47. Why does a turbine unit run at an excessive speed? What are the most serious consequences?

The main reasons for turbine overspeed are: (1) the generator loses load to zero, and the turbine speed control system does not function properly. (2) The speed gets out of control during the emergency stop device overspeed test. (3) After the generator is disconnected, the high and medium pressure main steam valves, throttle valves, extraction check valves, etc. become stuck or do not close properly. Handling of turbine overspeed: (1) Immediately break the vacuum to initiate an emergency shutdown, and confirm that the speed has decreased. (2) Check and open the exhaust valve of the high-pressure conduit. (3) If it is observed that the rotation speed continues to increase, decisive isolation and pressure relief measures should be taken. (4) Identify the cause of the overspeed and rectify the fault; only after a thorough inspection confirms that the turbine is operating normally can it be restarted. The critical safety devices and all overspeed protection systems must function properly before the turbine can be connected to the grid and operated under load. (5) During restart, close attention should be paid to and monitoring should be carried out on aspects such as turbine vibration, internal noises, bearing temperature, axial displacement, and thrust bearing temperature; if any abnormalities are detected, the startup process should be stopped

The main reasons for turbine overspeed are: (1) the generator loses load to zero, and the turbine speed control system does not function properly. (2) The speed gets out of control during the emergency stop device overspeed test. (3) After the generator is disconnected, the high and medium pressure main steam valves, throttle valves, extraction check valves, etc. become stuck or do not close properly. Handling of turbine overspeed: (1) Immediately break the vacuum to initiate an emergency shutdown, and confirm that the speed has decreased. (2) Check and open the exhaust valve of the high-pressure conduit. (3) If it is observed that the rotation speed continues to increase, decisive isolation and pressure relief measures should be taken. (4) Identify the cause of the overspeed and rectify the fault; only after a thorough inspection confirms that the turbine is operating normally can it be restarted. The critical safety devices and all overspeed protection systems must function properly before the turbine can be connected to the grid and operated under load. (5) During restart, close attention should be paid to and monitoring should be carried out on aspects such as turbine vibration, internal noises, bearing temperature, axial displacement, and thrust bearing temperature; if any abnormalities are detected, the startup process should be stopped

If the main fan and the compressor lose load (for example, if the inlet valve of the main fan suddenly closes, or if the inlet flow to the compressor decreases due to a lack of cracking gas or evacuation), and if the turbine speed control system fails or the emergency safety device becomes stuck, or even if the emergency safety device does function but the main steam valve and the speed control valves become stuck due to scaling, overly tight packing, bent valve stems, etc., it can lead to the unit running at excessive speeds. Overspeed occurs when the rotation speed exceeds 110% of the rated speed. When operating at excessive speeds, the tachometer indicates a speed **above the threshold at which the emergency shutdown device activates; the unit emits unusual noises associated with overspeed, vibration increases significantly, and there may be impacts or metallic sounds as well. The hazards of overspeed or runaway operation are extremely severe; it can cause deformation of the rotor, blades, and impellers, and in serious cases, it can lead to the rupture of these blades and impellers, which may then break free from the cylinders and damage the equipment or result in injuries or fatalities.