Analysis of pump overload causes
Thread Content
The selection parameters for the centrifugal pump are 30 m3/h flow rate and 25 m head, and the power of the pump purchased is 3 kW. This centrifugal pump functions as a unloading pump; the material it handles has a viscosity similar to that of water, with a relative density of 0.96. The pump inlet pipe is connected to the unloading arm, and the pump outlet pipe is connected to the storage tank; the height is 4 meters. The operator reported that the current was overloaded due to the pump outlet valve being fully open; operating with the pump outlet valve closed allowed the system to function, with the outlet pressure reaching 0.4 MPa. I would like to ask everyone what the possible reasons might be?
What is the flow rate when the valve on the pump outlet pipe is fully open?
The pipes connected to the pump’s inlet and outlet are both DN80, so the flow rate is 30 m3/h. The inlet and outlet of the pump body are DN40.
Excessive outlet flow causes overflow; reduce the outlet valve to keep the pressure around 0.3. When starting the pump, wait until the pump is filled with liquid before closing the outlet valve, then start the centrifugal pump. Once it is running steadily, open the outlet valve again to maintain an outlet pressure of around 0.3. In this way, the starting current will not be too high either
May I ask the teacher, what could be the reasons for the overflow? The inlet and outlet pipes are not long; together with the equivalent length of the pipe fittings, the total length is just over 20 meters. Moreover, the diameter of the pipes matches the flow rate.
If the resistance at the pump’s outlet and the back pressure are too low, the excess head of the pump is converted into flow rate; as a result, the output flow exceeds the rated value, the work done by the pump increases, and the current rises. Therefore, the resistance and back pressure should be adjusted to the rated head value through the pump outlet valve.
The pump’s motor is a bit small, or it has low efficiency.
The main reason is that the pump’s outlet pipe is too large and too short. In fact, the outlet pipe only needs to be one size larger than the pump’s outlet itself; in your case, it’s three sizes larger, which is a real waste. It doesn’t matter if the inlet pipe is a bit larger.
Thank you, teacher, for the explanation. There is a problem now: to meet a unloading flow rate of 30 m3/h, if flow control is applied at the pump outlet, the flow rate will be very low and the unloading time will increase significantly. Are there any other ways to improve this situation?