The most important characteristic of a centrifugal pump is that of converting energy from a source of motion (the motor) first into speed (or kinetic energy) and then into pressure energy. A pump's role is in fact to confer energy to the pumped liquid (energy later transformed into flow rate and head), depending on the structural characteristics of the pump itself and as a function of the specific system needs. The operation is simple: these pumps utilize the centrifugal effect to move the liquid and increase its pressure. A bladed wheel (impeller), the true heart of the pump, rotates within a hermetically sealed chamber provided with an inlet and an outlet(cochlea or volute). The impeller is the rotating element of the pump that converts the motor's energy into kinetic energy (the static portion of the pump, i.e. the volute, instead converts kinetic energy into pressure energy). The impeller is in turn secured to the shaft-pump, directly fit onto the motor transmission shaft or coupled to it using rigid coupling.
When the liquid enters the pump-casing, the impeller (fed by the motor) projects the fluid to the periphery of the pump-casing in virtue of the centrifugal force produced by the impeller's speed: in this way the liquid stores (potential) energy that will be transformed into flow rate and head (or kinetic energy). This centrifugal movement simultaneously creates a vacuum capable of sucking the fluid to be pumped. Then the liquid is easily channeled by connecting the pump with the discharge piping, leading to the pump exterior. The impeller of a centrifugal pump can be built according to many constructive variations: open impellers, closed impellers, single-channel impellers, single channel impeller, axial impellers, semi-axial impellers, semi-open vortex impellers, semi-open vortex impellers, spiral impellers, etc.
Centrifugal pumps may be single-stage, that is having only one flow and pressure generator (an impeller). A multi-stage centrifugal pump instead has more than one impeller (the first impeller discharges the liquid to the second and so on) and is characterized by the sum of the pressures delivered by each impeller. In addition to the initial priming action, the operation of a centrifugal pump also depends on the way in which the liquid suction itself is achieved: if in fact the pump is located below the vein from which it draws the liquid, the liquid spontaneously enters the pump (this is flooded installation). Whereas if the pump is located above the source to be pumped, then the liquid will have to be sucked: hence the pump (as for suction piping) must fore first be primed, i.e. filled with liquid (a self-priming pump will be used).
A centrifugal system has innumerable advantages as compared with other types of pumping: it ensures small clearance volumes, relatively silent service and easy operation with all types of electrical motors available on the market. It can also easily adapt to all liquid treatment problems in so far as, by adjusting to the particular conditions of use, it can meet the specific needs of the facilities where it will be used.
A centrifugal pump's performance may be graphically shown in a characteristic curve which usually shows data relative to the total geodetic height, to the actual motor power (BHP), to the efficiency, to the NPSHr and to the positive head, information indicated in relation to the pump capacity.
Every centrifugal pump is therefore characterized by its particular characteristic curve, i.e. the relations between its flow rate and its differential head. This graphical representation, i.e. the transposition of this relation on a Cartesian graph, is the best way to learn what flow rate is obtained at a given head (and vice versa).
For the cases in discussion, the curve consists of a line which starts from a point (equivalent to zero flow rate/maximum head) and that continuous until the end of the curve with head decreasing with increasing flow rate.
It is clear that other elements also interact to modify this representation, such as speed, motor power or impeller diameter. It must also be considered that a pump's performance cannot be known without knowing all the details of the system in which it will be used
The performance curve for each pump also varies with varying speed and is expressed by the following laws:
- the quantity of liquid transported changes as a function of speed
- the head varies as a function of velocity squared
- the power consumed varies as a function of the velocity cubed
The quantity of liquid pumped and the power absorbed are, roughly, proportional. The discharge of a centrifugal pump with constant speed can vary from a flow rate of zero (everything closed or valve closed), up to a maximum which depends on the design and on the working conditions. For example, if the amount of fluid pumped doubles, the speed and all others conditions remain equal, whereas the head increases 4 times and the power consumed increases 8 times with respect to the starting conditions.
The power absorbed by the pump can be located on the graph as point where the power curve intersects the pump curve at the operating point. But this does not yet indicate the size of the motor required.
There are many ways to determine the power of the motors for feeding the pump:
- a pump suitable for an operating speed or an operating range may be selected (the best and less costly method when the pump working conditions are relatively constant).
