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U.S. Department of Transportation Issues
Letter to NHA
by Karen Miller
Vice President of the National Hydrogen Association
TThe National Hydrogen Association has received a letter from
the U.S. Department of Transportation, dated November 26, 2001,
regarding a metal hydride hydrogen storage system. The
letter (23Kb PDF), forwards a letter sent to Coleman Powermate,
Inc. and warns of the potential of cylinder rupture due to expansion
and contraction of the metal hydride powder. This concern was
raised in June 2001 when a number of hydrogen safety experts
met with DOT representatives at DOT to determine whether existing
DOT classifications apply to these systems and presented technical
data indicating a potential problem. The purpose of the meeting
was to discuss DOT regulations and related technical issues
regarding the shipment of metal hydride containers used for
hydrogen storage.
A general discussion of current DOT regulations as presented
in 49 CFR Part 173 was held. A letter of interpretation from
DOT to Coleman Powermate, Inc. (DOT ref. No. 00-0335) was distributed.
This letter can be found on the DOT website at: http://hazmat.dot.gov.
In summary, DOT has existing regulations that cover the shipment
of compressed hydrogen gas (Class 2 - Division 2.1, Flammable
Gas) and metal hydrides (Class 4 - Division 4.1, Flammable Solid;
Division 4.2, Spontaneously Combustible Material; and Division
4.3, Dangerous when wet material). They do not have existing
regulations for the combination of compressed hydrogen gas and
metal hydrides together. In fact, DOT treats hydrides as a chemical,
separate from containers. A hydride storage container, as envisioned
by the NHA and various manufacturers and users, is not covered
as an article under DOT regulations currently.
The DOT letter of interpretation to Coleman stated that the
mixture of hydrogen gas and metal hydride should be classed
according to the highest applicable hazard class, which would
be Division 2.1, Flammable gas. A DOT representative stated
that the Canadian regulators have agreed with this interpretation,
and that they are planning to follow the same guidance.
There was a lengthy discussion about the technology of metal
hydrides and the unique properties of these materials. NHA Working
Group 2 members discussed the characteristic of metal hydride
powders to expand upon hydriding, which may introduce additional
forces on the container walls. The group presented photographs
and data depicting a metal hydride storage cylinder that had
been ruptured as a result of this phenomenon. The reactivity
and flammability of powders as a function of their state of
hydrogen charge, time and method of exposure to air, and method
of testing was also discussed. Concerns about the compatibility
of the metal hydride alloy and the container material, and the
potential for corrosion or metal-metal reactions, was briefly
mentioned.
There was general consensus that the current DOT regulations
and prescribed tests do not adequately address all of these
issues. A DOT representative stated that at the time of the
issuance of their letter of interpretation, that they were unaware
of these additional technical factors. DOT representatives stated
that they wanted to work closely with the industry in a collaborative
fashion, and they requested that additional technical information,
product descriptions or test reports, etc. be provided to them
as appropriate.
There was some discussion about methods utilized by metal hydride
container manufacturers to accommodate these special hydride
characteristics, particularly the expansion issue. Several of
the metal hydride manufacturers at the meeting stated that they
use proprietary methods, such as internal structures. DOT representatives
stated that they would like to be aware of these structures,
and that individual manufacturers could submit proprietary information
to DOT and receive confidential handling of the information
as long as it was properly identified.
DOT representatives stated that they preferred to see risk analyses
in order to establish proper safety requirements. When in doubt,
they always take the conservative approach.
On October 2, 2001, DOT issued another letter to Coleman Powermate,
Inc., replacing previous letters. This letter is attached.
Basically, the issue is that current DOT regulations do not
have appropriate provisions for transport of combination of
metal hydride and compressed hydrogen in cylinders. This includes
proper shipping name and adequate packaging (cylinder) requirements.
Therefore, an exemption, in accordance with 49 CFR 107.105 would
be required in order to transport these materials. It should
be noted that Coleman Powermate, Inc. has not done anything
wrong here and had addressed the safety concerns. In fact, Coleman
is a pioneer in this area, as they are the first company known
to have addressed the issue of transporting charged metal hydride
storage containers with DOT. Originally, it looked as though
no exemption would be needed, as it appeared to fit one of the
DOT classifications. However, upon further review, it has been
determined that the current regulations do not adequately take
into account potential problems with some hydrogen absorbed
in metal hydride storage systems.
The NHA has established Working Group 2 in order to draft a
standard for metal hydride storage systems that, among other
things, provides for manufacturing testing to assure safety
of the design. The group will be meeting in Hilton Head, South
Carolina in February. The draft would be submitted to ISO TC
197 to be considered as a new work item for international standardization.
Unfortunately, this process takes a couple years. Once a standard
is developed, regulatory bodies such as DOT will have something
to point to for approval of similar systems. In the meantime,
however, manufacturers of hydrogen storage systems comprised
of a cylinder containing metal hydride charged with hydrogen
gas will need to request an exemption from DOT for transporting
these systems. Information on requesting an exemption is available
on the DOT website.
If you are interested in joining Working Group 2 to develop
a draft standard for these hydrogen storage systems, please
send your contact information and stated interest to kmiller@ttcorp.com.
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