The NPSH available and required NPSH.

imagine a vertical tube of 1 meter height, filled with water.To raise the water column, there should be a depression at the top of the column, at least equal to the pressure (generated by the weight of water) at the foot ... More column is high, over this depression must be important.

If the depression needed to raise the water column breaks down the fluid pressure below the threshold " saturation vapor pressure "The liquid turns into gas in the area where the pressure is lower (usually at the point of maximum acceleration of fluid) Cavitation.

The water level at the suction limits the "possible pressure loss " to create without cavitating . If the water level is too important to be raised without cavitating, there should be intermediate pumps ...

This "possible pressure loss " before the pump is called cavitation NPSH available.

The NPSH available is a value meter column of fluid (mcf) and is linked to the water level at the suction.

The available NPSH is calculated automatically when you enter the suction height in the software mecaflux.

So far we have only raised the water column without giving speed. For moving a fluid to a speed, pump must generate an additional pressure drop. This pressure drop is proportional to the desired speed of the fluid

This additional pressure drop related to pump flow is called the NPSH required. The pump manufacturer provided a chart where we see the curve linking the NPSH required flow. in general these values vary from 0.5 to 3 mcf

The required NPSH of a pump is a value meter column of fluid (mcf)

If a column of water is so heavy it leaves the possibility of very low vacuum (NPSH available) before cavitating He will settle for a small additional suction (NPSH required) then a low-speed ...

If the water level is too important to be raised without cavitating should be on intermediate pumps ...

example of the binding curve NPSH required meter column of fluid (mcf) versus flow