Safe Filling of Cryogenic Liquid Storage Tanks

In industrial production, cryogenic liquid storage tanks play a crucial role. Whether it is liquid oxygen, liquid nitrogen, or liquid argon, the storage and transportation of these cryogenic liquids require strict operating procedures to ensure safety and efficiency. This article will provide a detailed introduction to the filling, operation, and maintenance processes of cryogenic liquid storage tanks, helping operators better understand and carry out the relevant steps.

Preparations Before Filling Cryogenic Liquid Storage Tanks

Before carrying out the filling work of cryogenic liquid storage tanks, adequate preparations are the key to ensuring that the entire filling process is safe and efficient. The following are some necessary preparations before filling, which help prevent potential safety hazards and ensure the smooth progress of the process.

1. Inspection of the Storage Tank

Before each filling of cryogenic liquid, careful inspection of the storage tank is essential. This step can effectively avoid potential safety hazards and ensure the smooth filling process. First, a visual inspection of the storage tank is required to check whether there is any damage to the tank surface, such as dents, cracks, or loose joints. Such damage may cause leakage of cryogenic liquids, leading to serious safety accidents. At the same time, check whether the storage tank is clean and whether it meets the operating conditions to be carried out. If any damage or non-compliance is found, inspection and repair must be carried out immediately, and filling operations can only proceed after the storage tank is confirmed to fully meet safety standards.

2. Nitrogen Replacement

Before leaving the factory, the storage tank is usually filled with low-purity, dry nitrogen and sealed under positive pressure of 0.05 Mpa. The purpose of this is to prevent moisture and air from entering the tank and to avoid contamination of cryogenic liquids during filling. Therefore, nitrogen replacement must be performed before filling cryogenic liquids to ensure that the oxygen content inside the storage tank meets the requirements. Only when the internal gas environment of the tank reaches the safety standard can the filling of cryogenic liquids begin.

3. Purging and Precooling

Before first use, after regular maintenance, and before reuse after long-term idleness, the storage tank must be purged and precooled with liquid nitrogen. This ensures that the cryogenic liquid inside the tank maintains purity and avoids contamination. During the purging process, the maximum purging pressure should be controlled within 0.35 Mpa. This step effectively removes impurities and moisture inside the tank, creating favorable conditions for the storage and use of cryogenic liquids.

4. Confirming the Transferred Substance

Before carrying out the transfer operation of cryogenic liquids, it is necessary to confirm that the substance contained in the transfer device is exactly the one to be transferred. This avoids errors during operation that could cause different types of cryogenic liquids to mix, which would be dangerous. Therefore, before connecting the infusion hose, carefully verify whether the substances in the transfer device and the storage tank are consistent.

5. Static Grounding Device

When connecting the infusion hose, a static grounding device must also be installed. This is because static electricity may be generated during the transfer of cryogenic liquids, and excessive accumulation of static electricity may cause sparks, leading to fire or explosion accidents. By installing a static grounding device, static electricity can be effectively discharged to the ground, preventing safety hazards caused by accumulation.

6. Inspection of Pipelines and Valves

Before starting filling, it is also necessary to check all pipelines and joints of the storage tank to ensure that they are securely connected. At the same time, check whether the positions of the valves are correct, confirm that the pressure gauge valve and liquid level gauge valve are in working condition, and that other valves are in a closed state. These inspections ensure the smooth progress of the filling process and avoid leakage or abnormal pressure caused by pipeline or valve problems.

7. Inspection and Discharge of the Liquid Level Gauge

At the start of filling, loosen the joints at both ends of the liquid level gauge, fully open the liquid phase valve and gas phase valve of the level indicator, and check whether the discharged airflow contains moisture. If moisture is found, continue discharging until no moisture remains. Then, tighten the joints at both ends of the liquid level gauge and close the balance valve, putting the liquid level gauge into normal working condition. This step ensures that the liquid level gauge can accurately display the liquid level inside the storage tank, providing reliable reference for operators.

8. Precautions for First Filling

When the tank is officially filled for the first time, the inner cylinder is at normal temperature, in a “warm storage tank” state, and therefore must be precooled. Thus, the first pre-filling will consume a certain amount of liquid, and the gas phase pressure will rise faster, which is normal. For a new storage tank or a tank used for the first time after maintenance, the inner container must be evacuated or replaced with liquid nitrogen, and direct filling is strictly prohibited. Only after confirming that the internal gas environment of the tank meets the requirements can the filling of cryogenic liquids be carried out.

Key Points of Cryogenic Liquid Storage Tank Filling

During the filling process of cryogenic liquid storage tanks, strictly following the correct operating steps is the key to ensuring safety and efficiency. The following are important operating points during filling that can help operators complete the filling task smoothly while minimizing safety risks.

