rapid quantitative risk analysis using a new software package...

________ ~'='._'='-~' L

Indian Journal of Chemical TechnologyVol. 4, July 1997, pp.l67-179

Rapid quantitative risk analysis using a new software package

MAXCRED-II

Faisal I Khan & S A Abbasi

Risk AssessmentDivision,Centre for PollutionControl & Bio-waste Energy, Pondicheny University,Pondicheny 605014, India

Received5 November 1996;accepted6 March 1997

In this paper, development of an elaborate software package for quantitative maximum credible acc;identanalysis (MAXCRED-ll) for simulating accidents such as explosion, fire, toxic release and or acombination of these, has been described. MAXCRED-II has been used in generating accident scenariosand estimating damage potential for a chemical industry producing Sulfolane. A number of differentscenarios have been generated for explosions, fires, as well as toxic releases in different process units andthe damage they would cause have been estimated. The studies indicate that a confmed vapour cloudexplosion followed by frre in hydrogen storage vessel would be the worst disastrous scenario and wouldalso have the highest probability of leading to cascading 'or domino effects. The keying of all the requiredinput information and getting directly usable output for the case study required approximately 2 h. Thisindicates the appropriateness of MAXCRED-II as a tool useful in conducting rapid quantitative riskassessment.

The Bhopal gas tragedy of 1984 has brought intosharp focus the hazards posed by chemical processindustries functioning in the midst of denselypopulated neighbourhoods. A very large number ofsuch industries either use hazardous chemicals

and/or operate reactors under such extremeconditions of temperature and pressure that even aminor human or equipment failure can lead to majorexplosions, fires and toxic leakages individually orsimultaneously.

As Bhopal gas tragedy\,2 was catastrophic in itsdimensions-leading to a death toll that is by far theworst ever in the history of industrial accidents-itis by no means an isolated instance. Indeed severalserious industrial accidents have taken place beforeand after Bhopal gas tragedy in India as well asabroad. Indeed, in India itself such accidents keepoccurring with frightening regularity.

Considering the fact that population as well asindustrialization are continually increasing in India,the frequencies of the accidents and the, damagecaused by them, are likely to increase. It, therefore,becomes essential to develop methodologies whichcan serve the following purposes:

(a) to forecast accidents: this is aimed forcreating opportunities to rectify problems (of man

and materials) before any harm can result;(b) to analyse consequences of likely accidents:

such consequence analysis fulfil two objectives:they help in sitting of industries and management

of sites so as to minimize the damage if accidentdoes occur;

they provide feedback for other exercises inaccident forecasting and disaster management.

(c) to help in development of managerialstrategies: for 'emergency preparedness' and'damage minimization'.

Maximum credible accident analysis (MCAAt5is an approach (for step b) to assess the damage achemical plant or industrial complex may cause toitself and surroundings if an accident occurs. In thismethodology, scenarios of worst yet plausibleaccidents are generated. The main aim is to foreseethe hazardous events (accidents) likely to cause

damage to industry and environment, and to assessthe extent of the likely damage. This analysis is veryuseful both for existing plants and for the ones atdesign stage because the analysis gives forewarning

168 INDIAN 1. CHEM. TECHNOL., JULY 1997

of the vulnerable spots enabling preventive steps tobe taken.

MCAA comprises of following main steps: (i)

study of the plant to identify hazardous material and

their capacities, (ii) identification of vulnerablesections, (iii) visualization of different accident

scenarios, (iv) damage calculation through mathe-

matical modelling, (v) delineation of maximumcreqible accident scenario.

MAXCRED, a software package dev€110pedbyauthors, is one such tool for enabling quantitativerisk assessment as a necessary prelude to riskprevention/minimization and disaster management6.The package incorporates state-of-the-art mathe-

matical models, including some developed or

modified by authors for rapid quantitative but

comprehensive MCCA7.

The Package MAXCRED-llMAXCRED-II is an improved version of the

software package MAXCRED (ver I) which wasdeveloped by authors in 1994. The package is codedin C++ language and is compatible with DOS aswell as WINDOWS working environments. Thesoftware is operable on personal computerrequiring a minimum 1 MB RAM and 1.5 MBROM. The MAXCRED-II algorithm is presentedin Fig. 1.

The software has four main options (modules):

,r-;;::-\\....::.."!-)

Fig. I-The MAXCRED-II algorithm

data, scenario generation, consequence analysis,

andfile processing. The data option handles generalinfonnation related to the properties of the various

chemicals, operations, and the surroundings, neededfor the execution of different models. The scenario

generation module enables development of accident

scenarios based on the properties of the chemicals

involved, operating conditions, and the likely waysof malfunctioning of the equipment or process thatwould cause an accident. The scenario generationoption has two sub-options (a) user defmed and (b)automatic. In the fonner, the user can defme theaccident scenario as per his/her own judgement. Inthe latter, scenarios are generated by MAXCRED-II

on the basis of the knowledge base attached to it.

The consequence analysis module takes the study toits next logical step, i.e., to forecast the nature andthe severity of the accident using advanced modelsof thermodynamics, heat transfer, and fluiddynamics. Thefile option enables the user to handleinput-output infonnation. It also provides facilitiessuch as consoling, printing, copying, etc. All-in-allMAXCRED-II is a versatile tool for QRA andenvisaged to be self-contained in the sense that itdoes not need other packages for data analysis orgraphics. The internal structure and working ofMAXCRED-II has been described in detailelsewhere7.

