How much is a Babjehec horizontal 30 cubic meter LNG storage tank?
30, 60, 100 cubic meter LNG storage tanks, LNG gasification station, and vehicle refueling station process and equipment selection
I. LNG, CNG, LPG – meaning
CNG: Compressed Natural Gas (CNG) refers to natural gas compressed to 20MPa and stored in high-pressure cylinders. Vehicles that use this as fuel are known as CNG cars. High-pressure cylinders are stored at room temperature: 20MPa.
LNG: Liquefied Natural Gas (LNG) is formed by cooling natural gas to -162℃ through cryogenic deep cooling. It is stored in low-temperature storage tanks, and vehicles powered by this fuel are known as LNG cars. Cylinders for low-pressure and low-temperature storage: 1.6 MPa.
LNG and CNG refueling, storage, and supply systems have certain differences, but natural gas ultimately enters the car engine in a gaseous state after being pressurized through a pressure regulator to 0.15 MPa for combustion and work.
LPG: Liquefied Petroleum Gas
Section 2: Applications of LNG Storage Tanks: Gasification Stations and Vehicle Fueling Stations
LNG liquefaction plant
(1) Peak load and load leveling for urban pipeline gas supply and accident adjustment
(2) Gas source for city municipal pipeline supply in areas without natural gas pipeline.
(3) Gas sources for community supply: boiler gas and domestic gas supply.
(4) Source for corporate gas supply; (Image)
2. CNG Fueling Station: Used for automotive fuel
III. Types of CNG Filling Stations:
LNG Fueling Station
1) Standard Station: Located at a fixed site, LNG is unloaded using unloading equipment, filled into storage tanks, and refueled for vehicles using fueling machines. (Image)
2) Mobile Filling Station: The relevant equipment and facilities of a CNG station are installed on a vehicle or skid, highly integrated for easy transportation and relocation, suitable for smaller-scale CNG stations. (Image)
CNG Vehicle Fueling Station
L-CNG Vehicle Gas Filling Station
Natural gas is widely recognized as a clean, environmentally friendly, and safe high-quality energy source. Once liquefied, its volume is reduced by approximately 600 times, which greatly benefits storage. Storage of liquefied natural gas (LNG) is done using atmospheric pressure and low-temperature tanks. Let's discuss the unique features of these LNG storage tanks.
What are the special requirements for LNG low-temperature storage tanks?
1
Low-temperature resistant
The boiling point of liquefied natural gas (LNG) at atmospheric pressure is -160°C. LNG is stored at low temperatures and atmospheric pressure, lowering the gas temperature below its boiling point, causing the operating pressure of the storage tank to be slightly above atmospheric. Compared to high-pressure, room-temperature storage, this method significantly reduces the tank wall thickness and enhances safety performance.
Therefore, LNG requires storage tanks with excellent low-temperature resistance and superior insulation properties.
2
High safety requirements
Due to the storage of low-temperature liquids inside the tank, in the event of an accident, the refrigerated liquid would evaporate in large quantities, with the vaporization amount being approximately 300 times that of the original refrigerated state, forming explosive gas clouds in the atmosphere.
Therefore, standards such as API and BS require double-walled tank structures, employing the containment concept. In the event of a leak in the first layer, the second layer can completely seal off the leaked liquid and evaporated gases, ensuring storage safety.
3
Special Material
The inner shell requires low-temperature resistance, commonly using materials like 9Ni steel or aluminum alloys, while the outer shell is prestressed reinforced concrete.
4
Insulation measures are stringent
Due to the temperature difference between inside and outside the tank reaching up to 200°C, the tank must have excellent thermal insulation properties to maintain a temperature of -160°C inside. High-performance thermal insulation materials must be filled between the inner and outer shells of the tank. The thermal insulation material at the bottom of the tank must also possess sufficient pressure-bearing capabilities.
5
Excellent seismic performance
General building seismic requirements are to crack but not collapse under specified seismic loads. To ensure the safety of storage tanks under unexpected loads, they must possess excellent seismic performance. For LNG storage tanks, it is required that they neither collapse nor crack under specified seismic loads.
Therefore, the selected construction site typically avoids seismic fault zones, and anti-seismic tests must be conducted on the storage tanks prior to construction. This analysis evaluates the structural performance of the tanks under dynamic conditions, ensuring that the tank body remains undamaged under the given seismic intensity.
6
Stringent construction requirements
Tanks must undergo 100% magnetic particle testing (MT) and 100% vacuum leak testing (VBT) for welds. Strict selection of insulation materials is required, and construction should follow established procedures. Post-tensioned prestressed construction is used to prevent concrete cracking, with strict control over the verticality of the tank walls.
The concrete exterior shell should have high compressive and tensile strength, capable of withstanding impacts from typical falling objects. Due to the thicker concrete at the bottom of the tank, hydration temperature should be controlled during pouring to prevent cracking caused by temperature stress.
Components characteristics of LNG low-temperature storage tanks?
1
Inner罐wall
The inner tank wall is a major component of the low-temperature storage tank, constructed from steel plates that are resistant to low temperatures and possess good mechanical properties. Typically, grades such as A5372, A516 Gr.60, Gr18Ni9, and ASME 304 are selected for special steel.
