DOT Regulations – Pressure
DOT UN tubes manufactured per DOT regulation and ISO standard 11120 shall be filled in accordance with 49 CFR Section 173.302b(b), which states: “UN pressure receptacles filling limits. A UN pressure receptacle is authorized for the transportation of non-liquefied compressed gases as specified in this section. Except where filling limits are specifically prescribed in this section, the working pressure of a UN pressure receptacle may not exceed 2/3 of the test pressure of the receptacle. Alternatively, the filling limits specified for non-liquefied gases in Table 1 of P200 of the UN Recommendations … are authorized. In no case may the internal pressure at 65 °C (149 °F) exceed the test pressure.” UN tubes manufactured in accordance with DOT 49 CFR and ISO 11120 are stamped with a working pressure and a test pressure. For example, a typical DOT UN jumbo tube for hydrogen service will have a working pressure (PW) marked on the tube as PW189 and a hydraulic/hydrotest pressure (PH) stamping of PH284BAR. You see that the test pressure is 284 bar. Two-thirds of that is 189 bar. Converted to psi, the maximum fill pressure shall be 2,748-psi at the standardized fill temperature. NOTE: 10% overfill that is allowed for some gases carried in DOT 3T and 3AAX tubes is not applicable to DOT UN tubes.
DOT Regulations – Temperature
The standardized fill temperature is defined in 49 CFR Section 173.301a(c) and states: “Cylinder pressure at 21 °C (70 °F). The pressure in a cylinder at 21 °C (70 °F) may not exceed the service pressure for which the cylinder is marked or designated, except as provided in §173.302a(b). For certain liquefied gases, the pressure at 21 °C (70 °F) must be lower than the marked service pressure to avoid having a pressure at a temperature of 55 °C (131 °F) that is greater than permitted.” It is mandatory to consider the temperature at the time of fill so the tube does not get overfilled. (Of course, you also don’t want to under-fill the tube and not take full advantage of its legal capacity.)
Before filling a tube trailer or multiple-element gas container (MEGC) it’s necessary to understand the stamp markings on the tubes. You must also inspect both ends of the tubes because many tubes are dual stamped with DOT 3-Series (e.g. 3T or 3AAX) markings on one end and DOT UN markings on the other end. In addition to these pressure markings, you will also find numerous other markings that are required by DOT regulations, including, but not limited to, manufacturer name or symbol, water volume, serial number, wall thickness, weight, initial test pressure, and country of origin (e.g. USA). The governing regulations (particularly with regard to fill pressure and requalification due date) will be dependent upon the country in which the equipment is being operated. For example, the People’s Republic of China and Korea are currently only recognizing the DOT 3-Series tubes.