Water Turbines

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Water Turbines

Water/hydraulic turbines are prime movers. Water turbines are machines

which convert the kinetic energy and the potential energies possessed by

water into mechanical rotary motion and further into electric power. Hydroelectric power can be developed whenever continuously flowing high

pressure water is available. By constructing dams across flowing rivers,

artificial reservoirs are created. Water is ducted from these reservoirs to the

turbine stations through large pipes called penstocks, and in the turbines

its hydraulic energy is converted into mechanical energy which in turn is

converted into electrical energy.

Classification of water turbines

The classification of water turbines are as follows:

1. According to the type of energy at inlet:

(i) Impulse or velocity turbine and

(ii) Reaction or pressure turbine

2. According to the direction of flow of water through runner

(i) Tangential flow turbine

(ii) Radial flow turbine

(iii) Axial flow turbine

(iv) Mixed flow turbine

3. According to the available water head at inlet

(i) High head turbine

(ii) Medium head turbine

(iii) Low head turbine

4. According to the disposition of the runner shaft

(i) Vertical shaft

(ii) Horizontal shaft

Impulse turbine:

In impulse turbine, the potential energy (Pressure) is converted to kinetic

energy by a nozzle and the jet of water is directed on the turbines curved

blades. The resulting impulse spins the turbine and leaves the fluid flow

with diminished kinetic energy. Newtons second law describes the transfer

of energy for impulse turbine. No pressure change occurs at the turbine

blades.

Pelton wheel:

Pelton wheel is a tangential flow impulse turbine, water flows along thetangent to the path of the runner. It operates under a high head of water


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and therefore requires comparatively less quantity of water. Water is

conveyed from the reservoir to the turbine through a penstock. The penstock

is connected to a branch pipe fitted with a nozzle. The whole of the pressure

energy of the water is converted into the kinetic energy in one or more

number of nozzles before it is passed on to the turbine wheel. The watercomes out of the nozzle in the form of a jet at very high velocities. This high

velocity jet is made to strike a series of curved blades mounted on the

periphery of a wheel keyed to the turbine shaft. The shape of the bucket is

that of a double hemispherical cup having dividing wall known as splitter at

the centre. The splitter divides the impinging jet into two halves, which are

deflected backward. The inner surfaces of the cups are smoothly finished.

The back side of the cups are designed for a deflecting angle of about 165.

Such buckets are evenly spaced and fitted around the runner. These are

shown in the figure 2.0

The three main purposes of such design are:

(i) For smooth flow of water with a minimum disturbed splash

(ii) To neutralise the axial thrust.

(iii) After the work, the water should not strike the back of the incoming

bucket and retard the motion of the wheel.

As there is no pressure variation in flow, the fluid partly fills the buckets

and it remains in contact with the atmosphere. The nozzle is provided with a

spear mechanism to control the quantity of water. The actual energy

transfer from jet to wheel is by changing the momentum of the stream. The

water after imparting its energy to the turbine is discharged into the tail-

race.

Figure 1.0 Flow of water on bucket


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Advantages of Pelton turbine

(i) Simple in construction and easy for maintenance

(ii) To derive more power, multiple jets may be used.

(iii) Can handle even muddy water.

Disadvantage:

1. A lot of head loss occurs when the river discharge is low.

Figure 2.0 Pelton turbine

Reaction Turbine:

Reaction turbines are acted on by water, which changes pressure as it

moves through the turbine and gives up its energy. They must be enclosed

in an air tight casing to contain the water pressure or they must be fully

submerged in the water flow. Newtons third law describes the transfer of

energy for reaction turbines. The turbine is located between the high

pressure water sources and the low pressure water exit, usually at the base

of dam. A reaction turbine requires high rate of flow of water but work well

even with low and medium head. The water supplied to the reaction turbine

posses both pressure as well as kinetic energies. All the pressure energy of

the water is not completely converted into kinetic energy as in case of the

impulse turbine.


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Francis Turbine:

It is a mixed flow reaction turbine. It requires medium quantity of water and

intermediate head.

Construction:

Figure 3 shows two views of a Francis turbine. Here a number of guide

vanes are fixed around the circumference of the runner. Each guide vane is

made to rotate about its pivot with the help of individual link and lever.

The runner is supported by a drive shaft or regulating shaft. The guide

vanes operate similar to engine valves, allowing only required quantity of

water. The runner is enclosed in a spiral casing or scroll casing. The exit end

of the runner is connected to the small end of the draft tube. The big end of

the draft tube is submerged deep in the tail race. Hence the entire waterpassage right from the head race up to tail race is totally enclosed. Some

smaller machines of this type have horizontal shafts, the majorities have

vertical shaft.

Working:

It is suitable for intermediate heads and intermediate rates of flow. The fluid

enters a volute casing which completely surrounds the runner. The cross

sectional area of the volute decreases along the fluid path in such a way as

to keep the fluid velocity constant in magnitude. From the volute, the fluid

passes between stationary guide vanes mounted all around the periphery of

the runner. The function of these guide vanes is to direct the fluid on to the

runner at required angle. This radial flow acts on the runner vanes, causing

the runner to spin. The guide vanes may be adjusted to alter the flow rate

through the machine. In its passage through the runner the runner blades

deflect the fluid so that its angular momentum is changed. From the centre

of the runner the fluid is turned into the axial direction and flows to tail-race

via the draft tube. The lower end of the draft tube must, under all conditions

of operation, be submerged below the level of water in the tail-race. Only inthis way it can be ensured that a hydraulic turbine is full of water.

Advantages:

(1)No head loss occurs even at low discharge of water.

Disadvantages:

(1)Eddy losses are inevitable

(2)Since the spiral casing is grounded, the runner is not easily

accessible. Hence dismantling is difficult.


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(3)Performance gets affected due to dirty/muddy water

Figure 3: Francis Turbine

Kaplan turbine:

Axial flow turbine is used for low heads, at high rotational speeds and large

rates of flow. Kaplan turbine is an axial flow reaction turbine having a very

few number of blades, usually from four to six and resembles a ships

propeller. The blade angle may be varied by turning the blades about their

own axes. The turbine is enclosed in a spiral casing which receives water

from the mains. From the casing the water is directed to the runner bladesby guide vanes. The arrangement of guide vanes is similar to that of a

Francis turbine. When both guide vane and runner blade angle may thus be

varied, a high efficiency can be maintained over a wide range of operating

conditions. Kaplan turbine is operated in an entirely closed conduit from

inlet to tail-race. Kaplan runner is as shown in figure 4.0 and Kaplan

turbine in figure 5.0

Advantages:

(a)

Simple in construction and requires less space(b)Eddy losses are almost eliminated


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Disadvantages:

(a)Cavitation is likely to occur due to high velocity flow of water.

(b)Cannot handle the dirty/muddy water efficiently.

Figure 4: Kaplan turbine runner

Differences between Francis and Kaplan Turbines:

Francis Turbine Kaplan Turbine1. It is a mixed flow turbine2. Medium head turbines

medium quantity of water.3. More number of vanes on the

runner (16 to 24)4. Requires large space.5. Eddy losses are inevitable.6. Free from cavitation.7. Guide vanes are used to

regulate the flow.

1. It is an axial flow turbine2. Low head turbine, requires

large water flow rate.3. Few number of vanes on the

runner( 4 to 6)4. Requires less space due to

sloped vanes.5. Eddy losses are almost

eliminated.6. Cavitation is likely to occur.7. Guide vanes and runner blades

are used to regulate the flow.


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Figure 5: Kaplan Turbine

Water Turbines

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