# Vapor Pressure

Written by Jerry Ratzlaff on . Posted in Fluid Dynamics

Vapor pressure, abbreviated as pv, of a substance is the pressure at a certain temperature when the liquid and vapor are in equilibrium.  Liquid vapor pressure is measured in the laboratory at 100 degrees fahrenheit and is referred to as the Reed Vapor Pressure.   As the temperature of a liquid increases, the vapor pressure also increases.

Boiling point is the temperature at which the vapor pressure equals atmospheric pressure.  In engineering, the vapor pressure is extremely important when sizing pumps.  When there is low net positive suction head available $$NPSHa$$, vapor pressure can play a large role in preventing (or assisting in) cavitation.

Vapor pressure of common hydrocarbon gasses are here

### Vapor Pressure Formula

$$\large{ p_v = x_s \; p_{v}{^o} }$$

$$\large{ p_v = - \gamma \; \left( NPSH - \frac{ v^2 }{ 2 \; g } - \frac{ p }{ \gamma } \right) }$$     (net positive suction head)

$$\large{ p_v = p \;-\; \frac {Ca \;\rho\; U^2} {2} }$$     (Cavitation number)

Where:

$$\large{ p_v }$$ = vapor pressure

$$\large{ Ca }$$ = Cavitation number

$$\large{ U }$$ = characteristic velocity

$$\large{ \rho }$$  (Greek symbol rho) = density of the fluid

$$\large{ g }$$ = gravitational acceleration

$$\large{ x_s }$$  (Greek symbol chi) = mole fraction of the solvent

$$\large{ NPSH }$$ = net positive suction head

$$\large{ p }$$ = pressure

$$\large{ \gamma }$$  (Greek symbol gamma) = specific weight

$$\large{ p_{v}{^o} }$$ = vapor pressure of pure solvent

$$\large{ v }$$ = velocity