STUDY AND PERFORMANCE ON
SINGLE STAGE CENTRIFUGAL PUMP
AIM:
To study single-stage centrifugal pump and to find out the efficiency and
performance characteristics of single-stage centrifugal pump.
APPARATUS:
Single-stage centrifugal pump setup, scale, hook gauge, V-notch, suction
pressure gauge, delivery pressure gauge, double acting reciprocating
pump (for priming), power supply, stop watch, etc.
THEORY:PUMP: Pump is a machine which when driven by power from external
source, raises water or any other fluid from lower level to higher level
(increases the pressure) i.e. it receives mechanical energy and raises the
potential energy of the fluid. Hence the pump is just the inversion of
hydraulic prime-mover. There are two main types of pump:
a) Centrifugal pump.
b) Reciprocating pump (positive displacement).
Centrifugal pump belongs to the category of dynamic pressure pumps
where in the pumping of liquid s or generation of head is effected by the
rotary motion of one or more rotating wheels called the impeller.
A centrifugal pump essentially consists of the following elements:
Rotating elements:
It consists of shaft and a vaned rotor called impeller. The vanes are curved,
cylindrical or have more complex surfaces. The unit has a finite number of
vanes. The number usually ranges between 6 and 12. The impeller is
mounted on the shaft coupled to a driving unit which may be an internal
combustion engine or an electric motor. By virtue of force interaction
between the vanes and the liquid, the mechanical energy of the driver is
transformed in to the energy of flow.
Stationary element:
It consists of casing, stuffing box and bearings. The casing is an air tight
chamber surrounding the pump impeller; it receives liquid from the suction
side which enters the casing-impeller space and leaves under high pressure
to the delivery side due to dynamic action of the pump. Packing, labyrinth
seals and glands are needed to reduce the leakage.
Suction pipe, strainer and foot valve:
Suction pipe connects the centre (eye) of the impeller to the sump, from
where the liquid is to be lifted. The pipe is laid air tight so that there is no
possibility of formation of air pockets.
The suction pipe is provided with a strainer at its lower end so as to prevent
the entry of solid particles in to the pump. The foot valve is a one-way valve
located above the strainer in the suction pipe. It serves to hold the liquid
upto pump suction before the pump is started and prevents back-flow when
the pump is stopped.
Delivery pipe and delivery valve:
A delivery pipe leads the liquid from the pump outlet to the point of use. A
regulating valve provided just above the pump outlet serves to control the
flow of liquid in to the delivery pipe.
PRINCIPLE OF OPERATION:
If a liquid is rotated with a sufficiently high velocity so as to enable it to raise
beyond the walls of container and if more liquid is constantly supplied at the
center of some suitable means, the tendency of the liquid will be to flow out
as illustrated in the figure. This is the principle of centrifugal pump.
The first step in the operation of pump is priming, i. e. suction pipe and
casing is filled with water so that no air pockets are left. Rotation of impeller
in casing provides a forced vortex which is responsible for imparting
centrifugal head to the water. Rotation of impeller effects a reduction of
pressure at the center. This causes the water in the suction pipe to rush into
the eye. The speed of the pump should be sufficiently high to produce
centrifugal head to initiate discharge against delivery head.
Mechanical action of the pump is to impart a velocity to the water. The
required relationship between centrifugal head and velocity head is given
by:
H=V2/2g
Where,
V = outlet velocity, H = delivery head
CLASSIFICATION:
The centrifugal pump possesses the following characteristic features on the
basis of which they are classified as follows:
1) Working head
2) Type of casing
3) Number of impellers for shaft
4) Relative direction of flow through impeller
5) Number of entrance to the impeller
6) Position of the shaft
7) Liquid handled
8) Specific speed
(1) Working Head:
(a) Low lift centrifugal pump:-They work against head up to 15 m. impeller is
surrounded by volute, no guide vanes are there.
