Both reciprocating and rotary pumps are positive displacement pumps. Positive displacement pumps differ from rotodynamic pumps in that they move a constant volume of fluid with each rotation of their shaft. The flow of a positive displacement pump is directly proportional to the rotational speed. For example, if 100 rotations per minute (rpm) results in 10 gallons per minute (gpm), 200 rpm will result in 20 gpm. For multistage pumps, a low net positive suction head required (NPSHr) first stage can be added to supply the second stage—which is especially applicable when the second stage has a higher NPSHr than the first stage.
A reciprocating pump uses a back-and-forth motion of a piston, plunger or diaphragm. Image 1 shows an air-operated double-diaphragm (AODD) pump, which uses two opposing diaphragms to draw fluid from the inlet manifold to the liquid chamber and then expel it to the discharge manifold. Check valves are used to prevent back flow from the discharge manifold into the liquid chamber or from the liquid chamber to the inlet manifold.
The reciprocating action results in a pulsating flow rather than a consistent flow, which may require devices on the inlet and outlet to dampen pulsations. Another type of reciprocating pump, the reciprocating power pump, is often a multiplex design, which has many pistons or plungers connected to a crank shaft. Each reciprocates out of phase from one another. Having more pistons or plungers in a multiplex design will increase the total flow rate and decrease the pulsation.
Rotary pumps have a chamber that contains meshing gears, cams, lobes, screws, vanes or similar elements that rotate relative to the casing. Image 2 shows a rotary lobe pump. In this design, the meshing lobes trap a volume of liquid and displace it from the inlet to the outlet, which is a similar concept to other rotary pump types. It is easy to see how the meshing action limits shearing of the liquid, and this is the reason they are often used with viscous and shear-sensitive liquids. The meshing action of rotary pumps does not create the same level of pulsating flow as reciprocating pumps, and they do not require inlet and outlet check valves like reciprocating pumps. Leakage is instead controlled by close running clearances.
Refer to the Positive Displacement Pump Basics Training at training.pumps.org.
