Hazard and Operability Study - ACS Symposium Series (ACS

5 Hazard and Operability Study A Flexible Technique for Process System Safety and Reliability Analysis Downloaded by GEORGE MASON UNIV on August 17, 2016 | http://pubs.acs.org Publication Date: March 14, 1985 | doi: 10.1021/bk-1985-0274.ch005

A. SHAFAGHI and S. B. GIBSON Columbus Division, Battelle Memorial Institute, Columbus, OH 43201

Safety and reliability of chemical process plants are such important issues, they deserve the best techniques to prevent problems occurring. To minimize risks resulted from operating problems and hazardous events, process system safety and reliability analysis is often employed. This is a rigorous approach undertaken to improve system reliability and safety. The approach consists of three main tasks; hazard identification, risk estimation, and risk control. The first task is crucial in process system safety analysis, because the effectiveness of the other two tasks depends on it. The technique of hazard and operability (HAZOP) study is a systematic approach to identifying most potential hazards and operating problems. The technique in contrast to the traditional methods is simple, creative, and flexible. D u r i n g t h e 1960's, t h e c h e m i c a l i n d u s t r y developed r a p i d l y , and t o a c h i e v e t h e b e n e f i t s o f s c a l e , c h e m i c a l p l a n t s became l a r g e r and more s o p h i s t i c a t e d . As communications and g r e a t e r p u b l i c awareness h e i g h t e n e d t h e e f f e c t o f i n c i d e n t s i n t h e i n d u s t r y d u r i n g t h e 1970's, g r e a t s t r i d e s had t o be made t o improve t h e t o o l s a v a i l a b l e t o i n c r e a s e p r o c e s s s a f e t y and r e l i a b i l i t y . I t s threefold purpose i s d e s c r i b e d below. The f i r s t t a s k i s t o i d e n t i f y p o t e n t i a l h a z a r d s and o p e r a b i l i t y problems. I n g e n e r a l , t h e r e e x i s t two t y p e s o f h a z a r d : i n h e r e n t , due t o t h e n a t u r e o f raw m a t e r i a l s used; and s u b t l e , due t o o m i s s i o n s and e r r o r s made i n d e s i g n . A f t e r t h e h a z a r d s have been i d e n t i f i e d , t h e second t a s k i s t o determine t h e r i s k s a s s o c i a t e d w i t h them. R i s k i s d e f i n e d a s t h e l i k e l i h o o d o f t h e hazardous event and t h e s e v e r i t y o f t h e a c c i dent. F a u l t t r e e s a r e o f t e n used t o q u a n t i f y t h e l i k e l i h o o d o f hazardous e v e n t s . The s e v e r i t y i s u s u a l l y d e f i n e d a s t h e degree, sometimes i n terms o f l i k e l i h o o d , o f exposure t o a c c i d e n t s .

0097-6156/85/0274-0033$06.00/0 © 1985 American Chemical Society

Hoffmann and Maser; Chemical Process Hazard Review ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Downloaded by GEORGE MASON UNIV on August 17, 2016 | http://pubs.acs.org Publication Date: March 14, 1985 | doi: 10.1021/bk-1985-0274.ch005

34

C H E M I C A L PROCESS H A Z A R D REVIEW

The u l t i m a t e g o a l i n p r o c e s s system s a f e t y and r i s k a n a l y s i s i s t o c o n t r o l t h e r i s k s . T h i s f i n a l t a s k i s c a r r i e d o u t by comp a r i n g t h e r i s k s c a l c u l a t e d w i t h r i s k c r i t e r i a s p e c i f i e d by an a u t h o r i t y . The c r i t e r i a c a n be s u b j e c t i v e l y d e t e r m i n e d , based on the p a s t e x p e r i e n c e o r t h e e x i s t i n g background r i s k s . However, many companies (1,2) have e s t a b l i s h e d n u m e r i c a l t a r g e t s f o r r i s k s . G i v e n c a l c u l a t e d r i s k s beyond t h e s p e c i f i e d l i m i t , d e c i s i o n s w i l l be made t o improve t h e d e s i g n o r o p e r a t i o n and maintenance p r o cedures t o reduce t h e r i s k s . The f i r s t t a s k , h a z a r d i d e n t i f i c a t i o n , i s c r u c i a l i n p r o c e s s system s a f e t y a n a l y s i s , because t h e e f f e c t i v e n e s s o f t h e o t h e r two t a s k s depends on i t . The t r a d i t i o n a l methods f o r i d e n t i f y i n g h a z a r d s d u r i n g t h e 1960·s ( i n c l u d i n g » p r o c e s s r e v i e w s ' , ·codes o f p r a c t i c e ' , c h e c k l i s t s », and s a f e t y a u d i t ) were no l o n g e r cons i d e r e d adequate i n t h e 1 9 7 0 s . There was a need f o r a t e c h n i q u e which c o u l d a n t i c i p a t e hazardous problems, p a r t i c u l a r l y i n a r e a s of n o v e l t y and new t e c h n o l o g y where p a s t e x p e r i e n c e was l i m i t e d . The t e c h n i q u e o f h a z a r d and o p e r a b i l i t y (HAZOP) s t u d y was developed t o f i l l t h i s need. HAZOP s t u d y has t h e f o l l o w i n g n o t e worthy f e a t u r e s : 1

