Module - 4

HYDRAULIC MACHINES

Mr. P. Jidhesh
Assistant Professor
Dept. of Mechanical Engineering
SREC, Coimbatore
CLASSIFICATIONS AND TERMINOLOGY
Pumps: Energy absorbing devices
since energy is supplied to them,
and they transfer most of that
energy to the fluid, usually via a
rotating shaft. The increase in fluid
energy is usually felt as an
increase in the pressure of the
fluid.
Turbines: Energy producing devices
they extract energy from the fluid
and transfer most of that energy to
some form of mechanical energy
output, typically in the form of a
rotating shaft. The fluid at the outlet
of a turbine suffers an energy loss,
typically in the form of a loss of
pressure. (a) A pump supplies energy to a fluid,
while (b) a turbine extracts energy 4
from a fluid.
The purpose of a pump is to add
energy to a fluid, resulting in an
increase in fluid pressure, not
necessarily an increase of fluid
speed across the pump.
The purpose of a turbine is to
extract energy from a fluid,
resulting in a decrease of fluid
pressure, not necessarily a
decrease of fluid speed across For the case of steady flow,
the turbine. conservation of mass requires that the
mass flow rate out of a pump must
equal the mass flow rate into the pump;
for incompressible flow with equal inlet
and outlet cross-sectional areas (Dout =
Din), we conclude that Vout = Vin, but
Pout > Pin.

5
Definition
Hydraulic Turbine is a prime mover that
uses the energy of flowing water and
converts it into the mechanical energy in
the form of rotation of runner wheel.
Also termed as ‘water turbines’.
Flow sheet of hydroelectric power plant
 Topics
1. Classification of Turbines
2. Selection of Turbines
3. Design of Turbines - Pelton, Francis, Kaplan
4. Draft Tube
5. Surge Tanks
6. Governing of Turbines
7. Unit Speed, Unit Discharge, Unit Power
8. Characteristic Curves of Hydraulic Turbines
9. Similitude or Model Anlysis
10. Cavitations
 Types of turbines
Turbines can be classified on the basis of:
• Head and quantity of water available
• Hydraulic action of water
• Direction of flow of water in the runner
• Specific speed of turbines
• Disposition of the shaft of the runner
 Classification of Turbines
1. According to type of energy at Inlet
a) Impulse Turbine - Pelton Wheel
Requires High Head and Low Rate of Flow
a) Reaction Turbine - Fancis, Kaplan
Requires Low Head and High Rate of Flow
2. According to direction of flow through runner
a) Tangential Flow Turbine - Pelton Wheel
b) Radial Flow Turbine - Francis Turbine
c) Axial Flow Turbine - Kaplan Turbine
d) Mixed Flow Turbine - Modern Francis Turbine
 Classification of Turbines
3. According to Head at Inlet of turbine
a) High Head Turbine - Pelton Wheel
b) Medium Head Turbine - Fancis Turbine
c) Low Head Turbine - Kaplan Turbine
4. According to Specific Speed of Turbine
a) Low Specific Speed Turbine - Pelton Wheel
b) Medium Specific Speed Turbine -Fancis Turbine
c) High Specific Speed Turbine - Kaplan Turbine
 Classification of Turbines
5. According to Disposition of Turbine Shaft
a) Horizontal Shaft - Pelton Wheel
b) Vertical Shaft - Fancis & Kaplan Turbines
 Classification of turbines
• Based on head and quantity of water
According to head and quantity of water available, the
turbines can be classified into
a) High head turbines
b) Medium head turbines
c) Low head turbines
a) High head turbines
High head turbines are the turbines which work
under heads more than 250m. The quantity of water
needed in case of high head turbines is usually small.
The Pelton turbines are the usual choice for high
heads.
 Classification of turbines
• Based on head and quantity of water
b) Medium head turbines
The turbines that work under a head of 45m to
250m are called medium head turbines. It
requires medium flow of water. Francis turbines
are used for medium heads.
c) Low head turbines
Turbines which work under a head of less than
45m are called low head turbines. Owing to low
head, large quantity of water is required. Kaplan
turbines are used for low heads.
 Classification of turbines
• Based on hydraulic action of water
According to hydraulic action of water, turbines can be
classified into
a) Impulse turbines
b) Reaction turbines

