Positive Displacement Pump Types

The purpose of a pump is to move fluids. The two types of pumps are dynamic pumps, which use velocity to move fluid, and positive displacement pumps, which move fluid by trapping a fixed amount of fluid and then forcing (displacing) that trapped volume into the discharge pipe.
According to Rensselaer Polytechnic Institute, the three main positive displacement pump types are reciprocating, metering and rotary pumps.
Reciprocating Pumps
Reciprocating pumps use a piston and cylinder with suction and discharge valves built into the pump. The pump draws a volume of liquid into the cylinder through the suction valve on the intake stroke and discharges the liquid under positive pressure through the outlet valves on the discharge stroke.
Classifications of reciprocating pumps include direct acting or indirect acting, single or double acting, high pressure or low pressure, and vertical or horizontal.
One type of reciprocating pump is the direct-acting steam pump, which has a steam cylinder end that is in line with a liquid cylinder end, with a straight rod connection between the steam piston and the pump piston or plunger.
The diaphragm pump, which uses use a flexible diaphragm to move liquid, is a direct-acting reciprocating pump, commonly used to pump water from a ditch or sump.Another type is the power pump, which converts rotary motion to low-speed reciprocating motion using a speed-reducing gear. Power pumps are generally very efficient and can develop high pressures but do tend to be expensive.
Metering Pumps
Metering pump provides precision control that delivers a specific amount fluid into a system. Metering pumps are available that are compatible with a variety of flows, pressures and fluids. According to Engineer's Edge, metering pumps are used to control contamination or leakage between fluid systems, provide specific control flow relative to discharge pressure, maintain an accurate fluid output capacity or isolate a fluids system.
Rotary Pumps
Rotary pumps use a rotating vane, screw or gear to progressively create one or more chambers with each revolution of the drive shaft. Clearances between rotating parts, and between rotating and stationary parts, are kept to a minimum to reduce slippage (leakage of fluid from the pump's discharge back to its suction). According to Engineer's Edge, the close clearances in rotary pumps makes it necessary to operate them at relatively low speed for reliable operation and to maintain pump capacity.
The many types of positive displacement rotary pumps generally fall into three categories: gear pumps, screw pumps and moving vane pumps.