1

1

f

• • • •

I I I I

t t t t

i s based on b r a i n s t o r m i n g t a k e s a m u l t i d i s c i p l i n a r y team approach i s s t r u c t u r e d by u s i n g g u i d e words i s cost e f f e c t i v e .

A b r i e f comparison between t h e t r a d i t i o n a l methods (such as t h e "what i f " method) and HAZOP s t u d y i s g i v e n i n T a b l e I .

T a b l e I . A Comparison Between T r a d i t i o n a l Methods and HAZOP

Study

T r a d i t i o n a l Methods

HAZOP

Experience-based Procedural Collective

Creative Systematic Collective with constructive interactions

C o n d u c t i n g a HAZOP

Study

Study

The approach t o i d e n t i f y i n g hazards and o p e r a b i l i t y problems by t h i s t e c h n i q u e i s t o s e a r c h f o r d e v i a t i o n s from d e s i g n i n t e n t s . The f i r s t s t e p i s t o p l a n t h e HAZOP s t u d y t o ensure t h a t t h e r e i s s u f f i c i e n t t i m e , e x p e r t i s e , and i n f o r m a t i o n a v a i l a b l e . Next, a m u l t i d i s c i p l i n a r y team, l e d by an e x p e r t i n t h e t e c h n i q u e , i s s e t up. Then, by means o f a f i x e d s e t o f a b s t r a c t words c a l l e d g u i d e words (such as 'more')» t h e team l e a d e r examines each p r o c e s s


5.

SHAFAGHI A N D GIBSON

Hazard and Operability Study


v a r i a b l e o r parameter o f i n t e r e s t (such as ' f l o w * ) and conveys the d e v i a t i o n s (such as 'more f l o w ' ) t o the team members. The o b j e c t i v e i s t o s t i m u l a t e the members i n t o c r e a t i v e t h i n k i n g about the consequences and causes o f the d e v i a t i o n s . F i n a l l y , the team a g r e e s on p o s s i b l e c a u s e s , consequences, and s o l u t i o n s o f the problems posed by the d e v i a t i o n s and recommends any f u r t h e r a c t i o n s t o be t a k e n . The s t e p s i n c o n d u c t i n g a HAZOP s t u d y a r e d e s c r i b e d i n g r e a t e r d e t a i l below.

P l a n n i n g the S t u d y . P l a n n i n g HAZOPs i s an i m p o r t a n t t a s k , and c o m p l i c a t e d by the f o l l o w i n g f a c t o r s : • • •

HAZOPs a r e time consuming and s h o u l d be done d u r i n g t h e d e s i g n p h a s e — n o r m a l l y a busy time f o r the p a r t i c i p a n t s HAZOPs s h o u l d be planned w e l l i n advance, because they a r e a b i g commitment and a heavy l o a d f o r each p a r t i c i p a n t The p i p i n g and i n s t r u m e n t (P&I) diagrams must be i n a s u f f i c i e n t l y f i n a l i z e d form.

E x p e r i e n c e has shown t h a t , p a r t i c u l a r l y on l a r g e p r o j e c t s , c o n d u c t i n g a p r e l i m i n a r y h a z a r d a n a l y s i s e a r l i e r i n the p r o j e c t i s a c o n s i d e r a b l e h e l p i n i n t e g r a t i n g the HAZOPs i n t o the p r o j e c t p l a n (JJ)· The p r e l i m i n a r y h a z a r d a n a l y s i s w i l l seek out t h e major, o b v i o u s h a z a r d and r e l i a b i l i t y problems.

A s s e m b l i n g the Team. F o r a c h e m i c a l p r o c e s s a t the d e s i g n s t a g e , the team members would i n c l u d e the p r o c e s s e n g i n e e r , the r e s e a r c h c h e m i s t , and the m e c h a n i c a l d e s i g n e n g i n e e r . On a h i g h l y a u t o mated p r o c e s s , t h e i n s t r u m e n t and c o n t r o l e n g i n e e r might be i n c l u d e d , and a d v i s o r s on e x p l o s i o n h a z a r d s , t o x i c o l o g y , and mater i a l s o f c o n s t r u c t i o n may be p r e s e n t when r e q u i r e d . An e f f e c t i v e team s i z e i s 5-7 p e o p l e , and i t i s i m p o r t a n t t h a t a l l members contribute. A team l e a d e r i s n e c e s s a r y t o ensure t h a t the methodology i s p r o p e r l y a p p l i e d and t o keep the meeting under c o n t r o l . T h i s l a t t e r t a s k s h o u l d by no means be u n d e r e s t i m a t e d .

