Compressor knowledge

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1. What is resonance? How to avoid resonance in the engine assembly? Answer: Resonance is a resonant phenomenon in which, when an object’s inherent natural frequency is equal to or a multiple of the frequency of an external exciting force, the amplitude of the object’s vibration increases suddenly. To avoid rotor resonance, it is sufficient to change the operating speed of the rotating body, so that its rotation frequency is different from its inherent frequency. Change the pipe connection method to avoid resonance between the unit and the pipes. Properly design support points on the pipeline to change its vibration frequency. 2. What are the hazards of resonance? Answer: During operation, the blades (or blade sets) are subjected to periodic excitation forces, which cause forced vibration. When the frequency of these periodic excitation forces is equal to or a multiple of the natural vibration frequency of the blades, it results in intense vibration of the blades (or blade sets), with a sharp increase in amplitude. This phenomenon is known as resonance. Resonance is a dangerous condition with the following hazards: 1) The amplitude of the blade increases sharply, causing fatigue cracks to form in the blade over a short period of time, or even leading to damage ； 2) After individual leaves break, their fragments can damage adjacent leaves ； 3) It may cause the rotor to lose balance and experience severe vibrations, leading to serious consequences. 3. Why do turbines avoid operating at critical speed? Answer: Every rotor has a certain natural frequency of vibration. When the operating speed equals this natural frequency or an integer multiple of it, resonance occurs, causing the amplitude of the rotor’s vibration to increase sharply. Beyond this speed, the vibration amplitude gradually decreases as the speed increases, and it stabilizes within a certain range. The speed at which the rotor’s vibration amplitude is at its maximum is known as the rotor’s critical speed. If the rotor operates at its critical speed, severe vibration will occur, and the bending of the shaft will increase significantly. Prolonged operation at this speed can lead to serious bending or even breaking of the shaft. Therefore, it is necessary to pass through the critical speed quickly during startup and shutdown to avoid operating the rotor at that speed.     4. What tasks need to be carried out when the unit exceeds criticality? Answer: Before reaching the critical speed, it is necessary to communicate promptly with all relevant departments, and pay attention to fluctuations in the steam pipeline network ； When exceeding the critical speed, pay close attention to monitoring the vibration and temperature changes of each bearing in the unit ； Operation status of the control valve. After passing the critical speed, close all the drain valves of the turbine and compressor casings, adjust the flow rate of water in the oil coolers promptly to maintain an appropriate oil temperature, and pay attention to checking the liquid level in the surface condenser as well as the liquid levels in the separators between various sections. 5. What should be noted during the process of the unit accelerating past its critical speed? Answer: 1) Pay attention to checking the vibration condition of the unit as well as the temperature of each bearing ； 2) Pay attention to changes in steam pressure, flow rate, and unit speed ； 3) Proceed quickly at the prescribed speed; do not stop ； 4) Check the flow and pressure at each stage of the compressor ； 5) Adjust the cooling water volume in a timely manner. 6. What are the hazards of operating at the critical speed? Answer: Operating continuously at the critical speed can, at the least, increase rotor vibration, and in severe cases, it can lead to accidents; especially when the rotor is poorly balanced, the vibration will be even greater. This can lead to blade damage or breakage, wear of bearings and steam seals, and even breakage of the main shaft. Therefore, during the start-up acceleration process, the unit should quickly pass through the critical speed; it is not allowed to remain at or near the critical speed for an extended period of time. 7. After passing the critical speed, the unit must operate stably for a period of time. What is the purpose of this? Answer: The reason for allowing the unit to operate stably for a period of time after reaching the critical speed is as follows: 1) After passing the critical speed, a thorough inspection of the unit is necessary to determine whether the excessive vibration at that speed has caused any abnormalities or damage. Special attention should be paid to the bearings, shaft seals, and the sounds emitted by the equipment; if vibrations lead to abnormalities, the process of increasing speed should be halted and appropriate measures taken ；     2) When passing through the critical speed, the speed increases rapidly, resulting in a significant increase in steam volume; this also leads to large temperature differences in the metal components. To avoid excessive temperature differences, uneven expansion, and thus thermal stress and vibration, stabilizing the warm-up process for a period of time after passing the critical speed can help reduce these temperature differences. 8. What are the factors that affect the critical speed? Answer: The critical speed of the rotor is primarily determined by the mass and stiffness of the rotor; in addition, it is also affected by the following factors: 1) the elasticity of the supports ； 2) Rotational torque of the impeller ； 3) Length of the rotor’s protruding end ； 4) Critical speed of the shafting ； 5) Damping (such as bearing oil film damping) ； 9. What is surge? Answer: Surge is a phenomenon that occurs when the gas flow rate changes due to external factors, resulting in the compressor outlet pressure being lower than the pressure in the pipeline system. This causes gas to flow back from the pipeline system into the compressor. When the gas pressure at the compressor outlet becomes higher again than the pressure in the pipeline system, the gas is sent back into the pipeline system via the compressor. Once the pressure in the pipeline system rises above the compressor outlet pressure, gas flow reverses once more, and this cycle repeats itself, leading to periodic fluctuations in gas flow and intense vibrations in the unit. This phenomenon is known as surge. 10. What are the hazards of surge? Answer: 1) Surge is extremely harmful to the unit; it causes intense vibration of the impeller when the rotor comes into contact with stationary components. It may damage compressor components such as sealing \"O\"-rings, exert force on the thrust bearings, disrupt the stability of the bearing oil film, and damage the bearings. It may damage the oil sealing system, disrupt the pressure difference required for the oil film seal, and lead to failures in the oil film seal. In severe cases, it can damage the seals and bearings, and even lead to the leakage of pressurized gas, causing catastrophic accidents such as explosions.     2) It may damage the installation quality of the machine, disrupt the clearance values between various components, and even cause deformation of the shafts, leading to increased vibration in the machine during subsequent operation. 3) When the compressor experiences surging, the vibration levels and displacements in each section increase significantly, while the inlet pressure, outlet pressure, inlet flow rate, and unit speed experience large fluctuations. This can cause some related instruments to fail or reduce their accuracy. 11. How to eliminate surge in a centrifugal compressor? Answer: Once surging occurs, the flow rate should be immediately increased (by opening the outlet relief valve or the bypass valve between the inlet and outlet—the anti-surge valve—to reduce the outlet pressure) to eliminate the danger. Subsequently, the cause of the surging must be identified and eliminated. It is also necessary to check whether there is any damage to the machinery before resuming normal operation. If it is required to maintain system pressure, after allowing gas to flow back by opening the anti-surge valve, the rotational speed should be appropriately increased so that the outlet pressure returns to its original level. 12. How is the surge control line determined? Answer: During design, on the curve that shows the inlet flow rate (Q) versus compression ratio (Pd/Ps) for all possible operating points of the compressor, there is a \"surge point\" on each curve, representing the maximum pressure head at a certain speed. The surge points at various speeds are connected to form what is known as the \"surge line\" (SLL). To prevent the unit from operating near this surge line, a protective line called the \"surge control line\" (SLC) is established. When the operating point reaches this line, the control system activates the anti-surge valve to keep the operating point away from unstable conditions. 13. Why should the anti-surge valve be fully opened when the compressor is shut down? Answer: When the compressor stops, the flow rate drops suddenly, which can easily cause the compressor to enter the surge zone and experience surging; in severe cases, this can damage the equipment. Therefore, during the shutdown process, the surge valve must be fully opened to prevent surging in the compressor. 14. What is the relationship between surging and the pipeline network? Answer: Centrifugal compressors and pipeline networks are connected to form a closed system for transporting gaseous media. The compressor and the pipeline network operate together within this system, thereby providing the necessary conditions for pressurizing and transporting the gaseous media. If the flow rate of the gas being transported by the compressor continues to decrease, and when the inlet flow rate drops to a certain level, separation vortices begin to form within the impeller’s flow channels. As the flow rate decreases further, these separation vortices expand, leading to severe rotational separation phenomena; the gas flow condition deteriorates significantly, and the discharge pressure of the compressor drops greatly. At this point, gas from the pipeline system flows back into the compressor, until the pressure at the compressor’s outlet exceeds that in the pipeline system, after which the compressor resumes gas discharge. This periodic oscillation of the gas flow within the system is known as compressor surge.     The reduction in compressor flow rate, along with severe rotational separation of the gas within the impeller flow channels, are the internal causes of surge. As the flow rate decreases, the pipeline network performance curve shifts continuously to the left and intersects the compressor performance curve in the surge region, resulting in surge conditions; therefore, the shift of the pipeline network performance curve to the left is a condition for surge to occur. Therefore, surge is not only related to the rotational separation of gas in the impeller flow channel, but also closely associated with the characteristics of the piping network; the larger the capacity of the piping network, the greater the amplitude of surge and the lower its frequency ； The smaller the network capacity, the smaller the amplitude of surge, and the higher the surge frequency. 