What is a Centrifugal Pump?

Centrifugal Pumps Overview

Centrifugal Pumps are the most popular and commonly used type of pump for the transfer of fluids. In simple words, it is a pump that uses a rotating impeller to move water or other fluids by using centrifugal force. These are the undisputed pump choice especially for delivering liquid from one location to another in numerous industries including agriculture, municipal (water and wastewater plants), industrial, power generation plants, petroleum, mining, chemical, pharmaceutical, and many others.

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Centrifugal Pumps are useful since they can generally handle large quantities of fluids, provide very high flow rates (which may vary with the changes in the Total Dynamic Head (TDH) of the particular piping system) and have the ability to adjust their flow rates over a wide range.

Centrifugal pumps are generally designed and suitable for liquids with a relatively low viscosity that pours like water or light oil. More viscous liquids such as 10 or 20 wt. oils at 68-70 deg F will require additional horsepower for centrifugal pumps to work. For viscous liquids of more than 30 wt. oils, positive displacement pumps are preferred over centrifugal pumps to help lower energy costs.

The following information shall help you to understand more about these pumps and enable you to select the best kind of pump for your operations.

Working of a Centrifugal Pump

Let us understand in detail, how a Centrifugal pump works. Centrifugal pumps are used to induce flow or raise a liquid from a low level to a high level. These pumps work on a very simple mechanism. A centrifugal pump converts rotational energy, often from a motor, to energy in a moving fluid.

The two main parts that are responsible for the conversion of energy are the impeller and the casing. The impeller is the rotating part of the pump and the casing is the airtight passage which surrounds the impeller. In a centrifugal pump, fluid enters into the casing, falls on the impeller blades at the eye of the impeller, and is whirled tangentially and radially outward until it leaves the impeller into the diffuser part of the casing. While passing through the impeller, the fluid is gaining both velocity and pressure.

The following chief factors affect the performance of a centrifugal pump and need to be considered while choosing a centrifugal pump:

• Working Fluid Viscosity &#; can be defined as resistance to shear when energy is applied. In general, a centrifugal pump is suitable for low viscosity fluids since the pumping action generates high liquid shear.
• Specific density and gravity of working fluid &#; The density of a fluid is its mass per unit of volume. A fluid&#;s mass per unit volume and gravity of a fluid is the ratio of a fluid&#;s density to the density of water. It directly affects the input power required to pump a particular liquid. If you are working with a fluid other than water, it is important to consider the specific density and gravity since the weight will have a direct effect on the amount of work performed by the pump.
• Operating temperature and pressure &#; Pumping conditions like temperature and pressures are an important consideration for any operation. For example &#; High-temperature pumping may require special gaskets, seals and mounting designs. Similarly, an adequately designed pressure retaining casing may be required for high-pressure conditions.
• Net Positive Suction Head (NPSH) and Cavitation &#; NPSH is a term that refers to the pressure of a fluid on the suction side of a pump to help determine if the pressure is high enough to avoid cavitation. Cavitation refers to the formation of bubbles or cavities in liquid, developed in areas of relatively low pressure around an impeller and can cause serious damage to the impeller and lead to decreased flow/pressure rates among other things. One must ensure that the system&#;s net positive suction head available (NPSHA) is greater than the pump&#;s net positive suction head required (NPSHR), with an appropriate safety margin.
• Vapour pressure of the working fluid &#; The vapor pressure of a fluid is the pressure, at a given temperature, at which a fluid will change to a vapor. It must be determined in order to avoid cavitation as well as bearing damage caused by dry running when the fluid has evaporated.

Owing to the use in the diverse range of applications, pumps come with different capacities and in various sizes. You should also consider the pressure and volume requirements of the specific operations for which you need the pump. The horsepower required is another important consideration when it comes to volume and discharge pressure.

