REACTION TURBINE

2.7.1 INTRODUCTION

In reaction turbine, a part of the head (H) acting on the turbine is converted into kinetic energy and the rest remains as pressure head. The water first enters a set of movable blades (guide vanes) and passes over a set of fixed runner blades. There exists a difference of pressure between these two sets of blades which is called ‘reaction pressure’ and is responsible for the motion of the runner blades.

Degree of reaction: it is defined as the ratio of the static pressure drop to the total pressure

drop in the stage. stagetheindroppressure

tionrotorindropessureR

secPr

By the application of Bernoulli’s equation to the inlet and outlet section of the runner blade,

g

pp

21 = Wnet -

g

VV

2

22

21

……..(1)

Where p1,p2 = pressure at inlet and outlet

V1, V2 = absolute of velocity at Inlet and outlet section

W net = work done by the turbine runner

Thus, if the pressure is constant at the inlet and outlet sections than such a turbine behaves purely as in impulse turbine. If on the other hand v1= v2 , then

W net =

g

pp

21 and this represents purely reaction turbine. Intermediate type of

turbine described by the degree of reaction.

R =

net

net

W

g

VVW

2

22

21

……..(2)

R =

netgW

VV

21

22

21 ……..(3)

Wnet = g

uVu 11

R =

121

1

22

21

uV

VV

u

…… (4)

2.7.2 COMPARISON BETWEEN REACTION AND IMPULSE TURBINE

Impulse Turbine Reaction Turbine

1. All the available hydraulic energy is converted into kinetic energy by

a Nozzle and it is the jet so ,

produced which strikes the runner

blades.

2. It is the velocity of jet which changes,

the pressure through remaining

atmospheric.

1. Only a fraction of the available hydraulic energy is converted into kinetic energy before the fluid enters the runner.

2. Both pressure and velocity change as the fluid passes through the runner, pressure at inlet is much higher than the outlet.

3. Casing is not necessary. Casing has no hydraulic function perform. It only

serves as a safe guards.

4. Water is admitted only in the form of

jets.

5. the turbine does not run full and air

3. The runner must be in closed within a water tight casing.

4. Water is admitted over the entire

circumference of the runner.

5. Water completely fills at the

has a free access to the buckets.

6. The turbine is always installed above the tail race and there is no draft tube used.

7. Flow regulations is done by means

Of a needle valve fitted into the

Nozzle.

passage between the blades and

while flowing between inlet and

outlet sections does work on the

blades.

6. The turbine is connected to the

tail race through a draft tube

which is a gradually expanding

passage. It may installed above

or below the tail race.

7. The flow regulation is carried out by means of a guide vane assembly , other component parts are scroll casing, stay ring, runner and the draft tube.

2.7.3 TYPES OF REACTION TURBINE

Mainly two types of reaction turbine: ( i ) Outward radial flow reaction turbine

(ii) Inward flow reaction turbine.

(i) Outward radial flow reaction turbine. Fig. 2.13 shows outwards radial flow reaction

turbine in which water from casing enters the stationary guide wheel. The guide wheel

consists of guide vanes with direct water to enter the runner which is around the stationary

guide wheel. The water flows through the vanes of the runner in the outward radial direction

and is discharged at the outer diameter of the runner. The inner diameter of the runner is inlet

and outer diameter is the outlet. The velocity triangle at inlet and outlet will be drawn by the

same procedure as adopted for inward flow turbine. The work done by the water on the

runner per second, power output developed and hydraulic efficiency will be obtained from

the velocity triangle. In this case as inlet of the runner is at the inner diameter of the runner,

tangential velocity at inlet will be less than of at outlet

u1 < V2 as D1<D2

Fig. 2.13 Outward radial flow reaction turbine

(ii) Inward flow reaction turbine: Fig 2.14 shows inwards radial flow reaction turbine in

which water from casing enters the ‘Guide Vanes Section’ through the guide vanes water

flows to the runner in the inward radial direction and discharge at the inner diameter of the

runner. The inner diameter of runner is outlet and diameter of runner is inlet.

Fig. 2.14. Outward radial flow reaction turbine.

2.7.4 COMPARISON BETWEEN OUTWARD AND INWARD FLOW

TURBINE

Outward Flow Turbine

Inward Flow Turbine

1. Water enters at the inner periphery and

discharges at the outer periphery

2. Flow outward

3 Flow rate increases

4. Turbine runner V1<V2, because of

V12-V2

2 imparted to water as it flow

2g through the turbine runner is

Positive.

5. very difficult to control the speed

6. Good for low medium heads

1. Water enters at the outer periphery

and discharges at the inner

periphery

2. Flow inward

3. Flow rate does not increases

4. Turbine runner V1>V2, because of

V12-V2

2 imparted to water as it flow

2g through the turbine runner is

Negative.

5. Easy to control the speed

6. Good for medium and high heads

and best suitable for large output units.