15. How do changes in the compressor inlet conditions affect its performance? Answer: The main factors that may change in the inlet conditions of chemical compressors are the inlet temperature, inlet pressure, and molecular weight of the inlet gas; these three factors have a significant impact on the compressor’s performance. 1) The effect of intake air temperature at constant speed and constant volumetric flow rate. a. The inlet temperature is proportional to the mass flow rate. b. As the temperature decreases, the pressure ratio will increase ； The opposite is true in reverse. c. Intake temperature is also inversely proportional to power: as the temperature rises, the power decreases, and vice versa. 2) The effect of inlet molecular weight: at a constant volumetric flow rate, an increase in molecular weight leads to an increase in the pressure ratio ； Conversely, as the pressure drop ratio decreases, the compressor power is proportional to the molecular weight. 3) Effect of intake pressure Similar to the molecular weight of the intake gas, the intake pressure is proportional to the mass flow rate. The intake pressure does not affect the pressure ratio; therefore, the exhaust pressure is proportional to the intake pressure, and the compressor power is also proportional to the intake pressure. 16. What types of maintenance should be carried out on centrifugal compressors? Answer: To achieve long-term fault-free operation of centrifugal compressors, proper use and maintenance are essential. Depending on the operating conditions of the unit, the maintenance tasks include the following aspects: 1) The lubrication system should have its various parameters such as temperature, pressure, pressure difference, and liquid level checked at intervals specified in the operational procedures. Whenever deviations from the specified parameters are detected, adjustments should be made promptly to ensure the proper functioning of the lubrication system. 2) According to the time specified in the procedural regulations, the sealing system should have its temperature, pressure, pressure difference, and liquid level readings checked at various points. If any deviation from the specified operational parameters is detected, adjustments should be made promptly to ensure the proper functioning of the sealing system. 3) The process and steam systems are inspected, at specified times and along designated routes, for the temperature, pressure, and liquid level readings at various points within these systems. Should any deviations from the specified operational parameters be detected, adjustments are made immediately to restore normal operation of the process and steam systems. 4) The main machine is the subject of inspection and maintenance; it is necessary to strictly check, at regular intervals, the vibration of various bearings, their temperature rise, oil return conditions, rotation speed, and shaft displacement readings. If any deviations from the specified operational parameters are detected, effective measures must be taken to eliminate the causes of the problems and restore normal operation of the main machine. 5) Take proper measures to prevent freezing and condensation of equipment, valves, and pipelines, so as to avoid damage to the equipment and blockages in the pipelines that could affect production. 6) Based on the inspection results, address any issues that are identified promptly, eliminate problems such as dirt, looseness, disorder, and shortages in the equipment, thereby improving its operational reliability. Regularly clean the working area and the equipment to ensure civilized production practices.     17. What is a warm pipe? Answer: When cold steam pipes are fed with steam, there is a process of thermal expansion. To ensure sufficient thermal expansion, there must be enough time to gradually increase the temperature and pressure (while taking care to drain condensate), so as to reach the rated values over time; this is known as pipe warming. 18. What is warm-up? Answer: There is a temperature difference of about 410°C between the cold state and the rated operating temperature of the turbine. The various components of the turbine’s moving and stationary parts need a time to adapt to such a large temperature difference (that is, a process of thermal expansion). To allow the unit to undergo sufficient and uniform thermal expansion, it is necessary to remain at low speed for 45–60 minutes in order to reduce the thermal stress on the materials; this period of time is known as warm-up. 19. What is the purpose of warming the tube? Answer: Before startup, the new steam pipes, flanges, valves, etc., are all in a cold state. Warm-up the pipe by gradually heating the steam pipeline and its accessories in front of the quick-close valve using steam. For units equipped with steam extraction and steam injection, the steam supply pipes for these devices should also be warmed up simultaneously. The temperature is increased gradually through the heating pipes, to prevent excessive thermal stress from occurring due to a sudden rise in temperature at startup, which could cause the pipes to deform or crack. Furthermore, warming the pipes preheats the steam pipes to the saturation temperature at the rated pressure, which prevents condensed water generated by the flow of fresh steam through the pipes due to low temperatures from entering the turbine along with the high-speed steam flow and causing water hammer accidents. 20. What issues should be considered when warming pipes? Answer: When warming pipes, the following points should be taken into consideration: 1) It is necessary to strictly control the rate of temperature increase in the pipe wall; the rate of temperature increase on the inner wall should be kept at 5°C/min to ensure uniform expansion of the pipe ； 2) It is carried out in two steps: low-pressure pipe heating and high-pressure pipe heating ；     First, low-pressure pipe warming is carried out, that is, steam at low pressure and high flow rate is used to heat the pipes. The steam pressure is controlled by the bypass valve of the main steam valve, and is generally maintained at 2.5–3 bar (5–6 bar for large-capacity turbines). In this way, at the beginning of the pipe warming process, the pipes are subjected only to temperature changes and not to pressure changes. When the wall temperature rises to the saturation temperature corresponding to the steam pressure of low-pressure pipe warming, pipe warming at increased pressure is carried out, that is, the steam pressure and temperature are gradually increased until the rated parameters are reached. For conventional medium-parameter turbines, the allowable temperature rise rate of the pipes is 5–10°C/minute, while for high-parameter turbines it should not exceed 3–5°C/minute. 3) Strictly control the time required to warm the pipes ； The time required to warm the pipe depends on factors such as pipe length, wall thickness, diameter, material of the pipe, and steam parameters. The pipe warming time for conventional medium-parameter turbines is 20–30 minutes, while it is 40–60 minutes for high-parameter turbines. 21. Why does steam leak and backflow into the vent when warming the pipes or after shutting down the system? Answer: The steam leakage from the valve stem is caused by medium-pressure steam leaking from the valve body of the quick-close valve along the valve stem; the temperature of this steam is 405 degrees. When the quick-close valve is in the closed position, steam enters the pre-chamber of the valve body, resulting in a higher leakage rate of the valve stem. When the quick-close valve is in the open position, steam enters the rear chamber of the valve body, and the leakage from the valve stem is minimal; therefore, the amount of steam that leaks is much greater when the system is shut down than when it is operating. The sealing steam for the steam turbine is low-pressure steam at a temperature of 175 degrees. When the leakage steam is injected into the sealing pipeline, the temperature of the sealing steam rises. The high-temperature steam enters the turbine through the shaft seal gap, resulting in a temperature difference between the upper and lower parts of the rotor due to static heating; in severe cases, this can cause the rotor to deform and bend. Overheated steam can also cause the shaft-end steam seals to overheat and become loose, posing a risk to the operation of the unit; therefore, during the unit’s warm-up process and after it is shut down, the steam leakage from the valve stems should be directed to the exhaust system. 22. What aspects require special attention to prevent water hammer accidents in steam turbines? Answer: To prevent water hammer accidents, special attention should be paid to the following aspects: 1) Try not to start the turbine when the temperature and pressure of the steam are unstable ； 2) When fluctuations in steam temperature and pressure occur during operation, pay close attention to the operating conditions of the unit, as water hammer is most likely to occur at such times ；     3) When the boiler is brought online or when the steam main is switched from one to another, the boiler operator should notify the turbine operator in advance and closely monitor the operation of the unit, as water hammer is most likely to occur at such times ； 4) In addition to paying attention to water drainage during warm-up/warming of the pipes, the direct drain valve should be opened every time the steam temperature drops or signs of water hammer are detected ； 5) The steam turbines and pipelines should be equipped with steam-water separators or other drainage devices, and their operational performance should be checked regularly; if any malfunction is detected, it should be repaired promptly ； 6) Before starting the turbine after it has been shut down, all drain valves should be opened, and then closed once the turbine has reached operating speed and is under load. 23. Why is it necessary to warm up the turbine? When starting, a steam turbine requires a certain amount of time for low-speed warm-up. During cold start, low-speed warming is intended to ensure that all components of the unit expand evenly as they are heated, thereby preventing deformation and loosening of components such as cylinders, diaphragms, nozzles, shafts, impellers, and shaft seals. For turbines that have not been fully cooled, especially those without a barring device, it is also necessary to perform low-speed warming up during startup. This is done to prevent the shaft from bending or deforming, thereby avoiding friction between the moving and stationary parts of the turbine. 24. What is saturated steam? Answer: The number of molecules that escape from the liquid surface per unit time is equal to the number of molecules that return to the liquid. The amounts of vapor and liquid remain constant, and the vapor and liquid phases reach equilibrium; this state is known as a saturated state. At this point, the pressure of the vapor and the liquid is called the saturated state. At this point, the pressure of the steam and liquid is called the saturation pressure, while their temperature is called the saturation temperature; for a given saturation pressure, there is a corresponding saturation temperature. Saturated steam is steam that is in equilibrium with a liquid. 25. What is superheated steam? Answer: Steam is considered superheated steam when its temperature is higher than the saturation temperature corresponding to its pressure. Superheated steam can be obtained by heating dry steam under reduced pressure; the difference between the temperature of the superheated steam and the saturation temperature corresponding to its pressure is known as the \"superheat.