MAXCRED – a new software package for rapid risk assessment in chemical process industries

Environmental Modelling & Software 14 (1999) 11–25

MAXCRED – a new software package for rapid risk assessment inchemical process industries

Faisal I. Khan, S.A. Abbasi*

Risk Assessment Division, Centre for Pollution Control & Bio-waste Energy, Pondicherry University, Pondicherry-605 014, India

Received 27 January 1997; accepted 14 July 1997

Abstract

A new software package for conducting rapid risk assessment (RRA) in chemical process industries and the system of method-ologies on which it is based are described. The objectives behind the development of the package are to achieve greater breadthand depth, sophistication, and user-friendliness in conducting RRA. In pursuit of these objectives we have incorporated in thepackage state-of-the-art models for generating accident scenarios and assessing their consequences. The package has been codedin C 1 1 using the concepts of object-oriented programming to enhance the tool’s speed of execution and ease of use. The paperalso demonstrates the applicability of MAXCRED with an illustrative example of a RRA conducted with its assistance. 1998Elsevier Science Ltd. All rights reserved.

Keywords:Hazard assessment; Consequence analysis; Risk assessment; Quantitative risk assessment

Software availability

Name of the product: MAXCREDDeveloped by: Faisal I. Khan and S. A.

AbbasiContact address: S. A. Abbasi,

DirectorCentre for PollutionControl & EnergyTechnologyPondicherry University,Pondicherry, 605 014,IndiaPhone:1 91 413 655267/655 262Fax: 1 91 413 655227/655 263

Available since: 1997Coding language: C1 1Hardware requirement: IBM PC AT 586 or

equivalent

* Corresponding author. Tel.: 0091 413 655 267/655 265; fax: 0091413 655 227/655 265.

1364-8152/98/$ - see front matter 1998 Elsevier Science Ltd. All rights reserved.PII: S1364-8152 (97)00031-5

1. Introduction

Chemical industries which often handle hazardouschemicals and operate reactors/storage vessels underextreme conditions of temperature and pressure are sus-ceptible to accidents. These accidents may be triggeredby material failure. The increasing density of industriescoupled with the increasing density of the human popu-lation have not only increased the frequency of accidentsbut also the extent of damage caused by the accidents.The most gruesome example is the Bhopal gas tragedywhich claimed over 20,000 lives.

The science of risk analysis has emerged to forecastthe likelihood of accidents, assess the consequences oflikely accidents, work out strategies to prevent accidentsand also to cushion the adverse impacts if an accidentdoes occur.

A total risk assessment exercise covering all steps(Greenberg and Crammer, 1992; Khan and Abbasi,1995a) exhaustively from beginning to end is expensivein terms of time as well as monetary and personnelinputs. It often becomes necessary to conduct rapid riskassessment (RRA) to draw the same conclusions that afully fledged risk assessment would lead to, albeit withlesser (yet practicable) accuracy and precision.

In this paper we describe a software package, and the

12 F.I. Khan, S. Abbasi /Environmental Modelling & Software 14 (1999) 11–25

system of methodologies on which the package is based,for conducting RRA in chemical process industries. Thepackage is named MAXCRED (MAXimum CREDiblerapid risk assessment) and is coded in C1 1.

In the past, software packages have been offered byothers for RRA; notably among these areWHAZAN(Technica, 1992),RISKIT (VTT, 1993) andEFFECTS(TNO, 1991). MAXCRED improves upon the existingpackages in the following areas:

(a) wider applicability; MAXCRED incorporates alarger number of models to handle a larger varietyof situations;

(b) greater sophistication; more precise, accurate, andrecent models have been incorporated inMAXCRED than handled by existing packages;

(c) greater user-friendliness;(d) scope for assessing second- and higher-order acci-

dents; whereas the existing RRA packages are cap-able of handling only the primary accidents,MAXCRED has provision for assessing the likeli-hood as well as magnitude of secondary and higher-order accidents triggered by the primary event.

The paper also illustrates the applicability of the newpackage in a real-life situation.

1.1. Methodology of MAXCRED

MAXCRED enables accident simulation and damagepotential estimation. The software has been developedin object-oriented architecture using C1 1 as a codingtool. The software is compatible with DOS as well asWINDOWS operating environments. It is operable oncomputers with a minimum of 4 MB RAM and 7 MBROM.

The sequence of actions or main steps involved inMAXCRED, its object architecture, and informationpathways are depicted in Figs. 1–4. Fig. 1 represents thesketch of the main menu and available options ofMAXCRED and Fig. 2 shows the object-oriented archi-tecture of MAXCRED. Fig. 3 depicts five essential stepsof MAXCRED, briefly described as follows.

1.1.1. The accident scenario generation stepIn this step accident scenarios are generated for the

unit under study. It is a very important input for the sub-sequent steps. The more realistic the accident scenario,the more accurate is forecasting the type of accident, itsconsequences, and associated risks. This would help indevelopment of more appropriate and effective strategiesfor crisis prevention and management.

Each accident scenario is basically a combination ofdifferent likely accidental events that may occur in anindustry. Such scenarios are generated based on theproperties of chemicals handled by the industry, physicalconditions under which reactions occur or

reactants/products are stored, geometries and materialstrengths of the vessel and conduits, in-built valves andsafety arrangements etc. External factors such as sitecharacteristics (topography, the presence of trees, ponds,rivers in the vicinity, proximity to other industries orneighborhoods etc.) and meteorological conditions arealso considered. In the available software packages suchas WHAZAN (Technica, 1992), EFFECTS (TNO,1991), RISKIT (VTT, 1993), and SAVE (TNO, 1992),this concept of risk assessment has been used to someextent. However, the level of sophistication needs to beenhanced subsequently by using advanced models ofthermodynamics, heat transfer, and fluid dynamics togenerate more realistic accident scenarios. Furthermore,the user-friendliness of these packages have limitations,as a result of which several real-life studies conductedon the basis of these packages seem to have major lacu-nae. This is illustrated in the following examples.

