LNG cryogenic tank pre-cooling scheme: reducing losses + ensuring safe operation

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This post was last edited by liuquan1100 on 2018-1-4 at 10:17. The necessity of pre-cooling: Before the low-temperature pipelines and LNG storage tanks in an LNG vaporization station come into contact with the low-temperature liquid, they must first be thoroughly cooled, that is, through a pre-cooling process. The LNG storage tanks and pipelines are made of austenitic stainless steel. Austenitic stainless steels possess excellent low-temperature performance, but have a high linear expansion coefficient. At LNG temperatures, the shrinkage rate of stainless steel is approximately 0.3%. For 304 material pipelines, at an operating temperature of -162°C, a 100m pipeline shrinks by about 300mm. 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. Effective measures have been considered in the piping design to provide compensation. In LNG equipment and pipelines, elbow compensators are 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 the low-temperature pipelines and equipment before introducing the low-temperature liquid, in order to ensure safe operation. The purpose of pre-cooling is to inspect 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) Inspect the welding quality. (3) Check the cold shrinkage of the pipeline and changes in support from pipe supports. (4) Check the sealing performance of the low-temperature valve. (5) Bring the storage tank into operational condition and test its vacuum performance. Piping purging before pre-cooling 1. The importance of piping purging before pre-cooling: The pipes 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. Replace the official gasket before the airtightness test. 2. Principles for pipeline purging: (1) Purging should be carried out in sections during construction, with each section 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 proper gaskets should be replaced before the airtightness test. (6) During purging, gently tap the surface of the pipes and the welded areas. (7) The construction unit shall formulate a specific purging plan in accordance with the process flow of this station and the requirements specified in the design. 3. The qualification standard for purging is that the airflow at a speed of 20 m/s is directed 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. 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 port of liquid nitrogen tank trucks (flange connection on one end, quick-connect fitting on the other end). (4) Frost-resistant work clothes, insulated frost-resistant rubber shoes, and frost-resistant insulated gloves required for pre-cooling personnel. (5) Watches and record sheets required for pre-cooling, with records taken every 15 minutes. Pre-cooling preparation work (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 the root valves of all safety valves. Open the root valve for the gas phase vent of the storage tank. (4) The automatic protection system tests passed and is fully operational. 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. (In the case of a Lanxi combined differential pressure level gauge, adjust the valve to the middle position.) (6) Replace the air in the pipes with dry nitrogen to prevent condensation water at the valve from freezing it during pre-cooling. Pre-cooling principle: During pre-cooling, the temperature of storage tanks and pipelines should be reduced gradually to avoid rapid cooling, thereby preventing damage to equipment and fittings caused by sudden temperature drops. Based on relevant operational experience, a cooling rate of 50°C/h is considered safe. 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 contaminants 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) Open the inlet valve at the top of the storage tank and slowly introduce low-temperature nitrogen into the LNG storage tank. Once the pressure in the tank rises to 0.2 MPa, close the vapor valve of the tanker. After insulating the tank for 15 minutes, open the manual vent valve on the vapor line 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 inlet valve. Close the level gauge balance valve and activate the level gauge (if it is a Lanshi combined differential pressure level gauge, open the valve fully). (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, it is necessary to promptly close the valve at the unloading station and open the manual valve for the gas phase of the storage tank in order to release pressure. Repeat this operation. (3) Release gas through the fill valve; once the temperature reaches a certain level, slowly open the inlet valve at the bottom of the LNG storage tank to allow fluid to enter from both the top and bottom. During the liquid feeding 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 storage 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 (2–4 m3 of liquid nitrogen), and the liquid feeding is completed. (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 hose carefully, handling it with care; personnel should stay clear of it. (7) Close the manual vent valve for the gas phase in the storage tank, as well as the liquid inlet valve at the bottom of the storage tank. The inlet valve at the top of the storage tank should be closed only after the LNG pipeline unloaded from the vehicle returns to normal temperature. (8) Use the liquid nitrogen in the storage tank to pre-cool the booster, ambient air vaporizer, and their cryogenic pipelines. 