Trends in steam generator designs for electric utility and indus trial applications

Texto completo

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Facultad de Ingeniería Mecánica y Eléctrica

de la U. N . L.

ASOCIACION MEXICANA DE INGENIOS MECANICOS Y ELECTRICISTAS, A. C

S E M I N A R I O D E I N G . M E C A N I C A

P o n e n c i a :

TRENDS IN STEAM GENERATOR DESIGNS FOR ELECTRIC

UTILITY AND INDUSTRIAL APPLICATIONS

12 7 9

3

o n t e r t e y , N . L A g o s t o d e 1 9 6 7

P r e s e n t a d a p o r :

W . W . S C H R O E D T E R

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N ú m . o í a s _ ^ ú m . A u t o r Ñ ú m . A d g . P r o c e d e n c i a P r e c i o

¿LÁ^ifcU b a s i f i c ó .

Catalogó

Facultad de Ingeniería Mecánica

y

Eléctrica

de la U . N . L.

ASOCIACION MEXICANA DE I N M P O S MECANICOS Y ELECTRICISTAS, A. C.

S E M I N A R I O D E I N G . M E C A N I C A

P o n e n c i a :

TRENDS IN STEAM GENERATOR DESIGNS FOR ELECTRIC

UTILITY AND INDUSTRIAL APPLICATIONS

M o n t e r r e y , N . L . A g o s t o d e 1 9 6 7 .

BIBLIOTECA UNIVERSITARIA "ALFONSO REYES"

.Oupñla Alt risina VS&Hoteca Universitaria

5 1 2 2 9

' r • fONOOUWVattlTAWft P r e s e n t a d a p o

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TRENDS I N STEAM GENERATOR D E S I G N S FOR ELECTRIC

UTILITY A N D INDUSTRIAL A P P L I C A T I O N S

The tremendous growth of e l e c t r i c power during t h e last two d e c a d e s , has

c r e a t e d a demand for steam g e n e r a t o r s of i n c r e a s i n g l y larger inputs a t i n c r e a s i n g l y

h i g h e r p r e s s u r e s . It has b e e n t h e successful m a n i p u l a t i o n a n d c o n t r o l of the t w o

d i s t i n c t processes of h e a t l i b e r a t i o n from f u e l and h e a t r e c o v e r y f o r steam g e n e r a t i o n

in large unit sizes w h i c h , in c o n j u n c t i o n w i t h larger t u r b i n e s , o t h e r p l a n t e q u i p m e n t

a n d d i s t r i b u t i o n systems, h a v e m a d e possible t h e r e m a r k a b l e f e a t of m o d e m , low

cost g e n e r a t i o n of e l e c t r i c p o w e r .

Among t h e c o n t r i b u t i o n s t o t h e d e v e l o p m e n t of low cost steam g e n e r a t i o n

p r o b a b l y n o n e has b e e n more u n i v e r s a l l y o b s e r v a b l e t h a n t h e d r u m - t y p e b o i l e r . This

is t h e oldest a n d y e t a l s o t h e newest t y p e of steam g e n e r a t o r . Of p a r t i c u l a r n o t e is

t h e c o n t r i b u t i o n m a d e by this c o m p a n y of t h e C o n t r o l l e d C i r c u l a t i o n design for high

s u b c r i t i c a l p r e s s u r e s . ilBti

"ilUMMA BtEtfS?

C o n t r o l l e d C i r c u l a t i o n i a * . l M

The r e l i a b i l i t y , a v a i l a b i l i t y , a n d s a f e o p e r a t i o n of C o n t r o l l e d C i r c u l a t i o n

boilers h a v e j u s t i f i e d t h e i r s e l e c t i o n in p o w e r plants t h r o u g h o u t t h e world as d e m o n s

-t r a -t e d by -t h e r e c o r d s . During -t h e pas-t 10 y e a r s C - E has sold 311 u -t i l i -t y b o i l e r s w i -t h

a t o t a l c a p a c i t y of 5 6 , 3 7 0 M w . Of t h e s e , 114 w e r e of t h e C o n t r o l l e d C i r c u l a t i o n

design with a t o t a l c a p a c i t y of 3 4 , 1 0 5 M w . w h i c h r e p r e s e n t s 6 0 p e r c e n t on a Mw

b a s i s .

r

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in t h e steam g e n e r a t i o n process a t s u b c r i t i c a l p r e s s u r e . The common c h a r a c t e r i s t i c

of a l l such boilers is t h e p r e s e n c e of t h r e e basic h e a t r e c o v e r y s u r f a c e s t h e e c o n o

-m i z e r , t h e e v a p o r a t o r a n d t h e s u p e r h e a t e r . For a l l of t h e s e s e c t i o n s w e -may d e f i n e

c i r c u l a t i o n as the movement of w a t e r or s t e a m , or a mixture of b o t h , through h e a t e d

t u b e s . C i r c u l a t i o n must be a d e q u a t e to a b s o r b h e a t from t h e tube m e t a l a t a r a t e t o

k e e p t h e tube t e m p e r a t u r e a t or b e l o w its design t e m p e r a t u r e . C i r c u l a t i o n should

a l s o k e e p t h e tube w i t h i n o t h e r physical a n d c h e m i c a l l i m i t a t i o n s required by the

inside a n d outside e n v i r o n m e n t . In c o n t r a s t t o t h e c i r c u l a t i o n in e c o n o m i z e r s and

s u p e r h e a t e r s , t h e c i r c u l a t i o n in e v a p o r a t o r s , w h i c h in modern high pressure units

a r e u t i l i z e d as f u r n a c e wall systems, may h a v e t h e t y p e of c i r c u l a t i o n i n v o l v i n g o n l y

t h e f l o w e n t e r i n g a n d l e a v i n g , known as t h e o n c e - t h r o u g h f l o w , and a l s o t h a t w i c h 1

involves r e c i r c u l a t i o n a f a f l o w g r e a t e r t h a n t h e t h r o u g h p u t of t h e steam g e n e r a t o r .

