Introduction To Francis Turbine –
- The Francis turbine is a mixed flow reaction turbine. This turbine is used for medium heads with medium discharge. Water enters the runner and flows towards the center of the wheel in the radial direction and leaves parallel to the axis of the turbine.
- Turbines are subdivided into impulse and reaction machines. In the impulse turbines, the total head available is converted into the kinetic energy.
- In the reaction turbines, only some part of the available total head of the fluid is converted into kinetic energy so that the fluid entering the runner has pressure energy as well as kinetic energy. The pressure energy is then converted into kinetic energy in the runner.
- The Francis turbine is a type of reaction turbine that was developed by James B. Francis. Francis turbines are the most common water turbine in use today. They operate in a water head from 40 to 600 m and are primarily used for electrical power production. The electric generators which most often use this type of turbine have a power output which generally ranges just a few kilowatts up to 800 MW
- Diagram Of Francis Turbine
Parts Of Francis Turbine
- parts of Francis turbine
Spiral casing or scroll Casing :
- The casing of the Francis turbine is designed in a spiral form with a gradually increasing area.
- Most of these machines have vertical shafts although some smaller machines of this type have a horizontal shaft. The fluid enters from the penstock (pipeline leading to the turbine from the reservoir at high altitude) to a spiral casing that surrounds the runner.
- This casing is known as scroll casing or volute. The cross-sectional area of this casing decreases uniformly along the circumference to keep the fluid velocity constant in magnitude along its path towards the stay vane. This is so because the rate of flow along the fluid path in the volute decreases due to continuous entry of the fluid to the runner through the openings of the stay vanes.
- The casing is made of a cast steel, plate steel, concrete, or concrete and steel depending upon the pressure to which it is subjected. Out of these a plate steel scroll casing is commonly provided for turbines operating under 30 m or higher heads.
Guide Mechanism :
- It consists of a stationary circular wheel all around the runner of the turbine. The stationary guide vanes are fixed on the guide mechanism. The guide vanes allow the water to strike the vanes fixed on the runner without shock at the inlet.
- The guide vanes( also called as wicket gates) are fixed between two rings. This arrangement is in the form of a wheel and called a guide wheel. Each vane can be rotated about its pivot center.
- The opening between the vanes can be increased or decreased by adjusting the guide wheel. The guide wheel is adjusted by the regulating shaft which is operated by a governor.
- The guide blades rest on pivoted on a ring and can be rotated by the rotation of the ring, whose movement is controlled by the governor. In this way the area of blade passage is changed to vary the flow rate of water according to the load so that the speed can be maintained constant. The variation of area between guide blades is illustrated in Figure
- The guide mechanism provides the required quantity of water to the runner depending upon the load conditions. The guide vanes are in general made of cast steel.
- Guide Mechanism For Francis Turbine
Runner and Turbine Main Shaft :
- Runner is a circular wheel on which a series of radial curved vanes are fixed. The surface of the vanes are made very smooth. The radial curved vans are so shaped that the water enters and leaves the runner without shocks.
- The flow in the runner of a modern Francis turbine is partly radial and partly axial.
- The runners may be classified as
- The runner may be cast in one piece or made of separate steel plates welded together. The runner made of CI for small output, cast steel, or stainless steel or bronze for large output. The runner blades should be carefully finished with a high degree of accuracy.
- The runner may be keyed to the shaft which may be vertical or horizontal. The shaft is made of steel and is forged it is provided with a collar for transmitting the axial thrust.
Draft Tube :
- The pressure at the exit of the runner of a reaction turbine is generally less than atmospheric pressure. The water at the exit cannot be directly discharged to the tailrace. A tube or pipe of the gradually increasing area is used for discharging water from the exit of the turbine to the tailrace. This tube of increasing area is called the draft tube
- The water after doing work on the runner passes on to the tall race through a tube called a draft tube.
- It is made of riveted steel plate or pipe or a concrete tunnel.
