Hydraulic Turbines

Chapter Seven
Chapter Seven
Hydraulic Turbines
Yimam A.(MSc.)
May 23, 2020
Yimam A.(MSc.) Chapter Seven May 23, 2020 1 / 68

Chapter Seven
Outline
1 Introduction
2 Classiﬁcation of Hydraulic Turbines
3 Construction and Working principle of Pelton and Francis turbines
4 Comparison of Hydraulic Turbines
5 Heads, Losses and Eﬃciencies of Hydraulic Turbines
6 Selection of Turbines

Chapter Seven
Learning Objectives
At the end of this chapter the students should be able to:
Understand function and classiﬁcation of hydraulic turbines
Understand comparison of hydraulic turbines
Understand heads, losses and eﬃciencies of hydraulic turbines
Understand various factors to be considered while selecting a hy-
draulic turbine for a given hydroelectric power plant.
Understand how to estimate the least number of turbines required
for a given hydroelectric power plant.

Chapter Seven
Introduction
Introduction
The hydraulic turbines(water turbines) is a prime mover (a machine
which uses raw energy of a substance and converts into mechanical
energy)that uses the energy of ﬂowing water and converts into the
mechanical energy(in the form of rotation of a runner).
This mechanical energy is used in running an electric generator
which is directly coupled to the shaft of the hydraulic turbine.
From this electric generator we get electric power which can be
transmitted power over long distances by means of transmission
lines and transmission towers.
Water turbines were developed in the 19th century and were widely
used for industrial power prior to electrical grids.

Chapter Seven
Classification of Hydraulic Turbines
Turbines can be classified on the basis of:
I. Head and quantity of water available
II. Hydraulic action of water
III. Direction of flow of water in the runner
IV. Specific speed of turbines
V. Disposition of the shaft of the runner
VI. Based on name of originator

Chapter Seven
I. Based on head of water and quantity of ﬂow
High head turbine :
When head is above 250 m
The quantity of water needed is usually small
e.g. Pelton wheel
Medium head turbine :
When head is between 60m – 250m
It requires medium ﬂow of water
e.g.- Francis turbine

Chapter Seven
Cont....
Low head turbine :
When head is between 15-60 m
large quantity of water is required.
e.g.- Kaplan turbine
Very low head turbine:
When head is less than 15m
e.g.- Propeller turbine

Chapter Seven
Cont....
The net head available to the turbine dictates the selection of type
of turbine suitable for use at a particular site.
The rate of flow determines the capacity of the turbine.
Impulse turbines have application in high head and small quantity
of flow.
Reaction turbines have application in low to medium head and high
rate of flow.

Chapter Seven
II. Based on hydraulic action of water on moving blades
1. Impulse turbine :
In the impulse turbine first all pressure energy of water convert into
the kinetic energy through a nozzle and generate a high speed jet
of water.
This water jet strikes the blade of turbine and rotates it.
This is done by passing the flow through nozzle or some guidelines.
The runner is rotated by the force of water and water passes over
the wheel at atmospheric pressure.
Where the flow hits the turbine as a jet in an open environment,
with the power deriving from the kinetic energy of the flow.

Chapter Seven
Cont....
No pressure drop across turbines.
Uses the velocity of the water to move the runner and discharges to
atmospheric pressure.
The water stream hits each bucket on the runner. No suction down-
side, water flows out through turbine housing after hitting.
Used in High head, low flow applications.
Example : Pelton turbine, Turgo turbine and crossflow turbines

Chapter Seven
Cont....
Figure: Pelton turbine arrangement

Chapter Seven
Cont....
2. Reaction Turbine:
In the reaction turbine there is pressure change of water when it
passes through the rotor of turbine.
So it uses kinetic energy as well as pressure energy to rotate the
turbine. Due to this it is known as reaction turbine.
These turbines work due to reaction of the pressure diﬀerence be-
tween the inlet and the outlet of the runner.
The turbine is totally embedded in the ﬂuid and powered from the
pressure drop across the device

