quantitative risk analysis in the chemical process industry

7/22/2019 Quantitative Risk Analysis in the Chemical Process Industry

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Reliability Engineering and System Safety 29 (1990) 55-68

Q u a n t i t a t i v e R i s k A n a l y s i s i n t h e C h e m i c a l P r o c e s sI n d u s t r y

Gary R Van SciverRohm and Haas Com pany, Box 584, Bristol, Pennsylvania 19007, USA

A B S T R A C TQu ant i ta t ive Ris k A nalysis Q R A ) i s a tool that i s being used increasingly inthe chemica l process i ndus try C P I ) t o he lp preven t rare bu t po ten t ia ll ycatastrophic events . The QRA methodology includes:

1) es tabl i shing QRA priori t ies ,2) identi fying accident scenarios,3 ) quan t i fying the f req ue nc y o f each scenario ,4 ) quan t i fying the consequences o f each scenario a nd5 ) quan t i fying to ta l r isk .

A wide vari e ty o f f ac to rs con t r ibu te t o t he ra ther l arge uncer ta in ty o f QR Aresults.

The resu l ts o f a Q RA can be expressed in terms o f abso lu te r i sk w h ich canbe com pare d to es tabl i shed levels o f unacceptabil ity. The resu l t s can also beexp resse d in term s o f rela tive r i sk , indicating the ef fect on r i sk o f variousdesign options. H owe ver, the m ost im portan t resul t o f Q R A is the operat ionalins igh t s revea led to t he ana lys t s wh ich l ead to r i sk reduct ion andunderstanding o f the sources o f residual r isk . This pap er describes the Q R Amethod as i t i s u sed in t he CPI .

I N T R O D U C T I O NI n t h e l a s t f e w y e a r s t h e r e h a s b e e n i n c r e a s i n g e m p h a s i s o n t h e p r e v e n t io n o fr ar e b u t p o t e n t i a l l y c a t a s t r o p h i c e v e n t s in t h e c h e m i c a l p r o c e s s i n d u s t r y( C P I) . S o p h i s t i c a t e d t e c h n i q u e s a r e b e i n g u s e d t o i d e n t i f y h i g h r i s k

55Reliability Engineering and System Safety 0951-8320/90/$03 50 1990 Elsevier SciencePublishers Ltd, England. Printed in Gre at Bri ta in


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56 Gary R Van Scivero p e r a t i o n s a n d t o i d e n t i fy m e a n s f o r r e d u c i n g t h e r is k o f ac c i d e n ts in t h e s eo p e r a t i o n s . Q u a n t i t a t i v e R i s k A n a l y s is ( Q R A ) is a m o n g t h e m o s t i m p o r t a n to f t h e s e t e c h n i q u e s .

R i s k is t h e l i k e l i h o o d o f su f f e ri n g l o ss o r i n j u r y . It is a f u n c t i o n o f t h r e ef a ct o r s: w h a t c a n g o w r o n g , t h e l i k e l i h o o d o f it g o i n g w r o n g , a n d t h ec o n s e q u e n c e s o f it g o i n g w r o n g . I n a Q R A , a n a n a l y s t i de n ti fi es a c c id e n ts c e n a r i o s a s s o c i a t e d w i t h a n o p e r a t i o n a n d q u a n t i f i e s t h e l i k e l i h o o d a n dc o n s e q u e n c e s o f t h e s e a c c i d en t s .

Q R A h a s b e e n i n e x i s t e n c e f o r m a n y y e a r s . B e f o r e it s u s e i n t h e C P I , it w a su s e d e x t e n s i v el y i n t h e n u c l e a r i n d u s t r y . 1 U n f o r t u n a t e l y , t h e a p p l i c a t i o n o fQ R A i n t h e C P I i s m u c h m o r e d i f fi cu l t t h a n i n th e n u c l e a r i n d u s tr y . T h i s isb e c a u s e o f t h e g r e a t e r d i v e r s i t y o f p r o c e ss e s , h a z a r d o u s m a t e r i a l s ,e q u i p m e n t t y p e s a n d c o n t r o l s c h e m e s i n t h e C P I . T h i s d i v e rs i ty r e q u ir e s n e wc a p a b i l it i e s o f Q R A , c a p a b i l i ti e s t h a t a r e c o n t i n u o u s l y ev o l v i n g .

I n th i s p a p e r , t h e m e t h o d o l o g y o f Q R A ( w i th a n e x a m p l e) , t h ei n t e r p r e t a t i o n o f Q R A r es u lt s, a n d t h e fu t u r e o f Q R A i n t h e C P I a r ed i s c u s s e d .

P R O C E S S S C R E E N I N GQ R A is a t o o l t h a t u s u a l l y re q u i r e s a l a rg e e x p e n d i t u r e o f t i m e a n d m o n e y .I n a n y g i v e n y e a r , i t m a y b e f e a si b l e t o l o o k a t o n l y a f e w o p e r a t i o n s w i t h i n ac o m p a n y . T h e r e f o r e , it is e s s e n t ia l t h a t Q R A b e s e le c t iv e l y a p p l i e d o n ap r i o r i t y b as is . It w o u l d b e a n u n f o r t u n a t e m i s a p p r o p r i a t i o n o f r e s o u r c e s t op e r f o r m a Q R A o n a n o p e r a t i o n w h i c h is r e la t iv e l y l o w r is k a n d i g n o r e o n ew h i c h i s r e l a t i v e l y h i g h r i s k .

