The concept of pump specific speed (NS)



CENTRIFUGAL-PUMP.ORG defines specific speed (Ns) as a dimensionless
number, or index, that identifies the hydraulic and geometric similarity of pumps.
Pumps with the same Ns, but are of different size, are considered  to be
hydraulically and geometrically similar - one pump being a size-factor, or model,
of the other.


The old definition of specific speed was simply a poor attempt at restating its equation. It states
that Ns is the speed in RPM at which a pump, if sufficiently reduced to a very small size, would
deliver a flow rate of one gallon per minute, at one foot of differential head. This definition is
meaningless and has no practical application.


Pump specific speed (Ns) is calculated from the equation:

Ns = [N x Q^0.50] / [H^0.75]

where:

N  =   pump speed, in RPM
Q  =   capacity at best efficiency point (BEP) at maximum impeller diameter, in GPM
H  =   head per stage at BEP at maximum impeller diameter , in FT

The equation has inconsistent units so Ns is simply considered to be dimensionless. For
simplicity, CENTRIFUGAL-PUMP.ORG uses the U.S. system of units in the calculations
through-out this web site.


Example: What is the specific speed of a two-stage pump whose capacity and
total head at BEP is 400 GPM, and 200 FT, respectively when the pump runs at
1780 RPM?

Solution:  Ns  =  [1780 x (400)^0.50  /  (200/2)^0.75] = 1126



Importance of pump specific speed

The concept of pump specific speed has many significance and practical
applications:

Specific speed identifies the type of pump according to its design and flow pattern.
According to this criteria a pump can be classified as radial flow, mixed flow, or
axial flow type. A radial flow pump is one where the impeller discharges the liquid
in the radial direction from the pump shaft centerline, an axial flow pump
discharges the liquid in the axial direction and a mixed flow pump is one that is a
cross between a radial and an axial flow pump design.

Specific speed identifies the approximate acceptable ratio of the impeller eye
diameter (D
1) to the impeller maximum diameter (D2) in designing a good
impeller.

Ns:     500 to   4000;    D
1/D2 < 0.5   -  radial flow pump
Ns:    4000 to   8000;   D
1/D2 > 0.5   -  mixed  flow pump
Ns:    8000 to 12000;   D
1/D2 = 1      -  axial  flow pump

These figures for Ns and D1/D2 ratio are not restrictive, rather, there is a big
amount of overlap in the figures as pump designers push the envelope of
operating range of the different types of pumps.

These types of pumps are also indicative of the manner energy is imparted by the
impeller into the liquid. In radial flow type, the pressure is developed by the
centrifugal action of the impeller. In mixed flow type, the pressure is developed
mainly by the centrifugal force in combination with the lifting action of the impeller.
And in axial flow type, the pressure is developed solely by the lifting action of the
impeller.

Specific speed is used in designing a new size pump by modelling , or
size-factoring, a  smaller pump with the same specific speed, or within the range
of + or - 10% of the specific speed. The performance and construction of the
smaller pump are used to predict the performance, and to model the construction,
of the new pump.

The specific speed is also a good indicator of pump efficiency. Over the years
charts have been developed showing plots of average pump efficiency versus
pump specific speed. These charts are valuable tool in comparing pump
efficiencies - whether a competitive pump is inferior, efficiency-wise, with another
pump, or whether a particular pump shows an usually high efficiency whose
accuracy might be doubtful.

Rule-of-Thumb: For similar pumps with about the same capacity at BEP, the
pump with the higher specific speed will typically have a higher efficiency also.



Discharge specific speed

Another term for specific speed is discharge specific speed.

Persons familiar with the term suction specific speed (Nss) know that Nss is
affected by parameters on the suction side of a pump, such as the impeller eye
area and eye diameter, the suction nozzle size, the suction area development of
the casing, etc., hence the word
suction in suction specific speed.

Similarly, the specific speed (Ns) of a pump is mainly affected by such factors as
the impeller width (or impeller "BA"), the volute throat area, and by the discharge
nozzle size.  To a lesser degree, Ns is also effected by the impeller discharge
angle and number of vanes. In short, Ns is affected by parameters on the
discharge side of the pump, hence those more familiar with pumps, including
centrifugal-pump.org, prefer to call it as discharge specific speed to highlight its
difference from suction specific speed.



R0609-THCO
Category:
File: NS definition
Related topics

Definition of specific speed (NS)
Pump comparison based on NS
Application of NS in pump selection
Application of NS in hydraulic re-rates
Application of NS in new pump design
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