86 CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
Although a steep slope minimises the penstock length, it will
be difficult to manually lay the penstock, construct support
piers and anchor blocks if the slope is greater than 1:1. There-
fore, for penstock alignments on slopes steeper than 1:1, the
added site installation cost may outweigh the savings made
on the pipe costs.
Avoid a penstock profile that starts at a gentle slope and then
becomes steeper, because of the risks of negative surge
pressures causing sub-atmospheric pressure. See Section 6.5.
For micro-hydro schemes with less than 20 kW of installed
capacity, the ground profile of the penstock alignment can be
measured using an Abney Level as discussed in Micro-hydro
Design Manual (Ref. 1). For larger micro-hydro schemes, the
use of a theodolite and a professional surveyor is recom-
mended. This is because if the prefabricated bends do not fit
at site due to survey errors, additional cost and time will be
required to amend these, especially if the site is located far
from the roadhead and the pipes are flanged. Note that some
slight adjustment can be made if the pipes are welded at site.
Errors in the design head calculation (due to survey errors)
will result in either oversizing or undersizing the electro-
Photo 6.3 Excavation to reduce the cost of the penstock and reduce
the need for anchors (Siklis)
Photo 6.4 penstock installation is often challenging and
requires safe and careful work.
mechanical units, which will also increase the project cost,
either in terms of lost power production or in extra cost for the
oversized units.
Minimum number of bends
Bends increase the headless and require additional anchor
blocks. Therefore the selected alignment should be as straight
as possible, both in plan and elevation. Note that small bends
can be avoided by varying the support pier heights for the
exposed section and the trench depth for the buried section.
Space for powerhouse area
The chosen alignment should be such that it is possible to
construct a powerhouse at the end of the penstock. A river
terrace well above the flood level is ideal for the powerhouse
area. A route that is otherwise suitable for the penstock
alignment but does not allow for the construction of the
powerhouse is inappropriate.
Stability
Since the penstock alignment is on steep ground slopes and
the pipe is under pressure, it is important for the alignment to
be on stable ground. Any ground movement can damage the