In this paper, MAXCRED-II has been used tostudy quantitative risk analysis of a typical chemicalindustry to be commissioned in the industrial area atThane, Maharashtra, India. The results of the studyhave been represented in terms of risk (damage-probability) profile; which represents the variationsin individual risk factor with distance from the

accident epicentre.

Nature of likely accidentsIn general, an industry may have four types of

accidents, namely (i) fire; (ii) 'explosion; (iii) toxicrelease and dispersion; (iv) a combination of theabove. The logical sequence and interdependence ofmost likely accidents that may occur in an inoustryis presented in Fig. 2.

Explosion and firesExplosions in the storage or process units can be

categorized in three main groups, accordini to modeof occurrence and damage potential. Theseexplosions are initiated either by the thermal

I 11 ! III II IllItII 1 !!t !; J ! ~ !

KHAN & ABBASI: RISK ANALYSIS USING MAXCRED-II 169

60~, II

B;~c:J We!'1 •• "'111Fig. 2- The diagram showing different accident events and their

interdependency

stratification of the liquid and vapour or by suchhigh explosion shock waves, which have sufficientstrength to rupture the reaction/storage vessels orconduits. An explosion mayor may not beaccompanied with fire; it depends upon the type ofexplosion and chemical involved in the explosion.

Confined vapour cloud explosion (CVCE)-CVCE as the name suggests, is condensed phaseexplosion occurring in confmement (equipment,building or/and congested surrounding)9,!o.Explosions in vessels and pipes, processing orstoring reactive chemicals at elevated conditions areexamples of CVCE. The excessive build-up ofpressure in the confmment leads to this type ofexplosions leading to high overpressure, shockwaves and heat loads (if chemical is flammable andget ignited). The fragments of exploded vessels andother objects hit by blast waves, become airborneand act as missiles. These missiles can lead to

further accidents by ramming into other processunits. The damage potential of the missiles isassessed on the basis of the momentum they attain.The extent of damage caused by a CVCE dependson the mass of the chemical and the explosionpressure.

Unconfined vapour cloud explosion (VVCE)-WCE generally occurs when sufficient amoWltofflammable material (gas or liquid having highvapour pressure) gets released and mixes with air toform a flammable cloud such that the,averageconcentration of the material in the cloud is higherthan the lower limit of explosionII. The resultingexplosion has high potential of damage as it occursin an open space covering large areas. The intensityof explosion mainly depends on the quantity of

material released and the strength of the ignitiOllsource.

The explosive power of a CVCE and WCE canbe expressed in terms of blast wave characteristics(overpressure, overpressure-impulse, reflectedpressure, duration of shock etc.). The overpressure isa very important parameter; its magnitude dependson the speed of flame propagation. Any obstructionin the flame propagation enhances the blast effect.

Boiling liquid expanding vapour explosion(BLEVE)-BLEVE is caused by a sudden release ofa liquidfrom confmement at a temperature above itsatmospheric pressure boiling point9. The suddendecrease in pressure results in explosivevaporization of a fraction of the liquid and a cloudof vapour and mist, with accompanying blasteffects. If the material is flammable and an ignitionsource is present, a fire ball may be fonned. The

broad defmition of BLEVE, as used bl the Kletz!!,National Fire Protection Association! and Societyof Fire Protection Engineers13 is used here.According to this defmition any liquefied vapour,flammable or non-flamniable, can produce aBLEVE. A fire ball/flash fire is not a part of thisdefinition, since a fire (fire ball/flash fire) wouldresult only if the material released is flammable andthus ignition' occurs. Nevertheless, it is a historicalfact that most BLEVEsinvolve flammable liquids,and most of these BLEVE releases are ignited by asurrounding fire, resulting in a fire ball/flash fire.Fires

Spillage of flammable material (liquid/gas) maylead to fire which could be triggered by any of theseignition sources: (a) an electric spark; (b) amomentary flame due to welding operation; (c)atmospheric friction; (d) burning of match stick. Incase of highly flammable materials the fire may bestarted even by the mild friction caused byatmospheric disturbances. Generally, the fire effectsare limited to areas close to the source· of fire(approx. -200 m radius). However, industrial firescan have a greater pervasive effect. The industrialfires are mainly characterized in' three groups,described.

Pool fire-Continuous or instantaneous releaseof flammable liquid on ignition results in a poolfireIO.12.Pool fire characteristics mainly depend onthe duration of release, saturation pressure, and theflammableproperties of materials.

170 INDIAN J. CHEM. TECHNOL., 1ULY 1997

Flash fire-Instantaneous ignition of a vapourcloud having concentration above lowerflammability limit gives a flash fIfe8,B. It occursonly with flammable chemical having boiling pointlower than ambient temperature. It is different from

fire ball in terms of speed of flame propagation,

duration of fire, and heat load generated. Flash fire

occurs with flammable chemical stored or processed

above ambient conditions. Damage associated with

flash or pool fire is assessed on the basis of the doseof heat radiation load received for a particular/giventime interval.

Fire bal/-Instantaneous combustion of

flammable vapour cloud due to radiation exposureabove material threshold levels, or missile and blast

. .. hara . d fi b 111415wave mteraction IS c ctenze as Ife a '.