The n a t u r a l c i r c u l a t i o n a n d t h e C o n t r o l l e d C i r c u l a t i o n units a r e i n c l u d e d in

t h e last g r o u p a n d share t h e f e a t u r e of a steam drum as a c h a r a c t e r i s t i c of s u b c r i t i c a l

r e c i r c u l a t i n g u n i t s .

For pressures u p to t h e 2 0 0 0 psig l e v e l , f u r n a c e w a l l systems w i t h n a t u r a l

c i r c u l a t i o n b o i l e r s a r e g e n e r a l l y a c c e p t e d as t h e best t e c h n i c a l a n d e c o n o m i c a l

a n s w e r to t h e various requirements f o r steam g e n e r a t i o n .

N a t u r a l c i r c u l a t i o n boilers e m p l o y t h e e f f e c t of t h e s a t u r a t e d d e n s i t y d i f f e r e j i

t i a l b e t w e e n w a t e r a n d mixtures of w a t e r a n d steam t o promote c i r c u l a t i o n .

C o n t r o l l e d C i r c u l a t i o n is a r e c i r c u l a t i n g system a t s u b c r i t i c a l pressures in w i c h

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p r o d u c e d by c o n s t a n t - s p e e d c i r c u l a t i n g pumps w i t h i n the down comer p i p i n g . A

comparison of n a t u r a l a n d C o n t r o l l e d C i r c u l a t i o n for a f u r n a c e wall system is shown

s c h e m a t i c a l l y in F i g . 1 .

It c a n be seen t h a t the most obvious d i f f e r e n c e b e t w e e n C o n t r o l l e d a n d n a

-tural c i r c u l a t i o n is in t h e i n t r o d u c t i o n of a c i r c u l a t i n g pump b e t w e e n t h e d o w n c o m e r

s y s t e m a n d t h e steam g e n e r a t i n g s u r f a c e a n d t h e i n t r o d u c t i o n of o r i f i c e s . The c i r c u

-lation through e c o n o m i z e r , s u p e r h e a t e r a n d r e h e a t e r is t h e same for both t y p e s .

The basic e l e m e n t s of a f u r n a c e w a l l system in a C o n t r o l l e d C i r c u l a t i o n b o i l e r

a r e shown in F i g . 2 . The system employs a drum w h i c h r e c e i v e s a mixture of steam a n d

w a t e r from t h e steam g e n e r a t i n g tubes a n d f e e d w a t e r from t h e e c o n o m i z e r . Steam drum

i n t e r n a l s s e p a r a t e t h e steam from t h e e x c e s s b o i l e r w a t e r . The s a t u r a t e d steam c o n t a i n i n g

a minimum of impurities d i s c h a r g e s from t h e top of t h e steam drum t o t h e s u p e r h e a t e r

portion of t h e u n i t . The s e p a r a t e d e x c e s s w a t e r mixes with t h e f e e d w a t e r in t h e steam

drum and is r e t u r n e d t o t h e f u r n a c e w a l l by t h e c i r c u l a t i n g p u m p .

A l l f u r n a c e w a l l tubes a r e a r r a n g e d in p a r a l l e l a n d in a s i n g l e upward p a s s .

They a r e f e d from w a t e r w a l l i n l e t h e a d e r s a t t h e lower e n d t e r m i n a t e in w a t e r w a l l

o u t l e t h e a d e r s a t t h e u p p e r e n d . From t h e r e t h e s t e a m - w a t e r mixture is r e l i e v e d t o t h #

steam d r u m .

With a design c i r c u l a t i n g r a t i o of 4 t o 1 , the system provides an a v e r a g e 3 lbs

of r e c i r c u l a t e d f l u i d for e v e r y pound of f l u i d e n t e r i n g a n d l e a v i n g t h e b o i l e r . This

f l o w is d i s t r i b u t e d a n d s t a b i l i z e d by a system of o r i f i c e s in t h e lower w a t e r - w a l l h e a d e r s

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Because the r e c i r c u l a t i n g system is i n t e g r a l w i t h t h e steam g e n e r a t o r a n d

i n d e p e n d e n t of o t h e r p l a n t e q u i p m e n t , t h e p r o t e c t i o n it a f f o r d s to t h e f u r n a c e - w a l l

system is present in f u l l f o r c e not only during normal o p e r a t i o n o v e r load r a n g e , but

a l s o f o r s t a r t u p , e x t r e m e l y low l o a d s , and s h u t d o w n . The f u r n a c e - w a l l system,

t h e r e f o r e , does not r e q u i r e a bypass system a n d is immune from f a i l u r e s d u e t o c o n

-trols a n d i n t e r l o c k s a s s o c i a t e d w i t h such systems. The r e c i r c u l a t i n g system is

i n d e p e n d e n t of pressure a b o v e a minimum a s d e t e r m i n e d by load a n d w i l l m a i n t a i n

c i r c u l a t i o n for l o w - a n d h i g h - p r e s s u r e o p e r a t i o n during s t e a d y - s t a t e as w e l l as

t r a n s i e n t c o n d i t i o n s »

mm

w m m m w

Basic Design P r i n c i p l e s T*2S r ^ .

