7/30/2019 Chapter 5 Steam Turbine

1/13

ME 4731 Power Plants

Dr.-Ing. Haitem Hichri

1

Salman UniversityCollege of Engineering at Alkharj

Mechanical Engineering Department

ME 4731 Power Plants

Lecture Notes

Chapter 5: Steam Turbines

Dr.-Ing. Haitem Salem Hichri

1

2

The Steam Turbine

The more modern method of extracting mechanical energy fromthermal energy is the steam turbine.

Steam turbines have been the norm in various land based powerplants for many years.

7/30/2019 Chapter 5 Steam Turbine

2/13

ME 4731 Power Plants

Dr.-Ing. Haitem Hichri

3

The Steam Turbine

The more modern method of extracting mechanical energy fromthermal energy is the steam turbine.

Steam turbines have been the norm in various land based powerplants for many years.

water turbines are categorized into impulse and reaction turbines.

4

How To Provide A Mass Flow Rate (Impulse Turbine)

Area for Flow of Fluid.

Proportional to the Length of the Blade.

More Number of Blade Spacing.

7/30/2019 Chapter 5 Steam Turbine

3/13

ME 4731 Power Plants

Dr.-Ing. Haitem Hichri

5

Analysis of Simple Stationary Impulse Blade

Consider a stationary 180 degreecurved blade.

A jet with a velocity Vi incidence onthe blade.

The blade deflects the jet along itssurface and finally the jet leaves with avelocity Ve.

The magnitude of velocity vectorremains unchanged.

However, the direction changesthrough 180 degrees.

Ve = - Vi The change in velocity : - 2 Vi. A jet with a finite mass flow rate will

experience a rate of change ofmomentum, FA:

( )iA VmF 2=

The force acting on the blade:

( )iR

VmF 2

=

However, this force cannot develop any motive power.

Vi

Ve

FA

FR

6

U

Vri

Vre

Vai

UVri

Vai

Inlet Velocity Triangle

U

VreVae

Exit Velocity Triangle

Analysis of Simple Moving Impulse Blade

7/30/2019 Chapter 5 Steam Turbine

4/13

ME 4731 Power Plants

Dr.-Ing. Haitem Hichri

7

Vri = Vai - U

Vre = -Vri

U

Vae = Vre -U

Vai

Analysis of Simple Moving Impulse Blade

( ) UVUUVUVV aiairiae 2===

( )UVVVV airire == 2

( )

( )UVmVmF

UVmVmF

aiR

aiA

==

==

2

2

( )UUVmP

VUmUFP

aib

Rb

=

==

2

8

Kinetic power lost by the jet :

( ) { }( )2222 222

UVVm

VVm

KP aiaiaeai ==

Power lost by jet = Power gained by the Blade

( )UUVmKP ai =

2

Thermodynamic efficiency of an impulse blade :2

2

= aimV

KPinput

Initial Power of the jet :

( ) ( )

=

=

=

=

2

22

. 44

2

2

aiai

aiai

input

bladeiV

U

V

U

V

UUV

Vm

UUVm

KP

KP

aiai

An efficient impulse blade is bulky Suitable for Dense fluids

7/30/2019 Chapter 5 Steam Turbine

5/13

ME 4731 Power Plants

Dr.-Ing. Haitem Hichri

9

Analysis of Simple Reaction Blade

Vai Vri Vre VaeU

UVV airi += UVV reae =

Change in velocity :rire VVV = ( )

( )

( )UVVmFUP

VVmF

VVmVmF

rireR

rireR

rireA

==

=

==

Motive Power Generated:

10

Thermodynamic efficiency of a Reaction blade :

( )

( )

( )

