The vacuum level fluctuates under reduced pressure; it’s not possible to achieve a sufficient vacuum level. Please advise on the reasons
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This post was last edited by dongliangsir on 2010-5-26 at 12:39. A vacuum distillation unit put into use in 1995. The gas from the degasification process, after passing through the water cooler, enters the primary vacuum pump; the gas from the output of the primary vacuum pump, after passing through another water cooler, enters the secondary vacuum pump, and the gas that emerges after passing through that water cooler is discharged. In March, fluctuations occurred in the vacuum level under reduced pressure, ranging from 4 KPa during normal production to 9 KPa. By adjusting the evacuator, it was possible to bring this level back to 4 KPa. However, the temperature at the top of the column fluctuated frequently, which in turn caused fluctuations in the vacuum level there as well. The primary evacuators consist of two sets arranged in parallel. Since fluctuations in vacuum level occurred, vapor leakage has started to occur in the non-condensable gas inlet line of the primary evacuators; this vapor enters the dehydration unit of the primary condenser. In the primary evacuators, one set always experiences vapor leakage while the other operates normally. By adjusting the amount of vapor supplied to the evacuators, the situation of vapor leakage can be reversed – the evacuator that was functioning normally before now experiences vapor leakage, while the one that had vapor leakage before now operates normally again. After the fluctuations in vacuum level occurred, the atmospheric leg pipeline was inspected, and no blockages or leaks were found ； The evacuator was inspected, and it was found to be loose; after tightening it, it was reinstalled ； The pressure relief system was inspected for leaks, and no leaks were found (the oxygen content in the gas coming from the pressure reduction process fluctuates significantly; it is not yet possible to determine whether there are leaks based on analysis, so further analysis will be conducted to make a judgment). We earnestly request experts with relevant experience to help diagnose the cause of this phenomenon and advise on how to address it. (After the evacuation pumps have been repaired and reinstalled, it is not possible to achieve a low vacuum level; in the first stage, one of the two sets of evacuation pumps still allows steam to leak through.)
There are quite a few factors that can affect the vacuum level; you can search for “vacuum level” on the forum to take a look. If there is air leakage when switching the pumps, it seems that the problem isn’t with the pumps themselves. Could it be due to high back pressure? Check the amount of non-condensable gas and the cooling conditions as well. The operation of the tower, such as the liquid level at the bottom and the temperature at the top, also needs to remain stable. In atmospheric-pressure towers, it’s advisable to remove more liquid first in order to reduce the load on subsequent stages. For vacuum towers, stop any steam injection processes first.
Consider checking the amount of non-condensable gas removed from the top of the column, thinking about increasing the draw rate of the atmospheric pressure tower, reducing the amount of material cracked in the vacuum furnace, and lowering the vacuum pumping load required
Reply to 2# dongliangsir: The amount of gas released from the venting process is very small. Under normal conditions, when the gas from venting is directed to the exhaust system, there is a slight odor in most parts of the facility; but now almost no such gas can be detected. Moreover, the amount of oil recovered by reducing the pressure also decreased from 4–5 tons per hour during normal periods to around 1 ton per hour. The temperature fluctuation at the top reduction is significant. During normal operation, the top reduction temperature is maintained between 100°C and 110°C; once fluctuations in vacuum level occur, this temperature can be reduced down to around 40°C. Later, by keeping the top reduction temperature above 120°C, the fluctuations in vacuum level were slightly reduced. Last week, the pump was checked for fluctuations in vacuum level; since switching to crude oil on Saturday, the top vacuum level has not been able to reach the desired level – it’s best to achieve -0.065 MP.
The bottom temperature should be controlled at a high level; generally, it’s between 70 and 100. What about the steam pressure and the cooling condition of the condenser? Try increasing the steam pressure, haha. The load within the tower needs to be uniform, so as to avoid excessive local pressure drops. If there are no issues with the vacuum pump nozzles, the possible reasons for air leakage are: 1. Several units operating in parallel at the same stage have different pumping capacities. 2. The temperature after cooling is high, resulting in an excessive inlet load that exceeds the designed compression ratio. 3. The evacuation pipeline at the last stage is not functioning properly, leading to increased pressure in the rear section.
During this maintenance work, all the flange surfaces of the vacuum pump were opened. During installation, the maintenance team was unable to achieve a concentricity of 0.5–2 mm for the vacuum pump; as a result, it is uncertain whether the vacuum pump installed by the team can function properly, and whether the issues of steam leakage and insufficient pumping capacity are caused by problems with the pump’s performance. The final cooler and the discharge line have been purged; they are definitely unobstructed ； New water is used for cooling; the temperature after cooling in the primary cooler is between 20~30°C, while that in the secondary and tertiary coolers is between 40~50°C. These temperatures remain at this level during normal operation as well. All of these coolers were recently overhauled last September ； As for the reason why insufficient parallel pumping capacity leads to steam leakage, it has not been clarified yet.
Based on your description, the amount of non-condensable gas has decreased; the possibility of leakage is low, the top temperature is not high, and the load is not a problem either. Let’s look for the cause in the equipment instead.
Conduct a leak test to see if there are any leaks in the equipment. Our facility uses water ring vacuum pumps for vacuum creation, and we find that they work well – the vacuum level is easy to control, and it remains quite stable
At present, the atmospheric pressure system is operating normally, so decisions regarding how to shut it down are still being made. It would be better to identify the issues before shutting it down; otherwise, if those problems remain unresolved after shutdown, things will get very complicated. Airtightness tests also need to be conducted after shutdown, and it is necessary to identify the problems first before deciding to shut down.
If the amount of non-condensable gas decreases, I think it’s because there is an obstruction in the backflow path of the evacuator, preventing proper discharge of the non-condensable gas. And from the other phenomena you provided, it also fits quite well. Check whether there are any filters or flame arrestors on the system.
Based on what the original poster said, here is my personal opinion in brief: \"The amount of gas that needs to be removed from the system is very small. When the temperature at the top of the system remains normal, it should be kept between 100°C and 110°C; however, when there are fluctuations in vacuum level, this temperature can drop as low as around 40°C.\" Later, by keeping the top reduction temperature above 120°C, the fluctuations in vacuum level were slightly reduced. Based on this analysis, I believe that the fluctuations in vacuum level are mainly caused by an excessively low amount of oil and gas at the top of the column. The amount of gas and oil being removed is too low, while the vacuum pump has a high suction power; the two vacuum pumps compete with each other, resulting in excessive gas flow in one direction and steam being drawn back into the inlet. One of the two vacuum pumps can be stopped; if the vacuum level can be maintained at around 4 KPa, then this can be considered the cause. Adjustments: 1. Reduce the draw rate of the atmospheric tower. 2. Slightly increase the bottom stripping temperature to boost the amount of gas and oil obtained from bottom stripping. 3. Slightly increase the blowdown volume at the tower bottom. If the above adjustments can stabilize the vacuum level, then this can also be considered the cause.