\" The working fluid in a steam turbine is superheated steam.     26. What is the impact of too low steam inlet temperature on the operation of a turbine? Answer: A decrease in the steam inlet temperature has the following effects on the operation of the turbine: 1) Effects on safety ① Increased steam humidity leads to an increased axial force exerted by the water droplets in the steam on the blades ； At the same time, the reaction degree at the high-pressure stage increases, which leads to an increase in the axial thrust of the turbine. This raises the temperature of the thrust bearing pads, and in severe cases, it can cause damage to these pads ； ② When the temperature drops too rapidly, it may be a precursor to a water hammer incident ； ③ When the load remains constant and the steam temperature decreases, the steam flow rate increases. Since the exhaust pressure is fixed, this increase in flow rate leads to a greater drop in enthalpy, which in turn increases the stress on the final stage blades and diaphragms; this could result in overload ； ④ The expansion process inside the machine shifts toward regions with higher humidity; the increased humidity in the final stages exacerbates erosion of the blades, thereby shortening their service life ； 2) Impact on economic efficiency ① As the temperature decreases, the enthalpy drop of the steam within the turbine decreases, resulting in a reduction in both the output power and thermal efficiency ； ② As the temperature decreases, the humidity of the exhaust steam increases, leading to greater moisture loss in the last stages of the turbine and reducing its operational efficiency. Therefore, from the perspectives of safety and economy, it is not permitted for steam turbines to operate under conditions where the steam temperature is lower than the normal level. 27. What are the effects of excessively high steam inlet pressure on the operation of a steam turbine? Answer: An increase in the steam inlet pressure of a steam turbine has the following effects on its operation: 1) The stress on pressure-bearing components such as steam pipes, steam chambers, and steam valves, as well as on the metal materials of fasteners, increases. When designing the turbine, the strength of each component is determined based on the rated main steam pressure. When the steam pressure exceeds this rated value, the main steam pipes, valves along the pipes, steam chambers of governing valves, and blades—all of which are subjected to steam pressure—become overloaded, potentially leading to damage of these components ；     2) An increase in the steam inlet pressure of the turbine leads to higher humidity of the steam during operation in the last stages of the turbine. This not only increases wet steam losses and reduces the efficiency of the unit, but also deteriorates the operating conditions of the blades in those last stages; the blades may suffer from erosion, and in severe cases water hammer incidents can occur, causing damage to the equipment and affecting the lifespan of those blades. 3) When the steam pressure increases during operation, if the opening degree of the control valve remains unchanged, both the flow rate and the enthalpy drop will increase. This leads to an excessive enthalpy drop in the control stage, resulting in overload and damage to the nozzles or blades ； 4) Throttling-type control turbines: An increase in steam pressure can cause overload of the blades and diaphragms in the earlier stages ； If the load is kept constant, it is necessary to reduce the size of the control steam valve, which will increase throttling losses ； 5) When the steam pressure rises within its permissible range, while the steam temperature and exhaust pressure remain unchanged, the ideal enthalpy drop of the steam turbine increases. If the load is kept constant, the required steam flow rate decreases, resulting in a lower steam consumption rate and improved unit efficiency. Therefore, from an economic perspective, increasing the steam inlet pressure can reduce steam consumption ； However, from a safety perspective, excessive steam pressure can damage the turbine; therefore, the steam pressure must be maintained at the design value. 28. What are the effects of excessively low steam inlet pressure on the operation of a steam turbine? Answer: When the steam pressure decreases, it has the following effects on the steam turbine: 1) It reduces the output capacity of the unit, resulting in a decrease in power output ； 2) When the steam inlet pressure is below the design value, the efficiency of the turbine decreases, and the amount of steam required at the same load increases; as a result, the axial thrust rises, and the stress on the blades in the subsequent stages increases. In severe cases, this can cause the blades to deform ； 3) The steam inlet pressure to the turbine is too low. If the cross-sectional area of the nozzles does not allow for an increase in steam flow rate, the turbine’s output power will decrease, causing its rotational speed to fall short of the rated value. This, in turn, affects the operation of driven machinery such as compressors. 29. What is vacuum? Answer: A vacuum, also known as negative pressure, is created when the gas pressure inside a container is lower than atmospheric pressure. A space with absolutely no materials at all (that is, a vacuum level of 100%, known as an absolute vacuum, which is very difficult to achieve). Typically, 760 millimeters of mercury (at 0 degrees) is used as the standard scale. If the reading indicated for the container is below atmospheric pressure, it is called vacuum level. The pressure value indicated by vacuum level is the pressure difference between the gas pressure inside the container and atmospheric pressure; it is also known as vacuum pressure or low pressure. The lower the atmospheric pressure inside the container, the higher the vacuum level ； Conversely, the higher the atmospheric pressure inside the container (up to 1 atmosphere), the lower the degree of vacuum ； If the gas pressure inside the container is equal to the atmospheric pressure, then the vacuum level is zero, which means there is no vacuum.     30. What is the function of a condenser? Answer: The condenser serves three main functions: 1) It is used to cool the exhaust steam from the turbine, causing it to condense into water ； 2) A high vacuum is created at the turbine exhaust outlet, so that as much of the heat contained in the steam as possible can be used to perform work ； 3) During normal operation, the condenser performs deoxygenation, reducing the amount of oxygen in the condensed water; this improves the quality of the boiler feedwater and prevents equipment corrosion. 31. Why does a condenser need a hot well? Answer: The function of a hot well is to collect condensate water, which facilitates the proper operation of the condensate pumps, and it also serves to monitor the level of condensate water. In the absence of a heat well, the water level is likely to rise intermittently, causing the condensate to become supercooled and affecting the economic efficiency of the unit’s operation. 32. How does a condenser work? How is a high vacuum created inside the condenser? Answer: The waste steam generated after the steam does work in the turbine enters the surface condenser, where it is cooled by cooling water and thus condensed into water. Its volume decreases dramatically by a factor of thousands, and the pressure drops sharply, thereby creating a vacuum. To maintain a high vacuum inside the surface condenser, an exhaust pump is installed to remove the non-condensable gases from it. Meanwhile, the condensate formed in the surface condenser is pumped to the deoxygenation tank as boiler feed water, thereby preventing the liquid level from rising too high and causing the tubes to be submerged, which would affect the heat exchange efficiency. 33. What parameters should be monitored regularly during the operation of a condenser?     Answer: During operation, the condenser should be monitored regularly for: 1) whether the vacuum level of the condenser is at its optimal value ； 2) Degree of supercooling of condensate water ； 3) Is the quality of the condensate water satisfactory? ； 4) Is the quality of the circulating water satisfactory? ； 34. Why are exhaust ports provided on the condenser? Answer: An exhaust port is provided on the lower side of the condenser shell; the air that leaks into the vapor space inside the condenser can be drawn out through this port by an exhaust pump, thereby maintaining a certain level of vacuum within the condenser. 35. What are the adverse effects of air entering the vacuum system on the operation of condensing equipment? Answer: When air enters the vacuum system, it has the following adverse effects on the operation of the condensing equipment: 1) A large amount of air entering increases the partial pressure of air in the condenser, thereby reducing the vacuum level in the condenser ； It increases the exhaust pressure and temperature, reducing the efficiency of the turbine ； In severe cases, the increase in exhaust temperature can cause deformation and vibration in the low-pressure cylinder of the turbine ； 2) The increased oxygen content in the condensate leads to accelerated metal corrosion in boilers, turbines, and pipelines ； 3) It hinders steam condensation, resulting in a decrease in the heat transfer coefficient; this leads to poorer heat transfer in the condenser, an increase in the amount of uncondensed steam drawn off by the extraction pump, and a decline in vacuum level ； 3) It increased the load on the extraction pump. Not only because the amount of air to be removed increases, but also the amount of uncondensed steam removed increases, causing the temperature of the mixture to rise ； 4) The subcooling of the condensate water increases. 36. What is the function of an air pump (exhaust pump)? Answer: The function of the exhaust pump is to create a vacuum during the startup of the turbine, and while the turbine is operating, it continuously removes any air that has leaked in as well as uncondensed steam from the condenser, thereby maintaining the vacuum level in the condenser.     37. What are the types of exhaust pumps? Answer: The common types of vacuum pumps are piston-type, centrifugal (rotary) type, water-jet type, and steam-jet type. 38. How does a steam jet pump work? What are its advantages and disadvantages? Answer: The working principle of the steam jet pump is shown in the figure on the right. A jet pump is mainly composed of three parts: the working nozzle A, the mixing chamber B, and the diffuser chamber C. As the working steam passes through nozzle A, the sudden reduction in the flow cross-section causes most of the pressure energy to be converted into kinetic energy of the steam stream; as a result, the velocity of the steam at the outlet of the nozzle reaches very high values, typically around 10,000 meters per second. Mixing chamber B is connected to the exhaust port of the condenser; the high-speed flow of steam creates a low-pressure area within mixing chamber B, one that is lower than the pressure at the condenser’s exhaust port. As a result, the mixture of air and uncondensed steam inside the condenser is drawn into the mixing chamber, where it is carried by the high-speed steam flow into diffuser C. Due to the trumpet-shaped design of the inlet section of the diffuser, the velocity of the steam and air mixture decreases gradually as it flows through the diffuser; part of its kinetic energy is converted into pressure energy, causing the pressure to rise. At the outlet of the diffuser, the pressure reaches a level slightly higher than atmospheric pressure, after which the gas is released into the atmosphere. Steam ejectors are widely used due to their advantages such as compact structure, reliable operation, low manufacturing cost, and the ability to achieve the desired vacuum in a short time (a few minutes). Its disadvantage is that it consumes a large amount of steam and has low efficiency. 