a) Impulse turbines
If the runner of a turbine rotates by the impact or
impulse action of water, it is an impulse turbine.
b) Reaction turbines
These turbines work due to reaction of the
pressure difference between the inlet and the outlet of
the runner.
Impulse Turbines
In an impulse turbine, the fluid is sent
through a nozzle so that most of its
available mechanical energy is converted
into kinetic energy.
The high-speed jet then impinges on
bucket-shaped vanes that transfer energy
to the turbine shaft.
The modern and most efficient type of
impulse turbine is Pelton turbine and the
rotating wheel is now called a Pelton
wheel.

Schematic diagram of a Pelton-type impulse

turbine; the turbine shaft is turned when high-
speed fluid from one or more jets impinges on
buckets mounted to the turbine shaft. (a) Side
view, absolute reference frame, and (b)
bottom view of a cross section of bucket n,
84
rotating reference frame.
A close-up view of a Pelton
wheel showing the detailed
design of the buckets; the
electrical generator is on the
right. This Pelton wheel is on
display at the Waddamana
Power Station Museum near
Bothwell, Tasmania.

A view from the bottom of an

operating Pelton wheel
illustrating the splitting and
turning of the water jet in the
bucket. The water jet enters from
the left, and the Pelton wheel is
turning to the right.
86
Reaction Turbines
The other main type of energy-producing
hydroturbine is the reaction turbine, which
consists of fixed guide vanes called stay vanes,
adjustable guide vanes called wicket gates,
and rotating blades called runner blades.
Flow enters tangentially at high pressure, is
turned toward the runner by the stay vanes as it
moves along the spiral casing or volute, and
then passes through the wicket gates with a
large tangential velocity component.

A reaction turbine differs significantly

from an impulse turbine; instead of
using water jets, a volute is filled with
swirling water that drives the runner.
For hydroturbine applications, the axis
is typically vertical. Top and side views
are shown, including the fixed stay 87
vanes and adjustable wicket gates.
There are two main types of reaction turbine—Francis and Kaplan.
The Francis turbine is somewhat similar in geometry to a centrifugal or mixed-
flow pump, but with the flow in the opposite direction.
The Kaplan turbine is somewhat like an axial-flow fan running backward.
We classify reaction turbines according to the angle that the flow enters the
runner. If the flow enters the runner radially, the turbine is called a Francis radial-
flow turbine.
If the flow enters the runner at some angle between radial and axial, the turbine is
called a Francis mixed-flow turbine. The latter design is more common.
Some hydroturbine engineers use the term “Francis turbine” only when
there is a band on the runner. Francis turbines are most suited for heads that lie
between the high heads of Pelton wheel turbines and the low heads of Kaplan
turbines.
A typical large Francis turbine may have 16 or more runner blades and can
achieve a turbine efficiency of 90 to 95 percent.
If the runner has no band, and flow enters the runner partially turned, it is called a
propeller mixed-flow turbine or simply a mixed-flow turbine. Finally, if the flow
is turned completely axially before entering the runner, the turbine is called an
axial-flow turbine.
88
Kaplan turbines are called double regulated because the flow rate is controlled in two
ways—by turning the wicket gates and by adjusting the pitch on the runner blades.
Propeller turbines are nearly identical to Kaplan turbines except that the blades are
fixed (pitch is not adjustable), and the flow rate is regulated only by the wicket gates
(single regulated).
Compared to the Pelton and Francis turbines, Kaplan turbines and propeller turbines
are most suited for low head, high volume flow rate conditions.
Their efficiencies rival those of Francis turbines and may be as high as 94 percent.
90
91
92
Typical setup and terminology for a hydroelectric plant
that utilizes a Francis turbine to generate electricity;
drawing not to scale. The Pitot probes are shown for
illustrative purposes only.

