C. Pressure In some systems it is necessary to increase the pressure of the liquid for and reasons. In addition to moving the liquid over changes in elevation through a piping system, the pressure of a liquid must often be increased in order to move the liquid into a pressurized vessel, such as a boiler or fractionating tower, or into a pressurized pipeline. Or, it may b necessary to overcome a vacuum in the supply vessel. D. Velocity There is another factor to be considered here, namely that not all of the velocity energy in a pump is converted to potential or pressure energy. The outlet or discharge connection of most pumps is smaller than the inlet or suction connection. Because liquids are, practically speaking, incompressible the velocity of the liquid leaving the pump is higher than it is entering the pump. This velocity head may need to be taken into account(depending on point of reference), when computing pump total head to meet system requirements. This is discussed further in Chapter 2, Section III