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1. What is the function of the shaft seal cooler? Answer ； When a turbine is equipped with an internal leakage shaft seal system, a shaft seal heater (shaft seal cooler) is generally installed to use heated condensate water in order to recover the steam leaking from the shaft seals, thereby reducing steam leakage and heat loss as well as improving the environmental conditions in the workshop. The air that enters through the shaft seal leaks is commonly led to the diffuser tube of the jet pump using a connecting pipe, and is removed by the negative pressure in the latter, thereby maintaining a micro-vacuum condition in the shaft seal heater. In this way, the first chamber of each shaft seal also remains in a slight vacuum state, preventing the shaft seal steam from leaking out. Function: It is used to extract the vapor-gas mixture from the turbine steam seal system, preventing steam from leaking from the end seals into the turbine room and oil system, thereby avoiding environmental contamination and damage to the quality of the oil. These gas mixtures enter the shaft seal cooler where they are cooled down to water; the condensed water is then heated, while the remaining uncondensed gases are released into the atmosphere. Operation of the 2-axis seal coolers. The activation and deactivation of the shaft seal cooler should be synchronized with the supply of steam to the main engine’s shaft seal; that is, the shaft seal cooler should be activated immediately when steam is supplied to the main engine’s shaft seal, and it should also stop operating when the steam supply to the shaft seal is stopped. When the shaft seal cooler is in operation, there must be sufficient cooling water flowing through it to ensure the proper functioning of the condensate pump. Primarily, before the unit starts operating at low load, the condensate flow rate is adjusted. After the water side is connected, install the shaft exhaust fan. During normal operation, monitor the negative pressure and water level of the shaft seal cooler to ensure it operates within specified ranges for optimal performance. 3 What is a regenerative heater? Answer ； It refers to a device that extracts a portion of the worked steam from certain intermediate stages of the steam turbine, which is then used to heat the boiler feedwater or condensate. 4 Why can the use of a regenerative heater improve the cycle thermal efficiency of the unit? Answer ； Regenerative heating system: In steam turbine equipment, the purpose of using a regenerative system that heats feedwater by extracting steam is to minimize cold-source losses and thereby improve the thermal efficiency of the unit. This allows the steam from the working part of the turbine to be drawn out from certain intermediate stages and used for regenerative heating of the feedwater to the heater and the main condensate, so that it no longer enters the condenser. The enthalpy of this portion of extracted steam is thus fully utilized, rather than being carried away by cooling. With the use of a regenerative heater, the total steam consumption of the turbine increases, while its heat consumption rate and coal consumption rate decrease. The increase in steam consumption rate is due to the reduced work done per kilogram of fresh steam entering the turbine, while the decrease in both the steam consumption rate and the coal consumption rate is attributable to the rise in feedwater temperature resulting from reduced cold loss; therefore, the thermal efficiency improves with the use of a regenerative heating system. Additionally, by using a regenerative heating system, the feedwater temperature is increased, which reduces the thermal stress on the boiler’s heat transfer surfaces caused by large temperature differences, thereby improving the reliability of the equipment. 5 What is the function of the cold oil cooler? Answer ； Function: During the normal operation of a steam turbine generator set, some of the energy is lost due to bearing friction; this energy is converted into heat, which raises the temperature of the lubricating oil in the bearings. If the oil temperature becomes too high, the bearings may soften, deform, or even be damaged. To ensure the proper operation of the bearings, the temperature of the lubricating oil must be maintained within a certain range. Generally, the temperature of the oil entering the bearings should be between 35–45°C, while the temperature rise of the oil exiting the bearings is usually around 10–15°C. Therefore, the oil that exits the bearings must be cooled before it can be recycled back to lubricate the bearings. A cold oil cooler is used to cool the lubricating oil of the main engine. The lubricating oil at higher temperature and the cooling water at lower temperature exchange heat in the oil cooler; by adjusting the flow rate of the cooling water, it is possible to control the temperature of the lubricating oil. (Since the rotor temperature is high, especially on the steam inlet side of the high-pressure cylinder, the journal also transfers heat outward, so the lubricating oil also serves to cool the journals.) 