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1. What is meant by “surge” in centrifugal compressors? Answer: At any given rotational speed, a centrifugal compressor has a minimum flow rate point at its maximum pressure. When the pressure exceeds this value or the flow rate falls below this point, vortices form within the gas in the compressor. The formation and dissipation of these vortices cause the flow passages in the impeller to become alternately blocked and unblocked, leading to low-frequency vibrations in both the airflow and the blades. This results in severe periodic vibrations and roaring noises inside the machine. This phenomenon is known as “surge” in centrifugal compressors. 2. What are the hazards of \"surge\" in centrifugal compressors? Answer: Surge is extremely harmful to compressors; it causes intense vibration of the blades due to strong pulsations and periodic oscillations in the airflow, which increases the stress on the impeller, raises noise levels, leads to severe vibrations throughout the unit, and may damage bearings and seals, resulting in shutdown or other accidents. 3. How to tell if a centrifugal compressor is experiencing “surge”? Answer: (1) Measure the noise of airflow in the compressor’s intake pipe ； Under stable operating conditions, centrifugal compressors produce low-level, continuous noise. However, as they approach the surge condition, periodic fluctuations in airflow occur throughout the system, resulting in noise levels that vary periodically in the exhaust pipeline. Once surge conditions are reached, the noise level increases sharply, and even explosive noises may occur. (2) Observe the changes in the compressor outlet pressure and inlet flow rate ； When a centrifugal compressor operates under stable conditions, the changes in its outlet pressure and inlet flow rate are minimal and regular, with very small variations in the measured data. When approaching or entering the surge condition, both of them change significantly, experiencing periodic and large-amplitude fluctuations; sometimes gas can even be observed being pushed back out from the compressor inlet. (3) Observe the vibration of the machine body and bearings. When approaching or entering the surge condition, both the machine body and the bearings experience intense vibrations, with their amplitude **increasing** compared to normal operation. 4. What measures should be taken when a centrifugal compressor experiences surge? Answer: (1) A backflow device is installed between the exhaust pipe and the intake pipe of the compressor; when the air supply volume supplied by this device drops to a specified value, the backflow amount is increased appropriately in a timely manner, causing a portion of the gas in the exhaust pipe to return to the compressor inlet – this phenomenon is known as backflow. This allows the centrifugal compressor to maintain its normal operating flow rate, keeping it within a stable operating range and thus preventing surge from occurring ； (2) When the air intake volume is low, in order to prevent surge and keep the compressor operating within its normal range, a portion of the gas from the compressor outlet can be vented or burned in a flare ； (3) Pressurization in the compressor outlet pipeline is prohibited; when instability occurs in the hydrogen-rich gas recovery device, causing an increase in pressure within the pipeline, it is necessary to reduce the pressure promptly to prevent surging ； (4) If the above measures prove ineffective, the machine can be stopped immediately for inspection and repair. 5. What is the “stagnation flow” of a centrifugal compressor? Answer: When the gas flow rate exceeds the rated value, the angle of the relative velocity of the airflow entering the impeller is too large; as a result, the airflow strikes the non-working side of the blades, causing flow separation on the working side of those blades. When the gas flow rate continues to increase until it reaches a certain maximum value, the flow velocity at the smallest cross-section within the flow channel will reach the speed of sound, and the flow rate can no longer increase. At this point, all the work done by the impeller on the gas is used to overcome the flow losses; mechanical energy is converted into thermal energy, and the gas pressure does not rise. This condition is known as \"stall flow rate\". 6. Under what operating conditions should a centrifugal compressor be used? Answer: The centrifugal compressor should operate within the range between surge conditions and stagnation conditions. 