Chapter Seven
Cont....
Runner placed directly in the water stream(totally immersed in wa-
ter) ﬂowing over the blades rather than striking each individually.
Lower head and higher ﬂows than compared with the impulse tur-
bines.
Unlike the Pelton turbine where the water strikes only a few of the
runner buckets at a time, in the Francis turbine the runner is always
full of water.
Example: Francis turbine, Propeller turbine, Kaplan turbine

Chapter Seven
Cont....
Figure: Schematic diagram of Kaplan Turbine

Chapter Seven
III. Based on direction of flow of water in the runner
A. Radial flow turbine:
Radial flow turbine is a turbine in which the water as it move along
the vane flow towards the axis of rotation or away from it (no pure
radial flow turbine is in use these days).Radial flow turbine is of two
types:
i) Inward flow turbine : If the flow of water is towards the axis of
rotation, its called inward flow turbine.
ii) Outward flow turbine : If the flow of water is away from the axis of
rotation , its called outward flow turbine.

Chapter Seven
Cont....
B. Axial ﬂow turbine :
It’s the turbine in which water enters the runner wheel parallel to
the direction of axis of rotation of runner.
Example - Kaplan turbine, Propeller turbine
C. Tangential ﬂow turbine :
It’s the turbine in which water strikes the runner wheel tangentially
to the path of rotation.
Example - Pelton turbine

Chapter Seven
Cont....
D. Mixed flow turbine :
It’s the turbine in which the direction of flow is partly radial and
partly axial.
Water enters the blade radially and comes out axially parallel to
the turbine shaft.
Example: Modern Francis turbine is a mixed flow turbine.

Chapter Seven
Cont....

Chapter Seven
IV. 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 horse power when working
under a unit head(by geometrically similar means that turbine is
identical in shape, dimensions, blade angles and gate opening etc.)
Ns =
N
√
Pt
H5/4
⇒ Ns =
1.165 × N P(kW)
H5/4
Where Ns = Specific speed of turbine
N = The normal working speed in r.p.m
Pt = Power output of the turbine in hp
H = The net or effective head in meters

Chapter Seven
Cont....
The speciﬁc speeds of the various types of runners are given below:
Type of turbine Types of runner Speciﬁc speed(Ns)
Pelton
Slow 10 to 20
Normal 20 to 28
Fast 28 to 35
Francis
Slow 10 to 20
Normal 20 to 28
Fast 28 to 35
Kaplan . . . 300 to 1000

Chapter Seven
Cont....
Speciﬁc speed Types of turbine
10 − 35 Pelton wheel with one nozzle
35 − 60
Pelton wheel with two or more nozzle,
Multi jet Pelton wheel
60 − 300 Francis turbine
300 − 1000 Kaplan turbine

Chapter Seven
Cont....
Low Specific Speed Turbine: If the specific speed is less than 50
the turbine is considered as low specific speed turbine. e.g. Pelton
wheel
Medium Specific Speed Turbine: If the specific speed is between 50
- 150, it is considered as medium specific speed turbine. e.g. Francis
Turbine
High Specific Speed Turbine: If the specific speed of turbine is above
250 it is known as high specific speed turbine. e.g. Kaplan Turbine
Turbines with low specific speeds work under a high head and low
discharge condition.
High specific speed turbines work under low head and high discharge
conditions.

Chapter Seven
V. Based on disposition of turbine shaft
Turbine shaft may be either vertical or horizontal.
In the modern practice, Pelton turbines usually have horizontal
shafts, where as other types have vertical shafts.

Chapter Seven
VI. Based on name of originator
Pelton turbine:
Named after Lester Allen Pelton of California(USA) in 1889.
It is an impulse type of turbine and is used for high head and low
discharge.
Francis turbine:
Named after James Bichens Francis in 1849.
It is a reaction type of turbine for medium high to medium low
heads and medium small to medium large quantities of water.