The fire ball is generally observed for highlyflammable chemicals processed or stored underextreme conditions. Evaluation of the consequencesof a fire ball requires the quantification of fIfe balltemperature, fire ball duration and fIfe ball size. Fireball temperature is dependent on the heat capacity ofthe fuel consumed and the means of combustion.

The fire ball temperature may vary from 1350 K forflammable gases to 5000 K for chemical explosives.

Toxic release and dispersionVapour clouds from industrial installations arise

principally from the accidental release of gases,

flashinft liquefied gases or evaporation of spilledliquids 6. The toxic vapour (gas) cloud is likely to bedangerous even at much greater distance from thepoint of release than their flammable counterparts.This is mainly due to the ease of their dispersioncor6pared to liquid (flammable or toxic) release,harmfulness at even very low concentrations (at ppblevels) and the high probability of coming in directcontact with the living systems.

Release conditions-To estimate the

characteristics of dispersion of gases due to anaccidental release, the following accidental releaseconditions (with appropriate models) have beenconsidered 16,17.

(a) Gaseous release.(b) Liquid release at atmospheric pressure. This

condition can further be categorized as: (i) liquidwith a boiling point above ambient temperaturewhich is processed/stored at a temperature below itsnormal boiling point; (ii) liquid with a boiling point

below ambient temperature which ISprocessed/stored at low temperature andatmospheric pres,Sure.

(c) Two phase release (liquid under pressure)16,17.This condition can also be further categorized in two

classes: (i) liquid having normal boiling point above

ambient temperature which is processed/stored

under high pressure and temperature above its

normal boiling point; (ii) liquid having normalboiling point below ambient temperature which isprocessed/stored under high pressure andtemperature above normal boiling point.

Dispersion-Dispersion is primarily governed bytwo facts: (a) momentum of release and (b) densityof the gas relative to air .

As long as the momentum of the escaping gas issignificant, the density factor does not becomeoperative but as soon as the momentum dies downto a level where the ambient air movements could

effect dispersion, the density factors take over toinfluence the shape of the plume.

When the gas escapes at high velocity as from ajet or a vent, the momentum effect is moreprominent and lasts longer (due to higher velecity ofrelease) than when the release velocity (ventingvelocity) is low.

According to Leesl8 releases in the form of jetscan be of four types: (i) turbulent momentum jet instill air, (ii) buoyant plume in still air; (iii) plumedispersed by wind and (iv) jet-turbulent plumedispersed by wind. The behaviour of such jets andvents is as relevant to the intended discharges as toaccidental discharges. The behaviour of thedispersion of such jets depends on the relativeimportance of discharge momentum, buoyancyeffects, and of wind turbulence. To estimate this

mode of dispersion, turbulent jet model along withplume dispersion, as in modified plume path

19 dtheory has been use .

Once the gas loses its momentum it is influencedby the density of the gas-air mixture relative to air.A difference in the molecular weight and/or in thetemperature between the gas and the ambient aircreates, in principle such a density difference, butthis density difference will affect the behaviour ofthe cloud only if the concentration of the gas issufficiently high. A large proportion of the liquid

droplets and a low l1irhumidi~favour the formationof a gas cloud heavier than air o.

I i'


A heavy cloud behaves differently from one of

neutral density in several important aspects. Itspreads not only downwind but also upwind, it isflatter in sh&peand the mechanisms of mixing withthe air are different21• Compared to dispersion oflighter-than-air or as-dense-as-air gases, heavy gasdispersion has been studied to a much lesser degree.Only a handful of models are available to handleh d" 2223 F h d"eavy gas IsperslOn .. or eavy gas IsperslOn,

2224Box model . has been adopted.

Dispersion of gases (gas-air mixture) havingdensity equal or less than air under the influence ofambient air movements is characterized by neutralbuoyancy dispersion. Even a heavy gas acquires adispersion pattern akin to that of neutral buoyancydispersion when density-driven turbulence becomesweak (in other words density differences betweenair-gas mixture and air becomes negligible) as moreand more ambient air is entrained in the cloud

causing atmospheric turbulence to dominate thedispersion process. For continuous and instanta-neous release conditions Gaussian plume and puffmodels have been used respectively, to predict thebehaviour of neutral dispersion.

Case studyA detailed risk assessment study has been carried

out for a typical chemical industry due to beinstalled in the industrial area at Thane25,

Maharashtra, India. The industry primarilymanufactures sulfolane which is a solvent used for

extraction of aromatic hydrocarbon; and is also usedas feed stock for man, chemical and petrochemicalindustries. The industry handles several hazardouschemicals such as butadiene, sulphur dioxide (S02),catechol, and sulfolene. A list of different processunits of the industry involving various hazardouschemicals is presented in Table 1.

Process summarySulfolane is prepared in two steps. First sulfolene

(2,4-dimethyl thiophene) is prepared by a reactionof butadiene and sulphur dioxide (S02); the productis then hydrogenated in presence of nickel (Ni)catalyst to yield sulfolane.