E n g i n e e r i n g a n d m a n a g e m e n t j u d g e m e n t p l a y a n i m p o r t a n t r o le in s e t ti ngp r i o ri t ie s f o r Q R A . F o r e x a m p l e , t h e r e m a y b e a p a r t ic u l a r o p e r a t i o n w h i c hh a n d l e s l a r g e q u a n t i t i e s o f a n e s p e ci a ll y h a z a r d o u s s u b s t a n c e o r t h e r e m a yb e a f a c i l i ty w h i c h h a s a h i s t o r y o f a c c i d e n t s o r o f n e a r m i s s es . T a b l e 1 is a li sto f t y p i ca l f a c t o r s t h a t m a n a g e m e n t c o n s i d e r s i n s e le c ti n g sy s t e m s f o r a

T A B L EFactors A ffecting the Need for Q RAIs a Q RA of the process needed to satisfy regulatory requirements?Is the process a new design?Are the m aterials in the process highly flammable or highly toxic?Does the process (or a similar process) have a history of accidents or near m isses?Is there public concern abou t th e process materials?Is the process likely to experience a runaway reaction?W ill extended process outages due to accidents result in lo ss of market share?


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uantitative risk analysis in the chem ical process industry 57

Q R A . T h e m o r e q u e s t i o n s i n t h i s t a b l e t h a t a r e a n s w e r e d y e s , t h e g r e a t e rt h e n e e d f o r a Q R A .A l t h o u g h e s t ab l i sh i n g Q R A p r i or i ti e s is u su a l ly a q u a l i t a t iv e p r o c e d u r e ,q u a n t i t a t i v e t e c h n i q u e s d o e x i st w h i c h c a n h e l p . F o r e x a m p l e , t h e D o w F i r ea n d E x p l o s i o n I n d e x , 2 t h e M o n d F i re , E x p l o s i o n a n d T o x i c i ty I n d e x 3 o ro t h e r i n d i c e s 4 c a n b e u s e d . T h e s e a r e q u i c k a n d s y s t e m a t i c p r o c e d u r e s f o rr a n k i n g p r o c e s s e s b y t h e i r in t r i n s ic h a z a r d s . O p e r a t i o n s w h i c h a r e a ss e s se dt h e h i g h e s t i n d e x v a l u e w o u l d b e c a n d i d a t e s f o r Q R A .

H A Z A R D I D E N T I F I C A T I O NT h e f i rs t s te p o f a Q R A i s h a z a r d i d e n t i fi c a t io n . T h e o b j e c t i v e o f t h is s t e p ist o a n s w e r th e q u e s t i o n : w h a t c a n g o w r o n g ? T h is i s t h e m o s t i m p o r t a n t s te pb e c a u s e h a z a r d s t h a t a r e n o t i d e n t i f i e d w i l l n o t b e q u a n t i f i e d , l e a d i n g t o a nu n d e r e s t i m a t e d r is k. I t is i m p o r t a n t t o d e v o t e a d e q u a t e r e s o u r c e s to t h i s s te pt o i n su r e a t h o r o u g h u n d e r s t a n d i n g o f t h e h a z a r d s.

S o m e o f t h e t e c h n i q u e s u s e d f o r h a z a r d i d e n t i f i c a t i o n i n c l u d e H A Z O PA n a l y s is , F a i l u r e M o d e a n d E f fe c ts A n a l y s i s ( F M E A ) , ' W h a t I f ' A n a l y si s,P r e l i m i n a r y H a z a r d A n a l y s is (P H A ) a n d C h e c k l is t A n a ly s is . T h e A I C h E -C C P S G u i d el in e s f o r H a z a r d E v a l u a t io n P r o c e d u r e s d e s c r i b e t h e s et e c h n i q u e s i n d e t a i l . T h e m e t h o d u s e d m o s t o f t e n i n t h e C P I i s H A Z O PAnalys i s .T h e w o r d H A Z O P is s h o rt f o r H A Z a r d a n d O P e r a b i l i t y . T h e t e c h n i q u ew a s i n v e n t e d b y t h e I C I i n t h e e a r l y 1 97 0s, 5 a n d h a s b e c o m e w i d e l y u s e d i nt h e U S A i n t h e l a s t 5 - 1 0 y e a r s . I n H A Z O P A n a l y s i s , b r a i n s t o r m i n gt e c h n i q u e s a r e u se d t o m e t h o d i c a l l y d e t e r m i n e t h e ca u s e s a n d c o n s e q u e n c e so f d e v i a ti o n s f ro m n o r m a l o p e r a t i n g co n d i ti o n s. A H A Z O P c o m m i tt e e ,n o r m a l l y m a d e u p o f 5 - 7 i n d i v i d u a l s w i t h d i v e r s e t e c h n i c a l b a c k g r o u n d s ,i d en t if ie s th e c a u s e s a n d c o n s e q u e n c e s o f p r o c es s u p s et s. A t y p ic a l H A Z O Ps t u d y m a y t a k e f r o m 2 0 t o 8 0 h , d e p e n d i n g o n i ts s c op e .T h e r e a r e b e n e fi ts o f p e r f o r m i n g a H A Z O P A n a l y s i s b e y o n d i d e n ti f y in gh a z a r d s f o r a Q R A . T h e h a z a r d s i d e n ti fi ed d u r i n g a H A Z O P m a k e i t e a si ert o a s se s s t h e r e l a ti v e ri s k o f t h e p r o c e s s s o t h a t Q R A p r i o r i ti e s c a n b ee s ta b l is h e d . A l so , m a n y r e c o m m e n d a t i o n s f o r r is k r e d u c t i o n r e su l t d i re c t lyf r o m a H A Z O P .A f t e r t h e h a z a r d s h a v e b e e n i d e nt if ie d , t h e s c o p e o f a Q R A c a n b e d e fi n ed .T h e Q R A w i l l n o r m a l l y f o c u s o n a c c i d e n t s c e n a ri o s a s s o c ia t e d w i t h s pe ci fi co p e r a t i o n s . F o r e x a m p l e , t h e Q R A m a y f o c u s o n t r a n s p o r t a t i o n a c c i d e n t si n v o l v i n g a v e r y h a z a r d o u s m a t e r i a l o r o n r u n a w a y r e a c t i o n s i n a p a r t i c u l a rr e a c t o r .


