Tanker Familiarization Tank Atmosphere Control 3 — Questions and Answers
Question 1: When purging a cargo tank from inert to gas-free, what is the correct procedure?
- Inject air directly into the inerted tank
- Ventilate the tank through the top while monitoring hydrocarbon levels at the bottom (Correct answer)
- Seal the tank and wait for vapors to dissipate naturally
- Flush the tank with seawater before ventilating
Correct answer: Ventilate the tank through the top while monitoring hydrocarbon levels at the bottom
Purging displaces hydrocarbon-rich inert gas from the top while monitoring at the bottom ensures vapors pass through the explosive range as briefly as possible.
Question 2: What is the danger of introducing air into a cargo tank that is in the 'rich' condition (above UEL)?
- The tank pressure will increase dangerously
- Diluting with air can bring the mixture into the explosive range (Correct answer)
- The oxygen will react with the cargo residue causing corrosion
- The temperature inside the tank will drop sharply
Correct answer: Diluting with air can bring the mixture into the explosive range
Adding air to a rich tank dilutes the hydrocarbon concentration, potentially passing through the explosive range between UEL and LEL.
Question 3: Which type of inert gas is considered the highest quality source for tanker IG systems?
- Flue gas from the ship's boiler
- Nitrogen gas from a dedicated nitrogen generator (Correct answer)
- Carbon dioxide from cylinders
- Engine exhaust gases
Correct answer: Nitrogen gas from a dedicated nitrogen generator
Nitrogen generators produce very high-purity inert gas with extremely low oxygen content, making them superior to boiler flue gas systems.
Question 4: What does a 'scrubber' do in an inert gas system?
- Removes hydrocarbon vapors from cargo tank vent gases
- Cools and cleans the flue gas by removing sulfur dioxide and soot particles (Correct answer)
- Measures the oxygen content of the inert gas
- Compresses the inert gas before injection into tanks
Correct answer: Cools and cleans the flue gas by removing sulfur dioxide and soot particles
The scrubber cools the hot boiler flue gas and removes corrosive impurities like SO₂ and particulates before the gas enters the cargo tanks.
Question 5: A ship's IG deck seal serves what primary function?
- To measure the flow rate of inert gas
- To prevent cargo vapors from flowing back into the IG system machinery (Correct answer)
- To regulate the pressure of inert gas supplied to tanks
- To filter oxygen out of the inert gas before delivery
Correct answer: To prevent cargo vapors from flowing back into the IG system machinery
The deck water seal acts as a non-return device preventing flammable cargo vapors from backflowing into the inert gas plant and ship machinery spaces.
Question 6: At what minimum oxygen content in the IG supply must operations be stopped immediately?
- 12% by volume
- 8% by volume (Correct answer)
- 5% by volume
- 15% by volume
Correct answer: 8% by volume
If the oxygen content of the inert gas supplied to tanks exceeds 8% by volume, inerting operations must be halted to prevent creating a flammable atmosphere.
Question 7: What is 'topping up' in the context of tanker inert gas operations?
- Filling cargo tanks to maximum capacity
- Adding inert gas to maintain positive pressure and correct oxygen levels in tanks (Correct answer)
- Adding water ballast to correct the ship's trim
- Refilling the IG plant fuel supply
Correct answer: Adding inert gas to maintain positive pressure and correct oxygen levels in tanks
Topping up replaces inert gas lost through normal pressure fluctuations to maintain the required positive pressure and oxygen levels in cargo tanks.
Tanker Familiarization Tank Atmosphere Control 3
When purging a cargo tank from inert to gas-free, what is the correct procedure?