Eddy Current Tubing Denting

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- " EVALUATION OF THE EDDY-CURRENT METHOD FORL THE INSPECTION OF STEAM GENERATOR TUBING

S.D. BROWN N D J.H. FLORABattelle Columbus Laboratories

SEPTEMBER 30, 1977

PREPARED FOR THE CORROSION GROIJPDEPARTMENT OF NUCLEAR ENERGY

BROOKHAVEN N ATION AL LABORATORYUPTON, NEW YORK 11 97 3

SPONSORED BYU.S. NUCL EAR REGULATORY CO MM ISS IO N

D I V I S I O N OF ENGINEERING STANDARDSUNDER CONTRACT NO. EY-76-C-02-0016

PROJECT 10-19-02-01


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DISCLAIMERThis report was prepared as an account of work sponsored by anagency of the United States Government. Neither the United StatesGovernment nor any agency Thereof, nor any of their employees,makes any warranty, express or implied, or assumes any legalliability or responsibility for the accuracy, completeness, orusefulness of any information, apparatus, product, or processdisclosed, or represents that its use would not infringe privatelyowned rights. Reference herein to any specific commercial product,process, or service by trade name, trademark, manufacturer, orotherwise does not necessarily constitute or imply i ts endorsement,recommendation, or favoring by the United States Government or anyagency thereof. The views and opinions of authors expressed hereindo not necessarily state or reflect those of the United StatesGovernment or any agency thereof.


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DISCLAIMERPortions of this document may be illegible inelectronic image products. Images are producedfrom the best available original document.


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DISCLAIMERThis report was prepared as an account of work sponsored by anagency of the United States Government. Neither the United StatesGovernment nor any agency Thereof, nor any of their employees,makes any warranty, express or implied, or assumes any legalliability or responsibility for the accuracy, completeness, orusefulness of any information, apparatus, product, or processdisclosed, or represents that its use would not infringe privatelyowned rights. Reference herein to any specific commercial product,process, or service by trade name, trademark, manufacturer, orotherwise does not necessarily constitute or imply i ts endorsement,recommendation, or favoring by the United States Government or anyagency thereof. The views and opinions of authors expressed hereindo not necessarily state or reflect those of the United StatesGovernment or any agency thereof.


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DISCLAIMERPortions of this document may be illegible inelectronic image products. Images are producedfrom the best available original document.


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EVALUATION OF THE EDDY-CURRENT METHOD FORTHE INSPECTION OF STEAM GENERATOR TUBING

-DENTING-S.D. BROWN N D J.H. F L OR A

Battel le Columbus Laboratories

SEPTEMBER 30,1977

PREPARED FOR THE CORROSION GROUPDEPARTMENT OF NUCLEAR ENERGY

BROOKHAVEN NATIONAL LABORATORYUPTON. N EW YORK 11 97 3 7OTICE

SPONSORED BYU.S. NUC LEAR REGULATORY COM MIS SIO N

D l V l S l O N . O F E N G I N E E R I NG S T A N DA R D SU N D E R CONTRACT NO. EY-76 -C -02 -0016 .

. PROJECT 10-1 '9 -02-01

PTRlBUTION OF'THIS DOC UMENT IS UNLIMITHBh


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. . . N O T I C EI .

' Thi s report was prepared as an account of work sponsored by the United StatesGovernment. Neither the Uni ted States nor the United States Nuclear RegulatoryCommission, nor any of their employees, nor any of their contractors, subcontractors,or their employees, makes any warranty , express or implied, o r assumes any legalliability or responsibility for the accuracy, co rnpl rter~ rs r usefulness of any informa-tion, app arat us, product or process disclosed, or represents that its use would notinfringe privately owned rights.

Printed in the United Stat& of America, . Available from , I I

' ~ & o n a l echnical Information Service: U.S. Department of Commerce .5285 Port Royal Road .'Springfield, VA 22161Price: Printed Copy $4.50; ~i crofic he 3.00December 1977 . 350 copies


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LEFT BLANKe


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D u r i n g t h e l a s t tw o y e a r s t h e p henomenon r e f e r r e d t o a s d e n t i n g h a s becomew id e sp re a d i n c e r t a i n PWR s te am g e n e r at o r s. . T h i s d e n t i n g i s c au se d b y a r u n-away o x i d a t i o n o f t h e ' c a r b o n s t e e l t u be s u p p o r t p l a t e s i n t h e c r e v i c e s b etw eent h e 1 n co n el t u b e s a nd t h e s u p p o r t p l a t e s : The m a g n e t i t e pr od u ce d b y t h i s r u n -away c o r r os i o n e x e r ts s u f f i c i e n t f or ce on t h e I nc o ne l t u b i n g t o d i s t o r t i t ( d e n ti t ' ) a nd t o d i s t o r t t h e ca rb o n s t e e l s u p po r t p l a t e s as . w e l l . As a r e s u l t o f b o t h .t he se d i s t o r t i o n s , s t r e s s c o r r o s i o n c r a c k i n g . ha sS be eno b s er v ed . i n t h e I n c o n e l .

. tu b in g , o r i g i n a t i n g fr o m t h e p r im a r y c o o l a n t s id e .I t i s i m p o r t a n t t o d e te rm in e t h e c a p a b i l i t i e s o f t h e c o m m er c ia l ly u se d eddy

c u r r e n t t e ch n iq u es t o d e t e c t t h e f o l l o w in g : 1, t h e p re se nc e o f m a g n et i t e f i l l i n g .t h e t u b & - t u b i ' s u p p o r t c r e v i c e s , 2, t h e d eg re e o f d e n t i n g p r es e n t, 3, t h e p r e s -. .ence o f s t r e s s c o r r o s i o n t y p e d e f e c t s i n t h e d e nt ed a re a, a nd 4, t h e p re se n ce o fc o r r o s i o n i nd u ce d d e f e c t s . o n t h e t u b es a bo ve t h e d e nt ed a r ea , e s p e c i a l l y wheret h e d e n t i n g i s t o o s e v er e t o p e r m i t pa ssa ge o f a ' s ta n d a r d ed dy c u r r e n t p ro b e.

D e t e c t i n g t h e . p re se nc e o f m a g n e t it e d e p o s i ts i n t h e c r e v i c e s g i v e s t h eo p e r a t o r . o f t h e 'steam ge n e r a to r a n i n d i c a t i o n t h a t c o n d i t i o n s 1e a d i n g t o d e n t i n gmay be developing. The a c c ur a cy i n m ea s ur in g t h e s i z e o f s m a l l d e n t s i n d i c a t e st h e a b i l i t y of t h e eddy c u r r e n t i n s p e c t i o n t o de te rm in e, b y in s p e c t i o n s a ts e l e ct e d t im e i n t e r v a l s , t h e r a t e o f p ro g re s s o f t h e d e n t in g r e a c t io n s , and,t h e r e f o r e , t h e r a t e o f s t r a i n i n g o f t h e I n c o n e l tu b es and t h e c a rb o n s t e e l sup-p o r t p la t es . The a b i l i t y o f t h e te ch niq ue t o d e t e c t d e fe c ts i n t h e pre sen ce o fde nts r e f l e c t s on i t s a b i l i t y t o p e rf or m i t s p r im ar y fu n ct io n : t h e d e t e c t i on o fw as ta ge o r c r a c k s b e f o r e t h e y p r o g r e s s t o t h e p o i n t ' a t w hi ch t h e y become as e r i o u s ha z ar d t o c o n ti n u e d s a f e o p e r a t i o n o f t h e ste am g e n er a to r . The a b i l i t yo f t h e t e c hn i qu e t o d e t e c t d e f e c t s i n a re a s o f t h e . t u b i n g abo ve t h e d e n t s a l s oa f f e c t s i t s c o nt in u ed u s e f u ln e ss i n p er f o rm i n g i t s p r i ma r y fu n c ti on .

C o ns e qu e nt ly , t h e e x p e r i m e n t a l p r og ra m a t B a t t e l l e-Columbus ~ a b o r a t o r i e s ,t h e f i r s t phase o f wh ic h was r e p o r t e d i n B N L - N U R E G - ~ O ~ I ~R ( ~, was ex tended t oi n c l u d e a st u d y o f t h es e c a p a b i l i t i e s o f t h e s t an d a rd eddy c u r r e n t i n s p e c t i o nequipment . I t i s a p p a r e n t t h a t t h e s i g n a l s p ro d uc e d b y t h e d e n ts a r e p ro no un ce d

- iii -


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so t h a t d e n t in g has a s u b s t a n t i a l , n e g a ti v e in f l u e n c e on t h e a b i l i t y o f t h es t a nd a r d eddy c u r r e n t i n s p e c t i o n t e c hn i q ue t o d e t e c t s m a l l , c o r r o si o n - i n du c e dd e f e c t s i n t h e d e nt ed a r ea s . A l t e r n a t e t e c h ni q u es a r e b e in g d e ve lo p ed t o o v e r -come t h i s p roblem, and a r e be in g tes te d on an expe r imen ta l bas i s ' by the i ndus -try; t h e i r c a p a b i l i t i e s we re o u t s i d e t h e s cope o f t h i s p ha se of t h e p rogram ..

S in ce t h e t u b i n g i s r i g i d l y su pp or te d by t h e m a gn e ti te d e p o si t s i n t h ed en te d. ar ea , t h e p re se nc e o f a s h o r t t hr ou g h- w al l c r ac k i s u n l i k e l y t o l e a d t or u p t u r e o f t h e tu be , and, t h e r e fo r e , t o m a ss iv e l ea k ag e d u r i n g t h e b r i e f p r e s-s ur e t r a n s i e n t s a n t i c i p a t e d t o o c cu r i n t h e e ve n t o f a d e si gn b a s i s a c c id e n t.F u r t h e r , the c r acks obse r ved i n the den ted a r eas o f t ubes t h a t have been r e -moved t o d a t e f o r e x a m in a ti o n, h av e a l l bee n p r i m a r i l y l o n g i t u d i n a l . The l e n g t h

. o f s uc h c r ac k s i s u n l i k e l y t o .e xc ee d t h e t h i c k n e s s o f t h e s u p p or t p l a t e , o r 314".Consequently, , po ss ib l e sa f e ty concerns ove r th e in a b i 1i y o f t h e e d d y c u r r e n tt ec hn iq ue t o d e t e c t such c r ac k s a r e co un te re d b y t h e l e s s e r p r o b a b i l i t y o f t h e i rc a u s in g p i p e r u p t u r e i n t h es e a re as .

T h i s r e p o r t r e pr e se n t s th e f i n d i n g s by t h e B a t t e l l e wo rk er s. The datat h a t i t c o n t a i n s s h o u ld b e u s e f u l f o r t h o s e e v a l u a t i n g i n - s e r v i c e i n s p e c t i o np rograms and the r e s u l t s o f t hese p rograms i n steam gene r a to r s i n wh ich den t i n ghas been observed.

John R. WeeksLeader , C orro s io n GroupBrookhaven Nat iona l Labora to ry

REFERENCE

1. J.H. F lo ra , S.D. Brown and J .R. Weeks, "E va lu at io n o f the Eddy Cu rren tMethod o f I n sp e ct in g Steam Gen erato r Tubing" , September 30, 1976.


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SUMMMARY REPORT

EVALUATION OF THE EDDY-CURRENT METHOD FORTHE INSPECTION OF STEAM GENERATOR TUBING

BROOKHAVEN NATIONAL LABORATORY

Septe mbe r 30, 1977

S . D . Brown and J. H. F lo ra

BATTELLECol umbus La bo ra tor , ie s505 King AvenueCol umbus, O hi o 43201


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TABL.E OF CONTENTS .. .

SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. .. . . . . . . . . . . . . . . . . . . . . . . . .ECHNICAL BACKGROUND; 2

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .e n t i n g .' 2Eddy C u r r e n t ' I n s e r v i c e I n s p e c t i o n. . . . . . . . . . . . . . . . . . . . 5

D e n t F a b r i c a t i o n . . . . . . . . . . . . . . . .. .I n s t r u m e n t a t i o n. . . . . . . . . . . . . . . . . . .D e t e c t i o n o f , D i s c o n t i n u i t i e s . i n D ented R eg io ns .E f f e c t s o f Reduced P ro be D i a m et e r f o r I n s p e c t i o nR e s t r i c t i n g D e n t s . . . . . . . . . . . . . . . .D e t e c t i o n o f S ma ll D en ts . . . . . . . . . . . . .D e t e c t i o n o f M a g n e t i t e F o r m at io n . . . . . . . .

E s t i m a t i o n o f D en t S i z e . . . . . . . . . . . . . . . .CONCLUSIONS . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . .o f Reg ions Beyond

. . . . . . . . . . . . . . . . . .UGGESTED AREAS FOR FURTHER RESEARCH ; 3 8. . . . . : . . . . . . . . . . . . . . . . . . . .ancake C o i l 38

. . . . . . . . . . . . . . . . . . . . . . . . . . .Cross-Wound C o i l 3 8. . . . . . . . . . . . . . . . .l e c t r o n i c A b s o l u t e / D i f f e r e n t i a l 3 9 .

L IS T OF FIGURES, .FIGURE 1. C r o s s e c t i o n a l V ie w o f D e n t i n g a t Tube S u p p o rt P l a t e s . . . . 3

FIGURE 2 . B l o c k D ia g ra m o f B a s i c E d d y - C u rr e n t S ys te m . . . . . . . . . . 7FIGURE 3 . D i f f e r e n t i a l E d dy -C u rr en t T e s t C o i l and C u r r e n t F lo w I n s i d e

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ub ing 8FIGURE 4 . Du a l -C o i l S ig na l Pa t t e r n Wh ich Res u l t s F rom Scann ing an OD

. . . . . . . . . . . . . . . . . . . . . . . . . .. . . .e f e c t : 8. .

. . . . . . . . . . . . .IGURE 5 . Typ ica l Eddy Cur ren t Response S igna ls 11. .

. . . . . . . . . . . . . . .IGURE 6 . Typ ' i ca l Den t D i e and Large Den t 1 3. . . . . . . . . . . . . . . . .IGURE 7'. L a b o r a to r y I n s t r u m e n t a t i o n 1 5

FIGURE 8 . S i g n a l P a t t e r n s f o r F l a t Bo tto me d H o le i n De nt . 400 KHz .. . . . . . . . . . . . . . . . . . . . . . .c a l e 0 .5 v / d i v 1 6


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L IS T OF FIGURES (C o n ti n u e d )

Page

FIGURE 9. S i g n a l P a t t e r n s f o r L o n g i t u d i n a l OD N ot ch , 1 I n c h Long i nD e n t s 4 0 0 K H z . . . . . . . . . . . . . . . . . . . . . . . . . 1 8

FIGURE 1 0. D e t e c t a b l l i t y o f 60% L o n g i t u d i n a l ID No tch 118 I nc h Longi n Dent, 400 KHz, Sc ale - 0 . 5 v I d i v . . . . . . . . . . . . . 1 3FIGURE 11. S i g n a l P a t t e r n s f o r L a r g e Cl e ar a nc e P r ob e Caused b y F l a tBottumed 1.101cs (FBH) 40, 60, 80, and 100% of Wa ll , 400 KHz,. .. . . . . . . . . . . . . . . . . . . . . . .c a l e - 0 . 5 v /d i v 2 1FIGURE 12. S i gn al Pa t t er ns f o r Dent and Tube Supp or t , 100 KHz, Sc ale -0 . 5 v / d i v . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3FIGURE 13. S i gn al P at te rn s f o r Dent and Tube Sup por t , 200 KHz, Sc ale -0 . 5 v / d i v . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 4FIGURE 14. S i gn al P a t t er ns f o r Dent and Tube Sup por t , 400 KHz, S ca le -0 . 5 v / d i v . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5FIGURE 15. S i gn al P at te rn s f o r Dent and Tube Sup por t , 700 KHz, S ca le -. . . . . . . . . . . . . . . . . . . . . . . . . .. 5 v / d i v 2 6FIGURE 16 . S t r i p C h a rt R ec or di ng s o f D i f f e r e n t i a l C o i l I l l u s t r a t i n gD e t e c t a b i l i t y o f D e nts U nde r S u p po r ts . . . . . . . . . . . . 2 8FIGURE 17 . S t r i p C h a r t R e co rd in gs o f A b s o lu t e C o i l I l l u s t r a t i n g D e t e c t-a b i l i t y o f D ents U nder S up po rt s. . . . . . . . . . . . . . . 2 9FIGURE .18. S i g n a l P a t t e r n s f o r T ub e S u p p o r t W i t h a n d W i t h o u t M a g n e t i t e . 3 1FTGURE 19 . Abs o lu te C o i l Response t o Tube Suppo r ts , 400 KHz . . . . . . 3 3FIGURE 20 . S t r i p C h a r t R e c or d i n gs o f D e nt s o f V a r i o u s S i z e s u s l r ~ yA b s o l u t e a n d D i f f e r e n t i a l C o i l s . . . . . . . . . . . . . . . . 3 4FIGURE 21. D en t S i z e C a l i b r a t i o n C h a r t s f o r B o t h A b s o l u t e a nd D i f f e r -e n t i a l C o i l s . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4

. . . . . . . . . .IGURE 22 . S t r i p C h a r t R e c o r d in g o f S e ve n -M i l D i n g . 3 6

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. .

EVALUATION O.F T H E EDDY-CURRENT METHOD FOR 'THE' .INSPECTION OF STEAM 'GENERATOR TUBING . .L

S. D . Broen and J:H. F l o r a

SUMMARY

C on tin ue d e v a l u a t i o n o f e x i s t i n g e d dy - cu r re n t i n - s e r v i c e i n -s p e c t i o n ( I S I ) me th od s f o r s te am g e n e r a t o r t u b i n g h as e mp ha si ze d t h ee f f e c t s o f d e n ti ng . D e nt in g i s a c i r c u m f e r e n t i a l d e fo r m at io n o f t h e t u b ei n t h e s u p po rt r e g i o n a s a r e s u l t o f t h e f o r m a ti o n o f a m ag n et it e ,Fe304, c o r r o s i o n p r o d u c t o n t h e c a rb o n s t e e l t u b e s u p p o rt p l a t e i n t h ec r e v i c e be tw ee n t h e s u p p or t p l a t e an d t h e t u b e w a l l . The e f f e c t s o f d e n t -i n g on e ddy c u r r e n t i n s p e c t i o n a r e t w o f o l d : ( 1 ) The d e te c t i o n and meas-u re me nt o f d e f e c t s i n t h e d e nt ed r e g i o n i s hampered by t h e d i s t o r t i o n o ft h e e d d y - c u r r e n t r e sp o n se s i g n a l s c au se d b y t h e d e n t s a nd ( 2 ) l a r g e ' d e n t smay p r e v e n t ' t h e pa ss ag e o f h i g h f i l l f a c t o r p r o b e s f o r c i n g t h e i n s p e c t i o nteams t o u se a s m a l l e r p r o be i n un de nt ed r e g i o n s . U n le s s a p p r o p r i a t em easures a r e ta ke n, l e s s r e l i a b l e d e t e c t i o n o f d e f e c t s and e s t i m a t i o n o ft h e i r d ep th can r e s u l t .

T h i s r e p o r t p r e s e n t s an ex p e ri m e nt a l e v a l u a t i o n o f e x i s t i n ge d d y - c u r r e n t I S 1 m e th od s when us e d t o i n s p e c t d e n t e d r e g i o n s . T ube sw ere m e c h a n i c a l l y de n te d t o s i m u l a t e v a r i o u s d eg re es o f s e r v i c e i n du c edd e n t i n g . These t u b e s we re t h e n i n s e r t e d i n ca rb o n s t e e l t u b e s u p p o r tp l a t e s and t h e c r e v i c e r e g i o n b etw ee n t h e s u p p o rt p l a t e a nd t u b e w a s. s ub -s e q u e n t l y p ac ke d w i t h p ow de re d m a g n e t i t e . D i a m e t r a l d e n t s i z e s c o n s i d e r e dd u r i n g t h i s p ro g ra m v a r i e d b e tw ee n 1 a nd 1 0 m i l s . E d d y - c u r r e n t s i g n a lp a t t e r n s and s t r i p c h a r t r ec o 'r di ng s w ere o b t a i n e d r u s in g a v a r i e t y o f t e s tp a r a m e t er s , s u c h a s f re qu en cy ,. p ha se r o t a t i o n . an d c o i 1 c o n f i g u r a t i d n , . o n .. tu be sa mple s c o n t a i n i n g v a r i o u s d eg re es o f d e n t i n g ' a n d w i t h s e v e r a l t y p e s 'o f d e f e c ts i n t h e d en te d r e g i o n .


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R e s u l t s o f l a b o r a t o r y e xp e ri m en ts s u g ge st t h a t d i a m e t r a l d e n t sw h i c h a r e 1 m i l and g r L e a t e r h a v e a n a d v e r s e e f f e c t o n. t h e d e t e c t i o n an'dm ea surem en t o f s m a l l vo lu me d e f e c t s , i . e . , n o t c h es a nd p i t s , i n t h e d e n te dr e gi on . R e l a t i v e l y l a r g e d en ts , 5 m i l s o r g r e a te r , p r ev e nt r e l i a b l e s ma llvolume d e f e c t d e t e c t i o n w i t h e x i s t i n g c o i l c o n f ig u r a ti o n s i n t h e d en te dr e g i o n . D i f f e r e n t i a l p ro be c o i l s o f r ed uc ed d i am e te r can be use d t o i n -s p e c t u n d en te d r e g i o n s i n t h e s te am g e n e r a t o r t u b e s , i f t he p robes can be

. c e n t e r e d and r e s t r i c t e d f r om e x c e s s i v e w ob ble .The d e t e c t i o n o f m a g n e t i te p r i o r , t o d e n t f o r m a ti o n a pp ea rs f e a -

s i b l e and may be b e s t a cc o m pl is h e d u s i n g a n a b s o l u t e p r ob e c o i l i n w h ic ht h e s u p p o rt s i g n a l h as been m i n im i z e d i n t h e v e r t i c a l c h a n n el . The p r e -sen ce o f m a g n e t i t e i s d e t e rm i ne d by a r e d u c t i o n i n a m pl it ud e and a f l a t -t e n i n g o f t h e s u p p o r t s ig n a l as o bs erv ed on t h e s t r i p c h a r t .

E a r l y d e t e c t i o n o f d e n t i n g i.s e a s i l y a cc om plis he d. L a b o ra t o ryi n v e s t i g a t i o n s s ug ge st t h a t an a b s o l u te p ro be c o i l f o r w hic h t h e t u b es u p p o r t s i g n a l h as be en m i n i m i z e d i n t h e v e r t i c a l c ha n ne l p r o v i d e s m orer e l i a b l e d e t e c t i o n a nd d e n t , s i z e m easurem ent. ~ e a s ur e m e n t f d e n t s i z ei s p o s s i b l e b y t h e e s t ab l is h m en t o f a p p r o p r ia t e c a l i b r a t i o n c u rv es a nd) . .p r e l m i n a ry s t u d ie s i n d i c a t e t h a t e r r o r s i n d i a m e tr a l d e n t s i z e e st im a te su s i n g ed d y -c u rr e nt t ec 'h n iq u es a r e on t h e o r d e r o f t w e n t y - f i v e p e r c en t .

TECHNICAL BACKGROUND

As shown i n F ig ur e 1 , the Se r ie s 51 Westinghouse des igned PWRsteam g e n e r a to r t u b e l t u b e s u p p o r t p l a t e i n t e r s e c t i o n c o n s i s t s o f a nom-i n a l l y 0.75 i n c h h i g h c i r c u m f e r e n t i a l gap w i t h a no min al r a d i a l c l e a r -ance o f 0 .0 14 i n c h . C o r ro s i o n o f t h e c ar bo n s t e e l s u pp o rt p l a t e haso c c u rr e d w i t h i n t h i s a n n u la r space p r o d uc in g a n o n - p r o t e c t iv e l a y e r o fm agn e t i t e , Fe304, co r ro s i on p roduc t . S i n ce t h e v olu me o f m a g n e t i t ef or me d i s a p p r o x i m a t e l y t w i c e t h e v olu me o f c a r b o n s t e e l consumed, t h e


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Muximum-Corros ion

Rote

F i g u r e 1. C r o s s e c t i o n a l v ie w of d e n t i n g a t t u b es u p p o r t p l a t e s .


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gap betw een t h e t ube and t ube s uppo r t becomes f i l l e d w i t h m agne t i t e ast h e t u be s u p po r ts c o r ro d e w i t h t h e g r e a t e s t r a t e o f p l a t e c o r r o s i o no c c u r r i n g a t m i d- pl an e . As m a g n e t i t e f o r m a t i o n c o n ti n u es , s u f f i c i e n tp r e ss u r e i s e x e r t e d on t h e t u b e t o c au se y i e l d i n g . T hi s y i e l d i n g i sg e n e r a l l y s ym me tric a bo ut t h e l o n g i t u d i n a l a x i s o f t h e t u be w i t h i n t h ec o n f i n e s o f t h e tu b e su p p o rt p l a t e and i s r e f e r r e d t o as d e n t i n g . Then e t e f f e c t o f d e n t i n g i s t o r ed uce t h e o v e r a l l d ia m et er o f t h e t u be w i t he s s e n t i a l l y n o r e d u c t i o n i n t h e t ub e w a l l t h ic k n e ss , i . e . , 5 2 p e r ce n t .

F i e l d da t a r e l a t i v e t o t h e d e n t i n g phenomenon su g ge st t h a t a te x t e n s i v e l y d en te d s i t e s , i . e . , S u r r y & T u rk ey P o i n t , d e n ts a r e i n d i -c a te d a t a l l t u b e- to - t u be s u pp o rt p l a t e in t e r s e c t i o n s . F or u n i t s w hic he x h i b i t m o de ra te d e n t i n g , i . e . , P o i n t B each & Ginna , t h e den t s a r e p r e -d o m i n a n t l y on t h e h o t l e g s i'd e. The d i s t r i b u t i o n o f d e nt s i z e a pp ea rst o be random a cr os s t h e s u p p or t p l a t e , w i t h d i f f e r e n t d i s t r i b u t i o n s ,i . e . , mean a nd s ta n d a r d d e v i a t i o n , on t h e h o t l e g and c o l d l e g s i d e s .Mean dep t h o f den t i n g may t y p i c a l l y v a r y betw een one and s i x t e en m i l sd i a m e t r a l . E xtre me d e n t i n g i s on t h e o r d e r o f 6 0 m i l s d i a m e t r a l .

The I S 1 i m p l i c a t i o n s o f d e n t i n g a r e a pp ar en t. I n s p e c t i o n f o rd i s c o n t i n u i t i e s i n t h e d en te d re g i o n i s d i f f i c u l t b ecause o f t h e s up er -p o s i t i o n o f t h e e d d y -c u r re n t d e n t s i g n a l and t h e d e f e c t s i g n a l . Thed e n t s i g n a l i n ge ne ra l i s o f l a r g e a m pl it ud e and may i n f a c t s a t u r a t et h e i n s t r u m e n t a t i o n o b s c u r i n g any d e f e c t s i g n a l . Large dents may pre-v en t t he pas sage o f t h e nor m a l 0. 720 i n c h d i am e t e r p r obe . Regions beyondt h e r e s t r i c t i n g d e nt may have t o b e i ns p ec te d w i t h s m a l le r f i l l - f a c t o rp ro be s r e s u l t i n g i n a l o w er s i g n a l - t o - n o i s e r a t i o , i . e . , d ec re as ed s e ns i -t i v i t y a nd i n c re a s e d w ob ble , u n l e s s a p p r o p r i a t e p r ob e c e n t e r i n g d e v i ce sa r e u t i l i z e d .

S in ce d e n t i n g c an be d e tr i m e n t a l t o t h e e f f i c i e n t and s a f eo p e r a t i o n o f s team g e n e ra t o rs , e a r l y d e t e c t i o n o f d e n t i n g and t h e a cc u-r a t e m easu re ment o f t h e a mount o f d e n t i n g a r e i m p o r t a n t c o n s i d e r a t i o n s .Fur thermore , i t i s d e s i r a b l e t o d e te c t t he p resence o f a m a g ne t i te f i l l e dc r e v i c e b etw ee n t h e t u b e and s u p p o r t p l a t e s b e f o r e a c t u a l d e n t i n g h asoc c u r r ed . T he r e f o r e , i t i s o f i n t e r e s t t o i n v e s t i g a t e t he d e t ri m e nt a l


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e f f e c t s o f d e n t i n g on e x i s t i n g I S 1 e d d y - cu r r e n t m etho ds. S p e c i f i c a l l y ,i t h as b een t h e p u r po se 0.f t h e e x p e r i m e n ta l ' i n v e s t i g a t i o n d e s c r i b e d i nt h i s r e p o r t t o ad dre ss t h e f o l l o w i n g a re as of i n t e r e s t :

P ro ce du re and c a p a b i l i t y f o r d e t e c t i n g e a r l y s t ag e so f d e n t i n g ; .

. 0 . A b i l i t y t o m eas ure t h e a mount ,o f d e n t i n g ;0 E f f e c t o f r e d u c i n g pr ob e d ia m et er r e q u i r e d t o c l e a r

l a r g e d e nt s on t h e m easurement o f d i s c o n t i n u i t i e s i n .. . . n o n -d e n te d a r e a s o f t h e t u b e s ;

0 A b i l i t y . t o d e t e c t d i s c o n t i n u i t i e s i n den ted a re as ;. 0 A b i l i t y t o d e t e c t m ag n e ti te fo r m a t io n b e f or e ' h . < ,

' d e n t i ' n g o c c u r s .

Eddy C u r r e n t I n s e r v i c e I n s p e c t i o n

E dd y-c ur re nt t e s t i n g i n v o l ve s p l a c in g a c o i l o f e l e c t r i c a l l yc on du ct in g i n s u l a t e d w i r e . i n c l o se p r o x i m i t y t o t h e m a t e r i a l t o be i n -s p ec te d. When t h e t e s t co i.1 i s e x c i t e d b y an' a l t e r n a t i n g . c u r r e n t ,u s u a l l y s i n u s o i d a l , an a1 t e r n a t i n g m a g ne t i c f l u x p e n e t ra t e s t h e m a t e r i a la nd ' i n d u c e s e d dy c u r r e n t s . . I n g e ne ra l, t he s e eddy c u r r e n t s f l o w a t r i g h t .*a ng le s t o t h e m ag n et ic f l u x l i n e s . :

. T h e i n d u ce d c u r r e n t s i n t u r n p ro d uc e 'a m ag n e ti c f l u x t h a t o p-'poses t h e ma gn et ic f l u x o f t h e t e s t c o i l . Changes i n th e magn i tude andph as e o f t h e e ddy c u r r e n t s c au se c o r re s p o n d in g c ha ng es i n t h e e l e c t r i c a li mp ed an ce s ee n a t t h e ' t e s t - c o i ' l t e r m i n a l s . F o r e xa mp le , i f a cr a ck i sp r es e n t a t t h e s u rf ac e o f t h e m a t e r i a l , i t w i l l a1 t e r t h e f l o w ' o f eddyc u r r e n t s when t h e t e s t c o i l i s n e ar t h e c r a c k. T h i s ' w i l l c hange ' t h e mag- .n i t u d e and phase of t h e ma gn et ic f l u x a nd i n t u r n a l t e r t h e e l e c t r i c a limpedance of t h e t e s t c o i l . S i m i l a r l y , c hanges i n e l e c t r i c a l r e s i s t i v i t y ,mag ne t i c pe rmeab i 1i y , a n d m a t e r i a l t h i c k n e s s w i 11 c a us e c ha ng es i n t h e 't e s t - c o i l i m p e d a n c e .


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.Eddy c u r r e n t i n s t ru m e n t s t r a n s l a t e t he s e. ch a ng es i n t e s t - c o i lim pedance i n t o o u t p u t v o l t a g e s w h ic hc c an be m o n i t o r ed a t t h e i n s t r u m e n t

' o u t p u t t e r m i n a l s . . F i g u r e 2 . i s a b l o c k .d ia gra m i l l u s t r a t i n g t h e compo-n e n ts o f t h e b a s i c . e d d y - c u r r e n t i n s t r u m e n t a t i o n us ed . t o i n s p e c t steamg e n e r a t o r t u b i n g .

An i n t e r n a l . d i f f e r e n t i a l . p ro be c o i l . i s u s e d . to i n s p e c t t h es te a m g e n e ra to r t u b in g . T h i s . i n c o r p o r a t e s t wo a nn u la r -s h ap e d t e s t c o i l sa d ja c en t t o e ach o th e r i n t h e a x i a l d i r e c t i o n so t h a t b ot h c o i l s i n -duce eddy c u r r e nt s i n th e t e s t m a t e r i a l , as i l l u s t r a t e d i n F i g ur e 3.The d i f f e r e n t i a l b r i d g e n et wo rk s u b t r a c t s t h e a l t e r n a t i n g vo l ta g esa s s oc i at e d w i t h o ne t e s t c o i l from t h e v o l t a g e a s s o ci a te d w i t h t h e o t h e rc o i l . O u tp u t s i g n a l s r e s u l t o n l y when t h e t e s t - c o i l im pedances a r e d i f -f e re n t . F o r e xa mp le , a b ru p t cha ng es i n t h e t u b in g such a s co r r o s io np i t s and s h o r t c r ac k s cause ,impedance d i f f e r en ce s and, the re fo re , a con-d i t i o n o f b r i d g e u nb ala nc e. T h is p ro du ce s a s i g n a l a t t h e o u t p u t o fthe b r id ge ne twork . Gradua l changes such as tempera tu re g ra d ie n t s andsome i n s id e d ia m e te r va r i a t i o n s cau se much sm a l l e r chan ges i n t h e o u t -p u t , d e pe n ding on th e d e s ig n an d sp a c in g be tw ee n th e t e s t co i l s .

The , v e r t i c a l ( V ) and h o r i z o n t a l ( H) o u t p u t s a r e s i m u l t a n e o u s lyre co rd e d on a s t r i p c h a r t r e c o rd e r an d 6M (F re q ue n cy M o d u lat i on , ). t a p er e c o r d e r so t h a t t h e s i g n a l s c a n be r e d i s p l a y e d f o r s ub se qu en t i n t e r -p r e t a t i o n . . The V and H o u t p u ts a r e a l s o d i s p l a y e d on a s t or a g e o s c i l -l o sco p e w i t h V . c on n e ct e d. t o t h e v e r t i c a l i n p u t and H c on ne ct ed t o t h e .h o r i z on t a l i n p u t . T h is p r ov i de s r e a l t i m e > i n d i c a t i o n s i g n a l . i n t e r -p r e t a t i o n a n d accom modates b a la n ce a d ju s tm e n t o f b o th V a nd H o u t p u t sd u r i n g t u b e sca n s . Ba lance adjus,tm,ents compensa te f o r . d r i f t and assu ret h a t th'e r e c o r d ed v o l ta g e s f a l l w i t h i n dynam ic r an ge o f t h e i n st r um e n ta -t i o n . The. FM t a pe r e c o r d i n g s f a c i l i t a t e d i s p l a y o f t h e V a nd H o u t p u tv o l t a g e s o n t h e s t o r a g e o s c i ' l o s c o pe a t a l a t e r t 'im e, so . t h a t d a t a , c anhe c a r e f u l l y a n al yz ed .


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F i g u r e 2 . Block d iagram of b a s i c e d dy - c u r re n t s y st em .

PhaseShifter Phnses p l i t t e roO

. .

5r v

Bridge S /\ * Detectorscillotor =L AmplifierNeiwork

R2 .

Test. Coils Detector

- OW passF i l t e r


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Test Coil Corrosion

/Eddy Current

F i g u r e 3 . D i f f e r e n t i a l e d dy -c ur re nt t e s t c o i l and' c u r r e n t f l ow i n s i d e t u b i n g.

F i g u r e 4 . D u al - co i l s i g n a l p a t t e r n w hi ch r e s u l t s f romscann ing an OD d e f e c t .


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The p ha se r o ta t .i .o n i s a d j u s t e d b e f o r e t e s t i n g s te am g e n e r a t o rt ub e s s o t h a t t r a n s v e r se movement and t i 1 o f t h e t e s t c o i l c au se a s i g -n a l v o l t a g e t h a t app ea rs p r i m a r i l y i n th e H o u t p u t . T i l t and w obblep r ov id e a t e s t - c o i l r es po ns e t h a t i s n e a r l y c o n st a n t i n phase, b u t p r o -v i d e s a c ha ng e ' i n t h e m a g n it u de o f t h e t e s t - c o i l im pe da nc e. T h i s r e s u l t si n a v o l t a ge a t t he H o u t p u t . A r e l a t i v e l y s m a ll v o l t a g e i s seen a,t t h eV o u t p u t f o r p r ob e w ob ble a f t e r ,phase r o t a t i o n has been a d j u s t e d p r o p e r l y .

' F la t -b o tt o m ed h o l e s d r i l l e d t o v a r i o u s d e p th s an d d i a m e te r s a r eused t o c a l i b r a t e each c o i l f o r measurement o f d e f e c t, d e p th . A f t e r i n i t i a lphase ad ju s tm e nt a nd c a l i b r a t i o n , t h e t e s t c o i l i s r ea dy t o s can t h e t ub es .The c h a r t r e c o r d e r p r o v i d es an i n d i c a t i o n o f t h e d e f e c t s and t h e i r a pp ro x-i m a t e ' l o c a t i o n . T h e V o u t p u t i s t h e be s t i n d i c a t o r , s i n c e i t d i s p l a y s aminimum amount o f outpu t when the p robe wobb les . Tube suppor ts se rve asr e f e r e n c e s i g n a l i n d i c a t i o n s o n b o t h th e V and H o u t p u t s f o r a p pr ox im a tel o c a t i o n o f d e f e c t s a lo n g t h e l e n g t h o f t h e t ub e.

The o u t p u t s i g n a l s f r o m d e f e c t i v e a r e as a r e r e ex am in ed a f t e rt h e s ca n by p l a y i n g t h e F M ' r e co r d e d V and H v o lt ag e s i n t o t h e . v e r t i c a 1and h o r i z o n t a l i n p u t s o f t h e s t o ra g e o s c i l l o s c o p e , r e s p e c t i v e l y . Each

. p o s i t i o n o f t h e r e s u l t a n t beam on t h e o s c i l l o s c o p e s cr ee n r e p r e se n t s t h em a g n i t ud e . a n d p ha se o f t h e im pe da nc e d i f f e r e n c e b etw ee n t h e tw o c o i l s .S i g n a l s c a u s e d b y d e f e c t s a n d t u b e s u p p o r t s m a k e f i g u r e - e i g h t p a t t e r n so n t h e s co pe a nd a r e r e t a i n e d u n t i l t h e y a r e e ra s e d by manual c o n t r o l .The f i g u r e -e i g h t- p a t t e r n i s a c l a s s i c r e s u l t o f t h e d u a l - c o i lr es p on s e. ,F or exam ple,. a s t h e f i r s t c o i l a p pr oa ch e s a f l a t - b o t t o m e dho l e i t s i mpedance c hanges i n m agn i t ude and phas e. The m agn i t ude . r eac hesa maximum when t h e t e s t c o i l i s c e n t e r e d u n de r t h e h o l e . F i g u r e 4 shows

.how t h e f i g u r e - e i g h t p a t t e r n i s t r a c e d by t h e s t o r a g e -o s c i l l o s co p e d i s -p la y . S in ce t h e s i g n a l f ro m t h e f i r s t c o i l i s i n v e r te d by t h e b r i dg en e tw o r k, t h e p h as e o f t h e i m pedanc e d i f f e r enc e v ec t o r s w eeps t h r ough ana ng le w i t h r e sp e c t t o t h e h o r i z o n t a l and r o t a t e s i n t o ' h e f i r s t ' q ua dra nti n t h e c o u n te r cl o ck w is e d i r e c t i o n as i t s c a n s o v e r t h e f l a t - b o t t o m e dh o l e . As t h e f i r s t c o i l l e a v e s t h e h o l e a nd t h e sec on d c o i l a pp ro ac he st h e h o l e , t h e p hase an g le c o n t in u e s t h e s h i f t i n t h e c o u nt e r cl o c k w is ed i r e c t i o n .


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S i nc e s i g n a l s f ro m t h e tw o c o i l s a r e s u b tr a c te d , t h e v e c t o rr a p i d l y de cr ea se s t ow a rd z e r o m ag ni tu de as t h e f i r s t c o i l l e av e s t h e h o l er e g i o n and t h e s econ d c o i l a pp ro ac he s t h e h o l e . The ze r o p o s i t i o n i s t h ec e n t e r o f . t h e o s c i l l o s c o p e d i s p l a y . Note t h a t t h e e f f e c t o f c o i l wobblea nd o t h e r u nw an te d v a r i a b l e s p r e v e n t an e x a c t z e r o f ro m o c c u r r i n g . As thes e c o n d c o i l s ca ns t h e f l a t - b o t t o m e d h o l e , t h e m i r r o r i mage o f t h e d e f e c ti s t r a c e d i n t h e f i r s t q u ad ra nt o f t h e o s c i l lo s c o p e sc ree n. E s t i m a t i o no f d e f e c t d e pt h i s p r o v id e d b y m ea su rin g t h e a ng le , , shown i n ~ i ~ u r e.The angle 4 i s measured fr o m t h e h o r i z o n t a l i n a c l o c k w i s e d i r e c t i o n . Ast h e f l a w d e p th decrease s, t h e e n t i r e s i g n a l p a t t e r n r o t a t e s i n t h e c lo c k -w i s e d i r e c t i o n .

E d d y - cu r r en t r es p on se s i g n a l s f o r v a r i o u s c o n d i t i o n s a r e s howni n F ig ur e 5.. The f requ enc y used was 400 KHz wh ich i s the norma l in spe c-t i o n f r e q ue n c y f o r t h e S e r i e s 51 I n c o n e l 0 . 87 5 - in c h OD, 0 . 0 5 0 -i n c h w a l lt h i c k n e s s t u b i n g . F i g u r e 5 ( b ) shows t h e s i g n a l p a t t e r n s f o r d i s c o n t i -n u i t i e s o f v a r io u s d ep th s. . A t hr ou gh -w a ll h o l e r e s u l t s i n a p a t t e r nwhose phas e ang l e i s no m i na l l y 40 deg rees , w he reas s ha l l ow e r OD d e f e c t sg i v e a p ha se a n g l e g r e a t e r t h a n 40 d eg re es . OD d e f e c t s t e n d t o g i v ephase ang l es w h i c h r ange f r om 40 t h r o ugh 160 deg rees . F i g u r e 5 ( b ) a l s oshows t h e p a t t e r n f o r a 48 p e r c e n t de ep I D d e f e c t . ID d e fe c ts w i l l g i v epa t t e r ns w hos e phas e ang l e v a r i es be t w een nor r l i na l l y 0 and 40 degrees .~ h & , d e f ec ts w hic h i n i t i a t e a t t h e ID s u r f a c e a n d p e n e t r a t e t h r o u g h t h ew a l l w i l l show a t o t a l p ha se s pr e ad o f a p p r o x i m a t e l y 40 de g re e s, w he re asd e f e c t s o r i g i n a t i n g o n t h e OD s u r f a c e and p r o p a g at i ng t h ro u g h t h e t u b ew a l l w i l l e x h i b i t a pha se sp r ea d o f a p p r o x im a t e l y 1 20 d eg re es . The im -p l i c a t i o n o f t h i s i s t h a t , because o f t h e phase co mp re ss io n f o r I D de-f e c t s , l e s s p r e c i s e e st im a te s o f d e f e c t d e pt h w i l l g e n e r a l l y be o b ta in ed .

I n t e r f e r e n c e s i g n a l s o f i n t e r e s t a r e shown i n F i gu r e 5 ( a ) . ,The mo st p r om i n e n t i n t e r f e r i n g s i g n a l s i n c l u d e t u b e s u p p or t s , w ob ble a ndden t s . F i g u r e 5 ( b ) p a t t e r n s a r e a t a f re q u en c y o f 400 KHz, a nd a t t h esame s e n s i t i v i t y as ( a ) . Thus, a d i r e c t c o mp ar is on o f t h e i n t e r f e r i n gs i g n a l s w i t h d i s c o n t i n u i t y s i g n a l s c an be r e a l i z e d .


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Figure 5 . -Typical eddy current response s igna l s .- -ir:.. ,?$ i;.; - Y?I> ; .I '_ - I

8 I > $pi . 8 ' . :


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EXPERIMENTAL INVESTIGATION. -I _ . , I . .# ..> = .'. _ . ;;";; > J.d" 8 -. c , - 1. . ):'";,,. . . I - , = : . . .. . - E L . , i , ,... L., . + . . ,

i t . , . * . -.7,., .. . . . ;:2:t ,Dent Fabrication. .-k, :. .. .

Experimental eva lua tion of th e ef fe c ts of denting on eddy-cur ren t IS1 t echn iques r equ i red the f abr ica t ion o f den t s by a r t i f i c i a lmethods. This was accompl ished by d r i l l ing , tap pin g and th re ad in g two3/4- inch th ick s teel p la tes approximately 2 .inc he s wide by 4 in ch es lo ng.Af te r th e p la te s were t i gh t l y f as tened by 1 /4- inch s t ee l b o l t s , a cen te rhole was dr i l l e d and reamed to a s i ze s l i g h t l y smal le r than the d iameterof the s team generator tubing. The exact center hole diameter was afunct ion of the dent s iz e being induced. A d i e and an extreme den t ar eshown i n Figure 6, -* , . ,= L - ,x--m -=Clamping th e dent d i e around t h e tube and t i g h t en i n g th e s t e e lb o l t s produced a den t approximately equal t o t h e d i f f e r en ce between th etube diameter and th e d i e diameter . The edges of t h e d i e hole wererounded so t h a t th e g r e a t e s t extens ion of th e den t would occur i n th ecen t e r por t ion of th e 3/4-inch t h i c k d i e . Diametral dents of 1 , 2 , 3,3-1/2, 9 and 100 mils were formed in samples of Inconel tubing.* Carbons t e e l tube suppor ts machined t o s imulate corresponding degrees of magne-t i t e formation were placed over the den t s . The annular cav i t y betweenth e tube and tube support p l a t e was f i l l e d w i t h powdered magneti te. The3/4-inch t h i c k tube suppor t p l a t e was capped on one end by a l u c i t e p las -t i c r ing which was held f i rmly i n place w i t h epoxy. Close t o l e r an cebetween th e l u c i t e r i n g and th e o u t s i d e sur face of t h e Inconel t u b i n gf a c i l i t a t e d f i l l i n g and packing t h e tube-to-tube suppor t p l a t e c a v i t ywith magneti te. A t h i n s t e e l cy l i n d e r s l i g h t l y l a r g e r in diameter thanth e Inconel t u b i n g b u t l e s s than the tube support p l a t e hole diameter ,was used t o pack th e magneti te in t h e tube cav i t y . Af te r packing,th e o t h e r end of th e tube suppor ts were sealed with a second l u c i t e r i n g .

*Inconel 600 t u b i n g was prov ided by W esti nghouse f o r this i n v es t i g a t i o nthrough courtesy of Mr. W. D. Fle tcher .


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Figure 6 . Typical d e n t , d i e and l arge dent ,8


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Instrumentation-

The laboratory test equipment employed was compatible with theASME Boiler and Pressure Vessel Code, Section XI , Rules for InserviceInspection of Nucl ear Power Plant Components, Appendix IV , Eddy-CurrentExamination Method for Nonferromagnetic Heat Exchanger Tubing, December31, 1975. A block diagram illustrating specific test equipmcnt is shown -in Figure 7. Zetec ID differential probe coils (OD 0.720 and 0.580 inch) d - .were used. Coil cable length was approximately 65 feet. A Nortec NDT-15eddy-current instrument was used to generate the conventional verticaland horizontal outputs for oscilloscope display and recording. The NDT-15provides continuous adjustment of frequency beyond the range of frequenciesusual ly incorporated in eddy-current instruments used for inspection ofsteam generator tubing. Instrumentation calibration procedures were per-formed in compliance with the Section XI inspection code.

Detection of Discontinuities' . -in Dented Regions $ P 8. . - ,I. Detection of simulated pits and cracks in the dented tube sup-

port region is illustrated in Figures 8 through 10. Figure 8 is a seriesof signal patterns caused by flat-bottomed holes in the tube support re-gion. The fslat-bottomed holes have a depth of 50 percent of the tube walland are 7164-inch in diameter. All holes are located in the cente r ofthe tube support region. The eddy-current measurements were taken withan excitation frequency of 40 0 KHz.

The sharp response from the flat-bottomed hole is clearly dis-tinguishable when no dents are present as indicated by the signal patternin the top left-hand portion o f Figure 8. However, the signal from asmall 0.001 -inch diametral dent partially obscur& the signal from theflat-bottomed hole. Larger dents provide even greater distortion in thesignal patterns so that th e 5 0 percent flat-bottomed holes are not evendetectable.

A series o f experiments were performed with Inconel -600 tubingcontaining long notches to simulate cracking in the tube support region.


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Probe Coil MotorDriverRubberWheel

NortecNDT-15 H V

Filter . " ektronixType 564 B

BrushModel 2 2 0Strip Chart

Figure 7. Laboratory instrumentation.


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50 % FBH

-7-- ube SupportI50%FBH + Tube Support

50% FBH + Tube Support + Fe30q + 3 M i l Dent

50 % FBH

50%FBH + Tube Support + Fe304 + 1 M i l Dent

50%FBH + Tube Suppcrc + r e 0 r 5 M i l u e n ~3 4Figure 8. Signal pat terns for f l a t bot toned ho le I ndent, 400 kHz, sc al e 0.5 v/div.


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They were f a b r i c a t e d by e l e c t r o n d i sch a rg e machin ing on th e tube OD. Thenotches were 1 i n c h l o n g and a p p ro x i ma te l y 0.010-inch wide. The notcheswere machined w i t h a gradual co n to u r t o s i mu l a te th e gradual i n c r e a s i n gdep th of s t r e s s c o r r o s i o n cracks which ca n occur i n steam g e n e ra to r t u b -ing . Shal low den ts 0.001- inch d i a m e t r a l were induced a t t h e l o n g i t u d i n a lc e n t e r o f t h e cracks. Tube suppor ts were then l o c a t e d over t h e c ra ckc e n t e r s and packed w i t h magne t i te .

The s i g n a l p a t t e r n s produced by these samples a r e i l l u s t r a t e di n F i g u re 9. O f s i g n i f i c a n c e i s t h e pronounced d i s t o r t i o n i n t h e c ra ckresponse by the combined e f f e c t o f th e d e n t and tube suppor t . The p re -sence of a d e f e c t i n t h e t u b e w a l l i s suggested by t h e l a r g e v e r t i c a lcomponent o f t h e s i g n a l p a t t e r n s . The s i g n a l p a t t e r n d i s t o r t i o n by t h eden t p r o h i b i t s a ccu ra te measurement o f d e f e c t depth. The presence o fl a r g e dents, i .e . , 5 m i l d i a me t ra l , co mp l e te l y obscures d e t e c t i o n andmeasurement o f l o n g i t u d i n a l OD cracks.

S i m i l a r r e s u l t s were ob ta ined w i t h notches l o c a t e d on t h e I Do f th e tube near t h e den ts i n th e tube su p p o r t r e g i o n . Sh o r t c ra cks o ft h i s t yp e can occur a f t e r d e n t i n g has caused s t r e s s on th e tube I D . Thes i g n a l p a t t e r n s f rom a l o n g i t u d i n a l I D notch, 118- inch long , a r e i l l u s -t r a t e d i n F i g u re 10. S i g n a l p a t t e r n s f rom t r a n s v e r s e ID notches o f com-p a ra b l e depth c o u l d n o t be d i s t i n g u i s h e d i n t h e presence o f s i g n a l scaused by tube suppor ts and dents. Narrow t r a n s v e r s e notches producesma l l s i g n a l s compared t o l o n g i t u d i n a l notches when t h e co n ve n t i o n a leddy-cu r ren t c o i l s a r e employed.

The s i g n a l p a t t e r n s a t t h e to p o f F i g u re 10 i n d i c a t e t h e d i f -f i c u l t y i n d e t e c t i n g I D c ra cks i n dented areas. Since th e s i g n a l p a t -t e r n s f rom both I D c ra ck and den ts produce sha l low angles w i t h r e s p e c tt o th e h o r i z o n t a l a x i s , i . e . , l e s s than 45 degrees, d i s c r i m i n a t i o n be-tween I D cracks and den ts i s most d i f f i c u l t . Dents g r e a t e r than 5 m i l sa r e expected t o obscure th e s i g n a l p a t t e r n s f rom I D d e f e c t s even thoughthey a re 50 p e rce n t o f t h e tube w a l l i n depth.


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Notch

Dent

Notch 40 % of Wall i n 1 MIL uent +Tube Support + Fe 03 4

*- 1 -. k -- . Notch

Notch 60; of Wall i n 1 M i l Dent +Tube Support + Fe 03 4 , = .

Notch

Notch 80%of Wall i n 1 M i l Dent +Tube Support + Fe304", ,

Figure 9. Signal patterns for longitudinal 13D n o & r1 inch long i n dents 400 kHz.


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Notch

Dent I

-mID Notch Only I D Notch + 1 M i l Dent

Notch

ID Notch + 1 M i l Dent + Tube Support + Feg04Figure 10. Dete ctab ility of 60% ongitudinal I.D.notch 118-inch long i n dent, 400 kHz, s c a l e -


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Effects of Reduced Probe Diameter ForInspection of Regions Beyond Restricting Dents

Denting greater than 0.03-inch will require inspection with aprobe of reduced diameter for clear passage. Probe coils of this sizehave relatively small fill factor and therefore a corresponding lowerlevel of sensitivity. If conventional probe coils are used, the smallfi 1 factor wi 1 1 a1 low greater lateral movement of the probe. This wi 1 1increase wobble and cause a higher level of noise signal during inspec-tion. Consequently, it is of interest to evaluate a small diameter probecoi and compare its performance to the conventional 0.720-inch diametercoi .

Tube samples have a nominal ID of 0.775-inch. Eddy-currentprobes having a diameter of 0.720-inch are usually employed to inspectstraight sections of tubing. An appreciably smaller probe of 0.580-inch in diameter was procured from Zetec for comparison with the 0.720-inch diameter probe. The 0.580-inch probe can be used to inspect tubeswith dents as large as 0.195-inch and, therefore, is, considered to berepresentative of a worst-case condition.

Figure 11 illustrates signal patterns produced with the 0.580-inch diameter probe coil compared to signals produced by the 0.720-inchdiameter coil. In each case the signal patterns caused by flat-bottomedholes, 40, 60, 80 and 100 percent of the tube wall, were recorded.

The signal patterns obtained with the 0.580-inch probe wereobtained with the probe located in three lateral positions. Signal pat-terns obtained with the probe coil windings in close proximity to theflat-bottomed holes were accomplished by inserting the probe in an ec-centric plastic sleeve. The probe was rotated in the tube so that thethin side of the sleeve, approximately 0.015-inch thick, was adjacent tothe flat-bottomed holes. A second set of signal patterns was obtainedby inserting the probe in a centering sleeve. The sleeve was machined withan ID of 0.590 inch and an OD of 0.720 inch so that the extent of wobble wasessentially equivalent to that of the conventional probe. A third setof data was obtained by inserting the probe in the eccentric sleeve androtating so that the coil windings were located at a maximum distancefrom the flat-bottomed holes.


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0.720 Inch Probe 3.580 Inch Probe Centered

,L------------------0.080 Inch Probe .%wayfrom FBH0.580 Inch Probe Near FBH

Figure 11. Signal patterns for large clearance probecaused by f l a t b o t t m e d h o le s (FBH) 40, 60, 80, and .100%of wall, 400 kHz, s c a l e - 0.5 v/div.


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With the exception of a small difference in phase angle and asmall decrease in amp1 itude, the signal patterns generated by th e 0.580-inch probe. are comparable to those generated by the 0.720-inch probe whenthe coil windings are located near the defects. However, considerablesensitivity is lost when the coil windings o f 0.580-inch coil are locatedon the opposite side of the tubing. Noise signals caused by probe wob-ble and variations in electrical properties along the length of the tub-ing can easily mask the signal patterns from defects.

When the 0.580-inch probe is centered in the tube, the sensi-tivity is about one-third that of the 0.720-inch probe. If lateral move-ment of the 0.580;inch probe could be restricted to minimize wobble, itis estimated that the 0.580-inch probe could approach the performance ofthe 0.720-inch probe in detecting OD defects. This could be accomplishedby development o f a suitable centering mechanism. Consideration shouldalso be given to optimization of excitation frequency since the coil dia-meter will effect eddy-current distribution of currents in the tube wall.

Detection of Small Dents

Initial eval uation of conventional eddy-turrent inspection o fdents confirmed the belief that relatively large dents, 0.050-inch to0.100-inch on the diameter, completely dominate the signal response ob-tained with the differential, annular coil. Detection of these largerdents is straight forward with the eddy-current test. Further studieshave emphasized the evaluation of smaller dents O. UU I - to U. U05- I fich onthe tub e diameter. It has been o f interest to determine detectabil ityof these small dents and to evaluate eddy-current operating parameterssuch as excitation frequency, probe coil configuration, i . e . , absoluteversus differential and choice of phase reference.

A series of eddy-current signal patterns recorded for frequen-cies ranging from 100 KHz to 700 KHz are illustrated in Figures 12through 15. It is instructive to observe the signal patterns caused byisolated dents and tube supports as well as those attributed to com-binations of these variables. The signal patterns produced by 7/64-inch


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2 - r i l Dent Only 3-mil Dent + Tube Support + Fe 03 4

Tube Support Only Flac Bottomed Holes - 40 , 60 , 80 , 100%~f Wall= - -_

Figure 12 . Signal patterns for dent and tube su?port,10 0 kHz, s c a l e - 0 .5 v/div.


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2-mi l Dent Only.r . :

- .

= # 3-mil Dent + "be Support + F e 0r - 3 4

Tube Support Only % ,': 4. .,. F la t Bottomed H 2 l-. . -q IW - lf 80 100%of Wall.- - Ff

'IC-Figure 13 . 'Signal patterns for dent and tube' su?pcrt,200 kHz, s c a l e - 0.5vIdiv.


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2-mil Dent Only

Tube Scpport t h l y ,-6;

3-mil Dent + Tube Suppcr t + Fe O3 4

F l a t Bottomed Hol e s - 40 , 60 , 80 , 100% of Wall. .Figure 14 . Signal patterns f o r dent and tube support,

40 0 kHz, s c a l e - 0.5 v/5iv.


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2-mil Dent Only 3-mil Dent + Tube Support + Fe 03 4

Tube Supp ort Only F l a t Bottomed Holes - 40, 60, 80, 100% of WallF igu re 15. S i g n a l p a t t e r n s f o r de n t and tube s uppor t ,

700 kHz, s c a l e - 0.5 v ld iv , ,. ,


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d iam eter ' f l a t - b o t t o m id ho les , 40, 60 , 80 and 100 pe rce nt was a1 so reco rde da t e a ch f r e q u e n c y f o r c o m pa ri so n . ,

: F i g u re s 1 2 t h r ou g h 1 5 show a m arke d in c r e a s e i n s e n s i t i v i t y t o ,t h e s m a l l 0 . 00 2 -i nc h d e n t as f re q u e n c y i s i n c re a s e d . conve rse l y , t h e sen-s i i v i . t y t o t h e c a r b o h ' s t e e l t u be s u p p o rt : d ec re as es r a p i d l y with f r equency .C o n se q u en t ly , t h e c o m b i n a t i o n o f a t u b e s u p p o r t a n d s m a l l d e n t i n ' t he , t u b e , .causes a s l g h t l y d i s t o r t e d t u b e s u p p o r t s , ib na l p a t t e r n a t. .1 00 W z . How- 'e v e r, a t 700 KHz, t h e s i g n a l p a t t e r n , shown i n t h e u p pe r r i g h t - h a n d o f :

. F ig u re 15, i s caused p r i m a r i 1y :by t he 0 ,002 -i nch den t . T .he refo re , i t i sc o n c lu d e d t h a t s m a l l d e n t s c an e a s i 1 .y -b e d e t e c t e d b y o p e r a t i n g t h e eddy-c u r r e n t i n s t ru m e n t a t i o n a t r e l a t i v e l y h i g h fr eq u en ci es . , Based, on t h e s eexper, iments, i t s h o u ld b e p o s s i b l e t o d e t e c t d e n t s on t h e o r d e r o f. 0.0 01 -i n ch u s in g ' t he 400 KHz o pe ra t i n g f r equency . A1 though th e f r eq uen c iesab ove 400 KHz p r o v i d e g r e a t e r s e n s i . t i v i y t o d e n t i n g , . c e r t a i n d is ad va n -t a ge s , s uc h as i n c r e a s e d s e n s i t i v i t y t o wo bb le an d p r ob l em s a s s o c i a t e dw i t h c a b l e c a p ac i ta n c e , may n o t w a r r a n t o p e r a t i o n a t h i g h e r f r e q u e n c i e s

. w i t h c o n ve n t io n a l i n s t r u m e n t a ti o n .A d d i t i o n a l e nha nce me nt o f t h e s m a l l d e n t s i g n a l c an a l ' s o ' b e

a c h i e v e d a s f o l l o w s . S in c e d e n t s o c cu r w i g h i n t h e c o n f i n e s o f t h e t u b e' suppor t a rea , t h e t ube supp or t can be cons ide red as an ex t raneous no i ses i g n a l . E x i s t i n g IS1 e d d y - c u r r e n t p r o c e d ur e s a r e s uc h t h a t a t 400 KHz,. .t h e t u b e. . suppo r t s i gn a l m akes an ang1.e o f , a pp ro x i m a te l y ,4 5 deg rees , i . -e. ,. .th e in -phas e and qu ad ra t ure compo,nents ar e equ a l . I f t h e e d d y - cu r r en tp h a s e r o t a t i o n. i s ch os en p r o p e r l y , t h e t u b e s u p p o rt s i g n a l c an be m i n i - :m iz ed i n , t h e v e r t i c a l c ha nn el and m ax im iz ed i n t h e h o r i z o n t a l c h an n el .D en t pr es en ce i s t h e n d e te rm in e d .by m o n i t o r i n g t h e o u t p u t o f t h e v e r t i c a l'channel .' he d e n t s i g n a l i s now a s i g n a l o f i n t e r e s t , whereas t h e t u be

. .s up p or t i s a n o i s e s i g n a l . B y m i n i m i z i n g t h e t u b e s u p p o r t s i g n a l . , t h ed e n t - t o - s u p p o r t s i g n a l - t o - n o i s e r a t i o i s m ax im iz ed e nh a nc in g d e n t d e t e c -. . .t a b i 1 t y .

A c om pa ris on o f t h e p r e v i o u s l y d e s c r i b e d p r oc e d ur e s f o r d e t e c -t i o n o f d e n ts u nd er s u p po r ts i s .shown i n F i g u r e s 16 and 1 7 f o r d i f f e r e n -t i a l and a b s o i u t e p ro b e c o i l s . . F ig u re 16 ill s t r a t e s r e s u l t s o b t a i n e dw i t h t he d i f f e r e n t i a l c o i l . A t t h e t o p o f t h e f i g u r e , t h e no rm al ISIs e t - u p i s shown w h i 'l e t h e b o t t o m o f t h e f i g u r e shows r e s u l t s when t h e


42/54

Dents Only Dents pl us Tube Sup port Tube Sup por t Onlyn

BRUSHACCUCHART1 I I I I I f I ; I

' CodeI . , II , , S e t u p

TubeSuppor t sMinimum

F i g u re 1 6. S t r i p c h a r t k e c or d i ng s o f d i f f e r e n t i a lc o i l . i l l u s t r a t i n g d e t e c t a b i l i t y of d e nt s u nders u p p o r t s . - 28 -


43/54

Dents Only Dents pl us Tube Sup por ts Tube Support Only

CodeS e t u p

I I I I I I I I I1 , I I l I I f ~ ~ ~ , I I I I I I I . t Tube~~~~~r~~

F i g u r e 1 7 . S t r i p . c h a r t r e c o r d i n g s of a b s o l u t e c o i li l l u s t r a t i n g d e t e c t a b i l i t y of d e n t s un d e r s u p p o r t s .

- -Minimum


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phase r o t a t i o n i s s e t t o m in im i ze t h e tu be s u p po rt s i g n a l i n t h e v e r t i c a lc ha nn el . L e f t ' t o r i g h t i n F i g u r e 16 a r e r e s p e c t i v e l y d e nt s o n ly . d en ts u n de rt ube s uppo r t s , and t ube s uppo r t s on l y . F i v e d e n t s i z e s w ere c o n s id e r e d;thes'e were 9 m i l s , 3 . 5 m i l . s , 3 m i l s , 2 m i l s and 1 m i l d i a m e t r a l . F i g u r e17 d a t a l a y o u t i s i d e n t i c a l w i t h F i g u r e 16, e xc ep t a l l d a t a w ere t a ke n

. w i t h a n a b s o l u t e p ro be c o i l . The a b s o l u t e c o i l c o n f i g u r a t i o n w a s, ac h ie v edby u t i l i z i n g th e f r o n t h a l f o f a d i f f e r e n t i a l c o i l a nd ba la nc in g a g a in s to n e- h al f o f a no th er d i f f e r e n t i a l c o i l p l a ce d i n a good s e c t i o n o f I n c o n e l -600 tub ing .

A d e t a i l e d c o n s i d e r a t i o n of F i g ur e s 16 and 1 7 i s r a t h e r i n v o l ve d ,b u t i n g en e ra l , t h e f o l l o w i n g c o n c l us i on s c an be rr~ade: ( 1 ) I n a l l f o u r

Icases cons idered , i e. , a b s o l u t e an d d i f f e r e n t i a l p r ob e c o i l s , n or rr ~a l S 1. s e t - up and t ube s uppo r t s m i n i m i z ed on t h e v e r t i c a l c hannel , den t s downt o 2 m i l s c an be r e a d i l y i d e n t i f i e d w i t h i n t h e t ub e s up po rts ; ( 2 ) F o rd e n t s on t h e o r d e r o f one m i.1, d i f f e r e n c e s e x i s t b etw ee n t h e d e n t s p l u st ube s uppo r t w av e f o r m and t he t ube s uppo r t w av e f o r m s o t ha t den t p r es enc ec an i n ge ne ra l be i d e n t i f i e d ; ( 3 ) T he maximum r a t i o o f d e n t s i g n a l t o t u bes u p p or t ; i . e . , maximum s i g n a l - t o - n o i s e r a t i o , i s a c hi ev e d f o r t h e c as ew i t h t h e a b s o lu t e p ro be w i t h t h e t ub e s u pp o rt m i n i n ~ iz e d n t h e v e r t i c a lc ha nn el . T h i s . l a t t e r i t e m c an be seen i n t h e l ow e r h a l f o f F i g u re 17.H e re a n o rm a l t u b e s u p p o r t s i g n a l s w in g s p o s i t i v e on t h e h o r i z o n t a l c ha n-ne l , whereas a d e n t s i g n a l s w in g s n e g a t i v e on th e ( v e r t i c a l c h an n el .

D e t e c t i o n. o f M a g n e t i t e F o r m at io n

E a r l y d e t e c t i o n o f d e n t i n g wo uld a l l o w im m ed ia te a l t e r a t i o no f t h e r e a c t o r o p e r a t in g c o n d i t i o n t o p r e ve n t f u r t h e r ch em ic al a t t a c kon t h e t ube s uppo r t s . T he r e f o r e , i t i s o f . i n t e r e s t t o d e t e c t t h e forma-t i o n o f m a g n e t i t e be tw ee n t h e t u b e ahd t u b e - su p p o r t b e f o r e d e n t i n g o cc u r s.F i gu r e 18 shows t he no rm a l I S 1 s e t - up edd y - c u r r en t r es pons e s i g na l s a t100 KHz 'and 400 KHz, f o r tube s upp or ts w i t h and w i t ho u t m ag net i te packed /w i t h i n t h e t u b e / t u b e s u p p o r t a n n u la r gap. A c om p ar is on o f t h e s i g n a lp a t t e r n s r e v e a l a s l i g h t c ha nge i n p h a s e 'a n g l e and a bo u t 30 p e r c e n t r e -d u c t i o n i n s i g n a l a m p l it u d e when t h e m a g n e t i te i s p r e se n t . A n a l y s i s o f


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. * ' . m w - . F.-' 8 -'. . . Tube Support Only

- Tube Support + Feg04

Tube Support Only

Tube Support + Fe304

400 kHz, scale - 3.5v;div100 kHz, scale - lv/div

Figure 18: Signal patterns for tube support withand without magnetite.


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the associated s t r ip chart recordings indicate the change or reduced ampl i -tude to occur in the horizontal component of the support signal. No changein waveform shape other than a reduction in amplitude was observed.

A magnetite packed tube-support and normal tube support was re-scanned using an absolute coil configuration and the tube supports mini-m zed procedure described previously. The resultant s t r i p chart record-ings are shown in Figure 19. Comparing the signal responses, one can againsee a reduction in support signal amplitude when magnetite i s present underthe tube support. The presence of the magnetite also appears to changethe signal shape slightly. The tube support with magnetite waveform i sless rounded than the support plate only case. Thus, the absolute coilmay offer advantages in the detection of magnetite because of changes innormal support plate signal ampl itude as we11 as signal wave shape.

Estimation of Dent Size

The fe asib il i ty of estimating dent size using eddy-current tech-niques was considered by scanning a se ries of known dents and observingthe resultant signal response. Both absolute and' di fferen ti al probe co il swere uti lized. In it ia ll y i t was reasoned tha t the dif ferenti al coilwould be sensitive t o axial dent contour changes since i t ac ts as a d if -fe ren tia tor. Hence, an absolute probe coil might be expected to providea more accurate measure of dent size.

The resultant eddy-current response signals are shown in Figure20, for five dent sizes, i.e., 1 , 2 , 3 , 3.5 dr~d9 mils, for both the abso-1ute and di fferenti a1 probe coi 1. A1 1 dents were positioned under tubesupports. The horizontal channel pos it ive peak deflection ampl i tude ofFigure 20 ( b ) and the vertical channel negative peak deflection amplitudeof Figure 20 (a ) are plotted in Figure 21. The relationship between peakamplitude and dent size i s approximately linear on a log-log plot over adent size range of 2 to 9 mils, for both the absolute and differentialco il . For the absolute coil there i s an apparent curvature below 2 mils.The same dents without tube supports were rescanned several times inde-pendently using absolute and differential probe coils and the resultant


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Vert ical(200 mvldiv)

BRUSH ACCUCHART1 1 1 1 I I ~ ' 1 I ~

Horizontal(200 mvldiv)

1 I I I I I I I L 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1I " " ' " " " "

Normal Tube Support Tube Support + Feg04Figure 1 9 . Absolute c o i l response to tubesuppoyts, 400 KHz.


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(a )A b s o l u t e Coil- TubeSupport s Minimum

. . . . . . . .- q . .- I :- '. . .. . . . . . . . . . . . . . . . . . . . . . . . . . . .. i . . . . . . ._. . . . . . .. . . . . _ . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . . . . . . : i . ; . . . ... ; ' ; : . I . : ::.::::.:.::.,.$.','.: ":.: *.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . : . :. . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. < . _ * . . ! * . - I -' : . " . . . .. . . . . . . . .. . . . . . . . . . . .. . . . . . . :.,_ . . : : .:i... . . - . . . . . . . . . . . . . . .. . . ". . . . . . . . . . . . . . ...... .... ..... f i . . .i.: . . . r ! . : : . ; : t : : : : i . : : . .

D i f f e r e n t i a l C o i l - NormalI S 1 Setup

Figure 20 . S t r i p chart recordings of dents ofvarious s i z e s using absolute and d i f f e r e n t i a lc o i l s .

DENT S l Z Eml l r DIAMETRALFigure 2 1 . Dent s i z e cal ibra t ion charts f or both

abso lute and d i f f ere nt ia l co i l s .


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c a l i b r a t i o n c u r v es a r e s om etim es 1 n e a r o r c u r v e d b elo w 2 m i l s d i a m e t r a l .T h is d i sc re p a n cy has n o t bcen re so l ve d b u t may be du e t o p ro b e p o s i t i o n in g ;

I n or de r t o i n v e s t i g a t e t h e a b i l i t y o f t h e e dd y- cu rr en t t ec h -n i q u e t o e s t i m a t e d e n t s i z e , t h e f o l l o w i n g e x p e r im e n t was c o nd u ct e d. Twoa d d i t i o n a l d e n ts were f a b r i c a t e d . The den ts were measured subsequent t ot h e e x pe r im e n t a nd f ou n d t o be 2 m i l s d i a m e t r a l . The e d d y 2 c u r r en t i n s t r u - .ment was s e t up u s i n g a d i f f e r e n t i a l p ro b e c o i l w i t h t h e s u p p o rt s s i g n a ln o n-m in im ize d , i .e ., '. n orm al IS 1 p ro ce d u re s . The d e n t c a l i b r a t i o n s ta n d a rdu se d t o e s t a b l i s h t h e c a l i b r a . t i o n c u r v e i n F i g u r e 21 was s c ann ed a nd t h ee d d y -c u r re n t s e n s i t i v i t y was a d j u s t e d u n t i l a s i , m i l a r c u r v e was o b ta i ne d .The tw o a d d i t i o n a l d e n t s we re t h e n sc ann ed f o u r t i m e s w i t h t h e r e s u l t a n tpeak s i g n a l a m p l i tu d e on t h e s t r i p - c h a r t r e c o r d e r n ot ed .

