Proper Usage Precautions for lng Liquefied Natural Gas Filling Station Storage Tanks:

Tanks with double root valves and valves near the tank should remain open.

2. During unloading, it is recommended that the tank not be filled beyond 90% of its effective capacity (manufacturers specify between 85%-95% of the geometric volume), with the remaining capacity not to be less than 10%.

3. High pressure in an LNG tank doesn't necessarily indicate an issue with the tank. Consider the entire system's heat loss and liquid retention time. Generally, when pressure exceeds 1.1 MPa, it's necessary to vent and relieve pressure. Long-term high pressure in the tank can affect the lifespan of the tank's vacuum insulation.

4. If the tank experiences extensive and prolonged "sweating" or frosting, and the pressure is rapidly and continuously increasing, consider the vacuum failure. Immediate action is required, including manual pressure relief and transferring the LNG liquid inside the tank.

5. Suggest conducting vacuum tests on the storage tank annually. For hot tanks, a vacuum level below 1Pa is considered acceptable. For cold tanks, a vacuum level below 0.5Pa (0.00375 TOF) is deemed qualified.

6. Tanks must not be transported with liquid; when not in use for extended periods, the LNG liquid inside the tank should be drained.

7. Storage tanks are pressure vessel equipment. According to the "Technical Supervision Regulations for Pressure Vessel Safety" implemented by the People's Republic of China, installation, maintenance, and modification of pressure vessels require that the units with corresponding qualifications notify the local quality supervision department. Permission must be obtained before proceeding with installation, maintenance, or modification.

LNG Storage Tanks

LNG Storage Tanks, Manufacturer of LNG Storage Tanks, Producer of LNG Storage Tanks

Natural gas is widely recognized as a clean, environmentally friendly, and safe high-quality energy source. Once liquefied, it reduces in volume by approximately 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 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 operation maintains the storage tank's pressure 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 will volatilize in large quantities, with the vaporization amount being approximately 300 times that of the original refrigerated state. This can form explosive gas clouds in the atmosphere.

Therefore, standards such as API and BS require double-walled tank structures and the use of containment principles. 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 able to withstand low temperatures, typically made of materials like 9Ni steel or aluminum alloys, while the outer tank wall is made of prestressed reinforced concrete.

4

Thermal insulation measures are strict

Due to the temperature difference between the inside and outside of the drum reaching up to 200°C, the drum must have excellent insulation properties to maintain an internal temperature of -160°C. High-performance insulation material should be filled between the inner and outer drums. The insulation material at the drum bottom must also possess sufficient pressure-bearing capacity.

5

Excellent seismic performance

The seismic requirement for general buildings is to crack but not collapse under specified seismic loads. To ensure the safety of storage tanks under unexpected loads, they must have excellent seismic performance. For LNG storage tanks, it is required that they neither collapse nor crack under the specified seismic loads.

Therefore, the selected construction site usually avoids seismic fault zones, and anti-seismic tests must be conducted on the storage tank prior to construction to analyze the structural performance under dynamic conditions, ensuring that the tank does not suffer damage under the given seismic intensity.

6

Strict construction requirements

Tanks must undergo 100% magnetic particle inspection (MT) and 100% vacuum leak tightness test (VBT). Careful selection of insulation materials is required, and the specified procedures should be followed during construction. Post-tensioned prestressed construction is employed to prevent cracks in concrete, with stringent control over the verticality of the tank walls.

The concrete outer shell should have high compressive and tensile strength, capable of withstanding impacts from ordinary falling objects. Due to the thicker concrete at the bottom of the tank, the hydration temperature must be controlled during pouring to prevent cracking caused by temperature stress.

Components characteristics of LNG low-temperature storage tanks?

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 specialty steels.

The inner tank bottom plate and hoop plate 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 polyurethane 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 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 drum bottom

Insulating the bottom of the drum is complex, requiring not only the application of polyurethane foam spray under the steel plate but also the design of a waterproof structure. The following image shows the insulation structure of a drum's bottom, including 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 outer drum concrete from the effects 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 leakage, thus, the reinforced concrete must possess adequate tensile strength.

For large storage tanks, to ensure the prestressed concrete tank walls are evenly loaded, the design methods of equal strength but different thickness, or equal thickness but different strength can be adopted.

What types of LNG storage tanks are available?

Various shapes

Cylindrical: Used in 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 liquefaction facilities, and LNG receiving stations.

Spherical: Used for civil gasification stations, LNG vehicle refueling stations.

Different configurations

On the ground

Semi-basement

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.

Challenges in storing 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 tanks. Generally, stratification is considered to have occurred within the tank when the vertical temperature difference in the liquid exceeds 0.2 degrees Celsius and the density exceeds 0.5 kg/m³.

aging phenomenon

LNG is a multi-component mixture, during storage, the evaporation rates of the various components differ, causing changes in the composition and density of LNG. This process is known as aging.

Individual stratified LNG convection cycles; Natural convection cycle 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 generation of vaporized gas instantaneously. At this point, the gasification of the 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 conditions. If not promptly released through safety valves, 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 storage tank, which result in stratification (Figure 1). The composition of the liquid has a significant impact on the timing and severity of evaporation and rollover.

In the long-term storage of LNG tanks, spontaneous boiling occurs as the lighter components (mainly N2 and CH4) evaporate first. 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 boiling phenomenon may occur. For continuously operating receiving stations, the occurrence of boiling in tanks is primarily categorized as the second type.

The LNG density at the top is lower, while it's higher at the bottom of the tank. As the LNG within 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 volatilization of BOG. Through mass transfer, the lower LNG rises to the top, the pressure decreases, and it becomes super-saturated liquid. The stored energy is then 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

Ballast Water Observation Gauge

Once the tank stratifies, pump out the LNG at the bottom of the tank first when exporting.

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 handle the 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 discharge pipe. Otherwise, load through the bottom discharge pipe, which promotes self-mixing of LNG of different densities in the tank and eliminates stratification.