Brief description of air separation operation problems and solutions 3(3)

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1. What are the precautions for the use and maintenance of oil coolers? Precautions during use: l) Check whether the connections to the system are secure, and inspect all attachments and monitoring instruments. 2) Before use, exhaust treatment should be carried out: loosen the plugs on the front cover (or rear cover) and the housing, slowly introduce water and oil, and then tighten the plugs once water or oil begins to overflow from them respectively. 3) First, introduce the cold medium water, and then add the hot medium oil; otherwise, thermal stress will be generated. 4) Coolant should be added slowly when introducing it; it is imperative to avoid adding a large amount at once to prevent the formation of a \"supercooled layer\" inside the pipes. Reduce heat transfer efficiency. 5) Adjust the amount of cooling water supplied to maintain the oil outlet temperature at the desired level. 6) The pressure on the oil side must be greater than the pressure on the water side. During maintenance, the following should be noted: l) Purified fresh water should be used for cooling water. 2) To prevent scaling and improve the heat transfer coefficient, the cooling water temperature should be as low as possible, and the water flow rate should be as high as possible. 3) In cold seasons, to prevent the heat transfer tubes of the cooler from freezing and cracking, it is necessary to drain the remaining oil and water from the cooler when it is not in use. 4) When out of use for a long period, fill the oil side with nitrogen at (0.02–0.04 MPa). 5) After long-term operation, dirt accumulates on the walls of the heat transfer tubes, which reduces their heat exchange efficiency; therefore, it is necessary to shut down the system periodically for cleaning. The cleaning interval depends on the quality of the water, with cleaning generally being carried out every 6 to 12 months. For cleaning, the water side is washed with clean, high-pressure water. The front and rear covers as well as the inner walls of the heat transfer tubes can also be cleaned mechanically, after which they are dried using compressed air ； The oil side is rinsed with a trichloroethylene solution at a pressure of less than 0.5 MPa; the flow direction of the solution is opposite to that of the oil. It is then rinsed with water, and finally dried using compressed air. After cleaning, a gas-tightness test must be conducted; it can only be put into use after a hydrostatic test. 2. How to carry out bare cooling work after the installation or repair of air separation equipment? After installation or repair, the air separation equipment should undergo a full-scale bare cooling process. Purpose of bare cooling: To check the quality of installation or repair of air separation equipment ； Cold deformation and compensation capacity of air separation units at low temperatures ； Are the equipment pipelines unobstructed and can they function properly? Preparations before cold shutdown: Have all unit equipment undergone multiple trial runs successfully? ； The instrumentation and electrical control systems are operating normally. Start the air separation unit according to the normal startup procedure, so that all the equipment and main pipelines inside the cryogenic tank are cooled, resulting in frost layers of varying thicknesses forming on them. The measurement points meet the technical requirements; in summer, the temperature at each reference point may be 5–10°C higher. During bare cooling, close the valve of the liquid hydrogen destillation and storage system. Working after cold exposure: Tighten the bolts at low temperatures quickly ； Remove frost in a timely manner ； After heating, inflate it to check for airtightness. Before the overall cold test, the fractionation tower should be thoroughly heated and purged. The air turbine compressor, turbine expander, air pre-cooling system, molecular sieve purification system (or switching system), and electrical and instrumentation control systems must be ready for operation. Procedure for the cold test: l) During the cold test, the main equipment such as the distillation tower and condensation evaporator should be cooled to the lowest possible temperature, so that frost forms on all containers, pipes, and their outer surfaces inside the cold box; this condition should be maintained for at least 2 hours. 2) During the cold test, personnel should be sent into the cryostat (wearing cold-protective hats, gloves, insulated clothing, etc.) to carefully inspect the leakage areas and mark them. For cold valves and low-temperature film control valves, special tools can be used to tighten the nuts, and care should be taken to ensure that the valves are not stuck. 3) After the cold test is completed (natural reheating is also possible, as long as the manhole is opened), a gas-tightness test should be conducted (a dynamic test is also an option). Therefore, all bolts, flange connections, valves, and other components need to be tightened again. Special care must be taken to ensure that the valve is not in a fully closed position during tightening. The airtightness test pressure is the same as the operating pressure. Generally, the overall cold test should be conducted twice. Based on the leakage and handling conditions during the test, it is decided on-site whether a further cold test is necessary. 3. How to clean and degrease? All pressure vessels, valves, and pipes of air separation equipment (that come into contact with oxygen or cryogenic media), as well as the areas surrounding these pipes, must be clean, dry, and free from oil contamination before installation. Those that have already been degreased by the manufacturer and are not contaminated need not be degreased again during installation. If contaminated by grease, degreasing treatment should be carried out. For the degreasing of aluminum components (pressure vessels, valves, pipelines), the use of carbon tetrachloride (CCl4) as a solvent is strictly prohibited; perchloroethylene or trichloroethylene solvents are recommended. The solvent must be oil-free and degreased. Decomposed solutions are not allowed to be used. Organic solvents are not suitable for components with rubber, plastic, or organic coatings; they are toxic and flammable, so safety precautions must be taken when using them. In addition to the aforementioned degreasing solutions, other methods such as alkali washing can also be used. However, an excessively high concentration of the alkaline solution can cause metal corrosion, especially in materials such as aluminum and magnesium. For parts made of aluminum or magnesium, the pH level should be kept at or below 10; the cleaning temperature should be between 70 and 80°C. After cleaning with an alkaline solution, the parts must be rinsed thoroughly with water until no residual alkalinity remains, followed by drying. After degreasing the pipeline, it should be welded within 24 hours, and secondary contamination must be strictly prevented. Whether the degreasing has been thorough can be visually and qualitatively checked using an ultraviolet lamp, or it can be examined by other methods. All components in contact with oxygen outside the cold box, such as carbon steel oxygen pipes and valves, must undergo thorough rust removal and degreasing before installation and use. Sandblasting (using only quartz sand), acid washing for rust removal, or carbon tetrachloride along with other effective non-flammable detergents can be used for this purpose. After rust removal and degreasing, the pipes should be passivated immediately or filled with dry nitrogen. Before installing and using the oxygen pipeline, the residues inside the pipeline should be blown clean using oil-free dry air or nitrogen, until no rust, dust, or other contaminants remain. The blowing speed should be greater than 20 m/s. It is strictly prohibited to use oxygen to blow clean pipes. On the surfaces of parts that come into contact with oxygen, as well as on those surfaces where residual oil from operation may introduce oxygen, the amount of grease residue must not exceed 125 mg/m2. 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