10% Overfill by SP
FIBA manufactures 3AA ton containers and 3AAX tubes. By DOT definition, both are cylinders. Both containers are suitable for hydrogen gas service. But, rather than being used for hydrogen, the 3AA tons are generally used for liquefied gases such as chlorine, HCl, silane, and ammonia. The 3AAX tubes are designed for hydrogen and this is, by far, the most common gas service for 3AAX tubes (followed by CNG). DOT regulations do not permit a 10% overfill allowance for flammable or poison gases. But, a long-time DOT special permit, DOT-SP 6530, remains active and is widely used to allow certain, 3-Series cylinders and tubes to be filled with hydrogen to 10% over their marked service pressure. At time of manufacture, any new FIBA, 3AAX tube will be stamp marked with a plus sign to indicate that it is qualified for 10% overfill in hydrogen service per DOT-SP 6530.
DOT 3-Series vs. DOT UN Tubes
A 10% overfill is not applicable to DOT UN tubes or ISO 11120 tubes. These tubes are manufactured new with a pressure rating that already provides an operating pressure that is greater than the pressure that a similar 3AAX tube would operate at when filled to 10% over the marked service pressure. For example, a DOT 3AAX-2400 tube with a plus sign will be filled with hydrogen to 2,640-psi (at 70°F). A DOT UN tube with the same wall thickness (i.e. 0.536” MW) will be stamp marked with a service pressure (which is also the maximum operating pressure) of 2,748-psi (189 bar).
“H” Mark Stamping
In addition to the mandatory markings applied to all UN or ISO tubes manufactured in accordance with ISO Standard 11120, the DOT UN tubes manufactured specifically for hydrogen service will have an “H” mark included in the stamp markings. The “H” mark indicates that the tube was manufactured and tested to meet the design requirements for embrittling gas service. Without the “H” mark the tube is not suitable for hydrogen service at any pressure!