CMC CMC Ventilation Systems & Gas Freeing

Question 1

What is the primary purpose of gas freeing a marine cargo tank before issuing a Safe for workers certificate?

Accepted Answer

To remove flammable, toxic, and inert gases to safe levels

Answer

Gas freeing removes hazardous vapors — flammable, toxic, and oxygen-displacing gases — to bring the atmosphere within safe entry parameters.

Question 2

Which ventilation method is most effective for gas freeing a deep cargo tank with multiple toxic gas layers?

Accepted Answer

Forced mechanical ventilation with ducting extended to the tank bottom

Answer

Mechanical ventilation with supply ducts extended near the tank bottom ensures fresh air displaces heavy vapors upward and out through exhaust points.

Question 3

What is "inerting" a cargo tank, and when is it used in marine operations?

Accepted Answer

Replacing the tank atmosphere with inert gas (N2 or CO2) to prevent ignition

Answer

Inerting introduces nitrogen or CO2 to reduce oxygen below the limiting oxygen concentration, preventing ignition of flammable vapor during cargo transfer or cleaning.

Question 4

What is the Limiting Oxygen Concentration (LOC) for most hydrocarbon vapors in marine tank operations?

Accepted Answer

12%

Answer

The LOC for most hydrocarbon vapors is approximately 12% oxygen; maintaining oxygen below this level prevents combustion regardless of flammable gas concentration.

Question 5

During forced ventilation of a marine tank, where should the exhaust be directed to prevent re-contamination?

Accepted Answer

Overboard, away from air intakes and ignition sources

Answer

Exhaust from gas freeing must be directed overboard and away from all air intakes, ignition sources, and personnel to prevent re-introduction of hazardous vapors.

(CMC) Certified Marine Chemist Practice Test

(CMC) Certified Marine Chemist Practice Test

CMC CMC Ventilation Systems & Gas Freeing