The vapor pressure of a liquid is the equilibrium pressure of a vapor above its liquid (or solid); that is, the pressure of the vapor resulting from evaporation of a liquid (or solid) above a sample of the liquid (or solid) in a closed container.
The vapor pressure of a liquid varies with its temperature, as the following graph shows for water. The line on the graph shows the boiling temperature for water.
In control-valve work, vapor pressure is an input for distinguishing cavitation from flashing and belongs in the process data required for sizing; use the fluid property at the relevant operating temperature rather than treating the water curve as universal.
