Centrifugal pump
Encyclopedia
A centrifugal pump is a rotodynamic pump
that uses a rotating impeller
to create flow by the addition of energy to a fluid. Centrifugal pumps are commonly used to move liquids through piping. The fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward into a diffuser or volute
chamber (casing
), from where it exits into the downstream piping. Centrifugal pumps are used for large discharge through smaller heads.
made one with straight vanes. The curved vane was introduced by British inventor John Appold
in 1851.
Like most pumps, a centrifugal pump converts mechanical energy from a motor to energy of a moving fluid; some of the energy goes into kinetic energy of fluid motion, and some into potential energy, represented by a fluid pressure or by lifting the fluid against gravity to a higher level.
The transfer of energy from the mechanical rotation of the impeller to the motion and pressure of the fluid is usually described in terms of centrifugal force
, especially in older sources written before the modern concept of centrifugal force as a fictitious force in a rotating reference frame was well articulated. The concept of centrifugal force is not actually required to describe the action of the centrifugal pump.
In the modern centrifugal pump, most of the energy conversion is due to the outward force that curved impeller blades impart on the fluid. Invariably, some of the energy also pushes the fluid into a circular motion, and this circular motion can also convey some energy and increase the pressure at the outlet. The relationship between these mechanisms was described, with the typical mixed conception of centrifugal force as known as that time, in an 1859 article on centrifugal pumps, thus:
The statement "the mass of water ... must necessarily exert a centrifugal force" is interpretable in terms of the reactive centrifugal force
—the force is not an outward force on the water, but rather an outward force exerting by the water, on the pump housing (the volute) and on the water in the outlet pipe. The outlet pressure is a reflection of the pressure that applies the centripetal force
that curves the path of the water to move circularly inside the pump (in the space just outside the impeller, the exterior whirlpool as this author calls it). On the other hand, the statement that the "outward force generated within the wheel is to be understood as being produced entirely by the medium of centrifugal force" is best understood in terms of centrifugal force as a fictional force in the frame of reference of the rotating impeller; the actual forces on the water are inward, or centripetal, since that's the direction of force need to make the water move in circles. This force is supplied by a pressure gradient that is set up by the rotation, where the pressure at the outside, at the wall of the volute, can be taken as a reactive centrifugal force
. This is typical of 19th and early 20th century writing, to mix these conceptions of centrifugal force in informal descriptions of effects such as that in the centrifugal pump.
Differing conceptions and explanations of how a centrifugal pump works have long engendered controversy and animadversion. For example, the American Expert Commission sent to the Vienna Exposition in 1873 issued a report that included observations that "they are misnamed centrifugal, because they do not operate by centrifugal force at all; they operate by pressure the same as a turbine water wheel; when people understand their method of operating we may expect much improvement." John Richards, editor of the San Francisco-based journal Industry, in his in-depth essay on centrifugal pumps, which also downplayed the signficance of centrifugal force in the working of the pump, remarked:
Modern sources say things like that the fluid "flows radially under centrifugal force", or "centrifugal force flings the liquid outward".
Others counter that "there is no force at all, and a great deal of confused thinking."
Some are more careful, attributing the outward force to the impeller, not to centrifugal force: "the impellers throw the water to the outside of the impeller case. This centrifugal action is what creates the pressure..."
Even serious texts that explain the working of the pump without mention of centrifugal force introduce the pump as one in which "the mechanical energy is converted, into pressure energy by means of centrifugal force acting on the fluid."
to seal the shaft but instead utilizes a "throttle Bushing". A common application for this style of pump is in a parts washer
.
If we need higher pressure at the outlet we can connect impellers in series.
If we need a higher flow output we can connect impellers in parallel.
All energy added to the fluid comes from the power of the electric or other motor force driving the impeller.
of the pipeline.