A p p l y i n g the Guide Words. HAZOP s t u d y i s a f r e e - w h e e l i n g t e c h n i q u e , but i t i s s t r u c t u r e d by means o f seven a b s t r a c t words, c a l l e d g u i d e words. The g u i d e words used i n HAZOP s t u d i e s , t o g e t h e r w i t h t h e i r meanings, a r e g i v e n i n T a b l e I I .


35


36 Table I I .

The

Guide Words

Guide Words


No,

(4)

Meanings

N e g a t i o n o f the d e s i g n intent Quantitative increase Q u a n t i t a t i v e decrease Qualitative increase Q u a l i t a t i v e decrease L o g i c a l o p p o s i t e o f the intent Complete s u b s t i t u t i o n

None

More Less As w e l l as Part of Reverse Other than

These g u i d e words are a p p l i e d t o p r o c e s s v a r i a b l e s and p a r a meters o f i n t e r e s t f o r the system under s t u d y . On c o n t i n u o u s chemical processes, process v a r i a b l e s i n c l u d e temperature, p r e s s u r e , f l o w , and c o n c e n t r a t i o n . F o r example, a s e t o f s p e c i a l i z e d g u i d e words ( d e v i a t i o n s ) f o r these v a r i a b l e s are g i v e n below:

Guide Word

Variable

Deviation

No Less More Part of

Flow Temperature Pressure Concentration

No f l o w Low temperature High pressure Low concentration

F o r b a t c h p r o c e s s e s , l e v e l , r e a c t i v i t y , and time might be a d d i t i o n a l parameters c o n s i d e r e d . F o r e l e c t r i c a l systems, v o l t a g e , c u r r e n t , phase, and f r e quency are among the parameters t o be c o n s i d e r e d . F i n d i n g the R e s u l t s . To conduct the s t u d y ( 5 ) , the HAZOP team examines P&I diagrams. Each diagram i s d i v i d e d i n t o d i s c r e t e homogeneous s e c t i o n s , u s u a l l y p i p e s , and each s e c t i o n i s cons i d e r e d i n t u r n . Each o f the g u i d e words i s a p p l i e d t o the s e c t i o n b e i n g s t u d i e d t o s t i m u l a t e the team i n t o i m a g i n i n g what c o u l d go wrong w i t h any p a r t o f the system g i v e n the suggested deviation at that section. One o f t h r e e outcomes i s p o s s i b l e f o r each g u i d e word application: (1)

No h a z a r d or problem e x i s t s

(2)

A h a z a r d o r problem e x i s t s . I n t h i s c a s e , a s u i t a b l e r e c o r d i s made t o t h a t e f f e c t , and the r e q u i s i t e s o l u t i o n w i l l have t o be found o u t s i d e the meetings


5.

S H A F A G H I A N D GIBSON

(3)


The team does not have s u f f i c i e n t i n f o r m a t i o n t o d e t e r mine whether a problem e x i s t s . I n t h i s c a s e , a r e c o r d i s made t o t h a t e f f e c t , and a g a i n the n e c e s s a r y i n f o r m a t i o n w i l l have t o be found o u t s i d e the m e e t i n g .


When each g u i d e word has been a p p l i e d t o each s e c t i o n o f each diagram, the HAZOP i s complete; but o f c o u r s e a l l the q u e s t i o n s r e c o r d e d s t i l l have t o be r e s o l v e d . The method i s s i m p l i f i e d and shown g r a p h i c a l l y i n F i g u r e 1. Note t h a t the o p e r a t i o n i n the d e c i s i o n box i n f a c t i n c l u d e s s e v e r a l i t e r a t i o n s , each f o r one s p e c i a l i z e d g u i d e word. Benefits The major c o n t r i b u t i o n HAZOPs make t o a p r o j e c t ' s e f f e c t i v e n e s s i s t h a t t h e y i d e n t i f y p o t e n t i a l problems a t the d e s i g n s t a g e r a t h e r than when they become i n c i d e n t s . T h i s p r o v i d e s a number o f benefits: • •

• • •

P o t e n t i a l problems a r e r e s o l v e d r e l a t i v e l y e a s i l y , and most s u b t l e h a z a r d s are i d e n t i f i e d a t the d e s i g n s t a g e P o t e n t i a l problems can be r e s o l v e d r a t i o n a l l y , whereas an i n c i d e n t u s u a l l y c r e a t e s an o v e r r e a c t i o n and e x p e n s i v e , ultra-conservative solutions E n g i n e e r i n g change o r d e r s d u r i n g c o n s t r u c t i o n and comm i s s i o n i n g are d r a s t i c a l l y reduced S t a r t u p i s more t i m e l y P l a n t s reach design r a t e s q u i c k e r .

