Suction Specific Speed is a function of speed and NPSHr but it is also a function of BEP flow. As the pump speed reduces both BEP flow and NPSHr are reduced. In this way Suction Specific Speed stays constant when you plug these numbers into the equation.
However it only remains constant if NPSHr changes as the ratio of speeds^2. This isn't always the case. NPSHr change with speed reduction is not always clean and predictable.
There is no scientific basis for imposing a single hard number of Suction Specific Speed without first considering the application, pump construction, modernity of the hydraulic design and most importantly the pump specific speed (Ns or nq). A low specific speed pump (e.g. 500 US, 10 metric) react very differently to high Suction Specific Speed compared to a higher specific speed pump (e.g. 3000 US, 60 metric).
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