39. Why use a multi-stage pump? Answer: Single-stage pumps have low pumping efficiency due to the significant pressure increase caused by the diffuser; they are unable to achieve high vacuums. Moreover, the mixture is released into the atmosphere, resulting in losses of thermal energy and condensed water. The compression process in the ejector is an adiabatic process; the temperature of the mixture rises during compression, which increases the amount of compression work required, meaning more steam is needed. An effective way to improve its efficiency is to use multi-stage compression with intercooling.     In a multi-stage pump, the vapor mixture is expanded through multiple diffuser stages, with each stage applying only a slight pressure increase, thereby achieving high pumping efficiency ； After being cooled in the intercooler, the steam-air mixture sees most of its steam condense; as a result, not only does its volume decrease, but its mass also drops significantly, which allows for reduced compression work by the second-stage extractor and less steam consumption ； The heat released by the condensation of the working steam in the cooler can be used to heat the main condensate water, thereby recovering the heat from the working steam and the condensate water and improving the efficiency of the system. 40. What is the difference between a start-up pump and a main pump? Answer: The task of starting the vacuum pump is to rapidly create a vacuum in the condenser before the turbine starts, in order to shorten the startup time. The start-up extractor operates for a short period of time, has a high pumping capacity and high steam consumption; therefore, it is designed as a single-stage unit without a cooler. Its structure is simple, and during operation it discharges the entire mixture of steam and air directly into the atmosphere, resulting in losses of both the heat and mass of the working steam. As a result, it is generally used only to create a vacuum in the condenser before startup. Once the vacuum reaches a certain level, the main extractor should be activated and the start-up extractor turned off. The task of the main exhaust pump is to remove the air from the condenser while the turbine is operating, in order to maintain its proper vacuum level. It is generally designed as multiple stages, with coolers between each stage to recover working steam, heat, and condensed water, thereby achieving high thermal efficiency. 41. Briefly describe the startup procedure for the main steam ejector. Answer: The startup procedure for the main steam ejector should follow these steps: 1) Feed main condensate water into the cooler ； 2) Open the steam valves in order, from back to front. First, open the steam valve of the last-stage ejector; once the vacuum level in the condenser reaches a certain value, open the steam valve of the previous stage, and finally open the steam valve of the first stage, thereby gradually removing the air from within the condenser. 3) Open the air valve – this helps prevent the copper tubes of the cooler from overheating, as well as avoiding sudden cooling when condensed water flows through them, which could damage the expansions at the ends of the copper tubes and cause leaks ； It also prevents air from entering the vacuum system before the injector starts operating. 42. Why should the condensate pump be started first before starting a condensing steam turbine? Answer: Because before starting a condensing steam turbine, the main exhaust pump must be started first. The main exhaust pump requires condensate water to cool the steam ejected from its nozzles; this prevents the copper tubes of the cooler from overheating, as well as avoiding sudden cooling when condensed water flows through them at high temperatures, which could damage the expansion joints in those tubes and cause leaks. Therefore, the condensate water pump needs to be started before the exhaust pump. 43. What aspects should be considered when using a vacuum pump in routine maintenance? Answer: The exhaust pump is an important component of the condensing system. Its proper functioning is crucial for maintaining the vacuum in the condenser, which in turn has a significant impact on the performance of the turbine. During routine maintenance, attention should be paid to the following aspects on a regular basis: 1) The working steam nozzles must be kept clean ； 2) The connection between the nozzle and the steam must be tight ； 3) The temperature and pressure of the working steam must be maintained at the rated values and remain stable ； 4) The steam pipeline of the exhaust pump must have sufficient flow area ； 5) For single-stage exhaust pumps, attention must be paid to the placement of the exhaust outlet ； 6) The heat exchange tubes of the extractor cooler must be kept clean. 44. What are the factors that cause abnormal operation of the exhaust pump? Answer: The main factors causing abnormal operation of the extractor are 1) insufficient supply of cooling water to the cooler ； 2) Leakage in the inner tube sheet and partition of the cooler, resulting in reduced cooling efficiency ； 3) Breakage of the cooler water pipes, loosening of the flanges on the pipe sheet, or blockage of the drain pipes, resulting in the pump being filled with water ； 4) Poor performance due to nozzle scaling, clogging, as well as wear and corrosion ；     5) The exhaust pump is under excessive load, which deteriorates its performance ； 45. What are the effects of too low or too high condenser vacuum? Answer: An excessively low vacuum in the condenser, that is, an increase in the exhaust pressure of the turbine, has the following effects on the turbine: 1) It reduces the turbine’s power output and lowers the efficiency of the plant ； 2) The increase in exhaust pressure leads to an increase in axial thrust, resulting in bearing damage ； 3) An increase in exhaust pressure leads to an increase in exhaust temperature; when this temperature exceeds the design value, it affects the safety of the various components at the turbine’s exhaust end as well as the condensing equipment. The vacuum in the condenser is too high, which means the exhaust pressure of the turbine decreases. Although the power output of the turbine increases, a lower exhaust pressure is not necessarily better. This is because: 1) There is a limit backpressure for steam expansion within the turbine stages; below this limit backpressure, the steam will expand suddenly after those stages, which only increases the losses in the turbine ； 2) The backpressure is too low, causing the volumetric flow rate of steam to become excessive, which results in a high exhaust velocity and increased flow losses ； 3) Excessively high vacuum is caused by an increased flow rate of the circulating water, which in turn increases the operating costs of the circulating water pump ； 4) When the vacuum is too high, the pressure of the condensing equipment falls below the design value, which increases the amount of air that leaks in and thus raises the load on the exhaust pump. Therefore, the most favorable vacuum level should be maintained during turbine operation. 46. What are the main factors affecting the vacuum level of the surface cooler? Answer: The main factors affecting the vacuum level of the surface cooler are: 1) Cooling water flow rate. A larger volume of cooling water and a higher flow velocity can enhance heat transfer, lower the condensation temperature, and thus increase the vacuum level. 2) With all other conditions unchanged, as the cooling water temperature decreases, the saturation temperature of the exhaust steam also drops, resulting in an increase in vacuum level.     3) Scaling on the water pipes of the surface cooler and on the heat transfer surfaces will reduce the heat transfer efficiency, thereby lowering the vacuum level. 4) Integrity of the vacuum system: If the vacuum system is not airtight, air will leak into the condenser, severely disrupting its vacuum level. This leads to an increase in exhaust pressure and temperature, thereby reducing the efficiency of the turbine. 5) Insufficient capacity of the extraction pump, the presence of dead zones in the condenser where non-condensable gases accumulate, or an increase in non-condensable gases carried in by the steam can also reduce the vacuum level of the condenser. 47. What measures should be taken if the vacuum level in the condenser drops sharply? Answer: If the vacuum level in the condenser drops sharply, the cause must be identified promptly and appropriate actions taken: 1) Check the inlet and outlet pressures of the condenser’s circulating water to determine whether there is a fault with the circulating water pump ； 2) Check the shaft seal steam pressure; insufficient shaft seal steam pressure can lead to air infiltration, resulting in a drop in vacuum ； 3) Check the pressure in the condensate pipeline to determine whether there is a fault with the condensate pump ； 4) In addition to checking the above components to identify the cause of the vacuum drop, efforts should be made to reduce the load; if the load is reduced to its minimum level and there is no sign of improvement, the machine should be shut down promptly. 48. What are the signs of scaling on the cooling surface of a condenser? Answer: The signs of scaling on the cooling surface of the condenser include: 1) An increase in the difference between the turbine exhaust temperature and the outlet temperature of the circulating water ； 2) The temperature of the steam-air mixture drawn out by the extractor increases. 3) The flow resistance on the water side inside the condenser increases ； 4) An air tightness test showed that the condenser’s air leakage had not increased. 49. What are the main causes of scaling on the cooling surfaces of condensers? How to solve the scaling problem? Answer: There are two types of scaling that can occur on the cooling surfaces of condensers: 1) The cooling water has a high hardness, containing large amounts of calcium and magnesium salts, which form hard carbonates once they accumulate inside the tubes ； 2) In DC water supply systems, soft mud deposits or organic matter often form ； Therefore, the main cause of scaling on the cooling surface of the condenser is poor quality of the cooling water. To address the problem of scaling, efforts should start with improving water quality and strictly controlling water quality parameters in order to reduce scaling and corrosion in cooling water. When there is excessive fouling and a significant drop in vacuum occurs, the condenser should be cleaned. 