Pumps are in general classified as Centrifugal Pumps (or Roto-dynamic pumps) and Positive Displacement Pumps.
[h=Centrifugal Pumps (Roto-dynamic pumps)]3[/h] The centrifugal or roto-dynamic pump produce a head and a flow by increasing the velocity of the liquid through the machine with the help of a rotating vane impeller. Centrifugal pumps include radial, axial and mixed flow units.
Centrifugal pumps can further be classified as

  • end suction pumps
  • in-line pumps
  • double suction pumps
  • vertical multistage pumps
  • horizontal multistage pumps
  • submersible pumps
  • self-priming pumps
  • axial-flow pumps
  • regenerative pumps

[h=Positive Displacement Pumps]3[/h] The positive displacement pump operates by alternating of filling a cavity and then displacing a given volume of liquid. The positive displacement pump delivers a constant volume of liquid for each cycle against varying discharge pressure or head.
The positive displacement pump can be classified as:

  • Reciprocating pumps - piston, plunger and diaphragm
  • Power pumps
  • Steam pumps
  • Rotary pumps - gear, lobe, screw, vane, regenerative (peripheral) and progressive cavity

[h=Selecting between Centrifugal or Positive Displacement Pumps]3[/h] Selecting between a Centrifugal Pump or a Positive Displacement Pump is not always straight forward.
[h=Flow Rate and Pressure Head]4[/h] The two types of pumps behave very differently regarding pressure head and flow rate:

  • The Centrifugal Pump has varying flow depending on the system pressure or head
  • The Positive Displacement Pump has more or less a constant flow regardless of the system pressure or head. Positive Displacement pumps generally gives more pressure than Centrifugal Pump's.

[h=Capacity and Viscosity]4[/h] Another major difference between the pump types is the effect of viscosity on the capacity:

  • In the Centrifugal Pump the flow is reduced when the viscosity is increased
  • In the Positive Displacement Pump the flow is increased when viscosity is increased

Liquids with high viscosity fills the clearances of a Positive Displacement Pump causing a higher volumetric efficiency and a Positive Displacement Pump is better suited for high viscosity applications. A Centrifugal Pump becomes very inefficient at even modest viscosity.
[h=Mechanical Efficiency]4[/h] The pumps behaves different considering mechanical efficiency as well.

  • Changing the system pressure or head has little or no effect on the flow rate in the Positive Displacement Pump
  • Changing the system pressure or head has a dramatic effect on the flow rate in the Centrifugal Pump

[h=Net Positive Suction Head - NPSH]4[/h] Another consideration is the Net Positive Suction Head NPSH.

  • In a Centrifugal Pump, NPSH varies as a function of flow determined by pressure
  • In a Positive Displacement Pump, NPSH varies as a function of flow determined by speed. Reducing the speed of the Positive Displacement Pump pump, reduces the NPSH