What are the differences between 'rotodynamic' or centrifugal pumps and displacement pumps?

Everything you need to know about 'rotodynamic' or centrifugal pumps and displacement pumps

The operating principle of a hydraulic pump is to provide energy to a fluid to displace or draw it. There are two main families of industrial pumps: centrifugal pumps and displacement pumps. Choosing between the two essentially depends on the viscosity of the fluid, which is defined according to its resistance to flow.

Centrifugal pumps generally make it possible to increase the fluid flow rate, while displacement pumps increase fluid pressure to enable transfer. Find out more about these two technologies, their operating principle and their main differences.

The rotodynamic or centrifugal pump

This is the most common type of industrial pump. They became popular with the advent of the combustion engine at the beginning of the 20th century and today represent nearly 75% of the international pump market. Their operating principle is based on the conversion of the mechanical energy of the impeller into kinetic energy.

What are centrifugal pumps?

They are used to transport fluids (their viscosity is limited). For example, in the cosmetic and agri-food industry, they are often used for the preparation of creams and product development. The particularity of these machines is to activate a centrifugal effect and transform mechanical energy into kinetic energy.

What are centrifugal pumps?

The operating cycle takes place in two steps:

Fluid suction is created by a depression in the centre of the wheel. The vane rotor or pump blades rotate clockwise at high speed. It propels the liquid, giving it speed and motion. This is known as kinetic energy, i.e. the energy of motion. As rotary speed increases, so does kinetic energy.

Fluid discharge is provided by the action of the pump's centrifugal force. Thanks to a spiral volute, a large part of the kinetic energy is transformed into driving pressure at the outlet.

A rotodynamic pump consists of an impeller housed inside an eccentric body, also called a volute. It is connected to a suction duct that is generally situated in the lower part, and a discharge nozzle, which is usually above.

Note that there is no separation between suction and discharge. It must be primed (or filled) to operate. A greater demand in pressure results in a lower flow rate.

Displacement pumps

Although this type of hydraulic pump also transforms mechanical energy into hydraulic energy, the mechanism is very different. Also called a volumetric pump, it uses variations in volume to suck in a fluid and force it to move in order to discharge it.

The basic principle of displacement pumps

They are used to move homogeneous fluids: thick or pasty fluids or abrasive fluids like gas. The viscosity of the fluid is essential for the proper operation of the pump. There are two types of displacement pumps: reciprocating pumps and rotary pumps. Their flow depends on the number of cycles or the rotary speed. A variation system enables the fluid flow to be modulated.

How does a displacement pump work?

The fluid is trapped in a chamber. Pressure forces it to move until it reaches the outlet. This is called hydrostatic energy transmission.

Reciprocating pumps

They operate via rectilinear displacement. Fluid is transferred by the alternating movement of one or more pistons or the deformation of an elastic diaphragm.

Operation is cyclical:

• First, the backward movement of the piston or the depression of the diaphragm causes the volume of the pump body to increase. As they are hermetically sealed, this creates a partial vacuum.
• The suction nozzle valve then opens to allow the fluid to fill this space.
• Next, the piston moves forward or the membrane moves upward and pressure increases in the pump body, creating a pulse that enables transfer.
• The suction valve then closes and the discharge valve opens to enable fluid outlet.
• The cycle is complete and can thus start over again

Rotary and semi-rotary pumps

Their principle is identical but these pumps operate by rotation to enable the fluid to circulate from the suction valve towards the discharge valve.

It is important to mention here that the operation of a displacement pump depends on the atmospheric pressure. This enables the arrival of the fluid to fill the vacuum.

There are several types of rotary pump:

• Vane pumps, equipped with an impeller fitted with vanes or blades.
• Screw pumps.
• External or internal gear pumps.
• Axial or radial piston pumps.
• Dosing pumps.

What are the differences between centrifugal pumps and displacement pumps?

Pumps adapted for different fluids

Centrifugal pumps are used with low-viscosity fluids like water. Indeed, the rotary movements of the pump can alter the nature of the transferred fluid. That is why they are not suitable for products that do not support being agitated or those that risk alteration. Displacement pumps are suitable for viscous products, such as oil, as well as delicate products. They are not very sensitive to the density of the fluid and counter-pressure. They are used at high pressure and low flow rates for dosing.

Differences in flow

For displacement pumps, which transfer an incompressible fluid, a regulating valve at the discharge is not useful or even dangerous. However, it is absolutely necessary for a centrifugal pump in which the flow is influenced by the pressure at the outlet.

Thus, the flow of a centrifugal pump is generally constant, while it is intermittent for a displacement pump. A suction phase follows a discharge phase. For this reason, this type of pump is equipped with a protection or discharge valve called a bypass. When the discharge line is plugged or obstructed, it creates a bypass (internal leakage).

Displacement pumps are often used for low pressures and medium flow rates, but certain models can reach very high pressure.

Priming

Centrifugal pumps are rarely self-priming. It is necessary to prime them before starting, i.e. they must be filled with the liquid to be pumped in order to expel the air inside the device.

There are several models that can be automatically primed if the suction height is not too high. On the other hand, displacement pumps are self-priming. It is therefore not necessary to fill them with liquid before starting. The vacuum created is sufficient to move the fluid.

Thus, the relatively simple mechanism of the centrifugal pump offers good performance at a reasonable price. In addition, they require little maintenance! However, they are only suitable for low-viscosity fluids. Displacement pumps are heavier for an equivalent flow. Their mechanism is more complex and bulkier. They are generally more expensive and require regular maintenance but are particularly suitable for dosing.