58 CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
Box 4.6 HDPE headrace pipes, the Ghandruk experience
The 50 kW Ghandruk micro-hydro scheme
was one of the first micro-hydropower projects
that Practical Action Nepal was involved in.
HDPE pipe has been used successfully for
the long headrace through forest, but lessons
should be learned from the problems experi-
enced:
Sticks and leaves entering the pipe at the
headworks get wedged at the weld beads,
causing pipe blockage.
Vandals throwing stones into the break
pressure tanks.
Pipe collapse due to negative pressure at a
high point (where the pipe is below the
hydraulic grade line).
Surging flow due to air being drawn into the
pipe at break pressure tanks.
At one short location, the hillside was not very
stable and the HDPE pipe has been supported
by galvanised wires tied to trees as can be
seen in Photograph 4.18. Also notice that the
HDPE pipe can be bent when the bend radius
is large. However, it would have been techni-
cally sounder if a gabion wall had been built
downhill of the pipe alignment and the pipe cov-
ered with soil as shown in Figure 4.8 Photo-
graph 4.19 shows a mitred bend on the
Ghandruk HDPE headrace pipe. This was
made by cutting pipe sections at an angle and
them by heat welding. It has started leaking at
the bend and the villagers have wrapped it
with plastic sheets and galvanised wire. Bends
that are constructed by cutting and welding
pipe sections require care during thejoining
process (i.e., the more joints, the higher the
likelihood of leakage). If there is some head
over the headrace pipe, then there can be sig-
nificant forces at the bend as discussed in
Chapter 7. Such forces can weaken the joints
and cause leakage. Also note that the pipe
section shown in the photograph should have
been buried.
Photo 4.18 HDPE headrace pipe along unstable alignment, Ghandruk micro-hydro
scheme, Nepal.
Photo 4.19 Bend prepared by cutting and welding the HDPE
headrace pipe at Ghandruk