What is a Centrifugal Pump?
What is a Centrifugal Pump?: Take Advantage Of Centrifugal Pump Classes To Maximise Pumping Efficiency.
Centrifugal pumps are commonly used in various industries for fluid transportation. They work by converting rotational energy into kinetic energy, propelling the fluid through the pump. These pumps are efficient, reliable, and suitable for a wide range of applications such as water supply, wastewater treatment, and chemical processing.
A centrifugal pump is the most common design of pump in use within pumping applications.
Used in over two thirds of pumping applications, it consists of a pump head containing the main working elements which rotate to produce flow and pressure when driven by a motor.
How Does a Centrifugal Pump Work?
The pump head is made up of several parts and in its simplest form consists of a Pump head, shaft, and an impeller. A motor coupled to the pump head rotates the pump head shaft, which in turn rotates the impeller.
This causes a vacuum to be created within the pump head, drawing fluid into the casing. The centrifugal force of the rotating impeller causes fluid to be thrown to the outside of the casing, leading it to being ejected from the pump casing which is usually via an outlet located at the top of the pump casing.
Advantages of a Centrifugal Pump
There are 6 advantages to using centrifugal pumps:
Smooth Flow – Centrifugal flow is laminar and non-pulsating allowing them to be used with flowmeters easily.
Low Maintenance & Easy to maintain - Due to their simple uncomplicated design, which are often maintained without special tools or equipment.
Efficient – As the internal elements are free to rotate, they are very efficient when handling low viscosity fluids
Versatile – They can be designed to handle a multitude of liquids and solids by changing the materials of certain parts, or impeller design.
Low Cost – Due to their simplicity of design and wide availability, they are one of the lowest initial costs of pump. If specified correctly they can also provide the lowest lifetime cost for an application.
Low Infrastructure costs – As Centrifugal pumps can be ran against a closed valve for short periods without damage, additional accessories such as pressure relief valves and other devices are not required.
Disadvantages of a Centrifugal Pump
There are 4 main disadvantages of a centrifugal pump:
Inaccurate Flow – As flow varies depending on the duty, and back pressure, the flow can vary significantly if more back pressure is experienced. As there is no fixed displacement per revolution as per positive displacement designs, the amount pumped is variable.
Limited Viscosity Handling – They are designed for low viscosity liquids, and if a liquid is being pump where viscosity changes with temperature, care must be taken to ensure that at the lowest temperatures and when a liquid is at its highest viscosity, that the pump can perform as expected
Limited Solid & Abrasive Handling – Whilst centrifugal pumps can handle suspended solids, due to their high speed of operation they are not always the best solution and the most cost effective. Some pumps require rubber lining for abrasive solids, and if the fluid has a high-density other pump technologies can work more reliably, at lower speeds with less maintenance.
Shearing – Centrifugal pumps operate at high speeds so they are not suitable for shear sensitive liquids such as beer, milk, cream, or other liquids which state may change if agitated.
Applications of a Centrifugal Pump
As centrifugal pumps are limited by the viscosity of the fluid they can handle, the applications they are most effective for are generally for the handling of liquids below 600cst, which can be clean or contain solids.
This can encompass a range of applications from container emptying or unloading, transfer, and circulation of low viscosity fluids similar to Water, to light oils, Glycol, Fuels, Chemicals, or low viscosity slurries.
Types of Centrifugal Pump
There are various types of Centrifugal pump available depending on the liquid being pumped, application, flow and pressure required with each design configured specifically for the fluid, and requirements of an application.
Single Stage
In a single stage Pump a single impeller rotates with the pump shaft producing flow. In this design pressures are limited to 150M Head, with Flows up to approximately 600M³H. They can be specified for clean or solid handling applications.
Single stage centrifugal pumps can be configured as close, or long coupled or vertically depending on the duty and installation requirements.
Close coupled
Close coupled pump designs have motors which are directly coupled to the pump head either utilizing an extended motor shaft in which to mount the impeller, or the motor and pump shaft connected via a set of stub screws.
Long Coupled
Long coupled designs have the pump mounted and perfectly aligned on a baseplate, with the motor shaft and pump shaft connected via a flexible or rigid coupling.
This design is better suited for heavy duty applications where the pump may be ran for extended periods without stopping, or at high flows or pressures.
Multistage
In a multistage pump, multiple impellers rotate within ring sections known as stages, with each impeller adding pressure to the receiving flow from the preceding impeller.
Each impeller produces increasingly high pressure (typically up to 1000M) with moderate flows (up to 1000M³H).
They are best suited for clean fluids, without solids, unless sufficiently oversized to accommodate any particles.
