What is the primary function of the turbine overspeed trip?

The primary function of the turbine overspeed trip is to close the steam stop valve to the turbine. This mechanism serves as a critical safety feature designed to protect both the turbine and the entire system from potential damage due to excessive rotational speeds. When the turbine speed exceeds a predetermined threshold, the overspeed trip activates, effectively cutting off the steam supply and preventing further acceleration.

This action is crucial because if a turbine spins too quickly, it can lead to mechanical failure, such as blade fracture or even catastrophic destruction of the turbine. By interrupting steam flow, the overspeed trip mitigates the risk of damage, ensuring safe operation under varying load conditions. The effectiveness of this safety mechanism underscores the importance of monitoring turbine speeds and maintaining the reliability of the steering and protection systems.

The other options, while important in their own contexts, do not address the specific function of the overspeed trip. For example, increasing steam velocity and improving lubrication are related to performance and efficiency but do not directly concern the safety function of controlling speed during overspeed conditions. Controlling turbine temperature is also significant for operational effectiveness, but again, it is not a role that the overspeed trip plays.