Dynamic pumps are one category of pumps under which there are several classes, two of
which are: centrifugal and axial. These pumps operate by developing a high liquid
velocity and converting the velocity to pressure in a diffusing flow passage. Dynamic
pumps usually have lower efficiencies than positive displacement pumps, but also have
lower maintenance requirements. Dynamic pumps are also able to operate at fairly high
speeds and high fluid flow rates.
Centrifugal Pumps
A centrifugal pump
consists of an impeller and an intake at its center. These are
arranged so that when the impeller rotates, liquid is discharged by centrifugal force
into a casing surounding the impeller. The casing is there in order to gradually
decrease the velocity of the fluid which leaves the impeller at a high velocity. This
velocity is converted to pressure which is needed to discharge the fluid. Some of the
advantages of cenrtifugal pumps are, smooth flow through the pump and uniform pressure
in the discharge pipe, low cost, and an operating speed that allows for direct connection to steam turbines
and electric motors. The centrifugal pump accounts for not less then 80% of
the worlds pump production because it is more suitable for handling large
capacities of liquids than the positive-displacement pump. Axial Flow Pumps
Axial flow pumps are also called propeller pump. These pumps develop most of their
pressure by the propelling or lifting action of the vanes on the liquid.
These pumps are often used in wet-pit drainage, low-pressure irrigation,
and storm-water applications. In general, vertical single-stage axial and
mixed-flow pumps are used however, sometimes two-stage axial-flow pumps are economically
more practical. Horizontal axial-flow pumps are used for pumping large volumes against
low pressures and usually employ siphonic action. When higher pressures are involved,
these pumps can be arranged to operate with siphonic action until the back pressure
places the hydraulic gradient above the pump.