Shutdown of the steam turbine

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Shutting down a turbine: 1. How many ways are there to shut down a turbine? How should one choose among these different methods? The ways to shut down a turbine include normal shutdown and emergency shutdown. A so-called normal shutdown refers to a planned shutdown. A fault shutdown refers to the process in which, in the event of an abnormal condition in a steam turbine generator set, protective devices are activated or a manual shutdown is carried out in order to prevent damage to the unit or to minimize losses. Fault shutdowns are further divided into emergency shutdowns and routine fault shutdowns. During normal shutdown, it is further divided into two methods based on the steam parameters during the shutdown process: ramp-down shutdown and rated-parameter shutdown. The method of shutting down is determined based on the purpose of the shutdown and the condition of the equipment. For a normal shutdown, if it is intended for maintenance purposes and it is desired to cool the unit as quickly as possible so that the maintenance can begin sooner, a reduced-parameter shutdown should be used whenever possible. Moreover, the duration of this reduced-parameter shutdown should be extended, with the parameters being lowered further. 2. What is sliding parameter shutdown? The turbine starts at rated parameters and load, with the high and medium pressure control valves fully open; the boiler adjusts the combustion process, gradually reducing the steam parameters, thereby causing the turbine load to decrease steadily. At the same time, the cylinder flange heating device is activated to gradually cool down the temperature of the cylinder flanges; once the parameters of the main steam reach a certain value, the generator is disconnected and shut down. This process is known as controlled-parameter shutdown. 3. What are the precautions for shutdown using a sliding parameter method? The precautions for shutdown via this method are as follows: (1) When shutting down using a sliding parameter approach, there are specific requirements regarding the rate at which the pressure of the fresh steam decreases; generally, for high-pressure units, the average rate of pressure drop for fresh steam is 0.02–0.03 MPa/min, with an average temperature drop of 1.2–1.5°C/min. At higher parameters, the cooling and pressure reduction rates can be faster ； At lower parameters, the cooling and pressure reduction rates can be slower. (2) During the sliding parameter shutdown, the new steam temperature should maintain a superheat of 50°C to ensure that the steam is free of water. (3) When the temperature of the new steam is lower than the temperature of the inner wall of the flange, the flange heating device can be activated. (4) Super-speed tests on the turbine must not be conducted during a slip parameter shutdown. (5) The high and low pressure heaters should shut down gradually during a sliding-parameter shutdown. 4. Why is it not allowed to conduct a turbine overspeed test during a slip parameter shutdown? Conducting an overspeed test when the steam parameters are very low is extremely dangerous. By the time of a smooth parameter-based shutdown leading to the disconnection of the generator, the steam parameters in front of the main steam valve are already very low; to conduct an overspeed test, it is necessary to close the control valve in order to increase the pressure in front of it. As pressure increases, the superheat of the steam decreases, which may cause the temperature of the new steam to fall below the saturation temperature at that pressure. This results in the steam containing water, leading to water hammer incidents in the turbine; therefore, it is stipulated that speed tests must not be conducted during the controlled shutdown of large-scale units. 5. What is a rotor coastdown curve? What is the purpose of drawing it? After the generator is disconnected from the grid, the time elapsed from the moment the main steam valve and control valves close until the rotor comes to a complete stop is known as the rotor coastdown time. The curve that shows the relationship between the rotor coastdown time and the rate of speed reduction is called the rotor coastdown curve. After a new unit has been in operation for some time and all its components are functioning properly, it is possible to measure the rotor’s coasting curve during shutdown periods. This curve can then be used as the standard coasting curve for that unit. When drawing this curve, it is necessary to control the vacuum level in the condenser, allowing it to decrease at a constant rate. Subsequent shutdowns should be recorded under the same conditions, so that the coasting curve can be drawn again, facilitating comparative analysis of any issues that may arise. If the coasting time decreases sharply, it may be due to bearing wear or friction between the moving and stationary parts of the turbine ； If the idle running time increases significantly, it indicates that the valves in the new steam or reheat steam pipes, or the extraction check valves, are not tight, allowing pressurized steam to leak into the cylinder. When the top shaft oil pump starts too early and the condenser vacuum is high, the coasting time also increases. 6. Why is it necessary to wait until the vacuum level reaches zero before stopping the steam supply to the shaft seal? If the steam supply to the shaft seal is stopped before the vacuum level reaches zero, cold air will enter the cylinder from the shaft end, causing localized cooling of the rotor and the cylinder. In severe cases, this can lead to friction at the shaft seal or deformation of the cylinder; therefore, it is required that the vacuum level reach zero before stopping the steam supply to the shaft seal. 7. Why is it specified that the vacuum level should be reduced after shutting down the turbine, so that it reaches zero when the rotor comes to rest? During the coasting process after a turbine is shut down, the best way to maintain a proper vacuum level is to gradually reduce it, aiming to bring the vacuum level to zero once the rotor has come to rest. This is because: (1) the shutdown coasting time is related to the vacuum retention time; the vacuum is reduced at a certain rate with each shutdown, which facilitates the comparison of the coasting curves. (2) If the vacuum drops too slowly during coasting, the unit will remain at its critical speed for a longer period of time, which is detrimental to the safety of the unit. (3) If the vacuum drops too rapidly during the pre-idle phase, and reaches zero even when there is still some rotational speed, the high heat generated by the blowing loss of the long blades in the subsequent stages can cause the exhaust temperature to rise. This also hinders the removal of water accumulated inside the cylinders, increasing the risk of corrosion of the turbine metal after shutdown. (4) If the rotor has already stopped but there is still a high vacuum, and the steam supply to the shaft seal cannot be stopped either, this will also lead to an increased temperature difference between the upper and lower cylinders, as well as uneven deformation of the rotor resulting in thermal bending. In summary, it is best to bring the speed to zero and the vacuum level to zero during shutdown; in practice, a vacuum break valve is used for control and adjustment. 