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1. Low temperature resistance
The boiling point of liquefied natural gas at normal pressure is -160°C. LNG chooses low-temperature and normal-pressure storage method, which reduces the temperature of natural gas below the boiling point, so that the operating pressure of the liquid storage tank is slightly higher than normal pressure. Compared with the high-pressure normal-temperature storage method, it can greatly reduce the thickness of the tank wall and improve safety performance.
Therefore, LNG requires liquid storage tanks to have good low temperature resistance and excellent cold retention performance.
2. High safety requirements
As the tank stores low-temperature liquids, once an accident occurs in the storage tank, the refrigerated liquid will evaporate in a large amount, and the amount of vaporization is about 300 times that of the original refrigeration state, forming an air mass that will automatically detonate in the atmosphere.
Therefore, API, BS and other specifications all require storage tanks to adopt a double-wall structure. Using the sealing concept, when the first layer of tank leaks, the second layer of tank can completely seal the leaking liquid and vapor to ensure storage. Safety.
3. Special materials
The inner tank wall requires low temperature resistance, and 9Ni steel or aluminum alloy is generally used, and the outer tank wall is prestressed reinforced concrete.
4. Strict insulation measures
Since the temperature difference between the inside and outside of the tank can be as high as 200&#;, to keep the temperature inside the tank at -160&#;, the tank body must have good cold insulation performance, and high-performance cold insulation materials are filled between the inner tank and the outer tank. The cold insulation material at the bottom of the tank must have sufficient pressure-bearing performance.
5. Good seismic performance
Generally, the seismic requirements of buildings are to crack without falling under the specified earthquake load. To ensure the safety of storage tanks under accidental loads, storage tanks must have good seismic performance. For LNG storage tanks, it is required to not collapse or crack under the specified seismic load. For this reason, the selected construction site should generally avoid the earthquake fracture zone. Before construction, the storage tank should be subjected to seismic tests to analyze the structural performance of the storage tank under dynamic conditions to ensure that the tank body is not damaged under a given seismic intensity.
6. Strict construction requirements
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Welds of storage tanks must be subjected to 100% magnetic particle testing (MT) and 100% vacuum tightness testing (VBT). It is necessary to strictly select cold insulation materials and follow the prescribed procedures during construction. In order to prevent the concrete from cracking, post-tensioned pre-stress construction is adopted, and the verticality of the tank wall is very strict.
The top of the concrete outer tank should have high compressive and tensile capacity, and be able to resist the impact of falling objects. Due to the thick concrete at the bottom of the tank, the hydration temperature must be controlled during pouring to prevent cracking due to temperature stress.
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:
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:
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 satellite peaking storage systemLNG 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
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