Side Channel
Side channel pumps are a hybrid design of pump in that they are like a multistage pump having multiple impellers, however they are self priming and can handle entrained gas making them suited for liquid fuels and in applications where there are very low inlet pressures (NPSHa) and low temperatures.
They are used for high pressures (up to 400M) and low flows (<42M³H) and are suitable for clean or gas entrained fluids only not containing solids or abrasive particles.
Self Priming
Centrifugal pumps are unable to self prime – which is to lift water from a tank located below the pump inlet, without an additional device such as a non-return valve on the inlet pipe.
Self-Priming Pumps are able to draw water from tanks, or bodies of water located below the inlet, without additional valves as well as handle entrained air.
Once the casing is filled with water on first use, it begins priming by using the fluid contained in the pump head to create a vacuum, expelling any air contained in the inlet pipework, until fluid enters the pump head. At this point the pump begins to function as a normal centrifugal pump.
They are usually less efficient than centrifugal or multistage pumps but can handle solids without issue. Maximum flow is around 700M³H, and head is 150M.
Magnetic Drive
Magnetic drive pumps have a different pump head design to traditional pumps.
Normally the pump shaft exits the pump head via a rear port sealed by a mechanical seal. Magnetic drive pumps ensure the pump head is completely sealed, with the pump rotated by a set of magnets within the pump head (drive magnet) and driven externally by a motor (driven Magnet), meaning any fluid is contained within the pump head as there is not a seal present for the fluid to leak from in the rear of the casing.
There is a casing O ring however, as this part is not in contact with rotating parts, the pump is said to be seal less.
Split Casing
Split casing also known as double suction pumps have an impeller which is designed for producing high volume flows.
This is due to the impeller being of back-to-back design, where its design acts like two impellers fixed together, mounted within a double volute.
They are built to deliver flows up to 6000M³H and heads less than 200M of low viscosity liquids.
Submersible / Borehole
Submersible designs are fit for submerging in a fluid which is utilized for cooling the motor, with borehole designs built to be installed within narrow boreholes drilled for extracting water from below ground. They can also be used in large bodies of water and providing a cooling sleeve is fitted, can be installed horizontally.
They produce moderate flows (up to 350M³H) at high pressures up to 650M and are suitable for clean and solid laden fluids with their most common application being that of sewage transfer.
Vertical Immersion / Immersed
Immersed, vertical immersion or Cantilever pumps are designs whereby the pump head is submerged in the fluid with the motor protruding out of the fluid. The pump head can contain a single impeller and be of single stage or contain multiple impellers and be of multistage design.
This design is more robust than that of submersible pumps as the motor is located outside of the fluid and is therefore not subject to attack from the fluid and can be easier to maintain.
As the pump head can be configured with either a single or multiple impellers their can reach high flows of up to 8000M³H, and pressures of 220M.
Pump as Turbine – Reverse Rotation
Pump as Turbine (PAT) is an advancement in pump technology that allows a pump to be used as a turbine to generate power, by operating it in reverse mode. It is driven by sources of excess flow and pressure which can be from excess pressure in pipelines where a pressure reducing valve may normally be used to reduce high pressures either from process, or due to changes in pipeline altitude in mountainous areas.
Parts of a Centrifugal Pump
A pump head contains 4 main parts but relies on up to 30 different parts for its correct function, working motion and longevity.
