Dust Explosion 51

8/2/2019 Dust Explosion 51

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Fire Safety Journal, 14 (198 9) 167 - 177 167

S a f e t y a n d F a u l t D e t e c t i o n i n P r o c e s s C o n t r o l S y s t e m s a n d S e n s o r s

R. K. CHOHAN124 Staines Road, Ilford, Essex IG1 2UY (U.K.)B. R. UPADHYAYAThe University of Tennessee, Knoxville, T N 37996 (U.S.A.)(Received May 1, 1988; in final form Jul y 29, 1988)

SUMMARYR e c e n t s e v e re a c c i d e n t s i n th e p r o c e s s

i n d u s t r y h a v e d e m a n d e d a c t i o n a n d i n v es t i-g a t i o n o f p r o c e s s s y s t e m s f o r s a fe t y . T h ep r e s e n t p a p e r a t t e m p t s t o i n t eg r a t e v a ri o u sa s p e c ts c o n n e c t e d w i t h s a f e ty o f s y s t e m s a n df a u l ts i n t h e e q u i p m e n t a n d s en so r s. E x c e p tf o r d u s t e x p l o s i o n s i n g r a in s t o r e s a n d o t h e rp r o c e s se s , m o s t o t h e r se r i o u s a c c i d e n t s h a v ei n v o l v e d e m i s s i o n s f r o m p o i n t f a i lu r e s s u c h a sv a lv e s, se a l s a n d p i p e c o n n e c t i o n s . W e a k l i n k sk n o w n t o h a v e c a u se d p a s t a c c i d e n ts ar ee m p h a s i z e d a n d s o m e s u g g e s t i o n s m a d e f o ra c c i d e n t p r e v e n t i o n . S e n s o r s h a v e b e e n id e n t i-f i e d a s a w e a k e l e m e n t i n a s a f e t y s y s t e m a n dv a r i o u s a s p e c t s a r e d i s c u s s e d . F a u l t d e t e c t i o n ,e s p e c ia l ly e a r l y f a u l t d e t e c t i o n , is i m p o r t a n tb o t h f o r e q u i p m e n t , s e n s o r s a n d a c tu a t o r s.V a r i o u s p r o c e d u r e s a r e r e v ie w e d . E a r l y d e t e c -t i on o f s y m p t o m s l e ad i ng t o a n a c c i d e n t isi m p o r t a n t , a n d t h e r e f o r e i n t e g r i ty o f al le l e m e n t s i n a d e t e c t i o n s y s t e m h a s t o b em a i n t a i n e d .

1. INTRODUCTIONT h e p r e s e n t c o n t r i b u t i o n a t t e m p t s t o i n t e -

g r a t e v a r i o u s a s p e c t s o f f a u l t s , fa i l u r e s a n ds a f e t y i s s u e s i n p r o c e s s e s . E m p h a s i s i s t h e nd i r e c t e d t o i n s t r u m e n t a t i o n , a n d f a u l t d e t e c -t i o n i n i n s t r u m e n t s a n d a c t u a t o r s .

T h e l as t t w o d e c a d e s h a v e w i t n e s s e d m a j o rp r o c e s s e x p l o s i o n s a n d h a z a r d s. A m o n gt y p i c a l a n d h i g h l y p u b l i c i z e d i n c i d e n t s w e r et h e r e le a se o f 5 0 m g o f c y c l o h e x a n e t o t h ee n v i r o n m e n t f r o m a p l a n t a t F l i x b o r o u g h ,E n g l a n d , l e a d i n g t o t h e l o s s o f 2 8 l iv e s o nJ u n e l , 1 9 7 4 ; a n d t h e e x p l o si o n in a p l a n tm a n u f a c t u r i n g 2 , 4 , 5 - tr i c h l o r o p h e n o l a t

S e v e s o , I t a l y , o n J u l y 1 0 , 1 9 7 6 . O n e o f t h em o s t s e v er e a c c i d e n t s h a p p e n e d v e r y r e c e n t l y ,o n N o v e m b e r 1 9 , 1 9 8 4 , i n M e x i c o C i ty a t ag a s p l a n t r e s u l t i n g i n t h e l o s s o f o v e r 3 0 0 l iv e sa n d o v e r 1 5 0 0 i n j u r ed . T h e m o s t s e r io u sa c c i d e n t a p p e a r s t o b e t h e r e le a se o f m e t h y li s o c y a n a t e g a s a t a p l a n t i n B h o p a l , I n d i a , o nD e c e m b e r 3 , 1 9 8 4 . T h i s i n c i d e n t c l a i m e d t h el iv e s o f m o r e t h a n 2 0 0 0 p e o p l e . A l l t h e s em a j o r i n c i d e n t s h a d o n e f a c t o r i n c o m m o n -t h e c a u s e o f e a c h i n c i d e n t w a s n o t c l ea r , a tl e a s t i m m e d i a t e l y a f t e r t h e in c i d e n t . T h e m o s ts e v e r e n u c l e a r a c c i d e n t a t a U . S . c o m m e r c i a lp o w e r p l a n t w a s t h e T h r e e M i l e I s la n d( T M I - 2 ) a c c i d e n t i n 1 9 7 9 . S o m e o f t h e s ei n c i d e n t s w i l l b e r e f e rr e d t o f u r t h e r i n t h ed i s c u s s i o n t h a t f o l l o w s .I n v i e w o f t h e a b o v e , i t is n o t s u r p r i si n gt h a t t h e r e h a s b e e n in c r e a s e d e m p h a s i s o ns a f e t y a n d r e l i a b i l it y o f p r o c e s s s y s t e m s inr e c e n t y e a r s . P u b l i c a t i o n s h a v e a p p e a r e d o nt h e p h y s i c a l p r o c e s s e s , s u c h a s e x p l o s i o n s ,e .g ., B o d u r t h a [ 1 ] , o n t o x i c a n d h a z a r d o u sw a s t e , e .g ., L a G r e g a a n d H e n d r i a n [ 2 ] , o nh u m a n d e t e c t i o n a n d d i a g n o s i s o f f a il u re s ,e .g ., R a s m u s s e n a n d R o u s e [ 3 ] , a n d o n f a u l td e t e c t i o n a n d d i a g n o si s , e .g ., H i m m e l b l a u[ 4 l .V a r i o u s a s p e c t s o f s a f e t y a n d f a u l t a n a l y s i st e n d t o b e e m p h a s i z e d f r o m t i m e t o t im e .F o r e x a m p l e , b e f o r e T M I - 2 , a g r e a t d e a l m o r ea t t e n t i o n w a s a t ta c h e d t o u t i li t y o f c o m p u t e rs u p p o r t t h a n t h e m a n - m a c h i n e i n te r fa c e . I ti s p r o b a b l e t h a t a t p r e s e n t , t h e r e i s m o r ec o n c e r n f o r h u m a n m a n a g e m e n t a n d o rg a n iz a -t i o n a l f a i l u r e s th a n p r o c e s s h a r d w a r e f a i l u re s[ 5 ] . O u r p u r p o s e i n t h e p r e s e n t p a p e r is t oe m p h a s i z e t h e i n t e g r a t i o n o f t h e s e m u l t i -d i s c i p li n a r y a s p e c t s. T h e o u t l o o k i s o n e o fp r e v e n t i o n r a t h e r t h a n c u r e . D i f f e r e n t a s p e c t so f s a f e t y a n d a c c i d e n t p r e v e n t i o n i n c h e m i c a l

