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I. Process of LNG vaporization stations II. Necessity of pre-cooling III. Purpose of pre-cooling IV. Pipeline purging before pre-cooling V. Materials required for pre-cooling VI. Preparatory work before pre-cooling VII. Principles of pre-cooling VIII. Main steps in pre-cooling IX. Safety precautions during pre-cooling X. Inspection items during pre-cooling I. Process of LNG vaporization stations LNG vaporization stations represent the main approach used in downstream natural gas applications; their primary functions are to store, vaporize, and transport LNG. It mainly includes a unloading platform, low-temperature storage tanks, a pressurization system, a gasification system, as well as pressure regulation, metering, and odorization systems. Process flow diagram of LNG vaporization station II. The necessity of pre-cooling: Before low-temperature liquids enter the LNG vaporization station, the low-temperature pipelines and storage tanks must first be thoroughly cooled, which is the pre-cooling process. LNG storage tanks and pipelines are typically made of austenitic stainless steel. Austenitic stainless steels exhibit excellent low-temperature performance, but have a high linear expansion coefficient. At LNG temperatures, the shrinkage rate of stainless steel is approximately 0.3 percent; for 304L piping, at an operating temperature of -162°C, a 100m length of pipe will shrink by about 300mm. Therefore, measures must be taken during design to prevent damage caused by cold shrinkage. The contraction and compensation of LNG pipelines is an important issue that requires careful consideration. Between two fixed points, the stress resulting from cold contraction can far exceed the material’s yield point. Especially, the requirements for pipelines inside LNG storage tanks are even stricter; any problems that arise can lead to serious consequences. Therefore, effective measures must be considered in pipeline design to compensate for this. In LNG equipment and pipelines, elbows and expansion joints are generally used to compensate for cold contraction. Although compensation for cold shrinkage is taken into account during design, when the rate of temperature change is high, there is still a risk of excessive temperature changes and excessive thermal stress, which can cause damage to the material or the joints. This requires pre-cooling operations to be carried out on low-temperature pipelines and equipment before they are exposed to low-temperature liquids, in order to ensure safe operation. III. Purposes of pre-cooling: To examine and test the low-temperature performance of low-temperature equipment and pipelines, including: (1) Checking whether the quality of low-temperature materials is satisfactory; (2) Assessing the quality of welding; (3) Verifying the degree of cold contraction of pipelines and any changes in support provided by pipe supports; (4) Testing the sealing performance of low-temperature valves; (5) Bringing the storage tank into operational condition and testing its vacuum performance. IV. Pipeline purging before pre-cooling: 1. Importance of pipeline purging before pre-cooling: The pipelines must be thoroughly cleaned before pre-cooling. If the purging is not thorough, it will cause the valve to freeze. Since most of the valves in low-temperature pipelines are welded with few flanges, it is not conducive to pipeline purging. Therefore, measures must be taken to strictly control purging. 2. The qualification standard for purging: The airflow is blown at a speed of 20 m/s toward a pad placed near the pipe opening and covered with a semi-wet white towel; it is considered qualified if there is no dust or impurities on the towel. 3. Principles for pipeline purging: (1) Purging should be carried out in segments during construction, with each segment defined by welded valves. It is important to seal the pipelines promptly after construction to prevent debris and rainwater from entering. (2) To prevent rust and slag from carbon steel pipes from entering the low-temperature pipes, carbon steel pipes cannot be used for purging the low-temperature pipes. (3) Blowing cannot be performed into the storage tank; it must be done from inside the tank outward. (4) No instrument equipment may be purged. (5) Due to the need for purging, temporary gaskets should be used during installation, and the proper gaskets should be replaced before achieving airtightness. (6) Tap the pipe surface and welding areas during purging. (7) Develop specific purging plans based on the process flow of each station. V. Materials required for pre-cooling: (1) Liquid nitrogen; (2) Portable thermometers and portable combustible gas detectors; (3) Copper fastening tools and quick-connect fittings for connecting to the unloading ports of liquid nitrogen tankers; (4) Work clothes, work shoes, and frost-resistant gloves required by the personnel responsible for pre-cooling; (5) Watches and record sheets needed for monitoring, with records to be taken every 15 minutes. VI. Preparatory work before pre-cooling: (1) Check the valves to ensure that all of them are in the closed position. (2) Confirm that all blind flanges in the vent system have been removed and that the vent system is unobstructed. (3) Open all root valves of the safety valves, and open the upstream and downstream valves of the two pressure reduction control valves. Open the root valve for the gas phase vent of the storage tank. (4) The automatic protection system tests were successful, and all functions are in operation. The nitrogen system was put into operation, and all emergency shut-off valves were opened. (5) All the