Turbine, Generator & Auxiliaries - Course 334

8/10/2019 Turbine, Generator & Auxiliaries - Course 334

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334.00-7

Tur bin e, G e ne ra to r

Aux i l i a r i e s

Course 334

THE CONDENSER

In th e

sec t ion

on th e stearn

tu rb ine

we discussed how

th e tu rb ine

conver ted

th e hea t

energy

of the stearn pass ing

through it to th e mecha nic al e ne rg y o f th e ro ta t in g sh af t .

t i s obviously to

o ur ad van tag e

to ex t ra c t as

much

work as

poss ib le from th e sa tu ra ted stearn

t h a t

i s genera ted in th e

stearn

gene ra to r .

In

the

s imples t terms, the lower the temp

e ra tu re and pre ssure a t

th e

ou t l e t

o f

the

low

pre ssure t u r -

bine the

g rea te r w i l l be

the amount

of

energy

which

can

be

ex t rac ted from th e stearn. In f a c t if

th e

exhaus t

of th e

tu rb ine i s near a pe r fec t vacuum ra the r t h a t a t atmospher ic

pres su re roughly

5

more

energy can be

ex t rac ted from th e

stearn pass ing through

the

tu rb ine . t i s

the condenser

which

prov ides

th e

means

o f ma in ta in in g

t h i s

low

a bs olu te p re ss ur e

a t the

exhaus t

of th e low pressure

t u rb ine .

The way

in

which

th e condenser provides t h i s low pre ssure i s th ro ugh c onde ns a

t i on of

th e

stearn i n to

water .

At th e ou t l e t

of

the

low

pressure

tu rb ine a ki logram of

wet steam

occupies cub ic meters . When t h i s steam

condens

es

to water th e one

ki logram

of

water

occupies

.001

cubic

m eters about one qua r t . Thus as one

ki logram

of steam con

denses ,

a lmost

38 cub ic meters

of empty space

which was

prev ious ly

f i l l e d

with steam

i s

l e f t

behind. t i s t h i s

crea t ion of

empty sp ace th ro ug h

th e

condensing

of steam which

provides

the

vacuum

in

th e

condenser .

Figure

7 .1 shows

a

sec t iona l view of

a

condenser . The

cool ing water

which

removes the l a t e n t hea t of vapour iza t ion

and causes th e steam to condense f lows through th e tubes .

This

condenser

cool ing water i s taken from the

lake

or r i ve r

pumped

through th e condenser tubes and then

f

lows back

to

th e lake or r i ve r . The condenser

cool ing

water i n l e t to the

condenser i s known as th e

i n l e t

water box and th e condenser

cool ing water ou t l e t as th e ou t l e t water box.

The type of

condenser

shown in Figure 7.1 i s known as

a

s ing le

pass condenser s ince th e coo ling

water

passes through

th e

condenser only

once.

Double

pass

condensers

are

also

found

in th e Nuclear

Generat ing

Divis ion but only on the

smal le r

un i t s .

The

tubes

a re

supported

a t

e i t he r

end by

tube

shee ts

and along t h e i r l eng th by sagging p la t e s .

In

add i t ion to g iv in g support

to

th e condenser tubes

the

sagging p l a t e s

are spaced

to ensure

t ha t the

tubes

are

damped a g ain st v ib ra ti on

induced

e i t h e r by

resonance with

the

tu rb ine running f requenc ies or by

steam pass ing

over the

March 1984

1


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334.00-7

tubes . The sagging

pla tes

are bolted or welded to the con

denser

she l l

and they cont r ibute to

the

condenser she l l

s t rength When

the

condenser

i s under

fu l l vacuum, the she l l

must

withstand an

externa l

pressure

of

almost

100 kPa.

The

sagging pla tes

h elp co un te rac t th i s pressure force which

is

tending

to

col lapse the condenser she l l Axial

support

i s

g i

ven

to

the

tube

sheets

by

staybars

which

run

pa ra l l e l

to

the

tubes .

These s taybars

tend

to prevent any s ign i f i can t

a xia l s tre ss es from being carr ied by the

tubes.