Chapter Seven
Cont....
Propeller /Kaplan turbine:
Named after Dr.Victor Kaplan in 1913.
It is a reaction type of turbine for low heads and large quantities of
ﬂow.
Dariaz turbine:
Patented by Dariaz in the year 1945 (reaction type)

Chapter Seven
Construction and Working principle of Pelton and Francis turbines
Construction of a Pelton turbine
1. Nozzle: A nozzle play main role of generating power from impulse
turbine. It is a diverging nozzle which converts all pressure energy of
water into kinetic energy and forms the water jet. This high speed
water strikes the blades and rotates it.
2. Casing: The main function of casing is to prevent discharge the
water from vanes to tail race. There is no change in pressure of water
from nozzle to tail race so this turbine works at atmospheric pressure.
It is used to prevent splashing of water and plays no part in power
generation.
3. Runner with buckets(Rotor): Rotor which is also known as wheel is
situated on the shaft. All blades are pined into the rotor. The force
exerted on blades passes to the rotor which further rotates the shaft.

Chapter Seven
Cont....
Runner is a circular disc on the periphery of which a number of evenly
spaced buckets are ﬁxed.
4. Blades: The number of blades is situated over the rotary. They are
concave in shape. The water jet strikes at the blades and change the
direction of it. The force exerted on blades depends upon amount of
change in direction of jet.
5. Braking nozzle: A nozzle is provided in opposite direction of main
nozzle. It is used to slow down or stop the wheel.

Chapter Seven
Cont....
Figure: Schematic diagram of a Pelton turbine

Chapter Seven
Cont....
Figure: Runner of a Pelton Turbine

Chapter Seven
Working Principle of Pelton turbine
High pressure water flow from dam (high head) to nozzle (low head).
This water flows through divergent nozzle where it’s all pressure
energy change into kinetic energy. It forms a water jet.
The water jet strikes the blade at high speed which rotates the rotor.
It transfers all kinetic energy of water to the rotor, which further
use to rotate the generator.
After transferring energy, water flows to the tail race.
This process run continuously until sufficient power generates.

Chapter Seven
Construction of a Francis turbine
1. Spiral Casing:
The runner is completely enclosed in an airtight spiral casing. The
casing and runner are always full of water.
It is a spiral casing with uniformly decreasing cross- section area
along the circumference. Its decreasing cross-section area makes
sure that we have a uniform velocity of the water striking the runner
blades, as we have openings for water ﬂow in-to the runner blades
from the very starting of the casing, so pressure would decrease as
it travels along the casing.
So, we reduce its cross-section area along its circumference to make
pressure uniform, thus uniform momentum or velocity striking the
runner blades.

Chapter Seven
Cont....
2. Guide Vanes(Wicket Gates)
Guide vanes are installed in the spiral casing.
Their most important function is to make sure that water striking
the runner blades must have a direction along length of the axis
of turbine otherwise the flow would be highly swirling as it moves
through spiral casing, making it inefficient to rotate runner blades.
The angle of these guide vanes is adjustable in modern turbines,
and we can adjust the water flow rate by varying the angle of these
guide vanes according to the load on the turbine.
These vanes direct the water onto the runner at an angle appropriate
to the design.

Chapter Seven
Cont....
3. Runner Blades:
Runner blades are said to be heart of a reaction turbine.
It is the shape of the runner blades which uses the pressure energy
of water to run turbine.
Their design plays a major role in deciding the eﬃciency of a tur-
bine. In modern turbines these blades can pitch about their axis,
thus can vary the pressure force acting on them according to the
load and available pressure.
It is a circular wheel on which a series of curved radial guide vanes
are ﬁxed.
The number of runner blades usually varies between 16 to 24.

Chapter Seven
Cont....
4. Draft Tube:
It is a gradually expanding tube which discharges water, passing
through the runner to the tailrace(connects the runner exit to the
tail race).
Its cross-section area increases along its length, as the water coming
out of runner blades is at considerably low pressure, so its expanding
cross-section area help it to recover the pressure as it ﬂows towards
tail race.
It is used for discharging water from the outlet of the runner to the
tail race.
It is applied at head ranges generally from about 15 to 750 meters
and in power ranges from about 0.25 to 800 MW per unit.