P r o t e c t i o n of t u b e s a g a i n s t f a i l u r e through o v e r h e a t i n g is a c h i e v e d , t h e n ,

by t h e s e p r i n c i p l e s :

1) R e c i r c u l a t i o n of w a t e r a t t h e r a t i o of 4 t o 1 , designs i n t o t h e system a large margin of s a f e t y for a l l system c i r c u i t s u n d e r a n y o p e r a t i n g c o n -d i t i o n o

2) Low steam q u a l i t y present o v e r t h e f u l l length of a l l tubes w h i c h a r e sized for a mass f l o w a d e q u a t e for a l l f l u x r a t e s , assures n u c l e a t e b o i l i n g c o n d i t i o n s with the bulk f l u i d t e m p e r a t u r e a t s a t u r a t i o n level

in e v e r y t u b e a t a n y e l e v a t i o n .

3 ) A system of o r i f i c e s e s t a b l i s h e s and m a i n t a i n s t h e s e s a f e t y f e a t u r e s by (a) Distributing t h e t o t a l f l o w through a l l t u b e s in r e l a t i o n t o t h e h e a t p i c k u p of i n d i v i d u a l c i r c u i t s or groups of c i r c u i t s , with t h e possibility of easy r e a d j u s t m e n t if required a n d , (b) S t a b i l i z i n g t h e f l o w under t r a n s i e n t f l u x c o n d i t i o n s by t h e n a t u r e of i n l e t o r i f i c e s w i t h as much pressure drop as is present in t h e h e a t e d c i r c u i t .

With t h e s e p r i n c i p l e s , t h e design is a b l e t o control the m e t a l t e m p e r a t u r e

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Because the r e c i r c u l a t i n g system is i n t e g r a l w i t h t h e steam g e n e r a t o r a n d

i n d e p e n d e n t of o t h e r p l a n t e q u i p m e n t , t h e p r o t e c t i o n it a f f o r d s to t h e f u r n a c e - w a l l

system is present in f u l l f o r c e not only during normal o p e r a t i o n o v e r load r a n g e , but

a l s o f o r s t a r t u p , e x t r e m e l y low l o a d s , and s h u t d o w n . The f u r n a c e - w a l l system,

t h e r e f o r e , does not r e q u i r e a bypass system a n d is immune from f a i l u r e s d u e t o c o n

-trols a n d i n t e r l o c k s a s s o c i a t e d w i t h such systems. The r e c i r c u l a t i n g system is

i n d e p e n d e n t of pressure a b o v e a minimum a s d e t e r m i n e d by load a n d w i l l m a i n t a i n

c i r c u l a t i o n for l o w - a n d h i g h - p r e s s u r e o p e r a t i o n during s t e a d y - s t a t e as w e l l as

t r a n s i e n t c o n d i t i o n s .

mm

w m m m w

Basic Design P r i n c i p l e s T*2S r ^ .

P r o t e c t i o n of t u b e s a g a i n s t f a i l u r e through o v e r h e a t i n g is a c h i e v e d , t h e n ,

by t h e s e p r i n c i p l e s :

1) R e c i r c u l a t i o n of w a t e r a t t h e r a t i o of 4 t o 1 , designs i n t o t h e system a large margin of s a f e t y for a l l system c i r c u i t s u n d e r a n y o p e r a t i n g c o n -d i t i o n .

2) Low steam q u a l i t y present o v e r t h e f u l l length of a l l tubes w h i c h a r e sized for a mass f l o w a d e q u a t e for a l l f l u x r a t e s , assures n u c l e a t e b o i l i n g c o n d i t i o n s with the bulk f l u i d t e m p e r a t u r e a t s a t u r a t i o n level

in e v e r y t u b e a t a n y e l e v a t i o n .

3 ) A system of o r i f i c e s e s t a b l i s h e s and m a i n t a i n s t h e s e s a f e t y f e a t u r e s by (a) Distributing t h e t o t a l f l o w through a l l t u b e s in r e l a t i o n t o t h e h e a t p i c k u p of i n d i v i d u a l c i r c u i t s or groups of c i r c u i t s , with t h e possibility of easy r e a d j u s t m e n t if required a n d , (b) S t a b i l i z i n g t h e f l o w under t r a n s i e n t f l u x c o n d i t i o n s by t h e n a t u r e of i n l e t o r i f i c e s w i t h as much pressure drop as is present in t h e h e a t e d c i r c u i t .

With t h e s e p r i n c i p l e s , t h e design is a b l e t o control the m e t a l t e m p e r a t u r e

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The t e m p e r a t u r e d i s t r i b u t i o n for a w e l d e d t u b e p a n e l with h e a t f l u x on t h e

f u r n a c e side is shown in F i g . 3 , isotherms a r e p l o t t e d on a t y p i c a l t u b e c o n f i g u r a t i o n

f o r a c o n s t a n t f l u x w i t h d i f f e r e n t inside film c o e f f i c i e n t s . W i t h i n c r e a s e d inside r e s i s

-t a n c e -t o -t h e f l o w of h e a -t , -t h e -t e m p e r a -t u r e level in -t h e crown region i n c r e a s e s , a n d

more h e a t flows towards t h e b a c k s i d e of t h e p a n e l . The d i s t r i b u t i o n a l s o shows t h a t

t h e thermal loading of the tube by t h e h e a t f l o w from t h e fin a l w a y s remains b e l o w

t h a t e x p e r i e n c e d by the crown of t h e t u b e .

Because t h e r e c i r c u l a t i n g system e s t a b l i s h e s a minimum inlet v e l o c i t y f o r a l l

tubes u n d e r a n y o p e r a t i n g c o n d i t i o n s , it p r o t e c t s t h e tubes a g a i n s t t h e d a n g e r of

o v e r h e a t i n g « v e n a t c o m p a r a t i v e l y low f l u x rates when t h e film c o e f f i c e n t d e p e n d s

on s u b c o o l e d w a t e r f l o w w i t h o u t b o i l i n g .

