A distinction is made according to the movement type of the displacer. The types are rotary and reciprocating positive displacement pumps.
Oscillating positive displacement pumps include
- Piston pump
- Diaphragm pump
The following constructions are counted among the rotating positive displacement pumps:
- Screw pump
- Progressing cavity pumps (eccentric screw pumps)
- Rotary lobe pump
- Gear pump
- Screw pump
- Peristaltic pump
In opposition to the centrifugal pumps the positive displacement pumps are suitable for a very high viscosity of the pumped fluid. Furthermore the reciprocating pumps can be suitable for metering and dosing applications.
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.
There are different types of piston pumps (reciprocating piston pumps):
- the submersible piston pump, also called plunger pump,
- the disc piston pump and
- the diaphragm pump.
By asynchronously opening and closing the inlet respective outlet of the pump chamber, the fluid will moved in the pump chamber and then pressed to the discharge side.
Reciprocating pumps are generally used for applications that require relatively low flows and high pressures. Since each cylinder delivers a definite volume of liquid to the system, these pumps are also applicable for dosing and metering tasks. The disadvantage against rotating pumps is the pulsating operation.
