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Section 5: Turbines and Centrifugal Compressors I. Turbines 1. Classification of turbines: Based on different thermodynamic processes, they can be divided into a) condensing types, whose exhaust pressure is lower than atmospheric pressure. b) Back-pressure type. The exhaust pressure is higher than atmospheric pressure. 2. Classified by working principle, they can be divided into: a) impulse turbines. The vapor expands only inside the nozzle. b) Reaction turbines, in which steam expands not only within the nozzles but also between the blade passages of the impeller. 3. According to the flow direction of the steam, they can be classified as a) axial flow steam turbines. b) Radial-flow steam turbine. 4. Working principle of the turbine: The high-temperature, high-pressure steam that enters the turbine expands within nozzles or blade channels, gaining great speed. The rapidly flowing steam strikes the turbine blades, causing them to rotate and thus driving the turbine. 5. Composition of the steam turbine a) Rotor: includes the main shaft, impeller, shaft seal sleeve, balance disk, and moving blades. b) Stator: It consists of the cylinder, diaphragm, nozzles, stationary blades, steam seal, and shaft seal, among other components. c) Control section: It includes components such as regulating devices, protection devices, and the oil system. 6. What is the speed stage, and why are speed-stage blades used? Answer: The first impeller in an impulse turbine has two rows of moving blades. This type of impeller is known as a velocity stage impeller. The reason for using speed-stage impellers is to reduce the number of turbine stages; a large reduction in pressure is applied at the first stage, resulting in a very high steam velocity at the nozzle exit. Within a row of moving blades, the kinetic energy cannot be fully utilized, and the residual speed remains quite high; therefore, a set of guide stator blades is used to change the direction of the steam flow. This allows the steam to be directed to a second row of moving blades, where its kinetic energy can be utilized again, thereby reducing losses due to residual speed. 7. What is the sluggishness rate of a turbine? The sluggishness rate refers to the time delay before the speed control valves respond to changes in load; this delay is particularly noticeable when the load changes direction or magnitude. At the same load level, the rotational speed can vary. The sluggishness rate is defined as the ratio of the maximum difference An in rotational speeds at the same load level to the rated rotational speed. 8. What is the speed variation ripple of a speed control system? Answer ； The speed variation rate refers to the ratio of the change in the turbine’s speed from full load to no load to its rated speed. Its calculation formula is: δ = (N1 – N2) / N × 100%. 9. What is coasting time, and what does the length of this time indicate? Answer: After the unit stops receiving steam, it continues to rotate due to inertia for a certain period of time; the time from when steam supply is stopped until the rotation comes to a complete stop is called coasting time. An increase in idle running time indicates that the main steam valve is not tight. A reduction in idling time indicates damaged shaft seals or dynamic/static friction. 10. What requirements should the speed control system meet? Answer: ① When the main steam valve is fully open, the speed control system should be able to maintain the turbine in no-load operation. ②When the turbine is suddenly reduced from full load operation to no-load operation, the speed control system should be able to keep the turbine speed below the operating speed at which the emergency speed governor activates. ③There are problems of sticking and looseness in the various movable connections on the main steam valve and throttle valve rods, the oil control valves, the hydraulic actuators, and the links of the speed control system. When the load changes, the speed control valve should move evenly and smoothly ； When the system load is stable, the load should not fluctuate. ④When the emergency safety device activates, it is necessary to ensure that the main steam valve closes tightly. ⑤It should be able to meet the sequential opening of the speed control valves. ⑥After the turbine is started, when its speed approaches the rated value, it should be possible to close the throttle valve completely, at which point the main steam valve is fully open. ⑦The sluggishness rate of the governor shall not exceed 0.2% of the set value. The sluggishness rate of the speed control system shall not exceed 0–5%. 13. What does it mean that a turbine cannot maintain no-load operation? Answer: After the turbine is started, when the main steam valve is fully open, the vacuum level is normal, and the load is zero, if the speed control system is unable to maintain the rated speed – or even if the speed exceeds the threshold at which the emergency shutdown device activates – then it is said that the turbine cannot maintain no-load operation. 