98 CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
6.73 FLANGE CONNECTION
This involves welding flanges (that have bolt holes) at both
ends of the pipes in the workshop, then joining them at site
by bolting them together. A rubber gasket should be placed
between the flanges for tightness and to prevent leakage. The
Galkot penstock (Photograph 6.8) is of flange connected type.
A comparison of these two methods along with general
6.7.4 HDPE AND PVC PIPES
For HDPE pipes the best method of joining them is by heat
welding as described in Chapter 4 (Box 4.7). Although special
flanges are available to connect HDPE pipes, they are gener-
ally more expensive than the cost incurred in heat welding
them. HDPE pipes are available in rolls for small diameter (up
to 50 mm) and for larger diameter they are available in discrete
lengths (3 m to 6 m in Nepal)
PVC pipes with small diameter (up to 200 mm) have socket at
one end such that another pipe can be inserted inside after
applying the solution at the ends. Larger diameter PVC pipes
are joined with a coupler, which is a short pipe section with
inside diameter equal to the outside diameter of the pipes to
be joined. The solution is applied on the connecting surfaces
of both the coupler and the pipes and then joined together.
6.8 Pipe lengths
Mild steel pipes can be manufactured at the workshop in
almost any length required. PVC and HDPE pipes are available
in fixed lengths (3 m to 6 m in Nepal). Although, shorter pipes
are easy to transport, additional costs will have to be incurred
in joining them at site (more flanges or welding work). It
should be noted that, unlike cement bags, animals (mules
and yaks) do not usually carry penstock pipes because of the
shapes and lengths involved.
Sometimes, due to the weight involved the only option for
transporting the generator and turbine to remote site is by a
helicopter. In such cases, it may be possible to transport the
penstock pipes in the same trip because the current transport
helicopter available in Nepal can carry up to three tons
(depending on altitude). The combined weight of the generator
charge is dependent on the flying hours and not on loads.
When this is the case longer pipes (up to 6 m lengths) can be
transported to site and hence joints can be minimised.
Recommendations for pipe lengths under various conditions
are discussed below:
Mild steel pipe
The following factors should be considered while sizing mild
steel pipes.
1.In general pipes longer than 6 m should not be manufactured
since they will be difficult to transport on trucks.
2.If the pipes need to be carried by porters from the roadhead,
the weight should be such that an individual length can be
carried by 1-2 porters. For example, if the pipe weight is about
50 kg, usually one porter can carry it. Similarly two porters may
be to able carry up to 90 kg. Therefore, it is optimum to size
pipes accordingly, especially if the penstock length is long and
the site is located more than a day’s walk from the roadhead.
3.For flat rolled pipes the manufacturing costs will be less if
the pipe length is a multiple of the available steel plate width.
For example if pipes are rolled from 1.2 m wide plates, lengths
of 1.2 m, 2.4 m or 3.6 m etc. will lower manufacturing costs.
HDPE and PVC pipes
As mentioned earlier, these pipes are manufactured in the
factory at fixed lengths, but they can be cut in half or one
third of the length for ease of transportation. However, a PVC
pipe with a socket at one end should not be cut since rejoining
will not be possible without a special collar. The following
factors should be considered while determining the length of
HDPE and PVC pipes.
1. In Nepal the maximum available length of these pipes is 6 m.
Even if longer pipes become available, such lengths should
not be used for micro-hydro schemes. Generally it is easier to
carry two 3 m pipes rather than one 6 m pipe of the same type.
2.If the pipes need to be carried by porters from the roadhead,
then the criteria outlined in No. 2 for mild steel also apply.
6.9 Exposed versus buried penstock
HDPE and PVC pipes should always be buried. This minimises
thermal movement and protects the pipe against impact,
vandalism and ultra-violet degradation. Flanged steel pipe
should be above ground. This is because the gaskets may need
to be replaced during the life of the scheme. Mild steel penstock
with welded joints can be either buried or above ground.
However, maintenance of buried pipe is difficult, therefore the
original painting and backfilling must be carefully supervised
to ensure that corrosion does not reduce the life of the penstock.
Sometimes part of the penstock alignment may be above ground
and part buried. In such cases, it is best to make the transition
at an anchor block, otherwise careful detailing is required. An
example of such detailing at the transition is the use of a
retaining masonry wall with a larger diameter mild steel pipe
through which the penstock comes out and can accommodate
thermal expansion and contraction, see Figure 6.5. An
expansion joint should normally be used immediately
downstream of the retaining wall. Note that the design of anchor
blocks is covered in Chapter 7. Table 6.4 compares the
advantages and disadvantages of buried penstock pipes.
For buried pipes, a minimum cover of 1 m should be provided
in all cases (i.e. HDPE, PVC and mild steel pipes). See Figure
4.8 for trench details. Buried pipes do not require support