Chary et al. (1995) studied two different accidentscenarios for the release of chlorine (stored in vesselsunder high pressure in a liquefied state) using SAVE.The scenarios are, two-phase release from 10 mm coppertube connected to a storage vessel (under high pressure)and two-phase release from 3/8" liquid chlorine line con-nected to the same vessel. The study considers only thetoxic dispersion effect generated when the atmosphericstability is in the "D" category (as per the atmosphericstability classification of Pasquill, 1971). The packagedoes not lead the user to study another major risk, ofsudden drop in pressure in the storage vessel if an acci-dental leak takes place. In such situations, of which theprobability of occurrence is as high as of other accidents,the storage vessel may develop a major rupture, theextent of which would depend upon the capacity of thevessel, construction material, and internal pressure. Suchcatastrophic failure of the chlorine vessel would generateshock-waves of high damage potential.

Contini et al. (1991) reported a benchmark exerciseundertaken with the aim of assessing the state-of-the-artin risk analysis. A study of the accidental release anddispersion of ammonia from a pressurized tank was per-formed by 11 different risk assessment teams drawnfrom different countries. The teams used differentsoftware for release and dispersion estimation. A totalof five different accident scenarios were generated bythe teams. Among these, the most common and mostdisastrous accident scenarios invoked by the packagesare:

I catastrophic failure of a pressurized storage tank,I release of ammonia through a large hole on the roof.

The first accident scenario has been modeled as a two-phase release of ammonia in bulk followed by denser-than-air-gas dispersion. The second scenario is modeledas continuous two-phase release followed by denser-than-air-gas dispersion. However, in the opinion of the

13F.I. Khan, S. Abbasi /Environmental Modelling & Software 14 (1999) 11–25

Fig. 1. Main menu of MAXCRED.

Fig. 2. Object-oriented architecture of MAXCRED.

authors, the two worst accident scenarios can be refinedfurther. The catastrophic failure of the vessel will occuronly through BLEVE (boiling liquid expanding vaporexplosion) or CVCE (confined vapor cloud explosion)

because ammonia is stored under high pressure in a liqu-efied state. The explosive release will lead to overpres-sure and shock-wave generation. These would createhigh turbulence in the atmosphere and strongly influence


Fig. 3. The MAXCRED algorithm.

the process of dispersion of ammonia. The adverseconsequences would be overpressure and toxic disper-sion under changed atmospheric conditions. However,the study team has totally neglected the overpressureeffect and change in atmospheric conditions evidentlybecause of the inadequacy of the software used by them.

A comparison of various models available with thedifferent software used by the 11 teams (Contini et al.,1991) to study the accidental release of ammonia(scenario 1) is presented in Table 1. A total of sevendifferent models are available in MAXCRED relevantto the study of the above-mentioned problem, whereassoftware such as RISKIT and SAFTI (Pitblado et al.,1990) have only four models. Moreover, onlyMAXCRED generates the scenario BLEVE, followed bytoxic release, while others are unable to do so.

A list of features available in different software arepresented in Table 2. MAXCRED is revealed as the mostversatile of the packages as it has 11 of the 14 possiblefeatures, whereas RISKIT has nine, WHAZAN and

DEGADIS (Havens and Spicer, 1985) have eight andother packages seven or less.

Raghvan and Mallikarjunan (1988) presented a riskassessment study of a chemical industrial complex deal-ing with hazardous chemicals such as liquefied pet-roleum gas (LPG), ammonia, propylene, ethylene oxide,and naphtha. The damage potential of different units andchemicals was assessed only for vapor cloud explosion.Such effects as explosive release of these chemicalseither through BLEVE or CVCE followed by ignitionleading to flash fire or fire ball were neglected. Similarly,the likely accident involving ammonia has been analyzedonly in terms of the gas dispersion. However, instan-taneous release of ammonia under high pressure maycause pronounced overpressure and shock-waves leadingto significant damage. Further explosive release ofammonia would influence the dispersion process as well.

We have also come across several other reports(NEERI, 1992; CISRA, 1993; TPL, 1993) in which oneof the existing risk assessment packages has been used.In all these reports, several credible accident scenarioshave not been considered, indicating a lack of rigorand/or user-friendliness of the packages. For example,in NEERI (1992), which has considered jet fire in a fuelstorage vessel, and fire ball formation due to rupture ofa propylene storage vessel, BLEVE in fuel oil storagevessel, and CVCE/BLEVE followed by fire ball in a pro-pylene storage vessel have been overlooked. In CISRA(1993), which has dealt with instantaneous release ofchlorine, and fire ball due to release of hydrogen fromthe storage vessel, the scenarios pertaining to BLEVEfollowed by fire ball due to release of hydrogen and con-tinuous release of chlorine from a vent valve have beenomitted; TPL (1993) reports release and fire of ethyleneand allyl chloride in the form of a flash fire, but hasomitted the credible scenario of explosion in an ethylenevessel as BLEVE.

