Head
Centrifugal pump curves show ‘pressure’ as head, which is the equivalent
height of water with S.G. = 1. This makes allowance for specific gravity
variations in the pressure to head conversion to cater for higher power
requirements. Positive Displacement pumps use pressure (ie; psi or kPa) and
then multiply power requirements by the S.G.

Static Head
The vertical height difference from surface of water source to centreline of
impeller is termed as static suction head or suction lift (‘suction lift’ can also
mean total suction head).
The vertical height difference from centreline of impeller to discharge point is
termed as discharge static head.
The vertical height difference from surface of water source to discharge point
is termed as total static head.

Total Head / Total Dynamic Head
Total height difference (total static head) plus friction losses & ‘demand’
pressure from nozzles etc. ie: Total Suction Head plus Total Discharge Head =
Total Dynamic Head.

NPSH
Nett positive suction head – related to how much suction lift a pump can
achieve by creating a partial vacuum. Atmospheric pressure then pushes
liquid into pump. A method of calculating if the pump will work or not.
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S.G.
Specific gravity. weight of liquid in comparison to water at approx 20 deg c
(SG = 1).

Specific Speed
A number which is the function of pump flow, head, efficiency etc. Not used in
day to day pump selection, but very useful as pumps with similar specific
speed will have similar shaped curves, similar efficiency / NPSH / solids
handling characteristics.

Vapour Pressure
If the vapour pressure of a liquid is greater than the surrounding air pressure,
the liquid will boil.

Viscosity
A measure of a liquid’s resistance to flow. ie: how thick it is. The viscosity
determines the type of pump used, the speed it can run at, and with gear
pumps, the internal clearances required.

Friction Loss
The amount of pressure / head required to ” force ” liquid through pipe and
fittings.