Pumps
• Machine that provides energy to a fluid in a fluid
system.
• Converts the mechanical energy supplied to it
externally to hydraulic energy and transfers it to
the liquid flowing through a pipe
• Flow is normally from high pressure to low
pressure
Pumps
• On the basis of mode of action of conversion of
mechanical energy to hydraulic energy, pumps
are classified as
– Rotodynamic pumps
– Positive displacement pumps
• In rotodynamic pumps, increase in energy level
is due to combination of centrifugal energy,
pressure energy and kinetic energy
• In displacement pumps, liquid is sucked and
then displaced due to the thrust exerted on it by
a moving member that results in the lifting of
liquid to a desired height.
Centrifugal Pumps
• Introduction:
Centrifugal pumps are the rotodynamic
machines that convert mechanical energy of
shaft into kinetic and pressure energy of water
which may be used to raise the level of water.
The wheel in which this conversion is to realized
is known as a impeller. A centrifugal pump is
named so, because the energy added by the
impeller to the fluid is largely due to centrifugal
effects.
Classification of Centrifugal
Pumps
Centrifugal pumps may be classified according to,
1. Working head
2. Specific speed
3. Type of casing
4. Direction of flow of water
5. Number of entrances to the impeller
6. Disposition of shaft
7. Number of stage
Classification of Centrifugal
Pumps
1. Working Head
Centrifugal pumps may be classified in to low,
medium and high-head pumps.
• Low-Head Centrifugal Pumps
These are usually single-stage-centrifugal
pumps and work below 15m head.
• Medium-Head Centrifugal Pumps
When the head lies between 15 and 45 m, the
pumps are called medium-head-centrifugal
pumps.
Classification of Centrifugal
Pumps
• High-Head Centrifugal Pumps
When the head exceeds 45m, the pumps are
known as high-head-centrifugal pumps. Usually
these are multistage pumps, and are provided
with guide vanes. These pumps may have
horizontal or vertical shafts. Vertical shafts are
useful in deep wells.
Classification of Centrifugal
Pumps
2. Specified Speed
Specific speed of a pump is defined as the
speed of a geometrically similar pump which
delivers unit discharge under unit head.
Ns = N√ Q / H3/4
--------------------------(10.4)
Classification of Centrifugal
Pumps
2. Specified Speed
Following table gives the values of specific
speed of different types of pumps:
Classification of Centrifugal
Pumps
3. Types of Casing
Pumps can be divided into following type
according to their casing:
a) Volute-Chamber Pump
b) Vortex-chamber Pump
c) Diffuser Pump
Classification of Centrifugal
Pumps
3. Types of Casing
a) Volute-Chamber Pump
Such type of casing is of spiral form, and has a
sectional area, which increase uniform ally from
the tongue to the delivery pipe as shown in
fig.10.1 more area is provided to accommodate
increased quantity of water as the water moves
towards the delivery pipe.
Classification of Centrifugal
Pumps
3. Types of Casing
a) Volute-Chamber Pump
Classification of Centrifugal
Pumps
3. Types of Casing
b) Vortex-chamber Pump
In a vortex chamber, a uniformly increasing
area is provided between the impeller outer
periphery and the volute casing as shown in fig.
10.2 water, on leaving the impeller becomes
free to adopt its path.
Classification of Centrifugal
Pumps
3. Types of Casing
b) Vortex-chamber Pump
Classification of Centrifugal
Pumps
3. Types of Casing
c) Diffuser Pump
In a diffuser Pump, the guide vanes are
arranged at the outlet of the impeller vanes.
Water enters the guide without shock. As the
guide vanes are of enlarging cros-sectional
area, the velocity of water decreased and
pressure increases Since the vanes provide
better guidance to flow, eddy losses are
reduced which increases the efficiency.
Classification of Centrifugal
Pumps
3. Types of Casing
c) Diffuser Pump
Classification of Centrifugal
Pumps
4. Direction of Flow of Water
Pumps can also have flow as under:
a) Radial Flow
b) Mixed Flow
c) Axial Flow
Classification of Centrifugal
Pumps
4. Direction of Flow of Water
a) Radial Flow
Radial flow is one in which the flow in the
impeller is completely in a radial direction. This
is used when the requirements are high and
low discharge.
Classification of Centrifugal
Pumps
4. Direction of Flow of Water
b) Mixed Flow
In a mixed-flow, by changing the direction of
flow from pure radial to a combination of a
radial and axial, area of flow is increased. Thus
mixed-flow pumps are used where medium
discharge is needed to raise the water to
medium heads. These are mostly used for
irrigation purposes.
Classification of Centrifugal
Pumps
4. Direction of Flow of Water
c) Axial Flow
These pumps find their use where high
discharge at low heads is required, as in
irrigation.
Classification of Centrifugal
Pumps
4. Direction of Flow of Water
Classification of Centrifugal
Pumps
5. Number of Entrances to the Impeller
Pumps can have either single or double
entrance according to the discharge needed:
a) Single – Suction Pump
b) Double – Suction Pump
Classification of Centrifugal
Pumps
5. Number of Entrances to the Impeller
a) Single – Suction Pump
Pumps which have suction pipe only on one
side of the impeller are called single-suction
pumps
Classification of Centrifugal
Pumps
5. Number of Entrances to the Impeller
b) Double – Suction Pump
In double suction pumps, the suction is made
from both sides of the impeller (Fig. 10.5b).
This increases the discharge considerably.
Classification of Centrifugal
Pumps
5. Number of Entrances to the Impeller
Classification of Centrifugal
Pumps
6. Disposition of Shaft
Usually, the centrifugal pumps are used with
horizontal shafts. Vertical shafts are used
where there is space limitation i.e. in deep
wells, mines etc.
Classification of Centrifugal
Pumps
7. Number of Stages
A centrifugal pump can have a single stage with
one impeller keyed to the shaft or it can be a
multi-stage pumps. A multistage pump has a
number of impellers mounted on the same
shaft and enclosed in the same casing.
Example No.1 A double-suction centrifugal pump delivers
2000 litres of water per second against a head of 25 m
while running at 725 rmp. What type of impeller should be
used for this pump?
Sol. Considering only one half of the impeller,
H= 25 m
Q = 2000/2=1000 lit/s = 1 m3/s
N= 725rmp
Ns = N√ Q / H3/4
= 725√1/ (25)3/4
= 64.8
Thus a high-speed radial impeller should be used.
Example No.2 A six-stage centrifugal pumps delivers 0.1
m3/s against a total head of 480 m. What is its specific
speed if it routes at 1450 rpm ? What type of impeller would
you recommended for the pump?
Sol.
Q = 0.1 m3/s
N= 1450 rmp
Head developed per impeller,
H = 480/6 = 80 m
Ns = N√ Q / H3/4
= 1450√0.1/ (80)3/4
= 17.14
For this specific speed, low-speed-radial impeller
is suitable.