6 Operation process of the cold oil cooler. Work process ； The lubricating oil enters from the lower part of the housing, and flows alternately toward the center or the periphery through the perforated middle partition and the non-perforated small partition; it moves in a zigzag pattern outside the tubes, before finally exiting through the oil outlet at the upper part of the housing. Cooling water enters through the water chamber and exits through the lower water chamber after passing through four processes. When filling the oil cooler with oil, the inlet and outlet valves as well as the air release valve are opened. The oil pressure is controlled using the outlet valve of the low-pressure oil pump to facilitate the filling process; care should be taken to ensure that the filling speed is not too fast. Once all air from the oil side has been removed, the valve is closed, completing the oil filling process. When filling with oil, be careful to prevent overpressure on the oil side, which could cause equipment damage. Cooling water can be supplied only after the oil cooler is filled with oil. The outlet valve remains fully open, and the oil temperature is adjusted using the inlet cooling water. Three cold oil coolers operate in parallel, with one as a spare. When performing the switch operation of the cold oil cooler during unit operation, the principle of turning it on first and then turning it off should be followed. The operation should be carried out by experienced personnel, with a dedicated person also present to supervise. Monitor the lubricating oil temperature, oil pressure, and bearing temperature to prevent misoperations that could lead to unit tripping and equipment damage. The operation should be carried out slowly; when introducing the oil cooler, gradually increase the inlet throttle to prevent fluctuations in oil pressure, while also removing any air from the system. When stopping, close the outlet throttle and the cooling water inlet valve. 7 Maintenance of the cold oil cooler. 1 ； Pay attention to the temperature at the outlet of the oil cooler and make adjustments as needed to keep it within the range of 38–42°C. Adhere to the principle of making minimal adjustments; be aware of how much the oil temperature changes depending on the degree to which the door is opened. Do not wait for a long time after adjusting the cooling water before checking, as this could lead to significant fluctuations in the oil temperature. At the same time, the oil temperature should be confirmed based on experience to prevent misjudgments and incorrect operations in the event of gauge failures. 2 ； Pay attention to the oil pressure (inlet and outlet oil pressures) to accurately determine internal leakage. 3 ； Make sure that the oil pressure is always higher than the water pressure to prevent coolant from leaking into the oil. 4 ； Strengthen inspections, identify and eliminate any deficiencies to prevent fire accidents. 8 Starting and stopping of the low-pressure heater Answers ； The preparatory work before starting the heater, as well as its operation, must be carried out in accordance with the plant operation procedures. Startup sequence ； (1) Check the relevant valves, main equipment instruments, etc.; they should be operating properly and in their correct positions. (2) Drain the air from the pipe. (3) Slowly open the steam inlet valve, control the rate of water temperature rise according to the specified values, and then activate the drain control device. (4) Check the shell-side air extraction system; it should be operating properly. Principles for starting and stopping the low-temperature heater ； Investment: The low-pressure heater is put into operation after the turbine has reached a certain load; that is, it is activated after the unit has started up and connected to the grid, and at a load corresponding to the temperature of the cylinder metal. It is turned on in order of increasing pressure, that is, in order of increasing operating temperature. Under normal circumstances, it is put into operation while the unit is being warmed up at an initial load (5–10% of the rated load). When starting up, pour water from the water side first; fully open the outlet valve on the water side and close the bypass valve, making sure to exhaust all air properly. The drains before and after the check valve in the extraction pipeline are in the open position; the drains from the low-temperature heater and the air pathways are connected sequentially, and the steam inlet valve of the low-temperature heater is slowly opened to supply steam to it. When the amount of drain water from the low-temperature heater reaches a certain level, at a load of around 15 MW, the drain pump is started and the water level is adjusted. At the same time, the operation of the drain control valve, water level sensors, check valves, etc., is checked. After the heaters are put into use, the drain flow is reduced or stopped depending on the conditions of the unit. Stopping is the reverse of starting: after appropriately reducing the load, shut down the system in order from high to low pressure, pay attention to its cooling condition, and open the corresponding air release valve. 