Integrated inherent safety index (I2SI): A tool for inherent safety evaluation

Integrated Inherent Safety Index(I2SI): A Tool for Inherent SafetyEvaluationFaisal I. Khan and Paul R. AmyotteFaculty of Engineering & Applied Science, Memorial University of Newfoundland, St. John’s, Newfoundland A1B 3X5, CanadaDepartment of Chemical Engineering, Dalhousie University, Halifax, Nova Scotia B3J 2X4, Canada; [email protected] (forcorrespondence)

Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/prs.10015

Inherent safety is a proactive approach for loss pre-vention and risk management. Considering the life-time costs of a process and its operation, an inherentsafety approach can lead to a cost-optimal option.Inherent safety may be achieved at any stage of processdesign; however, its application at the early stages ofprocess design yields the best results. Despite being anattractive and cost-effective approach, the inherentsafety methodology is not widely used. Many reasonshave been attributed to this lack of widespread use; thenonavailability of systematic tools for the application ofinherent safety principles is perhaps the most importantreason.

This paper presents a conceptual framework of anintegrated inherent safety index (I2SI). It is called anintegrated index because the procedure, when fullydeveloped, is intended to consider the life cycle of theprocess with economic evaluation and hazard po-tential identification for each option. The I2SI iscomposed of subindices which account for hazardpotential, inherent safety potential, and add-on con-trol requirements. An application of the I2SI is alsodiscussed. © 2004 American Institute of ChemicalEngineers Process Saf Prog 23: 136–148, 2004

Keywords: inherent safety, hazard indices, inher-ently safer design, loss prevention

INTRODUCTIONProcess safety has long been one of the main con-

cerns in industry and is currently recognized as an issueof vital importance. The main reason for the growingimportance of this problem is the constantly increasingscale of industrial operations. In the process industriesthe increase in scale can be especially dangerous be-

cause, unlike manufacturing industries where increasesin output are achieved by replication of productionunits, process plants achieve increased output by “scal-ing up.” Therefore, as process plants become larger,they have greater hazard potential. (Current effortsaimed at scale reduction through process intensifica-tion are therefore particularly noteworthy.) The num-ber of accidents reported in the process industries isconstantly growing, and the human and financial risk isrising dramatically (risk being defined as a measure ofhuman injury, environmental damage, or economicloss in terms of both the incident likelihood and themagnitude of the loss or injury) [1].

A traditional approach to managing process riskinvolves providing layers of protection between thehazardous agent and the people, environment, orproperty that is potentially impacted. The layers ofprotection are intended to reduce risk either by reduc-ing the likelihood of potential accidents or by reducingthe magnitude of the impact. The risk can be reducedto very low levels by providing a sufficient number oflayers of protection and by making each layer highlyreliable. Risk reduction strategies, whether aimed atreducing frequency or mitigating the consequences ofpotential accidents, can be separated into four catego-ries: (i) inherent, (ii) passive (engineered), (iii) active(engineered), and (iv) procedural. Among these fourclassifications, the first one—inherent safety—is of cur-rent interest and is the subject of discussion in thispaper.

Inherent safety is a proactive approach for risk man-agement. Many studies in the past have confirmed thatinherent safety is a cost-optimal option considering thelifetime costs of a process and its operation [2–6].Inherent safety can be incorporated at any stage ofdesign and operation; however, its application at the© 2004 American Institute of Chemical Engineers

136 June 2004 Process Safety Process (Vol.23, No.2)

earliest possible stages of process design (such as pro-cess selection and conceptual design) yields the bestresults. To fully exploit the benefits of inherent safety,the designer/process engineer requires an inherentsafety evaluation tool applicable to his or her situation.In a recent AIChE publication, Making EHS an IntegralPart of Process Design [7], inherent safety has beenidentified as having a primary focus on process safetywith a secondary focus on health and directly linkedbenefits to the environment. Inherent safety thus playsa key role in integrating environment, health, andsafety considerations throughout process design.

