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1. What is the function of the turbine oil system? The function of the turbine oil system is as follows: (1) To supply oil to each bearing of the unit in order to lubricate and cool them. (2) The supply regulation system and protective devices receive a stable and sufficient supply of pressure oil, enabling them to function properly. (3) Supply lubricating oil for various transmission mechanisms. Based on the functions of the turbine oil system, it is generally divided into two parts: the lubricating oil system and the control (protection) oil system. 2. What equipment does the steam turbine oil supply system mainly consist of? What roles do they each play? The turbine oil supply system mainly consists of a main oil pump, oil injectors, a steam-driven oil pump, oil coolers, oil filters, pressure reducing valves, an oil tank, etc. Their functions are as follows: The main oil pump serves as the power source for the oil system; during normal operation, it continuously supplies oil to both the lubricating oil and governing oil systems. The steam-driven oil pump or high-pressure electric oil pump (variable-speed oil pump) is also known as the auxiliary oil pump. It is used to supply power oil and lubricating oil when the main oil pump does not function properly during the startup or shutdown of the turbine. It is also used for testing the static characteristics of the system after it has been shut down. Low-pressure electric oil pumps and DC electric oil pumps are generally used to supply lubricating oil to the turbine during its barring operation or in emergency situations. An oil injector, also known as a fuel injector, is a type of pump that uses a small amount of high-pressure oil as power to draw in a large volume of oil, converting it into an oil stream at lower pressure, which is then supplied to the centrifugal main oil pump for feeding and to the bearings as lubricating oil. The tank is used for oil collection and storage, and it also serves to separate bubbles, moisture, impurities, and precipitates. The function of the oil cooler is to cool the lubricating oil that enters the bearings of the turbine. The high-pressure overpressure valve (reducing valve) is used to regulate the oil pressure entering the lubrication system by adjusting the oil flowing through the reducing valve, as the lubricating oil of the unit is supplied from the main oil pump via this reducing valve. The low-pressure overpressure valve (safety valve) operates to discharge some of the oil back into the tank when the lubricating oil pressure becomes too high, thereby maintaining a constant lubricating oil pressure. Oil filters are installed on the lubricating oil and control oil pipelines, primarily to prevent debris in the oil from entering the bearings and the control oil system. 3. What are the basic requirements for the oil system of a steam turbine? The oil supply system of a steam turbine must be safe and reliable; to this end, the oil system should meet the following basic requirements. (1) The design and installation are reasonable, with sufficient capacity and strength; the support structures are secure, the instruments are complete, and there is no vibration in the pipelines during operation. (2) Non-rising stem valves shall not be used in the system; instead, valves with fine-thread stems shall be employed. The check valves must operate smoothly and close tightly. The valve should be installed horizontally or upside down to prevent the valve core from falling out and causing a loss of oil flow. (3) Flange connections should be used as little as possible in the piping system; when flanges must be used, oil- and heat-resistant gaskets should be employed, and the flanges should be covered with metal boxes ； The oil pipe should be kept as far away as possible from heat sources; these heat sources must have solid and intact insulation, and should also be covered with iron sheeting. (4) The oil system must be equipped with an emergency oil tank, which should be located outside the main plant. The emergency oil discharge valve should be installed in a location far from the main oil tank for easy operation. (5) The pipelines, equipment, components, instruments, etc. of the entire system shall be kept clean and free of debris, and there shall be devices to prevent the entry of steam, water, and dust. (6) The oil distribution to each bearing should be reasonable to ensure proper lubrication of the bearings. 4. What is the main structure of a turbine oil tank? The turbine oil tank is generally welded from steel plates, and it contains two layers of filters as well as a clean zone filter to remove impurities from the oil and reduce its flow rate. The bottom is inclined to allow the separated water, sediment, or other impurities to be discharged quickly through the outlet at the very bottom. A fuel level gauge is installed on the fuel tank to indicate the level of fuel. The oil level gauge is also equipped with electrical contacts for the maximum and minimum oil levels; when the oil level exceeds these limits, these contacts close, triggering audible and visual alerts. On larger units, there are two oil level gauges: one is installed in front of the filter, and the other behind it, so as to allow for comparative monitoring. If the readings of the two gauges differ significantly, it indicates that the filter is severely clogged and needs to be cleaned promptly. To prevent the pressure inside the fuel tank from exceeding atmospheric pressure, vent holes are installed on the fuel tank lid; larger engines are equipped with dedicated exhaust fans for removing oil fumes. 