Federal Motor Vehicle Safety Standards; Compressed Natural Gas Fuel Container Integrity |
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Ricardo Martinez
National Highway Traffic Safety Administration
September 26, 1994
[Federal Register: September 26, 1994]
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DEPARTMENT OF TRANSPORTATION
National Highway Traffic Safety Administration
49 CFR Part 571
[Docket No. 93-02; Notice 05]
RIN [2127-AF14]
Federal Motor Vehicle Safety Standards; Compressed Natural Gas
Fuel Container Integrity
AGENCY: National Highway Traffic Safety Administration (NHTSA),
Department of Transportation (DOT).
ACTION: Final rule.
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SUMMARY: This rule establishes a new Federal motor vehicle safety
standard, Standard No. 304, Compressed Natural Gas Fuel Containers,
that specifies performance requirements applicable to compressed
natural gas (CNG) fuel containers: a pressure cycling test evaluates a
container's durability; a burst test evaluates a container's initial
strength; and a bonfire test evaluates a container's pressure relief
characteristics. In addition, the final rule specifies labeling
requirements for CNG containers. The purpose of this new standard is to
reduce deaths and injuries occurring from fires that result from fuel
leakage from CNG containers.
DATES: Effective Date: The Standard becomes effective March 27, 1995.
Incorporation by reference: The incorporation by reference of
certain publications listed in the regulations is approved by the
Director of the Federal Register as of March 27, 1995.
Petitions for Reconsideration: Any petition for reconsideration of
this rule must be received by NHTSA no later than October 26, 1994.
ADDRESSES: Petitions for reconsideration of this rule should refer to
Docket 93-02; Notice 5 and should be submitted to: Administrator,
National Highway Traffic Safety Administration, 400 Seventh Street SW.,
Washington, DC 20590.
FOR FURTHER INFORMATION CONTACT: Mr. Gary R. Woodford, NRM-01.01,
Special Projects Staff, National Highway Traffic Safety Administration,
400 Seventh Street, SW., Washington, DC 20590 (202-366-4931).
SUPPLEMENTARY INFORMATION:
Outline
I. Background
A. General Information
B. Previous Agency Rulemakings
II. Comments on the Proposal
III. Agency's Decision
A. Overview
B. Adopting Industry Standards
C. Pressure Cycling Test
D. Burst Test
1. Safety Factor
2. Hold Time Interval
3. Sequential Testing
4. Failure Criteria
E. Bonfire Test
1. Performance Requirements
2. Types of Pressure Relief Devices
3. Shielding
4. Test Gas and Pressure
5. Wind Velocity and Direction
6. Bonfire Fuel
7. Bonfire Test Fuel Pan Depth
F. Labeling Requirements
G. Leadtime
H. Benefits
I. Costs
VI. Rulemaking Analyses
A. Executive Order 12866 and DOT Regulatory Policies and
Procedures
B. Regulatory Flexibility Act
C. Executive Order 12612 (Federalism)
D. National Environmental Policy Act
E. Civil Justice Reform
I. Background
A. General Information
Natural gas is a vapor that is lighter than air at standard
temperature and pressure.\1\ When used as a motor fuel, natural gas is
typically stored on-board a vehicle in cylindrical containers at a
pressure of approximately 20,684 kPa pressure (3,000 psi). Natural gas
is kept in this compressed state to increase the amount that can be
stored on-board the vehicle. This in turn serves to increase the
vehicle's driving range. Since natural gas is a flammable fuel and is
stored under high pressure, natural gas containers pose a potential
risk to motor vehicle safety.
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\1\Standard temperature is 0 deg. Celsius or 32 deg. Fahrenheit
and standard pressure is 101.4 kiloPascals (kPa) or 147.7 pounds per
square inch (psi).
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Vehicles powered by CNG have not been numerous to date, although
they are increasing. The number of CNG vehicles in the United States
more than doubled from 10,300 in 1990 to 23,800 at the end of 1992. The
number of CNG vehicles is projected to again double to an estimated
50,800 vehicles in 1994. As discussed in detail in a final rule
regarding CNG vehicles published on April 25, 1994, recent Federal
legislation, as well as the need to meet environmental and energy
security goals, will lead to greater increases in the production and
use of these vehicles. (59 FR 19648).
B. Previous Agency Rulemakings
On October 12, 1990, NHTSA published an advance notice of proposed
rulemaking (ANPRM) to explore whether the agency should issue Federal
motor vehicle safety standards (FMVSSs) applicable to CNG fuel
containers and the fuel systems of motor vehicles using CNG or
liquified petroleum gas (LPG) as a motor fuel. (55 FR 41561). The ANPRM
sought comment about the crash integrity of vehicle fuel systems, the
integrity of fuel storage containers, and pressure relief for such
containers.
On January 21, 1993, NHTSA published a notice of proposed
rulemaking (NPRM) in which the agency proposed to establish a new FMVSS
specifying performance requirements for vehicles fueled by CNG. (58 FR
5323). The proposal was based on comments received in response to the
ANPRM and other available information. The NPRM was divided into two
segments: (1) vehicle requirements that focus on the integrity of the
entire fuel system, and (2) equipment requirements that focus on the
fuel containers alone.
NHTSA decided to model the proposed requirements applicable to CNG
fueled motor vehicles on Standard No. 301, Fuel System Integrity.
Standard No. 301 specifies performance requirements for vehicles that
use fuel with a boiling point above 32 deg.Fahrenheit (i.e., fuels
that are liquid under standard temperature and pressure). Vehicles
manufactured to use only CNG are not subject to Standard No. 301 since
CNG has a boiling point below 32 deg.F. Standard No. 301 limits the
amount of fuel spillage from ``light vehicles''\2\ during and after
frontal, rear, and lateral barrier crash tests and a static rollover
test. The Standard also limits fuel spillage from school buses with a
GVWR over 10,000 pounds after being impacted by a moving contoured
barrier at any point and any angle. By basing the CNG rulemaking on
Standard No. 301, the agency believed that passengers of CNG vehicles
would be afforded a level of safety comparable to that provided
passengers of vehicles fueled by gasoline or diesel fuel.
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\2\Light vehicles include passenger cars, multipurpose passenger
vehicles (MPV's), trucks, and buses with a gross vehicle weight
rating (GVWR) of 10,000 pounds or less.
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With respect to the ``vehicle'' requirements for CNG vehicles,
NHTSA proposed that the fuel system integrity requirements would
include frontal, rear, and lateral barrier crash tests for light
vehicles, and a moving contoured barrier crash test for large school
buses. The agency proposed that fuel system integrity would be
determined by measuring the fuel system's pressure drop after the crash
test rather than fuel spillage, since CNG is a vapor and not a liquid.
The allowable pressure drop for CNG fueled vehicles would be
equivalent, as measured by the energy content of the lost fuel, to the
allowable spillage of gasoline during Standard No. 301 compliance
testing.
With respect to the ``equipment'' requirements for CNG containers,
NHTSA proposed a definition for ``CNG fuel tank'' and performance
requirements that would apply to all such fuel containers manufactured
for use as part of a fuel system on any motor vehicle, including
aftermarket containers.\3\ Thus, while vehicles with a GVWR over 10,000
pounds (other than school buses) would not be subject to Standard No.
303, the CNG containers in those vehicles would be subject to the
equipment requirements. The agency proposed that each CNG container
would be subject to a pressure cycling test to evaluate container
durability and a pressure burst test to evaluate the container's
initial strength as well as its resistance to degradation over time. In
addition, the NPRM proposed requirements to regulate how the container
``vents'' its contents under specified conditions of elevated
temperature and pressure.
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\3\Among the terms used to describe CNG fuel tanks are tanks,
containers, cylinders, and high pressure vessels. The agency will
refer to them as ``containers'' throughout this document.
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II. Comments on the Proposal
NHTSA received a large number of comments to the docket addressing
the CNG proposal. The commenters included manufacturers of CNG
containers, vehicle manufacturers, trade associations, other CNG-
oriented businesses, research organizations, State and local
governments, the United States Department of Energy, and energy
companies. In addition, NHTSA met with the Compressed Gas Association
(CGA) and the Natural Gas Vehicle Coalition (NGVC) and had telephone
conversations and meetings with some of the commenters. A record of
each of these contacts may be reviewed in the public docket.
The commenters generally believed that a Federal safety standard
regulating the integrity of CNG fuel systems and fuel containers is
necessary and appropriate. In fact, some commenters, including the CGA,
the NGVC, and CNG container manufacturers stated that NHTSA should
issue a Federal standard as soon as possible to facilitate the safe and
expeditious introduction of CNG fueled vehicles. With respect to the
equipment requirements, the commenters generally believed that Federal
requirements about CNG fuel container integrity are needed and should
be implemented as quickly as possible. The CNG vehicle industry, led by
CGA and NGVC, expressed concern that lack of Federal regulations has
created a problem for the industry, given the issuance of potentially
conflicting industry and State regulations. Therefore, these commenters
stated that CNG container manufacturers may not know the appropriate
standards to which they should manufacture their containers. In
contrast, the American Automobile Manufacturers Association (AAMA)
stated that the vehicle system requirements are sufficient to regulate
the overall integrity of CNG fueled vehicles and that separate
requirements for CNG fuel containers are not needed. Nevertheless, AAMA
provided detailed comments about the container proposal in case the
agency decided to issue separate container requirements.
The commenters addressed a variety of issues discussed in the NPRM.
These issues include the appropriateness of adopting the American
National Standards Institute (ANSI) voluntary industry standard known
as NGV2;\4\ the pressure cycling requirements and test procedures; the
burst requirements and test procedures, including the proposed safety
factor, hold time interval, and need for sequential testing; the
pressure relief requirements and test procedures, including types of
pressure relief devices, shielding, test gas, test pressure, test fuel,
and fuel pan depth; labeling requirements; leadtime; costs; and
benefits.
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\4\NGV2 is a recently issued voluntary industry standard that
was adopted by the ANSI and addresses CNG fuel containers. It was
developed by an industry working group that included container
manufacturers, CNG users, and utilities.
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NHTSA issued an SNPRM proposing to pattern the burst requirements
more closely on NGV2, based on its consultation with other Federal
agencies, its review of comments to the January 1993 proposal, and
other available information. (58 FR 68846, December 29, 1993). NHTSA
proposed a burst test that would link the use of particular designs and
materials to compliance with safety factors tailored to those designs
and materials. NHTSA requested comment on the appropriateness of
requiring CNG containers to meet design and material requirements, such
as those specified in NGV2, and to meet safety factors tailored to
those requirements. As an alternative approach, the agency asked
whether it should specify a catch-all high end safety factor for any
container whose design and materials are not specified in NGV2.
Most commenters supported the proposal to incorporate NGV2 into the
Federal standard. However, AAMA and Ford opposed the design and
material specific approach of NGV2.
III. Agency's Decision
A. Overview
In today's final rule, NHTSA is issuing a new Federal motor vehicle
safety standard, Standard No. 304, Compressed Natural Gas Fuel
Containers, that specifies performance requirements applicable to a CNG
fuel container's durability, strength, and venting. A pressure cycling
test evaluates a container's durability by requiring a container to
withstand, without any leakage, 18,000 cycles of pressurization and
depressurization. This requirement helps to ensure that a CNG container
is capable of sustaining the cycling loads imposed on the container
during refuelings over its entire service life. A burst test evaluates
a container's initial strength and resistance to degradation over time.