7. Why are the shaft bearing designs of centrifugal compressor units more complex than those of centrifugal pumps? Answer: For radial supports of common units with rotational speeds exceeding 3,000 r/min, forced-lubrication sliding bearings, commonly known as bearing bushes, are frequently used. Centrifugal pumps generally use rolling bearings, while multi-stage pumps also use shaft bearings. However, the shaft bearings for pumps are usually cylindrical bearings divided horizontally into two halves. The bearing shells of large turbines with high rotational speeds are usually multi-piece composite types, known as oil wedge bearings. The sliding bearing supports the rotating shaft through the oil wedge formed by the oil film between the shaft and the bearing. The oil wedge causes the two centerlines of the shaft and the bearing bush to be out of alignment, while the center of the rotating shaft tends to align with the center of the bearing bush from time to time. The oscillation between concentric and eccentric conditions causes vibration of the rotating shaft, which is known as oil film vibration. The higher the rotational speed, the more severe the oil film vibration becomes. Large units with high rotational speeds use radial bearing bushings with multiple oil wedges, which prevents vibration of the oil film around the rotating shaft. 8. Why are centrifugal compressors not allowed to operate at their critical speed? Answer: Operating at the critical speed can cause resonance, which leads to an increasing amplitude of vibration in the equipment, ultimately resulting in its damage. 9. What is the function of a pressure compressor safety valve? Answer: When the pressure at the outlet of the compressor exceeds the specified value, the safety valve will open automatically to release the pressure, thereby ensuring the safety of the equipment. 10. What are the precautions for starting a screw pump? Answer: It is strictly prohibited to start the screw pump with the inlet and outlet valves closed or when the outlet flow path is blocked. 11. What is the function of a lubricating oil surge tank? Answer: It relies on the height difference to automatically replenish the oil pressure and volume in the main oil line, thereby ensuring the lubrication of the unit and its safety. 12. What are the precautions when filling the high-level fuel tank? Answer: After the lubricating oil pump starts, open the valve leading to the high-level oil tank; do not open it too wide. Do not leave the area during the oil filling process to prevent oil leakage. Fill the tank until oil returns through the sight glass, and then close the inlet valve. It should be noted: the unit must not be started if no return oil is observed in the high-level fuel tank. 13. How many types of shaft seals are there for centrifugal compressors? Answer: (1) Combed tooth labyrinth seal ； (2) Labyrinth seal plus vacuum seal ； (3) Floating ring seal. In reality, the floating ring seal is a combination of gas sealing + oil film sealing + double mechanical seal ； (4) Dry gas seal ； (5) Steam blockage seal. 14. What is the function of the turbine lubrication system? Answer: (1) Lubricate bearings and couplings ； (2) Carry away the heat generated by friction, cooling the lubricated areas ； (3) It can reduce wear at various friction points ； (4) Hydraulic speed control is available ； (5) As power for the turbine hydraulic valves ； (6) Flush out impurities from the lubricated area ； (7) Vibration damping and corrosion prevention effects. 15. Why are vent holes present on the engine lubrication oil tank and lubricating oil pipelines? Answer: During operation, the lubricating oil system of the engine generates some gas, and at the same time the oil temperature rises. Therefore, vent holes are necessary; they serve the following purposes: (1) To remove some of the gas from the oil, preventing gas blockages that could affect the return flow ； (2) Can release some heat ； (3) It can balance the tank pressure. 16. What is a labyrinth seal? Answer: Many thin metal sheets are fixed in the gaps between the shaft and the partitions to create tortuous gaps that resemble a maze. As the gas inside the compressor passes through this maze, it encounters significant resistance, which helps to prevent large-scale leakage of gas. However, this type of sealing still allows some amount of fluid to leak out. 17. What are the characteristics of a compressor rotor? Answer: The rotor is the part that rotates at high speed; the main shaft is equipped with an impeller, balance disk, thrust disk bushings, couplings, etc., and some also have labyrinth seal sleeves. For impeller blades, the exit angle is generally between 30° and 60°. Since gas is much lighter than liquid, the impellers have relatively large diameters and high speeds; this is necessary to achieve a high circumferential velocity and thus grant the gas greater kinetic energy. 18. What are the characteristics of the compressor stator? Answer: The stator is the casing, which consists of an intake chamber, stage partition plates, an exhaust chamber, shaft seals, inter-stage seals, inlet guides, and so on. 19. What is the critical speed? Answer: Resonance occurs when the frequency of the rotor’s forced vibration is the same as or coincides with the frequency of its natural vibration; at this point the amplitude of the rotor’s vibration is particularly large. The speed at which this happens is known as the critical speed. 