Pressure, S. Volume & S. Gravity

1.4 Pressure:

       Pressure of a fluid is the force per unit area of the fluid. In other words, it is the ratio of force on a fluid to the area of the fluid held perpendicular to the direction of the force.

     v Vapor Pressure and Cavitation

Saturation temperature, T_sat – the temperature at which a pure substance changes phase at a given pressure.

 Saturation pressure, P_sat – the pressure at which a pure substance changes phase at a given temperature.

 Vapor pressure, P_v of a pure substance – t he pressure exerted by its vapor in phase equilibrium with its liquid at a given temperature.

 Partial pressure – the pressure of a gas or vapor in a mixture with other gases.

Possibility of the liquid pressure in liquid-flow system dropping below the vapor pressure at some location, and resulting unplanned vaporization. A fluid vaporizes when its pressure gets too low, or its temperature too high. All centrifugal pumps have a required head (pressure) at the suction side of the pump to prevent this vaporization. Suction cavitation occurs when the pump suction is under a low pressure/high vacuum condition where the liquid turns into a vapor at the eye of the pump impeller. Cavitation must be avoided in flow systems since it reduces performance, generates vibration and noise and causes damage to the equipment.

Saturation vapor pressure of selected liquids at 20°C.

1.5 Specific Volume:

Specific volume is the volume of a fluid (V) occupied per unit mass (m). It is the reciprocal of density. Specific volume is denoted by the symbol (v). Its unit is (m³/kg).

1.6 Specific Gravity:

           Specific gravity SG is the ratio of a fluid density to a standard reference fluid, typically   water at 4˚C (for liquids) and air (for gases) :

For Example, the Specific Gravity of common fluids