part iv quantitative methods of risk assessment...

PART IV

QUANTITATIVE METHODS O F RISK ASSESSMENT -11 : Rapid risk assessment

INTRODUCTION TO PART IV

Risk assessment (RA) involves steps such as HAZOP (HAZard and OPerability study) and FTA (Fault Tree Analysis) which are expensive in terms of time as well as money. In order to achieve thoroughness, all industries must cany out total risk assessment involving all these essential steps. Yet, we offen encounter situations when it is required to conduct risk assessment quickly and inexpensively. These are situations when a certain degree of imprecision (which would come because of lesser thoroughness) is tolerable. For example in a reconnaissance of risks associated with a large industrial complex, rapid risk assessment (RRA) can be used to zero down on units requiring more elaborate RA. In this sense RRA is a methodology of which the sophistication, accuracy, and precision fall in-between index-based screening and total risk assessment.

RRA can be used for taking interim measures while a total RA is awaited. M is also very useful in choosing between alternative sites for a future industrial unit.

In this part we present a computer-automated tool MAXCRED developed by us for RRA (Chapter 11). The applications of MAXCRED are illustrated in Chapters 12 and 13.

Chapter 11

MAXCRED A NEW SOFTWARE PACKAGE FOR RAPID RISK ASSESSMENT IN CHEMICAL PROCESS INDUSTRIES'

A new softwam package for conducting rapid risk assessment (RRA) in chemical process industries and the system of methodologies on which it is based are described.

The objectives behind the development of the package are to achieve greater breadth and depth, sophistication, and user- friendliness in conducting RRA. In pursuance of these objectives we have incorporated in the package state-of-the-art models for generating accident scenarios and assessing their consequences. The package has been coded in C++ using the concepts of object oriented programming to enhance the tool's speed of execution and ease of use. The paper also demonstrates the applicability of MAXCRED with an illustrative example of a RRA conducted with its help.

Key words: Hazard assessment, consequence analysis, risk assessment, quantitative risk assessment.

SOFTWARE AVAllABlLlTY Name of the Product : MAXCRED Developed by : Faisal I Khan and S A Abbasi

' Accepted for publication in Journal of Environmental ModeNing and Software, The Netherlands (kindly see page A5)

contact address : S AAbbasi, Director Centre for Pollution Control 8 Energy Technology Pondicheny University, Pondicherry, 605 014, India

phone : +91 413 85267165262 Fax : +91 413 65227185263 Available since : 1997 Coding language : C++ Hardware requirement : IBM PC AT 586 or equivalent

INTRODUCTION

Chemical industries which often handle hazardous chemicals and operate reactorslstorage vessels under extreme conditions of temperature and pressure are susceptible to accidents. These accidents may be triggered by material failure. The increasing density of industries coupled with increasing density of human population have not only increased the frequency of accidents but also the extent of damage caused by the accidents. The most gruesome example is the Bhopal gas tragedy - which claimed over 20,000 lives.

The science of risk analysis has emerged to forecast the likelihood of accidents, assess the consequences of the likely accidents, work out the strategies to prevent the accidents and also to cushion the adverse impacts if an accident does occur.

A total risk assessment exercise covering all steps (Greenberg and Crammer, 1992; Khan and Abbasi,l995a) exhaustively from beginning to end is expensive in terms of time as well as monetary and personnel inputs. It often becomes necessary to conduct rapid risk assessment (RRA) to draw the same conclusions that a full fledged risk assessment would lead to, albeit with lesser (yet practicable) accuracy and precision.

in this paper we describe a software package, and the system of methodologies on which the package is based, for conducting RRA in chemical process industries. The package is named MAXCRED (MAXimum CREDible accident analysis) and is coded in C++.

in the past, software packages have been offered by others for RRA; notably among these are WHAZAN (Technica.1992). RlSKlT (VTT.l993), SAVE (TN0,1992) and EFFECTS (TN0,1991). MAXCRED improves upon the existing packages in the following areas :

a) wider applicability, MAXCRED incorporates a larger number of models to handle a larger variety of situations;

b) greater sophistication; more precise, accurate, and recent models have been incorporated in MAXCRED than handled by existing packages;

C) greater user-friendliness;

d) scope for assessing second and higher order accidents; whereas the existing RRA packages are capable of handling only the primary accidents, MAXCRED has provision for assessing the likelihood as well as magnitude of secondary and higher order accidents triggered by the Primary event.

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

METHODOLOGY OF MAXCRED

MAXCRED enables accident simulation and damage potential estimation. The software has been developed in object-oriented architecture using C++ as a coding tool. The software is compatible with DOS as well as WINDOWS operating environments. It is operable on computers with a minimum of 4MB RAM and 7 MB ROM.

The sequence of actions or main steps involved in MAXCRED, its object architecture, and information pathways are depicted in Figures 1-4. Figure 1 represents the sketch of main menu and available options of MAXCRED and Figure 2 shows the object oriented architecture of MAXCRED. Figure 3 depicts five essential steps of MAXCRED, briefly described as follows.

The accident scenario generation step

In this step accident scenarios are generated for the unit under study. It is a very important input for the subsequent steps. The more realistic the accident scenario, the more accurate is forecasting the type of accident, its consequences, and associated risks. This would help in development of more appropriate and effective strategies for crisis prevention and management.

Each accident scenario is basically a combination of different likely accidental events that may occur in an industry. Such scenarios are generated based on the properties of chemicals handled by the industry, physical conditions under which reactions occur or reactantslproducts are stored, geometries and material strengths of vessel and conduits, in-built valves and safety arrangements etc. External factors such as site characteristics (topography, presence of trees, ponds, rivers in the vicinity, proximity to other industries or neighboumoods etc.) and meteorological conditions are also considered. In the available software packages such as WHAZAN (Technica ,1992) EFFECTS (TN0,1991), RlSKlT (VTT,1993), and SAVE (TN0,1992), this concept of risk assessment has been used to some extent. But the level of sophistication needs to be enhanced subsequently by using advanced models of thermodynamics, heat transfer, and fluid dynamics to generate more realistic accident scenarios. Furthermore the user-friendliness of these packages have some limitations as a result of which several real-life studies conducted on the basis of these packages are seem to have major lacunae. This is illustrated by the following examples.

a) Chary eta/. (1995) studied two different accident scenarios for the release of chlorine ( stored in vessels under high pressure in liquefied state) using SAVE. The scenarios are, two-phase release from 10 mm copper tube connected to a storage vessel (under high pressure) and two-phase release from 318" liquid chlorine line connected to the same vessel. The study considers only toxic dispersion effect generated when the atmospheric stability is in the 'D' category (as per the atmospheric stability classification of Pasquill.1971). The package has not led the user to study another major risk, of sudden drop in pressure in the storage vessel if an accidental leak takes place. In such situations, of which the probability of occurrence is as high as of the other accidents, storage vessel may develop a major rupture, the extent of which would depend upon the capacity of the vessel, material of construction, and internal pressure. Such catastrophic failure of chlorine vessel would generate shock waves of high damage potential.

f DkTl ANALYSIS GRAPHICS FILE

Figure 1. Main menu of MAXCRED

Explosion

Fire

Toxic release

Documentation Output display

output

Figure 2. Object-oriented architecture of MAXCRED

Data input Chemical properties

Accident scenario Surrounding parameters

Consequence analysis I

Checking for higher degree of accidents

Are all units

Characterisation of worst accident scenario

Figure 3. The MAXCRED algorithm

b) Contini et a/. (1991) reported a benchmark exercise undertaken with the aim of assessing the state-of-the-art In risk analysis. A study of the accidental release and dispersion of ammonia from a pressurised tank was performed by 11 different risk assessment teams drawn from different countries. The teams used different software for release and dispersion estimation. A total of five different accident scenarios were generated by the teams. Among these, the most common and most disastrous accident scenarios invoked by the packages are:

0 catastrophic failure of a pressurised storage tank,

release of ammonia through a large hole on the roof.

The first accident scenario has been modelled as a two-phase release of ammonia in bulk followed by denser-than-air-gas dispersion. The second scenario is modelled as continuous two-phase release followed by denser-than-air-gas dispersion. However, in the opinion of the authors the two worst accident scenarios can be refined further. The catastrophic failure of vessel will occur only through BLEVE (boiling liquid expanding vapour explosion) or CVCE (confined vapour cloud explosion) because ammonia is stored under high pressure in liquefied state. The explosive release will lead to overpressure and shock-wave generation. These would create high turbulence in the atmosphere and strongly influence the process of dispersion of ammonia. The adverse consequences would be over pressure, and toxic dispersion under changed atmospheric conditions. But the study team has totally neglected the overpressure effect and change in atmospheric conditions evidently due to the inadequacy of the software used by them.

A comparison of various models available with the different software used by the 11 teams (Contini et a/. 1991) to study the accident release of ammonia (scenario 1) is presented in Table 1. A total of seven different models are available in MAXCRED relevant to the study of the above mentioned problem, whereas software such as RlSKlT and SAFETI (Pitblado et a/. 1990) have only 4 models. Moreover, only MAXCRED generates the scenario BLEVE, followed by toxic release while others are unable to do so.

A list of features available in different software are presented in Table 2. MAXCRED is revealed as the most versatile of the packages as it has 11 of the 14 possible features whereas RISKWIT has 9, WHAZAN and DEGADIS(Havens and Spicer, 1985) have 8 and other packages 7 or less.

c) Raghvan and Mallikajunan (1988) presented a risk assessment study of a chemical industrial complex dealing with hazardous chemicals such as liquefied petroleum gas (LPG), ammonia, propylene, ethylene oxide, and naphtha. The damage potential of different units and chemicals was assessed only for vapour cloud explosion. Such other effects as explosive release of these chemicals either through BLEVE or CVCE followed by ignition leads to flash fire or fire ball were neglected. Similarly the likely accident involving NH3 has been analysed only in terms of the gas dispersion. But instantaneous release of NH3 under high pressure may cause pronounced overpressure and shock- waves leading to significant damage. Further explosive release of NH3 would influence the dispersion process as well.

I. L i n t Of Wdmlm available wi th d l l f w r m t M f t w a n fo r a i m u l ~ t i n q ralaasa .nd dinprnion o f w n i a

............................................................................................ nodrnl. nv4ilLblw W i M CRUNCH O f W I 8 D m KEAVO-PLOW RISKIT MWAN m I EFFERS DllCIURA nUW prek.~..~ ........................................................................................... I , O U OUtflW Y Y Y Y Y Y Y Y Y 1 . C

2.hlD-PhaS*UUtflDU Y Y Y Y Y Y Y Y Y Y

3. Bvapcrltion but not Y Y Y Y If Y Y Y Y Y

tiam &p.ndat

4 . Light pas d inmrsion Y Y I Y Y Y Y Y Y Y

HodelsunLblnfor thn 2,s 2,5 2,s 2,s 2 4 5 4 , 2 4 2,s 1 , 5 , 7 *

study of ~ U l t r o p - d s

e adnpt.d f ro. C D ~ ~ M m t a1. (1901)

* Cnly UAXCIUm i s cay&:, of studying the pornnibla ra1nA.e of NH, u BLBVL followad by nvaparation

d d m u WU dinpmrnion

Di t f . 1P l t t m m o f Y Y I I I Y Y Y Y Y

nluu (1,C.V)

D.N. c1DUd Y Y Y Y Y Y Y Y Y Y

Jmt I I I Y Y Y Y Y N Y

W w t r a l cloud Y Y Y Y Y Y Y Y Y Y

hloyult ClOua I I I W Y Y Y Y Y Y

s v r f a a r o u g h m u Y Y Y Y Y Y Y Y Y Y

wind v a r i a t i o n I I I W m n N w N Y in tin .nd mp4-

-t d i s p r s i o n Y Y Y Y Y Y Y Y Y Y

-1 (B,G,K)

Dry o r j m t & p o l i t I( R I Y Y N I I(*) I N

B v U w t i o n Y Y Y I Y I N W Y

I : &Ut.ntu*oru 8 a box i r poeeibu m x t u u i o n

C = m n t i n r o u m (1 r g a u u i u ~ V a v a r i a b l e K s tllllric.1 8 a dontinrau

@ adopted fro. C o n t m m t r l . (1991)

We have also come across several other reports (NEER1,1992; CISRA,1993; TPL,1993) in which one of the existing risk assessment packages have been used. In all these reports several credible accident scenarios have been left unconsidered indicating a lack of rigor

user-friendliness of the packages. For example in NEERl (1992), which has considered jet fire in fuel storage vessel, and fire ball formation due to rupture of propylene storage vessel, BLEVE in fuel storage vessel, and CVCEIBLEVE followed by fire bail in propylene storage vessel have been overlooked. In ClSRA (1993), which has dealt with instantaneous release ~f chlorine, and fire ball due to release of hydrogen from the storage vessel, the scenarios pertaining to BLEVE followed by fire ball due to release of hydrogen and continuous release of chlorine from vent valve have been omitted; TPL (1993) r6orts release and fire of ethylene and allyl chloride in the form of flash fire, but has missed out the credible scenario of explosion in ethylene vessel as BLEVE.

Consequence analysis

Consequence analysis involves assessment of likely consequences if an accident, does rnaterialise. The consequences are quantified in terms of damage radii (the radius of the area in which the damage would readily occur), damage to property (shattering of window panes, caving of buildings) and toxic effects (chroniclacute toxicity, mortality). The assessment of consequence involves a wide variety of mathematical models. For example source models are used to predict the rate of release of hazardous material, the degree of flashing, and the rate of evaporation. Models for explosions and fires are used to predict the characteristics of explosions and fires. The impact intensity models are used to predict the damage zones due to fires, explosion and toxic load. Lastly toxic gas models are used to predict human response to different levels of exposures to toxic chemicals. A list of models included in MAXCRED for consequence estimation are given in Table 3. Several different types of explosion and fire models such as confined vapour cloud explosion (CVCE), unconfined vapour cloud explosion (UVCE), boiling liquid vapour cloud explosion (BLEVE), pool fire, flash fire and fire ball are included. Likewise, models for liquid release and two phase release have been incorporated. 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 description of different types of accident events is presented in a subsequent section.

Verification o f accident scenario

This feature is one of the specialities of MAXCRED. It verifies the plausibility of the proposed scenario. For example if the scenario envisages release of a highly flammable chemical on the basis of the characteristics of the chemical and quantities in which it is employed, this step checks whether the set of input conditions would indeed lead to the envisaged accident. For example the release of a flammable chemical may not lead to UVCE, if the meteorological conditions are unstable (highly turbulent atmosphere making dilution of gas faster) but the same type of release would have a high probability of causing UVCE if the atmospheric conditions are stable. Under such stable conditions mixing of the escaping chemical with ambient air would be inefficient ieading to build-up of the concentration of the chemical to the point of explosion.

If MAXCRED finds that an envisaged scenario is not within the realms of probability, it modifies the scenario to the extent that it becomes plausible. For example, user envisaged an accident scenario as CVCE followed by fire ball, however MAXCRED does not verify

I a h h 1, List of different ~ o d e l s used in XAICPBD --

Eveat Xodrl iacorporsted Bdfbreace

toxic releasb

Light gar di~persion

Bear! g a l dispersion

OVCE

CVCK

fire

Plash fire

Pool fir,

fire hall

Gaulaian dilperlion model with Pasquill and Slith,1983; Gifford,l961 Brbink ~odification turner,l910,1985; lrbink,1993

Box model, modified p l u ~ e path Van Olden,1974,1985; Dea?e6,1992 theor! for hearr gas Erbink,1995; Khan and Abbasi,l99lb

thernodynanic and beat transfer Baker et a1.,1983; Hartinsen et a1. ,1986; nodel Pruqb,l991; Vernart ct r1.,1993

condensed vapor cloud explosion Kleta,l971; Lees,l996; Prugh,1987

vapor cloud explosion in Leea,1996; Baker e t a1.,1983 confineaent [vessel or building] forcett and Uood,1993; Van den Berg el a1.,1991

flare ~ o d e l , fire torch and layes,1986; Greenberg and Crarmer,1992 spontaneous corbustion fovcctt and Wood,1993

co~hurtion of liquid pool Kayes,l986; Baker et a1.,1983; Davies,1993

spontaneou8 conbultion of vapor Kayee,1986; Boberts,1981; cloud Baker et a1.,1983; Davies,l993.

the same because the minimum conditions (limiting conditions) of CVCE occurrence are not satisfied. It further recommends the scenario to be modified as BLEVE, fire ball, or a combination of these according to the existing situation (data input to MAXCRED).

Checklng for higher degree o f accidents

An accident in a unit caused by another accident in another unit is termed as a 'second order accident' or 'secondary accident'. If the secondary accident causes another accident in a third unit such an accident is termed 'third order' or 'tertiary' accident. It is a speciality of MAXCRED that the package is capable of simulating second and higher order accidents. To do this, it uses models developed by Pietersen (1985, 1990), Clancey (1977), Eissenberg et el. (1975), Fowcett and Wood (1993) and Khan & Abbasi (1996). If the probability of occurrence of secondary accident is higher than a minimum value, the package will estimate the damage potential of the secondary accident and its likelihood of causing a third degree accident, and so on. To estimate the probability of occurrence and damage characteristics, the package uses information related to operating condition of secondary unit, chemical properties and topological parameters (wind velocity, roughness, obstruction etc.).

