Effect of Nitrogen-saturated water on NPSH for pumps 1

[JONZ]

Can somebody explain this phenomena:
NPSH for a pump can be significantly reduced when Nitrogen gas is added to the storage tank (as nitrogen blanket).

 

[butelja]

If the water is truly saturated with nitrogen, then you have only 1 way to guarantee adequate NPSH to prevent cavitation. That is to make sure that the pump is lower than the tank, AND that the increase in pressure due to elevation change exceeds the friction loss in the line.

This is because if the fluid is saturated with nitrogen, as soon as the pressure is reduced by any amount, nitrogen will start to come out of solution. This is a common problem on cooling tower applications, where the water is air saturated. Whether or not merely adding a nitrogen blanket leads to nitrogen saturation is a whole different argument. I feel it probably would not, and that it would take a much more intimate sparging or mixing action to saturate the water, as in a cooling tower. For a wealth of information regarding NPSH, go to:

http://www.pumps.org/public/pump_resources/

.