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FIGURE 2.1 natural gas is a flammable compound that exists in a controlled useful state and on occa- sion a dangerous uncontrolled state that is usually referred to as an emergency.


Natural gas is a naturally occurring hydrocarbon gas formed as a by-product of fossil decay. It is found in underground deposits of porous rock like sand, sandstone, and limestone deposits or mixed in crude oil. It is not an elemental gas, like hydrogen, but a mixture of many hydrocarbon gases whose major component is methane (CH4). In natural gas mined from the earth the methane concentration can range from 50% to 97%. Consumer grade natural gas usually has about 93% methane . Other major component gases of natural gas are ethane, butane and smaller percentages of other gases such as carbon dioxide (CO2), nitrogen, isobutane, butane and isopentane.

Odorizing Natural Gas

Natural gas by its nature is invisible, odorless, and tasteless, making it hard to detect. As a safety precaution it is odorized to aid in early detection. Based on population density transmission lines may or may not be odorized.

Although there are several sulfur-based odorants, the most common is a

liquid, tertiary butyl mercaptan. It has a very identifiable odor, often de- scribed as “cabbage-like” or “rotten eggs” due to its high sulfur content. (Sometimes a related compound thiophane is used that also has a rotten-egg smell.) Tertiary butyl mercaptan is added to the gas distribution system at the

gate stations in very minute trace amounts. It is injected by an operator using a metered device that adds the specific amount in the ratio of technically 0.002 grains/100 cu ft. This usually makes a recognizable warning odor at


FIGURE 2.2 tertiary butyl mercaptan is a common natural gas odorant.

Courtesy of Bill Hand

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