W h i l e t h e r e is h e a t transfer by film c o n d u c t a n c e of s u b c o o l e d w a t e r a t c o m

-p a r a t i v e l y low f l u x n e a r t h e tube i n l e t s in t h e lower -portion of t h e f u r n a c e , t h e

bulk of t h e h e a t a b s o r b e d in t h e f u r n a c e is t r a n s m i t t e d from t h e t u b e t o t h e f l u i d

by n u c l e a t e b o i l i n g .

The s t a b l e and high film c o e f f i c i e n t w h i c h is p r o d u c e d by n u c l e a t e b o i l i n g ,

is e s t a b l i s h e d by a s t e a m - w a t e r mixture of s u f f i c i e n t l y low q u a l i t y - a mean of 2 5

p e r c e n t a t t h e e v a p o r a t o r o u t l e t - a n d a mass f l o w w h o l l y a d e q u a t e for e v e n t h e

highest a n t i c i p a t e d f u r n a c e f l u x r a t e s . A l a r g e number of t h e t u b e m e t a l m e a s u r e

-ments on C o n t r o l l e d C i r c u l a t i o n units a n d e x t e n s i v e l a b o r a t o r y tests w i t h f u r n a c e

tubes m o d e l e d in a f u l l s i z e loop confirm t h e p r e d i c t i o n s based on p r e s e n t k n o w

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Sfit to >Hud

a at mm sun yd

t e m p e r a t u r e c o n t r o l e x e r t e d by a s s u r a n c e of proper c o o l i n g , t h e thermal loading

u n d e r a n y f u r n a c e c o n d i t i o n is w e l l w i t h i n t h e e l a s t i c range of t h e m a t e r i a l end

c o m p l e t e l y w i t h o u t i n f l u e n c e on f a t i g u e in case of c y c l i n g *

Drum I n t e r n a l s

The drum i n t e r n a l s a r e d e v i c e s used t o s e p a r a t e w a t e r from steam a n d t o

d i r e c t the f l o w of w a t e r a n d steam in a m a n n e r so a s to o b t a i n an optimum d i s t r i b u

-tion of drum m e t a l t e m p e r a t u r e in b o i l e r o p e r a t i o n .

The drum i n t e r n a l s f o r a C o n t r o l l e d C i r c u l a t i o n u n i t a r e shown in F i g . 4

w i t h c h a r a c t e r i s t i c a r r a n g e m e n t of primary a n d s e c o n d a r y s e p a r a t o r s a s w e l l as t h e

f i n a l dryers« F e e d w a t e r is i n t r o d u c e d b e l o w t h e w a t e r level a n d in this m a n n e r

mixes w i t h t h e r e c i r c u l a t e d , s a t u r a t e d w a t e r t o p r o d u c e t h e s u b c o o l e d f l o w t o t h e

c i r c u l a t i n g pumps«

O n e d i s t i n c t a d v a n t a g e of t h e C o n t r o l l e d C i r c u l a t i o n design is t h e i n t e r n a l

shrouding in t h e d r u m . This w a t e r - t i g h t shrouding d i r e c t s t h e return f l o w of steam

a n d w a t e r from the f u r n a c e a r o u n d t h e inside s u r f a c e of t h e d r u m , providing for

uniform h e a t i n g or c o o l i n g of t h e drum a n d drum h e a d s .

Because of t h e i m p o r t a n c e of o r i f i c e s t o t h e p r o t e c t i o n of t h e f u r n a c e - w a l l

system, c a r e is t a k e n t o p r e v e n t t h e i r p l u g g i n g by f o r e i g n m a t e r i a l . The o r i f i c e s

w h i c h a r e made of a u s t e n i t i c s t e e l a r e l o c a t e d in t h e i n l e t h e a d e r s a n d p r o t e c t e d

by s c r e e n p l a t e s e x t e n d i n g a l o n g t h e e n t i r e length of t h e h e a d e r . The 3 / 1 6 - i n .

o p e n i n g s in t h e s e s c r e e n s a r e s m a l l e r t h a n t h e smallest o r i f i c e d i a used in C o n t r o l l e d

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o r i f i c e s (in the order of 2 0 t o 3 0 f t per s e c o n d ) a l s o p r e v e n t s smaller p a r t i c l e s from

c l o g g i n g t h e o r i f i c e p r o p e r .

E x p e r i e n c e w i t h t h e very large number of C o n t r o l l e d C i r c u l a t i o n units in

o p e r a t i o n u n d e r t h e w i d e s t range of power p l a n t o p e r a t i n g c o n d i t i o n s has shown t h a t

c h e m i c a l b u i l d u p in o r i f i c e s does not o c c u r if proper f e e d w a t e r t r e a t m e n t is f o l l o w e d .

In a d d i t i o n , t h e d i f f e r e n t i a l h e a d across the pump w h i c h is measured a n d

monitored a c t s as a r e l i a b l e a n d c l e a r l y r e c o g n i z a b l e signal c o n c e r n i n g t h e a b s e n c e

or p r e s e n c e of a n y d e p o s i t i o n in t h e o r i f i c e or h e a t i n g s u r f a c e . If d e p o s i t i o n does

o c c u r , this d i f f e r e n t i a l w i l l g r a d u a l l y i n c r e a s e a n d i n d i c a t e t h e n e e d for a r e m e d i a l

a c t i o n long b e f o r e t h e tube c i r c u i t s t h e m s e l v e s f a i l .

C i r c u l a t i n g Pumps

Because c i r c u l a t i n g pumps a r e t h e key e l e m e n t in t h e f u r n a c e - w a l l system

a n d a r e e s s e n t i a l to t u b e p r o t e c t i o n under a l l o p e r a t i n g c o n d i t i o n s , a m u l t i p l e pump

a r r a n g e m e n t with shutoff v a l v i n g for e a c h pump is p r o v i d e d . This in itself insures

t h a t a v a i l a b i l i t y is not impaired by possible f a i l u r e of a n y i n d i v i d u a l p u m p .

