Hydraulic Turbines

Theory of operation

Flowing water is directed on to the blades of a turbine runner,

creating a force on the blades.

Since the runner is spinning, the force acts through a distance

(force acting through a distance is the definition of work). In this

way, energy is transferred from the water flow to the turbine.

Water turbines are divided into two groups; reaction turbines and

impulse turbines.

Reaction Turbines

Reaction Turbines are acted on by water, which changes

pressure as it moves through the turbine and gives up its energy.

They must be encased to contain the water pressure, or they

must be fully submerged in the water flow.

Newton's third law describes the transfer of energy for

reaction turbines.

Most water turbines in use are reaction turbines.

They are used in low and medium head applications.

Impulse turbines change the velocity of a water jet. The jet impinges

on the turbine's curved blades which almost reverse the flow. The

resulting change in momentum (impulse) causes a force on the

turbine blades.

Since the turbine is spinning, the force acts through a distance (work)

and the diverted water flow is left with diminished energy.

Prior to hitting the turbine blades, the water's pressure (potential

energy) is converted to kinetic energy by a nozzle and focused on the

turbine.

No pressure change occurs at the turbine blades, and the turbine

doesn't require a housing for operation.

Newton's second law describes the transfer of energy for impulse

turbines.

Impulse turbines are most often used in very high head applications.

Impulse Turbines

Pelton Turbine Runner

Buckets

Shaft

Runner

The main components of a Pelton turbine

Breaking jet

Jet striking the splitter and getting split in to two parts

Deflectionangle of jet

jet

Vane

Velocity Triangle at inlet and exit

u1 Vr1

V1=Vw1u

Deflection angle

Vr2

u2 Vw2

Vf2

V2

bf

Vf1=0

FRANCIS TURBINE : An Experimental Set up in the Lab

Various types of water turbine runners.

From left to right: Pelton Wheel, two types of Francis

Turbine and Kaplan Turbine

FRANCIS TURBINE RUNNER

Francis turbine runner, rated at nearly one million hp (750 MW),

being installed at the Grand Coulee Dam, United States.

Francis turbine

and generator

cut-away view

Head across a reaction turbine

Francis Turbine Cross-section

Guide vanes

Volute CasingVolute Casing

Guide vanes

Movingvanes

Draft Tube

ShaftRunner

Axial flow reaction turbine

This is a reaction turbine in which the water flows parallel to the

axis of rotation.

The shaft of the turbine may be either vertical or horizontal.

The lower end of the shaft is made larger to form the boss or the

hub.

A number of vanes are fixed to the boss. When the vanes are

composite with the boss the turbine is called propeller turbine.

When the vanes are adjustable the turbine is called a Kaplan

turbine.

An Axial Flow Turbine Runner

The function of the guide vane is

same as in case of Francis

turbine.

Between the guide vanes and the

runner, the fluid in a propeller

turbine turns through a right-

angle into the axial direction and

then passes through the runner.

The runner usually has four or six

blades and closely resembles a

ship's propeller.

schematic diagram of

propeller or Kaplan turbine.

R1R2

O

f b

V2Vr2

u2

Vf2

Vw2FG

H

E

aq

V1 Vr1

u1

Vf1

Vw1

B

D

CA

Wheel

Tangent

Tangent

Velocity Triangle at inlet and exit

Basic Parameters of a Francis Turbine

Speed ratio = where H is the Head on turbine

Flow ratio = where Vf1 is the velocity of flow at inlet

Discharge flowing through the reaction turbine is given by

Q = D1 B1 Vf1 = D2 B2 Vf2

Where D1 and D2 are the diameters of runner at inlet and exit

B1 and B2 are the widths of runner at inlet and exit

Vf1 and Vf2 are the Velocity of flow at inlet and exit

If the thickness (t) of the vane is to be considered, then the area through which flow takes place is given by ( D1­ nt) where n is the number of vanes mounted on the runner.