Characterisation of worst accident scenario

Arriving at worst accident scenario is the last step in the MAXCRED algorithm. The step determines the worst accident scenario based on the results of consequence analysis. This step holds the key to the final objective of the risk analysis excessive -devising strategies to avert a crisis or to minimise its adverse impact if the crisis does take place. It is possible that more than one 'worst accident scenario' emerges from the MAXCRED analysis because more than one sequence of events can lead to identical magnitudes of 'worst' damage. In such situations the control strategies would be developed by keeping all the 'worst' scenarios in the view.

DESIGN AND APPLICATION OF MAXCRED

In the following section MAXCRED is described in detail vis a vis design and application.

Data module

The main purpose of the module is to collect all relevant information needed for the execution of other modules. This module consists of three main objects derived from the main DATA object: toxic release, fire, and explosions. The explosion object is further divided into sub objects such as BLEVE, CVCE and UVCE. The fire object branches into flash fire, pool fire and fire ball. The object dependency and message flow of the DATA module is shown in Figure 4.

Accident scenario module

This module, dealing with the generation of accident scenarios, is based on the advanced concepts of hazard assessment proposed by Arendt (1990), Papazoglou et el. (1992), Vernart et at. (1993) and Khan and Abbasi (1995b,1995c). The accident scenarios are generated based on chemical properties, operating conditions, and details of the processlstorage units. Once an accident scenario has been developed, it can be processed for further verification and consequence assessment. For the same unit and same operating conditions various plausible accident scenarios can be visualised. Thus,

[[Fjj B L N E

DATA MODULE

Flash fire

Fire ball

Toxlc release rn Natural

Heavy gas

Filing t- - - - - - - - -- - - - -

- - - - -- - - - - 7

Heat of cornbust~on Vapour pressure

Temperature Pressure Capac~ty etc Mechanical data

. W~nd velocity Temperature

Figure 4. Object-oriented architecture of Data Module

this option helps in simulating the various likely accidents, and characterising the worst plausible ones. This module consists of two submodules (objects) - automatic and user defined.

The automatic submodule

This is a derived object to the main accident scenario object. it deals with the knowledge base which decides the accldent scenario for a set of information provided by the user. The knowledge base is a compendium of conditions and facts stored in if-and-else reasoning sequences. The information provided by the user is passed on to the knowledge base which examines whether the information satisfies the conditions necessary for a 'credible accident'; the latter is defined as 'the accident which is within the realm of possibility ( i.e, probability higher than l*eOB Iyr) and has a propensity to cause significant damage (at least one fatality)', This concept (Hagon, 1984; HSE,1988; Ale 1991; Lees, 1996) comprises of both probable damage caused by an accident and probability of its occurrence. There may be a type of accident which may occur very frequently but would cause little damage. And there may be another type of accident which may cause great damage but would have very low probability of occurrence. Both are not 'credible'. But accidents which have appreciable probability of occurrence as well as significant damage potential (as quantified above) come under the category of 'credible accidents'. For example the package tells us that the accident scenario for LPG storage under pressure 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*l O'atmospheric pressure) if(capacity 27000)

100% chances of BLEVE if(chemicai flammable)

80% chances of fire ball

The knowledge base has been developed in object-oriented architecture without using any expert shell, and by using heuristic and if-and-else reasoning. Forward chaining has been used to retrieve the information from the knowledge base while backward chaining is used to justify or check the retrieved information. The set of conditions on the basis of which the package decides whether an accident would occur or not for a given set of input parameters have been based on the reports of past accidents (Lees,1996; Pietersen,l985) and data generated by controlled experiments simulating accidents.

The user defined submodule

In this option the user defines the accident scenario on the basis of hislher knowledge and experience. For example, failure of a liquefied chlorine storage vessel can be visualised through various accident scenarios such as; BLEVE followed by dispersion, continuous release and dispersion, and instantaneous release and dispersion. The user considers these possibilities and chooses one or more of the likely modes of accident. These decisions become inputs to the subsequent analysis by MAXCRED. This option is very helpful for accident simulation study as a number of different accident scenarios can be generated for the failure of a unit, and on the basis of consequence analysis results, the most credible, accident scenario can be identified. For example, in the previous example

(failure of CI, Storage vessel) among the various scenarios, BLEVE followed by dispersion would be the most credible one.

Consequence analysis module

This module consists of state-of-the-art mathematical models for simulating the accidents chosen as credible in the previous step (Figure 5). This module works out the scale and the characteristics (type of accident, damage potential, percentage of lethality, and damage radii) of the 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 this module quantifies impacts such as peak overpressure, shock-wave velocity, shock wave duration, heat load, missile velocity, toxic load, damage radii of different impacts, and probabilities of causing lethality. The output of the consequence analysis has been so formatted that it can be directly used in reports without editing. Moreover, using these results makes it easy to draw damagelrisk contours.

The mathematical models used in this module are listed in Table 3. Brief explanatory notes on the phenomena simulated by these models are presented below.

Toxic release submodule

This submodule assesses the consequences of release of toxic gaslvapour. It simulates different types of release scenarios such as; continuous release, two-phase release, and instantaneous release. In conducting dispersion studies it takes into consideration, the densities of the gases or gas-air mixtures (because of the pronounced influence density exerts on the shape of the plume). The models can thus simulate dispersions of heavier- than-air, as light as air, and lighter-than-air gasestgas-air plumes. This module first estimates the concentration profile of the toxic gas that would develop consequent dispersion under the given meteorological conditions. It then works out the areas of toxic impact and the extents of toxicity that would be caused on the basis of exposure-based- toxicity data.

This submodule can handle the following options: heavy gas dispersion, light gas dispersion, lethality estimation and, damage estimation. Brief descriptions of these options are presented below.

Option heavy gas

This option estimates dispersion characteristics (concentration profile, distance travelled by the cloudlplume, and the dimensions of the cloud/plurne) of gases having effective density higher-than-air. It uses BOX model for instantaneous release and PLUME (heavy gas) model for continuous release (Bington,1986; Van Ulden,1974,1985; Deaves,1992; Erbink,1995; Khan and Abbsi,1997) to estimate gas concentrations and other dispersion characteristics. The results are then passed on to damage estimation options to calculate the percent likelihood of lethality and area under influence for various degrees of toxicity.

Option light gas

In this, similar operations are carried out for gases having density lighter-than-air orland as light as air in the previous option heavy gas done for heavier-than-air gases. This option incorporates with various dispersion models: Gaussian model (instantaneous and continuous), Plume model (continuous), and Puff model (instantaneous) to estimate the

Damage effect calculation

(FI-\(\ Toxic release

. Printing

. Consoling

. Filing

L - -

. Explosion energy

. Peak over pressure INSTANCES Flame dlmens~on

. Shock wave . Duration of fire

. Missile effects . Puff model

, Heat intensity . Plume model , Slab model . Disprsion characterstics

Figure 5, Internal architecture for Consequence Analysis Module

dispersion characteristics for different release scenarios (Pasquill and Smith,1983; ~urner,1985; Erbink,1993). The results are then used to estimate the toxic load

at particular location, chances of lethality at that location and radii of the areas which are under the influence of varying degrees of toxicity. Probit models proposed by Pietersen (1990), Greenbook (1992) and Clancey (1977) are used for estimating lethality.

Explosion submodule

An explosion is defined as sudden and violent release of energy. According to mode of occurrence and consequence, explosions have been further categorised as - boiling liquid expanding vapour explosion (BLEVE), confined vapour cloud explosion (CVCE) and unconfined vapour cloud explosion (UVCE).

BLEVE

A sudden release of pressurised gas or boiiing liquid processed or stored under high pressure leads to BLEVE. As highly energised molecules (due to high pressure) have high tendency to escape, a sudden release leads to a very fast movement (expansion) of molecules which in turn results in the generation of shock (blast) waves. If the material is flammable then there are chances of fire too. The velocity of the blast wave in BLEVE ranges from 330 to 450 mls and generates a positive overpressure of 0.5 to 1 atm. The duration of dynamic pressure and shock-wave is of the order of a few seconds. in general. damaging effect of BLEVE Is restricted to areas of 200-700 m radii.

UVCE

The delayed spontaneous ignition of a vapour cloud of flammable chemical in an unconfined or semi unconfined (congested boundary) space results in UVCE. The cloud generally forms either due to instantaneous release of gaslboiling liquid not having sufficient energy to cause BLEVE or by the continuous release of gaslboiling liquid. UVCE is also known as delayed vapour cloud explosion as explosion occurs some time after the release when the concentration of the material in the vapour cloud reaches its explosion (flammability) limit. The UVCE can produce very disastrous results as the vapour cloud may be transported to populated areas by wind before meeting an ignition source and explosion. The UVCE generates overpressure, shock-wave, heat load, and in some cases produces missiles/fragments of pipe, vessel, or other objects propelled by blast waves. The shock-waves it generates attain speeds reaching 400 mls and overpressure of the order of 2 atm. UVCE may have disastrous consequences especially as it has the potential to lead second and higher order of accidents.

CVCE

An Explosion in a confined space such as a vessel or a pipe - triggered by excessive pressure development either due to a runaway reaction process, overfilling, or absorption Of heat from external sources - is called as CVCE. Liquids of low boiling points, flammable gases, or highly reactive chemicals processed under extreme conditions, are most likely to generate CVCE. A CVCE occurs when the pressure in a confinement reaches a critical limits beyond the safety level. For example, a vessel will explode when the pressure goes beyond its design or bursting pressure. CVCE differs from BLEVE in three respects. First in CVCE the explosion occurs within a confinement, while in BLEVE the material

expands outside the boundary of the confinement (vessel) and can drift away before exploding. Secondly CVCE occurs at very high pressure, quite higher than the pressures adequate for BLEVE. Lastly CVCE could be more disastrous than BLEVE as it generates shock-waves of higher speeds and greater overpressure. The impact of CVCE can be observed over a much larger distance (1 to 3 km). Due to its large area of impact and more severe shock-waves CVCE has a greater potential of causing secondary, tertiary and higher order accidents.

Fire submodule

Uncontrolled combustion of any chemical in the presence of air is termed as fire. According to the mechanism of formation and the broad shapes it attains, the fire can be classified into three main types: pool fire, flash fire, and fire ball.

Release of low boilinglnon boiling liquid from a vessel may give rise to a pool of liquid which on ignition would yield pool fire. In certain situations a pool fire may also generate an explosive vapour cloud by supplying the required heat of evaporation to a liquid pool. However, possibility of occurrence of this phenomenon is limited to the boiling liquid processed I stored under liquefied or refrigerated conditions. There may be different ways of initiation of a pool fire but the ultimate destructive impact of a pool fire is caused by its heat load.

An instantaneous combustion of flammable gas or high boiling liquid (liquid of high vapour pressure) on ignition causes flash fire. Flash fire generally occurs when the quantity of chemical is not high enough to form an explosive cloud. The low flammability characteristics and rate of release of the chemical restrict flame speed precluding generation of a blast wave. However, the heat load generated by flash fire is quite high and its damaging effect can be observed over long distances. According to the different modes of release and ignition, flash fires can be characterised as flare, fire torch etc.

A spontaneous ignition of vapour cloud not having sufficient energy to explode leads to a fire ball. This phenomenon is generally observed for high boiling yet highly flammable liquids stored or processed under extreme conditions of temperature and pressure. In some cases (high capacity, stable conditions) these clouds may also generate blast waves. The fire ball is different from the flash fire in terms of flame speed, minimum capacity of chemical required, mode of release, and ignition. The destructive ability of fire ball is very high as the heat load generated by it is of the order of 1000 k~lm'. The fire ball radii generally vary from 100 meter to 300 meter and its impact duration from few seconds to few minutes. The fire ball characteristics depend on the type and the mass of the chemical involved.

Higher order accident submodule

This submodule of the consequences analysis module analyse the damage potential of the primary event at the point of location of the secondary unit and checks for the likelihood of occurrence of the secondary accident. This module has an independent set of information which should be provided by the user. This information, pertains to the operational details Of the secondary unit, chemicals used, meteorological conditions, and topological characteristics. This submodule consists of various sets of mathematical models (Pietersen,l985; Clancy,l977; Kletz,l977; Greenbook,l992; Prugh, 1987; Khan and Abbasi,1997a) to estimate the probability of occurrence of secondary accidents due to the

impact of primary ones. If the probability of secondary accident is estimated and found credible, the unit is processed for consequences estimation in a manner similar to the study of the primary accident. The same procedure can be repeated for higher order accidents.

Graphics module

This module enables visualisation of risk contours in the context of the site of accidents. The option has two facilities: (i) site drawing, and (ii) contour drawing.

The site drawing option enables the user to draw any industrial site layout using freehand drawing or using any already defined drawing tool. The contour drawing option has the facility for drawing various damagelrisk contours over the accident site.

Documentation module

This module deals with handling of data file, scenario file, output file, and flow of information. This module also works as 'information manager': it provides the necessary information to each module and submodule to carry out desired operations, and stores the results in different files. Besides this, it also provides all commonly used file operations such as copying, deleting, consoling and printing.

The applicability of the software is demonstrated with an illustrated example of its use in RRA as follows.

AN ILLUSTRATIVE EXAMPLE OF APPLICATION OF MAXCRED IN RRA

Problem Statement

Risk assessment study has been carried out for a typical chemical industry situated in an industrial area at Harmoli, Madya Pradesh, India. The industry primarily manufactures sulfolene which is a solvent used for the extraction of hydrocarbon: and is also used as feedstock for many chemical and petrochemical industries. The industry handles several hazardous chemicals such as butadiene, sulfur dioxide (SO,), catechol, and sulfolene.

Sulfolene is prepared by a reaction of butadiene and sulfur dioxide (SO,). The reaction is exothermic and is carried out in liquid phase under high pressure. If the reaction temperature goes even slightly above normal, it leads to undesirable side reactions (which cause further increase in the temperature and pressure in the reactor). The process also necessitate maintaining butadiene : sulfur dioxide concentration in the ratio 1:8 otherwise side reactions may occur generating high pressure and temperature in the reactor. Thus, precise control of material flow, temperature, and pressure are essential to prevent unwanted side reaction. Some other units in the industry, notably evaporation, stripping compressor and storage units prone to accidents. Of these the storage units (for butadiene and sulfur dioxide) are the most hazardous. Thus, a detailed risk assessment for the butadiene and sulfur dioxide storage units have been conducted. The set of data used in the present study is given in Table 4.

Accident scenario generation

The storage units pose the following three types of hazard: explosion hazard (due to butadiene and sulfur dioxide), fire hazard (due to butadiene), and toxicity hazard (due to sulfur dioxide). The plausible accident scenarios are:

able 4. Characteristics of the storage vessels of which risk assessment was conducted with MAXCRED

Variables

Chemical : Sulfur dioxide

storage capacity of the unit Actual chemical stored in the unit Storage pressure Storage temperature Physical state of the chemical Type of the vessel Design pressure of the vessel Material of construction of the vessel Percent degree of conjunction on-site Type of hazard present

Chemical : Butadiene

Storage capacity of the unit Actual chemical stored in the unit Storage pressure Storage temperature Physical state of the chemical Type of the vessel Design pressure of the vessel Material of construction of the vessel Percent degree of conjunction on-site Type of hazard present

Magnitude

7.5 ton 5.0 ton 6.5 atm -55Oc Liquefied gas Pressurized cylinder 7.25 atrn Mild steel 30% Toxic release

13.5 ton 10.0 ton 3.5 atm -25Oc Liquefied gas Pressurized cylinder 5.5 atrn Mild steel 30% Fire and explosion

Scenario 1: A UVCE followed by pool fi:e in butadiene tank which may damage the SO2 tank, in turn causing toxic release.

A continuous release of butadiene forms an explosive vapour cloud which on ignition leads to UVCE. The unexploded chemical in the dike or in the vessel burns as pool fire. Due to the consequent heat and overpressure load the other tank (containing SO2) is damaged causing the release of toxic gas.

Scenario 2: A CVCE followed by flash fire in butadiene tank which may later trigger and cause toxic release from the (SO j tank.

An instantaneous, explosive, release of butadiene under very high pressure sets off CVCE. The released chemical is ignited as a fire ball. As the heat and overpressure loads are very high the S02storage tank is damaged causing the release of its toxic contents.

Scenario 3: A BLEVE followed by fire ball.

A sudden release of butadiene as BLEVE on ignition turns into a fire ball. As the damage potential of a BLEVE is (generally) lesser than of CVCE or UVCE, the SO, storage tank is not affected.

Scenario 4: Toxic release of SO,

A continuous release of SO, from the SO2 storage tank.

Results

The output of MAXCRED for scenario 1 is shown in Table 5 . It is seen that lethal heat load and severely intense 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 secondary accident (release of SO,) is estimated as 23%. The damage potential (in terms of percentage damage) of various accidental events are presented in Table 6. It is evident that an area of -500 m radius is under high risk (individual risk factor greater than 1'10'~ lyr) due to lethal overpressure and toxic load.

