Question 542Steam Plants - Assistant Engineer

The correct answer is B) Excessive speed causes an oil pump to develop sufficient pressure to open a spring loaded relay valve which tends to close the steam control valve. This is the correct answer because the main propulsion turbine overspeed relay is designed to detect excessive turbine speed and initiate closing of the throttle valve to prevent damage to the turbine. When the turbine speed increases beyond a safe limit, the increased centrifugal force causes the oil pump to develop higher pressure, which opens a spring-loaded relay valve. This, in turn, tends to close the steam control valve, reducing steam flow and slowing down the turbine. The other answer choices are incorrect because they do not accurately describe the mechanism by which the overspeed relay closes the throttle valve. Excessive centrifugal force acting on spring-loaded flyballs (A), increased lube oil temperature actuating a solenoid valve (C), or a spring-loaded weight tripping a valve latch (D) are not the typical mechanisms used in a turbine overspeed protection system.