5. What is the structure and working principle of the turbine lubricating oil overpressure valve? The overpressure valve of the lubricating oil system, also known as a low-pressure overpressure valve, is usually installed on the outlet pipe of the oil cooler. Its main structure consists of a spool valve, springs, adjustment screws, etc. The lubricating oil enters at the lower part of the spool, exerting an upward force on it. This force balances the downward force exerted by the compression spring on the upper part of the valve. As the lubricating oil pressure increases, the spool moves upward, which increases the amount of oil that flows through the oil port back to the oil tank; as a result, the lubricating oil pressure decreases until it returns to normal. The same principle applies in reverse. Turning the adjustment screw allows the pre-tension of the spring to be changed, thereby altering the opening degree of the oil release port in order to set the lubricating oil pressure. Once the setting is complete, the nut is tightened to prevent the adjustment screw from loosening during operation. 6. Why is it necessary to study the use of fire-resistant oil as a medium in the oil systems of turbine-generator sets? As the unit power and steam parameters continue to increase, the lifting force of the control valves in the control system grows larger. Increasing the oil pressure of the oil-driven motor is one way to address this increasing lifting force of the control valves. However, an increase in oil pressure can lead to oil leaks, and conventional turbine oils have a low ignition point, which can easily cause fires. The auto-ignition temperature of fire-resistant fuel is relatively high, usually exceeding 700°C. In this way, even if it comes into contact with the surface of hot steam pipes, it will not catch fire; anti-fire fuel also makes it impossible for flames to be sustained or spread. Thus, **the threat of fires to the power plant is reduced. Therefore, it has become an inevitable trend in the development of turbines for ultra-high-pressure, high-power units to use fire-resistant oil instead of conventional turbine oil. 7. What are the characteristics of using fire-resistant fuel as the medium in an oil system? The main feature of fire-resistant fuel is its flame resistance, but it also has disadvantages such as being somewhat toxic, expensive, and having poor viscosity-temperature characteristics (that is, temperature has a significant impact on its viscosity). Therefore, the control system and the lubrication system are generally separated into two independent systems. The control system uses high-pressure fire-resistant oil, while the lubrication system uses ordinary turbine oil. 8. What determines the capacity of the main fuel tank? What is the circulation rate of turbine oil? The oil storage capacity of the turbine’s main oil tank is determined by the size of the oil system, and it should be sufficient to meet the oil requirements of the lubrication and control systems. The larger the engine, the more oil is required for the regulation and lubrication systems. The larger the fuel tank capacity, too. The circulation rate of turbine oil is equal to the ratio of the output volume of the main oil pump per hour to the total oil volume in the tank, and it should generally be less than 12. If the circulation rate is too high, the turbine oil stays in the tank for a short time, leaving no time for air and moisture to be separated, which causes the quality of the oil to deteriorate rapidly and shortens its service life. 9. What determines the speed control oil pressure and lubricating oil pressure of a steam turbine? The control system of a steam turbine typically uses oil to transmit signals and as power to drive the actuators, which in turn open and close the control valves and the main steam valve. To ensure rapid and sensitive adjustment, a certain speed control oil pressure must be maintained. The commonly used speed control oil pressures for steam turbines include 0.4–0.5 MPa, 1.2–1.4 MPa, 1.8–2.0 MPa, etc. Generally speaking, a high oil pressure enables the control system to operate with higher sensitivity, and reduces the physical size of the hydraulic actuators and throttle mechanisms. The speed control oil pressure has reached as high as 4.0 MPa in some cases. However, excessive oil pressure can lead to oil leakage and fire. The lubricating oil pressure for steam turbines is calculated during the design phase based on factors such as the rotor’s weight, rotational speed, the structure of the bearing bushes, and the viscosity of the lubricating oil. This ensures that a proper oil film can be formed between the journal and the bearing bushes, while also providing sufficient oil flow for cooling purposes. Therefore, the lubricating oil pressure in steam turbines typically ranges from 0.12 to 0.15 MPa. Excessively high lubricating oil pressure can cause oil leakage from the oil seals and bearing vibration. Low oil pressure results in a poor formation of the oil film, and may even cause loss of oil supply that damages the bearing shells. 10. What are the different types of main oil pumps for steam turbines? The main oil pumps for steam turbines are mainly divided into positive-displacement oil pumps and centrifugal oil pumps; positive-displacement pumps include gear oil pumps and screw oil pumps. Today, high-power units all use centrifugal oil pumps with direct spindle drive. 