TABLE OF CONTENTS Positive displacement pump performance has been included in this section on sewage pumps since most of the applications for these pumps in the water industries are in sewage plants. Some applications do exist in water treatment plants, but they do not affect appreciably the energy consumption of those plants.
As indicated in Chap. 7, positive displacement pumps can be divided into two categories, rotary pumps and reciprocating pumps. Rotary pumps can be any positive displacement pumps whose pumping device rotates around the pump shaft. Major classifications are sliding vane, flexible element, lobe, gear, progressive cavity, and screw pumps. Reciprocating pumps are also called power pumps and are those that depend on a linear action to move the liquid through the pump. Major classifications are plunger, diaphragm, and piston type pumps. Description of these pumps is included in Chap. 7.
All positive displacement pumps provide a volume of liquid with each rotation or stroke. Unlike a centrifugal pump, these pumps do not create any pressure themselves. The discharge piping and valves develop the pressure imposed upon the discharge of the positive displacement pumps. The positive displacement pumps have one affinity law in common with centrifugal pumps and that is that flow varies directly with speed. This is a general assumption, as the positive displacement pumps must...