The average peak am pl i tu de obt a i ne d was 69.25 mv, w i t h extremesbeing 65 mv and 72 mv. I f t h e a ve ra ge p eak a m p l i t u d e i s u s ed t o e n t e r t h el ow e r c a l i b r a t i o n c u rv e i n F i gu r e 21, a d ia m e t ra l d e n t s i z e o f a b ou t 2 .5m i l s i s o bt ain ed . T h i s g i v e s a n e r r o r on t h e o r d e r o f 25 p e r c e n t.

The i n t e ra c t i o n b etwe en a n e d d y -cu r re n t p ro be c o i 1 and a d e n ti s r e l a t i v e l y complex. The den t vo lume and ax ia l den t con tou r can beex pec te d t o a f f e c t t h e r e s u l t a n t d en t s i g n a l . B o t h t h e a b s o l u t e a n d d i f -f e r e n t i a l p ro be c o i l s a r e s e n s i t i v e t o d en t c o n to u r as c an be seen byc om pa rin g t h e % m i l d e n t s i g n a t u r e s o f F i g u r e 20, i . e . , h o r i z o n t a l cha n-n e l i n ( b ) and t h e v e r t i c a l c hannel o f ( a ) . The p e r t u r b a t i o n on t h e 3-m i l d e nt s i g n a t ur e i s p o s s i b l y due t o a x i a l v a r i a t i o n s i n t h e d en t.

The e f f e c t o f d e nt volum e i s i l l u s t r a t e d i n F ig u r e 22, where t h er es p on s e f r o m a 7 - m i l r a d i a l d i n g i s shown. A d i n g i s an i n d e n t a t i o n o ft h e t u b e w a l l w hi ch i s l o c a l i z e d i n a re a, as opposed t o a d e n t w h ic h e x i s t so v er t h e e n t i r e c i rc u m f er e n ce o f t h e t u b e. The r e s u l t a n t e d d y -c u r r e n t ,response i s s i m i l a r t o t h a t f r om a d e n t b u t t h e a m p l i tu d e o f t h e r es po ns ei s r educed s i g n i f i c a n t l y . The d i n g i n F ig u re 2 2 was sca nn ed a t t h e sa m e.s e n s i t i v i t y used t o e s t a b l i s h t h e d en t c a l i b r a t i o n c u rv e i n F i gu re 20 ( b ) .The s t r i p - c h a r t s e n s i t i v i t y i n F ig u re 22 i s t w i ce t h a t o f F i g ur e 20. Thus,i f t he d i f f e r e n t i a l c o i l c a l i b r a t i o n c h a r t i n F i gu re 20 ( b ) i s used t oe s t i m a t e t h e s i z e , an u n d e r e s t i m a t i o n w o ul d r e s u l t .


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Figure 22. Strip charc recording of seven mil ,Ding. (Localized tube indentation. )


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0 D e n t s g r e a t e r t h a n 2 m i l s d i am e t ra l a r e r e l i a b l y d e te c te du s in g e i t h e r a d i f f e r e n t i a l o r a b s o l u t e p robe c o i l . D en tson t h e o r d e r o f 1 m i l d i a m e t r a l a r e more r e l i a b l y d e t e c te du s i n g an a b s o l u t e pr ob e c o i l w i t h t h e phas e r o t a t i o n s uc ht h a t t h e t u be s u pp or t s i g n a l i s m in im iz ed i n t h e v e r t i c a lc ha nn el o f t h e e d dy - c ur re n t s t y i p c h a r t r e c o rd e r . A d e n td e t e c t i o n f r e q u e n c y o f 4 00 KHz, i . e . , t h e n o rm a l i n s p e c -t i o n f re qu en cy f o r @ ~ e r i e s 51 PWR s te am g e n e r a t o r s , i sa reas onab le compromise be tween enhancemen t o f t he sm a l ld e n t s i g n a l and m i n i m i z i n g e f f e c t s o f p ro be w ob ble .

0 L a b o r a t o r y e x p er im e n ts i n d i c a t e t h a t c o n v e n t i o n a l e dd y-c u r r e n t IS1 p r o c e d u re s ca n b e 'u s e d t o d e t e c t t h e p r e s e nc eo f m a g n e t i t e l o c a t e d b etw ee n t h e t u b e a nd t u b e s u p p o r tp r i o r t o th e fo r m a t io n o f d e n ti n g. A c t ua l i m pl em e nt at io ni n t o e x i s t i n g i n s e r v i c e i n s p e c t i o n s i s dep en de nt o n ad e-q u a te p re - s e r v i c e b a s e l i n e d a t a and c a r e f u l a n a l y s i s o ft u b e s u p p o r t s i g n a l s .

a D en ti ng has a s i g n i f i c a n t e f f e c t on t h e d e t e c t a b i l i t ya nd measurem ent o f d i s c o n t i n u i t i e s i n t h e d e nt ed r e g i o n .A 50 p e r c e n t t h r o u g h - w a l l FBH i s u n d e t e c t a b l e i n d en t s3 m i l s d i a m e t r a l o r g r e a te r . It i s e s ti m at ed t h a t s ma llvolum e d e f e c ts , i . e . , c ra c ks , w i l l n o t be r e l i a b l y de -t e c t e d a nd me as ured i n d e n t s g r e a t e r t h a n 1 m i l d i a m e t r a l .

a Pr ob e s o f r e d u c e d d i a me t e r , wh i c h c a n p a ss t h r o u g hs e v e r e l y d e n te d r e g i o n s , c an b e u se d t o d e t e c t d i s c o n -t i n u i t i e s i n u nde nte d s e c t io n s o f s team g e ne r a t o r t u b-i n g i f a c e n t e r i n g m ec hanis m i s em ploy ed t o m a i n t a i nc o l l i n e a r a l i g nm en t o f t h e c o i l c e n t ra l a x i s w i t h t h a to f t h e tu be . E xc es si v e wobble o r l a t e r a l s h i f t o f th ep ro be c o i l w i l l c aus e s i g n i f i c a n t changes i n p r ob e sen-s i i v i y.


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The a b s o l u t e p ro b e c o i l , i n c o n j u n c t i o n w i t h a n eddy-c u r r e n t s e tu p p r oc e du re i n w h ic h t h e t u b e s u p p or t s i g -n a l i s r o t a t e d s uch t h a t i s a pp ea rs p re d om in a nt ly ont h e h o r i z o n t a l a x i s o f t h e s t r i p c h a r t r e co r d er , a pp ea rst o o f f e r. advantages i n d en t d e t e c t i o n r e l i a b i l i t y . F ort h e c a se o f s m a l l d e n t s , i t s h o u ld a l s o ' o f f e r an a dvan-t a g e i n d e n t li le as ure me nt s i n c e t h e e f f e c t s o f t u b e s u p-p o r t s a r e m ore r e a d i l y m in im iz e d. The e f f e c t s o f te m-pe ra tu re changes, wobb le and tube ID changes need t o bem o r e c a r e f u l l y e v a l u a t ed . '

SUGGESTED AREAS FOR FURTHER RESEARCH

Wi t h emphas is on the d en t i n g phenomenon, sugges ted areas f o rf u r t h e r r es e ar ch a c t i v i t i e s a r e b e s t d i re c t e d t ow ard s t e s t c o i l d es ig n.S p e c i f i c c o i l c o n f i g u r a t i o n s a r e as f o l l o w s :

Pancake Co i l

Enhancement o f t h e d e t e c t i o n o f d i s c o n t i n u i t i e s i n d e nt ed r e g i o nsc an b e e x p e c t e d b y u s e o f a s m a l l d i a m e t e r p a nc ak e c o i l . T h i s t y p e o f c o i lw o u ld i n d u c e eddy c u r r e n t s b o t h p e r p e n d i c u l a r a nd p a ra 1 e l t o ' t h e l o n g i -t u d i n a l a x i s o f t h e t u b e and hence be e q u a l l y s e n s i t i v e t o t r a n s v e rs e andl o n g i t u d i n a l d e fe c ts . Use o f a s m a l l d i a l ,~ e t & c o i l w o u ld i n c r e a s e t h e d e-f e c t s i g n a l t o d en t s i g n a l r a t i o i m pr ov in g d e t e c t i o n r e 1 i a b i l i t . y . T h i st y p e o f c o i l w o uld be a p p l i c a b l e t o s m a ll a nd l a r g e d en ts .. .

Cross-Wound Coi l '

S ma ll d e n t s i g n a l s a nd t h e e f f e c t s o f t u b e s u p p o r t s may be r e -du ce d by t h e u se o f a c i r c u m f e r e n t i a l d i f f e r e n t i a l c o i l as o pposed t o, t h e e x i s t i n g a x i a l d t f f e r e n t i a l c o i l . S in ce s m al l de n t s and t ub e sup-

p o r t s . a r e c i r c u l a r s ym m etr ic , a c ro ss -w ou nd c o i l w o uld t e n d t o s u b t r a c to r m in i m iz e th e se i n t e r f e r i n g s i g n a l s e n ha nc in g a d e f e c t s i g n a l .


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E l e c t r o n i c A bs ol u t e I D i f f e r e n t i a 1--B oth t h e a b s o l u te and d i f f e r e n t i a l c o i l c o n f i g ur a t i o ns o f f e r

a dv an ta ge s i n t h e i n s p e c t i o n o f s te am g e n e r a t o r t u b i n g . R a t he r t h a np a r a l l e l i n g tw o s e pa ra te c o i l s ys te ms , a p p r o p r i a t e e l e c t r o n i c s i g n a lp r o c e s s i n g may ' ac h ie v e t h e d e s i r e d r e s u l t s . As an example , th e ou t - ,p u.t o f a s i n g l e a b s o l u t e p ro be may be e l e c t r o n i c a l l y d i f f e r e n t i a t e d ,i e ., h i gh - pa s s f i l t e r e d , a n d ' p a ra 1 e l e d . s .im ul a n e o u s l y w i t h t h e u n-f i l t e r e d o u t p u t . The o u t p u ts o f a d i f f e r e n t i a l c o i l can be i n t e g r a t e d ,i . e . , lo w-p as s f i l t e r e d and a g a i n paralleled~simultaneouslyw i t h t heu n f i l t e r e d o u tp u t . I n e i t h e r case, a s i n g l e c o i i c , on f i gu ra t i on i fa p p r o p r i a t e l y p ro c es s ed may o f f e r t h e a d v a n ta g e sv o f a t w o - c o i l S ys tem .


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