Power Plant Engineering

 Fig. 11.28
11.12.3 DIFFERENT TYPES OF DRAFT TUBES
In another type of draft tube, a circular plate concentric with draft tube is put into the elbow
section as shown in Fig. 11.28 (e) to improve the flow and efficiency of draft tube and for structural
reasons in large units. The draft tube plays more efficient role with the use of high specific speed runners
like Express Francis and Kaplan. Because the value of velocity energy at the runner exit increases with
an increase in specific speed and it will be nearly 45% in case of Kaplan turbine. Greater care is taken in
the design and construction of the draft tube when used with a runner of high specific speed.
The different types of draft tubes described are shown in Fig. 11.28 (a) to (g).
Draft Tube Liners. The pitting of the turbine runner and in the upper part of draft tube has
occurred in almost all types of runners and kinds of metals used. It is even reported that the runner
blades of 5 cm thick have been eaten in less than two years. In avoiding the pitting, the draft head is of
greatest importance. A poorly designed runner may be set with a low height above tailrace level and
show no pitting, while a properly designed runner may be set high enough above tailrace level but
pitting is unavoidable.
It is always desirable that at least the upper portion of the draft tube should be lined with either a
cast or plate steel liner to avoid the pitting on the surface of the tube. It has been reported that the unlined
concrete draft tube has been eaten by pitting to a depth of 30 cm to 50 cm just below the runner.
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POWER PLANT ENGINEERING
In high head plants (using reaction turbine as prime mover), where the velocity in the draft tube
may be considerably high, the upper part of the draft tube is usually made of steel plate.