Plunger pumps, also often just called piston pump by layman, belong to the group of oscillating positive displacement pumps - one of the oldest pump types. The displacers in plunger pumps are moved/pressed by crankshafts which is integral to the pump and the pump has one or more cylinders. The displacer of this pump is called “plunger”.

In contrast to piston pumps the plunger “dives” through a fixed seal into a sealed off working space, displacing the fluid. The seal in piston pumps is oscillating with the displacer (= piston) in the same time. Check valves in the inlet and outlet ports of the cylinder make sure that the fluid is being pushed/pressed in the right direction.

At least one plunger dives in its cylinder on each crankshaft’s rotation and gets withdrawn once taken back by the crankshaft. The plunger diving into the cylinder changes displaces a volume of fluid and, whilst withdrawing it draws in a volume of fluid i.e. the pump is pumping. In order to keep the pulsation of the pumped fluid as low as possible, an odd number of cylinders is used.