Question 8 DDE04 - Designated Duty Engineer - 1000-4000 HP

The Correct Answer is C **Explanation for C (Correct Answer):** The expansion tank serves several critical purposes in the engine cooling system: to accommodate the volumetric increase of the cooling water (jacket water) as it heats up to operating temperature, to provide a head pressure to the system, and to serve as a reserve to prevent air from entering the system. When the engine is cold, the level is typically lower (e.g., mid-glass or slightly above the minimum mark). As the engine reaches operating temperature (hot), the water expands significantly. Therefore, the **normal operating level when hot should be in the upper part of the sight glass** (often near the 'HOT MAX' line). This positioning ensures: 1. **Adequate Space for Expansion:** It confirms that the system has taken up the expanded volume of water. 2. **Maximum Reserve Capacity:** Placing it high maximizes the reserve capacity before the low-level alarm is triggered, ensuring the circulation pump suction remains flooded and preventing air entrainment. 3. **Proper Head Pressure:** Maintaining a high level ensures sufficient static head pressure across the system, aiding circulation and preventing localized boiling or cavitation. **Explanation of Incorrect Options:** **A) The level should be out of sight high in the sight glass when the engine is at operating temperature.** This indicates overfilling. If the level is too high, the expanded water may overflow the tank vent or relief valve, wasting coolant and potentially leaving insufficient air cushion for thermal expansion if the engine gets unusually hot (though expansion tanks usually have overflow pipes that address this, maintaining the level visibly high is the standard for operational checks, not completely out of sight). **B) The level should be in the lower part of the sight glass when the engine is at operating temperature.** This indicates a dangerously low level. If the level is near the bottom when the engine is hot, it means there has been significant coolant loss. The system is at high risk of drawing air into the circulation pump, which leads to poor cooling, cavitation damage, hot spots, and potential engine failure. **D) The actual level is unimportant as long as it is visible in the sight glass when the engine is at operating temperature.** This is incorrect because the specific level is critical. The sight glass usually has marked lines (Cold Min/Hot Max). If the level is visible but near the bottom (lower part), it signals a problem (coolant loss and potential air entrainment). The correct operational procedure requires maintaining the level within defined parameters (upper part when hot) to ensure system reliability.