2. Consequence analysis

Consequence analysis involves assessment of likelyconsequences if an accident scenario does materialize.The consequences are quantified in terms of damageradii (the radius of the area in which the damage wouldreadily occur), damage to property (shattering of windowpanes, caving in of buildings) and toxic effects(chronic/acute toxicity, mortality). The assessment ofconsequence involves a wide variety of mathematicalmodels. For example, source models are used to predictthe rate of release of hazardous material, the degree offlashing, and the rate of evaporation. Models forexplosions and fires are used to predict the character-istics of explosions and fires. The impact intensity mod-els are used to predict the damage zones due to fires,explosion and toxic load. Finally, toxic gas models are


Table 1List of models available with different software for simulating release and dispersion of ammonia

Models available CRUNCH DEGADIS DENZ HEAVG- RISKIT WHAZAN SAFTI EFFECTS DECRARA MAXCREDwith PLUMEpackagesa

1. Gas outflow Y Y Y Y Y Y Y Y Y I.C2. Two-phase Y Y Y Y Y Y Y Y Y Youtflow3. Evaporation but Y Y Y Y N Y Y Y Y Ynot time-dependent4. Light gas Y Y Y Y Y Y Y Y Y Ydispersion5. Heavy gas Y Y Y Y N Y Y N Y Ydispersion6. Liquid out flow N N N N Y Y Y Y N Y7. Explosive release N N N N N N N N N NModels usable for 2,5 2,5 2,5 2,5 2,4 3,4,5 3,4,5 2,4 2,5 1,5,7b

the study ofcatastrophy

aAdopted from Contini et al. (1991).bOnly MAXCRED is capable of studying the possible release of ammonia as BLEVE followed by evaporation and dense gas dispersion.

Table 2Comparison of features available with different software

Featurea CRUNCH DEGADIS DENZ HEAVG- RISKIT WHAZAN SAFTI EFFECTS DECRARA MAXCREDPLUME

Different types of Y Y N N N Y Y Y Y Yrelease (I,C,V)Dense cloud Y Y Y Y Y Y Y N Y YJet N N N Y Y Y Y Y N YNeutral cloud Y Y Y Y Y Y Y Y Y YBuoyant cloud N N N N Y Y Y N Y YSurface roughness Y Y Y Y Y Y Y Y Y YComplex terrain N N N N N N N N N NWind variation N N N N N N N N N Yin time and spaceAdvent dispersion Y Y Y Y Y Y Y Y Y Ymodel (B,G,K)Along-wind N Y Y N N N N Y N YdispersionVertical wind shear Y Y Y Y Y N N Yb YDry or jet deposit N N N N Y N N Nb N NDistance limit Y N N N N Y N N N NEvaluation Y Y Y N Y N N N Y

I, instantaneous; B, box; C, continuous; G, gaussian; V, variable; K, numerical; S, semicontinuous.aAdopted from Contini et al. (1991).bPossible extension.

used to predict human response to different levels ofexposure to toxic chemicals. A list of models includedin MAXCRED for consequence estimation are given inTable 3. Several different types of explosion and firemodels such as confined vapor cloud explosion (CVCE),unconfined vapor cloud explosion (UVCE), boilingliquid vapor cloud explosion (BLEVE), pool fire, flashfire and fire ball are included. Likewise, models forliquid release and two-phase release have been incorpor-ated. A special feature of MAXCRED is that it is able

to handle dispersion of heavy (heavier-than-air) gases,as-light-as-air and lighter-than-air gases. A brief descrip-tion of different types of accident events is presented ina subsequent section.

2.1. Verification of accident scenario

This feature is one of the specialities of MAXCRED.It verifies the plausibility of the proposed scenario. Forexample, if the scenario envisages release of a highly


Table 3List of different models used in MAXCRED

Event Model incorporated Reference

Toxic releaseLight gas dispersion Gaussian dispersion model with Pasquill and Smith, 1983; Gifford, 1961

Erbink modification Turner, 1970, 1985; Erbink, 1993Heavy gas dispersion Box model, modified plume path Van Ulden, 1974, 1985; Deaves, 1992

theory for heavy gas Erbink, 1995; Khan and Abbasi, 1998ExplosionBLEVE Thermodynamic and heat transfer Baker et al., 1983; Martinsen et al., 1986;

model Prugh, 1991; Vernart et al., 1993UVCE Condensed vapor cloud explosion Kletz, 1977; Lees, 1996; Prugh, 1987CVCE Vapor cloud explosion in Lees, 1996; Baker et al., 1983

confinement (vessel or building) Fowcett and Wood, 1993;Van den berg et al., 1991FireFlash fire Flare model, fire torch and Kayes, 1985; Greenberg and Crammer, 1992

spontaneous combustion Fowcett and Wood, 1993Pool fire Combustion of liquid pool Kayes, 1985; Baker et al., 1983; Davis, 1993Fire ball Spontaneous combustion of vapor Kayes, 1985; Robert, 1982;

cloud Baker et al., 1983; Davis, 1993.

flammable chemical on the basis of the characteristicsof the chemical and quantities in which it is employed,this step checks whether the set of input conditionswould indeed lead to the envisaged accident. Forexample, the release of a flammable chemical may notlead to UVCE, if the meteorological conditions areunstable (highly turbulent atmosphere making dilutionof gas faster) but the same type of release would havea high probability of causing UVCE if the atmosphericconditions are stable. Under such stable conditions mix-ing of the escaping chemical with ambient air would beinefficient, leading to build-up of the concentration ofthe chemical to the point of explosion.

If MAXCRED finds that an envisaged scenario is notwithin the realms of probability, it modifies the scenarioto the extent that it becomes plausible. For example, ifthe user envisaged an accident scenario as CVCE fol-lowed by fire ball, MAXCRED does not verify thisbecause the minimum conditions (limiting conditions) ofCVCE occurrence are not satisfied. It further rec-ommends the scenario to be modified as BLEVE, fireball, or a combination of these according to the existingsituation (data input to MAXCRED).