- The cross-section of the tube increases gradually towards the outlet. The draft tube connects the runner exit to the tailrace.
- This tube should be drowned approximately 1 meter below the tailrace water level.
Function of draft tube –
i) To decrease the pressure at the runner exit to a value less than atmospheric pressure and thereby increase the effective working head.
ii) To recover a part of electric energy into pressure head at the exit of the draft tube. This enables easy discharge to the atmosphere.
Types of draft tube:
i. Conical draft tube
ii. Simple elbow draft tube
iii. Moody spreading draft tube
iv. Elbow draft tube with circular cross-section at inlet and rectangular at outlet
(1) Conical Draft Tubes—
This is known as a tapered draft tube and used in all reaction turbines where conditions permit. It is preferred for low specific speed and Francis turbine. The maximum cone angle is 8° (a = 40°). The hydraulic efficiency is 90%.
(2) Simple Elbow Tubes-
The elbow type draft tube is often preferred in most of the power plants. If the tube is large in diameter; ‘it may be necessary to make the horizontal portion of some other section. A common form of section used is over or rectangular. It has low efficiency of around 60%.
(3) Moody Spreading Tubes-
This tube is used to reduce the whirling action of discharge water when the turbine runs at high speed under low head conditions. The draft tube has an efficiency of around 85%.
(4) Elbow with circular inlet and rectangular outlet—
This tube has circular cross-section at the inlet and rectangular section at the outlet. The change from the circular section to the rectangular section takes place in the bend from the vertical leg to the horizontal leg. The efficiency is about 85%.
Francis turbine Diagram :
Types Of Francis Turbines :
There are mainly two types of Francis turbines known as open flume type and closed type.
- In open flume type, the turbine is immersed underwater of the headrace in a concrete chamber and discharges into the tailrace through the draft tube. The main disadvantage of this type is that runner and the guide-vane mechanism is under the water and they are not open either for inspection or repair without draining the chamber.
- In the closed type, the water is led to the turbine through the penstock whose end is connected to the spiral casing of the turbine. The open flume type is used for the plants of 10 meters head whereas, closed type is preferred above 30 meters head. The guide vanes are provided around the runner to regulate the water flowing through the turbine The guide vanes provide gradually decreasing area of
flow for all gate openings, so that no eddies are formed, and efficiency does not suffer much even at part load conditions.
Working principles of Francis turbine
- The water is admitted to the runner through guide vanes or wicket gates. The opening between the vanes can be adjusted to vary the quantity of water admitted to the turbine. This is done to suit the load conditions.
- The water enters the runner with a low velocity but with a considerable pressure. As the water flows over the vanes the pressure head is gradually converted into velocity head.
- This kinetic energy is utilized in rotating the wheel Thus the hydraulic energy is converted into mechanical energy.
- The outgoing water enters the tailrace after passing through the draft tube. The draft tube enlarges gradually and the enlarged end is submerged deeply in the tailrace water.
- Due to this arrangement a suction head is created at the exit of the runner.
Francis Turbine Velocity triangle Diagram :
The majority of the Francis turbines are inward radial flow type and most preferred for medium heads. The inward flow turbine has many advantages over the outward flow turbine as listed below :
1. The chances of eddy formation and pressure loss are reduced as the area of flow becomes gradually convergent.
2. The runaway speed of the turbine is automatically checked as the centrifugal force acts outwards while the flow is inward.
3. The guide vanes can be located on the outer periphery of the runner, therefore, better regulation is possible.
4. The frictional losses are less as the water velocity over the vanes is reduced.
5. The inward flow turbine can be used for fairly high heads without increasing the speed of the turbine as the centrifugal head supports a considerable part of the supply head.
Vw1 = Velocity of whirl at inlet
Vw2 = Velocity of whirl at outlet
u1= Tangential velocity of whirl at inlet
u2= Tangential velocity of whirl at outlet
Vf1 = Velocity of flow at inlet
Vf2 = Velocity of flow at Outlet
V1 = Absolute velocity of water at the inlet of the runner
V2 = Absolute velocity of water at the Outlet of the runner
Vr1 = Relative Velocity at Inlet of the runner
Vr2 = Relative Velocity at the outlet of the runner
Φ = Vane angle at the exit.
θ = Vane angle at inlet
α = Guide vane angle
The velocity triangle at inlet and outlet of the Francis turbine is drawn in the same way as in case of inward flow creation turbine . as in case of Francis turbine, the discharge is radial at the outlet, the velocity of whirl at the outlet ( i.e. Vw2 ) will be zero. Hence velocity diagram for Francis turbine is shown below
We Know, absolute velocity at the outlet is 90° i.e. β= 90°
Flow ratio, Kf = Vf1 / √2gH
Flow ratio varies from 0.15 to 0.30
Speed ratio, Ku= u1 / √2gH
Speed ratio varies between 0.6 to 0.9
The ratio of width (B1) to the diameter of the wheel (D1), n= B1/D1
n ratio varies from 0.1 to 0.45
Specific speed (Ns): of a turbine is defined as the speed of a geometrically turbine which would develop unit power when working under a unit head.
Where P = shaft power, and H = net head on the turbine
Specific speed plays an important role in the selection of the type of turbine. Or, The suitability of a turbine for a particular depends on (a) head of water (b) rotational speed (c) power developed, which together fix a parameter called ‘specific speed’.
The formation, growth, and collapse of vapor filled cavities or a bubble in a flowing liquid due to local fall in fluid pressure is called cavitation. The critical value of cavitation factor (σc) is given by
σc = ( Ha – Hv – Hs ) / H
Ha = atmospheric pressure head in meters of water,
Hv = vapour pressure in meters of water corresponding to the water temperature.
H = working head of turbine (difference between head race and tail race levels in meters)
Hs = suction pressure head (or height of turbine inlet above tail race level) in meters.
The value of critical factor depends upon specific speed of the turbine.
If the value of σ is greater than σc then cavitation will not occurred in the turbine or pump.
Effect of cavitation:
(i) Roughening of the surface by pitting
(ii) Increase vibration due to irregular collapse of cavities.
(iii) The actual volume of liquid flowing through the machine is reduced.
(iv) Reduce output power
(v) Reduce efficiency
Method to avoid cavitation:
(i) Runner/turbine may be kept underwater
(ii) Design cavitation free runner
(iii) Selecting proper material, use stainless steel, alloy steel
(iv) Blades coated with harder material
(v) Selecting a runner of a proper specific speed
Efficiencies Of Francis Turbines :
1. Hydraulic efficiency :
It is defined as the ratio of the power produced by the turbine runner and the power supplied by the water at the turbine inlet.
2. Volumetric efficiency :
- It is possible some water flows out through the clearance between the runner and casing without passing through the runner.
- Volumetric efficiency is defined as the ratio between the volume of water flowing through the runner and the total volume of water supplied to the turbine.
3. Mechanical efficiency :
The power produced by the runner is always greater than the power available at the turbine shaft. This is due to mechanical losses at the bearings, windage losses and other frictional losses.
4. Overall efficiency :
This is the ratio of power output at the shaft and power input by the water at the turbine inlet.
Advantage of Francis Turbine :
2. The ratio of utmost and least operating head can even be two in the case of Francis Turbine.
3. The mechanical efficiency of the Pelton wheel decreases faster by wear than Francis turbine.
4. Francis turbine variation in operating head can be more simply controlled.
5. No head failure occurs still at the low discharge of water.
Disadvantage of Francis Turbine :
2. As spiral casing is stranded, the runner is not simply available. Therefore dismantle is hard.
3. The repair and inspection is much harder reasonably.
4. Cavitation is an ever-present hazard.
5. Current losses are certain
Application of Francis Turbine :
- Electricity production can be estimated with the help of flow rate and head.
- Francis turbine may be designed for a wide range of head and flow.
- It has high efficiency.
- They may be used as Pump.
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