0379 -7112 /89/ $3.5 0 Elsevier Sequoia/Pr inted in The Netherlands


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p r o c es s es a r e p r e s e n t e d b y F a w c e t t a n d W o o d[ 6 ] .A f t e r a d i s c u s s io n o f i n t e g r a ti o n o f d i ff e r -e n t s c e n a r i o s o f h a z a r d o u s a n d o t h e r u n d e si r-

a b l e e v e n t s , w e c o n c e n t r a t e o n a s p e c t s w h i c hw e f e e l ha v e b e e n s o m e w h a t n e g le c t e d ,n a m e l y , s e n s o r s , m e a s u r e m e n t , a c t u a t o r s ,c o n t r o l , a l a rm s a n d p r e d i c t i o n o f e v e n t s.S e n s o r s a r e c e n t r a l t o t h e p r o v i s i o n o f p r o c e s si n f o r m a t i o n f o r h a z a r d p r e v e n t i o n a n d c o n -t r o l . F a u l t d i a g n o s i s o f s e n s o r s , i n c l u d i n g t h a to f a c t u a t o r s , i s d i s c u s s e d . S u g g e s t i o n s a r e p u tf o r w a r d f o r s o m e n e w s e n s o rs a n d s ig n al p r o -c e s si n g f o r s o m e k n o w n p r o c e s s d e s ig n s .

2 . GENERAL CONSIDERATIONS ANDSCENARIOS

P r o c e s s s y s t e m s in c h e m i c a l , p e t r o c h e m i c a l ,f o o d a n d n u c l e a r t e c h n o l o g y t e n d t o b e c o m -p l e x . T h i s c o m p l e x i t y i s l a rg e l y a t t r i b u t e d t ot h e m a n y i n t e r a c ti n g s u b s y s t e m s o r p r o c e s s e s.P r o c e s s s y s t e m s u s u a l l y h a v e a ss o c i a t e d s a f e t ys y s t e m s w h i c h i n c l u d e a l a r m a n d p r o t e c t i o nf a c i li t ie s . B e s i d e s c o n s i d e r a t i o n s o f e q u i p -m e n t , m a n - m a c h i n e i n t e r a c t i o n s , e n v i r o n -m e n t a l a n d l e g a l c o n s i d e r a t i o n s , s a f e t ya n a l y s i s s h o u l d i n c l u d e i n s t r u m e n t a t i o n / c o n -t r o l a s p e c t s a s w e l l .

H a z a r d p r e v e n t i o n i n v o lv e s r e l ia b i l it y o fb o t h p r o c e s s e s a n d i n s t r u m e n t a t i o n . S a f e t ys y s te m s c a n b e i m p r o v e d b y r e d u n d a n c y{ s p a r e s ) , u p g r a d i n g c o m p o n e n t s , c o n t r o ls t r u c t u r e / s y s te m m o d i f i c a t i o n , a n d p e rh a p sd i s t r i b u t e d c o n t r o l . W h i l e h i g h e r r e l i a b i l i t yu s u a l l y i m p l i es h i g h e r c o s t s , t h e b e n e f i t s m a yb e s i g n i f i c a n t .

M o s t p r o c e s s e s o f s i g n i f i c a n t c o m p l e x i t yh a v e c o m p u t e r - b a s e d c o n t r o l a n d s a f e t y s y s -t e m s . F a i l u r e a n d f a u l t a n a l y s i s s h o u l d i n c l u d et h e f o l l o w i n g c o n s i d e r a t i o n s :

( a ) h a r d w a r e r e l i a b i l i t y( b } s o f t w a r e r e l i a b i l i t y( c ) s y s t e m i n t e g r i t y( d ) m a i n t e n a n c e( e ) s e n s o r / i n s t r u m e n t a t i o n r e l ia b i l it y( f ) a c t u a t o r r e l i a b i l i t y( g) m a n - m a c h i n e i n t e r f a c e i n c l u d in g

o p e r a t o r t r a i n i n g( h) i n f o r m a t i o n f l o w to m a n a g e m e n t .I d e a l l y , a ll t h e a b o v e s h o u l d b e f a u l t - t o l e r a n t .T h is m a y r e q u i r e s o m e c o m p e n s a t i o n f o r o n eo r m o r e f a il e d e l e m e n t , s e r v i c e o r a c t i v i t y .

C o m m o n l y e n c o u n t e r e d f a i lu r e e v e n tsi n c l u d e t h o s e i n v o lv i n g m e c h a n i c a l d e v i c ef a i l u r e s d u e t o e x t r e m e s i n p r o c e s s v a r i a b l e s{ e . g . , p r e s s u r e , t e m p e r a t u r e , c o n c e n t r a t i o n ) ,c o n t r o l / s a f e t y s y s t e m f a i lu r e d u e t o i ns tr u -m e n t a t i o n f a i l u re a n d / o r o p e r a t o r ' f a i lu r e '.

P r e v e n t i v e m a i n t e n a n c e m u s t a l so r e c e i v et o p p r i o r i t y . T h i s i m p l i e s i n s p e c t i o n , r e p a i r ,r e p l a c e m e n t , a n d d e t e r m i n in g t h e p r o b a b l ef r e q u e n c y o f f a il u re o f e q u i p m e n t o r a c o m -p o n e n t [ 7 ] . T i m i n g a n d a p p r o p r i a t e a c t i o na r e i m p o r t a n t a n d e a r l y f a u l t d e t e c t i o n s h o u l db e e n c o u r a g e d [ 8 ] . O p e r a t o r a n d m a n a g e m e n t; k i l l s h a v e t o b e u t i l i z e d .

W h e r e a s p r e v e n t i v e m a i n t e n a n c e is t h e? h i l o s o p h y b e h i n d s a f e t y s y s t e m s , p o s t -a c c i d e n t e v e n t s a l s o m u s t r e c e i v e a t t e n t i o n i na c o m p l e t e a s s e s s m e n t t o a i d i n t h e p r e v e n -t i o n o f f u t u r e a c c i d e n t s . A s s t a t e d e a r li e r , w es h al l e m p h a s i z e t h e f o r m e r .P l a m p i n g a n d A n d o w [ 9 ] p r e s e n t a p r o -c e d u r e f o r in f o r m a t i o n f l o w f o r a n al a rms y s t e m . T h i s p r o c e d u r e is d i v i d e d i n t o n i n es t a g e s :

( 1 ) d e t e r m i n e t h e f a il u r e e v e n t s o f t h ep l a n t ;

