Vertical multi-stage centrifugal pumps have a pressure of 5 kg at the atmospheric outlet, but no pressure at negative pressures

1. It’s possible that they say it’s an issue with cavitation； I tried running the pump at normal pressure first, and the pressure at the outlet was normal ； Gradually increase the vacuum, and it can be clearly seen that the outlet pressure gauge drops to 0 ； 2. The tank before the pump is the tower top condensate receiving tank, which functions like a buffer tank; the liquid is contained within a liquid seal

It’s probably a problem with your operation; the correct approach is to gradually increase the opening of the outlet valve while ensuring proper pressure. At the beginning of valve opening, since the outlet pipe contains gas, once the pipe is filled with liquid, the static pressure of the liquid can be reflected on the pressure gauge at the pump outlet, allowing the valve to be gradually opened wider. But one thing to keep in mind is that the outlet valve does not necessarily need to be fully open; it is best to refer to the rated current as a guide. Therefore, when starting the pump, wait until the pump outlet reaches the designed pressure, then open it slightly to fill the pipe with liquid, after which it can be controlled as needed. It should be noted that the pump’s outlet pressure must be set at a slightly low level, ideally just below the designed pressure. When the liquid fills the pipeline and creates a static pressure, the reading on the pressure gauge will gradually increase; as this pressure rises, the outlet valve is slowly opened until normal conditions are reached. Regarding the issue with the sealing ring, it is best to consult the manufacturer; if you decide to make modifications on your own, it would be helpful to send a photo for discussion. For reference.

Thank you for your reply! As for the operation procedure, I proceed as follows: 1. Keep the pressure inside the tank at atmospheric level, close the outlet valve, and start the pump; at this point, the pressure at the pump’s outlet is 0.5 MPa ； 2. Slowly release the vacuum inside the tank, and observe that the pressure before the pump decreases gradually again! At this time, the outlet valve is not open! Also, I installed a check valve in front of the pump outlet; I’m not sure if that has any impact ； Also, my tank contains 90% triethylamine and 10% low-boiling-point solvent; could it be that the low boiling point causes cavitation under high vacuum?

The valve opens as needed, but one thing is important: the outlet pressure of the pump must remain at the designed rated pressure in order for it to function properly. To be blunt, you may not understand how the process works. As I understand it, the opening degree of the pump outlet is used to control the flow rate required by the tower, and it has nothing to do with the pressure behind the outlet valve. The process requires that the pump be able to push the required liquid to the desired height, regardless of the pressure inside it. I’m not sure if this is the correct way to understand it; just for reference.

I’m not being presumptuous; if I don’t understand something, I’ll ask. Thank you for your guidance! I roughly understand what you mean: it’s the outlet opening that determines the pressure, and the pressure in turn determines the flow rate of the reflux at the top of the tower (if the pressure is low, the liquid can’t rise to the top, and thus there isn’t enough flow rate as required by the tower)! Is that what you mean? Actually, my question is: with the vacuum system active and the outlet valve completely closed, the pressure should theoretically be at its highest. My problem now is that there is pressure at the pump’s outlet under normal atmospheric conditions ； After turning on the vacuum, there was no pressure even with the outlet valve completely closed; as a result, no flow would reach the top of the tower no matter how slightly the outlet valve was opened!

Is the startup sequence of your negative pressure tower proper? If so, the outlet valve should be opened only after the negative pressure inside the tower and other conditions are met, as required. The prerequisite is that the pressure gauge for the pump outlet valve should be installed in front of the outlet valve. You might as well give it a try.

Actually, I first start up the negative pressure tower; once the condensation receiving tank at the top of the tower has accumulated a certain amount of liquid, the condensate is then pumped back to achieve full reflux! It was because when starting to prepare to return the material to the top of the tower for full reflux, it was found that there was no flow at the pump outlet flow meter! The first suspicion is that air lockage occurred due to no liquid inside the pump; empty the liquid in the tank and then open the pump’s drain valve to remove the liquid! Upon further debugging, it was found that there is still no flow at the top of the tower! It was only later that I thought of using a pressure gauge and normal pressure to check whether there was a problem with the pump’s performance! It was only later that I realized there is pressure at the pump outlet under normal pressure, but no pressure there when the pressure is reduced; that’s why I came here to ask for advice from fellow users! You mentioned that the storage tank and the pump are at the same level – could cavitation occur?

For now, this is sufficient until the process diagram is seen. There are many unknown factors; what I’ve said is merely for reference. Whether cavitation occurs at the same level is not a criterion for measurement. The pump inlet pressure and the operating conditions of the pump should be taken as a premise, and then the issue of cavitation should be considered.

Thank you for taking the time to guide me; it’s enough for me to learn for* some time! By the way, what are the operating conditions for the pump mentioned by you, namely the pump with pressure load at the inlet? Are there any other things to pay attention to regarding negative pressure pumps?

The operating conditions for the pump are indicated on the label, while the manual provides more detailed information. For example, head, temperature, flow rate, allowable net positive suction head,,, Oh, I recommend getting a book on Principles of Chemical Engineering; it will help you analyze and solve your problems.