20. Why is a throttle orifice plate installed on the bearing oil inlet pipe? Answer: It is mainly used to control the amount of oil entering the bearing, as well as to regulate the oil temperature, keeping the temperature rise within 13°C–15°C, thereby ensuring the normal operation of the bearing. 21. Why is it necessary to warm the pipes before starting a turbine? Answer: Before starting up, since the main steam pipes as well as various valves and flanges are at low temperature, if high-temperature, high-pressure steam is introduced immediately, the various pipe fittings in the pipeline will be heated unevenly, resulting in large temperature differences and thus significant thermal stresses that can damage the pipe fittings. Therefore, it is necessary to warm up the pipes first. When warming the pipe, use the hand valve to regulate the flow rate, and do so slowly. 22. Why is it necessary to drain water before starting the turbine? Answer: During the pre-startup pipe and machine warming-up process, some of the steam condenses into water. If this condensed water is not removed promptly, the high-speed flowing steam will carry the water into the cylinder, causing damage to the blades; therefore, it is necessary to drain all the water from the pipes before starting the machine. 23. Why does the oil pump need to keep running for a while after shutdown? Answer: Once the shaft comes to rest, the bearings and shaft journals experience rapid temperature increases due to the heat transmitted from the cylinders and rotor. If no cooling measures are taken, this can lead to deterioration of the oil quality in those areas, as well as damage to the babbitt lining of the shaft journals and bearings. To prevent this, the oil pump must continue to operate for a while to provide cooling. The duration for which the oil pump operates depends on how much the temperature of the cylinders and bearings drops; the pump can be stopped only when the cylinder temperature falls below 80°C and the bearing temperature falls below 35°C. 24. What are the common accidents that occur during turbine operation? Answer: There are six common accidents that occur during the operation of steam turbines, namely broken blades, overspeed, water hammer, severe vibration, oil deficiency and fire, or an increase in back pressure (in back-pressure type turbines). 25. What are the causes and hazards of compressor reversal? Answer: Reversal of the air compressor occurs during switching between two compressors or when a compressor is shut down. It is caused by improper operational procedures. The fundamental reason for this reversal is that after the compressor stops, the medium flows from the outlet back into the inlet. The possible causes are as follows: (1) During an emergency shutdown, the outlet valve does not close properly, the outlet check valve gets stuck, while the inlet valve remains open ； (3) After shutdown, the flare valve at the outlet was not opened or was closed too early, while the inlet valve was in the open position. The reversal of the compressor can increase the pressure at the compressor inlet, leading to an increase in the pressure throughout the system and affecting the stable and safe operation of the device. Furthermore, reverse rotation of the compressor can also deteriorate the lubrication of the compressor bearings (including the main bearings and thrust bearings), making it difficult for an oil film to form or keeping it unstable, which can lead to oil burning incidents. 26. Why does the engine oil system catch fire? Answer: Incidents of fires caused by leaks in the oil system of air compressors occur frequently. The main reasons for such fires are oil leaks from pipes and flange joints in the oil system, as well as leaks from oil deflector rings at bearings; when this oil comes into contact with hot surfaces, it can ignite. 27. Why do engines run at excessive speeds, and what are the hazards? Answer: When the compressor is subjected to a sudden load loss (such as when the flare valve suddenly opens fully), if the turbine’s speed control system fails or the emergency safety device becomes stuck, or even if the emergency safety device does function but the main steam valve gets stuck due to scaling, overly tight packing, a bent valve stem, or other reasons, it can lead to the compressor running at excessive speeds and spinning out of control. Over-speed operation is extremely hazardous; it can cause deformation of the unit’s rotor, blades, and impeller. In severe cases, the blades or impeller may break off from the cylinder and fly out, damaging the machine room or leading to other accidents. 