This requirement helps to ensure that a container's design and material
are appropriately strong over the container's life. A bonfire test
evaluates a container's pressure relief characteristics when pressure
builds in a container, primarily due to temperature rise. In addition,
the final rule specifies labeling requirements for CNG fuel containers.
As previously mentioned, the agency has issued a final rule
establishing a new Federal motor vehicle safety standard, Standard No.
303, Fuel System Integrity of Compressed Natural Gas Vehicles, that
specifies vehicle performance requirements applicable to the fuel
system of a CNG fueled vehicle. As explained in that final rule, the
fuel system integrity requirements are comparable to those requirements
in Standard No. 301. Like that Standard, the new requirements limit the
amount of fuel leakage in specified frontal, rear, and lateral barrier
crash tests for light vehicles and a moving contoured barrier crash
test for school buses with a GVWR over 10,000 pounds.
NHTSA believes that CNG containers must be evaluated in all
possible failure modes and environments to which they may be subjected.
Since the requirements contained in today's final rule do not address
all these situations, the agency is currently investigating other
possible requirements for CNG fuel containers and anticipates issuing a
SNPRM that would propose performance requirements applicable to such
characteristics as a CNG fuel container's internal and external
resistance to corrosion, brittle fracture, fragmentation, and external
damage caused by incidental contact with road debris or mechanical
damage during the vehicle's operation. The agency tentatively believes
that these additional performance requirements are critical for
determining a CNG container's safety. In addition, the agency
anticipates proposing additional labeling requirements that should
provide critical safety information about inspecting a CNG container
and its service life.
NHTSA notes that it has no statutory authority to regulate certain
aspects involving CNG containers, including inspection requirements
during the manufacturing process, in-use inspection requirements, and
retest requirements during use.
B. Adopting Industry Standards
In the NPRM, NHTSA explained its decision to propose pressure
cycling and burst tests and requirements. While the agency's proposal
was based on NGV2, the agency decided not to propose certain provisions
of the voluntary industry standard that the agency tentatively believed
might unreasonably restrict future designs. Similarly, NHTSA decided
not to propose regulations issued by the Research and Special Programs
Administration (RSPA)\5\ for CNG storage containers used on motor
vehicles, explaining that the RSPA regulations do not address the
conditions unique to the motor vehicle environment (e.g., increased
cycling due to refueling and pressure relief when the cylinder is less
than full). NHTSA further explained that in contrast to RSPA, NHTSA
does not typically regulate design and materials since NHTSA is
statutorily directed to issue performance-based safety standards.
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\5\RSPA is an administration within the United States Department
of Transportation that among other things regulates the
transportation of hazardous materials.
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NGVC and several CNG container manufacturers stated that NHTSA
should adopt the voluntary industry standard that has been developed by
the CNG industry working group. In support of this request, the
American Gas Association (AGA) cited a 1982 Office of Management and
Budget Circular that states ``It is the policy of the Federal
Government to (a) Rely on voluntary standards * * * whenever feasible
and consistent with law and regulation pursuant to law * * *.'' AGA and
NGVC believed that the voluntary standards provide a higher level of
safety than the regulations proposed by NHTSA. They further stated that
if NHTSA were unable to adopt NGV2 due to its prescriptive nature, then
NHTSA should still allow automobile and equipment manufacturers the
option of certifying to the industry standard by referencing NGV2 in
the regulations.
In promulgating a CNG container standard, NHTSA has sought to the
extent possible to adopt the tests and requirements set forth in NGV2.
NHTSA was limited in its ability to do this by the National Traffic and
Motor Vehicle Safety Act (Safety Act, 49 U.S.C. 30111), which commands
the agency to issue ``motor vehicle safety standards'' as minimum
standards of motor vehicle performance that are practicable, meet the
need for motor vehicle safety, and are stated in objective terms. NHTSA
found it necessary to modify certain elements of NGV2 to be consistent
with this statutory mandate. For instance, the agency has not
incorporated those aspects of NGV2 that are stated in nonobjective
terms (e.g., a container shall not show ``evidence'' of deterioration
or failure) NHTSA has decided to incorporate NGV2's design and material
requirements since the agency has been unable to find or develop a
meaningful dynamic performance requirement that would adequately
evaluate a container's initial strength and susceptibility to
degradation over time. The agency believes that the requirements are no
more specific than necessary to achieve these safety purposes.
NHTSA notes that it would be impermissible under the Safety Act for
the agency to adopt FMVSS provisions referencing NGV2 in its entirety
and stating that automobile and equipment manufacturers had the option
of certifying compliance to NGV2 by referencing this voluntary industry
standard. The Safety Act provides for manufacturer self-certification
with respect to FMVSSs only. To be part of a FMVSS, the provisions of a
voluntary industry standard must fully meet all of the requirements of
the Safety Act. Since all of NGV2 does not meet these requirements,
NGV2 may not be incorporated in its entirety. Even if NGV2 met these
requirements, NGV2 could not be incorporated in the FMVSS except to the
extent that the FMVSS made compliance with NGV2 mandatory.
C. Pressure Cycling Test
In the NPRM, NHTSA proposed pressure cycling requirements that
would require that the fuel container withstand a cycling test at
ambient temperature, without any leakage or deformation exceeding one
percent of any circumference. In the test, the container would be
hydrostatically pressurized to the service pressure, then to not more
than 10 percent of the service pressure, for 13,000 cycles. The
container would next be hydrostatically pressurized to 125 percent of
the service pressure, then to not more than 10 percent of the service
pressure, for 5,000 cycles. The cycling rate would not exceed ten
cycles per minute.
1. Number of Cycles
The proposed cycling requirements were intended to establish
minimum levels of safety performance for the durability of CNG fuel
containers used in motor vehicles. The agency stated its tentative
belief that the requirements are consistent with provisions in NGV2 and
with RSPA regulations for containers used to transport CNG. The agency
believed that the pressure cycling requirement would help to assure
that a CNG container is capable of sustaining the cycling loads imposed
on the container during refuelings. The number of cycles specified in
the proposal, 13,000 plus 5,000, is representative of four refuelings
per day, 300 days per year, for 15 years.
AAMA, Norris, and Thomas commented on the number of pressure
cycles. These commenters stated that the proposed number of cycles was
excessive and not representative of the actual operating conditions the
CNG containers would typically experience. AAMA and Norris stated that
cycling the container at 125 percent of service pressure for 5,000
cycles would be adequate. Thomas made inconsistent statements about the
appropriate number of cycles. On the one hand, it stated that 9,000
cycles at service pressure would be more reasonable than the proposed
number of cycles. On the other hand, it stated that the agency should
adopt NGV2 which specifies 18,000 cycles.
After reviewing the comments and other available information, NHTSA
continues to believe that the proposed number of pressure cycles
accurately represents the extreme conditions that CNG fuel containers
could experience during their lifetime, with a margin of safety. This
is based on the large number of cycles to which fleet vehicles are
subjected. The agency believes that the 5,000 cycles suggested by AAMA
and Norris would not ensure the safety of vehicles that experience
multiple refuelings each day, such as taxis and other fleets. NHTSA
further notes that the number of cycles being adopted is consistent
with the cycles in NGV2 and therefore establishes a minimum level of
safety that is consistent with NGV2, a standard supported by a large
majority of the commenters. Accordingly, the agency has determined that
a CNG fuel container will be subject to 18,000 pressure cycles.
2. Failure Criteria
In the NPRM, NHTSA proposed that a CNG fuel container would have to
meet two test criteria to pass the pressure cycling test: (1) No
leakage, and (2) no permanent circumferential deformation greater than
one percent. The agency proposed these two criteria to provide
objective means of evaluating a container's durability during
compliance testing. NHTSA adopted the no leakage portion of the
proposal from NGV2's pressure cycling test. The one percent deformation
level, which is not in NGV2's pressure cycling test, was based on the
Society of Automotive Engineers (SAE) Recommended Practice J10, August
1985, a requirement involving the performance of metal air brake
reservoirs. The agency proposed a limit on circumferential deformation
to aid in determining when a container's failure was impending.
No commenters objected to the no leakage criterion. Accordingly,
the agency has adopted the no leakage requirement in the final rule.
The agency believes that specifying that containers ``shall not leak''
provides an objective measure that will ensure that a container
maintains its integrity by retaining its contents under pressure.
Sixteen commenters addressed the issue of the allowable
circumferential deformation criterion. The commenters were NGVC,
Brunswick, Pressed Steel Tank (PST), Structural Composites Industry
(SCI), Tecogen, CGA, AAMA, Amoco, Alusuisse, Oklahoma Gas, ARC,
Flxible, Fiber Dynamics, Norris, Comdyne, and EDO. All the commenters,
except Brunswick, believed that the agency should not include a
deformation requirement in the pressure cycling or burst tests. The
commenters believed that the test requirement is not appropriate for
all container materials and designs. They stated that due to the nature
of the different materials used in these containers, and their
different rates of deformation under load, some materials such as
fiberglass, would deform more than others, such as steel. The
commenters also stated that deformation was not an indicator of
impending failure and that the SAE brake reservoir test was not
appropriate for a CNG fuel container application.
NHTSA has decided not to adopt the one percent circumferential
deformation requirement. In proposing this criterion, NHTSA tentatively
concluded that it would be an appropriate indicator of the fuel
container's durability characteristics. However, as the comments note,
it is not an appropriate criterion because of the differing
construction and materials used for CNG fuel container applications.
After reviewing the comments and other available information, the
agency now believes that limiting the circumferential deformation is
not a meaningful way to determine a container's strength or impending
failure, since the larger deformation experienced by some materials
does not necessarily represent these characteristics. Instead, the
agency believes that the no-leakage requirement, by itself, is the
appropriate criterion to define a container failure, after being
subjected to the pressure cycling test.
Brunswick further commented that some container designs, such as
full-wrapped composite containers, would deform in the axial direction
in addition to the circumferential direction. To account for axial
deformation, Brunswick recommended allowing a maximum five percent
volumetric expansion of the container.\6\ Brunswick stated that this
test is used to assure that the container material exhibits elastic
behavior at expected operating conditions.
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\6\Both RSPA's standards and NGV2 incorporate the concept of
volumetric expansion. In these standards, the volumetric expansion
is measured when hydrostatic testing is performed on the container
at 1.50 to 1.67 times the service pressure. This test is a non-
destructive one, i.e., the container may be put into service after
it is tested.
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NHTSA agrees with Brunswick's statement that some container designs
deform in the axial direction. Nevertheless, the agency believes that
measuring volumetric expansion would not provide an appropriate measure
of a container's impending failure in a destructive test (i.e., where
the container cannot be used again). In addition, the NPRM provided no
notice to amend the standard to measure such expansion in the axial
direction. Since the pressure cycling and burst tests being adopted in
this rule are capable of evaluating a CNG container's durability, the
agency believes that another non-destructive test would be redundant
and therefore is not needed. The agency further notes that the five
percent maximum level of expansion would not provide a meaningful
measure of a container's impending failure, since this level is based
on a container's performance under less stringent test conditions.
