OTHER COMPONENTS OF HYDROPOWER PLANTS 517
it also has the desirable effect of directing the flow in the direction of the
tailwater flow.
The elbow-type draft tube is divided into three parts, all three sec-
tions gradually expanding like diffusers:
- a vertical (entrance) part in circular cross-section, gradually expand-
ing; - a bend part (its aim being to minimize losses due to changes in the
direction of flow) in gradual transition from the circular section into
a rectangular section; - an almost horizontal part in rectangular section, gradually expanding
to direct the flow into the tail race with minimum losses.
Draft tubes of large discharge capacities (units operating under low
heads) are usually designed by model tests, investigated jointly with the
runner models. Considering the layout in Fig. 12.16, for example, the
energy equation between 1 and 3 gives:
Ysp 1 / g^21 /2gpa/ g^22 /2gHL (12.22)wherepais the atmospheric pressure and HLis the friction and eddy losses
in the draft tube.
Therefore the pressure head at the runner exit, p 1 / g, is given by
p 1 / gpa/ g Ys (^21 /2g ^22 /2g HL). (12.23)Denoting^21 /2g ^22 /2g HLHd, the head regained
Hdd(^21 /2g ^22 /2g) (12.24)wheredis the efficiency of the draft tube. By a proper design of the draft
tube, the exit velocity 2 can be reduced to 1 2ms^1 , with das high as 85%.
In order to avoid cavitation at the exit from the runner, the condition
p 1 / gpv/ g, where pvis the saturated vapour pressure (around 0.3 m of
water absolute), must be satisfied with a sufficient safety factor, since the
flow over parts of the runner will be at lower pressures.
The suggested dimensions of the draft tube used for high specific
speed turbines are shown in Fig. 12.17 (Mosonyi, 1988).
12.9 Other components of hydropower plants
The various appurtenances connecting the construction and operation of
hydroelectric power plants are as follows: dams (storage or diversion type
control works); gates; valves; intakes; waterways (open channel or low-
pressure tunnel head race); high-pressure pipes (penstocks); fish passes;