These parts and their function include:
Bearings These reduce friction between moving part and ensures rotation remains as per the required motion. | Bushing Or Spacer Sleeve Bushings or Sleeves act as a sacrificial part within pumps absorbing abrasion and shocks from thrust. | Circlip, Snap Ring, Retaining Rings or Seeger A fastener with open ends containing 2 holes which can be internal or external. They are placed in a machined groove to allow rotation but prevent axial movement. They can be used on a shaft for holding elements in place such as a motor fan blade, a bearing or a seal. |
Coupling A coupling is used to connect both the motor and pump shaft ensuring both shafts rotate at the same speed and in the same plain and is responsible for transferring both torque and speed between the two parts. The coupling can be rigid or flexible. | Drain Plug Drain plugs are situated at the lowest and highest point of pump casings and used to either fill the pump with fluid before startup or drain the casing of fluid before disconnection from the pipework and maintenance. | Diffuser A diffuser contains a series of vanes which surround the impeller which act to reduce the velocity of the fluid, resulting in an increase of pressure. |
Flange Used to attach pipework to the pump | Gasket / Paper Joint Sealing Paper joint sealing is used between the joints of multistage and side channel pumps ring sections as when the sections are pressed against each other they do not create a seal without another material. They accommodate the imperfections between the mating surfaces. | Grease Nipple Grease nipples are used to enable easy greasing of bearings. Grease is pumped through the nipple to feed into the bearing without having to dismantle the pump. |
Impeller The impeller rotates at the same speed as the pump shaft and is responsible for producing the flow and pressure in the pump. | Impeller Nut The end of the shaft where the impeller slots onto the shaft is usually thread with an impeller nut fastened to keep the impeller held onto the shaft. | Lip Seal These are used to seal around the shaft or bearings and are limited to low pressures. They are better suited for thin, non-lubricating liquids, used in smaller pumps, and held in place with a cover. |
Mechanical Seal / Packed Gland A pump head is sealed via the use of a mechanical seal or packed gland. A mechanical Seal seals around the shaft, and pump casing. This seal consists of two faces and a spring which press against each other forming a seal and are cooled by the pumped fluid. | Motor and dash; Electric, Hydraulic or Engine The motor is used to transfer power to the pump head by converting electrical, hydraulic, or mechanical energy into rotational mechanical energy, which is used to rotate the motor or output shaft, which in turn rotates the coupling, and pump shaft. This in turns rotates the impeller producing flow and pressure. | Oil Filling Plug Used for filling bearings with oil without dismantling of pump |
Oil Sight Gauge Used to view the amount of oil contained within a chamber such as an oil lubricated bearing housing, or gearbox. | O Rings O Rings are used to seal around metal parts such as within the pump casing, or around drainage ports as metal is unable to create a seal without an elastomer, silicone, or plastic, acting as a barrier. | Shaft The shaft is connected to the driving motor, and ensures the rotational movement is carried into the head of the pump, rotating the impeller at the same speed as the motor shaft. |
Shaft Key An impeller is held onto the shaft via the use of a key – a small rectangular bar piece of metal made in the same material as the shaft, which slots into a groove in the shaft and in a notch in the impeller. | Stud Bolt An externally threaded fastener with an appearance similar to a hexagonal bolt without the head. They can be used on flanges or as tie bolts. | Stuffing Box Where gland packing is placed to ensure sealing of the pump head. |
Suction & Discharge Stage Casing In a multistage and side channel pump, multiple impellers are housed with stage casings consisting of a suction and discharge casing. They are designed to fit around the impeller in a specific orientation, and contain an inlet port, equalization holes, outlet port and in a side channel pump a channel to handle both liquids and entrained gas. | Tiebolt Tiebolts are used in Multistage and side channel Pumps to keep ring sections of the pump tightly connected. | Thrower A raised ring located on a shaft to throw or scatter oil. |
Volute The volute is a piece of metal that forms part of the casing surrounding the impeller. It is responsible for ensuring pump efficiency and controls the amount of pressure generated by the impeller. | Washer Washers are used around bolts to ensure any tightening torque is evenly distributed around the fastener. | Wear Rings Wear rings are mounted either side of the impeller and help to maintain pump efficiency as well as protect the main pump casing from wear. A pumps Net Positive Suction Head Requirement (NPSHR) may increase by 50% due to a doubling of wear ring clearances. |
Centrifugal Affinity Law
Pump affinity laws are a set of formula used to determine a pumps performance when a change is made such as speed, or impeller diameter to predict the produced flow and pressure with high accuracy.
There are 3 affinity laws:
1) Flow is proportional to the shaft speed or impeller diameter
As the shaft speed or the impeller diameter is altered, the flow will change by the same amount.
If the speed of a pump is reduced by 20% the flow at the same head will also decrease by 20%.
2) The pressure produced is proportional to the square of shaft speed or impeller diameter
When the impeller diameter is altered or shaft speed is changed, pressure changes proportional to the square of the change in shaft speed or impeller diameter.
If a shafts speed increases by 10% then pressure at the same flow will increase by 21%
3) Power is proportional to the cube of shaft speed or impeller diameter
If the shaft speed increases by 10%, then due to power being proportional to the cube of shaft speed the pressure will increase by 33.3%.
Minimum Safe Continuous Flow
The Minimum Continuous Safe Flow is the minimum amount of a flow a centrifugal pump can do without experiencing issues such as cavitation, or excess wear.
It is often a figure used to design operating speeds, and the setting of bypass control valves in processes where pumps may be running continuously such as boiler feed applications, cooling or in lubrication applications.
P&ID Symbol
Centrifugal Pumps can be known by a number of different P&ID Symbols, listed below.
Troubleshooting
If you are having difficulty with your pump view our centrifugal troubleshooting guide.
