CETRIFUGAL PUMP
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.
How it works
Like most pumps, a centrifugal pumps 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
A centrifugal pump uses a spinning "impeller,"
which normally has backward-swept blades
that directly pushes water outward.
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 significance of centrifugal force in the working of the pump, remarked
John Richards's drawing of a theoretical shape for the volute casing around the impeller, which he calls a "mistake" due to the constriction at a
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 box to seal the shaft but instead utilizes a "throttle Bushing". A common application for this style of pump is in a parts washer.
Multistage centrifugal pumps
1) 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.
2) If we need higher pressure at the outlet we can connect impellers in series.
3) If we need a higher flow output we can connect impellers in parallel.
4) All energy added to the fluid comes from the power of the electric or other motor force driving the impeller.
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:
Pi = ρ g Q H / η
Where:
Pi is the input power required (W)
ρ is the fluid density (kg/m3)
g is the standard acceleration of gravity (9.80665 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 fluid. Power is more commonly expressed as kilowatts (103 W) or horsepower (multiply kilowatts 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
· Cavitations—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
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.
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.
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