( 2 ) a s s e ss th e i r c r e d i b i l i t y ;( 3) d e f i n e t h e m o d e s o f o p e r a t i o n ;( 4 ) d e t e r m i n e t h e r e s p o n s e p a t t e r n o f t h ep r o c e s s v a r i a b l e ;( 5 ) d e t e r m i n e t h e c o r r e c t i v e a c t i o n s a v ai l-

a b l e t o t h e o p e r a t o r ;( 6 ) d e t e r m i n e t h e l e ve l o f d i a g n os i sr e q u i r e d ;( 7 ) a n a l y s e e f f e c t p a t t e r n s a n d c o r r e c t i v ea c t i o n s t o g r o u p s i m il a r e v e n t s ;( 8 ) s e l e c t t h e s e t o f p r o c e s s v a r i a b l e s ;( 9 ) a s s es s t h e a l a r m s s e l e c t e d a n d i f u n s a t i s-f a c t o r y r e p e a t s t a g e s 6 t h r o u g h 8 .T h i s s e q u e n t i a l p r o c e d u r e a l l o w s a n a p p re -

c i a ti o n o f t h e d e p e n d e n c e o f m e a s u r e m e n ta n d s t a t e s o n f a i l u r e a n d c o r r e c t i v e a c t i o n .V a r i o u s s e t s o f s t a te s m a y b e d e f i n e d f o ra l a r m s g i v e n s o m e m e a s u r a b l e l i m i ts . T h e s el im i t s m a y b e d i c t a t e d b y r u l e s s u c h a s t h ef o l l o w i n g [ 9 ] :( a) f o r t h o s e s t a te s d e f i n e d t o s a f e g u a r de q u i p m e n t - u se a b s o lu t e l im i ts ;( b ) f o r t h o s e s t a t e s d e f i n e d t o d e t e c t w h e nt h e p r o c e s s v a r i a b le d e v i a t e s b e y o n d a b a n da r o u n d a s e t - p o i n t v a l u e - - u s e d e v i a t i o nl i m i t s ;( c ) f o r t h o s e s t a t e s d e f i n e d t o d e t e c t i n s t ru -m e n t f a i l u r e - - u s e z e r o o r f u l l- s c a le l i m i ts ;


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(d) for those states defined to dete ct pro-cess variables -- use rate-of-change limits.Measurement is essential to determine allsuch limits. The multi-disciplinary aspectsof any safety procedure, such as that ofPlampling and Andow [9], must be recog-nized.Hazards are connected with substances(gases, liquids, solids or mixtures) havingproperties such as toxicity, corrosivity, flam-mability, explosivity, reactivity with othersubstances, and radioactivity. Most hazardscan be classified as:(1) explosions including chemical and dustexplosions;(2) fires;(3) leaks, which eventually may lead to anexplosion or fire or a health hazard;(4) radiation.The discussion that follows will be based onthese four major causes.3. PROTECTION, CONTROL AND ALARM

Various relative time-scales can be identi-fied with various hazard and other criticalsituations. These can be conve nien tly classi-fied according to the possible modes ofcorrective action:(a) short time-scale, i.e., situatio ns wherecatastrophe or accident can occur once initi-ated:(b) inte rmed iate time-scale, i.e., catastro-phe or accident can occur but can be stoppedif suitable action is taken rapidly.(c) elemental time-scale, i.e., catastrophe oraccident can be avoided if proper controlaction is followed.It can safely be said that prediction of theimpending disaster is most urgent for case (a).Most disaster predictions are best described ascombinations of art and science. Risk analysisis part and parcel of these predictions. Morgan[10] states that risk is due to the followingprobabilistic factors:(1) The values of signific ant variables arenot or cannot be known, and accurate predic-tions cannot be made.(2) The physical, chemical and biologicalphen ome na involved are not fully understood.(3) The processes involved are inhe rent lystochastic, or at least complex enough toimpede the development of valid predictivemodels.

169Specific design details of a process or unitare of significance no t onl y with regard to thegeneral develo pment of models but also pecu-liarities as far as possibilities of leaks, exces-sive pressure fluctuations, etc., are concerned.Intermediate time-scales would involve

proper measurement followed by appropriatealarm and action such as system shutdown.This action must be rapid enough to minimizethe risk of a hazard or the extent of it. Ele-mental time-scales involve appropriate mea-surement and control. Both these time-scalesinvolve measurement sensors and predictionmay benefit from some measurement. There-fore, we shall concentrate hereafter on mea-surement and fault detection of instrumenta-tion and actuators. Actuators, especiallyinvolving moving parts, are subject to failureand leaks.

4. SENSORS IN THE TECHNOLOGIESKnowledge of operating conditions of aprocess and its surrounding is essential foranalysis of potential hazardous situations andtheir prevention. The required informationmay include one or more of the followingvariables: operating temperature and pressure,flow rate, density, concentra tion of substanceor impurity, moisture content, particle sizeand hardness, accumulation rates at local posi-tions.The availability, proper location and use ofappropriate sensors are all most importantfactors. The post-accident diagnostics duringthe accident at the Three-Mile Island Unit 2reactor became complicated because of thenon-availability of some important processinstrumentation [11]. This incident also

demonstrated the importance of properlyplacing instrument indicators on the controlpanel, thus emphasizing the need for betterhuma n factors considerations.A brief consideration of sensors of impor-tance to hazards and critical situations is inorder here. The parameters of interest arepressure, temperature, toxic substance con-centration, flow rate, and radiation levelsamong others.Pressure and temperature are the mostwidely measured process variables. Technol-ogy for these is highly advanced and welldescribed in textbooks and in symposia.


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170Recent discussions on high pressures andtemperatures are provided by Pegg [12], andAnderson and KoUie [13], respectively.Gas detection devices can be classified aspellistors, infrared cells, electrochemical cells,galvanic cells, semiconductor devices andacoustic devices. Some of the problemsinvolved are:(a) Flammable gas sensors such as pellistorsmay be non-specific and the catalyst getspoisoned. Semiconductive devices degrade tothe ex tent tha t the y are not routinely used.Further performance may be limited by asso-ciated electronics and physical arrangement ofthe sensing head. Long-term problems mayarise due to zero-drift and sensitivity fall dueto sensor degradati on [ 14].

(b) Infrared sensors have many virtues butare not cheap.(c) Electrochemical cells are not very sensi-tive to low conc entr ation s usually encoun-tered for toxic gases and may not be specific.The life of such sensors is also not appreciable[15].Besides the above, a number of gas ana-lyzers are used [16]. However, these are quitetime-consuming concentration determina-tions. They also are prone to unrepresentativesamples to be analysed.New sensor and device development is inprogress. Some examples are [17] :(a) zinc oxide surge arrestors to control thedamaging effects of lightning on transmissionequipment;(b) optical boiler-flame scanners to preventboiler explosions;(c) tagging compounds used in conjunctionwith an ionization chamber to detect hotspots in generators.Recent developments in acoustic and opti-cal instrumentation are showing promise forthe provision of reliable sensors. Such sensorsmay satisfy some desirable criteria as statedby Wangsmo and Riddervold [18]:(1) elimination of the poisoning problem;(2) automatic and immediate fault indica-tion through self-test capability;(3) discrimination between hydroc arbonand non-hydrocarbon gases or other contami-nants, i.e., selectivity capability;(4) fast response time;(5) no amb igu ity in response at high gasconcentration;(6) low mainte nance at infreque nt intervals.

