Question 22 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is D **Explanation for Option D (Correct):** Option D states: "The sea water side of the heat exchanger should be drained by opening the drain valves, while insuring that the vent valves remain open while draining." This represents the recommended operating procedure for securing equipment cooled by sea water for an extended period. 1. **Draining is essential:** Sea water is highly corrosive due to high salt content (chlorides) and dissolved oxygen, and can promote biological growth (fouling). Leaving the equipment filled with stagnant sea water for extended periods accelerates corrosion and pitting, which severely damages the tube bundle and shell. 2. **Opening both drain and vent valves is crucial:** To ensure complete draining of the heat exchanger (especially the tubes and water boxes), atmospheric pressure must be allowed into the system. Opening the vent valves breaks the vacuum that would otherwise form, allowing the water to flow freely out through the drain valves. Complete draining prevents corrosion associated with stagnant water and eliminates the risk of freeze damage if the equipment is secured in a cold environment. **Explanation for Incorrect Options:** **A) The sea water side of the heat exchanger should be left completely filled with sea water.** This is incorrect. Leaving stagnant sea water in the system for an extended period maximizes corrosion (pitting corrosion) and fouling (slime, barnacles), severely damaging the equipment. **B) The sea water side of the heat exchanger should be drained by opening the drain valves, while insuring that the vent valves remain closed while draining.** This is incorrect. If the vent valves remain closed, the draining process will be ineffective or incomplete. A vacuum will form inside the heat exchanger, preventing the water from flowing out fully, leaving pockets of stagnant, corrosive sea water behind. **C) The sea water side of the heat exchanger should be drained, then flushed with fresh water and left completely filled with fresh water.** This is incorrect for a standard long-term layup procedure. While flushing with fresh water is often a step used to remove corrosive chlorides (especially if the unit is going into dry storage or preservation), leaving it filled with fresh water is generally avoided unless anti-corrosion additives or biocides are introduced. If left filled with plain fresh water (which still contains oxygen), stagnant corrosion can occur, and biological growth (like algae) can still develop. Furthermore, filling the equipment and leaving it pressurized or completely full creates a significant risk of freeze damage if the ambient temperature drops. The standard procedure is to drain the system completely after any flushing or cleaning.