The net head of a turbine is defined as the difference

between the energy grade line just upstream of the turbine
and the energy grade line at the exit of the draft tube.
93
 Classification of turbines
• Based on direction of flow of water in the runner
Depending upon the direction of flow through the
runner, following types of turbines are there
a) Tangential flow turbines
b) Radial flow turbines
c) Axial flow turbines
d) Mixed flow turbines

a) Tangential flow turbines

When the flow is tangential to the wheel circle, it is
a tangential flow turbine. A Pelton turbine is a
Tangential flow turbine.
 Classification of turbines
• Based on direction of flow of water in the runner
b) Radial flow turbines
In a radial flow, the path of the flow of water
remains in the radial direction and in a plane
normal to the runner shaft. No pure radial flow
turbine is in use these days.
c) Axial flow turbines
When the path of flow water remains parallel to the
axis of the shaft, it is an axial flow turbine. The
Kaplan turbine is axial flow turbine
d) Mixed flow turbines
When there is gradual change of flow from radial to
axial in the runner, the flow is called mixed flow.
The Francis turbine is a mixed flow turbine.
 Classification of turbines
• Based on specific speed of turbines
Specific speed of a turbine is defined as the speed of
a geometrically similar turbine which produces a unit
power when working under a unit head.
The specific speed of Pelton turbine ranges between
8-30, Francis turbines have specific speed between
50-250, Specific speed of Kaplan lies between 250-
850.
• Based on disposition of shaft of runner
Usually, Pelton turbines are setup with horizontal
shafts, where as other types have vertical shafts.
Classification according to Specific Speed of Turbines
 IMPULSE TURBINE
• In an impulse turbine, the fluid is sent through a nozzle so that most of
its available mechanical energy is converted into kinetic energy.
• The high-speed jet then impinges on bucket-shaped vanes that transfer
energy to the turbine shaft.
• The modern and most efficient type of impulse turbine was invented by
Lester A. Pelton (1829–1908) in 1878, and the rotating wheel is now
called a Pelton wheel in his honor.
• The buckets of a Pelton wheel are designed so as to split the flow in
half, and turn the flow nearly 180° around (with respect to a frame of
reference moving with the bucket).
• According to legend, Pelton modeled the splitter ridge shape after the
nostrils of a cow’s nose.
• A portion of the outermost part of each bucket is cut out so that the
majority of the jet can pass through the bucket that is not aligned with
the jet to reach the most aligned bucket . In this way, the maximum
amount of momentum from the jet is utilized.
• The power output of the shaft is equal to Tshaft, where Tshaft is given by
PELTON WHEEL
PELTON
PELTON WHEEL WITH MULTILE JETS
 Design of Pelton Wheel
Guidelines:
1. Jet Ratio = Pitch Diameter of wheel / Dia. of Jet = D/d
2. Speed Ratio = Velocity of Wheel / Velocity of Jet = u/V

3. Velocity of Wheel,
4. Overall Efficiency , OR

5. Water Power, W.P. = ½mV2 = gQH

6. Shaft Power, S.P. =
7. No. of Buckets = (0.5 x Jet Ratio) + 15
 Design of Pelton Wheel
Problems:
1. A Pelton wheel has a mean bucket speed of 10 m/s with a jet of water flowing at
the rate of 700 lps under a head of 30 m. The buckets deflect the jet through an
angle of 160°. Calculate the power given by water to the runner and the
hydraulic efficiency of the turbine. Assume the coefficient of nozzle as 0.98.

