In contrast to the piston pump, however, the plunger does not completely fill the cross-section of the displacement chamber. In principle, the piston rod (plunger) is the displacement body here. It therefore does not move along the cylinder wall in a sealed manner. The cylindrical plunger piston is only sealed via a fixed stuffing box.
The inlet and outlet are each controlled by an automatic valve. During the suction stroke, negative pressure is created in the pump chamber. The valve to the suction line opens and liquid is sucked into the pump chamber. During the pressure stroke an overpressure is created in the working chamber, the valve opens to the pressure side and the piston pushes the liquid into the pressure line. In order to compensate for pressure surges, the pressure line in an air vessel contains an air cushion. It is compressed during the pressure stroke and also drives the liquid into the pressure line during the suction stroke due to its stored pressure force. This creates an even flow of liquid.