Why must centrifugal pumps operate in the descending section of the Q-H flow-head curve?
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Why must centrifugal pumps operate in the descending section of the Q-H flow-head curve?
A continuously rising one can’t be designed either
The parabolic-shaped drop curve of a centrifugal pump is the result of the combination of the relationship between pump flow rate and head and the losses within the pump. Within this curve lies the optimal operating range for the pump; however, there is no real choice regarding this curve – if one wants high efficiency and stable operation, along with constraints imposed by design limitations, then this is simply the way it has to be
It was because the pump chosen was too small, so the pump operated in the high-flow range.
You can also choose the previous operating condition; the problem is that the efficiency is low
Choosing the former will cause surge and unstable operation
It is generally selected in the high-efficiency region, referring to the Q-n curve
Since the impellers of commonly used centrifugal pumps are backward-curved impellers, the downward trend in the Q-H curve is determined by the inherent properties of these pump impellers; in my opinion, the relationship between flow rate and head follows a quadratic, parabolic pattern.
It’s not that work should be done during the decline phase; rather, that decline phase is the high-efficiency range of the pump. When it is too close or too far away, the pump’s efficiency decreases. You need to look at the efficiency curve as well, and at the same time consider the Wanfang equation.