There have been several efforts made by differentagencies to develop inherent safety evaluation tools.Examples include the INSET tool kit sponsored by theEuropean Community, the overall inherent safety indexprototype proposed by Edwards and co-workers atLoughborough University, UK, the inherent safety in-dex proposed by Heikkila and co-workers at VVT,Finland, and the fuzzy-based inherent safety index pro-posed by Gentile and co-workers at Texas A&M Uni-versity (details are provided later). In addition to thesetools, there are several programs or methods that havebeen adopted within different corporations to evaluatethe potential for inherent safety considerations in de-sign and operation. Examples include the Rohm andHaas major accident prevention program, ExxonChemical’s inherent safety review process at variouspoints in the process life cycle, Union Carbide’s index-based system for inherent safety reviews, and the soft-ware tool developed by Sandoz to assist chemists andengineers in identifying hazards and inherently saferprocess options [6, 8]. Recently, Amyotte et al. [9] pre-sented a study on the application of inherent safety indust explosion management. The study emphasizedwith illustrative practical examples the use of inherentsafety principles at the outset of process design orequipment selection to prevent or mitigate dust explo-sions.

There remains, however, the need for a systematicand easy-to-use tool that may answer most of the safetydesign questions. Khan et al. [10] presented an analysisof available tools for risk assessment and their effec-

tiveness in inherent safety evaluation. In another effort,Khan and Amyotte [6] discussed in detail the currentstatus of inherent safety practice in North America ingeneral and Canada in particular. Gupta and Edwards[11] presented an overall picture of inherent safety andfuture directions needed for advancement of its use.They identified the nonavailability of effective tools forinherent safety evaluation as one of the major limitingfactors restricting the application of inherent safety.The current authors attempted to bridge this gapthrough a new approach to inherent safety evaluation.This paper presents an integrated inherent safety index(I2SI) for inherent safety evaluation intended ultimatelyto be applicable throughout the life cycle of processdesign. The work at this point is largely conceptual,with the basic framework and desired features of theindex having been identified. Additional work is re-quired in several areas, particularly the aforementionedlife cycle and economic considerations.

AVAILABLE TOOLS FOR INHERENT SAFETY EVALUATION

Inherent Safety Principles/GuidewordsBefore reviewing available tools for the evaluation

of inherent safety (from [6]), a brief overview of theimportant principles of inherent safety is given. Theterminology of inherent safety varies somewhatthroughout the process safety community. Table 1,taken from Khan and Amyotte [6], presents commonlyused inherent safety principles or guidewords, the firstsix of which (along with the fundamental concept ofhazard elimination) are typically considered the mostgeneral and therefore widely applicable. The remain-ing principles in Table 1 are essentially subcategories(for example, avoiding knock-on effects may beviewed as a special case of limitation of effects; simi-larly, making incorrect assembly impossible may alsobe viewed as a form of simplification). With this ap-proach, the primary concepts displayed in Table 1 maybe summarized by four basic principles [12]: minimize(intensification), substitute (substitution), moderate (at-tenuation/limitation of effects), and simplify (simplifi-cation/error tolerance). In the current work, we have

Table 1. Inherent safety principles (from [8]).

Inherent Safety Principle Definition

Intensification Reduction in the quantity of hazardous materialsSubstitution Use of safer materialsAttenuation Operation at comparably safer operating conditions such as room temperature

and pressure and liquid phaseSimplification Avoidance of complexities such as multiproduct or multiunit operations or

congested pipe or unit settingsLimitation of effects Changing the design and operation for less severe effects (e.g., unit segregation)Error tolerance Making equipment robust, processes that can bear upsets, reactors able to

withstand unwanted reactions, etc.Avoiding knock-on effects Ample layout spacing, fail-safe shut down, open constructionMaking incorrect assembly

impossibleUnique valve or piping systems to reduce human error

Making status clear Avoidance of complicated equipment and information overloadingEase of control Less hands-on control

Process Safety Progress (Vol.23, No.2) June 2004 137

chosen to use the guidewords minimization, substitu-tion, attenuation, limitation of effects (or limiting of),and simplification.

Accident Consequence AnalysisAnalysis of the potential consequences of an acci-

dent is helpful in understanding the relative inherentsafety of process alternatives. The benefits of, for ex-ample, the principles of minimization and moderationcan be demonstrated by accident consequence analy-sis. One can point out in practical terms the impacts ofa release of different inventories of hazardous materialor a process incident occurring under various condi-tions of temperature and pressure (i.e., the factorslargely responsible for determining the system poten-tial energy). In this vein, the Rohm and Haas MajorAccident Prevention Program encourages inherentlysafer process development by requiring accident con-sequence analysis [13]. In North America, DennisHendershot of Rohm and Haas is one of the leadingpromoters of inherent safety [2, 3, 12, 14]. Hendershotand other process safety researchers and practitionersin North America have championed the cause of inher-ent safety.

