CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
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(d = 6Qdesign
) = 0.410 m or 410 mm
Ð c hbasin + h flush
( )d = 4Qdesign
Ðc h flush
= 0.386 m or 386 mm
Terefore, a diameter of 410 mm is required for the flush pipe.
The actual flush pipe diameter that was used for the Galkot scheme is 400 mm (slightly smaller than calculated). This was accepted
because it matches the penstock diameter and was easier to fabricate.
Box 5.1 Jhankre settling basin
The Jhankre mini-hydro scheme is
a rare example where it was pos-
sible to combine the gravel trap, set-
tling basin and forebay into one
structure as can be seen in Photo-
graph 5.8 and Figure 3.8 (Chapter
3). The intake is located at the left
bank of a wide pool and there was
adequate space to combine all three
structures here. Also, the topogra-
phy was such that it was possible
to start the penstock alignment right
at the intake.
The gravel trap is placed at the
curved section immediately behind
the intake. The straight section be-
hind the gravel trap works as a set- Photo 5.8 Gravel trap, settling basin and forebay of Jhankre mini-hydro
tling basin and the tank behind this
is the forebay. Since the initial pen-
stock alignment is buried it cannot be seen in the photo-
Platforms have been incorporated at the end of the gravel
graph. Also note that as discussed earlier, the structure
trap, settling basin and above the forebay to facilitate flush-
has been divided into two basins so that the plant need not
ing as well as cleaning of the trashrack. The flushing sys-
be completely shut during flushing.
tem for both gravel and sediment is a modification of the
“vertical flush pipe” method. As shown in Figure 5.7,
instead of a flush pipe a cylinder (tapered at the bottom)
is used which fits snugly into the hole on the basin floor.
Since the water depth is high, the cylinders are lifted
with spindle wheels (thus gaining mechanical advan-
tage) that rest on the slabs. However, note that with this
system, the excess flow cannot be diverted via the
cylinders since they are closed on top.
Figure 5.7 Flushing system of Jhankre mini-hydro
The trashrack at the forebay is cleaned by raking
through the bars (flat plates) with a special steel rake.
To prevent vandalism the spindle wheels are stored
separately and installed only during flushing. In the pho-
tograph the threaded ends (where the spindles are in-
serted) can be seen on the gravel trap and settling
basin platforms.