On

the steam

side of the

condenser,

the steam leaves

the

low pressure

turbine

through

an

exhaust trunk and

enters

the

she l l of

the

condenser where t

passes

around the tubes. The

condensed

water

o r condensate f a l l s in to

the

bottom of the

condenser and i s

col lec ted

in

a

hotwel l .

J

A la rge tu rb ine

uni t

general ly has one condenser

for

each low

pressure turbine

However,

to prevent var ia t ions

in

the

back

pressures of

each condenser, the she l l s

of

the

con

densers

are

joined

by

a large

say

2

metre

diameter

balance

This balance

pipe a lso

al lows a

condenser

ha l f

to be

shutdown

for

cleaning while the

turbine un i t

remains a t

power.

A

Exhaust

Trunk

B

Tubes

C Sagging Pla tes

D

n l e t

Water

Box

E. Tube

Sheet

F.

Out le t Water Box

G

She l l

H

Hotwell

Inspec t ion

Doors

J Staybars

K Expansion

Jo in t s

Single Pass Condenser

Figure

7.1

2


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334 00 7

xl ust Trunk

ir

xtraction

0 0

o

0 0

Steam

o

eating

o

Lane

00( )

0 0 0 ( 1 0

OOGOOOOOO

o

o

0 0

0 ( ) 0000

0 0 0 ,

OOO( )OOOOO

O

rr

c

\ \ . . .

j

Condenser

Cross Sect ion

Figure

7 2

Figure

7 2

shows

a cross

sect ion of

a

t yp i ca l

s ing le

pass

condenser The

exhaust trunk may be

bol ted

or

welded to

the bottom

of

the

turbine

cas ing

l t ernate ly may

be

supported

independently

and

connected

to

the

turbine

by

a

f l ex ib l e bel lows The la t ter method i s more typ i ca l of large

turbine un its s ince al lows the condenser to expand inde-

pendent

of the turbine

There

i s

a large

steam heating

lane or steam

excess

lane

down the cen tre of the condenser

sh e l l

which contains

no

cool ing

water tubes The purpose of th i s lane

i s

to ensure

tha t

a

port ion of the exhaust steam goes

to

the bottom of the

3


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condenser.

to reach

achieved:

334.00-7

By

allowing

a s ign i f ican t percentage of

the

steam

the

bottom

of the

condenser, the following

i s

1. The

water in

the hotw ell is kept a t

sa tura t ion

temper

a ture

I f the

water

in the h otw ell was al lowed to be

come

subcooled,

would be an

unnecessary

loss

of

heat

from the condensate. This unnecessary

loss

of

heat

energy

would make the s team/feedwater

cycle

less e f f i -

c i en t as th is lo s t hea t would have to be put back in to

the

condensate by e i ther the fee dh eatin g system

or

the

steam genera tor .

2.

The lower condenser tubes

are forced

to t ra nsfe r as much

steam

out of

the condenser

as

the upper tubes .

This

en

sures the en t i r e

tube

surface

i s

used

to

t r ans fe r

heat

3.

The

thermal

expansion

of the tubes i s

equal ized.

I f the

lower

tubes were

cooler

than

the upper tubes, i t

would

produce

a

bending

force

on

the

condenser

she l l

and

tube

shee ts

Condenser Air Extract ion

The steam

wi l l

continue to condense and

maintain

a good

vacuum as long as

four

c onditio ns a re fu l f i l l ed :

1.

a i r

enters the she l l steam s ide of the condenser.

2. The

tubes

car ry a

normal flow of

r e l a t ive ly cool lake

water .

3.

The

tubes

r emain expos ed to the steam.

4. The

tube surfaces are

not fouled

by

corrosion

products

or o ther

mater ia ls

ny

change

in the system which inva l ida te s one of these

sta tements

wi l l

re su l t

in

a decrease

in

the vacuum in the

condenser.

Apart

from

a

decrease

in

turbine

eff ic iency a

decreasing

vacuum w il l re su lt

in overheat ing of

the low

pres-

sure turbine blading as the

blades must pass

through higher

densi ty steam than they were designed to encounter .

I f a i r enters the condenser,

most

of

wi

remain

there

because

cannot condense with the

steam.

