7 LNG Storage Tanks
7.1 General Provisions
7.1.1 Selection of Liquefied Natural Gas Storage Tanks Shall Comply with the Following Regulations:
Assess the tank type of liquefied natural gas storage for risk, determine the impact on surrounding environment, personnel, and property safety.
Select the tank types shown in Appendix B of this specification for liquefied natural gas storage tanks.
In densely populated industrial areas or areas with numerous facilities, it is advisable to choose double containment tanks, full containment tanks, or film-lined tanks.
4. A single-container option is available when the required safety spacing is met.
7.1.2 The design of liquefied natural gas storage tanks shall comply with the following regulations:
During and after the OBE period, the tank system should continue to operate.
During and after the SSE, the storage capacity of the tank should remain unchanged and should be capable of isolation and maintenance.
The liquefied natural gas (LNG) storage tanks should undergo seismic calculations under both OBE and SSE conditions, ensuring the tanks can safely shut down under SSE conditions. The prestressed concrete outer shell of the full-volume tanks should be subjected to ultimate bearing capacity calculations under ALE conditions.
7.1.3 The pipeline openings of the liquefied natural gas storage tank should be located at the top of the tank.
7.1.4 The response spectra for OBE, SSE, and ALE should be determined according to the following requirements:
The OBE should represent seismic ground motion with a probability of exceedance of 10% within 50 years (return period of 475 years) and a damping ratio of 5%, and its response spectrum should not be less than the corresponding value for seismic fortification in the region as specified in the current national standard for seismic design of buildings, GB 50011.
SSE should represent seismic motion with a 2% probability of exceedance within 50 years (return period of 2475 years) and a damping ratio of 5%, and its response spectrum should not be less than the value corresponding to the rare earthquakes specified in the current national standard for seismic design of buildings, GB 50011, for the region in question.
The acceleration value of the 3 ALE spectrum should be half of the SSE spectrum acceleration value.
4 When vertical seismic response spectra are not available, the vertical seismic influence coefficient should not be less than 65% of the maximum value of the corresponding horizontal seismic influence coefficient.
7.1.5 The auxiliary structures of the liquefied natural gas storage tank should be designed in accordance with OBE.
7.1.6 During seismic action, the damping ratios for each design component of the liquefied natural gas storage tank should be selected according to the specifications of Table 7.1.6.
Table 7.1.6: Yarn Counts for Various Design Components
7.1.7 The permanent loads and variable loads of the liquefied natural gas storage tank shall comply with the specifications in Table 7.1.7.
Table 7.1.7: Permanent and Variable Loads on Liquefied Natural Gas Storage Tanks
Note: 1 a), b) Refer to Appendix B of this specification. "√" indicates consideration, while “—” indicates non-consideration.
"√* indicates that it only applies to the steel top of the outer tank for low-temperature steel tanks."
Section 7.1.8: The seismic and accidental loads for Liquefied Natural Gas (LNG) storage tanks shall comply with the specifications in Table 7.1.8.
Table 7.1.8 Seismic and Accidental Loads on Liquefied Natural Gas Storage Tanks
Note: a), b) Refer to Appendix B of this specification. "√" indicates consideration, while "—" indicates non-consideration.
7.1.9 The site of the liquefied natural gas receiving station should be evaluated for seismic and geological hazards.
Natural gas is widely recognized as a clean, environmentally friendly, and safe quality energy source. After liquefaction, the volume of natural gas is reduced by approximately 600 times, which greatly benefits storage. Storage of liquefied natural gas (LNG) is done using atmospheric pressure, low-temperature storage 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 resistance
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. This results in storage tanks operating at slightly above atmospheric pressure, which, compared to high-pressure, ambient-temperature storage, 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 will volatilize 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 and the application of sealing concepts. 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罐 wall requires low-temperature resistance, typically made of 9Ni steel or aluminum alloys, while the outer罐 wall is prestressed reinforced concrete.
4
Thermal insulation measures are stringent
The tank must have excellent insulation properties to maintain an internal temperature of -160℃ with an external temperature difference of up to 200℃, requiring high-performance insulation material between the inner and outer tanks. The insulation material at the tank bottom must also possess sufficient pressure-bearing capability.
5
Good seismic performance
General buildings are required to crack but not collapse under specified seismic loads. To ensure the safety of storage tanks under accidental loads, they must have 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 generally avoids seismic fault zones, and seismic tests must be conducted on the storage tank before construction to analyze the structural performance under dynamic conditions, ensuring that the tank does not sustain damage under the given seismic intensity.
6
Strict construction requirements
Tanks must undergo 100% magnetic particle inspection (MT) and 100% vacuum leak tightness testing (VBT). Select insulating materials strictly, and follow the specified procedures during construction. To prevent concrete cracking, post-tensioned prestressed construction is used uniformly, with stringent control over the verticality of the tank walls.