D. Burst Test
1. Safety Factor
With respect to the burst test, NHTSA proposed that a CNG fuel
container would have to withstand an internal hydrostatic pressure of
3.50 times the service pressure for 60 seconds, without any leakage or
circumferential deformation over one percent. The multiple of the
internal hydrostatic pressure, 3.50, is known as the safety factor. The
agency tentatively concluded that the burst test, together with a
pressure cycling test, would be sufficient to assure adequate levels of
safety performance for both the strength and durability of CNG fuel
containers used in motor vehicles.
The proposal of a burst test with a safety factor was based in part
on NGV2. NGV2 specifies several sets of detailed material and design
requirements. For each set of those requirements, NGV2 specifies a
unique safety factor for calculating the internal hydrostatic pressure
that the container must withstand. The safety factors range from 2.25
to 3.50, depending on the material and design involved. To satisfy this
aspect of NGV2, a container must meet both the material and design
requirements as well as the burst test.
NGV2 specifies four types of container designs. A Type 1 container
is a metallic noncomposite container. A Type 2 container is a metallic
liner over which an overwrap such as carbon fiber or fiberglass is
applied in a hoop wrapped pattern over the liner's cylinder sidewall. A
Type 3 container is a metallic liner over which an overwrap such as
carbon fiber or fiberglass is applied in a full wrapped pattern over
the entire liner, including the domes. A Type 4 container is a non-
metallic liner over which an overwrap such as carbon fiber or
fiberglass is applied in a full wrapped pattern over the entire liner,
including the domes.
The agency did not propose adoption of the material and design
requirements of NGV2. Instead, the agency proposed a single safety
factor of 3.50 for all containers, regardless of their materials or
design. It tentatively concluded that the factor would not impede
technological development, yet would assure an acceptable level of
safety for all containers.
CNG container manufacturers, CNG trade associations (NGVC and AGA),
utility companies, the American Automobile Manufacturers Association
(AAMA) and other commenters addressed the issue of the safety factor.
Most commenters disagreed with the agency's proposal to require that
all containers meet the same safety factor.
NGVC, AGA, and the CNG container manufacturers generally believed
that the material and design of the fuel container need to be taken
into account in establishing an appropriate safety factor, if safe,
cost-effective, and light-weight containers are to be produced.
Establishing an overly high factor for a given combination of material
and design could result in unnecessarily over-designed, heavy
containers, according to these commenters. They believed that some
materials, such as fiberglass, need a higher safety factor because they
degrade faster over time. In contrast, a material such as steel
maintains its strength for a longer time, and therefore containers made
of it could be made safely with a lower safety factor.
Many of these commenters recommended that NHTSA adopt the safety
factors specified in NGV2. They stated that compared to the regulations
proposed by NHTSA, the NGVC voluntary industry standard provides a more
appropriate level of safety, given the need to specify safety factors
based on the design and materials used.
However, several commenters disagreed with certain safety factors
specified in NGV2. CGA, PST, SCI, and NGV Systems supported a higher
safety factor for containers using unproven materials. In particular,
they were concerned with containers reinforced with carbon fiber
overwrap, for which NGV2 specifies a 2.25 safety factor for all carbon
reinforced containers, Types 2, 3, and 4.
NGV Systems stated that a safety factor of 2.25 constitutes an
``unacceptable safety risk,'' given the industry's limited experience
with carbon fiber and lack of a significant data base demonstrating
this materials safety and reliability. Accordingly, NGV Systems
supported a safety factor of 3.5 for what it termed unproven designs,
which may then be lowered as more experience and data accumulate. CGA
recommended safety factors of 2.5 for all Type 2 containers and 3.33
for all Type 3 and 4 containers, stating that these are used on all
fiber reinforced compressed gas containers now in commercial use. CGA
indicated that unlike other fiber overwrap used in the past for
transportation pressure vessels, there is no commercial experience with
the safety of carbon fiber reinforced containers for motor vehicle
applications to justify a 2.25 safety factor for such containers. CGA
stated that NGV2 does not adequately address damage tolerance concerns
for carbon reinforced fully wrapped containers with low safety factors.
PST recommended 3.33 for carbon fiber Types 3 and 4 containers. That
commenter recommended such conservative safety factors until
substantial data are accumulated on the use of carbon fiber containers
in actual service. SCI provided similar comments, and recommended
safety factors of 3.33 for the fully wrapped containers, which are
Types 3 and 4.
Three commenters stated that a single safety factor was
appropriate. CNG Pittsburgh, a consulting firm, stated that a safety
factor of 3.50 is conservative but reasonable for CNG fuel containers.
AAMA stated that adopting NGV2's approach with various safety factors
depending on the material and design involved would limit a
manufacturer's choice of container designs and materials. EDO
recommended a safety factor of 2.5 for all containers.
NHTSA decided to issue an SNPRM proposing to pattern the burst
requirement more closely on NGV2, based on its consultation with other
Federal agencies, its review of comments to the January 1993 proposal,
and other available information. In explaining its reason for issuing
the SNPRM, NHTSA stated that there did not appear to be any procedures
that could adequately test a container's susceptibility to degradation
over time. Therefore, it believed that specifying a single safety
factor would not protect in all instances against these problems since
the strength of some containers is dependent on the specific material
and method of design. Therefore, NHTSA decided to propose a burst test
that would link the use of particular designs and materials to
compliance with safety factors tailored to those designs and materials.
The agency tentatively concluded that such an approach might be
necessary to ensure the safe performance of pressure vessels used for
fuel containers. The agency further noted that international standards
addressing CNG fuel containers, including regulations of Transport
Canada and those being drafted by the International Standards
Organization (ISO) link the use of particular designs and materials
with strength requirements suitable for those designs and materials.
In the SNPRM, NHTSA requested comment on the appropriateness of
requiring CNG containers to meet design and material requirements, such
as those specified in NGV2, and to meet safety factors tailored to
those requirements. The agency also asked about the effect of adopting
NGV2 on future container technology, since the only way a container
manufacturer could comply with the Federal standard would be by
producing a container that uses those materials and designs specified
in NGV2 if the agency incorporated NGV2's material and design
provisions in the FMVSS. As an alternative approach, the agency asked
whether it should specify a catch-all high end safety factor for any
container whose design and materials are not specified in NGV2.
NHTSA received 18 comments to the December 1993 SNPRM about
adopting the design and material specific approach of NGV2. Sixteen
commenters, including NGVC/AGA, CGA, CNG container manufacturers,
public utilities, and two bus manufacturers supported the proposal to
incorporate NGV2 into the Federal standard. Eleven commenters supported
the safety factors in NGV2. Five others were concerned about the level
of some safety factors in NGV2 or the use of relatively new materials,
such as carbon fiber. CGA and SCI referenced their earlier comments to
the NPRM, again recommending safety factors of 2.5 for all Type 2
containers and 3.33 for all Type 3 and Type 4 containers. AAMA and Ford
opposed the design and material specific approach of NGV2. AAMA stated
that some of NGV2's requirements limit opportunities for future
development of advance container design or materials that may not fit
in the specifications in NGV2. No commenter favored having a catch-all
high end safety factor.
Based on the available information, NHTSA has decided to require
CNG containers to meet the safety factors applicable to the design and
material requirements specified in NGV2, except for carbon fiber.
Specifically, the agency is specifying separate safety factors for
containers using various materials (e.g., fiberglass, carbon, steel,
aluminum) and different designs (non-composite, hoop wrapped or full
wrapped composite containers, and welded). The agency believes that
this approach will result in the manufacture of safe containers for CNG
powered vehicles.
NHTSA has decided to adopt the specific safety factors and related
requirements set forth in NGV2, except for those safety factors
specified for carbon fiber. While NGV2 currently specifies a safety
factor of 2.25 for Type 2, 3, and 4 carbon fiber containers, NHTSA has
decided to specify a safety factor of 2.5 for Type 2 carbon fiber
containers and 3.33 for the Type 3 and 4 carbon fiber containers. The
agency is requiring a higher safety factor for Type 3 and 4 containers
since the fibers on those containers carry a greater proportion of the
load than on Type 2 containers.
NHTSA made this decision after reviewing all of the comments and
information obtained in response to both the NPRM and SNPRM; meetings
with container manufacturers, CGA and NGVC/AGA; and discussions with
other Federal agencies, including RSPA. Comments and information were
presented to support safety factors for carbon fiber containers,
ranging from 2.25 to 3.5. Brunswick, in particular, submitted
substantial test data and other technical information in support of
NGV2's 2.25 safety factor for carbon fiber, including testing it
performed on such containers which showed favorable results. RSPA
recommended a safety factor of not less than 3.0 for carbon fiber,
which is consistent with its FRP-1 and FRP-2 standards.
Notwithstanding comments supporting the 2.25 safety factor, NHTSA
has determined that under its statutory mandate, it is necessary to
specify higher safety factors for carbon fiber containers. In adopting
these more stringent requirements, NHTSA sought the advice of RSPA,
which has accumulated significant experience and expertise through its
efforts to regulate the safety of pressure vessels used to transport
hazardous materials. Specifically, NHTSA has adopted RSPA's
recommendation not to specify the 2.25 safety factor for carbon
composite containers.
The more stringent safety factors being adopted are consistent with
RSPA's longstanding approach to initially adopt conservative
requirements and subsequently modify the requirements, if further real-
world safety data become available supporting less stringent
regulations. NHTSA has determined that applying this approach to the
safety factors for carbon fiber containers is necessary, since carbon
fiber containers have not been used extensively in motor vehicle
applications. The agency believes that the higher safety factors are
justified until further data are developed and become available on the
use of carbon fiber containers in motor vehicle applications.
NHTSA acknowledges that using such a safety-oriented approach may
result in costlier and heavier carbon fiber containers. However, the
agency believes that the requirements being adopted will not preclude
the introduction and effective use of this new technology. Overall, the
agency believes that the safety factors being specified for carbon
fiber containers, along with the remaining safety factors it has
adopted from NGV2 for other materials, will result in safe CNG
containers.
As for AAMA's comment, NHTSA shares that association's concerns
about restricting future developments. However, based on comments by
the container manufacturers and other Federal agencies, the agency
believes that few, if any, designs beyond those accounted for in NGV2
are planned. If a new container technology is developed, the agency
will evaluate its safety in the context of a petition for rulemaking to
amend the Federal safety standard.
NHTSA has decided not to adopt the catch-all high level safety
factor, which could allow containers incorporating materials or designs
that have not been incorporated in NGV2 and thus might be detrimental
to safety. The agency further believes that it would be inappropriate,
at this time, to add a catchall factor. While such a proviso would
facilitate innovation and design change, the agency agrees with
commenters that specifying such a catchall might be detrimental to
safety, since untested designs and materials would be permitted.
2. Hold Time Interval
In the NPRM, NHTSA proposed that during the burst test, elevated
pressure would have to be sustained for 60 seconds. The agency noted
that while RSPA regulations also specify a 60-second period, NGV2
requires a 10-second hold time interval once the maximum pressure is
obtained. The agency believed that because NGV2 includes additional
tests to qualify container designs and the agency was not proposing
these additional tests, a shorter hold time would not be suitable.