The a i r

wi l l gradua l ly

f i l l

up the empty space l e f t by the

condensing

steam un t i l

the vacuum

in

the

condenser is destroyed. In

addi t ion some of

the

a i r

which leaks

in to the condenser wi l l

disso lve in

the condensing

water.

The oxygen in

the

a i r can

cause

con sid er ab le c or ro si on problems

in

the condensate

and

feedwater systems. Since the condenser operates below


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334.00-7

atmospher ic pres su re ,

there i s a tendency fo r a i r to

leak

in to

the condenser

and

t h i s a i r must be removed.

The

normal

method

of

s t a r t i ng up a tu rb ine un i t i s

to

evacuate th e condenser

she l l and

turb ine

to a pa r t i a l vacuum

and then

to

evacuate the

remaining a i r

a f t e r ro l l ing

th e

t u r

bine w ith

steam.

Once

the

condenser

i s a t

normal

operat ing

vacuum [about kPa a ] ,

th e a i r

ext rac t ion

system

must re

move only th e small amount of a i r which leaks

in to the

con

denser .

The

a i r

ex t rac t ion system has to be

capable of

deal ing

both with vacuum r a i s ing and normal maintenance of vacuum.

When ra i s ing vacuum, the

a i r ex t rac t ion system has to remove

not

only the a i r

which

f i l l s

the

condenser she l l during shu t

down, but a lso the a i r

in

th e

P and LP

tu rb ines

and pipework

back to the

emergency

sto p valves. The

volume of

a i r which

must

be removed i s

t yp i c a l ly

about

6500

cubic m eters. To

p rev en t ex cessiv e s ta r tup

t ime being

expended in drawing a

vacuum, t h i s

a i r

must

be

removed

in

something

on

the order

of

an

hour .

n the o ther hand, the maximum

amount of a i r

leakage

in to

an operat ing

un i t i s

something l ike one cubic

meter

per

minute. This

requires nowhere near the

a ir

removal

capaci ty t h a t i n i t i a l evacuat ion requ i res .

Whatever

the

method of

a i r

ex t rac t ion , there

wi l l

typ ica l ly be

a

la rge a i r

removal ca pa city fo r i n i t i a l evacuat ion and a smal le r capaci

ty

fo r ma in ta in in g

vacuum.

The

a i r

ext rac t ion

system removes

a i r and other non-con

d en sa ble g ase s from

the

condenser

by e i t he r a steam a i r e jec

to r

or

vacuum

pumps. Whatever

the

method of a i r

removal, the

system

c rea tes

area

of

lower

pressure

than

condenser

vacuum.

Air flows from the

condenser

to th i s low pressure area where

t can be removed. In Figure 6.2 , you can see the

a i r

ex

t rac t ion

po in t s

in the condenser.

They

are located within

the

tube

bundles

to ensure t ha t

gases moving toward these

poin ts must pass over many tubes pr io r

to

removal from the

condenser. The a i r ex t rac t ion system wi l l remove any gas and

would j u s t

as

soon suck

out steam as

air .However,

in

passing

over

the tubes ,

the steam

i s condensed and the a i r

ex t rac t ion

sy stem only removes non-condensable gases .

Steam Air Ejectors

Figure

7.3

shows

a

sec t iona l

view

of

a

t yp ica l

steam

a i r

e jec tor .

Steam a t about 2000 kPa g enters the nozzle where

steam hea t

energy i s conver ted to v elo ci ty . The high veloci

ty

steam

j e t i s d i rec ted i n to a di f fuse r . The d i f fuse r i n l e t

has

a much l a rge r

cross

sec t iona l area

than

t ha t of the high

speed

j e t

enter ing t This

crea tes

an extremely low p re s

sure

a t

the

i n l e t

to the di f fuse r .

