A positive displacement pump is a pump used in industry that provides a constant flow at a certain speed, regardless of any pressure changes. For this reason, they are sometimes known as constant volume pumps. They add energy to the fluid via a mechanical device for example gears, a piston or a plunger. So, what is a positive displacement pump exactly, and how does it work?
What is a Positive Displacement pump?
A positive displacement pump is an industrial pump that works by drawing fluid into the pump chamber at the inlet valve and then discharging it through the outlet valve, this ‘displacing’ the liquid.
This occurs by many different actions such as the turning of gears, the use of pistons or a diaphragm or reciprocating system that moves the fluid through. This adds energy to the liquid by applying force with the chosen mechanism. That’s why there are so many different types of positive displacement pump, because of the variety of mechanisms used in these pumps.
Types of Positive Displacement Pump
There are three main categories of positive displacement pump: rotary (cyclical or turning motion), reciprocating (forward or backward motion) and the linear type. Here are specific examples of these:
- Rotary Pumps - screw pump, gear pump, rotary vane pump
- Reciprocating pumps - piston pumps, plunger pumps, diaphragm pumps
- Linear pumps - Rope pumps, chain pumps.
Advantages a Positive Displacement Pump
A positive displacement pump is a good choice when you need a more or less constant flow regardless of pressure. These pumps are good in variable pressure situations.
A positive displacement pump also increases flow as viscosity goes up and so this type of pump is good for transporting high viscosity fluids. However, they are also excellent for transporting thinner liquids like solvents or ammonia.
Positive displacement pumps are a good choice for shear sensitive liquids, particularly gear pumps.
Because positive displacement pumps tend to operate at lower speeds than centrifugal pumps, this has a positive impact on the life of the seals. Seals therefore last longer in positive displacement pumps meaning less maintenance.
Displacement pumps are often used as ‘metering pumps’ because they can accurately control the flow rate by altering the speed at which it is driven. Some examples of this type of positive displacement pump include a leak-proof diaphragm pump or precision piston pump.
Disadvantages of a Positive Displacement Pump
On the downside, positive displacement pumps are low in efficiency and they can’ always deliver a pulsating free flow. A reciprocating pump or pulsating pump may be better for this. In general positive displacement pumps are often higher maintenance than other centrifugal pumps.
Where are Positive Displacement Pumps used?
- Progressive cavity pumps - to transport difficult liquids
- Gear pumps in ship engines
- Hydraulic systems to transport water
- A piston pump in a bicycle pump
- Lobe pump in chemical and food industries
- Van pump in automotive transition systems
- Screw pumps in oil production and fuel transfer
If you are interested in find out more about the different types of positive displacement pumps then you might also like to read about gear pumps, diaphragm pumps and vane pumps.
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