One of these d evelo pments is based on laserbeams [19]. This can be online and responsiveto low gas concentrations, is fail-safe and canbe self-calibrated. These characteristics areparticularly impor tant for some fugitive emis-sions.Hazardous substance detection and mea-surem ent can be based on tha t o f fugitive gasdetection systems [19]. Figure 1 shows arepresentation of the important methodsavailable at present. Another representationfor fugitive gas detection is shown in Fig. 2.The portable instruments enable detection at

REMOTESENSING.METHODS

i . ELECIRO~ETRICSENSORS2. SPECTROMETRCORELECTROOPTCALSENSORS3. CHEMLUMNESCENTSENSORS

CUMULATIVEMANUALSAMPLINGAND ANALYSIS

IMANUAL JGRABSAMPLINGMID ANALYSS

A) GAS ABSORPTION

B) INTEGRATEDSN~PLERC) GASADSORPTON

A) EVACUATEDFLASKB) DISPLACEMENTSYSTEMC) FLEXIBLEBAG

Fig. 1. Principle gas/liquid measurement systems.

DETECTIONMETHOD~, FLAME IONIZATIONDETECTION (FID)2. CATALYTIC OXIDATION(CAT OX)] . INFRA-RED ABSORPTION(I t )a. PHOTO-IO~IZATION (PID)

~,~ ,~ IONIZE THE GAS, THENMEASURETHE DEGREE OFIONIZATIONFig. 2. Fugitive gas detection.

I NSTRUI~NTATI NFOR

FUGITIVEGAS ~ETECTION

r NON-PORTABLE IIDETECTION~THOD

l , INFRA-RED2. LASER3. FIBER OPTICSh. SAMPLING ANALYSISETC


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v a r i o u s l o c a t i o n s s u c h a s v a l ve s a n d f a n s .V a l v e s a re n o t o r i o u s f o r l e a k s o f g a s es a n dl i q u i d s [ 2 0 ] a n d i t i s n o t g e n e r a l l y f e a s i b l e t oh a v e a s e n s o r f o r e a c h v a lv e . H o w e v e r , t h i ss h o u l d b e s e r i o u sl y c o n s i d e r e d w h e n h i g h l yd a n g e r o u s s u b s t a n c e s a r e i n v o l v e d .

A l l h a r d w a r e , i n c l u d i n g s e n s o r s , d e g r a d ea n d e x p e r i e n c e o t h e r f u n c t i o n a l p ro b l e m s .T e m p e r a t u r e a n d p r e s s u r e s e n s o r s n e e d c a li -b r a t i o n c h e c k s a n d r e s p o n s e t i m e t e s t i n g [ 2 1 ] .S o m e o t h e r p r o b l e m s w i t h i n s t r u m e n t a t i o nw e r e p o i n t e d o u t a b o v e . A n a l y t i c a l i n s t r u m -m e n t a t i o n h a s t w o m a j o r p r o b l e m s [ 2 2 ] :

( 1 ) l e a k s in p i p i n g / s a m p l e l in e s ;( 2) c o n t a m i n a t i o n o f t h e m e a s u r in g u n i t

d u r i n g s a m p l i n g .L e a k s i n s a m p l e l i n e s c a u s e d i l u t i o n , t h o s ei n t h e h y d r o g e n s u p p l y s y s t e m in a f l a m ei o n iz a t i o n d e t e c t o r m a y n o t b e d a n g e ro u s b u tm a y m a k e t h e s e n s o r n o n - f u n c t io n a l .

M a i n t e n a n c e o f p r o c e s s e s i n c lu d i n g in s t ru -m e n t a t i o n , a c t u a t o r s a n d c o m p u t e r s y s t e m sm u s t b e p r e s e n t e s p e c i a l l y f o r h i gh - ri sks i t u a t io n s . S y s t e m d e s i g n s h o u l d a l l o w f o rr e l a t i v e ly e a s y m a i n t e n a n c e . A n a l y t i c a li n s t r u m e n t s , f o r e x a m p l e , s h o u l d h a v e a f e wi n d i c a t o r s a n d a l l o w e a s y c h e c k i n g o fp l u m b i n g a n d e a s y s p o t t i n g o f l ea k s . T h i sw o u l d e n a b l e s i m p l e t e c h n i q u e s s u c h a s' s o a p i n g ' f o r l ea k d e t e c t i o n f o r i n s t r u m e n t sa t r o o m t e m p e r a t u r e s . S o a p b u b b l e s fr o m a na p p l i c a t i o n o f a s o a p s o l u t i o n w o u l d i n d i c a t ea l e a k [ 2 3 ] . V a l v e p a c k i n g s h o u l d b e p e r i o d -i c a ll y c h e c k e d a n d i f n e c e s s a r y , t h e p a c k i n gr e p l a c e d [ 2 0 ] .M a i n t e n a n c e a n d p r o p e r d e si g n ar e n e c e s -s a ry f o r t h e p r e v e n t i o n o f h a z a r d o u s a c c i d e n t sa n d p r o t e c t i o n , b u t a r e n o t s u f f ic i e n t. F a u l td e t e c t i o n a n d i t s a n a l y s is is a l s o a m o s t u r g e n ti ss ue . A n u m b e r o f p r o c e d u r e s a n d t e c h n i q u e se x i s t f o r t h e d e t e c t i o n o f f a u l t s i n p r o c e s s e s ,s e n s o r s a n d a c t u a t o r s . S i n c e s e n s o r s a n d a c t u -a t o r s a r e c e n t ra l t o t h e p r e v e n t i o n , c o n t r o l o fa n d p r o t e c t i o n f r o m a c c i d e n t s , t h e ir f a u ltd e t e c t i o n i s d i s c u s s e d i n t h e n e x t S e c t i o n .

5 . REVIEW OF INSTRUMENT AND ACTUATORFAU LT DETECTION TECHNIQUES5.1. Introduction

T h e m e t h o d s u s e d f o r m o n i t o r i n g a n d v er i-f i c a t io n o f p ro c e s s i n s t r u m e n t a t i o n a n d a c t u a -t i o n m a y b e c l a s si f ie d i n t o " s t a t i c " a n d" d y n a m i c " [ 2 4 ] . S t a ti c m e t h o d s a r e t h o s e