2.2. Checking for a higher degree of accidents

An accident in a unit caused by another accident inanother unit is termed as a "second-order accident" or"secondary accident". If the secondary accident causesanother accident in a third unit, such an accident istermed "third-order" or "tertiary" accident. It is a special-ity of MAXCRED that the package is capable of simulat-ing second- and higher-order accidents. To do this, ituses models developed by Pitersen (1985, 1990), Clan-cey (1977), Eissenberg et al. (1975), Fowcett and Wood

(1993) and Khan and Abbasi (1996). If the probabilityof occurrence of a secondary accident is higher than aminimum value, the package will estimate the damagepotential of the secondary accident and its likelihood ofcausing a third-order accident, and so on. To estimatethe probability of occurrence and damage characteristics,the package uses information related to the operatingcondition of the secondary unit, chemical properties andtopological parameters (wind velocity, roughness,obstruction etc.).

2.3. Characterization of worst accident scenario

Arriving at the worst accident scenario is the last stepin the MAXCRED algorithm. The step determines theworst accident scenario based on the results of conse-quence analysis. This step holds the key to the finalobjective of the risk analysis i.e. excessive-devising stra-tegies to avert a crisis or to minimize its adverse impactif the crisis does take place. It is possible that more thanone "worst accident scenario" emerges from theMAXCRED analysis because more than one sequenceof events can lead to identical magnitudes of "worst"damage. In such situations control strategies would bedeveloped by retaining all the "worst" scenarios in view.

3. Design and application of MAXCRED

In the following section MAXCRED is described indetail with regard to design and application.

3.1. Data module

The main purpose of the module is to collect all rel-evant information needed for the execution of other


modules. The module consists of three main objectsderived from the main DATA object: toxic release, fire,and explosions. The explosion object is further dividedinto subobjects such as BLEVE, CVCE and UVCE. Thefire object branches into flash fire, pool fire and fire ball.The object dependency and message flow of the DATAmodule is shown in Fig. 4.

3.2. Accident scenario module

This module, dealing with the generation of accidentscenarios, is based on the advanced concepts of hazardassessment proposed by Arendt (1990), Papazoglou etal. (1992), Vernart et al. (1993) and Khan and Abbasi(1995b, c). The accident scenarios are generated basedon chemical properties, operating conditions, and detailsof the process/storage units. Once an accident scenariohas been developed, it can be processed for further veri-fication and consequence assessment. For the same unitand operating conditions various plausible accident scen-arios can be visualized. Thus, this option helps in simu-lating the various likely accidents, and characterizing the

Fig. 4. Object-oriented architecture of "data" module.

worst plausible ones. This module consists of two sub-modules (objects)-automatic and user-defined.

3.2.1. The automatic submoduleThis is a derived object to the main accident scenario

object. It deals with the knowledge base which decidesthe accident scenario for a set of information providedby the user. The knowledge base is a compendium ofconditions and facts stored in if-and-else reasoningsequences. The information provided by the user ispassed on to the knowledge base which examineswhether the information satisfies the conditions neces-sary for a "credible accident"; the latter is defined as "theaccident which is within the realm of possibility (i.e,probability higher than 1*e-06/yr) and has a propensityto cause significant damage (at least one fatality)". Thisconcept (Hagon, 1984; HSE, 1988; Ale, 1991; Lees,1996) comprises both probable damage caused by anaccident and the probability of its occurrence. There maybe a type of accident which may occur very frequentlybut would cause little damage and there may be anothertype of accident which may cause great damage but


would have very low probability of occurrence. Both arenot "credible". But accidents which have an appreciableprobability of occurrence as well as significant damagepotential (as quantified above) come under the categoryof "credible accidents". For example, the package tellsus that the accident scenario for LPG storage underpressure is likely to be BLEVE followed by fire ball.This decision is arrived at as shown below:

if(release: instantaneous)if(pressure > 3.0*vapor pressure)if(pressure > 10*atmospheric pressure)if(capacity > 7000)100% chances of BLEVEif(chemical flammable)80% chances of fire ball

The knowledge base has been developed in object-oriented architecture without using any expert shell, andby using heuristic and if-and-else reasoning. Forwardchaining has been used to retrieve the information fromthe knowledge base, while backward chaining is usedto justify or check the retrieved information. The set ofconditions on the basis of which the package decideswhether an accident would occur or not for a given setof input parameters has been based on reports of pastaccidents (Lees, 1996; Pitersen, 1985) and data gener-ated by controlled experiments simulating accidents.

3.2.2. The user-defined submoduleIn this option the user defines the accident scenario

on the basis of his/her knowledge and experience. Forexample, failure of a liquefied chlorine storage vesselcan be visualized through various accident scenariossuch as: BLEVE followed by dispersion, continuousrelease and dispersion, and instantaneous release and dis-persion. The user considers these possibilities andchooses one or more of the likely modes of accident.These decisions become inputs to the subsequent analy-sis by MAXCRED. This option is very helpful for acci-dent simulation study as a number of different accidentscenarios can be generated for the failure of a unit, andon the basis of consequence analysis results, the mostcredible, accident scenario can be identified. Forexample, in the previous example (failure of Cl2 storagevessel) among the various scenarios, BLEVE followedby dispersion would be the most credible one.

3.3. Consequence analysis module

This module consists of state-of-the-art mathematicalmodels for simulating the accidents chosen as crediblein the previous step (Fig. 5). This module works out thescale and the characteristics (type of accident, damagepotential, percentage of lethality, and damage radii) ofthe accidents, the types of damaging impacts (shock-waves, heat loads, missiles, toxic dispersion etc.) they

may cause, and their area of impact. The output of thismodule quantifies impacts such as peak overpressure,shock-wave velocity, shock-wave duration, heat load,missile velocity, toxic load, damage radii of differentimpacts, and probabilities of causing lethality. The out-put of the consequence analysis has been so formattedthat it can be directly used in reports without editing.Moreover, using these results makes it easy to drawdamage/risk contours.

