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Principles and Operation of Turbines 1. Turbine stages: The basic working unit consists of a row of nozzle vanes followed by an adjacent row of rotor vanes. 2. Multiple-choice question: During expansion flow, the rate of change of velocity in the subsonic gas stream is greater than the rate of change of its specific volume; therefore, the cross-sectional area of the channel decreases as the velocity increases ； The rate of change of the speed of the supersonic gas flow is less than the rate of change of its specific volume, so the channel area increases as the speed increases. 3. Fill in the blank: (Nozzle loss) is the kinetic energy lost by steam due to friction within the flow channel. 4. Based on the characteristics of energy conversion of steam within the turbine, turbine stages can be classified as (pure impulse stage), (reaction stage), (impulse stage with reaction), and (compound stage). 5. Pure impulse stage: A stage in which expansion occurs in the inlet vanes, while the steam does not expand in the outlet vanes, is called a pure impulse stage. 6. Impulse stage with reaction: A stage in which most of the steam expansion occurs in the nozzle cascade, with only a small portion taking place in the rotor cascade, is called an impulse stage. 7. The optimal value maximizes the peripheral efficiency. 8. The optimal speed ratio is: (x1)op = 1/2 cosα1. 9. The optimal speed ratio for the reaction stage is: (x1)op = cosα1. 10. Briefly: External losses include (1) steam leakage through shaft seals ； (2) Mechanical losses: 11. The utilization of residual velocity and the reheat phenomenon in multi-stage turbines can make the ratio of their internal efficiency to that of a single-stage turbine greater than 1. 12. Fill in the blank: The internal power of a turbine minus the mechanical losses gives the (shaft power). 13. Noun: Peng-Tai-Men coefficient: The ratio of any ideal flow rate through the nozzle to the critical flow rate under the same initial conditions is the Peng-Tai-Men coefficient. 14. Fill in the blank: When the pressure is reduced, to maintain the same power of the turbine, it is necessary to open the control valve wider (to increase the steam inflow), and the axial thrust of the unit (increases accordingly). 15. As the initial temperature of the turbine increases, the average heat absorption temperature of the steam in the boiler rises, thereby improving the cycle efficiency (and reducing the heat consumption rate). 16. An increase in exhaust pressure leads to an increase in (exhaust temperature). 17. When the external load increases, it causes the speed of the steam turbine generator set to decrease. 18. The efficiency of a steam turbine mainly depends on the structure of its flow passage components and the load level under which the turbine operates. 19. The control methods for steam turbines include nozzle control, throttling control, sliding pressure control, and combined control. 20. (1) During the nozzle adjustment process, as each control valve is opened one after another. (2) Throttling regulation simultaneously changes the opening degrees of several control valves. (3) Slip regulation: Slip operation results in the lowest throttling losses at part load. (4) The composite control mode is a combination of constant-pressure operation and variable-pressure operation. 21. Noun: Static characteristics of the control system: The relationship between the unit’s power and its speed under stable operating conditions is referred to as the static characteristics of the control system. 22. Adjusting the system settings to synchronize the synchronizer does not change its static characteristics; it merely approximately shifts the static characteristic curve. 23. Noun: Slowing rate: It is the ratio of the maximum change in rotational speed to the rated speed, over a period during which the external load changes while the output power of the unit remains constant. 24. Dynamic quality of the control system: (1) Dynamic stability of the control system ； (2) Dynamic overshoot ； (3) Transition time 25. Noun: Dynamic stability refers to the ability of a unit to transition from one stable operating condition to another when subjected to disturbances, with the dynamic response curve of such disturbances being convergent. 26. The smaller the rotor rise time constant, the easier it is for the rotor to accelerate, and the greater the likelihood of overspeed. The value of the rotor rise time constant is inversely proportional to the ratio of the unit’s rated power. 27. Increasing the working oil pressure of the hydraulic actuator can reduce its piston diameter, thereby decreasing its time constant. 28. Fill in the blank: To compensate for the power lag of the unit caused by the volume of the reheater, a (dynamic correction device) can be added to the control system. 29. Short answer: The emergency trip system primarily monitors conditions such as excessive turbine speed, wear of thrust bearings, low lubricating oil pressure, low EH oil pressure, and low condenser vacuum. 30. Q&A: Power correction serves two purposes: one is to cause dynamic overshoot of the high-pressure control valve during the regulation process, in order to compensate for the lag in power changes in the medium and low-pressure cylinders ； The second is to make fine adjustments to the generator’s output power, in order to achieve precise control over the electrical power output of the unit. 31. The main purpose of coordinated control of unit systems is to adjust the boiler’s combustion rate and the turbine’s valve opening as quickly as possible in response to changes in external load, so as to achieve a new balance between energy supply and demand. 32. Why is a main regulator for coordinated control added? Answer: It is used to modify the control commands of the turbine and boiler control system, to coordinate the energy balance between them, and to control the switching of operating modes. 