However, the specific speed of a single impeller design does not change with a change in rotational speed. Specific speed is directly proportional to rotational speed when head and flow remain constant. At 1,200 rpm, specific speed is 703, and the impeller would look like a hybrid of the two impellers seen to the left of Figure 1. Its geometry would be similar to the radial vane impeller (beneath the 1,000 point). If the motor speed is lowered to 1,800 rpm, an impeller with a specific speed of 1,055 would be required for the same flow and head. The geometry would be similar to the Francis vane impeller (Figure 1 at the 2,000 point). If these values are entered as Q and H in the equation with a motor speed of 3,600 rpm, the specific speed is 2,110. Pump DesignĪn impeller is needed that will produces 900 gallons per minute (gpm) at 190 feet of head. The flow passages also increase in size at a corresponding rate. The cross sectional pictures in Figure 1 show that, as specific speed increases, the impeller inlet or eye diameter increases and eventually approaches or equals that of the vane outlet. This design discharges radially, but the transition from inlet to outlet is more gradual and results in the highest efficiency. ![]() Those between radial and mixed flow (1,700 to 3,500) are known as Francis vane impellers. Mixed flow impellers (4,000 to 8,000) exhibit both radial and axial characteristics, discharging between the radial and axial angles with a high-flow-to-moderate-head ratio. A radial vane impeller discharges 100 percent of its flow perpendicular to its suction, usually with a low flow-to-head ratio.Īn axial flow impeller discharges 100 percent of its flow along the same axis as its suction with a high flow-to-head ratio. ![]() Pump profile comparisons (courtesy of The lower values (500 to 1,500) on the left of the figure describe the geometry of the radial vane impeller while the higher values (9,000 and higher) on the right of the figure equate to true axial flow impellers (propellers). Viewed this way, specific speed can be useful when selecting a pump for a particular application and predicting premature failure due to off best efficiency point (BEP) operation.įigure 1. It is best to think of it as an index number that can predict certain pump characteristics. An Index NumberĪs Terry Henshaw stated in “Centrifugal Pump Specific Speed” (Pumps & Systems, September 2011), the definition of specific speed can be confusing. It allows the use of existing design and test data to design similar higher and lower flow pumps because the specific speed of a pump is independent of its size. Many pump designers see specific speed as the most important contributor to centrifugal pump design. Specific speed was first applied to centrifugal pumps in the latter 1800s and was a modified version of one developed for water turbines. For other articles in this series, click here.Įarly centrifugal pump design was heavily influenced by turbine technology during the mid to late 1800s. A truly dimensionless variation on the pump specific speed is available, but is less commonly used.Editor's Note: This is the second in a six-part series on centrifugal pump efficiency. The common pump specific speed is not dimensionless, however its dimensions are only important for converting between different unit sets (eg converting and comparing pump specific speeds calculated on a Metric basis and US Customary basis). Specific speed is the speed (in RPM) a geometrically similar impeller would need to operate in order to deliver 1 L/s of fluid at a head of 1 m (US units : 1 USgal/min at a head of 1 ft). Often pump characteristics such as NPSHr, suction lift and pump efficiency will be correlated against the pump specific speed. Pump Specific Speed is a method of characterising pumping conditions and it may be used to determine the most appropriate pump design for a given application. Volumetric flow rate at best efficiency point Impeller revolutions per second, measured in radians ![]() Definitionsĭifferential head across impeller at best efficiency point ![]() Pump specific speed may be used to determine an appropriate pump design for a given application when choosing between axial, radial or multistage centrifugal designs or positive displacement pumps. Pump Specific Speed, N S is a method of characterising a pump duty by head, flow rate and rotational speed.
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