How does the pump’s flow rate change when its impeller is cut?
Thread Content
According to online information, when all other conditions remain unchanged, there are differing opinions regarding the trend in pump flow rate when the impeller of a centrifugal pump is cut. Some say it increases, some say it decreases, and there are also those who say that within a certain cutting range, the flow rate first decreases before increasing again. How did it change? How can it be explained theoretically?
Imagine what it would be like if a backflow valve were installed at the pump’s outlet. Specifically, after cutting, the reduced cross-sectional area at the minimum distance from the pump casing represents the flow rate that is returned; the pressure will also decrease, for reference.
Impeller cutting is used to change the output characteristics of the pump; as for the final flow rate, it depends on the pipeline resistance curve
Does this resulting backflow mean that the flow rate delivered by the pump decreases after the impeller is cut? If we consider the pump’s flow curve: when the pump is operating at its maximum load, the flow rate is at its highest value. If the impeller continues to be cut, even down to almost the limit of the blades, then the pump’s output flow rate should decrease monotonically, right? Following this logic, after the impeller is cut, the external flow rate decreases monotonically.
I don’t understand what “monotonic” means; please explain it clearly. When discussing issues, one should start from the principles; it’s difficult to use extreme analogies in such cases. From the perspective of the flow curve, a decrease in diameter means that the resulting momentum decreases, and thus the pressure head generated also decreases. How small does it need to be to achieve the pressure you require? The best way is to conduct experiments on-site, as that’s the most effective approach. It’s best to prepare a few more impellers; then the answer will be clear. Often, errors in measurement lead to incorrect results; for reference only.
What I mean is that the efficiency-versus-flow rate curve of a pump is generally arch-shaped (including both increasing and decreasing curves); in actual operation, some pumps are not at their point of maximum efficiency. Therefore, after machining the impeller, it is possible for the flow rate delivered to increase. Monotonic refers to a curve that is either monotonically increasing or monotonically decreasing.
Just look at the cutting law of centrifugal pumps to find out
The pump performance curves should be referred to; pump manufacturers provide performance curves for different impeller sizes. Generally, for centrifugal pumps, if the operating conditions and piping remain unchanged and only the impeller size is reduced, the flow rate should decrease at the same head
If the operating conditions remain unchanged and the impeller becomes smaller, the centrifugal force will be lower; as a result, less shaft work performed by the motor is converted into kinetic pressure energy of the fluid. It can be inferred that the flow rate will decrease monotonically.
The centrifugal pump cutting theorem states that after cutting the impeller, both flow rate and head decrease. But after working in this field for many years, I’ve never heard of anyone cutting it, nor has anyone dared to do so.
It conforms to the cutting law, but this law has a condition: the cutting amount must not exceed 20 percent, otherwise it no longer holds true