2.7.5 CONSTUCTIONAL DETAILS OF THE FRANCIS TURBINE

The Francis turbine was developed by James B. Francis. It is an inward flow reaction turbine

that combines radial and axial flow concepts. Francis turbines are the most common turbine

in use today. They operate in a head range of thirty meters to several hundred meters and are

primarily used for electrical power production. The Francis turbine is a reaction turbine,

which means that the working fluid changes pressure as it moves through the turbine, giving

up its energy. A casement is needed to contain the water flow. The inlet is spiral shaped.

Guide vanes direct the water tangentially to the runner. This radial flow acts on the runner

vanes, causing runner to spin. The guide vanes (or wicket gate) may be adjustable to allow

turbine operation for a range of water flow conditions. As the water moves through the runner

its spinning radius decreases, further acting on the runner. Imagine swinging a ball on a string

around in a circle. If the string is pulled short, the ball spins faster. This property helps inward

flow turbines harness water energy. At the exit, water acts on cup shaped runner features,

leaving with no swirl and very little kinetic or potential energy. The turbine‘s exit tube is

specially shaped to help decelerate the water flow and recover kinetic energy. The Francis

turbines may be divided in two groups, the one group with horizontal and the other with

vertical shaft. In practice it is normal that turbines with comparatively small dimensions are

arranged with vertical shaft. The vertical arrangement is normally used also for small

dimensions if the tail race water level is above the turbine centre.

(a)

(b)

Fig. 2. 15. Francis Turbine

Components parts:

Penstock. It is a large sized conduit which conveys water from the upstream of the

dam to the turbine runner. Trash rack is provided at inlet of the penstock in order to

obstruct the entry of debris and other foreign matter.

Scroll Casing. The water from the penstock is conducted through the scroll casing

and distributed around the stay ring and the complete circumference of the guide vane

cascade. The decrease in area is in proportion to the decreasing volume of water to be

handled and that ensure velocity of water is constant along its path. The scroll casings

are normally welded steel plate construction for turbine at low, medium as well as

high heads. The stay vanes are given a favourable hydraulic shape to conduct the

water towards the guide vanes with minimum losses. The stay vanes also carry the

axial forces inside the scroll casing. The scroll casing is provided with taps for

pressure measurements, drain, air vent outlets and a manhole.

Guide Vanes . A series of air foil shaped vanes, called the guide vanes or wicket

gates, are arranged inside the casing to form a number of flow passages between the

casing and the runner blades. The vanes are shaped according to hydraulic design

specification and given a smooth surface finish. The bearings of the guide vane shafts

are lubricated with oil or grease. The guide vanes direct the water onto the runner at

an angle appropriate to the design. It provides the degree of adaptability to the

quantity of water to be admitted to the runner in the wake of load variations

Runner and Runner Blades. Runner is a rotor which has passages formed between

crown and shroud in one direction and two consecutive blades on the other. These

passages take water in at the outer periphery in the radially inward direction and

discharge it in a direction parallel to the axis of rotor. The driving force on the runner

is both due to impulse and reaction effects. It may either be of cast steel or a welded

construction where hot pressed plate blades are welded to the cast hub and ring. In

most cases the runner is made of stainless steel.The water flow through the labyrinth

seals is a leakage flow and is not utilized by the runner. On high head turbines the

leakage water is normally utilized as cooling water for the generator, transformers

and bearings.

Fig. 2.16. Francis Runner

Draft Tube. The draft tube forms the water conduit from the runner to the draft outlet.

It consists of the draft tube cone and the draft tube steel plate lining. It consists of the

draft tube cone and the draft tube steel plate lining. The aim of the draft tube is also to

convert the main part of the kinetic energy at the runner outlet to pressure energy at

the draft tube outlet. This is achieved by increasing the cross section area of the draft

tube in the flow direction. In an intermediate part of the bend however, the draft tube

cross sections are decreased instead of increased in the flow direction to prevent

separation and loss of efficiency.The draft tube cone is a welded steel plate design and

consists normally of two parts, the upper and lower cone. The inlet part of the upper

cone is made of stainless steel. It is normally provided with two manholes for

inspection of the runner from below. The lower part is designed as a dismantling

piece and is mounted to a flange on the draft tube bend top. This design is always

used for units where the runner is dismantled downwards. For units being dismantled

upwards the draft tube cone is made in one piece. The draft tube lining is completely

embedded in concrete.

Different types of Draft Tubes:

(a) Conical draft tubes (b) Simple elbow tubes

(c) Moody spreading tubes (d) Elbow draft tubes with circular inlet and

rectangular outlet

Fig. 2.17(a) Fig. 2.17(b)

Hs

Fig. 2.17(c) Fig. 2.17(d)

The conical draft tubes and Moody spreading draft tubes are most efficient while

simple and elbow draft tubes with circular inlet and rectangular outlet requires less

space as compared to other draft tubes.

2.7.6 POWER DEVELOPED BY FRANCIS TURBINE RUNNER

The velocity vector diagram at inlet and out of the runner of a Francis turbine is shown in Fig

7.8.