NHTSA received six comments addressing the appropriate hold time
interval. All commenters except EDO believed the 60 second hold time
requirement was not necessary. EDO stated that the requirement was
``tough but reasonable.'' NGVC, Brunswick, PST, and ARC stated that
specifying the hold time at 60 seconds instead of 10 seconds would not
compensate for the lack of other NGV2 required tests. NGVC stated that
the ten second hold time interval is not intended as a test of
container strength, but as the time for the pressure in the container
to stabilize. PST stated that along with the 3.5 safety factor, the 60
second hold time would make an already conservative test even more
stringent.
After reviewing the comments and other available information, NHTSA
has decided to specify a hold time of 10 seconds instead of 60 seconds.
The agency notes that the proposal was based on a misperception of the
hold time requirement's purpose. As the commenters stated, the hold
period is included only to stabilize the pressure. It is not used as a
surrogate for initial burst strength. Therefore, the reduction in hold
time will not affect the test's stringency. In addition, the agency
anticipates issuing a SNPRM that would propose additional performance
requirements to evaluate other aspects of a CNG fuel container's
integrity.
3. Sequential Testing
In the NPRM, NHTSA proposed that a container that passed the
pressure cycling test would then be subjected to the burst test. In
proposing that the same fuel container be used in both the pressure
cycling and burst tests, the agency believed that it would be
appropriate to establish that the fuel container maintained its initial
strength after being subject to the durability test.
Seven commenters addressed the issue of using the same container
for both the pressure cycling and burst tests. NGVC, AAMA, Comdyne,
Pressed Steel Tanks, and Amoco stated that requiring the same fuel
container for both tests would be unrealistic and overly stringent,
because in real world situations, a container would not be subject to
pressure cycling and burst conditions sequentially. They stated that
otherwise unnecessary material would have to be added to strengthen the
container so it could meet the burst test requirement after the
pressure cycling test. These commenters believed such additional
material would significantly increase the container's cost and weight
to the extent that the container would no longer be economically viable
to produce. They further stated that most containers that are currently
produced to meet NGV2 or RSPA requirements would not be able to meet
this requirement. In contrast, EDO and Metropolitan Suburban Bus
Authority (MSBA) favored the use of sequential testing.
After reviewing the comments and other available information, NHTSA
has decided not to require sequential testing. The agency believes that
using different containers in the pressure cycling and burst tests will
provide an adequate measure of both the container's initial strength
and its durability over its life, without imposing new cost burdens on
the industry. The agency notes that such testing is consistent with the
way in which industry currently tests under both NGV2 and RSPA
standards. The agency further notes that in testing for compliance with
some FMVSSs, the agency allows a manufacturer to use a separate vehicle
or component for different tests within a standard. For example, three
vehicles are crashed in Standard No. 301, and different brake hoses are
used for various tests in Standard No. 106, Brake Hoses.
4. Failure Criteria
In the NPRM, NHTSA proposed that to pass the burst test, a
container would have to meet the same two performance criteria as in
the pressure cycling test: (1) No leakage, and (2) no permanent
circumferential deformation of more than one percent. The purpose of
these requirements was to provide objective means to evaluate a
container's compliance strength. NGV2 includes the no leakage
criterion, but not the one percent circumferential deformation
criterion. As explained in the section on the pressure cycling test,
the deformation requirement was based on SAE Recommended Practice J10,
August 1985, which addresses the performance of metal air brake
reservoirs. The agency proposed a circumferential deformation limit to
aid in determining a container's impending failure.
After reviewing the comments, NHTSA is adopting the no leakage
criterion to evaluate failure of the burst test. The agency has decided
not to adopt the one percent deformation criterion because the agency
believes that circumferential deformation is not a meaningful measure
of a fuel container's impending failure in the burst test. See the
section above regarding the pressure cycling test for a more
comprehensive discussion about the agency's decision not to adopt the
pressure deformation criterion.
E. Bonfire Test
1. Performance Requirements
In the NPRM, NHTSA proposed performance requirements for CNG fuel
containers to address the need to withstand high temperatures and
pressures without catastrophic failure. Large pressure increases due to
exposure to flames could cause the CNG to escape catastrophically and
result in an explosive fire. The agency proposed that the ability to
withstand high temperatures and pressures be provided by a pressure
relief device. More specifically, it proposed that compliance would be
determined by first pressurizing the fuel container to 100 percent of
service pressure with nitrogen or air and placing it over a bonfire
until the container's contents are completely vented through a pressure
relief device. A pressure relief device can prevent a container from
experiencing high pressure for long periods of time. The agency
proposed a second test to be conducted in the same manner, except the
container would be pressurized to 25 percent of the service pressure.
The second test would evaluate container performance when containers
are partially filled. The purpose of the test is to reduce the
explosion potential of CNG containers when exposed to high temperatures
and pressures.
The proposed requirements were based on NGV2. However, there were
two differences between the agency's proposal and NGV2. First, under
the NPRM, the container would be pressurized with nitrogen or air; in
NGV2, it is pressurized by CNG. Second, under the NPRM, all fuel
containers would be required to use a pressure relief device to
completely vent the container's contents; in NGV2, the test is run for
20 minutes or until the container is completely vented, whichever comes
first. Therefore, under NGV2, a manufacturer could establish compliance
either by a container successfully withstanding the test conditions for
20 minutes without bursting or by completely venting its contents by
means of a pressure relief device at some point during that 20 minute
period. In the NPRM, the agency sought comment about whether to allow
an alternative way of demonstrating compliance with the bonfire test
that did not depend upon a pressure relief device. Under the
alternative, a container would be considered to have passed the test if
it did not burst during the test period. Compliance with the
alternative would be achieved by designing a container so that it has
sufficient strength to enable it to sustain the heat and pressure
buildup during the test.
Eleven commenters addressed the issue of whether containers should
be required to have a pressure relief device. NGVC, EDO, ARC, Flxible,
Manchester, Thomas, and MSBA agreed with the proposal to require
containers to be equipped with such a device. They stated that a
pressure relief device is an integral part of a CNG container and that
its importance warrants a requirement that each container have one. In
contrast, Brunswick, Comdyne, Pressure Technology, and AAMA stated that
containers should not be required to have a pressure relief device
because such a requirement would be design restrictive. Brunswick and
Pressure Technology stated that the container should be required to
``safely vent'' its contents without rupturing, whether the venting is
done through a pressure relief device or the container wall. AAMA
stated that a container should pass the requirement if it possesses
enough strength to retain its contents throughout the test. ARC
believed that the container sidewalls should not be permitted to
rupture during the bonfire test.
After reviewing the comments, NHTSA has determined that each CNG
container must be equipped with a pressure relief device. This is
necessary because each CNG fuel container needs to possess a means of
releasing its contents in case the internal pressure or temperature
reaches a dangerous level. By requiring containers to be equipped with
a pressure relief device, the agency will ensure the safety of
individuals, such as vehicle occupants and rescue personnel, who would
be near a CNG vehicle in a fire. The agency notes that the conditions
experienced in the bonfire test may be less severe than certain real-
world crash situations. Therefore, the agency is adopting a more
conservative approach and requiring a pressure relief device for all
containers. In addition, such a requirement is consistent with the
practice of most container manufacturers and NGV2 which requires such a
device on all containers.
Based on the comments, NHTSA has decided to adopt NGV2's test
criteria that allows the test to be completed after 20 minutes or when
the container has completely vented, whichever comes first. Adopting
these criteria alters the test in that while still requiring a pressure
relief device, a container could comply with the bonfire test if it
either completely vents its contents by means of a pressure relief
device at some point during that 20 minute period or by successfully
retaining the container's entire contents without bursting for the
duration of the bonfire test (i.e., 20 minutes). The agency believes
that each criterion appropriately measures a container's ability to
withstand high temperature and pressure because the bonfire test
represents extreme conditions. The agency emphasizes that in either
case the CNG container must be equipped with a pressure relief device.
NHTSA disagrees with the approach advocated by AAMA, Brunswick and
Pressure Technology to allow containers to ``safely vent'' their
contents from an area other than the pressure relief device such as the
sidewall. The agency acknowledges that, as an alternative to a pressure
relief device, pressure relief can be accomplished by allowing the
overpressurized container to vent its contents at a controlled rate,
without fragmentation, through the container's sidewall. However, there
would be significant problems with this approach. First, it would not
afford as high a degree of safety as requiring a pressure relief
device. The agency continues to believe that the safest way to release
CNG from an overpressurized container is through a pressure relief
device because some sidewall ruptures could result in fragments being
propelled from the container. Second, it would raise potential
enforceability problems since the concepts of ``release its contents at
a controlled rate'' and ``rupture without fragmentation'' are difficult
to define objectively. Based on the above considerations, NHTSA has
decided to require each CNG fuel container to either completely vent
its contents through a pressure relief device or not burst when tested
in accordance with the test conditions.
2. Types of Pressure Relief Devices
The proposal did not specify the use of a particular type of
pressure relief device. The agency is aware of three types of devices
currently being used: (1) The rupture disc, which is designed to
release CNG in the container when it reaches a specific pressure, (2)
the fusible plug, which is designed to release CNG in the container
when it reaches a specific temperature, and (3) a device that combines
these two devices.
Four commenters recommended the use of specific types of pressure
relief devices. EDO recommended that the agency require the fusible
plug device and prohibit the rupture disc device. EDO stated that a
combination of hot conditions and overfill at the refueling pump could
cause a rupture disc to activate, releasing CNG and causing a
potentially dangerous situation. It further believed that the safety
factor in the burst test would be sufficient to prevent over
pressurization and that the pressure relief device should only open in
a fire situation. Flxible stated that the agency should require a
fusible plug to ensure pressure relief of partially filled containers
subject to heat or fire. NYCFD stated that the agency should prohibit
the combination fusible plug and rupture disc devices, claiming that
over-charged containers exposed to high ambient temperature are likely
to fail whether or not they are exposed to fire. Thomas commented that
the agency should require the combination fusible plug and rupture disc
device because it is required by NFPA 52.\7\
---------------------------------------------------------------------------
\7\NFPA 52, Standard for Compressed Natural Gas (CNG) Vehicular
Fuel Systems, is a voluntary standard adopted by the National Fire
Protection Association that specifies guidelines for the ``design
and installation of CNG engine fuel systems on vehicles of all types
including aftermarket and OEMs and to their associated fueling
(dispensing) systems.'' (NFPA 52, Sec. 1-1)
---------------------------------------------------------------------------
After reviewing the comments, NHTSA has concluded that the standard
should not specify the type of pressure relief device with which a
container may be equipped. The NPRM and SNPRM did not provide
sufficient notice for the agency to adopt such a specification as part
of this final rule. Further, the agency believes that the bonfire test,
which is performed at both 100 percent of service pressure and 25
percent of service pressure, will adequately evaluate a container's
ability to vent its contents in a high temperature/pressure situation.
In the first test, the combination of the 100 percent service pressure
condition and the high heat from the bonfire will cause the container's
pressure to increase rapidly. This test evaluates a container's ability
to vent its contents at high temperatures and pressures. In the second
test, the 25 percent service pressure condition and the heat will cause
the container's temperature to increase before the pressure in the
container reaches a critical point. This test evaluates a container's
ability to vent its contents at high temperatures, where the container
is at a less than full condition.
