United Electric eGuide Hi Res

8/6/2019 United Electric eGuide Hi Res

1/17

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eGuide on Safety Instrumentation

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
http://www.ueonline.com/plant_safety.html


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Sponsor Profile Intelligenceat the Device

Practice Safe Sensing Process safety resourcesat your fingertips

Intrinsically Safeor Explosion Proof?

Simplifying Plant SafetyInstrumentation

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.
mailto:[email protected]:[email protected]://www.ueonline.com/plant_safety.html


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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.html


11/17

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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-


14/17

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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.


15/17

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Process safety resourcesat your fingertips

Practice Safe SensingIntelligenceat the Device

Sponsor Profile

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
http://www.ueonline.com/plant_safety.htmlhttp://www.ueonline.com/plant_safety.html


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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.

For more information go to

www.ueonline.com/plant_safety.htmlor email [email protected]
http://www.ueonline.com/plant_safety.htmlmailto:[email protected]:[email protected]://www.ueonline.com/plant_safety.htmlhttp://www.ueonline.com/plant_safety.html

Documents

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