Where v1 , v2 = absolute velocity of fluid at inlet and outlet

u1, u2 = blade velocity at inlet and outlet

Vr1, Vr2 = relative velocity of fluid at inlet and outlet

Vu1, Vu2 = whirl component or tangential component of absolute velocity at inlet

and outlet

Vf1, Vf2 = axial component of absolute velocity at inlet and outlet

ω = angular velocity

r1, r2 = runner radius at inlet and outlet

N = speed of rotation

α1, α2 = flow angle at inlet and outlet

β1, β1 = blade angle at inlet and outlet

Hs

Force exerted by the water, on the radial curved blade fixed on the wheel, from the force

exerted on the blades, power is developed.

Assumptions:

Flow is steady and one dimensional.

No energy loss in the runner due to friction and eddy formation.

Infinite number of blades, liquid flowing in the blades.

No loss due to shock at entry.

Fig. 2.18 Velocity triangle of Francis turbine.

From the second law of Newton momentum theorem,

Torque = rate of change of angular momentum

Torque(T) = m[Vu1 r1-( -Vu2 r2)]

Torque(T) = m(Vu1 r1+Vu2 r2) ...........(5)

Mass flow rate(m) = Q

β2 α2=90

V2=Vf2

V2

Vr

Fig 2.19

T = ρQ (Vu1 r1+Vu2 r2) ...........(6)

Power developed in one second.

Power (P) = T

P = ρQ(Vu1 r1+Vu2 r2) ω

P = ρQ(Vu1 r1 ω +Vu2 r2 ω)

Blade velocity

u1= r1 ω and u2= r2 ω

Power = ρQ(Vu1 u1+Vu2 u2)

P = ρQ(Vu1 u1+Vu2 u2) …….(7)

Equation (7) is valid when the α2 is less than 900, so Vu2 is negative because of direction

of Vu2 is opposite to that of blade motion.

If α2>900 than P = ρQ(Vu1 u1-Vu2 u2)

Means power developed by runner (P) = ρQ (Vu1 u1±Vu2 u2)

From the outlet velocity triangle. Water flow without whirl Vu2 = 0

So

P = ρQ(Vu1 u1-0Ҳu2)

Power Developed by Runner = ρQVu1 u1 …….. (8)

Hydraulic efficiency (ηH) = power developed by runner

power input to runner

ηH =

gQH

uVuVg uu

21 21

…….. (9)

ηH = 2

gH

uVuV uu

2

21 21 …….. (10)

ηH =

gH

u

gH

V

gH

u

gH

V uu

2

2

22

1

221

…….. (11)

Where Cv = and Cu = gH

u

2

ηH = 2 2211 uvuv CCCC …….. (12)

2.7.7 LOSSES AND EFFICIENCIES OF REACTION TURBINE

(FRANCIS AND KAPLAN)

The various losses that may occur in a Francis turbine units can classified under.

(i) Hydraulic Efficiency or Runner Efficiency (ηH). It is the ratio of power

developed by the turbine runner to the net power supplied by the water at the

entrance to the turbine.

ηH = turbinethetopliedpower

turbinethebydevelopedpower

sup

Hydraulic losses

(i) Blade friction losses

(ii) Eddy formation losses

(iii) Friction losses in draft tube

(iv) Disc friction losses

(v) Leakage losses

ηH = gQH

uVuVg uu

21 21

ηH = gH

uVuV uu 21 21 …….. (13)

(ii) Mechanical Efficiency ( ηMech ) . It is the ratio of power available at the turbine

shaft to the power developed by turbine runner. Mechanical losses are due to

bearing friction and other factor.

Mechanical losses

Turbine runner Shaft

Entrance of

Spiral casing

Turbine Runner

η Mech = turbinethebydevelopedpower

turbinetheatavailablepower

(iii) Volumetric Efficiency(ηV ). It is the ratio of quantity of water actually striking

the turbine runner and the quantity of water supplied to the turbine.

ηV = QQ

Q

…….. (14)

Where Q is the amount of water striking the runner VQ is the amont of water loss

due to leakage.

(iv) Overall Efficiency(ηO). It is the ratio of power available at the turbine shaft to

water power available at the entrane level.

HQQg

outputpower

powerinput

outputpowero

HMechvo …….. (15)

Fig.2.20. Power loss diagram for reaction turbine (Francis and Kaplan)

2.7.8 SOME DIMENSIONLESS NUMBER OF REACTION TURBINE

(i) Flow ratio (CV). It is the ratio of the velocity of flow (Vf) and the theoretical

velocity ( gH2 ).

Flow ratio (CV) = 2gH

Vf …(16)

Value of (CV) varies from 0.12 to 0 .30.

(ii) Speed ratio (Cu). It is the ratio of the blade peripherial velocity(u) and the

theoretical velocity ( gH2 ).

Cu =

gH2

u …(17)

Value of Cu varies from 0.60 to 0.90.

(iii) The ratio of width and diameter of the runner is known as width dia ratio(η).

η = runner(b) ofwidth …(18)

dia of runner (D)

Value of η varies from0.10 to 0.35.