33. Noun: Heat consumption Q0: The amount of heat consumed per unit of time (per hour) is referred to as heat consumption. 34. Steam consumption rate d0: Steam used per unit of electricity generated by the unit (KW). h) The amount of steam consumed (in kg) is referred to as ---- 35. The heat consumption rate is the amount of heat required per unit of electricity generated, and it can reflect the thermal efficiency of units with different capacities and parameters. 36. Q&A: Reasons for an increase in the temperature difference at the heater: (1) Water leaks out of the heater’s pipes due to cracks in those pipes, or the level of water on the steam side rises as a result of a malfunctioning trap, thereby flooding the heater’s pipes. This reduces the area available for steam to condense and release heat, resulting in insufficient heating and an increased temperature difference. (2) A fault in the heater evacuation system or severe leakage from the heater (in the case of a heater that is in a vacuum state) causes non-condensable gases to accumulate inside the heater. These gases adhere to the outside of the water pipes, causing heat transfer to deteriorate and the terminal difference to increase. (3) The surface of the heating water pipe is contaminated or scaled, increasing the heat transfer resistance and raising the terminal temperature difference. (4) Power plants often use the method of blocking tubes to temporarily address the issue of cracked water tubes in heaters, thereby avoiding the need to remove the entire heater. However, when too many tubes are blocked, it leads to a reduction in the heat transfer area, which in turn causes an increase in the temperature difference. 37. During unit operation, an increase in steam extraction pressure loss will cause the pressure inside the heater to decrease; if the temperature difference remains unchanged, the water temperature at the heater outlet will drop. 38. The thermal economy of the heater-cutout unit will decrease as a result. After removing the high-pressure heater, a considerable excess power can be obtained when the new steam flow rate remains unchanged and the flow path permits it, but the thermal efficiency decreases. 39. There are generally two methods for draining water from heaters: one is by gravity flow stage by stage, and the other is by using a drainage pump to send the water to the condensate pipeline at the outlet of the regenerator. 40. Excessively high condenser water level leads to reduced vacuum and increased subcooling. 41. Advantages of load regulation in sliding pressure operation: (1) Sliding pressure operation can improve the turbine’s adaptability to changes in external load. (2) Slip operation can extend the service life of the unit. (3) Slip operation improves the thermal efficiency of the unit to a certain extent. 42. Why, compared to constant-pressure operation, does the relative internal efficiency of the turbine increase at the same partial load under sliding-pressure operation? Answer: (1) The control valve that remains open under partial load is in its fully open position, resulting in a corresponding reduction in the throttling loss of the incoming steam. (2) The regulating stage possesses the characteristics of a pressure stage, enabling it to maintain constant stage efficiency at part load. (3) The humidity in each stage of the final stage group decreases relatively, which reduces moisture loss and thereby improves the stage efficiency of the final stage. 43. Why can the power consumption of the water pump be reduced under the same partial load, thereby decreasing the plant’s electricity usage? Answer: During low-load sliding pressure operation, the boiler feedwater flow rate and pressure decrease, allowing the feedwater pump to operate at a lower speed. Therefore, a variable-speed feedwater pump can be used, particularly one driven by a variable-speed small steam turbine. 44. Under high load conditions, the main steam pressure is controlled using a fixed-sliding-fixed method. 45. Peak shaving: The power grid adjusts the load in order to maintain balance between power generation and power supply; this adjustment process by the power grid is known as peak shaving. 46. The steam consumption rate is related not only to the effective steam consumption but also to the no-load steam consumption. 47. Fill in the blank: For unit systems including boilers, the economic allocation of load should be based on the principle of equal marginal increases in energy consumption; that is, when the total load is fixed, the load assigned to each unit system should result in equal marginal increases, thereby minimizing the overall energy consumption. 48. When subjected to large forces, metal materials may undergo plastic deformation, a phenomenon known as yield. The stress value of the specimen under tension is known as the yield limit of the material. 49. Metal materials will fracture under the combined action of certain temperature and tensile force. The higher the temperature, the greater it should be, and the shorter the load-bearing time before fracture. 50. In engineering, the amplitude of stress variation at which a material specimen breaks after undergoing 10^7 stress cycles is defined as the material’s fatigue limit. 51. The average stress on the cylinder wall is proportional to the pressure difference inside and outside the cylinder. 52. Scaling in the nozzle cascade flow channels will increase the pressure difference between the two sides of the partition. 53. Each layer within the rotor is subjected to the centrifugal force generated by the mass of the layers outside it; therefore, the tangential centrifugal tensile stress is greatest at the surface of its central hole and at the axial centerline of the impeller. 