Page 84 - Know and Understand Centrifugal Pumps

Pump Classification

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:

Where: 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 '/2
REP Flow. NPSHr = the net positive suction head required
by the pump at the REP.

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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 the 3/4
power. These mathematical manipulations simply give us Nss values that are easily
understood and recognizable. For example, the health inspector might judge a
restaurant's cleanliness on a scale from 1 to 100. We might ask you to rate this book
on a scale from 1 to 10. Those are easy numbers to deal with. How would you rate
this book on a scale from 2,369 to 26,426,851?This doesn't make sense. Likewise, the
mathematical manipulations in the Nss formula serve simply to convert weird values
into a scale from 1,000 to 20,000 that cover most impellers and pumps. Values at
1,000 and 20,000 are on the outer fringes. Most pumps register an Nss between
7,000 and 14,000 on a relative scale that is easily understood and comparable to
other Nss values of competing pumps, similar pumps, and totally different pumps.

The Nss value is a dimensionless number relating the speed, flow and
NPSHr into an operating window that can be expected from a pump. It
is an index or goal used by pump design engineers. Consulting
engineers use the Nss when comparing similar pumps for correct
selection into an application. Once the pump is installed, it becomes a
valuable tool for the process engineer, and for the operators interested
in keeping the pump running without problems. The Nss is an
indication of the pump's ability to operate away from its design point,
called the REP, without damaging the pump.
The Nss value is really simple, although often it is made to appear
complicated. The Nss is an equation with a numerator and a
denominator. The Nss value is obtained by dividing the numerator by
the denominator.