When the temperature difference reaches a certain value, it causes the upper cylinder to arch upward. As the upper cylinder arch deforms, the radial gap between the moving and stationary parts at the bottom of the lower cylinder decreases, which leads to radial friction between these moving and stationary components within the turbine. This results in wear of the diaphragm seal and the labyrinth seal located at the lower part of the lower cylinder. Additionally, the diaphragm and the impeller deviate from their normal position in the vertical plane, causing the axial gap to decrease as the rotor rotates; this, together with other factors, leads to axial friction. Friction causes the main shaft to bend, resulting in vibration. If not addressed promptly, it may cause permanent deformation, forcing the unit to shut down. 39. Why is it specified that the temperature difference between the upper and lower cylinders before startup should not exceed 50°C? When the turbine starts up and shuts down, the temperature of the upper half of the cylinder is higher than that of the lower half, and this temperature difference causes deformation of the turbine cylinder. It can cause the cylinder to bend upward, thereby damaging the blades and shrouds. The cylinder deflection of the turbine was calculated; when the temperature difference between the upper and lower parts of the cylinder reached 100°C, the deflection was approximately 1 mm. Measurements confirmed that this value is quite accurate. Experience shows that, assuming a temperature difference of 10°C between the top and bottom of the cylinder, the deflection of the cylinder is approximately 0.1 mm, while the radial clearance in typical turbines is 0.5 to 0.6 mm. Therefore, when the temperature difference between the upper and lower cylinders exceeds 50°C, the radial clearance is essentially gone; if startup occurs at this time, friction may occur in the radial steam seal. In severe cases, it can also cause the rivets of the belt to wear out, leading to more serious accidents. 40. How to reduce the temperature difference between the upper and lower cylinders? To reduce the temperature difference between the upper and lower cylinders and avoid deformation of the cylinder crown, the following measures should be taken: (1) Improve the drainage conditions of the cylinders, select an appropriate diameter for the drainage pipes, and prevent water from accumulating at the bottom. ⑵ During the startup and shutdown of the unit, operators should use each drain valve correctly and in a timely manner. ⑶ Improve the cylinder wind shields at high and medium pressures, enhance the insulation of the lower cylinders; the insulation bricks should not fall off to reduce cold air convection. ⑷ Use 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 cool the upper cylinder or heat the lower cylinder. 41. What is elastic deformation? What is plastic deformation? How to control the temperature differences across various parts of the cylinder during turbine startup in order to reduce cylinder deformation? When metal components are subjected to external forces, no matter how small those forces may be, the components will develop internal stresses and deform. When the external force is removed, if the component can return to its original shape and size, this type of deformation is known as elastic deformation. When an external force increases to a certain level, once that force is removed, the metal component cannot return to its original shape and geometric dimensions; this type of deformation is known as plastic deformation. For turbines, it is not allowed for any of their components to undergo plastic deformation. When starting the turbine, it is necessary to strictly control the temperature differences between the inner and outer walls of the cylinder, between the upper and lower cylinders, as well as between the inner and outer walls of the flanges and across the top, bottom, left, and right sides of these flanges, keeping them within specified limits in order to avoid the generation of unnecessary stresses. The specific temperature difference should be kept within the following range: (1) The temperature difference between the inner and outer walls of the flanges on the high and medium pressure inner and outer cylinders shall not exceed 80°C. ⑵ The temperature difference between the inner and outer cylinders at high and medium pressures (between the inner wall of the inner cylinder and the inner wall of the outer cylinder, and between the outer wall of the inner cylinder and the outer wall of the outer cylinder) shall not exceed 50–80°C. ⑶ The temperature difference between the upper and lower parts of the high-pressure and medium-pressure cylinders should not exceed 50°C, while the temperature difference between the upper and lower parts of the outer cylinder should not exceed 80°C. ⑷ The temperature difference between the bolt and the flange center should not exceed 30°C. ⑸ The temperature difference between the left