Centrifugal
pump mechanics
Professor
Anupam Dewan, PhD, elaborates on the various important considerations
that need careful attention while choosing pumps
A
pump uses energy to direct the flowing fluid. The flow in a pump is
usually turbulent and is characterised by three dimensionality and unsteadiness.
In a centrifugal pump, the fluid flows axially into the pump to the
centre of the rotating impeller (which is connected to a rotating shaft)
and is forced to flow outward by the centrifugal action. This results
in the fluid acquiring high kinetic energy and high pressure. The casing
shape is designed such that the fluid velocity is reduced as the fluid
is expelled out of the pump. However, this reduction of velocity occurs
as a result of the conversion of kinetic energy into static pressure.
The impeller consists of several curved blades, which are designed in
such a way that the fluid enters and is ejected as smoothly as possible
thus minimising the losses.
While choosing pumps for an application, certain important considerations
need to be made, which are discussed below:
Cavitation
The design of the pump should be such that there is minimal cavitation,
which occurs when the pressure of the fluid falls below the vapour pressure
at a given temperature. This process results in the formation of vapour
bubbles. These bubbles eventually collapse, resulting in a significant
loss in efficiency as well as structural damage to the pump. In order
to prevent cavitation, a pump with a high net positive suction head
(NPSH), which is defined as the difference between the suction pressure
and the saturation pressure of the fluid being pumped, should be chosen.
The user should be aware of the level of NPSH required for the operation.
He should ensure that NPSH available during the pump operation should
exceed NPSH required, so that there is little or no chance of cavitation.
Shutoff
One important characteristic of a pump is the shutoff pressure, which
is the maximum pressure a pump develops under the no-flow condition.
Head
size
One way to measure the fluid energy created
by the pump is by measuring the pump’s head size, which is directly
proportionate to the capacity or the flow rate of the pump.
Characteristic
curves
Another important aspect in the investigation
of a pump is to obtain characteristic curves, which is the reading of
the various processes. These can be used to evaluate the variation of
the head developed, efficiency, NPSH and brake horsepower with flow
rate for different impeller diameters.
Similarity
rules
Similarity rules are procedures which are
used to estimate the effect of changing the fluid, speed or size on
the performance of a pump within a geometrically similar family. The
use of similarity rules is essential in order to measure the behaviour
of the fluid.
