124 CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
The thrust due to hydrostatic force when the valve at the
powerhouse is closed or the total head including surge due
to sudden blockage of flow. If there is an expansion joint
upstream of the valve, the entire head will be transferred to
the machine foundation from the turbine housing.
The vertical force due to the weight of the foundation block,
the turbine and the generator.
Soil forces do not need to be considered because they are
balanced on each side of the foundation.
Since a homogeneous and rigid structure is required the
machine foundation should be constructed of reinforced
concrete. The design process is to tentatively size the machine
foundation and then check the structure against overturning,
sliding and sinking as in the case of anchor blocks.
The Jhankre mini-hydro machine foundation pits can be seen
under construction in Photograph 8.4. The machine
foundation of the Galkot scheme can be seen in Drawing
420/C/ 3C03 of Appendix C. Placing the turbine pit floor 0.3 m
below tailrace invert level helps to reduce abrasion by the
water leaving the turbine.
Example 8.1 illustrates the design principles of a machine
foundation.
Example 8.1 Design of machine foundation
Design a machine foundation to support a directly coupled tur-
bine and generator. The following information has been provided:
Penstock pipe diameter = 300 mm, mild steel.
The pipe centreline is 300 mm above the powerhouse floor.
Qdesign = 150 1/s
Gross head (hgross) = 51 m
Expected maximum surge head (hsurge) = 50 m
Water level in the tailrace channel = 0.25 m
Weight of turbine (WT) = 300 kg
Weight of generator (WG) = 350 kg
Site conditions reveal that the foundation needs to be constructed
on soil.
Calculations:
Try a reinforced concrete structure with dimensions as shown in
Figures 8.2 and 8.3.
htotal = hgross + hsurge = 51m + 50m = 101m
Force due to htotal, (FH)
or FH
= (Pipe area) x 101 m x unit weight
of water
= Ð 0.32/4 m2 x 101m x 9.8 kN/m3
= 69.960 N = 70.0 kN
Weight of turbine (WT)
= 300 kg = 300 x 9.8 = 2940 N =
2.94 kN
Weight of generator(WG)
3.43 kN
= 350 kg = 350 x 9.8 = 3430 N =
Place all forces on the machine foundation and divide the block in
three sections W1 , W2 and W3 as follows:
Calculate the weight of three sections of the block using 22 kN/
m3 for unit weight of concrete.
W, = 0.4x1.5m x 2.5m x22kN/m3 =33.00 kN
W2 = [(0.45 x 1.5 x 2.5)-(0.45 x 1 x 0.5)-(0.45 x 0.5
x 1)]x 22
= 27.23 kN
W3 = 2.35 x 1.5 x 2.5 x 22 = 193.88 kN
Check whether the block is safe against overturning:
Take sum of moments about point B (counter clockwise moments
as positive):
ΣM@b
= W1[(0.4/2) + 0.45 + 2.35] + (W2 + WT)x[(0.45/2 + 2.35)
+ (WG + W3) (2.35/2) - FH x 1.8
= 33.00(3.0) + (27.23+2.94)(2.575) 4- (3.43 + 193.88) (1.175)-
70 xl.8
or ΣM@B = 282.5 kNm
Sum of vertical forces,
V = W1+ W2+ W3+ WT+ WG
= 33.00 + 27.23 + 193.88 + 2.94 + 3.43
or, ΣV = 260.5 kN
Figure 8.2 Machine foundation section