Power Plant Engineering

11.9 POWER HOUSE AND TURBINE SETTING
According to the location of the hydel power station, the power houses are classified as surface
power house or underground power house. As the name implies, the underground power house is one
which is built underground. A cavity is excavated inside earth surface where the sound rock is available
to house the power station. A surface power house is one which is founded on earth’s surface and its
superstructure rests on the foundation.
The surface power house has been broadly divided into three subdivisions which is separated
from the intake as mentioned below :
(a) Substructure ;
(b) Intermediate structure ;
(c) Super-structure.
(a) Substructure. The substructure of a power-house is defined as that part which extends from
the top of the draft tube to the soil or rock. Its purpose is to house the passage for the water coming out
of the turbine. In case of reaction turbines, the hydraulic function of the sub-structure is to provide a
diverging passage (known as draft tube) where the velocity of the exit water is gradually reduced in
order to reduce the loss in pushing out the water. In case of impulse turbine, such a draft tube is not
required and only an exit gallery would serve the purpose.

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POWER PLANT ENGINEERING
The structural function of substructure is dual. The first function is to safely carry the superim-
posed loads of machines and other structures over the cavities. The second function is to act as transition
foundation member which distributes heavy machine loads on the soil such that the obtainable ground
pressures are within safe limits.
(b) Intermediate structure. The intermediate structure of a power house may be defined as that
part of the power house which extends from the top of the draft tube to top of the generator foundation.
This structure contains two important elements of the power house, one is the scroll case which feeds
water to the turbine. The generator foundation rests on the scroll-case which is embedded in the con-
crete. The other galleries, adits and chambers also rest on the same foundation. Scroll or spiral case is a
part of the turbine and it distributes water coming from penstock uniformly and smoothly through guide
vanes to the turbine. The scroll case is required only in case of reaction turbine. In case of impulse
turbine the place of scroll case is taken by the manifold supplying water to the jets.
(c) The structural function of the concrete around scroll case would depend upon the type of
scroll case used. If the scroll case is made of steel and strong enough to withstand internal loads includ-
ing the water hammer effects, the surrounding concrete acts more or less as a space fill and a medium to
distribute the generator loads to the sub-structure. If it is a concrete scroll case then this concrete should
be strong enough to withstand the internal hydrostatic and water hammer head as well as the external
superimposed loads on account of the machine etc. Many times, the steel scroll case is used as water
linear and in this case the surrounding concrete must be strong enough to withstand the internal hydrau-
lic pressures in addition to the superimposed loads.
The structural function of the generator foundation is to support the generator. Arrangements
may be made either to transmit the load 'directly to the substructure through steel barrel or through a
column beam or slab arrangement.
(c) Superstructure. The part of the power house above the generator floor right upto the roof is
known as superstructure. This part provides walls and roofs to power station and also provides an over-
head travelling crane for handling heavy machine parts.
The arrangement of the power house is shown in Fig. 11.10.
Arrangement of Reaction and Impulse Turbines. Factors affecting the choice between horizontal
and vertical setting of machines are : relative cost of plant, foundations, building space and layout of the
plant in general.
Vertical machines offer many advantages over horizontal especially when there are great varia-
tions in tail-race level. Horizontal machines turbine-house should be above the tail-race level or the
lower part of the house must be made watertight. In vertical machines, the weight of rotating parts acts
in the same direction as axial hydraulic thrust. This requires a thrust bearing capable of carrying consid-
erable heavy load. The efficiency of the vertical arrangement is 1 to 2% higher than for a similar hori-
zontal arrangement. This is due to the absence of a suction bend near the runner. As the alternator being
mounted above the turbine, it is completely free from flooding.
With the horizontal machines, there may be two turbines driving one generator and turbines
would operate at a higher speed bringing about a smaller and lighter generator. The horizontal machines
would occupy a greater length than the vertical but the foundations need not be so deep as required for
vertical machines. The horizontal shaft machines require higher settings to reduce or to eliminate the
cost of sealing the generator, the auxiliary electrical equipment and cable ducts against water.

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HYDRO-ELECTRIC POWER PLANTS
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Unit bay
Generator
floor
Crane
Galleries for
auxilleries
Penstock
Approach in
manhole
First stage
concrete
Runner
Draft tube
liner
Draft
tube
Gate
groove
Tailrace
Gantry for
draft tube gate
Second stage concrete
Third stage concrete
Generator
Super structure
Head race
Prime
mover
Draft tube
Tail race
Generator