Professional manufacturer of Liquefied Natural Gas Storage Tanks and Gas Steam Boilers
In the early stages of LNG tank development, tanks were all single-walled. They were insulated with block insulation, but had drawbacks such as the lack of a metallic moisture-proof layer and vulnerability to wind disasters. Subsequently, they evolved into double-walled, double-roofed tanks. Due to the ease of LNG vaporization, which can lead to overpressure inside the tank, they eventually developed into double-walled, single-roof tanks.
1. Tank Classification and Brief Introduction
LNG low-temperature storage tanks come in various types, including: underground tanks, spherical tanks, single-volume tanks, double-volume tanks, full-volume tanks, membrane tanks, and mother-child tanks. Currently, single-volume tanks and mother-child tanks are more commonly used in LNG plants, while underground tanks, full-volume tanks, and membrane tanks are primarily applied in large-scale LNG receiving stations.
(1) Single-Container Tank
Single-walled and double-walled containers are available, with a design pressure of 17-20kPa and an operating pressure typically at 12.5kPa. For large-diameter single-walled containers, the design pressure is relatively lower, potentially less than 14kPa. The equipment layout for single-walled containers requires a larger safety distance and land area, and firebreaks and embankments must be set up.
(2) Mother and Child Drums
The mother罐 consists of an inner and an outer tank. The inner tank is composed of multiple daughter tanks, assembled side by side within a large outer tank. It is a new type of low-temperature storage tank that has developed in recent years. It is suitable for both pressurized and atmospheric storage, with operating pressures ranging from 0.2 to 1.0 MPa, up to a maximum of 1.8 MPa. Like single-vessel tanks, it requires a larger safety distance, occupies a significant area, and is often equipped with firebreaks and cofferdams.
(3) Spherical Vessel
The inner and outer shells of the spherical storage tank are both spherical. In operation, the inner tank is an internal pressure vessel, and the outer tank is an external vacuum pressure vessel. The volume of the spherical tank is generally 200~1500 m³, with an operating pressure of 0.2~1.0 MPa.
(4) Underground Tank
An underground storage tank is a type of tank where the highest liquid level of the stored LNG is below ground, with the tank itself situated on an impermeable, stable foundation. Underground storage tanks are constructed using cylindrical metal containers, with a reinforced concrete outer shell. Due to their high investment cost and long delivery periods, they are generally not chosen for design.
(5) Full Capacity Vessel
The full-volume tank consists of an inner cylinder, outer cylinder, and a top cover, offering double containment for LNG and its vapor. The design pressure of the full-volume tank can reach 0.029 MPa, with a design temperature range of -196 to 60°C. The seismic design friction coefficient is generally 0.3 to 0.6, and the distance between the inner and outer cylinders should not exceed 2 meters. Compared to single or double containment tanks, it has the highest cost but the best safety reliability, making its application in receiving stations extremely widespread.
(6) Membrane Tank
The membrane tank utilizes stainless steel inner lining and concrete outer walls, with the same fire protection and safety distance requirements as full-volume tanks. The membrane tank offers greater operational flexibility compared to full-volume tanks and is particularly suitable for use in areas with frequent seismic activity and high population density. However, it is characterized by higher investment costs and a longer construction period.
(7) Double Container
The dual-container design has the same pressure as the single-container design, with both the inner and outer containers capable of independently containing the stored low-temperature liquid products. The outer container is made of low-temperature steel or concrete, while the inner container is constructed from 9% nickel steel or stainless steel. The distance between the inner and outer containers must not exceed 6 meters.
Important Usage Precautions for lng Liquefied Natural Gas Filling Station Tanks:
1. Tanks with dual-root valves, and valves near the tank's edges, should be kept open.
2. During unloading, it is recommended that the tank's maximum filling capacity not exceed 90% of its effective volume (manufacturers specify between 85%-95% of the geometric volume), and the remaining capacity should not be less than 10%.
3. High pressure in an LNG tank does not necessarily indicate an issue with the tank itself. Consider the entire system's heat loss and liquid storage duration. Generally, when the pressure exceeds 1.1 MPa, a pressure relief discharge is required. Long-term high-pressure conditions can affect the lifespan of the tank's vacuum insulation.
4. If the tank exhibits 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 within the tank.
5. Suggest conducting vacuum tests on the storage tank annually. For hot tanks, the vacuum level should be less than 1 Pa, and for cold tanks, the vacuum level should be less than 0.5 Pa (0.00375 TOF) to be considered合格.
6. Tanks cannot be transported with liquid. When not in use for an extended period, the LNG liquid inside the tank should be drained.
7. Storage tanks are pressure vessels. According to the "Safety Supervision Regulations for Pressure Vessel Technology" 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. Only after obtaining a permit can installation, maintenance, and modification proceed.
LNG Storage Tanks
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