Centrifugal pumps are a type of rotodynamic pump which uses a shaft-mounted impeller that rotates unidirectionally within the pump casing.
These pumps can be subdivided based on the type of impeller used:
- Axial flow pumps
Axial flow pumps (sometimes also known as propeller pumps) use an impeller typically driven by an electric motor with three or four vanes that direct the flow axially, which sets them apart from other types of centrifugal pumps where the flow is directed more radially. Some configurations can have their flow and head adjusted by altering the pitch of the impeller vanes.
Due to the axial orientation of the impeller vanes, these pumps produce very low pressure (head) (10-20 feet) compared to radially orientated centrifugal pumps, but they can also produce much higher flow rates, as high as several hundred thousand gallons per minute – the highest of a centrifugal pump type.
These pumps have different performance characteristics than other centrifugal pump types, with the curve of head versus capacity much steeper. Shut-off head can be as much as three times the head of its BEP (best efficiency point), and the required horsepower increases as the flow decreases, which is opposite to radial flow pumps.
These pumps are ideal for applications with high flow rate and low pressure requirements. They are commonly used in power plants for circulating water, sewage digesters, evaporators, and in the chemical for moving large amounts of fluid in vapours, and can be used in dewatering and irrigation applications where large amounts of water needs to be moved a short distance.
- Mixed flow pumps
Mixed flow pumps use a mixed flow impeller (also called diagonal impeller) that discharges fluid in a conical direction. Their application range covers the transition gap between radial flow pumps and axial flow pumps.
These pumps can have high or low specific speed. Low speed types combine impellers with an annular or volute casing and speeds between 35-80rpm. High speed types combine impellers with a diffuser or tubular casing and can reach speeds between 80-160rpm. Mixed flow pumps with a tubular casing are commonly installed in a vertical arrangement, and used in multistage configurations for high flow applications such as irrigation, municipal water supply and thermal power plants.
The main advantage of this type of pump is that they are a “jack of all trades”, combining the higher mass flow of axial pumps and the higher pressures achieved by radial pumps.
- Radial flow pumps
Radial flow pumps are the most common type of centrifugal pump. The radial outward movement of the flow in the impeller causes higher centrifugal forces which results in high discharge pressures but typically lower volume flow rates. The delivery head achieved is proportional to the impeller diameter, with several impellers in a series needed to achieve higher head.
The main benefit of this type of pump is that it can be used in a wide range of applications; there are various designs possible such as multi phases and self-priming; they’re pulsation free; high efficiencies can be achieved; their simple design reduces susceptibility and lowers maintenance costs; and because there is no axial thrust, they have a low bearing load and high service life.
Regardless of the impeller type, these pumps can be either single stage, where the total head is the result of one impeller, or multistage, with a series of impellers aligned in one casing.
Depending on requirements, centrifugal pumps can be either self or non self-priming and can have open, semi-open or closed impeller types.
Centrifugal pumps can also be identified by their basic mechanical configuration or characteristics. For example:
- Overhung impeller type where the impeller is mounted on the end of the shaft that is cantilevered from its bearing supports
- Impeller between bearings type where the impeller is mounted on a shaft between bearings situated at both ends
- Turbine type which usually refers to vertical multistage deep well type pumps with diffuser casings attached to one another
Here are some of the most common types of centrifugal pumps.
Types of centrifugal pumps
Submersible pumps are a centrifugal type pump designed to be either fully or partially submerged in water as the motor is hermetically sealed and close-coupled to the body, preventing it from being compromised.
As they do not use a motor above grade, floor space is better utilised. Furthermore, they tend to have lower maintenance costs, reduced sound levels, and are self-priming.
Many common types of pump can be designed to be submersible, with the exact type of motor and pump used dependent on the application and installation details. But generally, there are two main types, bore well, and those that operate in a sump, wet well or basin.
Depending on the pump design, they can be used in a number of different applications, from pumping large solids or grinding solids to smaller sizes, to transferring wastewater at large flow rates or high pressures, to pumping up water off the floor or the bottom of a tank. Some of the most common applications they are designed for are water, sewage, 12 volt, sand, irrigation, and solar.
API process pumps
API process pumps are designed to meet the American Petroleum Institute (API) 610 standard for pumps handling hydrocarbons. As they are intended to be used to handle hydrocarbons at extremely high pressures and temperatures, they are designed and manufactured very conservatively when it comes to casing thickness, flange design, weldments, allowable shaft deflection, bearing design, etc. to ensure hydrocarbons are not released which will cause them to burn or explode, and to prevent pump failure which can cause significant loss to the owner in terms of lost production.
