Hydropower Water Conveyance System (Part - 2) | Additional Documents & Tests for Civil Engineering (CE) PDF Download

Water Conducting System 

After flowing through the intake structure, the water must pass through the water conveyance system may be either of closed conduit type, as shown in Figure1 (tunnel off-taking from upstream of the river diversion) or could be open-channels as shown in Figure 2. High pressure intakes, for example as in the entry to penstocks (Figure 9) would be either reinforced concrete lined or steel lined. In this section we discuss the various types of water conducting passages. 

Figure 9. Alignment of a power canal along a hill slope

Open channels  

These are usually lined canals to reduce water loss through seepage as well as to minimize friction loss. The design of canals for hydropower water conveyance follows the same rules as for rigid bed irrigation channels, and are usually termed as power canals. 

A power canal that offtakes from a diversion structure (Figure2c) has to flow along the hill slope as may be observed from the alignment shown in Figure 9. A cross section of the canal would show that there would usually be high ground on one bank and falling ground on the other (Figure 10). It is important to stabilize the uphill cut-slope with some kind of protection in order to prevent fallout of loose blocks of stone into the canal. Some stretch of the canal could also be such that the bank with low ground needs to be supplemented with an artificially created embankment (Figure11). As observed from Figure 9, a power canal ends at a forebay, which is broadened to act as a small reservoir. From the forebay, intakes direct the water into the penstocks. There usually is a bye-pass channel which acts as a spillway to pass on excess water in case of a valve closure in the turbine of the hydropower generating unit. If such an escape channel is not provided, there are chances that under sudden closure of the valves of the turbines, surge waves move up the power canal. Hence, sufficient free board has to be provided for the canals.  

Figure 10. Cross section of a power canal in cutting.

Figure 11. Cross section of a power canal in partly cutting & partly filling.

Tunnels

As shown in Figure1, a river diversion structure may also direct water into a tunnel. A typical section through a tunnel is shown in Figure 12. The initial portion of the tunnel from the intake upto the Surge-Tank is termed as the Head Race Tunnel (HRT) and beyond that it houses the penstock or steel-conduits, which sustains a larger pressure than the HRT. The HRT may either be unlined (in case of quite good quality rocks) or may be lined with concrete. The surge tank is provided to absorb any surge of water that could be generated during a sudden closure of valve at the turbine end. Normally, the water level in the surge tank would be marginally lower than that at the intake (see Figure 12) and the difference of levels depends upon the friction loss in the HRT. Thus, when the HRT runs full, it is subjected to a much low pressure compared to the penstock. If a HRT is concrete lined, the reinforcement in the concrete may be nominal as the lining is only to assist in preventing fallout of rock blocks into the tunnel. However, if the rock mass above the tunnel is very weak, then the tunnel lining may have to support a larger rock weight, in which case the reinforcement has to be designed accordingly. A tunnel should also be designed for the empty condition, assuming the outside rock to be saturated with water. 

Figure 12, Section through a typical tunnel development for water conveyance of a hydropower system.

These aspects pertain to the structural design of a tunnel. Apart from this, there has to be a geometric design, finalising the shape of a tunnel. Section 5.2.3 discuss these issues of tunnel design. 

Surge tanks  

As explained, a surge tank (or surge chamber) is a device introduced within a hydropower water conveyance system having a rather long pressure conduit to absorb the excess pressure rise in case of a sudden valve closure. It also acts as a small storage from which water may be supplied in case of a sudden valve opening of the turbine. In case of a sudden opening of turbine valve, there are chances of penstock collapse due to a negative pressure generation. If there is no surge tank.  There are different types of surge tanks that are possible to be installed. The Bureau of Indian Standards code IS: 7396(Part1)-1985 “Criteria for hydraulic design of surge tanks” describes the most common types of surge tanks which are as follows: 

1. Simple Surge Tank: A simple surge tank is a shaft connected to pressure tunnel directly or by a short connection of cross-sectional area not less than the area of the head race tunnel (Figure 13). 

Figure 13. Simple surge lank

2. Restricted Orifice Surge Tank: A simple surge tank in which the inlet is throttled to improve damping of oscillations by offering greater resistance and connected to the head race tunnel with or without a connecting/communicating shaft (Figure 14). 

Figure 14. Restricted orifice surge tank

3. Differential Surge Tank: Differential Surge tank is a throttled surge tank with an addition of a riser pipe may be inside the main shaft, connected to main shaft by orifice or ports. The riser may also be arranged on one side of throttled shaft as shown in Figure 15. Port holes are generally at the bottom of the riser at the sides. 

Figure 15. Differential surge tank

In an underground development of hydropower system, tail race surge tanks are usually provided to protect tail race tunnel from water hammer effect due to fluctuation in load. These are located downstream of turbines which discharge into long tail race tunnels under pressure. The necessity of tail race surge tank may be eliminated by ensuring free-flow conditions in the tunnel but in case of long tunnels this may become uneconomical than a surge tank.

The Bureau of Indian Standards code IS: 7396(Part2)-1985 deals with the different types of surge tank that may be provided in the Tail Race Tunnel (TRT). A typical view of a surge tank in a TRT is shown in Figure 16. 

Figure 16. Surge tank in tail-race tunnel

Apart from the above types, there could be special types of surge tanks in multiple units which are discussed in IS: 7396 (Part3) and IS: 7396(Part 4) respectively. 

Penstock  

A penstock is a steel or reinforced concrete conduit to resist high pressure in the water conveyance system and may take off directly from behind a dam, from a forebay, or from the surge tank end of a head race tunnel as shown in Figure 17. Similar to a tunnel, a penstock needs to be designed for different types of loads. Further, they have to be equipped with different accessories, which may be different for overground or ground embedded types. These aspects of penstocks are thus discussed separately in Section 5.2.4.  

1=Dam;2=lntake;3=Embedded or concrete-jacket penstock: 4=Tunnel penstock;5=Power house at dam toe;6=lsolated power house;7=Exposed penstock;8=Head pond;9=Diver-sion conduit; 10=Diversion tunnel; 11=Surge tank

Figure 17. Layout of penstocks.
 a)    penstocks of power house built at dam toe;
 b)    penstocks of isolated power house;
 c)    penstocks of pressure diversion project;
 d)    penstocks of open-flow diversion project