Question 30

The Correct Answer is C ### Explanation for Option C (Correct Answer) **C) Low oxygen alarm:** An inert gas (IG) system's primary function is to replace the air (which contains oxygen) in cargo tanks with an oxygen-deficient gas (usually scrubbed flue gas from the main boiler or a dedicated generator) to prevent explosions. The system is intentionally designed to produce gas with a very low oxygen content (typically $<5 \%$ by volume, often maintained at $<1 \%$). Therefore, a low oxygen level is the *goal* of the system, not a fault condition that would trigger an alarm. The main alarms related to oxygen are **High Oxygen Alarms**, which activate if the oxygen content in the delivered gas or the tank atmosphere exceeds safe limits (e.g., $5 \%$ or $8 \%$ depending on the regulation and phase of operation). ### Explanation for Other Options (Incorrect Answers) **A) Low pressure alarm:** This alarm is crucial. If the pressure of the inert gas being delivered to the cargo tanks drops too low, it compromises the positive pressure blanket, allowing air (and thus oxygen) to be drawn into the tanks, creating an explosive atmosphere. This is a mandatory safety alarm. **B) Deck seal low water alarm:** The deck seal is a vital non-return device designed to prevent backflow of hydrocarbon vapors from the cargo tanks into the engine room or inert gas plant. It relies on a water seal. If the water level is too low, the seal is broken, creating a dangerous passage. This is a critical safety alarm. **D) Scrubber high water level alarm:** The scrubber cools and cleans the inert gas (flue gas) by spraying large amounts of seawater. If the water level in the scrubber exceeds the safe limit, there is a risk of the water carry-over into the downstream components (blowers, deck line) or even back into the boiler uptake. A high water level indicates a drainage failure or overfilling and necessitates an alarm and potentially shutdown.