HOW PUMPS WORK

HOW PUMPS WORK

In dynamic machines there is no closed volume, instead rotating blades supply or extract energy to or from the fluid. For pumps, these rotating blades are called impeller. For incompressible flow it is more common to use volume flow rate rather than mass flow rate. Volume flow rate is called capacity and is simply mass flow rate divided by fluid density.

Volume flow rate (capacity) v = m/ρ

Where v = volume flow rate

           m = mass flow rate

           ρ = density of the fluid

Performance of a pump is characterised additionally by its net head H, defined as the change in Bernoulli head between the inlet and outlet of the pump,

Net Head = H = (P/ρg + V²/2g +Z)out -  (P/ρg + V²/2g +Z)in

Dimension of net head is length, and it is often listed as an equivalent column height of water.

The net head of a pump H, is defined as the change in Bernoulli head from inlet to outlet for a liquid, this is equivalent to the change in the energy grade line, H  EGLout  - EGLin, relative to some arbitrary datum plane.

Consider the special case of incompressible flow through a pump in which the inlet and outlet diameters are identical, and there is no change in elevation.

H = P_out – P_in / ρg

Net head is simply the pressure rise across the pump expressed as a head (column height of the fluid). Net head is proportional to the useful power actually delivered to the fluid. It is traditional to call this power the water horsepower.

 water horsepower = W = mgH = ρgvH

All pumps suffer from irreversible losses due to friction, internal leakage, flow separation on blade surfaces, turbulent dissipation, etc. Therefore, the mechanical energy supplied to the pump must be larger than w water horsepower. In pump terminology, the external power supplied to the pump is called the brake horsepower, which we abbreviate as bhp. For the typical case of a rotating shaft supplying the brake horsepower.