centrifugal pumps, their types and working with examples

1. 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

4. • 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

5. • 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.

6. Pumping System and the Net
Head Developed by a Pump
• The word pumping, referred to a hydraulic
system commonly implies to convey liquid from
a low to a high reservoir. Such a pumping
system, in general, is shown in Fig . At any point
in the system, the elevation or potential head is
measured from a fixed reference datum line.
The total head at any point comprises pressure
head, velocity head and elevation head.
• For the lower reservoir, the total head at the
free surface is HA and is equal to the elevation of
the free surface above the datum line since the
velocity and static pressure at A are zero.

7. • Similarly the total head at the free surface in the
higher reservoir is ( HS + HA ) and is equal to the
elevation of the free surface of the reservoir
above the reference datum.
• The variation of total head as the liquid flows
through the system is shown in Fig. The liquid
enters the intake pipe causing a head loss hin for
which the total energy line drops to
point B corresponding to a location just after the
entrance to intake pipe. The total head at B can
be written as
HB = HA - hin

8. • As the fluid flows from the intake to the inlet
flange of the pump at elevation z1 the total head
drops further to the point C (Figure) due to pipe
friction and other losses equivalent to hf1. The
fluid then enters the pump and gains energy
imparted by the moving rotor of the pump. This
raises the total head of the fluid to a point D
(Figure) at the pump outlet (Figure).
• In course of flow from the pump outlet to the
upper reservoir, friction and other losses account
for a total head loss or hf2 down to a point E .
At E an exit loss he occurs when the liquid enters
the upper reservoir, bringing the total head at

9. the upper reservoir. If the total heads are
measured at the inlet and outlet flanges
respectively, as done in a standard pump test,
then
Total inlet head to the pump =
Total outlet head of the pump =
where V1 and V2 are the velocities in suction and
delivery pipes respectively.
Therefore, the total head developed by the pump,
….(1)

10. The head developed H is termed as manometric
head . If the pipes connected to inlet and outlet
of the pump are of same diameter v2 = v1 and
therefore the head developed or manometric
head H is simply the gain in piezometric pressure
head across the pump which could have been
recorded by a manometer connected between
the inlet and outlet flanges of the pump. In
practice, ( z2 – z1) is so small in comparison to
that it is ignored.
It is therefore not surprising to find that the static
pressure head across the pump is often used to
describe the total head developed by the pump.
The vertical distance between the two levels in

11. the reservoirs HS is known as static head or static
lift. Relationship between HS the static head
and H , the head developed can be found out by
applying Bernoulli's equation
between A and C and between D and F (Fig) as
follows:
……..(2)
Between D and F
……..(3)

12. substituting HA from Eq. (2) into Eq. (3), and then
with the help of Eq. (1), we can write
Therefore, we have, the total head developed by
the pump = static head + sum of all the losses.

14. Centrifugal pumps
• The pump which raises water or liquid from a lower level
to a higher level by the action of centrifugal force is
known as centrifugal pump.
• It will be interesting to know that the action of a
centrifugal pump is that of a reversed reaction turbine. In
a reaction turbine, the water at high pressure, is allowed
to enter the casing which gives mechanical energy at its
shaft; whereas in pump, the mechanical energy is fed
into the shaft and water enters the impeller (attached to
the rotating shaft) which increases the pressure energy
of out-going fluid. The water enters the impeller radially
and leaves the vanes axially.

15. Components of centrifugal
pumps
• Impeller
a rotating wheel fitted with a series of
backward curved vanes or blades mounted
on a shaft connected to the shaft of an
electric motor
• Casing
airtight passage surrounding the impeller
quite similar to the casing of a reaction
turbine.

16. • Casing can normally be of three types
– Volute casing (gradually increasing flow area)
– Vortex casing
– Volute casing with guide blades
• In a volute casing the impeller is surrounded by
a spiral casing. Such a casing provides a
gradual increase in the area of flow, which
decreases the velocity of water, with a
corresponding increase in pressure. A
considerable loss takes place due the formation
of eddies in this type of casing.

17. • In Vortex casing is an improved type of a volute
casing, in which the spiral ring is combined with
a circular chamber. In this type of casing, the
eddies are reduced to a considerable extent and
an increased efficiency is obtained.
• In a volute casing with guide blades, the guide
blades surround the impeller. These guide
blades are arranged at such an angle, that the
water enters without shock and forms a passage
of increasing area, through the water passes
and reaches the delivery pipes. The ring of the
guide blades is called diffuser.

