92 CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
If the turbine valve closure time, T, is less than Tc, then the
surge pressure wave is significantly high. Similarly, the longer
T is compared to Tc , the lower the surge effect.
Note that this calculation is based on the assumption that the
penstock diameter, material and wall thickness are uniform.
If any of these parameters vary, then separate calculations
should be done for each section.
Also note that when the T = Tc, the peak surge pressure is
felt by the valve at the end of the penstock. If a pressure
gauge is not installed upstream of the valve, a valve closure
time of at twice the critical time (i.e., T ≥ 2Tc) is
recommended.
The design engineer should inform the turbine manufacturer
of the closure time (T) so that if possible the manufacturer
can choose the thread size and shaft diameter such that it
will be difficult to close the valve in less than twice the
calculated closure time. The operator at the powerhouse should
be made aware of this closure time and the consequences of
rapid valve closure.
If the gross head of the scheme is more than 50 m, it is
recommended that a pressure gauge be placed just upstream
of the valve. Compared to the cost of the turbine and the
penstock, the cost of such a device is low (about US$50 in
Nepal) and is worth the investment. When the operator closes
or opens the valve, his speed should be such that there is no
observable change in the pressure gauge reading
6.5.3 CROSSFLOW TURBINE
In a crossflow turbine, instantaneous blockage of water is not
possible since there is no obstruction at the end of the
manifold (i.e. crossflow turbine has a rectangular bore opening
instead of a nozzle). Therefore, surge pressure can develop
only if the runner valve is closed rapidly. For a crossflow turbine
use the following method to calculate the surge head:
1. Calculate the pressure wave velocity ‘a’ (using the same
equation as for Pelton turbine).
2. Now calculate the critical time Tc, similar to the Pelton
turbine case: Tc = (2L)/a
3. Choose a closure time, T (in seconds), such that: T > 2Tc
Similar to the Pelton turbine case, the design engineer
should inform the turbine manufacturer of the closure time
(T) and the operator at the powerhouse should be made
aware of this closure time.
4. Now calculate the parameter ‘K1 using the following
equation:
K = [ L x V/g x hgross x T ]2
5. Calculate surge head by substituting the value of ‘K’ in the
equation below:
hsurge = [ K/2 ± K + k2 /4] hgross
If ‘K’ is less than 0.01 [i.e. closure time (T) is long enough],
then the following simplified equation can also be used:
hsurge
= hgross K
Note that if the valve is closed instantaneously, the entire
length of the penstock will experience a peak pressure as
follows:
hsurge = av/g (i.e., same as in the case of Pelton turbine
with one nozzle. )
However, in practice it will take at least five to ten seconds for
the operator to close the valve, therefore in a crossflow turbine
instantaneous surge pressure is not a problem.
If the gross head of the scheme is more than 50 m, a pressure
gauge should be placed upstream of the valve to control its
closing/opening speed, as in the case of a Pelton turbine.
6.5.4 QUICK METHOD FOR SMALL SCHEMES WITH
CROSSFLOW TURBINES
For small micro-hydro schemes using crossflow turbines (such
as milling schemes) where the power output is less than 20 kW
and the gross head is less than 20 m, this quick method may be
used. Add 20% to the gross head to allow for surge head,
i.e. hsurge = 1.2 x hgross . This results in a more conservative
value for the surge head but its contribution to the increase
in the thickness would be insignificant since the hgross is low.
6.6 Pipe wall thickness
6.6.1 POSITIVE INTERNAL PRESSURE
Once the surge head has been determined, the nominal wall
thickness (t) can be calculated as follows:
1. If the pipe is mild steel, it is subject to corrosion and welding
or rolling defects. Its effective thickness (teffective will therefore
be less than the nominal thickness. Therefore, for mild
steel, assume a nominal thickness (t) and to calculate teffective
use the following guidelines:
a) Divide the nominal wall thickness by 1.1 to allow for
welding defects.
b) Divide the nominal wall thickness by 1.2 to allow for rolling
inaccuracy of the flat sheets.
c) Since mild steel pipe is subject to corrosion,
The recommended penstock design life is 10 years for
schemes up to 20 kW, 15 years for schemes of 20-50 kW, and
20 years for schemes of 50-100 kW. These figures may be
adjusted on the basis of a financial analysis. For example the
effective thickness of a 3 mm thick mild steel pipe designed
for a 10 years life is:
t =effective
3
1.1 x 1.2
-1 = 1.27 mm
From this example it is clear that if a mild steel pipe used, the
nominal wall thickness (t) should be at least 3 mm.