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A centrifugal pump is a rotodynamic pump that uses a rotating impeller to increase the pressure of a fluid. Centrifugal pumps are commonly used to move liquids through a piping system. 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 system. Centrifugal pumps are used for large discharge through smaller heads.
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History
A water or mud-lifting machine that, according to the Brazilian historian of science Reti, "must be characterized as the prototype of the centrifugal pump" appeared as early as 1475 in a treatise by the Italian Renaissance engineer Francesco di Giorgio Martini.[1] True centrifugal pumps were not developed until the late 1600's, when Denis Papin made one with straight vanes. The curved vane was introduced by British inventor John Appold in 1851.
How it works
A centrifugal pump works by the conversion of the rotational kinetic energy, typically from an electric motor or turbine, to an increased static fluid pressure. This action is described by Bernoulli's principle. The rotation of the pump impeller imparts kinetic energy to the fluid as it is drawn in from the impeller eye (centre) and is forced outward through the impeller vanes to the periphery. As the fluid exits the impeller, the fluid kinetic energy (velocity) is then converted to (static) pressure due to the change in area the fluid experiences in the volute section. Typically the volute shape of the pump casing (increasing in volume), or the diffuser vanes (which serve to slow the fluid, converting the kinetic energy in to flow work) are responsible for the energy conversion. The energy conversion results in an increased pressure on the downstream side of the pump, causing flow.
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. The advantages to using a multistage centrifugal pump or a single stage pump include:
- To produce a higher pressure head, and
- To discharge a larger quantity of liquid.
If a higher pressure head is to be developed then the impellers are mounted on same shaft (series) while for large discharge of liquid, the impellers are mounted on different shafts (parallel).
To get high head, a number of impellers are connected in series. To get high discharge, the impellers should be connected in parallel. Water flows radially outward due to centrifugal force acting on it. The water is sucked in through the eye of the impeller due to low pressure created at the eye. The pump shaft is driven by an electric motor or similar.
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 characteristics of the pipeline. The power required to drive a pump (Pi), is defined simply using SI units by:
where:
- Pi is the input power required (W)
- ρ is the fluid density (kg/m3)
- g is the gravitational constant (9.81 m/s2)
- H is the energy Head added to the flow (m)
- Q is the flow rate (m3/s)
- η is the efficiency of the pump plant as a decimal
The head added by the pump (H) 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 water. Power is more commonly expressed as kW (103 W) or horsepower (multiply kW by 0.746). The value for the pump efficiency η 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
- Cavitation - The NPSH of the system is too low for the selected pump
- Wear of the Impeller - can be worsened by suspended solids
- Corrosion 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.
See also
- Axial flow pump
- Turbopump
- Pump
- Turbine
- Net Positive Suction Head (NPSH)
- Specific Speed ( Ns or Nss )
- Thermodynamic pump testing
References
- ^ Ladislao Reti, “Francesco di Giorgio Martini's Treatise on Engineering and Its Plagiarists”, Technology and Culture, Vol. 4, No. 3. (Summer, 1963), pp. 287-298 (290)
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