Why is it specified that the speed must reach zero and the vacuum level must reach zero when the turbine is shut down?

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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 coasting times. 2. If the vacuum drops too slowly during the coasting process, the unit takes a long time to reach its critical speed, which is detrimental to the safety of the unit. 3. If the vacuum drops too rapidly during the coasting process, and the vacuum reaches zero while there is still some rotational speed, the heat generated by the aerodynamic losses of the long blades in the subsequent stages increases, which can lead to an elevation in the exhaust temperature. This also hinders the removal of water accumulated inside the cylinders, increasing the risk of metal corrosion after the turbine stops operating. 4. If the rotor has stopped but there is still a high vacuum, the air supply to the shaft seal must not be stopped; otherwise, it will lead to an increased temperature difference between the upper and lower cylinders, as well as uneven deformation of the rotor resulting in thermal bending. When shutting down, it is best to bring the speed to zero and the vacuum level to zero; in practice, a vacuum break valve is used for control and adjustment.