LNG storage tank - Wikipedia

A liquefied natural gas storage tank or LNG storage tank is a specialized type of storage tank used for the storage of Liquefied Natural Gas. LNG storage tanks can be found in ground, above ground or in LNG carriers. The common characteristic of LNG Storage tanks is the ability to store LNG at the very low temperature of -162 °C (-260 °F). LNG storage tanks have double containers, where the inner contains LNG and the outer container contains insulation materials. The most common tank type is the full containment tank.[1] Tanks vary greatly in size, depending on usage.

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In-ground LNG tanks are also used; these are lined or unlined tanks beneath ground level.[2] The low temperature of the LNG freezes the soil and provides effective containment. The tank is sealed with an aluminium alloy roof at ground level. Historically there have been problems with some unlined tanks with the escape of LNG into fissures, the gradual expansion of extent of the frozen ground, and ice heave which have limited the operational capability of in-ground tanks. All piping connected to the LNG tanks, whether above ground or in-ground, are routed through the top of the vessel.[2] This mitigates against loss of containment in the event off a piping breach.

In LNG storage the pressure and temperature within the tank will continue to rise. LNG is a cryogen, and is kept in its liquid state at very low temperatures. The temperature within the tank will remain constant if the pressure is kept constant by allowing the boil off gas to escape from the tank. This is known as auto-refrigeration.[3]

The world's largest above-ground tank (delivered in ) is the 180 million liters full containment type for Osaka Gas Co., Ltd. The world's largest tank (delivered in ) is the 200 million liters Membrane type for Toho Gas Co., Ltd.[4]

See also

• Liquefied natural gas terminal

What is LNG?

LNG is an abbreviation for Liquefied Natural Gas. When natural gas is cooled at a liquefaction facility to approximately -260°F (-161.5°C) at atmospheric pressure, it condenses into a liquid.  One volume of liquid occupies approximately 1/600 the volume of natural gas. Careful purification and dehydration are necessary to remove impurities that would become solid at liquefaction temperatures.

LNG is composed almost exclusively of methane. It weighs about 45% that of water, so it floats on water. LNG is odorless, colorless, non-corrosive and non-toxic. LNG will NOT burn as a liquid. 

When LNG vaporizes, it burns in concentrations of approximately 5% to 15% gas in air. While LNG vapor is initially heavier than air, after LNG vapors (methane) become warmer than -160°F (-106.7°C), they become lighter than air and will rise and disperse rather than collect near the ground. LNG provides for efficient storage and transportation of natural gas, and is vaporized for use by customers.

LNG Storage Tanks

LNG is generally stored in large-volume above-ground low-pressure (less than 5 pounds per square inch gauge) tanks. The tank is double-walled, with a carbon steel outer wall and a nickel-steel inner tank. Nickel steel inner tanks are used because ordinary steel is too brittle to use in direct contact with the cryogenic LNG. The tanks are separated by a large blanket of insulation to help maintain the cryogenic temperatures.

Smaller LNG tanks, generally less than 70,000 gallons, store LNG in higher pressure (up to 250 pounds per square inch gauge) vacuum-jacketed double wall tanks with an inner wall of aluminum.

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Additional containment is afforded by large dike or by adding a third, outer concrete tank wall.

Although there is efficient insulation, heat still enters the tank. This causes a small amount of LNG top “boil off.” This keeps the LNG cold through a process called “autorefrigeration.” Small amounts of LNG are continually boiling and being removed from the tank. The cold vapor is heated and sent out to serve gas customers.

LNG Trucks

LNG trailers, like LNG storage tanks, are double-walled trailers with vacuum and insulation between the outer (carbon steel) and inner (aluminum) tank in order to maintain the low temperature. The trailers are approximately 42 or 48 feet long (look similar to other fuel trailers) and contain 11,000 to 13,000 gallons of LNG.

In addition to being used to transport LNG, LNG trailers can be connected to mobile vaporizers to provide temporary supply when needed for operation and maintenance on pipeline facilities or other contingencies. They have been used numerous times throughout Connecticut during planned construction and maintenance activities.

Temporary LNG Service in Winsted, CT

Types of Facilities

There are a variety of types of LNG facilities.

1. LNG export terminals (also known as liquefaction terminals) – receive natural gas from gas fields, liquefy (refrigerate) gas to LNG, store in LNG storage tanks, load LNG onto LNG tankers.
2. LNG receiving or import terminals (also known as vaporization of regasification plants) – receive LNG from LNG tankers, store it, vaporize it to natural gas and send it out by pipeline, or truck it out as a liquid.
3. LNG Peak Shaving Facilities – used to “shave the peak” off the coldest day demand by providing a supplemental supply of gas. Receive natural gas from the pipeline, liquefy it to LNG (or receive imported LNG), store LNG during warm weather, convert it to natural gas for customer use in cold weather.
4. LNG Base-Load Facilities – used for general year-round supply
5. LNG Satellites – small tanks which receive LNG by truck, store it during warm weather,   vaporize it to natural gas during peak demand
6. LNG temporary facilities - used to provide supply during planned construction and maintenance activities or in cases of supply emergencies.

Connecticut LNG Tanks – All Peak Shaving Facilities

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1. Connecticut Natural Gas (CNG) (left) and Southern Connecticut Gas Company (SCG) (right). Each plant has a large tank – inner tank made of 9% nickel steel tank - 96 feet 11½ inches high, outer tank made of carbon steel - 101 feet 9 inches high and 173 feet 4 inches in diameter.   Each tank holds 348,000 barrels of liquid (14.6 million gallons), the equivalent 1.2 billion cubic feet of gas.  Each tank is surrounded by a dike to retain any LNG in case of failure. The facilities can make LNG from natural gas, store LNG, and vaporize it for later use.  Depending on pricing, imported LNG may be purchased and trucked to the site.
   
   
2. Yankee Gas d/b/a Eversource Energy.  It has a large tank (left) – inner tank made of 9% nickel steel tank – 146 feet in diameter and 117 feet high, outer tank made of carbon steel lined concrete - 150 feet high and 158 feet in diameter. The tank holds 348,000 barrels (14.6 million gallons) of liquid, the equivalent 1.2 billion cubic feet of gas.  Because of the concrete outer tank (surrounding the double walled tank) there is no requirement for a large dike. The facility can make LNG from natural gas, store LNG, and vaporize it for later use.  Depending on pricing, imported LNG may be purchased and trucked to the site. The initial fill was largely accomplished by trucking in liquid (right). 
   
     
3. City of Norwich Department of Public Utilities has a small (55,000 gallons) LNG satellite with vaporizers for converting the liquid back into gas.
4. Safety jurisdiction for all intrastate LNG facilities resides with the PURA, administered through the Gas Pipeline Safety Unit.