28. Why are expansion joints installed on the compressor outlet pipe and the lubricating oil pipe? Answer: It mainly prevents the expansion of pipelines due to rising temperatures from exerting an additional force on the unit, which could cause vibration. By installing expansion joints, when the pipelines expand, the force is applied to these joints, causing them to deform; this **reduces the additional force on the unit, thereby contributing to its safe operation. 29. What is the purpose of a barring test? Answer: The main purpose of turning the machine is to check for any debris inside the machine and to ensure that all moving parts are tight. Additionally, turning the machine allows the lubricated parts to be initially lubricated. 30. Briefly describe the working process of a steam turbine? Answer: Steam under certain pressure and temperature enters the turbine. As it flows through the steam passage composed of nozzles and stationary blades, it expands, thereby acquiring high speeds. The high-speed steam impinges on the blades on the turbine rotor, causing it to rotate at a certain speed, which in turn drives the compressor or other working machines. 31. How many parts does a turbine structure consist of? Answer: The main components of a steam turbine include the cylinder, bearings, frame, nozzles, diaphragms, blades, coupling, main shaft, regulator, emergency safety device, steam seal, and main oil pump. 32. Briefly describe the working principle of a steam turbine? Answer: A turbine is a rotary prime mover that relies on steam to generate power. When steam passes through the turbine nozzles, thermal energy is converted into kinetic energy due to high-speed flow; as this high-speed flow passes over the working blades, it is transformed into mechanical energy that drives the rotation of the turbine rotor, thus causing the turbine to spin. 33. Explain the functions of the various components of a steam turbine? Answer: (1) Frame: Connects the turbine unit to the foundation, and supports and fixes the turbine. (2) Cylinder: Ensures that steam completes its work process within the steam turbine. (3) Nozzle: Converts the thermal energy of steam into kinetic energy, directing it in a specific direction so that the moving blades can do work. (4) Partition: It serves to separate the nozzles at different levels. (5) Blades: Divided into moving blades and stationary blades, which convert the kinetic energy of steam into mechanical energy. (6) Shaft: Fixes the impeller. (7) Shaft seal: It serves as a seal to minimize steam leakage outward. (8) Bearings: Turbine bearings include radial bearings and thrust bearings, which primarily serve to bear the weight of the rotor as well as axial thrust. 34. What is the function of the turbine speed control system? Answer: The function of the turbine speed control system is to maintain balance between the turbine’s output power and the load. When the load increases, the speed control system opens the valves more to increase the amount of steam supplied (the opposite happens when the load decreases). It is necessary to ensure that the turbine operates at its proper speed whenever the load changes. Additionally, when the load suddenly decreases, the speed control system must also prevent a rapid increase in rotational speed. The speed control system of a steam turbine serves to adapt to the load requirements and regulate the rotational speed. 35. What requirements should a speed control system meet? Answer: The speed control system shall meet the following requirements: (1) When the valves are fully open, the governor system shall be able to maintain the turbine at full load operation ； (2) When the turbine is suddenly reduced from full load to no-load, the speed control system should be able to maintain the turbine’s speed below the speed at which the emergency shutdown device activates ； (3) There is no sticking or loosening in the various moving connections of the main steam valve, such as those on the throttle valve rod, oil control valves, hydraulic actuation mechanisms, and the links of the speed control system. When the load changes, the throttle valve should move in a uniform and smooth manner; when the system load is stable, there should be no fluctuations in the load ； (4) After the emergency safety device activates, it must be ensured that the main steam valve closes tightly ； (5) When one or several parameters of the compressor (inlet and outlet pressure or flow rate) change, the speed of the turbine can be adjusted. 36. Why must the steam seal ejector and steam seal cooler be turned on before starting a back-pressure steam turbine? Answer: In back-pressure steam turbines, due to the high exhaust pressure, if the steam seal extractor and steam seal cooler are not activated first before starting up, a large amount of steam will leak out of the turbine through the shaft ends. Additionally, some of this steam will mix into the lubricating oil, causing it to become contaminated with water. Therefore, it is necessary to activate the steam seal extractor and steam seal cooler first to create a certain level of vacuum, which will draw the exhaust steam out and cool it into condensate water. 