The reaction producing sulfolene is highlyexothermic and is carried out in liquid phase underhigh pressure. A condition of slightly above normalreaction temperature favours undesirable side.reactions which further increase the temperature andpressure in the reactor. Moreover, concentration ofbutadiene and sulphur dioxide should be maintainedin a proportion of 1:8 otherwise the undesirable sidereactions may occur generating high pressure andtemperature in the reactor. Thus, precisemeasurement and accurate control of material flow,

temperature, and pressure are essential to control theunwanted reaction. Otherwise accident may occur,

Table I-The operating conditions and chemicals used in various process units of sulfolane plant

Units

ChemicalTemperature,OCPressure, atmQuantity, kg

Storage

Butadiene323.015000.0

S02

254.1625000.0Sulfolene

551.501??oo.0

Hydrogen

207.501??oo.0

Sulfolene reactor

Butadiene764.0890.0

S02

764.01220.0

Catechol

764.0500.0Sulfolene

764.01878.0

Evaporator

Sulfolene351.21878.0

S02

351.252.0

Butadiene

351.2222.0

Stripping unit

Sulfolene1401.21878.0

S02

1401.26.6

Ni

1401.2125.0

Butadiene

1401.21.5

Compressor unit

Butadiene454.01350.0

S02

305.254160.0

Sulfolene

501.56'1200.0

Hydrogenation reactor

H2401.45200.0

Sulfolene

401.451878.0

Sulfolane

401.451900.0

172 INDIAN J. CHEM. TECHNOL., JULY 1997

the probability of which is dependent upon thereliability of the temperature/pressure/flow controlsystems. Using fault tree analysis, a probability of3.5*£-05 yr-t for an unwanted situation in thereactor has been estimated. Similarly, theprobability of occurrence of undesirable event inother units have also been estimated. These

probabilities have later been used for risk factor

computation. A detailed description of fault tree

analysis and frequency estimation technique has427

been presented elsewhere '

Accident scenarios

Based on the history of major accidents in. d . 1 12 14 d th th' .process m ustnes" an e au ors expenence,

following scenarios have been visualized foraccidents in different units of the plant.

Storage unit-The main three raw materialssulphur dioxide, hydrogen, and butadiene are storedin large quantities while other raw materials such ascatechol and intermediates such as sulfolene are

stored in comparatively lesser quantities. A total offour most credible accident scenarios have beengenerated (one for each hazardous chemical). Forfailure of hydrogen vessel the scenario is confinedvapour cloud' explosion (CVCE) followed by fireball; for butadiene it is boiling liquid expandingvapour explosion (BLEVE) followed by flash fire;for sulphur dioxide it is BLEVE followed by toxicdispersion; and for sulfolene it ispool fire.

Sulfonation reactor-The sulfonation reactoroperates under high pressure and temperature forproducing' sulfolene. This unit deals with twohazardous chemicals in large quantities at severeoperating conditions. The accident scenario for thisunit is visualized as BLEVEfollowed byfire ball dueto butadiene, and release and dispersion of S02'

Rectification unit-This unit separates theunreacted raw material from the stream coming outfrom the reactor. This unit mainly handles S02 andsulfolene in large quantities. Two accident scenarioshave been visualized as continuous release of S02due to leak in pipe, equipment or from vent valve,and pool fire due to ignition of sulfolene.

Compressor unit-The main purpose of this unitis to increase the pressure of different streams. Theimportant chemicals dealt here are S02 andbutadiene. Two accident scenarios (one for eachchemical) have been visualized as unconfined

vapour cloud explosion (UVCE) due to butadiene,and release and dispersion 0[S02'

Hydrogenation reactor-The conversion ofsulfolene to sulfolane takes place in this reactor. Theunit handles hydrogen in large quantities. Theaccident scenarios for this unit are visualized as

BLEVE followed by fire ball due to hydrogen andpool fire due to sulfolene, sulfolane or both.

Discussion

The results of accident simulations performedwith MAXCRED-ll for different accident scenarios

mentioned above are presented in Tables 2 and 3. Itis evident from Table 2, that damage potential due

to explosion (CVCE) in hydrogen vessel is quite

high. The lethal overpressure (shock waves) havingpotentiality to cause 50% damage/fatality wouldinfluence an area of ~712 m radius. Furthermore,there would also be heat load due to fire ball

impacting an area of ~338 m radius. The accidentscenario for the release of S02 (Table 3) indicatesoverpressure as well as lethal toxic load over alarger area of ~1575 m radius. It is seen that damagepotential of explosion and fire in buti}.dienestoragevessel (Table 3) is comparatively less intense thanexplosion and fire in hydrogen vessel. Pool fire insulfolene tank: (Table 3) would lead to lethal heatload extending up to ~ 148 m from the accidentepicentre.

The risk profiles for accidents in variouschemical storage units have been plotted in Figs 3

and 4. The profiles for both hydrogen and su1~hurdioxide lie above the acceptable risk limits 9,30,while the profiles for other chemicals are mostlywithin the acceptable range.

The summarized output of MAXCRED-II for anaccident in sulfonation reactor is presented in Table4. It is evident from Table 4 that a lethaloverpressure load would occur up to a distance of~355 m. Moreover, a moderately intense heat loaddue to fIfe ball would also occur over the same

distance. The instantaneous release of S~ (Table 4)would lead to lethal toxic load over an area of ~1125

m radius. The risk profiles for accident insulfonation reactor is presented in Fig. 5; theprofiles for scenarios 1 and 2 are lying above theacceptable level of risk. The unit is accordinglycharacterized as 'highly hazardous'.