9 What are the effects of excessively high or low steam temperature on the unit? Answer ； When designing turbines, manufacturers select steel materials for components such as cylinders, diaphragms, and rotors based on the steam parameters. Each type of steel has a certain maximum allowable operating temperature; below this temperature, it retains certain mechanical properties. If the operating temperature exceeds the design value by a significant amount, it will inevitably lead to a deterioration in the metal’s mechanical properties, a decrease in strength, and an increase in brittleness. This can result in creep deformation of the cylinders, loosening of the impeller on the shaft, vibrations during turbine operation, or static and dynamic friction, all of which can cause severe damage to the equipment. Therefore, turbines must not operate at temperatures above their specified limits. When the new steam pressure and other parameters remain unchanged, a decrease in the new steam temperature leads to a reduction in the cycle thermal efficiency. If the load is kept constant, the steam flow rate increases, which in turn increases the wet steam losses in the turbine and reduces its internal efficiency. A decrease in the temperature of the new steam also reduces the enthalpy drop at all stages except the last stage, increases the reactivity at each stage, and raises the axial force on the rotor, which is detrimental to the safety of the turbine. A sharp drop in the temperature of the new steam can cause water hammer in the turbine, posing a serious threat to its safe operation. 10 What is the impact of an increase in the new steam pressure on the operation of the unit? Answer ； When the main steam pressure increases, the total useful enthalpy drop rises, and the work capacity of the steam increases. Therefore, if the load remains unchanged, the steam flow rate can be reduced, which is beneficial for the economic operation of the unit. However, the steam humidity at the last stages will increase, which is particularly detrimental to the operation of the final stage blades. For the regulation stage, the most hazardous operating condition occurs when the first regulation valve is fully open; at this time the initial pressure increases, resulting in an increase in both the enthalpy drop and flow rate in the regulation stage, which is unfavorable for it. However, when operating at rated load, the enthalpy drop in the regulation stage is not at its maximum, so the risk is generally low. If the main steam pressure increases without exceeding the limits, the unit operates at its rated load; as long as the humidity of the exhaust steam from the last stage remains within the allowable range, the control stages can be considered to be safe. However, the main steam pressure cannot be increased arbitrarily. Excessively high main steam pressure results in a large enthalpy drop across the control stages, which can damage the nozzles and blades over time. Additionally, when the main steam pressure rises above acceptable levels, the steam humidity at the last few stages increases, leading to erosion of those blades. An excessive increase in the new steam pressure can also lead to increased stress on pressure-bearing components such as steam ducts, steam chambers, and valves, posing a certain threat to the safe operation of the unit. 11 What is the impact on the operation of a turbine when the new uniform pressure decreases? If the new steam temperature and other operating conditions remain unchanged, a decrease in new steam pressure results in a decrease in load. If the load remains unchanged, the steam flow increases. When the pressure of the fresh steam decreases, the enthalpy drop in the regulating stage reduces while the reaction degree increases; meanwhile, the enthalpy drop in the final stage increases and the reaction degree decreases. There is no significant change, or only a minor change, in the overall axial thrust of the unit. A decrease in the pressure of the fresh steam leads to an increase in the steam consumption of the unit, resulting in reduced efficiency. When the pressure of the fresh steam drops significantly, in order to maintain the rated load, the flow rate exceeds the capacity of the final stage, which causes an increase in blade stress and axial thrust; therefore, the load should be limited. What impact does the level of the 12th exhaust pressure have on the unit? (Vacuum level) Answer ； Changes in exhaust pressure have a significant impact on the efficiency and safety performance of turbines. An increase in vacuum levels allows for reduced steam consumption by the turbine, thereby improving efficiency. The higher the vacuum level in the condenser, that is, the lower the exhaust pressure, the more thermal energy in the steam is converted into mechanical energy, and less heat is carried away by the circulating water. For every 1 KPa decrease in condenser pressure, the turbine’s load increases by approximately 2% of its rated load. A higher vacuum level is not necessarily better either; the higher the vacuum, the more energy the circulation water pump consumes. The higher the vacuum, the greater the humidity at the final stage, and the axial thrust increases. If the condenser vacuum deteriorates and the exhaust pressure rises, more thermal energy in the steam is carried away by the circulating water, resulting in greater heat loss. With the same steam flow rate and the same initial parameters, the load cannot reach its rated value. If the steam flow at rated load is increased, the blades will be overloaded and the axial thrust will increase; therefore, the plant should maintain an economic vacuum during operation to achieve better efficiency. 