Vapour pressure (pD) is a substance- and temperature-dependent gas pressure in a multiphase system. It is necessary to distinguish between vapourisation pressure (pressure where the substance starts to transition to the gaseous state) and saturation pressure (pressure when equilibrium between the phases is established). As the liquid phase ceases to exist, a gas pressure is measured instead of a vapour pressure.
In centrifugal pump technology, vapour pressure refers to the pressure at which the vapour and liquid phases are in equilibrium (see NPSH). The representation of vapour pressure as a function of temperature is known as the boiling point curve. It starts from the triple point (equilibrium condition of the possible phases: vapour, liquid, solid) right up to the so-called critical point (no phase difference between the vapour and liquid phases).
The unit of vapour pressure is the Pascal; the more widely adopted unit in pump technology is the bar. Starting from the triple point condition up to the critical condition, the vapour pressure (pD) and the density (ρ) of the water vary as a function of the temperature. See Fig. 1 Vapour pressure