The MAXCRED output for scenario 2 is shown in Table 7. Up to a radial distance of -500 rn from the accident epicenter, lethal heat load as well as damage causing shock- waves (overpressure) would be encountered. The probability of a secondary accident (release of SO,) is high : over 50%. The damage potentials of various impacts over different areas are shown in Table 8. An area of -700 rn radius would be at high risk.

The MAXCRED outputs of scenarios 3 and 4 are presented in Tables 9 and 10 respectively. Table 9 reveals that a moderately intense heat load having 50% probability of first degree burn as well as damage causing shock-waves would occur over an area of -150 m radius. As per scenario 4 (Table lo) , a lethal toxic load would be persistent over an area of -500 m radius. The probability of secondary impacts is low in both the scenarios. The damage potential of scenarios 3 and 4 as a function of distance from accident epicenter are presented in Tables 11 and 12 respectively. Severe damage due to overpressure and heat load as per scenario 3 would be limited to less than 200 rn radius while the toxic load as per scenario 4 would extend beyond 500 rn radius.

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

Pardme ter Magnitude

ma Peak over pressure (kpa) Duration of shock wave ( m s ) Shock wave velocity in air (m/s) Over pressure impulse (kpa/sec) Area covered by explosive mixture (sqm) Volume of vapor cloud (cu.m) Total heat released by W C E (kJ) Radiant heat intensity (kW/sq.m)

POOL PI= Mass release rate (kg/s) Uea of pool fire (sq.m) Burning area (sq.m) Flame velocity (m/s) Heat flux (kJ/sq.m)

TOXIC LOAD Box continous model Elevated source Concentration (kg/cu.m)

Heavy gas plume Characteristics Ground level conc. of plume (kg/cu .m) 6 -613-05 Ground level conc. of plume on axis(kg/cu.m) 7.243-05 Width of plume (m) 2.50Et02 Maximum ground level concentration (m) 3.493+01 Distance at which maximum concentration occurs(m) 10.0

Secondary Accident The probability of leading secondary 0.23 accident

Table 6. Probabilities of different types of damages at different distances from the epicentre of the accident for scenario 1

damaging effect at a distance of (meter) 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%

Tabla 7 . Impacts of scenario 2 at a distance of 500 meter from accident epicentre

parameter Magnitude

CVCg peak over pressure (kpa) Duration of shock wave (ma) Shock wave velocity in air (m/s) Over pressure impulse (kpa/sec) Total energy available (k~) Net energy available for bursting (kJ) Energy released by CVCE (kJ) Heat intensity (kW/sq.m)

MISSILB Initial velocity of fragment( lkg) (m/s) Kinetic energy associated with fragment (kJ) Penetration st. of mild steel at 500 m, (m)

F U S E PIRE Volume of vapor cloud (cu.m) Area under the fire effect (6q.m) Heat generated (kJ) Effective time of fire (s~c) Radiation heat intensity (kw/sq.m)

TOXIC LOAD Box ins tan taneous model Elevated source Concentration (kg/cu .m)

Heavy gas puff Characteris tics Ground level conc. of plume(kg/cu.rn) Ground level conc. of plume on axis(kg/cu.m) Cloud radius (m) Maximum distance traveled by the cloud (m) Maximum ground level concentration (m)

Secondary Accident The probability of leading secondary accident

Table 8. Probabilities of different types of damages at different distance from the epicentre of the accident for scenario 2


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% - -


le i F.I. Khan L S . A . Abb.1, P a t d l ~ h . 1 ~ . 6 0 5 011, India


Parameter Magnitude

BtKVE Peak over pressure (kpa) Duration of shock wave (sec) Shock wave velocity in air (m/s) Over pressure impulse (kpa/sec)

FIRE BACL Radius of fire ball (m) Duration of fire ball (sec) Heat generated (kJ) Radiation heat intensity (kw/sq.m)



Parameter Magnitude

TOXIC LOAD Box instantaneous model Elevated source Concentration (kg/cu .m)

Heavy gas puff Characteris tics Ground level conc. of plume(kg/cu.m) 2.853-06 Ground level conc. of plume on axis(kg/cu.m) 1.233-06 Cloud radius (m) 2.283+02 Maximum distance traveled by the cloud (m) 1.15E+03 maximum concentration at ground level (kg/cu.rn) 6.72


Table 11. Probabilities of different types of damages at different distances from the epicentre of the accident for scenario 3

damaging effect at a distance of (meter) 2 0 0 5 0 0 7 0 0 1 0 0 0

50% lethality by heat load 65% 38% 2 2 % 8 %

50% injury due to shock load 25% 8% -- - - 50% damage due to missile effect - - - - - - - -

50% lethality due to toxic load - - - - - - - -

Table 12. Probabilities of different types of damages at different distances from the epicentre of the accident for scenario 4


50% lethality by heat load - - - - - - - - 50P injury due to shock load - - - - - - - - 50% damage due to missile effect - - - - - - - -


DISCUSSION

The MAXCRED forecasts consequence of the scenario for the release of butadiene (UVCE followed by pool fire) reveal that the likely damage due to this event in terms of shock waves, missile, and heat load would be intense. It is evident from MAXCRED results (Tables 5 and 6) that even at a distance -500 rn away from the accident epicenter the intensity of shock waves and heat load would be severe enough to cause lethal damage. The high risk contour (individual risk factor > Ine-03) for more than 50% probability of damagellethality would be extended over an area of -500 m radius (Table 6). This scenario has 23% probability of causing secondary accident (release of sulfur dioxide), which could propel the gas in lethal concentrations over an area of -500 m radius.

The CVCE in butadiene tank as per scenario 2 would generate shock waves and missile effects (Table 7). The damage potential due to these shock waves and the missiles would cover wider area than likely as per scenario 1 (Table 8). The released butadiene from the vessel would form a vapour-air mixture which on ignition would cause flash fire lethally effecting an area of -700 m. The probability of a secondary accident (release of sulfur dioxide) due to this accident is about 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 to accident scenarios 1 and 2, the forecasts as per scenario 3 reveal moderate impacts; limited to within a periphery close to the accident site (Tables 9 and 11). The probability of secondary accident is negligible.

The results of accident scenario 4 (release of SO,) reveal build-up of lethal toxic concentration over an area of -500m radius (Tables 10 and 12) with near-zero probability of secondary accident.

In summary scenario 2 represents the worst likely disaster within the realm of credibility. It has the largest area-of-lethal-impact (shock waves, lethal heat load, and lethal concentration of SO, over an area of -700 m). Further, the most thickly populated areas (including neighbourhoods) lie within its range. This scenario is also the one most likely to cause domino effects as missiles, shock waves, and radiation loads would be generated simultaneousiy and units dealing with hazardous chemicals (flammable and toxic materials) are situated within the striking distance of the impact area of this scenario. Similar to scenario 2, scenario 1 is also likely to cause secondary accidents. In conclusion scenario 2 is the worst as far as the largeness of its impact area is concerned as well as in terms of its potentiality of causing secondary accidents.

CONCLUSION

A new software package MAXCRED (MAXimum CREDible accident analysis) has been developed as a comprehensive and user-friendly tool for rapid risk assessment in the chemical process industries. The package, coded in C++, has the following attributes:

i) it incorporates larger number of models to handle larger variety of situations useful in RRA;

ii) it includes more precise, accurate, and recent models than handled by existing commercial packages;

iii) greater user-friendllness;

iv) ability to forecast whether second or higher order accidents may occur.

The applicability of MAXCRED has been demonstrated with an illustrative example of RRA conducted in the storage unit of a typical chemical process industry. MAXCRED generated scenarios of four credible accidents and helped in assessing their consequences. It revealed that the scenario which forecasts a CVCE (confined vapour cloud explosion) followed by flash fire would be the worst of all as it would lethally effect the largest area as also would have the largest probability of triggering secondary, tertiary, and higher order accidents. The least harmful of four credible accidents was the scenario as per which a BLEVE (boiling liquid expanding vapour explosion) would occur followed by the formation of fire ball.

REFERENCES

1. Ale, B J M, (1991). Risk analysis and risk policy in the Netherlands and EEC, J Loss Prevention in Process Industries, 4(1), 58.

2. Arendt. J S. (1990). Using quantitative risk assessment in the chemical process industries, Reliability Engineering and System Safety, 29, 133-149.

3. Baker, W E, Cox, P A, Westin, P S, Kulesz, J J, and Strelow, R A, (1983). Explosion hazard and evaluation, Elesvier, Amsterdam.

4. Bington, P W M, (1986). Heavy gas dispersion from source inside building or in wake, Proceeding IChemE Symposium on refinement of estimate of the consequence of heavy toxic release, Manchester.

5. Chary, S V, Suryanarayana, G, Rao, K C K, (1995). Risk assessment in pesticide industry- a case study, Journal of IAEM, 23.27-35.

6. CISRA, (1993). Risk assessment of Chloralkali industry, Cell for Industrial Safety and Risk Analysis, CLRl -Madras, India.

7. Clancey, V J, (1977). Dangerous clouds, their growth and explosive properties, Chemical Process Hazards. 6.11 1-123.

8. Contini, C A, Amendal, A, and Ziomas, I, (1991). Benchmark study on major hazard study, Joint Research Centre -ISPRA, 134, Amsterdam.

9. Davies, P A, (1993). A guide to the evaluation of condensed phase explosion, Journal of Hazardous Materisls, 33, 1-33.

10. Deaves, D M, (1992). Dense gas dispersion modelling (HEAVG-PLUME), Journal of Loss Prevention Process Industries, 5(4), 219-222.

11. Eissenberg, N A, Lynch, C J, and Breeding, R J, (1975). Vulnerability Model: A Simulation System for Assessing Damage Resulting from Marine Spills, Report CG-D-136-75 (Enviro control), Rockville M D., USA.

12. Erbink, J J, (1993). The advanced Gaussian model STACKS, Procd. of ERCOFTAC work shop on Inter comparison of Advanced Practical Short range Atmospheric Dispersion Modelling, Manno, Switzerland.

13. Erbink, J J, (1995). Turbulent diffusion model from tall stacks, Ph.D. thesis submitted to VRN University, The Netherlands.

Fowcett, H H, and Wood, W S, (1993). Safety and accident prevention in chemical operation, (third ed.), John willey, New York.

Gifford, F A, (1961). Use of routine meteorological observation for estimate atmospheric diffusion, Nuclear Safety, 24, 67.

Greenberg, H R, and Crammer, J J, (1992). Risk Assessment and risk management in chemical process industries, Van Norstand Reinhold, New York.

Greenbook, (1992). Methods for determining of possible damage to people and objects resulting from release of hazardous materials, Report CPR-16E, Voorburg, Wamngton, UK.

Hagon, D 0, (1984). Use of frequency-consequence curves to examine the conclusion of published risk analysis and to define broad criteria for major hazard installations, Chem Engng Res Dev, 62, 381.

Havens, J A, and Spicer, T 0, (1985). (DEGADIS) Development of an atmospheric dispersion model for heavier-than-air gas mixtures, Report number CG-0-23-85, U S Coast Guard, Washington, D C.

HSE, (1988). The tolerability of risks formation from nuclear power stations, Health and Safety Executive , H M Stationary Office, London.

Kayes, P J, (1986). Manual of industrial hazard assessment technique, Technica Ltd., London.

Khan, F I, and Abbasi, S A, (1995a). Risk Analysis : An optimum approach, research report to CPCE //?A 01 I / 95, Pondicherry University.

Khan, F I, and Abbasi, S A, (1995b). Anatomy of industrial accidents, Chemical Engineering World, September, Bombay.

Khan, F I, and Abbasi. S A. (1995~). Accident simulation in CPI using MAXCRED, Indian Joumal of Chemical Technology, 3, 338-344.

Khan, F I, and Abbasi, S A, (1997a). Models for domino effect analysis in chemical process industries, Process Safety Progress (accepted; in press).

Khan, F I, and Abbasi, S A, (1997b). Modelling and simulation of heavy gas dispersion on the basis of modification in plume path theory, Atmospheric Environment, (in press).

Kletz, T A, (1977). Unconfined vapour cloud explosions, AlChE Loss Prevention, 11, 50.

Lees, F P, (1996). Loss prevention in CPI, Bufferworths, London.

Martinsen, W E, Johnson, D W, and Terell, W F, (1986). BLEVE : Their causes effects and prevention, Hydrocarbon Processing, 65(11), 141.

NEERl (1992). Risk assessment of Reliance petrochemical complex, National Environmental Research Institute. Nagpur, India.

Papazoglou, I A, Nivoliantou, Z, Aneziris, 0, and Christou, M, (1992). Probabilistic safety analysis in chemical process industries, Journal of Loss Prevention Process Industries, 5, 3-1 5.

Pasquill, F, ( I 971). Atmospheric dispersion of pollutants, J of Royal Meteorological Society, 97, 369-395.

Pasquill, F, and Smith, F B, (1983). Atmospheric diffusion, John willey, New York.

Pitblado, R M, Shaw, S J, and Stevens, G C, (1990). The SAFETI risk assessment package and case study application, Safety and loss prevention in the chemical and oil industries, 51.

Pietersen, C M, (1985). Analysis of the LPG-incident in San Juan Ixhuasapec- Mexico city, TNO report, The Netherlands.

Pietersen, C M, (1990). Consequence of accidental release of hazardous materials, Journal of Loss Prevention Process Industries, 3, 136-141.

Prugh, R W, (1987). Evaluation of unconfined vapour cloud explosions, Proceedings of lnternational Conference of Vapour Cloud Modelling, Boston.

Prugh, R W, (1991). Quantitative evaluation of BLEVE hazards, Journal of Fire Protection Engineers, 3(1), 9-24.

Raghvan, K V, and Mallikarjunan, M M, (1988). An approach to Maximum Credible Accident Analysis of cluster of industries, Procd. Envirotech lnternational conference, Bombay.

Roberts, A P, (1983). Thermal radiation hazard from release of LPG from pressurized storage, Fire & Safety Journal, 4, 197-21 2.

Technica, (1992). WHAZAN : A software for hazard assessment, Technica Ltd., London.

TNO, (1991). EFFECTS : a software for hazard assessment, TNO Prins Mauritis Research Laboratory, The Netherlands.

TNO, (1992). SAVE : a software for hazard assessment, TNO Prins Mauritis Research Laboratory, The Netherlands.

TPL, (1993). Risk analysis of storage unit of chemical process industry, Tamilnadu Petrochemicsl Ltd., Manali, Madras, India.

Turner, D B, (1970). Workbook on atmospheric dispersion estimates report Ap-26, US EPA, Qeseach Triangle park, NC.

Turner, D B, (1985). Pragmatic method for the estimating boundary layer parameter for diffusion calculation, Boundary Layer Metrology, 8, 93-100.

Van den Berg, A C, Van Wingerden, C J m, and The, H G, (1991). Vapour cloud explosion : experimental investigation of key parameters and blast modelling, Trans I Chem Engg, 690.

Van Ulden, A P, (1974). On spreading of a heavy gas cloud released near ground, Loss Prevention and Safety Promotion, 1,221.

49. Van Ulden, A P, (1985). Estimate of atmospheric boundary layer parameters for diffusion application. Journal of Climatology and Applied Meteorology, 25, 1609.

50. Vernart, J E S, Rutledge, G A, Surnathipala,K, and Sollows, K., (1993). To BLEVE or not to BLEVE ; Anatomy of boiling liquid expanding vapour explosion, Process Safety Progress, 2, 11 2-123.

51. VTT, (1993). RlSKlT : a software for risk assessment, The Technical Research Centre of Finland, Tempere. The Finalnd.

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Chapter 12

Risk analysis of an epichlorohydrin manufacturing industry using the new computer automated tool MAXCRED

Fnisnl I. Khan and S. A. Abbasi Risk Asscss~~ tc~ t t D i v i s i o r~ , Cc l i l rc for Pol lul ion Conlrol and Bio-Wnste Energy, Potldicllerry Un i ve rs l~y , Pondicherry 605 014, lnd ia

Received I Ocrober 1996

Studies pertaining to rlsk analysis, conducted on the storaue unlls of a typical chemical Industry angaged In tha menulactura of spichiorohydrln, are described. A comprahansive software pack. age MAXCRED IMAXimum CREDibie accident analysisl racently developed by us has been utli- ized for the purposa. Eight different sceneries, one lor each storage unit, have been studied, It is observed thal the nccldonl scenario for chlorine IBLEVE followed by dlpersionl leads to the largest 8raa.undar-1ethal.Impact while Ihe accidant lcensrio for propylens ICVCE followed by lira ball) fordcasls the most Intense damage per unit ares. Tho accidents Involving propylene, epichlorohydrin and fuel oil have high possibility of causing dominolsecondsry accidents as their destructive Impacts (shock waves, heat load1 would envelope other ntoraga and process units. Besides dernonatretinp tha utllizability of MAXCRED, this study eiso focusss sttentian on the nerd to bestow greater effort lowerds rlsk assessmenl/crlsis management. The authors hope the1 these sludies would hlghllght the asvarlty of rlsk posed by the industry and would lhus generate safely consciousness among plant managers. The studies may also help in developing accident.prevsnlion stretegies and lnslailalion of demega control devices. 0 1997 Elsevler Science Lld. All righls reserved

Kmywords: rtsk: Softwarm; MAXCRED: mplchlorohydrin: aecldane dammpms

Introduction Serious accidents cnn take place i n chemicnl process industries-in the form of explosions, flres and reiense of toxic che~nicals [1.2). Such accidents can taken heavy toll i n terins o f loss o f properly a~ td l lun~nn lives. The accidents-if they Involve release of large quantities o f toxic cbeni icnls-znn nlso contnrninnle inrge areas oiid render them useless for several years [3-51.