This

low pressure

area is

connected

to the condenser. The low

pressure

ai r -vapour mix

tu re

from

the

condenser comes in to p hy sic al c on ta ct with the


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334.00-7

Steam

Air

Ejec to r

F igure 7.3

high speed

j e t

and i s

bat ted

along by

the steam enter ing

the

di f fuser

The a i r

t rave l s through the di f fuse r with the

steam

and i s discharged

to a higher

pressure

region. This

cons tant removal of

low

p re ssu re airv ap ou r mixture a t the

di f fuser

i n l e t

encourages

more

a i r

to

flow

in to

the

d i f fus -

e r This es tab l i shes

a continuous

flow

from

condenser in to

e jec tor

For s ta r tup purposes when

la rge

volumes of

a i r

must be

removed a

high capaci ty

steam

e jec tor ca l led

a hogging

e jec tor

i s used. This high capaci ty e jec tor can evacuate the

condenser and tu rb ine un it to

normal

vacuum in

something

l ike

45 minutes.

The

hogging e jec tor uses l a rge

quant i

t i e s of

steam.

After

passing through the

e jec tor

the

steam

is

re -

leased

to

atmosphere. While th i s improves

the capac ity

of

th e

hogging

e jec tor

makes

the

e jec tor wastefu l of steam.

Thus

while

the hogging e jec tor

i s

des irable

for

rapid vacuum

ra i s ing is

fa r

too

inef f ic ien t

for

normal

operat ion .

During

normal

operat ion a much smal ler steam e jec tor

is

used.

This holding

e jec tor

as

may

be ca l led i s normally

a two

s tage steam

e jec tor

as shown in Figure

7.4 .

When the

a i r e j ec to r i s working

proper ly the

a i r from

the

main con

denser

i s drawn in to

the

f i r s t s tage

The

steam and

a i r

mixture from

the f i r s t s tage

pass in to

an in tercondenser

where most

of

the

steam

i s condensed. The a i r is then drawn


7/12

334.00-7

econd

Stage

jector

Water

Tubes

~ r i n

Vent To

Atmosphere

team

In l e t

Second

Stage I so la t ion

Valve

Drain to

Condenser

F i r s t Stage

s o l t i o n ~ v e

Suction from

c o n d e n s e r ~

F i r s t Stage

jector

Typical Two-Stage Condensing Air

Ejec to r

Figure 7.4

in to a

second

s tage

The

stearn and a i r

mixture

from the

second

s tage

pass

in to

an

aftercondenser

where

the

steam

is

condensed and the a i r

re leased

to atmosphere. The

cooling

water for both

th e

intercondenser and af tercondenser is main

condens at e t aken from the condensate extrac t ion pump pr ior to

the low

pressure

feedheaters . The advantage

of having two

s tages

in

the a i r e jec tor i s

t ha t

the

a i r is

ra i sed from

condenser vacuum to atmospheric pressure in two s teps ra the r

than one. The f i r s t s tage

ra ises

pressures

from

5 kPa a to

about 35 kPa a , while

the

second

s tage

ra i ses

pressure

from

35 kPa a to

normal

atmospheric pressure 101.3 kPa a .

Vacuum Pumps

a l t e rna t ive

method

of

a i r

extrac t ion i s

use

of

vacuum

pumps.

These pumps

are

of a

varie ty

of designs although they

are

normally r ot at in g p o si ti ve displacement pumps. The usua l

conf igurat ion

is

to have more

than

one

pump.

The number of

pumps

running depends on the vacuum

condi t ions .

Picker ing

NGS A fo r example, has th ree screw type pos i t ive disp lace-

ment vacuum

pumps fo r

each condenser. All th ree

pumps

are

used for i n i t i a l vacuum ra i s ing During

normal

operat ion ,

7


8/12

334.00-7

only one pump i s required . However, i f

backpressure r i ses

to

8 kPa a , a second pump wi l l

s t a r t ;

i f the

backpressure

r i ses

to 12 kPa a , the th i rd pump

wil l s t a r t .

Another possib le combination i s

to

use vacuum pumps fo r

normal

vacuum maintenance

and

a hogging

e j ec t o r

fo r i n i t i a l

vacuum

ra i s ing .

There i s rea l ly no c lea rcut advantage of one a i r ext rac -

t ion method

over the

o ther . Vacuum pumps allow eas ie r

on o f f

cont ro l and tend

to

increase p lan t e ff ic ie ncy s ince they

don

t

use steam. However, in p rac tice they requi re

more

maintenance and

are

g en er ally le ss

r e l i ab l e .