2.7.9 DESIGN ASPECT OF FRANCIS TURBINE RUNNER

A francis turbine runner is required to be designed to develop a known power output (Pt ),

speed of rotation (N) r.p.m., Head (H and ηH , η0 , n , CV , Cu value are assumed. Design of

runner involves the calculation the dimensions of runner and blades angles explain in

following steps.

(i) Calculate the flow rate

η0 = gQH

Pt

Q =0

t

gH

P

…(19)

(ii) If the (Y) is the number of blades; t is the thickness of blades; B is the width,

area of flow = (D-Yt)B

= D BD

Yt1

area of flow = K BD …(20)

flow rate Q= K BD Vf …(21)

where K=1-D

Yt

Thus assuming a suitable value of K ( between 0.85 to 1) and calculate the width and dia of

the runner wheel.

(iii) Peripherial velocity (u) is calculated by

u = 60

DN …(22)

(iv) Whirl component of velocity can be calculated by

ηH =

gh

uV 1u1 …(23)

(v) Vf is calculated by = Vf = Cv gH2 …(24)

(vi) With the help of inlet and outlet velocity triangle blade angle and flow angle

is calculated.

(vii) Thickness of the runner blades varies from 5 to 25 mm depending on

diameter (D).

(viii) Number of Runner blades varies from 16 to 24.

2.7.10 DIFFERENT TYPE OF REACTION TURBINE RUNNER

a) Slow speed runner

α1= 16 to26

β1= 90° to125°

C 1u =0 .6 to 0.69

Specific speed Ns = 60 to 110

b) Medium speed runner

α1= 26 to 33°

β1= 90°

C 1u = 0.69 to0 .73

Specific speed (Ns) = 110 to 170

c) High speed runner

Ns= 170 to 300

α1 = 33 to 38°

β1 = 60 to 90°

C 1u = .073 to

(c) High speed Francis runner

d) Kaplan runner

β1 < 90° , α1 < 90°

V1 Vr1

u1

Vu1

α1 β1

Vf

V1 Vr1

u1

Vu1

α1 β1

Vf1

(b) Medium speed Francis runner

(a) Slow speed Francis runner

Specific speed (Ns) = 300 to 1000°

C 1u = 1.21 to 2,45 (More than 1)

(d) Kaplan runner

Fig. 2.21(a), (b),(c),(d). Different velocity triangle of turbine runner.

2.7.11 RUN AWAY SPEED

The run away speed of a turbine is the maximum speed attained by the runner under

maximum head at full gate opening, when the external load (generator) is disconnected

from the system. All rotating parts must be designed to withstand the runaway speed

which varies among the manufacturers with the design of turbine and generator.

Runaway speed for various types of turbines is generally as follow:

Turbine Runaway speed

Pelton turbine 1.8 to 1.9 N

Francis turbine 2 to 2.2 N

Kaplan turbine 2.5 to 3 N

The exact value of runaway speed of any turbine can be predicted from the model tests

in the laboratory.

2.7.12 DRAFT TUBE THEORY

A pipe of gradually increasing area is used for discharging water from the outlet of the

turbine to tail race is called the draft tube. One end of the draft tube is connected to the outlet

of the runner and other end sub-merged below the level of water in tail race, draft tube, in

addition to serve a passage for water discharge, has the following two purposes also.

1. The turbine may be placed above the tail race and hence turbine may be

inspected properly.

2. The kinetic energy

g2

v 2

rejected at the outlet of the turbine is converted into

useful pressure energy.

Fig. 2.22. Draft tube arrangement of reaction turbine

h2 = height of runner from datum level.

Hs = suction height.

V2,V3= velocity of fluid at inlet and outlet section of draft tube

h4 = height of outlet of the draft tube from datum level.

h3 = height of outlet section od draft tube from tail race.

Applying the Bernoulli’s theorem between (2) inlet and (3) outlet of the draft tube.

4

233

2

222 h

g2

V

g

Ph

g2

V

g

P

…(24)

g2P

=

g3P

- ( h2- h4 )-

g2

VV 23

22 …(25)

g3P

=

ga

P

+ h3

Hs

h2

h3

h4

Draft tube

Pa (Tail Race)

Draft tube Outlet

Runner Outlet

2

3

Datum Level

g2P

=

ga

P

-( h2-h3 –h4)-

g2

VV 23

22 …(26)

From the Fig.2.22.

h2-h3 –h4 = Hs

g2P

=

ga

P

-

g2

VV Hs

23

22 …(27)

Hs= static suction head

g2

VV 23

22 = dyanamic suction head.

If frictional losses in the draft tube is hf

So g2P

=

ga

P

-

g2

VV Hs

23

22 + hf

…(28)

Draft tube efficiency is defined as ratio of actual conversion of kinetic head into pressure

head in the draft tube to the kinetic head at the inlet of the draft tube. Hence

Draft tube efficiency (ηd ) =

g2

V

hg2

VV

22

f

23

22

…(29)

g2P

=

ga

P

-

f

23

22

s hg2

VVH

2.7.13 GOVERNING OF REACTION TURBINE

The guide blades of a reaction turbine shown in Fig. 7.13 are pivoted and connected by levers

and links to the regulating ring. To the regulating ring are attached two long regulating rod to

a regulating lever.