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58 Gary R Van Sciver

F R E Q U E N C Y Q U A N T I F I C A T I O NA f t e r t h e s c o p e o f th e Q R A h a s b e e n d e f i n e d a n d t h e a c c i d e n t s c e n a r i o s a r ed e v e l o p e d , a s e c o n d k e y q u e s t i o n i s : h o w l i k e l y i s e a c h a c c i d e n t ? T h eo b j e c t iv e o f f r e q u e n c y q u a n t i f i c a t i o n i s t o c a l c u l a t e t h e p r o b a b i l i t y o f e a c ha c c i d e n t sc e n a ri o . T w o c o m m o n s o u rc e s o f th i s i n f o r m a t i o n a r e h i s to r ic a ld a t a a n d f a u l t t r e e a n a l y s i s . H i s t o r i c a l d a t a a r e b a s e d o n t h e a c c i d e n te x p e r i e n c e o f t h e s y s t e m w h i l e f a u l t tr e e a n a l y s i s is b a s e d o n t h e a c c i d e n te x p e r ie n c e o f th e c o m p o n e n t s .

T h e u s e o f h is t o r ic a l d a t a is t h e q u i c k e s t t e c h n i q u e o f q u a n t i f i c a t io n , i f t h ea p p r o p r i a t e d a t a a r e a v a i l a b l e . F o r e x a m p l e , h i s t o r i c a l d a t a m a y b ea v a i l a b l e f o r a t r a n s p o r t a t i o n r i s k s t u d y t o i n d i c a t e h o w m a n y t r a i nd e r a i l m e n t s p e r y e a r o n a v e r a g e c a n b e e x p e c t e d o n a p a r t i c u l a r r o u t e .A l t h o u g h t h e a v a i l a b l e d a t a a r e l i m i t e d , a c c i d e n t p r o b a b i l i t i e s a r e m u c he a s ie r t o c a l c u l a te d i r e c t ly f r o m h i s to r i c a l d a t a a n d t h e r e s u l ts a r e o f t e n m o r ea c c u r a t e . H o w e v e r , i t i s i m p o r t a n t t h a t t h e f a i l u r e d a t a u s e d a r e f r o ms y s t e m s t h a t a r e v e r y s i m i l a r t o t h e s y s t e m b e i n g a n a l y z e d . I f li tt le h i s t o r ic a lf a i lu r e d a t a e x is t, t h e n t h e u n c e r t a i n t y m a y b e la r ge .

F a u l t t re e a n a l y s is 5 is a n o t h e r m e t h o d f o r c a l c u l a t in g t h e p r o b a b i l i t y o fa n a c c i d e n t . T h i s t e c h n i q u e h a s b e e n u s e d e x te n s i v el y i n t h e n u c l e a r i n d u s t r y .I t in v o l v e s m o d e l i n g t h e f a il u re m e c h a n i s m s o f a d e v i c e o r a s y s t e m b y u s e o fa lo g i c d i a g r a m o f A N D g a t e s a n d O R g a te s . T h e f a i lu r e o f i n te r e st , c a l le dt h e T O P e v e n t , is e x p r e s s e d i n t e r m s o f l o g ic a l c o m b i n a t i o n o f b a s ic e v e n t s(e .g . c o m p o n e n t f a il u re s a n d o p e r a t o r e r ro r s) . T h e f r e q u e n c y o f fa i lu r e o f t h eb a s ic e v e n t s is e s t i m a t e d f r o m r e li a b il it y d a t a b a s e s s u p p l e m e n t e d b ye n g i n e e r i n g j u d g e m e n t . A f t e r t h e f a i lu r e d a t a o f a ll b a s ic e v e n t s h a v e b e e nd e t e r m i n e d , t he f r e q u e n c y o f t h e T O P e v e n t c a n b e c a l c ul a te d . D e p e n d i n g o nt h e s c o p e o f t h e Q R A , t h e r e m a y b e s e v e ra l f a u lt t re e a n a l y s e s o r s e v e ra lv a r i a t i o n s o f th e s a m e t re e . U l t i m a t e l y , t h e f r e q u e n c y o f e a c h a c c i d e n ts c e n a r i o u n d e r c o n s i d e r a t i o n m u s t b e d e t e r m i n e d .