The power required to drive a pump (
), is defined simply using SI units by:
where:
is the input power required (W)
is the fluid density (kg/m3)
is the standard acceleration of gravity (9.80665 m/s2)
is the energy Head added to the flow (m)
is the flow rate (m3/s)
is the efficiency of the pump plant as a decimal
The head added by the pump (
) is a sum of the static lift, the head loss due to friction and any losses due to valves or pipe bends all expressed in metres of fluid. Power is more commonly expressed as kilowatts (103 W, kW) or horsepower (hp = kW*0.746). The value for the pump efficiency, 
, may be stated for the pump itself or as a combined efficiency of the pump and motor system.
The energy usage is determined by multiplying the power requirement by the length of time the pump is operating.
Rotodynamic pump
A rotodynamic pump is a kinetic machine in which energy is continuously imparted to the pumped fluid by means of a rotating impeller, propeller, or rotor, in contrast to a positive displacement pump in which a fluid is moved by trapping a fixed amount of fluid and forcing the trapped volume into...
that uses a rotating impeller
Impeller
An impeller is a rotor inside a tube or conduit used to increase the pressure and flow of a fluid.- Impellers in pumps :...
to create flow by the addition of energy to a fluid. Centrifugal pumps are commonly used to move liquids through piping. The fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward into a diffuser or volute
Volute (pump)
The volute of a centrifugal pump is the casing that receives the fluid being pumped by the impeller, slowing down the fluid's rate of flow. A volute is a curved funnel that increases in area as it approaches the discharge port. The volute converts kinetic energy into pressure by reducing speed...
chamber (casing
Casing
Casing may refer to:* Cartridge , shell enclosing the explosive propellant in ammunition* Casing , metal tube used during the drilling of a well* Casing , decorative molding surrounding door or window openings...
), from where it exits into the downstream piping. Centrifugal pumps are used for large discharge through smaller heads.
History
According to Reti, the Brazilian soldier and historian of science, the first machine that could be characterized as a centrifugal pump was a mud lifting machine which appeared as early as 1475 in a treatise by the Italian Renaissance engineer Francesco di Giorgio Martini. True centrifugal pumps were not developed until the late 17th century, when Denis PapinDenis Papin
Denis Papin was a French physicist, mathematician and inventor, best known for his pioneering invention of the steam digester, the forerunner of the steam engine and of the pressure cooker.-Life in France:...
made one with straight vanes. The curved vane was introduced by British inventor John Appold
John Appold
John George Appold, FRS was a British fur dyer and engineer. Appold was the son of a fur-skin dyer, established in Finsbury...
in 1851.
How it works
The transfer of energy from the mechanical rotation of the impeller to the motion and pressure of the fluid is usually described in terms of centrifugal force
Centrifugal force
Centrifugal force can generally be any force directed outward relative to some origin. More particularly, in classical mechanics, the centrifugal force is an outward force which arises when describing the motion of objects in a rotating reference frame...
, especially in older sources written before the modern concept of centrifugal force as a fictitious force in a rotating reference frame was well articulated. The concept of centrifugal force is not actually required to describe the action of the centrifugal pump.
In the modern centrifugal pump, most of the energy conversion is due to the outward force that curved impeller blades impart on the fluid. Invariably, some of the energy also pushes the fluid into a circular motion, and this circular motion can also convey some energy and increase the pressure at the outlet. The relationship between these mechanisms was described, with the typical mixed conception of centrifugal force as known as that time, in an 1859 article on centrifugal pumps, thus:
The statement "the mass of water ... must necessarily exert a centrifugal force" is interpretable in terms of the reactive centrifugal force
Reactive centrifugal force
In classical mechanics, reactive centrifugal force is the reaction paired with centripetal force. A mass undergoing circular motion constantly accelerates toward the axis of rotation. This centripetal acceleration is caused by a force exerted on the mass by some other object. In accordance with...
