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Dense Gas
A gas with density exceeding that of air at ambient temperature. Sometimes the expression is used loosely to apply to aerosols such as flashing liquid ammonia. See also heavy gas and negatively buoyant vapors.
Flammable Gas (NFPA 55)
A gas that is flammable in a mixture of 13 percent or less (by volume) with air, or the flammable range with air is wider than 12 percent regardless of the lower limit, at atmospheric temperature and pressure.
Fuel Gas
Gaseous fuels consisting of natural gas and various manufactured or by-product gases.
Gas
The state of matter characterized by complete molecular mobility and unlimited expansion at standard temperature and pressure
Gas Enrichment
The addition of another flammable gas to a gas-air mixture to raise the concentration above the upper flammable limit.
Heavy Gas
A gas with density exceeding that of air at ambient temperature. See also dense gas and negatively buoyant vapors.
Inert Gas
A nonflammable, nonreactive gas that can be used to render the combustible material in a system incapable of supporting combustion.
Liquefied Flammable Gas (LFG)
Any flammable gaseous material or mixture of materials that is in liquid form under pressure.
Liquefied Natural Gas (LNG)
A fluid in the liquid state composed predominantly of methane and that can contain minor quantities of ethane, propane, nitrogen, or other components normally found in natural gas. (NFPA 59A)
Liquefied Petroleum Gas (LPG)
Any material having a vapor pressure not exceeding that allowed for commercial propane composed predominantly of the following hydrocarbons, either by themselves or as mixtures: propane, propylene, butane (normal butane or isobutane), and butylenes. (NFPA 58)
Neutral Buoyant Gas
A gas with density approximately equal to that of air at ambient temperature.
Nonflammable Gas (NFPA 55)
A gas that does not meet the definition of a flammable gas.
Oxidizing Gas (NFPA 55)
A gas that can support and accelerate combustion of other materials.
Passive Gas
A gas whose density is equal to that of the ambient air. Synonymous with neutral.
Positively Buoyant Gas
A gas with density less that of air at ambient temperature.
Purge Gas
A gas that is continuously or intermittently added to a system to render the atmosphere noncombustible. The purge gas can be inert or combustible.
Toxic Gas (NFPA 55)
A gas having a health hazard rating of 3 or 4 as defined in NFPA 704.
Toxic Gases
Materials that can cause physiological harm other than asphyxiation and that are immediately dangerous to life and health and can be fatal at relatively low concentrations, such as phosgene or hydrogen sulfide.