14: What are the reasons why the speed control system cannot maintain the rated speed under no-load conditions? Answer: The reasons are generally due to the throttle valves not closing properly or issues with the control system; there are roughly the following aspects involved ； ①It means that the valve seat contact is not tight, and the gap between the valve seat and the valve is too large. ②The linkage dimensions of the speed control system are installed incorrectly, or it increases the original tension of the speed control spring. ③The speed control system’s connecting rods, hydraulic actuators, misaligned valves, and cup valves are stuck. ④The drive lever or throttle linkage is loose. ⑤Besides the speed control valve, there are other areas where steam leaks. ⑥There is a large difference in temperature between the drive lever and the steam chamber, resulting in uneven thermal expansion, which causes the throttle valve to not be in its no-load position. 15. What is the static characteristic curve of a turbine? What are the requirements for such curves? Why? Answer: The static characteristics of a speed control system refer to the relationship between the load and the rotational speed of the turbine when it is operating in isolation. If this relationship is plotted on a graph with load on the horizontal axis and rotational speed on both the horizontal and vertical axes, the static characteristic curve of the control system is obtained. —The requirement for the static characteristic curve is that it should be a smooth, descending curve without any horizontal sections, with steeper slopes at both ends. Reason: To prevent speed fluctuations when the load is stationary. . 6. What are the reasons for oscillations in the speed control system? 7 Answer: ⑦ The speed control system has a high delay rate. ； ②The overlap ratio of the speed control valves is high. ⑤The throttle error overlap is large. ④Oil pressure fluctuation. ⑤The governor shaft is bent. ⑥The static characteristic curve of the speed control system is not satisfactory. o ⑦ Load fluctuations. 17. What is used to control the main steam valve and the throttle valve, and what is their function? Answer: The main steam valve is a two-position valve. It is the first valve through which steam enters the turbine; it is controlled by the shutdown oil pressure and the start-up oil pressure. Under normal conditions it remains fully open, but in an emergency situation, when the shutdown oil pressure is released, the valve closes completely, thereby stopping the flow of steam into the turbine and bringing the unit to a stop. The throttle valve controls the steam flow entering the turbine, thereby regulating its speed; it is controlled by the secondary oil pressure generated by an amplifier. 18. Working principle of the amplifier: The lifting height of the sleeve is used to control the oil leakage area, thereby regulating the oil pressure; the lever’s displacement signal is converted into an oil pressure signal. 19. Principle of incorrect throttle operation: Answer: In a balanced state, the two middle shoulders seal the oil ports on their sleeves. At this time, the forces acting on the upper and lower parts of the incorrect throttle are in balance. If the wrong throttle position causes the oil pressure to rise, the wrong throttle moves upward, which opens the lower oil discharge port controlled by the upper shoulder and causes the piston of the hydraulic actuator to move downward. As the piston moves downward, the feedback mechanism causes the throttle to return to its middle position; the oil port shoulder is sealed, the hydraulic actuator stops moving, and it reaches a new equilibrium position. 20. Why is it necessary to warm the pipes before starting a steam turbine? Answer: Before startup, since the main steam pipes, various valves, flanges, etc. are in a cooled state, it is first necessary to warm the pipes, allowing them to heat up gradually and expand evenly, thereby preventing excessive thermal stress. When warming the pipes, it is important to properly control the rate of temperature increase in the main steam pipes and the areas surrounding them. Under certain pressure, the temperature of the flanges, valves, and gas chambers connected to the steam pipeline is increased gradually and evenly, followed by an increase in pressure. Boosting voltage is also aimed at warming the tubes. During pressure increase, it is generally raised to the rated pressure at a rate of 0.1 MPa/min to 0.15 MPa/min per continent (i.e., 1 to 1.5 kgf/kg). The pressure increase while warming the pipes should not be too rapid, as this can cause the metal walls of the connection bolts, pipes, and valves to heat up too quickly and thus be subjected to excessive stress; it may also lead to cracks or damage due to a large temperature difference between the inside and outside of the pipes. 