Applications of Centrifugal Pumps:

The fact that centrifugal pumps are the most popular choice for fluid movement makes them a strong contender for many applications and as mentioned previously, they are used across numerous industries. Supplying water, boosting pressure, pumping water for domestic requirements, assisting fire protection systems, hot water circulation, sewage drainage and regulating boiler water are among the most common applications. Outlined below are some of the major sectors that make use of these pumps:

1. Oil & Energy &#; pumping crude oil, slurry, mud; used by refineries, power generation plants
2. Industrial & Fire Protection Industry &#; Heating and ventilation, boiler feed applications, air conditioning, pressure boosting, fire protection sprinkler systems.
3. Waste Management, Agriculture & Manufacturing &#; Wastewater processing plants, municipal industry, drainage, gas processing, irrigation, and flood protection
4. Pharmaceutical, Chemical & Food Industries &#; paints, hydrocarbons, petrochemical, cellulose, sugar refining, food and beverage production
5. Various industries (Manufacturing, Industrial, Chemicals, Pharmaceutical, Food Production, Aerospace etc.) &#; for the purposes of cryogenics and refrigerants.

Types of centrifugal pumps

Centrifugal pumps can be classified into several types depending on factors such as design, construction, application, service, compliance with a national or industry standard, etc. Therefore, one specific pump can belong to different groups and at times pump is known by its description itself. Some of these groups have been highlighted below:

Depending on the number of impellers in the pump, pumps can be classified as per the following:

• Single stage &#; A one impeller pump, single stage pump has a simple design and easy maintenance. Ideal for large flow rates and low-pressure installations. They are commonly used in pumping services of high flow and low to moderate TDH (Total Dynamic Head).
• Two-stage &#; This type of pump has two impellers operating side by side which are used for medium head applications.
• Multi-stage &#; pump has three or more impellers in series; for high head service.

What is Pump Head? In simple words, the pump head is pressure defined as the height to which the pump can raise the fluid to. It is important as it evaluates a pump&#;s capacity to do its job. The most important specifications of a pump are its capabilities regarding flow and pressure.

Type of case-split

The Orientation of case-split is another factor used to categorize Centrifugal pumps:

• Axial split &#; In these kinds of pumps, the volute casing is split axially and the split line at which the pump casing separates is at the shaft&#;s center-line. Axial Split Pumps are typically mounted horizontally due to ease in installation and maintenance.
• Radial split &#; Here, the pump case is split radially; the volute casing split is perpendicular to the shaft center-line.

Categorized by type of impeller design

• Single suction &#; This kind of pump has a single suction impeller that allows fluid to enter the blades only through one side; It has a simple design but impeller has a higher axial thrust imbalance due to flow coming in on one side of impeller only.
• Double suction &#; This particular type of pump comes with a double suction impeller that allows fluid to enter from both sides of the blades and has lower NPSHR than a single suction impeller. Split-case pumps are the most common type of pump with a double suction impeller.

If a pump has more than one impeller, the design of the first stage impeller will determine if the pump is of a single or double suction type.

On the basis compliance with industry standards

While choosing a centrifugal pump, the buyers should be selective based on the quality standards they have to achieve. They need to check for the following:

• ANSI pump &#; (American National Standards Institute) &#; ANSI standards refer to dimensional standards. The pumps are also required to meet ANSI B73.1 standards, also known as ASME B73.1 &#; (American Society of Mechanical Engineers).  The objective of this standard is to ensure interchangeability of ANSI process pumps of similar sizes. These centrifugal pumps are horizontal, end suction, single stage pumps and are comparable regardless of manufacturer.
• API pump &#; (American Petroleum Institute) API&#;s standard refers to the parameters of pump&#;s construction, design, and ability to handle high temperatures and pressures. API 610 specifications and a variety of API types include API VS4, API VS7, API OH3, API OH2, API OH1, API BB1, API BB2, API BB3 etc. Centrifugal pumps must meet the requirements of the American Petroleum Institute Standard 610 for General Refinery Service.
• DIN pump &#; DIN specifications. Centrifugal pumps satisfying these standards are used in installations requiring large flow rates, abnormally high working pressures or very high temperatures. Rarely used in mechanical building services.
• ISO pump &#; ISO , specifications, the international standard ISO specifies the requirements for class II end suction centrifugal pumps of single-stage, multistage, horizontal or vertical construction, with any drive and any installation for general application.
• Nuclear pump &#; ASME (American Society of Mechanical Engineers) specifications

By type of volute

Centrifugal pumps can also be categorized based on volute namely Single volute and Double volute:

• Single volute &#; This kind of pump Is usually used in small low capacity pumps where a double volute design is impractical due to a relatively small size of the volute passageway which makes obtaining good quality commercial casting difficult. Pumps with single volute design have higher radial loads.
• Double volute &#; This kind of pump volute has two partial volutes which are located 180 degrees apart resulting in balanced radial loads; most centrifugal pumps are of double volute design.

