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Sponsor Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Intelligence At The Device . . . . . . . . . . . . . . . . . . . 3
Practice Safe Sensing . . . . . . . . . . . . . . . . . . . . . . 7
Process Safety Resources At Your Fingertips . . 10
Intrinsically Safe Or Explosion Proof? . . . . . . . . . . 11
Simplifying Plant Safety Instrumentation . . . . . 15
Table of Contents
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Sponsor Profile
United Electric Controls, a privately held corporation headquartered in Watertown, Massa-chusetts, USA, is a manufacturer of durable and reliable pressure and temperature switch-es and sensors used in industrial applications throughout the world. UE products providea critical and independent layer of Plant Safety instrumentation that protects equipment,processes and personnel in a wide variety of challenging environments and operating con-ditions. Our reputation for dependable pressure and temperature solutions is a result ofinnovative design, superior manufacturing techniques, and a corporate focus on uncompro-
mising quality and service to our customers.
Innovative Design Solid-stateOneSerieselectronicswitchforplantsafetyapplications Cost-effectivesolutionstomeetandexceedcustomerrequirements Ruggedconstructionforthemostchallengingenvironments
Rapid Delivery Leanmanufacturingformaximumproductivity Practitionerofcontinuousimprovementandeliminationofwaste One-piece-flowforoptimummanufacturingefficiency
Uncompromising Quality ISO9001:2000certified
Customer-firstapproachthroughservice,deliveryandvalue Wideselectionofproductsthatmeetglobalagencycertification
Three Manufacturing Divisions UnitedElectricControls AppliedSensorTechnologies PrecisionSensors
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Intelligence At The Device
Cover Story: As Components GetSmarter And More Powerful, Manu-facturers Are Finding Themselves
Managing High-Performance Auto-mation And Control Systems Whose
Parts Are Capable Of MonitoringThemselves, Diagnosing Their Own
Problems, And Maybe Even Mak-ing A Decision Or Two, Economically
And Easily.
Jeanine Katzel for Control Engineering
Everythingissmarterthesedays,itseems:smartcars,smartappliances,smarttoys.Itsnotsurprising,then,thatintelligentdevicesareontheriseinindustrialautomationandcontrolaswell.Smartmotors,smartsensors,smartflowmetersareonlyafewamongmanycomponentspackedwithmicroprocessorpowerinatrendthathasalltheearmarksofusheringindustryintoanageofunprecedentedproductandsystemperformance.
Computingpowertodayisseeminglyinfiniteanddatastorageandmemoryrelativelyinexpen-sive,saysJeffWoolfolk,businessdeveloperandconsultingengineerforSiemensIndustryInc.ThenameofthegamenowishowmuchinformationcanIobtainefficientlyandhowcanIbestuseit.
Itappearsmanufacturersareobtainingandus-inginformationquiteefficientlyandeffectively.
Thankstotheadventoftechnologicaladvance-mentssuchassystemsonachip,multi-coreprocessing,andhigherlevelsofintegration,de-vicesaresmaller,lighter,faster,andsmarter.Theyarefindingtheirwayintoendlessapplicationsandreapingbenefitsinmanyareas.Theyareincreasinglymorecapable,asvendorsintroduceproductsthatareeasiertouse,energyefficient,self-diagnostic,andwireless;andtheyarefilled
withfeaturesthatarehelpingtodrivecompaniestothegreaterefficienciesandthrough-putsandhigher-levelprocessesthattheyneedtostaycompetitive.
Intelligent wireless devices analyze andtransmit information from remote installations
accurately and economically. Source: EmersonProcess Management
Use Those Brains!
Buildingcapabilityintocomponentsisnotentirelynew.Smartmotors,drives,andmotioncontrolsystemsarefrequentsightsinmodernfacilities.ThecomponentsinsidedrivestheIGBTs,theprocessors,saysRichMintz,productmanager,SEWEurodrive,werebuiltearlyontohandleharshplantfloorenvironments,andthatcapability,perhapsmorethanothers,has
enabledintelligencetomovetothedevicemorereadily.Forintelligencetomovetothedevicelevel,itmustbedesignedspecificallyforthatenvironment.Intelligentdevicesarenotthesamecomponentsfoundinstandardcontrolcabinetelectronicproducts.Manufacturershaveasked
forthesedevices,andsuppliershaveresponded,allowingtheprogressionfromcentralizedtode-viceintelligencetooccur.
Inaddition,thoseinitialdevicesdidnothave
thebenefitofthetremendouscomputingpower,memorycapacity,ordatatransmissioncapabilityemergingtoday.Currentmicroprocessorhorse-powercoupledwithlowpowerrequirementsarewhatsallowingustoembedmoreandmoredecision-levelfirmwareandsoftwareintode-vices,saysRayRogowski,managerglobalprod-uctmarketing,fieldinstrumentation,HoneywellProcessSolutions.Nottoomanyyearsago,youhadtogobacktotheDCStofindoutwhatwasgoingoninthefield.Nowyoucanpackageitrightupinthesensor.
Initially,microprocessorswentdowntothede-
vicetoimproveperformance,saysMarkSchum-acher,presidentofthe
Components such as Siemens Simocode Pro V
current/voltage monitoring modules add intel-ligence to motor control center applications.Source: Siemens Energy & Automation
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RosemountPressureBusinessforEmersonProcessManagement,todothingsliketempera-turelinearizationandcompensation.Itisthatperformanceimprovementandreliabilitythatcontinuetobethefoundationforthegrowingap-plicationofintelligencedevices.
Sincehiscompanyhashistoricallyfollowedaphilosophyofcentralizedcontrol,GrahamHar-ris,president,BeckhoffAutomation,arguesthatmulti-coreprocessingputsunprecedentedamountsofpoweronachiptocreateanall-in-oneautomation,control,andmeasurementplatformthatcandoeverythingfromperformingmachineconditioningandreal-timemeasure-menttomodifyingcontrolalgorithmsinrealtime.AccordingtoHarris,UsingonepowerfulCPU,itismoreefficienttomanagenumerousdevicesinsoftwareversuscontrolviadistributedCPUsineverymeasurementandmonitoringdevice.PC-andEthernet-basedtoolsaresimilarinapproachtothehumanbrain,managingcontrolofouror-gansandbodypartsviathenervoussystem.Thiskindofapproachinautomationiswhatwecall
scientificautomation,anexcitinggrowthareaoftechnology.Withinfiveyears,engineerswillseeitinareaslikeadaptivecontrolandconditionmoni-toring,andinthedevelopmentofmoreintelligentmachinesoverall.
Intelligent components,such as this microprocessor-based temperature trans-mitter, enhance measure-ment accuracy and reducemaintenance. Such devicesare capable of continuallymonitoring a sensor, track-ing operation, diagnosingfailures, and generating errormessages. Source: Moore
Industries-International Inc.
Benefits In Balance
Intelligentdevicesreapthreeprimarybenefitsforautomationandcontrol,saysRamiAl-Ashqar,motioncontrolproductmanager,BoschRexroth:
Thecontrollerdoesnthavetodoeverything.Componentintelligencefreesthecontrollertodoothertasks.
Electronicequipmentonamachinecanbemoreeasilymodularized,allowingbettermainte-
nanceandeasiertroubleshooting.Flexibilityisincreased.Distributedintelligence
facilitatestheeasierincorporationofmorede-vices.Onecontrollerinterfaceswithmanydeviceswhichdothework.
Theyalsocansaveenergy,addsMintz.Havingintelligenceinsidemotorsanddrives,forexample,givesyouclosercommunicationsandconnectionsthatallowthecomponentstofunctionbettertogether.Themoreacontrollercancommunicate
withadrive,andthemorethedriveknowswhatisgoingoninthemotor,thebetterableanopera-tionistomakethosesmallprocesschangesthatmakeadifference.
