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KDON2400/1000 Air Separation Unit: Short-Term Temporary (Fault) Shutdown, Cooling-Down Before Restart – Procedures and Precautions I. Shutdown Procedure: 1. Upon receiving the instruction for a short-term temporary shutdown, immediately inform all operators on duty to get ready for the shutdown； 2. Notify to start the instrument compressor and switch the instrument air source ； 3. The liquid oxygen and nitrogen products are vaporized and pressure-regulated before being supplied to the network; the compressors used for these products are stopped and the product gas is vented ； 4. Stop the operation of the expander and fully close the inlet and outlet main valves of the expander ； 5. Switch the regeneration gas source for the purification system (preheat the adsorber in advance for use) ； 6. Slowly close the main air inlet valve V1224 to the tower; release air from the air compressor (coordinate with the air compressor operator to prevent overpressure in the compressor) ； 7. Quickly close the nitrogen-oxygen product outlet valve, the vent valve, as well as all the manual low-temperature valves on V1, V2, and the cryogenic tank, in order to prevent loss of cold energy inside the tower. Monitor the pressure in the tower carefully, and control it using V1232 to avoid overpressure in the tower ； 8. Stop the program controller of the purification system, and manually close each switching butterfly valve in a timely manner to maintain a certain pressure in the two purifiers, thereby preventing wet air from entering; record the current operating status of the purifiers ； 9. Shut down all the normal-temperature and low-temperature water pumps in the pre-cooling system, as well as the chiller units, and promptly drain any excess water from the air-cooled towers, water-cooled towers, and pipelines ； 10. Shut down V1101, fully open the manual vent valve of the air compressor, and operate the air compressor at a reduced pressure and lower load. II. To reduce the time required to restart the unit after a short-term shutdown, cooling maintenance procedures should be carried out immediately following the shutdown. The steps for these procedures are as follows: 1. Start the booster for the liquid nitrogen storage tank; when the pressure inside the tower is less than 2 KP, use the low-temperature gas generated by vaporization in the storage tank to slowly refill the tower, maintaining a pressure of around 5 KP within the tower. This prevents negative pressure from forming, which could cause moisture from the air to be drawn in and lead to blockages in the tower trays, thereby affecting restart ； 2. Regularly check whether the pipes of the main heat exchanger at normal temperature are sweating or frosted ； 3. Check the temperature at the bottom of the main heat exchanger for air separation, to determine the correct startup procedure ； 4. When the main cooling liquid level is below 20%, the liquid must be completely drained before starting up the system ； 5. Air must be introduced into the air separation unit slowly, in order to prevent an excessive amount of air from entering and causing the level of the primary cooling liquid to evaporate rapidly. Especially when the liquid level is low, this can lead to the accumulation and concentration of C-H compounds, thereby affecting the safe operation of the primary cooler ； 6. Carry out the cold start in accordance with the operating procedures. During the start-up process, pay attention to the interrelationships among various thermal parameters inside the tower, make rational use of the cooling capacity, and strictly control the temperature in the middle section of the main heat exchanger. III. During the parking period, maintenance and routine upkeep of various equipment are carried out depending on the length of time it has been parked. For all moving equipment, it is necessary to turn them several times per shift to ensure that the transmission mechanisms remain flexible and to prevent rust and jamming. IV. Steps for cooling-down and restarting an air separation unit after a short-term shutdown: 1. Upon receiving the start-up instruction, check the instruments, lighting signals, and ensure that all control valves are functioning properly ； 2. Start the gauge compressor to supply air ； 3. Slowly increase the discharge pressure of the air compressor to 0.5 MPa, then slowly open V1101 to allow air to flow into the air cooling tower. Once the pressure in the air cooling tower reaches 0.5 MPa, open V1101 fully. 