Depending on where the bearing support is

Bearing support is also often used to categorize Centrifugal Pump:

• Overhung &#; where the impeller is mounted on the end of a shaft, supported by bearings on only one side. Further, the overhung pump type has a horizontal orientation of shaft or can be vertical in-line with bearing bracket.
• Between-bearing &#; where the impeller is mounted on a shaft that has bearing support on both ends, thus impeller is located in between-bearings. Examples are Axial Split Vertical Split Case

Depending on shaft orientation

Shaft orientation is another characteristic which distinguishes the type of Centrifugal pump:

• Horizontal &#; These are pumps with the shaft the in horizontal plane; popular due to ease of servicing and maintenance. It is sometimes overhung or placed between bearing design.
• Vertical &#; Vertical centrifugal pumps have their shaft in the vertical plane. They utilize a unique shaft and bearing support configuration that allows the volute to hang in the sump while the bearings are outside the sump. it is generally an overhung and of radial-split case type design.

The following highlights the difference between the above two:

Horizontal Centrifugal Pump 

• Easy availability of its rotor and internals makes it easier to install, inspect, maintain, and service.
• It can be coupled directly to a variety of drivers includian ng electric motor, engine, and turbine (steam, gas or power recovery hydraulic turbine
• It is available in either overhang design for low suction pressure service, or in between-bearing design for high suction pressure service.
• It is available in various nozzle configuration to simplify, or match the external site piping. The nozzle configuration can be of end suction top discharge, top suction top discharge, or side suction side discharge.
• Its low headroom requirement makes it suitable for most indoor installations.
• It has limited applications where the NPSHR exceeds the site NPSHA; Large pumps usually require an auxiliary booster pump. (With a vertical lineshaft pump, the NPSHA can be increased by lowering the setting of its impeller.
• Bigger footprint is required for horizontal designs.

Vertical Centrifugal Pump

• Most of them require large headroom for installation, servicing, and maintenance. Being of an overhang design, its hydraulic axial thrust is difficult to balance in high pressure service.
• Usually suitable for direct coupling to an electric motor. Using an engine or turbine, will require a right-angle gear drive and a universal shaft joint and a clutch.
• It can more easily withstand higher pressure service because of its simplified bolting and confined-gasket design
• It requires a smaller footprint and is suitable for installation where the ground surface area is limited, or is at a premium.
• With a vertical lineshaft pump, the impeller setting below the ground can be lowered to increase the site NPSHA.
• Vertical lineshaft turbine pumps, require large headroom for installation, servicing, and maintenance.
• Expensive sump pit and barrel in a multistage pump is usually required.
• There can be mechanical seal problems when pumping liquids with high dissolved or entrained gas which accumulates at the top of the stuffing box or seal chamber where venting can be difficult or less effective.

A good quality pump with superior components ensures efficiency and durability. Let&#;s now take a look at the major manufacturers of centrifugal pumps.

Leading Manufacturers of Centrifugal Pumps

Various Manufacturers provide a variety of Centrifugal pumps for different industry needs. A few leading examples of the manufacturer are Sulzer, Flowserve, Bingham, ITT, Goulds, SPX, National, Godwin, Aurora, Weir and Ebara.

A thorough research of these companies with respect to their range of products (types of pumps) and a good after-sales service (including training, preventive maintenance, spare parts, replacement pumps, retrofits, field services and repairs) is also required.

Power Zone is a leading global supplier of new, used and refurbished pumps of all leading brands. We stock an assortment of centrifugal pumps of all types and sizes, and can custom design and build pumps and packaged pump systems for your specific needs.

What are centrifugal pumps?