Accidents in process industries are generally caused by tile followitig factors:

I . Cracks i n sloragc or react io~~ vessels lending to mp- lure

2. Mnlfunclioning or conlrol equipment such as valves 3. Other type o f fnilures In instruments a i ~ d piping 4. Humnn errors 5. A coinbination o f two or more o f the nbove fnctors.

A brief review o f some of tlie recent accidents (3- 5 ) Ihnl have occurred i n different industries is presented below to illustrn~e the myriad ways i n wllicli ltu~rlnn or

equipment failure con cnuse nn nccident lending to loss of lives and property.

Illusrrarivc case rludies A l a refinery i n Fmnce, a spillnge occurred on 4 Jafii~nry 1966 when an operator was draining water from a 1200 m' pressurized propane sphere. The propane vapor spread over a radius o f 150 m n ~ i d was igiiited by a cur on the road. The pool o f propane below the sphere engulfed Ihe vcssel i n flames. The resultanl boiling. liquid-expanding-vapor explosion (BLEVE) killed the fireman and 17 olllers. The conflogrntion took 48 h to conlrol and caused extensive damage to the refinery.

On 22 June 1974 n 16" elbow of n pipe carrying potassium carbonole solution i n a lertilizer p la i~ l al Tam- iinadu, India, ruptured suddenly, splaslling the hot solution into the nearby control room. Tile toughc~ied glass panes shattered; eight people died i n tile control room instantaneously, one died i n hospilol nnd another sus. lairled grievous injuries.

92 Risk analysis using MAXCRED: F. I. Khan and S. A. Abbasi

On 8 March 1984, an explosion in a refinery at Kcr- ,la destroyed a fire tender along with the shed wherein

u.as housed, besides a chemical warehouse. cooling touer and other facilities. Later investigations revealed many ~hortcomings in the plant layout.

On I2 December 1987, a crude oil storage tank in , refinery at Maharashtra, India, started boiling over, sptlling the contents on the dike around it. Emergency Eenices and fire brigades, who were alerted, tried to e\acuate the contents. After 4 h of pumping out, the tank caught fire and exploded, spilling the contents. Eight hours of vigorous fire-fighting had to be corned out before the fire could be controlled. There was extensive damage to the property. A liberal stzing of the dike dnd providing a separate dike for a large tank like t h ~ s would have helped to prevent the spread of hrc to orher tanks.

On 5 November 1990, an explosion at the offside battey of compressors at a giis cracking plant in Mahar. ashtra. India, killed 35 people, besides causing heavy damages lo propeny and business interniption losses. Among the deficiencies in the layout. identtfied after the disaster. was the locatton of a contractor's shed danger- ousl! close to the gas compressors. Less publicized, but perhaps of greater consequence, was the lack of any Rcilir) to shut down the flow of hydrocarbon at the slte ttself The plant personnel had to run to the control room tailer than the vapor that followed them to close the feed valve.

.An acctdent took place on 18 Apnl 1989. tn a 13" natural gas ptpeltne In a g a company in India. The ptpe- llne Has carrying compressed natural gas at a pressure oi about 295-298 prig from the compressor ?tatton to \arious consumers. The location of the accident u a s about 730 feet from the compressor statton. A security personnel heard a loud sound at about 0950 hours and rau a huge cloud of black smoke emanating from the ruptured p~pcllne which caught fire immediately. The Adme rose as high as 150 feet dunng the initial stage

The fire-damaged bulldings consisted of the general {lures and the office of the materials department. Two employees died and six others received bum injurtes. Invest~gat~ons revealed that the portion of the p~pellne that had hlown off u a s extensively corroded as compared to other portions of the ptpeline. The underground plpeltne was close to the materials depanment where old lead cells were stored. The corrosion could be due to the leakage of spent weak acid which seeped through the lround and corroded the burled pipeline.

Bhopul dhusrer. The worst-ever disaster tn the history of the chcm~cal industry occurred in Bhopal. India, on 3 December 1984. A l e d of methyl isocyanate from a chemical plant. where 11 was used as an intermedtate in the manufacture of a pestictde, spread beyond the plant boundary and caused death by po~soning of over 2500 People-injuring about 10 times as many 141.

Methyl isocyanate boils at about 40rC at atmospheric pressure. According to press reports. the contents of the storage tank became overheated and boiled, causing the relief valves to lift. The discharge of vapor- about 25 tons-was too great for the capacity of the scrubbing system. The escaping vapor spread beyond the plant boundary where a shanty town had sprung up. The cause of the overheating was contamination of the methyl isoc)anate, by water or other materials, and several possible mechanisms were suggested. Accordtng to some reports cyantde uac produced. Had Union Carbide conducted risk analys~s icpecifically maxtmum credible accident analysis) during the design of the MIC system or even later. it would have learnt that in the ebent of a MIC leak the scrubbing system wc~uld be tnadequate. This would have enabled the ~ndustry to install better handling systems. thereby saving thousands of lives.

PEPCON a~plosion. On 4 May 1988. a massive explosion destroycd Pacific Engineering and Product~on Company (PEPCON) plant near Henderson. about 12 miles south of Las Vegas. USA.

PEPCON was one of the only two plants in USA that produced ammonium perchlorate (API: the other was the Kerr-McGee plant, also located in Henderson about 2 m ~ l e s from PEPCON plant. PEPCON reponedly produced ahout onc-lhird of the AP used as an oxidizer and propellant In so l~d, composite rocket fuels for NASA's space Ehuttle and micsiles.

Although .a fire staned the PEPCON explosion, the cause of the fire u a s not easy to explain. After the explosion. PEPCON blamed the fire on a leaking underground ptpeltne of Southwest Gas Company that tra. versed PEPCOK's propeny. But the natural gas plpellne had been installed about 10 years before PEPCON's plant, and although ruptured, it only conrributed to the fire and heat required to detonate the second and the largest e~plocion.

The fire was also attributed to a welder's torch hut one of the rcports absolved the welder of the blame Some blamed the batch dryer's fiberglass insulallon whlch had a history of AP cpills into the comhustihlc insulation.

The follou'lng were the tell-tale conditions In and around PEPCON.

I. lack of proper storage; 2 , combustible fiberglass inculariun and sources of fire: 3, glass panel walls in the batch house: 4, inadequate spacing between adjacent procesa vessela

and product storage ranks: 5. no alarm to warn plant personnel, fire depanments or

Henderson's other citizens: 6. no dependable fire-fight~ng arrangement a'tth cprtnk-

lers and deluge system: 7 . no modem, dependable, radio system to back-up

damaged telephone lines needed to call for help, coor- dinate response teams and warn the community:

Risk a n a l y s i s u s i n g MAXCRED. F. I . Khan and S. A. Abbasi 93

lack of an effective emergency response plan at PEP- CON within the surrounding industrial complex and ,,lth~n the town of Henderson.

The explosion caused about 5100 million in dam- ,.,, to [he surrounding community and completely kstroyed a neighboring marshmallow plant. About 350 ,rrsons weere injured. Two persons died-lhe plant man- ,ger and the controller.

rfir P I I I I I I ~ . ~ e.~pIosiof~. The explosion at the Phillips densir! polyethylene plant In Pasadena, Texas, on

!i October 1989 is one of the worst industrial accidents ,f the last 10 years.

The immed~ate cause was simpie: a length of pipe opened up to clear a choke without bothering to see

hat the isolation valve (which u'as operated by com- 2re\sed air) had not been duly closed. The air hosei ahich supplied power to the valve were connected up he wrong way round so the valve was open when i t ?

ictudtor uas In the closed position. Idenucal couplrngs vcrr used for the two connections so it was casy to revsrre them According to company procedure they ,hould ha\e been diwonnected during maintenance bur rne) were no1 The valve could be locked open or cloced hut this hardly mattered as the lock was missing. The :\plos~on occurred less than 2 min after the leak staned dnd two iso-butane tanks exploded 15 min later. The t~plostve force was equivalent to 2.4 ton* of TNT. 2.7 perlple-all employees-were killed and over 130 intored. Nearly 40 tons of ethylene gas leaked and cnpioded.

Tiff Sihvnori or,rrdenr. Perhap< the moqt macabre accrdent-next onl) to Bhopal pas tragedy in its severtty- occurrcd on 3 June 1989. near Nizhnevanovsh in West- ern Siberra Engineers stationed there nouced a sudden h o p in preswre ar the pumping cnd of an LPG pipeline Tllr pipeline was commissioned In 1985 to carry mixed LPG to rupply the industrial city of L'fa. Instead of liliestigaing the trouhle. the engineers responded by Increasing the pumping rate in order lo maintatn the rcquired pressure In the pipeline. The actual leakage Pllint wai about 890 miles downstrcsm between the loi\ns of Aima and Lfa where the pipeline was installed .~hout half a mile away to the side of the Trans Stherian Kallway. The smell of escaptng gas was reporled from tllr \alley settlements in the area but no one d ~ d anything .'bout it. Thc escaping liquefied gas formed two large pockets In the low lying areas along the railway line. The @as cloud then drifted for a dlstance of 5 miles. Some hours later, after the main leakage had started, a "an from Nizhnevanovsk destined for the Red Sea 'eilm of Alder was approaching the leakage area when the driver noticed a fog in the area that had a strong ""ell The driver of another train approaching from the "PPustte dlrectton (Alder to Nizhncvarrovsk) saw much 'he .;me as he approached the west bound train. Both

tratns were packed. with a total of I168 people on board. and as they approsched the area. the turbulence caused by them mixed up LPG mist and vapor with the overly- ing air to form a flammable cloud. Onc train or the other ignited the cloud. Several e ~ p l o i o n s took place in quick succession, followed by a ball of fire that was about I mile wide and which raced down the railroad tracks in both directions. Trees were flattened within a radius of 2.5 miles of the epicentre of explosions and wrndows Here broken up to 6 miles au'ay The accident leh 462 dead and 796 hospitalized with 7&802 hum tnjunes.

Prevo~riordcuf~rrol of ncciderrrs Coniidering the serious and eber-increa\ing risk posed by the chemical procesc tndustr~es as eluc~dated above. i t I S neceasary lo conduct risk assessment J6.71 and develop strntcgies to prevent the accidents or. if preyen- t t \e measures fail. to cushion their adverse impacts.

Such exercises in r i ~ k assessment would tnvolve the following essential steps [6].

I. to identify vulnerable spots or 'high risk' points in an tndustry:

2 . to stmulate accidents and acsess the damage they may cause:

3, ro use the results of the prevlous step in identifying rhc pnority area\ uhere prevendve meawrer need to he rntruduced;

4, to develop disaster management plane b n c d on (21 and (31 above.

It is evident that ctep (21 is very tmponant In quan- ttfying the risk poied We have, therefore, developed a softuare package. hlAXCRED The packase incorprrr- ares itate-of-[he-an malhemaucal models. including some de\eloped or modified b) the authors. for rapid quanr~tati\'e and cumprehensi\e risk asseccment. .A brief deccriptron of 1,lAXCRED i i prescntcd here.

The package MAXCRED MAXCRED is a soltuare package debeloped at Risk A\sersment Dtvision of Centre for Pollution Control 211.

Bto-Waste Energy [8]. The package enables simulntic of accidents and estimation ot their damagc potentid, The softnare is wntten In C?i language and is cornpat- thle u t ~ h DOS as well ar WINDOWS working e n w o n - imentr. The wftuare ts operable on personal computer requ~ring a minimum of I M B RA.M and 1.5 MB ROM space

The software has four main modules (opt111ns) scenario generation, concequence analysis. file. d r

graphics. The sccnar~o @eneration module enables 11, development of accident scenarios based on the properties of chem~cnls. uperatrng conditions and likely ways of malfunctioning that uould cause accidental releases. The consequence analysis module uses the previously developed accident scenarios and advanced models of thermodynnmics, heat transfer and fluid dynamics to

94 Risk analys is us ing MAXCRED: F. I. Khan and S. A. Abbasi

iorcca,t the nature of the accidents, and their poten- tl,~lller to cause damage. This module also enables the e,tlmat~on of the damage radii and the probabilities of dcFrees of damage. The file option enables the user to hdndle input/output information and the prilphics with

kVith the last-graphics optlon, visual scenartos of are generated. All-in-all MAXCRED is a ver-

saule tool for risk assessment and is envisaged t o be self- colltained in the sense that it does not need other packages for data LuKdly~iS or grdphics suppon.

In this paper. MAXCRED has been used to vis- ualtze different accident scenarios in a typical chemical ll~dustrq's storage unit. The induatry is a pan of large lndurtrial complex \ituated near Muzzafarnagar. Uttar IJr~desh. India. The results h a \ e been represented in the terms of risk (damage'probabil~ty) contour maps o \er tile accident site to enable eas) and swift asaersrnent of m,ixlmum credibility scenario.

Nature of likely accidents In general, an industry may have four types of accl- Jcnts-namely ( i ) fire. (iil explosion. iiiiJ ioxtc release ~ n d dispersion. and ( i \ J a combination of (il. (i iJ. and in) The logical sequence and dependency of most llkrly accidents that may occur in im industry IS

yrecnted In Figure I .

Case study 4 rirl, assewnent study has been carried out for a typical :hcrnical industry situated in a congested industrial com-

plex ( F i j i ~ , , ~ 2) and engaged in manufacturing of epichl- orohqdrrn (EPI). EPI IS a chemical that can cause fire as uell a5 to\lcity, hloreover, during the manufacture of EPI, industry deal, with various hazardous chemica1.i such as propylene, chlorine, and allylchlonde at elebated temperature\ and preshures. The industry and nearby area, are threatened becauie of different types of hazards (explosion. fire toxic ga\eous release, and corrosive liquid release) present In the industry.

Figure 2 Layout of the study area showing location of 8ndustrlss and thelr surround~ngs 1, Populour villages. 2. iesldentlal bulldmgs, 3 green belt, 4' chemtcal ~ndustiy: 5' ienillzer ~ndus- lry, 6: peUoIeum refinery, 7 petroleum ref~nery. 8: waste lands

I 1 I T O X U r ~ i ~ a s c

and Flrr Explorlon d < r p t r \ ~ u n

I

Figure 1 Tree dia~ram rhowlng different accidental events and thelr lnterdependencv

Risk analysis us ing MAXCRED: F. I. Khan a n d S. A . Abbasi 95

rlblc 1 The main unts of an ep~chiorohydrin IEPIl plant pflatlvely less hazardous. All these units have been funher wbiected to MAXCRED for hazard assessment

unltr Reference Operation and damage potential esumatlon. used ~n Figure For the sake of brevity, the present paper gives

3 details of the study of only the most hazardous units in propylene purification A F,actionation the plant (storage un~t) . proovlene chlorination B Raaction The storage unit commises of the vessels storing ~llyichloride purification Chlorine absorption in walar Chlorohydrination nydiolys~s nnd EPl purificat~on

Quenching Vent scrubbing Propylena bullets EPI tanks Fuel oil tanks Chiorlnated organic tanks Chlorine storage

C Fractionation D Absorption E Resct~on F Reaction and

dlstlllatlon G Heat transfer H Separation I Storage J Storage K Storage L Storape M Storage

Process .runrmaT Purified propylene in gaseous phase reacts with chlorine to give allylchloride. The crude allylchloride is punfied hy passing through a fractionating column. The pur~fied chemical subsequently passes through a chloroh)dration process at nearly ambient temperature to yield glycerol dichlorohydrin (GDH). GDH is further subjected lo hydrolysis at 10°C using lime as the sapontficat~on dgcnt. The crude EPI thus produced by hydrolysis passes through the fractionating column to yield pure EPI. hhich ir then stored in tanks The main unlts of an EPI plant are listed in Tebie I .

We have conducted risk analysis of the complete plant. In order to optimize the time and effon, a hazard identification and ranking exercise was first carried out. The exerc~sc is meant to identify the type of hazards present and a rough estimation of their hazard potential. The units are accordingly characterized and ranked in priority for the comprehensive study. Dow's Fire and Explosion Index [9] has been used for this step and a \urnmar). of results thus obtatned is presented in Figure 3. 11 is clear from the figure that storage of propylene and chlorine are the most hazardous. followed by the rtordge of fuel oil, and allylchloride Other process unlts: chlorination. chlorohydration and quenching. are com.

3 Damage potenlfal of different units in epichlorohvdrin Plant: the elaborations of A.0, ... M are given I" Table 1

\.arlous chemiials at elevaied temperature and (Table 2). Each slorage \,e*scl (for each hazardous chemical) has been subjected to MAXCRED for hazard arsessment and damage potential quantification, T o fel- icllate understanding of the use of MAXCRED, the results of the study are presented in steps that follow the algorithm of MAXCRED.