The need for easy-to-use smart devices iscritical. To show easily even non-engineers canlearn to develop intelligent systems, NationalInstruments has launched a robotics competitionusing Nitro. The Nitro control system platform isdesigned to help students elarn to use tools suchas NIs LabView and CompactRio to build intel-ligent robotic devices and solve challenging engi-neering problems. Source: National Instruments
Amongthemoreimportantaspectsofincor-poratingintelligentdevicesintoasystemiseaseofuse.SaysEmersonsSchumacher,Puttingmoreintelligenceintofielddevicesunquestion-ablymakesthemmorecomplicated.Thatswhy
itiscriticaltokeepdeviceseasytouse,adelicatebalanceandatoughengineeringchallenge.Noinformationfromadeviceisimportantifitisnoteasilyaccessible.Manyplants,today,arestaffedbyfewer,lessexperiencedworkers.Ontheonehand,vendorsmustconcentrateonbuildingeaseofuseintotheirintelligentproducts.Ontheother,end-usersmustbedisciplinedaboutwhatinfor-mationtheyreallyneedtoknow.
Partoftheburden,addsArvesStolpe,Compac-tRioProductManager,forNationalInstruments,fallstosoftwaredeveloperstocreateapplicationsformodernmulti-processingsystemsthatare
easytouse.Todayscomplexsystemsareex-tremelypowerful,butwithouttherighttools,theyarealsodifficulttoprogram,design,andmanage.
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Weneedproductsthatdontrequireamulti-processorembeddedsystemdevelopertoapplythem,heinsists.Ifwedontdothat,industrialandautomationengineerswillneedtobemorethancontrolexperts,theywillneedtobeembed-
dedexperts.Andthatisreallyaskingalot.
Integrated motor and drive mounted directlyon a machine make it easy to configure a systemin different ways. Continued integration of intelli-gent components is expected to increase moni-toring and control accuracy even further. Source:Bosch Rexroth
Buildingintelligenceintodevicesalsohasdrivendiagnosticstothecomponentlevel.Itreducesmaintenancebecausethemachinecanbemodularized,saysAl-Ashqar.Iftheelectron-icsandthemachinearemodularized,itiseasiertoidentifyaproblem.Errormessagescanbedisplayedandproblemspinpointed.
Firstgenerationsmartinstrumentscouldfailin
unpredictablewaysduetoundetectedsoftwarefaults,addsCharlesLarson,directoroftechnologyforMooreIndustries-InternationalInc.Butincreasedprocessingpoweravailableatlowpowerhasalloweddevicestoperformmoreextensivediagnostics.Sometemperaturetransmitterscandetectand
identifybrokenleadwiresandhighleadresistanceconditionswhenmulti-wiresensorconfigurationsareused.Temperaturetransmittersthatacceptdual-sensorinputsallowfallbacktosinglesensoropera -tionwhenasensorfailureisdetected.Whencoupled
withexternalwatchdoghardware,adevicethatisnotoperatingproperlycanbeguaranteedtogener -ateafaultsignal.Thatkindofreliabilityallowssmartinstruments,suchasintelligentsafetyalarmtrips,tobeusedeveninsafetyapplications.
Manyuserstodaywanttoembracedevice-levelin -telligence,saysSiemensWoolfolk,butarestillthink -ingintermsofoldschoolcapabilities.Productstodayaresocapable,hesays.Themoreyouletthemdo
foryou,themorevaluablewillbethereturn.Intel -ligentcomponentsdomorethanmonitoracontactclosure.Theydiagnoseadevelopingproblemandletuserspreemptivelyheadoffafaultorafailurethatcouldshutdownproduction.
John Deere saw a 25% savings in energy use onits storage retrieval system just by carefully coor-dinating movement of the X and Y axes. The actionallowed the company to re-use the potential energystored in the rack locations. Source: SEW Eurodrive
Technology Of Choice
What,then,istheultimateinfielddeviceintel-ligence?Wireless,agreemost.Itmaystillbeagiantleapoffaithformany,butcomfortwiththe
technologyisgrowingandmoreapplicationswillemergeaswirelessisproventowork.Itwillbetheeasiest,mosteconomicaloption,whichwillmakeitthetechnologyofchoice,saysEmersonsSchumacher.Thecostofdevicesismuchlessthanthecostofinstallingthem,addsHoney-wellsRogowski.Ifyoucaneliminatethewiringandstillhavedevicesthatarereliable,safe,androbust,thesavingsarehuge.
Microprocessorperformanceandlowerpowerrequirementshaveenabledwirelesstechnologiestoexpandsignificantly,saysMarkMuldowney,fieldinstrumentstechnology,productarchitect,
HoneywellProcessSolutions.Advancesinsignalprocessingletdevicesnotjustgivedatatoendusers,buttoanalyzethedataandgenerateinfor-mationthatisactionablegivingusersinforma-tiontheyneedandinformationtheycantrust.
Theconceptofintelligentsensorsevery-whereorubiquitoussensornetworksasUCLAresearcherscallthemcouldonedaybecomepartofourcontrolprocesses,musesStolpe,whosecompanyhasjustreleasedwirelesssensornodes.Thecloserthesensorcanbetothedecision-makingprocess,thehigherqualitysignalpossible.Andreally,controldependsontheaccuracyofthemeasure -
ments.Ifyouhaveanaccuratemeasurementwithlownoiseatthenode,youcanrespondtoanyinputquicklyandaccurately.
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To Make The Impossible,Possible And The Possible, Easy
Tohearthoseclosesttothetopic,thepo-tentialforintelligentdeviceshasanexciting,varied,andlimitlessfuture.Weseefurtherintegrationcoming,saysMintz.Wevegotalgorithmsthatuseasensoronthegearboxandintelligenceinthedrivetotellyouwhentodoanoilchange.Weseethenextgenera-tionintegratingfunctionsintosinglepack-ages:agearboxwitheverythinginsidethatyouplugintoanEthernetport.Itgivesmanypauseforthought.
Thepossibilitiesareincredible,saysAl-Ashqar,recallingthatfewwouldhavepredicted20yearsagotheimpactofmobilephones.AremotePLCmightonedayactasamastercontrollertakinginformationfrommultipleplants,communicatingittotherightpersonnel,andperhapsevenmakingdecisionsorrunningmachinesinmultiplelocations.
Theabilitytoaddfunctionsmoreeas-ilywillcontinuetoexpand,offersWoolfolk.Safetyfunctionswillbecomemoreandmoreimportant,heobserves,andarestartingtofindtheirwayintotheintelligentdevice.
Ofcoursehurdlesremain,amongthem
poweringintelligentdevicesandensuringwirelesslinkreliability.Ubiquitoussensorswillhavetobeverylowpowerdevicesthatarenonethelessverypowerful;andwirelessstillmustsolveissueswithcommunicating
backtoacentralhost,especiallyinmissioncriticalapplications,saysStolpe,buthebelievesthechallengeswillbeovercome.Ifwecanmakeintelligentproducts
sufficientlycapableandeasytoapply,wewillallowoperationsandmanufacturingpersonneltobesuccessfulinwhattheyneedtodo.Wewilldriveintel-ligentdevicetechnologytothepointofbeingincrediblyusableandinsanelypowerful,makingtheimpossiblepossible,andthepossibleeasy.
Author InformationJeanine Katzel is a contributing ed-
itor to Control Engineering. Reachher [email protected]
Intelligence In Action: Smart Devices Save Energy
Puttingintelligenceintoadevicecanimproveperformanceandsaveenergy,asaretrofitprojectatatractormanufacturershowedrecently.AJohnDeerefacilitywantedtomodernizea25-year-oldstorage-and-retrievalmachinetoincorporate
energy-savingtechnologies,andturnedtoSEWEurodriveforassistance.
