Specific Speed
- It may be defined as the speed of a geometrically similar pump.
Mathematically,
Â
Ns = NQ1/2/ H3/4
Â
Where, Ns = specific speed, rpm
            N = pump speed, rpm
            Q = pump discharge, m3/sec
            H = total head
Â
Note: Geometrically similar pump have similar performance characteristics and identified specific speed regardless of their size.
Â
Â
Effect of speed and impeller diameter on pump performance
Â
- A) Effects of change of pump speed (N)
- a) The capacity varies directly as the speed.
- b) The head varies as the square of the speed
- c) The BHM varies as the square of the speed.
Â
Affinity law:
- Let characteristics curve speed for the given pump be N1 in rpm and N2 be the new desired speed of the impeller in rpm.
Â
Thus mathematically,
- Q2 = Q1 (N2/N1)
Or, N2/N1 = Q2/Q1
Â
- H2 = H1 (N2/N1)2
Or, N2/N1 =
Â
- BHP2 = BHP1 (N2/N1)3
Or, N2/N1 =
Â
Or, N2/N1 = Q2/Q1 = Â =
Â
Where,
N2= New speed desired rpm
N1 = Speed at which the characteristics are known rpm
Q2 = Capacity at the desired speed N2 , lit/sec.
Q1 = capacity at speed N1, lit/sec
H2 = Head at desired speed, N2 for capacity Q2 , m
H1 = Head at capacity Q1 and speed N1, m.
BHP2 = BHP at desired speed N2 at H2 and Q2.
BHP1 = BHP at speed N1 at H1 and Q1.
Â
Effects of change in impeller diameter
- Changing the impeller diameter has the same effect on the pump performance as changing the speed. So, the following relationship apply:-
Â
- capacity varies directly as the diameter
- The head varies as the square of the diameter
- The BHP varies as the cube of the diameter
Â
- The relation goes on like this:
Â
D2/D2 = Q2/Q1 = vH2/H1Â =3vBPH2/BPH1