Working of Centrifugal Pumps
Centrifugal pumps are the most preferred hydraulic pumps used in
domestic and industrial world. In this video we will have a conceptual
overview of the working of centrifugal pumps.
Impeller - The Heart of Centrifugal Pumps
Centrifugal pumps are used to induce flow or raise pressure of a
liquid. Its working is simple. At the heart of the system lies impeller.
It has a series of curved vanes fitted inside the shroud plates. The
impeller is always immersed in the water. When the impeller is made to
rotate, it makes the fluid surrounding it also rotate. This imparts
centrifugal force to the water particles, and water moves radially out.
In Fig.1 this process is illustrated.
Since the rotational mechanical energy is transferred to the fluid, at
the discharge side of the impeller, both the pressure and kinetic energy
of the water will rise.
At the suction side, water is getting displaced, so a negative pressure
will be induced at the eye. Such a low pressure helps to suck fresh
water stream into the system again, and this process continues.
From foregoing discussions it is clear that, the negative pressure at
the eye of the impeller helps to maintain the flow in the system. If no
water is present initially, the negative pressure developed by the
rotating air, at the eye will be negligibly small to suck fresh stream
of water. As a result the impeller will rotate without sucking and
discharging any water content. So the pump should be initially filled
with water before starting it. This process is known as priming.
The impeller is fitted inside a casing. As a result the water moves out
will be collected inside it, and will move in the same direction of
rotation of the impeller, to the discharge nozzle.This is shown in the
Fig.3.
From the illustrations of the pump so far, one speciality of the
casing is clear. It has an increasing area along the flow direction.
Such increasing area will help to accommodate newly added water stream,
and will also help to reduce the exit flow velocity. Reduction in the
flow velocity will result in increase in the static pressure, which is
required to overcome the resistance of pumping system.
Impeller Design
As we have discussed earlier impeller is the most vital part of a
centrifugal pump. Successful impellers have been developed with many
years of analysis and developmental work. Fig.4 shows one of such
impeller with its one shroud plate removed for better view of vanes.
These vanes are backward curved. Backward curved vanes have the
blade angle less than 90 degree. Backward curved vanes are the most
preferred vane type in the industry due to its self stabilizing power
consumption characteristics. This means with increase in flow rate power
consumption of the pump stabilizes after a limit. Forward and radial
blades are less common in the industry. The eye configuration of the
impeller shown is state of the art. This vane is extracted from a
Kirloskar pump model. Such projecting eye section induces better swirl of flow and guarantees high negative pressure at the suction.
If pressure at the suction side of impeller goes below vapor pressure
of the water, a dangerous phenomenon could happen. Water will start to
boil forming vapor bubbles. These bubbles will move along with the flow
and will break in a high pressure region. Upon breaking the bubbles will
send high impulsive shock waves and spoil impeller material overtime.
This phenomenon is known as cavitation. More the suction head, lesser
should be the pressure at suction side to lift the water. This fact puts
a limit to the maximum suction head a pump can have.
However Cavitation can be completely avoided by careful pump selection.
The term NPSH (Net Positive Suction Head) helps the designer to choose
the right pump which will completely avoid Cavitation. NPSH is defined
as follows.
Where
Pv is vapor pressure of water
V is speed of water at suction side
For a given pumping system it will have an NPSH called
'Available NPSH'. Pump manufacturer will specify the minimum NPSH required for each pump for its safe operation, known as
'Required NPSH'. If the pump needs to work without Cavitation the
'Available NPSH' should be greater than
'Required NPSH'.
Types of Impeller
The impeller type we have used for the discussion so far is called as
an enclosed type. Here vanes are closed from both the ends with shroud
plates. Other types of impeller which are used in industry are Semi open
and open impellers. If the working fluid is cloggy in nature it is
preferred to use an open kind of impeller. But they are slightly less
efficient.
The mechanical design of centrifugal pump is always challenging. A
shaft is used to connect between the impeller and motor. Since water
pressure inside the casing is huge, a proper sealing arrangement is
imperative in arresting the water leakage through the shaft casing
clearance. Mechanical seal or stuffing box based mechanism is used for
this purpose.
Impeller is mounted on the bearings. But at the suction side of impeller
it is not advisable to fit a bearing, since it will block the flow. As a
result the bearings have to be fitted at the other end. This means
impeller is mounted like a cantilever. For high flow rate pumps, a
bearing housing with cooling oil is necessary for improving the life of
the bearings.
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