ThesysteminquestionincludedanXaxisthattravelsastorage-and-retrievalcraneandaYaxisthatmovesthepalletsupanddown.Typically,explainsRichMintz,productmanager,Ifyouconfigureastorage-and-retrievalsystemtogofrompointAtopointBtoretrieveapalletofftherack,bothaxesstartmovingatonce.TheXaxisstartstomoveandtheYaxisstartstolift.Bothmotorsstartatthesametime.Butdotheyhaveto?IftheYaxisissettowaitasecondortwobeforeitstartslifting,itcanavoidasimultaneousinrushandreducedemandontheelectricalsystem.
Thevendoralsolookedatwhathappenswhenthedrivesdecelerate.ContinuesMintz,WhenyoudeceleratetheXaxis,youputenergybackintothedcbus.WhenyoudeceleratetheYaxiswhenthesystemisloweringthepallet,itisbleedingoffenergy.Itisbeingburnedupoverthebrakingresistor.Nowwhatifwetimedtheoperationsothatwhenonemotorisregeneratingenergy,insteadofburningitovertheresistor,wewaitanduseittomovetheothermotor?Whatifwetimewhenthesetwomotorsactsothatwhenoneisregeneratingpower,theotherisusingthatregenerativeenergyinsteadofgivingitupoverthebrakingresistor?
Theanswersarethatenergyissavedandefficiencyisincreased.TheconceptswereappliedtotheexistingsystemusingSEWsMOVIPLCmotioncontroller.Asaresult,Deerewasabletoreduceenergyconsumptionbythesystemsome25%withoutchangingthecycleratesandtheydiditjustbyaddingintelligencetothecomponents.
WetookaPLCcontrollerandputitinsideoneofthedrives,explainsMintz,andwereabletoachieveasignificantenergyreductiononalargesystemforamajormanufacturer.DoingitinsidethemainPLCwouldhaverequiredamore
robustandexpensiveunitaswellasadditionalprogramming.ButifyoutaketheloadoffthePLC,letataskcontrollertellintelligentdeviceswhichtaskstoper-form,andleteachintelligentdevicecontrolitself,bigsavingsintimeandmoney
arepossible.
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With Process Safety And Sensors,
Sometimes Garbage In Equals MoreThan Garbage Out. Because These
Devices Measure Pressure, Tem-perature, Flow, Level, And Other
Process Parameters, They Play APivotal Role In Determining A Pro-cess Units Output. A Wrong Read-ing Can Produce Waste Or Create A
Catastrophe That Costs Lives AndMakes National News.
Hank Hogan for Control Engineering
Withprocesssafetyandsensors,sometimesgarbageinequalsmorethangarbageout.Be-causethesedevicesmeasurepressure,tempera-ture,flow,level,andotherprocessparameters,theyplayapivotalroleindeterminingaprocessunitsoutput.Awrongreadingcanproducewasteorcreateacatastrophethatcostslivesandmakesnationalnews.
Now,thankstoadvancesintechnology,sensorsystemshavegreaterintelligenceandthereforemorediagnosticcapabilities.Today,sensorscanbecertifiedbythirdpartiestomeetsafetyinteg-ritylevels,orSIL,designationsfoundinIEC61508.Onepositiveresultofthisisthepotentialtousefewersensorswithoutcompromisingsafety,
leadingtoadecreaseinwiringandinstallationcosts.Anotherpositiveeffectisthepotentialforimprovedprocesscontrol,largelyduetoincreas-inglyintelligentsensors.
Certifiedproductsdeliverhigherdesignqual-ity,assertsWilliamGoble,principalpartnerandco-founderofexida,acompanythatoffersfunctionalsafetytraining,technicalsupport,andexpertisefromitsNorthAmericanheadquartersinSellersville,PA.(ThecompanysEuropeanbaseisinFischbachau,Germany.)Thisbetterdesignisduenottosuccessbutrathertofailure,notesGoble.Nearlyhalfoftheproductsthatattemptcertificationfailonthefirsttry.Onlywhenthedesignprocessisimprovedanddiagnosticsare
addeddotheypass.
Goblesdatashowsthatthenumberofcerti-fiedsensorshasexplodedinrecentyears,withthecumulativetotalofsuchdevicesmovingfromfivein2003toeightin2005and24in2007.Therehavebeenindicationsthatvendorsareconvert-ing,orwillsoonconvert,entirefuturesensorlinestocertifiedproducts.Goblenotesthattakingfulladvantageofsuchsensorswillrequirethatsystemsbeconfiguredcorrectly.Forexample,runningdiagnosticsincertifiedsensorsmaysendthereadingoutofrange.Anincorrectlyconfig-uredcontrollerwillinterpretthisasafault,shut-
tingdowntheprocessunnecessarily.Abetterap-proachwouldbetoimplementahold-last-valuestrategy,withactiontakenonlyifareadingstaysoutofrangeforlongerthanaspecifiedtime.
A loading bay for Shell petroleum productsin the U.K. makes use of safety-certified massflowmeters. Courtesy Endress+Hauser.
Known FailuresOneofthefirstcompaniestoofferacertified
sensorwasboughtafewyearsagobySiemensEnergyandAutomation(SEA).LouisDiNapoliisapplicationengineeringandtechnicalsupport
managerfortheSEAprocessinstrumentationunitthatmakessafetycertifiedsensors.DiNapolisaysthatthesesensorsdifferfromnon-certifiedequivalents.
Forexample,theyoftencontaintwomicro-processors,fromdifferentvendors,whicharebasedondifferenttechnologyandimplementa-tions.Eachmicroprocessortakesinputfromthetransducer,theunitthattranslatesthephysicalparameterintoanelectricaloutput.Theresults
Practice Safe Sensing
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ofthetwodifferentcalculationscanthenbecompared,withsimilarresultsensuringthatthemicroprocessorsareperformingcorrectly.An-otherfailsafefeatureofthenewsensorsincludestheabilitytoforcethesensorviasoftwaretoa
predeterminedlevel,suchas80%offullscale.Thisresultcanthenbemeasuredandcomparedtowhatsexpected,providinganotherindicationthatthesensorisworkingcorrectly.
Akeypointtobeawareofregardingthesefeaturesisthattheydonotfunctiontoensureacorrectsensorreading.Theyalsodontpreventafailure.But,notesDiNapoli,thatsnotwhatasafety-certifiedsensorisdesignedtoachieve.Youdontcarethatitfails.Youcarethatitfailsinaknownfashion,hesays.Whatistransmittedinthecaseofsensorfailureisaknown,pre-determinedquantity,allowingthatfailuretobe
detected.Ofcourse,thesystemhastobeconfig-urednotonlytodetectthefaultbutalsonotactonthevalue,sinceitistheresultofafailure.
Whileinternalcomputationsandvaluestransmittedtotheoutsideworldarechecked,sensorsystemsmayhaveonlyonetransducerinthem.
Thisfrontendoftheentirechain,saysDiNapoli,turnsouttobetheareawhereitismostdiffi-culttoassurereliableoperation,inpartbecausetheresonlyoneprocessmeasurement.Sincetheresnothingtoprovideacheckorreference,assurancehastobeinferred.
However,increasingintelligence,alongwithsomesophisticatedalgorithms,maymakesuchindirectcheckingeasiertodoso.Fiveyearsago,sensorpack -ageswererelativelydumb.Today,theyremuchmoreintelligentandprovideinformationthathelpsensuresafeoperationaswellasimprovetheprocess.
