【Weekly Topic】Week 26, 2011——Air Separation Emergency Plan——Expander Trip
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Could everyone first briefly describe the process, and talk generally about what adjustments the air separation unit needs to make after an emergency shutdown of the expander?
We have a system in which expanded air is compressed externally and fed into the upper tower; a failure of the expander does not have a significant impact on the entire system, as long as the backup expander can be started as quickly as possible. Pay attention to the system pressure during the expander trip and the startup of the standby expander; it will be fine soon!
The amount of fluid that is manually and quickly passed through the check valve to the expander remains unchanged; this reduces the load on the air separation unit and lowers the liquid production volume. The cause is investigated, and reverse pouring is initiated.
Check whether the emergency shut-off valve is closed; manually close the adjustable nozzle and open the booster pump’s return valve. Start another set of expanders and adjust the amount of expansion air to meet the current operating conditions. Then identify the cause of the shutdown and address it. After resolving the issue, conduct a trial run, and once everything is fine, decide whether this repaired expander will be used as a backup unit or as a backup unit until the existing units are taken out of service. . .
The internal compression process is rather complicated: when the expander trips, Stage 1 of the booster stops operating, and Stage 2 stops as well. If there is high vibration, the booster shuts down and the entire system comes to a halt, requiring treatment in accordance with the procedures for a full shutdown of the air separation unit. If the booster does not trip, it is necessary to adjust the air compressor right away, as otherwise there will be significant air leakage from the system. It is also required to adjust the high-pressure plate to reduce the amount of oxygen and nitrogen present, and to close the valves for the product liquids (O, N) inside the tower ； Based on experience, without stopping the oxygen supply, the liquid in the main cooler and the upper column cannot be maintained for even an hour; therefore, it is necessary to promptly investigate the reason for the expansion machine to shut down, while also making preparations on site (mainly by reducing pressure, bringing the pressure at the pressurization end to the same level as the suction pressure in the expansion machine) ； The adjustment strategies inside the tower are similar; for the argon tower, it is possible to reduce the amount of liquid air and the volume of crude argon extracted, among other things
Reply to 1# gader: First, open the valve for pressure equalization by 5-10% to build up pressure in the lower column; do not open the valve leading to the upper column. If the pressure becomes too high, release nitrogen from the upper column through the corresponding valve. When the temperature is low, adjust it by opening the high-pressure air throttle valve. As soon as the pressure in the lower column approaches normal levels, start the expansion machine for cooling – the expansion machine operates more efficiently in a cold state, and after mixing with the hot air entering the lower column, the temperature drops. At this point, gradually open the valve leading to the upper column to prevent overpressure; the pressure will remain low. With proper operation, the pressure in the upper column will not exceed 42 KPA throughout the process. In my experience, after an emergency shutdown, the liquid level in the main cooler is around 4.5 M, while the liquid level in the lower column does not exceed 1.1 M. Starting up in a cold state, it takes no more than two hours from gas introduction to the production of qualified oxygen and nitrogen. The valve for releasing waste nitrogen from the low-pressure heat exchanger must be set to automatic mode, so that the pressure in the upper column can be adjusted promptly! Valves that can and must operate automatically should indeed be set to automatic mode; relying on operators to make adjustments is unrealistic – humans are not omnipotent, and it’s impossible for them to monitor all the screens every second!
Since the expanded gas accounts for about 10% of the air used in processing, the pressure of the air compressor should be controlled first. Another thing is to reduce oxygen supply and maintain the pressure in the upper tower; most importantly, it’s necessary to start the backup unit as soon as possible.
The external compression process is relatively easy to handle: adjust the air compressor and maintain the pressure in the upper tower, while also paying attention to the performance of the plate heat exchanger. Activate the backup expansion machine as soon as possible; if none is available, identify the cause of the shutdown and resolve it promptly before restarting the system. The internal compression process is rather complicated, mainly due to the control of the pressure booster and the plate-type cooling system. It would be better if the air compressor could operate automatically; if not, manual adjustment is required. At the same time, it is necessary to promptly investigate the reasons for the expansion machine to shut down, and make the necessary preparations on site ； During this period, close the liquid product discharge valve to reduce heat loss inside the tower
1. In the internal compression mode, the expander shuts down after one cycle, and the booster is unloaded in its second stage; if vibration is high, the backup expander is activated. 2. When the expander shuts down after one cycle, it is necessary to add a high-pressure liquid air valve. 3. Reduce the expansion volume.