50. Why can a high water level in the steam side space of the condenser cause a drop in the condenser vacuum? Answer: Because when the water level in the steam side space of the condenser rises and submerges part of the copper tubes below, it reduces the cooling area of the condenser. This leads to a decrease in the condenser’s efficiency, causing the exhaust pressure of the turbine to rise and resulting in a drop in vacuum. When the water level in the condenser becomes too high, submerging the air chambers and air extraction ports on both sides of the condenser, air can no longer be extracted from within it. As a result, air accumulates increasingly inside the condenser, preventing the exhaust steam from condensing in a timely manner. At this point, the vacuum gauge at the throat of the condenser indicates a drop in vacuum, while the vacuum indicator on the air ejector shows an increase. . Therefore, too high a condenser water level can affect its vacuum. 51. What causes the condenser to fill with water? Answer: The following reasons can cause the condenser to fill with water: 1) Failure of the condensate pump ； 2) Rupture of the condenser copper tubes ； 3) The check valve of the standby condensate pump is damaged or not functioning properly, allowing condensate to flow back from the standby pump into the condenser ； 4) During normal operation, the recirculation valve for condensate was accidentally opened wider, causing the condensate to flow back into the condenser. 52. During the normal operation of the unit, why must the water level in the condenser not be too high? Answer: When the water level is too high, it will flood part of the condensation space in the condenser, reducing its cooling efficiency, lowering the vacuum level, and increasing the subcooling of the condensed water. If the water level rises above the air pipes, the air in the condenser cannot be removed, the exhaust pump loses its function of removing air, and the vacuum level drops rapidly. If the water level becomes too high and overflows into the turbine, it can cause damage to the rotor blades and permanent bending of the rotor.     53. Why is the cooling water for the condenser introduced from below and discharged from above? Answer: Because the temperature of the cooling water exiting the system is higher than that of the water entering it, and water with a higher temperature has a lower density, in order to ensure that the convection caused by this density difference flows in the same direction as the flow of the water, water generally enters from below and exits from above. 54. Which indicators should be regularly checked and analyzed during the operation of an air cooler? Answer: The performance of the surface cooler is reflected in the vacuum level. However, vacuum is the result of various factors; to facilitate analysis, the following parameters should be checked regularly: 1) Temperature rise – the difference between the inlet and outlet temperatures of the cooling water. When the turbine load remains constant, that is, when the exhaust volume is the same, an increase in temperature rise indicates a decrease in water flow, which may be caused by blockages in the tube sheet or contamination of the tubes. 2) Terminal difference: the difference between the exhaust temperature and the outlet temperature of the cooling water. An increase in the terminal difference indicates a decrease in the heat transfer coefficient, which is usually caused by factors such as fouling of the heating surfaces or air leakage. 3) Subcooling: the difference between the exhaust temperature and the condensate temperature. A high degree of regular subcooling indicates a problem with the structure of the surface cooler; a sudden increase in subcooling is usually caused by leaks of cooling water or air in the surface cooler. 55. What are the main causes of subcooling in condensate water? Answer: The supercooling of condensate is mainly caused by the following factors: 1) During operation, the water level of the condensate in the hot water well of the condenser is too high, causing the condensate to flood the lowest rows of tubes; as a result, the cooling water in these tubes absorbs some of the heat from the condensate, leading to supercooling ；   2) An increase in the amount of air that leaks into the condenser, or poor performance of the exhaust pump, leads to an increase in the total pressure and gas partial pressures within the condenser. As a result, the vapor partial pressure decreases, meaning that the vapor condenses at a lower temperature, which causes supercooling ； 3) Poor fouling of the condenser, as well as too dense or poorly arranged tubes that result in high air resistance, are also common causes of supercooling of the condensate water. 56. How to reduce the subcooling of condensate water? Answer: The main methods to reduce the subcooling of condensate water are as follows: 1) Closely monitor the water level in the condenser during operation to ensure it does not exceed the lowest row of tubes, or use a low-water-level operation method (by opening the outlet valve of the condensate pump to full capacity during operation, without using any manual or automatic regulators to control the water level in the condenser; the working characteristics of the condensate pump are utilized to maintain a lower liquid level in the condenser) ； 2) Monitor the integrity of the vacuum system to prevent air from leaking in ； 3) Keep the vacuum pump operating properly ； 4) Modify the old-type condensers to reduce air resistance. 57. Why is a high vacuum not required when starting a condensing steam turbine? Answer: A very high vacuum is not required when starting the turbine. Because the higher the vacuum, the less steam is required by the impulse turbine; too little steam will not allow for an adequate warm-up effect. A vacuum level of -60 to -80 kPa is appropriate; if the vacuum decreases, that is, if the back pressure increases, then at the same turbine speed, the amount of steam entering the turbine increases and the exhaust temperature rises slightly, all of which help to achieve a better and faster warming-up process. 58. Why does the vacuum drop when a condensing steam turbine is started up? Answer: During startup, the vacuum is generally kept at a low level; as a result, some air remains in the cylinders and pipes and is carried by the steam flow toward the condenser. Therefore, the vacuum in the condenser always decreases during startup. 59. Why is it necessary to evacuate the vacuum before starting a condensing steam turbine?     Answer: Before the turbine is started, there is air inside it, and the pressure within the turbine is equivalent to atmospheric pressure. If vacuum is not created, the exhaust pressure will increase because air cannot be condensed. In this situation, when starting up, a large amount of steam is required to overcome the frictional forces and inertial forces in the bearings of the turbine and compressor, in order to drive the rotor; this results in an increased steam impact force on the blades ； Additionally, after the rotor is impulsed, the presence of air in the condenser weakens the heat exchange between the exhaust steam and the cooling water. As a result, the exhaust steam temperature rises, causing deformation of the internal components of the low-pressure cylinder of the turbine. The increase in back pressure within the condenser may also trigger its safety devices or interlocks. Therefore, it is necessary to evacuate the condenser before starting a condensing steam turbine. 60. Why does the exhaust temperature of the turbine increase when the vacuum in the surface cooler decreases? Answer: Because water vapor has a certain saturation temperature at a specific pressure, and this saturation temperature increases as the pressure rises. The exhaust temperature of the turbine is essentially corresponding to the saturation temperature of the surface cooler; therefore, when the vacuum inside the surface cooler decreases (that is, when the back pressure of the turbine increases), the exhaust temperature also rises. 61. What is the ultimate vacuum of a steam turbine? Answer: For a running turbine, if the steam flow rate remains constant, as the back pressure decreases, the turbine’s power does not increase in exactly the same proportion. As the back pressure continues to decrease, the specific volume of the steam exhaust increases. For a given turbine, the exhaust area of the last stage remains constant; as a result, the residual velocity loss in the exhaust from that last stage keeps increasing. When the increase in effective heat drop due to reduced back pressure equals the increase in residual velocity loss, the power of the turbine no longer increases. The corresponding back pressure at this point is referred to as the turbine’s ultimate back pressure or ultimate vacuum. Therefore, the increase in vacuum is limited by the expansion capacity of the last stage blades of the turbine; beyond this limit, increasing the turbine’s power does not yield economic benefits. 62. What is the most favorable vacuum for a steam turbine? Answer: With the same steam parameters, increasing the vacuum can raise the turbine load (up to the limit vacuum). When the temperature of the cooling water remains constant, to increase the vacuum it is necessary to increase the amount of cooling water used (i.e., increase the cooling ratio), which in turn increases the energy consumption of the circulation pump. The so-called optimal vacuum is the one at which the difference between the increased power output of the turbine due to the higher vacuum and the additional power consumed by the circulation pump is maximized.     63. How to maintain the most favorable vacuum? Answer: To improve the economic efficiency of turbine operation, the turbine should operate under the most favorable vacuum condition, as this results in the lowest total amount of steam equivalent to the steam consumption of the turbine and the electricity consumed by the circulation water pump. A common phenomenon in actual operation is a low vacuum level, which fails to reach the optimal value; this is particularly evident in summer. Therefore, increasing the vacuum level of the turbine and achieving the most favorable vacuum value possible is an important issue to consider for economic operation, and the following main measures can be taken. 1) Lowering the temperature of the circulating water: When the temperature of the circulating water is 22°C, for every 1°C decrease in this temperature, the vacuum level can increase by 0.3%, which allows for a 0.3–0.5% reduction in fuel consumption. In systems that use circulating water, measures should be taken to improve the efficiency of cold water towers and cooling ponds. 2) Increasing the circulating water volume can reduce the temperature difference between the inlet and outlet of the circulating water. The main measures to increase the circulating water volume are to reduce pipeline resistance and enhance the output of the circulating water pump. 3) Keep the condenser copper tubes clean to improve cooling efficiency. 64. What is the function of the turbine’s shaft seal? Answer: Since the turbine shaft must pass through the cylinder, a certain radial clearance must exist between the shaft and the cylinder. Moreover, as the steam pressure inside the cylinder differs from the external atmospheric pressure, it is inevitable that the high-pressure steam inside the turbine will leak out through this clearance, or that external air will enter. To improve the efficiency of the turbine, such steam leakage must be prevented or reduced as much as possible. To this end, steam seals are installed at both ends of the rotor where it passes through the cylinder; these steam seals are known as shaft-end steam seals, or simply shaft seals. The shaft seal at the steam inlet end is used to prevent steam from leaking out of the cylinder. If high-pressure steam leaks from the cylinder, it leads to a loss of working fluid, deteriorates the operating conditions, raises the temperature of the shaft journals, and may allow steam and water to enter the bearings, thereby deteriorating the quality of the lubricating oil. In severe cases, this can even cause the bearing materials to melt, posing a threat to the safety of the turbine. The exhaust side shaft seal is used to prevent air from entering the cylinder, which could disrupt the proper operation of the condenser, increase the exhaust pressure of the turbine, and reduce the efficiency of the unit. The shaft seals of large steam turbines are relatively long and are usually divided into several sections; there is an annular chamber between adjacent sections, which can be used to accommodate pipes for introducing or extracting steam.                        