3. Shielding
NHTSA notes that there are two types of shielding that can affect
the performance of pressure relief devices in bonfire tests: (1)
``Vehicle-based protective shielding'' that is placed around the
container in the vehicle to protect the container from surrounding
heat, and (2) ``test shielding'' that is placed over the pressure
relief device to prevent flames from contacting the device. Test
shielding is, as the name suggests, installed only for the purpose of
conducting bonfire tests. Unlike vehicle-based protective shielding, it
is not used to affect real world performance.
In the NPRM, NHTSA recognized that some CNG vehicles may have
vehicle-based shielding installed to protect the containers from
exposure to heat. Nevertheless, the agency proposed that no vehicle-
based shielding be used during the bonfire test because Standard No.
304 is an equipment standard, and applies to CNG containers, not to
vehicles. Further, since the presence or amount of shielding could vary
from vehicle to vehicle, the agency tentatively concluded that the
containers should be tested in the worst case situation, i.e., without
any vehicle-based shielding. Nevertheless, the agency stated that it
did not want to discourage vehicle manufacturers from including
shielding in CNG vehicles as an added safety feature.
NHTSA received six comments addressing the use of vehicle-based
shielding during the bonfire test. PST, EDO, ARC, Ontario, and NGVC
agreed with the agency that vehicle-based shielding of the container
should not be used during the bonfire test. They believed that such
shielding could detract from or mask the results of the test. In
contrast, AAMA stated that ``[i]f a manufacturer chooses to add the
additional expense to protect the fuel tank from exposure to potential
flame, the protection ought to be allowed in any test as representative
of the tank's use in the vehicle.''
After reviewing the comments, NHTSA has decided not to permit
vehicle-based shielding of the container during the bonfire test. As
explained in the NPRM, the bonfire test is intended to evaluate the
container and not the vehicle. Since this is an equipment standard, the
tests are designed to ensure that the containers are safe for
installation in any vehicle, regardless of the amount of protective
vehicle shielding, if any, with which it is equipped. The agency
disagrees with AAMA's contention. Using vehicle shielding in compliance
testing would not ensure that a container could perform safely under
worst case conditions (i.e., no vehicle-based shielding of any type or
extent) that the container could encounter during its service life
(e.g., if the container is subsequently placed in a different vehicle).
Test shielding consists of a metal plate over the pressure relief
device and is permitted, but not required, under NGV2 for purposes of
the horizontal bonfire test. In the horizontal test, the CNG container
is positioned over the bonfire with its longitudinal axis in a
horizontal position. In the NGV2 vertical bonfire test (container
longitudinal axis in a vertical position), pressure relief device
shielding is also permitted, but not required, except where the CNG
container is fitted with a pressure relief device on both ends. In that
case, the bottom pressure relief device must be shielded. The goal is
to not allow flames to impinge directly on any relief device. This may
be done through test shielding, or by orienting the container so as to
avoid flame impingement on any pressure relief device. Without this
metal plate, the flames could contact the pressure relief device,
possibly causing it to vent the container prematurely. If this
occurred, the bonfire test results would neither evaluate the CNG
container as a whole nor accurately reflect the container's pressure
relief characteristics.
CGA and PST opposed allowing shielding of the pressure relief
device during the bonfire test. They commented that shielding the
pressure relief device during the bonfire test would not be
representative of a real-world crash fire situation. CGA stated that
allowing, but not requiring shielding to be placed around pressure
relief devices could produce non- repeatable results. PST stated that
excessive shielding around the pressure relief device could cause an
otherwise acceptable design to fail the test, but did not elaborate as
to how this could occur.
NHTSA has decided to require test shielding of the pressure relief
device during the horizontal bonfire test. The agency notes that the
purpose of this test is to replicate the effect of fires on the
pressure relief device and the fuel container as a system. Requiring
shielding will assure that the bonfire test is evaluating the fuel
container as a whole, rather than merely the pressure relief device,
since a flame that impinges on the pressure relief device, could
activate prematurely. Requiring shielding, rather than simply allowing
it, will assure repeatable and consistent test results. The rule also
requires shielding of the pressure relief device during the vertical
bonfire test, except where the container is fitted with a pressure
relief device on only one end. In that case, the container is
positioned with the pressure relief device on top, so as to avoid
direct contact with the flame.
4. Test Gas and Pressure
In the NPRM, NHTSA proposed that the CNG container be pressurized
with either nitrogen or air to 100 percent of service pressure for the
bonfire test. The agency acknowledged that NGV2 specifies the use of
CNG, but tentatively concluded that using nitrogen or air as the test
gas would be safer than using CNG.
AAMA and Tecogen recommended that CNG be used as the test gas.
Tecogen further commented that the container manufacturers have
historically conducted such tests using CNG and are therefore well
aware of the necessary safety precautions. It further stated that using
CNG as the test gas would reveal the pressure relief valve's
effectiveness with respect to the discharge rate. AAMA commented that
CNG should be used as the test gas because the thermal properties of
CNG differ from those of nitrogen and air and NGV2 specifies the use of
CNG as the test gas. AAMA also recommended that the CNG containers be
pressurized at the start of the test to 95 to 100 percent of service
pressure, but offered no rationale.
After reviewing the comments, NHTSA has determined that using CNG
as the test gas would better reflect the real-world conditions in a
fire, since the test gas would be the same as the gas used in CNG
containers. The agency notes that the bonfire test addresses the
responsiveness of the pressure relief device and that air and nitrogen
have different thermal properties than CNG. Therefore, the pressure
relief device might perform differently if air or nitrogen were used
instead of CNG. In the NPRM, the agency explained that using CNG as a
test gas might not be safe. These initial concerns have been allayed by
the comments indicating that manufacturers are aware of and accustomed
to taking the necessary safety precautions when using CNG as a test gas
to evaluate a container. NHTSA notes that it decided not to specify CNG
as the test gas in the CNG vehicle standard. Nevertheless, the agency
believes that differences in reaction to heat are important for the
bonfire test, which involves high temperatures, but not for crash
tests, which do not involve such temperatures.
NHTSA continues to believe that it is necessary to pressurize the
CNG container to 100 percent of service pressure at the outset of the
test. The agency has determined that the containers need to be tested
at full service pressure to represent the worst case scenario.
5. Wind Velocity and Direction
In the NPRM and SNPRM, NHTSA did not address the allowable wind
velocity and direction. The agency received comments from NGVC, CGA,
and PST stating that a limit should be placed on wind velocity to
increase the bonfire test's repeatability.
After reviewing the comments, NHTSA has decided to specify that the
average wind velocity at the container during the test may not exceed
2.24 meters per second (5 mph). The agency believes that permitting
higher crosswinds would vary or reduce the flame's heat. Therefore,
placing limits on the crosswind assures the test's repeatability and
the level of stringency that the agency anticipated in proposing this
test.
6. Bonfire Fuel
In the NPRM, NHTSA proposed that the fire for the bonfire tests be
generated using No. 2 diesel fuel. This fuel type was proposed so that
the standard would be consistent with the bonfire test in NGV2, which
also specifies this type of fuel.
NGVC, CGA, AAMA, and Norris commented that the agency should
specify a different fuel to generate the bonfire that is more
environmentally sound. CGA stated that the large amounts of smoke that
would be created by burning the diesel fuel are contrary to the
environmental objectives of developing CNG vehicles. NGVC and Norris
suggested using a CNG or propane grill for the test.
After reviewing the comments and other available information, NHTSA
has decided to specify the use of No. 2 diesel fuel in the final rule.
The agency is aware of the environmental problems associated with this
type of fuel and will further study whether other fuels should be used
to generate the bonfire test. However, until the agency can determine
that a different fuel is an appropriate replacement for diesel fuel,
the Standard will specify No. 2 diesel fuel for use in the bonfire
test.
7. Bonfire Test Fuel Pan Depth
In the NPRM, NHTSA proposed that the bonfire test pan containing
No. 2 diesel fuel be at least 100 centimeters (cm) deep. The agency
specified a depth to ensure that there would be an adequate amount of
fuel to run the test.
AAMA, Comdyne, CGA, Alusuisse, and PST commented that the fuel pan
depth was excessive. Alusuisse stated that a pan of the proposed size
would contain more than 1,000 liters of fuel. PST stated that a 100
millimeter (mm) depth would be more reasonable. CGA, AAMA, and Comdyne
stated that the depth of the fuel pan should not be specified so long
as a sufficient quantity of fuel is provided for the test.
The agency intended to propose a depth of 100 mm. However, due to a
typographical error, it proposed a depth of 100 cm. NHTSA agrees that a
fuel pan with a depth of at least 100 cm would be too deep. NHTSA also
agrees that the fuel pan's depth does not need to be specified,
provided that there is a sufficient amount of fuel to maintain the fire
for the duration of the test. Accordingly, the agency has removed the
requirement for fuel pan depth and has replaced it with the provision
that there be ``sufficient fuel to burn for at least 20 minutes.'' The
agency believes that this provision is consistent with the test's
purpose of simulating a severe fire by raising the container's
temperature and pressure by completely surrounding it with flames
produced by a specific fuel type.
F. Labeling Requirements
In the NPRM, NHTSA proposed to require that container manufacturers
certify that each of their containers complies with the proposed
equipment requirements and permanently label the container with the
following information: the symbol ``DOT'' to constitute a certification
by the manufacturer that the container conforms to all requirements of
the standard; the date of manufacture of the container; the name and
address of the container manufacturer; and the maximum service
pressure. The agency stated that labeling the container would provide
vehicle manufacturers and consumers with assurance that they are
purchasing containers that comply with the Federal safety standards. In
addition, the agency believed that the proposed requirement would
facilitate the agency's enforcement efforts by providing a ready means
of identifying the container and its manufacturer.
EDO, NGVC, Thomas, NYCFD, and Volvo GM addressed the proposed
labeling requirements. EDO and NYCFD stated that the label should
include the maximum fill pressure at a location close to the fill
receptacle. NGVC recommended that a blank area for the container
installation date be included in the label to be filled in by the
installer. Volvo GM stated that only containers that are manufactured
after the standard's effective date, and therefore actually subject to
the standard, should be entitled to display the DOT symbol as
certification of compliance with the standard. Thomas stated, without
elaboration, that the labeling requirements of NGV2 should be adopted.
NHTSA's proposal did not include certain additional information
included in NGV2, including the type of container, inspector symbols,
trademarks, manufacturer's part number, and serial numbers.
After reviewing the comments, NHTSA has decided to adopt the
proposed labeling requirements with a slight modification from the
proposed format. In item (a), the agency has modified the proposal
which states ``The tank manufacturer's name and address'' to state the
following: include the statement that ``If there is a question about
the proper use, installation, or maintenance of this container, contact
[manufacturer's name, address, and telephone number].''
The agency has decided not to require the other additional items of
information in NGV2 since the agency did not propose the inclusion of
such information in the NPRM. Notwithstanding the agency's decision not
to require this additional information, a manufacturer may list such
information on the label, provided the additional information does not
obscure or confuse the required information. In particular, NHTSA
encourages manufacturers to include the container type, e.g., Type 1,
2, 3 or 4, since the agency has decided to adopt NGV2's design and
material specifications in this final rule. Specifying the type of
container should facilitate oversight of compliance tests since each
type of container is required to undergo hydrostatic burst tests, but
with different safety factors.
