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Ammar Aziz
Ammar Aziz • Jan 16, 2016

Turbine overspeeding reason

Dear all, consider a simple steam turbine system with some feed-water heaters installed before the steam generator. Can you please explain this paragraph in a more understandable manner:
1.In case of load rejection, the steam pressure in the turbine decreases. Water in the heaters evaporates and in absence of any protection the steam can enter in to the turbine and result in turbine over speed.
2.A source of steam i.e. Auxiliary steam for Deaerator is connected through extraction steam pipe. The steam can enter into the turbine by leakage and result in turbine over speed.
Steam turbine over speed is usually caused by a sudden 100% loss of the driven load. This can be due to the coupling failure or in cases where the load is an electric generator loss of load on the generator. This load loss leads to the loss of pressure. When the back pressure decreases more water tends to boil in the preheaters generating steam. This steam can cause an increase in the exit velocity at the turbine nozzle leading to increase in turbine speed. Since the driven load had failed totally, this becomes a runaway catastrophy.
Modern turbines have sophisticated electronic governors which at least prevents hazardous failures and limit the damage.

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