Specific speed is a number characterizing the type of impeller in a unique and coherent
manner.
Specific speed is defined by the equation,
s
3/4
()
NQ
N
H
=
Where:
N = pump speed
Q = flow at BEP at maximum impeller diameter (no corrections even if it is a double
suction impeller)
H = head per stage at BEP at maximum impeller diameter
It states that N
s
is the speed in rpm at which a pump, if sufficiently reduced in size,
would deliver (in US units) 1 gpm at a head of 1 ft. This definition is of little practical
utility.
Specific speed is for an impeller, hence for multistage pumps only the first impeller ...
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