Free Tanker Familiarization Tank Atmosphere Control Questions and Answers 1 — Questions and Answers
Question 1: According to SOLAS regulations, what is the maximum permissible oxygen content by volume for a cargo tank's atmosphere to be considered effectively inerted?
- 15%
- 11%
- 8% (Correct answer)
- 5%
Correct answer: 8%
SOLAS Chapter II-2 requires that inert gas systems maintain the atmosphere in any cargo tank at an oxygen content not exceeding 8% by volume. This provides a crucial safety margin, as most hydrocarbon vapors cannot burn in an atmosphere with less than approximately 11% oxygen.
Question 2: A tanker is discharging cargo when the Inert Gas (IG) system fails, causing the pressure in the tanks to drop. What is the most critical and immediate hazard created by this situation?
- Structural damage to the tank deck from over-pressurization.
- The cargo pumps overheating due to loss of back-pressure.
- A significant increase in toxic vapor concentration at the vent mast.
- Air being drawn into the tank, potentially creating a flammable atmosphere. (Correct answer)
Correct answer: Air being drawn into the tank, potentially creating a flammable atmosphere.
When the IG system fails during discharge, the removal of liquid creates a vacuum. If this is not compensated for by inert gas, air will be drawn into the tank from the atmosphere. This introduction of air (oxygen) can raise the oxygen level above 8%, potentially bringing the tank atmosphere into the flammable range, which creates a severe risk of explosion.
Question 3: Which of the following correctly describes the primary purpose of 'purging' a cargo tank with inert gas immediately after cargo discharge and before commencing gas-freeing?
- To introduce fresh air until the oxygen level reaches 21% for safe entry.
- To use high-pressure water jets to clean the tank bulkheads of cargo residue.
- To reduce the hydrocarbon vapor concentration to a level below 2% by volume before introducing air. (Correct answer)
- To increase the tank pressure significantly above atmospheric pressure to test for leaks.
Correct answer: To reduce the hydrocarbon vapor concentration to a level below 2% by volume before introducing air.
Purging with inert gas is a critical intermediate step. Its purpose is to displace or dilute the hydrocarbon vapors, reducing their concentration to a safe level (typically below 2% by volume) so that when fresh air is subsequently introduced for gas-freeing, the tank's atmosphere never passes through the flammable range.
Question 4: When preparing a tank for man-entry, what is the primary objective of the 'gas-freeing' operation that follows purging?
- To displace the inert gas with fresh air, raising the oxygen level to 21% and ensuring hydrocarbon levels are safe. (Correct answer)
- To reduce the hydrocarbon gas content below the critical dilution line using more inert gas.
- To fill the tank with water to remove any remaining cargo residue.
- To lower the tank temperature to ambient conditions after a heated cargo.
Correct answer: To displace the inert gas with fresh air, raising the oxygen level to 21% and ensuring hydrocarbon levels are safe.
After purging has reduced hydrocarbon levels, gas-freeing is the final step to make a tank safe for human entry. This process involves ventilating the tank with fresh air to displace the oxygen-deficient inert gas, with the goal of achieving a stable 21% oxygen level and ensuring any remaining toxic or flammable vapors are below safe occupational exposure limits.
Question 5: A tanker is loading at a terminal equipped with a Vapor Emission Control System (VECS). A high-pressure alarm for the cargo tanks sounds. Which of the following is the most probable cause related to the VECS operation?
- The cargo loading rate is too slow.
- The inert gas system is supplying gas with a high oxygen content.
- A blockage or closed valve exists in the vapor return line to the shore facility. (Correct answer)
- The P/V breaker on the mast riser has lifted.
Correct answer: A blockage or closed valve exists in the vapor return line to the shore facility.
A VECS is designed to safely transfer vapors from the ship's tanks back to the shore facility to prevent their release into the atmosphere. If the vapor return line is blocked, or a valve is mistakenly closed, the vapors displaced by the incoming cargo cannot escape, causing a rapid and dangerous build-up of pressure in the tank, which would trigger the high-pressure alarm.
Question 6: When purging a tank using the 'dilution' method, what is the key principle for it to be effective?
- Introducing inert gas at a very low velocity to create a stable interface with the tank vapors.
- Injecting inert gas at a high velocity to promote thorough mixing with the existing tank atmosphere. (Correct answer)
- Using an inert gas that is significantly denser than the hydrocarbon vapors.
- Simultaneously introducing fresh air from the bottom of the tank.
Correct answer: Injecting inert gas at a high velocity to promote thorough mixing with the existing tank atmosphere.
The dilution method relies on creating turbulence. By injecting inert gas at a high velocity, it mixes with the tank's atmosphere (containing hydrocarbon vapors), gradually reducing the overall concentration of the original gases as the mixture is vented. This is in contrast to the displacement method, which relies on density differences and low velocity to create a stable interface.
Free Tanker Familiarization Tank Atmosphere Control Questions and Answers 1
According to SOLAS regulations, what is the maximum permissible oxygen content by volume for a cargo tank's atmosphere to be considered effectively inerted?