Designing an Ultra-low Specific Speed Centrifugal Pump

Traditional commercial centrifugal pump designs have specific speeds (Ns) greater than 500 because low values of Ns are usually associated with low efficiency, pulsation, and a drooping head characteristic. In this instance, acceptable hydraulic performance is achieved with a unique impeller design coupled with direct current (DC) motor characteristics. When the specific speed of a centrifugal pump falls below 500, the ratio of friction loss to entire loss grows extremely large and satisfactory performance can no longer be expected. Consequently, positive displacement pumps dominate this specific speed regime. In this development, a combination of trial calculations, using a method of performance estimation for an impeller, and the idea of machining the flow passage and ditching a lot of shallow spiral grooves into the impeller face to maintain a small vane tip clearance (the axial gap between the impeller and liner) was adopted. Together with the adoption of a DC-canned-motor, miniaturization of the pump unit was achieved. The test results of two similar impeller designs show peak efficiencies of 28 percent and 25 percent are attained for the specific speeds of 370 and 340, 16 DESIGNING AN ULTRA-LOW SPECIFIC SPEED CENTRIFUGAL PUMP