—the force is not an outward force on the water, but rather an outward force exerting by the water, on the pump housing (the volute) and on the water in the outlet pipe. The outlet pressure is a reflection of the pressure that applies the centripetal force
Centripetal force
Centripetal force is a force that makes a body follow a curved path: it is always directed orthogonal to the velocity of the body, toward the instantaneous center of curvature of the path. The mathematical description was derived in 1659 by Dutch physicist Christiaan Huygens...
that curves the path of the water to move circularly inside the pump (in the space just outside the impeller, the exterior whirlpool as this author calls it). On the other hand, the statement that the "outward force generated within the wheel is to be understood as being produced entirely by the medium of centrifugal force" is best understood in terms of centrifugal force as a fictional force in the frame of reference of the rotating impeller; the actual forces on the water are inward, or centripetal, since that's the direction of force need to make the water move in circles. This force is supplied by a pressure gradient that is set up by the rotation, where the pressure at the outside, at the wall of the volute, can be taken as a reactive centrifugal force
Reactive centrifugal force
In classical mechanics, reactive centrifugal force is the reaction paired with centripetal force. A mass undergoing circular motion constantly accelerates toward the axis of rotation. This centripetal acceleration is caused by a force exerted on the mass by some other object. In accordance with...
. This is typical of 19th and early 20th century writing, to mix these conceptions of centrifugal force in informal descriptions of effects such as that in the centrifugal pump.
Differing conceptions and explanations of how a centrifugal pump works have long engendered controversy and animadversion. For example, the American Expert Commission sent to the Vienna Exposition in 1873 issued a report that included observations that "they are misnamed centrifugal, because they do not operate by centrifugal force at all; they operate by pressure the same as a turbine water wheel; when people understand their method of operating we may expect much improvement." John Richards, editor of the San Francisco-based journal Industry, in his in-depth essay on centrifugal pumps, which also downplayed the signficance of centrifugal force in the working of the pump, remarked:
Modern sources say things like that the fluid "flows radially under centrifugal force", or "centrifugal force flings the liquid outward".
Others counter that "there is no force at all, and a great deal of confused thinking."
Some are more careful, attributing the outward force to the impeller, not to centrifugal force: "the impellers throw the water to the outside of the impeller case. This centrifugal action is what creates the pressure..."
Even serious texts that explain the working of the pump without mention of centrifugal force introduce the pump as one in which "the mechanical energy is converted, into pressure energy by means of centrifugal force acting on the fluid."
Vertical centrifugal pumps
Vertical centrifugal pumps are also referred to as cantilever pumps. They utilize a unique shaft and bearing support configuration that allows the volute to hang in the sump while the bearings are outside of the sump. This style of pump uses no stuffing boxStuffing box
A stuffing box is an assembly which is used to house a gland seal. It is used to prevent leakage of fluid, such as water or steam, between sliding or turning parts of machine elements.-Boats:...
to seal the shaft but instead utilizes a "throttle Bushing". A common application for this style of pump is in a parts washer
Parts washer
A parts washer is a piece of equipment used to remove contaminants or debris, such as dirt, grime, carbon, oil, grease, metal chips, cutting fluids, mold release agents, ink, paint, and corrosion from workpieces...
.
Multistage centrifugal pumps
A centrifugal pump containing two or more impellers is called a multistage centrifugal pump. The impellers may be mounted on the same shaft or on different shafts.If we need higher pressure at the outlet we can connect impellers in series.
If we need a higher flow output we can connect impellers in parallel.
All energy added to the fluid comes from the power of the electric or other motor force driving the impeller.
Efficiency of large pumps
Unless carefully designed, installed and monitored, pumps will be, or will become inefficient, wasting a lot of energy. Pumps need to be regularly tested to determine efficiency.Energy usage
The energy usage in a pumping installation is determined by the flow required, the height lifted and the length and friction characteristicsDarcy friction factor formulae
In fluid dynamics, the Darcy friction factor formulae are equations — based on experimental data and theory — for the Darcy friction factor. The Darcy friction factor is a dimensionless quantity used in the Darcy–Weisbach equation, for the description of friction losses in pipe flow as well as open...
of the pipeline.
The power required to drive a pump (
), is defined simply using SI units by:where:
is the input power required (W) is the fluid density (kg/m3) is the standard acceleration of gravity (9.80665 m/s2) is the energy Head added to the flow (m) is the flow rate (m3/s) is the efficiency of the pump plant as a decimalThe head added by the pump (
) is a sum of the static lift, the head loss due to friction and any losses due to valves or pipe bends all expressed in metres of fluid. Power is more commonly expressed as kilowatts (103 W, kW) or horsepower (hp = kW*0.746). The value for the pump efficiency, , may be stated for the pump itself or as a combined efficiency of the pump and motor system.The energy usage is determined by multiplying the power requirement by the length of time the pump is operating.
