Despite the name, pistons have nothing to do with this pump’s operation. Like a gear pump, two rotors rotate in a close fitting housing – however, the rotors are driven independently on two rotating shafts. External timing gears ensure that the rotors do not contact.

As the rotors rotate, a cavity is formed at the inlet side of the pump – creating a vacuum that pulls liquid into the spaces between the tips of the rotor.

The liquid is trapped between the rotor and casing wall – and is carried around to the outlet. The tight clearance between the outside of the rotor and the hub of the other rotor ensures liquid does not pass between them.

At the outlet the reduction in size of the pumping cavity forces liquid through the outlet.