Power Plant Engineering

4. Part Load Efficiency. Full load is defined as the load under which a turbine develops its
maximum efficiency anything above that is known as overload and anything below that is known as part
load.
The part load efficiency differs greatly for different specific speed and types of turbines.
Fig. 11.31 shows the variations in part load efficiencies with different types of wheels.
Fig. 11.30

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384
POWER PLANT ENGINEERING
In case of Pelton wheel, only the jet diameter through which the water flows is reduced by the
governing mechanism when the load on the turbine is reduced below full load. The velocity diagrams at
inlet and outlet remain practically unaltered in shape at all loads except for very low and very high loads.
Thus the absolute velocity at inlet does not change and discharge loss remains same. Therefore, the part
load efficiency curve is more flat in case of Pelton turbine.
100
90
80
70
60
50
40
30
20
10
0
10
20
30
40
50
60
70
80
90
100
Percentage of full load
P
er
c
en
tage of
E
ff
ic
ienc
y
Im
pu
ls
e 
N
=
2
5
S
Kaplan Turbine
Fr
an
cis
N
=
8
0
S
Fra
nc
is 
N
= 
40
0
S
Fix
ed 
Pro
pel
ler 
N
= 9
00
S
Fig. 11.31
EXERCISES
1. Draw a neat sketch of a power house and describe the main features of sub-structure and
superstructure.
2. Describe the advantages and disadvantages of underground power stations compared with
overground power stations.
3. What topographical and other conditions decide the setting of turbine either vertical or hori-
zontal (a) in case of reaction turbine (b) in case of Pelton wheel ?
4. What do you understand by open flume setting ? Draw the neat sketches of horizontal and
vertical open flume setting for reaction turbines. When the open flume setting is more prefer-
able ? What are its disadvantages ? Which factors are considered in deciding the setting of
Pelton wheel (a) in horizontal plane (b) in vertical plane. Discuss the advantages of one over
the other.
5. Discuss the differences between Pelton, Francis and Kaplan turbines and type of power plants
they are suitable . Why the inward flow reaction turbines have superseeded the outward flow
turbines ?
6. What do you understand by the term specific speed of a water turbine ? What information
does it give and how it is made use in practice ? Indicate how the form of a reaction turbine
depends upon “specific speed”.
7. Find out the expression for the specific speed of a water turbine in terms of H.P. developed,
the speed and the head available.
Further show that the specific speed of a Pelton wheel is 2450 d/D, where d and D are the
diameter of jet and diameter of mean circle bucket of Pelton wheel in meters respectively.
Assume that mean bucket speed = 0.46.
Maximum efficiency = 0.88% and coefficient of velocity of jet = 1.

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HYDRO-ELECTRIC POWER PLANTS
385
8. Explain why the discharge conditions for a high specific speed runner are less favourable
than those for a low specific speed runner both being assumed to be running at their points of
maximum efficiency 4.18. Explain why the discharge conditions at part load are less favour-
able for the high specific speed runner than for the low specific speed runner.
9. For a given head and stream flow available at a certain power plant, what quantities may be
changed so as to permit the use of various types of turbines ? Which type of turbine will give
the smallest number of units in the plant ? Which type will run at the lowest speed ?
10. How does the maximum efficiency of a reaction turbine vary with the type of the turbine ?
For what type it is the highest ? Why ? For what type it is lowest ? Why ?
11. What are the disadvantages of a very low specific speed reaction turbine ? What are its ad-
vantages ? How does the efficiency of the Pelton wheel vary with its speed ? Why ?