Size Effect
For sufficient high NSPH (or sufficient low suction specific speed); and low viscosity for high Reynolds Number), real pump also posses a strong size effect on efficiency. This is because in normal manufacturing processes, the clearance and preventing internal leakage do not scale up in proportion to the size, nor do the surface roughness height. Thus a larger pump tends to be more efficient. Strictly speaking, however, the geometry of the larger pump is not the same as that of the smaller pump, and this forces one to modify.
Viscosity Effect
Centrifugal pump geometries have not generally been optimized versus Reynolds number-often because the effect on hydraulic shape is not very great except for the highest viscosity of the pumpage, and a given application can sometime experience a substantial range of viscosity. Studies of conventional centrifugal pump over a range of Reynolds number have been combined in monographic chart in the hydraulic Institute Standards, which yield correction factor to the head, efficiency and flow rate of the BEP of a low viscosity pump in order to obtain the BEP of that pump when operating in higher viscosity.
NSPH Effect
In many excess, the available NSPH is low enough, or the suction specific speed, at which the pump stage must operate is high enough for significant two phase activity to exist within the impeller. This is to expected in centrifugal impeller of water pump if the available speed is greater than about 3 to 4.