The summarized output of MAXCRED-II fordifferent accident scenarios in the rectification unit

I III I III I I It"III 'I', i~' I~

Parameters

KHAN &. ABBASI: RISK ANALYSIS USING MAXCRED-II

Table 2-MAXCRED output for an accident scenario in hydrogen storage vesselValues

173

Results of MAXCRED simulation for accident scenari~VCE and fire ball

Explosion: CVCEEnergy released during explosion

Peak overpressure

Variation of overpressure in air

Shock velocity of airDuration of shock wave

Missile characteristics

Initial velocity of fragmentKinetic energy of fragmentFragment velocity at study pointPenetration ability at study pointConcrete structureBrick structureSteel structure

Damage radii (DR) for various degrees of damage due to overpressure

DR for 100% complete damageDR for 100% fatality or 50% complete damageDR for 50% fatality or 25% complete damage

Damage radii (DR) for the various degrees of damage due to missile

DR for 100% damage or 100% fatilityDR for 50% damage or 100"10fatilityDR for 100% offatality or 10% damage

Fire: Fire ballRadius of the ftre ballDuration of the ftre ball

Energy released by ftre ballRadiation heat flux

Damage radii due to thermal loadDamage radii for 100% fatality/damageDamage radii for 50% fatality/damage

Damage radii for 100% third degree of bumsDamage radii for 30% third degree of bums

(kJ) :2.466242e+08(kPa)

338.44141

(kPais)

235.58842

(mls)

:604.92639(ms)

:37.953421

(m/s)

:3238.17529(kJ)

:1.048578e+07(mls)

:1207.36307

(m)

:0.996405(m)

1.273148

(m)

:0.091060

(m)

:255.456(m)

:467.342(m)

:712.455

(m)

2198.087402

(m)

:2313.660400(m)

:2662.127686

(m)

:154.178947(s)

:85.134566(kJ)

:7.67077e+09(kJ/m2)

:172.897638

(m)

:225.463(m)

:338.674(m)

:451.432(m)

:507.674

1.000E+Ol

1iI:J..•

~ 1.000E-070 0.:1.5 1~ 2 2~ 3 3.:1 4

'tlistance. km

1.()OOE+Ol

1.000E+008

. ---- -

~ l;~ 1.000E-04

.•. "."18 1.000E-05:J..,~ 1.000E-06..,£ UlOOE~71.000E-Oe

00.511.522.533.5

Disl ance, km

- Acceplllllie timit +Hydrogen *" Sulfur dioxide

Fig. 3-Risk proftle showing severity of an accident occurringin storage tinit of hydrogen and SulphUr dioxide

~AoceptBble limit +Butadiene *Sul!olene

Fig. 4-Risk proftle showing severity of an accident occurring instorage unit of butadiene and sulfolene

174

Parameters

INDIAN 1. CHEM. TECHNOL., JULY j 997

Table 3-Summarized output ofMAXCRED for an accident in butadiene, S02' and sulfolene storage vessels

Values

(kJ)

(kPa)

(kPals)(m/s)(ms)

Accident scenario for butadiene storage vessls-BELEVE and flash fireExplosion: BLEVE

Total energy released

Peak overpressure

Variation of overpressure in airShock velocity of airDuration of shock wave

1.455438e+05

125.4348286

71.41065590

545.214562730.4833162

Damage radii (DR) for various degrees of damage due to overpressure

DR for 100% complete damageDR for 100% fatality or 50% complete damageDR for 50% fatality or 25% complete damage

Fire: Flash fire

Volume of vapour cloudDuration of fire

Radiation heat flux

(m)(m)(m)

(m3)

(s)

(kJ/m2)

128.143234.523367.453

11629.24023

3876.3666

141.97184

(k1)

(kPa)(kPals)(m/s)(ms)

Damage radii due to thermal load

Damage radii for 100% fatality/damage (m)Damage radii for 50% fatality/damage (m)

Damage radii for 100% third degree of bums (m)

Damage radii for 50% third degree of burns (m)

Accident scenario for sulphur dioxide storage vessel-BLEVE and toxic re[eIlseExplosion: BLEVE

Total energy releasedPeak overpressureVariation of overpressure in airShock velocity of airDuration of shock wave

55.455112.243

194.245

245.443

1.363438e+0412.38482867.86065590343.95562730.4833162

Damage radii (DR) for various degrees of damage due to overpressureDR for 100% complete damageDR for 100% fatality or 50% complete damageDR for 50% fatality or 25% complete damage

Toxic release & dispersion

Gaussian instantaneous: ModelConcentration at distance 350 mConcentration at cloud axis

Value of source height

Damage radii for various degrees of damageDamage radii for 100% lethalityDamage radii for 50% lethalityDamage radii for 10% lethality

Accident scenario for sulfolene storage vessel-pool fire

Fire: PoolfireInstantaneous model

Radius of the pool fireBurning areaBurning rateHeat flux

Damage radii due to thermal loadDamage radii for 100% fatality/damageDamage radii for 50% fatality/damageDamage radii for 100% third degree of burnsDamage radii for 50% third degree of burns

(m)(m)(m)

(kglm3)

(kg/m3)

(m)

(m)(m)(m)

(m)(m2)

(kg/s)

(kJ/m2)

(m)(m)(m)(m)

53.454105.466178.465

3.845254e-037.487667e-025.000000

1575.0812048.4322854.264

5.00000078.5374938.42223

i]60.191

'~354: ..18.433

206.154":87.245

I fI I f. III II '11111"11 "I " I H 111'!·III;

Parameters

KltAN & ABBASI: RISK ANALYSIS USING MAXCRED-II

Table 4-Surnmarized output ofMAXCRED for an accident in sulfonation reactor

Values

175

Accident scenario for butadiene-BLEJIE andfire ballExplosion: BLEJIETotal energy released

Peak overpressure

Variation of overpressure in air

Sliock velocity of airDuration of shock wave


Initial velocityKinetic energy of fragmentFragment velocity at study point

(kJ)

(kPa)

(kPa/s)

(m!$)

(ms)

(m/s)(kJ)(m1s)

3.06956Ie+o7

211.67897

140.89212

491.7827

37.95345

847.34125525224.1875377.14608

Damage radii (DR) for various degree of damage due to overpressureDR for 100% complete damage (m)DR for 100% fatality or 50% complete damage (m)DR for 50% fatality or 25% complete damage (m)

Fire: Fire ballRadius of the fire ballDuration of the fIre ball

Energy released by fire ballRadiation heat flux

195.632355.672534.456

81.6771724.67305l.l 3452 Ie+o865.32177

Damage radii due to thennalload

Damage radii for I()()% fatility/damageDamage radii for 50% fatality/damageDamage radii for I()()% third degree of bumsDamage radii for 50"10 third degree of bums

Accident scenario for sulphur dioIide-taxic relase

Toxu: release & dispersion

Gaussian instantaneous: ModelConcentration at distance 350 mConcentration at cloud axis


Damage radii for various degrees of damageDamage radii for 100% lethality

Damage radii for 50% lethalityDamage radii for 10% lethality

1000E+01

(m)(m)(m)(m)

(m)(m)(m)

1oooE+01

134.354668243.021343349.676769563.810963

3.257244e-047.1 1394 Ie-035.o00ooo

1125.4321795.4322225.241

l.oooE+O ~

M 1OO0E-011 ~o. I',~ 1000E-02~,", ,

~ 1000E.03/ ~ ~~ 1000E-O'Ii ~,i 1OO0E~1..• ,~ ; ooOE-06[£ 2 25

1oo0E-070 0.5 1 1.5 koiatllllC 8. m3.5

1 OOOE+0'

1.000E-Ol

:;IIIII

~ 1oo0E-O'I'"

~ 1000E·Q5'0'>

~ 1.000E-06

1.oo0E-07o 0.5 15 2 2.5 3 3.5 4

Distence, km

-- Acceptable lImit +BuldlQne *Sulfur dioxIdeFig. 5-Risk profIle showing severity of an accident occurring in

sulfonation reactor due to butadiene and sulphur dioxide

-Acceplable limit +Sulfolene "Sulfur dioxide

Fig. 6-Risk profIle showing severity of an accident occurring in

rectification unit due to sulfolene and sulphur dioxide


Parameters)

Table 5-Summarized output ofMAXCRED for an accident in rectification unit

Values

(m1s) 3.500000

(kg/m~

8.752341e-05

(m)

875.642

(m)

1178.243

(m)

1422.345

Damage radii for various degrees of damage

Damage radii for 100% lethalityDamage radii for 50% lethality

Damage radii for 10% lethality

Accident scenario for sulphur dioxidetoxic release

Toxic release & dispersionGaussian continuous: Model

Value of wind speedGround level X concentration

Accident scenario for sulfolene--pool fire

Fire: Pool fireContinuous Model

Burning area

Burning rateHeat flux

223540.781250

150484.828125206.848602


Damage radii for 100% fatality/damage

Damage radii for 50% fatality/damage

Damage radii for 100% third degree of bums

Damage radii for 50% third degree of bums

(m)(m)

.em)

(m)

251.342

321.467

402.645

4R7.125

Parameters

Table 6-Summarized output ofMAXCRED for an accident in compressor unit

Values

Accident scenario for sulphur dioxidetoxic release

(kg/m3) 6.642633e-06(kg/m3)

5.335456e-04(m)

2.000000

(m)

:280.654(m)

336.142

(m)

475.244 ,Toxic release & dispersion

Gaussian instantaneous: Model

Concentration at distance 350 mConcentration at cloud axis


Damage radii for various degrees of damage

Damage radii for 100% lethalityDamage radii for 50% lethalityDamage radii for 10% lethality

(kJ) :2.496782e+06(kPa)

52.485844

(kPals)

36.535244

(mls)

385.69592

(ms)

37.953491

(mls)

670.388489

(kJ)

449420.718

(mls)

:249.954208

(m)

110.235

(m)

207.432

(m)

324.125

Accident scenario for butadien~UVCE

Explosion: UVCETotal energy releasedPeak overpressureVariation of overpressure in airShock velocity of airDuration of shock wave

Missile charilcteristics

Initial velocityKinetic energy of fragmt;ntFragment velocity at study point

Damage radii (DR) for various degrees of damage due to overpressureDR for 100% complete damageDR for 100% fatality or 50% complete damageDR for 50% fatality or 25% complete damage

I 'II I I I III IIIII' ~~1~I


.l.OOOE+Ol , .OOOE+Ol ~---_.