171w h e r e t h e s e n s o r p e r f o r m a n c e is n o t c o r re -l a te d w i t h s y s t e m d y n a m i c s . T h e s e i n cl u d el i k e - s e n s o r c o m p a r i s o n , l i m i t c h e c k i n g , in s t r u -m e n t - l o o p i n t e g r i t y c h e c k i n g , s ig n a l c l ip p i n g ,c a l i b r a t io n , e t c . I n t h e d y n a m i c a p p r o a c h t h es e n s o r v a l i d a t i o n i s p e r f o r m e d u s i ng s i gn a la n a l y s is i n c o n j u n c t i o n w i t h a p h y s i c a l o r a ne m p i r ic a l s y s t e m m o d e l . T h e l a t te r a p p r o a c hi s m o r e v e r s at i le b e c a u s e o f i t s a p p l i c a t i o nd u r i n g p l a n t o p e r a t i o n .A n o n i n v a s iv e t e s t , s u c h as r a n d o m d a t aa n a l y si s , c a n b e u s e d i n s o m e a p p l i c a t io n sw i t h o u t t h e n e e d f o r a p h y s ic a l m o d e l . S t a ti ca n d e m p i ri c a l m e t h o d s a r e ea s y t o i m p l e m e n ta n d a r e c o s t - e f f e c ti v e . I t i s p o s s i b l e t h a t m o s to f t h e s e n so r s c an b e m o n i t o r e d b y t hi sa p p r o a c h . O f t e n m e t h o d s a r e d e v e l o p e d th a ta r e su i t a b l e f o r m o n i t o r i n g a s p e c i f ic d e t e c t o r .I n t hi s p h i l o s o p h y a s e p a r a te m e t h o d m u s t b ea v a i l a b l e f o r e a c h t y p e o f s e n s o r . A t h i r da p p r o a c h f o r s e n s o r v a l i d a t i o n i s t o u s e s ig n a lst h a t a r e a c q u i r e d f r o m d i s si m i l ar s e n so r s . T h i sp r o v i d e s a d i v e rs i ty o f m e a s u r e m e n t t h u so v e r c o m in g t h e p r o b l e m o f c o m m o n - c a u s ef a il u re s . I n t h i s S e c t i o n w e re v i e w d y n a m i cs e n s o r v a l i d a t i o n m e t h o d s a n d s t a t e a f e we x a m p l e s .5.2. Statement of sensor failure detectionproblem

T h e m e t h o d s d e v e l o p e d f o r s e n s o r f a u l ta n a l y s is m u s t b e a b l e t o d i s ti n g u i sh b e t w e e ns e n s o r f a i l u r e a n d c h a n g e s i n t h e s y s t e mb e h a v i o r . F o r a g e n e r a l s o l u t i o n o f t h e p r o b -l e m , a s y s t e m d y n a m i c e q u a t i o n a n d a m e a -s u r e m e n t e q u a t i o n a r e n e c e s s a r y . T h e l in e a rf o r m o f t h e s e e q u a t i o n s is g i v en b y~((t) = AX ( t) + BU(t ) + W(t) ( c o n t i n u o u s t i m e )

( 5 . 1 )Y(k) = HX(k ) + V(k) + S F ( k ) ( d i s c r e t e t i m e )

( 5 . 2 )w h e r eX i s a n n - d i m e n s i o n a l s t a t e v e c t o rU is a p - d i m e n s i o n a l p e r t u r b a t i o n v e c t o rY is a n m - d i m e n s i o n a l m e a s u r e m e n t v e c t o rS F i s a v e c t o r o f s e n s o r e r r o r sk d e n o t e s t h e c u r r e n t s a m p l e d d a t a i n d e xW a n d V a r e p r o c e s s a n d m e a s u r e m e n t n o i s ec o m p o n e n t s

S e n s o r f a i l u r e s c a n b e m o d e l e d a s c h a n g e si n t h e m e a s u r e m e n t m a t r i x H , a b i a s i n t h e


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172sensor output, and additional sensor noise.Other fault estimation must be performed byfirst isolating the sensor and using appropri-ate fault estimation algorithms. Empirical andphysical model descriptions must also beconsidered.5 .3 . D y n a m i c m e t h o d s f o r s e n so r fa u l t d e te c -t io n a n d e s t i m a t i o nThe fundamental considerations in thisapproach are:(1)generate hardware and/or analyticalredundancy of the measurements;(2) a c ompa riso n logic for isola ting thesensor error from system behavior;(3) quantitative estimation of sensor errors.Analytical redundancy methods usingKalman filtering techniques [25] or dedicatedobservers [26] have been applied in the aero-space indust ry [27] and in nuclear powerplant models [28]. The Kalman filter is appli-cable to both steady-state and transientconditions, and for linear and nonlineardynamics.A generalized likelihood ratio method ofcomparing various fault conditions wasapplied to a pressurized water reactor (PWR)pressurizer level detection system [29]. Thisapproach is computationally slow. A moregeneral method of isolating faulty sensorsusing a parity-space representation was usedfor a boiling water reac tor (BWR) suppressionpool signal validation [30]. All these algo-rithms make use of physical models requiringvalidation. Such models m ay fail in a transien tsituation.A data-dependent modeling strategy andestimation of sensor errors by appropriatemeasurement model has recently been devel-oped [31] and applied for faulty estimationof core-exit thermocouples in the loss-of-fluidtest (LOFT) reactor. A subsystem of thereacto r (in this case the reactor core) was con-sidered fo r analysis a n d a dual-hypothesestesting procedure was applied to the modelgenerated from the in-core neutron detectorand core-exit thermocouple signals. Sensorerrors such as bias, excess noise, changes inresponse time can be estima ted using propermeasurem ent models.Utilization of a pair of signals not relatedto the sensor under study could provide infor-mation about sensor validation. Such a tech-nique for monitoring fluid flow velocities has

been reported. A pair of thermocouples [32]or a pair of 16N det ect ors [33] ma y be corre-lated to determine the fluid transit time andhence the velocity. This provides an indepen-dent met hod of verifying flow meters andthus provides diversity against common modefailure of redundant sensors.The choice of a particular approach for sig-nal validation and instrument fault detectiondepends on the implementational aspect ofthe m et hod and the available measurem ents.Distinction must also be made between slowdegradation and transient failures.The above discussion on fa ult dete ctio nand identification concentrated on signal pro-cessing. However, other procedures are alsoto be encourage d such as physical/ chemica leffects on component aging, use of mechan-istic models for syste m behavior and historicalrecords of components.

6 . P O T E N T I A L P R O C E D U R E S F O R P R E V E N T I O NOF ACCIDENTSPrevention of and protection from acci-dents can be visualized as decisions andactions made at various hierarchical levels.The highest of these levels would be at thenational level (such as the EPA in the UnitedStates) and lowest level would be a plantoperator. Generally speaking, as one ascendssuch a hierarchy, data becomes somewhatimprecise, infrequent and incomplete result-ing in a greater or lesser degree of risk associ-ated with decisions and actions.A serial procedure for accident preventionis shown in Fig. 3. The first step in Fig. 3attempts to classify a system, region or spaceinvolving a hazardous material (solid, liquid,

gas or mixed phase). Factors which are to beconsidered in such a grading of flammabilityand toxicity include the following [34] : com-ponents present in the space, volume of space,relative quantity of each component present,phase of component, pressure, temperature.Some substances can easily be classified ashaving a high hazard rating, for example,meth yl isocyanate.Suitable action(s) will be required for sub-stances having a high hazard rating. Furthermeasures or quantification may be carriedout (e.g., see the Appendix in ref. 34).A literature search of all known physical,


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I L O N F ~ I L I I Y ~ O N I C I T YO R I ~ -~ssoNs- MO~BATEEm~U~Y~OXCTY'STORES) ~

~ H I O N F ~ I L I I Y / T O X I E I T Y

2. LOCATEPOSITIONS IN SYSTEM MOST LIK ELY TO(A) ACCUMULATE(0) ESCAPE(C) HAVE CONDITIONS COHDUCIVETO A UTOIG INHION OR OTHER OPERATIONS

(SUCH AS WASHING OIL TANRER CARGOSWITH WATERJETS (JONES & BOND(198L~)).3. ~I.COL AIE~S TIi"IATE THRESHOLDCONDITIONS E.G. ELECTROSTATIC HRESHOLD) IN

A C, AIN STORE OR OIL TANKERS CARGO)BASED ON WORSTCASE ASSUM PTIONS SUCHAS i'II NIM ~ IGIN IT]O N ENERGY, FLAI,It~AI~LEVAPOR CONCENTRATION, ET C.

