Low-temperature storage tanks are primarily used in the industrial gas sector, with a wide range of applications across various industries. These include steel mills, chemical plants, oil refineries, food processing, and aerospace, among others, all of which require gases. Industrially, gases such as oxygen, nitrogen, and argon, which are in high demand, are separated from air by air separation equipment. This involves compressing and liquefying the air, then separating the oxygen, nitrogen, and argon based on their different boiling points, and storing them in low-temperature storage tanks.
The company boasts two shot blasting rooms, with all tanks utilizing the pneumatic steel shot blasting process for rust removal, ensuring both exterior quality and paint quality. (After the overall shot blasting, the paint's 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 equipment 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.
Heze Boiler Factory Co., Ltd.'s Pressure Vessel Division specializes in manufacturing 5-200 cubic meter LPG storage tanks, liquid ammonia tanks, etc. Our LPG storage tank series is designed, manufactured, and inspected in accordance with the GB150-2011 standard, utilizing Q345R steel plates for pressure vessels. The entire tank is rolled using CNC coiling equipment and焊接 with automatic gas shielded welding technology, ensuring strong and aesthetically pleasing welds. After undergoing heat treatment in an overall annealing furnace, the internal stresses of the tank are better eliminated, enhancing the toughness and hardness of the material, offering superior results compared to resistance wire heat treatment. The LPG storage tanks undergo 100% radiographic inspection to ensure that the steel plates and welds meet the technical requirements. High-pressure jet peening for rust removal is more effective than manual or abrasive wheel polishing. Then, coated with four layers of paint—two anti-rust and two topcoats—the tanks are guaranteed to not require repainting for years of use. Each tank is inspected and certified by the Quality and Technical Supervision Bureau before shipment, ensuring nationwide applicability.
Natural gas is widely recognized as a clean, environmentally friendly, and safe high-quality energy source. Once liquefied, its volume is reduced by about 600 times, which greatly benefits 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
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 an operating 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 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 the use of double-walled tank structures and the application of containment 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 tank wall must be capable of withstanding low temperatures, typically made of 9Ni steel or aluminum alloys, while the outer tank wall is made of pre-stressed reinforced concrete.
4
Strict thermal insulation measures
The tank must have excellent insulation properties to maintain an internal temperature of -160°C, given the maximum temperature difference between inside and outside the tank can reach up to 200°C. High-performance insulation material must be filled between the inner and outer tanks. The insulation material at the tank bottom must also have sufficient pressure-bearing capacity.
5
Excellent seismic performance
The seismic requirements for general buildings 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抗震试验 must be conducted on the storage tanks before construction to analyze their structural performance under dynamic conditions, ensuring the tank body remains undamaged under the given seismic intensity.
6
Strict construction requirements
Tanks must undergo 100% magnetic particle testing (MT) and 100% vacuum leak testing (VBT) for welds. Select insulating materials strictly and follow the prescribed procedures during construction. To prevent concrete cracking, post-tensioned prestressed construction is used universally, 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 ordinary falling objects. Due to the thicker concrete at the bottom of the tank, hydration temperature should be controlled during casting to prevent cracking caused by temperature stress.
Characteristics of the components of an LNG low-temperature storage tank?
1
Inner can wall
The inner tank wall is a key component of the low-temperature storage tank, made of steel plates that are resistant to low temperatures and possess good mechanical properties, typically using A5372, A516 Gr.60, Gr18Ni9, and ASME 304 special steels.
The inner bottom plate and hoop plate of the can are made of 16mm thick steel plate with material A537 CL2, while the rest of the plates can be made of 6.35mm thick steel plate with material A537 CL1.
2
Insulation layer
Insulated罐body
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.
Insulated Can Lid
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 Bottom of Can
Insulation at the bottom of the drum is complex, requiring not only polyurethane foam spray beneath the steel plate but also a waterproof structure design. The illustration below shows the insulation structure of a drum's bottom, which includes a 65mm thick layer, 60mm thick dense concrete, 2mm thick waterproof felt, two layers of 100mm thick foam glass, and finally, 70mm thick concrete to cover and protect the exterior drum concrete from the impact of low temperatures.
3
Concrete exterior shell
The exterior wall and roof of the concrete tank are composed of prestressed reinforced concrete and low-temperature-resistant steel lining plates. The concrete strength should be ≥25 MPa. The exterior roof and wall must withstand internal pressure in case of unexpected gas leakage, thus the reinforced concrete must possess adequate tensile strength.
For large storage tanks, to ensure uniform stress distribution on the pre-stressed concrete walls, designs with equal strength but varying thickness or equal thickness but varying strength can be adopted.
What types of LNG tanks are there?
Various shapes
Cylindrical: Used for industrial gasification stations, small-scale LNG production facilities, satellite liquefaction units, civil gasification stations, and LNG refueling stations for vehicles.
Large cylindrical: Used for base load, peak-shaving type liquefaction facilities, and LNG receiving stations.
Spherical: Used for civil gasification stations, LNG refueling stations for vehicles.
Different settings
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.
50-100 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 plants.
40,000 to 200,000 m³: For LNG receiving stations.
Storage issues for LNG
Liquid Stratification
LNG is a multi-component mixture, and due to changes in temperature and composition, the differences in liquid density can cause stratification within the storage tank. Generally, stratification is considered to occur within the tank when the temperature difference in the vertical direction of the liquid exceeds 0.2°C and the density exceeds 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 convection cycles, natural convection cycle diagram within the LNG storage tank
Rolling Phenomenon
The rolling phenomenon refers to the rapid up and down movement of two layers of LNG with different densities inside the storage tank, resulting in a surge of vaporization gas. At this point, the vaporization amount of LNG in the tank is 10 to 50 times higher than the normal evaporation rate, causing the tank's pressure to rapidly rise and exceed the set safety pressure, leading to overpressure conditions. 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 phenomenon is the differing densities of liquid layers within the storage tank, which results in stratification (Figure 1). The composition of the substances significantly impacts the timing and severity of evaporation and boiling.
In the long-term storage of LNG tanks, due to the evaporation of lighter components (mainly N2 and CH4) first, spontaneous boiling occurs. After a period (hours to even days) of filling with new LNG of different densities and temperatures into tanks originally containing LNG, a sudden boiling phenomenon may occur. For continuously operating receiving stations, the occurrence of boiling in 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 within the tank stratifies, with the introduction of external heat, the bottom LNG's temperature rises, causing its density to decrease. The top LNG becomes heavier due to the vaporization of BOG. Through mass transfer, the lower LNG rises to the top, the pressure decreases, and it becomes supersaturated liquid, releasing stored energy rapidly, producing a large amount of BOG, resulting in the rolling phenomenon.
Note that LNG stratification is the prerequisite for rolling.
Methods for Detection and Elimination of Delamination
Temperature Monitoring
Density Monitoring
Ballast Oil Group Monitoring
Once the tank stratifies, the LNG at the bottom of the tank is first pumped out during the export process.
After LNG stratification, the top entry device should be used for circulation operations to promote mixing of LNG within the storage tank and prevent rolling. However, this also increases the amount of vapor and the cost to process the additional vapor (as shown in Figure 4).
During unloading, if the LNG density on the ship is heavier than the density in the storage tank, feed the LNG into the tank through the top loading pipe. Otherwise, feed it through the bottom loading pipe to promote self-mixing of LNG with different densities within the tank, thus eliminating stratification.
