Seeking advice: How should the air cooling and circulating water systems be managed during the winter shutdown of atmospheric and vacuum distillation units?
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Our facility is a refinery in the north, with the main unit being a vacuum distillation plant. We plan to shut down during this Spring Festival. The planned shutdown procedure involves first using crude oil to flush out all the heavy oil pipelines, and then replacing the crude oil with DST0# fuel oil; this way, there is no need for line cleaning. When operations resume next year, the crude oil will displace the DST0# fuel oil, and that will be sufficient. However, since all the process pipelines will be filled with fuel oil, how should we handle the wet air coolers at the top of the atmospheric tower (currently only the fans are in use, while the water pumps were stopped back in November) to prevent freezing? What precautions should also be taken regarding the circulating water system in the utility systems? I hope experienced experts can provide some guidance
To prevent freezing, we use unpurified air or nitrogen to blow out all the water in the circulating water system; this is done in stages, device by device, to avoid any remaining water. The circulating water pump is usually used to drain all the water remaining in it, and the fresh water in the utility steam lines must also be completely purged.
Reply to 2# aa78564721: We plan to divert the circulating water to a bypass line before dealing with the oil water coolers on each side, and then use compressed air to remove any remaining water inside the equipment. However, there is one issue: the outside temperature is very low (–16°C), and as the cleaning process progresses, the tiny amounts of water remaining inside the equipment may freeze due to the low temperature of the compressed air, making it impossible to remove them. Additionally, the largest volume of circulating water used in the vacuum distillation process is for cooling the water coolers in that system; these pipes are quite large, with a diameter of DN250, so it seems not suitable to use compressed air for cleaning them
It should work if it’s possible to ensure that there are no leaks at the valves at either end, but the branch lines all have three-way welds; if these three-way connections are long, they create dead zones where freezing can occur locally. Generally, if the temperature is too low, it’s advisable to use steam for drying first; once it’s almost dry, then switch to air drying. It’s okay if there’s a little water inside – just keep the drain open. Equipment that gets damaged due to freezing usually does so because water freezes inside and causes expansion, leading to damage. For DN250 pipes that are underground, blowing air at them is sufficient; even if they freeze, they won’t get damaged. For pipes that are above ground, drain the water from the lowest points first before blowing air, and disconnect some flanges if necessary. Don’t turn off things like the reverse flushing; keep them on all the time.