Chapter Seven
Cont....
It is an inward-ﬂow reaction turbine
that combines radial and axial ﬂow
concepts.
Normally they have horizontal shaft
but vertical shaft may also be used for
small size turbines.

Chapter Seven
Cont....
Figure: Main parts of reaction turbine

Chapter Seven
Cont....
Figure: Runner of reaction turbine

Chapter Seven
Working principle of Reaction Turbine
Low head and high velocity water enters the spiral casing,and as
it enters the casing it starts flowing through guide vanes into the
runner blades.
Guide vanes guides the flow of water to strike the runner blades
at proper angle, to produce maximum power output. The water
flowing through spiral casing is able to keep its pressure energy
consistent throughout the circumference of spiral casing due to its
uniformly decreasing cross-section area.
These guide vanes can change their angle to increase or decrease the
flow rate of water into turbine.

Chapter Seven
Cont....
The runner blades are also made adjustable, as when the flow of
water is fast and energy demand is less than they would pitch them-
selves to incline at a smaller angle with the axis of turbine.
And when the load on the turbine is more and flow of water is less,
they would adjust themselves at a greater angle with the axis of
turbine.
Two factors which determines the efficiency of a reaction turbine
are the angle of attack of water when it strike runner blades, and
the profile of runner blade over which water glides.
Due to the adjustability of both the guide vanes and runner blades,
we are now able to use this turbine over a wide range of water
potential and load demands.

Chapter Seven
Comparison of Hydraulic Turbines
Impulse Turbine:
In impulse turbine only kinetic energy is used to rotate the turbine.
In this turbine water ﬂow through the nozzle and strike the blades
of turbine.
All pressure energy of water converted into kinetic energy before
striking the vanes.
The pressure of the water remains unchanged and is equal to at-
mospheric pressure during process.Water may admitted over a part
of circumference or over the whole circumference of the wheel of
turbine.
In impulse turbine casing has no hydraulic function to perform be-
cause the jet is at atmospheric pressure. This casing serves only to
prevent splashing of water.
This turbine is most suitable for high head and lower ﬂow rate.Pelton
wheel is an example of this turbine.Yimam A.(MSc.) Chapter Seven May 23, 2020 41 / 68

Chapter Seven
Cont....
Reaction Turbine:
In reaction turbine both kinetic and pressure energy is used to rotate
the turbine,and there is no change in pressure energy of water before
striking.
In this turbine water is guided by the guide blades to ﬂow over the
turbine.
The pressure of water is reducing after passing through vanes.
Casing is absolutely necessary because the pressure at inlet of the
turbine is much higher than the pressure at outlet. It is sealed from
atmospheric pressure.
This turbine is best suited for higher ﬂow rate and lower head sit-
uation.Francis turbine is an example of this turbine.

Chapter Seven
Heads, Losses and Efficiencies of Hydraulic Turbines
Heads of Hydraulic Turbines
Heads are defined as below:
a) Gross Head: Gross or total head is the difference between the head-
race level and the tail race level when there is no flow.
b) Net Head: Net head or the effective head is the head available at
the turbine inlet. This is less than the gross head, by an amount,
equal to the friction losses occurring in the flow passage, from the
reservoir to the turbine inlet.

Chapter Seven
Losses of Hydraulic Turbines
Various types of losses that occur in a power plant are given below:
(a) Head loss in the penstock: This is the friction loss in the pipe of a
penstock.
(b) Head loss in the nozzle: In case of impulse turbines, there is head
loss due to nozzle friction.
(c) Hydraulic losses: In case of impulse turbines, these losses occur due
to blade friction, eddy formation and kinetic energy of the leaving
water. In a reaction turbine, apart from above losses, losses due to
friction in the draft tube and disc friction also occur.

Chapter Seven
Cont....
(d) Leakage losses: In case of impulse turbines, whole of the water may
not be striking the buckets and therefore some of the water power may
go waste. In a reaction turbine, some of the water may be passing
through the clearance between the casing and the runner without
striking the blades and thus not doing any work. These losses are
called leakage losses.
(e) Mechanical losses: The power produced by the runner is not
available as useful work of the shaft because some power may be lost in
bearing friction as mechanical losses.
(f) Generator losses: Due to generator loss, power produced by the
generator is still lesser than the power obtained at the shaft output.