11. What are the advantages and disadvantages of positive-displacement oil systems? The greatest advantage of positive-displacement oil pumps is their reliable oil suction. The drawback is that its operating speed is low; it cannot be driven directly by the main shaft. When the oil motor operates and consumes a large amount of oil, the oil pressure at the pump outlet drops significantly, affecting the rapid response of the control system. 12. What are the characteristics of centrifugal oil pumps? The advantages of centrifugal oil pumps are: (1) high speed, as they can be driven directly by the turbine shaft without the need for any reduction gear. (2) The characteristic curve is relatively flat; when the control system operates with a large amount of oil, the oil pump’s output increases while the outlet oil pressure drops only slightly, which meets the requirement for rapid operation of the control system. Disadvantages of centrifugal oil pumps: The inlet of the oil pump is under negative pressure, and any entry of air can cause the pump to malfunction. Therefore, a dedicated oil injector must be used to supply oil to the main oil pump in order to ensure its reliable and stable operation. 13. How does the oil injector work? The oil injector consists of a nozzle, filter screen, diffuser tube, mixing chamber, etc. An oil injector is a type of jet pump whose working principle is as follows: high-pressure oil is ejected at high speed through an oil nozzle, creating a vacuum in the mixing chamber; as a result, the oil in the tank is drawn into the mixing chamber. The high-speed oil flow drives the surrounding low-speed oil flow, and after mixing in the mixing chamber, it enters the diffuser tube. As the oil flows through the diffuser tube, its velocity decreases and the pressure rises, until it exits at a certain pressure to be supplied to the system. The filter installed at the inlet of the oil injector is intended to prevent debris from clogging the nozzle. 14. What are the two ways in which oil injectors can be arranged in a system? The oil injector can be connected in parallel or in series within the oil system. In series oil injectors: the oil from the outlet of the first-stage injector is supplied, one path to the inlet of the main oil pump and the other path to the second-stage injector; the oil from the outlet of the second-stage injector is used as oil for the lubrication system. In parallel configuration, as in the case of 100MW and 125MW units, the oil injectors are arranged as follows: the first stage of injectors supplies oil for the inlet of the main oil pump, while the second stage of injectors provides oil for lubrication purposes. This connection method avoids throttling losses, is cost-effective, and is widely used. 15. Why is a smoke exhaust fan installed in the turbine oil tank? The function of an exhaust fan installed in the fuel tank is to remove gases and water vapor from the tank. This prevents water vapor from condensing in the fuel tank on one hand ； On the other hand, it keeps the pressure in the tank below atmospheric pressure, allowing the bearing return oil to flow smoothly into the tank. Conversely, if the fuel tank is sealed, a large amount of gas and water vapor will accumulate inside the tank, creating positive pressure that can affect the return flow of oil to the bearings; it also makes it easy for water to accumulate in the fuel tank. Range hoods also help to remove harmful gases, preventing the oil quality from deteriorating. 16. Why is a drain pipe installed at the bottom of the fuel tank? During the operation of the turbine, water can end up in the turbine oil due to reasons such as excessive steam leakage from the shaft seals, poor drainage from the steam-driven oil pumps, and excessive leakage at the water-inlet flanges of the water-cooled generator rotor. Once these water-containing oils return to the fuel tank, due to the higher density of water, the water separates from the oil and settles at the bottom of the tank. Removing this water in a timely manner prevents the separated water from mixing back with the oil, thereby avoiding a deterioration in the quality of the oil. That’s why there are drain pipes at the bottom of the fuel tanks. 17. What are the hazards of degraded turbine oil quality? The quality of turbine oil is crucial for the proper operation of turbines.**** The deterioration of the oil quality alters the properties of the lubricant as well as the strength of the oil film, resulting in inadequate lubrication of various lubricated components; this in turn causes the bearing materials to melt and get damaged ； It can also cause the components of the control system to corrode and rust, leading to sticking and serious consequences such as the failure of the control system and protective devices to function properly. Therefore, it is necessary to pay attention to the supervision of the quality of turbine oil. 18. What are the quality indicators for turbine oil? There are many indicators for the quality of turbine oil, with the main ones being viscosity, acid value, acidity and alkalinity reactions, emulsification resistance, and flash point. In addition, transparency, freezing point temperature, and mechanical impurities are also criteria for assessing oil quality. 