Which device protects a steam turbine from excessive speed, preventing overspeed conditions?

• A. Governor assembly
• B. Overspeed switch
• C. Throttle valve
• D. Flyball governor

Answer: (A)

The governor assembly is the component that automatically keeps the turbine spinning at the desired speed by sensing how fast it’s turning and adjusting the amount of steam admitted. When the speed tends to rise, it reduces steam flow; when the speed drops, it increases flow. That real-time regulation is what prevents overspeed in normal operation, making the governor the primary protective control over turbine speed.

The overspeed switch is a separate protective device that kicks in only if speed already exceeds its limit, serving as a backup rather than the main means of preventing overspeed. The throttle valve itself controls steam flow but doesn’t sense speed or automatically correct it. A flyball governor is a type of speed-governing mechanism, but the broader governor assembly encompasses the entire speed-control system.