The concrete outer tank roof should possess 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 must be controlled during pouring to prevent cracking caused by thermal stresses.
What are the features of the components of an LNG low-temperature storage tank?
1
Inner can wall
The inner wall of the low-temperature storage tank is the main component, constructed from low-temperature-resistant steel plates with good mechanical properties, typically using A5372, A516 Gr.60, Gr18Ni9, and ASME 304 special steel grades.
The inner bottom plate and annular plate of a can are made of 16mm thick A537 CL2 steel plate, while the rest of the 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, a density of 40-60 kg/m³, and a thickness of approximately 150 mm.
罐顶Insulation
The inner tank top features a suspended rock wool insulation layer. For instance, if a tank's top is equipped with 4 layers of glass fiber insulation, each layer is 100 mm thick, with a density of 16 kg/m3 and a thermal conductivity of 0.04 W/(m·K).
Insulated Tank Bottom
Insulation at the bottom of the drum is quite complex, requiring not only the application of polyurethane foam spray under the steel plate but also the design of a waterproof structure. The following illustration depicts the insulation setup for the bottom of a drum, featuring a 65mm thick cushioning layer, 60mm thick dense concrete, 2mm thick waterproof bitumen felt, two layers of foam glass each 100mm thick, and finally, a 70mm thick concrete cover to protect the exterior drum concrete from 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 due to unexpected gas leakage, thus, the reinforced concrete must possess adequate tensile strength.
For large storage tanks, equal strength but varying thickness or equal thickness but varying strength design methods can be adopted to ensure uniform stress distribution on the prestressed concrete tank walls.
What types of LNG storage tanks are there?
Various shapes
Cylindrical: Used for industrial gasification stations, small-scale LNG production units, satellite liquefaction plants, civil gasification stations, and LNG refueling stations for vehicles.
Large cylindrical: Used for base load, peak-shaving type liquefaction facilities, and LNG receiving stations.
Spheres: Used for civil gasification stations, LNG refueling stations for vehicles.
Different settings
Ground
Semi-subterranean
Underground
Various structural styles
Single包容罐、Double包容罐and Full包容罐。
Varying capacities
5 to 50 m3: Commonly used for civil LNG vehicle refueling stations and civil gas liquefaction stations, etc.
50-100 m3: Commonly used in industrial gas liquefaction stations.
100 to 1,000 m3: Suitable for small-scale LNG production facilities.
10,000 to 40,000 m3: Suitable for basic load and peak-shaving type liquefaction units.
40,000 to 200,000 m³: For LNG receiving stations.
Challenges in storing LNG
Liquid Stratification
LNG is a multi-component mixture, and due to variations in temperature and composition, differences in liquid density can cause stratification within storage tanks. Generally, stratification is considered to occur within the tank when the temperature difference in the vertical direction 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, and during storage, the evaporation rates of the various components differ, leading to changes in the composition and density of LNG. This process is known as aging.
Individual stratified LNG convective circulation, natural convective circulation diagram within LNG storage tanks
Rolling phenomenon
The rolling phenomenon refers to the rapid up and down movement of two layers of LNG with different densities within the storage tank, resulting in a significant amount of vaporization gas being produced instantaneously. At this point, the vaporization amount of LNG in the tank is 10 to 50 times higher than the normal natural 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 the safety valve, it may cause mechanical damage to the storage tank, resulting in economic losses and environmental pollution.
The fundamental cause of rolling phenomenon is the differing densities of liquid layers within the storage tank, which result in stratification (Figure 1). The composition of the liquid significantly impacts the timing and severity of evaporation and rolling.
LNG storage tanks, during long-term storage, spontaneously develop boiling due to the evaporation of lighter components (mainly N2 and CH4). After a period (hours to even days) of filling with new LNG of different densities and temperatures, the original LNG in the tank suddenly starts to roll. For continuous operation receiving stations, the phenomenon of rolling in storage tanks mainly falls under the second category.
The LNG density at the top is lower, while it's higher at the bottom of the storage tank. As the LNG inside the tank stratifies, with the introduction of external heat, the bottom LNG temperature increases, 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, pressure decreases, and becomes supersaturated liquid, rapidly releasing stored energy, producing a large amount of BOG, resulting in the boiling phenomenon.
Note that LNG stratification is the prerequisite for rolling.
Methods for Testing and Eliminating Delamination
Temperature Monitoring
Density Monitoring
Ballast Water Monitoring
Once the storage tank stratifies, pump out the LNG from the bottom of the tank first when exporting.
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 vapor gas and the cost of processing the additional vapor gas (as shown in Figure 4).
During unloading, if the LNG density on the ship is heavier than the density in the storage tank, discharge through the top loading pipe. Otherwise, discharge through the bottom loading pipe to facilitate self-mixing of LNG of different densities in the tank and eliminate stratification.
