The saturation vapor pressure is the pressure at which fluid passes from the gaseous to liquid state (or from liquid to gas) for a given temperature.

If the fluid temperature increases, the pressure at which fluid passes from liquid to gas (saturation vapor pressure) increases. Thus a liquid like water can turn into steam at ambient pressure by supplying heat, but it is possible to make this transformation without changing the temperature by lowering the ambient pressure below the vapor pressure saturation.

When the liquid is sucked into a pipe a pressure drop is created, if this downward pressure is lower than the saturation vapor pressure, the liquid begins to boil. (Steam), this phenomenon is called cavitation.

The cavitation is a formation of vapor bubbles due to pressure drop. In forming these bubbles increase the volume of fluid present in the area of low pressure which in effect increase the pressure in certain places where gas bubble condenses violently imploding. The shocks created by the bursting of bubbles destroy the walls of the organs in contact with the fluid. Cavitation quickly wears pump .

Calculation of the saturation vapor pressure

The approximate calculation of the saturation vapor pressure can be done using the Clapeyron equation, taking as hypotheses - among others - that the vapor behaves as an ideal gas and the enthalpy vaporization does not vary with temperature in the range considered.

• with:

  • T 0 : Boiling temperature of the substance at a pressure P 0 given K

  • P sat : Saturation vapor pressure in the same units as P 0

  • M: molar mass of the substance, kg / mol

  • L v : Latent heat of vaporization of the substance, J / kg

  • R: gas constant, equal to 8.31447 J / K / mol

  • T: steam temperature, K

  For water, for example:

  • M = 0.018 kg / mol

  • L v = 2.26 × 10 6 J / kg

  • P 0 = 1013 mbar

  • T 0 = 373 K