the required NPSH and NPSH available
Imagine a vertical tube one meter in height filled with water, to raise the water column should be a depression to the top of the column, at least equal to the pressure (generated by the weight of water) at the bottom… the higher the colomn of water is, the higher the depression should be..
If the negative pressure necessary for raising the water column, drops the liquid pressure below the threshold of "saturation vapor pressure" the liquid turns into gas in the region where pressure is the lowest (usually at the point of maximum acceleration of the fluid), the fluid cavitate.
The water level at the suction limits the depression it is possible to create without cavitating
If the water level is too important to be raised without cavitation, it will be necessary to place intermediate pumps ...
This possible depression before cavitation is called NPSH (net positive suction head) available.- Available NPSH value is a meter column fluid (mcf) and it is linked to the water level at the suction
In mecaflux NPSH available is calculated automatically when you enter the suction head.
So far we have only raised the water column without giving it a speed, to moves liquid at a certain rate, the pump must generate additional depression. This pressure must be even greater than the desired flow rate is important.
This additional pressure related to the flow of the pump is called the required NPSH.The pump manufacturer provided a graph where we see the curve binding NPSH required to the flow. Usualy the required NPSH values, vary from 0.5 to 3 mcf
- The required NPSH of a pump is a value in meters of fluid column (mcf)
If the water level is too important to be raised without cavitation, it will be necessary to place intermediate pumps ...
Example curve binding required NPSH, in meters of fluid column (mcf) to flow rate: 