Centrifugal pumps are often utilized for pumping of low viscosity liquids e.g. water. The pump is particularly well adapted for pumping large volumes of low viscosity liquids.

Due to their high capacity flow rates, ease of maintenance and general robustness, the centrifugal pump is used extensively in many industrial applications all over the world.

Radial Centrifugal Pump With Volute Casing

 

Centrifugal Pump Components

 

Centrifugal pumps are relatively simple in design and have few components. Some of the most common components are listed below.

 

Volute Casing and Diffuser

A volute casing or diffuser is used to convert kinetic energy to pressure.

Volute Casing With Impeller (a)

An impeller rotates and imparts kinetic energy onto the liquid surrounding it due to friction. As the impeller rotates, the liquid moves towards the outer periphery of the impeller and its kinetic energy is changed to pressure.

 

Compression Packing and Mechanical Seals

Sealing of the pump is achieved using either compression packing (a.k.a. packing), or, a mechanical seal. Packing is a very old design (several thousand years) whilst mechanical seals are quite a recent development.

Mechanical Seal

Packing is the cheaper of the two options although it has several disadvantages compared to a mechanical seal. These include:

• Gland packing presses physically against the shaft and slowly creates a groove on the shaft which may later need to be repaired.
• Contact between the gland and shaft creates friction which manifests itself as heat. This heat must be removed to prevent damage to the shaft and gland.
• Friction between the gland and the shaft means that the pump motor requires more amps for the same amount of work completed; thus an overall drop in efficiency occurs.
• Packing must be periodically tightened to maintain the correct leakage rate.
• The service life of packing is generally far shorter compared to a mechanical seal.

 

Stuffing Box Arrangement

In addition to the above, a lantern ring may also need to be installed to allow cooling of the packing.

Mechanical seals do not suffer from the same problems associated with packing although they are more expensive and prone to leakage if the primary sealing faces are not completely clean.

 

Wear Rings

Wear rings are used to seal the space between the impeller and casing. Wear rings installed on the impeller are called &#;impeller wear rings&#;, whilst wear rings installed on the casing are referred to as &#;casing wear rings&#;.

Impeller Wear Ring

If no wear rings were installed, the process liquid would be able to flow from the discharge to suction side of the pump in relatively large quantities and this leads to a drop in pump efficiency (as much as 4%). It is possible to operate a pump without wear rings, but resultant damage to the impeller and casing will occur and the cost of replacing an impeller or casing is far greater than the cost of replacing the wear rings.

Rubbing of an impeller against the casing may also lead to a process known as &#;galling&#;, which means the two metals have micro-welded together. In severe instances, galling can lead to the pump seizing completely.

 

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How Centrifugal Pumps Work

The below video is an extract from our Introduction to Centrifugal Pumps Online Video Course.

 

Liquid is drawn in through the suction port (center hole in the casing) and discharged through the discharge port (hole on the top of the casing).

 

Suction and Discharge

Once the liquid has passed through the suction port, it passes through an impeller and is discharged radially away from the eye of the impeller (center of the impeller). The liquid flows away from the eye of the impeller due to centripetal force, although this flowing motion is almost always attributed to centrifugal force, which is incorrect as centrifugal force is actually an imaginary (not real) force.

Centrifugal Pump Impeller

The impeller has vanes separated by channels, the liquid flows through these channels. Each channel has an increasing flow path area. As the flow path area expands, the liquid&#;s velocity decreases and the pressure increases. The relationship between area, velocity and pressure, is described by Bernoulli&#;s Principle.

 

Impeller Channels and Vanes

Due to the unique shape of the volute casing, the liquid undergoes a further velocity decrease and pressure increase. The liquid is then discharged through the discharge port.

 

Classification by Flow

 

Centrifugal pumps are classified as either radial, axial or mixed flow.

Radial flow pumps discharge liquid perpendicular to the main pump shaft (90 degrees apart from the main pump shaft's orientation); this type of pump is ideal for many pressure and flow applications.

Radial Flow Centrifugal Pump

Mixed flow pumps discharge liquid at an angle exceeding 90 degrees relative to the pump shaft.