4. Start the pre-cooling system ； 5. Start the purification system (after startup, open the expander seal gas outlet valve and adjust it to 0.25 MPa) ； 6. Start the oil pump of the expansion unit, and check and adjust the oil temperature, oil pressure, and oil level ； 7. Turn on V1224 and slowly introduce gas into the distillation tower; at this time, pay attention to fluctuations in air compressor pressure to prevent excessive pressure swings that could damage the molecular sieve adsorber bed as well as the distillation tower trays ； 8. Start the expansion unit ； 9. Fully utilize the cooling capacity of the expansion mechanism: increase the setting of V448 and decrease that of V447. Take into account the relationship between high temperature and high enthalpy drop on one hand, and the quality of the cooling capacity on the other; in the final stage, aim to ensure that no liquid is produced at the expansion machine’s outlet, thereby minimizing the temperature at its outlet as much as possible ； 10. Accelerate the accumulation of primary cooling liquid and adjust production conditions ； 11. Once the main cooling liquid level stabilizes and the oxygen and nitrogen products meet the required standards and are ready for production, the oxygen and nitrogen compressors are started to supply gas to the users. V. Precautions for operations at various startup stages during the short-term shutdown, cooling, and restart process of air separation units: 1. Precautions for starting the expansion oil pump: A. The oil level in the tank must reach 800 mmH2O, and the oil temperature should be ≥25°C; otherwise, the electric heater needs to be activated ； B. Test the oil pump pressure interlock device to ensure its reliability ； C. The sealing air for the expansion unit must be supplied properly, with a pressure of 0.25 MPa. 2. Precautions for starting the pre-cooling system: A. The V1138 pressure-controlled return water automatic control valve should not be activated while air is being introduced into the air-cooling tower; the liquid level in the air-cooling tower within the system must not be at “0”, to prevent air from entering the circulating water system and affecting its operation ； B. The water pump may have difficulty starting when there is air backpressure; in such cases, the start valve can be closed to eliminate the effect of this backpressure as well as any residual air in the pipes. After rotating the pump manually, it can be started, and once pressure is built up, the outlet valve should be opened slowly to prevent a rapid drop in pressure ； C. During the initial operation phase, the system experiences significant fluctuations; during this time, the level interlocks for the various control automatic regulating valves can be temporarily disabled. These interlocks can be reactivated once the system is operating normally. Throughout this process, the DCS operator is responsible for monitoring the system’s conditions ； D. During the initial stage, it is difficult to control the liquid level in the water cooling tower, and this is due to the lagging effect of the packed tower. Manual control can be used to stabilize the level at the design value, after which automatic control can be activated to adjust the setting value of V1111 ； E. To start the chiller, it is necessary to power on the chiller and heat the compressor oil before starting it. 3. Precautions when starting the purification system: A. The system cannot be switched into operation directly; excessive movement may cause air currents to impact the molecular sieve bed and lead to water contamination in the molecular sieve ； B. The electric heater should be controlled in manual mode. Ensure that gas is supplied first and electricity is provided afterward; cut off power first and then gas, to avoid damaging the electrothermal generator ； C. The timing for switching the molecular sieve purifier control mode from manual to automatic should be chosen during the heating and cooling phases, when valve operations are minimal. This should be done in coordination with the instrumentation staff to ensure that the current operating status matches the program control steps, and no mistakes should occur ； D. The automatic switching time can be appropriately adjusted according to the charging, depressurizing, heating, and cooling times, based on the actual conditions at the site ； E. At startup, check whether the opening positions of the inlet and outlet valves of the molecular sieve purifier are correct; if not, make adjustments. The valves should operate slowly. 4. When introducing gas into the distillation tower, the following points should be noted: A. To ensure the safety of the expander, it is necessary to check whether the supply of seal gas to the expander is proper, adjust the pressure reducing valve so that the seal gas meets the required standards, start the oil pump system for the expander, and adjust the oil pressure to meet the required levels ； B. Slowly open V1224 to introduce gas into the fractionation tower; at this time, pay attention to fluctuations in the pressure of the air compressor, in order to prevent excessive pressure swings that could damage the molecular sieve adsorber bed as well as the tray sheets in the fractionation tower ； C. The adjustment of V1 and V2 should be done slowly to prevent an excessive amount of air intake, which could lead to the accumulation and evaporation of primary coolant and thus cause fluctuations. 