These pumps come in many forms, may employ a number of pumping mechanisms and come in different configurations. The API 610 standard is limited to centrifugal pumps, with end suction pumps, horizontal split case pumps, multistage pumps, vertical inline pumps and vertical turbine pumps some of the types that can be used as an API 610 Pump. There are some types of centrifugal pumps such as close coupled pumps which cannot be used for this purpose as they are not designed conservatively enough or are unable to handle the high temperatures and pressures required.
These pumps often have explosion-proof motors, robust enough that frequent servicing is not required, and are always supplied with closed impellers which have the wear rings pinned or welded to them to make sure they don’t slip off from differential thermal expansion.
These pumps are primarily found in refineries as they are designed and certified to be used in the petroleum industry to handle oil, gasoline and other hydrocarbons, but they can also be found in other industries where a conservative and robustly designed pump may be desired such as in the power industry.
Canned motor pumps
Canned motor pumps are designed to be sealless and have no leakage. They have a motor that is close coupled to the pump and contained in a hermetically sealed chamber. The rotor is exposed to the pumped fluid to lubricate the sleeve bearings and thrust surfaces, with the motor stator isolated from the fluid by a metallic can to ensure it stays dry. As there is no need for a mechanical seal, these pumps are cheaper to maintain than other types.
The design of these pumps make them ideal for moving dangerous or difficult fluids like radioactive coolant, heat transfer liquids that are proven to evaporate when using mechanically sealed pumps, and extremely corrosive or noxious liquids. They are commonly found in the nuclear power and chemical industries, and other applications where the fluid cannot be released into the environment.
As the name suggests, circulator pumps are designed to circulate water through a closed system where the fluid runs in a loop between the discharge line and the suction without ever being exposed to atmospheric pressure. Where they are used in a completely closed system, there is usually also an expansion tank attached to the pump’s suction line to prevent the pipes from becoming over pressurised and bursting as the water expands.
They are most commonly single stage. Small versions that are found in homes and small commercial hydronic heating systems are designed to be close coupled to eliminate the need for a mechanical seal, and include a centrifugal wet rotor. On the other hand, high flow versions are designed as vertical inline pumps with mechanical seals.
These pumps are commonly used in hot water heating and cooling systems, and applications where chemicals need to be regularly mixed into the fluid such as in swimming pools and spas.
Drum pumps are designed to eliminate the need of turning the pump drum to remove the fluid inside for applications where fluid weight makes the manual tipping of the drum impractical. These pumps have a vertical shaft inside a narrow tube that can fit into the top opening of a standard 55 gallon drum. At the top of the tube there is a small AC or DC electric motor, often with a trigger switch, that is connected to the shaft. At the bottom of the tube is the pumping element which may be a small centrifugal impeller for thin liquids, or a progressive cavity rotor and stator for thicker, more viscous fluids.
These pumps are generally limited to flows of about 70 gallons per minute, and to pressures (heads) of about 70 feet, although positive displacement types can produce higher pressures. They are typically limited to immersion depths of about four feet.
These pumps are usually portable, but they can also be hard piped within the system. They are commonly used in chemical, food processing, and other manufacturing food and processing plants that purchase fluids or pasty products in large containers and need to pump small quantities of the contents out of the container. Hand-cranked drum pumps are also available and common in the food and agricultural industries.
End suction pumps
End suction pumps are the most common and least expensive types of centrifugal pumps that have a casing with the suction on one end and the discharge at the top. They are usually single stage with one impeller and a volute type casing. The impeller raises the velocity of the fluid as it moves along its vanes. The fluid then moves into the casing, where the velocity is converted to high pressure.
These pumps normally use an AC motor, but other motor types are available; and can be either close coupled or frame mounted.
As these pumps come in a range of sizes, materials and designs, they can be used for a wide variety of industries such as residential, commercial and industrial for applications ranging from water at ambient temperature, to high pressure and temperature viscous oils, abrasive slurries, and corrosive chemicals.
Grinder pumps are equipped with a cutting system that enables it to grind solids present in the fluid to prevent the pump and adjacent pipe from clogging, aiding in the transportation of the fluid. These pumps are typically a submersible configuration, with the motor submerged in the sump and attached to an impeller which has grinding teeth to macerate any solids and pass the slurry through the volute to build pressure.
Once the solids have been macerated, the slurry is then pumped under high pressure through a force main to a septic tank, pump station or sewer line.