65. Why is it not possible to stop the shaft seal steam supply immediately when the turbine is shut down? Why is it necessary to wait until the vacuum drops to zero before stopping the shaft seal steam supply? Answer: After the turbine stops running, the cylinder remains under vacuum for a certain period of time. If the steam supply to the shaft seal is turned off immediately, cold air will enter the cylinder through the shaft seal, causing it to cool down and deform rapidly. This, in turn, will lead to wear of the shaft seal and vibration during subsequent operation. Therefore, it is necessary to wait until the vacuum is reduced to zero and the pressure inside the cylinder equals the external pressure before shutting off the steam supply to the shaft seal. In this way, cold air will not leak into the cylinder through the shaft seal, thereby preventing deformation and damage to the equipment. 66. Why is a certain level of vacuum required as a condition for starting a turbine? Answer: A certain level of vacuum is necessary before starting a turbine; this value is usually around 60 kPa. If the vacuum level is too low, more fresh steam is required to rotate the rotor. The excessive waste steam being discharged into the condenser suddenly causes the pressure on the steam side of the condenser to rise significantly, which may result in a positive pressure on that side. This can damage the safety film that prevents air from entering, and it also causes significant thermal shock to the cylinder and 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 startup time. 67. Why is it necessary to start the shaft turner before supplying sealing steam to the turbine? Answer: During vacuum extraction, the vacuum level inside the cylinder gradually increases. If steam is supplied to the shaft seal at this time, a large amount of steam will enter the cylinder through the shaft seal. The heat from this steam is transferred to the rotor and the cylinder. Due to the principle of rising hot air, the upper part of both the cylinder and the rotor becomes hotter than the lower part; as a result, the upper part of the rotor gradually bends. The longer the vacuum extraction lasts, the greater the bending of the shaft. When the rotor starts to rotate, friction occurs between the moving and stationary parts, which leads to vibration. In general, it is prohibited to supply steam to the shaft seal until the shaft turner has been started.     68. What is the purpose of turning the disk? Answer: Cranking the machine is very important before starting the unit, especially after it has been shut down. The purpose of turning the rotor before startup is: 1) to straighten the rotor ； 2) Prevent thermal deformation caused by steam leakage from the valves into the turbine ； 3) Shaft seal steam can be supplied in advance ； 4) Lubricate the bearing shells ； 5) Reduce the inertial force when the rotor is impulse-driven ； 6) Check whether all rotating parts are functioning properly ； The main purpose of turning the rotor after shutdown is: 1) to prevent the rotor from bending due to the temperature difference between the upper and lower cylinders ； 2) Ensure that the turbine can be started at any time after shutdown. In turbines equipped with a turning gear, once the turbine rotor has come to a complete stop during shutdown, the turning gear must be activated immediately. Generally, it is required that the turbine temperature drop below 100°C before the turning gear can be stopped. 69. Why is low-speed warming required during turbine startup? Answer: During the cold start of a turbine, high-temperature steam comes into contact with the turbine components at normal temperature, which inevitably causes certain thermal stresses and thermal deformations within the turbine. To reduce the temperature difference and avoid excessive thermal stress and thermal deformation, during the startup of the turbine, the steam pressure, temperature, and flow rate entering the turbine are controlled to allow the turbine to heat up gradually and expand evenly. Another advantage is that it provides operators with an opportunity to conduct a thorough inspection of the unit’s operation after it has started up.     70. What does a sharp change in coasting time indicate? Answer: A sharp reduction in idle time indicates that the rotor is experiencing rubbing or jamming, and this issue must be addressed promptly. The increase in idle time indicates that either the valves in the steam pipeline or those in the extraction pipeline are not tight, allowing steam to leak into the cylinder. 71. Why are cylinder drain pipes installed in the cylinders of steam turbines? Answer: During the startup and warming-up of the turbine, steam in the cylinders condenses into a liquid state; if this liquid is not removed promptly, it can cause erosion of the blades. During shutdown, condensate also forms inside the cylinder, corroding its interior. During operation, improper operation of the boiler and piping systems can result in steam containing water, which may also cause liquid to enter the cylinder. To ensure the safety of the equipment, a cylinder drain must be installed to continuously drain the condensate from the cylinder. 72. What requirements must be met for starting a steam turbine? Answer: 1) The vacuum of the turbine must be above -500 mmHg gauge. 2) The oil temperature and oil pressure meet the process specifications. 3) The steam temperature and pressure must be within acceptable limits. 4) Auxiliary equipment: all types of gauges and signaling devices must be in good working order and in their operational positions. 5) All protective devices have been confirmed to be qualified. 73. While evacuating, why isn’t the speed increased? Answer: An increase in the condenser vacuum leads to an increase in the turbine speed. Increasing the vacuum while simultaneously raising the speed can cause the turbine to overspeed or enter a critical region, resulting in failures. 74. Why does the vacuum level drop when the turbine is started up? Answer: Before startup, the vacuum is generally kept at a low level; as a result, some air remains in the cylinders and pipes and is not completely removed. After startup, this residual non-condensable gas is carried by the airflow toward the condenser, which is why the vacuum in the condenser drops during startup.     75. When operating under no load for an extended period, why does the exhaust steam temperature of the turbine rise? Answer: There are two reasons for the increase in turbine exhaust temperature during no-load operation: 1) During no-load operation, since the steam flow into the turbine is low, the steam expands to a very low pressure before reaching the rear part of the unit, approaching the dry exhaust pressure, but with a high degree of superheat. 2) During no-load operation, the turbine does not produce work, and the amount of steam entering the turbine is very small. This small amount of steam is struck and disturbed by the rapidly rotating impeller, creating a kind of blowing effect. This mechanical impact and blowing effect, along with frictional heat generation, cause the exhaust temperature to rise. For the above two reasons, the exhaust temperature increases during no-load operation. 76. Why is it necessary to wait until the rotor has stopped rotating before reducing the vacuum in the condenser to zero after the turbine is shut down? Answer: During shutdown, unless it is an emergency shutdown that requires breaking the vacuum to stop the operation immediately, the vacuum level should be reduced gradually. By the time the rotor stops rotating, the vacuum level is close to zero; this allows one to determine whether the equipment is functioning properly by observing the rotor’s coasting time after each shutdown. Furthermore, maintaining a certain level of vacuum also helps to keep the cylinder dry after shutdown, preventing static corrosion. 77. What is the cold start of a steam turbine? What is hot start? Answer: The cold start of a turbine refers to the process of gradually bringing a stationary turbine at normal temperature into its normal operating state. Cold start is the most complex and comprehensive process in operation; the machine has to go through a sequence that takes it from a stationary state to its rated speed, from room temperature to high temperatures, from no load to rated load, from low flow rates to high flow rates, and from low pressure to high pressure. The hot start of an industrial steam turbine refers to starting the turbine while it is not fully cooled. The classification between hot and cold states varies depending on the specific unit. Generally, the metal temperature at the rated speed during a cold start of a turbine is used as the benchmark; if the external temperature of the lower cylinder is higher than this value, it is considered a hot start ； Below it is called the cold state. The metal temperature at the rated speed during cold start varies among different units, generally ranging between 150 and 200°C. Therefore, it is generally stipulated that a cylinder outer wall temperature above 150–200°C indicates a hot state.     78. What is the significance of recording the idle time of a steam turbine? Answer: It refers to the period during which the turbine continues to rotate due to inertia after the steam supply is completely cut off, until it finally stops. An excessively long idling time indicates that the main steam valve is not closing properly, allowing steam to leak in ； If the idle time is too short, it indicates friction between the moving and stationary parts. 79. What is axial displacement? What are the hazards of changes in axial displacement? Answer: During the operation of compressors and turbines, the movement of the rotor along the axis of the main shaft is referred to as axial displacement. The axial displacement of the turbine unit should be kept within the allowable range, generally between 0.8 and 1.0 mm. Exceeding this value can lead to frictional collisions between the moving and stationary parts, resulting in serious damage such as shaft bending, breakdown of partitions and impellers, and fracture of numerous turbine blades. For a single blade or a set of blades, when an instantaneous impact force is applied to them, the blade or the set of blades will deviate from their equilibrium position and oscillate periodically; this is the free vibration of the blade. The number of vibrations per second of the blade is called the free vibration frequency of the blade ； The maximum distance from the leaf’s equilibrium position is called the amplitude. The natural vibration frequency of a blade depends only on its material, structural design, size, and method of fixation; it is also known as the natural frequency. 