100 CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
TABLE 6.4 Advantages and disadvantages of buried penstock pipe
ADVANTAGES
DISADVANTAGES
Protects the penstock against adverse effects of
temperature variations.
Protects the water from freezing due to low air
temperatures.
Protects the pipe from falling debris and trees.
Protects the pipe from tampering and vandalism.
Eliminates support piers.
Pipes are less accessible for inspection, and fault
finding becomes difficult.
Repair and maintenance of the pipes is difficult.
Installation is expensive in rock and where soil
cover is thin.
Impracticable on steep slopes (>30°).
Small bends do not need anchor blocks.
Does not change the landscape.
rings are tightened which compresses the packing and
prevents leaking. Jute or other similar type of fibre is used for
packing. When the pipes expand or contract, the change in
lengths is accommo dated inside the joint section since there
is a gap between the pipes. An advantage of an expansion
joint is that it reduces the size of the anchor blocks since they
will not need to withstand forces due to pipe expansion.
Another advantage is that they can accommodate slight
angular pipe misalignment. Expansion joint requirements for
various penstock conditions are discussed below.
Mild steel pipes
An expansion joint should always be incorporated
immediately downstream of the forebay and immediately
downstream of each anchor block, for both above ground and
buried steel pipe. One is also recommended immediately
downstream of a transition from buried to above ground pipe.
HDPE pipes
Expansion joints are not necessary for HDPE penstock pipes
provided that they are buried (which should always be the
case). This is because HDPE pipes are flexible and can bend
to accommodate the expansion effects due to the differences
in temperature between installation and operational phases.
Photo 6.9 Jhankre mini-hydro penstock alignment
Figure 6.4 Jhankre mini-hydro penstock alignment at downstream end