HAZEXPERT- an integrated expert system to support hazard analysis

in process plant design

M. H. Goringand H. G. Schecker

AGPhysikalisch Chemische VerfahrenstectmikDepartment of Chemical Engineering

University of DortmundPostfach500500

D- 4600 Dortmund 50Germany

Tel: 49-231-7552309; Fax: 49-231-7555222

ABSTRACT

HAZEXPERT (Hazard Analysis ExpertSystem) is a software tool developed to aidhazardanalysis inprocess plantdesign and is basedon process flowsheets and piping& instrumentation diagrams (PID's).Asone of the keypoints, a software-interface (OFI=Object llriented flowsheetlnterpreter, to providefeatures of an "intelligent flowsheet") was developed to enableautomatic transformation of the graphicalrepresentation of CAD-Systems intodynamic objectorientednetworks that are accessed by modulargenericknowledge-bases. Therefore, a fundamental objectoriented datamodelfor chemical plantswasdeveloped basedon graphtheorywhichprovides the basis for the integration of the differentsoftwarecomponents and supports processing andrepresentation of multiple flowsheets. The systemis integratedwith additional databases for equipment, process, and substance data.A new systematic methodtoperform hazard analysis was developed to combine usefulprocedures of currentlyavailable manualmethodologies with structured processing of the large amount of data belonging to a wholeprocessplantin processdesign. The integrated approach improves completeness andefficiency of the wholeanalysisprocedure considerably.

KEYWORDS

Safetyanalysis; CAD; expertsystem; objectoriented programming; objectorientednetwork; graphtheory; processengineering; processflowsheets; PIO; HAZOP

1.0biectiyes

Objective of the tool is to offer the process plantdesigner,whoonly has limitedknowledge of identifyingtechnical safetyproblems, help in systematic hazardidentification andmovethese procedures towardsearlierphases in processplantdevelopment.Due to the complexity of chemical plants, hazardanalysis is commonly carriedout usingsystematicmanualmethodslike Hazardand Operability Studies(HAZOP),which are,however, very labourintensive andfor complex plants,very time consuming. An alternative approach for the systematicexamination and the application of specific expertknowledge to perform hazardanalysis in chemicalplantscan be effectively achieved usingprogramming stylesand toolswhich are found in AI­technology.The systempresented reduces the time required for the whole analysis, carriesout a systematicexamination and results in an unified standard of safety analysis.A significant point is the integration withexistingsoftware tools (especially CAD)in order to makeeffective use of the large amountof process information already available in generalprocessengineering.A structured writtenreportis createdautomatically, used to allowa briefexamination of the resultsbyan expert,for preparation of safetydiscussions and documentation for furtherprocessplant development.Earliersoftware developments in that domain weremostlyimproved text processing systems, whichsupported the analysis usinga specific method, but whichwerenot directlyconnected to the actualplantunderexamination. Attempts at converting the usualmanualmethods ( for examplethe widelyusedHAZOP- methodology) werenot very successful, as significant sections of thesemethods are notknowledge based.As a resultof this, a new systematic method had to be developed for the systempresented.

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2.ApplicatioD DescriptioQ

The complete designof thesystem is shownin Fig. 1, combining the advantages of AI andconventionalsoftware. Theshaded sections represent thecomponents that form HAZEXPERT.

process flowsheet, PID(CAD-System)

equipment, process andsubstance data

(Database)

fig. 1:The complete design of HAZEXPERT for integrated safetyanalysis

The whole system wasdeveloped to run as hardware-independent as possible withfocus on AppleMacintosh (development platform) andIBM-PC.CAD-systems are widely distributed in the chemical industry andare usedfor the drawing up andinspection of flowsheets. In the currentversion all CAD-systems whichhavea DXF-software interfacecan be used. MICROSTATION by Intergraph wasusedin the development of this system.The coreof HAZEXPERT consists of modular knowledge bases, which actually carryout the safetyanalysis. The expertsystem-shell NEXPERT OBJECT by Neuron Datais usedfor development andimplementation. The thusavailable frame-concept enables a realistic representation of a chemical plantin a dynamic object-oriented network, in which thetypical relationships ocurring in a chemical plantarereproduced.A rulebasedsafety examination is thencarried outon thisobjectnetwork. Further significant criteria fortheuse of this shellare the integration via interfaces to standard databases (in thisprojectORACLE andEXCEL by Microsoft) andthe support of various platforms. The use of built-in Hyperbidge makes thedevelopment of the user-interface relativly easy, SUPERCARD by Silicon Beachbeingthesystemcurrently implemented as an "intelligent" user-interface on the Apple Macintosh.To permitconnection to CADsystems, an interface designated OFI(Objectoriented FlowsheetInterpreter) wasdeveloped in ANSI-C andis currently available on Apple Macintosh andIBM-PC.