Forexample,sensorscanhaveregistersthatcounteverytimethepressuregoesaboveoneofthreesetpoints.Oneofthosepointscouldbetheprocessdesignmaximum.Fromthosecounts,asimplehistogramcouldrevealimportantinfor-mation,suchastheprocessbeingabovedesignmaximumpressureamajorityofthetime.Thesetypesofresultsmayindicateadesignflaworacontrolsystemisntworkingwell,saysDiNapoli.
Anothercompanywithalonghistoryinsafety-ratedsensorsisEndress+HauserofReinach,Switzerland.Thecompanyhaslocalsalescenters
andrepresentatives,withtheU.S.headquartersinGreenwood,IN.Endress+HauseralsopartnerswithRockwellAutomation.
Safety-certifiedmass flowmetersare used in theoil and gas in-dustry. Courtesy
Endress+Hauser
GeroldKlotz-Engmannisheadofdepartmenttechni-calsafetyforthecompanysGer-mansalescenter.HenotesthatthecompanydesignsproductstoconformtoIEC61508;allnew
productdesignswillmeetthisstandard.Achievingthatrequiresselfdiagnosticssothatpassiveinternalcomponentfaultscanbespotted,alongwithstepstoensurenosoftwareglitchesorproblems.
NearlyallcompanyproductshaveundergonethirdpartyassessmentandcanbeusedinSIL2orSIL3safetyinstrumentedsystems.Thesys-temarchitecturedetermineshowmanycompli-antsensorsshouldbeused.Forexample,ifaSIL2levelisrequired,thenthesituationisrelativelysimple,saysKlotz-Engmann.NormallyasinglechannelarchitecturewithaSIL2compliantsen-sorissufficient.Thingsgetmorecomplicatedifahigherlevelofsafetyisrequired,headds.Thenitmaybenecessarytousetwoorthreesensors,withthesensorsvotingtoarriveataconsensusmeasurement.Theadvantageofthethreesen-
sorarrangementisincreasedsafetyandgreateravailability,sincethetriowillcontinuetoprovideinformationevenifoneortwosensorsfail.
CarlSonoda,marketingmanagerforfield
instrumentsolutionsattheYokogawaElectricCorporationofTokyo,saysthecompanysfirstcertifiedsensorappearedin2003.Today,Yokoga-wahasaseriesoftemperaturesensors,pressuretransmitters,andmultivariabletransmitters,allsafety-ratedanddesignedtoachieveaSIL2or3performance.
Thebestpracticeforasafetyloopdesign,henotes,istousemultipletransmitters,witheachvoting.Addingtoredundancycostsarethenum-berofdevices,wiring,post-installationtesting,andongoingmaintenance.
Asafety-rateddevicecanhelpreducethenum-
berofsensorsrequiredandtherebycutsuchex-penses.OurIECSIL2/3certifiedtransmittercanprovidefunctionalitysothatitspossibletouseonesafetytransmitterinsteadoftwostandardtransmitters,saysSonoda.
Deployment Choice: Many Or Few
Thequestionabouthowmanysensorstouseisntclearcut.DalePerry,pressuremarketingmanagerfortheChanhassen,MN-basedRose-mountdivisionofEmersonProcessManagement,notesthatusingfewersensorsisamixedbless-
ing.Fewersensorsincreasethepossibilityofafalsealarm,whichcarriesacostsinceitmightshutdownaprocessneedlessly.Thus,thetotaloutlayofanysolutionwillhavetobecarefullyconsidered.Thecorrectanswerabouthowto
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implementasensorstrategywilldependuponmanyfactorsinadditiontoinstallationandcom-missioningcosts.
A safety-certified transmitter helps keep a phar-maceutical process running smoothly and safelyCourtesy Siemens Energy and Automation
Rosemountshistorywithsafetycertifiedsensorsparallelsthatofothermanufactur -ers.Initially,safetycertifiedproductswereunique,andtheyfacedadistinctiverequire -mentinthattheyhadtomeettheapplicable
standard.Overtime,incorporatingthefea -turesneededforcertificationintostandarddevicesbecameacompanybestpractice.Thisevolutionexplainswhythepricedifferential
betweenstandardandsafety-ratedsensorshasdiminishedsignificantly.
However,itmayneverdisappear
entirely.Acertifieddevicecarriesextraexpenses,notallofwhichcanbefoundinhardwareorsoft-ware.AtRosemount,forexample,Perrysaysincomingordersarecheckedtoensurethatalloptionsorderedwiththedevicearewithinthesafetycertificationscope.
Also,acopyoftheproductsafe-tycertificate,adocumentlistingthecertifiedfailuremodes,effects,anddiagnosticanalysis(FMEDA),aswellasthedeviceserialnum-
berandfailuredataareshippedwitheachtransmitter.DoingsoprovidesrequireddocumentationsaysPerry.Thesameintelligencethatmakessensorssaferincreas-inglysuppliesothercapabilities,hesays.Usersdemandpredictivedi-agnosticsbeyondthesensor.Theywantthisfunctionalitybecausemoreinsightintoaprocesshelpspreventabnormal,andpotentiallyunprofitableordangerous,situa-tions.Thesedemandscouldleadtochangesinsafety-ratedsen-
sors,saysPerry.Weseetheseadvancedprocessdiagnostics,aswellasloopdiagnostics,beingincludedinfuturesafetycertifiedproducts.
Year Cumulativetotal
Source: Control Engineering and exida
1996 11997 1
1998 2
1999 2
2000 2
2001 4
2002 4
2003 52004 7
2005 8
2006 12
2007 24
2008 34
2009 39
2010 46
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Recently It Seems That Everyone
Is Talking About Process Safety,But More Than That, Theyre DoingSomething About It.
Peter Welander, Control Engineering
Recentlyitseemsthateveryoneistalk -ingaboutprocesssafety,butmorethanthat,theyredoingsomethingaboutit.ARCAdvisoryGroupsaysthatstronggrowthofsafetyinstrumentedsystemswillcontinuein2008.Currentsoftconditionsmaytemperitinsubsequentyears,buttheworldwidemarket,whichwasaround$1.4billionin2007,isex -pectedtogrowatacompoundannualgrowthrateofover12%peryeartoover$2.5billionin2012.ThisisfromanewARCstudy,ProcessSafetySystemWorldwideOutlookMarketAnalysisandForecastThrough2012.
Thesafetysystemmarkethasexperi-encedunprecedentedgrowthforthelasttwoyears.IncreaseddemandforoilandgasduetotheeconomicgrowthofChinaandIndia,alongwiththehighpriceofcrudeoil,isfuel -inginvestmentsinoilandgasproductionand
inrefining,leadingtoincreaseddemandforsafetysystems,accordingtoARCvicepresi -dentAsishGhosh,theprincipalauthorofthestudy.
Thecomplexityofsafetysystemshasin -creasedovertimebecauseofeffortstoadd
capabilitiesandmakethemeasiertoworkwithonanoperationalbasis.(Systemarchi -tectshaveperhapslearnedsomelessonsfromtheover-proliferationofalarmfunctionsincontrolsystems.)Theresultisthatcompaniesfrequentlyhavetoengageoutsidehelpwhendesigningnewsystemsorupgradinganexist -ingfacility.Thiscaninvolveaconsultinggroup,systemintegrators,oraprimaryautomationsupplier.
Majorprocessautomationsuppliersgen -erallyhavedepartments,divisions,orevenwholesubsidiarycompaniesdedicatedto
processsafety.Examplesinclude:
ABBSafetySystems;GEFanucSafetyNet;EmersonSmartSIS;HoneywellSafetyManager;InvensysTriconex;RockwellICSTriplex;SiemensSIS;andYokogawaProSafeRS.
Thislistisjustabeginning,andthereare
manymoresafetysuppliersavailable.(Readthisarticleonlineat www.controleng.com/ar-chiveforlivelinkstoallthesecompanies.)
