The outlet pressure of atmospheric pressure countercurrent pumps is often low
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Our plant uses a typical three-column distillation system; the atmospheric pressure reflux tank often experiences negative pressure. The reflux temperature is generally maintained at around 60°C. After the reflux pump operates for some time, its outlet pressure decreases – the normal outlet pressure is 0.7 MPa, but it can drop to 0.5–0.6 MPa. The same situation occurs when another pump is used instead. However, when both pumps operate simultaneously, the outlet pressure rises back to the normal level of 0.7 MPa. This is true for both pumps. I would like to ask everyone for help in explaining the following: 1. Is a decrease in the outlet pressure of the recirculation pump caused by cavitation? ? II. If it is cavitation, then why, when both pumps are running simultaneously, do their outlet pressures remain the same and normal? ?
It should be caused by inlet erosion. When only one unit is in operation, the outlet valve cannot be opened fully; otherwise, cavitation will cause a drop in pressure. When two units are in operation, the rated flow rate can be met as long as cavitation does not occur, which is why the aforementioned phenomenon takes place. It is mainly related to the pump’s characteristic curve.
I also think it’s caused by cavitation; otherwise, the outlet temperature wouldn’t rise that much. In ordinary reflux tanks, methanol is cooled by water, so the temperature doesn’t reach 50 degrees; the fact that it’s now so high is likely due to a large amount of mechanical energy being converted into heat. As for the pressure, I think it’s normal. In typical piping systems, pumps are designed to operate one at a time with another as a backup; therefore, the diameter of the outlet pipe is determined based on the flow rate of a single pump. Now that both pumps are operating simultaneously, the diameter of the pump outlet decreases. Can it be understood that this means the outlet valve of the pump needs to be closed slightly, and it’s normal for the pressure at the pump outlet to increase as a result? Personal opinion...
Is the outlet control valve set too high? The pump outlet pressure is low; it is necessary to maintain a slight positive pressure in the return tank. If that is not possible, nitrogen should be used to raise the liquid level in the return tank appropriately. The return water flow to the pre-cooler before the return tank should be set to full capacity
It is unknown whether the flow rate is stable; if it is stable, then everything is fine, but if it is not stable, it is necessary to check for reasons such as cavitation.
The water cooler valve is opened too wide, causing negative pressure in the return tank; I have encountered this situation before
Make the pipe connecting the return tank to the cooler’s air outlet larger
I. What is the typical reflux temperature for atmospheric pressure columns, in degrees Celsius? The pressure equalization line valve between the condenser and the reflux tank of our atmospheric pressure tower is kept open and generally remains unchanged. II. If the reflux temperature at atmospheric pressure remains at 60°C for an extended period of time (with the cavitation temperature at atmospheric pressure being 64.5°C), will cavitation gradually develop over time? Is it useful to reduce the pressure by closing the outlet valve in the event of cavitation? Won’t this further reduce the traffic?
As long as the pump works fine under these conditions and the product is of qualified quality, that’s sufficient
The outlet valve of the pump is opened too wide. If you observe carefully, you’ll see that the pressure is high at first; as the valve is gradually opened, the pressure decreases. Additionally, the current levels may fluctuate and exceed the normal range. Try reducing the opening of the outlet valve, especially the manual valve. If you operate two pumps, what will happen if one of them fails? It’s better to find a way to adjust the settings – there must be a solution. The design doesn’t allow for such situations; this problem occurs frequently during operation, and it’s mainly due to operational errors. It’s necessary to discuss the issue with those in charge of the equipment and the manufacturing process to determine the cause. Generally speaking, it’s unlikely that there’s a problem with the equipment itself
The reflux temperature is so high that methanol is even vaporizing; it must be cavitation. When both pumps are used together, under normal conditions the outlet pressure should be twice the original value. It’s normal for them to achieve 0.7 MPa when operating together. By the way, what is the typical reflux temperature for atmospheric-pressure columns?