FRANCIS TURBINE

INTRODUCTION:

Hydraulic (or water) turbines are the machines which use the energy of

water (Hydro-power) and convert it into mechanical energy. Thus the turbine

becomes the prime mover to run the electrical generators to produce the

electricity, Viz., Hydro-electric power.

The turbines are classified as Impulse & Reaction types. In impulse

turbine, the head of water is completely converted into a jet, which impulses the

forces on the turbine. In reaction turbine, it is the pressure of the flowing

water, which rotates the runner of the turbine. Of many types of turbine, the

Pelton wheel, most commonly used, falls into the category of turbines. While

Francis & Kaplan falls in category of impulse reaction turbines.

Normally, Pelton wheel (impulse turbine) requires high heads and low

discharge, while the Francis & Kaplan (reaction turbines) required relatively

low heads and high discharge. These corresponding heads and discharges are

difficult to create in laboratory size from the limitation of the pumps availability

in the market. Nevertheless, atleast the performance characteristics could be

obtained within the limited facility available in the laboratories. Further,

understanding various elements associated with any particular turbine is

possible with this kind of facility.

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DESCRIPTION :

While the impulse turbine is discussed elsewhere in standard text books,

Francis turbine, the reaction type which is of present concern consists of main

components such as propeller (runner) scroll casing and draft tube. Between

the scroll casing and the runner, the water turns through right angle and passes

through the runner and thus rotating the runner shaft. When guide vane angles

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

conditions.

The actual experimental facility supplied consists of a centrifugal pump

set, turbine unit, sump tank and Venturimeter arranged in such a way that the

whole unit works on recirculating water system. The centrifugal pump set

supplies the water from the sump tank to the turbine through gate valve. The

water after passing through the turbine unit enters back to the sump tank

through the draft tube. The water then flows back to the sump tank through the

Venturimeter with pressure gauges for the measurement of flow rate.

The loading of the turbine is achieved A.C. Generator. The provision for

; measurement of brake force (voltmeter and ammeter), turbine speed (digital

RPM indicator), head on the turbine (pressure gauge), head over the

Venturimeter (pressure, vacuum gauge, 2 Nos) are built-in on to the control

panel.

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

The cross sectional area of 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. Each vane is pivoted and by a suitable mechanism all may be

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turned in synchronism so as to alter the flow rate of the machine. In its passage

through the runner the fluid is deflected by the runner blades so that angular

momentum is changed. From the centre of the runner the fluid is turned to 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 in this way it can be ensured that a turbine is full of

water.

THEORY:

Francis turbine is a inward mixed flow reaction turbine named after the

American Engineer James B. Francis. In a Francis Turbine, water enters the

runner at its outer periphery and flows out axially at its centre. This

arrangement provides a large discharge area with the given diameter of the

runner. A part of the net available energy of the water is converted into kinetic

energy and the rest of the major portion remains as pressure energy, as water

enters the runner. The runner rotates due to reaction pressure caused by the

pressure difference at the runner entry and exit.

The principal component parts of Francis Turbine are:

1. Scroll casing: It’s a spiral shaped closed passage of gradually reducing

cross-sectional area, enclosing the runner. Its function is to distribute the

flow uniformly along the periphery of the runner in such a way that the

velocity remains constant at every point.

2. Guide Mechanism: There are two main functions of the guide mechanism

(a) To regulate the quantity of water supplied to the runner and (b) To

adjust the direction of flow so that there is minimum shock at the

entrance to runner blades. It consists of a series of guide vanes of

aerofoil section fixed between to rings, in the form of a wheel known as

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guide wheel. Each guide vane can be rotated about its pivot centre,

which is connected to a regulating ring by means of a link and lever. By

operating the regulating ring the guide vanes can be rotated, varying the

width of the passage between adjacent vanes, thus altering both the flow

angle as well as the quantity of flow.

3. Runner: The runner consists of a series of curved vanes arranged evenly

around the circumference, in the annular space between two plates. It

may be cast in one piece or made of separate steel plates welded together.

The runner vanes are so shaped that water enters radially at the outer

periphery and leaves it axially at the inner periphery. This change in the

direction of flow from radial to axial as it passes over the curved vanes

changes the angular momentum of the fluid thereby producing the torque,

which rotates the runner. The runner is keyed to shaft of the turbine.

4. Draft tube: It is a gradually expanding closed passage connecting the

runner to the tailrace (collecting tank). The lower end of the draft tube is

always kept submerged in water. The function of a draft tube is to

convert the high kinetic energy of flow at runner exit into pressure

energy, thus increasing the efficiency of the turbine. It also enables the

turbine to be installed above the tail race level without any loss of head.

