8/12/2019 Turbine Part 3

1/21

Lecture 3

Steam Turbine (Part III)

8/12/2019 Turbine Part 3

2/21

Steam Turbine

Axial Thrust Management by Double Flow Turbine Design

In large turbines, managing large axial thrust on rotor is not feasible by

thrust bearing alone

It is therefore common to build double flow HP and LP turbines having

reaction blading

The axial thrust roduced at one end cancels out nearly same axial thrust

develoed in oosite direction due to design

8/12/2019 Turbine Part 3

3/21

Steam Turbine

8/12/2019 Turbine Part 3

4/21

Steam Turbine

Management of axial thrust by the use of:

1 Im!ulse bla"ing

The imulse stage has no ressure dro across the moving blades

The axial thrust is therefore eliminated

# Dummy !iston

"ot commonly used in nuclear installations

Seen in some single flow HP turbine at conventional ower stations

Here the axial thrust on the blade wheels is balanced by the dummy

iston

8/12/2019 Turbine Part 3

5/21

Steam Turbine

Labyrinth packing to limit:Steam leakage

Past balancing

Piston

Balancing

piston

Steam pressure

balance example

HP steam inlet

8/12/2019 Turbine Part 3

6/21

Steam Turbine

Tan"em $om!oun"ing

The steam exands roughly $%% times from the inlet to the exhaust of the

turbine

Therefore in large nuclear ower stations, it may not be feasible to

accommodate large increase in the volume of the steam in only one LPturbine

"ormally one HP turbine exhausts its steam into two or three LP turbines

with the double flow design

In one of the $#% &'e Indian "PP, one double flow HP sulies steam totwo double flow LP turbines

The arrangement of all the turbines in the turbine unit on a common shaft is

(nown as a tandem comounded turbine unit

8/12/2019 Turbine Part 3

7/21

Steam Turbine* Tandem +omounding

Illustration of Tan"em $om!oun"ing

8/12/2019 Turbine Part 3

8/21

Steam Turbine

Im!ulse %s &ea'tion Turbine: $om!arison an" Present Tren"

"early all turbines of resent age are imulse reaction turbine -commonly

called as reaction turbine.

It is established that the efficiency of the turbine deends on enthaly dro

er stage/ 0or reaction stage, this efficiency is efficiency of imulse stage,rovided that enthaly dro er stage is (et small/

2fficiency also deends on the ratio between blade velocity and steam

velocity/ Imulse stage is the most efficient when this ratio is about one half,

while reaction stage is the most efficient when this ratio is nearly one/

8/12/2019 Turbine Part 3

9/21

Steam Turbine

8/12/2019 Turbine Part 3

10/21

Steam Turbine

8/12/2019 Turbine Part 3

11/21

Steam Turbine

Im!ulse %s &ea'tion Turbine: $om!arison an" Present Tren" ('ont")

In view of large turbines, re5uiring high steam velocities and therefore high

velocity ratios, the advantage of reaction stage are obvious excet with

following limitations

This efficiency is offset by the ossible wastage of energy due to assing ofsteam through the clearance saces between the moving blades and the

casing from ustream side of moving blade to downstream without doing

useful wor( in case of reaction turbine

0or a given ercentage of moisture in the steam assing through a stage,

reaction turbines suffer a loss in efficiency almost twice as great as inimulse turbine

8/12/2019 Turbine Part 3

12/21

Steam Turbine

Im!ulse %s &ea'tion Turbine: $om!arison an" Present Tren" ('ont")

6s the reaction turbines have ressure dro across moving blades, the

cumulative force of axial thrust of moving blades in HP turbine will be so

large that the thrust bearing would be extremely large and costly/ Seeing

this, HP turbine are designed as reaction turbine as double flow turbine to

be considered with imulse stages/

Thus, a reaction stage should be located in the low ressure region of

turbine

There is a general rule to use a greater ercentage of imulse on the HP

end and greater ercentage of reaction on the LP end

8/12/2019 Turbine Part 3

13/21

Steam Turbine

ffe't of xhaust Pressure $hange on Turbine x!ansion

6s noted from the 7an(ine cycle, lowering the bac( ressure on turbine

from P! to P3 will increase the wor( done by the area shown hatched

2xhaust ressure deends on the temerature of the available cooling water

&oreover wor( done er unit dro in ressure of steam is more at lower

ressures

2xansion to lower ressure results in increased wetness fraction, which

causes erosion of last stage blades

Increased secific volume at lower exhaust ressure demand greater heightof blades in the last stages of turbine to rovide larger area for the flow of

increased volume of steam

8/12/2019 Turbine Part 3

14/21

Steam Turbine

ffe't of xhaust Pressure on Turbine x!ansion

8/12/2019 Turbine Part 3

15/21

Steam Turbine

The &eheat $y'le

Steam at a given initial temerature is artially exanded and leads to wor( on

turbines

This artially exanded steam at some stages of turbine -rocess e8f. is again

heated

This is done first by bleed steam and then by live steam u to its original

temerature

Process figure indicates temerature rise and figure on slide 3# for reheating

arrangement

The reheated steam is then exanded in the remaining stages of the turbine

-rocess g8h. before being condensed

The reheat cycle incororates an imrovement in thermal efficiency and reduces

moisture content at exhaust

8/12/2019 Turbine Part 3

16/21

Steam Turbine

The &eheat $y'le

8/12/2019 Turbine Part 3

17/21

Steam Turbine

&egenerati%e Fee" *eating

Steam is extracted at intermediate stages

This is to heat feedwater in feedwater heaters before sending to boiler

Higher thermal and cycle efficiency due to such bleed steam releasing all its heat in

feedwater

It is universally used in central ower generating station

6 small loss of wor( is exected from the bleed steam not exanding in the turbine

This loss is outweighed by*

9ain in cycle efficiency

:uantity of exhaust steam is less

Si;e of condenser decreases

8/12/2019 Turbine Part 3

18/21

Steam Turbine

&egenerati%e Fee" *eating

8/12/2019 Turbine Part 3

19/21

Steam Turbine

ffe't of Fee" *eating

It may be seen that feed heating has raised the feedwater temerature from

T) to T1 in T8S diagram

So, the boiler needs to increase the temerature from T1 to T! before

steam roduction begins

Thus, the contribution of heat addition from boiler side becomes less,

leading to cycle efficiency imrovement

0eedwater tyically enters a steam generator of "PP at around 1$

8/12/2019 Turbine Part 3

20/21

Steam Turbine

ffe't of Fee" *eating ('ont")

+ycle efficiency is also better as the extraction steam leaving the feed

heater and the feedwater leaving the same feed heater is tyically $

8/12/2019 Turbine Part 3

21/21

Steam Turbine

ffe't of Fee" *eating