These impeller designs can work well as long as flow remains at or near the best efficiency point (BEP). It is this reduction in inlet velocity that causes the NPSHr to drop as flow moves to the left of a typical H/Q curve and when the rotational speed of the same pump is reduced. Increasing the eye diameter decreases the inlet velocity and, therefore, reduces the NPSH required to maintain uniform flow. During the past year, I have received several requests to revisit this topic and its application to wastewater pumps.Ĭlear water impellers are usually designed for high efficiency, but they can also be designed for low NPSHr. In the February 2010 issue of Pumps & Systems, I wrote an article on my Excel-based Suction Specific Speed and Suction Energy calculators and how they can be used to predict the onset of suction recirculation.