Fig. 2.23. Governing of reaction turbine

The regulating lever is keyed to a a regulating shaft which is turn by the servomotor piston of

oil pressure governor. The penstock, feeding the turbine inlet, has a relief valve better known

as pressure regulator; when the guide vanes have to be suddenly closed, the relief valve opens

and delivers the water direct to the tail race. Its function is therefore, similar to that of the

deflector in Pelton turbines. Thus the double regulation, which is the simultaneous operation

of two elements is accomplished by moving the guide vanes and relief valve in francis

turbine, by the governor

2.7.14 CAVITATION

Cavitation definition and other basic theory explained in the Centrifugal pump unit. Turbine

parts should be properly designed in order to avoid Cavitation because damaging the metallic

surface, Cavitation also decrease the efficiency of the turbine. Cavitation depends upon:

Vapour pressure (Pv) which is a function of temperature of flowing water.

Barometric pressure (Pb) due to the location of turbine above the sea level.

Suction pressure (Hs) which is the height of runner outlet above tail race level.

Absolute velocity of water at outlet. Prof. Thoma (1881-1943) suggested a Cavitation

factor ( ) to determine the zone where turbine can work without being affected from

Cavitation.

Critical Cavitation factor ( crit) = H

)H-(H sb

H

)H-H-(H svatm … (30)

Where Hb= barometric pressure in metre of water = Ha – Hv

Ha= Atomspheric pressure in metre of water

Hv = Vapour pressure in m of water

Hs = Suction pressure in m of water

H= Working head.

According to the Prof. Thoma, Cavitation can be avoided if the value of are not

less than the critical value given above. Prof. Rogger suggested the following empirical

relation for Francis turbine.

crit = 0.0317 2

S

100

N

… (31)

The maximum permissible specific speed can be calculated by

Ns = 1000317.0

crit

= 562 H

H-H sb …(32)

Method to Avoid Cavitation

turbine installed below the tail race level

Outlet pressure of turbine (P2 )

g2P

=

ga

P

-

hf

g2

V Hs

23

22 V

Means outlet pressure of turbine is the function of suction head (Hs). if the Hs decrease

than the PQ pressure at the outlet of turbine increase and value will increase so that

chance of Cavitation minimize which means the turbine work in the safe zone so that

suction heat decrease to negative value means turbine installed below the tail race level.

Other method of avoid the cavitation explained in the centrifugal unit.

design cavitation free runner

selection of speed

use of machining

η

σ σ crit

Fig. 2.24. Graph Efficiency vs Cavitation factor.

KAPLAN TURBINE

2.8.1 INTRODUCTION OF PROPELLER AND KAPLAN TURBINES

A propeller turbine is an axial flow reaction turbine suitable for low head installations (upto

30 m) where high discharges are available. The blades of a propeller turbine remain fixed in

position under all conditions of operation. Propeller turbines operate at high efficiency under

fall load conditions but their efficiency is considerably reduced when they are subject to

changing loads, Kaplan is a type of propeller turbine which has adjustable blades whose

position may be varied with load on the turbine and hence with the flow rate. As the blades

are adjusted for a shock-free entry of flow, a high efficiency is maintained over a wide range

of turbine output. A Kaplan runner is designed for a given head and discharge also, therefore,

it works efficiently under much lower heads and at reduced discharge. Except for the runner

itself, the main component parts of a Kaplan turbine are spiral casing, guide vanes and draft

tube which are the same as for a Francis turbine. Between the guide vanes and the runner, the

water turns through a right-angle and flows parallel to the turbine shaft. The number of vanes

of a Kaplan runner varies from 3 to 6 which are made of stainless steel and mounted on the

hub. Kaplan turbines are also capable of taking over loads up to 15 to 20% and give high

efficiency at all gate openings.

Fig. 2.39 shows the velocity triangle for a Kaplan turbine at full and part-load conditions

(speed-head remains constant). As the discharge changes consequent upon a change in the

turbine output, the blade angles get simultaneously adjusted in such a way that the flow enters

the runner blades without shocks. Expressions for work done, efficiency and power

developed for propeller and Kaplan turbines are obtained in a similar manner as for a Francis

runner.

Fig 2.39 Velocity Triangle for a Kaplan turbine runner at full and part loads

2.8.2PERFORMANCE OF REACTION TURBINES (LOAD EFFICIENCY CURVES)

The part load efficiency of a propeller (fixed blade) turbine is less than those for a Francis and

adjustable blade (Kaplan) turbine. This is mainly due to turbulence produced at the entrance (entry

with a shock) and the discharge at the runner outlet being far from radial. Kaplan conceived the idea

of making the blades movable about a pivot, thereby making it possible to adjust them to produce

tangential entry (shock-free) for a wide range of gate movement. For such a runner the efficiency will

decrease but slightly as the load- decreases from 40 m to 30% of normal load efficiency curves for

reaction turbine shown in Fig. 2.40.

