102 CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
Figure 6.7 Thermal expansion of a penstock pipe
temperature. Different materials expand at different rates.
The values of this coefficient for mild steel, HDPE and PVC
are shown in Table 6.2.
Thot = highest temperature in °C that the pipe will experience.
Note that this can even be during mid-summer afternoon
when the pipe is empty (either during installation or repair
work).
Tcold = lowest temperature in °C that the pipe will experience.
This can be during winter when the water temperature is just
above the freezing point. Note that if freezing temperatures
are expected, the pipe should either be emptied or provision
Example 6.3 Calculating penstock pipe
expansion length
A mild steel penstock pipe is 45 m long between the forebay
and the first anchor block. The steel temperature during
installation was 40°C, and the expected lowest temperature
during the operational phase is 4°C during winter. What
expansion gap should be recommended to the manufacturer?
Also, if the temperature during installation is 20°C, what gap
should be provided?
α = 12 x 10-6 m/m °C
Thot = 40 ° C
Tcold = 4 ° C
L = 45m
L = a(Thot-Tcold)L
or L = 12 x 10-6 (40 - 4) x 45
or L = 0.019m
or L = 19 mm
Therefore minimum recommended expansion gap
= 19.4 x 2 = 38.8 mm, say 40 mm.
If the temperature during installation is 20°C
L = 12x1O-6(40-20) x 45
or L = 0.011 m
or L = 11 mm
Therefore, during installation an expansion gap of 11 mm x
2 = 22 mm should be provided.
should be made for constant flow. If the water in the penstock
stagnates during freezing temperature, ice will form inside
the pipe and could burst it, because when water freezes, the
volume expands. L = pipe length in m. Since it may be difficult
to determine when the expansion joint will be installed at
site, the manufacturer should be asked to allow an expansion
gap of 2 L Then, during installation, the temperature should
be noted and the gap left accordingly.
Photo 6.12 An expansion joint should be located just below an
anchor block to protect the block from forces which it may not be
designed to resist (Siklis)
6.11 Painting
Since mild steel pipes are subjected to corrosion, appropriate
coats of paint should be applied before dispatching them to
site. Proper painting of mild steel pipes significantly increases
their useful lives.
The pipes should be sand blasted if possible, otherwise they
should be thoroughly cleaned using a wire brush and a piece
of cloth. Prior to painting, the pipe surface should be clean
from oil, dust and other particles. When applying subsequent
coats of paint, the previous coat must be dry.