A liquid's vapor pressure limit is defined as the absolute pressure below which it flashes into its gas phase (vapor or steam for water) for the fluid temperature at which the system is operating. Vapor pressure is a fundamental parameter for any hydraulic transient analysis. Low transient pressures can cause a liquid to vaporize and, once one or more of these vapor pockets collapse later on, result in very large transient pressures, which may break pipes or other system components.

The vapor pocket collapse process is analogous to the well-known tip-cavitation phenomenon, which causes pitting damage at pump impellers; however, vapor pockets can be orders of magnitude larger than cavitation bubbles and can result in system-wide transients.

Heating or pressurizing a fluid increases its vapor pressure-an important consideration in industrial applications. Consider both operating temperature and pressure when determining a liquid's vapor pressure limit. (For example, water boils at a lower temperature at high altitudes due to the lower atmospheric pressure and lower absolute vapor pressure. Similarly, water boils at a higher temperature in a pressure cooker and this increased steam temperature accelerates the cooking process.) This is why the parameter library provided with HAMMER often provides values for liquids at different temperatures.