2. A Pelton wheel has to develop 13230 kW under a net head of 800 m while
running at a speed of 600 rpm. If the coefficient of Jet C y = 0.97, speed ratio is
0.46 and the ratio of the Jet diameter is 1 /16 of wheel diameter. Calculate
i) Pitch circle diameter
ii) the diameter of jet
iii) the quantity of water supplied to the wheel
 Design of Pelton Wheel
Problems:
3. Design a Pelton wheel for a head of 80m. and speed of 300 RPM. The Pelton
wheel develops 110 kW. Take co-eficient of velocity= 0.98, speed ratio= 0.48 and
overall efficiency = 80%.
4. A double jet Pelton wheel develops 895 MKW with an overall efficiency of 82%
under a head of 60m. The speed ratio = 0.46, jet ratio = 12 and the nozzle
coefficient = 0.97. Find the jet diameter, wheel diameter and wheel speed in
RPM.
 REACTION TURBINE

• The other main type of energy-producing hydroturbine is the reaction turbine, which consists of fixed guide
vanes called stay vanes, adjustable guide vanes called wicket gates, and rotating blades called runner blades
• Flow enters tangentially at high pressure, is turned toward the runner by the stay vanes as it moves along the
spiral casing or volute, and then passes through the wicket gates with a large tangential velocity component.
• Momentum is exchanged between the fluid and the runner as the runner rotates, and there is a large pressure
drop.
• Unlike the impulse turbine, the water completely fills the casing of a reaction turbine. For this reason, a reaction
turbine generally produces more power than an impulse turbine of the same diameter, net head, and volume
flow rate.
• The angle of the wicket gates is adjustable so as to control the volume flow rate through the runner. (In most
designs the wicket gates can close on each other, cutting off the flow of water into the runner.)
• At design conditions the flow leaving the wicket gates impinges parallel to the runner blade leading edge (from
a rotating frame of reference) to avoid shock losses.
• Note that in a good design, the number of wicket gates does not share a common denominator with the number
of runner blades. Otherwise there would be severe vibration caused by simultaneous impingement of two or
more wicket gate wakes onto the leading edges of the runner blades. For example, in Fig. 14–87 there are 17
runner blades and 20 wicket gates. These are typical numbers for many large reaction hydroturbines. The
number of stay vanes and wicket gates is usually the same. This is not a problem since neither of them rotate,
and unsteady wake interaction is not an issue.
• There are two main types of reaction turbine—Francis and Kaplan.
 FRANCIS TURBINE – KAPLAN TURBINE
• The Francis turbine is somewhat similar in geometry to a centrifugal or mixed flow pump, but with the flow in
the opposite direction. Note, however, that a typical pump running backward would not be a very efficient turbine.
• In contrast, the Kaplan turbine is somewhat like an axial-flow fan running backward. If you have ever seen a
window fan start spinning in the wrong direction when a gust of wind blows through the window, you can
visualize the basic operating principle of a Kaplan turbine.
• There are actually several subcategories of both Francis and Kaplan turbines, and the terminology used in the
hydroturbine field is not always standard.
• we classify dynamic pumps according to the angle at which the flow exits the impeller blade—centrifugal (radial),
mixed flow, or axial. In a similar but reversed manner, we classify reaction turbines according to the angle that the
flow enters the runner.
• If the flow enters the runner radially as in, the turbine is called a Francis radial-flow turbine.
• If the flow enters the runner at some angle between radial and axial, the turbine is called a Francis mixed-flow
turbine.
• Francis turbines are most suited for heads that lie between the high heads of Pelton wheel turbines and the low
heads of Kaplan turbines.
• A typical large Francis turbine may have 16 or more runner blades and can achieve a turbine efficiency of 90 to 95
percent. If the runner has no band, and flow enters the runner partially turned, it is called a propeller mixed-flow
turbine or simply a mixed-flow turbine.
• Finally, if the flow is turned completely axially before entering the runner, the turbine is called an axial-flow
turbine. The runners of an axial-flow turbine typically have only three to eight blades, a lot fewer than Francis
turbines. Of these there are two types: Kaplan turbines and propeller turbines.
• Kaplan turbines are called double regulated because the flow rate is controlled in two ways—by turning the
wicket gates and by adjusting the pitch on the runner blades. Propeller turbines are nearly identical to Kaplan
turbines except that the blades are fixed (pitch is not adjustable), and the flow rate is regulated only by the
wicket gates (single regulated). Compared to the Pelton and Francis turbines, Kaplan turbines and propeller
turbines are most suited for low head, high volume flow rate conditions. Their efficiencies rival those of Francis
turbines and may be as high as 94 percent.
FRANCIS TURBINE
FRANCIS TURBINE
 Design of Francis Turbine
Guidelines:
1. Velocity of Wheel,