3. Utilization of venting low-temperature nitrogen: When pre-cooling with liquid nitrogen, it is necessary to vent the low-temperature nitrogen through gas pipes. Safety precautions during pre-cooling: (1) In a confined space, liquid nitrogen absorbing external heat can cause a rapid increase in pressure; therefore, attention must be paid to the sequence in which valves are closed, to prevent the situation where cryogenic liquids become trapped. (2) Be sure to check for leaks at the hose connections, and people should stay away from this area. (3) Pay close attention to the increase in pressure in the pipelines and storage tanks. (4) Make sure to check whether the safety valve is frosted. (5) Inspection items during pre-cooling: 1) Check whether the low-temperature materials show any signs of low-temperature cracking. 2) Check the welded areas of low-temperature pipelines for cracks, especially at the flange welds. 3) Check the pipe cold shrinkage and changes in support from pipe supports. 4) Check the sealing performance and flexibility of low-temperature valves; check whether they are 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 detects the changes in the vacuum level of the storage tank before and after pre-cooling, thereby evaluating the performance of the tank. Method 2 for pre-cooling LNG storage tanks: ① Before pre-cooling, check whether all valves except the safety valve are in a closed state, and verify whether there is still a large amount of nitrogen remaining inside the LNG storage tank. ②Connect the unloading pipeline; first, use cooler gas to perform preliminary pre-cooling in order to purge the pipeline required for unloading. ③The cooler nitrogen from the liquid nitrogen tank is introduced into the LNG tank through the bottom of the storage tank, and the vent valve at the top of the LNG tank is opened to use nitrogen for preliminary pre-cooling of the tank. During this process, all threaded and flanged connections are inspected to check for any leaks. ④After the initial precooling of the LNG storage tank is completed, close the vapor-phase valve of the tanker truck to expel the nitrogen from the natural gas storage tank. Then, slowly open the liquid-phase valve to begin transferring liquid nitrogen into the LNG storage tank. When the pressure in the natural gas storage tank rises to 0.2–0.3 MPa, stop the liquid transfer and open the vent valve to discharge the relatively hot nitrogen from the tank. Once the pressure drops to 0.1 MPa, transfer more liquid nitrogen into the natural gas storage tank. This process is repeated several times until the rate of pressure increase in the natural gas storage tank slows down; at this point, the precooling of the LNG storage tank is complete. Pre-cooling with nitrogen: ① After the liquid nitrogen has been drained, open the pump’s return air valve and the LNG storage tank’s return air valve. Also open the drain valve of the pump chamber and use low-temperature nitrogen to pre-cool the pump chamber and its pipelines. During this process, check all the connections in the pre-cooled pipelines to ensure there are no leaks; if any are found, address them promptly. ②After the initial pre-cooling of the pump tank is completed, close the pump tank drain valve. Then open the valve between the pump tank and the refueling machine, as well as the drain valve of the refueling machine. Use cryogenic nitrogen to preliminarily pre-cool the refueling pipes and the refueling process. During this process, inspect all connections for any leaks; if any are found, address them promptly. ③After the liquid filling machine has been preliminarily pre-cooled and deflected, the liquid outlet valve of the natural gas storage tank, as well as the inlet and outlet valves of the pump chamber, are opened. Liquid nitrogen is then used to pre-cool the pump and the pump chamber, and all connections are checked again to ensure there are no leaks. ④After the pump has been pre-cooled, insert the filling gun of the liquid filler into the corresponding port and press the pre-cooling button to start the pump’s pre-cooling process; at the same time, check all the connections in the pipes for any signs of leakage. ⑤After the large-cycle pre-cooling of the liquid filler is completed, the liquid nitrogen and nitrogen in the pipelines and pump chambers are drained, completing the pre-cooling of the skid and the process pipelines of the liquid filler.