Mixed-Flow Centrifugal Pump

Axial flow pumps are used for low pressure, high flow, applications. Almost no radial force is imparted onto the liquid, but the pump is still classified as centrifugal because some small part of the liquid's movement is radial.

Axial Flow Centrifugal Pump

 

Classification by Impeller

 

Impellers are available in three common designs, these are closed, semi-open and open.

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Open, Semi-Open and Closed Impellers

Closed &#; closed type impellers have two shrouds. The vanes of the impeller are completely sandwiched between the two shrouds. This type of impeller is very efficient for pumping low viscosity liquids with few suspended bodies (water, sea water, etc.). The closed type impeller has the greatest mechanical strength of all impeller designs due to the support given to the vanes from the shrouds.

 

Closed Type Impeller

 Semi-Open/Semi-Closed &#; this type of impeller is also known as a &#;partially open&#; or &#;partially closed&#; type impeller. Semi-open impellers have only one shroud. This type of impeller is used for pumping liquids with a moderate amount of suspended bodies. Semi-open impellers are not as efficient as fully enclosed impellers because the pumped liquid is not guided directly along the vanes.

 

Semi-Open Type Impeller

Open &#; open type impellers have no shrouds. This type of impeller is ideally suited for pumping high viscosity liquids (providing they do not foam when agitated) and liquids with many suspended bodies. Typical applications for this type of impeller would include sewage and paper pulp. Sometimes the centre of the impeller will also be fitted with a serrated knife to chop suspended bodies as they are drawn into the eye of the impeller.

Open Type Impeller

 

Classification by Stage

Centrifugal pumps can be either single or multi-stage.

 

A single stage impeller is a pump with only one impeller. Single stage pumps typically use a volute casing, although a diffuser may be used if space is limited.

 

Single Stage Pump

A multi-stage pump is a pump with more than one impeller. Multi-stage pumps typically use diffusers because installing a volute casing for each impeller is impractical due to size considerations.

Multi-stage pumps are referred to by the number of impellers installed on a common shaft, for example, a five stage pump is a multi-stage pump with five impellers (shown below).

Multi-Stage Pump (5 stage)

Multi-stage pumps allow the pumped liquid to be passed through multiple impellers and diffusers before being discharged; this allows the pressure to be gradually increased at each pump stage.

 

Classification by Suction Type

Impellers can be either single suction or double suction.

Single suction impellers have only one suction inlet.

Double suction impellers have two suction inlets.

Single and Double Suction

 

Between Bearings or Overhung

Centrifugal pump impellers can be supported at one end in a cantilever arrangement, or, at both ends.

Impellers supported at only one end are referred to as overhung pumps, because the impeller is &#;hanging&#; within the casing.

Overhung Pump

Impellers supported on both sides are referred to as between bearings pumps, because the impeller(s) is installed between the shaft bearings. Large centrifugal pumps and multi-stage centrifugal pumps are almost always between bearings pumps.

Between Bearings Pump

 

Pump Curves

Pump curves are used to find the optimum pump conditions based upon certain characteristics, such as the liquid being pumped, pressure desired and flow rate desired. The characteristics desired depends upon requirements; some pumps have many conditions that must be satisfied before they are put into service.

Two of the most important characteristics of a pump curve are the shut-off head and pump runout.

The shut-off head represents the maximum amount of static head that can be generated by the pump.

The runout value is the maximum permissible flow through the pump without the pump incurring damage.

Pump Curve

Shut-Off Head &#; is the maximum amount of static head (sometimes called &#;total head&#;) that can be generated by the pump when operating at a certain speed. Once the shut-off head is reached, there is no flow.

Example

Imagine pumping a liquid through a 10m long vertical orientated pipe. You do not know the shut-off head, so you begin to drill holes in the vertical pipe until water flows-out. You drill at 10m in height, then 9m in height and water flows-out when you drill at 8m in height. You then cut the top 1 metre of the vertical pipe off and can see that the water level is at approximately 8.5m. You see that there is no flow. The water level you see represents the shut-off head.

In our example, the &#;height&#; can be classified as the &#;discharge head&#;, this is the pressure head that lifts the liquid from the pump discharge pipe to the final outlet.