Condenser Cooling

Water

The condenser cool ing water system, or system as t

i s most often

ca l led

i s

used

to remove the l a ten t heat of

vapour izat ion

from

the exhaust

steam

enter ing the

condens

e r . The r i s e in

temperature across the condenser i s

l imi ted to about 10C for th ree

reasons:

1. The

higher the temperature r i s e

of

across the con

denser , the

higher

condenser pressure and therefore the

lower cyc le e f f ic iency .

2.

The

h igher the

temperature r i s e

the

grea ter

the

t enden

cy for

a i r

to come out of

solut ion and

co l l ec t in the

high poin ts

of

the

system.

3.

I f

the

water

ente r s

there

warm

outf low.

has re su l ted

r i s e .

leaves the

condenser

much

warmer

than

t

i s a rapid

growth in marine l i fe in

the

The prevention of

t h i s

b io lo g ic p ol uti on

in

l ega l

l imi t s

on

condenser temperature

The

hea t energy which must be removed from

each

kilogram

of exhaust

steam

i s about 2450 KJ. In increas ing i t s temper

a ture

by 10C, a ki logram of water can remove about 4

KJ. Thus t requi res

about

58 kilogram s of water

2450

KJ/42 KJ to condense one kilogram

of

exhaust

steam.

I t re -

qui res about

60 cubic

meters of flow

per

second fo r a

s ingle la rge tu rb in e

uni t .

This extremely l a rge flow ra te

presents

two

problems:

1. an extremely l a rge amount of

pumping

power must be

con

sumed, and

2.

the re

can

be

l i t t l e placed in

the

system fo r

water

t r ea tmen t

and pur i f i ca t i on

which

impedes flow.

8


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334.00 7

For

a la rge NDU

genera t ing

s t a t i on ,

th e req uired

W

pumping power i s on the order o f 4 pe r un i t . To keep t h i s

p um pin g p ow er

within economic l imi t s , the

pumps

are genera l ly

of high capac i ty

and

low discharge

head.

Since th e

condenser

i s

normally

loca ted above th e l eve l of the lake

or

r i ve r ,

apprec iable

pumping

power

may

be expended to ra i se the water

up

to

the

condenser .

Much of

t h i s

power

requirement

can

be

e l imina ted

by opera t ing the

W

system as a s iphon. This

means

the

pump

i s only requi red to overcome the f r i c t i on flow

lo s ses .

When a W

system

i s opera ted as a

siphon the W

s ide

of

the

condenser

operates

below atmospher ic pres su re . As

water

passes

through

the condenser

a i r

which

has

dissolved

in the

W

water comes out of

so lu t ion .

These

gases

tend to

co l l e c t

in th e

condenser tubes and water

boxes

blocking flow

and

des t roying the

siphon. The W system

i s

pro tec ted

ag ain st the a i r

binding of the

tube s ide of

the

condensers by

an a i r

removal system.

This vacuum pr iming system

as it i s

ca l l ed , removes a i r from

the

condenser

water

boxes.

t i s

worth mentioning t ha t

th i s

system i s

complete ly

separa te from

the a i r removal system fo r

the

she l l

s ide

of th e

condenser .

Because of

the

extremely

l a rge

flow ra te of W water ,

the re

i s

littl water

t rea tment

except i n t e rm i t t en t ch lo r ina -

t ion and

t r a sh removal. Water

f i r s t

passes

through a coarse

sc reen. The spacing between bars must be small

enough to

prev en t lo gs ,

boa t s , people

or

ice from being swept i n to

the

W

system.

On

the othe r hand the spacing must

be

wide

enough

to prevent

a

l a rge

quant i ty of sm all f i sh or seaweed

from r ap id ly plugging

the

screen . The c oa rse scre en spacing

i s

genera l ly

2

to 15

cm with

exact

spacing

l a rge ly

determin-

ed

by

the water

condi t ions

near

the

s t a t i on :

the

d i r t i e r

the

water

th e

f i n e r

th e mesh

of

the coarse sc reen.

The

W

flow

next

passes

through a

band screen

or

t r ave l l i ng

screen

as it

i s

a l t e rna t ive ly ca l led . This screen

has

a

f ine mesh typ ica l ly

cm

or

l e s s ) and i s

driven

by a

motor.