F a u l t t r ee a n a l y s is h a s s e v er a l w e a k n e s s e s , b u t t w o o f t h e m o r e i m p o r t a n to n e s a r e m o d e l i n c o m p l e t e n e s s a n d f r e q u e n c y u n c e r t a i n t y . F i r st , t h e a n a l y s isi s o n l y a s g o o d a s t h e c o m p l e t e n e s s o f t h e l i s t e d f a i l u r e m e c h a n i s m s . A ne x h a u s t i v e s e a r c h f o r t h e s e m e c h a n i s m s is es s e n ti a l f o r h i g h q u a l i t y r e s ul ts .N e v e r t h e l e s s , t h e p o s s i b i l i t y a l w a y s e x i s t s t h a t a n i m p o r t a n t f a i l u r em e c h a n i s m h a s b e e n o v e r l o o k e d a n d n o t i n c l u d e d i n t h e f a u l t t r e e .S e c o n d l y , t h e a b s o l u t e v a l u e c a l c u l a t e d f o r T O P e v e n t f r e q u e n c y isd e p e n d e n t o n t h e q u a l i t y o f th e b a s ic e v e n t r e li a b il it y d a ta . E v e n t h o u g h t h eq u a l i t y o f t h e r e l ia b i li ty d a t a t h a t a r e a v a i la b l e i n t h e C P I i s c o n t i n u o u s l yi m p r o v i n g , d a t a o n s p ec if ic a p p l i c a t i o n s a r e o f t e n n o t a v a i la b l e. I f t h is i s t h ec a s e, e n g i n e e r i n g j u d g e m e n t m u s t b e u s e d t o e s t i m a t e r e li a b il it y d a t a .A b i g a d v a n t a g e o f f a u l t tr e e a n a l y s i s is t h e i n s i g h t s it p r o v i d e s i n t o t h eo p e r a t i o n . T h e p r o c e d u r e r e q u ir e s t h e a n a l y s t to c o n t i n u a l l y as k : h o w c a n


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t h is h a p p e n ? I t r e q u i r e s e x te n s i v e k n o w l e d g e o f th e m e c h a n i c s o f a s y st e m .F u r t h e r , i t e n a b l es t h e a n a l y s t t o i d e n ti f y t h e m o s t i m p o r t a n t c o n t r i b u t o r s t oT O P e v e n t f r e q u e n c y , a n d o f t e n l e a d s to t h e m o s t e f fe c ti ve r is k r e d u c t i o nm e a s u r e s . A l s o , f a u l t t r e e a n a l y s i s t a k e s i n t o a c c o u n t t h e s p e c i f i c s y s t e md e s i g n , e . g . e m e r g e n c y s y s t e m s , p r o c e s s c o n d i t i o n s , a n d o p e r a t i n gp r o c e d u r e s .

C O N S E Q U E N C E A N A L Y S I ST h e t h i r d e l e m e n t o f ri s k is c o n s e q u e n c e s . T h e a n a l y s is o f c o n s e q u e n c e s int h e C P I i s v e r y c o m p l e x d u e t o t h e g r e a t v a r i e t y o f m a t e r i a l s, c h e m i c a lr e a c t io n s a n d t e c hn o l o g ie s . C o n s e q u e n c e a n a l ys is is th e a s p e c t o f Q R A t h a ti s g r o w i n g m o s t r a p i d l y .

T h e o b j e c t i v e o f c o n s e q u e n c e a n a ly s i s is t o q u a n t i f y t h e n e g a t i v e i m p a c to f a n e v e n t. C o n s e q u e n c e s a re n o r m a l l y m e a s u r e d in t e r m s o f t h e n u m b e r o ff a ta li ti e s, a l t h o u g h t h e y c o u l d a l so b e m e a s u r e d i n t h e n u m b e r o f in j u ri e s o ri n p r o p e r t y lo s s. T h e t h r e e t y p e s o f ef fe c ts t h a t a r e n o r m a l l y c o n s i d e r e d a r et h e r m a l r a d i a t i o n , e x p l o s i o n o v e r p r e s s u r e a n d t o x ic e x p o s u re .

A w i d e v a r ie t y o f m o d e l s a r e u s e d i n c o n s e q u e n c e an a l ys is . S o u r c em o d e l s 6 a r e u s e d t o p r e d i c t t h e d i s c h a r g e r a t e , t h e d e g r e e o f fl a sh i ng , t h ea m o u n t o f a e ro s o l fo r m a t i o n , a n d t h e a m o u n t o f e v a p o r a ti o n . V a p o r c l o u dm o d e l s 6 a r e u s e d t o p r e d i c t t h e a m o u n t o f d o w n w i n d d i s p e rs i o n , t a k i n g i n t oa c c o u n t m e t e o r o l o g i c a l c o n d i t i o n s a n d v a p o r d e n s it y . I m p a c t i n te n s i tym o d e l s T a r e u s e d t o p r e d i c t t h e c o n s e q u e n c e z o n e s o f fi re s a n d e x p l o s i o n s.T o x i c g a s m o d e l s a a r e u s e d t o p r e d i c t h u m a n r e s p o n s e t o a r a n g e o fe x p o s u r e s t o a t o x i c m a t e r ia l . O t h e r m o d e l s a r e u s e d t o p r e d i c t t h e e ff ec ts o nh u m a n s o f e x p o s u r e t o f ir es o r e x p l o s io n o v e r p r e s s u re .