Safety-on-fieldbusplatformsforprocessappli-
cationsexistorareindevelopmentandincludeAS-interface,ProfiSafe,andFoundationFieldbusSIF.Provenindiscreteapplications,theseeffortswillrequireadditionaltimetocom-pleteandbeadoptedbyendusers,butthepromiseisthere.
Resources Galore
Ifyouwanttodosomeinitialresearchonthetopic,youwonthavetogofar.Asearchatwww.controleng.comonprocesssafetywillbringyousome10,000resultsthatcanbedividedintovarioussubtopics.
Also,Siemens,inpartnershipwithControlEngineeringandPlantEngineering,haslaunchedanewWebsiteatwww.safetybase.com whichcontainsahugeamountofinformationonbothprocessanddiscretemanufacturingsafetyissues.
And,ofcourse,thereistheOSHAprocesssafetysiteatwww.osha.gov/SLTC/processsafetymanagement,whichlistsallrelevantstandardsrelatedtothepotentialre -leaseoftoxic,reactive,orflammableliquidsandgasesinprocessinvovlinghighlyhazardouschemicals.
Ifyoupreferface-to-facecontact,nextmonthsISAExpoinHoustonwillhaveadedicatedconferencetrackcalledSafetyExchange.ISApromisesthatitwillincludepeer-reviewedtechnicalsessionscoveringtopicssuchassafetyinstrumentation,SISapplication,alarmmanage -ment,competency,andmore.Sessionsareplannedduringtheconference,includingSISApplicationsinOilandGasProduction,apresentationthatexploresthenewsafetyinstrumentedsystemengineeringtechniquesrequiredtomeetHSEstandardsforenergyfacilities.
Allthattosay,thereisnoshortageofresourcesasyouexplorethiscriticalarea.Thereismuchtodiscoverandlearnatyourfingertips.
Process Safety Resources At Your Fingertips
http://www.controleng.com/archivehttp://www.controleng.com/archivehttp://www.controleng.com/archivehttp://www.controleng.com/http://www.safetybase.com/http://www.osha.gov/SLTC/processsafetymanagementhttp://www.osha.gov/SLTC/processsafetymanagementhttp://www.safetybase.com/http://www.controleng.com/http://www.controleng.com/archivehttp://www.controleng.com/archivehttp://www.ueonline.com/plant_safety.html8/6/2019 United Electric eGuide Hi Res
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The Distinctive Ring From The Ra-
dio Phone Told The Control RoomOperator That The Person On TheOther End Would Be The Tank Level
Technician. He Also Knew That IfA Call Was Coming In, Something
Was Amiss At The Remote SiteTank Farm. I Cant Get On The LastTank Said The Voice On The OtherEnd.... Here Are Considerations ForDeciding Between Intrinsically Safe
And Explosion-Proof.Jeanine Katzel for Control Engineering
Wireless temperature transmitters are among
the intrinsically safe equipment used by chemicalplants to reduce the hazards of explosive envi-ronments. Such devices are simple to install andmaintain. Source: Emerson Process Management
Enraf radar level gauges measure productlevel and other parameters in bulk storage tanks,combining explosion-proof and intrinsically safetechnologies to achieve a cost-effective, yet safesystem for tanks containing hazardous sub-stances such as gasoline, crude oils, and liquidchemicals. Service technician uses an intr insicallysafe, handheld device equipped with extensivediagnostics to commission a gauge safely whilethe tank is in full servic e. Source: HoneywellProcess Solutions
Thedistinctiveringfromtheradiophonetoldthecontrolroomoperatorthatthepersonontheotherendwouldbethetankleveltechnician.He
alsoknewthatifacallwascomingin,somethingwasamissattheremotesitetankfarm.
Icantgetonthelasttanksaidthevoiceontheotherend.Theresacatonthesteps.Well,
justshooitaway,thecontrolroomoperatorstartedtosay,butstoppedabruptlyasthetech-niciancurtlyinterrupted.ItsamountainlionandImnotgoingnearit,hesnapped.Imcomingin.Onetanklevelwouldremainunread.
Thesituationisunusual,andadmittedlyrareintodaysworldofautomatedmeasurementsystems,butthiscaseistrue.Beforeautomat-
ingtankgaugingata
distantlocation,thiscompanyactuallyhadtodealwithamoun-tainliononthetanksteps.
Usingtoautomatedtankgaugingsystems,whichmanyhavedone,eliminatespersonnelhazardssuchastankclimbing,exposuretoproduct,andinclem-entweather,butraises
otherissuesamongthem,thetypeofprotectiontoselectforsuchhazardousap-plications.
Intrinsically Safe Or Explosion Proof?
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Thedangersinvolvedintheproductionandpro -cessingoffuels,suchasexplosiveatmospheres,mayrequiretheuseofintrinsicallysafe(IS)devicesundersomecircumstances,orexplosion-proofenclosuresinothers.Whichoptionisbestforyourapplication?Whenshoulditbewireless?Whatbenefitsdoesonetechnologyholdovertheother?
Thedecisionisnotalwayssimpleoreasy.
What Type Of Hazard Is It?
Decidingwhichtypeofprotectiontoapplyisrarelycut-and-driedforavarietyofreasons,fromequipmentinvolvedandtypeofhazardtocompanypolicyorpreference.Intrinsicallysafeandexplosion-proofenclosures,devicesandpracticeshavemorerelevanceincertainindus-triesthaninothers.Here,examplesfocusontheoilandgas,refining,andchemicalindustries,butsimilarsafetyrisksexistelsewhere.Inwaterandwastewateroperations,forexample,digestersreleasehazardousmethanegas.Agricultureandminingindustriesfacepotentiallyexplosivedusts.Butinoilandgasproductionandrefiningorinchemicaloperations,hazardousenvironmentsarecommon,andagiven.Inothercircumstances,hazardousmayindicatepoisonous.
Choosingexplosion-prooforintrinsicallysafedevicesisreallymoresite-thanapplication-spe-cific.Inpractice,suchdevicesareunderconsid-erationinonlyasmallpercentageofcases(less
than5%).Therefore,amongthefirststepsanyfacilitywouldtakeinitshazardanalysiswouldbetoascertainifintrinsicallysafeorexplosionproofpracticesanddevicesareindeedrequired.
Intheanalysis,safetycompliancepersonnelsurveyanapplicationwhileitisoperatingusingsniffersorothersensingdevicestodetectthepresenceofhazardousgasesanddeterminethedegreeofthehazard[byNECclassanddivision].
Ifhazardousgasesarepresentnormally,aClass1,Div.1situationexists.Iftheyarepresentonlyoccasionallyorunderunusualcircumstances,theareaisdesignatedClass1,Div.2.WithClass1,Div.2situations,non-incendivedevicestypicallyaresufficient.Anoverwhelmingmajorityofappli-cationsfallintothiscategory.Itisfortheremain-ingcasesthoseClass1,Div.1situationsthatISorexplosion-proofdevices(EXD)andpracticesareintended.
Briefly,explosion-prooftechnologyplacesadevicewithinanenclosuresturdyenoughtowithstandanexplosionofaspecifiedlevel.
Shouldafireorexplosionoccurintheequipment,theeventwillbecontainedwithintheenclosureandanygaseswillcoolbelowignitionlevelbeforetheycanescapetotheatmosphere.
Explosion-proofenclosuresareratedandtestedbyindependentlaboratories(suchasFac-toryMutual)forspecifiedpressuresandflamelengths.Typicalapplicationsincludelightfixtures,engineswitches,pumps,andotherequipmentsuchasmotorsthatmustbeplacedinhazardousareas.Explosion-prooftechnologyshouldbethesoleoptiononlywhenanIS-rateddeviceisun-available.Apumporagitatorwithalargemotor,
forexample,issimplyofasizethatcantbemadetobeintrinsicallysafe.Explosionproofequipmentandpracticesthenwouldbemandatory.