- the power can be read at the end of the curve (the most common solution which guarantees sufficient power under almost all operating conditions).
- the power can be read in correspondence with the operating point summing adding an additional 10% (a system generally used only in refineries or in other applications where the installation characteristics are constant).
- using the curves, all operating conditions may be considered (the best method to use when trap effects are present, large variations in the geodetic height, long pipes to be filled...)
A pump's performance, and especially rotodynamic pumps, are normally illustrated with a similar curve, which clearly demarcates the relation between the liquid pumped per unit time and the increase in pressure.
But curves for different pump categories have very different characteristics. Positive-displacement pumps, for example, show a flow rate virtually independent of the differential pressure (and the corresponding curve is almost always a vertical line), whereas centrifugal pumps have a performance curve which, as we have seen, opposes the decrease in flow rate as the head developed increases (and vice versa). The curve for regenerative pumps instead has a performance half-way between both pump categories.
A general rule which can always be used to understand the forces developed by a centrifugal pump is: a pump does not create pressure but only provides flow rate. Pressure is simply a measurement of resistance to flow.
Comparison of curves
Principles of hydraulicsCentrifugal pump
A pump that exploits the rotary motion of a bladed wheel (impeller) inserted in the pump casing itself. The impeller, moving at high speed, projects the water previously sucked outwards in virtue of the centrifugal force developed, channeling the liquid in the fixed casing and then into the discharge pipe.Submerged pump
A submerged pump is a vertical axis pump, designed to reach great depths thanks to the length of its suction pipe. This should not be confused with a submersible pump which is characterized by its perfectly sealed motor immersed in the very liquid to be pumped.Flow rate
Quantity of liquid (in volume or in weight) which must be pumped, transferred or raised in a certain time interval by a pump: normally expressed in liters per second (l/s), liters per minute (l/m) or cubic meters per hour (m?/h). Symbol: Q.Head
Height by which a liquid is raised: pumping implies the raising of a liquid from a lower level to a higher level. It can be expressed in meters of a liquid column or in bars (pressure). In this latter case the liquid pumped does not change height but is delivered exclusively at ground level at a given pressure. Symbol: H.Performance curve
A special graph that indicates a pumps performance: in fact the diagram shows curve formed by flow rate and head values, indicated in reference to a specific type of impeller, a diameter and a particular pump model.Flooded suction
A particular pump installation, located below the vein from which the water is drawn: in this way the water spontaneously enters the pump without any difficulty.Priming
Filling of a pump or piping to displace the air within. In some cases self-priming pumps can bus used, i.e. pumps equipped with an automatic mechanism which facilitates priming and therefore the starting of the pump that would otherwise be impossible or in any case very slow.Cavitation
A phenomenon that derives from unstable current flow. Cavitation manifests itself with the formation of cavities in the pumped liquid and is accompanied by noisy vibrations, a reduction in flow rate and, to a lesser degree, in pump efficiency. It is caused by the rapid passage of small vapor bubbles through the pump: their implosion generates micro-jets that can also result in severe damage.Friction head losses
Energy losses due to liquid friction along the pipe walls, proportional to pipe length. These are also proportional to the square of the flow velocity and vary as a function of the liquid pumped. Any occasion to slow the normal flow of the fluid pumped in any case is a source of friction head loss such as sudden changes in direction or in cross-sections of pipes.
In order to properly size a pump, the sum of these losses are added to the differential head already anticipated.Mechanical seal
Mechanical seal for rotating shafts. Used whenever external dripping of the liquid cannot be tolerated. It is composed of two flat-surfaced rings, one stationary and the other rotating: the two faces press against each other leaving only a very thin hydrodynamic film composed of the liquid to be retained having the function of lubricating the sliding parts.Viscosity
This is a characteristic of the fluid pumped: it represents the capacity of a fluid to oppose motion. Viscosity varies as a function of temperature.Specific gravity
Every fluid has a characteristic density.
Water, used as a basis for comparison, has by convention a specific gravity (or density) equal to 1 (at 4°C and at sea level). The specific gravity is the value used to compare the weight of a certain volume of liquid with the weight of the same amount of water.
SAVINO BARBERA SRL
Via Torino 12
P.O. Box 58
10032 Brandizzo (To)
Cap. soc. 48000 i.v.
Tel +39 011 913.90.63
Fax +39 011 913.73.1