MIL-HDBK-1022A
is colorless, tasteless, and non-toxic. It is a light gas,
weighing about two thirds as much as air. It tends to rise
and diffuse rapidly in air when it escapes from the system.
Natural gas burns in air with a luminous flame. At
atmospheric pressure, the ignition temperature of natural gas
mixtures has been reported to be as low as 900 degrees F
(482 degrees C). The flammable limits of natural gas-air
mixtures at atmospheric pressure are about 5 percent to 15
percent by volume of natural gas. While natural gas consists
principally of methane, it also contains ethane, small amounts
of propane, butane, and higher hydrocarbons and may contain
small amounts of nitrogen, carbon dioxide, hydrogen sulfide,
and helium which will vary from zero to a few percent
depending upon the source and seasonal effects. As
distributed in the United States and Canada, natural gas also
contains water vapor. This "pipeline quality" gas can contain
7 pounds or more of water per million cubic feet of gas (112
kg/106 m3). Some constituents of natural gas, especially
carbon dioxide and hydrogen sulfide in the presence of liquid
water, can be corrosive to carbon steel, and the corrosive
effect is increased by pressure. The pressures used in CNG
systems covered by NFPA 52 are substantial and well above
those used in transmission and distribution piping and in
other natural gas consuming equipment. As excessive corrosion
can lead to sudden explosive rupture of a container, this
hazard must be controlled. Pressures in CNG fueling stations
are typically less than 5,000 psi (35 000 kPa).
2.3.7.2
Special Precautions for CNG
a) Provide venting for safety relief in areas where
CNG is to be stored.
b) CNG is a highly flammable substance. Therefore,
in design of facilities, use the following precautions to
prevent fires from becoming uncontrollable:
(1)
Do not directly extinguish fires with
water.
(2)
Do not extinguish large fires.
(3)
Allow large fires to burn while cooling
adjacent equipment with water spray.
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