Question 11

The Correct Answer is A. **Explanation for A (centrifugal force):** The overspeed tripping device on an auxiliary turbine (like a turbo-pump or turbo-generator) is a safety mechanism designed to shut down the turbine automatically if its rotational speed exceeds a safe limit. This mechanism typically employs a flyball governor or an eccentric weight mounted directly on the turbine shaft. As the turbine speed increases, the **centrifugal force** acting on these weights increases quadratically. When the speed reaches the preset trip limit, the centrifugal force overcomes the resistance (usually a spring or retaining mechanism), causing the weight to move outward. This outward movement directly or indirectly (often via linkages or a pilot valve) triggers the closure of the main steam inlet valve, thereby tripping the turbine and preventing catastrophic failure due to overspeed. **Explanation for why other options are incorrect:** * **B) pneumatic force:** While pneumatic systems (using compressed air) can be used in the control or regulation systems of a turbine, they are generally used for remote actuation, manual trips, or part of the larger speed control system (governor). They are not the **automatic actuating force** (the trigger) that senses the actual shaft speed; that role belongs to centrifugal force. * **C) high back pressure:** High back pressure (pressure in the turbine exhaust line) is a condition that generally reduces the power output and speed of the turbine. It might cause a trip through other protection systems (like a low-oil-pressure trip if the turbine slows excessively), but it is a symptom of an external operational problem, not the direct, automatic trigger for an **overspeed trip**. * **D) hydraulic pressure:** Hydraulic pressure (using oil) is frequently the **motive force** used to physically close the large steam valve (the trip valve) once the trip has been initiated. However, hydraulic pressure itself is controlled by a pilot valve, which is actuated by the **centrifugal force** (the primary speed sensing element). Therefore, hydraulic pressure is the means of actuation, not the automatic sensing force that initiates the trip due to overspeed.