Air separation plant shutdown plan

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A company plans to shut down its production system for a major overhaul starting July 1, 2022. To ensure a safe and smooth shutdown of the gas production area, a shutdown plan for the air separation unit has been formulated. General requirements for air separation unit shutdown: 1. Operate strictly in accordance with the shutdown plan, operating procedures, and shutdown operation tickets; prior to shutdown, conduct thorough inspections and verifications using valve confirmation sheets and similar documents. 2. Organize meticulously, clarify respective responsibilities according to the division of labor, strictly enforce the dispatch order system, and conduct repeated checks and final confirmations prior to shutdown. 3. Operate with care, strictly enforce and monitor the control of various process parameters to prevent overheating, overpressure, dry cooking, etc., and shut down the system in reverse order according to the process flow. 4. Throughout the parking process, strict control must be exercised over key areas, and regular inspections should be carried out to identify problems early. A two-person operation and supervision system, with one person operating and another monitoring, should be implemented to ensure safety. 5. The site is clean and tidy, with proper lighting, a functional fire protection system, and adequate safety facilities. 6. After parking, thoroughly check the installation of all blind flanges; the blind flange diagram and shutdown plan should be posted. 7. Before the system is shut down, the Safety and Environment team shall report to the park management committee in advance to ensure proper communication. One day before shutdown, an application for abnormal emissions must be submitted; during the shutdown process, emissions must be strictly controlled to minimize the impact on the surrounding environment. yunrun.com.cn/tech/4350.html Organizational Structure and Command System: In accordance with the principle of unity of duties, responsibilities, and powers, as well as the rule that those in charge are responsible for their tasks, a parking organization structure and command system are established. Items to be confirmed before shutting down the air separation unit: 1. The mechanical, electrical, and instrumentation systems of each individual system are ready for shutdown. 2. All confirmation tasks must be signed in writing and records must be kept. 3. During the system shutdown and replacement period, no work such as working at heights, hot work, confined space operations, or contractor activities are permitted on site; unauthorized personnel are not allowed to remain inside the facility. Public utility conditions required before shutting down the air separation unit: 1. External power supply is normal, and the internal substation is operating properly. 2. The air compressor station is operating normally, and the supply of instrument air and plant air is satisfactory. 3. The primary water, circulating water, demineralized water, and boiler feed water throughout the plant are operating normally. 4. The nitrogen pipeline system is operating normally: the nitrogen gasification compressors and the liquid nitrogen vaporizers in the air separation unit are ready for use, enabling the supply of nitrogen at different pressure levels. Air separation unit shutdown steps: 1. Air separation unit shutdown: ① Reduce the expansion amount of the expander in advance to consume the level of the main cooling liquid. ②Notify the dispatch to prepare for parking and to prepare to switch to instrument air. ③Open the nitrogen vent valve FCV23305 and the oxygen vent valve PCV23302, close the nitrogen and oxygen delivery valves FCV23303 and FCV23302, stop the outward transfer of the product, and reduce the load on the liquid oxygen pump. ④Open the booster return valves BV24212 and BV24231. ⑤Shut down the pressurized turbine expansion unit C2301. ⑥Shut down the high-pressure liquid oxygen pump ； Liquid argon pump, argon shutoff system. ⑦Discharge liquid. ⑧Open the purge valve on the pipeline inside the cold box (as appropriate). ⑨Heat the device ； After the device is heated to room temperature and the dew point analysis shows it is within acceptable limits, preparation for shutdown begins. ⑩Switch the instrument air system to the backup pipeline. ⑪Open the vent valve BV24121 on the outlet pipe of the air turbine compressor. ⑫Shut down the air pre-cooling system. ⑬Shut down the molecular sieve purification system and record the shutdown status of the molecular sieve program. ⑭Shut down the air compressor unit (in accordance with the operating procedures for compressor units). ⑮Close all valves other than those mentioned above. ⑯When shutting down the pressurized turbine expansion unit and opening the return valve of the pressurized compression unit, close attention should be paid to the outlet flow rate of the air turbine compressor; if necessary, the compressor’s vent valve should be opened slightly to prevent surging in the compressor. 2. Shutdown of the air compressor unit
① Reducing the compressor load
a. Notify the downstream systems in the air separation unit to prepare for gas venting. As the argon system, liquid oxygen pump, and turboexpander in the air separation unit gradually reduce their loads, promptly adjust the anti-surge valves at each stage of the booster compressor. When the reflux valve of the liquid oxygen pump and the reflux valve on the boosting side of the booster turboexpander are both fully open, the gas supply valves for Stage 3 and the intermediate extraction gas supply valve for Stage 2 can be closed, while the anti-surge valves for Stages 2 and 3 should be opened wider. b. When the booster compressor is reducing its load and the low-pressure main heat exchanger of the air separation unit is being purged, the air compressor’s vent valve is opened or the inlet guide vanes are partially closed to maintain an outlet pressure of 0.517 MPa. After the air regulating valve leading to the low-pressure main heat exchanger is completely closed, the air compressor continues to vent while maintaining pressure. Once the molecular sieve system and pre-cooling system are taken out of service, HV23101 is closed; thereafter, the inlet guide vanes of the air compressor are gradually closed to 10%, and the anti-surge valve of the air compressor is fully opened. c. During the compressor’s load reduction process, if the steam pressure is too high, i.e., greater than 8.83 MPa, slightly open the high-pressure steam vent valve. ②Shutting down the turbine: a. Directly press the shutdown soft switch on the central control DCS to stop the turbine, and verify that both the quick-shut valve and the control valve are closed. Confirm that the jacking oil pump starts automatically when the speed drops to 400 rpm. b. Pay attention to monitoring changes in the level of liquid in the hot well, replenish deionized water into it as needed, and open all drain valves and warming valve outlets on the steam pipeline. c. Start the turning gear: If the jacking oil pump does not have an automatic function, it should be started first. When the unit speed dropped to zero, it was confirmed on site that the rotor had stopped rotating; then the barring motor was started on site to verify that the barring function was working properly. d. Stop the main pump: First, close the air valve of the first-stage pump, then close the steam valve of the first stage ； Then close the air valve of the second-stage vacuum pump, and close the second-stage steam valve. Once the vacuum has dropped to zero, close the shaft seal steam valve and isolate the shaft seal control valve. e. Close the main steam electric isolation main valve (HV24003). f. After the exhaust pump has been shut down for 1–2 hours, disable the interlock associated with the standby condensate pump, stop the condensate water pump, close the control valve as well as the shut-off valve, and ensure that the bypass valve is also closed. It is strictly prohibited for water from the condensate system to flow back into the condenser due to leaks in the check valves. g. Oil shutdown system: Ensure that the temperature of the turbine exhaust cylinder is below 50°C, stop the disk brake, shut down the top shaft oil pump, disable the oil pump interlock, stop the main oil pump, and stop the oil mist fan after 20 minutes. Disengage the automatic control interlock of the fuel tank electric heater; turn off the heater in case of power outage. h. Strictly follow the items and requirements specified in the maintenance work order to disconnect and isolate the equipment that needs maintenance. Specific shutdown times for the air separation unit: 1. After confirming that nitrogen supply for gasification has been stopped, the shutdown time for the air separation unit is 2 hours. 2. Time required to drain liquid from the air separation unit: 12 hours. 3. Time needed to warm up the cold box: 12 hours. 4. Time required to shut down the entire unit: 2 hours. 5. Time needed for the turbine to cool down: 48 hours. 6. Time required for further cooling after removing the insulation from the turbine: 24 hours. 7. In the event of a smooth shutdown of the air separation unit, the total time is approximately 100 hours