The number of pumps s e l e c t e d for a n y system may v a r y b e t w e e n two a n d

f o u r . Because e x p e r i e n c e w i t h C o n t r o l l e d C i r c u l a t i o n units has e s t a b l i s h e d t h e

r e l i a b i l i t y of modern c i r c u l a t i n g pumps, spare pumps a r e not normally p r o v i d e d .

Each i n s t a l l e d pump c a n be isolated by a m o t o r o p e r a t e d s u c t i o n v a l v e a n d a d i s

-c h a r g e s t o p - -c h e -c k v a l v e . In -c a s e of f a i l u r e , a pump may be isolated for a

s u f f i c i e n t length of time to e f f e c t repairs w i t h o u t impairing t h e f u l l - l o a d c a p a c i t y

of t h e u n i t . It is recommended t h a t f o r o v e r a l l s a f e t y of t h e f u r n a c e - w a l l system,

BIBLIOTECA UNIVERSITARIA 0 5 9 3 4 6

(14)

o r i f i c e s (in the order of 2 0 t o 3 0 f t per s e c o n d ) a l s o p r e v e n t s smaller p a r t i c l e s from

c l o g g i n g t h e o r i f i c e p r o p e r .

E x p e r i e n c e w i t h t h e very large number of C o n t r o l l e d C i r c u l a t i o n units in

o p e r a t i o n u n d e r t h e w i d e s t range of power p l a n t o p e r a t i n g c o n d i t i o n s has shown t h a t

c h e m i c a l b u i l d u p in o r i f i c e s does not o c c u r if proper f e e d w a t e r t r e a t m e n t is f o l l o w e d .

In a d d i t i o n , t h e d i f f e r e n t i a l h e a d across the pump w h i c h is measured a n d

monitored a c t s as a r e l i a b l e a n d c l e a r l y r e c o g n i z a b l e signal c o n c e r n i n g t h e a b s e n c e

or p r e s e n c e of a n y d e p o s i t i o n in t h e o r i f i c e or h e a t i n g s u r f a c e . If d e p o s i t i o n does

o c c u r , this d i f f e r e n t i a l w i l l g r a d u a l l y i n c r e a s e a n d i n d i c a t e t h e n e e d for a r e m e d i a l

a c t i o n long b e f o r e t h e tube c i r c u i t s t h e m s e l v e s f a i l .

C i r c u l a t i n g Pumps

Because c i r c u l a t i n g pumps a r e t h e key e l e m e n t in t h e f u r n a c e - w a l l system

a n d a r e e s s e n t i a l to t u b e p r o t e c t i o n under a l l o p e r a t i n g c o n d i t i o n s , a m u l t i p l e pump

a r r a n g e m e n t with shutoff v a l v i n g for e a c h pump is p r o v i d e d . This in itself insures

t h a t a v a i l a b i l i t y is not impaired by possible f a i l u r e of a n y i n d i v i d u a l p u m p .

The number of pumps s e l e c t e d for a n y system may v a r y b e t w e e n two a n d

f o u r . Because e x p e r i e n c e w i t h C o n t r o l l e d C i r c u l a t i o n units has e s t a b l i s h e d t h e

r e l i a b i l i t y of modern c i r c u l a t i n g pumps, spare pumps a r e not normally p r o v i d e d .

Each i n s t a l l e d pump c a n be isolated by a m o t o r o p e r a t e d s u c t i o n v a l v e a n d a d i s

-c h a r g e s t o p - -c h e -c k v a l v e . In -c a s e of f a i l u r e , a pump may be isolated for a

s u f f i c i e n t length of time to e f f e c t repairs w i t h o u t impairing t h e f u l l - l o a d c a p a c i t y

of t h e u n i t . It is recommended t h a t f o r o v e r a l l s a f e t y of t h e f u r n a c e - w a l l system,

BIBLIOTECA UNIVERSITARIA 0 5 9 3 4 6

(15)

* »

the repaired pump be returned to service as soon as p o s s i b l e . It is a l s o recommended

t h a t a spare rotor assembly be a v a i l a b l e a t the p l a n t .

F u r n a c e - w a l l - s y s t e m p r o t e c t i o n by t h e c i r c u l a t i n g pump is f u r t h e r insured by

various interlocks a n d o t h e r p r o t e c t i v e d e v i c e s » The d e t a i l s of these depend on the

p a r t i c u l a r tupe of pump i n s t a l l e d . A d e v i c e common t o all C o n t r o l l e d C i r c u l a t i o n

units is a firing interlock w h i c h will not permit h e a t input w i t h o u t having the minimum

number of pumps in o p e r a t i o n . This is e v i d e n c e d by the measured pressure d i f f e r e n t i a l

b e t w e e n the pump suction h e a d e r a n d the wall i n l e t h e a d e r . This a l s o provides a

signal on loss of a n y p u m p .

W e l d e d Walls

The w e l d e d w a l l d e s i g n , F i g . 5 , has proven p r a c t i c a l in f a b r i c a t i n g panels

in large q u a n t i t i e s , with c o m m e r c i a l l y a v a i l a b l e t u b i n g . The panels have proven

t h e i r long-term r e l i a b i l i t y , under p r a c t i c a l l y a l l possible o p e r a t i n g c o n d i t i o n s .

E x p e r i e n c e a n d d e v e l o p m e n t h a v e permitted d e s i g n , support, a n d e r e c t i o n t e c h n i q u e s

to grow simultaneously with the rapidly increasing s i z e a n d pressure r e q u i r e m e n t s .

With increased use of pressurized f i r i n g and b e c a u s e of design simplification

as w e l l a s e a s i e r e r e c t i o n p r o c e d u r e s , t h e past d e c a d e has seen a continuous increase

in t h e use of w e l d e d f u r n a c e w a l l s until t o d a y it has b e c o m e a standard C - E component

for a l l u t i l i t y c o n t r a c t s ( F i g . 6 ) .