Fig 2.40 Load efficiency curves for reaction turbines

2.8.3 DIFFERENCE BETWEEN FRANCIS AND KAPLAN TURBINES

Francis turbine Kaplan turbine

1. Number of blades in runner varies between 16-24

1. Number of blade varies to only 3-6

2. Profile type fixed blade runner. 2. Aerofoil type adjustable blade runner

3. Ordinary governor is sufficient as servomotor is of large size.

3. Heavy duty governor is essential due to smaller size of servo motors.

4. Servomotor is kept outside the runner shaft.

4. Servomotor is kept inside the runner shaft.

5. Only guide vanes are controlled, high efficiency is obtained only at full load.

5. Due to simultaneous control of both guide and runner blades.

6. Only guide vanes regulation is done. 6. Guide vanes and runner vane regulation is done.

7. System may have one or two servomotors depending on the size of units.

7. Governing is always done with the servomotors irrespective of the size of the unit.

8. Radial inward/outward turbine. 8. Axial Flow turbine.

9. Medium heads (30-180m) and medium 9. Low head (upto 30 m ) and high flow

flow rate turbine. rate turbine.

10. Specific speed range 60 – 300. 10. Specific speed range 300 -1000.

11. Horizontal or vertical disposition of shaft.

11. Only vertical shaft disposition.

2.8.4 CONSTRUCTIONAL DETAILS OF KAPLAN TURBINE

(a) Penstock: It is large sized conduit which conveys water from the upstream of the dam to

the turbine runner. Because of the large volume of water flow, size of the penstock required is

larger than that of a Pelton wheel. It is made of steel and is imbedded inside the dam.

(b) Scroll casing: Penstock is connected to and feeds water directly into an annular channel

surrounding the turbine runner. The channel is spiral in its layout and is known as the spiral

or scroll casing. Casing constitutes a closed passage whose cross-section area gradually

decreases along the flow direction; area is maximum at inlet and nearly zero at exit.

(c) Guide vanes or wicket gates: A series of airfoil shaped vanes, called the guide, the

casing and the runner blades. The guide vanes direct the water into runner at an angle

appropriate to the design. They direct the flow just as the nozzle of the Pelton wheel. The

configuration and arrangement is such that energy of water is not consumed. Between the

guide vanes and the runner, the water turns through right angle and subsequently flows

parallel to the shaft. The purely axial flow arrangement provides the largest flow area; even at

larger flow velocities are not too large.

Fig 2.41 (d) Runner: The runner is in the form of a boss which is nothing but extension of bottom end

of the shaft into a bigger diameter. On the periphery of the boss are mounted equidistantly 3

to 6 vanes made of stainless steel. Thus compared to the Francis turbine which has 16 to 24

number of blades, a propeller turbine with only 3 to 6 vanes will have less constant surface

with water and as such a low value of frictional resistance. Furthermore, the runner blades are

directly attached to the hub and this feature estimates the frictional losses which are caused

by the bend provided in a Francis turbine.

Fig 2.42 (e) Draft tube: The pressure at the exit of the runner of a reaction turbine is generally less

than tail race. A pipe of gradually increasing area is used for discharging water from the exit

of the turbine to the tail race. This pipe of gradually increasing area is called draft tube. One

end of the draft tube is connected to the outlet of the runner while the other end is submerged

below the level of water in the tail race.

Fig 2.43

2.8.5 DESIGN OF A PROPELLER (ON KAPLAN) TURBINE

Let a Kaplan turbine be required to be designed to develop P watts of power while working under a

head (H) metre and running at N rpm. The appropriate values of 0' Cu' C u and n are assumed for the

purpose of design.

Being an axial flow turbine, the area of flow remains same at its inlet and outlet and, therefore, the

velocity of flow remains constant throughout the runner 21 ff VV . Let the quantities of the boss be

indicated by (L) and that at outer rim by 2.

(i) Calculate the required flow rate (Q)

gQH

P

0

pgH

PQ

0 … (1)

(ii) Assuming the suitable value of K and velocity of flow

ghCV f 2 … (2)

(iii) Assume the suitable value of (n) (ratio of outer diameter to inner diameter). Calculate the

hub diameter D1 and outer diameter (D2) from flow rate relation.

fVDDQ ]''[4

21

21

… (3)

(iv) Calculate the U1 and U2

60

'

602

1'1

1

NDUand

NDU

… (4)

(v) Assume 02 uV at the outlet and suitable values )( H , calculated 1uV .

g

uVu

gH

V uuH

1111 '' … (5)

(vi) With help of velocity triangle at inlet and outlet, calculate all ,,, 121 VV 12 , and

21 ',' VV .

(a) Outlet Periphery (b) At Boss

Fig. 2.44

2.8.6 Specific speed of Propeller/Kaplan Turbines

ghCDDVDDQ f 2)(4

)(4

22

21

22

21

… (6)

Where D1 , D2 are the diameters of runner and the boss

ghCDN

U u 260

D

ghCN u

260

… (7)

gQH

P

0

or ghCDDgHgQHP 2)(4

22

2100

= 322

210 2)(

4HgCDDg

Substituting the value of N and P in specific speed expression.

