72
When a vehicle is involved in an accident or fire causing damage to any part of
the CNG fuel system, the system shall be retested before being returned to service.
Damaged supply lines must be replaced, not repaired.
The owner or user, or both, shall maintain all containers, container appurtenanc-
es, piping systems, venting systems and other components in a safe condition.
As a precaution to keep pressure relief devices in reliable operating condition,
care shall be taken in the handling or storing of compressed natural gas containers
to avoid damage. Care shall also be exercised to avoid plugging by paint or other dirt
accumulation of pressure relief device channels or other parts that could interfere
with the functioning of the device. Only qualified personnel shall be allowed to ser-
vice pressure relief devices. Only assemblies or original manufacturer’s parts shall be
used in the repair of pressure relief devices unless the interchange of parts has been
proved by suitable tests.
CODE OF FEDERAL REGULATIONS #49 states: In addition to the requirements of
this paragraph, cylinders marked DOT-3HT must be qualified in accordance with CGA
pamphlet C-8 and must comply with the following:
Cylinders built prior to implementation of FMVSS 304 must be subjected, at least
once in three years, to a test by hydrostatic pressure in a water jacket, for the determi-
nation of the expansion of the cylinder. A cylinder must be condemned if the elastic
expansion exceeds the marked rejection elastic expansion.
Cylinder service life must not exceed fifteen years.
Each cylinder must be inspected and hydrostatically tested every three years in
accordance with 49 CFR 173.34(e) as prescribed for DOT-8HT cylinders, except that
the rejection elastic expansion criteria does not apply, permanent volumetric expan-
sion must not exceed 5 percent of total volumetric expansion at test pressure and
retest dates must be imbedded in the epoxy coatings in a permanent manner other
than stamping. Retest dates may be steel stamped on the shoulder of the top head
in accordance with 178.BB-15(c). Re-heat treatment or repair of cylinders is not au-
thorized.
Fuel System Description
The fuel system begins with DOT-certified tanks designed to hold compressed natu-
ral gas (CNG) up to 3,600 psi at standard day temperature (70° F, or 21° C). The tanks
are high strength steel wrapped with fiberglass for additional strength.
The fuel flows from the tank to the engine through manually controlled shutoff
valves. These include an integral pressure relief valve consisting of a combination
rupture disk (for pressure) and fuse plug (for temperature) to vent the contents of
the tank should high pressure and high temperature occur, such as in a fire. The rated
temperature for relief is 212° F.
From the valve, fuel flows into a common fuel line through high pressure stain-
less steel compression fittings. The tanks are joined to the common line through high
pressure stainless steel Tees and crosses. Any open tank is connected to any other
open tank, so for fueling, the fuel flows through the common line to all tanks. The
tank shutoff valves isolate the tanks; the Tees on the valves still have the high line
pressure.
To Next Page
