Low-temperature storage tanks are primarily used in the industrial gas sector, with a wide range of applications across various industries such as steel mills, chemicals, oil, food, and aerospace. These industries require gases for their operations. Industrial gases like oxygen, nitrogen, and argon, which are in high demand, are separated from air using air separation equipment. The air is compressed and liquefied, then separated based on the different boiling points of oxygen, nitrogen, and argon, before being stored in low-temperature storage vessels.
The company boasts two sandblasting rooms, with all tanks utilizing the pneumatic steel shot sandblasting rust removal process, ensuring both the appearance and paint quality. (After the overall sandblasting and rust removal, the paint adhesion is enhanced, significantly extending the lifespan of the paint.)
Each unit undergoes hydrostatic testing to ensure equipment strength safety; each unit is tested for airtightness to guarantee sealing performance. After hydrostatic testing, the internal air is blown clean; and plastic plates are used to seal off all pipe openings to prevent secondary contamination from the outside.
Huzhou Boiler Factory Co., Ltd.'s Pressure Vessel Department mainly produces 5-200 cubic meter LPG storage tanks, anhydrous ammonia storage tanks, etc. Our LPG storage tank series is designed, manufactured, and inspected in accordance with the GB150-2011 standard, utilizing Q345R steel plates specific for pressure vessels. The entire tank is rolled using CNC coil winding equipment and is焊接 with automatic gas shielded welding technology, ensuring strong and aesthetically pleasing weld joints. Post-weld heat treatment is conducted in an overall annealing furnace to better eliminate internal stress within the tank, enhancing the material's toughness and hardness, providing superior results compared to resistance wire heat treatment. The LPG storage tanks undergo 100% radiographic testing to ensure that the steel plates and welds meet the technical requirements. High-pressure jet shot peening is used for rust removal, yielding far superior results to manual or abrasive wheel sanding. After this, the tanks are coated with four layers of paint, two rust-inhibiting and two topcoats, ensuring they will not require repainting for many years of use. Each tank is inspected and certified by the Quality and Technical Supervision Bureau before leaving the factory, making them universally acceptable nationwide.
Natural gas is widely recognized as a clean, environmentally friendly, and safe high-quality energy source. Once liquefied, the volume of natural gas is reduced by approximately 600 times, offering significant benefits for storage. Storage of liquefied natural gas (LNG) is done in atmospheric pressure, 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
At atmospheric pressure, the boiling point of liquefied natural gas (LNG) is -160°C. LNG is stored at low temperatures and atmospheric pressure, lowering the gas temperature below its boiling point. This results in an operational pressure of the storage tank slightly above atmospheric, 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 would evaporate in large quantities, with the vaporization rate being approximately 300 times that of its 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 gas, ensuring storage safety.
3
Special Material
The inner tank wall requires low-temperature resistance, typically made of 9Ni steel or aluminum alloys, while the outer tank wall is made of pre-stressed reinforced concrete.
4
Thermal insulation measures are stringent
Due to the maximum temperature difference between inside and outside the can reaching up to 200℃, in order to maintain a temperature of -160℃ inside the can, the vessel must have excellent thermal insulation properties. High-performance thermal insulation materials are filled between the inner and outer cans. The thermal insulation material at the bottom of the can must also have sufficient load-bearing capacity.
5
Excellent seismic performance
General building seismic requirements are to crack but not collapse under specified earthquake 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 the specified earthquake loads.
Therefore, the selected construction site generally avoids seismic fault zones, and seismic tests must be conducted on the storage tanks prior to construction to analyze the structural performance of the tanks under dynamic conditions, ensuring that the tank body does not sustain damage under the given seismic intensity.
6
Strict construction requirements
Tanks must undergo 100% magnetic particle testing (MT) and 100% vacuum leak testing (VBT) on welds. Careful selection of insulation materials is required, and construction should adhere to specified procedures. To prevent cracking in concrete, post-tensioned prestressed construction is used universally, with strict control over the verticality of the tank walls.
The concrete outer tank roof should have high compressive and tensile strength, capable of withstanding impacts from general 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.
What are the characteristics of the components of an LNG low-temperature storage tank?
1
Inner can wall
The inner tank wall is a major component of low-temperature storage tanks, 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's 304 stainless steel are selected.
The inner bottom plate and hoop 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body
The inner side of the outer tank liner is coated with polyurethane foam, typically requiring a thermal conductivity of ≤0.03 W/(m·K) for the polyurethane foam, with a density of 40-60 kg/m³ and a thickness of approximately 150 mm.
罐顶Insulation
The inner tank top is equipped with a suspended rock wool insulation layer. For instance, if a tank's top is fitted 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 Bottom for Jars
Insulating 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 illustration below shows the insulation structure of a drum's bottom, which includes a 65mm thick cushion layer, 60mm thick dense concrete, 2mm thick waterproof bitumen felt, two layers of expanded glass each 100mm thick, and finally, a 70mm thick concrete cover to protect the outer drum concrete from the impact of low temperatures.
3
Concrete exterior shell
The outer tank wall and roof are constructed 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 the internal pressure generated by accidental 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 even stress distribution on the prestressed concrete tank walls.
What types of LNG storage tanks are there?
Various shapes
Cylindrical: Used in industrial gasification stations, small-scale LNG production facilities, satellite liquefaction units, residential gasification stations, and LNG fueling stations for vehicles.
Large cylindrical: Used for base load, peak-shaving type liquefaction facilities, and LNG receiving stations.
Spheres: Used for civil gasification stations and LNG refueling stations for vehicles.
Different settings
Ground
Semi-subterranean
Underground
Different structural styles
Single包容罐, double包容罐, and full包容罐.
Varying capacities
5～50 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.
100-1,000 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 stations.
LNG storage issues
Liquid Stratification
LNG is a multi-component mixture, and due to variations in temperature and composition, differences in liquid density can cause stratification within the storage tank. Generally, stratification is considered to occur within the tank when the vertical temperature difference in the liquid exceeds 0.2°C 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 components vary, 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 inside LNG storage tank
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 instantly. At this point, the vaporization of the LNG inside the tank is 10 to 50 times greater than 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 released promptly 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 phenomena is the differing densities of liquid layers within the storage tank, resulting in stratification (Figure 1). The composition of the liquid significantly influences the timing and severity of evaporation and rolling.
LNG 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 and then topped up with new LNG of different densities and temperatures, rolling can suddenly occur after a period (hours to even days). For continuously operated receiving stations, the occurrence of rolling in tanks primarily falls under the second category.
The LNG density at the top is lower, while it's higher at the bottom of the tank. Once the LNG inside the tank stratifies, with the introduction of external heat, the bottom LNG's temperature rises, decreasing in density. The top LNG becomes heavier due to the vaporization of BOG. Through mass transfer, the lower LNG rises to the top, pressure decreases, and it becomes supersaturated liquid, releasing accumulated energy rapidly, producing a large amount of BOG, resulting in the rolling phenomenon.
Note that LNG stratification is a prerequisite for rolling.
Methods for Detection and Elimination of Delamination
Temperature Monitoring
Density Monitoring
B.O.G. Monitoring
Once the tank has stratified, pump out the LNG from the bottom of the tank first during the export process.
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 handling the increased vapor (as shown in Figure 4).
During unloading, if the LNG density on the ship is heavier than that in the storage tank, load from the top unloading pipe. Conversely, load from the bottom unloading pipe. This promotes natural mixing of LNG with different densities within the storage tank, eliminating stratification.
