Posted: Posted on January 22, 2018 UPDATE: Fill Pressure in Hydrogen Service

NOTE:  DOT regulatory changes require the March 25, 2013 posting to be updated.

SP No Longer Required for 10% Overfill in Hydrogen Service

As noted in a prior blog, FIBA manufactures 3AA ton containers and 3AAX tubes. By DOT definition, both are cylinders. Both containers are suitable for hydrogen gas service. 3AA tons are generally used for liquefied gases, rather than hydrogen. The 3AAX tubes are designed for hydrogen and this is, by far, the most common gas service for 3AAX tubes (followed by CNG). Generally, DOT regulations do not permit a 10% overfill allowance for flammable or poison gases.

DOT Publishes Revision to Special Filling Limits

However, regulations incorporated on January 21, 2016 into 49 CFR § 173.302a(c), now allow certain, 3-Series cylinders and tubes to be filled with hydrogen to 10% over their marked service pressure. The new DOT regulations have incorporated into the CFR the same cylinder and pressure relief device requirements that had been part of long-time DOT special permit, DOT-SP 6530. Due to this incorporation of safety control measures and special provisions in DOT-SP 6530, it is no longer necessary to be party to this special permit, mark it on your tube trailers and maintain a copy of the permit on board the trailer and at your fill site.  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 49 CFR § 173.302a(c).

DOT 3-Series vs. DOT UN Tubes

As stated in the March 25, 2013 blog, 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!  (Please note that FIBA is also seeking a revision to 49 CFR to require this “H” mark on tubes that are operating in compressed natural gas [CNG] service.)