Suction specific speed, Nss

The way that a pump receives the liquid into the impeller determines the available combination of discharge flow and head that the pump can generate. Hssentially, it determines the operating window of the pump. 

This operating window is quantified or rated by the term Suction Specific Speed, Nss . The Nss is calculated with three parameters, the speed, the flow rate, and the NPSHr. These numbers come from the pump s performance curve, discussed in Chapter 7. The formula is the following  

Wliere N = the speed of the pump/motor in revolutions per minute Q = the square root of the flow in gallons per minute at the Best Efficiency Point BEP. For double suction pumps, use A BEP Flow. NPSHr = the net positive suction head required by the pump at the BFiP. 

For the purposes of understanding this concept and formula, there s nothing mathematically significant about the square root of the flow, or the NPSHr to 1 power. These mathematical manipulations simply give us Nss values that are e. y understood and recognizable. For example, the health inspector might j. . a restaurant s cleanliness on a scale from 1 to 100. We might ask you to rate this Lc on a scale from 1 to 10. Those are easy numbers to deal with. How would yc- this book on a scale from 2,369 to 26,426,851 This doesn t make sense. Likewi , the mathematical manipulations in the Nss formula serve simply to convert weird v . into a scale from 1,000 to 20,000 that cover most impellers and pumps. Values at 

000 and 14,000 on a relative scale that is easily understood and com lai )le to other Nss values of competing pumps, similar pumps, and totally different pumps. 

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Another dimensionless group, analogous to the specific speed, that relates directly to the cavitation characteristics of the pump is the suction specific speed, Nss ... 

A rotational speed of 6000 rpm was selected for optimum efficiency at a specific speed of 500. The radial type of impeller element determined by this specific speed was expected to operate satisfactorily at suction specific speeds nss of beyond 8000 and up to 20,000. For reliability, a value of 12,000 was selected. The gap between this capability of the impeller and an ascertained nss requirement of 70,000 (36-in. liquid level at the inducer inlet) was equivalent to 0.75 psi. Therefore, the inducer was designed for a head rating of at least this amount. 

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