Centrifugal pumps - Useful Information

What is a centrifugal pump?

A centrifugal pump is a mechanical device designed to move a fluid by means of the transfer of rotational energy from one or more driven rotors, called impellers.  Fluid enters the rapidly rotating impeller along its axis and is cast out by centrifugal force along its circumference through the impeller’s vane tips.  The action of the impeller increases the fluid’s velocity and pressure and also directs it towards the pump outlet.  The pump casing is specially designed to constrict the fluid from the pump inlet, direct it into the impeller and then slow and control the fluid before discharge.

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How does a centrifugal pump work?

The impeller is the key component of a centrifugal pump.  It consists of a series of curved vanes.  These are normally sandwiched between two discs (an enclosed impeller).  For fluids with entrained solids, an open or semi-open impeller (backed by a single disc) is preferred (Figure 1).

Fluid enters the impeller at its axis (the ‘eye’) and exits along the circumference between the vanes.  The impeller, on the opposite side to the eye, is connected through a drive shaft to a motor and rotated at high speed (typically 500-rpm).  The rotational motion of the impeller accelerates the fluid out through the impeller vanes into the pump casing.

There are two basic designs of pump casing: volute and diffuser.  The purpose in both designs is to translate the fluid flow into a controlled discharge at pressure.

In a volute casing, the impeller is offset, effectively creating a curved funnel with an increasing cross-sectional area towards the pump outlet.  This design causes the fluid pressure to increase towards the outlet (Figure 2).

The same basic principle applies to diffuser designs.  In this case, the fluid pressure increases as fluid is expelled between a set of stationary vanes surrounding the impeller (Figure 3).  Diffuser designs can be tailored for specific applications and can therefore be more efficient.  Volute cases are better suited to applications involving entrained solids or high viscosity fluids when it is advantageous to avoid the added constrictions of diffuser vanes.  The asymmetry of the volute design can result in greater wear on the impeller and drive shaft.

What are the main features of a centrifugal pump?

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.

What are the limitations of a centrifugal pump?

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.

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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 

What are the main applications for centrifugal pumps?

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 

Summary

A centrifugal pump operates through the transfer of rotational energy from one or more driven rotors, called impellers. The action of the impeller increases the fluid’s velocity and pressure and directs it towards the pump outlet.  With its simple design, the centrifugal pump is well understood and easy to operate and maintain.

Centrifugal pump designs offer simple and low cost solutions to most low pressure, high capacity pumping applications involving low viscosity fluids such as water, solvents, chemicals and light oils.  Typical applications involve water supply and circulation, irrigation, and the transfer of chemicals in petrochemical plants.  Positive displacement pumps are preferred for applications involving highly viscous fluids such as thick oils and slurries, especially at high pressures, for complex feeds such as emulsions, foodstuffs or biological fluids, and when accurate dosing is required.

What is a centrifugal pump?

Centrifugal pumps are classified as a dynamic pump type that use centrifugal force to turn mechanical energy into hydraulic energy. They are by far the most commonly used pumps on the market because they are reasonably priced and offer a good price/performance ratio for high flow applications. However, centrifugal pumps have a number of limitations that make them less than ideal for a range of industrial and hygienic applications. Electric diaphragm pumps solve many of these common issues.

Disadvantages of centrifugal pumps

Centrifugal pumps have several disadvantages compared to diaphragm pumps. Graco’s QUANTM pump is an electric double diaphragm pump (EODD) that addresses many of these common disadvantages of centrifugal pumps.

• Narrow optimum operating range and efficiency point
  Centrifugal pumps have a specific impeller diameter, which means they only operate at optimum efficiency at a specific flow. Any flow range below or above this best efficiency point (BEP) is quite narrow. Not only does working outside the preset range significantly reduce the pump’s efficiency, but eventually, it also leads to cavitation, vibration, impeller damage, suction and discharge recirculation, or reduced bearing and seal life forcing complete rebuilds of the whole unit. QUANTM electric diaphragm pumps are well suited to varying flow, pressure, and applications. It poses no risk for shear-sensitive liquids and can easily handle abrasives and solids.
• Impact of changes in viscosity
  Some liquids can vary in viscosity due to temperature, chemical reaction or separation (varied layers or solids). The performance of a centrifugal pump is especially impacted when pumping more viscous liquids, because of the increased resistance to flow as the impeller rotates. The volume pumped by a centrifugal pump depends on head (back pressures) viscosity and the homogeneity of the material. If these values vary dynamically in a certain application, it is difficult to keep a centrifugal pump operating close to its best efficiency and also without prematurely wearing out the pump.
• Damage caused by deadheading
  When a discharge valve is closed or a line blockage occurs, it causes the fluid to recirculate in the pump, and pressure to continue to build resulting in a build-up of motor temperatures. When a pump is run in a deadheaded condition for too long, the excess heat damages the expensive seals and reduces pump life. Without purchasing additional sensors and controllers, a centrifugal pump will continue to run until something fails. The extra cost of these additional sensors and controllers needs to be included in the total cost evaluation.
• Not self-priming
  Centrifugal pumps are not self-priming. For the pump to work properly, its casing must be filled with liquid before start-up. When the casing fills with vapors or gases, the pump impeller becomes gas-bound and incapable of pumping. To make sure the pump remains primed and does not become gas-bound, centrifugal pumps need to be installed below the fluid level, from which the pump takes its suction. Alternatively, the pump can be primed by supplying liquid under pressure through another pump placed in the suction line. QUANTM pumps are self-priming and have excellent suction capabilities.
• Unsuitable for abrasives and solids
  Particularly in the chemical industry, selecting a centrifugal pump for pumping solids is quite a challenge, as process owners need a pump that is not only compatible with the chemicals being processed, but also capable of handling solids without choking the impeller and thus causing increased pump wear and potential blockages. The centrifugal pump types that are suited to this specific set of applications tend to have lower performance areas and be more expensive.
• Unsuitable for shear-sensitive liquids
  High-speed systems, such as the centrifugal pump, tend to shear liquids which is why this technology is not the best choice for shear-sensitive liquids.
• Unsuitable to run dry
  A centrifugal pump cannot run dry without causing damage to the system. The system needs to have resistance to dissipate rotation speed of the impeller. The QUANTM electric diaphragm pump can run dry without causing any damage to the system to avoid expensive repairs.

Common centrifugal pump applications

While centrifugal pumps are commonly used in various industries because of their efficiency with low viscosity materials and high flow rates, they may not always be the best fit for every application. Here’s a look at how centrifugal pumps are used in specific industries and QUANTM’s potential advantages:

• Food and Beverage Manufacturing
  Centrifugal pumps are commonly used to transfer beverages and other low-viscosity liquids in the food industry. But the forces generated by centrifugal pumps can sometimes damage sensitive fluids, altering quality. QUANTM pumps are able to maintain the integrity of shear-sensitive products like creams and yogurts. Additionally, their hygienic design ensures compliance with food safety standards.
• Water Treatment and Transfer
  Centrifugal pumps are often used for their ability to move large amounts of water, but they can struggle with varying pressure conditions and the presence of solids. QUANTM pumps are able to handle a wider operating range, including higher solid content fluids without the risk of clogging. Their resistance to harsh conditions makes them an ideal alternative for water treatment applications.

Centrifugal pumps vs diaphragm pumps