Hazard IndicesSeveral hazard indices have been developed as tools

for chemical process loss prevention and risk manage-ment. These indices measure one or more aspects ofinherent safety, usually implicitly (e.g., fire, explosion,and/or toxic hazards), and it is generally necessary touse several indices to obtain a full understanding of theoverall process characteristics. These indices can becalculated fairly quickly for a number of process op-tions and design variations. Each gives a dimensionlessindex value that is defined relatively and may be com-bined with a decision analysis tool for setting priorities.Available indices include the following:

• Dow Fire and Explosion Index [15]: fire and ex-plosion hazards;

• Dow Chemical Exposure Index [16]: acute chem-ical exposure hazards;

• Safety Weighted Hazard Index (SWeHI) [17]: fire,explosion, and toxic release hazards. Provision isalso made in the calculation procedure for evalu-ation of safety control measures;

• Environmental Risk Management Screening Tool[18]: environmental hazards, including air, groundwater, surface water (human), surface water (eco-logical), and waste water;

• Transportation Risk Screening Model [19]: risk topeople and the environment from chemical trans-portation operations;

• Hazardous Waste Index [20]: flammability, reac-tivity, toxicity, and corrosivity hazards of wastematerials;

• Mond Index [21]: fire, explosion, and toxicity haz-ards;

• Toxicity Hazard Index [22].

In addition to using quantitative hazard indices,many of the more qualitative hazard analysis tools such

as Safety Review, Checklist, What-If?, What-If?/Check-list, and HAZOP [23] can be adapted to incorporate theprinciples of inherent safety. The general aims of thesequalitative tools are to identify hazards, examine theadequacy of existing safeguards, and, if needed, sug-gest additional safeguards on a risk-determined basis.Inherent safety considerations can play a key role infulfilling these goals; Bollinger et al. [24] and CWRT/CCPS [7] provide further thoughts on this point.

The INSIDE Project ToolkitThe INSIDE (INherent SHE In DEsign) project was a

European government/industry project sponsored bythe Commission of the European Community to en-courage and promote inherently safer chemical pro-cesses and plants. The project has developed a set oftools, the INSET Toolkit, to identify inherently saferdesign options throughout the life of a process and toevaluate the options.

The tools of particular interest with respect to mea-suring the inherent safety of chemical processes are theISHE performance indices. The inherent safety perfor-mance indices intentionally involve relatively simplecomputations. This enables a large number of processoptions to be rapidly evaluated. The various inherentsafety, health, and environmental aspects of a processare evaluated using separate indices; no attempt ismade to combine the indices into a single overall mea-sure.

The INSET Toolkit instead recommends a multiat-tribute decision analysis technique to evaluate theoverall inherent safety aspects of the various processoptions. The INSET Toolkit is particularly interesting asan inherent safety measurement tool for two reasons[25, 26]. First, it represents the consensus and combinedexpertise of a number of companies and organizations.Additionally, it is intended to consider safety, health,and environmental factors in one set of tools, an ap-proach consistent with the recommendations of CWRT/CCPS [7].

Overall Inherent Safety IndexEdwards and Lawrence [27] and Edwards et al. [28],

of Loughborough University, have developed an over-all inherent safety index to characterize the inherentsafety potential of different process routes for the sameproduct. The index is therefore designed to enableselection of inherently safer process routes in the pro-duction of a particular product. It considers sevenchemical- and process-related factors characterizing in-herent safety, namely: (i) inventory, (ii) flammability,(iii) explosiveness, (iv) toxicity, (v) temperature, (vi)pressure, and (vii) process yield.

As described by Edwards and Lawrence [27], thecomputation procedure starts by dividing the completeprocess into various substeps. Subsequently, the chem-ical score is calculated for inventory, flammability, ex-plosiveness, and toxicity attributes of each chemicalused in the substeps. From all the chemical scores, thehighest is considered the representative value of chem-ical score for this substep. The process score is com-puted for pressure, temperature, and yield attributes ina later substep. Chemical and process scores are com-


bined to give a single value for each substep. Thesevalues are subsequently combined to produce a singleindex for a given process route. Finally, a ranking ofprocess routes is done, implying that the higher thevalue of a process route, the less inherently safe is thatroute.

This work is significant for a number of reasons. It isone of the first attempts to develop an indexing meth-odology to incorporate inherent safety in the designprocedure. Additionally, the focus is on selection of thechemical synthesis route which, as previously dis-cussed, is the life cycle stage when inherent safety canoffer its greatest benefits. Finally, as a prototype index,the methodology employed can serve as a guide tofurther development of such indices.