13 Why is the shaft seal supplied first and then evacuation carried out when the unit starts up? Answer ； During hot start-up, the temperatures of the rotor and cylinder metals are high. If vacuuming is carried out 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 barring gear from functioning properly and cause the main shaft to bend. Therefore, during hot start-up, the shaft seal should be activated first before vacuuming is done. 14 Why is it strictly prohibited to supply steam to the shaft seal when the rotor is at rest? Answer: Because steam is supplied to the shaft seal when the rotor is at rest, this 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 also leads 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. 15 What is differential expansion? Answer ； The relative expansion of the turbine rotor and the cylinder is called thermal expansion difference. *It is conventionally specified that when the rotor expansion is greater than the cylinder expansion, the differential expansion is considered a positive differential; whereas when the cylinder expansion is greater than the rotor expansion, the differential expansion is considered negative. Based on cylinder classification, it can be further divided into high difference, medium difference, low I difference, and low II difference. The expansion difference value is a very important operating parameter; if it exceeds the limit, the thermal protection system will activate and cause the main unit to trip. The main factors that cause the expansion difference to increase in the positive direction are briefly described as follows: Answer ； (1) The warm-up time at startup is too short; the speed or load increases too rapidly. (2) The heating temperature of 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 slide pin system or bearing plate is poor, prone to sticking. (4) Excessively high shaft seal steam temperature or excessive supply of steam to the shaft seal causes excessive elongation of the shaft journal. (5) When the unit starts up, parameters such as steam inlet pressure, temperature, and flow rate are too high. (6) Wear of the thrust bearing, resulting in increased axial displacement. (7) The insulation layer of the cylinder does not provide adequate insulation, or it has fallen off; in seasons when such conditions are strictly prohibited, the temperature inside the turbine room is too low, or there is cold air flowing through the room. (8) Cold steam (or cold water) is introduced into the interlayer of the double-cylinder structure. (9) An inaccurate zero point of the differential pressure 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) Effect of speed variation. (13) The impact of changes in the extraction volume at various levels: if the primary extraction is shut down, the effect on the pressure difference is significant. (14) The bearing oil temperature is too high. (15) During the coasting-down process of the turbine unit, due to the effect of the “Poisson effect”. 17 What is differential expansion? What do positive and negative values represent? Answer ； When the steam turbine is started or shut down, 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 it is negative, it means that the axial expansion of the rotor is less than that of the cylinder. When the steam turbine starts up, the rotor heats up rapidly; this value is generally positive ； When the steam turbine shuts down or sheds load, differential expansion tends to become negative. 18 What are the factors affecting the bearing oil film? Answer: The factors that affect the oil film of a bearing rotor include ； (1) Rotational speed (2) Bearing load (3) Viscosity of the oil (4) Clearance between the shaft diameter and the bearing ； (5) Dimensions of the bearing and shaft diameter ； (6) Temperature of lubricating oil ； (7) Lubricating oil pressure (8) Diameter of the bearing oil inlet. 19 How is the zero position of the turbine’s axial displacement determined? Answer ； In the cold state, the zero position for axial displacement is determined by moving the rotor’s thrust disc toward the working surface of the thrust bearing, pressing it tightly against that surface; at this point, the instrument should indicate zero. 20 How is the zero point set for high-pressure differential expansion? Answer ； The zero-point setting method for high-pressure differential expansion is the same as that for axial displacement. When the turbine is in a fully cooled state, the rotor is pushed toward the generator side, with the thrust disc pressing against the working surface of the thrust bearings; at this point, the instruments indicate zero. During the barring process, the high-pressure differential gauge should indicate a certain negative value (–0.3~0.4 mm). 