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Two different hydraulic functions are required for Intake/Outlet, since the flow direction is reversed in generation and pumping up modes. Therefore, following issues should especially be considered in hydraulic design of Intake/Outlet in PSPP.
Adoptable measures are as follows:
Designs are conducted as follows based on hydraulic model tests by Central Research Institute of Electric Power Industry in Japan and design and construction experiences of PSPP in Japan.
Fan shape with partition wall is the basic design of Lateral Type Intake/Outlet, to control average intake velocity and maximum outlet velocity. To prevent separation and reverse of water flow, each element of the structure is designed as follows.
Structures are designed to meet requirement of average Intake velocity as follows.
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| The shape based on Intake velocity |
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Control of maximum Outlet velocity is necessary for preventing trash rack from resonance fracture and erosion of opposite bank slope of reservoir. Structure is designed to meet requirement of maximum Outlet velocity as follows.
| $v_{max}$ | : Maximum Outlet velocity at trash rack |
| $v_0$ | : Mean velocity of Headrace or Tailrace tunnel |
| $L$ | : Length of gradual-expansion section |
| $d$ | : Diameter of Headrace or Tailrace tunnel |
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| Relationship between the length of gradual-expansion section and maximum Outlet velocity at trash rack (by CRIEPI inJapan) |
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Velocity at opposite bank slope of the reservoir should be limited to required level that has no possibility of erosion and slope collapse. The length of gradual-expansion section is needed to be extended to reduce maximum Outlet velocity. To control velocity at the opposite bank slope of reservoir, length of gradual-expansion section is set based on the following graph which shows relation between distance to opposite side and its velocity. This graph is cited from the result of study on three-dimensional jets. (Turbulent Jets : N.Rajaratnam)
| $v$ | : Velocity at opposite bank (m/s) |
| $v_{max}$ | : Maximum Outlet velocity at trash rack (m/s) |
| $x$ | : Distance to opposite bank (m) |
| $A$ | : Square measure of one separated area (m$^2$) |
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| Relationship between the distance to opposite bank and its velocity |
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The allowable mean velocity of the 1mm's non-adhesive grain is considered to be 0.55m/s. (Source: Hideo KIKKAWA, KASEN-KOUGAKU [River engineering], Asakura Publishing Co., Ltd. Japan)
| Allowable velocity of non-adhesive grain | ||
|---|---|---|
| d (mm) | v (m/s) | Remarks |
| 0.005 | 0.15 | $d$ : Particle size $v$ : Mean allowable velocity |
| 0.05 | 0.20 | |
| 0.25 | 0.30 | |
| 1.00 | 0.55 | |
| 2.50 | 0.65 | |
| 5.00 | 0.80 | |
| 10.00 | 1.00 | |
| 15.00 | 1.20 | |
| 25.00 | 1.50 | |
| 40.00 | 1.80 | |
| 75.00 | 2.40 | |
| 100.0 | 2.70 | |
To prevent occurrence of vortex, the average Intake velocity is limited to less than 1.0m/s, and anti-vortex girders are installed. It is applied for the reservoir which has deep available drawdown range and little space between LWL and top elevation of Intake/Outlet. Anti-vortex girder works for preventing vortex occurrence focusing on the following features of vortex.
Vortex goes around in a specific area and doesn't stay in a same place, and it hardly occurs while the center of vortex flow is obstructed. Therefore, girders are set in appropriate spans in the vortex moving area, where the center of flow strikes.
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| Design of anti-vortex girder (by CRIEPI in Japan) |
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If the velocity is too high, Karman vortex occurs and vibrates the trash rack steel bar at the right angle to the water flow. To prevent trash rack from resonance fracture, the characteristic frequency of trash rack steel bar should be more then 2.5 times of that of Karman vortex. (Technical Standards for Gates and Penstocks: Hydraulic Gate and Penstock Association, Japan) The following measures are taken.
| $f_n$ | : Natural frequency of trash rack steel bar (Hz) |
| $f$ | : Frequency of vibratory force by Karman's vortex (Hz) |
| $V$ | : Flow velocity at trash rack (mm/s) |
| $t$ | : Thickness of steel bar (mm) |
| $S_t$ | : Strouhal number (=0.2) |
| $\alpha$ | : Coefficient of supporting condition |
| ($\alpha=\pi^2$ for simple support, $\alpha=4\pi^2/\sqrt{3}$ for fixed support) | |
| $E$ | : Elastic modulus of steel bar (=206,000 N/mm$^2$) |
| $I$ | : Second moment of steel bar area (mm$^4$) |
| $g$ | : Gravitational acceleration (=9,800 mm/s$^2$) |
| $L$ | : Supporting interval of steel bars (mm) |
| $W$ | : Weight of steel bar and water around it (N) |
| $Q$ | : Volume of steel bar (mm$^3$) |
| ($Q=t\cdot B\cdot L$) | |
| $\gamma$ | : Unit weight of steel bar (=76.93*10$^{-6}$ N/mm$^3$) |
| $\gamma_w$ | : Unit weight of water (=9.8*10$^{-6}$ N/mm$^3$) |
| $B$ | : Width of steel bar (mm) |
| $b$ | : Effective interval of steel bars (mm) |
| ($b=c - t$) | |
| $c$ | : Interval of steel bars (center to center) |
| (Reference) Example of calculation of fn/f | |||
|---|---|---|---|
| $L$ (mm) | 600 | 500 | 400 |
| $f_n$ (Hz) | 71.8 | 103.4 | 161,6 |
| $t$=16mm, $B$=100mm, $c$=150mm | |||
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| Relationship between maximum Intake/Outlet velocity and $f_n/f$ |
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Standard of placement and figure of the partition wall is suggested by the study of Central Research Institute of Electric Power Industry in Japan as follows.
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| Detailed shape of the partition wall (by CRIEPI in Japan) |
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There is another type of intake of PSPP, Morning Glory Type. The type has the following advantages and disadvantages compared with lateral type.
According to the characteristics above, the morning glory type is generally applied especially to upper reservoirs which has small sedimentation inflow and can be made totally empty, and is artificially excavated pond type.
There are not enough examples to make a design standard of the morning glory type intake. Therefore, basic design are referred to the intake which has similar conditions. Sample requirements are shown as follows:
| Features | Requirements |
|---|---|
| Degree of the Screen | 1:0.3 |
| Velocity at the Screen | Less than 0.5m/s |
| Velocity at the beginning of Bellmouth | Less than 0.7m/s |
| Shape of the Bellmouth | Curve : $(x/a)^2+(y/b)^2=1 , \quad b/a\doteqdot 3.6$
Where, $a$ : difference between the radiuses of the beginning and the end of the bellmouth $b$ : length of the bellmouth |
| Width of screen peer | 1.0m |
| Thickness of the top part | 1.0m |
| Elevation of the Inflow Top | L.W.L$-$0.5m |
| Length of Vertical Waterway | More than 36.0m
(beginning of the bellmouth - waterway VIP) |