37. Why must the backpressure steam be directed in front of the exhaust isolation valve before starting a backpressure turbine? Answer: The exhaust steam from a backpressure turbine is discharged into the steam pipeline network before being sent to the users. Therefore, a steam warming process is also required from behind the turbine exhaust isolation valve up to the transition section of the pipeline network. The requirements for this steam warming are the same as those for warming the steam before the main steam isolation valve at the turbine inlet. Thus, before starting a backpressure turbine, it is necessary to direct the backpressure steam to the area behind the turbine exhaust isolation valve; care must be taken during operation to manage the condensate flow and prevent water hammer effects. 38. Which components on the cylinder generate thermal stress during the startup and operation of a turbine? How to control it? Answer: When a turbine starts up, the steam temperature inside the cylinder rises sharply, resulting in a large temperature difference between the inner and outer walls of the cylinder. This causes the inner wall to experience thermal compressive stress while the outer wall experiences thermal tensile stress; when the temperature difference is large, the thermal stress is also high. When the stress exceeds the yield limit of the cylinder material, plastic deformation occurs, and cracks may even form. The thickness of the cylinder flange is much greater than that of the cylinder itself, which means that thermal stress has a greater impact. The heat is transferred to the cylinder bolts through the flange, and the temperature of the flange is always higher than that of the bolts. As a result, the bolts are subjected to additional thermal stress. If this additional stress exceeds the strength limit of the bolts, there is a risk of them breaking. Thermal stress is generally related to the heating rate; for medium and low-pressure steam turbines, thermal stress is controlled by regulating the heating rate, that is, by controlling the startup warm-up time. 39. What are the causes of unit vibration? Answer: The main causes of vibration in the unit are: (1) Low bearing oil pressure ； (2) Oil temperature too high or too low ； . (3) Degradation of oil quality ； (4) The main steam temperature is too high or too low ； (5) Main steam with water carryover ； (6) Oil at the compressor inlet ； (7) Failure of the sliding pin mechanism, causing the cylinder to deviate from its center due to expansion ； (8) The spindle is bent or the coupling between the impeller and the spindle is loose ； (9) Blade breakage or detachment, disrupting dynamic balance ； (10) Debris enters the machine inlet, blocking the impeller flow channel ； (11) Damage to the labyrinth shaft seal, collision between the comb teeth or friction with the shaft ； (12) Cylinder deformation caused by excessive thermal stress due to inappropriate temperature changes ； (13) Misalignment of the coupling centers ； (14) The bearing shell clearance is not within specification ； (15) The connecting bolts between the unit bearings and the base are loose. 40. What are the effects of excessively high or low inlet steam pressure on the operation of a turbine? Answer: Since the turbine is designed with the strength of its various components taken into account based on the rated main steam pressure, when the main steam pressure exceeds this rated value, it can cause overload in the main steam as well as in the valves on the pipelines, the steam chambers of the throttle valves, and their blades; this may even lead to damage to these components. Furthermore, when the steam pressure exceeds the rated value, it increases the operating temperature of the latter stages of the turbine, thereby worsening the operating conditions of the blades in those stages. When the steam pressure is below the design value, the efficiency of the turbine decreases, the amount of steam required at the same load increases, which leads to an increase in axial thrust. At the same time, it increases the stress on the subsequent blades, and in severe cases, it can cause the blades to deform. Furthermore, too low steam pressure will cause the nozzle to become blocked, preventing the turbine’s power from reaching the rated value. 