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Learn about Liquefied Natural Gas. How is it stored? How is it kept cold?
When natural gas is cooled to a temperature of approximately -260°F [-160°C] at atmospheric pressure it condenses to a liquid called liquefied natural gas (LNG). One volume of this liquid takes up about 1/600th the volume of natural gas at a stove burner tip. LNG weighs less than one-half that of water, actually about 45% as much. LNG is odorless, colorless, non-corrosive, and non-toxic. When vaporized it burns only in concentrations of 5% to 15% when mixed with air. Neither LNG, nor its vapor, can explode in an unconfined open environment.
Natural gas is composed primarily of methane (typically, at least 90%), but may also contain ethane, propane and heavier hydrocarbons. Small quantities of nitrogen, oxygen, carbon dioxide, sulfur compounds, and water may also be found in “pipeline” natural gas. The liquefaction process removes the oxygen, carbon dioxide, sulfur compounds, and water. The process can also be designed to purify the LNG to almost 100% methane.
How is it stored?
LNG tanks are always of double-wall construction with extremely efficient insulation between the walls. Large tanks are low aspect ratio (height to width) and cylindrical in design with a domed roof. Storage pressures in these tanks are very low, less than 5psig [0.3 barg]. Smaller quantities, 70,000 gallons [265 cubic meters] and less, are stored in horizontal or vertical, vacuum-jacketed, pressure vessels. These tanks may be at pressures anywhere from less than 5 psig [0.3 barg] to over 250 psig [16 barg]. LNG must be maintained cold (at least below -177°F [-83°C]) to remain a liquid, independent of pressure.
How is it kept cold?
The insulation, as efficient as it is, will not keep the temperature of LNG cold by itself. LNG is stored as a “boiling cryogen,” that is, it is a very cold liquid at its boiling point for the pressure it is being stored. Stored LNG is analogous to boiling water, only 470°F [243°C] colder. The temperature of boiling water (212°F [100°C]) does not change, even with increased heat, as it is cooled by evaporation (steam generation). In much the same way, LNG will stay at near constant temperature if kept at constant pressure. This phenomenon is called “autorefrigeration”. As long as the steam (LNG vapor boil off) is allowed to leave the tea kettle (tank), the temperature will remain constant.
If the vapor is not drawn off, then the pressure and temperature inside the vessel will rise. However, even at 100 psig [6.7 barg], the LNG temperature will still be only about -200°F [-129°C].
Have there been any serious LNG accidents?
First, one must remember that LNG is a form of energy and must be respected as such. Today LNG is transported and stored as safely as any other liquid fuel. Before the storage of cryogenic liquids was fully understood, however, there was a serious incident involving LNG in Cleveland, Ohio in 1944. This incident virtually stopped all development of the LNG industry for 20 years. The race to the Moon led to a much better understanding of cryogenics and cryogenic storage with the expanded use of liquid hydrogen (-423°F [-253°C]) and liquid oxygen (-296°F [-183°C]). LNG technology grew from NASA’s advancement.
In addition to Cleveland, there have two other U.S. incidents sometimes attributed to LNG. A construction accident on Staten Island in 1973 has been cited by some parties as an “LNG accident” because the construction crew was working inside an (empty, warm) LNG tank. In another case, the failure of an electrical seal on an LNG pump in 1979 permitted natural gas (not LNG) to enter an enclosed building. A spark of indeterminate origin caused the building to explode. As a result of this incident, the electrical code has been revised for the design of electrical seals used with all flammable fluids under pressure.