There are two main families of pumps: centrifugal and positive displacement pumps. In comparison to the latter, centrifugal pumps are usually specified for higher flows and for pumping lower viscosity liquids, down to 0.1 cP. In some chemical plants, 90% of the pumps in use will be centrifugal pumps. However, there are a number of applications for which positive displacement pumps are preferred.
The efficient operation of a centrifugal pump relies on the constant, high speed rotation of its impeller. With high viscosity feeds, centrifugal pumps become increasingly inefficient: there is greater resistance and a higher pressure is needed to maintain a specific flow rate. In general, centrifugal pumps are therefore suited to low pressure, high capacity, pumping applications of liquids with viscosities between 0.1 and 200 cP.
Slurries such as mud, or high viscosity oils can cause excessive wear and overheating leading to damage and premature failures. Positive displacement pumps often operate at considerably lower speeds and are less prone to these problems.
Any pumped medium that is sensitive to shearing (the separation of emulsions, slurries or biological liquids) can also be damaged by the high speed of a centrifugal pump’s impeller. In such cases, the lower speed of a positive displacement pump is preferred.
A further limitation is that, unlike a positive displacement pump, a centrifugal pump cannot provide suction when dry: it must initially be primed with the pumped fluid. Centrifugal pumps are therefore not suited to any application where the supply is intermittent. Additionally, if the feed pressure is variable, a centrifugal pump produces a variable flow; a positive displacement pump is insensitive to changing pressures and will provide a constant output. So, in applications where accurate dosing is required, a positive displacement pump is preferred.
The following table summarises the differences between centrifugal and positive displacement pumps.
Pump Comparison: Centrifugal vs Positive Displacement
| Property | Centrifugal | Positive Displacement |
| Effective Viscosity Range | Efficiency decreases with increasing viscosity (max. 200 Cp) | Efficiency increases with increasing viscosity |
| Pressure tolerance | Flow varies with changing pressure | Flow insensitive to changing pressure |
| Efficiency decreases at both higher and lower pressures | Efficiency increases with increasing pressure | |
| Priming | Required | Not required |
| Flow (at constant pressure) | Constant | Pulsing |
| Shearing (separation of emulsions, slurries, biological fluids, food stuffs) | High speed damages shear-sensitive mediums | Low internal velocity. Ideal for pumping shear sensitive fluids |
Centrifugal pumps are commonly used for pumping water, solvents, organics, oils, acids, bases and any ‘thin’ liquids in both industrial, agricultural and domestic applications. In fact, there is a design of centrifugal pump suitable for virtually any application involving low viscosity fluids.
| Type of centrifugal pump | Application | Features |
| Canned motor pump | Hydrocarbons, chemicals where any leakage is not permitted | Sealless; impeller directly attached to the motor rotor; wetted parts contained in can |
| Magnetic drive pump | Sealless; impeller driven by close coupled magnets | |
| Chopper/grinder pump | Waste water in industrial, chemical and food processing/ sewage | Impeller fitted with grinding teeth to chop solids |
| Circulator pump | Heating, ventilation and air conditioning | Inline compact design |
| Multistage pump | High pressure applications | Multiple impellers for increased discharge pressures |
| Cryogenic pump | Liquid natural gas, coolants | Special construction materials to tolerate low temperatures |
| Trash pump | Draining mines, pits, construction sites | Designed to pump water containing solid debris |
| Slurry pump | Mining, mineral processing, industrial slurries | Designed to handle and withstand highly abrasive slurries |