CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
93
Note that this does not apply for HOPE pipes: their effective
thickness is the nominal wall thickness of the pipe. A low
temperature correction factor may apply to PVC pipes, refer to
the pipe manufacturer: if the temperatures are sub-zero, teffective
may be as low as 0.5t. Apart from protection from ultraviolet
degradation, this is another reason to bury PVC pipes at high
altitude.
2. Now calculate the safety factor (SF) from the following
equation:
SF=- 200 x teffective x S
htotal x d
where:
teffective is the effective thickness and d is the internal diameter
of the pipe. Note that same units (m or mm) should be used
for both teffective and d since they cancel out in the above equation.
S is the ultimate tensile strength of the pipe material in N/
mm2. Values of S and other useful parameters are shown in
Table 6.2.
htotalis the total head on the penstock as follows:
htotal = hgross + h surge
3. For mild steel or PVC pipes:
If SF < 3.5, reject this penstock option and repeat calculation
for thicker walled option. However, SF ≥ 2.5 can be accepted
for steel pipes if the surge head has been calculated accurately
and all of the following conditions are met:
a) There are experienced staff at site who have installed
penstock pipes of similar pressures and materials.
b) Slow closing valves are incorporated at the powerhouse
and the design is such that a sudden stoppage of the entire
flow is not possible.
c) Damage & safety risks are minimal. For example even if the
pipe bursts, it will not cause landslides or other instability
problems in the short run.
d)Careful pressure testing to total head has been performed
before commissioning.
Then from the manufacturer’s catalogue the actual thickness
should be chosen such that it is equal to or larger than the
calculated t .effective The Safety Factor should then be checked
using the actual thickness. For HDPE pipes, it is recommended
that the Safety Factor always be at least equal to 1.5.
6.6.2 NEGATIVE INTERNAL PRESSURE
Check the pipe wall thickness for buckling if the negative surge
can produce negative internal pressure in the pipe. Note that
the negative pressure must not exceed 10 metres head, see
Section 6.5.1. The shape of the negative surge pressure profile
cannot be accurately determined: assume it is horizontal in
the lower half of the penstock, and diminishes gradually in the
upper half to zero at the forebay, see Figure 6.3.
In order to provide an adequate factor of safety against
buckling, the minimum pipe wall thickness is given by:
teffective ≥ d(FP / 2E)0.33
where:
teffective is the effective pipe wall thickness, mm
d is the pipe internal diameter, mm
F is factor of safety against buckling (2 for buried penstock
and 4 for exposed penstock)
P is the negative pressure, N/mm2 (10 m head ≅ 0.1 N/mm2)
E is Young’s modulus for the pipe material, N/mm2 (from Table 6.2).
If the steel quality is uncertain it is best to ask for samples and
have them independently tested at laboratories. Properties of
PVC and HDPE vary considerably; they should be confirmed
from manufactures’ catalogues or by laboratory tests.
6.7 Pipe jointing
6.7.1 GENERAL
Individual mild steel penstock pipes can be joined at site by
two conventional methods, namely site welding and via
flanges. Each of these methods has its own advantages and
disadvan-tages as discussed in Table 6.3.
For HDPE pipes:
HDPE pipes are available in discrete thicknesses based on the
pressure ratings (kg/cm2) or static heads. The designer should
set SF ≥. 1.5 and calculate teffective (note that t = teffective for HDPE).
6.7.2 SITE WELDING
This involves transporting a welding machine and diesel or
petrol to site, then joining the pipes by welding together the
ends as shown in Photograph 6.10.
TABLE 6.2 Physical characteristics of common materials
MATERIAL
Steel (ungraded)
Steel to IS 226 / 75
or IS 2062/84
PVC
HDPE
YOUNG’S
MODULUS (E) N/MM2
2.0 x 105
2.0 x 105
2750
1000
COEFFICIENT OF LINEAR
EXPANSION (á ) °C
12.5x10-6
12.5x10-6
ULTIMATE TENSILE
STRENGTH (S) N/mm2
320
410
UNIT WEIGHT (Y)
KN/m3
77
77
(20 - 60) x 10-6
(140 - 240) x 10 -6
35-55
20-35
14
9.3