Intrinsicallysafedevicesandassociatedwiringlimittheamountofpower(voltage,current,in-
ductance,capacitance)toandinadevice,sothatasparkcannotbegenerated.TheyarepoweredfrombeyondaZener-diodeorgalvanic-isolationbarrierinasafeareaawayfromthehazard,typicallyacontrolroom.Intrinsicallysafedevices
canbeopened,maintained,orotherwisehandledwhilepowered.Suchaninstrumentmightbeusedtomeasurethetemperatureinsideatankorapipeline,forexample,whereanenclosurewouldbeimpractical.ISoptionsareoftenlessexpensivethanexplosion-prooftechniques,buttendtobemorecomplicatedtoapply.
Ifariskassessmentrevealsanareaoraprocesstobeahazardousatmosphereinneedofoneoftheseprotectivetechnologies,man-agementmustthoroughlyfamiliarizeitselfwithallappropriatecodesandstandards(NEC,Atex,IEC)atthecorporate,local,national,andinter-
nationallevelsandfollowthemscrupulously,understandingthatstandardsaredynamicandthat,althoughprogressisbeingmadetowardglobalharmonization,variationsarestillsignifi-cant.Further,codesaresubjecttointerpreta-tion,andthelocalauthorityhavingjurisdictiontypicallyhasthefinalsayinwhatstandardsareapplicableandhow.
Acompanywithmultiplesitesshouldconsiderthatidenticalsituationsinplantsinseparategeographicallocationsmaybeinterpreteddif-ferently.Aswithanysafetytechnology,asafetyprofessionalwithexpertiseinthetechnologies
andallapplicablecodesandstandardsshouldbeconsultedbeforemovingaheadwithanyhazard-ous-areaproject.
Tank Farm Example
Withthisbackgroundinmind,letslookathowthesetechnologiesplayoutinreal-liferefineryoperations.MarkMenezes,PEng.,ismeasure-
mentbusinessmanagerCanadaforEmersonProcessManagement.Heoffersonereasonwhyrefineriesfacegreater-than-normalhazards:Inthelast10yearsorso,manyrefinerieshaveupgradedtheirinstallationstooperateonheaviercrudeoils,suchasthosefoundintheCanadiantarsandsorinVenezuela.So,ifyouwanttooper-atearefineryusingsomeofthesefeedstocks,youneedtocrackthisheavyoilwithalotofsteamunderhighpressure.
Alotofhydrogenisinusehere,usedtomaketheseheavyoilslighter.Hydrogenisoneofthemosthighlyflammablegasesthereare.Any
hydrogenleakathighpressureisverydangerous.Inaddition,sulfursarecommonlystrippedoutofthesesourheavyoils.Anyplantofthisnatureisgoingtopayalotofattentiontohazardousenvironments.
Oneareaofspecialconcernisthetankfarm.Operationsneedstoknowhowmuchfuel,chemical,orotherliquidresidesineachtank.The
job,explainsFrankVanBekkum,seniorproductmarketingmanager,HoneywellProcessSolutions,canbedonemanuallybysendingsomeonetothetank(asdescribedabove).Butthisapproachisdifficult,dangerous,andundesirable.Inaddition,
hesays,Tankfarmsareoftensituatedindeso-
lateareas.Oil,gas,andchemicalproductsmaybehazardoustohealth;tankscanbedifficulttoreachthroughice,snow,or,inanotheractualcase,aneaglesnest.
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Modernrefineriesuseavarietyofequipmentformeasuringtanklevel.Options,saysVanBek-kum,includeradargaugesystemsthataretypi-callypoweredbythemainservice,equippedwithcommunications,andpackagedinanexplosion-
proofbox.Inaddition,headds,anintrinsicallysafetemperaturegaugingdeviceonthetankgoesintotheproduct.Sowhatwehaveisaseparatedevicesuppliedinanexplosion-proofboxwithaconnectionandtemperatureprobesthatgointothetankthatareintrinsicallysafe.Eveniftheprobewhichisbasicallyastainlesssteelhoseshouldleakorfail,theinstallationisstillsafe.
Theapproachisstraightforward,butthecomplexityofmanyoperationsmakesuchsafetysystemsdifficulttoapplycorrectly,warnsVanBekkum:Intrinsicallysafedevicesarentalways
anoptioninatankfarm,becauseyouhavepumpsandcontrolvalvesthathavepowerandcommunicationrequirementsthatgobeyondthedefinitionofintrinsicallysafe.IfyouareusingISinstrumentationinthefield,itmustbekeptsepa-ratefromapowereddevicetoavoidanyconfu-sionastothetypeofdeviceitis.
Intrinsically safe devices and practices are crticialat loading racks. Here, a load computer (bottom),and tang grounding equipment help achieve safeand efficient hydrocarbon loading. Groundingequipment for the road and rail tankers helps pre-vent accidents by slowly discharging static electric-
ity without creating dangerous sparks. Once thedischarge is complete, a permissive signal is trans-mitted to the system controlling the load process.Source: Honeywell Process Solutions.
Historical Views,Future Developments
Regardlessofthetechnology,addsVanBek-kum,mostproblemsresultfromhumanerror,and
mostoccurnotwhennewequipmentisinstalled,butwhenmodificationsaremadewithoutad-equateplanningandcommunication.
Forexample,anexistingtransmitterisre-placedwithanewtechnology,perhapsfromadifferentmanufacturer,andthemaintenancede-partmentisawareofthechangebuttheproduc-tiondepartmentisnot,heexplains.Caremustbetakentoensurespecificationsmatchandthatalltechniciansareinformedofanychanges.
Oneconcernisthattheuseofbothtechnolo-giesinthesameareacouldleadtoconfusion,
error,and,therefore,danger.Workersmustbetrainedspecificallyfortheareainwhichtheywillwork;indeedmanytodayarebeingcross-trainedtohandlebothtechnologiesasmorecompaniesfindthemselvesusingbothapproachesatthesamesite.MenezesandVanBekkumagreethat,inmostcases,theuseofonetechnologyortheotherisnotnecessarilyapplicationdriven.Rather,itisasite-by-sitedecision,influencedby,amongotherthings,habitandtraditionratherthantech-nologyandhazard.
IntheopinionofEmersonsMenezes,explo-sion-proofpracticeshavehistoricallybeenmore
commoninNorthAmericavs.intrinsicallysafepracticesinEurope.Intrinsicallysafeislikelytohaveaslightlylowerinstalledcost,butismorecomplextoimplement.YouhavetothinkaboutalotmorethingswithIS.Explosion-prooftech-
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nologyisfairlysimpletoapply.Youbuyrobustdevicesthatwillcontainthepotentialexplosionandinstallthemusingrobustconduit.Suchprac-ticesensureifanexplosionoccurswithinadevice,itiscontained,andbythetimethegaseshave
escapedtheyhavecooledtothepointthattheyarenolongerdangerous.Productsusedinoilandgasoperationsandrefineriesarerobusttostartwithbecauseofthenatureoftheindustry.MostNorthAmericaoperationsaresousedtoinstall-ingrobustdevicesthattheyassumeallproductsareexplosionproof.
Influence Of Bus Networks, Wireless
BothMenezesandVanBekkumagreethattheworldismovingtowardintrinsicallysafeprac-ticesandawayfromexplosion-prooftechniques.