Chapter Seven
Efficiencies of a Hydraulic Turbine
Various types of efficiencies of a turbine are:
(a) Hydraulic efficiency: It is the ratio of the power developed by the
runner to the actual power supplied by water to the runner. It takes
into account the hydraulic losses occurring in the turbine
ηh =
Runner output
Actual power supplied to runner
ηh =
Runner output
(ρgHQ)
Where, Q = Quantity of water actually striking the runner blades
H = Net head available at the turbine inlet

Chapter Seven
Cont....
(b) Volumetric efficiency: It is the ratio of the actual quantity of water
striking the runner blades to the quantity supplied to the turbine. It
takes into account the volumetric losses.
Let ∆Q Quantity of water leaking or not striking the runner blades
ηv =
Q
(Q + ∆Q)
(c) Mechanical efficiency: The ratio of the shaft output to the runner
output is called the mechanical efficiency and it accounts for the
mechanical losses.
ηm =
Shaft output
Runner output

Chapter Seven
Cont....
(d) Overall efficiency: Ratio of shaft output to the net power available
at the turbine inlet gives overall efficiency of the turbine.
ηo =
Shaft output
Net power available
ηo =
Shaft output
Runner output
×
Runner output
ρQgH
×
Q
(Q + ∆Q)
♣ Thus all the three types of losses i.e. mechanical, hydraulic and
volumetric have been taken into account to know the overall efficiency
of the turbine.

Chapter Seven
Selection of Turbines
The various factors to be considered while selecting a turbine are
as follows:
1 Working head
2 Speed and speciﬁc speed
3 Power output and eﬃciency
4 Nature of load
5 Type and quantity of water available
6 Number of units and overall cost
7 Cavitation

Chapter Seven
1) Working head
For low heads propeller or Kaplan turbines are used
Kaplan turbines for low heads of 60 m or less are used. These may
be used under variable head and load conditions.
Propeller turbines are used for very low heads of 15 m or less, and
they work satisfactorily when the head remains constant.
For medium head of between 60 m and 150 m, Francis turbines with
low speciﬁc speed are generally used.
For heads of 150 m to 350 m, again Francis turbines are used and
For very high heads of 350 m or above Pelton turbines are used

Chapter Seven
2) Speed and Specific speed
High specific speed is essential where head is low and output is large,
because otherwise the rotational speed will be low which means cost
of turbo generator and power house will be high.
It is better to choose turbines of high specific speeds. High spe-
cific speed turbines mean small sizes of turbines, generators, power
house,etc.and are therefore, more economical.
Rotational speed depends on specific speed, also the rotational speed
of an electrical generator with which the turbine is to be directly
coupled, depends on the frequency and number of pair of poles.

Chapter Seven
Cont....
The range of speciﬁc speeds of the turbines should correspond to
the synchronous speed of the generator,
N =
120 × f
p
Where f is the frequency and
p is the number of poles.
First indication for the type of turbine to be used is given by the
head available, which is certainly the most important factor govern-
ing the selection.
Second most important criterion is the speciﬁc speed which deter-
mines the type of the turbine to be used.

Chapter Seven
Cont....

Chapter Seven
Cont....
The runaway speed of a water turbine is its speed at full flow, and
no shaft load.
The maximum speed governor being disengaged at which a turbine
would run when there is no external load but operating under design
head and discharge is called runaway speed.
The practical values of runaway speeds for various turbines with
respect to their rated speed N are as follows:
Pelton Wheel 1.8 to 1.9N
Francis turbine (mixed flow) 1.65 to 1.8N
Kaplan turbine (axial flow) 1.9 to 2.2N

Chapter Seven
3) Power output and eﬃciency
The higher the speciﬁc speed for a given head and horse power the
smaller the size of the turbine and the generator and consequently
lower the cost of the installation.

