Suction specific speed



Q  -  What is suction specific speed?


A  -  Suction specific speed (NSS) is a dimensionless number, or index, that
defines the suction geometry, or suction characteristics, of a pump. It is calculated
from the equation:

NSS  =  [ (N x Q^0.5)  /  (NPSHR)^0.75 ]

where:

N            =   rotative speed in RPM
Q            =   capacity in GPM at BEP (For double suction impeller, Q=GPM/2)
NPSHR =   NPSHR in Feet based on 3% head loss at BEP

In multi-stage pump the NPSHR is based on the first stage impeller NPSHR.


Example: What is the suction specific speed of a double suction pump operating
at 3560 RPM if it required 18 FT of NPSH at its BEP of 800 GPM?

Solution:  NSS = [3560 x (800/2)^0.50 / (18)^0.75] = 8148


Some practical applications of NSS are:

1.  It is that it is commonly used as a basis for estimating the safe operating flow
range for a pump. The higher the NSS is, the narrower is its safe operating flow
range relative to its BEP. Many users prefer their  pumps to have an NSS in the
range of 8000 to 11000 for improved MTBF or trouble-free operation.

2.   It is commonly used as an indicator if a pump were susceptible to develop
internal flow recirculation problem. The higher the NSS number, the higher is its
susceptibility.


As the term implies, the suction specific speed of a pump is mainly influenced by
the suction design geometry of the pump including its suction nozzle diameter, the
casing suction area development, the impeller suction eye area, eye inlet
diameter, and suction vane angle.
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