PowerEngineeringThirdClassTest3rdClassOldPartBPaper1ReviewQuestionsandAnswers动力工程三级考试试题试卷

1.Consideralargemodern(15000-kpa)steamgeneratorfiringpulverizedcoal.Explainin

detailhowthehotgasesgiveuptheirheatinpassingfromthefireballinthefurnacetothe

stackexit.Indicateinyourexplanation,thevariouspartsoftheunitcontactedbythegases

intheircorrectsequence.(15marks)

ANSWER:

2.List5characteristicsoffueloilthatmustbeconsideredwhenselectingapowerplantfuel.

Explainhowtheseimpactontheoperationoftheplant.(15marks)

ANSWER

A.THESPECIFICGRAVITY

-enablestheengineertodetermineiftheoilislightorheavy

-generallythehigherthespecificgravity,thelowerthehydrogencontentandtheharderit

istoignitethefuel

-lightfueloilavoidsthecostofkeepinganoiltankwarmed

-heavyfueloilsarethecheapesttopurchasebutitbecomescriticaltoknowthewater

contentoftheheavyfueloil.Someheavyfueloilsrequiretobeheatedtoashighas135

degreesCattheburnersupplybeforetheywillbumcompletely

B.THEHEATINGVALUE

-isrequiredtobeknown

-enablestheengineertocalculateboilerefficiencywhentheboilerisrunningsteadywith

aminimumofinterference

-boilerefficiencyistheheatsuppliedtothesteamperhourdividedbytheheatavailable

fromthefuelperhour

C.THEFLASHPOINT

-enablesengineertojudgeiffuelissafetostorenearabuilding

-thehighertheflashpoint,thesafertheproductwilltendtobetohandleandstore

-normallyfueloilswithaflashpointoflessthan66degreesCwouldnotbestored

-allfueloilsnormallyhaveaflashpointofaround90-95degreesC

-fueloilsaresafertohandlethankeroseneorgasolinewhichhaveamuchlowerflash

point

Flashpoint:thetemperaturewhereaproductgivesoffenoughvaportobeignitedbutnotsustain

aflame

D.CONGEALINGORPOURPOINT

-enablestheengineertoknowatwhattemperaturetheoilwillrefusetoflow

-tracersteamlineswouldneedtobeinstalledonallfuellinesinareasthatexperience

sub-zerotemperatures

-toknowthepourpointwouldindicatewhenthetracersteamlinesshouldbebrought

backintoserviceinthefall

E.VISCOSITY

-isameasureoftheresistancetoflow

-itisexpressedintimeinsecondsthattheoilatacertaintemperaturewilltaketoflow

throughastandardsizedholebygravity

-thehighertheviscositythemoretroubleonecanexpectwhenburningthefuel

-iftheviscosityistoohigh,atomizationispoorandexcessivecarbondepositswillresult

onthefurnacefloorandintheboilerheatingareas

theflamepatternwillbepoorwithincompletecombustionandalowerheatrelease.Operatingcostsasa

resultwillbehigher

3.Asashiftengineerinchargeofalargehighpressure(15000kpa)multiburnergasfired

boilerequippedwithasuperheater,economizer,forceddraftfans,andautomatic

combustioncontrols.Describethepossiblecausesofandtheactionsthatyouwouldtakein

thefollowingcircumstances.Theboilerisonsteadyload.

a.Alowdrumwaterlevelcondition.(8marks)

ANSWER

A.BOILERFEEDWATERLEVELCONTROLVALVE

-cause:isolationvalveclosedoninletoroutlet

-remedy:openvalve

-cause:failed

-remedy:openby-passaroundcontroller

B.BOILERFEEDWATERPUMP

-cause:failed

-remedy:startbackuppump

-cause:isolationvalveclosed

-remedy:openvalve

-cause:cavitation

-remedy:drainandbleedpumptoriditoftheairlock;thenrestartpump

C.INSTRUMENTATION/EQUIPMENTFAILURE

-cause:failureofswitch

-remedy:replaceswitch

-cause:electricalwire

-remedy:checkforlooseconnectionsandtighten

-remedy:ifshortcircuitingduetobarewiresorbrittleinsulation:replace

wire

-cause:mercuryswitchglassiscracked

-remedy:replacemercuryswitch

-cause:sedimentinfloatchamber

-remedy:blowoutthechamberbymeansofdrainvalveprovided

-cause:lowwaterfuelcutoffswitch

-remedy:testregularlytoensureitisworkingproperly.

Immediatelyreplaceifnotworking.Thisisacritical

pieceofequipmentinstrumentation.

b.Aflamefailure.(7marks)”

ANSWER

-cause:fuelsupplypressuredrops

-remedy:shutdownmainburners.Allalarmsmustberesetandall

parametersmetbeforereignitingburners.

-cause:lowwaterfuelcutoffhasactuated

-remedy:findcauseoflowwaterlevelandaddressthatissuebefore

attemptingtore-establishflame

-cause:ifboilerisnormallyrunatslightlynegativepressureanditgoestopositive

pressure一boilerwillshutdown

-remedy:louversoninducedfanshaveopenedtoorapidlycausing

suddenpressurechangefromnegativetopositive.

Operator:resetalarm,thenclosethelouversslightlytore­

establishnegativepressurewithinthefurnace.

-cause:burnermalfunction

-remedy:isolatethatburnerandreplaceitwhentheunitisshutdownfor

maintainancetoallowcontinuedoperation

-cause:tubefailure

-remedy:catastrophictubefailure:waterhasextinguishedflamesatthe

burners.Operator:shutdownentireunitimmediately.Ensure

inducedfansarerunningtopurgethefurnaceofcombustiblegases

orpocketsofgases.Watertubeswillneedtobereplacedandunit

willneedtobeinspectedbyaqualifiedboilerinspectorbeforeunit

isbroughtbackintoservice.

-cause:fireeyesnotfunctioningproperly

-remedy:cleanfireeyesaccordingtoprocedure

4.Awatertubeboilerhasjustbeentakenofflineandisintheprocessofcoolingdown.

Describetheproperprocedureforpreparingtheboilerforinspectionandthesafety

precautionstobetakenduringtheinspection.(10marks)

ANSWER4

1.Reduceloadonboiler.

2.Atabout50%load,operatethesootblowerstocleanthefiresidesurfacesasmuchaspossible.

3.Whenthefiresareout,allowtheboilertocooltoapressureof35-70Kpaandthenopenthe

drumvents.

4.Isolatetheboilerbyclosingthesteamstopvalves;feedwaterstopvalves;fuelvalves;air

dampersonFDfans;steamtosootblowervalves;chemicalfeedvalves;steamtofanorfeed

pumpturbines;atomizingsteamvalves;continuousblowdownvalvesandbyopeningcircuit

breakersoffeedpumpmotors,fanmotors,chemicalfeedpumpmotors,retractablesootblower

motors,rotaryairheaterdrivesoranyvalveorcircuitbreakerwhichmayallowanygas,liquid

orsolidtoentertheboilerorcouldcausemotionofequipmentwithintheboiler.Tagallvalves

withbrightlycolored"DONOTOPERATE,,tagsandrecordallthesesotheycanbeaccounted

forwhentheyareremovedinpreparationforstartinguptheboileragain.Thesetagsarenotto

removedortamperedwithuntilallworkissignedoffandaretoberemovedonlybytheoperator.

5.Whenboilerwaterhascooledtoabout90degreesC,opentheblowdownvalvesanddrainboiler.

6.Ifitwasformaintenance,theboilerwouldnowbeflushedoutwithahighpressurewaterhose.

Theinspectorwillwanttoinspectsludge;scale;buildup;etcsoavoidthisstepuntilapprovalby

theinspectorisreceived.

7.Whentheboilerhasdrained,closeandtagtheblowdownvalve.

8.Openfiresideaccessdoor.

9.Ifextensionlightsaretobeused,atransformermustbeprovidedwhichchangespowerto12Vto

eliminatethedangerofelectrocutiontopersonnelintheboiler.

10.Provideprotectiveclothing,goggles,faceshieldandgloveswherenecessary.

SAFETYPRECAUTIONS:

1.Properpersonalprotectiveequipment:coveralls,glasses/goggles,gloves,steeltoedworkboots,

faceshield,radio,flashlight,personalairmonitor.

2.Trainedpersonnelaretheonlypersonsallowedtoentervessel:CONFINEDSPACE.

3.Preplanningmeetingtoplanallaspectsoftheconfinedspaceentry:allinvolveddepartmentsor

personnelshouldbeinattendanceatallrelatedmeetings.Thisincludestheemergencypersonnel

aswell.

4.Safeworkplansorproceduresmustbeauthorizedpriortoanyentryorworkbeingdone.

5.Meetingjustpriortoentrytogooverallaspectsofentry.

6.Allpermitsareinorderandapproved.

7.Ongoingairqualitytesting.Dependingonvesselmaybeeverysixhoursorcontinual.If

continualallpersonnelenteringvesselmustwearSCBAgear.Itisrecommendedthatall

personnelwearsomekindofpersonalairmonitorsuchasthe“RATTLER“tomonitorany

changesinairquality.

8.SCBAequipmentifnecessary.

9.Qualifiedstand-bypersonattheentryofthevessel.Thispersoncanatnotimeleavethevessel

unattended-mustberelievedbyanotherqualifiedpersonbeforeleavingtheareaforanyreason.

10.Recordofallpersonnelwhoenterthevessel.Sign-insheetaswellasidentificationtagslefton

boardatentrytovessel.

11.Confinedspaceentrytagshungatentrytovessel.

12.Radiocontactbetweenpersonnelenteringvessel,stand-bypersonandemergencypersonnelat

alltimes.

13.Updatingpennitsandairsamplingasnecessary.

14.Iftheentryisleftopenatanytimewithnopersonnelaround,a"DONOTENTER“signshould

behungdirectlyovertheopeningoftheentry.

IfdangerousgassuchasH2Sispresentintheareaorhasthepotentialofbeinginthearea:signsshould

behungindicatingthesame.

5.a.Whatisthepurposeofawatercolumnonaboiler.(3marks)

ANSWER

Actsasareservoirtodampenagitationofthewater.

-makesthelevelinthegageglasseasiertoread

Trapanysludgeorsedimentandpreventitfromcollectingintheglassconnections

Providesaplacefortheinstallationofalarmfloats

Providesvisibleindicationofwaterlevelinboiler.

Allowsoperatortocheckorverifywaterlevelinboilershouldthegageglassbebroken.

-usingtri-cocksattachedtowatercolumn.

b.Describethebestprocedureforverifyingtheproperoperationofawatercolumnandgauge

glassonanoperatingboiler.(6marks)

ANSWER

1.Closethegagesteamvalveandthegagewatervalveinordertoisolatethegagebeforechecking

thecolumnconnections

2.Closecolumnwatervalveandopencolumndrainvalvethusblowingthroughthesteam

connectionandthecolumnandprovingthemclear

3.Closecolumnsteamvalveandopencolumnwatervalvethusblowingthroughthewater

connectionandprovingitclear

4.Closecolumndrainvalveandopencolumnsteamvalveputtingcolumnbackinoperating

condition

5.Opengagedrainvalveandgagesteamvalvethusblowingthroughthesteamconnectionandthe

gageglassandprovingthemclear

6.Closegagesteamvalveandopengagewatervalvethusblowingthroughthewaterconnection

andprovingitclear

7.Opengagesteamvalveandclosegagedrainvalveputtinggagebackinoperatingcondition

NOTE:allconnectionstobothcolumnandgagehavenowbeenblownthroughseparatelytoprove

themclear.

c.Describethecorrectprocedureforreplacingabrokentubulargaugeglassonanoperating

boiler.(6marks)

