At present, the LNG cryogenic storage tanks commonly used at home and abroad have three methods: atmospheric storage, pressure storage of mother and child tanks and pressure storage of vacuum tanks. Which storage method is used depends mainly on the size of the storage.
① Vacuum tank
Vacuum tank is a double-layer metal tank, the inner tank is a low temperature resistant stainless steel pressure vessel, the outer tank is made of carbon steel material, the interlayer is filled with insulation material, and vacuum is drawn. The vacuum tank is transported to the site as a whole after the factory manufactures the pressure test.
The total storage capacity of LNG is less than 1000m? Generally, multiple vacuum tanks are used for centralized storage. At present, the maximum volume of single tank used in China is 150m ?. The vacuum tank process is relatively simple. Generally, a booster is used to pressurize the storage tank. The material enters the gasifier by self-flowing pressure. No power equipment is used and the energy consumption is low. Therefore, small and medium-sized LNG gasification stations at home and abroad basically use vacuum. Pot form.
② Mother-child tank
The inner tank of the mother-child tank is a plurality of low-temperature resistant stainless steel pressure vessels. The outer tank is a large carbon steel container covered with multiple inner tanks. The inner and outer tanks are also filled with thermal insulation materials. Air enters. The inner tanks of the mother and child tanks are manufactured at the factory and shipped to the site after pressure testing, and the outer tanks are installed on site.
The storage and distribution station with a storage scale of 1000m? To 5000m? Can be selected for storage according to the situation. Due to the internal tank transportation requirements, the current domestic single tank can be up to 250m ?. The gasification process is roughly the same as the vacuum tank. Because the interlayer needs to be vented with nitrogen, a set of liquid nitrogen is added to the device.
③Atmospheric pressure tank
The structure of atmospheric pressure tanks includes bimetallic tanks, and the outer tanks use prestressed concrete structures; there are aboveground tanks, and underground tanks, most of which are under 20,000m? Are bimetallic tanks.
Both the inner and outer tanks of atmospheric pressure tanks are installed and manufactured on site, and the production cycle is longer.
LNG low-temperature atmospheric storage tank has an operating pressure of 15KPa and an operating temperature of -162 ℃， and is a flat-bottomed double-walled cylinder. The tank body is composed of two layers, inside and outside, with a heat insulation structure between the two layers, which is a cold insulation layer. Inner tank for
Stores liquefied natural gas, and the outer shell plays a role of protection and cold preservation. In order to reduce the introduction of external heat into the tank, the internal and external tanks are designed to be separated and independent. The tank item is a self-supporting vault, and the inner tank item must have sufficient strength and stability to withstand the external pressure caused by the cold insulation material, etc. and the internal pressure generated by the internal gas.
The storage tank uses pearlescent sand as a cold insulation material and is filled with dry nitrogen to ensure a slight positive pressure of the interlayer. The insulation material is isolated from the atmosphere to avoid the influence of atmospheric pressure or temperature changes and the humidity air enters the cold insulation layer between the inner and outer tanks The service life of the cold insulation material is effectively guaranteed and the use effect of the cold insulation material is effectively guaranteed and improved. When designing and manufacturing thermal insulation structures, attention must be paid to the use of moisture-proof measures.
Through technical and economic comparison, the one-time investment of the low-temperature atmospheric storage tank program is lower than that of the mother-child tank, but the operating cost of the low-temperature atmospheric storage tank is much higher than that of the mother-child tank.
Although the one-time investment of the low-temperature atmospheric storage tank scheme is low, there are the following problems in the scheme:
① Low temperature atmospheric storage tank is more suitable for liquefaction plant and receiving terminal station
Usually low-temperature and atmospheric storage tanks are used in both the liquefaction plant and the receiving terminal station. This is because the volume of LNG storage tanks in the receiving terminal station is very large （single tank volume is greater than 50,000 m?）, And other types of storage tanks cannot be achieved. The liquid received is transported by the LNG tanker, and the storage tank on the LNG tanker is also in the form of low temperature and normal pressure, and the pressure does not change much. Furthermore, a BOG condensing device is installed in the receiving terminal station, which can turn BOG into LNG again.
Because of the liquefaction method in the liquefaction plant, the BOG generated during storage can be transformed into regeneration gas for liquefaction, so the low-temperature atmospheric storage tank is used, and the amount of BOG evaporation has little effect on it.
Because the LNG used in this station is transported by tank trucks, the LNG in the tank trucks are all under pressure, and a large amount of gasification and a large amount of BOG gas will be generated when discharged into a low-temperature atmospheric tank. And because BOG is atmospheric gas, the investment and operating costs of BOG compressors are high. The operating costs of LNG pumps have also increased.
②The low-temperature atmospheric storage tank system is complicated
Low-temperature atmospheric storage tanks must consider the problem of overpressure of the storage tank, as well as the evacuation of the storage tank （the formation of negative pressure）, and air supplementation measures are required. Breathing valve, etc. An automatic dry powder fire extinguishing system and a lifting device shall also be provided on the top of the storage tank.
Because the general working pressure of the low-temperature atmospheric storage tank is 15KPa, in order to maintain this pressure, the pressure regulating valve of the storage tank needs to be opened frequently, and the BOG gas is recovered to the liquid chemical section or torch burned in the station.
This site does not have this part of the facility. If a low-temperature and atmospheric pressure tank is used, only part of the BOG can be compressed into the pipe network for transportation, and the rest must be released, causing waste and polluting the environment. Or build a comparative BOG storage tank in the station to store BOG, so it is necessary to increase a large investment.
③LNG pump needs to be set, which has high requirements on LNG pump
Since the liquid stored in the low-temperature atmospheric storage tank has no pressure, in order to meet the net positive suction head of the LNG pump, the foundation of the storage tank （elevated foundation） should be improved. In addition, in order to prevent cavitation, the inlet pipe of the LNG pump should consider the use of vacuum cooling pipes.
④ The construction period is long, and the maintenance is difficult
Both the inner and outer tanks of atmospheric pressure tanks are installed and manufactured on site, and the production cycle is longer. If the storage tank fails, all production needs to be shut down for maintenance.
⑤ Low temperature and pressure storage tanks need to consider stratification and boiling
The LNG used in this station is the gas source of several liquefaction plants such as Xinjiang Guanghui. Due to the different gas components, the problem of stratification and boiling needs to be considered. Once this problem occurs, the problem is much more serious than the mother-child tank .