Key points of steam turbine operation at critical speed
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Key points for operating a steam turbine at over-critical speed. Key points for operating a steam turbine at over-critical speed. During the startup and shutdown process of the steam turbine, the rotor speed will pass through the critical speed range. In this interval, the centrifugal force generated by the eccentricity of the rotor's center of gravity resonates with the natural vibration frequency of the rotor, which will cause the vibration of the unit to increase sharply. To ensure equipment safety, the following operating specifications must be strictly observed. 1. Preparation before passing criticality Before passing criticality, the status of the unit must be fully confirmed. Focus on checking the lubricating oil temperature, oil pressure, top shaft oil pressure, bearing temperature, vibration value and hydrophobic system to ensure that all parameters are within the normal range. Any anomalies should be eliminated before crossing the threshold. 2. Operation when crossing the critical speed. When crossing the critical speed, the principle of "fast and stable" should be followed. It is strictly forbidden to stay, warm up or wander in the critical speed range. It must be passed quickly, smoothly and in one go. In order to achieve this goal, the rising rate can be appropriately increased (usually required to be 300-500r/min), and a dedicated person should closely monitor the shaft vibration, tile vibration, bearing temperature and unit sound. At the same time, the vacuum destruction door should be closed and a higher vacuum maintained to reduce resistance and help the unit pass faster. During this process, no other operations are allowed, and it is strictly prohibited to fine-tune, return the equipment, or repeatedly increase or decrease the speed to prevent interference. 3. What to do after crossing the critical point. If the vibration does not return to normal after passing the critical point, it is strictly prohibited to continue to increase the speed. Surveillance must be stepped up immediately and the causes analyzed. If the vibration reaches the alarm value, follow it closely ； Once the trip value is reached, it must be opened and shut down immediately. The cause must be identified and dealt with before restarting. When the critical speed is passed again during idle running, the vibration must also be closely monitored to prevent dynamic and static friction or shaft bending. 4. Basic Principles All operations must be based on the factory's operating procedures and TSI protection settings as the highest standards to ensure safe and stable operation of the unit.
The critical speed is indeed a key link in the process of starting and stopping the steam turbine. The author's operation points are summarized very comprehensively. Two additional points of personal experience: It is recommended to manually turn the car 2-3 times before passing the critical point to confirm that the rotor is not stuck. Last year, our factory encountered a situation where the critical speed vibration exceeded the standard due to thermal deformation during shutdown. In actual operation, the rising rate can be appropriately reduced (it is recommended to control it within 300rpm/min), so that the vibration monitoring system can have a more sufficient response time. However, the specific parameters should still be subject to the technical specifications of the equipment manufacturer. When you usually record vibration data, it is recommended to separately mark the shaft vibration and tile vibration values when the critical value is exceeded, which is very helpful for subsequent fault analysis.