IntrinsicallysafepracticesaregainingpopularityinNorthAmericafortworeasons:increasinguseofbustechnologiesandproliferationofwirelesssystems.NotesMenezes,IfIhave,forexample,aFoundationfieldbusinstallation,Icanputonebarrierinthesafeareaandconnectfivetrans-mittersonthesamebus.Mycostisdividedby5.Manyuserswhohavehistoricallyusedexplo-sion-proofwiringpracticesturntointrinsicallysafewhentheyadoptFoundationfieldbus.Bustechnologiesarealsocatchingonwithmotorsnotthepowertothemotor,butthesignal.Eightmotors,forinstance,connectedtogetheronanISlow-powerProfibusorDeviceNetbus,canbe
startedatonce.
Wirelessalsoispromotingtheadoptionofintrinsicallysafepractices.Manywirelessdevices
arebatteryoperatedand,inonesense,inherentlyintrinsicallysafe.Ifaplantneedstomonitorthetemperatureinareactor,itisrelativelyinexpen-sivetoinstallawirelesstemperaturetransmit-teronit.Thepowerlevelisjustnotsufficient
togenerateahazard,saysMenezes.Iftheresaproblem,forexample,withabatteryinsideatransmitter,thetechniciangoesintothefield,unscrewsthecap,removesthebattery,replacesitwithafreshone,andscrewsthecapbackonrightinthehazardousenvironment.Inmanyrespects,itisanISdesign.
Menezesalsooffersamoredetailedexample:Thefailureofaprimaryseal.Apressuretrans-mittermaybecorrectlyinstalledusingeitherintrinsicallysafeorexplosion-proofpractices,heexplains,butifahazardousfluidburststhroughtheprimaryseal(thetransmitterdiaphragm
thatconnectsthedevicetotheprocess)andtheprocessfluidfindsitswayintothetransmitter,itsnotaleaptoseethattheprocessfluidmayburstthroughintoeverythingelseandfollowtheconduitintothecontrolroom.Youcouldconceiv-ablyhave2,000-psihydrocarbonsquirtingontothefloorofwhatissupposedtobeasafearea.Awirelessdevicewouldmeannoconnectionsfromthetransmitterinthehazardousareatothesafearea.Headmitsthereareotherwaystosolvethisproblemusingdualsealingdevicesandcon-duitseals,buttheyaddcostandcomplexity.
Withlegacysystems,theexplosionproofap-
proachhasbeensimpler,morecomfortable,saysMenezes.Butonceyouhavemultipledevicesonabusorinstallationsusingwirelesstechnol-
ogy,intrinsicallysafebecomesmorecompelling,particularlywithinstrumentation.
VanBekkumagrees,hisviewsonwirelessaresharedbycolleagueJeffBecker,HoneywellPro-
cessSolutionsglobalwirelessdirector.Custom-
ersareapplyingwirelesstechnologytomeasurepotentiallycriticalprocessvariablesinhazardouslocationsthatwerepreviouslyinaccessible,saysBecker.Applicationsarerelatedto,amongoth-ers,butanespherepressuremonitoring,inven-torymanagement,safetyreliefvalvemonitor-ing,safetyshowermonitoring,andrupturediscmonitoringallcriticalpointsinarefineryorchemicalplant.Wirelesshasempoweredandenabledtheseapplicationsandmetregulatoryneedsrelatedtoenvironment,explosiveclassifiedareas,safetyregulatory,andbottomlinecom-panyprofitabilityobjectives,headds.
Althoughtherearestillsomeemotionalissuesassociatedwithwireless,theindustryismovingfasterandfastertowardwirelessandtherearealotofbenefits,addsVanBekkum.Whenmakingwirelessdecisions,itscriticaltodevelopaninfra-structurefortheentireplantthatmeetscurrentandfutureneeds.Wirelessismorethanaone-to-onereplacementofcopperwire.Ifyoubuildagoodsystemaccordingtocurrentstandards,itwillbesafeandsecure.Nothingis100%foolproof,butyoumakethingsassafeaspossible.Youreducetherisktoanacceptablelevel.
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Contributed by: Richard Frauton,Product Manager, United Electric Controls
Safetyimplementationtypicallyisdonebyagroupthatincludesplantinstrumentengineersandtechnicians,whoarechargedwithfindingsimpleandreliablesolutions.Often,thesesitu-ationsinvolvethequestionofwhentoshutaprocessdown.Suchdecisionsfrequentlyhingeonkeyprocessvariablessuchasflow,level,temper-atureandpressure.Thesemustbeinaspecifiedrangeatvariouslocationswithinchemicalplants,
petrochemicalrefineries,powerplants,criticalprocessvesselsandevenateyewashstations.
Forsuchpointsafetyapplications,aproperlydesignedandimplementeddigitalswitchwithself-diagnosticscanbeanimportantpartofthe
answer.Asanelementofamultipletechnologysolution,adigitalswitch-basedapproachcanhelpeliminatecommon-modefailures,significantlyimproveresponsetime,achieveneededsafetyintegritylevels(SILs),andsimplifyplantsafetyinstrumentation.
Switch background
Yearsago,manyswitcheswereblindmechanicaldevicesactuatedelectromechanicallyorbypneu-matics.Theyofferednoindicationofreliability,suchassuccessorfailureinresponsetoacommand.
Thislackoffeedbackwasparticularlyworrisomeinsafetyapplications.Theresultcouldbecatastrophic,
shouldamalfunctionoccurinplaceoftheproperre-sponsetoatrippedpressureortemperaturealarm.
Partlybecauseofthispossibility,therehasbeenageneraltrendtowardothersolutions.Inparticular,onepopularimplementationhasbeentouseatransmittertogetherwithadedicatedcontrolsystem,onethatisseparateanddistinctfromthebasicprocesscontrolsystem.Abenefitofthisapproachisthattransmitterscancon-veyagreatdealofrelevantprocessinformation,whichcanbeusefulforsafety,controlandlateroptimization.Thistechnologyalsoensuresthatconnectionsareactiveandthetransmitteris
working,twocriticalrequirementsforanysafetyapplication.
Insuchsetups,transmittersprovideprocesscontinuousdata,andanalarmorprotection
systemactsuponthisinformation.Informalsurveyshaveshownthatsomewherebetween25and33percentoftransmitterstodayareinsuchloops.Thus,nearlyathirdofthetime,theresultisapointsafetysolutionofferingbinary,on-offactionatthecontrolroomthatiseffec-tivelyequivalenttowhathadbeenprovidedbyatraditionalswitch.
Whiletransmittershavebeenevolving,switch-eshavebeenundergoingtheirownrevolution.Switchesnowaredigital,withprogrammablesetpointsanddeadbands.Theyoffersuchcapabilitiesasself-diagnosticsolid-stateelectronics,plugportdetectionandnuisancetripfiltering.Theyalsohavefail-safe-openprogram-mingmodesthateliminatetheproblemofundetectedfailure.
Anexampleofhowthiscapabilitycanbeimplement-edisfoundinUnitedElectricControlsOneSeriesofdigitalpressureandtemperatureswitches.Whencondi-tionsarenormal,theswitchpulsesat20Hzbetweena1and0,sendingacontinu-ousheartbeatsignaleasily
detectedbyacontroller.Aprocessupsetwillbemarkedbyclosingtheswitchandtheoutputofacontinuous1.Iftheinstrumentdetectsa
fault,theswitchopensandtheoutputdropstoasteady0,thesameresultasisthecaseforawiringfault.In
thisarrangement,bothsuccessandfailureareclearlyindicated.
a. Whenprocessandinstrumentconditonsarenormal,apulsedoutputindicatesso.
b. Whenasetpointisreached,theswitchcloses.
c. Asconditonsreturntonormal,pulsedoutputresumes
d. Adetectedinstrumentfaultcausestheswitchtoopenandstayopen.