21. When does the critical operating condition occur in the regulating stage of a condensing steam turbine with nozzle regulation? Answer: The enthalpy drop in the regulating stage of such a turbine reaches its maximum value when the first regulating steam valve is fully open and the second regulating steam valve is closed; at this point, the steam flow rate through the first set of nozzles is at its highest. Due to the impact force of steam on the moving blades, the product of the steam flow rate and the enthalpy drop is proportional, and the stress on the moving blades of the control stage is at its maximum at this time. This is the critical operating condition for the control stage of a condensing steam turbine with a nozzle regulator. 26. What are the advantages of using nozzles for regulation in steam turbines? Answer: Compared to throttling regulation, nozzle regulation results in lower throttling losses at low load conditions, higher efficiency, and more stable operation. The disadvantage is that when the load changes, the steam temperature in the high-pressure section of the turbine varies significantly, which can lead to large thermal stresses at the regulating stages, resulting in poor adaptability to load fluctuations. 27. What is combined throttling-nozzle control? What are the advantages of using this type of control? Answer: In order to take advantage of both throttling control and nozzle control, any large-capacity unit that operates at basic load levels uses throttling control at low loads, while employing nozzle control at high loads. This type of regulation is known as combined throttling-nozzle regulation; its advantage is that it reduces the range of temperature variations in the control chamber, thereby improving the speed and reliability with which loads can be adjusted. 28. What is a slant-nozzle? Answer: In steam turbines, due to structural requirements, the axis of the nozzles is at an angle to the direction of movement of the blades; therefore, the outlet portion of these nozzles is designed to be slanted, and such nozzles are called slant-nozzles. 29. What factors are related to the velocity coefficient of turbine blades? Answer: The original degree coefficient of the moving blade is related to factors such as the blade profile, blade height, inlet and outlet angles of the blade, reaction degree of the blade, and surface roughness. The blade velocity coefficient is determined through experiments, and it is usually taken as φ = 0.85–0.95. 30. When a turbine operates in supercritical or subcritical conditions, what is the relationship between flow rate and the pressures before and after a turbine stage when the operating conditions change? Answer: (1) When both the conditions before and after a turbine stage are critical, the amount of fluid passing through that stage is proportional to the pressure before that stage. That is: G1/G0 = P01/P0. (2) When the turbine stage does not reach a critical state before or after the change in operating conditions, the flow rate through that stage is proportional to the square root of the pressures before and after the stage. 31. Briefly describe the basic working principle of a hydraulic centrifugal governor. Answer: By utilizing the principle of centrifugal force generated when a liquid column rotates, the sensed rotational speed signal is converted into a change in oil pressure signal. 32. Briefly describe the working principle of a rotary damping speed regulator. Answer: In a rotary damping speed regulator, the turbine shaft drives the damper body directly; several damping tubes are fixed on it. The pressure oil from the main oil pump enters the oil chamber after being throttled by needle valves. Some of this oil flows into the front bearing box through the damping tubes, leakage pipes, and drain holes. As the turbine rotates, the oil column in the damping tubes experiences centrifugal force, thereby creating a primary oil pressure in the oil chamber. As the rotational speed changes, the centrifugal force of the oil column in the damping tube changes as well, and the primary oil pressure also changes; the oil pressure in the oil chamber is proportional to the square of the rotational speed. A change in the oil pressure corresponds to the change in the oil pressure signal output by the rotary damper. 33. Briefly describe the working principle of the speed regulator for radial drilling pumps. Answer: A radial drilling pump, also known as a pulse oil pump, consists of a impeller, pump casing, flow-stabilizing mesh, and sealing ring. The oil pressure difference between the inlet and outlet of the pump is proportional to the square of its rotational speed, and this change in oil pressure serves as the signal representing the output oil pressure. 