A s u b s t a n t ia l a m o u n t o f r e se a r c h is b e i n g c o n d u c t e d t o m i n i m i z e t h eu n c e r t a i n t y w h i c h e x is ts w i t h m a n y o f t h e s e m o d e l s . N e v e r t h e l e s s , b e c a u s em u c h is sti ll u n k n o w n a b o u t t h e se p h e n o m e n a , s ig n if ic a n t e r r o r ( a n o r d e r o fm a g n i t u d e ) c a n b e e x p e c t e d in m o s t c o n s e q u e n c e a n a ly s es .

C o n s e q u e n c e a n a l y s is c a n b e a n e f fe c ti ve r i sk r e d u c t i o n t o o l b y i ts el f. T h es e n s i ti v it y o f th e r e s u lt s t o i n v e n t o r y s ize , s t o r a g e t e m p e r a t u r e , s t o r a g ep r e s s u r e , b u f f e r z o n e s a n d m a t e r i a l p r o p e r t i e s c a n p o i n t th e w a y t oi n h e r e n t l y s a f e r p l a n t s .

M E A S U R E S O F R I S KT h e l a st s te p o f a Q R A is t o c a l c u l a t e t h e a c t u a l r is k . A n u m b e r o f a c c i d e n ts c e n a r i o s h a v e b e e n i d e n t i f i e d . F r e q u e n c y a n d c o n s e q u e n c e s h a v e b e e nc a l c u l a t e d f o r e a c h a n d r i s k c a n n o w b e d e t e r m i n e d .


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F i g 2 T y p i c a l r is k c o n t o u r s


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u a n t i t a t i v e r i s k a n a l y s i s in t h e c h e m i c a l p r o c e s s i n d u s t r y 61O n e m e t h o d o f p r e s e n t i n g t h e r e s u lt s o f a Q R A i s a r i sk p r o fi le . T h i s i s ap l o t o f fr e q u e n c y v e r su s c u m u l a t i v e n u m b e r o f f a ta l it ie s a n d is a m e a s u r e o f

r i s k t o s o c i e t y . A t y p i c a l p r o f i l e i s s h o w n i n F i g . 1 . T h e a d v a n t a g e o fp r e s e n t i n g t h e r e s u l t s i n a r i s k p r o f i l e i s t h a t t h e p o p u l a t i o n d i s t r i b u t i o na r o u n d t h e p l a n t a n d t h e lo c a l m e t e o r o l o g i c a l c o n d i t i o n s a r e t a k e n i n t oa c c o u n t .A n a l t e r n a t i v e m e t h o d f o r p r e se n t i n g t h e re s u lt s o f a Q R A is i n th e f o r mo f a r i s k c o n t o u r ( se e F i g . 2) w h e r e i n d i v i d u a l r is k i s m e a s u r e d i n s t e a d o f r i s kt o s o c i e ty . I n d i v i d u a l r is k is d e f i n e d a s t h e l i k e l i h o o d t h a t a n i n d i v i d u a l a t as p e c i f i c l o c a t i o n w o u l d b e e x p o s e d t o t h e c o n s e q u e n c e s o f a c c i d e n t sa s s o c i a t e d w i t h t h e o p e r a t i o n u n d e r s t u d y . W h i l e t h e p o p u l a t i o nd i s t r i b u t i o n i s n o t t a k e n i n t o a c c o u n t w i t h i n d i v i d u a l r is k, t h e c o n t o u r s a r es ti ll v e r y u s e f u l b e c a u s e t h e r i sk a t a p a r t i c u l a r p o i n t c a n b e s h o w n , e.g . a t t h ep lan t f ence l ine .

E X A M P L E C P I Q R AT o i l lu s t ra t e t h e t e c h n i q u e s u s e d i n Q R A , c o n s i d e r t h e f o l lo w i n g e x a m p l e .

A p r o p o s a l h a s b e e n m a d e f o r t h e c o n s t r u c t i o n o f a n e w p r o c e ss w h i c hf e a t u re s a 3 0 00 g a l l o n C F S T R ( c o n t i n u o u s f l ow s ti rr e d t a n k r e ac t o r) . T h er e a c t i o n i s c a t a l y z e d a n d e x o t h e r m i c . A s s h o w n i n F i g . 3 , t h e h e a t o f r e a c t i o nis r e m o v e d b y c o o l i n g w a t e r p a s s in g t h r o u g h t h e r e a c t o r j a c k e t .M a n a g e m e n t is c o n c e r n e d b e c a u s e a s m a l l e r e x is ti n g r e a c t o r u n d e r w e n t ar u n a w a y r e a c t i o n l as t y e a r w h i c h r e l ea s e d t o x ic m a t e r i a l t o t h e a t m o s p h e r e .N o o n e w a s i n j u r e d , b u t i t i s f e a r e d t h a t a s i m i l a r i n c i d e n t w i t h t h is l a r g e rr e a c t o r m a y h a v e m o r e s e ve r e c o n s e q u e n c e s . T h e r e f o re , it is d e c i d e d t h a t aQ R A s h o u l d b e p e r f o r m e d .T h e f ir st s te p o f t h e Q R A is a H A Z O P s t u d y o f th e p r oc e ss . T h e s e ve np a r t i c i p a n t s i n t h e s t u d y i n c l u d e r e p r e s e n ta t i v e s f r o m t h e p l a n t , e n g i n e e r in ga n d r e s ea r c h . T h e s t u d y t a k e s a w e e k to c o m p l e t e . A n u m b e r o f a c c i d e n ts c e n a r io s a r e i d e n ti f ie d t h a t c o u l d l e a d t o a n a t m o s p h e r i c r el ea s e. S o m e o ft h e s e a r e l i s t e d b e l o w :

1. lo s s o f co o la n t f low,2 . a g i t a t i o n i n t e r r u p t i o n ,3 . c a t a l y s t f l o w i n t e r r u p t i o n ,4 . t e m p e r a t u r e i n d i c a t o r f a il u r e .