19. What is the viscosity of turbine oil? What is the viscosity index? Viscosity is the standard for determining the blending and thinning of turbine oil. When viscosity is high, the oil becomes thick and doesn’t flow easily ； When viscosity is low, the oil becomes thin and fluid; it’s easy to flow. Viscosity is measured in Eng units, with the viscosity of commonly used turbine oils ranging from 2.9 to 4.3 Eng units. Viscosity has a significant impact on the lubrication performance of bearings; too high viscosity can cause the bearings to overheat, while too low viscosity can lead to the breakdown of the oil film. When the oil quality deteriorates, its viscosity increases. 20. What is the acid number of turbine oil? What is an acid-base reaction? The acid value indicates the amount of acids present in the oil. It is calculated based on the number of milligrams of potassium hydroxide required per gram of oil to neutralize it. The acid value of the new turbine oil should not exceed 0.04 KOH mg/g of oil. When oil deteriorates, the acid value rises rapidly. The acidity or alkalinity reaction refers to whether the oil is acidic or alkaline. Good turbine oil should be neutral in nature. 21. What is anti-emulsification degree? What is the flash point? Anti-emulsification degree is the ability of an oil to separate rapidly from water, and it is expressed by the time required for separation. The good emulsification resistance of turbine oil should be no more than 8 minutes; when organic acids are present in the oil, this emulsification resistance decreases. The flash point refers to the temperature at which, when turbine oil is heated to a certain degree, part of the oil turns into gas and can catch fire upon being exposed to flame; this temperature is known as the flash point (also called the ignition point). Since the temperatures in turbines are very high, the flash point cannot be too low. A good turbine oil should have a flash point of no less than 180°C. When the oil quality deteriorates, its flash point decreases. 22. Why must ventilation holes and steam pipes be installed on the turbine bearing cover? Generally, a negative pressure exists inside bearings, usually due to the suction effect of the oil flowing out of them. Due to the negative pressure formed inside the bearing, steam is drawn in along with water droplets, which then condense. To prevent negative pressure from forming inside the bearing, ventilation holes or vents are provided on the bearing cover to connect with the atmosphere. On the other hand, providing an air vent on the bearing cover can also serve to remove the vapor generated by the turbine oil in the bearings due to heating, thereby preventing the pressure inside the bearing housing from exceeding atmospheric pressure. During operation, care should be taken to keep the ventilation holes unobstructed to prevent blockages. The ventilation hole of the front bearing cover of a turbine in a factory became blocked, causing combustible gases to accumulate in the bearing housing; these gases were ignited by electrical sparks inside the bearing housing, resulting in an explosion of the front bearing housing. 23. What is the task of the turbine control system? The basic task of the turbine control system is to promptly adjust the power output of the steam turbine in response to changes in external load, so as to meet the varying electricity demands of users; simultaneously, it ensures that the operating speed of the turbo-generator set remains within the normal permissible range. 24. What requirements should a control system generally meet? The control system shall meet the following requirements: (1) It should be able to maintain no-load operation when the main steam valve is fully open. (2) When the load drops suddenly from full load to zero, it enables the turbine speed to remain below the speed at which the emergency shutdown device activates. (3) When the load is increased or decreased, the control system should operate smoothly without any oscillations. (4) When the emergency safety device activates, it is necessary to ensure that the high- and medium-pressure main steam valves as well as the control steam valves close quickly. (5) The vehicle with a speed regulation system should meet the requirements (generally 4%–6%); the lower the delay rate, the better, with it generally should be below 0.5%. 25. How many types of steam inlet control methods are there for turbines? What are their respective advantages and disadvantages? There are three methods for regulating the steam supply to a turbine: (1) Throttling regulation: Throttling regulation is also known as mass regulation. The steam supply to the turbine enters all nozzles through one or several control valves that are switched on simultaneously. This regulation method only works when the rated load is applied, with the control valve fully open to minimize throttling losses; this is when the turbine achieves its highest efficiency. When the load decreases, the control valve is closed to a smaller degree, allowing throttling of the steam within that valve and thereby reducing its pressure before it enters the turbine. Due to throttling, there is throttling loss, which also reduces the efficiency of the turbine. (2) Nozzle adjustment method: also known as flow interruption adjustment method. The amount of steam entering the turbine is controlled by several control valves that are opened and closed in sequence, entering the first-stage nozzles of the turbine to adjust its load. Each control