Classification of flammable gas

This is in response to your letter requesting clarification of the Hazardous Materials Regulations (49 CFR; HMR Parts 171-180) pertaining to the transportation of a Class 3 (flammable liquid) material overcharged with a Division 2.2 (non-flammable) gas in a DOT 39 cylinder. You state that the Class 3 material (hexane) has a flashpoint of -18º F and a boiling point of 113º F and is overcharged with nitrogen at a pressure ranging from 100-135 psi. Specifically, you ask whether the filled cylinder may be transported under the proper shipping description "Compressed gas, n.o.s. 2.2, UN1956," and whether the package that is marked and labeled for a Division 2.2 primary hazard class may also be marked and labeled for the Class 3 subsidiary hazard.

Under the HMR, you must analyze the cylinder contents based on the form in which the material will be transported. It is our determination that the contents of your cylinder meet the defining criteria for a Division 2.1 (flammable gas) material and would most appropriately be assigned "Compressed gas, flammable, n.o.s. (contains hexane), 2.1, UN1954" for the basic description. We base this determination on the fact that the cylinder is filled with hexane and then pressurized with nitrogen to 100-135 psi, causing a portion of the hexane to stay in the gas phase. Based on our calculations, the compressed gas mixture will produce flammable vapors in the range of 3 - 6% depending on the temperature and pressure. This is within the range of the flammability limits of hexane, which is 1.2 - 7.7%. Therefore, the gas phase of the vaporized hexane in the cylinder makes it a Division 2.1 material.

Although not applicable to the above scenario, in response to your question concerning subsidiary hazards, a material that is additionally determined to have a flammable liquid subsidiary hazard, regardless of whether the subsidiary hazard is shown in Column (6) of the Hazardous Materials Table (HMT), must be identified on the shipping paper in accordance with § 172.202(a)(3) and shown in the basic description with the numerical subsidiary hazard in parentheses following the primary hazard class or division number. With respect to the marking and labeling for subsidiary hazards, if the subsidiary hazard is not assigned in Column (6) of the HMT, the package may, nevertheless, be marked for the subsidiary hazard and must be labeled for the subsidiary hazard.