T h e s e a c c i d e n t s c e n a r io s w i ll f o r m t h e b a s is o f th e Q R A .T h e n e x t s te p i s q u a n t i f i c a t i o n o f f re q u e n c y w h i c h w i l l b e d o n e w i t h f a u l tt re e a n a l y s is ( F T A ) . T h e T O P e v e n t o f t h e f a u l t t r e e is a t m o s p h e r i c r e l e a sef r o m t h e r e a c t o r r u p t u r e d i s c d i s c h a r g e l in e . T h e u p p e r l e v el s o f t h e f a u l t t re e


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6 2 Gary R Van Sciver

m

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a r e s h o w n i n F i g. 4. B a s e d o n t h e s e r e su lt s, a n u m b e r o f r e c o m m e n d a t i o n sb e l o w ) t h a t s h o u l d s i g n i fi c a n t l y r e d u c e t h e p r o b a b i l i t y o f a r e l e a se a r e

i d e n t i f i e d .R e c o m m e n d a t io n s f ro m t h e F T A

1. I n s t a ll a r e d u n d a n t t e m p e r a t u r e i n d i c a t o r .2 . I n t e r l o c k t h e c o o l a n t f l o w a n d t h e r e a c t a n t f e e d s .3 . I n t e r l o c k t h e c a t a l y s t f l o w a n d t h e r e a c t a n t fe e d s .4 . I n t e r l o c k t h e a g i t a t o r r o t a t i o n a n d t h e r e a c t a n t f e e d s .

T h e n e x t s te p o f t h e Q R A i s t h e c o n s e q u e n c e a n a ly s is . A w i d e v ar i e ty o fm o d e l s a re u se d t o e st im a t e t h e c o n s e q u e n c e s o f a r u n a w a y r e a ct io n . S o m eo f t h e t h i n g s t h a t a re c o n s i d e r e d i n c l u d e t h e r e a c t i o n k i n e t i c s , t h e f l o w


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u a n t i t a t i v e r i s k a n a l y s is i n th e c h e m i c a l p r o c e s s i n d u s t r y 63

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Fig 4 QR A example : fault t r e e .

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HIGHWAYFig 5 QR A example : consequ ence analysi s re sul ts


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6 4 Gary R Van Sciver

CUMULATIVEFRE UENCY ORIGINAL

CASE

MODIFIEDCASE

DAMAGEFig. 6. QR A exam ple: risk profiles.t h r o u g h t h e r u p t u r e d i s c l i n e , t h e v a p o r c l o u d b e h a v i o r , a n d t h e w e a t h e rc o n d i t i o n s . T h e r e s u l t s fo r o n e s e t o f c o n d i t i o n s a r e s h o w n i n F i g . 5.

F i n a ll y , t h e o v e r a l l r is k is c a l c u l a t e d f r o m t h e f r e q u e n c y a n d c o n s e q u e n c ee s t i m a t e s . T h e r e s u l t s a r e p r e s e n t e d i n r is k p r o f i le s ( F ig . 6) a n d r i sk c o n t o u r s( F ig s 7 a n d 8) f o r th e p r o c e s s w i th a n d w i t h o u t t h e r e c o m m e n d a t i o n s f o ri m p r o v e m e n t i m p l e m e n t e d . T h e s e c o m p a r i s o n s c le a rl y s h o w t h e q u a n t i-t at iv e i m p r o v e m e n t g a in e d b y i m p l e m e n t i n g t h e r e c o m m e n d a t i o n s .

U N C E R T A I N T YT h e d e v e l o p m e n t o f Q R A i n th e C P I i s s ti ll i n i ts in f a n c y . S o m e a s p e c t s o fr is k a r e i n h e r e n t l y d if f ic u lt t o q u a n t i f y d u e t o a l a c k o f k n o w l e d g e o r d a t a .A s s u m p t i o n s m u s t o f t en b e m a d e t o r e d u ce t h e sc o p e o f t h e Q R A t o am a n a g e a b l e l e v e l , b u t t h i s o f t e n e l i m i n a t e s s o m e r i s k c o n t r i b u t o r s . A l l o f


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Quantitative risk analysis in the chemical pro cess industryl n 6

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10 4

PROCESS

FENCELINE

HGHWAY

Fig. 7. QRA example: original risk contour.these factors contribute to the uncertainty o f Q R A in the CPI. Tables 2 and 3list specific sources o f uncerta inty.Some of the l isted i tems contribute to the error of b o t h frequencyand consequence estimates. Some of the listed items are very significantcontributors. This list is not complete but it still indicates how QRAestimates can be in error by an order of ma gnitude or m o r e

I - I ~ ~ _~ ~ . _ 1

FENCELINE~H GHWAY

HOUSE S

Fig. 8. QRA example: modified risk contour.