The inner bottom plate and annular plate of a certain can are made of 16mm thick A537 CL2 steel plate, while the remaining plates can be made of 6.35mm thick A537 CL1 steel plate.
2
Insulation layer
Insulated罐壁
The inner side of the outer shell is coated with polyurethane foam, typically requiring a thermal conductivity of ≤0.03 W/(m·K) for the foam, with a density of 40-60 kg/m³ and a thickness of approximately 150 mm.
罐顶Insulation
The inner tank lid is equipped with a suspended rock wool insulation layer. For instance, if a tank's lid is set with 4 layers of glass fiber insulation, each layer is 100 mm thick. The density of the glass fiber wool is 16 kg/m³, and its thermal conductivity is 0.04 W/(m·K).
Insulated bottom for cans
Insulation at the bottom of the tank is quite complex. In addition to spraying polyurethane foam under the steel plate, a waterproof structure must also be designed. The following illustration shows the insulation structure of a specific tank bottom, including a 65 mm thick layer, 60 mm thick dense concrete, 2 mm thick waterproof felt, two layers of expanded glass each 100 mm thick, and finally covered with 70 mm thick concrete to protect the external tank concrete from the impact of low temperatures.
3
Concrete exterior shell
The outer tank wall and roof are composed of prestressed reinforced concrete and low-temperature-resistant steel lining plates. The concrete strength should be ≥25 MPa. The outer tank roof and walls must withstand internal pressure from accidental gas leaks, thus the reinforced concrete must possess sufficient tensile strength.
For large storage tanks, to ensure even stress distribution on the prestressed concrete tank walls, designs with equal strength but varying thickness or equal thickness but varying strength can be adopted.
What types of LNG storage tanks are there?
Various shapes
Cylindrical: Used for industrial gasification stations, small-scale LNG production facilities, satellite liquefaction units, residential gasification stations, and LNG refueling stations for vehicles.
Large cylindrical: Used for base load, peak-shaving liquefaction units, and LNG receiving stations.
Spherical: Used for civil gasification stations and LNG refueling stations for vehicles.
Different configurations
Ground
Semi-basement
Underground
Different structural styles
Single包容罐、Double包容罐 and Full包容罐.
Various capacities
5～50 m3: Commonly used for civil LNG vehicle refueling stations and civil gas liquefaction stations, etc.
50 to 100 m³: Often used in industrial gas liquefaction stations.
100 to 1,000 m3: Suitable for small-scale LNG production facilities.
10,000 to 40,000 m3: Used for base load and peak-shaving type liquefaction units.
40,000 to 200,000 m3: for LNG receiving station.
LNG storage issues
Liquid Stratification
LNG is a multi-component mixture. Due to variations in temperature and composition, differences in liquid density can cause stratification within the storage tank. Generally, it is considered that stratification has occurred within the tank when the vertical temperature difference of the liquid is greater than 0.2 degrees Celsius and the density is greater than 0.5 kg/m³.
aging phenomenon
LNG is a multi-component mixture; during storage, the evaporation rates of the various components differ, leading to changes in the composition and density of LNG. This process is referred to as aging.
Individual stratified LNG convective circulation, natural convection circulation diagram within LNG storage tanks
Rolling phenomenon
The rolling phenomenon refers to the rapid up and down movement and mixing of two layers of LNG with different densities within the storage tank, which results in the sudden generation of a large amount of vaporized gas. At this point, the vaporization of LNG inside the tank is 10 to 50 times the normal evaporation rate, causing the tank's pressure to rapidly rise and exceed the set safety pressure, leading to overpressure in the tank. If not promptly released through a safety valve, it may cause mechanical damage to the storage tank, resulting in economic losses and environmental pollution.
The fundamental cause of rollover phenomena is the differing densities of liquid layers within the tank, which result in stratification (Figure 1). The composition of the liquid plays a significant role in the timing and severity of evaporation and rollover.
LNG storage tanks can spontaneously develop rolling due to the evaporation of lighter components (mainly N2 and CH4) during long-term storage. When the tanks are initially filled with LNG, after a period of time (hours to even days) with new LNG of different densities and temperatures being added, a sudden rolling phenomenon may occur. For continuously operated receiving stations, the rolling phenomenon in storage tanks is primarily of the second type.
The upper part of the LNG tank has lower density, while the bottom part has higher density. Once the LNG inside the tank stratifies, with the introduction of external heat, the bottom LNG's temperature rises and its density decreases. The top LNG becomes heavier due to the vaporization of BOG. Through mass transfer, the lower LNG rises to the top, the pressure decreases, forming supersaturated liquid, and the accumulated energy is rapidly released, producing a large amount of BOG, resulting in the churning phenomenon.
Note that LNG stratification is a prerequisite for rolling.
Methods for Detection and Elimination of Delamination
Temperature Monitoring
Density Monitoring
BOG Monitoring
Once the tank stratifies, pump out the LNG from the bottom of the tank first during the export.
After LNG stratification, the top-loading device should be used for cyclic operations to promote mixing of LNG within the storage tank and prevent rolling. However, this also increases the amount of vaporized gas and the cost of processing this additional vapor (as shown in Figure 4).
During unloading, if the LNG density on the ship is heavier than that in the storage tank, load through the top unloading pipe. Otherwise, load through the bottom unloading pipe, which promotes self-mixing of LNG of different densities in the tank and eliminates stratification.