80. What is water hammer? What are the harms? How to prevent it? Answer: In pressure pipelines, sudden changes in the flow velocity of the liquid cause significant, repeated, and rapid variations in the pressure of the liquid within the pipes. This creates a \"hammering\" effect within the pipes, and this phenomenon is known as water hammer. Water hammer can be divided into positive water hammer and negative water hammer. Their hazards include the following: in the case of positive water hammer, the pressure in the pipeline increases, and this pressure can reach dozens to hundreds of times the normal pressure within the pipeline, resulting in high stresses on the pipe walls. The repeated changes in pressure cause vibration in the pipelines and equipment, and these alternating stresses lead to damage to the pipelines, fittings, and equipment.     During negative water hammer, the pressure in the pipeline decreases, which can cause vibration in the pipeline and equipment. Changes in stress transfer have an adverse effect on the equipment; moreover, during negative water hammer, if the pressure drop is too low, it can create an unfavorable vacuum within the pipes, and under the influence of external pressure, the pipes may be flattened. To prevent water hammer, measures such as increasing the opening and closing time of valves, minimizing the length of the pipes, and installing safety valves or air chambers in the pipes can be taken to limit excessive increases in pressure or excessive drops in pressure. 81. What are the main signs of water hammer in a steam turbine? Answer: When a water hammer occurs in a turbine, the main signs are as follows: 1) A sharp drop in the inlet steam temperature ； 2) White steam emerges or water droplets splash out from the steam pipe flanges, shaft seal steam detection tubes, shaft seals, cylinder joint surfaces, etc ； 3) Water hammer noise can be clearly heard inside the steam pipe ； 4) Increased unit vibration ； 5) Increased axial displacement of the rotor; 6) Sound of water droplets hitting metal inside the turbine ； 7) Increased temperature of the tungsten carbide tiles in the thrust bearing ； 8) Increase in the oil outlet temperature of the thrust bearing ； 9) Increase in pressure inside the condenser (degradation of vacuum) ； The above signs do not necessarily appear simultaneously. 82. What is the main function of lubricating oil? Answer: 1) Lubrication function – it lubricates the shaft journals to prevent direct friction between the moving and stationary parts, and it removes the friction heat generated by their relative movement as well as the conductive heat transferred from other hot components, thereby preventing the bearing shells from overheating and getting damaged. 2) Supporting function: By utilizing the viscosity of the lubricating oil, it enters the bearing clearance driven by the rotor, forming a rigid fluid that supports the rotating rotor. 3) Transmission mechanism: By utilizing the fluidity of liquids and the ability of pressure to be transmitted, and by adjusting the system’s pressure level, control commands can be conveyed to effectively manage the unit. 83. What is the function of a pressure accumulator in the lubricating oil system? Answer: The functions of the accumulator in the lubricating oil system are as follows: 1) When the operating oil pump fails and stops, it maintains the oil pressure for a moment once the backup pump starts up, thereby preventing the unit from stopping due to low oil pressure ； 2) Maintain stable oil pressure when it changes. 84. What is the impact of oil temperature control levels on the unit? Answer: At low oil temperatures, the viscosity is high, the oil film is thick, and the load-carrying capacity is strong; however, the fluidity is poor, which may lead to semi-dry friction in the bearings and cause accidents. If the oil temperature is too low, the viscosity becomes low, resulting in poor load-carrying capacity. The frictional heat cannot be dissipated, which also affects the operation of the bearings. 85. What impact does an excessively high or low oil level in the lubricating oil tank have on the lubrication of the machine? Answer: An excessively high oil level in the lubricating oil tank can lead to poor return flow of oil, causing significant blockage in the pipes used for returning oil as well as in the spaces designated for venting air from the bearing box. This disrupts the slight negative pressure within the bearing box, resulting in oil leakage. Additionally, an excessive oil level can cause oil to flow over the edges of the filter slots and enter the tank without being filtered first. If the oil level is too low, the operation of the main oil pump may be unstable. As the oil level drops significantly, the number of oil circulation cycles increases, reducing the time the oil stays in the tank and thereby weakening the air separation effect. 86. Why is an exhaust pipe installed in the fuel tank? What are the effects if the fuel tank is sealed?     Answer: The tank exhaust pipe allows gases and water vapor to be discharged, preventing the water vapor from condensing inside the tank and thus keeping the pressure in the tank close to zero. This enables the oil returning from the bearings to flow smoothly into the tank. If the tank is sealed, then a large amount of gas and water vapor will accumulate inside it, creating positive pressure that makes it difficult for the oil to return. 87. What is the function of a high-level fuel tank? How to ensure that the high-level fuel tank functions as intended? Answer: A high-level fuel tank is a protective device. When the unit is operating normally, the lubricating oil in the upper oil tank enters from the bottom and exits from the top to return to the tank. When the main oil pump fails and the auxiliary oil pump does not start in time, the lubricating oil in the upper oil tank will flow to the lubrication points along the inlet pipe under the force of gravity, in order to meet the lubrication requirements during the unit’s idle operation. The oil storage capacity in the upper oil tank should generally be sufficient to provide lubrication for at least 5 minutes. To ensure the effectiveness of the overhead oil tank, the following technical measures should be taken: 1) Installation location of the overhead oil tank – It should be placed at a height of no less than 5 meters above the axis of the machine unit, directly above one end of this axis, in order to minimize the length of the pipelines and the number of bends, thus reducing the resistance encountered by the lubricating oil as it flows back to the bearings. 2) A breather hole or some other air supply mechanism should be provided at the top of the high-level oil tank. When lubricating oil flows from the high-level oil tank to the bearings, air is drawn in through the breather hole to fill the volume inside the tank, thereby preventing a negative pressure from forming in the tank and ensuring that the lubricating oil can flow out of the high-level oil tank due to gravity. When the lubrication system is a closed-loop system (such as the lubrication system of an ammonia compressor), there are no breather holes at the top of the oil reservoir. During a shutdown due to a fault, the lubricating oil in the oil reservoir flows through the inlet pipe to the bearings. As the lubricating oil gradually flows out, the space inside the oil reservoir increases. This increasing space is promptly filled with air through the return pipe that is connected to the oil reservoir. 3) A check valve should be installed on the main lubricating oil pipeline at the outlet of the lubricating oil pump. On the pipeline that runs from the outlet of the lubricating oil pump to before the lubricating oil enters the compressor, a check valve is necessary; once the main oil pump stops operating and the auxiliary oil pump does not start up in time to supply oil, the check valve closes immediately. This ensures that the lubricating oil from the high-level tank must flow through the bearings and then back into the oil tank, thereby preventing the lubricating oil from taking a shortcut and avoiding damage to the bearing bushes during the period when the unit is not in operation. 88. What checks and verifications should be done before starting the lubricating oil pump? Answer: Before starting the lubricating oil pump, the following checks and confirmations should be carried out: 1) The accumulator has been filled with nitrogen to the specified pressure ； 2) The fuel tank level is at the stop line ； 3) Control the oil temperature in the tank at 30–40℃ ； 4) Open the nitrogen filling valve for the fuel tank and the oil filling valve on the high-level tank; ensure that the seal gas valve for the compressor shaft bearings is open ； 5) Set the selection switches for the two lubricating oil pumps to the “manual” position, and contact the electrical department to supply power ； 6) Confirm that the pump outlet and inlet valves are fully open ； 7) Confirm that the upstream and downstream valves as well as the bypass valve of the pump outlet pressure control valve are open. 89. What is the purpose of purging nitrogen on top of the fuel tank? Answer: The purpose of filling with nitrogen is to separate the oil from air and prevent the oil from oxidizing ； Additionally, a protective layer is formed on the top of the fuel tank to reduce the risk of explosion or combustion caused by fuel vapor at high temperatures. 90. What are the factors that affect the lubrication performance of lubricants? Answer: After entering the bearing, the lubricating oil, due to the effect of adhesion, rotates along with the rotating shaft and forms a pressurized oil film. This oil film lifts the rotating rotor, enabling liquid lubrication between the bearing bush and the shaft neck. This, in turn, ensures the long-term safe operation of the unit. As the operating cycle of the unit lengthens, impurities in the lubricating oil will undoubtedly increase over time, leading to a significant deterioration in its lubricating properties and effectiveness. The factors that affect the lubricating performance of the oil include the following: 1) During the installation of the equipment in the lubricating oil system, as well as when assembling valves and pipelines, inadequate rust removal and cleaning can result in residual particulate and flaky impurities remaining in the system, thereby contaminating the lubricating oil ； 2) During long-term operation of the equipment, abrasive particles generated by frictional wear enter the oil ； 3) Due to improper storage of the fueling facilities, they are contaminated by the natural environment; during fueling, contaminants enter the lubricating oil tank and thus make their way into the oil circulation system ； 4) During long-term operation of the unit, since the gas medium being transported contains corrosive gases such as H2S, process fluctuations or operational errors can cause this gas to enter the lubrication system, thereby corroding the equipment, valves, and pipelines in that system; the resulting corrosion products then contaminate the lubrication system ； 5) During the operation of the unit, condensate from the turbine seals, as well as condensate carried in by gaseous media due to failed compressor seals, can enter the lubrication system, causing contamination of the lubricating oil ； As can be seen from the above, there are various factors that cause lubricant contamination and affect its lubricating performance and effectiveness. In terms of the form of pollutants, there are only three types: solids, liquids, and gases. Therefore, technical measures such as degassing, liquid removal, and filtration should be employed to eliminate pollutants from lubricating oil, thereby maintaining its lubricating properties and effectiveness. 