and right, as well as the top and bottom, of the high and medium pressure inner and outer cylinder flanges should not exceed 30°C. During the startup process of the unit, it is necessary to closely monitor the temperature changes at various metal measurement points, adjust the amount of steam used for heating as appropriate, and ensure that both the main steam temperature and the reheat steam temperature remain within appropriate ranges. By carrying out these tasks properly, the safe startup of the unit can be ensured, thereby extending its service life. 42. Why does the turbine rotor bend after friction occurs? Due to the temperature differences in the metal of the cylinder flanges, the cylinder deforms, causing the radial dynamic and static clearances to disappear. As a result, when the rotor rotates, friction occurs radially 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 on both sides of the shaft to increase rapidly. The increase in temperature difference causes the rotor to bend. This repeats over and over; the greater the temperature difference on either side of the main shaft, the more the rotor bends. 43. What is the cause of an increase in rotor bending and fluctuations in the barring current after the turbine is shut down or before it is started while still hot? How to deal with it? After the turbine is shut down or before it is started while still hot, an increase in rotor bending and fluctuations in the barring current are observed. The cause is often a temperature difference between the upper and lower parts of the high- and medium-pressure cylinders that exceeds the specified limit, leading to cylinder deformation and seal friction, which in turn causes the main shaft to bend. If an increase in the rotor bending value and fluctuations in the turning current are detected, the cause should be investigated first. If the temperature difference between the upper and lower cylinders is excessive, it is necessary to check whether all the steam drain valves of the turbine are functioning properly, and to ensure that no cold water or steam is flowing into the cylinders. Depending on the temperature difference between the high-pressure and medium-pressure cylinders, either the lower cylinder should be heated or the upper cylinder cooled with air, in order to minimize this temperature difference. The shaft should also be aligned, and the rotor bending value should be restored to its original level. 44. Why is it required that the temperature of the new steam be 50–80°C higher than the cylinder temperature during hot start? When the unit starts under hot conditions, it is required that the temperature of the new steam be 50–80°C higher than the temperature of the cylinder. It can be ensured that, after the new steam is throttled by the control valve, cooled in the steam guide tubes, and expanded through the nozzles of the control stage, its temperature remains above the metal temperature of the cylinder. Because the startup process of the unit is a heating process, it is not allowed for the cylinder metal temperature to drop. If the temperature of the new steam is too low during hot start-up, it will cause excessive stress on the cylinder and flange metals. The rotor will contract sharply due to sudden cooling, resulting in a negative value for the high-pressure differential expansion. This leads to the disappearance of the axial clearances between the moving and stationary parts, thereby causing friction that can damage the equipment. 45. What are the reasons why the cylinder cannot expand during the turbine startup process? During startup, the reasons why the cylinder cannot expand include: (1) Improper control of main steam parameters and condenser vacuum. ⑵ Improper use or incorrect operation of the cylinder and flange bolt heating devices. ⑶ The sliding pin system is stuck. ⑷ The load increase speed is fast, and warm-up is insufficient. ⑸ The drain valves on the body and related extraction pipes are not open. 46. Why are the cylinder and flange heating devices activated after the turbine is started up? For units with high parameters and large capacity, the thickness of their cylinder walls and flanges reaches 300–400 mm. After the turbine is started up, the metal that comes into contact with steam first experiences a rapid temperature rise, while the increase in the overall metal temperature is primarily due to heat transfer. As a result, the inner and outer surfaces of the cylinder flange are heated unevenly, which easily leads to high thermal stresses between the upper and lower cylinders, as well as on the inner and outer walls of the cylinder flange and between the flange and bolts. Meanwhile, the deformation of the cylinder and flange can cause friction between moving and stationary parts, leading to vibration in the unit. In severe cases, it can cause damage to the equipment. Therefore, after the turbine is started up, the cylinder and flange heating systems should be activated based on the actual temperatures of the cylinder and flanges. 