4

5

H

PNN t

s

2

1

2

3

02

22

1

4

52)(

4

)2(60

HgCDD

DH

HgCN u

s

… (8)

2

1

21

0 11.576

D

DCCN us … (9)

For Kaplan turbine, assuming

90.00

70.035.02

1 CandD

D us CN 3.428

2.8.7 CAVITATION IN PROPELLER AND KAPLAN TURBINE

The Cavitation phenomenon and method to avoid it have been explained in Francis turbine

and centrifugal pump. There is applicable also for propeller or Kaplan turbine. The runner of

Kaplan turbine Cavitation affected the following places.

Outer edge of the runner blade, it is known as cleavage cavitation because it takes

place at the clearance of the runner blades.

Surface of the runner blade.

Base of the runner blade.

Fig 2.45 Cavitation on Kaplan turbine blade

2.8.8 CHARACTERSISTICS CURVES OF HUDRAULIC TURBINE Characteristics curves of a hydraulic turbine are the curves, with the help of which the exact behaviour and performance of turbine under different condition can be known. These curves are plotted from the results of the test performed on the turbine under the different working condition.

The important parameters which are varied during the test on turbine are :

1. Speed (N) 2. Head (H)

3. Discharge (Q) 4. Power (P)

5. Overall efficiency (η0) 5. Gate opening

Out of the above six parameters, three parameters namely speed (N), Head (H) and discharge (Q) are independent parameters. Out of the three independent parameters (N,H,Q), one of the parameter is kept constant (say H) and variation of other four parameters with respect to any one of the remaining two independent variables (say N and Q ) are plotted and various curves are obtained. These curves are called characteristic curves. The following are important.

Characteristics curves of a turbine:

1. Main Characteristics Curves or Constant Head Curves. 2. Operating Characteristic Curves or Constant Speed Curves. 3. Muschel Curves or Constant Efficiency Curves.

2.8.8.1 Main Characteristics Curves or Constant Head Curves

Main characteristics curves are obtained by maintaining a constant head and a constant gate opening (G.O.) on the turbine. The speed of the turbine is varied by changing load on turbine. For each value of the speed, the corresponding value of the power (P) and discharge (Q) are obtained. Then the overall efficiency (η0) for each value of speed is calculated. From these reading the value of unit speed (Nu ), unit power (Pu) and unit discharge(Qu) are determined. By changing the gate opening, the values of Qu , Pu

and η0 and Nu are determined and taking Nu as abscissa, the values of Qu , Pu and η0 are plotted. Fig. 2.46 shows the main characteristics curves for reaction (Francis and Kaplan) turbines.

(a) (b)

(c) (d) Fig.2.46 Main Characteristic curves of Pelton turbine

Fig 2.47 Main Characteristic curves of Reaction turbine

2.8.8.2 Operating Characteristics Curves or Constant Speed Curves

Operating Characteristics Curves are plotted when the speed on turbine is constant. In case of turbine, the head is generally constant. There are three independent parameters namely N, H and Q. For operating characteristics N and H are constant and hence the variation of power and efficiency with respect to discharge Q are plotted. The power and efficiency curves are slightly away from the origin X-Axis, as to overcome initial friction certain amount of discharge will be required. Fig. 8.10 shows the variation of power and efficiency with respect to discharge.

Fig 2.48 Operating Characteristic Curves

2.8.8.3 Efficiency Curves or Muschel Curves or ISO-Efficiency Curves

These curves are obtained from the speed vs. efficiency and speed vs. discharge curves for different gate openings. For a given efficiency from Nu vs. η0 curves, there are two speeds. From the Nu vs. Curves corresponding to two values of discharge. Hence for given efficiency there are two values of discharge for a particular gate opening. This means for a given efficiency there are two values of speed and two values of discharge for a given opening. If the efficiency is maximum there is only one value. These two value of speed and two values of discharge corresponding to a particular gate opening are plotted as shown in Fig. 2.49

The procedure is repeated for different gate openings and the curves Q vs. N are plotted. The points having the same efficiency are joined. The curves having same efficiency are called iso-effeciency curves. These curves are helpful for determining the zone of constant efficiency and for predicting of the turbine at various efficiencies.

Fig 2.49 Constant efficiency curve

2.8.9 DERIAZ TURBINE

A Deriaz machine is similar to a Kaplan machine but has inclined blades. It is particularly suitable for head range between Kaplan and Francis turbines. It utilized the idea of Kaplan Turbine in which maximum efficiency is attained at variable loads condition, the use of movable blade runner. In a Deriaz runner the blades instead of being at right angles to the hub will be inclined at an angle of 45º, it is stated that a runner of this type can be used for a head upto 200m.

