CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
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2. Design water level in gravel trap
This should be at least 0.49 m + 10% (safety margin) i.e. 0.54 m below the low river level at the intake, or “a” = 0.54 m in Figure 4.11.
3. Flood flow through intake pipes
Flood level is 1.5 m above low river level.
Allow 0.2 m head over gravel trap spillweir (i.e. b = 0.2 m in Figure 4.11).
Then net head on intake pipes = 0.54 + 1.5 - 0.2 = 1.84 m., or “c” = 1.84 in Figure 4.11.
Trial and error solution for flow:
a) Try
Q = 200 1/s in each pipe
or, 1.2Q / d = 1.2x0.2 / 0.262 = 0.916
or, f = 0.0152 (from Moody Chart)
or, hwall loss = 0.0152x40x0.202 / 12x0.2622 = 1.64m
V = 4Q / IId2 = 4x0.2 / IIx0.2622 = 3.71 m/s
Inlet & exit losses (0.8 + 1.0) V2 / 2g = 1.26m
Total head loss = 2.90m
This is more than 1.84 m (net head on intake pipes for flood level),
... assumed flow is too high.
Try interpolating : 200 1/s (1.84/2.90)1/2 = 159 1/s
b) Check for Q = 159 1/s in each pipe.
or, 1.2Q / d = 1.2x0.159 / 0.262 = 0.730
or, f = 0.0154 (from Moody Chart)
or, hwall loss = 0.0154x40x0.1592 = 1.05m
Inlet and exit losses (0.8+1.0) V2 / 2g = 0.80m
Total head loss = 1.85m
This is near enough to 1.84 m
Therefore flood flow through the two intake pipes is: 2 x 159 1/s =
318 1/s
4. Spillway length at gravel trap
Weir equation:
Qspillway = Cw x Lspillway x (hovertop)1.5
... Required spillway weir length,
Lspillway = Qspillway / Cw x (hovertop)1.5
Take Cw = 1.6 and Qspillway = 318 1/s to allow the entire flood flow to
be spilled over in case the turbine needs to be closed during such
floods.
hovertop = 0.20 m (head over weir, assumed earlier)
... Lspillway = 0.318 / 1.6x0.201.5 = 2.22m
or ‘d‘ = 2.22 m in Figure 4.11.
Note the following:
1. To limit the inlet velocity, two parallel pipes have
been used from the intake to the gravel trap. Down-
stream of the gravel trap there is only one pipe
and the velocity is doubled. The reason for using
a lower velocity in the initial pipe length was to
lower the submergence head so that excavation
work at the intake could be minimised. With lower
inlet velocity there less chance of attracting float-
ing debris which could block the inlet.
2. The spillway weir length is reasonable, but note
that an alternative assumption for head over the
weir would give a different answer. The walls
around the gravel trap must be high enough to
contain the flood flow over the spillweir.
3. The gravel trap has to be located sufficiently far
downstream of the intake to ensure that the gravel
trap spillweir is above the maximum river flood
level at that point: in this case the 40 m intake pipe
length is more than enough to meet this criterion.