valve operates a set of nozzles, and the number of control valves that are opened is determined based on the level of load. When each control valve is not fully open, there is also throttling loss, but this constitutes only a portion of the total fresh steam; therefore, at low loads, it results in less throttling loss compared to throttling control, offering better economic efficiency. The downside is that adjustment during maintenance and installation is rather complex ； The temperature of the steam chamber changes significantly under varying operating conditions; therefore, the load must not change too rapidly. (3) Bypass regulation method: This method is widely used in turbines that employ throttling regulation, especially reaction turbines. Usually, at the economic load of the turbine, the main control valve is fully open. When the economic load is exceeded, the bypass valve is opened to direct the new steam to the subsequent stages of blades. Its advantage is that it achieves the highest operating efficiency at economic (design) load conditions, with the least throttling loss. Its drawback is that when the economic load is exceeded, steam bypasses into the system; consequently, the proportion of high-quality metal materials increases, and its efficiency decreases due to the throttling losses of the bypass valve and the rise in pressure within the bypass chamber. 26. Why must a feedback device be installed in the turbine control system? In the turbine control system, the displacement of the slide valve causes the hydraulic actuator to move. The operation of the hydraulic actuator, in turn, affects the displacement of the spool valve; this effect is called feedback. Feedback is an important component in the automatic control of turbines, ensuring that the control actions remain stable. Without feedback, the control system will not be able to function. Common feedback mechanisms include lever feedback, window feedback, and spring feedback. 27. What is the overlap of the regulating valves? Why must there be overlap? Turbines with nozzle regulation generally have several control valves. When the preceding control valve has not yet been fully opened, the subsequent control valve is opened, which means that the control valves have a certain degree of overlap. The overlap degree of the control valves is usually around 10%; in other words, when the previous control valve opens to a back-pressure that is about 90% of the front-pressure, the subsequent control valve then opens. If there is no overlap in the control valves, the characteristic curve of the actuator will have irregularities; as a result, the static characteristics of the control system are not a smooth curve. Such a control system cannot operate smoothly, which is why there must be some overlap between the control valves. 28. What are the static and dynamic characteristics of a control system? The operating characteristics of a control system are of two types: dynamic characteristics and static characteristics. Under stable operating conditions, the relationship between the turbine’s power and speed is the static characteristic of the control system. The characteristics of the transition process from one stable operating condition to another are known as the dynamic characteristics of the control system; they refer to the patterns of change over time in parameters such as the unit’s power, speed, and the opening degree of the control valves during this transition. 29. What is the static characteristic curve of a control system? What are the requirements for the static characteristic curve? The static characteristic curve of the control system is the curve that shows the relationship between load and speed under stable conditions. The static characteristic curve of the regulating system should be a smoothly decreasing curve; there should be no horizontal portions in the middle, and both ends of the curve should be relatively steep. If there is a horizontal section in the middle, it will cause spontaneous fluctuations or instability in the load during operation. The left end of the curve is steeper, which mainly helps the turbine to stabilize at a certain speed for the paralleling and disconnection of the generator. It also reduces the impact of external load fluctuations on the unit under low loads after grid connection. The steeper slope on the right side is intended to maintain a stable and economical load for the unit; when the grid frequency drops, it allows the turbine to handle a lower load, thereby preventing overloading of the turbine. 30. What is the speed variation rate of a control system? What are the requirements for the rate of speed change? From the static characteristic curve of the control system, it can be seen that when the unit operates alone, from no-load to rated load, the speed of the turbine decreases from n2 to n1. The ratio of this speed change to the rated speed n0 is known as the speed variation rate, denoted by δ; that is, δ = (n2 – n1)/n0 × 100%. Control systems with a smaller δ have the advantage of being able to respond flexibly to changes in load, making them suitable for units used for frequency regulation ； Regulation systems with a larger δ exhibit good load stability and are suitable for units that provide basic load ； If δ is too large, the unit is prone to overspeeding during load rejection ； If δ is too small, the control system may experience oscillations; therefore, it is generally set between 4% and 6%. The speed variation rate is related to the static characteristic curve; the steeper the curve, the greater the speed variation rate, and vice versa. 