White Paper: Simplifying Plant Safety Instrumentation
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Voting for an improvedSafety Integrity Level
Digitalswitchtechnologyalsosatisfiesrequirementsneededto
achieveagivenSIL,whichisameasureoftherelativerisk-reductionprovidedbyasafetyfunction.AsdefinedbytheInternationalElec-trotechnicalCommissionsstandardIECEN61508,SILincludesbothahardwareandsystemcomponent.Onthehardwareside,integrityisdeterminedbyaprobabilisticanalysisofthedevice,withparticu-larSILsmeetingthetablebelowfordiscreteorlowdemandopera-tion.
Inthecaseofhighdemandorcontinuousoperation,thefol -lowingtableapplies.
DeterminingtheSILforasystemisamulti-steppro-cess.Itbeginswitharigorousriskanalysisofthesystem,followedbycalculationswith
theSIL,orpreferablyrawfailureprobabilitydata,forthedevicesdeterminedtobeinthecriticalpath.Fromthatanoverallreliabilityfigureisdetermined,whichthenyieldsthesystemSIL.
KnowingthisprovidesmethodsneededtoachieveagivenSIL.Forinstance,avotingschemeinvolvingthreeSIL2-compliantcomponentscanleadtoaSIL3-system,as
demonstratedbythefollow-ing:
Probs=Prob1xProb2xProb3
whereProbsistheprobabil -ityofsystemfailureinavotingschemebasedonindependentcomponentsandProbxistheprobabilityofcomponentxfail-ing
SinceratioofoneSILto
anotheris10:1,threecomponentvotingraisesthesystemSILfromoneleveltothenext.Italsoistheminimumnumberofcom-ponentsneededtobreakties.
Point safety applications
SowhataresomeoftheapplicationsthatdemandagivenSILandapointsafetysolution?Exam-plescanbefoundinprocessingplants,transportationandworkersafety.
Thefirstgroupinvolvesactiveprocessing,withinstancesfoundinchemicalandpetrochemicalplants,refineries,andoilandgasfacilities.Insuchsituations,thereisatleastone,andpossiblysev-eral,criticalprocessvessels,inwhichareactionoccursthatmustbemonitoredforlevel,flow,pres-sure,temperature,oracombinationofthese.
SIL Probabilityoffailureondemand Riskreductionfactor
1 0.1-0.01 10-100
2 0.01-0.001 100-1000
3 0.001-0.0001 1000-10,0004 0.0001-0.00001 10,000-100,000
SIL Probabilit yoffailureondemand Riskreductionfactor
1 0. 00 001-0. 00 00 01 100,000-1,000,000
2 0.000001-0.0000001 1,000,000-10,000,000
3 0.0000001-0.00000001 10,000,000-100,000,000
4 0.00000001-0.000000001 100,000,000-1,000,000,000
S d b
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Inpetrochemicalrefining,forexample,incomingcrudeoilundergoesdistillationwiththeoutputprocessedthroughanisomeriza -tionunittoalteritsstructurebeforeemergingasafuel.Isomerizationofteninvolvesheating
productinthepresenceofacatalyst,suchasplatinumoranothermetal.Thecombinationofheatandachemicalreactioncanspiraloutofcontrol,ruiningproductandpossiblyleadingtoanexplosion.Thesameistrueforthesulfurremovinghydrotreaterunitsfoundinmultipleplaceswithinarefinery.Thus,thetempera -tureandpressuremustbemonitoredatmanylocations,and,ifneedbe,theprocessstopped.
Asecondsetofapplicationsinvolvestrans -portationorstorageofflammablematerials.Examplescanbefoundingrainelevatorsandpowerplantcoaldustconveyors.Inthe
firstcase,grainmustbemovedintoandthenwithinastructure,whichisaccomplishedbyagrainelevator.However,anyfineairbornesus -pensionoforganicmaterialiscombustible.Forthatreason,stonesandmetallicfragmentsareremovedbeforegrainistransportedormilled.Still,theelevatoritselfcanbeasourceofheatorsparks.Thusthetemperaturewithinthemechanismhastobemonitoredandtransporthaltedifdangerousconditionsdevelop.
Thesamesituationprevailsinpowerplantsorotherfacilitieswithcoaldustconveyors.If
aconveyorbearingorrollerbeginstooverheatandthesafethresholdexceeded,thismustbedetectedandtheconveyorshutdown.
Anexampleofafinalapplicationcategorycanbefoundineyewashorsafetystations.Theseareinstalledtoensureworkersafetyandmustfunctionflawlesslywhenneeded.Consequently,itiscriticalthatthewashsolutionbecoolenough
nottoscaldandwarmenoughnottofreeze.Thesestationsaresituatedinindustrialsettings,wheretemperatureextremesarepossible.Thus,amethodtomonitorthesituationandsignalacriticalalarmisimportant,intheeventthatthewashtemperatureistoohighorlow.
Considerations
Astheseexamplesshow,thereoftenisarequirementtomonitorandreacttocriticalprocessvariables,suchastemperatureandpressure.Inimplementingasolution,engi -
neersshouldkeepinmindthatmultipletech -nologiesarebetterthanone,speedcansave,andthatsafetysystemsmustbeindependentofthebasicprocesscontrolsystem.
Thefirstpointisimportantbecausemultipletechnologiesavoidcommon-modefailures.
Takethecaseofatransmitter-controllerloopversusaswitch.Theformerwillsufferfromthepotentialofacommon-modefailureduetotherelianceonapossiblydistantcontrollerforaction.Becauseitisself-contained,theswitchcontinuestoworkregardlessofwhathappenstosomeothercomponent.
Aswitchisalsosignificantlyfaster.Forin -stance,theOneSerieshasaresponsetimeof
lessthan60mS,fivetimesbetterthanwhatcanbedonewithatransmitter.Anytimesav -ingscanbecrucialinpreventingormitigatinganunsafecondition.
Theself-containednatureofaswitchalsoensuresthatthesafetysystemcanbeinde -pendentofthecontrolsystem.Aswitchwilltakeitsownreadingsoftemperatureandpressure,forexample,andthenreactinre -sponsetoitsownprogramming.
However,itisimportantthatthisinde -pendencenotbetotal.Thatis,anysafetyinstrumentmustbeabletoreportonitsownconditionandinterfacewiththerestofaplantnetwork.InthecaseoftheOneSeries,thisisdonethroughself-diagnosticsthatallowextensivefaultdetection,includingpluggedportsandpowersupplyout-of-rangecondi -
tions.Italsooffersmultipleoutputs,withbothaswitchfunctionanda4-20mAanalogout -put.Finally,itreportsfaultslocallyonitsowndisplayandremotelyviamultipleoutputs.
AfinalandimperativeconsiderationforanytechnologyinvolvesdeterminingtheSILratingforanentiresystem.Asdiscussedearlier,theuseofvotingcanallowSIL2componentstocreateaSIL3system.Insucharrangements,acriticalpieceofinformationisindependentverificationofsafetyratings.Partofsuchverificationwillincludefailuremodes,effectsanddiagnosticsanalysis.FEMDAdataispart
oftheIECEN61508certificationandcanbeusedincalculatingoverallsystemSIL.
Conclusion
Ashasbeenshown,advancesintechnol-ogyhavemadeswitchescapableofmonitor-ingtemperatureandpressurewhileconductingself-diagnostics,therebyremovingtheperilsof
blindmechanicalaction.Thesedevelopmentsconstitutegoodnewsforengineerswhotodaywanttosimplifyinstrumentationinpointsafetyapplicationscommoninpetrochemicalplants,coaldusttransportation,criticallineorvesselprotection,oreyewashsafetystations.Suchswitchesofferimprovedsolutionsthroughavoid-anceofcommonmodefailures,fasterresponsetimes,independencefrombasicprocesscontrolsystems,andtheabilitytoachievedesiredSILviaavotingscheme.
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