When

the

screen accumulates

su f f i c i en t t ra sh

to

ra i se

the d i f f e r en t i a l pres su re ,

th e

motor i s

turned on

and

the

screen

s lowly ro t a t e s to a new

pos i t ion .

The screen i s wash-

ed

with high pressure water

j e t s

near th e

top

of i t s

t r ave l .

The

purpose

of the screen i s

to block any

t ra sh l a rge

enough

to block the tubes .

Some

marine l i f e

may

pass

both

th e

coarse and

t r ave l l i ng

screens and accumulate on

the

condenser

tube

shee ts

and

tube

su r faces . The

warm condi t ions

and

cons tant

renewing

of

wate r

r e su l t in rap id

growth of

th i s

marine

l i f e .

This growth can

r ap id ly

r e su l t

in tu be p lu gg in g.

To r e s t r i c t the

growth

ra te

chlor ine i s in te rm i tte nt ly in je cte d in to the

W

in take . Th e

chlor ine

in je ct io n r ate

must

be low enough to p re ve nt p ois on -

ing

of

the

marine

l i f e in the

W

outf low.


10/12

334.00 7

UHscpn D

WATEI

llItIv

UNIl

r

..

TENtlO.....

>CII WI

'-

>CIIU

~ P ' H I L S

I

: : l

F

-

-

I

I

I

.

I

i

-

..

f:I

.

I

. . ~

.

C1l1fH D

.AUII

Trave l l ing Screen

Figure

7 .5

Regardless of what

methods

are used to r e s t r i c t tube

and

tube shee t fou ling , even tua lly the

tube

shee ts pa r t i cu l a r ly

the i n l e t tube sheet must be c leaned. To f ac i l i t a t e c lean-

ing

of

condenser CCW s ides while

ope ra t ing , the c i rcu la t ing

water

system

of

la rge s t a t i ons

supply

ind iv idua l condenser

ha lves which can be i s o l a t ed

from

each o ther . The ar range

ment

shown

in

Figure 7 .6

al lows

ha l f

of

one

condenser to be

i s o l a t ed and

opened

fo r

cleaning.

The balance piping between

condensers

ensures

the

remaining

f ive

condenser

ha lves are

su f f i c i en t to

keep

the

un i t

a t

power.

10


11/12

334.00-7

Discharge

Line To ommon

Discharge

Channel

I

l l

I

l l

l l

Q

I

Q

Q

I

I

~

Q l l

I

r

Q l l

N

Q

l l

M

0 Q

=l l=

O Q

=l l=

OQ

=l l=

~

~

~

o

~

Al

o

Al

o

~

Al

UO

H

UO

H

UO

H

I

I

uct ion

From ommon

Screen

House

and

hlor ina to r

W System For

ingle

Turbine

Uni t

with Three P

Turbines

Figure 7 .6

11


12/12

334.00-7

SSIGNM NT

1 .

Why does the

LP turbine exhaust

to

a

vacuum

ra ther

than

to atmosphere?

How

i s

th i s vacuum

maintained?

2.

raw a

cross sec t ion of

a typ ica l

condenser showing:

a

exhaust

t runk

b tubes

c

tube

sheets

d i n l e t

and

out le t water boxes

e sagging

pla tes

f s taybars

g

she l l

h

hotwel l

i

expansion

jo in t s

j waterbox inspect ion doors

3. Why

are tu rb ine

uni ts

with more than one

condenser

f i t -

ted with balance pipes between condensers?

4.

Why

a re

the

a i r

e xtra ctio n p oin ts for

the

condenser

she l l surrounded by

tubes?

5.

What

i s

the

purpose

of

a

steam

heat ing

lane .

6. Why

must

a ir be extrac ted from a

condenser

shel l?

From

condenser waterboxes?

7. What

is

a

hogging

ejector?

8.

How

does

a

steam

a i r e jec tor work?

9.

What

provisions

are

made

to

prevent

blockage of condens

er

tubes?

R.O. Schuelke

12

Turbine, Generator & Auxiliaries - Course 334

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Transcript of Turbine, Generator & Auxiliaries - Course 334