34. What are the static characteristics of the speed sensing mechanism in a governor? Answer: Under stable operating conditions, the relationship between the output signals of the speed sensing mechanism (displacement, oil pressure, oil pressure difference) and the input signal (rotational speed) is known as the static characteristics of the speed sensing mechanism in a governor. The curve representing this relationship is called the static characteristic curve of the sensory mechanism. 35. What is the operating range of a typical synchronizer? Answer: The range within which a typical synchronizer can change the speed is from -5% to 7% of the rated speed. 36. Why is it necessary to control the rate of change of the metal temperature of the turbine during its startup, shutdown, and operation under varying conditions? Answer: Practice has shown this to be necessary. Thermal pressure is related to the degree of rapid heating. For a given turbine, during startup, shutdown, and operation under varying conditions, the faster the rate of temperature rise (or fall), the greater the temperature difference in the metal components and the greater the thermal stress generated. Therefore, it is necessary to control the rate of increase and decrease of the metal temperature. 37. What is the function of the low vacuum protection device in a turbine? Answer: When the vacuum level in the turbine falls below the normal value, the low vacuum protection device emits an alarm signal; when the vacuum level drops to its critical value, this device activates and automatically stops the turbine. 38. Why is a low vacuum protection device necessary in a turbine? Answer: When the vacuum level in the turbine decreases, the turbine’s output drops, its thermal efficiency declines, and it also causes an increase in axial thrust as well as a rise in the exhaust temperature. It poses a serious threat to the safe operation of the turbine ； Therefore, the turbine is equipped with a low vacuum protection device. 39. During the slip-parameter start-up of a high-pressure steam turbine, when does the metal heat up more intensely? Answer: The metal heats up more intensely after the impulse start and during the load-in process after connection to the grid, especially at low loads. 40. What is the impact of impurities in steam on the safe operation of boiler and turbine equipment? Answer: Excessive impurities in steam can cause salt deposits to form on the heating surfaces of the superheater, in the flow passages of the turbine, or within the steam pipes. If scale deposits on the tubes of the heater’s heating surfaces, it will reduce the heat transfer efficiency ； At the least, it reduces heat absorption, increases the flue gas temperature, and lowers the boiler efficiency ； In severe cases, it can cause the wall temperature of the tube to exceed the allowable limit for the metal, resulting in the tube being damaged due to overheating. If salt deposits accumulate in the flow-through parts of the turbine, it will reduce the steam flow cross-section, increase the roughness of the blades, and even alter their shape. This leads to increased resistance in the turbine, reduced output and efficiency. It also causes increased stress on the blades and axial thrust, as well as heightened vibration in the turbine, potentially resulting in turbine failures. Salt deposits in the valves of steam pipes can cause the valves to stick, fail to operate properly, and leak steam. 41. What are the hazards of connecting generators out of synchronization? Answer: Connecting generators out of synchronization poses serious hazards; it causes significant damage to the generators themselves, as well as to the three-phase main transformers and circuit breakers connected in series with them. In severe cases, it can even destroy the generator windings. Deform the end. If a large generator experiences such an accident, power oscillations occur between that generator and the system, affecting the stable operation of the system. 42. Why does a centrifugal pump generate axial thrust? How can the axial alignment of a centrifugal pump be monitored during normal operation? Answer: The reasons and methods for monitoring are as follows: (1) During operation, the pressures on both sides of the impeller in a centrifugal pump are not equal; simultaneously, a reaction force is generated as the fluid leaves the impeller. Together, these forces exert an axial thrust on the impeller, which is why a centrifugal pump generates axial thrust. (2) During normal operation, the axial shift of the pump is monitored through the following aspects: metal friction sounds inside the pump ； Thermal changes in the bearing thrust bearing) lead to a rapid increase in temperature ； The current fluctuates and increases ； Pump outlet pressure fluctuation ； Visually observe the movement of the oil. 43. What is the suction vacuum degree of a water pump? Why is this value specified? The allowable suction vacuum for a water pump refers to the permissible vacuum level at the inlet of the pump. Because when the vacuum at the pump inlet is too high, the liquid at the inlet vaporizes, resulting in cavitation. Cavitation causes great harm to turbines, and it should be avoided at all costs. 