5. Precautions for starting the expansion unit: A. Before starting, the purge valve should be opened to perform intermittent purging; if water is present, it must be removed by heating to prevent water from freezing and blocking the pipes and nozzles, which could reduce the unit’s operational efficiency ； B. Before starting up, attention should be paid to the liquid-air level in the fractionation tower, to prevent this level from rising above the bubble cap trays in the lower tower; otherwise, liquid may enter the air pipelines of the recirculation system and the expander. If liquid enters the expander, the inlet valve of the expander should be closed immediately, and the liquid-air control valve should be opened wide to allow the liquid to flow quickly into the upper tower. Once the temperature in the middle section of the main heat exchanger returns to normal, the operation of the expander can be resumed ； C. When starting the expansion unit, it is necessary to check for changes in the resistance of the suction filter of the brake fan, in order to prevent the fan side of the unit from entering the surge zone when adjusting the expansion amount ； D. When fully utilizing the cooling capacity of expansion refrigeration, it is necessary to control the temperature before the machine properly to prevent liquid slugging accidents caused by excessively low temperatures after the machine. 6. The following points should be taken into account regarding the accumulation of primary cooling liquid and adjustments to operating conditions: A. As liquid accumulates, a large amount of air gets liquefied, resulting in an increasing volume of gas entering the tower; therefore, it is necessary to maintain the pressure supplied by the air compressor according to changes in operating conditions, while gradually reducing the amount of gas released ； B. Regularly open V1253 to check whether there is free water in the air purification system and blow it out promptly ； C. To accumulate liquid more quickly, it is necessary to prevent an increase in the temperature difference at the hot side of the main heat exchanger; for this reason, the temperature in the middle section of the main heat exchanger must not be too low, and it should not drop below –120 °C, with the normal operating range being –116 °C ℃ ； D. The CO2 content in the air entering the molecular sieve adsorber shall not exceed 2×10-6 (2 ppm). If the CO2 level is too high, it is possible to increase the volume of nitrogen gas used for cleaning, raise the temperature of the regenerating gas, or adjust the switching cycle ； E. When the main cooling liquid level approaches 2600 mmH2O, shut down one expander, fully close the inlet and outlet valves of that stopped expander, and promptly warm it using dry air ； F. When the liquid oxygen adsorber is started up for pre-cooling, attention should be paid to its pressure changes; it must not rise sharply to prevent overpressure in the upper tower. VI. Safe operating procedures after startup: 1. C-H compounds in the main cooling liquid oxygen must be strictly controlled, and 1% of the liquid oxygen must be discharged at the end of each shift ； 2. The level of the main cooling liquid must be strictly maintained within the normal range of 2200–2600 mmH2O; it should neither be too low nor too high ； 3. When the content of C-H compounds in liquid oxygen is too high, the following measures should be taken: (1) Increase the discharge volume of liquid oxygen ； (2) Identify the cause of the increase and take measures to eliminate it ； (3) Check whether the molecular sieve purifier is operating properly ； (4) If, after taking measures, the acetylene or hydrocarbon content still increases and reaches the shutdown limit of 1×10-6, the equipment should be shut down immediately, the liquid should be discharged, and the equipment should be heated for purging. 4. To prevent humid air from entering the cryostat and hazardous gases from accumulating within it, the cryostat must be filled with dry air under pressure; this air comes from the air after purification and is fed into the cryostat via V203 ； 5. When the oxygen purity of the product decreases, it is necessary to analyze the difference in purity between liquid oxygen and gaseous oxygen in order to determine whether there is a leak in the main cooling system; a purity difference of greater than 5% indicates a possible leak ； 6. Regularly inspect the purification system to check whether the specified temperature is reached during regeneration and cold blowing, and analyze the trends of various parameters of the purification system to ensure that the heating regeneration temperature curve is smooth, without multiple peaks or flat sections ； 7. Regularly check V1253 to determine whether there is water in the air entering the molecular sieve purifier, in order to prevent water from entering the adsorber and ensure the safety of both the adsorber and the distillation tower ； 8. Valves used in low-temperature applications and those on oxygen pipelines must be free of oil and grease; degreasing should be carried out when maintaining or servicing these valves, and contact with oil and grease is strictly prohibited ； 9. Conduct thorough inspections of all operating equipment and systems, and keep detailed records ； 10. Carefully record the operating parameters of various equipment and systems, ensuring neat handwriting and accurate data, without any falsification.