CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
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piers, and the savings made on the piers may equal or even
exceed the cost of excava-tion and backfill. Since this is a
site-specific case, a cost calculation should be done if buried
mild steel pipe is being considered. Both exposed and buried
penstock pipes require anchor blocks at significant bends.
However, for relatively low head and flow, as well as small
bend angles, the 1 m depth of well compacted soil cover on
buried pipe may be adequate (see Chapter 7, Anchor blocks
and support piers). The nature of the terrain and the soil
depth may also govern whether to bury or expose the penstock
pipe. Buried penstock is not practicable on routes steeper than
30° because the backfill will be unstable. Where top soil is
thin or rock is exposed, the costs involved in excavating the
rock may make burial of the pipe impossible.
6.10 Expansion joints
Penstock pipes are subjected to temperature variations due to
changes in the ambient temperature. When the ambient
temperature is high the pipes will expand and when it drops,
the pipes will contract. Such thermal expansion causes stresses
in the pipes if they are not free to expand.
An above ground penstock is subjected to greater temperature
variations resulting in higher thermal expansion. The thermal
expansion or contraction is highest when the penstock is
empty, such as during installation or repair work. The
temperature variation is relatively low when the pipe is full
since the flow of water with fairly constant temperature
prevents the pipe from rapidly heating up. As long as pipes
are free to move at one end, thermal expansion does not
cause additional stresses. However, a penstock pipe section
between two anchor blocks is kept fixed at both ends. In such
a case thermal expansion could cause additional stresses and
the pipe can even buckle. Therefore, provision must be made
for the penstock pipe to expand and contract, by installing an
expansion joint in a penstock pipe section between two anchor
blocks. The most common type of expansion joint used in
Nepal is of sliding type. This is shown in Figure 6.6 and
Photograph 6.13. Such an expansion joint is placed between
two consecutive pipe lengths and bolted to them. The stay
TABLE 6.3 Comparison between site welding and flange connection
PIPE JOINING METHOD ADVANTAGE
DISADVANTAGE
GENERAL RECOMMENDATIONS AND
COMMENTS
Site welding
Flange connection
Easy to fabricate at
workshop since
flanges do not
have to be welded
at pipe ends.
Higher degree of precision
work required at site to
weld the pipe ends.
Improper welds can cause
leaks and pipe can burst at
high heads.
A properly welded
pipe will not leak
and requires less
maintenance.
Need to transport a
welding machine and a
generator at site. Also
requires supply of petrol/
diesel to site.
Difficult logistics if the site is more than a
day’s walk from the road head.
Generally not economic for small
schemes and/or short penstock lengths.
Select this option only if the site staff are
experienced, site is less than a day’s
walk from the roadhead and penstock
length is more than 50 m.
Easy to install at Fabrication cost is high
site. Site installa- since flanges need to be
tion work involves welded at ends. Also there
placing a gasket is some wastage since the
between the
flange is prepared by
flanges and bolting removing the central disc
them.
of a diameter equal to the
external pipe diameter.
The pipe alignment and
the bends can not be
adjusted at site.
Can leak if the bolts are
not well tightened or if
gaskets are of poor quality.
Flange connection is appropriate for
schemes that are located more than a
day’s walk from the roadhead and/or
have a relatively short penstock length.
Minimum flange thickness should be at
least twice the penstock wall thickness or
8 mm whichever is larger.
A minimum bolt diameter of 12 mm is
recommended.
A minimum gasket thickness of 5 mm is
recommended.
Should be above ground.
Higher risk of vandalism
since the bolts can be
removed.