31. What is the delay rate of a control system? During operation, the control system must overcome the frictional forces within the various moving components. Additionally, due to factors such as gaps between components and their degree of overlap, the static characteristics differ during acceleration and deceleration, resulting in two curves that are almost parallel to each other. In other words, the rotational speed must be varied by a certain amount; only after overcoming the resistance and clearance does the adjustment of the valve’s reverse movement begin. The ratio of the maximum possible speed variation △n under the same load to the rated speed n0 is called the sluggishness rate (also known as insensitivity), which is usually denoted by the letter ε, i.e., ε=△n/n0×100%. 32. What is the impact of an excessively high delay rate in the control system on the operation of the turbine? The effects of an excessively high delay rate in the control system on the operation of the turbine are as follows: (1) when the turbine is operating at no load ； An excessive delay rate of the control system will cause instability in the turbine speed, thereby making parallel operation difficult. (2) After the turbine is connected to the grid, excessive droop rate will cause fluctuations in the load. (3) When the unit load is suddenly reduced to zero, an excessive rate of change prevents the control valves from closing immediately, resulting in a sudden increase in speed and thus triggering the emergency shutdown device. If the emergency safety device fails to function, it can lead to serious accidents such as a vehicle running at excessive speed. 33. Why can’t the delay rate be equal to zero? (1) The actual delay rate of the control system cannot be made equal to zero. Due to the friction and other resistances present in the various components of the control system during operation, the spool valve of the hydraulic actuator must always have a certain degree of backlash, which results in a delay in the response of the system to changes in speed, leading to corresponding delays in changes in the opening degree of the control valves. (2) Theoretically, a control system with a zero delay rate is unstable, as this leads to excessive sensitivity in regulation, causing the control valves to be in constant motion. Especially for hydraulic control systems, maintaining a slight degree of delay is beneficial for improving control performance. In a hydraulic control system, fluctuations in the control oil pressure are inevitable, and these fluctuations can cause the control valve to shift. This is also why a certain degree of over-tightening is necessary for the throttle valve, so as to counteract the effects of pressure fluctuations and prevent any movement of the valve stem. The optimal delay rate is ε=0.3%–0.4%. 34. What are the advantages of using an electro-hydraulic control system? The use of an electro-hydraulic control system has the following advantages: (1) The use of electrical components increases the precision of the control system, reduces the lag rate, enables rapid reduction of power output in the event of load rejection, and improves dynamic overspeed performance. (2) Achieve full-range adjustment of rotational speed to control the steady acceleration of the turbine. (3) It can participate in primary frequency regulation of the power grid based on the selected static characteristics (the slope of which can be easily improved, as well as the maximum amplitude of frequency modulation), in order to meet the requirements of the generators, boilers, the power grid, and other elements. (4) The use of a power system provides resistance to internal disturbances and improves the frequency regulation dynamics, thereby enhancing the unit’s adaptability to loads. (5) It can be easily integrated with machines, furnaces, and main control devices to achieve automatic control of machinery, electricity, and furnaces. 35. Why must turbines be equipped with protective devices? To ensure the safety of turbine equipment and prevent damage to it, in addition to requiring a reliable operation of the control system, it is also necessary to have appropriate protective devices. These devices enable the turbine to respond promptly and shut down quickly in case of control system failures or other accidents, thereby avoiding damage to the equipment. The protection devices themselves must be particularly reliable; moreover, the greater the capacity of the turbine, the more severe the consequences of any accident, which means that higher reliability is required for these protection devices. 36. What is the function of the automatic main steam valve? The function of the automatic main steam valve is to quickly cut off the steam supply and stop the operation of the turbine after the turbine protection system activates. Therefore, it is the actuator of the protective device. 37. What are the requirements for the automatic main steam valve? To ensure safety, it is required that the automatic main steam valve act quickly and close tightly. For high-pressure steam turbines, under normal steam inlet conditions and exhaust pressure, after the main steam valve is closed automatically (with the control valve fully open), the turbine speed should be able to drop below 1000 r/min. The time from the activation of the turbine protection system to the complete closure of the main steam valve is generally required to be no more than 0.5–0.8 seconds. 