Question 9 QMED01 - Junior Engineer

The Correct Answer is C **Why option C ("prevent steam from leaking into the astern element while at full sea speed") is correct:** Guardian valves (also known as sentinel valves or astern-guardian valves) are critical components of steam turbine propulsion systems. Their primary function is to serve as an absolute shutoff barrier, ensuring that no steam accidentally enters the astern (reverse) element of the turbine while the vessel is moving forward at high speed (full sea speed). If steam were to leak into the astern element while it is spinning rapidly in the forward direction, it would cause severe overheating and mechanical damage to the astern blading and shrouding due to windage friction and thermal stress. The guardian valve is typically locked closed when steaming ahead and only opened when astern power is deliberately needed (i.e., when maneuvering or reversing). **Why the other options are incorrect:** * **A) provide a means to supply steam directly to the astern element of the turbine:** While the guardian valve *must* be opened to supply steam to the astern element, it is not the *means* of supply itself; it is the ultimate safety stop. The astern maneuvering valve (or the main throttle depending on the specific design) controls the flow rate. The guardian valve's function is protective closure, not primary supply control. * **B) provide an emergency means of quickly closing the throttle:** The main emergency stop valve or the quick-closing mechanism on the main throttle valve performs this function. The guardian valve is designed for positive isolation of the astern element, not for stopping the entire turbine plant during an overspeed or emergency. * **D) prevent steam from leaking into the astern element while the vessel is maneuvering:** While it certainly prevents leaks when maneuvering, the most critical scenario the valve is designed to guard against is the high-speed forward operation (full sea speed). During maneuvering, the main throttle is frequently used to direct steam to the astern element, requiring the guardian valve to be open. Therefore, its essential protective role is specifically highlighted during sustained ahead operation at full speed, making option C the more precise and definitive answer regarding its protective function.