Natural gas is widely recognized as a clean, environmentally friendly, and safe high-quality energy source. After liquefaction, its volume 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 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 slightly above atmospheric for the storage tanks, 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 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 containment principles. In the event of a leak in the first layer, the second layer can completely contain the leaked liquid and evaporated gases, ensuring storage safety.
3
Special Material
The inner tank wall must withstand low temperatures; typically, materials like 9Ni steel or aluminum alloys are used. The outer tank wall is made of pre-stressed reinforced concrete.
4
Thermal insulation measures are stringent
The tank must have excellent thermal insulation properties to maintain an internal temperature of -160°C, considering the maximum temperature difference between inside and outside the tank can reach up to 200°C. High-performance thermal insulation material must be filled between the inner and outer tanks. The thermal insulation material at the bottom of the tank must also have sufficient load-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 chosen construction site typically avoids seismic fault zones, and抗震试验 must be conducted on the storage tanks prior to construction to analyze their structural performance under dynamic conditions, ensuring that the tank does not sustain damage under the specified seismic intensity.
6
Strict construction requirements
Tanks must undergo 100% magnetic particle testing (MT) and 100% vacuum tightness testing (VBT). Strict selection of insulation materials is required, and the prescribed procedures must be followed during construction. Post-tensioning pre-stressed construction is used to prevent concrete cracks, with strict control over the verticality of the tank walls.
The concrete exterior tank top should possess high compressive and tensile strength, capable of withstanding impacts from typical falling objects. Due to the thicker concrete at the tank bottom, hydration temperature should be controlled during casting to prevent cracking caused by thermal stress.
What are the features 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 304 are used.
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, 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 the tank top is designed with four layers of glass fiber insulation, each layer is 100 mm thick. The density of the glass fiber wool is 16 kg/m³, and the thermal conductivity is 0.04 W/(m·K).
Insulation at the bottom of the can
Insulating the bottom of the tank 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 tank's bottom, featuring 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 outer tank concrete from the impact of extremely low temperatures.
3
Concrete outer shell
The outer tank wall and roof are composed of prestressed reinforced concrete and low-temperature-resistant steel linings. The concrete strength should be ≥25 MPa. The outer tank roof and walls must withstand internal pressure from accidental gas leakage, hence the reinforced concrete must possess sufficient tensile strength.
For large storage tanks, to ensure uniform 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 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 settings
Ground
Semi-basement
Underground
Different 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: Often used in industrial gas liquefaction stations.
100-1,000 m3: Suitable for small-scale LNG production facilities.
10,000 to 40,000 m³: Suitable for base load and peak-shaving type liquefaction units.
40,000 to 200,000 m³: for LNG receiving stations.
Storage issues of LNG
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's liquid when the temperature difference in the vertical direction is greater than 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 individual components vary, causing changes in the composition and density of LNG. This process is known as aging.
Individual stratified LNG convective circulation, natural convection circulation diagram inside LNG storage tank
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 instantaneous generation of a large amount of vaporized gas. At this point, the vaporization of LNG in 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 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, leading to stratification (Figure 1). The composition has a significant impact on the timing and severity of evaporation and rolling.
In the long-term storage of LNG tanks, spontaneous boiling occurs due to the evaporation of lighter components (mainly N2 and CH4), resulting in rolling. After a period (hours to even days) of filling with new LNG of different densities and temperatures into tanks that originally contained LNG, a sudden rolling phenomenon may occur. For continuously operating receiving stations, the occurrence of rolling in tanks primarily falls under the second category.
The LNG at the top of the tank has lower density, while the LNG at the bottom has higher density. Once the LNG inside the tank stratifies, as external heat is introduced, the bottom LNG's temperature increases, causing its density to decrease. The top LNG becomes heavier due to the volatilization of BOG. Through mass transfer, the lower LNG rises to the top, the pressure decreases, turning it into supersaturated liquid. The accumulated energy is rapidly released, producing a large amount of BOG, resulting in the churning phenomenon.
It's worth noting that LNG stratification is a prerequisite for rolling.
Methods for Testing and Eliminating Delamination
Temperature Monitoring
Density Monitoring
Ballast Oil Gas Monitoring
Once the tank stratifies, pump out the LNG from the bottom of the tank first during the export process.
After the LNG is stratified, a top entry 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 and the cost to process the additional vapor (as shown in Figure 4).
During unloading, if the LNG density on board the ship is heavier than that in the storage tank, unload through the top discharge pipe. Conversely, unload through the bottom discharge pipe. This promotes self-mixing of LNG of different densities within the storage tank, eliminating stratification.
