francis turbine manual
DESCRIPTION
francis turbineTRANSCRIPT
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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