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?
12.What effects the efficiency of a reaction turbine on part-load? Is the part load efficiency is a
function of specific speed?
13.What are the advantages and disadvantages of very high specific speed turbine runners?
14.What are the advantages of a high speed runner under very low heads. What are the advan-
tages of medium speed runner under the same conditions?
15.What are the advantages of Pelton for very high heads? What are the disadvantages of low
speed reaction turbine for the same conditions?
16.For the same power under the head compare impulse wheels and reaction turbines with re-
spect to efficiency, speed, space occupied, freedom from breakdown, ease of repairs and
durability with silt laden water. Describe the characteristics of various types of turbines used
in hydro-electric power stations with reference to (a) head (b) part load efficiency and maxi-
mum efficiency and (c) specific speed, and, state how these factors help in the choice of the
turbine.
17.What factors are considered in selecting a prime-mover for a hydro-electric power plant?
18.What factors are mainly considered in selecting a prime-mover for (a) run-off river plant (b)
storage plant (c) pump-storage plant.
When the supply is drawn direct from the river instead of storage reservoir, would you go in
for Pelton turbine or a reaction turbine? State the reason for your choice. The head and flow
are same in both cases. How do the rotational speed and cavitation effect the selection of a
water turbine for hydro-electric power plants? What are the advantages of reaction turbine
over the Pelton wheel in respect of efficiency, size, cost and maintenance?