reduced considerably by housing individual generators only (outdoor
power house), although it has the disadvantage that maintenance works
have to be restricted to good weather conditions only. Under certain
topographic conditions, particularly when the power plant is situated in
narrow canyons with no convenient site for a conventional type of power
house, this may be located underground. Many examples of this type exist
in Europe and elsewhere (e.g. the Cruachan and Dinorwic plants in the
UK).
It is essential to equip a power house with a crane to lift and move
equipment for installation and maintenance purposes. Travelling cranes
spanning the width of the building and capable of traversing its entire
length are normally used. The crane rail elevation depends on the
maximum clearance required when the crane is in operation which, in
turn, determines the overall height of the superstructure (Fig. 12.26).
12.9.7 Tail race
The tail race is the waterway into which the water from the turbine units is
discharged (through draft tubes if reaction-type units are used). It may be
very short and if the power house is close to the stream the outflow may be
discharged directly into it. On the other hand, if the power house is situ-
ated at a distance from the stream the tail race may be of considerable
length. Proper tail race design ensures, especially in low-head plants, that
more of the plant gross head is available for power development.
The tail race in the vicinity of the draft tube exit (head of tail race)
must be properly lined, as it may otherwise degrade and cause lowering of
the tailwater elevation of scouring of the channel bottom. Should this be
allowed to progress the designed turbine setting level would alter, thus
causing reduced efficiency of the turbine (cavitation in the turbine
runner), and remedial measures (artificial raising of the water level) would
have to be taken. The tail race channel may sometimes aggrade, in which
case the gross head at the plant decreases, with a resulting reduction in
power output. This situation may arise if the main spillway outflow is close
to the tail race without an adequate separating wall.
Gates, with an appropriate hoisting mechanism, must be provided at
the draft tube outlet (between the piers and tail race) to isolate the draft
tube for maintenance works.
The tail race of the underground power house is invariably a hori-
zontal tunnel into which the turbine units discharge the water. Such tunnel
flow could sometimes take place under pressure, calling for the necessity
of a surge tank close to the turbine units (i.e. at the head of the tail race
tunnel).
524 HYDROELECTRIC POWER DEVELOPMENT