root valves of the pressure gauge are opened. The root valve and gas-liquid equilibrium valve of the tank level gauge are opened. (6) Replace the air in the pipeline with dry nitrogen to prevent condensation water at the valves during pre-cooling from freezing them shut. VII. Pre-cooling principles: During pre-cooling, the temperature of storage tanks and pipelines should be reduced gradually to avoid sudden cooling, thereby preventing damage to equipment and fittings caused by sharp temperature drops. Based on relevant operational experience, a cooling rate of 50°C/h is considered safe. VIII. Main steps for pre-cooling 1. First, pre-cool using low-temperature nitrogen (1) Check that the unloading hose is in good condition, with no rainwater, debris, or other impurities inside it. The hose is connected to the tank truck, and it is checked to ensure the connection is secure. (2) Increase the pressure in the tank truck, open the vapor valve of the tank truck, and check for any leaks at the hose connections. (3) Slowly introduce low-temperature nitrogen into the storage tank; once the pressure in the tank rises to 0.2 MPa, close the liquid discharge valve on the unloading platform. After insulating the tank for 15 minutes, open the manual vent valve for the gas phase of the tank to release the nitrogen. Rising and falling pressures occur repeatedly. (4) Determine the internal temperature of the storage tank by releasing gas through the fill valve and measuring it with a thermometer; once the desired temperature is reached, the gas pre-cooling process is complete. 2. Liquid nitrogen pre-cooling: (1) Release the pressure in the storage tank to a slight positive pressure, and close the lower liquid inlet valve. Close the level gauge balance valve and activate the level gauge. (2) Slowly open the liquid-phase valve of the tank truck to a smaller opening, and slowly close the gas-phase valve of the tank truck, allowing a small amount of liquid nitrogen to enter from the upper part of the storage tank. Control the opening degree of the unloading platform valve; keep it slightly open at a low degree to maintain the pressure at 0.3 MPa. When the pressure in the storage tank rises to 0.2 MPa–0.3 MPa, the valve at the unloading station should be closed promptly, and the manual vent for the gas phase of the storage tank should be opened to relieve pressure. Repeat this operation. (3) Release gas through the fill valve; once the temperature reaches a certain level, or when the level gauge indicates a certain liquid level, slowly open the inlet emergency shut-off valves at the bottom of the storage tank, allowing liquid to flow in from both the top and the bottom simultaneously. During the liquid inlet process, closely monitor and record the pressure in the storage tank to prevent it from rising. When the pressure rises, the lower inlet valve should be closed promptly. Feel the temperature of the tank’s exterior with your hand to confirm that there are no issues with the tank. (4) The liquid level gauge of the storage tank reaches a certain value, and the liquid feeding stops. (5) After the liquid storage task is completed, close the liquid-phase valve of the tank truck and open the gas-phase valve of the tank truck to purge the liquid discharge pipeline into the storage tank. (6) Close the tank truck valves and the liquid discharge valves on the unloading platform, remove the hoses, handling them with care, and ensure that people stay away. After closing the unloading station valve, the liquid between this valve and the check valve should be drained. (7) Close the manual vent valve for the gas phase in the storage tank, as well as the manual valve before the emergency shut-off valve for liquid inlet at the bottom of the storage tank. The inlet valve at the top of the storage tank should be closed after the LNG pipeline for unloading returns to normal temperature. (8) Use the liquid nitrogen in the storage tank to pre-cool the booster, air-bath vaporizer, and their cryogenic pipelines. 3. Utilization of vented low-temperature nitrogen: When pre-cooling with liquid nitrogen, it is necessary to vent the low-temperature nitrogen through gas pipes. This low-temperature nitrogen can be used to pre-cool other storage tanks via gas pipes connected to those tanks, thereby saving liquid nitrogen. IX. Safety precautions during pre-cooling: (1) In a confined space, liquid nitrogen absorbing external heat can cause a sharp rise in pressure; therefore, attention must be paid to the sequence in which valves are closed, and it is strictly forbidden for low-temperature liquids to become trapped. (2) Make sure to check for leaks at the hose connections, and people should stay away from that area. (3) Pay close attention to the increase in pressure in the pipelines and storage tanks. (4) Make sure to check for frost behind the safety valve. X. Inspection items during pre-cooling (1) Check whether the low-temperature materials show signs of low-temperature cracking. (2) Check the welded areas of low-temperature pipelines for cracks, especially at the flange welds. (3) Check the cold shrinkage of the pipeline and changes in bracket support. (4) Check the sealing performance and flexibility of the low-temperature valve, and verify whether it has frozen. (5) Check whether there is any leakage at the flange connection, and whether the bolt tension has decreased due to cold shrinkage. (6) The liquid nitrogen is stored in the tank for 2 to 3 days. Observe the changes in liquid level and pressure rise. It also measures the changes in the vacuum level of the storage tank before and after pre-cooling, in order to evaluate the tank’s performance.