Hydel lecture-2

Classification of Hydraulic Turbines
(Prime Movers)
1

Hydraulic Turbines
• The prime-mover in the hydraulic power plant converts the energy of
water into mechanical energy and further into electrical energy.
• These machines are classified on the basis of the action of water an
moving blades. As per the action of water on the prime-mover, they
are classified as impulse turbine and reaction turbine.
2

Hydraulic Turbines
• In impulse type turbine, the pressure energy of the water is converted
into kinetic energy when passed through the nozzle and forms the
high velocity jet of water. The formed water jet is used for driving the
wheel.
• In case of reaction turbine, the water pressure combined with the
velocity works on the runner. The power in this turbine is developed
from the combined action of pressure and velocity of water that
completely fills the runner and water passage.
• Summary
• Impulse turbines: High-head, low flowrate devices.
• Reaction turbines: Low-head, high-flowrate devices.
3

Hydraulic Turbines
• The casing of the impulse turbine operates at atmospheric: pressure
whereas the casing of the reaction turbine operates under high
pressure. The pressure acts on the rotor and vacuum underneath it.
This is why the easing of reaction turbine is made completely leak
proof.
• The details of few turbines which are commonly used in hydro-
electric power plants are given below.
• Pelton
• Francis
• Kaplan
5

Pelton Turbine
• Figure shows the layout of the Pelton turbine. This was discovered by
Pelton in 1880. This is a special type of axial flow impulse turbine
generally mounted on horizontal shaft, as mentioned earlier A number
of buckets are mounted round the periphery of the wheel as shown in
Fig.
6

Pelton Turbine
• The water is directed towards the wheel through a nozzle or nozzles.
The flow of water through the nozzle is generally controlled by
special regulating system. The water jet after impinging on the
buckets is deflected through an angle of 160° and flows axially in
both directions thus avoiding the axial thrust on the wheel.
• The hydraulic efficiency of Peltan wheel lies between 85 to 95%.
Now-a-days, Pelton wheels are used for very high heads upto 2000
meters.
• Arrangement of jets. In most of the Pelton wheel plants, single jet
with horizontal shaft is used. The number of the jets adopted depends
upon the specific speed required.
7

Pelton Turbine
• Any impulse turbine achieves its maximum efficiency when the velocity of
the bucket at the center line of the jet is slightly under half the jet velocity.
Hence, for maximum speed of rotation, the mean diameter of the runner
should be as small as possible. There is a limit to the size of the jet which
can be applied to any impulse turbine runner without seriously reducing
the efficiency.
• In early twenties, a normal ratio of D/d was about 10 : 1. In a modern
Turgo impulse turbine, it is reduced upto 4.5 to 1. The basic advantage of
Turgo impulse turbine is that a much larger jet could be applied to a runner
of a given mean diameter. The jet of pelton turbine strikes the splitter edge
of the bucket, bifurcates and is discharged at either side.
8

Reaction Turbine
• The reaction turbines are further divided into two general types as
Francis and Propeller Type.
• The propeller turbines are further subdivided into fixed blade
propeller type and the adjustable blade type as Kaplan Turbine.
9

Francis Turbine
• In Francis turbine, the water enters into a casing with a relatively low
velocity, passes through guide vanes located around the
circumference and flows through the runner and finally discharges
into a draft tube sealed below the tailwater level.
• The water passage from the headrace to tail race is completely filled
with water which acts upon the whole circumference of the runner.
• A large part of the power is obtained from the difference in pressure
acting on the front and back of the runner buckets, and only a part of
total power is derived from the dynamic action of the water.
10

Propeller Turbine
• The propeller runner may be considered as a development of a Francis
type in which the number of blades is greatly reduced and the lower
band omitted.
• It is axial flow turbine having a small number of blades from three to
six as shown in Fig. 11.22.
11

Propeller Turbine
• The propeller turbine may be fixed blade type or movable blades type
known as Kaplan Turbine.
• The fixed blade propeller type turbine has high efficiency (88°l0) ; at
full load but its efficiency rapidly drops with decrease in load.
• The efficiency of the unit is hardly 50% at 40% of full load at part
load operation. The use of propeller turbine is limited to the
installations where the units run at full load conditions at all times.
The use of propeller turbine is further limited to low head installations
of 5 to 10 meters.
12

Specific Speed
• The design engineer has a variety of turbine types available for any given
application.
• It is necessary to determine which type of turbine would best fit the job
before detailed design work is attempted.
• As with pump, the use of a specific speed parameter can help provide this
information.
• It is defined as the speed of a turbine which is identical in shape,
geometrical dimensions, blade angles, gate opening etc., with the actual
turbine but of such a size that it will develop unit power when working
under unit head
• • This is the speed at which the runner of a particular diameter will develop
1kW (1hp) power under 1m(1 ft) head.
14

Specific Speed
• It is defined as the speed of a turbine which is identical in shape,
geometrical dimensions, blade angles, gate opening etc., with the
actual turbine but of such a size that it will develop unit power when
working under unit head
• • This is the speed at which the runner of a particular diameter will
develop 1kW (1hp) power under 1m(1 ft) head.
15
4
/
5
)]
(
[
)
(
)
(
)
(
m
H
kW
P
rpm
N
rpm
Ns 

Specific Speed
• A river has a discharge of 400 liters per second and the available head
is 80 meters. Will it be advisable to use a single jet pelton wheel
running at 700rpm. If not what is your recommendation? Assume
efficiency of the machine as 87%.
16
75
QH
P


kW
P 273
80
400
9810
87
.
0






Specific Speed
• The specific speed of the machine is given by
17
4
/
5
)]
(
[
)
(
)
(
)
(
m
H
kW
P
rpm
N
rpm
Ns 
34
.
48
)]
(
80
[
273
)
(
700
)
( 4
/
5


m
rpm
rpm
Ns

Specific Speed
• Typical turbine cross
sections and maximum
efficiencies as a function
of specific speed.
• Provide a guide for
turbine-type selection.
• The actual turbine
efficiency for a given
turbine depends very
strongly on the detailed
design of the turbine.
18

Specific Speed
• Since the specific speed of a pelton wheel is varies between 10 to 40rpm, the
above speed is not within the range of pelton wheel. So either two wheel should
be used or two jets with one wheel should be used if H>300m. Therefore
Francis turbine could be used.
19

Hydel lecture-2

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