Typical 3 7 0 Mw C o n t r o l l e d C i r c u l a t i o n Unit

(16)

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g e n e r a t o r p u r c h a s e d in t h e e a r l y 1 9 6 0 ' s . It is d e s i g n e d for 2 . 5 m i l l i o n Ibs/hr of

steam a t 2 5 0 0 psig a n d 1005 F w i t h r e h e a t of 1005 F . The f u r n a c e is 5 5 f t w i d e ,

w i t h a division w a l l a t t h e c e n t e r a n d a c o m p l e t e t a n g e n t i a l f i r i n g c i r c l e in e a c h

h a l f . The s u p e r h e a t e r has a h o r i z o n t a l s e c t i o n , i n t e r m e d i a t e p l a t e n a n d finishing

p e n d a n t s e c t i o n s w h i l e t h e r e h e a t e r is composed of a h o r i z o n t a l a n d a finishing

p e n d a n t s e c t i o n . The f u r n a c e - w a l l system is f a b r i c a t e d u t i l i z i n g fusion w e l d e d

p a n e l c o n s t r u c t i o n consisting of 1 3 / 4 - i n . dia t u b e s on 2 1 / 8 - i n . c e n t e r s . The

f u r n a c e is a gas t i g h t e n c l o s u r e i n c l u d i n g t h e roof w h e r e w e l d e d seals a r e provided

for e n t r a n c e of s u p e r h e a t e r a n d r e h e a t e r e l e m e n t s .

Industrial Boilers

In t h e industrial b o i l e r f i e l d , a v a r i e t y of drum t u p e b o i l e r s a r e b e i n g

m a n u f a c t u r e d in t h e U n i t e d S t a t e s . The i n c r e a s i n g l y p o p u l a r shop assembled or

p a c k a g e t u p e h a v e now r e a c h e d steam c a p a c i t i e s of o v e r 2 0 0 , 0 0 0 I b s / h r . They

a r e b e i n g used w h e r e previously only c o n v e n t i o n a l f i e l d e r e c t e d boilers w e r e

a v a i l a b l e . Mass p r o d u c t i o n methods a n d o t h e r e c o n o m i c s of f a c t o r y assembly h a v e

brought costs a n d e r e c t i o n time of such b o i l e r s down well below t h a t of c o m p a r a b l e

c a p a c i t y f i e l d e r e c t e d t u p e s .

O u r e x p e r i e n c e with t h e s h o p - a s s e m b l e d boilers brought a b o u t our m o d u l a r

c o n c e p t of p a r t i a l shop assembly a n d s t a n d a r d i z a t i o n of c o m p o n e n t parts f o r t h e

larger f i e l d e r e c t e d industrial b o i l e r s .

M o d u l a r Design C o n c e p t

(17)

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various building blocks or m o d u l e s . Each module must perform its intended f u n c t i o n

within a d e f i n e d d e g r e e of f l e x i b i l i t y and must be c a p a b l e of being f i t t e d to a d j a c e n t

modules t o foim a n i n t e g r a t e d final p r o d u c t . The main purpose of applying this design

c o n c e p t is, of c o u r s e , to produce a b e t t e r product a t lower c o s t . With a properly

c o n c e i v e d a n d e x e c u t e d program, b e t t e r q u a l i t y and lower cost a r e c o m p a t i b l e .

For this modular design c o n c e p t o , w e l d e d - w a l l construction was i d e a l l y suited

b e c a u s e it a l l o w e d t h e f a b r i c a t i o n of large tube panels in t h e s h o p . Backed u p by t h e

necessary b u c k s t a y s , the w e l d e d w a l l , b e c a u s e of its c o n t i n u i t y , provides structural

strength f o r support of the b o i l e r and also a t i g h t gas e n c l o s u r e for pressurized firing

of f u e l . It e l i m i n a t e s t h e need f o r , and a t t e n d a n t problems w i t h , a pressure c a s i n g .

Welding t h e modular i n l e t a n d o u t l e t headers t o the sidewall tube p a n e l s in t h e shop

minimized t h e f i e l d w e l d i n g to a limited number of large dia f e e d e r a n d relief t u b e s ,

completing this portion of the c i r c u l a t i o n s y s t e m . Fins on e d g e tubes of t h e individual

shop-assembled p a n e l s a r e w e l d e d t o g e t h e r in the f i e l d t o c o m p l e t e a n integral

pressure-tight e n v e l o p e around not only the f u r n a c e , but the b o i l e r s e c t i o n a s w e l l .

This g a s t i g h t e n v e l o p e , made up of tubes a n d f i n s , o p e r a t e s a t a saturation t e m p e r a

-ture corresponding to the o p e r a t i n g pressure of the b o i l e r . This t e m p e r a t u r e is w e l l

a b o v e the d e w p d i n t o f t h e corrosive gases in t h e products of c o m b u s t i o n , insulation

on the w e l d e d w a l l s is isolated from this corrosive atmosphere on the hot side a n d

c o v e r e d by a preformed metal casing on t h e cold s i d e .

"Alfonso wmr,

V U - 6 0

This modular c o n c e p t was a p p l i e d in d e v e l o p i n g the V U - 6 0 in the e a r l y

(18)

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various building blocks or m o d u l e s . Each module must perform its intended f u n c t i o n

within a d e f i n e d d e g r e e of f l e x i b i l i t y and must be c a p a b l e of being f i t t e d to a d j a c e n t

modules t o fonri a n i n t e g r a t e d final p r o d u c t . The main purpose of applying this design

c o n c e p t is, of c o u r s e , to produce a b e t t e r product a t lower c o s t . With a properly

c o n c e i v e d a n d e x e c u t e d program, b e t t e r q u a l i t y and lower cost a r e c o m p a t i b l e .