Discharge flowing through the reaction turbine is given by

Q = ( D1­ nt) B1 Vf1 = ( D2­ nt) B2 Vf2

Hg

u

2

1

Hg

V f

2

1

60

11

NDu

60

22

NDu

secondper striking water ofWeight

secondper doneWork

2211

2211 1uVuV

ggQ

uVuVQww

ww

11

1uV

gw

Work done per second on the runner = a V1 (Vw1u1 Vw2u2)

= Q (Vw1u1 Vw2u2)

Work done per unit weight =

=

If the discharge at the exit is radial, then Vw2 = 0 and hence

Work done per unit weight =

Hydraulic efficiency =

2211

2211 1

..

..uVuV

HgHQg

uVuVQ

PW

PRww

ww

KAPLAN TURBINE - SUMMARY

Peripheral velocities at inlet and outlet are same and given by

where Do is the outer diameter of the runner

Flow velocities at inlet and outlet are same. i.e. Vf1 = Vf2

Area of flow at inlet is same as area of flow at outlet

where Db is the diameter of the boss.

6021

NDuu o

22

4bo DDQ

Governing of Reaction Turbines

Governing of reaction turbines is usually done by altering the

position of the guide vanes and thus controlling the flow rate by

changing the gate openings to the runner.

The guide blades of a reaction turbine are pivoted and connected

by levers and links to the regulating ring.

Two long regulating rods, being attached to the regulating ring at

their one ends, are connected to a regulating lever at their other

ends.

The regulating lever is keyed to a regulating shaft which is turned

by a servomotor piston of the oil

Governing of reaction turbines

Bulb Turbine The bulb turbine is a reaction turbine of Kaplan type which is used for

extremely low heads.

The characteristic feature of this turbine is that the turbine components as

well as the generator are housed inside a bulb, from which the name is

developed.

The main difference from the Kaplan turbine is that the water flows in a

mixed axial-radial direction into the guide vane cascade and not through a

scroll casing.

The giude vane spindles are normally inclined to 600 in relation to the

turbine shaft and thus results in a conical guide vane cascade contrary to

other types of turbines.

The runner of a bulb turbine may have different numbers of blades

depending on the head and water flow.

The bulb turbines have higher full-load efficiency and higher flow capacity

as compared to Kaplan turbine. It has a relatively lower construction cost.

The bulb turbines can be utilized to tap electrical power from the fast

flowing rivers on the hills

Schematic of Bulb Turbine Power Generating Station

Reaction turbine Impulse turbine

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

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.

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

It is the velocity of jet which changes, the pressure throughout remaining atmospheric.

3 The runner must be enclosed within a watertight casing (scroll casing).

Water-tight casing is not necessary. Casing has no hydraulic function to perform. It only serves to prevent splashing and guide water to the tail race

4.

Water is admitted over the entire circumference of the runner

Water is admitted only in the form of jets. . There may be one or more jets striking equal number of buckets simultaneously.

5.

Water completely fills at the passages between the blades and while flowing between inlet and outlet sections does work on the blades

The turbine does not run full and air has a free access to the buckets

6.

The turbine is connected to the tail race through a draft tube which is a gradually expanding passage. It may be installed above or below the tail race

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

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

Flow regulation is done by means of a needle valve fitted into the nozzle.

A kaplan turbine is to be designed to develop 7,350

kW. The net available head is 5.5 m. Assume that the

speed ratio is 2.09 and flow ratio is 0.68 and the overall

efficiency as 60%. The diameter of the boss is ⅓rd of the

diameter of the runner. Find the diameter of the runner,

its speed and its specific speed.

68.02

1

Hg

V fm/s 13.75.510268.01 fV

09.22

1 Hg

um/s 07.235.51022.21 u

5.5101000

1073506.0;

3

0

QHQg

P

72.22213.7344

2

2

1

22

o

ofbo

DDVDDQ

m/s07.2360

69.6

60

NNDu o

(Ans) rpm37.6705.5

735086.65

45

45

H

PNNs

P = 7350 kW, H = 5.5 m

Q = 222.72m3/s

N=65.86 rpm (Ans)

Do = 6.69 m (Ans)