A positive displacement pum(PD) moves a fluid by enclosing a fixed volume repeatedly and moving it mechanically through the pump system. The pumping action of this pump is cyclic and it can be driven by gears, screws, pistons, diaphragms, vanes and rollers.
How does a PD pump work?
There are different types of PD pumps and they come in a wide variety of designs but the majority can be placed into two main categories: rotary and reciprocating pumps.
Reciprocating PD pumps
Reciprocating PD pumps work by the repeated strokes of either a plunger, piston or diaphragm. These cycles are known as reciprocation.
Piston PD pumps
In piston PD pumps, the first piston stroke creates a vacuum. It then opens an inlet valve while closing the outlet valve at the same time. It then draws the fluid into the piston chamber. As the piston’s motion reverses, the inlet valve is closed and the outlet valve is opened. This allows the fluid in the piston changer to be discharged. The bicycle pump is a very basic example that uses this technique. Piston pumps can be double acting with outlet and inlet valves on both sides of the piston. Whilst the piston of the pump is in suction on one side, on the other side, it is in compression. We have radial versions of these valves that are more complex and are often used in industrial applications.
Plunger PD pumps
Plunger positive displacement pump types operate in a similar way to piston pumps. The volume of the fluid moved by the piston pump depends on the volume of the cylinder but in a plunger pump, the volume depends on the size of the plunger. The seal around the plunger or the piston is important to maintain the pumping action. This helps to avoid leaks. Generally, a plunger pump seal is very easier to maintain since it is stationary at the top of the cylinder of the pump while the seal around the piston is moving up and down repeatedly inside the pump chamber.
The diaphragm PD pumps
Diaphragm PD pumps use a flexible membrane instead of a plunger or piston to move fluid. By expanding the diaphragm, the pumping chamber volume is increased and the fluid is drawn into the diaphragm pump. Compressing the diaphragm reduces the volume and expels some fluid. Diaphragm PD pumps are hermetically sealed and this makes them ideal for hazardous fluids pumping.
Reciprocating pumps have a cyclic action that creates pulses in the discharge with the fluid accelerating during the compression phase. It slows during the suction phase. This can sometimes cause damaging vibration in the installation. Often, some form of smoothing or damping is employed. Pulsing can be reduced by using two or more diaphragms, pistons, plungers with one in its compression phase while the other in the suction phase.
The predictable and repeated action of a positive displacement pump makes it ideal for applications where accurate dosing or metering is required. By altering the rate of strokes, it is possible to provide measured fluid quantities.