Inherent Safety IndexHeikkila et al. [29], of the Helsinki University of

Technology, proposed a new index for inherent safetyevaluation. The index is relatively simple and is de-signed to consider a range of factors affecting the in-herent safety of a process. These factors are groupedinto two main categories: chemical and process inher-ent safety. The chemical inherent safety index de-scribes the effect of the choice of raw materials andother chemicals on the inherent safety of the processthrough consideration of heats of reaction, flammabil-ity, explosiveness, toxicity, corrosiveness, and incom-patibility of chemicals. The process inherent safety in-dex describes the effect of the type of processequipment and process conditions on inherent safety.The parameters considered here are inventory of chem-icals, process temperature and pressure, the type ofprocessing equipment, and the structure of the process.The separate chemical and process indices are summedto yield a total inherent safety index.

It appears that this work attempts to build on theprototype index of inherent safety of Edwards andLawrence [27] by extending beyond the choice of rawmaterials and the sequence of reaction steps. In a seriesof subsequent publications the group at the VTT andthe Helsinki University of Technology have describedtheir use of the following:

• Case-based reasoning (using a design case data-base) to determine a process structure subindex[30];

• A genetic algorithm for optimization of a processsynthesis model [31];

• Accident statistics and layout data to determine anequipment safety subindex [31].

Heikkila [32] provides a summary of the progress tothat date.

Fuzzy Based Inherent Safety IndexGentile et al. [33], of the Mary Kay O’Connor Process

Safety Center at Texas A&M University, have attemptedto improve on some of the subjective factors in theinherent safety index of Heikkila et al. [29] by usingfuzzy set theory. The modifications were essentiallyaimed at improving the sensitivity (either excessive or

insufficient) in the ranges selected for each of thevarious index parameters.

The fuzzy logic system was used to calculate a pro-posed inherent safety index based on if–then rules thatdescribe knowledge related to inherent safety. In thisapproach each factor is described by a linguistic vari-able whose range of interest is divided into fuzzy sets.For each set, a membership function is defined whichhas a specific shape describing the physical behavior ofthe set [33].

As noted by Gentile et al. [33], the fuzzy-based ap-proach eliminates the problems presented by a tradi-tional interval approach for parameter ranges and isseen as a first step toward a more reliable and simplemethodology for inherent safety evaluation. Furtherwork identified by the authors [33] includes: parame-terization of inherent safety, unified membership func-tions, development of if–then rules, and reliable androbust methods of quantification.

Expert System for Inherently Safer ProcessesPalaniappan and co-workers [34, 35] developed a

systematic methodology and automated tools for inher-ently safer route selection and flowsheet development.Palaniappan et al. [34] describe the knowledge requiredfor inherent safety analysis during route selection andhow it can be formalized in the form of expert rules.The authors illustrate a new inherent safety index forranking of process routes and a graphical method foranalyzing reaction networks. In the second part of theirwork [35], they proposed a methodology for imple-menting inherent safety during flowsheeting. Themethodology is based on ranking of flow sheets usingindices that are based on ranking patterns proposed byHeikkila et al. [29]. These methodologies are imple-mented in automated tools that provide significant helpin evaluating the inherent safety of alternate processes(route selection and/or flowsheeting).

Despite the significant indexing work conductedworldwide, there remains the need to develop a sys-tematic, rigorous, and reliable method of inherentsafety quantification, which builds upon use of themain inherent safety principles. Objective rankingmethods are of critical importance. The method shouldbe aimed to help designers and engineers in inherentlysafer process design and related decision making.

INTEGRATED INHERENT SAFETY INDEX

Conceptual FrameworkThe conceptual framework of the I2SI is shown in

Figure 1. As illustrated, the I2SI is composed of twomain subindices: a hazard index (HI) and an inherentsafety potential (ISPI) index. The hazard index is in-tended to be a measure of the damage potential of theprocess after taking into account the process and haz-ard control measures.

The inherent safety potential index, on the otherhand, accounts for the applicability of the inherentsafety principles (or guidewords) to the process. TheHI is calculated for the base process (any one processoption or process setting will be considered the baseoperation or setting) and remains the same for all other


possible options. The HI and ISPI for each option arecombined to yield a value of the integrated index asshown in Eq. (1).

I2SI �ISPI

HI(1)

Both the ISPI and the HI range from 1 to 200; therange has been fixed considering the maximum andminimum likely values of the impacting parameters.This range gives enough flexibility to quantify the in-dex. As evident, an I2SI value greater than unity de-notes a positive response of the inherent safety guide-word application (inherently safer option). The higherthe value of the I2SI, the more pronounced the inherentsafety impact.

