PUMP SUCTION HEAD

NPSH (Net Positive Suction Head)

A pump installed above the water level can suck the water due to the
atmospheric pressure on the surface itself, which is equivalent to about 10 m of water column. This means that, however great the suction power of the pump, the height from which it can draw up the fluid remains 10 m (for water).
In reality, the limit is lower due to pressure drops in the suction line,
the kinetic height of the current and the dynamic effect of the pump’s impeller.
Trying to suck up the fluid from a greater height leads to cavitation in the pump, which is not only seriously damaging to the pump but also prevent further increases in flow rate.
This consists in the sudden creation and collapse of cavities, composed primarily of vapour, as the fluid flows. These cavities are formed, at the operating temperature, in zones in which the fluid pressure approaches the vapour pressure at that temperature. In centrifugal pumps, this occurs mainly at the entrance to the impeller blades where the sudden acceleration of the current leads to a reduction in pressure. The cavities of vapour which result are conveyed onwards by the flow and implode at points where the fluid pressure increases. The implosion of these bubbles of vapour is accompanied by a shock wave, which causes hammering on the surface in question. This can result in fatigue, plastic deformation and removal of material from the surface. The effect can be accelerated by the corrosive action of the fluid being handled.
To characterise the behaviour of a pump in response to cavitation, one determines the NPSH (Net Positive Suction Head) which represents the absolute height or load, net of the fluid’s vapour tension, which must exist in suction to ensure that cavitation does not occur.
It will be immediately evident how important it is to check that the
system available NPSH is greater (by at least 1 m) than that required by the pump.

The available NPSH is calculated as follows: