Turbine rotor speed decreases during shutdown when which event occurs?

The correct choice highlights that the turbine rotor speed decreases during shutdown when the fuel valve closes. This is because closing the fuel valve stops the combustion process that generates the heat and energy necessary to keep the turbine rotor spinning at its operational speed. Without fuel, the turbine no longer has the energy input required to maintain rotation, leading to a gradual decrease in speed as the rotor begins to coast and eventually come to a stop due to friction and mechanical losses.

During a controlled shutdown, closing the fuel valve is a critical step that directly affects the turbine's operation. As the combustion process ceases, the turbine experiences a reduction in thermal energy and, consequently, in mechanical output. This process is integral to safely bringing the turbine down from operational speed to prevent potential damage and ensure a smooth transition to a shutdown state.

Other events, such as the cool-down timer timing out or the turning gear taking over, may influence post-shutdown procedures or maintenance protocols but do not directly cause the immediate reduction in rotor speed. Similarly, while the shaft speed decreasing below a preset limit may indicate a shutdown process, it is the action of closing the fuel valve that initiates the immediate drop in rotor speed.