In the upcoming SNPRM, NHTSA anticipates proposing additional
requirements about the CNG fuel container's label, including the
container type. In addition, the agency anticipates proposing that the
label include an additional statement addressing the container's
inspection and maintenance. Specifically, the label would state that
``This container should be visually inspected after an accident or fire
or at least every 12 months for damage and deterioration in accordance
with the applicable Compressed Gas Association guidelines.'' The agency
believes that such a statement would alert owners to the desirability
for reinspection over time or in the event of an accident. NHTSA will
also propose requirements related to the label's location, in response
to EDO's and NYCFD's comment that the maximum service pressure should
be labeled in an area close to the fill receptacle.
G. Leadtime
In the NPRM, NHTSA proposed to make the equipment requirements
effective on September 1, 1994. The agency believed that this would
provide a reasonable time period for manufacturers to make minor
modifications in container design. This proposal was based on the
agency's belief that the proposed requirements were similar to RSPA
standards currently in effect. The agency requested comment on the
feasibility of this effective date.
NHTSA received eleven comments about the proposed effective date
applicable to the container requirements. The commenters were TMC, the
U.S. Department of Energy, TBB, Oklahoma Gas, NGVC, EDO, Volvo/GM,
AAMA, ARC, Navistar, and NGV Systems. EDO and Navistar requested that
the final rule be issued as early as possible. DOE and Oklahoma Gas
recommended an effective date of September 1, 1995. NGVC recommended an
effective date of September 1, 1996, unless NGV2 were adopted which
would permit an immediate supply of containers. NGV Systems stated that
an earlier effective date would be difficult to meet since the rule, as
proposed, would require new tooling, process development, and perhaps
equipment modification. ARC stated that the rule, as proposed, would
require major modifications, since its containers have been designed to
comply with NGV2. AAMA and Volvo/GM stated that the effective dates for
the vehicle requirements and the equipment requirements should not be
concurrent.
NHTSA notes that these comments were based on the requirements, as
proposed in the NPRM. Since the final rule has been made essentially
consistent with NGV2 (with the exception of carbon fiber containers),
the agency anticipates that container manufacturers can for the most
part already certify that containers, other than carbon fiber ones,
comply with the new standard. This belief is based on comments on the
NPRM and meetings with NGVC, the CGA, and CNG container manufacturers.
With regard to manufacturers of carbon fiber containers, EDO indicated
that it already complies with the standard and Brunswick indicated that
it would need less than one month lead time for a safety factor greater
than 2.25. Brunswick further stated that it would need an unspecified
time period to modify the mounting brackets and other hardware. The CNG
industry groups have informed the agency that they want a CNG fuel
container standard to be effective as quickly as possible. In addition,
they favor having an opportunity to ``voluntarily certify compliance''
to the standard once the final rule is published. The CNG industry
groups believe that it is necessary for Federal standards to be in
place as soon as possible, given the expected increased demand for CNG
containers in light of Federal and State fleet programs for clean fuel
vehicles. They also favor quick adoption of a Federal standard to
preempt state regulations that otherwise may be promulgated and to
ensure that substandard CNG containers are not marketed.
After reviewing the comments, NHTSA has decided to establish an
effective date six months after the final rule is issued. As explained
above, most CNG containers can be certified to comply with the new
Federal motor vehicle safety standard since they already comply with
NGV2 or can be modified so that they comply within six months.
Nevertheless, the agency believes that it is necessary to provide a
leadtime of six months to allow manufacturers time to make whatever
design changes are necessary and to conduct testing so that they can
certify that their containers comply with the new standard. In the
meantime, prior to the standard's effective date, the industry is free
to advertise containers as meeting the CNG equipment standard that will
take effect in six months.\8\ Manufacturers have taken the approach of
seeking early compliance with respect to other agency requirements such
as those relating to dynamic side impact protection and air bags.
Therefore, the agency encourages manufacturers to seek, to the extent
feasible, to manufacture their CNG containers to meet these new
requirements before the date the standard takes effect.
---------------------------------------------------------------------------
\8\However, the agency emphasizes that a manufacturer may not
certify a container as meeting the equipment standard until the
standard goes into effect. Under the Vehicle Safety Act, a
certification is a statement that a vehicle or item of equipment
meets all applicable Federal Motor Vehicle Safety Standards that are
then in effect. Therefore, until a standard is effective,
manufacturers may not certify compliance with it.
---------------------------------------------------------------------------
With regard to the concern expressed by AAMA and Volvo GM that the
effective date of the container regulation should precede that of the
vehicle regulation, AAMA based its comments on the belief that it will
need to know the performance of the containers it will use in the fuel
systems of its vehicles. NHTSA notes that CNG containers now typically
meet NGV2 and thus should comply with NHTSA's standards. Therefore,
AAMA members already have access to and detailed knowledge about
containers that should meet the new requirements.
H. Benefits
In the NPRM, NHTSA addressed the proposal's benefits with respect
to CNG vehicles. The notice did not directly address the benefits of
regulating the CNG fuel containers.
NHTSA received no comments directly addressing the benefits of
regulating CNG containers. Brunswick criticized the proposal, believing
that it would place carbon fiber containers at a competitive
disadvantage. Brunswick stated that the proposed single burst factor
would provide less benefits than if the agency adopted NGV2.\9\
---------------------------------------------------------------------------
\9\Because NHTSA is adopting Brunswick's request for multiple
safety factors, that commenter's concern about a single safety
factor is moot.
---------------------------------------------------------------------------
NHTSA anticipates that the number of CNG fuel vehicles will
increase greatly in the near future, in light of directives by the
Clinton Administration\10\ and legislation by Congress to develop
vehicles powered by cleaner burning fuels. This final rule will
increase the safety of this growing population of vehicles.
---------------------------------------------------------------------------
\10\Executive Order 12844 increased by 50 percent the number of
alternatively fueled vehicles to be acquired by the Federal
Government from 1993 through 1995. (April 21, 1993) In addition, in
1993, the President established the Federal Fleet Conversion Task
Force to accelerate the commercialization and market acceptance of
alternative fueled vehicles throughout the country.
---------------------------------------------------------------------------
I. Costs
In the NPRM, NHTSA stated that each container would cost $600. The
agency further stated that the container testing costs would range from
approximately $4,050 to $8,600 for each model of container.
NGVC, NGV Systems, PST, Brunswick, ARC, Thomas Built, and Flxible
addressed the costs of the proposal with respect to CNG containers.
NGVC and the CNG container manufacturers stated that the proposal,
especially given the single safety factor in the burst test
requirements, significantly understated the costs of the rulemaking.
Brunswick stated that container manufacturers would incur significant
costs since they would have to redesign and requalify their currently
designed tanks. As a result, it believed that the CNG containers would
be more expensive and heavier. It estimated that the proposal would
increase costs between 10 percent and 55 percent, depending on the
material and method of construction. Brunswick further stated that this
proposal would add many millions of dollars on an industry-wide basis.
NGVC commented that the qualification tests could cost $20,000 for
each model of container since many tests will be required on prototype
containers. It stated that some manufacturers estimate that the design,
manufacture, and qualification costs could approach $150,000 per
container model, a figure that greatly exceeded NHTSA's estimate of
$74,000.
NHTSA believes that the basis for the comments about the costs of
this rulemaking have been largely eliminated except in connection with
carbon fiber tanks. The comments were based on the proposal for a
single safety factor of 3.5 for all types of tanks. As noted above, the
agency has decided to specify multiple safety factors that are
consistent with NGV2 except in the case of the factors for carbon fiber
containers. Since all the container manufacturers commenting on the
proposal either already certify to or can comply with NGV2 without any
design changes, the cost to manufacturers will be minimal for noncarbon
fiber tanks.
V. Rulemaking Analyses
A. Executive Order 12866 and DOT Regulatory Policies and Procedures
NHTSA has considered the impact of this rulemaking action under
Executive Order 12866 and the Department of Transportation's regulatory
policies and procedures. This rulemaking document was not reviewed
under E.O. 12866, ``Regulatory Planning and Review.'' This action has
been determined to be ``nonsignificant'' under the Department of
Transportation's regulatory policies and procedures. NHTSA has
estimated the costs of the amendments in a Final Regulatory Evaluation
(FRE) which is included in the docket for this rulemaking. As discussed
in that document, NHTSA estimates that the cost for the pressure
cycling, burst, and bonfire testing will range from $9,000 to $21,725
per container size and type. In addition, the cost of the containers
used in the test is estimated to range from $1,800 to $6,600. Since the
safety factors in the burst test applicable to carbon fiber containers
are more stringent than those in NGV2, the cost of those containers
will increase. Based on comments by Brunswick and other information,
the switch from carbon fiber containers meeting a 2.25 safety factor to
carbon fiber containers meeting the factors adopted in this final rule
will increase the container cost and the lifetime fuel costs about 8.75
percent for vehicles equipped with Type 2 containers. Those costs would
be range from $115 for passenger cars to $602 for heavy trucks. The
switch would increase costs about 37.1 percent for vehicles equipped
with Type 3 and Type 4 containers, resulting in a cost increase ranging
from $496 for cars to $2,560 for heavy trucks.
B. Regulatory Flexibility Act
NHTSA has also considered the effects of this rulemaking action
under the Regulatory Flexibility Act. Based upon the agency's
evaluation, I certify that this rule will not have a significant
economic impact on a substantial number of small entities. Information
available to the agency indicates that businesses manufacturing CNG
fuel containers are not small businesses.
C. Executive Order 12612 (Federalism)
NHTSA has analyzed this rulemaking action in accordance with the
principles and criteria contained in Executive Order 12612. NHTSA has
determined that the rule will not have sufficient Federalism
implications to warrant the preparation of a Federalism Assessment. No
state has adopted requirements regulating CNG containers.
D. National Environmental Policy Act
In accordance with the National Environmental Policy Act of 1969,
NHTSA has considered the environmental impacts of this rule. The agency
has determined that this rule will have no adverse impact on the
quality of the human environment. On the contrary, because NHTSA
anticipates that ensuring the safety of CNG vehicles will encourage
their use, NHTSA believes that the rule will have positive
environmental impacts. CNG vehicles are expected to have near-zero
evaporative emissions and the potential to produce very low exhaust
emissions as well.
E. Civil Justice Reform
This final rule does not have any retroactive effect. Under 49
U.S.C. 30103, whenever a Federal motor vehicle safety standard is in
effect, a State may not adopt or maintain a safety standard applicable
to the same aspect of performance which is not identical to the Federal
standard, except to the extent that the State requirement imposes a
higher level of performance and applies only to vehicles procured for
the State's use. 49 U.S.C. 30161 sets forth a procedure for judicial
review of final rules establishing, amending or revoking Federal motor
vehicle safety standards. That section does not require submission of a
petition for reconsideration or other administrative proceedings before
parties may file suit in court.
List of Subjects in 49 CFR Part 571
Imports, Incorporation by reference, Motor vehicle safety, Motor
vehicles.
PART 571--[AMENDED]
In consideration of the foregoing, 49 CFR Part 571 is amended as
follows:
PART 571--[AMENDED]
1. The authority citation for Part 571 continues to read as
follows:
Authority: 49 U.S.C. 322, 30111, 30115, 30117 and 30166;
delegation of authority at 49 CFR 1.50.
2. Section 571.5 is amended by redesignating (b)(7) as (b)(10) and
adding new paragraphs (b)(7) through (b)(9), to read as follows:
Sec. 571.5 Matter incorporated by reference.