Indexing ProcedureThe conceptual framework and sequence of steps

involved in the computations of the HI and ISPI areshown in Figures 2 and 3, respectively. A detaileddescription of each is presented in the following sec-tions.

Hazard IndexAs shown in Figure 2, the HI is composed of two

subindices: a damage index (DI) and a process andhazard control index (PHCI). The damage index is afunction of four important parameters, namely fire and

explosion, acute toxicity, chronic toxicity, and environ-mental damage. The DI is computed for each of theseparameters using the curves in Figures 4a–c and 5a–c,which effectively convert damage radii to damage in-dices by scaling up to 100. Figure 4a–c was developedfor the scenarios of fire and explosion, and toxic re-lease and dispersion for acute as well chronic cases. Inorder to get DI, the damage radii need to be known.The damage radii for fire and explosion and/or toxicrelease may be calculated using the SWeHI approach.The SWeHI approach involves three main steps: (i)quantification of core factors (energy factors in the caseof fire and explosion hazards and “G factor” in the caseof toxic hazards) according to process unit type, i.e.,reaction, storage, (ii) assignment of penalties consider-ing external forcing factors such as operating condi-tions and environmental parameters, and (iii) estima-tion of damage radii using core factors and penalties.Full details of SWeHI are available in the paper byKhan et al. [17].

The DI calculation for environmental damage (Fig-ure 5a–c) is adapted from the previous work of Khan etal. [36]. Here, the environmental damage index is char-acterized by considering the impairment impact for air,water, and soil. Monographs were developed for eachenvironment (air, water, and soil) as shown in Figure5a–c. It may be seen in Figure 5a–c that for eachenvironment, there are three lines classified as A, B,and C. This classification was done to account for thecharacteristics of the released chemicals according tothe NFPA rankings for toxic/corrosive and reactive

Figure 1. Conceptual framework of the I2SI.

Figure 2. Framework for hazard index (HI)computation.


chemicals. Chemicals with an NFPA ranking less than 2were classified as A, 2 and 3 as B, and 4 as C. Thesethree DI’s—one for each environment—are combinedthrough Eq. (2) to give the DI for environmental dam-age.

DIen � Min�100, ��DIair�3 � �DIwater�

3 � �DIsoil�3�1/3� (2)

The damage indices computed for each of theseeffects (fire and explosion (fe), acute (ac) toxicity,chronic (ch) toxicity, and environmental (en) impair-ment) are combined as per Eq. (3) to give the final DI.

DI � ��DIfe�2 � �DIac�

2 � �DIch�2 � �DIen�

2�1/ 2 (3)

The other subindex, PHCI, is calculated for variousadd-on process and hazard control measures that arerequired or are present in the system. The conceptualframework of the PHCI calculation is illustrated in Fig-ure 6. This index is quantified subjectively based on ascale that is mutually agreed upon among processsafety experts. The index ranges from 1 to 10 for anycontrol arrangement and is quantified based on thenecessity of this control arrangement in maintainingsafe operation. This necessity is divided into ninegroups as listed in Table 2. For any given controlsystem, based on necessity (from Table 2), an index(PHCI) may be derived from Figure 7. This process isrepeated for all possible control systems (currently 10control systems have been identified as shown in Fig-

ure 6). The PHCIs of these 10 different control systemsare finally combined through Eq. (4) to give the finalPHCI,

PHCI � �PHCIp � PHCIt � PHCIf � PHCIl � PHCIc

� PHCIiv � PHCIb � PHCIfr � PHCIs � PHCId�, (4)

where p stands for pressure, t for temperature, f forflow, l for level, c for concentration, iv for inert venting,b for blast wall, fr for fire resistance wall, s for sprinklersystem, and d for forced dilution.

Finally, the DI is divided by the PHCI to calculate avalue of HI as shown in Eq. (5).

HI �DI

PHCI(5)

Inherent Safety Potential IndexSimilar to the previous case (HI), the ISPI is com-

posed of two subindices: an inherent safety index (ISI)and a PHCI. (Note: At the conceptual level of thecurrent discussion, the same nomenclature is retainedfor the PHCI here as with the hazard index, HI). Toquantify the ISPI, the first step is to calculate an ISI. Theconceptual framework for this procedure is shown inFigure 3.