The &#;static head&#; (a.k.a. total head) is the difference in vertical height from the top of the liquid being pumped to the highest point where the liquid is discharged.

The shut-off head is the total head value when no flow occurs.

Total Pressure Head (static head)

Runout &#; is the maximum permissible flow through the pump without damaging the pump. High flow rates often lead to cavitation which is not desired.

 

Cavitation

Cavitation occurs due to pressure variations encountered by the liquid as it travels through the pump impeller. Entrained vapour bubbles form and collapse due to this sudden pressure change. Whilst cavitation is inconsequential on a small scale, it is very damaging to the pump when repeated thousands of times per second.

Effect of Pressure Change On A Vapour Bubble (cavitation)

Cavitation often makes the pump sound as if marbles are being shaken inside the pump casing. If cavitation is suspected, remedial action must be taken to reduce or stop the cavitation as soon as possible.

 

Gas Binding

The liquid being pumped can be thought of as an essential part of the pump. Without liquid, the pump will not operate correctly. Gas binding refers to a situation where too little liquid is present within the pump and a negative suction pressure cannot be obtained. If no negative suction pressure can be obtained, no liquid can be drawn into the pump and no flow will occur.

Note: Positive displacement pumps do not suffer from gas binding because these types of pump are self-priming (can pump air).

 

Priming

Pumps that can pump air are referred to as 'self-priming'. Unlike positive displacement pumps, a centrifugal pump cannot pump air and thus is not self-priming. It is normally a requirement that a head of pressure is available when the pump is started, this often means the pump is installed below the level of liquid being pumped (the liquid is drawn into the pump due to gravity). Another means of priming the pump is to use an additional pump to feed the main centrifugal pump until suction is obtained.

Pump Installed Below Liquid Being Pumped

 

3D Model Details

This 3D model shows all major components associated with a typical centrifugal pump, these include:

• Impeller
• Volute Casing
• Shaft
• Mechanical Seal
• Suction and Discharge Ports
• Shaft Key
• Bearing
• Nuts and Bolts
• Wear Rings
• Compression Packing
• Lantern Ring

This is a 3D model of a Centrifugal Pump.  

 

3D Model Annotations

Discharge/Outlet

Fluid is discharged through this connection.

Suction/Inlet

Fluid is drawn into the impeller through this connection.

Wear Ring

An impeller wear ring is installed to reduce the clearance between the casing and impeller. Reducing the clearance reduces the amount of leakage from the discharge to suction side of the impeller; this ultimately improves the efficiency of the pump.

Impeller

Fluid flows into the eye of the impeller and then outwards radially. As the fluid moves outwards through the impeller vanes, its kinetic energy is converted to pressure energy. There are three types of centrifugal impeller, these are the closed, partially closed and open types; the type used depends upon what fluid is being pumped.

Volute Casing

Centrifugal pump casings are of the diffuser or volute type. Single stage pumps (one impeller) almost always utilise volute casings, whilst multistage pumps (>1 impeller) usually utilise diffuser casings.

Compression Packing

Compression packing seals the space between the shaft and casing. Compression packing is usually referred to simply as &#;packing&#;. An alternative to compression packing is the mechanical seal.

Stuffing Box

The area where the packing and lantern ring are installed is known as the &#;stuffing box&#;. The packing is literally &#;stuffed&#; into this space. On this model, the annotation marker has been placed above the stuffing box.

Lantern Ring

Lantern rings are used to distribute cooling liquid to the packing. The liquid cools and lubricates the packing, which helps prevent it overheating.

Gland Follower

A gland follower is used to compress the packing, but it is important a leak rate be maintained through the packing. The leakage rate should be measured in drops per minute and the gland follower should be adjusted if the leakage rate becomes excessive.

Ball Bearings

Bearings carry the axial and radial loads generated by the pump when it is stationary and in service. The type of bearing used depends upon many factors, although ball bearings are considered a suitable bearing for many service applications. Ball bearings are a type of anti-friction bearing.

 

Additional Resources

https://en.wikipedia.org/wiki/Centrifugal_pump

https://www.powerzone.com/resources/glossary/centrifugal-pump

https://www.introtopumps.com/pumps-101/what-is-a-centrifugal-pump

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