19. Piping System of a Centrifugal
Pump
In general a centrifugal pump has (a) Suction pipe
and (b) delivery pipe
• Suction pipe
– Pipe connecting the inlet of the pump and the
sump is the suction pipe
– Dipping end is provided with a strainer (to
avoid entry of foreign matter) and foot valve
– Since the pressure at the inlet of the pump is
suction (negative) and its value is limited to
avoid cavitation. It is therefore essential that

20. Piping System of a Centrifugal
Pump
that the losses in the suction pipe should
as small as possible. For this purpose,
bends in the suction pipe are avoided and
its diameter is often kept larger.
Sometimes, to reduce the axial thrust, the
suction pipe is branched into two parts
and the liquid is allowed to enter the
impeller from both sides. Such a pump is
called double suction pump.

21. • Delivery pipe
– Used for delivery of liquid
– One end connected to the outlet of the pump
while the other delivers the water at the
required height to the delivery tank
– A check valve is provided in the delivery pipe
near the pump, in order to avoid pump from
hammer and also to regulate the discharge
from the pump.

22. Working of centrifugal pump
• Works on the principle that when a certain mass of fluid
is rotated by an external source, it is thrown away from
the central axis of rotation and a centrifugal head is
impressed which enables it to rise to a higher level.
• First step toward the operation of the pump is primming
• Suction pipe, pump and portion of delivery pipe up to the
delivery valve is filled with water to remove any air or
vapour pocket.

23. • Done by pouring water through the inlet and
releasing the air release pin.
• Pump is started by electric motor to rotate the
impeller.
• Rotation of impeller in casing full of water
produces forced vortex which creates a
centrifugal head on the liquid

24. • The delivery valve is opened as the centrifugal
head is impressed.
• This results in the flow of liquid in an outward
radial direction with high velocity and pressure
enabling the liquid to enter the delivery pipe.
• Partial vacuum is created at the centre of the
impeller which makes the sump water at
atmospheric pressure to rush through the pipe

25. • Delivery of water from sump to delivery
pipe continues so long as the pump is on.
• It is normally considered as the reverse of
a radially inward flow reaction turbine.

26. Work done by the centrifugal
pump
Consider a centrifugal pump lifting water from a
lower level to a higher level. Draw inlet and
outlet velocity vector triangles as shown below.

27. Work done by the centrifugal
pump
Let
V = Absolute velocity of the entering water
D = Diameter of the impeller at inlet
v = Tangential velocity of impeller at inlet ( also
known as peripheral velocity at inlet)
Vr = Relative velocity of water to the wheel at inlet
Vf = Velocity of flow at inlet
V, D1, V1, Vr1, Vf1 = corresponding values at the
outlet
N = Speed of the impeller in rpm

28. Work done by the centrifugal
pump
Let
θ = Vane angle at inlet
Ф = Vane angle at outlet
= Angle at which the water leaves the impellerᵝ
Since water enters the impeller radially with α=90
and then Vw at inlet =0.
(Work done per kN of water), where, E is the energy
transfer to the rotor per unit mass of the fluid =

29. 0=wV
( )11
1
uV
g
w=
( )uVuVmE ww −= 11/
( )uVuVgmgE ww −= 11/1/
As

30. Discharge Q = area x velocity
111 ff VBDDBVQ ππ ==
B and B1 are the widths of the impeller at inlet
and outlet and Vf and Vf1 are the velocities of flow
at inlet and outlet

31. Speed ratio
mgH
u
2
1
=φ
Flow ratio
m
f
gH
V
2
1

32. Head
• Suction head is the vertical distance from sump
level to the centre line of pump (hs)
• Vertical distance from centre line of pump to water
surface in delivery tank is delivery head (hd)
• Sum of suction head and delivery head is static
head.
dss hhH +=

33. • Manometric head is the actual head against
which the pump has to work
• For zero losses, manometric head is given by
g
uV
H w
m
11
=
But losses do occur in the impeller and casing so
g
uV
H w
m
11
= -(loss of head in impeller and casing)

34. Hm = total head at outlet of pump- total head
at inlet of pump






++−





++= i
ii
m z
g
Vp
z
g
Vp
H
22
2
0
2
00
γγ
where
dh
p
=
γ
0

35. g
V
2
2
0 Is the velocity head at outlet
g
Vd
2
2
=
Zo is the vertical height of outlet from datum

36. s
i
h
p
=
γ






++++=
g
V
hhhhH d
fdfsdsm
2
2

37. Efficiencies
• Power is shifted from shaft of electric motor to
the shaft of pump and then to impeller.
• From impeller, shifted to water
• Following efficiencies are involved

38. Manometric efficiency
=η Manometric head/head imparted by impeller to water
1111 uV
gH
g
uV
H
w
m
w
m
mano ==η

39. Mechanical efficiency
(energy available at impeller /energy given
to the impeller)
Overall efficiency
ratio of power output of the pump to power
input to the pump