47. What is the purpose of warming up? The purpose of warming up is to ensure that the metal components of the turbine are properly preheated, thereby reducing the temperature differences between the inner and outer walls of the cylinder flanges, as well as between the flanges and bolts. It also reduces the temperature differences on the surface and at the center of the rotor, which in turn lowers the internal stresses in the metal. This allows the cylinder, flanges, and rotor to expand uniformly, with the pressure differences remaining within safe limits. As a result, the dynamic and static clearances inside the turbine are maintained, preventing friction from occurring. Additionally, it enables the speed to increase when the turbine is under load, shortening the time required to reach full load and thus helping to save energy. 48. What are the reasons for the increase in exhaust temperature during the startup and acceleration of a steam turbine? When a turbine starts up and speeds up, the reasons for the increase in exhaust temperature are: (1) A decrease in vacuum inside the condenser, with air not being completely removed, resulting in a mixture of steam and air. The poor thermal conductivity of air leads to an increase in exhaust pressure, as well as a higher saturation temperature. ⑵ A large amount of drain water from components such as the main steam pipes, reheat steam pipes, and the cylinder itself is directed to the expansion tank; the steam that emerges from this expansion tank is then sent to the throat of the condenser. The temperature of this drain water and steam is 4 to 5 times higher than the saturated temperature inside the condenser. ⑶ During the warm-up process, the steam flow is low and the flow velocity is slow, so the heat generated by friction in the blades cannot be removed in a timely manner. 49. Why is it necessary to enhance the drainage of the turbine itself as well as the main and reheat steam pipes during turbine startup and shutdown? During the startup process of the turbine, the metal temperature of the cylinder is low. Although the temperature of the main steam and the reheat steam entering the turbine are set at lower levels, they are still significantly higher than the temperature of the inner wall of the cylinder. The difference between the steam and cylinder temperatures exceeds 200°C. During the initial stage of warming up, the steam causes condensation within the cylinders, releasing heat and generating a large amount of condensed water. This process of heat release due to condensation continues until the temperatures of the cylinder walls and the inner surfaces of the steam pipes reach the saturated temperature at that pressure; only then does the amount of water formed through condensation start to decrease. During the shutdown process, the steam parameters gradually decrease; especially in a sliding-parameter shutdown, after the steam does work in the first few stages, it contains wet steam, which is thrown toward the periphery of the cylinder under the effect of centrifugal force. The lower the load, the higher the moisture content in the steam. Furthermore, after the turbine is stopped by braking, a considerable amount of residual steam in the cylinder and steam pipes still condenses into water. Due to the presence of hydrophobicity, it can cause water erosion of turbine blades, unit vibration, temperature differences between the upper and lower cylinders, as well as corrosion inside the cylinders. Therefore, when starting or stopping the turbine, it is necessary to enhance the drainage of hydrophobic substances from the turbine itself and its steam pipelines. 50. What are the requirements for oil temperature when starting a turbine or when it operates above its critical speed? The viscosity of turbine oil is greatly affected by temperature. At too low temperatures, the oil film becomes thick and unstable, exerting a pulling force on the shaft, which can easily lead to vibration or even oil film oscillations. However, if the oil temperature is too high, its viscosity decreases significantly, resulting in an excessively thin oil film. An overly thin oil film is unstable and prone to breakdown; therefore, there are certain requirements for both the upper and lower limits of the oil temperature. At the beginning of startup, the linear speed on the surface of the shaft journals is low and the specific pressure is high; if the viscosity of the turbine oil is too low, a stable oil film cannot be established, which is why a lower oil temperature is required. Beyond the critical speed, the rotational speed increases rapidly; the viscosity of the turbine oil should be lower at this point, which means a higher oil temperature is required. The oil temperature should be above 30°C when starting the turbine, and it should range from 38 to 45°C once the critical speed is exceeded. 