SPECIFICATIONS :

Supply Pump / Motor Capacity : 10hp, 3ph, 440V, 50Hz, AC.

Turbine : 150mm dia Impeller.

: Guide vane angles adjustable from maximum to minimum.

: Run-away speed- 1900 rpm (approx.)

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: Maximum flow of water- 1200 lpm (approx.)

: Maximum head – 14 mts (approx.)

Loading : AC Alternator connected with Electrical Switches

Provisions : a) Flow rate by Venturimeter, Cd = 0.91

Two gauges to measure head on Venturimeter.

b) Head on turbine by pressure gauge of range : 0-2 and Kg/cm2 and vacuum gauge : 760 mm of Hg.

c) Electrical load changed by Alternator assembly connected to electrical switches.

d) Electrical load measurement by energy meter

e) Propeller speed by digital RPM indicator.

f) Supply water control by butterfly valve.

Electrical Supply : 3 ph, 440V, AC, 30A, with Neutral & Earth.

NOTE: Volume of water required for operation unit - 2500 ltr (approx.)

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OPERATION (General):

1. Connect the supply pump-motor unit to 3 ph, 440V, 32A, electrical supply,

with neutral and earth connections and ensure the correct direction of pump-

motor unit.

2. Ensure that all the three indicators are glowing.

3. Keep the gate closed and switch on the MCB.

4. Keep the electrical load at zero, by keeping all switches in off position.

5. Keep the guide vane for the required position by adjustable wheel (1/4, ½, ¾

and full open).

6. Press the green button of the supply pump starter and then release.

7. Slowly, open the gate so that turbine rotor picks up the speed and Attains

maximum at particular opening of the gate.

8. Apply load by switching on each switch one at a time. (Or in a bunch)

9. Change the position of the guide vane angles and repeat the readings. If

necessary, the gate valve(butterfly valve) also can be used for speed control.

10.Note down the Venturimeter pressures, speed, pressure and vacuum -on the

meters at the control panel and tabulate results.

11.After completion of experiment remove the load by switching off all the

electrical switches.

12.Close the gate & then switch OFF the supply water pump set.

13.Follow the procedure described below for taking down the reading for

evaluating the performance characteristics of the Francis turbine.

14.Finally change the position of the belt by using adjustable movement and

repeat the experiment for Francis turbine.

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A. TO OBTAIN CONSTANT SPEED CHARACTERISTICS:

(Operating Characteristics)

1. Keep the guide vane opening at a particular position.

2. For loading the turbine, operate the Electrical switches.

3. Note down all the readings in observation column for all loadings. Speed can be maintained constant with help of butterfly valve.

4. Vary the gate opening setting to different position and repeat (2) for different speeds and tabulate the results.

5. Similarly vary the vane position and note down different speed readings. The above readings will be utilized for drawing constant speed characteristics.

B. TO OBTAIN CONSTANT HEAD CHARACTERISTICS:

(Main Characteristics)

1. Select the guide vane angle position.

2. Keep the gate closed and start the pump.

3. Slowly open the gate and set the pressure on the gauge.

4. For different loads, change the guide vane angle position, and maintain the constant head and tabulate the results as given in Table.

C. TO OBTAIN RUN-AWAY SPEED CHARACTERISTICS:

1. Switch OFF all the load on the turbine.

2. Keep guide vane angle at optimum position.

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3. Slowly open the gate to maximum and note down the turbine speed. This is

the run-away speed which is maximum.

NOTE: Run-away speed is also influenced by the tightening in gland packing

of the turbine shaft. More the tightness, less the run-away speed.

D. PERFORMANCE UNDER UNIT HEAD-UNIT QUANTITIES:

In order to predict the behavior of a turbine working under varying conditions

and to facilitate comparison between the performances of the turbines of the

same type but having different outputs and speeds and working under different

Heads, it is often convenient to express the test results in terms of certain unit

quantities. Unit quantities refer to the turbine parameters which are obtained

when a particular turbine operates under a unit head, discharge and power

output. Thus making it possible to predict the behavior of a turbine working

under different conditions and compare the performance of turbines of different

sizes but of same type. The different unit quantities are:

1. Unit Speed: It is the theoretical speed at which a given turbine would

operate under a given head (i.e. at 1m) unit speed, Nu=N/H½.

2. Unit Discharge: It is the theoretical discharge at which a given turbine

would operate under a unit head and unit speed, Qu=Q/H½.

