CHAPTER 2 UNDERSTANDING UTILITY NATURAL GAS Another unique physical property of cryogenic LNG is that the gas is super
-cold and therefore heavier than air, even though the chemical’s vapor density is actually lighter than air. It will take some time for the gas to reach warmer temperatures, so it may not rise immediately. When the gas warms to critical temperature, it will begin to rise and dissipate. Until that point the natural gas vapors will hug the ground and pose a special hazard to responders, very similar to the hazards of heavier-than-air flammable products.
CHEMICAL PROPERTIES — FLAMMABILITY, HEALTH, AND REACTIVITY
Physical properties affect the way chemicals will move, sink, mix or vaporize. Because they don’t change the ma- terial they are universal to all chemicals. Chemical prop- erties, on the other hand, determine how the chemical will behave (e.g., flammable, toxic, or explosive) in an emergency. Natural gas has several chemical hazards; the
most common is flammability. Its ease of combustion and high heat output are the very reasons it is used in the utility gas industry as a fuel gas.
Flammability
Flammability comprises several chemical properties. Flashpoint, flammable range, and ignition temperature are the most important in emergencies. Flash- point is the minimum temperature at which a liquid is capable of producing vapors sufficient to create a flammable atmosphere. Since natural gas is a flammable gas at normal temperatures flashpoint is not applicable; it is more critical when responders are dealing with flammable liquids. Since natural gas exists as a gas the important properties to consider are flammable range and ignition temperature. Understanding where the flammable range exists and the temperature at which natural gas will ignite is crucial in assessing a natural gas emergency.
Flammable Range STREET SMART TIP
Whatever the concen- tration of the LEL of natural gas — 4%, 4.9% or 5% — the noise will be the same when it ignites—
Whoomph!!!!
The flammable range of natural gas is 4% to 15% in air. Depending on the mixture, some authorities list natural gas’s lower explosive limit (LEL) as 5, 4.5% or 4.9%. For our discussion we will use 4% to 15%.
The flammable range actually means that only mixtures of natural gas with
more than 4% gas in air or less than 15% can form an ignitable mixture. Less than 4% in air is considered "too lean” to burn, and more than 15% is "too rich” to burn. Beware, however: always exercise extreme caution around con- centrations of natural gas in excess of 15%. Being “too rich” does not mini- mize or eliminate the flammable hazard. Air can be introduced by opening a window or door and the concentration will come back down into the flam- mable range, and ignition could still occur.
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