(b) Medium lift centrifugal pump:-They work against the head of as high as
40 m. they are provided with guide vanes.
(c) High lift centrifugal pump:-They deliver the liquid at the heads of above
40 m. High lift pumps are generally multi stage pumps and used.
(2) Types of Casing:
(a) Volute or spiral casing: It has a volute casing and volute is of a spiral form.
The cross-sectional area of the moving stream is gradually increases
towards the delivery pipe. The cross sectional area at any point is
proportional to quantity of water flowing across that section. The losses of
kinetic head are avoided due to volute casing.
(b) Vortex or whirlpool chamber: Here an annular space is provided
between the volute and impeller. This arrangement arrest the formation
of eddies and gives an improved performance.
(c) Volute casing with guide vanes: Here fixed guide vanes are provided
along the impeller periphery. When the liquid flows through the diverging
passages formed between the guide vanes, conversion of dynamic into
static head occurs. Liquid leaves the vanes where it is then collected in
the volute chamber where further diffusion occurs before the liquid is
discharged to the delivery pipe. Pump fitted with guide vanes are called
diffuser pump or turbine pump.
(3) Number of Impellers:
(a) Single stage centrifugal pump: In this type, one impeller keyed to the
shaft. It is usually low lift pump.
(b) Multi stage centrifugal pump: In this type, two or more impellers are
keyed to the shaft and enclosed in some casing. Pressure is built in
steps/stages. The water is led through the by-pass channel from there
outlet of one stage enters the inlet of the next until it is discharged into a
wide chamber. These pumps have number of stages depending on
head required.
(4) Relative Direction of Flow through Impeller:
(a) Radial flow pump: In radial flow impellers, the head is developed by the
action of centrifugal force upon the liquid which enter the impeller axially
at the center and flows radially towards the periphery.
(b) Axial flow pump: It is a roto-dynamic pump and hardly called as a c.f
pump because centrifugal force is not called into play for the generation
of pressure. In these pumps, the head is developed by the propelling or
lifting action of the vanes on the liquid which enter the impeller axially
and discharged axially. These pumps have very large discharge and are
best suited for irrigation purpose.
(c) Mixed flow pump: Flow through a mixed flow pump is a combination of
axial and radial flow pump. The head is developed partially by the action
of centrifugal force and partly by axial propulsion as a result of which the
fluid enter the impeller axially at the center and is discharged in an
angular direction. These impellers resemble the shape of screw and are
sometimes called screw impellers. This is best suited for irrigation purpose.
(5) Number Entrances to the Impeller:
(a) Single suction pump: it has a single entry and water is admitted from a
suction pipe on one side of the impeller.
(b) Double suction pump: it has double entry and admits the water from
both sides of impeller. It is suitable for pumping large quantities of fluid.
Here axial thrust is neutralized.
(6) Position of Shaft:
The shaft may be disposed horizontally or vertically. The centrifugal pump
is generally designed horizontally. Vertical position affects the economy
of space and hence suitable for deep wells and mines, it may also be
used for irrigation purposes.\
(7) Liquid Handled:
(a) Closed impeller pump: An ordinary centrifugal pump is equipped with a
closed impeller in which the vanes are covered with shrouds on both
sides. This type is meant to handle non-viscous liquids such as ordinary
water, hot water, hot oil and chemicals like acids, etc. Material of
impeller should be selected according chemical properties of the liquid
handled.
 For hot water above 1500C temperature - use cast steel impeller.
 For chemicals which are liable to corrode a ferrous surface.
 For acids - use coated impellers.
(b) Semi-open impeller pump: It is also known as non-dug pump. The
impeller is provided on one side only. This pump is used for viscous liquid.
The design is adopted for the industrial purpose which requires rugged
design for the viscous liquids such as sewage water, paper pulp, sugar
molasses, etc.
(c) Open impeller pump: In this type no shrouds or plates are provided on
either side i.e. the vanes are open on both the sides. Such pumps are
used where the pump has very rough duty to perform such as to handle
abrasive liquid.