54. In a straight-blade profile with constant cross-section, the bending moment is greatest at the root section, and so is the bending stress. 55. The direction of the force exerted by steam on the moving blade is such that the bending stress on the back arc of the blade profile is compressive. 56. Factors affecting the relative expansion difference of the rotor: (1) The rate of change of steam temperature at each stage in the flow passage. (2) Shaft seal steam supply temperature. (3) Temperature difference between the inner and outer walls of the cylinder flange. (4) Steam temperature in the cylinder interlayer. (5) Cylinder exhaust temperature. (6) Friction blower loss. (7) The rotational effect of the rotor. 57. For a moving blade, the impact force of the steam on it changes once for each passage through a nozzle flow channel. This alternating steam force is called high-frequency excitation force. 58. Classification of rotor blade vibrations: In Type B vibration, the peak equilibrium position remains unchanged and the amplitude is zero. 59. Frequency dispersion is the ratio of the difference between the maximum natural frequency fmax and the minimum natural frequency fmin of blades in the same stage to their average value. 60. The safety criterion for unfrequency-modulated blades is: Ab≥［Ab］. 61. Blades or groups of blades that are not allowed to operate under resonance conditions are called frequency-tuning blades. 62. Comprehensive question: Causes of centrifugal force in a balance: (1) Unbalanced mass of the rotor. (2) Rotor bending causes mass imbalance. (3) Quality imbalance caused by loose components fitted on the rotor. Characteristics of the vibrations caused by rotor mass imbalance: Since the unbalanced centrifugal force is proportional to the square of the rotational speed, the amplitude of the forced vibration is also proportional to the square of the rotational speed. For the mass imbalance caused by rotor bending, the direction of the unbalanced centrifugal force is the same as the direction of the rotor bending. With the rotational speed unchanged, the angle between the phase of its maximum amplitude and the phase of the shimmy remains constant ； As the rotation speed increases, this angle also increases. For the quality imbalance caused by loose assembly parts, the positioning key restricts the direction of mass eccentricity, and the phase of the maximum amplitude is related to the phase of its positioning key. As the rotational speed increases, the angle between the phase of the maximum amplitude and the phase of its positioning key grows, and the play increases; moreover, the increase in amplitude with rising rotational speed is greater than the square of the rotational speed. 63. A rotor with an operating speed higher than the critical speed is a flexible rotor. 64. Turbines that are started using rated parameters and connected via a header system. 65. For units that use sliding-parameter startup, an appropriate vacuum should be established in the condenser before the boiler is ignited. 66. The method to reduce rotor eccentricity before starting up is to perform continuous turning for a prolonged period of time. 67. The superheat of the steam entering the turbine during startup should be greater than 50°C. 68. Methods of starting a turbine by impulse starting: (1) Impulse starting controlled by the main steam valve or its bypass valve. (2) The control valve regulates the start-up. (3) The medium-pressure control valve controls the start-up. 69. Startup of the medium-pressure cylinder: During the start-up impulse, the high-pressure cylinder is in an isolated state, and the main steam enters the reheater via the high-pressure bypass, thereby ensuring that the temperature of the reheated steam meets the requirements specified for the steam inlet parameters during the hot-start of the medium-pressure cylinder. Before and after the unit is connected to the grid, the steam supply is switched to the high-pressure cylinder; this type of startup is known as medium-pressure cylinder startup. 70. The so-called “grid connection” refers to connecting the output terminal of the generator to the power grid through an isolating switch, so that the electrical power generated by the generator can be fed into the grid for use by consumers. Grid connection conditions: The voltages on both sides of the isolating switch are equal, their phases correspond to each other, and the frequencies are the same. 71. The frequency of the output voltage of a synchronous generator is the same as the frequency of the power supply in the grid. 72. The closure of the main steam valve is an important indicator in the shutdown process. After the main steam valve is closed, the steam supply to the turbine is cut off; the process in which the rotor’s rotational inertia overcomes friction to slow down is known as the \"free-run process\". 73. Compared to cold start, hot start requires a higher temperature of the steam supplied to the shaft seal. 74. Optimization principles (objectives) for the turbine startup process: The objective of optimizing the startup process is to accelerate the startup speed as much as possible, while ensuring the safety of the unit. 75. During a major overhaul shutdown, the shutdown period should be utilized as much as possible to cool the cylinders and rotor; therefore, a sliding-parameter shutdown is commonly used for such shutdowns. 76. Causes of turbine rotor failure: (1) Repeated exposure to alternating stresses leads to the embrittlement of the rotor material, resulting in fatigue cracks. (2) Another reason is the high-temperature creep of the material. 77. The number of cycles of low-frequency alternating stress experienced from the appearance of the first detectable crack in the rotor of a new machine until the rotor fails is defined as the residual life.