41. What effects does an excessively high or low inlet steam temperature have on the operation of a turbine? Answer: An excessively high steam temperature, above the design value, is advantageous from an economic perspective, but it is not permissible from a safety standpoint. At high temperatures, the mechanical properties of metals decline rapidly, which reduces the service life of various components in the turbine, such as control valves, nozzles and blades in the speed stages and pressure stages, shaft seals, and bolts. It may also cause the impeller casings in the earlier stages to become loose. Therefore, an excessively high steam inlet temperature is not allowed. A steam temperature lower than the design value increases the blade reaction degree, thereby increasing the axial thrust. Operating at too low a temperature increases steam consumption and affects economic efficiency. Furthermore, a decrease in steam temperature will cause water erosion of the blades in the latter stages of a condensing steam turbine, reducing its service life. 42. What problems can occur when a turbine operates under overload? Answer: When a turbine is overloaded, the following problems generally occur: (1) Due to the increased steam inflow, the bending stress on the blades increases; at the same time, the deflection caused by the stress on the diaphragms and stationary blades also increases ； (2) As the steam inflow increases, the axial thrust rises, which leads to an increase in the temperature of the babbitt in the thrust bearings; in severe cases, this can result in the destruction of those bearing components ； (3) When the opening degree of the speed control valve approaches its limit position, the hydraulic actuator also reaches near its maximum stroke, which deteriorates the performance of the speed control system. Both the rate of speed variation and the lag rate increase, leading to a decline in operational stability. Due to the aforementioned issues, it is not allowed for the turbine to operate under overload for extended periods. 43. What is the function of a vapor seal cooler? Answer: The mixed gas (water vapor and air) discharged by the cooling steam extractor reduces the impact on the environment. 44. How is the change in axial displacement generated? What are the harms? 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 reasons for this axial displacement include the following: (1) When the compressor starts up or the turbine loses load, the direction of the axial force changes, and since there are gaps between the main thrust blocks and auxiliary thrust blocks and the thrust discs on the main shaft, this results in movement of the rotor and thus axial displacement ； (2) Excessive axial thrust leads to the breakdown of the oil film, causing the babbitt on the bearing shells to wear or melt, which results in axial displacement ； (3) As the load on the unit increases, the bearing shims and pad frames behind the thrust bearings undergo elastic deformation due to axial forces, which also causes axial displacement. This type of axial displacement is known as elastic displacement of the bearing housing. The amount of elastic displacement depends on the structure and the load applied, and it is generally between 0.2 and 0.4 mm. The axial displacement of the turbine unit should be kept within the allowable range, generally 0.8 to 1 mm. Exceeding this value can lead to frictional collisions between the moving and stationary parts, resulting in serious damage such as shaft bending, rupture of partitions and impellers, and breakage of numerous turbine blades. 45. What is a hard shaft? What is a flexible shaft? Why are flexible shafts used in compressors? Answer: The first critical speed of the rotor applies to an axially stiff shaft, or rigid shaft, whose operating speed is above this critical speed. A shaft whose first critical speed of the rotor is below the operating speed is called a soft shaft, or flexible shaft. Due to the high rotation speed of the compressor, if a solid shaft is used, it will inevitably lead to an increase in the size of the shaft and the components mounted on it, as well as the entire unit, which in turn raises costs and may sometimes be difficult to implement. Therefore, flexible shafts are generally used in air compressors with very high rotational speeds. 46. Why is it necessary to warm up the turbine at low speed? Answer: 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 or loosening of parts such as cylinders, diaphragms, nozzles, shafts, impellers, steam seals, and shaft seals. For turbines that have not been fully cooled, especially those without a barring device, low-speed warming is also necessary during startup; this is done to prevent the shaft from bending and deforming, which could otherwise lead to friction between the moving and stationary parts of the turbine. The speed during warm-up should not be too low. Because the rotation speed is too low, it is difficult to establish a bearing oil film, resulting in bearing wear. At the same time, if the rotational speed is too low, control becomes difficult, and shutdowns are likely to occur when steam temperature and pressure fluctuate. If the warm-up speed is too high, it will result in a too rapid warm-up.