( ) 22. 2

2

2

aeae

VUVV

UVV

VmUVVm

UVVm

KPKP

KP

rire

rire

rire

rire

exitbladei +

=

+

=

+

=

( )( ) 2. 22

22

aeVUUVV

UUVV

aiae

aiaebladei

+

=

Motive Power Generated: ( )UUVVmFUP aiaeR 2==

A compact Reaction blade is inefficient Suitable for Thin fluids

7/30/2019 Chapter 5 Steam Turbine

6/13

ME 4731 Power Plants

Dr.-Ing. Haitem Hichri

11

12

Need for Multi Staging

Temperature Pressure

Specific

Volume Enthalpy Entroypy Del h

Increse in

velocity

C Mpa cu. M kJ/kg kJ/kg K KJ/kg m/s

550 15 0.02293 3449 6.52

50 0.01235 9.492 2089 6.52 1360 1650

Mean Peripheral Speed of the Blade = 825 m/s

7/30/2019 Chapter 5 Steam Turbine

7/13

ME 4731 Power Plants

Dr.-Ing. Haitem Hichri

13

Strategy for Multi Staging

Temperature Pressure

Specific

Volume Enthalpy Entroypy Del h

Increse in

velocity

C Mpa cu. M kJ/kg kJ/kg K KJ/kg m/s

550 15 0.02293 3449 6.52 93 431.3

500 11.37 0.02845 3356 6.52 91 426.6

450 8.487 0.03575 3265 6.52 92 429.0

400 6.215 0.04559 3173 6.52 91 426.6

350 4.448 0.05914 3082 6.52 92 429.0

300 3.098 0.07832 2990 6.52 91 426.6

250 2.087 0.1063 2899 6.52 92 429.0

200 1.35 0.1489 2807 6.52 195 624.5150 0.4759 0.3679 2612 6.52 248 704.3

100 0.1013 1.442 2364 6.52 275 741.6

50 0.01235 9.492 2089 6.52

14

Current Practice

Purely multistage impulse turbines are mainly preferredin medium capacities of power generations.(30 60 MWunits).

The main advantages are simplicity of construction, lowcosts, reliability and convenience of operation.

The height of blades in last stages of multistage turbinerapidly increase. As the volume increases, the blade height increases,

and the base of the blade spins at a slower speedrelative to the tip. This change in speed forces adesigner to change from impulse at the base, to a high

reaction style tip.

7/30/2019 Chapter 5 Steam Turbine

8/13

ME 4731 Power Plants

Dr.-Ing. Haitem Hichri

15

HP Turbine Rotor

16

LP Turbine Rotor

7/30/2019 Chapter 5 Steam Turbine

9/13

ME 4731 Power Plants

Dr.-Ing. Haitem Hichri

17

Velocity compounding isaccomplished in the first twostages by two rows of moving

blades between which isplaced a row of stationaryblades that reverses thedirection of steam flow as itpasses from the first to thesecond row of moving blades. The velocity is reduced in twosteps through the first two

rows of moving blades. In the moving blades, velocitydecreases while the pressureremains constant. Four pressure-compoundedstages.

Turbine with

Impulse Blading

18

3 Reaction stages arepreceded by an initialvelocity-compoundedimpulse stage where a largepressure drop occurs.

This results in a shorter, lessexpensive turbine.

In the reaction blading of thisturbine, both pressure andvelocity decrease as thesteam flows through theblades.

The graph at the bottomshows the changes inpressure and velocitythrough the various stages.

Reaction turbine

7/30/2019 Chapter 5 Steam Turbine

10/13

ME 4731 Power Plants

Dr.-Ing. Haitem Hichri

19

Simple Impulse-Reaction Blade

Jet will lose power both by Impulse and Reaction.

One important and essential element in all these cases is a nozzle.

Vai

Vae

Vri

Vre

U

20

Impulse-Reaction turbine

This utilizes the principle of impulse and reaction.

There are a number of rows of moving blades attached to therotor and an equal number of fixed blades attached to thecasing.

The fixed blades are set in a reversed manner compared to themoving blades, and act as nozzles.

The fixed blade channels are of nozzle shape and there is asome drop in pressure accompanied by an increase in velocity.

The fluid then passes over the moving blades and, as in thepure impulse turbine, a force is exerted on the blades by thefluid.

There is further drop in pressure as the fluid passes through themoving blades, since moving blade channels are also of nozzleshape.

The relative velocity increases in the moving blades.

7/30/2019 Chapter 5 Steam Turbine

11/13

ME 4731 Power Plants

Dr.-Ing. Haitem Hichri

21

U

VriVai

VreVae

ii

e

e

p

vavr

The reaction effect is an addition to impulse effect.

The degree of reaction

stagein thedropenthalpyThebladesmovingin thedropenthalpyThe=

22

First law for fixed blades:

0 1 22

2

0

2

101

VVhh

=

First law for moving blades:

2

2

1

2

21

2 rVV

hhr

=

22

2

1

22

0

2

2021 r

VVVVhh r

+

=

22

2

1

22

0

2

120

2 raVVVV

hh r

+

=

7/30/2019 Chapter 5 Steam Turbine

12/13

7/30/2019 Chapter 5 Steam Turbine

13/13

ME 4731 Power Plants

Dr.-Ing. Haitem Hichri

25

Compound Turbines

Compound turbines have more than one cylinder:1. a high-pressure and2. a low-pressure turbine.

The low-pressure cylinder is usually of the double-flow type to handlelarge volumes of low-pressure steam (due to limitations on the length ofthe blades).

Large plants may have an intermediate pressure cylinder and up to fourlow-pressure cylinders.

The cylinders can be mounted along a single shaft (tandem-compound),or in parallel groups with two or more shafts (cross-compound).

Reheating is usually done between the high- and intermediate-pressureturbines.