A tank carrier designed to carry flammable liquids, combustible liquids, Class B poisons, and liquid food products with vapor pressures up to 4 psi is which designation?

• A. MC 331/DOT 407
• B. MC 312/DOT 412
• C. MC 306/DOT 406
• D. MC 307/DOT 407

Answer: (C)

The main idea here is how vapor pressure determines the tank type. A liquid cargo tank that is designed for liquids with vapor pressures up to about 4 psi is a non-pressurized design. That corresponds to the designation MC 306 / DOT 406. This class handles flammable liquids, combustible liquids, and various other liquids (including some poisons and edible liquids) as long as their vapor pressure stays at or below 4 psi.

Other designations cover different pressure regimes and uses. Tanks labeled DOT 407 (and MC 331 or MC 307 in that family) are low-pressure tanks designed for liquids at higher internal pressures, not limited to a 4 psi ceiling. Tanks labeled DOT 412 (MC 312) are for corrosives and involve different construction for pressurized or semi-pressurized service. So, the combination of carrying liquids with a vapor pressure up to 4 psi fits the non-pressurized MC 306 / DOT 406 designation.