As an example o f the c o s t e f f e c t i v e n e s s o f the t e c h n i q u e , an assessment o f the v a l u e o f HAZOPs on a $38 m i l l i o n p r o j e c t was done. T a b l e I I I shows the c o s t e f f e c t i v e n e s s o f the HAZOP s t u d y t e c h n i q u e f o r a new p r o j e c t c l e a r l y . Flexibility As we have s e e n , the methodology i s b a s i c a l l y v e r y s i m p l e , and because the g u i d e words a r e g e n e r a l , the HAZOP can be a p p l i e d t o any type o f system. Some o f the systems s t u d i e d i n c l u d e : • • • • • • • •

C o n t i n u o u s c h e m i c a l s and p e t r o c h e m i c a l s p r o c e s s e s B a t c h o r g a n i c s , s p e c i a l t y c h e m i c a l s and p h a r m a c e u t i c a l processes P i l o t plants Bench r e s e a r c h p r o c e s s e s E l e c t r i c a l i n t e r l o c k systems Computer i n s t a l l a t i o n s D r a i n a g e systems Molecular genetics research laboratory.


37


38

Divide P&l Diagram Into Sections, Usually Pipes


Select a Pipe

Record: The Consequences, Causes and Suggest Remedies

Need More Information

F i g u r e 1. The Method o f HAZOP S t u d y .

T a b l e I I I . P l a n t HAZOP Study

Total plant c a p i t a l cost Cost o f HAZOP study

$000's

%

38,000

100

60

0.2

647

1.7

Cost o f m o d i f i c a t i o n s t o problems r e v e a l e d by HAZOP study Cost o f c o r r e c t i o n s i f study n o t done C a p i t a l savings Other s a v i n g s on o p e r a t i n g c o s t s where problems c o u l d n o t have been c o r r e c t e d p r a c t i c a b l y a f t e r t h e d e s i g n stage

1,487

3-9

780

2.1

262/yr

0.7


5.

SHAFAGHI A N D GIBSON


HAZOPs c a n be a p p l i e d e q u a l l y w e l l t o new d e s i g n s o r e x i s t i n g s i t u a t i o n s , a l t h o u g h i n the l a t t e r case s o l u t i o n s t o the p o t e n t i a l problems i d e n t i f i e d by HAZOP a r e u s u a l l y more d i f f i c u l t and c o s t l y t o implement. Areas where HAZOPs c o u l d be a p p l i e d e f f e c t i v e l y i n c l u d e : • • •

Manufacturing processes Maintenance a c t i v i t i e s Product l i a b i l i t y .


Conclusion The h a z a r d and o p e r a b i l i t y s t u d y i s a s i m p l e and e f f e c t i v e t e c h nique f o r i d e n t i f y i n g p o t e n t i a l problems i n a wide v a r i e t y o f systems and a c t i v i t i e s . I t s advantages o v e r more t r a d i t i o n a l approaches a r e t h a t i t i s more c r e a t i v e and r i g o r o u s and i s p a r t i c u l a r l y w e l l s u i t e d t o new t e c h n o l o g i e s and a r e a s where some n o v e l t y e x i s t s . A l t h o u g h easy t o a p p l y , c o n d u c t i n g HAZOPs r e q u i r e s t r a i n i n g and e x p e r i e n c e t o s e l e c t the b e s t approach, p l a n the a n a l y s i s , g e t the b e s t o u t o f the team, and r e c o r d the r e s u l t s . Literature Cited 1. 2. 3. 4.

5.

Helmers, E. N.; Schaller, L. C. Plant/Operations Progress, Vol. 1, No. 3, July, 1982, p. 190. Gibson, S. B. Chemical Engineering Progress, Vol. 76, No. 11, November, 1980. Gibson, S. Β. I . Chem E. Symposium Series, No. 47, 1976. Chemical Industries Association, "A Guide to Hazard and Operability Studies", Publications Department, Alembic House, 93 Albert Embankment, London, SE1 7Tu, England, 1977. Lawley, H. G., Chemical Engineering Progress, Vol. 70, No. 4, A p r i l , 1974, p. 45.

RECEIVED November 3, 1984


39

Hazard and Operability Study - ACS Symposium Series (ACS

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