Issues regarding abnormal current in vertical centrifugal pumps
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There are three vertical centrifugal pumps on site, with two in use and one as a spare; the medium is gasoline, the flow rate is 100 cubic meters per hour, the head is 70 meters, the rated current is 55A, and all three pumps are of the same model. A couple of days ago, one of the centrifugal pumps tripped twice in one day. The operating current of the motor was 56 A (this was the current when the operator reduced the outlet valve from 30% to 20%; the current of the other pump, whose outlet valve was set at 30%, was around 38 A) ； The three-phase currents are balanced, at 56.1/57.1/56.8 respectively ； The resistance of the entire circuit with respect to ground is infinite. The current when the motor connected to the coupling rotates alone is 16A, but it’s also very easy to turn the pump shaft by hand. I would like to ask the experts: what aspects should be considered to determine the cause of this issue, or what could be the possible reasons?
Abnormal current in vertical centrifugal pumps can be caused by various factors. Based on your description, the following possibilities exist: 1. The opening and closing of valves affect the pump’s efficiency. The load on the pump is directly related to the degree of valve opening. When the opening degree of the outlet valve is reduced from 30% to 20%, the system head increases and the current rises. This could be an important factor contributing to the increase in current. Since the system requires higher pressure to maintain a stable flow rate, the motor load increases, which in turn raises the current. 2. The pump or motor may be faulty. Since you mentioned that the current per rotation of the motor is 16A, and it is easy to turn the pump shaft by hand, the operating current of the entire system is on the high side. This may indicate some problem with the motor or pump. Possible causes include bearing wear, seal failure, impeller wear, or motor coil faults. 3. Excessive flow rate or changes in medium properties. If the flow rate increases or the properties of the medium (such as viscosity) change, this can also lead to an increase in current. You can first conduct a check based on the points mentioned above to see if there are any corresponding issues. If the outlet valve is adjusted back to 30%, can the current be reduced to the normal range? If not, it may be necessary to further inspect the condition of the pump and motor. It is recommended to contact professional mechanical and electrical maintenance personnel for a thorough inspection and repair. .
If such a problem arises, there’s no need to struggle with making a decision. As long as it is used to distinguish processes and is mechanical, that’s fine. In terms of the process, it’s about controlling the opening degree of the outlet valve; if it’s normal, then everything is fine. If it doesn’t fall under mechanics, it’s best to solve the problem directly and then check it. As per the requirements, if there is a sudden surge in current, it is necessary to disassemble it for inspection. It’s likely a mechanical issue, for reference.
One reason is that this situation has happened several times, and I really want to know the approach to making a judgment； Secondly, in our area, the mechanical equipment and electrical systems are handled by two separate teams from different specialties. The electrical team says that the no-load current is within normal limits, so it’s not their fault; the mechanical equipment team says that the cranking operation is working properly, so again it’s not their fault. The passing of responsibility between the two teams is extremely severe~
Why does impeller wear cause an increase in the current of a pump? I don’t quite understand... Also, based on the rotation of the pump and the operation of the motor without any load, I thought it would be possible to identify problems with the pump or the motor... So why does it suggest that there are issues with the pump or motor instead? In the end, by closing the outlet, although the pressure increases, the flow rate decreases, right? Overall, the load should decrease... Otherwise, why would one need to close the outlet when starting a centrifugal pump?
It would be best to send a video for easier discussion. If the crank can be turned, check to see if there is an excessive axial clearance. The fact that the turning gear is working fine does not mean that the machinery is in good condition, as there is an issue with axial forces. The axial force in the stopped state is not necessarily the same as that in the operating state. As for responsibility, it is best to combine operation and inspection. Because the technical complexity of maintaining such things isn’t very high; with just a basic understanding, one can handle it. The current situation has three issues, namely in terms of process, machinery, and electrical (pneumatic) systems. As for how to coordinate, the party with the most say is the process team. If the process is guaranteed to work properly, then there are only two aspects left. For electrical appliances, as long as there is no overcurrent when they are not in use, it is considered normal. Finally, there’s the mechanical aspect; in this case, it’s necessary to convince the other party using principles of mechanics, or apply pressure through higher-level supervisors so that they can check the solution. For reference.