112


CHAPTER 5 Gas Pipeline Operations


4H MEDIC ANNA


H Hydrogen (VD = 0.069) H Helium (VD = 0.14) H Hydrogen cyanide (VD = 1.0) H Hydrogen fluoride (VD = 0.34)


M Methane (VD = 0.6) E Ethylene (VD = 0.97) D Diborane (VD = 0.96) I


Illuminating gas (10% Ethane and 90% Methane mixture, VD = 0.6) C Carbon monoxide (VD = 0.97)


A Anhydrous ammonia (VD = 0.588) N Neon (VD = 0.7) N Nitrogen (VD = 0.96) A Acetylene (VD = 0.90)


Figure 5.2 An easy way to remember the common hazardous gases and simple asphyxi- ants that are lighter than air is the acronym 4H MEDIC ANNA.


Natural gas is the most common gas that emergency responders will en-


counter that is lighter than air. For more information on other lighter than air gases, see Figure 5.2. Boiling Point—Boiling point is the temperature at which a liquid changes


its phase to a vapor or gas, or, in other words, the temperature at which the vapor pressure of the liquid equals atmospheric pressure. The lower the boil- ing point, the more vapors that are produced at a given temperature.


Natural gas has an extremely low boiling point of –259°F (–161°C). In com-


parison, propane has a boiling point of –44°F (–42°C). Natural gas may be liq- uefied into a cryogenic liquid known as liquid natural gas (LNG) by reducing its temperature and pressure. The purpose of changing natural gas from a gas to a liquid is entirely to facilitate its transportation and storage. In some U.S. locations, LNG is shipped via marine tanker into a terminal and then stored in its liquid state. It may then be either shipped as LNG in a cryogenic cargo tank truck (MC-338) or vaporized and shipped from the terminal facility as natural gas. LNG has a liquid to vapor expansion ration of 1:420. Responders can expect to see increasing usage of LNG as an energy source within the next decade. Vapor Pressure—The vapor pressure of natural gas at 60°F (15.5°C) is ap-


proximately 120 psi. In comparison, butane has a vapor pressure of 11.5 and propane a vapor pressure of 102 psi at 60°F (15.5°C). Natural gas can be under extreme pressures when transported via transmission and distribution pipeline systems. Solubility—Natural gas is slightly soluble in water. Water spray can be used to move or redirect natural gas vapors due to the strong air currents cre-


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