iiw==-

:s

1.000E-C3

t510 1.000E-04.;;; "."iii 1.000E-05" "0.;;'6 1.000E-06E

1.000E-1l70

0.511.522.533.54

Distance, km

M0'UJ. 1.1lO0E·030 t) 1.000E·04.l!!~

1.000E·o:liii :J 1.000E·06..., :~..., 1.000E·07.£ 0

0.5 1 1.5 2 2.3 3 3.5 4

Distance. km

~ AOCQPtablQ limit +502 *ButjOdiQnQ - Acceptable limit +Sulfolene ..- HydrogenFig. 7-Risk profile showing severity of an accident ~ in Fig. 8-Risk profile showing severity of an accident occurring in

compressor unit due to butadiene and sulphur dioxide hydrogenation reactor due to sulfolene and hydrogen

7.873726e+07276.639988109.426933589.7643737.953491

(kJ)(kPa)(kPals)(mls)(s)

Table 7-Swmnarized outputt ofMAXCRED for an accident in hydrogenation reactorValuesParameters

Accident scenario for hydrogen-BLE~ and fire IHlll

Explosion: BLEVE

Total energy releasedPeak overpressureVariation of overpressure in airShock velocity of airDuration of shock wave


Initial velocityKinetic energy of fragment

Fmgment velocity at Study point

(mls)(kJ)(mls)

819.215393517270.781

468.159302

Damage radii (DR) for various degrees of damage due to overpressureDR for 100% complete damage (m)DR for 100% fatality or 50% complete damage (m)DR for 50010fatality or 25% complete damage (m)

Fire: Fire ballRadius of the fire ballDuration of the fire ball

Energy released by fire ballRadiation heat flux

210.434

383.214565.145

146.17894781.56245666.78657e+09167.897638

(m~:7.5000ooo

(m)

125.53749

(legis)

98.42223

(kJ/m2):1980.191

(m)

:250.432(m)

:345.673(m)

:474.765(m)

:565.214


Damage radii for 100% fatalitY/damageDamage radii for 50% fatality/damageDamage radii for 100% third degree of bumsDamage radii for 50% third degree of bums-

Accident scenario for sulfolane---pool fire

Fire: Pool fireInstaneous model

Radius of the pool fire

BumingareaBumingrateHeat flux


Damage radii for i00% fatality/damageDamage radii for 50010fatality/damageDamage radii for 100% third degree of bumsDamage radii for 50010third degree of bums

(m)(m)(m)(m)

210.263302.145418.233497.764


is presented in Table 5. The fIrst accident scenario--release of SOz-would lead to lethal toxic load overan area of ~875 m radius, while the second

scenario--a pool fIre due to sulfolene-would resultin lethal heat load over a zone of ~321 m radius. Inother words an accident in rectifIcation unit would

either cause toxic load due to S02, or lethal heat

load due to sulfolene, or both. The risk profile of

such an accident· is given in Fig. 6. It may be seen

that the predicted risk profIles overlap the maximumacceptable risk level. This unit is, therefore,characterized as 'hazardous'.

The results for the two most credible accident

scenarios, visualized for accident in compressorunit, are presented in Table 6. The damage potentialdue to toxic release (Table 6) is lower compared tothe previous units and lethal toxic load is restrictedonly up to an area of ~280 m radius. The possiblereason is that this unit processes lesser amount ofSOz at moderate conditions of temperature and

pressure. The second accident scenario--explosiondue to instantaneous combustion of butadiene

vapour cloud (UVCE)--would lead to overpressureload and missile effects (Table 6). The maximumdistance up to which there would be potentiality ofcausing 50% damage due to explosion is estimatedas 207 m.

The risk profIle for the compressor unit (Fig. 7)lies below the maximum acceptable risk level.Hence, this unit is characterized as 'moderatelyhazardous' .

The summarized output of MAXCRED-II for thetwo scenarios generated for the hydrogenationreactor is given in Table 7. Overpressure load andmissile impacts would be caused up to ~350 m fromthe acciden: epicentre. The maximum damagedistances due to overpresswe and missiles havebeen estimated as ~383 m and ~1765 m

respectively. The lethal heat load due to fIre ballwould reach a distance of ~210 m. The second

scenario--pool fIre due to sulfolane--would resultin lethal heat load extending up to ~250 m (Table 7).The risk profIle for this unit (Fig. 8) lies just abovethe maximum acceptable risk level. Hence, this urlitis characterized as 'hazardous'.

In brief, the MAXCRED-II simulations revealthat the storage and sulfonation units are the mosthazardous. The hydrogenation reactor and therectifIcation unit are progressively less hazardous

whereas the compressor unit is the least hazardousof all accident-prone unit.

Conclusion

This paper demonstrates the applicability ofsoftware package MAXCRED-II in rapidly

performing quantitative risk analysis and generatingdifferent accident scenarios which can then be used

in developing damage prevention and mitigation

strategies. The applicability has been illustrated witha case study of a chemical industry coming up atThane industrial area near Thane, Maharashtra.

Of the different scenarios generated withMAXCRED-II for likely credible accidents indifferent units of the industry, the scenario of

'boiling liquid vapour cloud explosion followed bytoxic dispersion of SOz' is likely to cause the worstimpact over a large area. The scenario 'confInedvapour cloud explosion in hydrogen storage unitfollowed by fIre ball' has the maximum potential of

causing cascading (domino) effects. The study also

reveals that storage and sulfonation units are themost hazardous of the constituents of the industryand greatest attention be bestowed upon them. Thestudy also identifIes the hazards associated withother units and ranks them in order of priority forimplementing risk reduction measures.