As they are able to produce higher pressures than other centrifugal designs, they are able to transport thicker fluids through smaller discharge outlets and for longer distances.
These pumps are typically utilised by small communities, new developments or private individuals for pumping sewage, sludge, manure, and other liquids that contain large or tough solids.
Chopper pumps are similar to grinder pumps, in that they are designed with a cutting system to macerate solids in the fluid for easier pumping through the system.
With these pumps, the fluid being pumped moves into the impeller inlet and passes through a rotating chopping mechanism to cut any solids present into small pieces that can be handled by the impeller. They may have a cutter at the back side of the impeller and a filter at the pump inlet to block any solids that are too large.
They come in a variety of configurations including horizontal end suction, self-priming, vertical sump and submersible types.
The main differences between chopper pumps and grinder pumps, is that chopper pumps are larger than grinder pumps and able to macerate large solids such as bones, therefore they are generally owned and operated by private companies and designed for specific industrial and agricultural applications. They can also be used for wastewater treatment.
Horizontal split case pumps
Horizontal split case pumps are designed with the casing split on a horizontal plane into two separate chambers, and the suction and discharge connections in the lower half, opposed to each other. They are more economical for high flow applications as the impeller can be supported by bearings on both sides.
These pumps can be used in a variety of applications including HVAC, water and wastewater, and fire.
Booster pumps are used to increase the pressure of the fluid in order for higher flow rates to be achieved and overcome high system head. They use one or more impellers to draw the fluid into the intake and boost its pressure as it passes through the impeller(s) and the volute or diffuser casing.
These pumps can be single stage where the additional pressure required is not significant, or multistage where higher head is needed such as where the fluid needs to be moved to higher elevations or through long pipelines.
They are used in conjunction with other pumps as they are designed to boost the performance of an existing pumping system, and come in a range of configurations including end suction, horizontal split case to vertical turbine depending on the flow and pressure required.
They generally have a flow rate range between 5-10,000gpm, a total head (pressure) range between 200-7,500ft, and a horsepower range between 1-5,000hp.
They are typically used in water systems or applications where there is low waste contamination. Single stage versions are found in private residences or buildings far from the municipal water supply, or where water pressure at the building is inadequate. They can also be found in irrigation applications. Multistage versions are used to boost water supply in hilly areas for agricultural, commercial and residential uses; as well as in tall buildings and in industrial applications.
Cantilever pumps are designed so only the impeller and casing are submerged in the fluid being pumped, with the joints, seals, bearings, bushings and suction check valves all located outside the fluid. This means the impeller(s) is cantilevered from the motor instead of being supported by lower bearings.
Most cantilever pumps are designed to be mounted vertically, but some are designed to be mounted horizontally depending on the placement needs of the application. Their usage is limited to shallow fluid depths of less than 8-10ft as pump costs increase exponentially as depth increases due to the need for larger shaft diameters to cantilever the impeller.
These pumps are ideal for moving slurries and abrasive solutions where the fluid could degrade or interfere with submerged joints and for handling high temperature fluids, and are the standard choice for use with abrasive fluids. They are commonly found in sump pump applications, as well as mineral processing plants, steel mills, chemical plants and industrial wastewater facilities.
Syringe pumps are available to two types (infusion and withdrawal) and are used to deliver precise amounts of fluid at specific time intervals. Infusion type syringe pumps are designed to administer small amounts of fluid at high, controlled pressures, while withdrawal type syringe pumps are used to remove fluid samples automatically such as is required in the medical or pharmaceutical industries.
Syringe pumps are designed to differ depending on the application and the features required. While general purpose types are not designed for a specific industry, specialised versions are rated for different industries such as agricultural, dairy, food and beverage, maritime, medical, processing, etc. Features can include explosion-proof, corrosion resistance, hygienic, portable and dry run.
Trash pumps are heavy duty machines, designed to handle fluids with a high level of solids such as trash, mud, natural debris, waste products, sand, gravel, crushed stone or concrete chips. These pumps are able to process fluids with suspended particulates that would clog other centrifugal pumps and can move hundreds or even thousands of gallons per minute, while not all trash pumps can grind up the solids entering the pump, some do come with a cutting mechanism
Most trash pumps are submersible type pumps, but can also be sump pumps.
These pumps can be used in a variety of applications, including dewatering on construction sites, trash processing as waste facilities, and industrial or agricultural applications where large amounts of waste that cannot be easily pumped is produced.