What is the effect of the degree of saturation of dissolved gasses on NPSH?

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 ... more.

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