44. What are the main reasons for supercooling of condensate water? Answer: The reasons for supercooling of condensate water include: (1) Air accumulation on the steam side of the condenser, which reduces the vapor partial pressure. As a result, the condensate temperature decreases. (2) The condenser water level is too high during operation. Some cooling water pipes were submerged, resulting in subcooling of the condensate water. (3) The cooling water pipes of the condenser are poorly arranged or too densely packed, resulting in a layer of water film forming outside these pipes. The temperature of the outer layer of this water film is close to the saturation temperature of steam, while the inner layer of the film is in close contact with the outer wall of the copper tube, and thus is close to or equal to the temperature of the cooling water. When the water film thickens and hangs down as droplets, the temperature of these droplets is the average temperature of the water film, which is clearly lower than the saturation temperature, resulting in supercooling. 45. What are the advantages and disadvantages of operating coolers in series and in parallel? Answer: The advantages and disadvantages are as follows: (1) The advantages of operating coolers in series include good cooling performance and uniform oil temperature. (2) Disadvantages of series operation of cold oil coolers: high oil pressure drop, and inability to isolate in case of oil leakage. (3) Advantages of parallel operation of coolers: minor drop in oil pressure, easy isolation, and the ability to maintain one set while it is still in operation. (4) Disadvantages of parallel operation of oil coolers: poor cooling effect and uneven oil temperature. 46. Why is an overspeed protection device installed in turbines? Answer: Turbines are high-speed rotating equipment, and the centrifugal stress on their rotating components is proportional to the square of the speed; that is, as the speed increases, the centrifugal stress rises rapidly. When the turbine speed exceeds 20% of the rated speed, the centrifugal stress should be approximately 1.5 times the stress at the rated speed ； At this point, not only will the components that are pressed together within the rotating parts become loose, but the centrifugal stresses will also exceed the strength limit of the material, causing the components to be damaged. To this end, steam turbines are equipped with overspeed protection devices. 47. Explain the droop rate of the speed control system? Answer: The percentage difference between the stable speed N2 of the turbine at no load and its stable speed N1 at full load, relative to the rated speed, is called the speed variation rate of the speed control system; it is denoted by δ, namely δ = (N2 – N1) / N0 × 100%. 48. Explain the speed variation rate of the speed control system? Answer: Due to friction, gaps, and issues such as incorrect throttle setting within the various components of the speed control system, its operation becomes sluggish. In other words, the static characteristic curves of these components are no longer a single line; instead, they are two approximately parallel lines. The percentage difference in rotational speed between the static characteristic curve during acceleration and that during deceleration, at the same power level, is referred to as the sluggishness rate, denoted by ε. ε = △n/n0 × 100%. 49. Explain the basic working principle of impulse steam turbines. Answer: When steam under certain pressure and temperature enters the nozzle, due to the change in the cross-sectional shape of the nozzle along the direction of the steam flow, the pressure and temperature of the steam decrease, its specific volume increases, and its flow velocity rises. That is, the steam expands and accelerates within the nozzle, with thermal energy being converted into kinetic energy. Steam at high velocity flows out of the nozzle and enters the flow channels of the rotor blades; within these curved flow channels, the direction of the steam flow is changed. The steam imparts a thrust on the rotor blades, generating a torque that causes the blades to rotate, thereby driving the shaft to spin and producing mechanical work by converting kinetic energy into mechanical energy. 50. Why do the high- and intermediate-pressure cylinders of large steam turbines adopt a double-layer structure? Answer: Currently, the high- and intermediate-pressure cylinders of large steam turbines produced in China utilize a double-layer structure for two main reasons: (1) In large steam turbines, due to the significant increase in inlet steam pressure, the pressure difference between the inside and outside of the high- and intermediate-pressure cylinders also increases. With a double-layer cylinder structure, this large pressure difference can be shared between the inner and outer layers of the cylinder. This allows the thickness of the flanges on the inner and outer cylinders to be reduced, which is beneficial for the operation of the turbine. (2) The outer cylinder does not come into contact with the hot new steam, thereby reducing the thermal stress on the turbine. In this way, the outer cylinders can be made from lower-grade steel, saving high-quality steel.