38. What is the function of the automatic main steam valve actuator? To prevent sticking caused by the main steam valve remaining stationary for long periods, a moving mechanism is generally installed on the main steam valve. For medium-pressure main steam valves equipped with hand wheels, when operating them, turn the hand wheel a few turns to close the valve slightly, and then open it fully. The dynamic test of the high-pressure automatic main steam valve produced by Shanghai Steam Turbine Factory involves releasing a portion of the oil pressure in the control room through a pilot valve, thereby closing the main steam valve. When the piston of the hydraulic actuator moves to a certain stroke (usually 5 mm), the oil drain port is blocked, and the main steam valve will not close any further. After closing the test valve, the main steam valve was opened fully once again. 39. What are the advantages and disadvantages of combining the medium-pressure main steam valve and the medium-pressure control steam valve into a combined valve? In intermediate reheat units, the main steam valve and the control steam valve after reheating (i.e., the medium-pressure main steam valve and the medium-pressure control valve) are combined within the same valve body to form a combined valve. During operation, first open the main steam valve equipped with a pre-actuation valve, and then open the control steam valve. This structure is compact, resulting in minimal steam flow losses. However, the protruding end of the main steam valve stem is relatively long, making it prone to jamming; additionally, the pre-opening valve is susceptible to wear. 40. What are the advantages of a pre-actuation valve structure on the main steam valve? The valve disc of the main steam valve in a high-pressure steam turbine is relatively large, and the steam pressure is very high. Before the valve opens, there is a significant pressure difference across it; thus, a great amount of hydraulic power is required to open it. Consequently, the servo-motor for the actuator must also be designed to be quite large. After the main steam valve is equipped with a pre-opening valve structure, the lifting force required to open the main steam valve is significantly reduced, resulting in a more compact control device structure. 41. Why are the main steam valves usually opened by hydraulic pressure and closed by spring force? This is because in any accident scenario, including when the oil supply is cut off, the automatic main steam valve should still be able to close quickly. Therefore, generally, main steam valves are designed to be closed by spring force. For reliability, a double-spring structure is generally used as well. To achieve a sufficiently large closing force and rapid closure, generally, when the main steam valve is fully closed, the spring still exerts a compressive force of 5,000–8,000 kN on the main steam valve. 42. What are the specific requirements regarding the tightness of the automatic main steam valve? For high-pressure steam turbines, it is required that at the rated initial steam parameters and back pressure, with the main steam valve fully closed (and the control valve fully open), the unit’s speed can be reduced to below 1000 r/min. For medium- and low-pressure steam turbines, it must be ensured that after the main steam valve is fully closed (and the control valves are fully open), the turbine speed can drop to zero. 43. Why are overspeed protection devices installed on steam turbines? A steam turbine is a high-speed rotating device, and the centrifugal force on its rotating components is proportional to the square of the rotational speed; in other words, as the speed increases, the centrifugal stress rises rapidly. When the turbine speed exceeds the rated speed by 20%, the centrifugal stress approaches 1.5 times the stress at the rated speed. At this point, not only do the components that are fitted together with interference fit become loose, but the centrifugal stress also exceeds the allowable strength of the material, leading to component damage. For this reason, superspeed protection devices are installed in all turbines; these devices activate when the speed exceeds the rated speed by 8% to 12%, quickly shutting off the steam supply and causing the turbine to stop operating. 44. What are the two types of emergency stop devices? Based on their structural differences, emergency safety devices can be divided into pendulum-type and ring-type. Their working principles are exactly the same. The basic principle is that when the turbine speed reaches the speed at which the emergency shutdown device is designed to activate, a flying weight (or flying ring) is ejected to strike the trip mechanism, causing the emergency shut-off valve (emergency shut-off throttle) to operate and closing the main steam valve and control valves, thereby bringing the turbine to a rapid stop. 45. What are the structural differences between a flying-ring type emergency safety device and a flying-weight type emergency safety device? Turbines produced by Shanghai Turbine Works and Beijing Heavy Electrical Machinery Factory typically use fly-ring type emergency safety devices. The main difference between it and the flyweight type emergency protector is that it uses a flying ring with an eccentric weight, mounted on the turbine shaft, in place of an eccentric