References1 Khan F I & Abbasi S A, Chem Eng World, September,

(1996).2 Kletz T A•. What went wrong (Gulf Publication, London),

1986.3 MallikaJjunan M M, Raghavan K V & Pitersen C M, An

approach to maximum credible accident analysis of cluster

of industries, in Proceedings of Envirotech IntemationalConference, Bombay, September, 1988,21-24.

4 Khan F T & Abbasi S A, Risk analysis: An optimum schemefor hazard identification and assessment, in Proceeding ofIX National System Conference, Coirnbatore, (1995) 112-119.

5 Greenberg H R & Cranuner J J, Risk assessment andmanagement for chemical process industries, (Van NostrandReinhold, New York), 1991.

6 Khan F T & Abbasi S A, MAXCRED-A software packagefor quantitative risk analysis, Report'No CPCEIR&D 5/94,Pondicheny University, Pondicheny, 1994.

7 Khan F I & Abbasi S A, MAXCRED-H: An advancedversion of MAXCRED, Environmental Software (in press),1996.

8 Khan F I & Abbasi S A, Anatomy of Industrial Accidents,Report No CPCEIR&D 1/94, Pondicherry University,Pondicherry, 1994.

9 Prug R W, Fire Prot Eng, 3(1) (1991) 9.

I Ii I I I "1'I "11"11""1 .~~!~,I !.: I . !


10 Pitersen C M, J Loss Prevent Process Ind, 3 (1990) 136-141.

11 Kletz T A, AIChE Loss Prevent, 11 (1977) 50.12 Fire protection handbook, National Fire Protection

Association, 16th ed., 1986.

13 The SFPE handbook offire protection engineering, Societyof Fire Protection Engineers, 1988.

14 Scielly N F & High W G, The blast effects of explosions, inProceeding of Fifth international symposium on loss

prevention and safety promotion, Cannes, 1986.

15 Roberts A F, Fire SafetyJ, 4 (1982) 197-212.16 Shaw P & Brisco F, Evaporation from spills of hazardous

liquids, United Kingdom Atomic Energy Report, SRD-lOO,UK.

17 Kayes P J, World bank manual of industrial hazardassessment techniques (Technica Ltd, London), 1985.

18 Lees, F P, Loss prevention in the process industries, Vol I(Butterworths, London), 1980.

19 Khan F I & Abbasi S A, Atmos Environ, (in press).20 Lees F P, Loss prevention in the process industries, Vol 2

(Butterworths, London), 1980.

21 Longl0 G K & Schatzamann M, Atmos Environ, 25A (1989)1189.

22 Van Ulden A P, A new bulk model for dense gas dispersion,'Two-dimensional spread in still air, 1UTAM Symposium onatmospheric dispersion of heavy gas and small particles,Delft, 1983.

23 Deaves D M & Hall R C, The effect of sloping terrain ondense gas dispersion, in Proceeding of First internationalconference on loss of containment, 1989.

24 Weatley C J & Webber D M, Aspects of the dispersion ofdenser-than-air vapours relevant to gas £loud explosions,SRD report ST/OO7/809/&K/H, Safety and ReliabilityDirectorate, Warrington, 1984.

25 Khan F I, Feasibility report for the manufacture of

sulfolane, B Sc Engg Project submitted to Dept of ChemicalEng, Aligarh Muslim University, Aligarh, 1992.

26 Khan F T & Abbasi S A, Indian J Chem Technol, 3 (1996)338.

27 Khan F I & Abbasi S A, Application of fault tree analysis

with fuzzy probability to estimate the probability of

occurrence of an accident, Report No CPCF1R&D 9/94,

Pondicherry University, Pondicherry, 1995.28 Pasman H J, Duxbury H A & Bjordal E N, Hazardous

Mater, 30 (1992) l.

29 Guidelines for chemical process quantitative risk analysis,American Institute of Chemical Engineers, New York, 1989.

30 National Science Council, Issues in risk assessment

(National Academy Press, Washington, DC), 1993.

Annexure A

Damage Effect Calculations for the Accident ScenariosThe explosion, fIres and toxic dispersions eventually cause

damage in four ways. The potential of these effects can be

expressed in terms of probit function, which relates percentageof the affected people in a bounded region due to particularaccident event by a normal distribution function.

(1) Heat Radiation Effect

The probit function for 100% lethality for heat radiation isgiven as10,15.

Pt=-36.38+2.56.ln [t·q4l3]Where q is defIned as therma1load (kW/m2); t is time of

exposure (s); and Pr is probit value.(2) Toxic Effect

Lethality of a toxic load is expressed in terms of probitfunction as

Pr=a+b In (C'. t)Where a, b, n, are constants; C is concentration in ppm, and t

is time of exposure (s). The values of the constants for differentgases are available in literature 10,26,28

(3) Pressure and Shock Wave Effect

The probit· equation for likelihood of death due to shockwave (lung rupture) is given byll,I4

Pt=-77.I+6.91 ·lnP"For injury, the equation isPf=-15.6+l.93 In P"

Where P" is peak overpressure (N/m2).(4) Missile EffictThe probit function for fatality in human beings or damage to

vessels is expressed as9,14,t7:Pt=-17.50+5.30 In S

Where, S is the kinetic energy of the missile (J).

rapid quantitative risk analysis using a new software package...

Documents