(8)Specific Speed:
It is the term used for classifying the pump on the basis of their
performance and dimensional proportions regardless of their actual size
or the speed at which they operate.
It is defined as the speed of pump when delivering 1m3/s against a head
of 1m
Specific speed=Ns= (N √Q)/ (H)¾
where,
Q=quantity of water in m³/s
H=Delivery head, total or manometric head in m
N=Pump speed in revolution per min
PERFORMANCE CHARATERISTICS OF CENTRIFUGAL PUMP:
Main characteristics:
To obtain the test data for main characteristics, the pump is run at a
constant speed and the discharge is varied over the desired range.
Measurements are taken for suction head, delivery head, shaft power and
height above V-notch for each discharge and calculations are made for
the pump efficiency. Curves are then plotted for discharge V/s head, power
and efficiency.
Operating characteristics:
Pumps are generally designed for maximum efficiency and that occur when
the pump operates at the designed speed. A particular part of main
characteristics corresponding to design speed are called the operating
characteristics.
ADVANTAGES OF CENTRIFUGAL PUMP:
 Smooth and even pump flow.
 Low initial cost.
 Compact, occupies less floor space.
 Gross weight is small.
 Installation is easy.
 Efficiency of low head pump is high.
 Construction is simplified by the elimination of many parts such as nonreturn valves, glands, air vessels, etc., therefore less numbers of spare
parts are required.
 Low maintenance cost, periodical checkup is sufficient.
 High speed can be coupled directly through the flanged coupling to
the electric motors or steam turbine.
 Uniform torque.
 It can also handle fluids such as muddy water, sewage water,
chemicals, etc.
DISADVANTAGES OF CENTRIFUGAL PUMP:
 The centrifugal pump is not able to build up high pressure as
compared to reciprocating pump.
 Priming is a must.
 The centrifugal pump is not able to handle small discharge of high
head.
PROCEDURE:
 Priming of the centrifugal pump is carried out using a double acting
reciprocating pump which fills water in centrifugal pump taking water
from reservoir.
 After the priming is complete, the power is supplied to the centrifugal
pump and the input power is measured in terms of KWh at regular
intervals of time.
 Also suction and the delivery head are measured with the help of
gauges mounted on the suction and delivery pipes itself. The former is
obtained in terms of mm of mercury while the latter is obtained in
terms of meter of water column.
 The discharge is measured in terms of the height of water column
above the V-notch. The height is measured in a square column in
which hook gauge is inserted to observe the level rise. As the sump
and square column are connected the level shown are same as in
sump. Observations of all above measurements are noted in
observation table.


The discharge, input power, output power and efficiency are
calculated according to the formulas and based on this result table is
made.
Graphs are plotted for - discharge v/s output power, discharge v/s
efficiency and discharge v/s total head and discharge v/s Cd.
SPECIFICATIONS:
 Efficiency of motor
 V-notch angle
 Coefficient of discharge
 Height of V notch above datum level
CALCULATION:
Suction head in m of water (Hs)
Total head
Qactual
Output power
Input power
%η
Sr.
Suction head
Hs
In mm
of hg
In m of
water
ηmotor =90 %
θ
=90˚
Cd
= 0.6
hdatum = 0.264 m
=ρm х H/ ρw
=Hs + Hd
=8/15хCd х tanθ/2 х √2g х (h) 5/2
=Qactual х ρw х g х H
=E х 36х105 х 1/(t х1000) х ηm
=(output /input) x 100
Delivery
head
Hd
m of
water˚
Total head
H=Hs +Hd
Height of
water (Hr)
over
V-notch
m
Qact
m³/sec
1
2
3
E
kWh
initial
Final
Time
sec
Output power
kW
Input power
kW
%η
Diff.
CONCLUSION:
Based on the number of observations, observation table, result table and
graphs, the following conclusions can be derived: