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Nu c l e a r En g i n e e r i n g a n d De s i g n 1 1 2 ( 1 9 8 9 ) 2 4 3 - 2 5 7N o r t h - H o l l a n d , A m s t e r d a m

24 3

S H A F T S O F M A I N C O O L A N T P U M P S - F A I L U R E A N A L Y S I S A N D R E M E D I E SR . L O E H B E R G 1, W . U L L R I C H i a n d K . G A F F A L 2I Siemens AG-UB KWU, Erlangen, Fed. Rep. Germany2 KSB, Frankenthal, Fed. Rep. GermanyRe c e i v e d No v e m b e r 1 9 8 7

The pape r desc r ibes f a i lu re ana lys i s and r emed ies wh ich were e l abo ra ted a f t e r the occu rance o f seve ra l f a i lu re ca ses in sha f t so f m a i n c o o l a n t p u m p s . Th e i m p r o v e d d e s i g n o f r e p l a c e m e n t s h a f t s a s we l l a s p r o c e d u r e s f o r o p e r a t i o n a l m o n i t o r i n g a n din - se rv ice in spec t ions a re shown .

1 . I n t r o d u c t i o nT h e r e a c t o r c o o l a n t p u m p s o f a p r e s s u ri z e d w a t e r

r e a c t o r c i r c u l a t e t h e p r i m a r y c o o l a n t t h r o u g h t h e r e a c -t o r c o re , t h e p r i m a r y c o o l a n t p i p e s a n d t h e s t e a m g e n e r -a t o r . F i g . 1 s h o w s a t y p i c a l d e s i g n o f a r e a c t o r c o o l a n tp u m p f o r a 1 30 0 M W p r e s s u r i z e d w a t e r r e a c t o r . F ig . 2l is ts t h e i m p o r t a n t p o w e r a n d d e s i g n d a ta .

D u e t o th e i m p o r t a n c e o f t h e r e a c t o r c o o l a n t p u m p sf o r t h e o p e r a t i o n o f t h e p l a n t , t h e r e c e n t l y f o u n dd a m a g e s t o t h e p u m p s h a f t s o f a c e r t a i n p u m p t y p e a ret h e r e f o r e o f p a r t i c u l a r i m p o r t a n c e . E x t e n s i v e in v e s t i g a -t i o n s a n d d a m a g e a n a l y s e s h a v e b e e n p e r f o r m e d . T h i sw o r k a n d t h e d e ri v e d i m p r o v e m e n t m e a s u r e s a r e t h es u b j e c t o f t h i s p a p e r .

t iBearing shaft!xiRemovab!e coupling

I

I

Pump shaft

Horsepower ating cold 7200 kWhot 5250 kW

BPM 1 450 minSpecific speed nq : 130m inTorque cold 45 502 NmShaft diameter(Impeller seat) 171 mmTorsion stress (undisturbed) 47 N/ram2

without motor 6300 mmTotal hightwi th motor 10300 mm

Nozzle diameter 750 mmwithout motor 63 twi th motorhousing

Total weight

Material shaftimpeller

lOOt20 MnM oNi5 5clador GS 18 NiMoCr 3 7 clad

X42 CrMo 14or X5 CrNi 13 4G-X5 CrNi 13 4

F i g . 1. Re a c t o r c o o l a n t p u m p f o r a 1 3 0 0 - M W P W R .0 0 2 9 - 5 4 9 3 / 8 9 / $ 0 3 . 5 0 E l s ev i er S c i e n ce P u b l is h e r s B .V .

Fig . 2 . Typ ica l power and des ign da ta .

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244 R. Loehberg et a L / Shafts of main coolant pumps

R a d i a l b e a r i n g B e a r i n g s h a f tA x i a l b e a r in g 1 5 7 0 m mR a d i a l b e a r i n g

S p a c e r c o u p l i n gF a c e t e e t h - - - - I 8 2 0 m m

S h a f t s e a l s e c t io n ~ = ~

I P u m p s h a f tR a d i a l b 2 1 8 5 m mS t o p s e ;K e yI m p e l l e r

i S h a ft d ia m e t e r 1 7 0 m mF i g . 3. Sh a f t a s s e m b l y o f a r e a c t o r c o o l a n t p u m p f o r a 1 3 0 0 - M W

P W R .

P l a n t! (N o . o f R C P s )O b r i g h e i m ( 2 )S t a d e ( 4 )

B i b l i s A ( 4 )N e c k a r - I 3 )~ w e s t h e i mB i b l i s B ( 4 )U n t e r w e s e r ( 4 )G r a f e n r h e i n -r e i d ( 4 )G r o h n d e ( 4 )P h i l i p p s b u r g 2 ( 4 )

MiJhlh.-K/~rlicd (4)' ( N S S S u p p l i e r )(3)(41(4)est- (4)

Design ype a: Sha~im pellerhub/keysb : I m p e l l e r s t u d / r a d i a l p i n sc : I m p e l le r s t u d / f a c e t e e t hFig. 5. Design

S h a f t m a t e r i a l

( a ) X 4 2 C r M o 1 4( a ) X 4 2 C r M o 1 4( a ) X 4 2 C r M o 1 4 ( 3 x /X 5 C r N i 1 3 4 ( l x )

X 4 C r N i C u N b 1 6 4(b )( a ) X 5 C r N i 1 3 4( a ) X 5 C r N i t 3 4( a ) X 5 C r N i 1 3 4( a ) X 5 C r N i 1 3 4

X 5 C r N i 1 3 4(b )( c ) X 5 C r N i 1 3 4( a ) X 5 C r N i 1 3 4(c ) 1X22CrNi 17( a ) ~ X 5 C r N i 1 _ 3 4( c) ~ N ! ? ? 4( b ) X 5 C r N 1 1 3 4

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

2 . D e s i g n o f t h e p u m p s h a f tT h e s h a f t a s s e m b l y s h o w n i n f i g . 3 h a s a w a t e r

l u b r i c a t e d r a d i a l b e a r i n g i n s i d e t h e p u m p a n d a n e x t e r -n a l o il - l u b ri c a t e d c o m b i n a t i o n r a d i a l / a x i a l b e a r in g . T h e

s h a f t s e a l b e t w e e n t h e m e d i u m a n d t h e a t m o s p h e r ec o n s i s ts o f a , m e c h a n i c a l s e a l. I n o r d e r t o b e a b l e t oc h a n g e t h i s se a l as s i m p l y a s p o s s ib l e , t h e p u m p s h a f tc o n s i s t s o f s e v e r a l s e c t i o n s : t h e p u m p s h a f t i t s e l f, t h eb e a r i n g s h a f t a n d a r e m o v a b l e s p a c e r c o u p l i n g t o f a c i l i -t a t e s e a l r e p l a c e m e n t .

D e s i g n ( a ) D e s i g n ( b )

S t o p s e a l :O r i g i n a ld e s i g nI m p e l l e r h u b .K e y .

P u m p s h a f tS t o p s e a l :O p t i m i z e d/ d e s i g n

/ S h a f t /I ~ l m p e l l e r f i t

.... !

C h r o m e p l a te d

H u b / k e y sK S B d e s i g n

D e s i g n ( c )

U --Pump h a f t [ P u m p s h a f ti

i , iF a c e t e e t h ~ !~ S topseal" ~ l i I ~ ~ m p e , e r t u dl e e v e

i l / ; ~ ,, ' / I ' ~ I 1 \ 1

" N u t I m p e l l e r

I m p e l le r \ ~ . ~ ; , , S t o p s e a l~ \ i 2 " ~ / R a d i a l p in

il

I m p e l l e r s t u d N u t w i t hh o u s i n gImpeller stud/radial pin Impeller stud /fac e tee thM F A ( A n d r i t z ) d e s i g n K S B d e s i g n

F i g . 4 . I m p e l l e r a t t a c h m e n t a n d t o r q u e t r a n s m i s s i o n .

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24 6 R. L oehberg et a L / Shafts of main coolant pumpsPlant Inspection Operating(No. of RCP's) hoursG6sgen 3) 6/85 47 000

Grafenrheinfeld(4)

Biblis B (4)

Unterweser 4)Neckarwestheim(3)Biblis A (4)

6/87 15 OOO

7/86 37 00012/86 4 000

7/87 4 000

6/86 64 000

8/86 640009/86 74 000

10/86

Result, measures1 pump shaft fractured at upper keyway radius(new pump shaft)2 pum p shafts: indications on upper keyway radius(ground out)Center bore added3 pu mp shafts with central bores, as installedUT result: no indication4 pu mp shafts: indications on upper keyw ay radius(1 pump shaft new , 3 shafts ground out)1 pum p shaft fractured at imp eller hub end3 pum p shafts: indications on impeller hub end(1 pump shaft new , 3 shafts ground out)2 pum p shafts inspected: no indications2 pum p shafts replaced(inspection ater)2 pum p shafts: indications on upper keywa y radius(ground out)2 pum p shafts: no indications4 pum p shafts: indications on upper keyway radius(ground out)3 p ump shafts inspected: no indications

820004700047000

5/87 5 000

MOIheim- 11/86 400 0K~ irlich (4)Stade (4) 3/87 116 000Grohnde 4) 5/87 22 000Obrigheim 2) 6/87 140000Philippsburg (4) 6/87 22 000

2 p ump shafts (X42 CrMo 14)inspected: no indications1 pum p shaft (X42 CrMo 14)inspected: no indication1 pump shaft (1,4313): indications on upper keyway radius(ground out)2 pum p shafts: indications on impeller hub end (area notrespected n 10/86), ground out, no indications2 pum p shafts inspected: no indications2 pum p shafts with indications at upper keyway atonly 1 key2 pum p shafts inspected: no indications1 pum p shaft inspected: no indication4 pump shafts replaced. Visual: indications on all 4(detailed nspection ater)2 pum p shafts inspected: no indications

!4 pum p shafts inspected: no indicationsFig . 7 . In spec t ions o f pump sha f t s .

i n d i c a t e d a s e p a r a t io n o f t h e p u m p s h a f t f ro m t h ei m p e l le r . S i n c e th e p u m p m o t o r i n it i al l y c o n t i n u e d t or u n , t h e re w a s n o r o d in s e r ti o n . W h e n t h e D N B l im i tv a l u e b e c a m e t o o l ow , t h e r e a c t o r p r o t e c t i o n s y s t e mi n i t i a t e d a r e a c t o r t r i p .

W h e n t h e p u m p w a s d i s a ss e m b l e d , t h e f ra c t u r e o ft h e s h a f t w a s n o t a s e x p e c t e d ( b a s e d o n t h e f a i l u r e a tG o e s g e n a n d t h e p r e l i m i n a r y i n s p e c t i o n r e s u lt s ) o n t h eu p p e r k e y w a y , b u t r a t h e r a t t h e e n d o f th e i m p e l l e r h u b .F i g . 6 s h o w s t h i s f r a c t u r e a s c r a c k t y p e 2 .

3 .4 . Pr oo f o f t he in t egr i ty o f t he pr e s s ur e boundar y dur ings ha f t f r ac tur eT h e s h a f t f r a c t u r e s w h i c h h a v e o c c u r r e d h a v e n o t

c a u s e d a n y d a m a g e t o t h e p r e s s u r e c o n t a i n i n g p u m ph o u s in g . T h e d e s i g n o f th e p u m p i s s u c h t h a t d u r i n g af r a c t u re o f t h e p u m p s h a f t t h er e c a n b e n o c h a l l e n g e t ot h e p r e s s u r e b o u n d a r y . A s i s s h o w n i n f i g . 9 , t h e i m -p e l le r is g u i d e d o n t h e u p p e r e n d b y t h e h o u s i n g a n d o nt h e l o w e r e n d b y t h e s u c t i o n n o z z l e , s o t h a t t h e r o t a t i o n

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R. Loehberg e t aL / Sha f t s o f main coo lan t pump s 24 7

!~mt 1160 040 020 0

I ~ ShaftorbitmeasurementpumpYD40x-direction

Shaft orbitmeasurementpumpYD40y-direction

! Shaft orbit00400200

measurementpumpYDlOx,y-direction(comparison)SupporL L t l i I25. 26 27. 28. 29. 30. 1. 2 3. 4.

- - - - NOV.86 . . . ~!-" Dec 86i

F i g . 8. C h a n g e i n s h a f t o r b i t d u r i n g f r a c t u r e o f p u m p s h a f t Y D40 .

a b

F i g . 9. G u i d e o f i m p e l l e r .

-Housing

- Supportplane

)oft plane

F i g . 10 . S c a n n i n g e l e c t r o n m i c r o s c o p e p h o t o s o f c h r o m i u m l a y e r.

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24 8 R. Lo ehberg et al. / Shaftso f t h e i m p e l l e r w i t h o u t t i l t i n g is as s u r e d . A f r a c t u r e o ft h e p u m p s h a f t d o e s n o t p r e s e n t a n y s a f e t y - r e l a t e dp r o b l e m .

3 .5 . I nv e s t i ga t i on r e s u l ts f r om t he pu m p s ha f t sA c c o r d i n g t o t h e d e s i g n s i n u s e , a d i f f e r e n t i a t i o n c a n

b e m a d e b e t w e e n t h e t w o c r a c k t y p e s (f ig . 6) . C r a c k s o ft y p e 1 o c c u r a t t h e u p p e r e n d o f t h e k e y w a y . I n t y p e 2t h e c r a c k s a r e s u m m a r i z e d w h i c h l i e a b o v e t h e i m p e l l e rh u b i n t h e v i c i n i t y o f t h e s t o p s e a l .

I n t h e c a s e o f a l l c r a c k s , t h e c h r o m e l a y e r in t h ef r a c t u r e l o c a t i o n e x h i b i t s a c r a c k n e t w o r k w i t h a " m e s hs i z e " o f a b o u t 0 . 2 t o 0 . 4 m m ( f i g 1 0 ) . T h e c r a c k s i n t h eb a s e m e t a l a r e c o n t i n u a t i o n s o f t h e c r ac k s i n t h e c h r o m el a y er . D u r i n g f a b r i c a t i o n t h e c h r o m e l a y e r is fr e e o fc r a c k s . A t s e v e r a l c o m p r e s s i v e o r f r i c t i o n w e a r p o i n t sc l o s e - m e s h e d c r a c k n e t w o r k s h a v e b e e n o b s e r v e d i n t h ec h r o m e l a ye r . H e r e , h o w e v e r , n o c r a c k s t a r t s i n t o t h eb a s e m e t a l h a v e b e e n f o u n d .

of m ain coolant pump,s(1) lm~es t igat ions o f (vpe 1 cracks

T h e c r a c k s r u n p r i m a r i l y i n t h e c i r c u m f e r e n t i a l d i -r e c t i o n . E x c e p t f o r t h e s h a f t f r a c t u r e d e s c r i b e d i n se c -t i o n 3 .1 , th e s e c r a c k s a t t a i n l e n g t h s u p t o 4 5 m m a n dd e p t h s u p t o a b o u t 2 0 m m . s u c h c r a c k s h a v e b e e n f o u n do n t h e l o a d e d a n d u n l o a d e d s i d e o f th e k e y ra d i u s . T h ec r a c k s t a r t i n e a c h c a s e i s o n t h e s h a f t s u r f a c e , a b o u t5 1 0 m m n e a r t h e k e y w a y . M o s t o f t h e c r a c k s e n db e f o r e o r a t t h e e d g e o f t h e k e y w a y . T h e c r a c k l e n g t h i sc h a r a c t e r i s t i c o f t h e e f f e c t s o f c o m b i n e d s t r e s s ( b e n d i n g ,t o r s i o n , r e s i d u a l s t re s s es , t h e r m a l s t r e s s es ) . A s s h o w n i nf i g . 1 1 , t h e c r a c k s i n v o l v e d a r e f a t i g u e c r a c k s w i t h t h et y p i c a l c h a r a c t e r i s t i c s s u c h a s s t r i a t i o n s a n d c r a c k a r r e s tl in e s . T h e o x i d a t i o n u p t o t h e t i p o f t h e c r a c k a n d t h ec r a c k a r r e s t l i n e s p o i n t t o t h e f a c t t h a t t h e c r a c k so r i g i n a t e d l o n g b e f o r e t h e y w e r e d e t e c t e d a n d h a v eg r o w n g r a d u a l l y a n d i n st a g e s.( 2) I nv e s t i ga t i ons o f t y pe 2 c r ac k s

T h i s c r a c k t y p e w a s i n v e s t i g a t e d c l o s e ly o n t h e s h a f to f p u m p Y D 4 0 o f th e G r a f e n r h e i n f e l d p l a n t. F i g. 1 2

b

~ Y

1

I

' t>"

dFig. 11 . Cracks in r eg ion o f keyway (c rack type 1 .

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R. Loehberg et aL / Shafts o f main coolant pum ps 24 9

Fig. 12. Crack s in region o f stop seal (crack ty pe 2).s h o w s t h e f i nd i n g s f r o m m e t a l l o g r a p h y a n d f r a c to g r a -p h y . T h e c r a c k r u n s p r i m a r i l y p e r p e n d i c u l a r t o t h e s h a f ta x i s. T h e c r a c k s t a r t i s i n t h e c h r o m e p l a t e d s u r f a c ea b o u t 9 0 d i s p l a c e d i n t h e r o t a t i o n a l d i r e c t i o n t o t h ek e y w a y . T h e d u c t i l e f i n a l f r a c t u r e i n v o l v e d o n l y a b o u t5 % o f t h e e n t i r e f r a c t u r e s u r f a c e . N o i n d i c a t i o n s h a v eb e e n f o u n d o f d e v i a t i o n s i n t h e q u a l i t y o f t h e b a s em a t e r i a l . A l s o i n t h e c a s e o f t h i s f r a c t u r e i t i s a f a t i g u ef r a c t u r e w i t h f a t i g u e s t r i a t i o n s . O n t h e s u r f a c e t h e r e a r e5 c r a c k a r r e s t l i n e s . I n c o m p a r i s o n t o c r a c k t y p e 1 , t h i sw a s a r a p i d c r a c k p r o p a g a t i o n .

4 . Fa i l ure ana l y s i s

Stress com bination Torsional stresseAxia ls t ress Bending stress

Stress com bination Torsional stress Axial stresse Bending stress

0Stress com bination Torsional stress Axial stress Bending stress Thermal stress

4 . 1 . Load i ngsT h e h y d r a u l i c f o r c e s a n d m o m e n t s w h i c h a c t o n t h e

i m p e l l e r a r e s h o w n i n fi g . 1 3 . F o r t h e d e t e r m i n a t i o n o ft h e o p e r a t i o n a l s t r e n g t h , t h e l o a d i n g s o c c u r r i n g d u r i n gf u ll p o w e r - c o l d o p e r a t i o n h a v e b e e n c o n s e r v a t iv e l y c o n -s i d e r e d . F u r t h e r m o r e , t h e t h e r m a l l o a d i n g s o n t h e s h a f t

Hydraulic Q ~ . A ~ /ax ia l t h rus t - a ~ a , ~ - ~ ~ '~= bending m om ent ~ ~rad ia l thrus t t "~ L ~ j ~f /~Torque ~ V ~ ,~

Fig. 13. Hydraulic forces and moments (on impeller).

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25 0 R . L o e h b e r g e t a L / S h a f t s o f m a i n c o o l a n t p u m p sm u s t b e c o n s i d e r e d . S p e c i f i c a l l y , t h e r e a r e t h e f o l l o w i n gl o a d i n g s :(1) Axial thrust

T h e d o w n w a r d a x i a l t h r u s t r e s u l t s f r o m t h e p r e s s u r ed i f f e r e n t i a ls a n d t h e p u l se s f r o m t h e f l o w m e d i u m b e -t w e e n t h e i m p e l l e r in l e t a n d t h e o u t le t . U n d e r o p e r a t i n gc o n d i t i o n s t h e d o w n w a r d h y d r a u l i c t hr u s t o n t h e p u m ps h a f t is l o w e r t h a n t h e u p w a r d f o r c e f r o m t h e p r o d u c to f s h a f t c r o s s - s e c t i o n a l a r e a a n d s y s t e m p r e s s u r e . T h ea s y m m e t r i c d i s c h a r g e f ro m t h e p u m p h o u s i n g t h r o u g ho n l y o n e p r e s s u r e n o z z l e c a u s e s a d i f f e r i n g p r e s s u r ed i s t r i b u t i o n i n t h e s p a c e s a r o u n d t h e im p e l l e r . T h e r e -f o r e , t h e d o w n w a r d a x i a l th r u s t i s n o t a p p l i e d e x a c t l yc e n t r a l l y . T h i s m e a n s t h a t t h e r e s u l t i n g a x i a l t h r u s te f f ec t s a d y n a m i c b e n d i n g m o m e n t o n t h e s ha f t.(2) Rad ial thrust

T h e r a d i a l t h r u s t r e s u l t s f r o m t h e u n s y m m e t r i c a lp r e s s u r e d i s t r i b u t i o n o n t h e c i r c u m f e r e n c e o f t h e i m -p e l l e r . I t c o n s i s t s o f t h e s p a t i a l c o m p o n e n t s w h i c h a c t a sa d y n a m i c b e n d i n g m o m e n t o n t h e s h a ft , a n d o f c o m p o -n e n t s w h i c h r o t a t e w i t h t h e s h a f t , w h i c h l e a d t o a s t a t i cb e n d i n g m o m e n t o n t h e s h a ft . T h e l e v e r a r m w i t h w h i c ht h e r a d i a l t h r u s t a c t s o n t h e p u m p s h a f t , i s t h e d i s t a n c ef r o m t h e m i d d l e o f t h e i m p e l l e r i n l e t t o t h e w a t e r -l u b r i c a t e d r a d i a l b e a r i n g .(3) Torque

T h e m o t o r t o r q u e a c t s s t a t i c a l l y w i t h a d y n a m i cc o m p o n e n t o f a fe w p e rc e n t w h i c h i s d e t e r m i n e d f r o mo p e r a t i n g m e a s u r e m e n t s a n d is ta k e n i n t o a c c o u n t i nt h e s t r e s s c a l c u l a t i o n s .(4) Thermal loadings

T h e s e a l i n j e c t i o n w a t e r s y s t e m k e e p s t h e p u m p s h a f td u r i n g s t e a d y s t a t e o p e r a t i o n i n t h e v i c i n i t y o f t h e s e a lsa n d t h e w a t e r - l u b r i c a t e d b e a r i n g a t a n o p e r a t i n g t e m -p e r a t u r e o f a b o u t 5 0 C . T h e i m p e l l e r i t s el f i s e x p o s e dt o a n o p e r a t in g t e m p e r a t u r e o f 2 9 0 C i n t h e a r e a o f t h ev a n e s .

T h e r m a l s t r e s s e s t h e r e f o r e o c c u r o n t h e p u m p s h a f ts u r f a c e d u e t o t h e d i f f e r e n t t h e r m a l e x p a n s i o n c o e f f i -c i e n t s b e t w e e n t h e c h r o m e l a y e r a n d t h e b a s e m e t a l .

I n a d d i t i o n t o t h e s e s t e a d y - s t a t e t h e r m a l l o a d i n g s ,t h e r e a r e a l s o t r a n s i e n t s t r e s s p r o c e s s e s t o b e c o n s i d -e r e d , w h i c h r e s u l t f r o m t h e o p e r a t i o n a l e v e n t s " L o s s o fs e a l i n j e c t i o n " a n d " S e a l i n j e c t io n f l o w r e s t o r a t i o n " .4.2. Loadings a nd explanat ion of fa i lure(1) Calculation method

T h e s t re s s e s in t h e p u m p s h a f t w e r e c a lc u l a t e d w i t ht h e a i d o f d i f f e re n t c o m p u t e r p r o g r a m s i n c l u d in g a 3 - D

F i n i t e E l e m e n t p r o g r a m . F o r t h e s t e a d y - s t a t e l o a d c a s e ,c o l d w a t e r o p e r a t i o n w a s u s e d a n d f o r t h e t r a n s i e n tl o a d c a s e h o t w a t e r o p e r a t i o n w a s u s e d . F o r t h e s t r e s se s ,t w o s e c t i o n s o f t h e p u m p s h a f t w e r e c o n s i d e r e d i np a r t i c u l a r. A l s o , t h e c r a c k s h a d o c c u r r e d i n t h e s e c o n -s i d e r e d s e c t i o n s :S e c t i o n I = e n d o f k e y w a y ( c r a c k t y p e 1 ),S e c t i o n I I = e n d o f i m p e l l e r h u b ( c r a c k t y p e 2 ).(2) Normal operat ion

T h e d e t a i l e d s t r e s s d i s t r i b u t i o n i n S e c t i o n I c a n b es e e n i n f i g . 1 4 . C r a c k i n i t i a t i o n c a n n o t b e e x p l a i n e df r o m t h i s s t r e s s d i s t r i b u t i o n s i n c e t h e c r a c k s t a r t p o i n tl ie s i n t h e c o m p r e s s i v e s t r e ss r e g i o n . I n a d d i t i o n , c r a c k sh a v e b e e n f o u n d i n a s i m i l a r s c o p e o n t h e u n l o a d e dk e y w a y s i d e . T h e a d d i t i o n a l i n c r e a s e i n t h e s t r e s s c o m -p o n e n t s i n t h e Z - d i r e c t i o n f r o m t h e s t e a d y - s t a t e t e m -p e r a t u r e l o a d i n g ( h o t o p e r a t i o n w i t h f l o w i n g se a l i n j e c -t i o n w a t e r o r a l o n g l o s s o f s e a l i n j e c t i o n f l o w ) i s i n t h es h a f t b a s e m e t a l u n d e r t h e c h r o m e l a y e r o r i n t h ec o m p r e s s i v e r e g i o n o r a t o n l y a l o w te n s i l e lo a d < 10N / m m 2 ; i .e ., a l s o th e s e l o a d i n g s d o n o t c a u s e c r a c ki n i t i a t i o n .(3) Lo ss o f seal injection, sea l injection f lo w restoration

D u r i n g l o s s o f s e a l i n j e c t i o n , a n d e v e n m o r e s od u r i n g s e a l w a t e r r e v e r s e f l o w , tr a n s i e n t t h e r m a l s t r es s e so c c u r w h i c h l e a d t o h i g h t e n s i l e s t r e s s e s i n t h e k e y w a yr e g i o n . F i g . 1 5 s h o w s t h e t e m p e r a t u r e d i s t r i b u t i o n i n t h ej u n c t i o n r e g i o n s h a f t / i m p e l l e r d u r i n g v a r io u s o p e r a t in gc o n d i t i o n s . D u r i n g n o r m a l h o t o p e r a t i o n w i t h s e a l i n -j e c t i o n , t h e t e m p e r a t u r e d i s t r i b u t i o n i n t h e i m p e l l e r h u ba n d s h a f t s h o w s t h e t y p ic a l " b e l l s h a p e " o f t h e i s o -

38.8

bLoaoeo Keyway f lank Z729 101

7 3 2

ress com~ ~nents 7 N/ram z

6 6 8 " ]/ CTens~ler a n g e j j

Fig. 14. St ress dis t r ibut ion a t upp er end of keyway.

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R. Loehberg e t aL / Sha f t s o f main coo lan t pum ps 25 1Impeller hub Stop seal

,

I Keyway ShaftI 24 4218 166 114 62,3Normal operationwithseal njection

]T [C]

T[*C] 275 253 20 9 165 121 98,5Normal operationwithoutseal njectionT[C] 271 245 193 141 88,7Transientoperatingconditionwith seal njectionflow reversalafter a certain period Of ime

Fig. 15. Tem perature dis t r ibut ion in shaf t - impel ler hub region.

t h e r m s . I t is c a u s e d b y t h e s t r o n g c o o l i n g e f f e c t o f th es e a l i n j e c t i o n w a t e r ( i n l e t t e m p e r a t u r e a p p x . 4 0 C ) ,w h i c h c o o l s t h e i m p e l l e r h u b a n d t h e s h a f t i n t h i sr e g i o n . D u r i n g a s u f f i c i e n t l y l o n g l o s s o f s e a l i n j e c t i o nt h e m i d d l e o f t h e b e l l c u r v e w a n d e r s f r o m t h e o u t l e t

p o i n t o f t h e s e a l i n j e c t i o n w a t e r o n t h e e n d o f t h eh o u s i n g i n t o t h e r e g i o n o f t h e m i x i n g c h a m b e r .

A f t e r r e s t o r a t i o n o f t h e f l o w o f t h e s e a l i n j e c t i o nw a t e r , t h e i m p e l l e r h u b i s r a p i d l y c o o l e d b y t h e i n t e n -s i v e c o o l i n g i n t h e t h r o t t l i n g g a p . T h e s h a f t, o n t h e

Stress ntensity N/mm2T

O l . . . . . II " *'o ; '8 1 0 240 320 400 480 560 s ~c~ Time after restoration of seal injection

( ) Keyway radius chrome plating( ~ Keyway adius base metal

( ~ Impellerhub plane chrome plating(~ ) Impellerhub plane base metal

=cation I ~ :atic

137Fig. 16. S tresses in chrom e layer and shaf t d ur ing res torat ion o f seal in ject ion (examp le for a cer ta in shaf t geom etry) .

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25 2 R. Loehberg et al . / Sha fts of main coolant pum pso t h e r h a n d , i s c o o l e d m o r e s l o w l y . A s a f u n c t i o n o f t i m ea f t e r s e a l w a t e r f l o w r e s t o r a t i o n d i f f e r i n g h i g h s t r es s e so c c u r i n t h e c h r o m e l a y e r a n d i n t h e s u r f a c e r e g i o n o ft h e s h a f t b a s e m e t a l . T h e a p p r o x i m a t e l y h i g h e s t t r a n -s i e n t s t r e s s e s o c c u r 1 8 0 s a f t e r s e a l i n j e c t i o n f l o w r e -s t o r a t i o n .

T h e t e m p e r a t u r e d i f f e r e n c e s i n t h e r a d i a l d i r e c t i o ng e n e r a t e c o m p r e s s i v e s t re s s e s i n t h e i n te r n a l o f t h e s h a fta n d t e n s i l e s t r e s s e s o n t h e s u r f a c e . T h e s h a f t s i n t h isr e g i o n a r e c o v e r e d w i t h a c h r o m e l a y e r a p p x . 1 5 0 f f mt h i ck . I t s e x p a n s i o n c o e f f i c i e n t a n d m o d u l u s o f el a s -t i c i t y a r e c l e a r l y l o w e r t h a n t h o s e o f t h e b a s e m e t a l .H i g h t e n s i l e s t r e s s e s o c c u r i n t h e c h r o m e l a y e r d u e t ot h e h i g h o p e r a t i n g t e m p e r a t u r e i n t h e e v e n t o f a l o n gl o ss o f s e al i n j e c t i o n a n d t h e i n t e n s i v e c o o l i n g d u r i n gr e v e r s a l o f s e a l i n j e c t i o n f l o w .

A n o t h e r c o n s e q u e n c e o f t h e s t ro n g c o o l i n g is t h eg r o w t h o f t h e a s s e m b l y r a b b e t g a p b e t w e e n t h e im p e l l e rh u b a n d t h e s h a f t , s o t h a t a le s s e r s h r i n k f i t r e s u l t s . I na d d i t i o n t o t h e r a d i a l c o m p r e s s i v e s t r e ss e s , s tr e s s es r e -s u l t f r o m t h e a x i a l d e f o r m a t i o n r e s t r i c t i o n f r o m t h er u b b i n g b e t w e e n t h e h u b a n d t h e s h a f t d u r i n g t h ec o o l i n g o f t h e i m p e l l e r h u b . A l l t h e s e l o a d i n g s g e n e r a t es t r e s s e s i n t h e c h r o m e l a y e r a n d i n t h e s h a f t b a s e m e t a l .T h e i r c o u r s e a s a f u n c t i o n o f t i m e a f t e r r e s t o r a t i o n o fs e a l in j e c t i o n f l o w f o r a c e r t a i n s h a f t g e o m e t r y i s s h o w nin f ig . 16.( 4 ) S t r e s s s u p e r p o s i t i o n s

T h e s u p e r p o s i t i o n o f t h e s t re s s c o m p o n e n t s f r o m a l lt y p e s o f l o a d i n g s w a s p e r f o r m e d f o r S e c t i o n s I a n d I I .

A t m a x i m u m t e n s i l e s t re s s e s i n t h e c h r o m e l a y e r( d e p e n d i n g o n s h a f t g e o m e t r y ) o f a b o u t 5 0 0 N / m m 2a n d i n th e b a s e m e t a l u p t o a b o u t 2 0 0 N / m m 2 t het r a n s i e n t s t r e s s e s h a v e a s i g n i f i c a n t p o r t i o n . A t t h em a x i m u m d y n a m i c a l l y a c t i n g s t r e s s e s f r o m t h e s t e a d y -s t a t e c o l d w a t e r o p e r a t i o n w i t h h i g h n u m b e r s o f c y c l e s,t h e r e a r e a m p l i t u d e s o f a m a x i m u m o f _+ 1 7 .5 N / m m 2a t R = 0 . 93 .

I n s e c t io n I I " e n d o f t h e im p e l l e r h u h " t h e s tr e s se sa r e l o w e r t h a n a t t h e e n d o f t h e k e y w a y . T h i s a p p l i e sb o t h t o t h e t r a n s i e n t t h e r m a l s t r e s s e s ( f i g . 1 6 ) a s w e l l a st o th e s u p e r i m p o s e d m a x i m u m s t re s s es . O f c o u r s e a tt h i s l o c a t i o n t h e e f f e c t s o f t h e s h r i n k f i t s t r e s s e s f r o mt h e s t o p s e a l m u s t b e t a k e n i n t o a c c o u n t .

F r o m t h is i t c a n b e d e r i v e d t h a t a c r a c k i n i t i a t i o n i nt h e c h r o m e l a y e r a t t h e e n d o f t h e k e y w a y a n d a t t h ee n d o f t h e h u b i s p o s s i b l e . I n o r d e r t o d e t e r m i n e t h ef u r t h e r c r a c k p r o p a g a t i o n i n t o t h e b a s e m e t a l , a d -d i t i o n a l f r a c t u r e m e c h a n i c s c a l c u l a t i o n s a r e n e c e s s a r yf o r t h e i n d i v i d u a l s h a f t s .

4 . 3 . F r a c t u r e m e c h a n i c sA f r a c t u r e m e c h a n i c s a n a l y s i s o f t h e s h a f t f r a c t u r e s

a s s u m e s t h a t t h e f o l l o w i n g p r o p e r t i e s a n d e f f e c t s a r ek n o w n :- M a t e r i a l c h a r a c t e r i s t i c v a l u e s f o r t h e p r e v a i l i n g l o a d -

ing .- T h e c o l l e c t i v e l o a d i n g s o f t h e c o m p o n e n t r e s u l t i n g

f r o m a l l o p e r a t i n g c o n d i t i o n s a n d t h e r e s u l ti n gs t r e s s e s . I n t h i s p r o c e s s , t h e h e i g h t o f t h e a v e r a g es t r e s s a n d t h e q u o t i e n t o f t h e a l t e r n a t i n g s t r e s s e s( m i n i m u m s tr e ss d i v i d e d b y m a x i m u m v a l u e ) ( s tr e ssr a t i o R ) a n d t h e t i m e - r e l a t e d s t r e s s v a r i a t i o n s m u s tb e i n c l u d e d .

- T h e e f f e c t o f th e w a t e r c h e m i s t r y o n t h e c ra c k i n i -t i a t io n a n d c r a c k p r o p a g a t i o n , a l s o i n c o n n e c t i o nw i t h t h e p o s s i b l y p r e s e n t s u r f a c e l a y e r o n t h e m a t e r i a l .W i t h r e g a r d t o t h e s p e c i f i c s t re s s e s in t h e R C P s h a f tr e s u l t i n g f r o m s t a t i c a n d d y n a m i c , h i g h a n d l o w - c y c l e

l o a d i n g s , t h e m a t e r i a l p r o p e r t i e s w e r e m e a s u r e d . T h e yd e t e r m i n e t h e o p e r a t i o n a l s t r e n g t h o f t h e s h a f ts :

t h e m a t e r i a l f a t i g u e b e h a v i o u r d u r i n g a l t e r n a t i n gb e n d i n g l o a d i n g ,

- t h e c r a c k p r o p a g a t i o n .T h e m o s t s i g n i f i c a n t i n f l u e n c e f a c t o r s f o r t h e f a t i g u e

s t r e n g t h h a v e b e e n q u a n t i f i e d ( f i g . 1 7 ) f r o m t h e f a t i g u ee x p e r i m e n t s o n s h a f t m a t e r i a l 1 . 4 3 1 3 a t K S B , a tF r a u n h o f e r I n s t it u t e in D a r m s t a d t ( L F B ) , a t A l li a n z( A Z T I s m a n in g ), a n d a t S i e m e n s / U B K W U . I n a d-

S t r e s s a m p l i t u d eT O 0 0 I . . . . . .N / r a m 2 ~

tO 0

PM = M ed i umCr - Chrome plat ingR = S t ress rat io (Tu

1010 4 1 ; 5 1 ; 6

M Cr R q1 :

~ +~ +

+ + 0,6+ + 0 , 7+ _ l0 7 108 N 109

Load cyclesFig. 17. Trend cu rve of fa t igue s t rength o f shaf t mater ia ls .

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R. Loehberg et al. / Shafts o f main coolant pumps 253diti on to the negative effects of the hard chrome plati ngand of the medium, primarily the high stress ratio Rleads to a severe reduction of the fatigue strength at thetarget number of cycles of 101. KWU also performedthe corresponding investigations for the materialX42 CrM o14 . No signific ant differences were foundbetween the two materials.

It can be deduced from the inspectio n results (fig. 7)that crack type 1 (crack at end of keyway) in theindivi dual shafts occurred after very different operatingtimes. When an initial crack depth corresponding to theinfluence zone of the chrome plating is assumed in thebase metal (a = 0.15 mm), then the crack propagationto the order of magnit ude in the millimeter range can beexplained from the calcu lated stresses o n the surfacesand the measured crack propagation rates with consid-eration of the scattering of the measured data points.Corresponding crack propagation estimates have al-ready been performed for certain shafts.

5 . R e m e d i a l a c ti o n sBased on the fracture appearances and the fracture

analysis, in a second step, replacement shafts with cer-tain improvements in the areas of the seating of theimpeller have been fabricated for the directly involved

plants. Fig. 18 shows an overview of these basic im-provements. In particular they are:(1 ) S top s ea lThe stop seal was originally shrunk on to a chromeplated portion of the shaft. In order to reduce theeffects of the shrink fit in the area of the shaft surfacealso in connection with the chrome plating, the shaftdiameter in the seating area was increased and was leftwithout chrome plating. The shrink fit was designed for0.3 mm. Another change in the stop seal is an improve-ment in the temperature conduction of the shaft in theseating area. The stop seal was lengthened downwa rd sothat it reaches into the corresponding cylindrical recessin the impeller hub and shields the keyway. The closegap between the impeller hub a nd the stop seal preventsinterchange flow.(2 ) Op t im iz a t ion o f t he key l eng th

Starting out from the fact that the typical failureshave occurred at the keyway end in conne ction with thechrome plated surfaces, calculations were initiated forthe optimization of the key. The result is a shorter key.The end of the key following this optimization is belowthe upper chrome plated seating of the impeller hub. Itnow lies not only in the non-chrome-plated region, theloadings resulting from the alternating bending stressesand the torqu e transmis sion have become more favoura-ble. Fig. 19 shows the favourable distribution of the

j - -

chrome plated1 Stop seal

Extended into impel ler Shaft increased, not chrom e plated Shrink fi t 0,3 mm2 Optimized key length

Key shor tened Key end in not chrome plated areaof impel ler hub

3 Optimized chrome plating

4 Sealed surface priorto chrome plat ing byrol l ing

5 Center bore for UTFig. 18. Improvement in seating area of impeller.

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25 4 R. Loehbe rg et al. / Shafts of main coolant pum psl o a d s o v e r t h e le n g t h i n q u e s t i o n ( c o n t a c t p r e s s u r eb e t w e e n t h e k e y w a y a n d t h e k e y ) . A t t h e u p p e r e n d o ft h e k e y t h e c o n t a c t p r e s s u r e i s d e f i n i t e l y l o w e r . I n t h el o w e r s h a f t s e c t i o n t h e i n c r e a s e i s f a v o u r a b l e i n a n a r e ao f t h e l o w e s t a l t e r n a t i n g b e n d i n g s t r e s s e s .(3 ) Op t im iz a t ion o f t he chr ome p la t ing

T h e c h r o m e p l a t in g i s d e s i g n ed f o r a n o p t i m i z e dp l a t i n g t h i c k n e s s. T h e t r a n s i t i o n b e t w e e n t h e c h r o m ep l a t e d a n d u n p l a t e d s e c t i o n s o c c u r s w i t h i n t h e r e g i o n o ft h e h u b , i n o r d e r t o a v o i d p r e s s i n g t h e e d g e o f t h e h u bo n t h e c h r o m e p l a t i n g l a y e r . T h e s h o r t e n i n g o f t h e k e ye l i m i n a t e d a n i n t e r r u p t i o n o f t h e c h r o m e p l a t i n g l a y e r( b y t h e k e y w a y ) a t t h e u p p e r f i t . I n t h e l o w e r f i t , t h es h a f t s u r f a c e w a s n o t c h r o m e p l a t e d i n t h e r e g i o n o f t h ek e y w a y .(4) Rol l ing

I n o r d e r t o r e d u c e t h e R - r a t i o i n t h e s u r f a c e r e g i o n ,r e s i d u a l c o m p r e s s i v e s t r e s s e s w e r e g e n e r a t e d b y r o l l i n g .T h e m o s t f a v o u r a b l e r o l l in g p a r a m e t e r s w e r e f ir s t d e -t e r m i n e d o n a t e st s h a f t w i t h t h e a i d o f i n t e r n a l s t r e ssm e a s u r e m e n t s . F i g . 20 s h o w s t h e s i g n i f i c a n t te s t r e s u lt s .I t c a n b e s e e n t h a t c o m p r e s s i v e r e s i d u a l s t r e s s e s r e m a i na l s o u n d e r t h e c h r o m e l a y e r . I n t h e t r a n s i t i o n r e g i o nb e t w e e n t h e r o l l e d a n d u n - r o l l e d s e c t i o n s , g r a d u a l l yd e c r e a s i n g c o m p r e s s i v e f o r c e s w e r e a p p l i e d . T h i s a v o i d st e n s i l e r e s i d u a l s t r e s s e s c l o s e t o t h e s u r f a c e i n t h e t r a n s i -t i o n a r e a .

S t r e s s a l o n g l e n g t h o f k e y pI 3 5 0 0 -

N / m m3 0 0 0 -

2 5 0 0 -

2 0 0 0 -

1 5 0 0 -

1 0 0 0 -

5 0 0 -

g !I II L . ~

II

] ' ~ ' - - " 1

o

O r i g i n a l d e s i g n- - - - - S h o r te n e d k e y

r -r - - Jp ~ JI'r ' ~ J I- - ' 1 I II - - ' - - i J

C l e a r a n c e i n i m p e l l e r h u b

/V\/). II m p e l l e r s i d e K e y s h o r t e n e d

Fig. 19. Distribution of stresses along key (surface compressionk e y w a y / k e y ) .

S t r e s s i n N / r a m 2

- 4 0 0 1 !- 6 0 0 4 , , , - - ]2 0 0 4 0 0 6 0 0

2 0 0 t . .4 16 0 0 , , ,

2 0 0 4 0 0 6 0 0+ 2 0 1 " ~ "

- 2 0 0 ~4 0 0 ~ \ . . . . ~ ,"6 0 0 ~ ,

2 0 0 4 0 0 6 0 0 D e p t h u n d e r t h e s u r f a c e in p m

Fig. 20.

t u rn e d a n d c h r o m e p l a te d

- - a x i a l- - - t a n g e n t i a t

r o i le d a n d c h r o m e p l a t e d

a x i a t- - - t a n g e n t i a l

r o l le d w i t h o u t c h r o m e p l a t in g

a x i a l- - - t a n g e n t i a l

Depth dis t r ibut ion of res idual s t resses in var ioussections of test shaft .

(5) Cen ter bore fo r ul trasonic inspect ionU s e o f u l t r a s o n i c i n s p e c t i o n t e c h n i q u e s o n t h e a s - i n -

s t a l l e d c o n d i t i o n o f t h e t o p f a c e o f t h e p u m p s h a f tp r o v i d e s o n l y a l i m i t e d i n s p e c t i o n s e n s i t i v i t y i n t h es h a f t a r e a s w h i c h a r e o f i n t e r e s t ( i m p e l l e r s e a t i n g a n dk e y w a y ) b e c a u s e o f t h e g e o m e t r y . I n o r d e r t o o b t a i n a ni m p r o v e d t e s t s e n s i t i v i t y w i t h t h e u s e o f t h e u l t r a s o n i ct e c h n i q u e , t h e s h a f t s w e r e p r o v i d e d w i t h a c e n t e r b o r e .T h e i n s p e c t i o n w a s t h e n p e r f o r m e d w i t h s p e c ia l l y d e v e l -o p e d t e s t p r o b e s .(6 ) C om pr ehens ive eva lua t ion o f t he op t im i z ed s ha f t de -s ign

T h e a b o v e l i s t e d c o r r e c t i v e m e a s u r e s f o r e l i m i n a t i n gt h e f r a c t u r e s s h o u l d r e s u l t i n a t r o u b l e - f r e e o p e r a t i o n o ft h e r ea c t o r c o o l a n t p u m p s h af ts . O p e r a t i o n a l m o n i t o r -i n g a n d p e r i o d i c i n s p e c t i o n s s h o u l d f u r t h e r a i d i n th i sg o a l .

6 . O p e r at i o n a l m o n i t o r i n gD u r i n g t h e o p e r a t i o n o f th e p u m p s t h e se a l i n j e c t i o n

a n d s e a l l e a k a g e ( f l o w a n d t e m p e r a t u r e ) a s w e ll as t h eb e a r i n g t e m p e r a t u r e o f th e o i l - l u b r i c a t e d a x i a l -r a d i a lb e a r i n g a r e m o n i t o r e d . I n a d d i t i o n , a l l r e a c t o r c o o l a n tp u m p s i n P W R p l a n t s a re e q u i p p e d w i t h i n s t ru m e n t a -t i o n f o r m o n i t o r i n g s h a f t o r b i t s . T w o e d d y - c u r r e n t

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R. Loehberg et aL / Shafts of main coolant pumps 255probes are mount ed at 90 to each other in the regionof the removable shaft coupling. They measure therelative motion of the outer cylinder surface of thespacer coupling with respect to the housing. Usually theSmax value is monitored; in some cases also the individ-ual signals in the X an d Y directions.

A pre-alarm a nd set-poi nt alarm are generated. Typi-cal values for the pre-alarm and pump shutdown are,for example, an increase in the Sm~ value to 1.5 or2 x the values observed during normal operation. I n theevent of a pre-alarm, additional manual analyses areperformed. When the set point alarm is attained, thepump is shut down. The supplementary manual analysesusually include the preparation of frequency spectrums.In order to reduce the manu al activities, some equipme ntis used to provide automatic spectrum comparisons, inorder to facilitate a frequency-selective evaluation of theshaft orbit signals and to generate additional early-warnin g systems. In newer PWR plants the signals fromthe shaft orbit measurements can be included in thevibration monitoring system (Loose Parts MonitoringSystem) and are processed there.

7 . I n -s erv i ce ins pec t i onsFig. 7 shows that in the course of the refueling

operations, pump shafts are inspected in some plants.At present this is performed through the disassembly ofthe pump shaft. Surface crack inspections involve MPinspections, the eddy current process and shop and fieldmetallography with replication techniques. The plan isto perform these inspections in the future with the

pump shafts still installed inside the pumps using ultra-sonic techniques. The no n-destr uctive ultrason ic inspec-tion of the shafts in the assembled conditions is possi-ble: Ultrasonic signals can be introduced either throughthe end face of the pump shaft (motor side) or throughthe special center bore machined just for this purpose.(1 ) U l t ras on ic examina t ion f r o m the face end

The shaft surface provided for the coupling has aring with typical dimensions of OD = 70 mm and ID =40 mm. Commercial test heads, because of the unavoi-dably large bundle divergence at the long sonic paths,have a low sensitiv ity and resolution (fig. 21). They canbe expected to indicat e evidence of crack depths greaterthan 30% of the shaft diameter, with the circumferentialposition and the length not being exactly measurable.Furthermore, the test suffers from the difficult-to-inter-prete signals due to the geometry and the multiplesignals generated by the wave transformation in thestepped shape.(2 ) U l t r ason ic in s pec t ion o f t he s ha f t f ace s ur face w i thfocus ed beam t r ans ducer

When the entire shaft face surface is used as aper-ture, a focused beam transducer (circular-array ultra-sonic unit) can attain a significant improvement inresolution and in the suppression of false signals: thedetectable crack depth corresponds to a 3 mm deep, 5mm long sawcut; the peripheral resolution is about 10 .Detections of inclined cracks and cracks on edges re-main as problems: in order to be able to detect suchcracks the focusing may not be hard, in order not toobtain a full reflection already on the shaft shoulders.The sawcut characteristic signal and angel dynamics(fig. 22) correspond to focusing that produces a corn-

Process used until now

UT test hea d MB2SN

Further developed process withimproved focusing and angle beam

N n mFocus beam ransducer

Fig. 21. The principle of sonic fields.

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25 6 R. Loehberg et aL / Shafts of main coolant pumps

C har ac te r i s t ic cu r ve fo r key w a y ( saw cu t )Eh (sonic wav e length s = 1570 turn)T :

/ I t 0- ] ~ R=1 5mm- , o - / I I d_ B 24 / / t = ' 5 r a m- / - I I / , w a ,

I - 6 q 2 s ~ . / j 4 R : 8 m m ! R = 4 2 m m/ I I I t = S m m ! ! t = , m r

t _ t o _ , s o , o v o , ) ./ I I I I0 5 10 15 rnm 20- - I ~ K e y w a y d e p t h ( s a w c u t)

Primaryma x

J~ Ad jace ntma xI

190 360

(Saw cut)R = Keyway radiust = Keyway depth

Fig. 22. Test sensitivity and resolution of UT focused beam transducer.

p l e t e r e f l e c t io n f r o m a 1 4 m m c r a c k d e p t h . T h e e x -a m i n a t i o n i s s i m p l e a n d c a n b e p e r f o r m e d a c c u r a t e l y .T h e s e n s i t i v i t y i n t h e k e y w a y r e g i o n c a n b e c o n s i d e r e dt o b e a d e q u a t e .

RCP e st block

Section A-A

R=13t = 8R = 6,5 f-" / '~ 7"~ ,~ ~, = 0,1

R = 3,25 " ~ t ~1t =6,5 ~ '2~t = 0 , 3 R = K e y w a y r a d i u s

t = K e y w a y t J e p t h

Angle dynamic (saw ut)Keywaydepths n mmEh 6,5 8,0 4,0 6,50 t,O

dB 0,3

o , t

"Z"

Detail "Z"Saw cut O.t mm deep/2.Omrn wide1

n R L ~ 4 I W b O 3 h l . g O B U a u

Fig. 23. Test sensi t iv i ty and resolut ion of UT tes t head (centerbore test).

(3) Ultrasonic examination using center boreT h e s e n s i t i v i t y o f t h e t e s t c a n b e i m p r o v e d w i t h t h e

u s e o f a c e n t e r b o r e - p a r t i c u l a r l y a t s t e p s i n t h e s h a f t .A r e l i a b l e t e s t t e c h n i q u e m u s t b e i n s e n s i t i v e t o s u r f a c es c r a t ch e s . I t m u s t e v a l u a t e c r a c k in d i c a t i o n s a n d m u s td i f f e r e n t i a t e t h e m f r o m f a l s e i n d i c a t i o n s . W i t h t h e u s eo f l o w - f r e q u e n c y u l t r a s o n i c w a v e s w i t h a s u i t a b l e f o c u s -i n g e v e n t h e sm a l l e s t i n c i p i e n t c r a c k s ( 0 .1 m m d e e pn o t c h ) c a n b e r e l i a b l y d e t e c t e d i n t h e s h a f t g e o m e t r ya n d c a n b e e v a l u a t e d ( f ig . 2 3 ). A c o r r e s p o n d i n g I n -S e r v i c e I n s p e c t i o n t h r o u g h a c e n t e r b o r e w a s p e r -f o r m e d , f o r e x a m p l e , i n J u n e 1 9 8 7 a t G o e s g e n .

8 . C o m p a r i s o n t o s h a f t s w i t h a x i a l p o w e r t r a n s m i s s i o nF r a c t u r e s i n t h e r e g i o n o f t h e c o n n e c t i o n b e t w e e n

s h a f t a n d i m p e l l e r , a s d e s c r i b e d i n t h i s p a p e r , h a v e s of a r o c c u r r e d o n l y i n c a s e s o f s h a f t s w i t h i m p e l l e r sf a s t e n e d w i t h k e y s t o i m p e l l e r h u b ( t y p e ( a )) . D e s i g nt y p e (b ) ( im p e l l e r s t u d / r a d i a l p i n s) a n d ( c) ( im p e l l e rs t u d / f a c e t e e th ) w i t h p o w e r t r a n s m i s s i o n a t t h e fa c e,h a v e t h e f o l l o w i n g a d v a n t a g e s:- P r e - l o a d e d a n d t h e r e f o r e p l a y - f r e e a t t a c h m e n t o f t h e

i m p e l l e r b y m e a n s o f fa c e t e e t h / s t u d o r r a d ia lp i n s / s t u d .

- F a v o u r a b l e t h e r m a l c o n d i t io n s i n th e p o w e r t r an s -m i s s i o n a r e a . T h e c o o l i n g o f t h e sh a f t o c c u r s o n t h ee n t i r e l e n g t h .

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R. Lo ehberg et al . / Sha fts of main coolant purnps 25 7I n s e n s i t i v i ty to t e m p e r a t u r e g r a d i e n t s ( n o g a pc h a n g e s , o n l y c h a n g e s i n p r e l o a d ) .L a r g e r s h a f t d i a m e t e r a n d s m a l l e r i m p e l l e r o v e r h a n g .C o m p r e s s i v e p r e - l o a d s in t h e i m m e d i a t e p o w e r t r a n s -m i s s i o n a r e a , s o t h a t a l t e r n a t i n g b e n d i n g s t r e s s e s d on o t r e a c h i n t o t h e t e n s i l e s t r e s s r e g i o n .N o c h r o m e p l a t i n g i n th e i m m e d i a t e p o w e r t r a n s m i s -s i o n a r e a ( r a d i a l p i n s o r f a c e t e e t h ) .

F o r t h e s e r e a s o n s t h e i n i t i a t i o n o f c r a c k s i n t h e r e g i o no f t h e c o n n e c t i o n b e t w e e n s h a f t a n d i m p e l l e r i s n o te x p e c t e d i n t h e s e d e s i g n s . P u m p s h a f t s o f d e s i g n t y p e( b ) h a v e a l r e a d y a t t a i n e d o p e r a t i n g t i m e s o f 7 4 0 0 0h o u r s ( N e c k a r w e s t h e i m ) . A t B o r s se l e t h e i m p e l l e r a t -t a c h m e n t i s p e r d e s i g n t y p e ( c ) a n d h a s b e e n i n u s ea l r e a d y f o r 1 07 0 0 0 h o u r s .