3. Unit Power: It is the theoretical power at which a given turbine would

develop under a unit head (i.e. at 1m) unit power, Pu = P/H3/2.

4. Specific Speed: It is the speed of a geometrically similar turbine (i.e. a

turbine identical in shape, blade angles etc.) which would develop unit

power when working under a unit head. The Ns is usually computed dor

the operating conditions corresponding to the maximum efficiency.

Ns = N P½ H5/4

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Specific speed provides a basis in which different types of turbines can be

compared irrespective of their sizes.

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TABLE OF CALCULATIONS

Output Power

, BPELEC

in watts

Output Power, BPSHAFT

in watts

Head on Venturimeter, HV in m of

water

Head

‘H’ in m

Discharge In ‘Q’ in m3/s

Hydraulic Input, PHYD in Watts.

Efficiency in %

Specific speed

NS

% of Full Loa

d

Unit Quantities under Unit Head

Unit speed, Nu

Unit Power

, Pu

Unit Discharge

, Qu

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TABLE OF READINGS

Vane Position

Voltmeter reading ‘V’ volts

Ammeter reading ‘A’ amps

Speed ‘N’ in RPM

Delivery pressure,

‘P’ in Kg/cm2

Suction Pressure, ‘Pv’ in mm of Hg.

Venturi meter Readings

Pressure on Inlet side, ‘PI’ in Kg/cm2

Pressure on Throat, ‘PT’ in Kg/cm2

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FORMULAE USED

1. Electrical Power as indicated in energy meterBPelec = n 60 60 1000 -------------------------- in KW Ec t

WHERE : n = No of revolution of energy meter disc Ec = Energy meter constant =3200

t = Time taken by energy meter for “n” revolution 0.7 = Transmission efficiency

2. Discharge Rate, Q: Through Venturimeter

Q = Cd (A1A2(2gHv)½)/(A12-A2

2)½ in m3/s

Where, Cd = Coefficient of discharge

A1 = Inlet area of Venturimeter (100mm diameter) = 7.8510-3m2

A2 = Throat area of Venturimeter (50mm diameter) = 1.9610-3 m2

g = 9.81 m/s2

Hv = Head on Venturimeter, m=10h = 10 (P1-(PT)

PI = Pressure at Venturi inlet,

PT = Pressure at venturi throat.

1. Hydraulic input to the turbine in W.

PHYD = WQH

Where, W = 9810 N/m3

Q = Flow rate of water in m3/sec from formulae-2.

H = Head on turbine in m from formulae-4.

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2. Head on the Turbine, H

H = 10(PI+PV/760) Where, P = Pressure on the turbine = P1

PV = Vacuum at the turbine

3. Turbine Efficiency, %tur = BPSHAFT/HPhyd100

4. Unit quantities – under unit head,

a) Unit speed, Nu = N/(H)½

b) Unit power, Pu = P/H3/2

c) Unit discharge, Qu = Q/(H)½

5. Specific speed,N (P)½

NS = H5/4

Part load P8. Percentage full load = 100

Max. load P

GRAPHS:

A) For constant head characteristics

a. Turbine efficiency Vs Unit speed.

b. Unit power Vs Unit speed.

c. Unit discharge Vs Unit speed.

B) For constant speed characteristics:

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a. Turbine efficiency Vs Percentage of full load.

b. Efficiency Vs discharge.

c. BPSHAFT Vs discharge.

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PRECAUTIONS AND THINGS TO REMEMBER1. Do not start pump set if the supply voltage is less than 300V (phase to phase

voltage)

2. Do not forget to give electrical earth & neutral connections correctly.

Otherwise, the RPM Indicator gets burnt if connections are wrong.

3. Frequently, atleast once in three months, grease all visual moving parts.

4. Initially, fill-in the tank with clean water free from foreign material. Change

the water every six months.

5. At least every week, operate the unit for five minutes to prevent any

clogging of the moving parts.

6. To start and stop the supply pump, always keep gate closed.

7. Gradual opening and closing of the gate is recommended for smooth

operation.

8. In case of any major faults, please write to manufacturer, and do not attempt

to repair.

9. Fill the water enough so that the pump does not choke.

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LIST OF REFERENCES:

1. Fluid Mechanics and Machinery by H.M. RAGHUNATH.

2. Hydraulics & Fluid Mechanics by Dr. P.N. MODI & Dr. S.M. SETH.

3. 3. Flow Measurement Engineering Hand Book by R.W. MILLER.

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