What is the purpose of operating a turbine at idle for a period before shutdown?

Operating a turbine at idle for a period before shutdown is primarily done to cool the turbine case and avoid blade seizure. When a turbine operates at high temperatures and then is suddenly shut down, the rapid cooling can lead to thermal stress, which may cause the blades and other components to contract unevenly. This uneven contraction can increase the risk of seizure between the turbine blades and their respective housings, leading to potential mechanical failures.

By allowing the turbine to idle, the temperatures slowly decrease, promoting a more uniform cooling process across the components. This practice helps to reduce the risk of any component binding as they contract, extending the operational life of the turbine and improving reliability. The idle period also allows any residual heat to dissipate in a controlled manner, preventing thermal shock and subsequent damage from occurring.

While the other options may seem relevant in terms of general turbine operations, they do not specifically address the direct mechanical and thermal benefits gained from idling the engine prior to shutdown.