Gas supply mode and basic requirements for LNG storage tanks

The LNG storage tank is a jacketed vacuum powder adiabatic pressure vessel specially designed for storing and supplying low-temperature liquefied gases such as liquid nitrogen, liquid oxygen, liquid argon, liquid carbon dioxide, and the like. It has been widely used in industrial production and daily life.
Analysis of hazard characteristics of cryogenic liquids
Cryogenic liquids have lower boiling points, greater expansion, stronger suffocation and strong oxidizing properties.
 
1) The boiling point of the cryogenic liquid at a pressure of 101.3 KPa: liquid nitrogen at -196 ° C, liquid oxygen at -183 ° C, and liquid argon at -186 ° C. When in contact with the human body, it can cause severe frostbite to the skin and eyes. When a small amount of cryogenic liquid leaks or the inner leakage of the pipe valve, it will absorb the heat of the surrounding environment, and the leaking point will quickly dew condensation, and if it is serious, it will freeze.
2) The cryogenic liquid receives high heat or a large amount of leakage from the surrounding environment to absorb the surrounding energy, and its volume will expand due to rapid gasification. At 0 ° C and 101.3 KPa, the volume of gas after gasification of 1 L of cryogenic liquid: 674 L for nitrogen, 800 L for oxygen, and 780 L for argon. In a closed container or pipe, the internal pressure rises due to the vaporization of the cryogenic liquid, which may cause the explosion of the container or the pipeline.
3) In the environment around the cryogenic liquid storage tank, the low temperature liquid leaks easily forms a rich gas region after gasification. If the concentration of nitrogen, argon or carbon dioxide is large, it is easy to cause suffocation injury. In addition, when the oxygen concentration is large, oxygen-rich damage also occurs.
4) Oxygen is a strong combustion improver with extremely strong oxidizing properties. Liquid oxygen is close to combustibles. It is easy to cause burning when exposed to open flames. It is easy to produce knocking due to vibration and impact when it comes into contact with combustible materials. It is potentially explosive when mixed with combustible materials. Liquid oxygen can adhere to the fabric of the clothes, and it is easy to cause flashing and injure the human body in case of ignition.
 
LNG storage tank gas supply mode and basic requirements
According to the use occasions and user needs, the gas supply modes of LNG storage tanks mainly include: high pressure gas cylinder filling, low temperature insulated gas cylinder dispensing, pipe network centralized gas supply and low temperature liquid spraying liquid supply.

1) High pressure gas cylinder filling
The LNG cryogenic storage tank is used as a gas supply source for high-pressure gas cylinder filling, which is suitable for the needs of many scattered sporadic gas users. Generally, it must be standardized and implemented by professional production filling units. According to the current national administrative licensing requirements, the filling unit must hold the designated production (storage) approval certificate, safety production license and cylinder filling license for dangerous chemicals, that is, “one book and two certificates” before high pressure cylinder filling Installed. At the liquid outlet of the LNG storage tank, a low temperature liquid pump and a high pressure gasifier are arranged, and the high pressure gas is filled into the special gas cylinder by a high pressure filling system. In the cylinder filling process, the pipeline low temperature and overpressure automatic parking protection system must be set up. When there is still low temperature liquid in the gas outlet of the gasifier or the pipeline pressure exceeds the maximum working pressure of the cylinder, the cryogenic liquid pump should stop automatically, and the low temperature liquid storage tank should stop supplying gas, so as to prevent the low temperature liquid from directly filling into the gas cylinder or gas. The bottle is overfilled and caused an explosion.

2) Low temperature insulated cylinders
From the liquid outlet of the LNG cryogenic storage tank, the cryogenic liquid is directly dispensed into a special small container such as a low temperature insulated gas cylinder by means of the storage pressure in the tank, which can be used for relatively independent and large users. In the case of low temperature insulated cylinders, if there is a vent valve on the cylinder, the vent valve should be opened and vented. When disassembling, the operator should pay attention to standing on the side, and the liquid outlet or the vent should not stay for a long time. In the process of packing, if the pipe valve is frozen, it is better to use heat hydrolysis at 70 °C to disable electric baking or forcible tapping. At the dispensing site, the air should be kept clear to avoid accumulation of cryogenic liquid or gas.

3) Centralized gas supply in the pipe network
It is connected to the low-temperature liquid low-pressure gasifier and the terminal pipeline regulating system by the LNG cryogenic storage tank liquid outlet, and then connected to the gas use pipe network, which can be concentrated to supply a large area or more users. According to the gas demand of the pipe network, after determining the gas use pressure, the working pressure is adjusted by the terminal pipeline regulating system, and the gas is used to output the gas at an automatic constant pressure to meet the regional concentrated gas demand. The cryogenic liquid in the cryogenic liquid storage tank should always maintain a certain amount of stock, so that the inner container is kept at a low temperature state, and the gas supply system is ensured to be in a low pressure constant operating condition.

4) Low temperature liquid spray supply
LNG cryogenic storage tank are used to directly supply low-temperature liquid (the common medium is liquid nitrogen) to the external objects through the low-temperature pipeline for low-temperature processing (such as low-temperature pulverization, food quick-freezing, etc.). LNG low temperature storage tanks are equipped with low temperature liquid spray systems. The distance between the two should be shortened as much as possible to reduce the length of the low temperature pipeline and reduce the loss of low temperature cooling. The pipeline for conveying cryogenic liquids shall have insulation protection measures and moisture-proof treatment structures, and generally adopt vacuum insulated pipes or externally applied white metal pipes with polyurethane insulation materials with better thermal insulation effect.