Vapor Pressure

Definition

Vapor pressure is a fluid property that rises with temperature: the hotter the liquid, the higher the pressure at which it vaporises. It is expressed as an absolute pressure at a given temperature. In a flowing system, vaporisation occurs wherever the local absolute (static) pressure drops to the vapor pressure of the liquid at the local temperature — for example at a pump suction, across a restriction, or at a high point in the line.

Engineering context

Vapor pressure sets the lower limit on the absolute pressure a liquid system can sustain before it vaporises, which is why it is the reference pressure in two of the most common liquid-system checks. In a net positive suction head (NPSH) calculation, the available suction margin is measured above the liquid’s vapor pressure; in cavitation, vaporisation begins when the local pressure reaches the vapor pressure. FluidFlow uses the vapor pressure of the modelled fluid at its operating temperature when it reports NPSH available at pumps and when identifying where in a network the liquid is at risk of vaporising.

Related definitions

NPSH · Cavitation · Pump curve

See it in FluidFlow

FluidFlow accounts for fluid vapor pressure when it reports NPSH available at pumps and flags locations where the liquid may vaporise, so suction and two-phase risks surface as part of the network solution.