11. Why does a condenser need a hot well?
Answer: The function of the hot well is to collect the condensed water, which is beneficial for the normal operation of the condensate pump. The hot well stores a certain amount of water to ensure that the condensate pump does not immediately run dry when the load is reduced. The volume of the hot well is generally required to be equivalent to the amount of condensate collected in about 0.5 to 1.0 minutes at full load.
12. What issues should be paid attention to during the turning process?
Answer: (1) Monitor whether the current of the turning motor is normal and whether the ammeter needle is fluctuating; (2) Regularly check if there are any changes in the rotor bend indication values; (3) Regularly listen for any rubbing sounds inside the cylinder and at the high and low-pressure steam seals; (4) Regularly check the operating condition of the lubricating oil pump and the thrust bearing oil pump.
13. What is the task of the steam turbine control system?
Answer: It is to keep the output power of the steam turbine balanced with the external load. That is, when the external load changes or the grid frequency changes, the turbine control system correspondingly adjusts the turbine's power to match the external load, establishing a new balance and keeping the speed deviation within the specified limit. In addition, it ensures that the external load matches the turbine output power and maintains stable turbine operation. When an external fault causes the turbine generator to suddenly lose load, the control system reduces the turbine governor valve to ensure that the turbine speed rise stays below the critical safety device activation value, maintaining the turbine running at no load.
14. Why is it necessary to specify the superheat degree of steam when starting and stopping a steam turbine?
Answer: If the steam is not sufficiently superheated, during the startup process, the temperature in the earlier stages may drop significantly, and the temperature in the later stages may even fall to the saturation temperature at that stage's pressure, turning into wet steam. Steam containing water can be extremely damaging to the blades, so during startup and shutdown, it is safer to maintain the superheat degree of the steam between 50~100°C.
15. Why should the deaerator in operation maintain a certain water level?
Answer: If the water level is too high, it may cause water ingress through the shaft seal, resulting in unit vibration, and excessive water running through the overflow pipe. If the overflow pipe cannot discharge in time, it can lead to deaerator head vibration, water hammer in the extraction steam pipe, and in severe cases, backflow accidents in the turbine extraction pipeline. If the water level is too low, it may cause a decrease in inlet pressure of the feedwater pump, leading to vaporization and, in severe cases, affect boiler water supply. When the water tank level is too low, the following measures can be considered: 1) limit turbine load; 2) control non-production steam use, reduce steam and water consumption, and decrease boiler evaporation; 3) appropriately increase the make-up water.
16. What are the causes of an increase in the condenser circulating water outlet temperature?
Answer: (1) An increase in the inlet water temperature, which correspondingly raises the outlet water temperature. (2) An increase in turbine load. (3) Fouling or blockage of the condenser tube sheet and copper tubes. (4) A reduction in circulating water flow. (5) Blockage of the secondary circulating water filter. (6) An increase in exhaust volume. (7) A drop in vacuum.
17. What are the dangers of excessively high new steam temperature to a turbine?
Answer: When manufacturers design turbines, the cylinder, diaphragm, rotor, and other components are selected based on the type of steel suitable for the steam parameters. Each type of steel has a maximum allowable operating temperature. Below this temperature, it maintains certain mechanical properties. If the operating temperature significantly exceeds the design value, it will inevitably lead to the deterioration of the metal's mechanical properties. Strength decreases, brittleness increases, causing cylinder creep deformation, looseness of the impeller fitting on the shaft, vibration or friction during turbine operation, and in severe cases, equipment damage. Therefore, turbines must not operate above the rated temperature.
18. What are the hazards of a decrease in steam turbine vacuum? Answer: (1) The exhaust pressure increases, which can reduce the enthalpy drop, making it uneconomical, and at the same time, it reduces the unit output; (2) The exhaust cylinder and bearing housing undergo thermal expansion, which may cause misalignment and vibrations; (3) Excessive exhaust temperature may lead to compressor condenser copper tube relaxation, compromising tightness; (4) It may increase the axial thrust of a pure impulse steam turbine; (5) A decrease in vacuum reduces the volumetric flow of the exhaust, which is unfavorable for the last-stage blades. The last stage may experience flow separation and swirling, and at certain parts of the blades, increased exciting forces may occur, potentially damaging the blades and causing accidents.
19. What are the main causes of increased axial displacement in a steam turbine? Answer:
(1) A decrease in steam temperature and pressure, overload in the flow path, and shutdown of the feedwater heater.
(2) Increased leakage due to wear in the diaphragm shaft seals.
(3) Poor steam quality causing scaling in the flow path.
(4) Occurrence of water hammer.
(5) Load changes. In general, the axial thrust of a condensing steam turbine increases with increasing load. For extraction or back-pressure steam turbines, the maximum axial thrust may occur at an intermediate load.
20. What is condensate subcooling? What are the causes of high subcooling?
Answer: The subcooling of condensate is the saturation temperature at the condenser pressure minus the temperature of the condensate. Causes: (1) There are structural defects in the condenser; there are not enough passages for the steam to reach the lower part of the condenser between the tube bundles. This causes the condensate to flow down from the top of the tubes, fall onto the upper tubes below, cool again, and not encounter accelerated airflow. As a result, when the condensate reaches the hot well, excessive subcooling occurs. (2) When the condenser water level is high, part of the copper tubes are submerged in condensate, causing subcooling. (3) Air leakage on the steam side of the condenser or malfunction of the vacuum system. (4) Excessive condenser cooling water flow or too low water temperature. (5) Rupture of condenser copper tubes, causing condensate to leak into the circulating water.
