Continuing Qualification and Maintenance (28)
Criticality: Low
Progress: Addressed, Not Adequately
Score: 4
DOT Relevance: ยง180
Description of Key Area
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.
Discussion of Criticality
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.
Discussion of Progress
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.
Recommendations
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.
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