Fig 2.50 Deriaz turbine

2.8.9.1 Bulb (Tabular Turbine)

Aron Fischer in 1973 developed in Germany a modified axial flow turbine which was known as tabular turbine. The turbo-generator set using tabular turbine has an outer casing having the shape of bulb. Such a set is now termed as bulb set and the turbine used for the set is called a bulb turbine. The bulb unit is a water tight assembly of turbine and generator of horizontal axis, submerged in stream of water. Such a revolutionary concept has led to complete modification of the usual arrangement of various elements that constitute a low head unit. The axis of rotation coincides with the axis of the passage of the water, which is generally straight. The economical harnessing of fairly low heads on major rivers is now possible with high-output bulb turbines. The idea of such axial flow rectilinear draft tube machine is quite old, but its application could not be developed until the electrical engineering succeeded in producing a highly compact and alternator and hydraulic experts took full advantage of bulb turbine forms to increase output and specific speed.

Advantages of Bulb set over the use of Kaplan turbines

The bulb sets can be employed at very low head sites. The width of plant is less because of absence of spiral casing. Reduction of cost up to 30% as compared to conventional Kaplan installation is

possible. A high speed of rotation can be secured through suitable gearing and thus the size of

the generator unit and consequently that of the bulb can be reduced. Maximum turbine efficiency is increased by about 30% due to almost straight flow

and straight draft tube. Under equivalent conditions of head, the runner diameter and efficiency, the bulb

units are capable of passing higher discharge than the conventional Kaplan turbine. In view of 4, 5 and 6 stated above, the specific speed of bulb turbine is higher than

the Kaplan turbine. There is reduced loss of efficiency at part loads. The unit is better suited for operation on widely varying loads.

Disadvantages of Bulb set over the use of Kaplan turbines

Since the bulb set is completely submerged under pressure, water leakage into the generator chamber and condensation are source of trouble. Although improvements in water-proofing and sealing techniques have reduced this problem considerably, this can still become maintenance problem due to high humidity inside the chamber resulting in gradual deterioration of electrical insulation.

The electrical techniques also involve considerable amount of double handling of equipments and it is doubtful whether any saving is the erection time can be effected as compared to the conventional Kaplan type.

Field of Application of Bulb Set

On the higher head, it is limited to about 15m, beyond this head, bulb turbines have little advantage over Kaplan type.

On the low head, bulb set can offer the greater benefits; the limit is economic in character. The large unit discharge at low head resulting in very low rotational speed and excessive plant cost. The higher rotational speed can be obtained by subdividing the discharge in number of bulb set.

The profitability limit for bulb set is in the region of few hundred HP For 2m head and10,000 to 30,000 HP for 5 to7 head.

2.8.10 SPECIFIC QUANTITIES AND UNIT QUANTITIES

(a) Specific Flow rate : Flow a centrifugal pump or reaction turbine.

Q = (п D B) Vm

Where width (B) is the function of the D (diameter)

Or B = K1 D

θ = п k1 D2 V m

and Vm α √ H

Q α п K1 D2 √H

Q α D2 √H ……………….(26)

Q = Qs D2 √H ……………(27)

Where Qs is the constant and is known as specific flow rate

Qs = Q / D2 √H ……………(28)

If D = 1 H = 1

Qs = Q

So for unit head, unit dia. Of runner required flow rate is called specific flow rate.

(b) Unit flow rate : Equation (26) given below

Q α D2 √H

If D = Const.

Q α √ H

Q = Qu √ H …………………(29)

θu is known as unit flow rate.

Qu = Q / √ H

(c) Specific power : Power(p) α QH Q α D2 √H

p α D2 √H H

p α D2 √H3/2 …………(30)

p = Ps D2 H2/3 …………(31)

specific power (Ps) = P/(D2 H3/2)

(d) Unit power : Equation (30) given below. P α D2 H3/2 if D = Const.

P α H3/2

P = Pu H3/2 …………(32)

Unit power Pu = P / H3/2

(e) Specific force : F = ρQ (Vu2 – Vu1)

Or F = ρ Q D Vu

Q α D2 √ H and Vu2 α √ H

F α d2 h ………..(33)

F = Fs D2 H ………..(34)

Specific force Fs = F /D2 H

(f) Unit force : Equation (33) given below.

F α D2 H (If D = Const.)

F α h

F = Fu H

Unit force Fu = F / H

(g) Unit Speed : For a turbine or centrifugal pump.

Blade velocity (V) = п D N /60

or N α U /D

and U α √H, N α √H / D if D = const.

N α √H

N = No √H

Where Nu = unit speed

Unit speed Nu = N /√H

(h) Specific speed of turbine Blade velocity (V) = п D1 N / 60

u α √H

D1 α √H

From the equation

p α D2 H 3/2

p α (√ H / N)2 H3/2 , p α HH5/2/N2

N α √ (H5/2 / p)

N = Ns √H5/2 / √Pt

Or Specific speed (Ns) = N√Pt / h 5/4

Specific speed of turbine is defined speed of turbine working under unit head and delivering unit horse power output.

If H = 1 and p = 1 H.P.

Ns = N