51. What should be noted when exceeding the critical speed? When passing the critical speed, the following points should be noted: (1) When reaching the critical speed, it is generally necessary to cross it quickly and smoothly, but it is also not advisable to rush past it at high speed to avoid adverse consequences; current regulations specify that the acceleration rate when passing the critical speed should be around 500 r/min. ⑵ During the transition past the critical speed, it is necessary to monitor vibration and speed conditions in order to determine the type of vibration and avoid misjudgment. ⑶ The vibration noise should be normal; if the vibration exceeds acceptable levels or abnormal noises such as knocking or friction are heard, the machine should be stopped immediately. The cause must be identified, and it can only be restarted after it is confirmed that there are no abnormalities. ⑷ After reaching the critical speed, the rate at which the speed increases should be controlled. 52. What are the reasons for excessive positive and negative differential expansion values in steam turbines? Reasons for a high positive value of turbine differential expansion: (1) Insufficient warm-up time during startup, and too rapid acceleration or load increase. ⑵ The temperature of the cylinder interlayer or the flange heating device is too low, or the flow rate is too low, resulting in insufficient heating. ⑶ The steam inlet temperature increases. ⑷ The temperature of the steam supplied to the shaft seal has increased, or the amount of steam supplied to the shaft seal is too high. ⑸ The vacuum decreases, causing an increase in the steam flow entering the turbine. ⑹ Speed variation. ⑺ As the valve opening is increased, the throttling effect decreases. ⑻ The slip pin system or bearing plate is stuck, preventing the cylinder from expanding. ⑼ The bearing oil temperature is too high. . ⑽ The stress on the non-working surface of the thrust bearing increases, leading to wear, and the rotor moves in the direction of the machine head. ⑾ The cylinder insulation has come loose, or there is cold air flowing through. ⑿ Changes in the differential expansion of other related cylinders in a multi-cylinder unit cause changes in the differential expansion of this cylinder. ⒀ Cold steam or cold water is introduced into the interlayer of the double-cylinder structure. ⒁ The zero point of the differential expansion indicator is inaccurate, or it is affected by changes in frequency and voltage. Reasons for a large negative differential expansion value: (1) Too rapid load reduction or sudden load shedding. ⑵ The steam temperature dropped sharply. ⑶ Water impact. ⑷ The temperature of the shaft seal steam has decreased. ⑸ Overheating in the cylinder interlayer and flange heating device. ⑹ The steam inlet temperature is lower than the metal temperature. ⑺ The axial displacement changes to a negative value. ⑻ The bearing oil temperature has decreased. ⑼ High-temperature steam flows into the double-cylinder interlayer (leakage from the steam inlet stub). ⑽ Changes in cylinder differential expansion related to multi-cylinder units. ⑾ The zero point of the differential gauge is inaccurate, or it is affected by changes in frequency and voltage. 53. Why is it prohibited to start the turbine when its rotor bending exceeds the specified value? Generally speaking, most turbines indirectly monitor the extent of rotor elastic bending by monitoring changes in rotor wobble. When the rotor’s deflection exceeds the original value by a significant amount, it indicates that the elastic bending of the rotor is quite substantial; at this point, the deformation of the cylinder is also likely to be significant. The radial clearance between the moving and stationary parts of the turbine may disappear. If the turbine is started under such conditions, the bent portion of the rotor will come into contact with the diaphragm seal, causing friction. This friction not only leads to wear of the seal but also generates high temperatures in the bent portion of the rotor. These local high temperatures further increase the bending of the rotor, intensifying the friction. This vicious cycle can result in permanent bending of the rotor; therefore, starting the turbine is prohibited when its bending exceeds the specified limits. 54. Why are units whose control systems cannot maintain operation at no-load conditions prohibited from being started? If the turbine cannot operate at no load, it indicates serious defects in the control system. Forcing it to start will result in difficulties in connecting to and disconnecting from the power grid; even if it is possible to connect to the grid, it will not be able to reduce its load to zero freely. Moreover, the unit will experience severe overspeeding when the load is suddenly removed. Shutting down a steam turbine 1. How many ways are there to shut down a turbine? How to choose among the various shutdown methods? There are normal shutdown and emergency shutdown methods for steam turbines. A so-called normal shutdown refers to a planned shutdown. A fault shutdown refers to the