Progress: Addressed, Not Adequately
DOT Relevance: 180
DOT addresses continuing qualification and maintenance in 49 CRF 180. This section applies primarily to specification cylinders, which are metallic. This section also addresses special permit cylinders, which would include composite cylinders. Inspection intervals for specification cylinders range from three to twenty years. Inspection intervals for composite cylinders have generally been three years, but DOT has recently been allowing five year inspection intervals. The industry, particularly CGA, has standards that are used for visual inspection. CGA standards include C-6, C-6.1, C-6.2, C-6.3, and C-6.4. Continuing qualification generally involves a hydrostatic pressure test. DOT has issued special permits to allow alternate non-destructive evaluation (NDE). Cylinders in transportation service have generally been at pressures of 41.4 MPa (6000 psi) or lower.
Appropriate inspection intervals and qualification methods are important, particularly as service pressure increases with hydrogen gas contents. The current experience with specification and special permit cylinders offers assurance of safety within the current limits of pressure and design constraints. As pressures increase, and as design requirements may change, the inspection intervals and qualification methods must be evaluated. This is especially true if any of the materials of construction are adversely affected by exposure to high pressure hydrogen.
Hydrogen cylinders with service pressures from 48.3 to 89.6 MPa (7000 to 13000 psi) are now entering service. These cylinders will be subject to periodic inspection and qualification in accordance with current guidelines. This will give some indication of adequacy of current guidelines.
The industry and DOT should monitor safety of high pressure hydrogen cylinders as they enter service. Adjustments in continuing maintenance and qualification guidelines should be made as data is gathered. The industry and DOT should continue to investigate the effects of high pressure hydrogen on materials of construction and alternate NDE methods that could be used to identify potential damage in cylinders made from materials that are subject to hydrogen embrittlement.