How long is the barring gear engaged on a cold turbine?

The correct choice indicates that the barring gear is engaged on a cold turbine for a duration ranging from 5 to 30 minutes. This time frame is important because barring gear is used to rotate a turbine slowly, allowing for an even heat distribution within the rotor and other components. Engaging the barring gear helps to prevent thermal stresses that can occur if one portion of the turbine heats up more quickly than another.

The flexibility within the range (5 to 30 minutes) allows for adjustments based on specific operational procedures and the type of turbine. It's crucial for operators to closely monitor temperatures during this process to ensure that the components are adequately warmed up before full operational speeds are reached. Engaging the barring gear for too short of a time may not sufficiently equalize temperatures, while too long could lead to unnecessary wear on the machinery.

The other options specify durations that either do not align with standard practices in turbine operations or provide an unrealistic timeframe for this specific task. Understanding this range equips power engineers with essential knowledge for ensuring the longevity and safety of turbine operations.