The mathematical models used in this module arelisted in Table 3. Brief explanatory notes on the phenom-ena simulated by these models are presented below.

3.3.1. Toxic release submoduleThis submodule assesses the consequences of release

of toxic gas/vapor. It simulates different types of releasescenarios such as: continuous release, two-phase release,and instantaneous release. In conducting dispersion stud-ies it takes into consideration the densities of the gasesor gas-air mixtures (because of the pronounced influencedensity exerts on the shape of the plume). The modelscan thus simulate dispersions of heavier-than-air, as lightas air, and lighter-than-air gases/gas-air plumes. Thismodule first estimates the concentration profile of thetoxic gas that would develop consequent dispersionunder the given meteorological conditions. It then worksout the areas of toxic impact and the extents of toxicitythat would be caused on the basis of exposure-based tox-icity data.

This submodule can handle the following options:heavy gas dispersion, light gas dispersion, lethality esti-mation and damage estimation. Brief descriptions ofthese options are presented below.

3.3.1.1. Option heavy gas This option estimates dis-persion characteristics (concentration profile, distancetraveled by the cloud/plume, and the dimensions of thecloud/plume) of gases having an effective densityhigher-than-air. It uses the BOX model for instantaneousrelease and the PLUME (heavy gas) model for continu-ous release (Bington, 1986; Van Ulden, 1974, 1985;Deaves, 1992; Erbink, 1995; Khan and Abbasi, 1998) toestimate gas concentrations and other dispersion charac-teristics. The results are then passed on to damage esti-mation options to calculate the percent likelihood oflethality and area under influence for various degreesof toxicity.

3.3.1.2. Option light gas In this, similar operations arecarried out for gases having densities lighter-than-airor/and as light as air in the previous option: heavy gasselected for heavier-than-air gases. This option incorpor-ates various dispersion models: the Gaussian model(instantaneous and continuous), Plume model(continuous), and Puff model (instantaneous) to estimatethe dispersion characteristics for different release scen-


Fig. 5. Internal architecture of "consequence analysis" module.

arios (Pasquill and Smith, 1983; Turner, 1985; Erbink,1993). The results are then used to estimate the toxicload (concentration) at a particular location, the chancesof lethality at that location and radii of the areas whichare under the influence of varying degrees of toxicity.Probit models proposed by Pitersen (1990), Greenbook(1992) and Clancey (1977) are used for estimatinglethality.

3.3.2. Explosion submoduleAn explosion is defined as a sudden and violent

release of energy. According to the mode of occurrenceand consequence, explosions have been further categor-ized as: boiling liquid expanding vapor explosion(BLEVE), confined vapor cloud explosion (CVCE) andunconfined vapor cloud explosion (UVCE).

3.3.2.1. BLEVE A sudden release of pressurized gasor boiling liquid processed or stored under high pressureleads to BLEVE. As highly energized molecules (due tohigh pressure) have a high tendency to escape, a suddenrelease leads to a very fast movement (expansion) ofmolecules which in turn results in the generation ofshock-(blast) waves. If the material is flammable thenthere are chances of fire too. The velocity of the blastwave in BLEVE ranges from 330 to 450 m/s and gener-ates a positive overpressure of 0.5–1 atm. The durationof dynamic pressure and the shock-wave is of the order

of a few seconds. In general, the damaging effect ofBLEVE is restricted to areas of 200-700 m radii.

3.3.2.2. UVCE The delayed spontaneous ignition of avapor cloud of flammable chemical in an unconfined orsemi-unconfined (congested boundary) space results inUVCE. The cloud generally forms either due to instan-taneous release of gas/boiling liquid not having sufficientenergy to cause BLEVE, or by the continuous release ofgas/boiling liquid. UVCE is also known as delayedvapor cloud explosion, as the explosion occurs sometime after release when the concentration of the materialin the vapor cloud reaches its explosion (flammability)limit. The UVCE can produce very disastrous results asthe vapor cloud may be transported to populated areasby wind before meeting an ignition source andexplosion. The UVCE generates overpressure, shock-waves, heat load, and in some cases producesmissiles/fragments of pipe, vessels, or other objects pro-pelled by blast waves. The shock-waves it generatesattain speeds reaching 400 m/s and overpressure of theorder of 2 atm. UVCE may have disastrous conse-quences, especially as it has the potential to lead tosecond- and higher-order accidents.

3.3.2.3. CVCE An explosion in a confined space suchas a vessel or a pipe-triggered by excessive pressuredevelopment either due to a runaway reaction process,


overfilling, or absorption of heat from external sources-is called CVCE. Liquids of low boiling point, flammablegases, or highly reactive chemicals processed underextreme conditions, are most likely to generate CVCE.A CVCE occurs when the pressure in a confinementreaches a critical limits beyond the safety level. Forexample, a vessel will explode when the pressureexceeds its design or bursting pressure. CVCE differsfrom BLEVE in three respects. First, in CVCE theexplosion occurs within a confinement, while in BLEVEthe material expands outside the boundary of the con-finement (vessel) and can drift away before exploding.Secondly, CVCE occurs at very high pressure, muchhigher than the pressures necessary for BLEVE. Thirdly,CVCE could be more disastrous than BLEVE as it gen-erates shock-waves of higher speeds and greater over-pressure. The impact of CVCE can be observed over amuch larger distance (1–3 km). Owing to its large areaof impact and more severe shock-waves CVCE hasgreater potential to cause secondary, tertiary and higher-order accidents.

