The main steps of pre-cooling

　　1. Connect the liquid nitrogen tanker and the low-temperature atmospheric tank with a metal hose, and the tanker is boosted by self-pressurization;

　　2. Slowly flush the low-temperature nitrogen on the top of the tanker into the atmospheric tank. At this time, open the manual and automatic vent valve of the gas phase of the atmospheric tank to vent the nitrogen;

　　3. Slowly open the liquid phase valve of the tanker to a smaller opening, close the gas phase valve of the tanker, and use low-temperature liquid nitrogen to pre-cool. The first time the storage tank is filled, the body is a "hot tank". If the valve opening is large, it may cause the pressure of the storage tank to rise rapidly and cause damage to the tank.). This process needs to continuously discharge gas to the top gas phase pipe, and control the pressure of the inner tank to not exceed 12KPa.

　　4. As the pre-cooling process progresses, the temperature of the inner tank continues to drop until the temperature at the lower part of the inner tank drops below -100°C, the liquid intake can be appropriately increased to fully pre-cool the storage tank.

　　5. Closely observe and record the pressure of the storage tank during the liquid feeding process to prevent the pressure from rising. When the pressure rises, close the inlet valve in time.

　　6. ​​The above process involves the refilling procedure of re-feeding liquid after a tank truck has finished unloading the liquid, which should be carried out in accordance with the above procedure (the pre-cooling procedure of low-temperature gas is omitted).

　　7. When the storage tank is pre-cooled to a certain level, liquid will begin to accumulate in the tank, and a small amount of liquid will be stored to fully pre-cool the pipelines in the system.

　　8. After the above process is over, stop the liquid supply and the pre-cooling ends.

　　What to check during pre-cooling

　　1, check the outer pipe material and weld seam for cracks and other damage;

　　2. The pre-tightening force of the bolt may be reduced due to cold shrinkage, and the flange connection part will leak. Therefore, it is necessary to focus on leak detection and cold-tighten the flange;

　　3, check the tightness and flexibility of the cryogenic valve, whether it is frozen;

　　4, tighten the tube support;

　　5. After pre-cooling and cold-tightening are completed and qualified, the external pipeline can be insulated and wrapped.