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The tanks are full at 3,000 psi, or 3,600 psi at 70° F. The pressure will vary with
temperature, decreasing with lower temperature and increasing with higher tem-
peratures. Filling apparatus compensates for the temperature effect.
[WARNING] Never fill to more than permissible pressure. Contact the manu-
facturer if an accidental overfill occurs.
Specific Gravity
The specific gravity of natural gas relative to air (air = 1.00) is 0.56 to 0.62, depending
on gas composition. This means that natural gas is lighter than air. In the event of a
natural gas leak, the gas will rise and dissipate given open conditions. There is no
possibility of CNG accumulating in pools on the ground beneath a spill.
Odorants have been added that allow natural gas to be detected before reaching
dangerous concentrations.
In the case of LNG releases, the cold vapor is initially heavier than the surrounding
warmer air, so it stays low near the ground (a visible vapor cloud is often formed from
the condensation of water in the cold air gas mixture). As the vapor cloud warms, it
will increase in volume, rapidly rise and dissipate in an open environment.
Fuel Toxicity And Safety
Natural gas is a nontoxic gas. However, it is flammable under proper conditions. Also,
it can cause suffocation if it displaces enough oxygen. LNG has the added safety con-
cern of being a cold (-260° F) liquid under pressure. Contact with LNG or associated
cold components may cause severe frostbite. Furthermore, many common materials
change their strength characteristics when exposed to LNG temperatures, thus pre-
senting additional hazards.
[WARNING] LNG tanks have the potential for explosions under circumstanc-
es such as those described for LPG explosions.
Although natural gas has odorants to aid in detection of leaks, these odorants
are removed during liquefaction; thus LNG vapors cannot be detected by smell. LNG
odorants have been developed but are not commonly used due to the relatively re-
stricted use of LNG at this time.
Flammability
Auto-ignition temperature for natural gas at atmospheric pressure is 1,004° F com-
pared to an auto-ignition temperature range of 442° to 880° F for gasoline and ap-
proximately 500° F for diesel fuel. The risk of fire in the presence of an ignition source
exists when the ratio of air to fuel is within flammability limits (i.e., fuel can not ignite
if it is mixed with too much or too little oxygen). The flammability limits for natural
gas are 5.3 to 15 % volume of gas in air. For comparison, the flammability limits of
unleaded gasoline are 1 to 7.6 % volume of gasoline in air. As a practical matter, there
is no oxygen present in CNG cylinders or LNG tanks, therefore ignition within the
cylinder or tank is not possible. In the event of a fuel leak, there will be a small area in
which the air/fuel ratio is within the flammability limits. In a closed garage, or within
the passenger compartment, ignition conditions are more likely to be met. Odorants
used in CNG allow its detection before the lower flammability limit is reached.
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