Chapter Seven
Cont....
Kaplan turbine gives high efficiency over a considerable range of
load.
Pelton wheel has somewhat lower, but sustained good efficiency over
a wide range of load.
Francis turbines give lower part efficiencies as their specific speeds
increases.

Chapter Seven
4) Nature of load
Pelton turbine has the advantages that it works quite satisfactorily
even under variable load conditions.
The choice of a Kaplan turbine and a propeller turbine depends
upon the character of the load.
If the load varies, a Kaplan turbine is suitable as its blades adjust
themselves to the varying conditions;
but a propeller turbine, gives its best performance only when the
load is fairly constant.

Chapter Seven
Cont....
Figure: Eﬃciency of diﬀerent turbines

Chapter Seven
5) Type and quantity of water available
If the water used has excess amount of dirt or sand, Francis turbine
cannot be used as its runner cannot withstand the erosive action of
the water.
Moreover water ways are of very small sectional area and easily
get choked by ﬂoating debris and also ﬂuid frictional losses become
relatively high, these factors again restricts the choice of Francis
turbine.
The Pelton wheel would do the duty fairly well.

Chapter Seven
6) Number of units and overall cost
The average overall efficiency of large plant is influenced by the
number of units installed.
A plant with two similar units would have better value of average
efficiency than one unit of double the size of the single unit.
It is desirable to select units of the same size for flexibility of oper-
ation.
A multiunit plant can work better at part load also and can meet
the large variations of load more economically.
The present trend is to use a single unit of big size instead of two
or more units to reduce the capital cost and running cost.

Chapter Seven
Cont....
The various factors inﬂuencing the choice between horizontal and
vertical type of turbines are as follows:
Relative cost of the plant.
Site conditions, i.e. space available for foundations and buildings.
Layout of the plant.
Vertical shaft machines need deeper excavation and higher buildings
than do horizontal shaft machines; the latter need more ﬂoor area.
Experience shows that vertical machines are better for large sizes.

Chapter Seven
In vertical shaft machines, the weight of the rotating parts act in
the same direction as the axial hydraulic thrust.
This necessitates the use of thrust bearing capable of taking such
heavy load and working on runaway speeds even.
As far as the eﬃciency of the machine is concerned, there is hardly
any diﬀerence.

Chapter Seven
7) Cavitation
The formation of water vapour and air bubbles on the water surface
due to the reduction of pressure is known as cavitation.
Cavitation factor determines whether the turbine is to be placed
above or below tail water level. Placing below needs extra excava-
tion, also water has to be pumped out for inspection and repairs.
Deeper foundation increases excavation cost.
Diﬀerence in the pressure of water entering the turbine and that ex-
ists after striking the runner blades is too high, due to this pressure
diﬀerence the air molecules which are relatively at high pressure
than water coming out, enters the turbine casing in the form of
bubbles.

Chapter Seven
Cont....
These bubble keeps on exploding near the surface of the runner
blades continuously causing a shock wave, which produces a kind of
defect at runner’s surface called cavitation, thus causing a serious
problem for turbines eﬃciency.
So, what can we do is to prevent blades from cavitation?
One solution is to use a really hard surface material like stainless
steel or we can also go with surface Hardening of the runner blades,
to prevent them from cavitation.

Chapter Seven
Turbine selection chart

Chapter Seven
Number of Turbines In a Plant
The least number of turbines requirement can be estimated if the
following data is available.
Discharge rate of water
Head of water
Eﬃciency of turbine
Speciﬁc speed of turbine
Speed of the generator directly coupled with turbine.
Example:

Chapter Seven
Ethiopia’s Hydroelectric Power plant turbines
EEP’S Turbines
Koka: Francis turbine
Awash II: Vertical Francis
Awash III: Vertical Francis
Melka Wakena: Vertical Francis
Tis abay I: Francis turbine
Tis abay II: Francis turbine
Fincha: Vertical Pelton
GGI: Vertical Francis
GGII: Vertical Pelton
GGIII: Francis
Tekeze: Vertical Francis

Chapter Seven
Question
Thank You!

Hydraulic Turbines

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