2. Work done per second or Power,

3. Velocity of Wheel,

4. Discharge,
 Design of Francis Turbine
Problems:
1. A reaction turbine works at 450 rpm under a head of 120 m. Its diameter at inlet
is 1.2 m and the flow area is 0.4 m2 . The angle made by the absolute and
relative velocities at inlet are 20º and 60º respectively with the tangential
velocity. Determine
(i) the discharge through the turbine
(ii) power developed (iii) efficiency.
Assume radial discharge at outlet.
2. A Francis turbine has inlet wheel diameter of 2 m and outlet diameter of 1.2 m.
The runner runs at 250 rpm and water flows at 8 cumecs. The blades have a
constant width of 200 mm. If the vanes are radial at inlet and the discharge is
radially outwards at exit, make calculations for the angle of guide vane at inlet
and blade angle at outlet
KAPLAN TURBINE
KAPLAN
FRANCIS
 Design of Kaplan Turbine
Guidelines:
1. Velocity of Wheel, where

2. Work done per second =

3. Velocity of Flow at Inlet and Outlet are equal

4. Discharge,

5. Flow Ratio =
 Do

Db

Kaplan Turbine
Selection of Turbine
Franci
s

Pelton

Kapla
 Draft Tube
The water after working on the turbine, imparts its energy to the vanes and
runner, there by reducing its pressure less than that of atmospheric Pressure. As
the water flows from higher pressure to lower Pressure, It can not come out of the
turbine and hence a divergent tube is Connected to the end of the turbine.

Draft tube is a divergent tube one end of which is connected to the outlet Of the
turbine and other end is immersed well below the tailrace (Water level).

The major function of the draft tube is to increase the pressure from the inlet to
outlet of the draft tube as it flows through it and hence increase it more than
atmospheric pressure. The other function is to safely Discharge the water that
has worked on the turbine to tailrace.
Draft Tube
Types of Draft Tube
 Surge Tanks
Surge tank (or surge chamber) is a device introduced within a hydropower water
conveyance system having a rather long pressure conduit to absorb the excess
pressure rise in case of a sudden valve closure. The surge tank is located
between the almost horizontal or slightly inclined conduit and steeply sloping
penstock and is designed as a chamber excavated in the mountain.

It also acts as a small storage from which water may be supplied in case of a
sudden valve opening of the turbine.

In case of a sudden opening of turbine valve, there are chances of penstock

collapse due to a negative pressure generation, if there is no surge tank.
Surge Tank
 Governing of Turbines
Governing means Speed Regulation.

Governing system or governor is the main controller of the hydraulic turbine. The
governor varies the water flow through the turbine to control its speed or power
output.

1. Impulse Turbine
a) Spear Regulation
b) Deflector Regulation
c) Combined

2. Reaction Turbine
Governor of Pelton Wheel
Performance of Turbines under unit quantities
The unit quantities give the speed, discharge and power for a particular
turbine under a head of 1m assuming the same efficiency. Unit quantities
are used to predict the performance of turbine.

