Power Plant Engineering

Fig. 11.10
Fig. 11.11
In actual cases, the arrangement of the machine (vertical or horizontal) is so chosen which will
give the lowest cost of the station. The majority of impulse turbines are of the horizontal shaft types. The
horizontal arrangement is simpler than vertical from constructional and maintenance point of view.
The overall height and width of the station will be relatively greater in case of vertical arrange-
ment. The floor space occupied by horizontal shaft units is in general greater than that required for
vertical shaft machines. Horizontal shaft arrangement is adopted in most cases, for Pelton wheels, mainly
because this type of setting lends itself readily to the use of multiple runner units and secondly, because
the resulting hydraulic conditions are not favourable with vertical machines.
There are mainly two principal types of setting as :
(1) open flume and
(2) cased turbines.
The open flume setting as shown in Fig. 11.12 and Fig. 11.13 (Rewalls power plant on black river
at Watertown in U.S.A.) are chiefly used for low heads with concentrated falls or with a short canal.
Open penstock setting is one where the entry to the runner has no casing but is placed in an open
forebay. The runner should be placed at a convenient depth below the water surface such that eddies and
suction of air through vertices will not take place. The turbine is completely submerged which results in
a simple and comparatively cheap plant. The disadvantage of this arrangement is that the pit must be
drained to enable inspection and maintenance to be carried out on the turbine and guide vane mecha-
nism. The turbine should have an adequate water head above it, otherwise a sudden increase in load may
draw the water to a dangerous level and allow air to enter. Such condition would break the vacuum in the
draft tube and stop the turbine.
The cased turbines are further divided as concrete casing or steel plate casing as mentioned
earlier. The width of the concrete flume should be kept as small as possible as design permits because
the concrete approach flume often fixes the machine spacing. The concrete scrolls are limited to low
head installations upto 20 metre heights. The complicated form work and reinforcement required for a
concrete flume makes it expensive so that other methods of construction have to be used.
Steel plate scrolls are used for heads ranging from 10 m to 120 m. The arrangement of steel scroll
is shown in Fig. 11.13.

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POWER PLANT ENGINEERING