For this modular design c o n c e p t o , w e l d e d - w a l l construction was i d e a l l y suited

b e c a u s e it a l l o w e d t h e f a b r i c a t i o n of large tube panels in t h e s h o p . Backed u p by t h e

necessary b u c k s t a y s , the w e l d e d w a l l , b e c a u s e of its c o n t i n u i t y , provides structural

strength f o r support of the b o i l e r and also a t i g h t gas e n c l o s u r e for pressurized firing

of f u e l . It e l i m i n a t e s t h e need f o r , and a t t e n d a n t problems w i t h , a pressure c a s i n g .

Welding t h e modular i n l e t a n d o u t l e t headers t o the sidewall tube p a n e l s in t h e shop

minimized t h e f i e l d w e l d i n g to a limited number of large dia f e e d e r a n d relief t u b e s ,

completing this portion of the c i r c u l a t i o n s y s t e m . Fins on e d g e tubes of t h e individual

shop-assembled p a n e l s a r e w e l d e d t o g e t h e r in the f i e l d t o c o m p l e t e a n integral

pressure-tight e n v e l o p e around not only the f u r n a c e , but the b o i l e r s e c t i o n a s w e l l .

This g a s t i g h t e n v e l o p e , made up of tubes a n d f i n s , o p e r a t e s a t a saturation t e m p e r a

-ture corresponding to the o p e r a t i n g pressure of the b o i l e r . This t e m p e r a t u r e is w e l l

a b o v e the d e w p d i n t o f t h e corrosive gases in t h e products of c o m b u s t i o n . Insulation

on the w e l d e d w a l l s is isolated from this corrosive atmosphere on the hot side a n d

c o v e r e d by a preformed metal casing on t h e cold s i d e .

"Alfonso wmr,

V U - 6 0

This modular c o n c e p t was a p p l i e d in d e v e l o p i n g the V U - 6 0 in the e a r l y

(19)

p r e s s u r i z e d , two-drum b o i l e r designed for firing oil and gaseous fuels in a c a p a c i t y

range from 1 0 0 , 0 0 0 to 7 5 0 , 0 0 0 lbs of steam per h r , with design pressures to 1600

psig a n d steam temperatures to 960 F .

It is worth mentioning a t t h i s p o i n t t h e names of various c o n t r a c t s our s u b s i

-diary company C E - r r e y S . A . of M o n t e r r e y has in various stages of c o n s t r u c t i o n .

These units a r e among t h e largest industrial boilers t o t a l l y f a b r i c a t e d in M e x i c o .

The V U - 5 0 , the predecessor of t h e V U - 6 0 , is a f i e l d e r e c t e d b o i l e r . Units

of this type h a v e been f a b r i c a t e d for various industrial firms in M e x i c o by C E - r r e y

S . A . Recent t y p i c a l installations a r e a 3 4 ton/hr c a p a c i t y unit o p e r a t i n g a t

42 K g / c m2 a n d 4 0 0 ° C for C e l a n e s e M e x i c a n a ; a 6 3 toi^/hr b o i l e r a t the same

operating conditions installed a t Industries del A l c a l i . C E - r r e y has a l s o f a b r i c a t e d

for C I A . Fundidora de Fierro y A c e r o de M o n t e r r e y , S . A . two V U - 5 0 with 6 0 t o i y t i r

a t 43 K g / c m2 a n d 4 4 5 ° C steam o u t l e t c o n d i t i o n s .

2

Recent c o n t r a c t s i n c l u d e two modified V U - 6 0 , 180 t o r / h r , 42 K g / c m and

400°C f o r P e m e x , C . D . M a d e r o , T a m a u l i p a s .

The latest c o n t r a c t has been a w a r d e d t o C E r r e y by Westinghouse in c o n n e c

-tion to t h e expansion of the CIA Fundidora de Fierro y A c e r o de M o n t e r r e y , S . A .

This c o n t r a c t c a l l s for a blast f u r n a c e gas a n d n a t u r a l gas fired modified V U - 6 0

with a c a p a c i t y of 110 t o r / h r a t 6 4 K g / c m2 and 4 9 0 ° C steam o u t l e t c o n d i t i o n s .

The V U - 6 0 has e v o l v e d from previous VU designs b u i l t over a 4 0 - y e a r p e r i o d .

(20)

b e e n r e t a i n e d . To f u r t h e r i n c r e a s e t h e r e l i a b i l i t y of this u n i t , s e r v i c e reports w e r e

a n a l y z e d t o e s t a b l i s h t h e n a t u r a l of a l l p r e v i o u s p r o b l e m s , h o w e v e r m i n o r , a n d , a s

n e c e s s a r y , design m o d i f i c a t i o n s w e r e m a d e . In a d d i t i o n , t h e latest t e c h n o l o g y a n d

m a n u f a c t u r i n g t e c h n i q u e s w e r e used w h e n e v e r a p p l i c a b l e .

Based on a c c e p t e d p e r f o r m a n c e p a r a m e t e r s , t h e r a n g e of p h y s i c a l dimensions

of t h e steam g e n e r a t o r w a s e s t a b l i s h e d . It w a s d e s i r a b l e t o p r o v i d e maximum flexiW

l i t y t o m e e t v a r y i n g o p e r a t i n g c o n d i t i o n s a n d possible o n - s i t e s p a c e l i m i t a t i o n s . The

m a j o r v a r i a b l e s of w i d t h , d e p t h , a n d h e i g h t a s w e l l as steamdrum dia w e r e set u p to

b e v a r i e d i n d e p e n d e n t l y of e a c h o t h e r a n d w e r e e x a c t l y d e f i n e d . For e x a m p l e , the

f u r n a c e width is v a r i e d in 1 6 - i n . increments and f u r n a c e d e p t h in 1 2 - i n . i n c r e m e n t s .