91. Why are monitoring and protection measures necessary for bearing temperature? Answer: Two support points are provided at both ends of the centrifugal compressor rotor, on which corresponding radial bearing bushings are installed. The high-speed rotation of the rotor is made possible by the support provided by these bearing bushings as well as by adequate lubrication. Due to the high speed of rotation, a pressure oil film is formed in the gap between the rotor shaft and the bearing bushings; this film provides both supporting capacity and lubrication. A continuous and sufficient supply of lubricating oil provides the necessary conditions for the formation of a pressure oil film. However, the formation of this oil film as well as its load-carrying capacity are also affected by the temperature of the lubricating oil. If the temperature of the lubricating oil is too high, both its lubricity and viscosity decrease. An oil film with reduced lubricity and viscosity is prone to local damage under the effect of continuous oscillations in the rotor loads. The breakdown of the oil film is a prerequisite for lubrication failure and wear of the bearing surfaces; therefore, it is not permissible for the rotor to operate at excessively high lubricating oil temperatures. Upper limits for the lubricating oil temperature should be established, along with monitoring and protection measures, to prevent disruptions in the formation of the oil film and a decline in its load-carrying capacity.   An excessively low lubricant temperature is also dangerous; when the temperature is too low, the viscosity of the lubricant increases and its fluidity decreases, which reduces its lubricating capacity. In some cases, a low oil temperature can also cause oscillations in the oil film, leading to issues such as cracks in the bearing shells and damage to the gas seals. Therefore, monitoring and protective measures should also be in place for the minimum allowable oil temperature before it enters the machine. Based on practical experience and relevant information, it is recommended that the upper limit for the lubricating oil temperature entering the machine should not exceed 55°C, while the lower limit should not be below 42°C. 92. Why are monitoring and protection measures required for lubricant pressure? Answer: Lubricant pressure is a key parameter that ensures an adequate supply of lubricant. When the compressor rotor is operating at high speeds, sufficient lubricant is needed to create a pressure oil film between the shaft journals and the bearing shells, thereby maintaining fluid friction between them. This oil film also helps to dissipate the heat generated by friction, improving the working conditions of the bearing shells and ensuring the smooth operation of the compressor. The increase in lubricant pressure not only raises energy consumption but also makes it more difficult to maintain the oil seals, increasing the risk of lubricant leakage and damage to those seals, which is highly detrimental to the long-term safe operation of the equipment ； The lubricating oil pressure drops and the amount of oil decreases; the heat generated by friction is difficult to dissipate, and it is also hard to maintain the pressure oil film required for fluid friction in the bearing shells. This can even lead to damage to the bearing shells, wear of seals, and destruction of the impeller. To ensure an adequate supply of lubricating oil and maintain the pressure oil film necessary for liquid friction in the bearing shells, it is essential to equip large-scale units with lubricating oil pressure monitoring and protection measures. The oil supply pressure for the lubrication system of centrifugal compressors is generally set at 0.10–0.15 MPa, while a higher oil supply pressure is required for gearboxes, typically set at 0.20–0.30 MPa. To ensure the pressure oil film necessary for the proper operation of the bearing shells, as well as to remove the heat generated by friction, a minimum supply pressure of 0.06 MPa is typically set. When the lubricating oil pressure drops to 0.06 MPa, the auxiliary oil pump starts automatically. If the measures taken do not yield any effect and the lubricating oil pressure continues to drop, an automatic shutdown interlock is activated at a pressure level of 0.05 MPa.     93. To prevent reverse rotation after the compressor stops, what actions must be taken before reducing speed and shutting it down? Answer: To prevent the compressor from reversing rotation after it stops, the following must be done before reducing speed or shutting down: 1) Open the vent valve or return valve to allow the gas to be vented or returned ； 2) Ensure that the check valves on the system pipelines are properly closed ； 3) After completing the above tasks, gradually reduce the speed and shut down the machine. 94. What are the hazards of excessive vibration in steam turbines?   Answer: When the vibration of a turbine exceeds the specified limits, it can cause damage to the equipment, and even lead to serious consequences: 1) The stress on rotating components such as blades, shrouds, and impellers increases, resulting in high alternating stresses that cause fatigue damage ；   2) Wear occurs in the moving and stationary parts of the unit; in mild cases, this leads to wear of the end shaft seals and diaphragm steam seals, an increase in gaps, higher steam leakage losses, and a reduction in the economic efficiency of the unit’s operation ； In severe cases, it can cause the spindle to bend ；   3) It causes the various connecting components to loosen. When vibration is severe, it can cause resonance in the bearings, bearing housings, main oil pumps, transmission gears, condensers, pipes, and other components connected to the unit. This can lead to loose connection bolts and broken foundation bolts, thereby causing serious accidents ； 4) Excessive vibration at the high-pressure side may cause the emergency shutdown device to activate erroneously, resulting in the shutdown of the unit. 95. What are the causes of abnormal vibration in steam turbines?   Answer: Abnormal vibration in steam turbines can be classified into the following three main categories based on their causes: 1) Structural reasons, which are related to defects in the machine’s design and manufacturing.   2) Reasons related to installation: Caused by defects in the machine’s assembly or on-site installation.   3) Operational reasons: Caused by incorrect operating procedures, machine damage, or excessive wear.     96. What are the main causes of abnormal vibration in turbines due to equipment problems?   Answer: The main installation-related causes of abnormal vibration in steam turbines are: 1) Incorrect balancing of the rotating parts ；   2) Poor alignment of the turbine with the compressor, couplings, gearbox, etc ；   3) The rotating components attached to the unit, such as the governor, the oil pump driven by the main shaft, and the emergency safety devices, are not properly balanced and have been installed incorrectly ；   4) The heated components are not installed correctly; when installed in the cold state, no account is taken of their free expansion and thermal deformation under operating conditions at high temperatures. As a result, these components cannot expand freely when heated, which leads to bending and disruption of balance ；   5) The fit dimensions of certain components do not meet the requirements ；   6) The bearing installation does not meet the requirements; the fit between the bearing bush, bearing sleeve, and bearing housing is improper ；   7) The unit foundation does not meet the requirements or the foundation has sunk ；   8) The connection between the pipelines and the unit does not meet the requirements; improper pipeline installation generates stress, causing the unit to deform or shift, or resulting in vibrations in the piping system ；   9) Poor insulation of the cylinder or damaged insulation layer leads to uneven thermal expansion ； 97. What are the reasons for sudden abnormal vibration in a turbine during operation?   Answer: Abnormal vibration in the turbine during operation is usually caused by improper operation and maintenance: 1) Rotor imbalance, that is, an uneven distribution of mass within the rotor, such as erosion of the blades, scaling on the blades, or loss of blades ；   2) Insufficient or inadequate bearing lubrication, unstable operation of the oil pump, excessively high or low temperature of the lubricating oil, and low oil pressure can all affect the formation of the bearing oil film ；   3) Excessively high steam temperature causes excessive thermal expansion and deformation of the cylinder ；   4) The decrease in vacuum causes the exhaust temperature to become too high, resulting in abnormal expansion of the exhaust cylinder ；   5) Too low steam temperature can cause water hammer in the turbine, among other issues.       6) Debris falling between the rotating and stationary parts of turbines, compressors, transmissions, and couplings ；   7) Some rotating parts fixed to turbines, compressors, transmissions, and couplings become loose, deformed, or displaced, causing the center of gravity of the rotating body to change ；   8) The sliding pin system is stuck and cannot expand freely ；   9) Unstable airflow in the compressor, with increased vibration of the compressor ； 98. What are the possible reasons for an increase in axial thrust during compressor operation? Answer: There are many factors that affect the axial thrust of a compressor, some of which are related to the structure and others to operation. The main reasons for an increase in axial thrust during operation are: 1) Overpressure at the compressor outlet ； There are many reasons for overpressure, such as an increase in rotational speed or reduced production at the same rotational speed. 2) Damage to the wheel cover seal and spacer sleeve seal, resulting in increased thrust at the higher stages ； 3) The balance disc sealing device is damaged or the balance tube is blocked, resulting in a decrease in the axial force on the balance disc. 99. What are the hazards of axial force? Answer: The axial thrust acting on the rotor has the following hazards: 1) Under the action of the axial force, the rotor will undergo axial displacement in the direction of that force. This axial displacement will cause relative sliding between the shaft journal and the bearing shells, which may result in damage to those bearing shells or the shaft journal ； 2) Excessive thrust can affect the lifespan of the bearings; in severe cases, it can cause the bearing shells to burn out, leading to rotor misalignment ； 3) Excessive rotor displacement can lead to friction, wear, and collision between the rotor assembly and the stator assembly, resulting in machine damage. 100. What phenomena occur when a cylinder becomes deformed? Answer: If the cylinder becomes deformed, the following phenomena will occur: 1) Air leakage will occur in the horizontal or vertical cross-sections of the cylinder, usually near the steam seals at the high and low pressure ends ；     2) The vacuum cannot be established or the vacuum is damaged ； 3) Occurrence of steam seal wear, abnormal noises resulting from friction between moving and stationary components inside the cylinder, or increased vibration of the unit ；