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66 G aG R. Van SciverT A B L EContributors to Frequency Uncertainty

1 Incomplete hazard identification study2 Incomplete knowledge of the chemical hazards3 Incomplete fault tree analysis4 Incomplete reliability data5 Human behavior assumptions6 Incomplete list of common mode failures7 Plant construction errors8 Process changes9 Procedural changes10 Maintenance schedule changes11 Incomplete operator training12 Management errors

Par t of the u nce rta inty stems from the difficulty of verifying results. Sincethe accident scenarios under considera tion are such rare events we wouldnot expect to actually experience any during the entire operating life of theplant. In addition ma ny of the individual assumptions which contribute touncert ain ty are difficult to verify. Fo r example the hu ma n response toexposure of toxic materials is virtually impossible to test.

Usually QRA models are conservative so that the calculated risk ofidentified hazard s is overstated. Of course the calculated risk of u n i d e n t i f i e dhaza rds is not included. These two effects tend to cancel each other buteither may be dominating.

T A B L E 3Contributors to Consequence Uncertainty1 Assumptions regarding the quantity temperature pressureand composition of the hazardous material2 Source model assumptions3 Vapor cloud model assumptions4 Impact intensity model assumptions5 Toxicityassumptions6 Thermal radiation model assumptions7 Explosion model assumptions8 Ignition source assumptions9 Meteorological assumptions10 Population distribution assumptions

11 Vulnerability of population assumptions12 Mitigating factor assumptions


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u a n t i t a t i v e r i s k a n a l y s i s in t h e c h e m i c a l p r o c e s s i n d u s t r y 67

Q R A B E N E F I T ST h e e s t i m a t e o f r is k is ot t h e m o s t i m p o r t a n t r e su l t o f a Q R A . Q R A is a to o lf o r e x a m i n i n g t h e e l e m e n t s o f ri sk . T h e r e a r e a m u l t i t u d e o f w a y s t o r e d u c er i s k , b u t t h e a n a l y s t s s h o u l d b e a b l e t o d e t e r m i n e w h i c h m e a s u r e s w i l l b em o s t e f f e c t i v e . T h i s i s h o w Q R A c a n b e s t s e r v e t h e C P I .D u r i n g t h e p ro c e s s o f p e r f o r m i n g a Q R A , t h e a n a l y s ts a r e r e q u i r e d tol e a r n t h e c h e m i c a l o p e r a t i o n i n d e t a i l . T h e c h e m i c a l h a z a r d s m u s t a l l b et h o r o u g h l y u n d e r s to o d . E v e r y i m p o r t a n t f a il u re m e c h a n i s m m u s t b eu n c o v e r e d . A l l o f t h e f a c t o rs w h i c h d e t e r m i n e i f p e o p l e c a n b e h a r m e d b y t h eo p e r a t i o n a r e c o m p l e t e l y a n a ly z e d . B y f o l l o w i n g th i s e x h a u s t iv e p r o c e d u r e ,t h e a n a l y s t s w i l l l i k el y d e v e l o p i n s i g h t s i n t o h o w t o r e d u c e t h e r i sk s o f t h eo p e r a t i o n i n t h e m o s t c o s t - e f f e c t i v e m a n n e r .

F o r e x a m p l e , d u r i n g t h e c o u r s e o f t h e Q R A t h e a n a ly s t s m a y o b s e r v e t h a ta p a r t i c u l a r p r o c e s s is i n h e r e n t l y t o o h a z a r d o u s t o b e p l a c e d n e a r a la r g ep o p u l a t i o n c e n t e r. T h e y m a y o b s e r v e t h a t t h e u s e o f a k e y s a fe t y i n t e r l o c kc a n d r a m a t i c a l l y r e d u c e t h e l ik e l i h o o d o f a r u n a w a y r e a c t i o n . T h e y m a yo b s e r v e t h a t t h e h a z a r d s o f s to r i n g a l a r g e q u a n t i t y o f a t ox i c m a t e r i a lc a n n o t b e s u b s t a n ti a l ly r e d u c e d b y e n g i n e e r i n g c o n t r o ls , a n d t h e r e f o r e t h ei n v e n t o r y o f t h e m a t e r i a l m u s t b e r e d u c e d . T h e s e i n s ig h t s a r e t y p i ca l o f t h o sea n a n a l y s t w i l l l e a r n d u r i n g t h e c o u r s e o f h i s r e v i e w .

F U T U R E O F Q R AO n e q u e s t i o n w i l l b e a s k e d o f e v e r y c h e m i c a l p r o c e s s t h a t i s b u i l t: i s i t sa fe ?T h e a n s w e r is n e v e r s t r a ig h t f o r w a r d . T h e r e w i ll a l w a y s b e a d e m a n d f o rm o r e i n f o r m a t io n . Q R A is a n o b v i o u s s o ur c e o f t h a t i n f o r m a t i o n . T r e n d si n d i c a t e th a t i n t h e f u t u re , Q R A w i ll p l a y a n i n c r e a s i n g ly i m p o r t a n t r o l e i nr i s k m a n a g e m e n t t h r o u g h o u t t h e C P I .