1. Filling Method

The filling process of cryogenic liquids must be strictly carried out according to correct methods. Liquids always flow from higher-pressure containers to lower-pressure containers. Therefore, during filling, the liquid phase interface of the source is connected to the liquid phase interface of the storage tank with an infusion hose. During input of liquid into the tank, the top inlet pipe of the tank plays a key role. This inlet pipe is actually a spray pipe that sprays the incoming cryogenic liquid onto the evaporated gas inside the tank. The advantage of this is that the cryogenic liquid absorbs the evaporated gas, thereby reducing the pressure inside the container. In addition, the bottom filling inlet/outlet valve of the tank can quickly input liquid into the tank. By fully opening the bottom inlet/outlet valve and opening the top inlet valve, lower and stable pressure can be achieved, keeping the tank pressure at a stable level. When liquid splashes out from the full analysis valve, it indicates that the container is full, and filling should be stopped immediately, and the full analysis valve closed.

2. Pressure Balance and Valve Operation

During filling, if the pressure of the receiving tank is higher than that of the transfer tank, it is necessary to depressurize the receiving tank or increase the pressure of the transfer tank. The specific operation steps are as follows: First, connect the gas and liquid phase interfaces of the source (or receiving tank) with the liquid phase interface and gas phase interface of the storage tank using infusion hoses. Then open all valves on the gas phase connection loop between the source (or receiving tank) and the storage tank. When gas phase pressure reaches equilibrium, open all valves on the liquid phase connection loop between the source (or receiving tank) and the storage tank. At this time, the cryogenic liquid pump can be turned on to transfer cryogenic liquid into the tank. When the mass flow meter reaches the rated value, filling must be stopped immediately. Finally, close all gas and liquid phase valves in time, remove the infusion hoses, and separate the storage tank from the source (or receiving tank).

Normal Output Procedures of Storage Tank Cryogenic Liquids

The output of cryogenic liquids is a key step in the use of storage tanks. Correct procedures not only ensure smooth transfer of liquids but also effectively prevent potential safety risks. The following are detailed steps of normal cryogenic liquid output, divided into two common methods: pressure transfer and cryogenic liquid pump transfer.

1. Pressure Transfer

When carrying out pressure transfer of cryogenic liquids, it is first necessary to confirm that the substance contained in the tank is indeed the cryogenic liquid to be transferred. Then, connect the infusion hose to the receiving tank joint and install the static grounding device. Before this, purge and replace the metal hose with cryogenic liquid. Next, connect the pressurizer liquid and gas phase interfaces with the pressurizing vaporizer using infusion hoses. Slowly open the pressurizing input and output valves to increase the pressure inside the storage tank until the pressure difference is at least 0.1 Mpa higher than the receiving tank. Open the liquid outlet valve, and when the cryogenic liquid inside the storage tank is nearly discharged, close the pressurizing input and output valves. Observe the liquid level gauge of the receiving tank, and if liquid sprays from the side full analysis valve, stop filling and close the liquid outlet valve. Finally, open the residual liquid discharge valve to discharge the residual cryogenic liquid inside the infusion hose, confirm that normal pressure is reached, and then remove the hose (defrosting of the infusion hose is recommended).

2. Cryogenic Liquid Pump Transfer

When using a cryogenic liquid pump for transfer, it is also necessary to confirm that the substance contained in the tank is indeed the cryogenic liquid to be transferred. Connect the gas and liquid phase interfaces of the receiving tank with the gas and liquid phase interfaces of the storage tank using infusion hoses, and install the static grounding device. Before this, purge and replace the metal hose with cryogenic liquid. Make the necessary connections to the pump, slowly open the transfer filling valve of the receiving tank, and maintain the pump filling pressure 0.1–0.2 Mpa higher than the tank pressure. Open the liquid outlet valve, and when the liquid level gauge of the receiving tank reaches the rated height or the mass flow meter reaches the rated value, stop filling and close the liquid outlet valve. Finally, open the residual liquid discharge valve to discharge the residual cryogenic liquid inside the infusion hose, confirm that normal pressure is reached, and then remove the hose (defrosting of the infusion hose is recommended).

Conclusion

The filling and operation of cryogenic liquid storage tanks is a complex and rigorous process that requires operators to strictly follow operating procedures and ensure that every step is performed accurately. Through the detailed introduction in this article, it is hoped that operators can better understand and master the knowledge of filling, operation, and maintenance of cryogenic liquid storage tanks. In actual work, safety is always the most important. Only by strictly following regulations can the storage and transportation of cryogenic liquids be ensured to be safe and reliable, providing strong support for industrial production.