The normal planned shutdown of the cryogenic air separation unit follows the principle of "reverse process operation, load reduction first and then shutdown, and safety priority", which is divided into six stages, and the whole process is strictly controlled to eliminate equipment damage and potential safety hazards.
Prior to shutdown, the downstream gas user shall be notified in advance to confirm the product pipe network switching scheme and prepare the emergency gas source.
The first step is to reduce the load and shut down the product system: gradually reduce the output of oxygen, nitrogen and argon, close the main product delivery valve, fully open the product vent valve to vent locally, and synchronously reduce the load of liquid oxygen pump and liquid nitrogen pump until shutdown, so as to avoid sudden changes in pipe network pressure.
The second step is to shut down the expansion unit by reducing the cooling capacity: gradually turn down the nozzle, open the return valve to reduce the load, operate strictly according to the speed reduction curve, shut down after the speed drops to a safe value, close the inlet and outlet valves, and shut down the oil pump after the shaft temperature returns to normal temperature. The booster shall be unloaded by fully opening the anti surge valve synchronously, and it is strictly prohibited to stop with load.
The third step is liquid withdrawal and pressure relief in the distillation system: slowly discharge the liquid oxygen in the main cooling system and the liquid in the upper and lower towers, strictly control the liquid discharge speed, and prevent the high-speed vaporization of liquid from causing water hammer impact on the equipment. Open all purge valves and vent valves, slowly relieve the pressure of the upper and lower towers to normal pressure, continuously monitor the hydrocarbon content of the main cooling throughout the whole process, and completely discharge the excessive hydrocarbon.
Step 4: stop the molecular sieve and precooling system: stop the switching procedure, stop the electric heater after pressure relief and cold blowing of the adsorber, and close the main inlet and outlet valves of the purifier. Then shut down the chilled water pump, chiller and normal temperature water pump in turn to empty the liquid level of the air cooling tower and water cooling tower.
Step 5 stop the compressor unit: the air compressor fully opens the air valve to unload, stops after running for a period of time without load, and continues to run the cooling water for half an hour until the oil temperature drops.
Finally, the storage mode is selected according to the parking time: short-term parking, filling dry nitrogen with micro positive pressure to keep cold; After a long-term shutdown, a comprehensive heating and thawing shall be carried out, and the heating rate shall be ≤ 10 ℃/h. All measuring points shall be sealed and sealed with nitrogen after reaching normal temperature, so as to ensure the safety and reliability of the next start-up.