Liquefied Natural Gas (LNG) storage tanks are specialized products for storing LNG, classified as special equipment and type III pressure vessels. They are made of 06Ni9DR material, undergo non-destructive testing, hydrostatic and pneumatic testing, and are inspected on-site by the Technical Supervision Bureau, with a pressure vessel inspection certificate issued. The manufacturing process also includes external rust removal and painting. The LNG storage tanks undergo strict quality assessment for the material, dimensions, and weld quality of the pressure components, operational quality, installation quality, internal equipment, and safety accessories.
Routine physical and chemical tests for the material of the drum, such as mechanical properties and chemical composition.
The weld joints, weld seams, tank end caps, and the mutual geometric positions of all pressure components are rigorously inspected through X-ray non-destructive testing and magnetic particle inspection. Tests are conducted on the product's sealing, pressure resistance, and all technical indicators that could affect the safe operation of the product.
Liquefied Natural Gas Storage Tanks Category: Products Common Structure: Vertical LNG Storage Tanks, Horizontal LNG Storage Tanks Atmospheric Storage Tanks Atmospheric LNG Storage Tanks and Extra-Large Atmospheric LNG Storage Tanks
Table of Contents
1 Common Structure
2 Points of Attention
Common Structure Editing
Common structures for LNG tanks include: vertical LNG tanks, horizontal LNG tanks, vertical mother-child tanks, and atmospheric pressure tanks.
Vertical LNG Storage Tank
Volume options include 50 cubic, 100 cubic, 150 cubic, and 200 cubic.
Horizontal LNG Storage Tank
Volume options include 60 cubic and 100 cubic.
Vertical mother and son tanks
A mother tank consists of multiple interconnected daughter tanks to meet large storage capacity requirements, with several daughter tanks assembled side by side within a large outer tank. The number of daughter tanks ranges from 3 to 7, generally not exceeding 12. The volume of individual daughter tanks should not be too large, usually between 100 to 150 cubic meters, with the maximum reaching 250 cubic meters. Common sizes include 1000 cubic meters, 1750 cubic meters, and 2000 cubic meters.
Atmospheric Storage Tank:
We offer large and medium-sized atmospheric LNG storage tanks, as well as extra-large atmospheric LNG storage tanks.
Cautionary Notes
1. Cylinders must be stored in well-ventilated areas, with a distance of not less than 1.5 meters from any fire or heat source. Cylinders are strictly prohibited from being heated with fire, boiled in hot water, or left exposed to direct sunlight. Regular inspections should be conducted on cylinder valves and pipeline joints to ensure airtightness and prevent gas leaks. Leaks can be checked using soap and water, but open flames are strictly forbidden for leak testing.
2. When lighting, always ignite the primer first, followed by opening the gas; do not reverse this order. There should be someone supervising during use; do not leave the area to prevent boiling water from spilling and extinguishing the flame, which could cause the gas flow to surge and lead to an explosion. After using the gas cylinder, ensure the valve is tightly closed to prevent gas leakage.
3. Do not completely exhaust the liquefied gas inside the cylinder; a certain residual pressure should be maintained. The residual pressure should generally be greater than 49.03 kPa (equivalent to 0.5 kg/cm², gauge pressure) to prevent air from entering the cylinder. After the liquefied petroleum gas is used up, the remaining residue inside the cylinder is also a flammable substance and should not be poured out自行. This is to prevent fires from being caused by the residue's flow and evaporation.
4. LPG cylinders are pressurized containers that require proper maintenance and regular inspections. During handling and use, they should be prevented from falling or being struck. It is prohibited to use metal tools to敲击 open the valve, and they should be shielded from direct sunlight and prolonged exposure to rain. Cylinders are typically inspected every 2 years.
5. Although the explosive range of liquefied petroleum gas (LPG) is not very wide, its lower limit is small, making it easy to ignite and explode upon leakage. Additionally, as LPG is heavier than air, it tends to flow downward in the air, accumulating in low-lying areas and posing a hidden danger of gas explosion. Therefore, in areas where gas is prone to leakage, relying solely on window ventilation is insufficient; attention must also be given to proper ventilation at the lower level.
6. When liquefied gas leakage is detected indoors, promptly open windows and doors for ventilation, allowing it to disperse in the direction away from any open flame. Strictly prohibit any fire nearby until the fault is resolved and the unique odor has dissipated. Dispose of any leaked fluid on the ground by covering it with sand and soil, then remove it to a safe location. In the event of a gas cylinder fire, immediately close the valve, move it to an open area outside, and extinguish it using dry powder fire extinguisher, CO2 fire extinguisher, or by covering it with a damp sack.
7. Educate children not to tamper with gas cylinders at will, and users must understand the safety knowledge of using gas cylinders.