Gerrerarion of accident scenarios Based on the history of major accidents in process ~ n d u s - tries and the authors' experience, the following scenarios have been visualized for accidents In different storage unlts.

Sceriario I . An excessive pressure development in the storage vessel of propylene (under high pressure and temperature) leads to confined vapor cloud explosion ICVCE). The vapor cloud generated by CVCE on ignition turns into a fire ball and consequently damages the other storage vessels (chlorine, allylchloride, epichlorohydrin. fuel oil).

Scer~arro 2. A sudden release of pressunred chlorine triggers a boilinp-liquid+xpanding-,.apor explosion (BLEVE) and dispersion of toxic vapor cloud; in other words explosive release followed by toxic dispers~on.

Scenario 3. Unconfined vapor cloud explosion (UVCE) occurs in the epichlorohydrin unit accompanied by a pool fire. This w ~ l l happen if a small leak In the storage vessel rclcuscs material at a moderate Row rate and forms a vapor cloud in the atmosphere, which on ignition leads to UVCE. Due to the heat generated by UVCE or b) an external source of ignit~on the rema~ning matenal in the vesvel or dike catches fire caustng a pool fire. A UVCE followed by a pool fire can damage neighhonng vessels, due to excess shock waves and heat load and trigger secondary and h~gher order accidents.

Scenurio 4. Instantaneous release is followed by pool fire in the allylchloride rtorage unit.

Scenario 5. There I S a pool fire in the fuel oil storage unlt.

Scenarros 6 and 7. Toxic release of chlorine and chlori- nated organics is followed by evaporation and dispersion.

These scenmios have been processed for damage estimatiu~i through MAXCRED. A brief note on the

96 Risk analysls us i ng MAXCRED: F. I. Khan a n d S. A. Abbas i

~ ~ b l 1 2 Operaling conditions of different storage units - - --

Hazardous chem~cals No of tanks Capaclty of each rank Operattng pressdre latmi Operating tamperalure ltonlm3! 1°C)

Chlorlne Allylchloride ~~chlarohydrln Fuel oii 1 chlorohydrlno 1 ChIor8nated organin 1

damage-effect calculation models used for the detailed crud) is presented below.

UII~~IU,~~ rflecr calculation^ for the acciderlr scenorloy The explosion, fires and l o u c dispersions eventually t.ause damage four ways. The potential o f these effects can be expressed i n terms of probit function [1&1?]. u hich relates percentage of people affected i n a bounded r e ~ i o n due to a particular accldent event to a normal distribution function.

Hpar rud~ution effect. The probit function for 1009 lethality for heat radiation is given as:

Pr = 36.38 + 2,56*1n[1*~~ '~ ]

\!here y ii defined as thermal load (kWlm2): t is time of exposure lsl: and Pr is probit value.

To.tir effect. Lethality o f a toxic load is expressed i n lerinr of probit function as:

Pr = a + hln(C'*t)

u here a. h. n are constants: C is concentration i n ppm. r IS []me of exposure (s). The !slues o f the constants for

Tablr 3 presents the summab of calculation (output o f MAXCRED) for Scenario I . The missiles generated by CVCE may hit nearby targets and can lead to second. ary exploiions or toxic releases. The vapor cloud generated by CVCE on ignition may cause a fire ball and hence sewre heat radiation effect. The shock wave generated due to CVCE caused injur). as well as second order accidents by ~eriously damaging other vessels. I t has been estimated that shock waves wlth a 50% probability o f causing injury would he observed o\,er an area of 500 m radius. The heal radiation effect with 100% probab~llp o f lethality uou ld be observed, over an area of 300 m radius and missile effects with 50% chances of damage would be observed across 750 m radius.

The sudden release of chlorine and the consequent BLEVE ;L\ per Scenario ? would lead to severe shock waves, and toxic releaaes (Table 4 ) . I n this scenario the damage potential o f the shock waves is estimated to be cornparati\ ely lesser than Scenario I. A 100% damage-

--

Tabla3 The output of MAXCRED for Scanarlo 1

Parameters Values different gaqcs are available i n the literature [ I 1.1?].

D~stance from accident epicenter lml: 200

I'rrsslrru and hock w v e rfic!. The probit equation for EXDOsiOn' CVCE

l~kelihood of death due to shock wave (lung rupture) is Energy released durjng explorlon I~J) : 1.733260e+8 w e n hy Peak overpressure IkPal: 5230.851562

Variation of OVCrprBSSUIe in air IkPaisl. 2339.458984 P r = - 7 7 I + 6,91*111P' Shock veiocjty of a~ r lmisl: 1978 708984

Duration of shook wave Is!. 33 019657 For in juy , the equation is M~sslle character~st!cs

Pr = - 15.6 + 1.931np"

where p" is peak overpressure lN/m2).

Afisile pflececr. T h e probit function for fatality in human being\ or damage to vessels is expresred as:

Pr = - 17.50 t 5.301115

u here S 1s the kinetic energy of the missile (I).

Hn:ard y~~ortt$cnrion The reyultr o f the calculations for different accident scenarios are summarized below.

inltlal veloc8tv of fragment imlsl 1098.601074 K~netic energy of fragment IkJl' 7.860266e-05 Fragment veloc~ty at study polnt Imlsl' 984.562378 Penetration ability at study point

Concrete srructure (mi: 0.975414 Brlck structure (mi: 0.992585 Steel structure (mi: 0.094620 F~re. Fire ball

Radius of the fire bell lm): 278.059906 Duratnon of the fire ball (rl: 113.623726 Energy released by flre ball (kJI' 2 468228e-10 Radatlon heat intensitv IkJlm'i: 23 570.539062

Risk analysis using MAXCRED: F. I. Khan and S. A. Abbasi 97

1.bl.4 Tho output of MAXCRED for Scenario 2 Table5 The output of MAXCRED for Scenario 3

parameters Values Parameters Values

Distance from acctdant 8pbcenter lm): 200 Expioslon: BLEVE

~ o t a i energy rsloastrd IkJl: 5.786538e+05 peak overprossura IkPal: 134.8982 Variatton of overpressure In alr IkPa/al: 57.98675 cknrk veloel t~ of air lmlsl: 343.9556 , ~

nuratton of shock wave IS): 33.48331 - - No m~ss~ le offen ~ o x i c release and dtsperslon

Light gas disp~rslon charaner~stics Gausr8en ~nstanteneous: model

Concentret~on at diaance 200 m lkg/mal: 1.845254e-04 Concentratlon at cloud axis ikglm31: 1 345267e-02 Value 01 source helght lml. 5.000000 Puff character,sttcs:

Puff concentrat~on at center of lkg/m31: 8 1699450-04 cloud Concentratlon at cloud edges Ikg/m31: 8.139317e-04 D~stance along downwind Iml: 200.00 Dosage at study polnt Ikg/mal: 0.078989

causing ?hock wave u o u l d he operative over an area o f 400 m radius. A s chlorine is noncombustible, n o heat n d i a t i u n effects wou ld be observed. but a b u ~ l d - u p o f lethal concentration wou ld take place over an area o f 1500 m rad~us .

A UVCE as per Scenano 3 wou ld g ive rise to heat rndiation effects, shock wares. and m ~ s s i l e etfects. I n addillon there wou ld be s e c o n d 0 lmpact o f burning o f released mater ia l I n the dikeluesael w h i c h wou ld agaln lead to addi t ional heat load. I n Scenario 3, mts<ile effect 15 not significant (Table 5). However, shock waves o f intensity h i g h enough to damage a l l ob jecu coming I n their h a y w o u l d be persistent i n an area o f 300 m rad~us . The combined impact o f heat rad~at ion. UVCE, and poo l fire wou ld be lethal o \e r an area o f 600 m radius. Due to intense heat load some o f the chemical w o u l d evapor- ate and disperse causing its bu i ld up, to a lethal concentration over an area o f 200 m radius.

The mode l results for Scenano 4 are prehented in T ~ h l r 6. Lethal heal load u o u l d be observed over Ihe study area; the max imum damage distance w o u l d envelope an area o f 400 m radlus.

The output o f M A X C R E D fo r Scenario 5 ( fue l o i l ) is tabulated ir. Table 7. Lethal heat load wou ld be observed over an area o f 350 m radius.

The results o f Scenarios 6 and 7 have been Presented i n Tables 8 and 9, respectively. I n bo th cases the lethal concentration L 1 0 0 6 chance o f fatal i ty) is confined l o an area o f 200 m radius.

Risk esrimarivn Using the results obtained by M A X C R E D and the prob- :lbil ity o f occurrence o f the accident scenurios, i n d i v ~ d u a l

D~stance from accldent epicenter (ml: ZOO Expioslon: UVCE

Total energy released by explosion ikJI: 6.628088e+05 Peak overpressure IkPal: 115.6722 Varlatlon of overpressure In air IkPaIsl' 42.56866 Shock veloctty of alr imlsl. 453.5449 Duration of shock wave IsI: 21.90854 Missile characteristics:

ln~tlai velociry of mirslle Imls!: 876.561 Ktnet~c energy associated wlth IkJI' 1988426.3750 mlsslle Fragment veloclty at study point Imlsl: 234 740 Penetrat~on ability at study point

Concrete structure Brick structure Steel structure Fire: Pool flre

Im! 0.2165 lm l 0.2456 Iml: 0.0021

Instantaneous model

Radlus of the pool flre (mi: 5.0 8urning area lm'l: 78.53749 Burning rate Ikg/sl: 38.42223 Heat ~ntensity IkJ/m21' 1160.191 Toxtc release and dtsperslon

Lrght gas dlspers!on character~st#cs Gavss~an Instantaneous: model

Concentrat~on at d~stsnce 200 m lkg/mll: 6 M2933e-07 Concentration at cloud 3x1s ikg/m31 5.335456e-05 Puff characterlsrtcs:

Puff conceotratlon at center of lkgim'l 2 932708e-05 cloud Concentration at cloud edges lkglm3i: 2.972075e-05 D8stsnce along downwtnd im!: 200.0 Dosaoe at studv ~ o l n t lkalmal. 0.0008844

Table 6 The output of MAXCRED tor Scenario 4

Parameters Valuer

Dlstance from acciaenl cplcenter (mi' ZOO Flre: Pooi fire

Contmuous model Burning area Burning rate Heat Intensity

fatal r l sk factors have becn estimated. The probabi l i ty of occurrence o f an indiv idual event has been adopted from the literature [13-171. T h e r ~ s k factor is a direct representation o f the threat ( taking in to consideration bath damage potent ia l and probabi l i ty o f occurrence) to an indiv idual i n a n area.

98 Risk analysis using MAXCR!

r.hf.7 The output of MAXCRED for Scenario 5

parameters Values - DlSrance from accident epicenter iml: ZOO ~i,. pool ftre

Instantaneous model ~ ~ d i u r of the pool tire lml. 5.00 Burnlng area im21: 78.53749 surning rate (kglsl: 38.42223 neat ~ntansity IW/m2l: 1160.191

able 8 The output of MAXCRED tor Scenario 6

Parameters Values -

Dtsrsnce from accident epicenter Iml: 200 Flre. Pool flre

Cont~nuous model Burning area Im21: 75 00 Burning rate Ikglhl. 11 842.82 Heat intensity 1W/m2l. 336.84860

Tabl.9 The output of MAXCRED for Scenarlo 7

Parameters Values

Distance from accident rmi. 200 epicenter Fire Pool ftre

Conllnuous model Burn~ng area Burning rate Heat lnienslty

I n order t o enable visualization o f accident scenarios. h e i r r isk contour5 have been drawn over the stud) area (F~yurcs 4-81, I t m a y be seen that the risk contours for Scenarios 1-3 (Fisures 4-51 are extended beyond the houndanes o f the ~ n d u s t r y and enve lop~ng other tndus- lnes and neilrby populated areds. The n5h conlourr for other 5enarios are conf ined to the campus of the EPI plant but the r isk contours fo r Scenarios 4 and 5 extend Up to other storage vessels i n the factory ( F i g u r r ~ 7 and 8) . This may cause secondary accidents, the impacts o f which may g o beyond the factory confines.

I n summary Scenario 2 represents the worst l ~ k e l y disaster w i th in h e realm o f credibi l i ty . I t has the largest area-of-lethal-impact shock wave over an area of radius 500 m and lethal concentmtion across an area o f radius 1500 m). Further. the most thickly populated areas (lncludlng residentla1 areas o f Mahro l l and Ramput) l i e * l h l n its range. I f one considers the cumulat ive effects. the Scenar~o 1 w o u l d come out as the worst, as more Intense impacts ( in terms o f heat radiation. shock waves,

I: F. I . Khan and S. A. Abbasi

Figure4 Rfsk contours ndtcatlng the impact area for an acci. dent occurring 10 propyiene storage vessel IScenerlo l i due to severe rlsk IAI, hiQh risk 161, and moderate risk lC1

Figure5 Risk contours >nd~cat~ng The lrnpan area for an accident occurring In chlorine storage vessel IScenurlo 21 due to severe risk IAI hlgh risk 181. and moderare risk ICI

missiles) are observed per uni t area i n this scenario. O f the eight scenarios. Scenario 1 IS the most l ikely to cause cascading cffects as m~ssiles. shock \rates, and radiation effects wou ld be generated simultaneously and other industries o r units dealing w i t h flammable and toxic materials are situated within the str ik ing distance o f the prirnar) acc~dent o f Scenario I . Scenarios 3 .4 and 5 also have the potential to lead to secondary accidents as r e \ - ere heat load generared i n thcse w o u l d encompss, other storage tessels. A l l I n all, Scenario ? i s the worst as far as pr imary effects are concerned, whereas Scenario 1 is the worst i n terms o f its potent ia l i ty o f causing cascading (donii~ir,) effects.

Risk analysis using MAXCRED: F. I. Khan and S. A. Abbasi 99

Flgure6 Risk contours lndtcat~ng the lrnpact area for an accl- Figur.8 Risk contours indrcatlng The lmpacr area for an acc- dent occurring In ap~chlorohvdr~n storage vessel 1Scenario 31 dent OCCurr$ng n fuel 0 1 storage vessel (Scenarfo 51 due to sev. due to revere r~sk IAl. hlgh r~sk (01, and moderate r~sk (Ci ere rtsk iA1, hlgh rtsk iBI. and moderate risk it1

Flour01 Rfsk contours Indicating the impact area for an ecci. dent occurr8ng In alivchlortde storage vessel 1Scenarlo 41 due :O severe rlsk (A), hlgh rlsk (01, and moderate risk lC1

Conclusions Thl5 paper demonstrates the applicahilit) o f the softuare Fdckage ~ I A x c R E D for performtng quantrtative rtsk ""Ysts. The package generates dtfferent credtble acci-

Scenarios, and quanttfies the damage they cause. Thi5 informatron can then he used in de\,eloping slra- ' e w for preventing accidents and to dampen thetr

impacts i f the accident\ do take place. The applr- c~b l l t t? has been illustrated with a case study o f a typrcal ~hemical industry aituuted i n Muzzafamagar. Uttar Pra- de\h. Indta.

In the first s t ephaza rd identificatton and rank- tnP-storage and process units involving chlorinatron. chlorohy~ration, and quenching were identified as the nl'O't vulnerable i'rs.ir.~,is propensity o f causing acci-

dents. A detailed study of the credihle accidents in storage units and their Impacts was then camed out with the help of h IAXCRED. The stud~cs reveal that the storage untts of propylene. chlonne and epichlorohgdrin are the most hazardous, and acctdents In these units ma) cause relere damage to the factory and tts rurruundrngs.

To reduce the hazards assocra!ed with the storape of chlorine. propylenc and epichlorohydnn. proper hazard mlnimira~iodmit igation measures should he taken. A few suggestion5 are made in thts context:

I, Instead of one or tho largc-capacity \ essels, se\,eral vessels o f smaller capacit) should he u5ed for storage.

? Adequate space should be kept between the storage vessels and huffers provided hetween them so that adverse consequences of failure rn one of them do not cause second or htgher order acctdents.

3. Sen~it ibe pas-dctecung de\ices for flammable and toxrc gases should be installed i n the premises o f storage area and other units.

4. There should he regular and thorough inspections o f electronic cuntrol equipment followed by meticu- lous maintenance.

5 . Sufficient quanttties o f tnen gases should be readily available to dilute the concentrations o f roxiclflammable gascr i f they escape to the atmosphere. and to cuntrol fire.

6. A thorough emergency preparedness strategy should always he kept in positron. fortified by periodic drills or ' d ~ runs' qo tnar the damage is contained i f an accldcnr does occur.

References I l l Ahhlrl. S A ,tnd Vcnllla. \' Rnh Anrrrmrr!!. E~rr<loprdrii f ,r

Et~ii~rlr>ir~niiai Etlalrtrcrr,i~ Enblor Med~a. K;!red. 1994 121 Khan. F. I and Ahha',. S A Ar~rrrr,m! oflrrl~r~rr~ol A~.rn<iri~r\.

