Vapor Pressure of Fluid

The vapor pressure, which influences boiling and cavitation, is the pressure exerted by a vapor in equilibrium with its liquid at a given temperature and indicates the tendency of molecules to escape from the liquid surface.

In a closed, evacuated container, liquid molecules escape into space, raising the pressure until equilibrium is achieved, where the rate of molecules escaping equals the rate of returning. This equilibrium pressure is the vapor pressure.

Vapor pressure depends on temperature. As temperature rises, molecules gain kinetic energy, overcoming intermolecular forces and escaping into the vapor phase more easily, which increases vapor pressure.

For example, water at 100°C has a vapor pressure of 101.3 kPa,  which matches atmospheric pressure at sea level, causing it to boil. At higher altitudes, with lower atmospheric pressure, water boils at lower temperatures because its vapor pressure matches the reduced atmospheric pressure more quickly.

Monitoring vapor pressure is crucial in designing and operating pumps and turbines to prevent cavitation.

Cavitation occurs when local fluid pressure drops below the vapor pressure, forming vapor bubbles that collapse violently, damaging equipment.