21. What is primary pipe warming and secondary pipe warming? What precautions should be taken during pipe warming? Primary pipe warming refers to the warming process of the pipe between the main steam isolation valve and the main steam takeoff valve ； Secondary pipe warming refers to the pipe warming from the main steam isolation valve to the main steam valve. When warming pipes, proper drainage must be ensured, as well as attention paid to the rate of increase in steam pressure and temperature. 22. Why is it necessary to warm up the turbine at low speeds? What are the disadvantages of using too high or too low speeds for warming up? Answer ； The purpose of low-speed warm-up is to ensure uniform thermal expansion of all components of the unit, thereby preventing components such as cylinders, diaphragms, nozzles, shafts, impellers, steam seals, and shaft seals from deforming or becoming loose. For turbines that have not been fully cooled, especially those without a barring device, low-speed warming-up is necessary during startup to prevent shaft deformation, which could lead to friction between the moving and stationary parts at the steam inlet area. If the speed is too high during warm-up, it can lead to excessively high warm-up temperatures; whereas if the warm-up speed is too low, proper bearing oil pressure cannot be established. Therefore, the warm-up speed must be appropriate. 23. During startup, why sometimes can’t the rotor rotate? ① The speed control oil pressure is too low, and the probe of the vacuum limiter is not in proper position. ②Valves that should be opened in case of improper operation were not opened, such as the automatic main steam valve and the governor valve. ③The steam parameters and condenser vacuum are too low. ④When starting with the main valve bypass door, in cold weather, due to the low temperature, steam remains in the pipes and cylinders. It cools and solidifies quickly, making the rotor less prone to movement. ⑤When using the synchronizer to turn the start wheel back to its original position, the air release valve behind the automatic main steam valve opens too wide, resulting in a large pressure difference before and behind the automatic main steam valve. ⑥Friction occurs in the mechanical parts. 24. Why does the turbine rotor bend in four directions after shutdown? Answer: After shutdown, as the turbine cools down, convection causes the hot air to remain in the upper part of the cylinder. As a result, the upper parts of the cylinder and rotor cool more slowly, creating a temperature difference between the upper and lower sections that causes the rotor to bend upward. The extent of this bending varies depending on the structure of each turbine. 25. Why do turbine blades break? ① When the turbine operates under overload, it causes the blades to exceed their allowable stress and thus break. ②The frequency of the turbine blades is not within acceptable limits, resulting in resonance and blade breakage during operation; this problem is particularly severe when operating at high or low frequencies. ⑦The blades break off due to a decrease in their mechanical strength caused by factors such as impact from mechanical impurities, wet steam erosion, and chemical corrosion. 26. Phenomena of turbine blade fracture ⑦ Metallic noises or impacts inside the cylinder. ②Turbine vibration may increase (when blades break off and get trapped between the rotating and stationary parts, causing friction or an imbalance). ⑦The final stage blades fall off, which may break the first row of copper tubes in the condenser and cause leakage. 27. Causes and phenomena of water hammer. ① Poor boiler water quality and improper operation ； ②Proper pipe warming and water drainage were not carried out before startup ； ⑦The steam temperature and pressure are too low. Signs: ⑦ Drop in steam temperature and pressure ； ②Vibration increases ； ⑦White smoke is coming from the shaft seal. Or white smoke coming from the steam pipe flange ； ④The sound of metal hitting or being struck. 28. What is the function of a damper? It prevents pressure fluctuations in the secondary oil from being transmitted to the oil-driven motor, which could otherwise cause fluctuations in speed ； 29. High-level fuel tank operation (lubricating oil): Provides fuel supply to the unit for 15 minutes in case of an accident, when the auxiliary fuel pump starts. Maintain stable oil pressure. 30. What is the function of the high-level oil cylinder for seal oil? It provides seal oil in emergency situations, and its liquid level helps to maintain the seal oil pressure 5 KPa higher than the reference air pressure. Control the leakage rate of the sealing oil.