Repon So CPCEiR and D 1/94, Pond~cherr) Cn>vcrvty, PonJ tchrrr). 1994.

Risk analysis using MAXCRED: F. I. Khan and S. A. Abbasi

.. . ... F P lnrs Prr\,cnlron tn rhr Procrrr Induriner. But. nioues and the# a~olicntion Prxcedfnes o i Nntlonal Confcrcncc i . ' ,Icrl l Ieno'~s.~~ndon. \olume 1, 1980 o f ~ ~ a f c t ? ~ys!ern.'krw Delh~, 1989 - K I ~ ~ Z . T. A. Who1 \\'em1 H'mnR. Gul l Publlcauon. London. 1986 1 I I I P~clci\cn. C M.. Cuniequcnccr of accidcnul releare of hazardour

Icl F. I and Ahbari. 5. A Mojrrr occ.tdmi corr rn,dzn la malcnal k,arriol of i Prevmrton in rlrr Proerrr l,idvr,nei, v/,e,,,aul process tnd~s~rtes. Chemical Enpxncenng Harld. IYW. 3. 136141 ~~ptcrnhci. 1996 I I? ] Kryss. P. J Uorid Borii Mania1 OJ hidusrnol Ho:ordArsrr rnwii

lbl &ha", F I, and Abbui. S. A Rtrk analy,ir: on opi~rnurn wheme Terbntqurr Tcchnicu Lid. London. 1985. hr hlwrd tdcnufication and asrcrbrncm Rwcedlnpr nih'altonol I131 Conunl. S.. rnendols. A snd Z~ornas. I Bmchmarl rxercl'r an

Conference. Co~mbaton. 1995. malor hazard anaiyrls. Jola Kcicari-h Centre-ISPRA. 1991 r,, k',,:onherp. H R, and Crammer. J I Risk Arrcrrmrnr atld Llaa- I141 keq . F P. Lou Prrir,ar~on 1 ~ 1 rlir Prorm ~~idarinrr. Fviit,rr , , -.-- . - m

,,pcnll.t,r jar Chrm,rrl Procrrr Irrdurrncs. Van Sostrand Rc~n- iind El.e!ir DUIU Bultewnnha. London \oiume 2. IOXO hold. Yew York. 1991. 1151 Reliabll~ty Dlrecloralc. Failure frcquenc) uf harduorr cornp-

111 &hm. F I, and Abbui . S. A MAXCRED--A rofmure p o r i o g ~ nenlr Rcpon KO 87-?YRIR.?7h'Vt, 1992 ill, qlrnnrriurrvc rrsl cnei\rts. Rcpon No CPCEI7(&D 5/94. Pond- 1 1 61 Europan Cornmun~t!, Counol d~tect~on on rhr rnvlor accident , ~ h ~ r i \ Lln~vcmty. Pondlcherry. 1994 Ent~mnrnenlnl soiluarc. hunrdous --- . of cenuxn . - ~nduslnlil ncil\~ticr. Repon So 8!150/501 1990 t t ~ . London. IYY?

19, 008 Chcmical Company. Duw hrt . ondfiplorron Index Huard 1171 Khan F I and Ahhart. S A Accident r,mulatton ~n chemicvi ~ l ~ , ~ ~ f i c v i ~ o n Gmdc lbih ed~i~on). 1980 prweah ~ndujtrir* u\>n! sofluare WAXCKED Iridio,, jcri~nial o(

1 1 0 ) \I~illlar,unan, M. \ I and Raghuan. K V K ls l ar5ervnrnl tech. Clrrali<ui T r ~ l ~ t l r i l o ~ , . 1996. 3. 33R-344

Chapter 13

Risk Analysis of a Cldoralkali Industry Situated a Populated Area Using th; Software Package MAXCRED-11 Faisal I. Khan and S. A. Abbarl

Kirk haen~ticnl Division, Cenae for Pollulion Conlrol& Energy Tecltnology,

Pondiclterry Universily, Pondiclterry @5 014, lltdia

Risk asse.~.sriierrl, based o i l q r ra i i l i fa r~w r,znrir,r~rrrt crrd. ible accideirr arralysts (MdM), hm hccrt co i rd~rc lcd jor n chlomlkalt t r i d w l n ~ sifrrarrd iir l l i c rrlid.si o f dcrtselv

si.i r.rvxio;i 2 ~l~ariceforrh i v e n r d ro a.s IIAXCR1:iI-11) ;P c r r ~ i l y derx.t(~/n,d Ity its.

Airiort(! Ihc six dij jcrci i l nro.cl c r r d i hk acc id~ i t f s c r rrar9os d c w d o / ~ d 1rsirrg AfAXCRED-11, fhc oi ls ertuuag- i t ig 'cor!firred uaporcloird eq~ los~o r i foilotied hj,Jlr~ bal l ' (111 I ~ C hvdir~geri sforage t~.~.scl) corircs orrf lo bc lhc i i c ~ r s ~ i i t fenrrs o//bc h f ~ b a f [ ~ t v / x i r s i ~ j ~ for d n i i r o ~ c ( u m i ~ i r s - srrrv, iitissrle, heal load) If also bar the pofcir i ial of carrsrr* clor~rirto @fled (cbairl of ucc~doi fs ) 7be sce- rrario uf ca i rs i i r~ dvi i i i r io qficf lcbutir rfaccrrbrrfs). 7be scerton'o of 'cottliriuotu release of ch lo~ l r r e j , v~~ i sroraxe ~ r s s r l ' g l lrc .secorid riiosl dkflsfmirs, 111 rr!Trr.c o j Clhal l u r i c lood liI.v!ll oucr a l a v e dtsiarice (3252 irrorrs). h t sutrlrriary r l ~ e slrrdy retxtals ihar giyeri lhe aia$sc,s o j riiarcnals ~ I o r ~ d , a n d Ibc coridilioirs i i t rt'lrich the)' urn slorcd, fbere Lc a l i w tick ojaccidcnls i i t Ibe siorwge LVS-

scLr fba l t l au l d have far-machtrg cotueqtterrces 7be f i idrulrv Ihirs wscs a nrcal risk l o Iaiuc arras o r s1rP mtriidi;w tircl;rdritg d~m lypopu la l od ;;rlla8ss (pir l icrc l a d y Cht t r t iaka lap l a i ~ d K a l a p l l a n d rbe cariiprrses of I'orrdicbcrry U i~ iw rs i l y ar td f'ortdicbcny ErtgirteetiriC( Col lqc .

7hrr p a [ w donottsrrn~cs (,Ire i r r i l i z ~ ~ b ~ i i ( ~ ~ o/ AMX- CRID-11 a i t d also focuses artertrtoti orr the !teed fo Iw slotu .qrcntm ofloti l ou~a~ r t s riSb fl.cres,srir~i~I II hr hn[~cd f/ifr/ f,Iit3,~v ,sftrrfics toill ~ i t r r k /~Iartf rii,#ir(r#1~5 co r~~~c i~~ i r . s of IIJE seriot~s coitseqffcttces i ba l car, rwrr l r f~v~rr acci- r h l l s ill lbc i r vu l i io~ablc ~1111s. ApfJrrciaflort o j f l rc risk, i t r I i o l ~ , trdll pmrripl then: lo dcrr lop accidr~ri l p f rc r i i -

INIRODUClION

A l a r ~ c ntlmhcr ofcllclnicnl process il~lltlstrics r,illrL,r llsc Ilnznnlous ~~iatcrisls or opernc reaclon under con(l i l i~l~lc (I (

l i i ~ l ~ ~c~npcralure and prcsi lre. Sitcl~ Inih~rtries nrc Ixone

to accirlen~s in wl i ic l~ Iiamrdous n~aterial can leak out of tile storage/reactlon vcsscls itllci CuiIIalillnate liic enviru~i. nicnt or reactors nlny crnck/cxpiwlc Ic~~dinp to sln,ck a,nvcn, nrcs or Ix,ll~. l l l c re I!:~vc Ix.en ntlnicrous ~~cc i r l c~~ t s of this type all over tile world resuhinp in massive Inwes or nlonclary resources nnd hu1n:111 livcs, illuslrarlvc ex:l~nj,lus of ;I few :lccIdencs arc prcsc~llcd 111 l':tl~lc I .

Over tile last lew years a new Iirancl~ of science-risk assesslnen( in cllelllical procers intiustries (RACPl)-has emerged lo deal will1 !Ire Iiazrrds llced by a chel~~ical process inrluslry. RhCl'l ai~iis to dcvelop tcchniqi~ec and tm~ls rv l t l~ wl~icl i an I i idulry can nr~licipatc tile l lkcl i l im~d 01 accidents, Identify 'soft spots' or 'rlanger zones' in ils production system, work out prol>nl>ilities of lnilr~res that ni:ty occrtr in its equip~tienl or 111e mistakes In tile Iinnrllinp. o f the equipment, assem Ilie damnpe polcnt~al of llkeiy ac- ciclents, work out preventive 8ieps and, linally, develop crisis Inanageinent str:ltrpies (if an accident does occur in spilc of all precnutions).

One 01 tlte nlost polenl I)r:~nclics of IMCI'I is qttantita live risk assessment ( O M ) o f wl~icl i a malor coniponent is mnximum credible aciident analysis (MCM). In MCAA 14, 51 we idcntify I iaza~lu t~s situ:~tii~ns, iievelup different accident scenarios and escilmte d:llnnpe poten1i;lI of likely accidents. MCAA cornnrises 01 tile lollowinp main steps, (i) study of Ihe plant l o Identify liazardvur malcrials and tlieir capacities: (11) identilica~ion of vulnerable seclions: (Ill) vl- snalimtion o f dlllerenl nccidcnt scenarios: (iv) (lanlnge cnl- cuiation t l l rougl~ n~at l~e~t~nt ica l ~i~odcling; (v) dclincati<~n of nmximunr credlble accldent scenarios.

'~'II~Is, so far no l~i(llgcn~,t~s p:~rk:~gc o l adcclt~:lIc rupllls- tlcatlon or user.frientllll~rss wns av:~il;~lrlc in Indi;~ lo tun-

duct M C M . 1'111s lias cofnpcllcd llie industr~es and c o n ~ ~ l l - lancy ntms lo depend on irnpnned packages such as UVIAZAN [9] offered by l'echnica Lilnlted. RISKWT (101 tflered by v17 Tccl~nirnl Itescarcli Centre 01 rinlantl and I!I'TECIS ill] overed by MO-Mmlr ib Lnlx,ntoty, l l i e Ncllicrl:ln(ls. 1'0 overcome I l i iq pap as n lw to lncrcare tile lrvcl (,I s~,p l~ is~ lcn~lc~~i (:IS :I~SII t~scr-fricndllness) a package M.L%C1181> (MAXInrunl Cl\l!l)ihlc arcitlent ~nnlysls) (12) Iia. i ~cen ( leval irm~l I,y us I~;~scrl on L4CAA. I l i c p:~ck:~ac 111. c<rr(i<,~.;l~cq ~I:II~-<I~-~II~-:II~ III : I I~~~I~:II~~:I~ l l l f ~ l ~ l 7 , I ~ i ~ l ~ ~ i l \ n ) : sr,tl>c devclomd or ni,xlilicd I,y autllrrrs k r rapid quanlitr- live and c~mi;>relirnsive MCAA We lhave recently mr,rlilic.d

I'rrncqr Sakly I'r<*,~w (M,I Ib, NIB 3 )

' 38

# TABLE 1. Sorm Major Accidents In tho PrMeu Industries Durino lad Twontv Ywn . . - - .

Year Location Chemical Event Deaths Financial Loss' ,971 Houston, USA Vinyl chloride BLEVE* 1 145 milion S

Brazil West Virginia, USA Potschefstroom,

South Africa Srate Island. USA

Fllxborough, UK Seveso, luly

Columbia Waverly, USA Monranas. Mcxrco Spencer. USA Houston, USA Mexico City,

Mexico Bhopal, lndia Kennedy Space

Center, USA P~per Alpha, UK

Illino~s. USA Pasadena. USA ,Antnerp, Belgium L'fa, USSR

Bombay, lndia

Porto de Leutoes, Ponugal

Bangkok, Thailand

Panipat, lndia

Dronka, Egypt Madrds, lndia

Gujnt, lnd~a Mumbai, lndia

---

'liu~llnp lkgu>J cxpndmp. vrwr ekplmmn ,::i nconllnrd \rpur cluud explorlon

,Canfmcd <lpur cluud rxpla*lon ' n P ~ m x m l c d ions m lr- ol US I.

monomer Butane Gas .m 3

Cyclohexane Tern chloradibenze

paradioxin hW Propane Chlonne H 2 0 Methyl bromide Propane

Methyl-i-dnate Hydrogen

Hydrogen

Propane Isc>butane Ethylene oxide Ammonia

Propane

Ammonia

Chlomdred gas Benzene

Natural gas Hydrocarbon

LVCER* CVCEw** Toxic releax

Fire in emprj storage lank

LVCE" Toxlc releav

Toxic release B E E BLEVE BLEVE BLE\Z BLE\E

TOXIC relea= HLEVE

Explos~on B Firr

BLEb'E BELT CVCE Exploa~on 6

Tomc release F~re B

Exploa~un Firr B . Exploston Fire B

Tomc releabe Explos~on B

Tomc reledbr Toxic releme Enplos~on d F~re

F~re Fire

90 million S 110 million S 95 million 5

350 m~li~on 5

450 mlI110n S 35 mill~on %

35 miilion 5 47 million 5

103 million 5 8 miliion S

10 millton S 50 million S

350 million 5 250 mill~on S

300 million S

835 million S 255 million % '00 rmliion S 570 miliion 5

30 mill~an 5

h i million 5

55 mili~on 5

'5 miilion S

25 million 5 45 million 9

10 mill~on 5 15 m~llion S

'L~%XCSD and the hrpher version hl4XCRED-11 15 noa requinnp a mirumum of 1 hlB RkM and 1 5 MB ROM. The a\\"llable 1131. A brief description of hlAXCRED-I1 is pre- hIAXCRED-I1 dgorithm is presented in Figure 1. enled belo*, The sofraue has four main options (modules): data.

scenano genenuon, consequence analys~s and file proc- esslng. The &ta opt~on handles general information re.

PACKAGE MAXCRED-II lared-to the properties of the varxous chemicals, o p m - lions. and their surround~ngs, needed for the execution of

AmCRED-~~ is an improved vers~on of the s o h a r e different models. me scenario generatlon module enables Package MAXCRED (ver 1) (121 which was developed by development of accident scenar~os based on the properries us 1" 1994, The package is coded in C++ language and is of the chemicals mvolved, operating cond~t~ons and the cumPatible w~th W S as well as WINDOWS work~ng en\,^- ways malfuncuon~ng of the equipment or process could ""menls. The software is operable on personal computcr cause an accident. The scenano generatlon option has two

Fall, 1937 173

FIGURE 1 The MAXCRED-I1 algorithm

urrhcr options a) user defined and b) automatic. In the mner h e user can defme the accident scenario as per %\/her own ludgment, In the lamer. scenarios are gener- itrd by ~W;CRED.II on the basis of the knowledge base inached to 11. The consequence analysts module takes the study of tu nen logical step, I.c., to forecast the nature and hr sevenry of the accident using advanced models of ther- nQ+namiw, heat uansfer, and fluid dynamics. Tle file VUon enables the user to handle input-outpul informa- 10" I1 also provides facilities such as mnsoiing, printing, :OPYmg etc. All-in-all MAXCRED-I1 is a v e ~ l i l e tool for 3-4 and a envisaged to be self-contained tn the sense that 1 dws not need orher ~ackages for data analysis or graph- " -4 derailed description regarding internal stsucturc and xOrbng of MAXCRED-11 is available wlth the authors.

thts paper, ~VXCRED-11 has been used to study quan- 'btlve mk analysis of a typical hazardous industry- :hloralksli. The indusuy is situated on the coastal line (Bay ?f Limpal) and surrounded by densely populated areas ~v~~la8es-~hinnakab,pet, klapet , university hostels, erc.).

The results of the study have been represented in terms of risk (damage * probabiltry) contour maps on the acctdent site to enable easy and saifr assessment of maximum credible scenano and damage potential.

NATURE OF LIKELY ACCIDENTS

Explmionr and F L s

Explosions m the storage or process units can br categorized in three main groups, according to mode of occurrence and damage potent1alll4- 171 These explosions are mittated ether by the thermal stratiftauon of the liqu~d and vapor or by such high exploston shock waves, which have suff~cicnt strengrh to rupture the reaction/storage vesseis or conduits, hn explosion may or may not be accompanted with fire; it depends upon the type of explosion and the chemical involved in the explosion.

Conpned Vapm aoud E r p b s h (CVCE)

CVCE [I41 as the name suggests, is condensed phase explosion occurring in confinement (equipment, buildmg or/and congested surroundng) Explosioru tn vessels and pipes, processing or storing reactive chenucais at elevated conditions are examples of CVCE. The excessive budd-up of pressure m the cofltnemcnt leads ro ihis ~ ' p e of cxplo- sions leading to high overpressure, shock waves and heat loads (if chemical is flammable and get ~gnited). The fragments: of exploded vessels and other objects hlt by blast waves, become airborne and act as missiles These msiles can lead to further accidents: by ramming into other process untu. The damage potential of the mssiles is assessed on the basis of the momentum they anam. The cxrent of damage caused by a CVCE depends on the mass of the chcrmcal and the explosion pressure.