I t c o u l d b e a r g u e d t h a t t h e s t a te o f t h e a r t o f Q R A is n o t s u f fi c ie n tl yd e v e l o p e d t o p r o v i d e a u s e f u l p r o d u c t . H o w e v e r , i t i s l ik e l y t h a t i n t h e f u t u r et h e u n c e r t a i n t y o f Q R A e s t im a t e s w i l l c o n t i n u a l l y b e r e d u c e d . T h e m o d e l ss h o u l d b e c o m e m o r e a n d m o r e a c c u r a t e a s t h e y a r e v e r if ie d w i t h r e s e a rc h .A l s o , a s p e o p l e b e c o m e m o r e f a m i l ia r w i t h t h e s tr e n g t h s a n d w e a k n e s s e s o ft h e t o o l , t h e y w i l l p r o b a b l y p u t i t t o b e t t e r u s e .Q R A is a l r e a d y b ei n g u s e d b y s o m e E u r o p e a n g o v e r n m e n t s a s a to o l t om e a s u r e a b s o l u t e r i s k . 9 T h e r i s k m u s t m e e t s t a n d a r d s t h a t h a v e b e e ne s t a b li s h e d f o r a c c e p t a b il i ty a n d u n a c c e p t a b i l it y . T h o s e w h o u n d e r s t a n d t h eu n c e r t a i n t ie s i n h e r e n t t o Q R A m a y o b j e c t t o s u c h a r ig i d a p p l ic a t i o n o f t h et o o l . T h e r e i s a l e g i t im a t e f e a r t h a t e x c e s si v e r e s o u r c e s w i ll b e s p e n t o n a ne x e rc is e w h i c h e n c o u r a g e s o v e r l y s im p l if ie d r i sk m a n a g e m e n t . N e v e r t h e le s s ,


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68 Gary R. Van Scivert h e u s e o f Q R A i n t h e p u b l i c a n d p o l i ti c a l a r e n a is l ik e ly t o e x p a n d . T h e n e e dt o d e v e l o p s kills i n ris k m a n a g e m e n t a n d i n th e a p p l i c a t io n o f Q R A m u s t b er e c o g n i z e d .

C O N C L U S I O N SI n d i c a t i o n s a r e t h a t Q R A w i ll b e u s e d in c r e a s i n g l y i n t h e f u tu r e . H o w e v e r ,t h e r e is a l w a y s a d a n g e r o f u s i n g t h e r e su l ts o f a Q R A b li nd ly , w i t h o u t s o u n de n g i n e er in g a n d m a n a g e m e n t j u d g e m e n t . B u t t h e n e e d f or s o u n d j u d g e m e n tis e v i d e n t w h e t h e r Q R A is u se d o r n o t . W i t h o u t Q R A , t h e e ff ec ts o f p o o rj u d g e m e n t m a y b e w o r se .

W i t h t h e e x p e c t e d r a p i d g r o w t h o f Q R A i n t h e C P I , t h e re a r e l i ke ly to b ei m p r o v e m e n t s to t h e m e t h o d a n d r e d u c t i o n in t h e u n c e r ta i n ty . N e v e r th e le s s,t h e t o o l m u s t b e k e p t i n p e r s p e c ti v e . I t is o n l y o n e s m a l l p a r t o f a t o t a l r i skm a n a g e m e n t p r o g ra m . T h e m o s t i m p o r t a n t p a r t is h a v i ng c o m p e t e n te n g i n e e r s , o p e r a t o r s a n d m a n a g e r s .

R E F E R E N C E S1. W AS H- 140 0 , R eactor Sa jeO S tud) ; An A ssessm en t o f Acc iden t Ri sks in U SCommercia l Nuclear Plan t s . US Nuclear Regula tory Commiss ion , Rockvi l l e ,MD, October 1975.2. D ow s Fire and Exp losion In de x H az ar d Classi fication Guide, 6th edn . AIChE,New York , 1987.3. Lewis , D. J. , The M on d F i re , Explos ion and Toxic i ty In de x- -A De velop me nt oft he Do w Index, p resen ted a t t he A IC h E Loss P reven t i on S ym pos i um , Hou s t on ,Texas, Apri l 1979.4 . M uda n, K. S ., Ha zard Ra nkin g for Chemical P rocess Facil it ies, p resented a t theAS ME Wi n t e r Annua l Mee t i ng , B os t on , MA, Decem ber 1987 .5. Guidel ines for Hazard Evaluat ion Procedures . P repared by B a tt el le C o l um bu s

Di v i si on fo r C C P S / AIC hE, AIC hE , New Y ork , 1985.6. Guidel ines fo r Use o f V apor Clou d Dispersion Mod els . Prepared by S teven R .Hanna and P e t e r J . Dr i vas fo r C C P S / AIC hE, AIC hE, New York , 1987 .7. T h e As s e s s m en t o f M a j o r H a z a r d s . The Ins t i tu t ion of Chemical Engineers ,London, 1982.8. Risk Analysis in the Chemical Industry. Chemical Manufacturers Associa t ion ,G ov ern m en t Ins ti tu tes Inc ., Rockvi l le , M D, 1985.9 . Envi ronmen ta l Program o f The Nether lands 1986-90 , t h e G o v e r n m e n t o f T h eNetherlands, 1985.

quantitative risk analysis in the chemical process industry

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