What is the effect of the degree of saturation of dissolved gasses on NPSH? Compare 100 deg F de-aerated water in a tank with a bladder pressured to 10 psig with a tank without a bladder for the same temperature and pressure, with the pressure provided by, say, a nitrogen bottle causing the water to be saturated with nitrogen.

Answer:  

             There is definitely an effect. The dissolved gas changes the molecular interaction of the liquid in which it is dissolved. Chemical engineers are familiar with this phenomenon via Henry’s Law, and Oswald coefficient, which relates the V/L (void fraction – the freed-up gas volume to liquid volume ratio) as function of saturation pressure and actual pressure of the mixture. This is not to be confused with the effect of free gas on pump suction performance, and neither it has anything to do, directly, with cavitation (which is caused by vaporization of liquid and subsequent collapse of vapor bubbles). The dissolved (not free) gas affects the “ability” of a liquid to become vapor when the pressure drops.

In practice, a good example are cooling water tower double-suction pumps, where the incoming water has been so well aerated when passing through the tower - that a significant amount of air stays dissolved, and reduces the NPSHA. The NPSH margin (NPSHA-NPSHR) for these pumps is not significant to begin with, and with air affecting the NPSHA, the propensity for these pumps to “get in NPSH trouble” is real. As an estimate, the reduction of NPSHA for these pumps is about 1-3 feet.

In your case, you should be OK if NPSH margin is good. Also, even if some nitrogen dissolved in water, it will probably stay dissolved and will not come out of the solution at the low pressure inlet areas, because of the time delay – it flows through quickly. In the cooling tower example, the water stays well dispersed in order to get cooled, i.e. the surface area is extremely enlarged, and air can easily get in.