3.3.3. Fire submoduleUncontrolled combustion of any chemical in the pres-

ence of air is termed as fire. According to the mechanismof formation and the broad shapes it attains, the fire canbe classified into three main types: pool fire, flash fire,and fire ball.

Release of low boiling/non-boiling liquid from a ves-sel may give rise to a pool of liquid which, on ignition,would yield pool fire. In certain situations a pool firemay also generate an explosive vapor cloud by supplyingthe required heat of evaporation to a liquid pool. How-ever, possibility of this phenomenon occurring is limitedto the boiling liquid processed/stored under liquefied orrefrigerated conditions. There may be different ways ofinitiating a pool fire but the ultimate destructive impactof a pool fire is caused by its heat load.

An instantaneous combustion of flammable gas orhigh boiling liquid (liquid of high vapor pressure) onignition causes flash fire. Flash fire generally occurswhen the quantity of chemical is not high enough toform an explosive cloud. The low flammability charac-teristics and rate of release of the chemical restrict flamespeed, precluding generation of a blast wave. However,the heat load generated by flash fire is quite high and itsdamaging effect can be observed over long distances.According to the different modes of release and ignition,flash fires can be characterized as flare, fire torch. etc.

A spontaneous ignition of a vapor cloud with insuf-ficient energy to explode leads to a fire ball. Thisphenomenon is generally observed for high boiling, yethighly flammable, liquids stored or processed underextreme conditions of temperature and pressure. In somecases (high capacity, stable conditions) these clouds mayalso generate blast waves. The fire ball is different from

the flash fire in terms of flame speed, the minimumcapacity of the chemical required, mode of release, andignition. The destructive ability of a fire ball is very highas the heat load generated by it is of the order of1000 kJ/m2. Fire ball radii generally vary from 100 to300 m and their impact duration from a few seconds toa few minutes. The fire ball characteristics depend onthe type and mass of the chemical involved.

3.3.4. Higher-order accident submoduleThis submodule of the consequences analysis module

analyzes the damage potential of the primary event atthe point of location of the secondary unit and checksfor the likelihood of occurrence of the secondary acci-dent. This module has an independent set of informationwhich should be provided by the user. This informationpertains to the operational details of the secondary unit,the chemicals used, meteorological conditions, and topo-logical characteristics. This submodule consists of vari-ous sets of mathematical models (Pitersen, 1985; Clan-cey, 1977; Kletz, 1977; Greenbook, 1992; Prugh, 1987;Khan and Abbasi, 1996) to estimate the probability ofoccurrence of secondary accidents due to the impact ofprimary ones. If the probability of a secondary accidentis estimated and found credible, the unit is processed forconsequence estimation in a manner similar to the studyof the primary accident. The same procedure can berepeated for higher-order accidents.

3.4. Graphics module

This module enables visualization of risk contours inthe context of the site of accidents. The option has twofacilities: (i) site drawing, and (ii) contour drawing.

The site drawing option enables the user to draw anyindustrial site layout using freehand drawing or usingany already defined drawing tool. The contour drawingoption has the facility for drawing various damage/riskcontours over the accident site.

3.5. Documentation module

This module deals with handling data, scenario andoutput files, and flow of information. This module alsoworks as an "information manager": it provides thenecessary information to each module and submoduleto carry out desired operations, and stores the results indifferent files. Besides this, it also provides all com-monly used file operations such as copying, deleting,consoling and printing.

The applicability of the software is demonstrated withan illustrated example of its use in RRA.


4. An illustrative example of application ofMAXCRED in RRA

4.1. Problem statement

A risk assessment study has been carried out for atypical chemical industry situated in an industrial area atHarmoli, Madhya Pradesh, India. The industry primarilymanufactures sulfolene, which is a solvent used for theextraction of hydrocarbon; and is also used as feedstockfor many chemical and petrochemical industries. Theindustry handles several hazardous chemicals such asbutadiene, sulfur dioxide (SO2), catechol, and sulfolene.

Sulfolene is prepared by a reaction of butadiene andsulfur dioxide (SO2). The reaction is exothermic and iscarried out in liquid phase under high pressure. If thereaction temperature rises slightly above normal, it leadsto undesirable side reactions (which cause furtherincrease in the temperature and pressure in the reactor).The process also necessitates maintaining butadiene:sul-fur dioxide concentrations in the ratio 1:8 otherwise sidereactions may occur generating high pressure and tem-perature in the reactor. Thus, precise control of materialflow, temperature, and pressure are essential to preventunwanted side reactions. Some other units in the indus-try, notably evaporation, stripping compressor and stor-age units are prone to accidents. Of these, the storageunits (for butadiene and sulfur dioxide) are the most haz-ardous. Thus, detailed risk assessments for the butadieneand sulfur dioxide storage units have been conducted.The set of data used in the present study is given inTable 4.

4.2. Accident scenario generation

The storage units pose the following three types ofhazard: explosion hazard (due to butadiene and sulfurdioxide), fire hazard (due to butadiene), and toxicity haz-ard (due to sulfur dioxide). The plausible accident scen-arios are:

4.2.1. Scenario 1: A UVCE followed by pool fire inbutadiene tank which may damage the SO2 tank, inturn causing toxic release

A continuous release of butadiene forms an explosivevapor cloud which, on ignition, leads to UVCE. Theunexploded chemical in the dike or in the vessel burnsas pool fire. Owing to the consequent heat and overpres-sure load the other tank (containing SO2) is damaged,causing the release of toxic gas.

