Question 16 OSE01 - Chief Engineer - OSV

The Correct Answer is C **Explanation for Option C (Correct):** Option C, "Maintaining relatively low day tank levels when the relative humidity of the atmosphere is high," is the condition most likely to result in water-contaminated fuel. 1. **Low Tank Level:** When the fuel tank level is low, there is a large volume of empty space (ullage) above the fuel. This large ullage space is filled with air. 2. **Atmospheric Humidity:** When the relative humidity of the atmosphere is high, the air entering the tank (through vents or during cooling cycles) contains a significant amount of water vapor. 3. **Condensation:** Marine diesel fuel day tanks often experience temperature fluctuations (e.g., cooling down at night, or when fuel is consumed and replaced by ambient air). When the moist air (high relative humidity) inside the tank cools, the water vapor reaches its dew point and condenses out of the air, forming liquid water droplets on the cool interior surfaces of the tank (the tank "sweats"). This condensed water runs down the walls and collects at the bottom of the tank, contaminating the fuel. A larger ullage space (low fuel level) provides a larger surface area and volume of moist air for condensation to occur. **Explanation for Incorrect Options:** * **A) Maintaining relatively high day tank levels when the relative humidity of the atmosphere is high:** A high fuel level means the tank has very little ullage space (air). While the incoming air is humid, the small volume of air and limited surface area minimizes the amount of water that can condense compared to a low tank level. * **B) Maintaining relatively high day tank levels when the relative humidity of the atmosphere is low:** This condition involves minimal ullage space (high level) and minimal water vapor in the air (low humidity). Both factors significantly reduce the risk of condensation and water contamination. * **D) Maintaining relatively low day tank levels when the relative humidity of the atmosphere is low:** While a low fuel level provides a large ullage space, the low relative humidity means the air inside the tank contains very little water vapor. Therefore, even if cooling occurs, minimal condensation will take place.