flyweight. When the turbine speed rises to the operating speed, the centrifugal force of the eccentric ring overcomes the spring force and causes it to fly outward. The spinning speed of the flying ring can also be adjusted by changing the spring force via the adjusting nut. 46. What is the function of the turbine axial displacement protection device? What are the different types? The axial clearance between the turbine rotor and stator is very small. When the axial thrust on the rotor becomes excessive, causing the bearing material to melt, the rotor will experience unauthorized axial movement, which leads to friction between the rotating and stationary parts and results in severe damage to the equipment. Therefore, turbines are equipped with axial displacement protection devices. Its function is to emit an alarm signal when the axial displacement reaches a certain value ； When the axial displacement reaches a dangerous level, the protection device activates, shuts off the steam supply, and stops the machine. Based on the structure of their sensing elements, axial displacement protection devices can be divided into three main categories: mechanical, hydraulic, and electrical. The first two types are commonly used in small and medium-sized steam turbines, while the latter ones are used in high-power steam turbines. 47. What is the function of the low oil pressure protection device? Too low lubricating oil pressure can lead to the breakdown of the lubricating oil film, which not only damages the bearing shells but also can cause serious problems such as friction between moving and stationary parts. Therefore, lubricating oil low-pressure protection devices are installed in the oil systems of steam turbines. Low oil pressure protection devices generally serve the following functions: (1) When the lubricating oil pressure falls below the required level, they first send a signal to alert the operators to take action promptly. (2) When the oil pressure continues to drop to a certain value, an auxiliary oil pump (AC or DC oil pump) is automatically activated to increase the oil pressure. (3) After the auxiliary oil pump starts, if the oil pressure continues to drop to a certain level, the machine should be shut down; if it drops any further, the crankshaft rotation should also be stopped. When the oil pressure at the outlet of the turbine’s main oil pump is too low, it can compromise the operation of the control and protection systems. Generally, when this oil pressure drops to a certain level, the high-pressure auxiliary oil pump (speed control oil pump) starts up automatically to ensure the proper operation of the turbine. 48. What is the function of a low-vacuum protection device? A decrease in vacuum during turbine operation not only affects the turbine’s output and efficiency, but an excessive drop in vacuum can also compromise the turbine’s safety due to high exhaust temperatures and increased axial forces. Therefore, turbines with higher power are equipped with low-altitude air protection devices. When the vacuum level drops to a certain value, an alarm signal is issued; when it reaches the specified limit, the machine will shut down automatically. When the vacuum drops to zero and positive pressure is generated in the exhaust of the condensing steam turbine, it also causes the exhaust cylinder safety valve (atmospheric valve) to activate, thereby protecting the turbine from damage. 49. What is the purpose of the emergency safety device oil filling test apparatus? Due to the strength constraints of the unit, overspeed tests should not be conducted frequently. To ensure the proper functioning of the overspeed shutdown devices and to avoid excessive overspeed testing of the units, large and medium-sized units are equipped with oil-filling testing devices on their emergency safety devices, allowing for functional tests during normal operation as well as tests to verify that overspeed does not occur during startup. 50. Why is differential expansion protection required for steam turbines? During the startup, shutdown, and under abnormal operating conditions of a turbine, a temperature difference arises because the rotor heats (or cools) faster than the cylinder, resulting in a differential expansion (referred to as differential expansion). Whether it is positive differential expansion or negative differential expansion, once a certain value is reached, the stationary and moving parts of the turbine will come into contact and experience friction. To prevent static and dynamic friction caused by excessive differential expansion, large-scale units are generally equipped with differential expansion protection. When the positive or negative differential expansion reaches a certain value, this protection activates, closing the main steam valve and control valves to achieve an emergency shutdown. 51. What is the function of the steam extraction check valve interlock? The function of the extraction steam check valve interlock is primarily to prevent the turbine from speeding up due to steam from the extraction pipelines and regenerative heaters flowing back into the cylinder after the main steam valve and control valves are closed. This is even more important for large units. The commonly used extraction steam hydraulic check valve operates when the main steam valve is closed and the generator is disconnected, using the force of pressurized water or springs to quickly and forcefully close the extraction steam check valve.