8. Why is it necessary to activate the oil pump interlock switch during the turbine’s barring process? Although the turbine barring device has interlock protection that triggers shutdown when the lubricating oil pressure drops to a certain level, thereby protecting the bearings of the turbine, this protection system can sometimes fail. If the lubricating oil pump fails to supply oil or malfunctions, it can lead to friction between the turbine bearings and their damage. Once the oil pump interlock is activated, if the AC oil pump fails, the DC oil pump can be started automatically to prevent shaft bearing damage. 9. What issues should be noted during the barring process? The following points should be taken into account during barring: (1) Monitor whether the current of the barring motor is normal, and whether the ammeter reading fluctuates. (2) Regularly check whether there are any changes in the rotor bending indication value. (3) Regularly listen for any friction sounds inside the cylinder and at the high and low pressure seals. (4) Regularly check the operation of the lubricating oil pump. 10. Why must the lubricating oil pump continue to run for a while after the machine has stopped and the turning of the rotor is complete? The main purpose of keeping the lubricating oil pump running is to cool the shaft journals and bearing shells; even after the machine stops, the temperature of the rotor’s metal remains high, and heat is transferred through the bearings along the direction of the shaft journals. If there is not enough lubricating oil to cool the rotor journal, the temperature of the bearing shells will rise; in severe cases, this can cause the bearing material to melt and lead to bearing damage ； Excessively high bearing temperatures can also cause the remaining oil in the bearings to oxidize rapidly, leading to smoking and even fire. During the operation of the low-pressure oil pump, the cold oil cooler must also remain in operation to keep the lubricating oil temperature below 40°C. After the high-pressure steam turbine is shut down, the lubricating oil pump should operate for at least 8 hours. Of course, each unit should be determined specifically according to the circumstances. 11. What maintenance tasks should be carried out after shutdown? Maintenance work after shutdown is very important; in addition to monitoring the operation of the crankshaft turning device, the following tasks also need to be performed: (1) Strictly cut off the sources of steam and water connected to the cylinders, to prevent steam and water from entering the cylinders, which could lead to an increase in the temperature difference between the upper and lower cylinders and even damage to the equipment. (2) Closely monitor the exhaust temperature of the low-pressure cylinder and the water level in the condenser as well as that in the heaters; full water levels are strictly prohibited. (3) Pay attention to the cooling water of the generator rotor water inlet seal support to prevent a disruption in the cooling water, which could damage the packing. (4) After the boiler is depressurized, all the drain valves and vent valves of the unit should be opened ； During winter, take proper frost protection measures to ensure that no water accumulates in any equipment or pipes. 12. Where is the maximum bending of the rotor after the turbine is shut down? When is it most dangerous to start the turbine? After the turbine is stopped, if the barring gear cannot be used for some reason, the rotor will gradually bend due to temperature differences between the upper and lower parts of the cylinder or other factors. The area with the greatest bending is usually near the regulating stage, and the maximum degree of bending occurs within 2 to 10 hours after shutdown; therefore, starting the turbine during this period is the most dangerous. 13. Why hasn’t the load been reduced to zero, preventing the generator from being disconnected? If the load cannot be reduced to zero during shutdown, it is usually due to faulty or stuck control valves, a malfunctioning extraction check valve that does not close properly, or an excessive amount of steam flowing back from the heating system. If the generator is disconnected at this time, an overspeed accident will occur. Therefore, it is necessary to first eliminate the fault by using methods such as closing the automatic main steam valve and the electric isolation valve to reduce the load to zero, before disconnecting and shutting down the generator. 14. Why is it better to first reduce the steam temperature before reducing the steam pressure during a controlled shutdown? During normal operation of the turbine, the superheat of the main steam is relatively high; therefore, during a controlled shutdown, it is better to keep the steam pressure constant while appropriately reducing the steam temperature, thereby lowering the superheat of the main steam. This helps with the cooling of the cylinder, results in a lower temperature of the cylinder after shutdown, and can shorten the time required for rotating the shaft. 15. What are the requirements for the sealing oil system of a hydrogen-cooled generator when it is in the state of cranking after shutdown? The sealing oil system of a hydrogen-cooled generator must operate properly both during cranking and when it has stopped rotating but remains under pressure. Since the seal oil system is connected to the lubricating oil system, hydrogen-containing seal oil may enter the main oil tank through the connected pipelines, and the hydrogen in the oil will be separated out in the main oil tank. If hydrogen accumulates in the main fuel tank, there is a risk of a hydrogen explosion and a fire in the main fuel tank; therefore, the exhaust fans and hydrogen exhaust fans used in the fuel system and the main fuel tank system must also remain in continuous operation.