DESIGN GUIDELINES
Chart 2: Flow Chart for Tunnel Design Required flow
1
Velocity
Diameter of Tunnel
Pressure
Rock Quality 2 Hydraulic
gradient
3 Required
Tunnel Support
Tunnel cross- section
Roughness value ‘n’
Total Head Loss
Non
pressure 2
Area
Profile
Hydraulic
Radius 3
Roughness value ‘n’
Slope of Tunnel
4
Discharge ‘Q’, Velocity ‘v’
Required Tunnel Support
Tunnel cross section Rock Quality
3.1.1.6 Cross Drainage Works General
The water conveyance system (canal or pipe) laid down on a ground surface or buried for hydropower development have to cross generally the streams, drains and sometimes road, railway, valley or depressions, the structural works necessary to be incorporated in the water conveyance system for such crossings are called cross drainage works. They include aqueducts, siphons and flumes. The design guides for these cross drainage works are briefly described in the following sub-sections.
Aqueducts
An aqueduct literally means a channel for conveying water and it may be either above or below the ground. In water resources engineering the term is confined to mean a structure carrying canal over a drainage channel without having to lower down the bed of the drainage channel for the crossing.
Hydraulic designs of the aqueducts are similar to the open canal using Manning’s formula. A typical section of the pipe aqueduct is shown in Fig. 3.18.
Fig. 3.18: Pipe Aqueduct Siphons
Siphons are usually proposed to cross a large drainage channel, rivers along the canal alignment. It is usually more economical to carry the canal water under the channel in an inverted siphon than to carry the drainage water under the canal through a culvert. Siphon provides excellent reliability, as the accuracy of the cross drainage flow predication is less critical where siphons are used. However, the use of siphon is contingent upon availability of head for siphon losses. Other factors which will affect the results of a cost comparison are the width and depth of the drainage channel. A typical section of a pipe siphon is shown in Fig. 3.19.
Fig. 3.19: Typical Section of Pipe Siphon
Hydraulic Consideration: Available head, economy and allowable pipe velocities determine the size of the siphon pipe. Thus it is necessary to assume internal dimensions for the siphon and compute head-losses such as entrance, friction, bend, exit and trashrack. The sum of all the computed losses should approximate the difference in energy grade elevation between the upstream and downstream ends of the siphon. Adequate submergence below upstream water level should be provided to conduit at the inlet.
Design Procedure:
• Determine the structures required at inlet and outlet and size of pipe
• Compute siphon head losses in the layout. If the head losses do not agree with the available head, it may be necessary to make some adjustment such as pipe size, etc.
If the computed head losses are greater than the available head, i.e. greater than the difference in upstream and downstream water level, the siphon will cause backwater effect upstream. In this case either the size of the pipe has to be increased to reduce the head losses or revised the canal profile. If the computed head losses are appreciably less than the difference in upstream and downstream water surface, it may be possible to decrease the size of pipe or the canal profile may be revised so that the available head is approximately the same as the head losses
Flumes
Flumes are used to convey canal water along steep side hill terrain, or to convey canal water over other waterways, or natural drainage channels. Flumes are also used at locations where there is restricted right of way or where lack of suitable material makes construction of canal banks undesirable or impracticable. Flumes supported on a bench excavated into a hillside are called bench flumes and flumes supported above the ground with reinforced concrete, structural steel, or timbers are called elevated flumes. Flumes are hydraulically designed as an open canal flow using Manning’s formula as described above.
3.1.1.7 Structural Elements to be Incorporated in the Water Conveyance System for Controlling Water-Hammer
General
The sudden closure of the terminal closing mechanism, wicket gate or valve guarding the turbines is followed by inertia effects resulting in over pressures and depressions in the penstock or pressure conduit, which should be allowed too in the structural design. Such pressure changes induce the effect of water-hammer in the pressure conduit. During load demand, low-pressure waves are formed at the turbine as the water column is accelerated. This could result into water column separation creating vacuum zone, subsequent refilling of this vacuum zone or rejoinder of the water columns can create extremely high pressure.
There are number of ways for controlling water-hammer. They are listed below:
• Reduce penstock velocities (enlarge conduit diameters);
• Reduce length of waterways. Profile changes can also alleviate some problems;
• Reduce valve closure times or opening times;
• Vary machine hydraulic characteristics;
• Increase WR2 (a constant called flywheel effect);
• Change wave velocity;
• Install pressure control valves;
• Add a surge tank;
• Add a cushioning stroke on the turbine (two closure rates);
• Add air chambers.
Since the present study is concerned only with the design of water conveyance system, the methods of controlling water-hammer related to the structural elements to be incorporated in the water conveyance system such as forebay and surge tank / shaft only have been dealt with here.
Forebay (Headpond)
The forebay (headpond) is essentially a broadened section of the canal in which a gated spillway is installed. The purpose is to distribute evenly, over a proper transition, the water conveyed by the power canal among the penstocks and, at the same time, to regulate the water flow into the latter, as well as to ensure the disposal of excess water. At the forebay sediment still carried by the water settles down. The storage capacity of headpond tends also to reduce the drop of water-level in case of sudden load increase. Headponds having a great storage capacity may even provide daily pondage for the plant. Thus the forebay acts as a regulating pondage to cushion the impact of sudden load rejection or load acceptance.
The upsurge in the forebay caused by the load rejection is estimated using following equation.
In case of sudden closure, the maximum height of the surge is given by the expression known as:
E. Feifel’s equation:
h
g V g
V g
h V ⎟⎟⎠ + ×
⎜⎜ ⎞
⎝ + ⎛
=
Δ 22
2 2
2 2 2 2
max --- (3.78) For the gradual and complete closure
g V h g h =V +
Δ 4
2
max --- (3.79) where
V = water velocity (mean velocity of flow) h = water depth – effective depth
hmax
Δ = height of the surge
In general total capacity of the forebay is fixed so that the total live storage volume in it is equivalent to the total volume required for three minutes of operation. Three minutes of operation is adopted from the criteria of realistic water starting time, which is in the order of 1 to 2 minutes and an extra minute for the forebay. Realistic water starting time is the time required accelerating the water in the water conductors from rest to the steady state velocity at full gate discharge with all units on a given penstock of power conduit in operation∗.
A schematic sketch of a forebay showing plan, profile and sections is presented in Fig. 3.20 and the flow chart for the design of forebay is shown in Chart 3 below.
∗ Reference: Davi’s Handbook of applied Hydraulic- fourth edition
Fig. 3.20: Forebay Plan and Profile with Cross-section
Pressure penstock pipe starts from the end of the forebay chamber. The portion of forebay from where the penstock starts often called as a penstock chamber. A sill between the penstock bottom part and the penstock chamber is maintained which is often half a meter. The upper part of the penstock is located with sufficient suction head with the water surface in the chamber.
The suction head requirement is calculated using the formula (3.41).
Spillway releasing excess water from forebay to some natural channel is designed using following formula:
2 3
CLH
Q =
--- (3.80) Where,Q
- Discharge in m3/sC - Discharge Coefficient depends on the shape of the structure H - Water head in meter
L - Spillway Length
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