Practice Test Geeks(CMC) Certified Marine Chemist Practice Test
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CMC Ventilation Systems & Gas Freeing Flashcards

6 cards from real CMC practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

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

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

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

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

    Answer: Forced mechanical ventilation with ducting extended to the tank bottom

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

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

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

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

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

    Answer: 12%

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

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

    Answer: Overboard, away from air intakes and ignition sources

    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.

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

    Answer: 20–30 air changes/hour

    Effective initial gas freeing of hydrocarbon-laden tanker tanks typically requires 20–30 air changes per hour to rapidly dilute and displace vapors.