The ISI computation follows the same procedure asa HAZOP study in which guidewords (in the presentcase, inherent safety principles such as minimizationand substitution) are applied to the process system.Based on the extent of applicability and the ability toreduce the hazard, an index value is computed for eachguideword. Figure 8a–d shows index values for variousranges of applicability for four of the guidewords. Todecide on the abscissa values (extent of applicabilityand ability to reduce the hazard), guidelines are devel-oped as reported in Table 3. For the guideword sim-plification—where the current authors experienced dif-ficulty in quantifying this subjective parameter—theindex value can be decided using the guidelines pre-sented in Table 4 (which admittedly are subjectivethemselves).

Figure 8a, b, and d presents the characterization ofthe ISI for minimization, substitution and limiting of,respectively. In the case of attenuation, there are threemain operating parameters identified that control theattenuation process: (i) temperature, (ii) pressure, and(iii) toxicity/corrosiveness of the chemicals. Mono-graphs were developed and are shown in Figure 8c forthese operating parameters; the index value may beestimated based on the extent of applicability of theguideword to these operating conditions. It is worthmentioning that the extent of attenuation guidewordapplicability in the case of toxicity may be measured interms of reduction in LC50 values (LC50intial /LC50changed). The final inherent safety index for atten-uation may be estimated by combining the three indi-ces through Eq. (6).

ISIa � Min�100, ��ISItemp�3 � �ISIpres�

3 � �ISItoxi�3�1/3� (6)

Figure 3. Framework for inherent safety potentialindex (ISPI) computation.


Figure 4. (a) Damage index graph for fire and explosion. (b) Damage index graph for acute toxicity. (c)Damage index graph for chronic toxicity.

Figure 5. (a) Damage index graph for air pollution. (b) Damage index graph for water pollution. (c) Damageindex graph for soil pollution.


Subsequently, the ISI values calculated for the dif-ferent guidewords (as defined in Figure 3) are com-bined through Eq. (7) to give a final overall ISI.

ISI � Min�200, ��ISIm�2 � �ISIsu�2 � �ISIa�

2 (7)

� �ISIsi�2 � �ISIl�

2]1/ 2}

After analysis and implementation of inherent safetymeasures, there may still be a requirement to installadd-on process and hazard control measures. Theseare accounted for using the same procedure as utilizedin the HI calculation procedure and discussed earlier.This index is quantified subjectively based on a scalethat is mutually agreed upon among process safetyexperts. The index ranges from 1 to 10 for any controlarrangement and is quantified based on the necessity ofthis control arrangement in maintaining safe operation.

This necessity is divided into nine groups as listed inTable 2. For any given control system, based on neces-sity (from Table 2), an index (PHCI) may be derivedfrom Figure 7. This process is repeated for all possiblecontrol systems (currently 10 control systems havebeen identified as shown in Figure 6). The PHCIs ofthese 10 different control systems are finally combinedthrough Eq. (4) to give the final PHCI.

Finally, the ISPI is computed in a manner similar tothe HI by dividing the ISI with the PHCI as shown inEq. (8).

ISPI �ISI

PHCI(8)

Fuzzy Probability Theory in Index ComputationIn accordance with Gentile et al. [33], and considering

the fact that there is subjectivity in the quantification of thesubindices and the final index, the use of fuzzy probabil-ity space (membership function) theory for index com-putation is recommended. A trapezoidal membershipfunction (�x) may be defined for each subindex, withsubsequent computations carried out using these mem-bership functions instead of crisp values.

� x � a i, bi, ci, di (9)

� x � �0.75x i, 0.50xi, 1.50xi, 1.75xi� (10)

where �(xi) � fuzzy probability such that (Scheme 1)

�� xi� � 0 forai � x � 0

� 1 � �ai � x�/�ai � bi� forbi � x � ai

��xi� � 1, forci � x � bi

� 1 � �x � di�/�di � ci� fordi�x � ci

� 0 forx � di

The fuzzified values of the subindices and/or finalindex may then be converted to crisp values. There areseveral approaches to do so, as briefly discussed be-low.

Figure 6. Framework for process and hazard controlindex (PHCI) computation.

Table 2. Guidelines to decide extent of requirementof control arrangements.

DescriptionExtent ofRequirement

Essential 10Very important 9Important 8Not greatly important but required 7Required 6Requirement is moderate 5Good if available 4Requirement does not affect process 3Not required 1–2

Figure 7. Monograph for process and hazard controlindex (PHCI).


Chen’s Ranking MethodThe first step in Chen’s ranking method [37] is to

estimate the final composite L (x) of an index value Ai.

