Inert gas systems principles Practice Questions

1. The primary purpose of an inert gas system (IGS) on a tank vessel is to:

• A.Maintain the tank atmosphere in a non-flammable condition by displacing oxygen below the explosive range✓
• B.Cool the cargo vapors to reduce their vapor pressure during loading
• C.Provide a supply of breathable air for personnel entering cargo tanks
• D.Pressurize the cargo tanks to prevent vapor loss during heated cargo operations

Why: An inert gas system maintains the oxygen content in cargo tanks below approximately 8% by volume, keeping the atmosphere outside the flammable range regardless of hydrocarbon vapor concentration, as required by SOLAS and 46 CFR 32.

2. The maximum permitted oxygen content of inert gas delivered to cargo tanks under SOLAS requirements is:

• A.5% by volume✓
• B.8% by volume
• C.11% by volume
• D.2% by volume

Why: SOLAS Chapter II-2 requires that inert gas delivered to cargo tanks must contain no more than 5% oxygen by volume, ensuring the atmosphere in the tanks remains well below the minimum oxygen concentration needed to support combustion.

3. The deck water seal on an inert gas system serves to:

• A.Prevent cargo vapors from flowing back into the inert gas supply piping and machinery spaces✓
• B.Maintain a positive pressure in the cargo tanks at all times
• C.Filter particulate matter from the inert gas before it enters the tanks
• D.Provide an alarm when oxygen levels in the inert gas exceed 5%

Why: The deck water seal is a non-return device that prevents backflow of flammable cargo vapors from the cargo tanks into the inert gas supply system (which leads to machinery spaces), preventing the risk of fire or explosion in those spaces.

4. When purging a cargo tank with inert gas prior to gas-freeing, the process is complete when:

• A.The hydrocarbon content is reduced below 2% by volume throughout the tank✓
• B.The oxygen content reaches 21% and is uniform throughout the tank
• C.The tank pressure reaches atmospheric pressure
• D.The inert gas flow rate equals the vapor generation rate

Why: Per ISGOTT, purging introduces inert gas into an already-inerted tank to lower the hydrocarbon content below 2% by volume; in that low-oxygen condition the tank atmosphere lies below the critical dilution line on the flammability composition diagram, so when air is later admitted during gas-freeing the dilution path passes outside the flammable envelope rather than crossing it. The safety comes from this dilution-line geometry in an oxygen-deficient atmosphere, not from 2% being below the LEL (the LEL of most petroleum vapors is about 1% by volume, which is below 2%, not above it).

5. If the oxygen content of the inert gas being delivered to cargo tanks rises above the permitted maximum, the tankerman should FIRST:

• A.Stop cargo operations and isolate the tanks until the fault is corrected✓
• B.Increase the inert gas flow rate to dilute the oxygen concentration
• C.Ventilate the cargo tanks to remove the excess oxygen
• D.Notify the terminal and continue operations at a reduced transfer rate

Why: A high oxygen reading in delivered inert gas means the protection is compromised and tanks may enter the flammable range. Operations must stop immediately and tanks isolated until the IGS fault is identified and corrected, per SOLAS and company procedures.