4.2.2. Scenario 2: A CVCE followed by flash fire inbutadiene tank which may later trigger and causetoxic release from the (SO2)tank

An instantaneous, explosive, release of butadieneunder very high pressure sets off CVCE. The released

Table 4Characteristics of the storage vessels of which risk assessment wasconducted with MAXCRED

Variables Magnitude

Chemical: Sulfur dioxide

Storage capacity of the unit 7.5 tonActual chemical stored in the unit 5.0 tonStorage pressure 6.5 atmStorage temperature 2 55°CPhysical state of the chemical Liquefied gasType of vessel Pressurized cylinderDesign pressure of the vessel 7.25 atmMaterial of construction of the vessel Mild steelPercent degree of conjunction on-site 30%Type of hazard present Toxic release

Chemical: Butadiene

Storage capacity of the unit 13.5 tonActual chemical stored in the unit 10.0 tonStorage pressure 3.5 atmStorage temperature 2 25°CPhysical state of the chemical Liquefied gasType of vessel Pressurized cylinderDesign pressure of the vessel 5.5 atmMaterial of construction of the vessel Mild steelPercent degree of conjunction on-site 30%Type of hazard present Fire and explosion

chemical is ignited as a fire ball. As the heat and over-pressure loads are very high, the SO2 storage tank isdamaged causing the release of its toxic contents.

4.2.3. Scenario 3: A BLEVE followed by fire ballA sudden release of butadiene as BLEVE on ignition

turns into a fire ball. As the damage potential of aBLEVE is less than that of CVCE or UVCE, the SO2

storage tank is not affected.

4.2.4. Scenario 4: Toxic release of SO2

A continuous release of SO2 from the SO2 storagetank.

4.3. Results

The output of MAXCRED for scenario 1 is shown inTable 5. It is observed that a lethal heat load and severelyintense overpressure load would occur over an area of| 500 m radius. Lethal toxic load due to release of SO2

(secondary accident) would occur over an area of|700 m radius. The probability of a secondary accident(release of SO2) is estimated as 23%. The damage poten-tials (in terms of percentage damage) of various acciden-tal events are presented in Table 6. It is evident that anarea of| 500 m radius is under high risk (individual risk


Table 5Impacts of scenario 1 at a distance of 500 m from accident epicentre

Parameter Magnitude

UVCEPeak overpressure (kPa) 21.0583Duration of shock-wave (ms) 41.0981Shock-wave velocity in air (m/s) 358.1821Overpressure impulse (kPa/s) 0.69359Area covered by explosive mixture (sqm) 253.312Volume of vapor cloud (cu.m) 20265.007Total heat released by UVCE (kJ) 1.3e06Radiant heat intensity (kW/sq.m) 0.25809Pool fireMass release rate (kg/s) 73.7941Area of pool fire (sq.m) 69.9700Burning area (sq.m) 15372.82Flame velocity (m/s) 1.6447Heat intensity (kJ/sq.m) 20.8043No missileToxic loadBox continuous modelElevated sourceConcentration (kg/cu.m) 6.65111E2 05Heavy gas plume characteristicsGround level conc. of plume (kg/cu.m) 6.61186E2 05Ground level conc. of plume on axis (do) 7.24479E2 05Width of plume (m) 2.509362E2 02Maximum ground level concentration (do) 3.495698E2 01Distance at which maximum concentration 10.000occurs (m)Secondary accidentThe probability of leading secondary 0.23accident

factor greater than 1*10-3/yr) due to lethal overpressureand toxic load.

The MAXCRED output for scenario 2 is shown inTable 7. Up to a radial distance of| 500 m from theaccident epicenter, lethal heat load as well as damagecausing shock-waves (overpressure) would be encoun-tered. The probability of a secondary accident (releaseof SO2) is high; over 50%. The damage potentials ofvarious impacts over different areas are shown in Table8. An area of| 700 m radius would be at high risk.

The MAXCRED outputs of scenarios 3 and 4 arepresented in Tables 9 and 10, respectively. Table 9reveals that a moderately intense heat load having 50%

Table 6Probabilities of different types of damage at increasing distance from the epicentre of the accident for scenario 1

Damaging effect At a distance of (m)

200 500 700 1000

50% lethality by heat load 100% 75% 35% 10%50% injury due to shock load 100% 85% 11% —50% damage due to missile effect — — — —50% lethality due to toxic load 100% 100% 40% 16%


Parameter Magnitude

CVCEPeak overpressure (kPa) 206.1113Duration of shock wave (ms) 41.0981Shock wave velocity in air (m/s) 546.560547Over pressure impulse (kPa/s) 6.7876Total energy available (kJ) 2.3325E08Net energy available for bursting (kJ) 1.5170E08Energy released by CVCE (kJ) 1.5991E07Heat intensity (kW/sq.m) 9.16729MissileInitial velocity of fragment(1 kg) (m/s) 305.45Kinetic energy associated with fragment (kJ) 6.3289E03Penetration strength of mild steel at 500 m 0.0045(m)Flash fireVolume of vapor cloud (cu.m) 16330.556Area under the fire effect (sq.m) 280993.065Heat generated (kJ) 1.24562E06Effective time of fire (s) 51.16843Radiation heat intensity (kW/sq.m) 15.9623Toxic loadBox instantaneous modelElevated sourceConcentration (kg/cu.m) 3.45231E2 07Heavy gas puff characteristicsGround level conc. of plume (kg/cu.m) 5.224523E2 07Ground level conc. of plume on axis (do) 7.432341E2 07Cloud radius (m) 1.167534E02Maximum distance traveled by the cloud (m) 7.874571E02Maximum ground level concentration (do) 4.345431Secondary accidentThe probability of leading secondary 0.54accident

probability of first-degree burn, as well as damage caus-ing shock-waves, would occur over an area of| 150 mradius. As per scenario 4 (Table 10), a lethal toxic loadwould be persistent over an area of| 500 m radius. Theprobability of secondary impacts is low in both thesescenarios. The damage potential of scenarios 3 and 4 asa function of distance from accident epicenter arepresented in Tables 11 and 12, respectively. Severe dam-age due to overpressure and heat load as per scenario 3would be limited to less than 200 m radius, while the