~. ASSESS F LIKELY/POSSIO~CO NDITIONS EXCEED THOSE OF THEESTI~TED/EALCULATED THRESHOLDSFROil STEP ( 3) .

5. M~J~AGERIALAETION

MANAGEMENTIPRE-ACCIDENT ACTION IPOST-ACCIOENT ACTION

IB) EXPLOSI ON/ F I REONTAINMENTC) INSURANCELOCATION CONSIDERATIONI I I ISEN SO R S AC T U ATO R S, D ES IG N O PER ATIO Nr FLANGES 4=THER MOV NG PARTS i

IIII I 1PREVENTIVE CONTROL ALARM'4- .. .. .. . LIMITING CONDITIONACTION(AUTOMATED/OTHERWISE)

Fig. 3. Procedure for accident prevention.

c h e m i c a l , a n d t o x i c p r o p e r t i e s m u s t b e c a r ri e do u t u s i n g r e f e r e n c e s s u c h a s S a x [ 3 5 ] . T h e s ew o u l d a c c o u n t f o r t h e c h e m i c a l h a z ar d s . I t i si m p o r t a n t t o c o n s i d e r t h e p h y si c a l a n d e q u i p -m e n t h a z a r d s s u c h a s h i g h t e m p e r a t u r e , p r es -s u re , c o n s t r u c t i o n m a t e r i a l , e q u i p m e n t a g e,m a i n t e n a n c e , r e l i a b i l it y o f i n s t r u m e n t s / a c t u -a t o r ] c o n t r o l s y s t e m s a n d h u m a n e rr o r. T h er e le a se o f to x i c / f l a m m a b l e s u b s t a n c e f r o m av a lv e , l e ak s f r o m p i p e f la n g e s a n d p u m p s e al s,f a i lu r e o f p r o c e s s v e s s el s d u e t o c o r r o s i o n o re x c e s s p r es s ur e a re e x a m p l e s o f e q u i p m e n th a z a r d s. E q u i p m e n t h a z a r d s t e n d t o b e s it e -s p e c i f i c . S t r i c t l y s p e a k i n g , t h e a n a l y s i s w o u l dr e q u i r e r e f e r e n c e t o a p a r t i c u l a r p l a n t . H o w -e v e r , s o m e g u i d e l i n e s a r e a v a i l a b l e f o r a f r e -q u e n t l y o c c u r r i n g e q u i p m e n t h a z a r d , n a m e l y ,l e a k s f r o m v a l v es [ 3 6 , 3 7 ] . D e c i s i o n s b a s e do n m o d e l s s u c h a s le a k d e t e c t i o n a n d re p a ir( L D A R ) [ 3 6 ] a n d re g u la r m a i n t e n a n c e a r er e c o m m e n d e d .

173T h r e s h o l d c o n d i t i o n s f o r a u t o i g n i t i o n o r

i n t o l e r a b l e f u g i t i v e e m i s s i o n s a r e d e s i r a b l e a n ds o m e p r o c e d u r e s a r e a v a i la b l e a s p o i n t e d o u te a r l i e r ( s e e a l s o r e f . 1 ) . I f t h e p r o c e s s c o n d i -t i o n s a r e s u c h t h a t t h e r e i s a f a ir p r o b a b i l i t yo f e x c e e d i n g s u c h a t h r e s h o l d , s u i t a b l e a n dt i m e l y a c t i o n i s r e q u i r e d . P r o c e s s m a n a g e m e n ti s c e n t r a l l y i n v o l v e d a n d p r e , a c c i d e n t / p o s ta c c i d e n t a c t i o n s a r e s h o w n i n F i g . 3.

A n u m b e r o f d i sa s te r s, e x c e p t d u s t e x p lo -s io n s , a p p e a r t o b e d u e t o e m i s s i o n s t o t h ea t m o s p h e r e a n d c a n b e c a u s e d b y :

( a ) r e l i e f v a l v e o p e r a t i o n ;( b ) e q u i p m e n t f a il u r e su c h a s p i p e r u p t u r e ;( c ) l e a k s i n v a lv e s , f a n s , p u m p s e a ls , e t c .

T h e d i s a s t e r in B h o p a l i n v o l v e d t h e o p e r a t i o no f a r e l i e f v a l v e d u e t o i n t e r n a l o v e r - p r e s s u r ec a u s e d w h e n w a t e r c o n t a m i n a t e d m e t h y l is o-c y a n a t e . T h e S e v e s o i n c i d e n t a l so in v o l v e dl i q ui d e s c a p e f r o m a r e l i e f v a l v e c a u s e d b y ar u n a w a y r e a c t io n . V a l ve s a n d o t h e r u n i ts w i t hm o v i n g p a r t s h a v e t o b e p r e s e n t i n a p r o c e s s .F a i l u r e s o f su c h c o m p o n e n t s i s a l so p o s s i b l ea n d s a f e t y / p r o t e c t i o n s y s t e m s i n c lu d i n gm a n a g e m e n t s h o u l d a l l o w fo r th i s.I n v i ew o f t h e a b o v e , a n y c o m p o n e n t s w i t hm o v i n g p a r t s o r w e a k p o i n t s s u c h a s s e al sm u s t r e c ei v e a t t e n t io n . E a r l y d e t e c t i o n o fe m i s s i o n s i s h i g h ly d e s i r a b l e f o l l o w e d b yp r o m p t a c t i o n . A p p r o p r i a t e s e n s o rs t o g e t h e rw i t h a l a r m / c o n t r o l / p r o t e c t i o n a c t i o n is a b s o -l u t e l y n e c e s s a r y w h e r e h i g h l y h a z a r d o u ss u b s t a n c e s , s u c h a s m e t h y l i s o c y a n a t e , a rei n v o lv e d . T h i s a n d t h e d e g r e e o f r e d u n d a n c yo f s e n s o r s , a c t u a t o r s , e t c . , s h o u l d r e f l e c t t h er is k a n d h a z a r d o u s n a t u r e o f t h e s u b s t a n c e si n v o l v e d a n d p r o c e s s c o n d i t i o n s .

A u t o m a t i o n i s d e s i r a b le a s it t a k e s n eg li -g i b le t i m e f o r s p e c i f i e d a c t i o n s, a s w e l l a sa v o i d i n g h u m a n e r ro r s. H o w e v e r , t h e n e e d f o rh u m a n - c o m p u t e r i n t e r a c ti o n i s t h e re . A s L e e s[ 3 8 ] p o i n t s o u t , t h e r e i s l i tt l e t h a t a c o m p u t e rc a n d o t o c o m p e n s a t e f o r f a il u r e s a n d m a l-f u n c t i o n s i n p l a n t e q u i p m e n t v a l v e s a n ds e ns o rs . O p e r a t o r / m a n a g e m e n t a c t i o n isr e q u i r e d a n d s h o u l d b e ti m e l y , s u i ta b l e a n ds u f f i c i e n t . C o m p u t e r s u p p o r t w o u l d c e r t a i n l ye n h a n c e r e l i a b il i ty a n d a c c i d e n t p r e v e n t i o nb u t i t s h o u l d b e r e c o g n i z e d t h a t i t is o n l y o n es u b s y s t e m c o m p r i si n g t h e w h o l e s y s t e m ( th ep r o c e s s p l a n t ). I n t e r a c t i o n s b e t w e e n t h e s e isa n i m p o r t a n t c o n s i d e r a t i o n . L e e s [ 3 8 ] p o i n t so u t t h e i m p o r t a n c e o f t h e o p e r a t o r i n r e co g -n i z i n g p a t t e r n s , d i a g n o s i n g f a u l t s , d e c i s i o n -


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174making and input commands to a computer.Information presentation to operators, eitherverbal or from instruments or computer out-put, should be simple and minimal.

We describe below some possible post-design procedures in specific cases. In themanufacture of carbon monoxide, hydrogenis a by-product. Because hydrogen has a wideflammable range in air and a low ignitionenergy of 0.002 mJ, it is important that it isseparated from carbon monoxide [34]. Ifsuch a mixture flows as a stream near person-nel or a residential district, then an arrange-ment shown in Fig. 4 may be employed.Because the thermal conductivities of hydro-gen and carbon monoxide are easily differ-entiable, a thermal conductivity gas analysiscan be employed for hydrogen gas detection[39, 40]. Regular tests should be carried outon this instrument, such as soap-bubble leakdetection using a soap solution. If a Wheat-stonebridge is employed, then a step currentinput may be used to analyse the instrumentin the same way that immersion-type thermo-meters are analysed [21]. Alarms should besufficiently audible and be coupled with oth erprotective measures. Control action shouldallow for enhanced removal of hydrogeneither immediately downstream of the mea-surement location or at an upstream point.Worst-case situation should be used to designand implement the protection systemincluding personnel training and awareness.Leaks fr om seals and valves should be assessedregularly using a portable instrument .For highly hazardous substances such asmethyl isocyanate, there are at least two

CONTROL ANDPROTECTONSYSTEM~ SAMPLELINE

I CARBON ONOXIDETREAMCONTAININGYDROGENMPURITY PZFig. 4. Prevention and protection for a carbonmonoxide stream containing hydrogen as impurity.

levels of post-design decision-making. One isthe national level, and the other is the plantoperational level. The national level may pos-sess the power to shutdown a plant but sucha decision is usually a pre-design one. Shouldthe plant concerned be operational, then thefollowing should be located and possibleprovision of sensors, positioned in theirproximity, be considered:(a) control valves;(b) re lief /other valves;(c) seals, connecti ons, other weak loca-tions.

Equipment design usually allows for afactor of two or more times the highest pres-sures and temperatures attainable. This hasproved to be desirable as the Seveso incidenthas shown that unexpected pressures and tem-peratures can be attained. Components withmoving parts and potential weak spots such aspipe connec tions must be isolated for atten-tion. Possible actions may be depicted asshown in Fig. 5. A det ect or should be locatednear the potential opening area of a valveor seal. A gas absorber unit surrounding thevalve should be provided, and should be easilyremovable. Alternatively, the emitted gasescan be vented to a flare as suggested by Hust-vedt [41]. Absorption and flaring may notbe suitable for all gases. Other control devicesinclude those based on thermal oxidation, theuse of emissions as fuels, carbon adsorptionand condensation [42].Sensors are to be located at various areasaround the plant with signals activating, upon

SPATALLY~ aLOCATEDGAS

TOXCFLA~ABLE GASABSORBERORFLAREVA,~r I- ~TOXC/F~MMABLE.... ~ ~ DETECTOR

Fig. 5. Provision for leak detection, containment andprotection.


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r e a d i n g t h r e s h o l d v a l u e s , al a r m s f o r p e r s o n n e la n d s u r r o u n d i n g r e s i d e n t i a l a re a s . A l l s e n s o r ss h o u l d b e r e g u l a r l y a n a l y s e d f o r f a u l t s e i t h e rb y m e t h o d s d e s c r i b e d in S e c t i o n 5 o f b yd i r e c t t e st i n g . S i t e p e r s o n n e l s h o u l d c a r r y p o r -t a b l e g a s d e t e c t o r s . T h e f r e q u e n c y o f i n s p ec -t i o n o f v a lv e s , s e al s a n d c o n n e c t i o n s f o r i m m i -n e n t f a i lu r e s a n d l e a k s s h o u l d r e f l e c t t h ed e s ig n , k n o w n f a il u re r a te s o f t h e c o m p o n e n ta n d d a m a g e d u e t o p r o c e s s o p e r a t i o n a l c h a r-a c t e r i s t i c s o r i n t e r n a l f o r c e s . F u r t h e r , t h i s f r e -q u e n c y s h o u l d i n c re a s e w i t h a g e o f th e c o m -p o n e n t . P l an t s h u t d o w n d o e s n o t i m p l y th a tt h e a l a rm , c o n t r o l a n d p r o t e c t i o n s y s t e m s a r et o b e d i s a b l e d . S e v e s o s h o u l d b e a l e s s o n t h a ta n e m i s s io n c a n t a k e p l a c e a f t e r a p l a n ts h u t d o w n .

P r o c e s s e s w i t h h i g h l y f l a m m a b l e s u b s t a n c e sm u s t a l so b e p r o v i d e d w i t h f ir e a n d e x p l o s i o np r o t e c t i o n . T h e s e m u s t r e s p o n d q u i c k l y . N e wt e c h n o l o g y s h o u l d b e a s s e ss e d a n d b e u t i l iz e dd e p e n d i n g u p o n t h e c o s t s a n d b e n e f i t s. M i c ro -c o m p u t e r - b a s e d f ir e a n d s e c u r i ty s y s t e m s h a v et h e f o l lo w i n g b e n e f i t s [ 4 3 ] ;( a ) e n h a n c e d r e s p o n s e , s u c h a s v o i c e m e s -s a ge s t a i lo r e d t o t h e l o c a t i o n o f f ir e a n d z o n e so f s a f e t y ;( b ) r e l a t i v e r e d u c t i o n i n h a r d w a r e ;

( c ) r e l a ti v e r e d u c t i o n i n s y s t e m c o s t ;( d ) e a s i ly u p g r a d e d .D u s t e x p l o s i o n s h a v e a l so r e c e i v e d a t t e n -

t i o n d u e t o i n c r e a s e d i n c i d e n c e , e s p e c i a l ly inc o n n e c t i o n w i t h f o o d d u s t s a n d p o l y m e r i cp o w d e r s [ 4 4 , 4 5 ] . S u c h e x p l o s i o n s i n vo l v ed u s t p a r t i c le s o f a p p r o p r i a t e s iz e s, a ir o r o x y -g e n e n v i r o n m e n t , a n d a n ig n i t io n s o u r c e .M i n i m i z i n g t h e i n c i d e n c e r e q u i r e s c o n s i d e r -a t i o n s s u c h a s:

