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Understand Centrifugal Pumps for Fountains... Dry & Submersible Centrifugal Pump Basics

Centrifugal PumpsCentrifugal pumps generate pressure from a process of continuous flow through a rapidly rotating impeller arrangement (see brass coloured impeller in sketch) driven by an electric motor.

Centrifugal pumps when sued in water fountain designs can be either submersible or run dry

An impeller wheel, fitted with vanes, rotates in order to circulate the fluid. Energy transfer ceases as soon as the fluid leaves the impeller wheel duct and water pressure gradually decreases the further along the pipe the water travels (some of the energy is lost through friction with the pipe walls).

Velocity of the rotating impeller is converted into pressure. This happens in an annular chamber without vanes or in a spiral casing, or even round the impeller in a system of gradually widening ducts arranged around the impeller. Impeller and distributor together form one stage. The sucking action of fluid displaced by the impeller causes the same volume of fluid to re-enter the pump through the inlet connection, so that during the revolution of the impeller a continuous flow is maintained.

If the suction of a centrifugal pump is obstructed then cavitation might occur which shows itself in poor pump performance and even excessive noise.

Widely varying requirements regarding lift height and flow rate, while endeavouring to make the centrifugal pumps as economical as possible, have lead to very many different pump models.

For small heights (pressure head) up to c 40 m, single stage pumps are used.

Higher heads generally require multi stage versions. Fluid is then pumped successively through a series of similar impeller/distributor assemblies (stages) arranged one after the other. Final pressure achieved is equal to the sum of individual stage pressures.

Apart from the lift height, the number of stages is also dependent on the speed at which pumps rotate and on displacement size, so that the same head or height can be achieved with quite a different number of stages.

The Relationship Between Pump Head (H) and Pump Flow (Q) for Centrifugal Pumps

Pump Flow and Pump Head Diagram
  • With any type of centrifugal pumps, flow rate “Q” depends on head “H”, and this relationship is illustrated by the Q-H curve, also called a throttle curve
  • The Q-H curve for the centrifugal pump is determined by the position of a metering valve and by simultaneous measurement of head “H” and corresponding flow “Q” values.
    This is shown in the sketch opposite.
    When the valve after the outlet from a centrifugal pump is completely closed, head equals zero (fully throttled).
  • The sketch shows a typical relationship between head “H” and flow “Q”.
    If all valves are closed, the column of water in the vertical line rises to a height of 10,0 m (but now there is zero flow).
  • By opening the valve at 8 m high a quantity of water “Q” of 150 litres is delivered into the collecting basin in 1 minute.
  • Likewise at 6 m high there would be a quantity of water “Q” of 250 l/min, at 4 m 312 l/min and at 2 m 360 l/min.
  • These values can be plotted to form a Q-H curve
  • Summary... the higher you pump a fixed amount of water then the greater is the pressure required to achieve this. This principle is very important in fountain selection.
    In general it is normally best to select a pump based upon its mid operating data.
    By putting a restriction in the delivery side of a centrifugal pump, eg a valve, it becomes possible to artificially increase the head thereby reducing the flow from the pump.