Glossary term
Vapor Pressure
The equilibrium pressure exerted by a vapor above its condensed phase at a given temperature.
Definition
quantityVapor pressure is the equilibrium pressure of a vapor in contact with its liquid or solid phase at a specified temperature.
Vapor pressure measures a substance's tendency to evaporate. It is strongly temperature dependent and governs boiling, cavitation, evaporation losses, distillation, drying, storage tank emissions, vacuum operation, pump suction limits, and phase-equilibrium calculations in chemical and energy systems.
In a closed container with liquid and vapor at equilibrium, molecules continuously leave and return to the liquid surface. The vapor pressure is the pressure at which those rates balance for a given temperature. As temperature rises, vapor pressure rises; when vapor pressure equals the surrounding pressure, boiling can occur.
Vapor pressure is a property of a pure substance at a given temperature, but mixtures require phase-equilibrium treatment. For engineering estimates, vapor-pressure correlations are used only within their valid temperature range and should not be extrapolated casually.
Engineering use
Vapor pressure affects pump suction, cavitation margin, vacuum drying, distillation, refrigeration, storage tank design, relief calculations, solvent handling, emissions, and process safety. In a pump inlet, if local absolute pressure drops near liquid vapor pressure, vapor bubbles form and can collapse downstream, damaging impellers and reducing performance.
In storage and process equipment, high vapor pressure can increase evaporation losses, flammability risk, pressure buildup, and ventilation requirements. Low vapor pressure can make drying or degassing slower unless temperature or vacuum conditions are adjusted.
Common mistakes
A common mistake is comparing vapor pressure to gauge pressure instead of absolute pressure. Another is using a room-temperature vapor-pressure value in equipment that heats, cools, flashes, or operates under vacuum. A strong vapor-pressure review states substance or mixture, temperature, pressure basis, correlation source, valid range, impurities, dissolved gases, and whether the value is used for boiling, cavitation, emissions, or phase equilibrium.