3. MetbodoloKY

An approach to incorporate all safety relevant tasksintoonesystem seems not to be verypromising dueto thecomplexity andsizeof the whole knowledge domain. Therefore, the safetyrelevant taskswerestructured to provide the basis for a modular design withspecial emphasis to work out the fundamentalprocedures that are commonly used in different specific subdomains.

HAZEXPERT usesmainly a top-down method sincethereareonlya limited numberof possibledangerous consequences following certaincomponent failures in a process plant. This makesit easiertostructure theknowledge domain andallows grouping into"generic hazards" which can be chosen aspossible "topevents" by the user (such as "dangerous overpressure","explosion"). Starting from that "topevent" the system performs systematic hazard analysis working on the underlying modelof the processplant underinvestigation. From theuser'spointof view, a bottom-up approach is stillpossible, sincedataof process plantcomponents can be changed andsubsequently be processed.In the currentversion, HAZEXPERT contains generic knowledge basesconcerning hazards thatlead todangerous ovemressure in process equipment.

Experiences from manual hazard analysis show twobasicheuristics for efficient systematic assessmentof complex process plants using flowsheets:

1.Process equipment is investigated starting withthe beginning of the process in orderof flow ofproduct:

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fig. 2: proposed order for hazardanalysis. boxesrepresenting itemsof equipment

2. Investigation is focussed on one itemof equipment at a certainpointof time andconsiders a minimumsearchspace including neighbouring itemsof equipment connected by streamlines:

fig. 3: plant item 3 is currentlybeinganalyzed, plant items2,4.5formthe boundary of currentsearchspace

These heuristics seem to be rathertrivialbut a detailed analysis showsthat this approach is veryefficientto minimize the necessary inputof data and to keepthe focus of attention whilemoving througha plant.Therefore, these procedures have beenconsidered in the basic designof the expertsystem. Additionalprocedures have been included to aid the user wheninvestigating non-linear structures (e.g. recycleloops).

4, Software Interface for CAD·Systems (OFI>

The expertsystem shouldbe able to use the latest available flowsheets to examinate the actual state inprocess development. Furthermore. there should be no need for the process plantdesigner to draw thewhole flowsheet againsolelyas input for theexpertsystem(evena graphical input is verytimeconsuming).Therefore, a softwareinterfacewas developed designated OFI (Objectoriented FlowsheetInterpreter)that provides features of an "intelligent flowsheet" for standard CAD-Systems.

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fig4: transformation of the graphical representation into an object oriented network

A fundamental classdescription of process plantswasdeveloped to form the basisof an objectorientednetwork that represents the graphicrepresentation as close as possible. This modelallows astraightforward description of anycomplex structure withthreeparameters only (fig. 4) andtakes careaboutspecific items(piping, complex equipment, controlstructures) that are characteristic to processplantdesign. Additional overhead fordrawinga design(e.g. "knots" forTvbranches. special"connectionpoints")was avoided to have a realistic representation and no additional restrictions for thedesigner.

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User

~Ero:ram:read fi eS1 print lists of

Items

results

CAD-System :draw or modify

process flowsheet / PID

HAZEXPERT:read objects, build objectoriented model, generichazard knowledge bases

tg. : working withthe

OFIuses CAD-Files in DXF-fonnat andproduces output tables in a commonly useddatabase fonnat(inthecurrent version SYLKDB) thatare read directly by theexpertsystem. Thisprocedure has beenchosen forcompatibility reasons, withregard to different CAD-Systems anddifferent hardware­platforms,OFIprovides a complete description of theplant, consisting of topology, equipment andprocess datarequired by the expert system.

5. Design of HAZEXPERT

Theknowledge basesforming thecoreof HAZEXPERT are implemented completely in NEXPERTOBJECT. General design is shown in fig. 6.

PLANTDES (plantdescription)In HAZEXPERT, a process plantis represented asa directed graph. Eachcomponent of a process plantis represented as an object thatbelongs to a certain generic class. Different kinds of relationship betweensingle objects suchas flow direction, signal-lines, andspecific itemsof equipment are represented by asubobject relationship.Using this representation, a model of anycomplex process plant canbe builtas a dynamic objectoriented network. Furthermore, PLANTDES allows simultaneous useof several PID'srepresenting alargerplantenabeling analysis across several flowsheets. Sources of failures, thatotherwise mighteasilybe overlooked, canthenbe detected.The representation of a process plantallows a verydetailed description (component level). All levelsthatcontain less information canbe used without having to make anychanges in the knowledge bases.PLANTDES automatically provides a description on a higherlevel(unitlevel)usingthe detailedinformation of the component level. Thathigherleveldescription has advantages sincethehigh-levelinfonnation can be useddirectly by generic hazard knowledge bases. Generic knowledge bases canbebuiltin a much moreconvenient manner without having to follow streamlines eachtime. Ifnecessary,

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especiallyif only weakconclusions can be drawnon unit level, there is still the detaileddescriptionavailable to confirmreasonin .