Problems of centrifugal pumps
- CavitationCavitationCavitation is the formation and then immediate implosion of cavities in a liquidi.e. small liquid-free zones that are the consequence of forces acting upon the liquid...
—the NPSHNPSHNPSH is an initialism for Net Positive Suction Head. In any cross-section of a generic hydraulic circuit, the NPSH parameter shows the difference between the actual pressure of a liquid in a pipeline and the liquid's vapor pressure at a given temperature.NPSH is an important parameter to take into...
of the system is too low for the selected pump - Wear of the ImpellerImpellerAn impeller is a rotor inside a tube or conduit used to increase the pressure and flow of a fluid.- Impellers in pumps :...
—can be worsened by suspended solids - CorrosionCorrosionCorrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
inside the pump caused by the fluid properties - Overheating due to low flow
- Leakage along rotating shaft
- Lack of prime—centrifugal pumps must be filled (with the fluid to be pumped) in order to operate
- Surge
Centrifugal pumps for solids control
An oilfield solids control system needs many centrifugal pumps to sit on or in mud tanks. The types of centrifugal pumps used are sand pumps, submersible slurry pumps, shear pumps, and charging pumps. They are defined for their different functions, but their working principle is the same.Magnetically coupled pumps
Small centrifugal pumps (e.g. for garden fountains) may be magnetically coupled to avoid leakage of water into the motor. The motor drives a rotor carrying a pair of permanent magnets and these drag round a second pair of permanent magnets attached to the pump impeller. There is no direct connection between the motor shaft and the impeller so no gland is needed and, unless the casing is broken, there is no risk of leakage.Priming
Most centrifugal pumps are not self-priming. In other words, the pump casing must be filled with liquid before the pump is started, or the pump will not be able to function. If the pump casing becomes filled with vapors or gases, the pump impeller becomes gas-bound and incapable of pumping. To ensure that a centrifugal pump remains primed and does not become gas-bound, most centrifugal pumps are located below the level of the source from which the pump is to take its suction. The same effect can be gained by supplying liquid to the pump suction under pressure supplied by another pump placed in the suction line.See also
- Axial flow pumpAxial flow pumpAn axial flow pump, or AFP, is a common type of pump that essentially consists of a propeller in a pipe. The propeller can be driven directly by a sealed motor in the pipe or mounted to the pipe from the outside or by a right-angle drive shaft that pierces the pipe.The main advantage of an AFP is...
- TurbopumpTurbopumpA turbopump is a gas turbine that comprises basically two main components: a rotodynamic pump and a driving turbine, usually both mounted on the same shaft, or sometimes geared together...
- PumpPumpA pump is a device used to move fluids, such as liquids, gases or slurries.A pump displaces a volume by physical or mechanical action. Pumps fall into three major groups: direct lift, displacement, and gravity pumps...
- TurbineTurbineA turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and...
- Net positive suction head (NPSHNPSHNPSH is an initialism for Net Positive Suction Head. In any cross-section of a generic hydraulic circuit, the NPSH parameter shows the difference between the actual pressure of a liquid in a pipeline and the liquid's vapor pressure at a given temperature.NPSH is an important parameter to take into...
) - Specific speedSpecific speedSpecific speed Ns is a quasi non-dimensional number used to classify pump impellers as to their type and proportions. In Imperial units it is defined as the speed in revolutions per minute at which a geometrically similar impeller would operate if it were of such a size as to deliver one gallon per...
(Ns or Nss) - Thermodynamic pump testingThermodynamic pump testingThermodynamic pump testing is a form of pump testing where only the temperature rise and power consumed need to be measured to find the efficiency of a pump. This is measured by means of temperature and pressure probes fitted to tapping points on the pump’s inlet and outlet...