situation where, in the event of an abnormal condition in a steam turbine generator set, protective devices are activated or a manual shutdown is carried out in order to prevent damage to the unit or to minimize losses. Fault shutdown is further divided into emergency shutdown and routine fault shutdown. During normal shutdown, it is further divided into two methods based on the steam parameters during the shutdown process: reduced-parameter shutdown and rated-parameter shutdown. The method of shutting down is determined based on the purpose of the shutdown and the condition of the equipment. For a normal shutdown, if it is intended for maintenance purposes and it is desired to cool the unit as quickly as possible so that the maintenance can begin sooner, a reduced-parameter shutdown should be used whenever possible. Additionally, the duration of this reduced-parameter shutdown should be extended, with the parameters being lowered further. 2. What is a sliding parameter shutdown? Starting from rated parameters and rated load, the steam turbine fully opens its high- and intermediate-pressure control valves; meanwhile, the boiler adjusts its combustion, gradually reducing the steam parameters and thereby progressively decreasing the turbine’s load. At the same time, the cylinder flange heating device is activated to gradually cool down the temperature of the cylinder flanges; once the parameters of the main steam reach a certain value, the generator is disconnected and shut down. This process is known as sliding parameter shutdown. 3. What are the precautions for shutdown using a sliding parameter method? The precautions for shutdown using a sliding parameter method are as follows: (1) When shutting down with a sliding parameter method, there are specific requirements regarding the rate at which the pressure of the fresh steam decreases; generally, for high-pressure units, the average pressure drop rate of the fresh steam is 0.02–0.03 MPa/min, while the average temperature drop rate is 1.2–1.5°C/min. At higher parameters, the cooling and pressure reduction rates can be faster ； At lower parameters, the rates of temperature and pressure reduction can be slower. ⑵ During the sliding-parameter shutdown process, the temperature of the new steam should be maintained at a superheat of 50°C to ensure that the steam is free of water. ⑶ When the temperature of the new steam is lower than that of the inner wall of the flange, the flange heating device can be activated. ⑷ During the gradual-parameter shutdown process, no turbine overspeed tests shall be conducted. ⑸ The high and low pressure heaters should shut down gradually during a sliding-parameter shutdown. 4. Why is it not allowed to conduct a turbine overspeed test during a sliding parameter shutdown? Performing overspeed tests when the steam parameters are very low is extremely dangerous. By the time of a smooth parameter-based shutdown leading to the disconnection of the generator, the steam parameters in front of the main steam valve are already very low; to conduct an overspeed test, it is necessary to close the control valve in order to increase the pressure in front of it. As pressure increases, the superheat of the steam decreases, which may cause the temperature of the new steam to fall below the saturation temperature at that pressure. This results in the steam containing water, leading to water hammer accidents in the turbine; therefore, it is stipulated that speed tests must not be conducted during the controlled shutdown of large-scale units. 5. What is a “free-run curve”? What is the purpose of drawing it? After the generator is disconnected, the time elapsed from the closure of the automatic main steam valve and control valves 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 carried out under the same conditions, with the coasting curve being recorded each time, so as to facilitate 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 idle time increases significantly, it indicates that the valves in the new steam or reheat steam pipes, 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. 6. Why is it necessary to wait until the vacuum reaches zero before stopping the steam supply to the shaft seal when shutting down the machine? If the supply of steam 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 specified that the vacuum level must reach zero before the supply of steam to the shaft seal can be stopped. 7. 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? During the coastdown phase after shutting down a turbine, the best way to maintain vacuum is to gradually reduce it, striving to bring the rotor to a complete stop and achieve a vacuum level of zero. 