What occurs to turbine rotor speed at shutdown?

The correct answer reflects the process that occurs during the shutdown of a turbine. When a turbine is brought to a stop, a critical step in this process is the closure of the fuel valve. This action effectively cuts off the fuel supply, leading to a decrease in combustion within the turbine. Consequently, the temperature and pressure drop, causing the turbine's rotor speed to begin decreasing as the energy input is halted.

As the turbine speeds decline, various protective mechanisms are in place to ensure the rotor's safe deceleration. The turning gear, for example, can be engaged to help control the rotor's speed and prevent excessive stress during the cooling process. However, the immediate action of closing the fuel valve is essential to initiate the shutdown sequence.

In this context, the cool-down timer and the preset limit for shaft speed are aspects related to monitoring and ensuring safe operational parameters but do not directly cause the rotor speed to decrease. Similarly, while the turning gear's function is important for maintaining the rotor in a safe state post-shutdown, it is not the first action taken during the shutdown sequence. The shutdown process begins with stopping fuel flow, making the closure of the fuel valve integral to turbine shutdown operations.