The head for the duty point of the pump is composed of
- the static head (static = independent of the flow rate)
- height difference between suction side and discharge side liquid level (geodetic head)
- pressure difference between discharge side and suction side tank (for closed tanks)
- the required outlet pressure, if any
- the friction loss head from the pressure losses in the piping system as a function of the flow rate
The useable mechanical work transferred from the pump to the fluid being pumped, related to the weight force, is called head H of the pump. At constant speed n and constant flow Q, it is independent of the density of the pumped liquid, but dependent on its viscosity.
It can be calculated by the pressure difference divided by the density of the pumped fluid and the local gravitational constant.
For Newtonian fluids one can consider the head independent from the pumped fluid for kinematic viscosities less than 20 mm²/s. By this reason it is especially suitable to present the characteristic curve of centrifugal pumps.
For pumping water the value is equal to the pressure given in meters of water column.