CHAPTER 2 UNDERSTANDING UTILITY NATURAL GAS Vapor Density
Vapor density is the weight of a given volume of vapor or gas compared to the weight of an equal volume of dry air at the same temperature and pressure. Air has a Vapor Density (VD) of 1. Natural gas and its principal component, methane, are lighter than air, with a vapor density of 0.55, and will rise when re- leased. But propane and butane are heavier than air and any flammable liquid will have vapor density heavier than air.
VAPOR dENsity
NAtURAL GAs .55
PROPANE 1.5
FIGURE 2.6 natural gas is lighter than air. Propane is heavier than air. air = 1.
One exception to this is when the vapors are from cryogenic or liquefied
natural gas (LNG). The supercold gas will hug the ground until the gas reaches normal or ambient temperatures. Then it will rise.
Specific Gravity
Although specific gravity is not a critical issue in normal natural gas response, LNG does have a specific gravity, which is the weight of a substance com- pared to the weight of an equal amount of water which is 1. Substances with a specific gravity less than 1 float and greater than 1 sink in water. The spe- cific gravity of liquefied natural gas (LNG) is .42, or lighter than water.
Water Solubility
Natural gas is not soluble in water. However, water spray can be used to move or redirect a vapor cloud due to the strong air currents created by the fog streams. The air currents move the hazard but the water will not absorb or reduce the hazard.
Vapor Pressure
Another physical property of natural gas is its vapor pressure, defined as the pressure exerted within a container by the vapor that is in equilibrium with the liquid at a given temperature. It is measured by many scales or methods: millimeters of mercury (mm Hg) usually used for liquids, pounds per square inch (psi), or atmospheres (ATM). For example, atmospheric pressure at sea level is 1 atmosphere, 760 mm Hg or 14.7 psi.
The vapor pressure of natural gas at 60ºF is about 8 atmospheres or 120
psi. This allows it to be transported under extreme pressure in pipelines and distribution systems without liquefying. Transportation and distribution sys-
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