Tanker Familiarization Inert Gas Systems 2 — Questions and Answers
Question 1: What does 'gas-freeing' of a cargo tank involve?
- Filling the tank completely with inert gas to purge residues
- Ventilating the tank with fresh air to replace inert gas and reduce hydrocarbon vapors for safe entry (Correct answer)
- Purging hydrocarbon gas with pure nitrogen before loading
- Testing the tank atmosphere for flammable vapors only
Correct answer: Ventilating the tank with fresh air to replace inert gas and reduce hydrocarbon vapors for safe entry
Gas-freeing ventilates a cargo tank with fresh air to displace inert gas and dilute hydrocarbon vapors to safe levels, restoring sufficient oxygen for personnel entry.
Question 2: What is the primary function of the scrubber in an inert gas system?
- To remove oxygen from the inert gas before it reaches the tanks
- To cool and clean the flue gas by removing sulfur dioxide and soot particles (Correct answer)
- To compress the inert gas to the required delivery pressure
- To continuously monitor hydrocarbon levels in the gas supply
Correct answer: To cool and clean the flue gas by removing sulfur dioxide and soot particles
The scrubber cools the hot flue gas and removes harmful impurities including sulfur dioxide (SO₂) and soot before the gas is delivered to the cargo tanks.
Question 3: Before personnel enter a tank that has been held in an inerted condition, what action is absolutely essential?
- Top up the inert gas supply to maintain positive tank pressure
- Confirm that the oxygen analyzer shows 8% before entry
- Gas-free the tank until oxygen is at least 19.5% and hydrocarbon levels are below the LFL (Correct answer)
- Seal all tank openings except one entry hatch
Correct answer: Gas-free the tank until oxygen is at least 19.5% and hydrocarbon levels are below the LFL
Before entry, the tank must be fully gas-freed so oxygen is restored to a breathable level (≥19.5%) and hydrocarbon concentrations are confirmed below the lower flammable limit.
Question 4: What is the correct procedure before loading flammable cargo into a tank that currently contains a normal air atmosphere (21% oxygen)?
- Load directly since the cargo will displace air naturally
- Purge with additional fresh air first to ensure cleanliness, then load
- Inert the tank to reduce oxygen below 8%, then proceed with loading (Correct answer)
- Gas-free the tank to remove all traces of previous cargo, then load with air inside
Correct answer: Inert the tank to reduce oxygen below 8%, then proceed with loading
Before loading flammable cargo into a tank containing air, the tank must be inerted (oxygen reduced to below 8%) to eliminate the risk of an explosive atmosphere forming during loading.
Question 5: What does a pressure/vacuum (P/V) valve on a cargo tank protect against?
- Backflow of inert gas toward the engine room
- Both excessive over-pressure and excessive vacuum within the cargo tank (Correct answer)
- Oxygen ingress into the inert gas supply line
- Sulfur dioxide contamination of the cargo
Correct answer: Both excessive over-pressure and excessive vacuum within the cargo tank
P/V valves protect cargo tanks against structural damage by opening to relieve both excessive positive pressure buildup and dangerous negative pressure (vacuum) within the tank.
Question 6: What minimum oxygen percentage must be present in a tank atmosphere before personnel may safely enter?
- 8% or above
- 12% or above
- 16% or above
- 19.5% or above (Correct answer)
Correct answer: 19.5% or above
OSHA and international maritime standards require a minimum of 19.5% oxygen before personnel may safely enter an enclosed space such as a cargo tank.
Question 7: What is 'purging' in the context of an inert gas system?
- Adding oxygen to a tank to bring it to normal atmospheric levels
- Removing inert gas from tanks using fresh air before personnel entry
- Using inert gas to dilute and displace hydrocarbon vapors until they fall below the lower flammable limit (Correct answer)
- Cleaning the inert gas scrubber with seawater to remove soot deposits
Correct answer: Using inert gas to dilute and displace hydrocarbon vapors until they fall below the lower flammable limit
Purging uses inert gas to dilute and displace hydrocarbon vapors inside a tank until the hydrocarbon concentration falls safely below the lower flammable limit (LFL).
Tanker Familiarization Inert Gas Systems 2
What does 'gas-freeing' of a cargo tank involve?