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Unit Level

Plant Descriptiong 6: generaldesign 0 HA

HAZ Manager(Hazard Analysis Manager)This knowledge base is used for controland managing the generalstructureof hazard analysis. Due tothe natureof the problem, two differentlevels are provided to be the startingpoint of analysis:

1.Plant -Level for searching and analyzing components and plant item combinations, whichmightform potential sourcesof danger

2. Unit -Levelexamination of individual unit hazards

Besidesfull automatic examination, HAZ_Manager provides a flexible strategyto give the user controloflocation (focuson equipment> and domain (focus on generic hazard) of the current investigation. Bythat meansexperience and knowledge aboutthe plant whichis not availableto the expert systemcan beused to avoid trivial conclusions andshorten processing time.On the other hand, full responsibility forthe result is on the user's side.

SSYS (substancesystems)This knowledgebase enables the representation of differentsubstance systemsfor each item ofequipment and providehelp in automatic generating of lists of substances. This feature is neccessary forconvenient handling of

-design states-deviations (user inputor results of reasoning)-phases of batch processes

PROPAGATEFollowing streamlines is a very important featurein carryingout hazardanalysis.In manualhazardanalysis, the analystfollows streamlines on paper,tries to find out possible failuresandsubsequentprocess deviations whichare evaluated undersafetycritical aspects. The problem of keepingfocus issolvede.g. by highlighting the currentstreamline. A procedure that performs analysis line by line is usedvery often. This methodology is also used in HAZEXPERT. Startingfrom the current item of equipment,pipingis analyzed systematically as to whetherdeviations of processparameters such as pressure,temperature. substances can occur, leading to a dangerous situation.

A fundamental procedure performs systematic traversing of the directedgraph and consistsof a goal­oriented search and a controlled propagation of possibledeviations along streamlines. Algorithms weredeveloped that perform backward and forward propagation and are independent from generichazardknowledge bases.Anycomplexstreamlines can beanalyzed. therefore a systematic and completeanalysis of the processplant can beachieved.

Each singlegenericknowledge base usingPROPAGATE contains knowledge concerning-relevantprocess parameter(objective)-knowledge about type of process equipment at the boundary of currentsearchspace-decisions to be madeat branches-experience-based knowledge aboutfailures (whatcan go wrong?)and how to changethe relevantprocess parameterfor components (pumps. valves, ...) in streamlines

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Severaldifferent types of relevant process parameters doexistand require different useof thepropagation methods, two simple examples areshown in fig.7 (triangles represent objects, linesthesubobiect relationshi ):

81 subst 8subst 0

subst Rsubst 8subst Csubst 0

sUbst Rsubst C

LQQfAlmost anychemical plantcontains streamlines that areforming a loop (e.g. recycle loop formass,energy, control, ...). Theseloopscanhavesignificantly different consequences in the contextof a certaingeneric knowledge basecompared to a lineararrangement. Therefore, LOOPis able to find simple andcomplex loopsandprovides that information for thegeneric knowledge bases. This allows specificevaluation of loopsandcomponents beingpartof a loopin different contexts.

Generic KBs(generic hazard knowledge bases)The undesired top event"dangerous overpressure" wasstructured due to the physical characteristics offailure andprocess deviations. Maincauses are deviations in mass- and!or energybalance. Analysisshowsfivemainprinciples which can be usedfor structured hazard identification:

-increased massflow intothe system (failure of pumps, valves, ...)-reduced massflow out of the system (lineblocking, failure of pumps, ...)-increased energy flow intothe system (failure of heatexchanger, )-reduced energyflowoutof the system ((failure of heatexchanger, )-increased energy release in the system itself(unwanted chemical reaction)

At leastoneknowledge base wasbuiltfor systematic hazard identification concerning eachof thesedomains. Quantitative outputis produced to define inputdatafor subsequent designor calculationprograms. In the current version, this is demonstrated for the design of emergency reliefsystems.

A casedependent explanation component wasdeveloped andis beingusedin mostof the genericknowledge basesthat creates structured documentation of user input, reasoning and results of the expertsystem.

6. Application Use and Payoff

The development of the system resulted partly withtheco-operation of 7 companies in the germanchemical industry. Significant advantages area unified quality of the safetyexamination andthedocumentation, whichdueto the underlying plantmodel, can alsobe usedduring plantoperation.The connection withthe CAD-system leadsto significant savings in timeand avoids the mistakes whichcanoccurwithmanual datatransferal.

7. Maintenance

The maintenance of the system wasa veryimportant aspect of its development, as the system is usedbythe participating companies to form the basisfor theirfurther development.Eachindividual section of the application (Knowledge bases, OFl, UserInterface) has a modular design,in orderto support structured development as wellas maintenance. Interfaces between the knowledgebasesweredefined as wellas the use of databases for storing component specific expertknowledge. Theplantmodelfollows technical standards (DIN) andindustrial company standards.At the moment maintenance is carried out by the original developers.