Figure 8. (a) Inherent safety index for minimization guideword. (b) Inherent safety index for substitutionguideword. (c) Inherent safety index for attenuation guideword. (d) Inherent safety index for limiting ofguideword.

Table 3. Guidelines to decide the extent ofapplicability of inherent safety guidewords.

DescriptionExtentindicator

Completely applied and hazardeliminated

10

Completely applied and most significanthazard reduced

9

Completely applied and hazard reduced 8Completely applied and hazard

moderately reduced7

Significantly applied and hazardeliminated

6

Significantly applied and hazardreduced

5

Applicable and hazard may beeliminated

4

Applicable and hazard may be reduced 3May be applicable and hazard may be

eliminated2

May be applicable and hazard may bereduced

1

Table 4. Guidelines to decide ISI value forguideword simplification.

DescriptionInherentsafety index

Process simplified to large extent andhazard eliminated

100

Process simplified to large extent andmost significant hazard reduced

90

Process simplified to large extent andhazard reduced

80

Process simplified to large extent andhazard reduced moderately

70

Process simplified and hazardeliminated

60

Process simplified and hazardreduced

50

Process simplified moderately andhazard reduced

40

Process simplified moderately andhazard reduced moderately

30

No significant process simplificationand hazard reduced moderately

20

No significant process simplificationand no substantial hazardreduction

10


With the help of index values, Lh � 1 (x) and Lh �0 (x),the membership �(x) can be estimated. If there are mindex values, there are m fuzzy numbers [L (x), x � 1,2,. . . , m]. The ranking of these m fuzzy numbers(represented by a trapezoidal number [ai, bi, ci, di]) isdone by maximizing and minimizing the set. Chen [37]has generalized the results for trapezoidal fuzzy num-bers as shown in the previous schematic and Eq. (11),

U i�x� �1

2 � �di � Lmin�

�Lmax � Lmin� � �bi � di�� 1 (11)

��Lmax � ci�

�Lmax � Lmin� � �ai � ci��,

for triangular fuzzy number, bi � ci.

Simple Averaging MethodCheng and Lin [38] discussed a simple averaging

method for the defuzzification of a trapezoidal fuzzynumber:

U i�x� ��ai � bi � ci � di�

4. (12)

Weighted Averaging MethodThe weighted averaging method is similar to the

simple averaging method except that it assigns weightsto each fuzzy number set. Generally weights wMLI (typ-ically 60%) and wLLI (typically 40%) are assigned to themean of the most likely interval (MLI � ci – bi), and 40%to the mean of the largest likely interval (LLI � di – ai).Therefore, the utility value can be determined by

U i�x� �wMLI�MLI� � wLLI�LLI�

2(13)

�0.6�c i � bi� � 0.4�di � ai�

2.

The present authors suggest the use of a simpleaveraging or weighted averaging method for the index-ing procedure described in the current paper. Thisaspect of the I2SI is the subject of current work in ourresearch group.

APPLICATION OF THE PROPOSED INDEX TO MMA CASE STUDYAs an initial step in concept validation, the I2SI

indexing procedure was applied to a literature casestudy involving six process routes for methyl methac-rylate (MMA) production [27]. As previously described,Edwards and co-workers [27, 28] developed an inher-ent safety indexing procedure for the selection of pro-cess routes; MMA production was used as the casestudy in their work.

We used the same case study with the reported datato provide an initial test of the effectiveness of the I2SIin evaluating inherent safety aspects of chemical pro-cess routes. The details of the six processes are pre-sented in Table 5. The acetone cyanohydrin (ACH)process was taken as the base process, and the varioussubindices were computed for each alternative. Theresults of this exercise are given in Table 6. The ACHprocess has the maximum HI and nearly the lowestISPI, resulting in low values of the I2SI. In contrast, thei-C4 and TBA routes have relatively high I2SI valuesthroughout, mainly due to the high ISPI values. Thelack of variation in HI values is due to all the optionsbeing evaluated against a common base process(ACH). The results achieved by the current indexingprocedure (I2SI) are well in line with the case studyresults published by Edwards and Lawrence [27].

