Hydropower Water Conveyance System (Part - 1) | Additional Documents & Tests for Civil Engineering (CE) PDF Download

Introduction

A dam or diversion structure like a barrage obstructs the flow of a river and creates a potential head which is utilized by allowing the water to flow through the water conducting system upto the turbines driving the generators and then allowing it to discharge into the river downstream. Right from the intake of the water conducting system, where water enters from the main river, up to the outlet where water discharges off back into the river again, different structural arrangements are provided to fulfil certain objectives, the important ones being as follows:

1. The water inflowing into the conveyance system should be free from undesirable material, as far as possible, that may likely damage the turbines or the water conducting system itself.
2. The energy of the inflowing water may be preserved, as far as possible, throughout the water course so that the turbine-generator system may extract the maximum possible energy out of the flowing water.

As an example of the first case, it may be cited that in hilly rivers, there are good chances of sand, gravel, and even boulders getting into the water conducting system along with the flowing water. The bigger particles may choke the system whereas the smaller ones may erode the turbine blades by abrasive action. Apart from these, floating materials like trees or dead animals and in some projects in the higher altitudes ice blocks may get sucked into the system which may clog the turbine runners.

The main components of a water conveyance system consists of the following:

1. Intake s tructure
2. Water conducting system comprising of different structures
3. Outflow structure, which is usually a part of the turbine tail end

The water conducting system, again, may be of two types

1. Open channel flow system
2. Pressure flow system

In the pressure flow system, there could be further classification into the two types, as:

1. Low-pressure conduits and tunnels
2. High-pressure conduits, commonly called the penstocks

In either of the above cases, some provision is usually made to prevent the undesirable effects of a power rejection in the generator that may cause the turbine to spin exceedingly fast, resulting in a closure of the valves controlling the flow of water at the turbine end. If the closure is relatively fast, high pressures may develop in pressured systems conducting water to the turbine. For open channel systems, this may lead to generation of surges in the water surface which may even cause spillage of the channel banks if adequate freeboard is not provided.

This chapter discusses the important issues related to the different components of a hydropower Water Conveyance System.

Intakes 

An intake is provided at the mouth of a water conveyance system for a hydropower project. It is designed such that the following points are complied, as far as possible:

1. There should be minimum head loss as water enters from the reservoir behind a dam or the pool behind a barrage into the water conducting system.
2. There should not be any formation of vortices that could draw air into the water conducting system.
3. There should be minimum entry of sediment into the water conducting system.
4. Floating material should not enter the water conducting system.

The position and location of an intake in a hydropower project would generally depend upon the type of hydropower development, that is, whether the project is of run-of-river type or storage type. For each one of these hydropower projects, there are a few different types, the important ones of which are explained in the following paragraphs.

Run-of-river type intake 

• Intakes adjacent to a diversion structure like a barrage. Here, an intake for a tunnel is placed upstream of the diversion structure to draw water from the pool (Figure 1). For a canal intake (Figure 2), the head regulator resembles that of an irrigation canal intake. It may be observed from Figure 3 that the canal conveying water, also called the power canal, leads to a Forebay before leading to the turbine unit. The exit passage from the turbines is called the Tail Race Channel. There is also a Bye-Pass Channel to release water when the turbines shut down suddenly.  

Figure 1. Intake adjacent to a barrage leading to a tunnel (a) Plan , (b) Section X-X 

Figure 2. Intake adjacent to a barrage leading to a canal (a) Plan (b) Section through head regulator

Figure 3. (a) Power canal leading to fore-bay at head of turbine unit, (b) Detail ‘A of head regulator for canals in hilly region equipped with silt deflector for preventing boulder entry & settling tank to remove sediment that too entered the canal.

• Intakes for in-stream power house. These are used for powerhouses located across rivers or canals to utilize the head difference across a canal drop. Here, the intake length is kept quite short and leads to either a vertical axis Kaplan turbine or a horizontal axis bulb turbine (Figure 4). 

FIGURE 4. Intakes for river or canal fall power house integrated with turbine unit, (a) Kaplan turbines, (b) Bulb turbines. 

Reservoir type intakes 

• Intakes for concrete dams are located on the upstream face of the dam as shown in Figure 5. The face of the intake is rectangular and is reduced to a smaller rectangular section through a transitory shape known as the bell-mouth. From the smaller rectangular section, another transition is provided to change the shape to circular. 

Figure 5. Intake from a reservoir upstream of a storage dam (a) Sectional elevation X - X. 

Figure 5. intake from a reservoir upstream of a storage dam (b) Sectional plan Y - Y.

• Intakes for embankment dams are usually in the form of a conduit, which is laid below the dam and whose intake face is inclined (Figure 6) or are provided in the form of a tower (Figures 7 and 8). A tower type intake is constructed where there is a wide variation of the water level in the reservoir. 

! -Trashrack ; 2 - Membrane valve ; 3 - Aeration pipe ; 4 - Gate room ; 5 - Winch ; 6 - Crane Figure 6. Sloping intake for an embankment dam

Figure 7. Tower type intake for an embarkment dam with flow control by a vertical lift or radial gate in conduit

Figure 8. Tower type intake with cylindrical gate for flow control

• Intakes which have pressure tunnels off-taking from a storage reservoir and where the intake is located at a distance from the dam, say through the abutments, then the intake structure of such layout may be of inclined type or tower type as was provided in conjunction with the dam itself. 

The choice and location of the intake structure depends upon the following factors. 

• Type of development, that is, run-of-the-river or storage dam project;
• Location of power house vis-à-vis the dam ;
• Type of water conductor system, that is, tunnel, canal or penstock;
• Topographical features of area;
• In cases where there is a considerable movement of boulders, stones and sand in the downstream direction, the intake should be arranged so that the effect of such movement will not lead to a partial restriction or blockage of the intake. In respect of storage reservoir intakes the sill level of the intake should be aimed to be kept above the sedimentation level at or near the dam face arrived at; and 
• The intake can often be located so as to enable it to be constructed before the level of the reservoir is raised.  

 Detail about the design of hydropower intakes may be obtained from the Bureau of Indian Standards code IS: 9761-1995 “Hydropower intakes-criteria for hydraulic design”. 

In all the above intakes it may be noticed that a Trash Rack Structure is provided at the entry. A trash rack is actually a grill or a screen for preventing entry of suspended or floating material into the water conducting system. It is made usually of metallic strips welded in vertical and horizontal directions at regular spacings.