The advantage of LNG is that the significant reduction in volume when natural gas is liquefied makes it practical to store. LNG is stored in cryogenic tanks designed to keep the LNG below the vaporization temperature. The job of the tank is to contain the gas and to insulate it from warming due to heat from the surrounding air. Storage is most common in four points in the gas delivery system:
- For LNG transported by tanker, LNG is stored in the production region in large tanks at the liquefaction facility until it is pumped into a tanker.
- For LNG transported by tanker, LNG is stored in the delivery region in large tanks at the regasification terminal until it is regasified and injected into the gas delivery system.
- For local LNG peaking storage, often called peak shaving facilities, LNG is stored in tanks connected to gas transmission or distribution facilities on a pipeline or utility distribution system.
- For use as a fuel for trucking, locomotives, or shipping, LNG is stored in tanks at a fuel facility.
Storage at import/export terminals
LNG storage tanks at a liquefaction facility
Source: Freeport LNG The Isle E-Magazine
Several types of LNG storage tanks are used at liquefaction and regasification terminals. The most common are above ground tanks that include:
- Single containment tanks that are doubled walled (9% nickel inner tank, carbon steel outer tank).
- Double containment tanks with primary and secondary tanks (9% nickel inner tank, carbon steel outer tank with a carbon steel roof, surrounded by free standing pre-stressed concrete wall).
- Full containment tanks with primary and secondary tanks that are hardened to withstand impacts from missiles or flying objects (9% nickel inner tank, prestressed concrete outer tank with carbon steel liner, concrete roof).
The differences between these types of tank reflect the level of protection against spillage and projectiles built into the design. In all designs the inner tank is made of steel consisting of 9% nickel. A single containment tank has only a carbon steel outer tank surrounding the inner tank. The double containment tank adds the protection of a steel roof and a concrete wall. If a leak were to occur, the concrete wall would prevent the LNG from spreading and thus would limit the vaporization and size of an LNG pool. The full containment tanks add the protection of a concrete roof and full concrete outer tank designed to withstand impacts from missiles or flying objects. Most new storage tanks are constructed with full containment for safety and security.
Tanks can also be built below ground or partially below ground. Buried tanks are usually built using a design called a membrane tank. This type of tank has a pre-stressed concrete outer tank, a lining of polyurethane foam insulation, and a thin steel inner tank called a membrane layer. Partially below-ground tanks are similar in design to full containment tanks. While below-ground or partially below-ground tanks are considered safer than above-ground tanks, they are also more expensive to construct so their use is less common than above-ground tanks.
Since the inner tank has direct contact with the LNG, the specialized steel used must be able to withstand extreme temperatures. To accomplish this, the nickel steel inner tank is insulated with a non-flammable type of insulation such as perlite. A vacuum system is also employed so that any vapors that might escape from an inner tank weld or crack are recovered for safe handling. All penetrations through the tank for vapor management and for injecting/removing the LNG are through the roof thus reducing the risk of leaks. Fire and vapor detection systems are used throughout the storage facility so that any leaks or fires can be quickly detected and automatic fire-suppression response can be triggered promptly. Tanks are placed in a walled and bermed impoundment system that will collect and drain spilled LNG in the event of a tank failure. Normally tank impoundments are designed to handle a volume of at least 110% of the tank volume.
Storage at gas peaking facilities
LNG peaker storage tanks with liquefaction equipment on the right and vaporization equipment on the left. (Source: manufacturer website)
Peak shaving and fuel facilities consist of smaller tanks that hold limited amounts of LNG. The cryogenic tanks are typically double-walled nickel alloy steel that are vacuum insulated to limit warming from the surrounding air. They also include safety equipment such as pressure relief valves, and safety shut off valves. The tanks may be vertical or horizontal. Peak shaving and fuel facilities may or may not include on-site liquefaction equipment. If liquefaction is included then the facility takes natural gas directly from gas pipelines, liquefies it, and then stores in tanks. Liquefaction is commonly performed by use of liquid nitrogen that cools the feed gas below the liquefaction point. Storage facilities without liquefaction, also called satellite facilities, rely on LNG tanker trucks to refuel the storage tanks. Vaporization occurs using air heat exchangers that warm the LNG until it returns to gas using the temperature of ambient air. The facility also includes controls to ensure the gas is stored and vaporized in a safe manner.
Map of LNG storage facilities in the U.S.
Source: U.S. Department of Transportation, Pipeline and Hazardous Materials Safety Administration