* * * * *
(b) * * *
(7) Standards of Suppliers of Advanced Composite Materials
Association (SACMA). They are published by Suppliers of Advanced
Composite Materials Association. Information and copies may be obtained
by writing to: Suppliers of Advanced Composite Materials Association,
1600 Wilson Blvd., Suite 1008, Arlington, VA 22209.
(8) Standards of the American Society of Mechanical Engineers
(ASME). They are published by The American Society of Mechanical
Engineers. Information and copies may be obtained by writing to: The
American Society of Mechanical Engineers, 345 East 47th Street, New
York, NY 10017.
(9) Computer Analysis by the National Aeronautics and Space
Administration (NASA). This was conducted by the National Aeronautics
and Space Administration. Information and copies may be obtained by
writing to: National Aeronautics and Space Administration, 600
Independence Avenue SW, Washington, DC 20546.
* * * * *
3. A new Sec. 571.304, Standard No. 304; Compressed Natural Gas
Fuel Container Integrity, is added to Part 571, to read as follows:
Sec. 571.304 Standard No. 304; Compressed Natural Gas Fuel Container
Integrity.
S1. Scope. This standard specifies requirements for the integrity
of compressed natural gas (CNG), motor vehicle fuel containers.
S2. Purpose. The purpose of this standard is to reduce deaths and
injuries occurring from fires that result from fuel leakage during and
after motor vehicle crashes.
S3. Application. This standard applies to containers designed to
store CNG as motor fuel on-board any motor vehicle.
S4. Definitions.
Brazing means a group of welding processes wherein coalescence is
produced by heating to a suitable temperature above 800 deg.F and by
using a nonferrous filler metal, having a melting point below that to
the base metals. The filler metal is distributed between the closely
fitted surfaces of the joint by capillary attraction.
Burst pressure means the highest internal pressure reached in a CNG
fuel container during a burst test at a temperature of 21 deg.C (70
deg.F).
CNG fuel container means a container designed to store CNG as motor
fuel on-board a motor vehicle.
Fill pressure means the internal pressure of a CNG fuel container
attained at the time of filling. Fill pressure varies according to the
gas temperature in the container which is dependent on the charging
parameters and the ambient conditions.
Full wrapped means applying the reinforcement of a filament or
resin system over the entire liner, including the domes.
Hoop wrapped means winding of filament in a substantially
circumferential pattern over the cylindrical portion of the liner so
that the filament does not transmit any significant stresses in a
direction parallel to the cylinder longitudinal axis.
Hydrostatic pressure means the internal pressure to which a CNG
fuel container is taken during testing set forth in S5.4.1.
Liner means the inner gas tight container or gas cylinder to which
the overwrap is applied.
Service pressure means the internal settled pressure of a CNG fuel
container at a uniform gas temperature of 21 deg.C (70 deg.F) and
full gas content. It is the pressure for which the container has been
constructed under normal conditions.
Stress ratio means the stress in the fiber at minimum burst
pressure divided by the stress in the fiber at service pressure.
S5 Container and material requirements.
S5.1 Container designations. Container designations are as
follows:
S5.1.1 Type 1--Non-composite metallic container means a metal
container.
S5.1.2 Type 2--Composite metallic hoop wrapped container means a
metal liner reinforced with resin impregnated continuous filament that
is ``hoop wrapped.''
S5.1.3 Type 3--Composite metallic full wrapped container means a
metal liner reinforced with resin impregnated continuous filament that
is ``full wrapped.''
S5.1.4 Type 4--Composite non-metallic full wrapped container means
resin impregnated continuous filament with a non-metallic liner ``full
wrapped.''
S5.2 Material designations.
S5.2.1 Steel containers and liners.
(a) Steel containers and liners shall be of uniform quality. Only
the basic oxygen or electric furnace processes are authorized. The
steel shall be aluminum killed and produced to predominantly fine grain
practice. The steel heat analysis shall be in conformance with one of
the following grades:
Table One--Steel Heat Analysis
----------------------------------------------------------------------------------------------------------------
Chrome-Molybdenum Carbon-Manganese
Grade element percent Carbon-Boron percent percent
----------------------------------------------------------------------------------------------------------------
Carbon..................................... 0.25 to 0.38......... 0.27 to 0.37......... 0.40 max.
Manganese.................................. 0.40 to 1.05......... 0.80 to 1.40......... 1.65 max.
Phosphorus................................. 0.015 max............ 0.015 max............ 0.025 max.
Sulfur..................................... 0.010 max............ 0.010 max............ 0.010 max.
Silicon.................................... 0.15 to 0.35......... 0.30 max............. 0.10/0.30
Chromium................................... 0.80 to 1.15......... N/A.................. N/A
Molybdenum................................. 0.15 to 0.25......... N/A.................. N/A
Boron...................................... N/A.................. 0.0005 to 0.003...... N/A
Aluminum................................... 0.02 to 0.07......... 0.02 to 0.07......... 0.02/0.07
----------------------------------------------------------------------------------------------------------------
\1\``N/A'' means not applicable.
(b) Incidental elements shall be within the limits specified in the
Standard Specification for Steel, Sheet and Strip, Alloy, Hot-Rolled
and Cold-Rolled, General Requirements for ASTM A 505 (1987).
S5.2.1.1 When carbon-boron steel is used, the test specimen is
subject to a hardenability test in accordance with the Standard Method
for End-Quench Test For Hardenability of Steel, ASTM A 255 (1989). The
hardness evaluation is made 7.9 mm (\5/16\ inch) from the quenched end
of the Jominy quench bar.
S5.2.1.2 The test specimen's hardness shall be at least Rc
(Rockwell Hardness) 33 and no more than Rc 53.
S5.2.2 Aluminum containers and aluminum liners. (Type 1, Type 2
and Type 3) shall be 6010 alloy, 6061 alloy, and T6 temper. The
aluminum heat analysis shall be in conformance with one of the
following grades:
Table Two--Aluminum Heat Analysis
------------------------------------------------------------------------
6061 alloy
Grade element 6010 alloy percent percent
------------------------------------------------------------------------
Magnesium......................... 0.60 to 1.00...... 0.60 to 1.20
Silicon........................... 0.80 to 1.20...... 0.40 to 0.80
Copper............................ 0.15 to 0.60...... 0.15 to 0.40
Chromium.......................... 0.05 to 0.10...... 0.04 to 0.35
Iron.............................. 0.50 max.......... 0.70 max.
Titanium.......................... 0.10 max.......... 0.15 max.
Manganese......................... 0.20 to 0.80...... 0.15 max.
Zinc.............................. 0.25 max.......... 0.25 max.
Bismuth........................... 0.003 max......... 0.003 max.
Lead.............................. 0.003 max......... 0.003 max.
Others, Each\1\................... 0.05 max.......... 0.05 max.
Others, Total\1\.................. 0.15 max.......... 0.15 max.
Aluminum.......................... Remainder......... Remainder.
------------------------------------------------------------------------
\1\Analysis is made only for the elements for which specific limits are
shown, except for unalloyed aluminum. If, however, the presence of
other elements is indicated to be in excess of specified limits,
further analysis is made to determine that these other elements are
not in excess of the amount specified. (Aluminum Association Standards
and Data--Sixth Edition 1979.)
S5.2.3 Structural reinforcing filament material shall be
commercial grade E-glass, commercial grade S-glass, aramid fiber or
carbon fiber. Filament strength shall be tested in accordance with the
Standard Test Method for Tensile Properties of Glass Fiber Strands,
Yarns, and Rovings Used in Reinforced Plastics, ASTM D 2343 (1967,
Reapproved 1985), or SACMA Recommended Test Method for Tow Tensile
Testing of Carbon Fibers, SRM 16-90, 1990. Fiber coupling agents
(sizing) shall be compatible with the resin system. If carbon fiber
reinforcement is used the design shall incorporate means to prevent
galvanic corrosion of metallic components of the fuel container.
S5.2.4 The resin system shall be epoxy, modified epoxy, polyester,
vinyl ester or thermoplastic.
S5.2.4.1 The resin system is tested on a sample coupon
representative of the composite overwrap in accordance with the
Standard Test Method for Apparent Interlaminar Shear Strength of
Parallel Fiber Composites by Short-Beam Method, ASTM D 2344, (1984,
Reapproved 1989) following a 24-hour water boil.
S5.2.4.2 The test specimen shall have a shear strength of at least
13.8 MPa (2,000 psi).
S5.2.5 For nonmetallic liners, the permeation of CNG through the
finished container's wall at service pressure is less than 0.25 normal
cubic centimeters per hour per liter water capacity of the container.
S5.3 Manufacturing processes for composite containers.
S5.3.1 Composite containers with metallic liners. The CNG fuel
container shall be manufactured from a metal liner overwrapped with
resin impregnated continuous filament windings, applied under
controlled tension to develop the design composite thickness. After
winding is complete, composites using thermoset resins shall be cured
by a controlled temperature process.
S5.3.1.1 Type 2 containers. Type 2 containers shall have a hoop
wrapped winding pattern.
S5.3.1.2 Type 3 containers. Type 3 containers shall have a full
wrapped ``helical or in plane'' and a ``hoop'' wrap winding pattern.
S5.3.2 Type 4 containers. Composite containers with nonmetallic
liners shall be fabricated from a nonmetallic liner overwrapped with
resin impregnated continuous filament windings. The winding pattern
shall be ``helical or in plane'' and ``hoop'' wrap applied pattern
under controlled tension to develop the design composite thickness.
After winding is complete, the composite shall be cured by a controlled
temperature process.
S5.3.3 Brazing. Brazing is prohibited.
S5.3.4 Welding. Welding shall be done in accordance with the
American Society of Mechanical Engineers (ASME) Boiler and Pressure
Vessel Code, Section IX, Article II, QW-304 and QW-305 (1992). Weld
efficiencies shall be in accordance with ASME Boiler and Pressure
Vessel Code, Section VIII, UW-12 (1989). Any weld shall be subject to
full radiographic requirements in accordance with ASME Boiler and
Pressure Vessel Code, Section VIII, UW-51 thru UW-53 (1989). For Type 2
and Type 3 liners, longitudinal welds and nonconsumable backing strips
or rings shall be prohibited.
S5.4 Wall thickness.
S5.4.1 Type 1 containers.
(a) The wall thickness of a Type 1 container shall be at least an
amount such that the wall stress at the minimum prescribed hydrostatic
test pressure does not exceed 67 percent of the minimum tensile
strength of the metal as determined by the mechanical properties
specified in S5.7 and S5.7.1.
(b) For minimum wall thickness calculations, the following formula
is used:
TR26SE94.000
Where:
S = Wall stress in MPa (psi).
P = Minimum hydrostatic test pressure in Bar (psig).
D = Outside diameter in mm (inches).
d = Inside diameter in mm (inches).
S5.4.2 Type 2 containers.
S5.4.2.1 The wall thickness of a liner to a Type 2 container shall
be at least an amount such that the longitudinal tensile stress at the
minimum design burst pressure does not exceed the ultimate tensile
strength of the liner material as determined in S5.7 and S5.7.1.