( a) e q u i p m e n t d e s ig n( b ) c a r e f u l o p e r a t i o n s u c h a s a v o i d in g a n y

n a k e d f l a m e .A s f o r t h e o t h e r p o t e n t i a l a c c i d e n t s i t u a -t i o n s, p r o t e c t i o n m u s t b e p r o v i d e d . M e t h o d sa r e a v a i la b l e a n d d i s c u s s e d b y P a l m e r [ 4 5 ] .T h e s e a r e b a s e d o n m i n i m i z i n g c l o u d f o r m a -t i o n , c o n t a i n m e n t , s e p a r a t i o n o f p l a n t , v e n t -i n g , i n e r t i n g a n d a u t o m a t i c s u p p r e s s i o n . A sP a l m e r p o i n t s o u t , i t m a y b e w i se t o u s e th e s em e t h o d s i n c o m b i n a t i o n r a t h e r t h a n s in g ly .

7 . CONCLUSIONT h e p r e s e n t p a p e r ha s a t t e m p t e d t o in te -

g r a t e t h e v a r i o u s a s p e c t s r e l a t e d t o s a f e t y i n

175p r o c e s s, f o o d a n d o t h e r s y s t e m s . T h e f o l lo w -i n g fa c t o r s a i d i n m a k i n g d i f f i c u l t t h e t a s k o fa c c i d e n t p r e v e n t i o n :

( a) i n c o m p l e t e , a n d i m p r e c is e k n o w l e d g e o fp r o c e s s e s i n v o l v e d ;( b ) h u m a n e r ro r s ;

( c) c o m p o n e n t f a il u re ;( d ) la c k a n d / o r a b s e n c e o f s e n s o rs a t p o t e n -t i a l s o u r c e s o f a c c i d e n t s ;( e) u n s a t i s f a c t o r y m a n - m a c h i n e i n te r f ac e s ;

( f) l ac k o f p r o p e r / e f f e c t i v e c o m m u n i c a t i o na n d a c t i o n w i t h i n a p l a n t, a n d b e t w e e n t h ep l a n t a n d t h e p u b l i c .

T h e r e is r o o m f o r p r o g r e s s i n a ll t h e s ef a c t o r s . P o s t - a c c i d e n t s m u s t b e u s e d t o p r o -v i d e i n f o r m a t i o n f o r a n a l y s is a n d i m p r o v e -m e n t o f s a f e t y s y s t e m s . P o s t - a c c i d e n t s t u d i e ss u c h as t h a t o f T h e o f a n o u s [ 4 6 ] a re o f v a l u ea n d s h o u l d b e u s e d f o r f u r t h e r a n a l y s i s a n d / o rd e c i s i o n - m a k i n g i n c o n n e c t i o n w i t h a c c i d e n tp r e v e n t i o n a n d p r o t e c t i o n .T h e m o s t s e ve r e a c c id e n t s h a v e s h o w n t h a tt h e f o l lo w i n g w e r e in v o l ve d : c o m p o n e n t f ail -u r e o r v a l v e o p e n i n g d u e t o e x c e s s p r e s s u r e ,l a c k o f e a r l y d e t e c t i o n d u e t o a b s e n c e o r n o n -a v a i l a b il i ty o f a p p r o p r i a t e s e n s o rs , a n da b s e n c e o f a l a rm s a n d a p p r o p r i a t e a c t i o n .D i s a s t e r s i n v o l v e s h o r t t i m e - s c a l e s a s d i s c u s s e di n S e c t i o n 3 . P r o p e r s y s t e m a n d c o m p o n e n td e s i g n , p e r s o n n e l t r a i n i n g , e a r l y f a u l t / l e a kd e t e c t i o n f o l l o w e d b y p r o m p t , s u i ta b l e a c t i o nis t h e r e f o r e m o s t i m p o r t a n t . R e l i a b i l i t y o fv a r io u s c o m p o n e n t s i n a s a f e t y s y s t e m ,h u m a n e r ro r s a n d c o m m u n i c a t i o n i s a ls oi m p o r ta n t . T h e e c o n o m y o f m i c r op r o c e ss o r sd o e s a l lo w h a r d w a r e r e d u n d a n c y o f c o m p u t e rs u p p o r t t h u s p r o v id i n g s t a n d b y p r o c e s s o r s i ne m e r g e n c y s i t u a t i o n s .

R e c o m m e n d a t i o n s w h i c h c an b e p r o v id e da r e :

( 1) c o m p u t e r i z e d c o n t r o l s y s t e m w i t hm a n u a l o v e r r id e ;( 2 ) c o m p o n e n t s s h o u l d h a v e a m a n u a l o v e r -

r i d e d u e t o , f o r e x a m p l e , p o w e r fa i l u re o r lo s so f c o m p r e s s e d a i r t o p n e u m a t i c v a l v e s ;

( 3 ) f i x e d f u g i t i v e g a s s e n s o r s , w i t h h a r d -w a r e r e d u n d a n c y i n c r i ti c a l s i t u a t io n s , a tp r o b a b l e l o c a t i o n s o f g a s i n c a se o f re l e as e .P e r i o d i c t e s t i n g t o b e p r o v i d e d ;( 4 ) a l a r m s y s t e m s w h i c h s h o u l d b e s u f fi -c i e n t b u t n o t g i ve i n to l e r a b l e f a ls e a la r m s ;

( 5 ) s y s t e m d e s i g n s h o u l d i n c l u d e i n t r i n s i cs a f e t y c a p a b i l i t ie s [ 4 7 ] . F o r h i g h l y e x p l o s i v em e d i a , t h i s s h o u l d a l so a p p l y t o p o w e r -


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176c o n s u m i n g d e v i c e s ; d e s i g n o f s o l e n o i d v a l v e sf o r i n t r i n s i c a l l y s a f e s y s t e m s i s d i s c u s s e d b yJ o h n s o n a n d C o l o n y [ 4 8 ] .

A l l p r o c e s s e s i n v o l v i n g h ig h l y h a z a r d o u sm a t e r ia ls s h o u l d b e f l e x i b le e n o u g h f o r te c h -n o l o g y u p d a t i n g . T h i s c o u l d i n v o l v e , f o re x a m p l e , i m p r o v e d d e s i g n v a l ve i n s t a l la t i o na n d s o f t w a r e a d d i t i o n t o t h e c o m p u t e rs u p p o r t. A c t i o n f o r im p l e m e n t a t i o n d u r in ga n d i m m e d i a t e l y a f te r a p o t e n t i a l a c c id e n tm u s t b e t h o r o u g h l y e x a m i n e d a n d p e r i o d -i c a l l y a s s e s s e d . A l l p r e v e n t i v e a n d p r o t e c t i v em e a s u r e s s h o u l d b e c o n s i d e r e d f o r s a f e t y o ft h e p l a n t , p e r s o n n e l a n d t h e p u b l i c .

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