What type of pump is classified as constant (positive) displacement?

• A. Gear type pump
• B. Centrifugal pump
• C. Piston pump
• D. Diaphragm pump

Answer: (A)

A constant (positive) displacement pump moves a fixed volume of fluid with each cycle, so the output depends on speed rather than the system’s pressure. A gear-type pump achieves this by trapping fluid in the spaces between intermeshing gears and the housing. As the gears rotate, those trapped pockets are carried from the inlet to the outlet, delivering the same amount of fluid per revolution. Therefore, at a given rotational speed, the flow is essentially constant, until back pressure reaches the pump’s limit. This behavior contrasts with centrifugal pumps, where flow is highly sensitive to system resistance and back pressure. Piston and diaphragm pumps are also positive-displacement devices, but gear pumps are the classic, straightforward example of constant displacement due to their fixed-volume trapping mechanism.