T h e r e a r e s e v e r a l i n c r e m e n t a l d r u m c e n t e r d i s t a n c e s , b o i l e r t u b e s i z e s , and a c o n s i

-d e r a b l e v a r i a t i o n in -design p r e s s u r e s . C o n s i -d e r i n g t h e w i -d t h , -d e p t h , h e i g h t , -drum

d i a m e t e r s , b o i l e r t u b e s i z e s , a n d design pressure v a r i a b l e s , t h e r e a r e o v e r 1 0 , 0 0 0

u s a b l e c o m b i n a t i o n s .

F l e x i b i l i t y

To i l l u s t r a t e t h e a p p l i c a t i o n of t h e m o d u l a r c o n c e p t t o a c h i e v e w i d e f l e x i

-b i l i t y , c o n s i d e r a section of t h e V U - 6 0 as shown in F i g . 9 . These f u r n a c e sidewall

modular p a n e l s consist of t u b e s , whose t e r m i n a l ends a r e w e l d e d i n t o p i p e h e a d e r s .

The t o p illustration shows t h r e e b a s i c modules; A , B, a n d C ; e a c h h a v i n g

a p a r t i c u l a r w i d t h . W h e n w e l d e d t o g e t h e r in t h e f i e l d , t h e y m a k e up t h e s i d e w a l l

of a u n i t h a v i n g a f u r n a c e d e p t h of 19 f t . The lower i l l u s t r a t i o n uses t h e same basic

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c a s e u t i l i z i n g a g r e a t e r number of B m o d u l e s . This e x a m p l e w a s not s e l e c t e d a s a n

over s i m p l i f i c a t i o n , but r a t h e r t o i l l u s t r a t e t h e r e p e t i t i o u s use of t h e s e modules a s

t h e y a r e a c t u a l l y used in this b o i l e r d e s i g n . The V U - 6 0 is c u r r e n t l y a v a i l a b l e in

2 2 d i f f e r e n t f u r n a c e d e p t h s in w h i c h modules such as A , B a n d C a r e used in v a r y i n g

q u a n t i t i e s . In a c t u a l p r a c t i c e , modules A a n d B a r e e a c h a v a i l a b l e in s e v e r a l widths

a n d e a c h module A , B, a n d C may a l s o be f u r n i s h e d in t h r e e d i f f e r e n t h e i g h t d i m e n

-s i o n -s .

As m e n t i o n e d b e f o r e , e a c h module consists of tubes a n d p i p e h e a d e r s w h i c h

a r e shop assembled i n t o f i n n e d p a n e l e d w a l l s . There a r e 6 b a s i c tubes a n d 5 b a s i c

h e a d e r s w h i c h may be used in v a r y i n g q u a n t i t i e s t o m a k e 2 4 d i f f e r e n t v a r i a t i o n s of

assembled modules similar t o A , B, a n d C . W h e n used in c o m b i n a t i o n , modules A ,

B, a n d C w i l l p e r m i t 6 6 v a r i a t i o n s in f u r n a c e h e i g h t a n d d e p t h .

Figure 10 i l l u s t r a t e s two V U - 6 0 units of d i f f e r e n t f u r n a c e w i d t h and f u r n a c e

d e p t h . As m e n t i o n e d b e f o r e , with t h e modular a p p r o a c h , steam g e n e r a t o r s of v a r y

-ing s i z e may b e c o n s t r u c t e d us-ing standard c o m p o n e n t s . In t h e i l l u s t r a t i o n , modules

i d e n t i f i e d with a subscript a r e i d e n t i c a l t o those w i t h o u t t h e subscript e x c e p t f o r a

l i n e a r d i m e n s i o n .

The largest single cost r e d u c t i o n r e a l i z e d is in e r e c t i o n . E r e c t i o n time is

c o n s i d e r a b l y r e d u c e d by t h e use of t h e large m o d u l a r w e l d e d p a n e l s . I n s u l a t i o n is

a t t a c h e d t o , a n d supported b y , t h e w e l d e d - w a l l e n v e l o p e by impaling pins w e l d e d t o

t h e f i n s . A p p l i c a t i o n of t h e preformed m e t a l lagging is s i m p l e . The s t r u c t u r a l s t r e n g t h

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in f o u n d a t i o n c o s t s . Because of t h e p h y s i c a l s i z e of t h e b o i l e r b a n k , t h e tubes a r e

i n d i v i d u a l l y e x p a n d e d i n t o t h e d r u m s .

W i t h standard m o d u l e s , it is f a r simpler a n d r e q u i r e s c o n s i d e r a b l y less time

to process t h e n e c e s s a r y f a b r i c a t i o n i n f o r m a t i o n . This is e s s e n t i a l s i n c e many units

a r e sold for short d e l i v e r y . W i t h t h e n e c e s s a r y e n g i n e e r i n g i n f o r m a t i o n c o v e r i n g

a l l possible assemblies a n d parts a v a i l a b l e in t h e s h o p , e n g i n e e r i n g processing c a n

f r e q u e n t l y b e r e d u c e d t o i n d e n t i f y i n g r a t h e r t h a n c o m p l e t e l y d e s c r i b i n g t h e s e i t e m s .

To p r e - e n g i n e e r a l l modules represents a c o n s i d e r a b l e i n i t i a l investment in

e n g i n e e r i n g c o s t s . When d i s t r i b u t e d o v e r a large number of b o i l e r s , h o w e v e r , t h e

cost a g a i n s t e a c h b e c o m e s nominal a n d obviously s u b s t a n t i a l l y less t h a n t h e cost

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