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. ⑵ If the vacuum drops too slowly during coasting, the unit will remain at its critical speed for a longer period of time, which is detrimental to the safety of the unit. ⑶ If the vacuum drops too rapidly during the pre-idle phase, with the turbine still running at a certain speed, the vacuum can already reach zero. The high amount of heat generated by the aerodynamic losses in the longer blades of the subsequent stages can cause the exhaust temperature to rise, and it also hinders the removal of water accumulated inside the cylinders, thereby increasing the risk of corrosion of the turbine’s metal components after shutdown. ⑷ If 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, resulting in thermal bending. 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 and adjustment. 8. Why is the oil pump interlock switch activated during the turbine barring process? Although the turbine barring device is equipped with interlock protection that causes it to shut down when the lubricating oil pressure drops to a certain level, thereby protecting 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 dry friction of the turbine bearings, resulting in damage. 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. 9. What issues should be noted during the turning of the shaft? The following points should be noted during the barring process: (1) Monitor whether the current of the barring motor is normal and whether the ammeter reading fluctuates. ⑵ Regularly check whether there are any changes in the rotor bending indication value. ⑶ Regularly listen for any friction sounds inside the cylinders as well as at the high and low pressure seals. ⑷ Regularly check the operation of the lubricating oil pump. 10. Why must the lubricating oil pump continue to run for a while after the machine has stopped and the turning gear operation is complete? The main purpose of the continuous operation of the lubricating oil pump is to cool the shaft journals and bearing shells; even after shutdown, the temperature of the rotor metal remains high, and heat is transferred through the bearings in 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 bearing damage ； Excessively high bearing temperatures can also cause the remaining oil in the bearings to oxidize rapidly, leading to smoking and even fires. During the operation of the low-pressure oil pump, the cooler must also remain in operation to keep the lubricating oil temperature 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. 9. What maintenance tasks should be carried out after shutdown? Maintenance work after shutdown is extremely important. After shutting down, in addition to monitoring the operation of the cranking device, the following tasks must also be carried out: (1) Strictly cut off the source 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. ⑵ Closely monitor the exhaust temperature of the low-pressure cylinder and the water level in the condenser as well as that of the heaters; full water levels are strictly prohibited. ⑶ Pay attention to the cooling water of the generator rotor water-inlet seal support to prevent a disruption in the cooling water, which could cause the packing to burn out. ⑷ 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. 10. Where is the maximum bending of the rotor after the turbine is shut down? When is it most dangerous to start? After the turbine is shut down, 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 maximum bending occurs near the regulation stage, and the peak value of this bending is reached within 2 to 10 hours after shutdown; therefore, starting the turbine during this period is extremely dangerous. 11. 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 caused by loose or stuck control valves, a malfunctioning extraction check valve that does not 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. 12. Why is it better to first reduce the steam temperature before reducing the steam pressure during a sliding parameter shutdown? Since the superheat of the main steam is relatively high during the normal operation of the turbine, when shutting down the plant under sliding-parameter conditions, it is best to maintain the steam pressure constant while appropriately reducing the steam temperature, thereby decreasing the superheat of the main steam. This facilitates the cooling of the cylinder, results in a lower temperature of the cylinder after shutdown, and helps to shorten the time required for cranking the turbine. 13. What are the requirements for the sealing oil system of a hydrogen-cooled generator when it is in the barring mode after shutdown? The seal oil system of a hydrogen-cooled generator should remain in normal operation mode during cranking or when it stops rotating but is still under pressure inside. Since the seal oil system is connected to the lubricating oil system, hydrogen-containing seal oil may enter the main oil tank through the connected pipelines, and the hydrogen in the oil will be separated out in the main oil tank. If hydrogen accumulates in the main fuel tank, there is a risk of a hydrogen explosion and a fire in the main fuel tank; therefore, the exhaust fans and hydrogen exhaust fans used in the fuel system and main fuel tank system must also remain in continuous operation.