200 500 700 1000

50% lethality by heat load 100% 100% 85% 65%50% injury due to shock load 100% 100% 75% 45%50% damage due to missile effect 50% 22% 3% —50% lethality due to toxic load 100% 100% 80% 65%

Table 9Impacts of scenario 3 at a distance of 500 m from the accident epi-centre

Parameter Magnitude

BLEVEPeak overpressure (kPa) 11.78460Duration of shock-wave (s) 23.54321Shock-wave velocity in air (m/s) 53.67607Overpressure impulse (kPa/s) 2.74131E2 02Fire ballRadius of fire ball (m) 98.56723Duration of fire ball (s) 10.89234Heat generated (kJ) 1.05807E06Radiation heat intensity (kW/sq.m) 11.452321Secondary accidentThe probability of leading secondary 0.023accident


Parameter Magnitude

Toxic loadBox instantaneous modelElevated sourceConcentration (kg/cu.m) 1.132552E2 06Heavy gas puff characteristicsGround level conc. of plume (kg/cu.m) 2.853264E2 06Ground level conc. of plume on axis (do) 1.239232E2 06Cloud radius (m) 2.285449E02Maximum distance traveled by the cloud (m) 1.150000E03Maximum concentration at ground level 6.722849(kg/cu.m)Secondary accidentThe probability of leading secondary 0.00accident

toxic load as per scenario 4 would extend beyond500 m radius.

4.4. Discussion

The MAXCRED forecasts the consequence of thescenario for the release of butadiene (UVCE followedby pool fire) reveals that the likely damage due to this

event in terms of shock-waves, missile, and heat loadwould be intense. It is evident from MAXCRED results(Tables 5 and 6) that even at a distance| 500 m awayfrom the accident epicenter the intensity of shock-wavesand heat load would be severe enough to cause lethaldamage. The high risk contour (individual risk factor >1*e-03) for more than a 50% probability ofdamage/lethality would be extended over an area of|500 m radius (Table 6). This scenario has a 23% prob-ability of causing a secondary accident (release of sulfurdioxide), which could propel the gas in lethal concen-trations over an area of| 500 m radius.

The CVCE in the butadiene tank as per scenario 2would generate shock-waves and missile effects (Table7). The damage potential due to these shock-waves andthe missiles would cover a wider area than is likely inscenario 1 (Table 8). The butadiene released from thevessel would form a vapor-air mixture which, onignition, would cause flash fire lethally affecting an areaof | 700 m. The probability of a secondary accident(release of sulfur dioxide) due to this accident is approx.54%. The secondary accident would cause toxic build-up of SO2 over a wide area. All-in-all, a region of|700 m radius would be at great risk due to heat load,shock-waves, and toxic load.

Compared with accident scenarios 1 and 2, the fore-casts as per scenario 3 reveal moderate impacts; limitedto within a periphery close to the accident site (Tables9 and 11). The probability of a secondary accident isnegligible.

The results of scenario 4 (release of SO2) reveal build-up of a lethal toxic concentration over an area of| 500mradius (Tables 10 and 12) with a near-zero probabilityof a secondary accident.

In summary, scenario 2 represents the worst likely dis-aster within the realm of credibility. It has the largestarea-of-lethal-impact (shock-waves, lethal heat load, andlethal concentration of SO2 over an area of| 700 m).Further, the most thickly populated areas (includingneighborhoods) lie within its range. This scenario is alsothe one most likely to cause domino effects, as missiles,shock-waves, and radiation loads would be generatedsimultaneously and units dealing with hazardous chemi-cals (flammable and toxic materials) are situated within




200 500 700 1000

50% lethality by heat load 65% 38% 22% 8%50% injury due to shock load 25% 8% — —50% damage due to missile effect — — — —50% lethality due to toxic load — — — —



200 500 700 1000

50% lethality by heat load — — — —50% injury due to shock load — — — —50% damage due to missile effect — — — —50% lethality due to toxic load 100% 80% 60% 45%

the striking distance of the impact area of this scenario.Similar to scenario 2, scenario 1 is also likely to causesecondary accidents. In conclusion, scenario 2 is theworst as far as the size of its impact area is concerned,as well as in terms of its potential to cause secondaryaccidents.

5. Conclusion

A new software package MAXCRED (MAXimumCREDible accident analysis) has been developed as acomprehensive and user-friendly tool for rapid riskassessment in the chemical process industries. The pack-age, coded in C1 1, has the following attributes:

1. it incorporates a larger number of models to handlea wider variety of situations useful in RRA;

2. it includes more precise, accurate, and recent modelsthan handled by existing commercial packages;

3. greater user-friendliness;4. ability to forecast whether second- or higher-order

accidents may occur.

The applicability of MAXCRED has been demon-strated with an illustrative example of RRA conductedin the storage unit of a typical chemical process industry.MAXCRED generated scenarios of four credible acci-dents and helped in assessing their consequences. Itrevealed that the scenario which forecasts a CVCE(confined vapor cloud explosion) followed by flash firewould be the worst of all, as it would lethally affect thelargest area and also would have the greatest probabilityof triggering secondary, tertiary, and higher-order acci-

dents. The least harmful of four credible accidents wasthe scenario in which a BLEVE (boiling liquidexpanding vapor explosion) would occur, followed bythe formation of a fire ball.

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MAXCRED – a new software package for rapid risk assessment in chemical process industries

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