When should the overspeed tripping device on a steam turbine be activated?

The activation of the overspeed tripping device on a steam turbine is primarily designed to protect the turbine from excessive rotational speeds that could lead to catastrophic failures. During the shutdown process, if the turbine is still rotating and not decelerating as expected, the overspeed tripping device becomes critical. It ensures that if the turbine's speed exceeds safe operational limits, it will automatically disconnect the steam supply, effectively bringing the turbine to a halt and preventing damage.

In contrast, during startup, normal operations, or cleaning processes, the turbine is either being carefully monitored or not under the same risk of uncontrolled speed as during a shutdown scenario. Thus, the overspeed tripping device is specifically meant to safeguard against the risks associated with the turbine losing control over its rotational speed, especially during the transition out of active operation.