Uncoqfined Vapm aoud Explosion (wCE)

W C E [14. I R generally occurs when sufficient amount of flammable material (gas or itqutd having high vapor pressure) gets released and mixes wth air to fom a flammable cloud such that the avenge concentration oi the material in the cloud is higher than the lower lirmt of exploston. ?he resulting explosion has high potenuai of damage as 11 occurs m an open space covering large areas. The intensiry of explos~on mainly depends on the quanur). of matenal released and the srrength of the ignition source

The expiostve power of a CVCE and L1'CE can be expressed in t e r n of blasr ndve characrerisrtcs (overpressure, overpressure-impulse. reflected pressure, duratton of shock, erc.). The overpressure IS a very tmpomnt panme- ter, its magnitude depends on the spced of flame propagation. Any obsmction tn dte flame propagation enhances the blasr effect.

&#ling liquid Expanding Vnpm Exphsion (BLEW?)

BLEVE [If] n caused by a sudden release from confinement of a Itquid at a temperature above its aunospheric pressure boiltng polnt. The sudden decrease in pressure results in explosive vaporization of a fraction of the liquid and a cloud of vapor and mist, with accompanytng blast effects. If the matenal is flammable and an ignition su rce

present, a flre ball may be formed. The broad defmnion n ~ ~ e as used by the Pwgh [I51 is used here. Accord-

to this deflnllion any liquefied vapor, flammable or :nnambie, can produce n BLE\Z A f ~ e ball/flash fire not a part of this definition, since a fue (tire ball/flash

result only if the material released is flammable d bus ignition scur s . Nevertheless, it is a historical fact

most BLEVES involve flammable liquids, and most of ,,, B ~ T releases are ignited by a surrounding f m , re- I L, lung in a fire ball/flash fire

Fires

spillage of flammable material (liquid/gas) may lead to :,re [ j 4 . I 71 which could be tnggered by any of these igni-

sources (a) an electric spark; (b) a momentar)- flame due to welding operation; (c) atmospheric fricuon: (dl lumlngof match stick In case of htghly flammable maten- 11s b e fire may he sraned even by the mld friction caused ,y atmospheric d a ~ r b a n c e s . Generdlly the fire effects are lhted to areas close to the source of flre (approx. - 200 n ndlus) Hoare\.er. industrial fires can have a greater per- ;asive effect The induslrial fires are mainly charactenzed n three groups, described below

mwl Fln

Conlinuous or in~tantaneous release of flammable liquid ,n ipn'tion results in a pool ftre. Pcal fire charncreristics ~lamiy depend on the duration of release, saturation pre5- ,ure, and the flammablr propenles of materiais

insranvanrous ignition of a vapor cloud having concen- ra:ion above lower flamrnabiliv limit gives a flash fire bur iis$n't explode It occurs only wth flammable chemical iavlng boiling pomt lower than ambtent temperature. It L? i~firrcnt from fire ball In terms of flame propagatton, dura- lun oi fire. and heat load generated. Flash fire occurs with ilRhiy flammable chemcal stored or processed at ambient 'ond~tions. Damage associated with flash or pool fue is as- cssed on the basis of the dose of heat radiation load re- 'ri\ed for a panicular/given time interval.

inrlantaneous combustion of flammable vapor cloud due 0 rrdiation exposure above matenal threshold levels or lllsilie and bhst wave interactton IS characterized as fue ' ~ l l The fire bail is generally observed for hlghly flammable :tlernlcals processed or stored under extreme condiuuns. ii.aluation of the consequences of a fue ball requires the !uantfication of fre ball temperature, fire ball duration and Ire hali slze F~re ball temperature is dependent on the heat :aPaclr). of the fuel consumed and the means of combus- . ' U f i r fire ball temperature may vary from 1350 K for lammabie gases to 5000 K for chemcal explosives

greater distance from the pomt of release than their flammable counterpans. This is mamly duc to the ease of their dispersion and the high probability of corning in dl- rect conuct with the living systems

Release Condltlons

To estimate the charactcrlstics of dispersion of gases due to an accidental reiease, h e follow~ng accidental release condiuons (with appropriate models) have been considered b, 18-20],

a) Gaseous release. b) Liquid release at aunoaphenc pressure. This cundttion

can funher be categorized as: (i) liquid with a bo~llng

FIGURE 2 Layout of the study area showing the location of CAL (Chem-Feb AUralies Limited-marked wlrh slanted lines). The location of authors research cenur is marked with an anon

TABLE 2. Storag. DbtaL of Hazardour Chemicals

I'urnber Caoacim Pressure Temperature Chemical o f ~ G k s (ma) ' (bar) ('C)

Hvdroeen 1 150 150 31 chlonne I 50 IT 10 30 Fumace oil 1 15 1 35 ~ ~ l f , , ~ ~ 2 10 2 Ambient Arnb~ent

r O ~ l c Release and Dlspwsion HCI 2 125 Arnbient Ambient N ~ O H (3346) 3 325 Ambient Ambicnt

clouds from industrial installations arise princl- N ~ O H (48%) 1 325 Ambient Ambient pal'! from the accidental release of gases, flashing lique- ~l~~~~ 2 15 Ambient Ambient

Rases or waporailon of spilled liquids. ?he tosic \'a- hgpcrchlorate por (gas) cloud is likely to be dangerous even at much

Prwcss Safety Progress (~01.16, No 3) 2 4 1 Fall, 1997 175

point above ambient temperature which 15 processed/ at a temperature below lo normal boiling pornt.

(,,) liquid with a boiling point below ambrent tempera- ,,, which is processed/stored at low temperature and junospheric pressure.

,) 7 . ~ 0 phase release (liquid under pressure), This condi- tlon can also be further categorized in mro classes: (i) liquid having normal boiling point above ambient tern- peramre which is processed/stored under high pressure and temperature above im normal boillng point, (id llquid having normal boiling point below ambrent trm- peramre which is processed/stored under high pressure and temperature above nonnal boiling pomt

!nrperston

Drsperslon is primardy governed by mo facrs:

TABLE 3. Values ol the Constonh o, b, n (U, 24 of Ea. 2 for DNerenf Chemlcak . - .-.

Chemical (Gaspapor) a b n

Acroline -993 2.05 1.0 Ammonra - 12.4 1.0 2.0 Brom~ne -124 1.0 2 0 Chlor~ne - 5 3 0 5 2 7 Hydrogen fluonde - 26.4 3 3 j 1.0 Hydrogen chloride - 21.76 2.65 1.0 Methyl bromide -19.92 5.16 1.0 Methyl-rsoqanate - 1.2 1 0 0.7 Nlvogen oude - 18.6 1 0 3 7 Phosgene -0.80 1.0 0 9 Sulfur dloxide -19.2 1.0 24

J) ilornentum of release. j ) Densiv of the gas relatlve lo air.

soon as the momentum dles down to a level where the .Ir iong as h e momentum of thr escaping gas IS sign& ambtent air movcmenrc could effect dlspersron, the denstry

:ant the densry facror does not become operattve but as fdcton uke osrr to influence the shape of the plume.

TABLE d. ReruHs of MAXCRED Simulation lor Accident SconarioCVCE and Fire Ball

Parameten Values

Explosion C K E Energy released dumg explos~on (kJ).X.BE + 0.8 Peak overpressure (kPa) 782 5 Variatton of overpressure ~n air (kPa/s) 806 8 Shock velocity of alr (rn/s). 10'9.4 Durdtion of shock wave (ms) 233 1

.\irrrr/e Resub lnrt~al velocrr) of fragment (rn/s).665 6 Kinetrc energy of iragment (kj) 133668.2 Fragment velocir) at ZOO m (rn/s) : 308.0 Penetration abilrtv at 200 m (based on emplncal models) Concrete stmcture (m): 0.071 Brick strucmre (m). 0.091 Steel strucrure (m):0.015

DamageRadir (DRi for I'uriotrs Lkgrees ofDamage due to Oi~rptriSufE DR for 1 W a cornpiere damage (m) 975 DR fur 10Pt famliy or iW>h complete damagr (m). 1315 DR for 5Wo holly or ?in% complete damage (m): 1934

Damage Radli (DRl for the lhnous Lkgrces o/Du~~logr drrc lo .llrssrle DR for 10046 damage or 1Wh fataliry (m): 1845 DR for 5G% damdge or 100% fataliy (m) 1931 DR for 1Wh fatalit) or 10% damage (rn) 200:

FIW F r 4 Ball Radius of fire hall (rn). 89 5 Duration of fire ball ( 5 ) 54.3 Energy released by fire bill (k)) ?.2E + 0- hdiation hear inrenslt) (kJ/sq m) 217 5

Damage Radii due lo Thrrmal Load Damage radir for IW/o fatal~w/damage (m): 125 Damage radii for 5Wo C;luliry/damage (m). 175 Damage radii for 1W?b rh~rd degree hums (m).229 Damdue rddil for 500h third degree of bums (ni) ,276

Process Safery Progreu (Vo1.16, No.3)

ahfn the gas escapes at high velocity as from a jet or a capacity and storing cond~tions of each is presented ~n :,,, he momenhlm ~ffec( is more .prominent and lasu Table 2. ,nger (due to higher velocity of release) than when the re- We have conducted MCAA for each of the storage ves- ase ,.elociry (venring velocity) is low. sels with MAXCRED-I1 but for the sake of brevity we hccordlng to Lees W r e h s e ~ In rhe form of lets can be present below the resulu of the study of the accidents : four wpes: (i) hlrbulent momentum jet in still air, (li) involving lhrn most hazardous chemicals (hydrogen, ,"van[ plume in sriil air; (U) plume dispersed by wind and chlorine, and furnace oil) ,,) ,et.rurbulent plume dispersed by wind. The behavlor

jers and venu IS as relevant to the intended dls- larges as to accidental discharges. The behav~or of the jpenion of such jets depends on the relative Importance d~sharge momenrum, buoyancy effects, and of wlnd

rbulence We have used turbulent jet model along wnh ume dispenlon, as in modified plume path theory [I81 to tlmate this mode of dmpersion Once the gas loses its momenrum it is ldluenced by the qsln of the gas-air m u r e relauve to air. A difference In e molecular a,e~ghr and/or in the temperature berween e p a and the ambient air creates. in pnnc~ple. such a .nslr). difference. bur this density difference will affect the ,hav~or of the cloud only if the concentrat~on of the gas IS

ff~crendy high. A large proponion of thc liquid droplets d a Ina air humidlty favor the formation of a gas cloud a\ ler than au A heavy cloud behaves differently from one of neuml nsin in several Important aspects. It spreads nor only ,ana.~nd but also upwind, it is flatter in shape and the ?<hanlsrn$ of m~xlng u,rth the air are different Compared dlspnlon of lighter-than-air or as-dense.as.aa gases.

rivv .~zc dispersion has been studied to a much lesser grer Only a handful of models are availabie lo hdndie a1-v g35 dlspersion 118. 201 We have adopted Box model 11 for hea\? gas dispersion. D~spen~on of gases (gas-air mirmrc) having dens17 ~s.1 or Icss than air under the ~ d u e n c e uf ambirnr au Jvemenu is characrerized hy neutral buoyancy disper- ln E\.en a heay gas acqulres a dlspers~on panem akm that of neutral buoyancy dspenlon when dens~ry-dnven 'hulence becomes weak (in other words density differ- ce bemeen alr-gls m u r e and air become negligible) more and more ambient air is entramed in the cloud

Jsmg amosphenc mhuience to donunare the dlspersion Xcss We havc u x d Gaussian plume and puff models

conl~nuous and instantaneous release condltlons re- -cr~vel!. to predlcr the behavior of neutral dlsprslon

I[ STUDY

ChemFab hlkalis Limited 1211 (W) a a Chlonlkall in- SIT sltuated above 11 km Nonh of Pondichew on [he '1 Corn on lndzan Peninsula. It m engaged in the manu- 'lure of chlonlkali products such as: caustic soda, liquid i o ~ e . hydrogen. and hydmchloricacid. The indusy lies the mdst of densely populated villages (Chinnakalapet, laper. Pdlalchavady, and Kanagachenykularn) Several ucauonal ~ns t i~ t ions including Pond~cherry Universmty. nd1chrn-y Engineering College, and several schools are Jaled in close pmxirmty of the lndusuy (Figure 2).

have subjected the enfire W plant to MCM. In the step of the study we have identified the set of stor-

Unlu (central tank f a d as the most vulnerable part of ' plant. The farm stores large quantities of hazardous 'Ticals under high pressure, a list of such chemicals with

Accldent Scenarios

Based on the hlstory of major accidents in the chloralkah industries [14. 22, 231 and authors' experience, following scenarios have been visualized for accidents m the vessels storing different chemicals.

An excessive high pressure development m the storage vessel of hydrogen leads to CVCE. The unbumt vapor cloud released due to explosion on ignition leads lo fire ball.

A h~gh pressure development or a leak in the vessel leads to release of flammable hydrogen vapor. whlch forms a va- Dor cloud. The vawr cloud on Instantaneous lenltlon yields LVCE. The remaining unburnt vapor cloud concen- uation below the iou~ exdosme lunir on lpnillon elves nse . . ro flash fire

FIGURE 3 ksk contours indicating the impact area for an acc~dent occurring in hydmgen storage vessel (scenario 1) due to severe nsk (A), h~gh risk (B), and moderate risk (C).

up high pressure tn the vessel leads to sudden re- , of 11qu1d chlorine as BLEW The released chermcal lrrse In the atmosphere under sl~ghlly stable atmc- mc cond~tion (KCU~S most frequently, around 65% in a .) .u release is of rwo phase and dens~ty of gas is higher , heavy gas dispersion model under coastal mete- ylcal cond~tions is operative.

concznuous release and d~spers~on of chlonne. In this also h e a q gas dispersion model w~th coastal effects

lid be used.

lace OU

n mstanuneous release of furnace oil (high dens~r) Ilq- fuel) on Ignluon leads to pool fire Onstantaneous lel)

A continuous release of furnace oil on ignition leads to pool fue (conunuous model)

These scenarios have been processed for damage esu. mation through \W[CRED-11. A brief note on h e damage- effect calculation models used for the detailed rmdy Is pie- sented below.

Dnmnee Ef&a c3lcuhtions for the AcCidcnt Sa&o8

The explosion, fires and toxrc dispersions eventually cause damage in four ways. The potential of chese effects can be expre& in terms of ptu&r/unciion 1.2, 22-24]. which relates percentage of rhc people affected in s bounded region due 10 a panicular accident event by a normal distribution function.

The probit function for 1000h lethally for hear rad~at~on 15 glven as [2].

TABLE 5 brunt of MAXCRED S~rnuld~on for Accident ScenanWVCE and Flash Flre

Parameters Values

hplosion LYCE Total energ? released by csplosion Peak overpressure \'analion of overpressure In alr Shock veioclty of alr Duration of shock nave

.\fiZZ~/e Characlmf~cs lninal veioclry of missile Kinel~c enerm associated a ~ t h mlsslle Fragment velocity at 200 m Penetrat~on abil~ry at 200 m (based on emp~rical models) Concrete stmcture Brick svucture Steel svucrure

Damage Radri (DRIfor I'anous Dc'gmes of Damage due to Chq3essure DR for 1 W e complete dam~ge DR for I@% fatally or ~CPA complete damage DR for 5090 fataliy or 25% complete damage

LMnrage Radii (DR)/or rhe Varyng Ee8ret-s ofDamage due to Alwile DR for 100% damage or 1000~ faraliy DR for 5Wt damage or 10090 farally DR for lOWb fatailry or 10% damage

Fire Flash Fire Volume of vapor cloud Effective time of fire Effective thermal load

Danzage Radii due lo 7hermal Lmd Damage ndii for 10096 fataliiy/damagc Damage ndii for 50% faraliry/damage Damage radii for 10096 rh~rd degree of bums Damape radi~ for 50% th~rd degree of bums

mere q is defined as thermal load (KW/m2); r is time ,f exposure (s); and Pr is probir value.

~ttxelir). of a toxic load is expressed in remu of prob~t funcuon as [ 21241:

m e r e a, b, n are constants; Cis concentration in ppm, t u ~rme of exposure (s). The values of the constants

for d~fferent gases are presented in Table 3

The probit equaoon for Ilkellhood of death due ro shock aa\,e (lung rupture) s glven by [231

For inlun., the equauon s

Where F' 1.5 peak overpressure (N/m2)

The probit function for fatal~cy ln human belngs or damagr to vessels is expressed as [231.

