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

To cool the tank for entry

Answer

To allow painting without primer

Answer

To comply with Coast Guard docking regulations only

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

Natural convection ventilation only

Answer

Opening all hatches simultaneously without mechanical assist

Answer

Using portable fans placed at the deck level only

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

Filling the tank with fresh water before entry

Answer

Painting the internal surfaces with corrosion inhibitor

Answer

Pressurizing the tank above atmospheric pressure

Question 4

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

Accepted Answer

12%

Answer

6%

Answer

10%

Answer

16%

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

Back into the engine room for filtration

Answer

Into adjacent tanks for dilution

Answer

Through the bilge pumping system

Question 6

How many air changes per hour are generally recommended for initial gas freeing of a tanker cargo tank with hydrocarbon residue?

Accepted Answer

20–30 air changes/hour

Answer

2–5 air changes/hour

Answer

6–10 air changes/hour

Answer

The number is irrelevant — only final readings matter