N2 is 'slightly' soluble in water. Henry's Law can be used to estimate its solubility.
Quantifying the impact on dissolved gases on NPSH is not straightforward, I saw one article on it years ago which I either didn't copy for my files or which I've mis-filed and can't now locate >:-<
Yes, nitrogen is soluble in water. Solubility at atmospheric pressure is:
~18 ppm @ 10ºC
~14 ppm @ 25ºC
~10 ppm @ 60ºC
I don't really see how it affects NPSH, though. I don't think the amount of dissolved nitrogen in water will change the NPSHR of a pump at all. I also don't see how it will affect the head, unless you add nitrogen blanket a storage tank such that the addition of nitrogen increases the pressure in the tank. In that case, the head (NPSHA) will increase.
Air is mostly nitrogen. There is dissolved air in water, in proportion to the temp of the water. Two places this causes problems are 1/ steam boilers, which often require deaerators to strip the air (oxygen in particular) out of the water, and 2/ hot water heating systems. Under system operating conditions, air will pop out of solution. This results in air binding occuring at upper parts of a system, even though the system can be significantly above atmostpheric pressure (air at atmospheric pressure cannot leak in). In hot water system, this is the source of many problems. Hydraulically, the arrangement of the components doesn't really matter. From a dissovled air removal perspective, it matters a LOT.
I should state that the listed solubility numbers are based on water exposed to air (partial pressure ~0.79), not a pure nitrogen environment... actual solubility where the partial pressure of nitrogen is 1.0 will be higher than the values I previously listed.
A conservative approach for centrifugal pump NPSHR in water is to determine the partial pressure needed to keep the volume of gas dissolved in the water and add this pressure to the water vapor pressure over the temperature range of the pumped fluid. You need to determine the total gas volume present in the water and the fractions of the constituent gases (eg. air has 79%N2 and 21%O2 whatever its total volume in the water). For nitrogen in water, Henry's Law coefficients in psia/cc/Kg are as follows:
Temp(F) Henry_N2
75 1.042
100 1.202
150 1.331
175 1.335
200 1.314
250 1.216
300 1.061
400 0.673
500 0.336
