Vapor Pressure

Vapor pressure, abbreviated as p_v, 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.

• See Articles  -  Properties of Hydrocarbon Gasses

 

 

 

Vapor Pressure Formula
\( p_v \;=\; x_s \;  p_{v}{^o} \)     (Vapor Pressure) \( x_s \;=\;  p_v \;/\; p_{v}{^o} \) \( p_{v}{^o} \;=\;  p_v \;/\; x_s  \)
Symbol	English	Metric
\( p_v \) = Vapor Pressure (psi)	\(lbf\;/\;in^2\)	\(Pa\)
\( x_s \)  (Greek symbol chi) = Solution Mole Fraction	\( dimensionless \)	\( dimensionless \)
\( p_{v}{^o} \) = Pure Solution Vapor Pressure (psi)	\(lbf\;/\;in^2\)	\(Pa\)