1. Unit speed (Nu) - Speed of the turbine, working under unit head

2. Unit power (Pu) - Power developed by a turbine, working under a unit head

3. Unit discharge (Qu) - The discharge of the turbine working under a unit head
Specific Speed of Turbine
 Unit Quantities & Specific Speed
Problems:
1. Suggest a suitable type of turbine to develop 7000 kW power under a head
of 20m while operating at 220 rpm. What are the considerations for your
suggestion.
2. A turbine is to operate under a head of 25m at 200 rpm. The discharge is 9
m3/s. If the efficiency is 90%, determine:
i) Power generated ii) Speed and Power at a head of 20m
 Characteristics Curves of Turbine
These are curves which are characteristic of a particular turbine which helps in
studying the performance of the turbine under various conditions. These
curves pertaining to any turbine are supplied by its manufacturers based on actual
tests.

The characteristic curves obtained are the following:

a) Constant head curves or main characteristic curves
b) Constant speed curves or operating characteristic curves
c) Constant efficiency curves or Muschel curves
 Constant head curves or main characteristic curves
Constant head curves:
Maintaining a constant head, the speed of the turbine is varied by admitting different
rates of flow by adjusting the percentage of gate opening. The power P developed is
measured mechanically. From each test the unit power Pu, the unit speed Nu, the
unit discharge Qu and the overall efficiency are determined.

The characteristic curves drawn are

a) Unit discharge vs unit speed
b) Unit power vs unit speed
c) Overall efficiency vs unit speed
 Constant speed curves or operating characteristic curves

Constant speed curves:

In this case tests are conducted at a constant speed varying the head H and
suitably adjusting the discharge Q. The power developed P is measured
mechanically. The overall efficiency is aimed at its maximum value.

The curves drawn are

 Constant efficiency curves or Muschel curves

Constant efficiency curves:

These curves are plotted from data which can be obtained from the constant
head and constant speed curves. The object of obtaining this curve is to determine
the zone of constant efficiency so that we can always run the turbine with
maximum efficiency.

This curve also gives a good idea about the performance of the turbine at
various efficiencies.
 Similitude of Turbines

Dimensionless Numbers:

Where
Q = Discharge
N = Speed of Wheel
D = Dia. of Wheel
H = Head
P = Shaft Power
 Similitude of Turbines - Problems

Problems:
1. A hydraulic turbine develops 120 KW under a head of 10 m at a speed of
1200 rpm and gives an efficiency of 92%. Find the water consumption and
the specific speed. If a model of scale 1: 30 is constructed to operate under a
head of 8m what must be its speed, power and water consumption to run
under the conditions similar to prototype.
2. A model turbine 1m in diameter acting under a head of 2m runs at 150 rpm.
Estimate the scale ratio if the prototype develops 20 KW under a head of 225
m with a specific speed of 100.
 Cavitations
If the pressure of a liquid in course of its flow becomes equal to its vapour pressure
at the existing temperature, then the liquid starts boiling and the pockets of vapour
are formed which create vapour locks to the flow and the flow is stopped. The
phenomenon is known as cavitation.

To avoid cavitation, the minimum pressure in the passage of a liquid flow, should
always be more than the vapour pressure of the liquid at the working temperature.
In a reaction turbine, the point of minimum pressure is usually at the outlet end of
the runner blades, i.e., at the inlet to the draft tube.
Methods to avoid Cavitations
 Reference
Chapter 18
A Textbook of Fluid Mechanics and
Hydraulic Machines
Dr. R. K. Bansal
Laxmi Publications
PUMPS
Pumps
• A pump is a machine which is used to raise or
transfer the fluids.
• It is also used to maintain the constant flow
rate or constant pressure.
• It is normally driven by a engine or a motor.
• Pumps are rated by the horse power.
Important specifications for pump maximum
discharge flow, maximum discharge pressure,
inlet size and discharges size.
Classification of pumps:
It is classified into positive displacement pumps and
Roto dynamic pumps.
• In positive displacement pumps, fluid is drawn or
forced into a finite space and it is sealed.
• It is then forced out and the cycle is repeated.

