Which types of pumps are examples of positive displacement pumps?

Positive displacement pumps are designed to move fluid by trapping a fixed amount of it and forcing (displacing) that trapped volume into the discharge pipe. This is in contrast to centrifugal pumps, which use rotational energy to move fluid without trapping any specific volume.

Diaphragm pumps and piston pumps are both prime examples of positive displacement pumps. Diaphragm pumps use a flexible diaphragm that moves back and forth to create a vacuum that draws fluid in and then pushes it out as the diaphragm moves in the opposite direction. Piston pumps operate by using a piston to create a high-pressure seal that displaces a specific volume of fluid with each stroke.

By focusing on the way these pumps function, it's clear that they fit the definition of positive displacement because they consistently move a specific amount of fluid, regardless of the fluid’s viscosity or pressure in the system. In contrast, other types of pumps listed do not operate on the principle of displacing a defined volume.