SUMMARY AND CONCLUSIONThe inherently safer approach to loss prevention

and risk management is intended to avoid or eliminatehazards or reduce their magnitude, severity, or likeli-hood of occurrence by careful attention to fundamentaldesign and layout issues. In this approach, less relianceis placed on add-on engineered safety systems andfeatures and procedural controls, which can and dofail. Unlike these add-on approaches, which increasecost and are maintenance intensive, some applicationsof inherent safety can lead to enhanced safety andlower capital and operating costs. The inherent safetyapproach uses basic design measures to achieve hazardelimination, prevention, and reduction. Classically, aninherently safe plant or activity cannot (under any cir-cumstances) cause harm to people or the environment.In practice, such an ideal plant or process rarely exists.It is therefore more helpful to think in terms of inher-ently safer plants or processes, which in comparisonpose less inherent risk. Professor Trevor Kletz [4] wasthe first to formalize the principles of inherent safety.

This paper presented a conceptual framework of anintegrated inherent safety index. It is ultimately in-tended to consider the life cycle of a process witheconomic evaluation (not discussed in this paper) andhazard potential identification for various options. TheI2SI is composed of subindices that account for damagepotential, inherent safety (guideword approach), andadd-on process and hazard control requirements. Themain thrust behind such guideword-based approaches

Scheme 1


Table 5. Basic information for the MMA process routes.a

Process Process StepMajor HazardousChemicals

Temperature(° C) Pressure (psia)

Acetone cyanohydrin(ACH)

Hydrogen cyanideproduction

Hydrogen cyanide 1200 50

MethaneAmmonia

ACH production Acetone 29–38 15Hydrogen cyanideACH

HMPA/HMPASE production ACH 130–150 103Sulfuric acid

MMA production HMPA/HMPASE 110–150 103Methanol

Gas production Sulfuric acid 980–1200 15Fuel gasSulfur dioxideAmmonium bisulfate

Sulfur trioxide production Sulfur trioxide 405–440 15Sulfur dioxide

Ethylene based viapropionaldehyde(C2/PA)

Propionaldehydeproduction

Ethylene 130 220

HydrogenCarbon monoxide

Methacrolein production Propionaldehyde 160–185 720FormaldehydeMethacrolein

Methacrylic acid production Methacrylic acid 350 54Methacrolein

MMA production Methacrylic acid 70–100 100–110Methanol

Propylene based(C3)

Isobutyryl fluorideproduction

Propylene 70 1322–1469

Carbon monoxideHydrogen fluoride

Isobutyric acid Isobutyryl fluoride 40–90 147Isobutyric acid

Methacrylic acid Isobutyric acid 320–354 37–44Methacrylic acid


Ethylene based viamethyl propionate(C2/MP)

Methyl propionateproduction

Ethylene 100 1469

Carbon monoxideMethyl propionate

Methylal production Methanol — —Methylal

MMA production Methyl propionate 350 —Methylal

Isobutylene based(i-C4)

Methacrolein production Isobutylene 395 —Methacrolein

Methacrylic acid production Methacrolein 350 54Methacrylic acid


Tert butyl alcohol(TBA) based

Methacrolein production TBA 340 71Methacrolein

Methacrylic acid production Methacrolein 350 54Methacrylic acid


aMMA, methyl methacrylate; ACH, acetone cyanohydrin; HMPA, 2-hydroxy-2-methyl propionamide; HMPASE,2-hydroxy-2-methyl propionamide sulfur ester.


is to help designers strike a balance among hazardavoidance, prevention, control, and mitigation. Theproposed indexing system has been applied to an MMAcase study. The application demonstrates that it hasgood applicability in the initial phase of process design(process selection and design decision making).

The authors agree that there are certain aspects ofthe indexing procedure that necessitate subjectivejudgement. To deal with these and other uncertainties,we suggested the use of fuzzy mathematics. Additionalwork is clearly needed to refine the mathematical treat-ment. There is also substantial additional work re-quired on life cycle and economic aspects. Such workis planned within our research group. We welcomecomments and suggestions on our concept of an inte-grated inherent safety index.

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Table 6. Results of I2SI application to six MMA production processes.a

Process Process Step

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I2SIDI PHCI ISI PHCI

Acetone cyanohydrin(ACH)

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Propylene based (C3) Isobutyryl fluoride production 174 71 35 59 0.2Isobutyric acid production 174 71 102 55 0.7Methacrylic acid production 174 71 118 51 0.9MMA production 174 71 156 43 1.5

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Tert butyl alcohol(TBA) based

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aMMA, methyl methacrylate; ACH, acetone cyanohydrin; HMPA, 2-hydroxy-2-methyl propionamide; HMPASE,2-hydroxy-2-methyl propionamide sulfate ester; DI, damage index; PHCI, process hazard and control index; ISI,inherent safety index; I2SI, integrated inherent safety index.


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Integrated inherent safety index (I2SI): A tool for inherent safety evaluation

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