In Roto dynamic pumps, centrifugal force is used to

move the fluid into a pipe.
Reciprocating Pumps:
• It is a positive displacement pump
• It uses a piston and cylinder arrangement with
suction and delivery valves integrated with the
pump.
• It can be single acting and double acting
• There may be single or multi cylinders also.
• It converts Mechanical energy into Pressure
Energy
Single acting Reciprocating Pump:
 During suction stroke the piston moves to the left,
causing the inlet valve to open.
 Water is admitted into the cylinder through the
inlet valve.
 During the discharge stroke the piston moves to the
right closes the suction valve and opens the out let
valve.
 Through the outlet valve the volume of liquid
moved out of the cylinder.
Layout of Single acting Reciprocating
Pump:
Double Acting Reciprocating Pump – Working:
• Each cycle consists of two strokes.
• Both the strokes are effective, hence it is known
as double acting pump
• Liquid is filled at one end and discharged at
other end during forward stroke.
• During the return stroke, end of cylinder just
emptied is filled and the end just filled is emptied.
Advantages of reciprocating pump:
• Relatively compact design
• High viscosity performance
• Ability to handle high differential pressure
Centrifugal Pumps:
•hydraulic machine with a rotating part.
•It uses a piston and cylinder arrangement
with suction and delivery valves integrated
with the pump.
•It converts Mechanical energy into Pressure
Energy by means of centrifugal force acting on
liquid.
Classification of Centrifugal Pumps:
•According to Type of casing
1. Volute casing
2. Vortex casing
3. Diffuser casing
• According to Number of Stages
1. Single Stage
2. Multi Stage
• According to Type of Impellers
1. Single Suction Impeller
2. Multi Suction Impeller
• According to Shape of Vanes (Blades) of Impellers
1. Radial Flow Impeller
2. Axial Flow Impeller
3. Mixed Flow Impeller
Working
Components of Centrifugal pump:
 A rotating component comprising of an impeller and a
shaft.
 A stationery component comprising a volute (casing),
suction and delivery pipe.

Working Principle of Centrifugal pump:

Principle: 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.

Working:
 The delivery valve is closed and the pump is primed, so
that no air pocket is left.
 Keeping the delivery valve still closed the electric motor
is started to rotate the impeller.
 The rotation of the impeller is gradually increased till
the impeller rotates at its normal speed.
 After the impeller attains the normal speed the
delivery valve is opened when the liquid is
sucked continuously upto the suction pipe.
 It passes through the eye of the casing and
enters the impeller at its centre.
 The liquid is impelled out by the rotating vanes
and it comes out at the outlet tips of the vanes
into the casing.
 Due to the impeller action the pressure head as
well as the velocity heads are increased.
 From the casing the liquid passes into the pipe
and lifted to the required height.
Volute Casing
• An air-tight spiral
passage surrounding the
impeller.
• In this type of casing the
area of flow gradually
increases from the
impeller outlet to the
delivery pipe .
• Increase in area of flow
decrease the velocity of
flow.
Vortex or Whirlpool Casing
• To convert velocity head of
liquid into pressure head.
• Its gradually transformed
into pressure head.
• If a circular chamber is
provided between the
impeller and volute
chamber the casing is
known as Vortex Chamber
Diffuser Casing
 The impeller is surrounded
by a diffuser.
 The guide vanes are
designed in such a way
that the water from the
impeller enters the guide
vanes without shock.
 It reduces the vibration of
the pump.
 Diffuser casing, the
diffuser and the outer
casing are stationery parts.
 Reciprocating Pump Vs Centrifugal Pump
Sl.No Characteristics Reciprocating Pump Centrifugal Pump

1. Type of pump Positive Displacement Pump Rotodynamic Pump

Suitable for More discharge

2. Discharge Suitable for Less discharge

3. Capacity Range Small Capacity Any Capacity

Operating High, hence used for lifting oil Comparatively less operating
4.
Pressure from deep oil well Pressure
Can run at higher speeds
5. Running Speed Can run at low speed only
without Cavitation
6. Priming Does not need priming Needs priming
7. Efficiency More Less

8. Drive Mostly Belt Driven Coupled Directly to motor