Specific Speed

• It may be defined as the speed of a geometrically similar pump.

Mathematically,

N_s = NQ^1/2/ H^3/4

Where, N_s = specific speed, rpm

             N = pump speed, rpm

             Q = pump discharge, m³/sec

             H = total head

Note: Geometrically similar pump have similar performance characteristics and identified specific speed regardless of their size.

Effect of speed and impeller diameter on pump performance

1. A) Effects of change of pump speed (N)
2. a) The capacity varies directly as the speed.
3. b) The head varies as the square of the speed
4. c) The BHM varies as the square of the speed.

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.

Thus mathematically,

1. Q₂ = Q₁ (N₂/N₁)

Or, N₂/N₁ = Q₂/Q₁

2. H₂ = H₁ (N₂/N₁)²

Or, N₂/N₁ =

3. BHP₂ = BHP₁ (N₂/N₁)³

Or, N₂/N₁ =

Or, N₂/N₁ = Q₂/Q₁ =  =

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.

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:-

1. capacity varies directly as the diameter
2. The head varies as the square of the diameter
3. The BHP varies as the cube of the diameter

• The relation goes on like this:

D₂/D₂ = Q₂/Q₁ = vH2/H1  =3vBPH2/BPH1