S5.4.2.2 The wall thickness of a liner to a Type 2 container
shall be at least an amount such that the compressive stress in the
sidewall of the finished container at zero pressure shall not exceed 95
percent of the yield strength of the liner as determined in S5.7 and
S5.7.1 or 95 percent of the minimum design yield strength shown in
S5.7.3. The maximum tensile stress in the liner at service pressure
shall not exceed 66 percent of the yield strength.
S5.4.2.3 Stresses at the end designs at internal pressures between
no more than 10 percent of service pressure and service pressure shall
be less than the maximum stress limits in the sidewall as prescribed
above.
S5.4.3 Type 3 containers. The wall thickness of a liner to a Type
3 container shall be such that the compressive stress in the sidewall
of the finished container at zero pressure shall not exceed 95 percent
of the minimum yield strength of the liner as determined in S5.7 and
S5.7.1 or 95 percent of the minimum design yield strength shown in
S5.7.3
S5.4.4 Type 4 containers. The wall thickness of a liner to a Type
4 container shall be such that the permeation rate requirements of this
specification are met.
S5.5 Composite reinforcement for Type 2, Type 3, and Type 4
Containers.
S5.5.1 Compute stresses in the liner and composite reinforcement
using National Aeronautics and Space Administration (NASA) NAS 3-6292,
Computer Program for the Analysis of Filament Reinforced Metal-Shell
Pressure Vessels, (May 1966).
S5.5.2 The composite overwrap shall meet or exceed the following
composite reinforcement stress ratio values shown in Table 3.
S5.6 Thermal treatment.
S5.6.1 Steel containers or liners.
S5.6.1.1 After all metal forming and welding operations, completed
containers or liners shall be uniformly and properly heat treated under
the same conditions of time, temperature and atmosphere prior to all
tests.
S5.6.1.2 All containers or liners of steel grades ``Chrome-
Molybdenum'' or ``Carbon Boron'' shall be quenched in a medium having a
cooling rate not in excess of 80 percent that of water. ``Carbon-
Manganese'' steel grades shall be normalized and do not require
tempering after normalizing.
S5.6.1.3 All steel temperature on quenching shall not exceed
926 deg.C (1700 deg.F).
S5.6.1.4 All containers or liners or steel grades ``Chrome-
Molybdenum'' or ``Carbon Boron'' shall be tempered after quenching at a
temperature below the transformation ranges, but not less than
482 deg.C (900 deg.F) for ``Carbon-Boron'' steel or 565 deg.C
(1050 deg.F) for ``Chrome-Molybdenum'' steel. ``Carbon Manganese''
steel grades do not require tempering after normalizing.
S5.6.2 Aluminum containers or liners (seamless and welded). After
all forming and welding operations, aluminum containers or liners shall
be solution heat treated and aged to the T6 temper. The liner and
composite overwrap shall meet the cycle life and strength requirements
set forth in S7.1 and S7.2 of this standard.
S5.7 Yield strength, tensile strength, material elongation (metal
containers and metal liners only). To determine yield strength, tensile
strength, and elongation of the material, cut two specimens from one
container or liner. The specimen either has (a) a gauge length of 50 mm
(2 inches) and a width not over 38 mm (1.5 inches), or (b) a gauge
length of four times the specimen diameter, provided that a gauge
length which is at least 24 times the thickness with a width not over 6
times the thickness is permitted when the liner wall is not over 5 mm
(3/16 inch) thick. The specimen shall not be flattened, except that
grip ends may be flattened to within 25 mm (1 inch) of each end of the
reduced section. Heating of specimens is prohibited.
S5.7.1 Yield strength. The yield strength in tension shall be the
stress corresponding to a permanent strain of 0.2 percent based on the
gauge length.
S5.7.1.1 The yield strength shall be determined by either the
``offset'' method or the ``extension under load'' method as prescribed
by Standard Test Methods for Tension Testing of Metallic Materials,
ASTM E8 1993.
S5.7.1.2 In using the ``extension under load'' method, the total
strain or ``extension under load'' corresponding to the stress at which
the 0.2 percent permanent strain occurs may be determined by
calculating the elastic extension of the gauge length under appropriate
load and adding thereto 0.2 percent of the gauge length. Elastic
extension calculations shall be based on an elastic modulus of 69 GPa
(10,000,000 psi) for aluminum, or 207 GPa (30,000,000 psi) for steel.
If the elastic extension calculation does not provide a conclusive
result, the entire stress strain diagram shall be plotted and the yield
strength determined from the 0.2 percent offset.
S5.7.1.3 For the purpose of strain measurement, the initial strain
is set while the test specimen is under a stress of 41 MPa (6,000 psi)
for aluminum, and 83 MPa (12,000 psi) for steel. The strain indicator
reading is set at the calculated corresponding strain.
S5.7.1.4 Cross-head speed of the testing machine is 3.2 mm (1/8
inch) per minute or less during yield strength determination.
S5.7.2 Elongation. Elongation of material, when tested in
accordance with S5.7, shall be at least 14 percent for aluminum or at
least 20 percent for steel; except that an elongation of 10 percent is
acceptable for both aluminum and steel when the authorized specimen
size is 24t gauge length x 6t wide, where ``t'' equals specimen
thickness.
S5.7.3 Tensile strength. Tensile strength shall not exceed 725 MPa
(105,000 psi) for ``Carbon Manganese'' and 966 MPa (140,000 psi) for
``Chrome-Molybdenum'' and ``Carbon-Boron.''
S6 General requirements.
S6.1 Each passenger car, multipurpose passenger vehicle, truck,
and bus that uses CNG as a motor fuel shall be equipped with a CNG fuel
container that meets the requirements of S7 through S7.4.
S6.2 Each CNG fuel container manufactured on or after March 27,
1994, shall meet the requirements of S7 through S7.4.
S7 Test requirements. Each CNG fuel container shall meet the
applicable requirements of S7 through S7.4.
S7.1 Pressure cycling test at ambient temperature. Each CNG fuel
container shall not leak when tested in accordance with S8.1.
S7.2 Hydrostatic burst test.
S7.2.1 Each Type 1 CNG fuel container shall not leak when
subjected to burst pressure and tested in accordance with S8.2. Burst
pressure shall be not less than 2.25 times the service pressure for
non-welded containers when analyzed in accordance with the stress ratio
requirements of S5.4.1, and shall not be less than 3.5 times the
service pressure for welded containers.
S7.2.2 Each Type 2, Type 3, or Type 4 CNG fuel container shall not
leak when subjected to burst pressure and tested in accordance with
S8.2. Burst pressure shall be no less than the value necessary to meet
the stress ratio requirements of Table 3, when analyzed in accordance
with the requirements of S5.5.1. Burst pressure is calculated by
multiplying the service pressure by the applicable stress ratio set
forth in Table Three.
Table Three--Stress Ratios
------------------------------------------------------------------------
Material Type 2 Type 3 Type 4
------------------------------------------------------------------------
E-Glass...................................... 2.65 3.5 3.5
S-Glass...................................... 2.65 3.5 3.5
Aramid....................................... 2.25 3.0 3.0
Carbon....................................... 2.50 3.33 3.33
------------------------------------------------------------------------
S7.3 Bonfire test. Each CNG fuel container shall be equipped with
a pressure relief device. Each CNG fuel container shall completely vent
its contents through a pressure relief device or shall not burst while
retaining its entire contents when tested in accordance with S8.3.
S7.4. Labeling. Each CNG fuel container shall be permanently
labeled with the information specified in paragraphs (a) through (d).
The information specified in paragraphs (a) through (d) of this section
shall be in English and in letters and numbers that are at least 12.7
mm (\1/2\ inch) high.
(a) The statement: ``If there is a question about the proper use,
installation, or maintenance of this container, contact
________________.'' inserting the CNG fuel container manufacturer's
name, address, and telephone number.
(b) The statement: ``Manufactured in ____________.'' inserting the
month and year of manufacture of the CNG fuel container.
(c) Maximum service pressure ________ kPa (________ psig).
(d) The symbol DOT, constituting a certification by the CNG
container manufacturer that the container complies with all
requirements of this standard.
S8 Test conditions: fuel container integrity.
S8.1 Pressure cycling test. The requirements of S7.1 shall be met
under the conditions of S8.1.1 through S8.1.4.
S8.1.1 Hydrostatically pressurize the CNG container to the service
pressure, then to not more than 10 percent of the service pressure, for
13,000 cycles.
S8.1.2 After being pressurized as specified in S8.1.1,
hydrostatically pressurize the CNG container to 125 percent of the
service pressure, then to not more than 10 percent of the service
pressure, for 5,000 cycles.
S8.1.3 The cycling rate for S8.1.1 and S8.1.2 shall not exceed 10
cycles per minute.
S8.1.4 The cycling is conducted at ambient temperature.
S8.2 Hydrostatic burst test. The requirements of S7.2 shall be met
under the conditions of S8.2.1 through S8.2.2.
S8.2.1 Hydrostatically pressurize the CNG fuel container, as
follows: The pressure is increased up to the minimum prescribed burst
pressure determined in S7.2.1 or S7.2.2, and held constant at the
minimum burst pressure for 10 seconds.
S8.2.2 The pressurization rate throughout the test shall not
exceed 1,379 kPa (200 psi) per second.
S8.3 Bonfire test. The requirements of S7.3 shall be met under the
conditions of S8.3.1 through S8.3.10.
S8.3.1 The CNG fuel container is filled with compressed natural
gas and tested at (1) 100 percent of service pressure and (2) 25
percent of service pressure. Manufacturers may conduct these tests
using the same container or with separate containers.
S8.3.2 The CNG fuel container is positioned so that its
longitudinal axis is horizontal. Subject the entire length to flame
impingement, except that the flame shall not be allowed to impinge
directly on any pressure relief device. Shield the pressure relief
device with a metal plate.
S8.3.3 If the test container is 165 cm (65 inches) in length or
less, place it in the upright position and subject it to total fire
engulfment in the vertical. The flame shall not be allowed to impinge
directly on any pressure relief device. For containers equipped with a
pressure relief device on one end, the container is positioned with the
relief device on top. For containers equipped with pressure relief
devices on both ends, the bottom pressure relief device shall be
shielded with a metal plate.
S8.3.4 The lowest part of the container is 102 mm (4 inches) above
the liquid surface of the diesel fuel at the beginning of the test.
S8.3.5 The CNG fuel container is tested with the valve and
pressure relief device or devices in place.
S8.3.6 The fire is generated by No. 2 diesel fuel.
S8.3.7 The fuel specified in S8.3.6 is contained in a pan such
that there is sufficient fuel to burn for at least 20 minutes. The
pan's dimensions ensure that the sides of the fuel containers are
exposed to the flame. The pan's length and width shall exceed the fuel
container projection on a horizontal plane by at least 20 cm (8 inches)
but not more than 50 cm (20 inches). The pan's sidewalls shall not
project more than 2 cm (0.8 inches) above the level of fuel.
S8.3.8 Time-pressure readings are recorded at 30 second intervals,
beginning when the fire is lighted and continuing until the container
is completely tested.
S8.3.9 The CNG fuel container is exposed to the bonfire for 20
minutes or until its contents are completely vented.
S8.3.10 The average wind velocity at the container is not to
exceed 2.24 meters/second (5 mph).
Issued on September 16, 1994.
Ricardo Martinez,
Administrator.
[FR Doc. 94-23571 Filed 9-21-94; 1:13 pm]
BILLING CODE 4910-50-P
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