Vchere. S IS the b e u c energy of the m 1 1 c 0)

Dlscul~lo~

The results of the alculations for different accident sce. narios are presented beloup:

Table 4 represents the ourput of MAXCRED-11 for scenario I . Lethal overpresrure load, as well as missiles would be observed over a m of - 975 and - 1,845 meters, re- rpcctively The vapor cloud generated by CVCE on Ignition would lead to a fire baU which would cause scorching Such lethal heat load would be observed over an area of 125 meters radius. The s h ~ k waves and heat load would also tn~ger secondary accidents by seriously damaging other vessels. It s evidenced from Table 4, ttus scenario has very high dama8.e potential (shock waves, mssile and heat load) Over an area of - 3OO meters radius and as other stonge units (chlorine. HCI, NaOH) are situated within this range. llcnce, there is a high probability of second gr high order

(domino effect) accidents Moreover, an area of around - 1OOO meters radius from the site of accident, which con- tains densely populated pockets, comes under the nnge of lethality

Figure 3 represents nsk contours over the slre of accident. The risk contours have been plotted on the bas~s of frequency of occurrence of panicular event (taken from literature [2. 24-263) and damage potential calculated by MAXCRED-11. The 'severest risk contour' (nsk factor > 1. lo-') has been observed over an area of around - 1050 meters radius, while 'high risk contour' (risk factor > 1 lo- ' ) and 'moderate risk contour' (risk factor > 1 r have been observed over areas of - 1,750 and - 2.200 meters radii respectively.

The results for scenano 2 (UVCE followed by flash fire) are presented in Table 5. In this case the damage potential of the shock waves and missiles is lower compared to the scenario 1. The unbumt m m r e of vapor-air havlng con- cenvation lower than its explosive limits would lead to flash fire. Lethal overpressure load would be generated up to a ndial disrance of - 600 meters, while missrles would cause damage up to - 865 meters (wlthout considering the probab~iity of impact of missile) all around the epicenter of the accident. The lethal heat load would encompass an area of - 45 meters radius. This scenario is less likely to cause secondary accidents as its zone of d u e n c e is smaller. Moreover, the probabil~ty of occurrence of this scenario is 2 rimes lower than that of scenario 1. The risk contours for scenario 2 are shown in Figure 4. The severest nsk contour s emended up to - 300 meters radius while hgh and moderate risk contours emend up to - 450 and - 625 m e rers radii respect~vely.

FIGURE 4 Risk contours indicating the impact area for an accident occurring in hydrogen storage vessel (scenario 2) due to severe risk (A), high risk (B), and moderate risk (C).

Procur Safety ~mgru5 (Vo1.16, No.3) 245 Fall. 1997 179

TABLE 6. Resub cl MAXCRED Simulation for Accldent Scenario-BLEVE and Toxic Relaam

Parameten Values - Eplosron BLEVE

Total energy relased (kJ): 3.2E + 06 Peak o v e r p m r e (kPa): 150.5 Variation of overpressure in air (@ah): 99.0 Shock velocity of air (m/s): 4i2.1 Duration of shock wave (ms): 33.0

Mmik Cbamctoisrics Initial velocity of fragment (m/s). 287.6 Kinetic energy of fragment (kj): 975446 0 Fragment velocity at 200 m (m/s):95.l Penetration ability at 200 m (based on emp~ncal models) Concrete stmcrure (m) : 0.006 Brick swcture (m). 0.007 Steel s t ruc tu~ (m): 0.001

Damage Radu (DRl for Vanow. Degmes o/Damage due to Chmpesure DR for 100% complete damage (m): 323 DR for 1000/0 fatahty or 5 W complete damage (m). 494 DR for 50% fatality or 25% complete damage (m) 731

Damage Radir (Dm for the rirriour D e g w of Damage due to .!fssile DR for lo09a danuge or 108/0 fatality (m) : 498 DR for 50% damage or 100% fatality (m). 683 DR for 103% fatality or 1136 damage (m) ,860

T m c Dtrposron Heav gas dispersion charanenstics

Box Imfantanwur Afcdel Concenrration at study p ~ n t (kg/cu.m): 4.2E-07 Concentnt~on at cloud axe (kg/cu.m). 9.7E-03 Value of source height (m).75

Damage Radiijor Vartous Degw of Damage Damage radii for 100% lerhaiit). (m) 2529 Damage radii for 50% lethality lm),3?52 Damage radu for 10% lethabty (m). 5825

Puff Cbaracrensrics Puff concentnuon at centre of cloud (kg/cu.m). 5.4E-04 Concenuat~on at cloud edges (kg/cu.m). 5 4E-04 Distance along downwind (m), 200.0 Dosage at srudy point (k~/cu.m): 0.0356

4 HLEcT u per scenario 3 would lead to overpressure, The output of WCRED-I1 for the continuous reieasc of dnd instantaneous toxic release. As the chemical is non- toxc gas chlorine a presented m Table 7 Lcthal toxic load flammabie no heat load would be obsen.ed hut toxc d~s. would be observed over an area of - 3,252 meters radius. Prrsiun would occur. The output of MAXCRED-11 for dus which covers four villages (Chlnnakalapet. Kalapet. Pd- 'cenano, presented in Table 6, shows that lethal overprer ialchavady. and ~ana~acherrykulam) w~th a total popula- SUrr iuad would be observed over an area of - 323 meters tion of around 20.000. The frequency of occurrence rad~us. while lethal toxic load (as per shon term exposure (4.5 * 10-i/).r) of thls scenano IS 8 tlmes fugher than that of Ilmll. SEL) would be observed over an area of - 2,529 scenano 2. The .severes: nsk contour' (F18ure 6 ) extends mrlers radius. The risk contours for this scenario are pre- up to - 3.500 meters rad~us, whlle 'fugh' and 'moderate' <enled in Figure 5; the 'severest risk contour, enends up to nsk contours mend! up to areas of - 4.750 and - 6,530 * 1.250 meters (due to toxic load) while 'high' and 'mod- meters radii. enle' risk contours extend up to - 1,710 and - 2,550 me- The output hom MAXCRED-I1 for insununeous pool fire

rddii respect~vely. The severest risk contour envelopes of furnace oil (xenano 5). given in Table 8, reveals that [he densely populated villages (Chinnakalapetand Kalapet) lethal heat load would be observed over an area of around ' 5 well as residential academic institutions (Pondicherry 188 meter ndius. Thls area encompases parts of the Lnlvers~ty campus and schools). nearby acadermc insutution (Pondcherry University) and

!@ Pall. 1W7 246 -. C . L . -. &I ,L kt.. a\

FIGURE 6 Risk contours mdrcaung, the lmpact area for an FIGURE 5 Risk contours indrcating the impact area for an accident occurring in chlorine sioraRe vessel

acc~dent occurrine m chlorine storage vessel (scenario 4) due to severe nsk (A). htah risk (scenario 3) due to severe nsk (A). hzgh wk (R), and moderate nsk (C ) . (8). and moderate risk (C)

the village Chlnnaltllapet The .severest r~sk contour' (Rg- ure -) exrends up to 375 meters radir, while 'h~gh' and mtderate nsk contours envelop areas wlth radu of - 300

znd - 475 meten. respectlvel) This scenarlo also be- spraks of the threats of nearby storage vessels by secondary accidents.

The results of scenario 6 are presented in Table 9 In thrs sirnano, severest nsk contour would be confined to - 130 nlrrers radrus and the h~gh and moderate nsk contours

would emend up to circles of - 250 and - 475 meters radii respecnvely (F~gure 8)

In surnmarj scenarlo 1 has the h~ghest nsk factor (Table 10) and represents the worst duaster. It has the largest area-of-lethal-impan (overpressure zone of - 975 meters radius. m s i l e zone of 1845 meten radius, and hear load zone of - 125 meters radius). Densely populated areas (four villages) and sensitrve locattons (two acadenuc instr. rules) would come under rts lethal impact range The next mosr undesirable likely event is scenario 4.11 has the largest

TABLE 7. Rbwb of MAXCRED Simulobn for Accident Scenario-Toxic Reloam Parameters Values

TMC DLSpemon Heavy gas dhpers~on characterisucs

Bar Continuous Model Value of wind speed (m/s) : 3 5 Ground level X concenmt~on (kg@. m) 0 0023

Dotaage Radti for Vanous Degrre of Damage Damage radh for 1000/0 le~haliry (m). 3252 Damage radii for 50% lethal~ty (m) :4615 Damage radii for 1W lethalrty (m), 10404

Pllrrne Cbamaerirricr Ground level conc of plume (kg/cu m). 0.0027 Ground level conc of plume on axis (kg/cu m),0.0044 Ground level conc of plume at border (kg/cu m) : 0 0004 Tidrh of plume (m). 112.0 Cloud concentrarion (kg/cu m):0.0042 hlaximum ground level concentration (kg/cu m). 0.0505

TABLE 0. Resub ol MAXCRED Simulation for Accident Scenario-Pool Fire

Parameters Values

FIre: POol Fire instantaneous modd

Radius of pool fire Burning area Burning rate Heat flux

Damage Radii Due to 3h~mal Load Damage radii for lOC% fatalicy/damage (m): 188 Damage radii for 5046 fauiiry/d.amage (m). 236 Damage radii for 1 W a third degree of bums (m)- 273 Damage n d i ~ for 5046 hid degee of bums (m): 350

FIGURE 7 Rrsk contoun uidlcat~ng the unpacr area for an acc,dcnt wcumg od storage \essel FIGURE 8 ILsk contours m d ~ c a t ~ n ~ the unpact area for an

(scenano 5 ) due to severe nsk (A), h~gh nsk acc~dent wcurnng m furnace od storage vessel

(B), and mcderare risk (C) (scenano 6) due to setere nsk (.4), hlgh nsk (B), and moderare nsk (C)

damage area (due to likely lethal touc load) of around - 3.252 meters radius, but there IS low probability of other Ivpes of accidents (donuno effect) In this scenario, and the Overall risk facror is lesser than scenano 1 Funhermore, scenario 1 would be the most likely to cause cascading effects as missile, shock wave, and ndlation impacts would he occurring simulraneousiy. Several other vessels storing flammable, toxic, and cormsive materials are situated ulthin the striking dlsiance of the area of Influence of this hcenario.

This paper demonswares the appllcabilit). of sofware package hlAXCRED-I1 for performing quantlurive b l W . The package can be used to generate different acc~dent scenarios and simulate likely accidents The outpur of the packagc can be used in developing strategies for hazard prevention and mltigat~on.

A chemlcal Industry situated in the midst of populalrd villages and academic instlrutions has been studied ~n de-

7

TABLE 9, Rewb ol MAXCRED Simulation foc Accident Seenorlo-Pod Fire - Parameters \'slues

F I ~ . Pool FIR Continuous model

Radius of pool fire Burning area Burning nte Hear flux

Damage Radii Due to 7hennaI Laad Damage radii for 1004b fatality/&mage Damage radh for 5 M faralrty/&mage Damage ndli for I W h thud degree of bums Damage radii for 5(Ph third degree of bums

TABLE 10. Rkk Focton toc DMerent Scenarios Chemical Process Industries," XiE h'atronal Confer. ence on Environmental Genotouns, p. 54 (February

Manmum Damage Ind~v~dual 1996). Drstance. MDD Probabil~ty' Risk Factor 4 Mnllikarjunan,M. M.. and K. V. Raghvm, "Risk .45-

Scemno (meters) (vear-') at MDD sessment Technioues and Then Aoolication.' Proceed-

1 975 4.5e-05 3.214e-04 rngr o/ hhtionai ConJmce o/'~a/ep System. New 2 600 1 0e-03 1 562e-06 Delh~ (1989) 3 2529 1 k.05 7 42de-0j 5 Greenbag, H. R, and J. J. Crammer, Rsk Asses

4 3252 1 k -05 1 l2re-04 ma,! and Management for Chemical Roc= l n d m IM c no n z 2 A"?.. nc tnes \an hosvand Remhold. \ ea York (1991) JW > "7-"2 2 77,=-u>

6 250 Oe.03 6. Khan, P. I., and S. A AbbPri, "hsk Analysis A Sys- tematic Method of Hazard Assessmen: and Control." Jr, of lndwtnal Pollution Control. ll(2). p. 89 (1995)

\ > r u n arc ,drptrd frorn LC., 1199bl cununl (19911~t ' Van .5Civerr G. R, 'Quantitative hsk Anaiysls In the .a): rmdtr>oru Chemical Process Industries." Rellabilrh Enflneenn~

311 Of Lhe SV[ most credible renanos generated urlth \LUCRED-11 :he scenano whlch forcsees confined vapor iloud expiosron folioa,rd by fie ball ponenu %,orst duas. Per (Table 10). Its effecu could permeate wide areas at a high probability It also has the greatest likelrhood of caus- Ing second and hrgher order accrdene leadrng to chain re- 2c:lons or 'domino' effects Another scenarro (number 4) which focuses on the release of chlonne has the highest I~hrlv damage radrus of all the scrnarios even though its or.erdl1 nsk factor is lesser than of scenario 1 (Table 10) due In lesser probabrlrrv of occurrence Other three scenanos. ulilch all are verv'much wrthin the realms of cossibilrw, a150 have significantly hlgh rrsk potenual

.

All-in-all the srudv reveais Lhdt the universln. cdmous and [he \Illages (panlc;larly Chrnnakalapet and Kalapei) are at 'En h~gh nsk due to the presence of C4Ls ctonge uniu

1 K l e q T. A, ,,ucha~ Went Krong." Gulf Publication. London (1986)

2 La, P. P., Loss Pleventron m the Process industries, Yols 1-3. Bunerwonhs, London (1996).

3 KbuS F. I., and S. A Abbasl. "Disaster Forecasting and Consequence Analysis (Risk Assessment) rn

P r * e ~ Safely Progress (Vo1.16, iio.3)

. - - B Svstem Safep. 29, p 55 (1990)

6 Arendt, J. S., "Management of Quanutative hsk As- sessment in the Chemical Pmess Induslr,.," Phnt /Op matron Proglrs, 9(4). p. 262 (19W).

9 WHAZAN, Techmca Luruted. London (1990). 10 RISKWIT, \IT-The Techmcal Research Centre of

Finland. Tempere (1094) 11 EFFECTS, TKO-Pnns Maunu Laboratory, Rilswiik

(1992). 12 Khan, F. I., and S. A Abbasi, "hWCRED--A soft-

ware Package for Quantitative bsk Analvsu." Repon KO CPCE/R B D 5/94, Pondicherry Unirersrfv. Pondrcherry (1994). EnmmtrnrentaISoftware m press

13 Khan, F. I., and S. A. AbbiWL ".httTCR.ED.II .4n Ad- vanced Yersron of hKXCRED." Repun No CPCE/R&D 14/96, Pondlcherry Univenry. Pondrchem (19%)

14 P i t a n , C. M., "Consequences of Accidental Release of Hazardous Matenals," J of Loss Prpwnlron Pmcess lndwtnes, 3, p 136 (1990)

15. Rugh, R W., "Quant~tatr\.e Evaluarron of 'BELT' Hazards." FIR Pmt Eng~ . . 3(1), p 3 (1991).

16 Sdelly, N. F.. and W. G. High, "The Blast Effects of Explos~ons." In Pmceedrngs of Fy?h Inlernarional Sjrnposlum on Los h ~ t ~ t i o t i and Safeet)~PIOmotion. Cannes (1986).

17. Roberts, A. F., "Thermal Radiation Hazards from Re. leases of LPG from Pressurized Storage," F14 Sa/ep Jouml , 4. p 197 (1982).

18 Khan. F. I., and S. A. Abbvl, "Modeling of Heavy

249 Fall. 1997 183

Dispersion Using Modified Plume Path Theory" ~r~ospber i c Envlronmenf, in press (1997).

19 Van Uldea A P., "A Kew Bulk Model for Dense Gas ~ s ~ e r s i o n : Two Dimensional Spread m Still Air," 1~7.W Symposium on Atmospheric Dispersion of Heary Gas and Small Panicles, Delh (1983).

20 Wealley, C. J., Yld D. M. Webber, "Aspecu of the Disprslon of Denser-than-&r Vapors Relevanr to Gas Cloud Explosions,'' SRD Repon ST/W7/80/UK/H. Safer). and Reliability Directorate, Warringon (19%).

21. chemFab hlLPllr Ud, "Risk Analysis for the Chloral- ka11 Plant," Cell for Industrial Safely and Rlsk Analy sir, p. 5 (1993).

22 Khan, F. L, and S. A Abbui, "Accrdent Simulation in Chemical Process Industries using Software hW(-

CRED." l~?dran/oumal of Chemical Technolog): 3 , p' 339 (1996).

23. Pasman, H. J., et d, "Major Hazards in the Process Indusuies. Achievements and Challenges in Loss Pre- vention." p~unral of Hazardour hialpriak. 30, p. 1 (1992).

24 ContlaL S., m al, "Benchmark Exercise on hlajor Hazard .Analysis," Joint Research Centre, insutute of System Engineering & Informatrcs, Ispra (1991).

25. Reliability Dkctorate, "Failure Frequency of Hard- ware Cornponenw," Repon KO. 87-298lR.Z?/hTT, Re- liabiliry Directorate (1992).

26 European Community, "Council Duecuon on the Major Accident Hazards of CeKaln Industrial Acn\.nlcs," Repon So. 82/50/501 EEC, London (1992).

Process Safely Progress (Vol16, h'o 3)

part iv quantitative methods of risk assessment...

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