ANSWER

1.Shutoffthesteamandwatervalvesonthegage

2.Openthegagedrainvalve

3.Removebrokengageglass.Eachendoftheglassisheldinplaceinthegagefittingbymeansof

anutandawasher.Byunscrewingthenuts,thewashersandglassmayberemoved

4.Opengagevalvestoblowoutsmallfragmentsofglassandthencloseagain

5.Cutnewglasstocorrectlengthusingaglasstubecutter

6.Placenutsandnewwashersontheglassandinstallinthegagefittings.Somepowderedgraphite

maybeusedasalubricantbetweenwashersandnuts.Nutsshouldbehandtightenedonly

7.Heatuptheglassslowlybycrackingopengagesteamvalve.Thenclosegagedrainvalveand

crackopengagewatervalve.Whenwaterlevelinglassstabilizes,openthegagesteamandwater

valvesfully

NOTE:operatorsshouldwearproperPPEtoprotecteyes,face,handsetcincasenewglassburstswhen

putintoservice.Useextremecautionwhenpressurizingthegageglass

6.Describetheprincipleofoperationandthemethodoftestingofeachofthefollowingsafety

devicesfoundonapackagedboiler.

a.Highlimitpressurecontrol.(5marks)

ANSWER

HighLimitControl(HighSteamPressureFuelCut-Off)

TheBoilerCoderequiresthatautomaticallyfiredsteamheatingboilersnotunderconstantsupervision

mustbeequippedwithahighlimitpressurecontrolthatshutsoffthefuelsupplyataboilerpressurenot

toexceed103kPa(15psi).

Thiscontrolmustbeconnectedtotheboilerwithoutanyshut-offvalvesanditshallbeprotectedbya

syphonorasimilarmeansofmaintainingawatersealtopreventsteamfromenteringthecontrol.

Thepurposeofthehighlimitcontrolistoshutoffthefiringequipmentshouldtheoperatinglimit

pressurecontrolfailtoshutofftheburnerwhenitreachesitscut-outpointandpressurecontinuestorise.

Thehighlimitcontrolthusactsasasafeguardtopreventexcessiveboilerpressureandwillpreventthe

pressurefromrisingtothepointatwhichthesafetyvalveswouldopenupandreleasethesteamto

atmosphere.

Indesignandoperationthehighlimitcontrolisquitesimilartotheon-offpressurecontrolbutitisnot

equippedwithadifferentialspringsothatthecut-outandcut-inpointsofthecontrolareveryclose

together.Also,thecontrolafterhavingshutofftheburneronreachingitscut-outpoint,locksoutand

doesnotautomaticallystarttheburnerupagainwhenthepressuredrops.Instead,ithastobemanually

resetbyaresetleverorbuttonontheoutsideofthecontrolbeforetheboilercanbestartedagain.The

reasonforthisarrangementistoindicatetotheboileroperatorthattheoperatingcontrolis

malfunctioning.Theoperatingcontrolmustberepairedorreplacedimmediately.

Itisgenerallyrecommendedtosetthecut-outpressureofthehighlimitcontrolsufficientlyabovethe

cut-outpressureoftheoperatinglimitcontroltoavoidunnecessarylock-outsshouldtheoperating

controlbeslightlyoutofcalibration.Ideallythecut-outpressureofthehighlimitcontrolshouldnot

exceedninetypercentofthesafetyvalvesetting.

4thcllOPressureControl

TEST

Thepressureandtemperaturehighlimitcontrolscannotbetested

duringnormalboileroperationsincetheyonlyoperateabovethe

cut-outpointoftheoperatingcontrol.Totestthehighlimitcontrol,

disconnectthepowertotheboilercontrolsandplaceatestlead

acrosstheterminalsoftheoperatingcontrol.Checkthesettingofthe

highlimitcontrol.Restorepowertothecontrolsandstarttheboiler.

Allowtheboilertofireuntilthesteampressureorwatertemperature

reachesthesettingofthehighlimitcontrol.Thecontrolshould

operateatthispointandshutdownthefiringequipment.Ifthetestis

O.K.,disconnectthepowerandremovethetestlead.Resetthehigh

limitcontrolandputtheboilerbackintooperation.

Thehighlimitcontrolofalow-pressuresteamheatingboilershouldopenatapressurehigherthanthe

cut-outpressureoftheoperatingcontrol,butlowerthan103kPa(15psi),thepoppingpressureofthe

safetyvalve.Onlow-pressurehotwaterboilersthecut-outtemperatureofthehighlimitcontrolshould

besetabovethecut-outsettingoftheoperatingcontrolbutneverhigherthat121℃(250°F),the

maximumallowablewatertemperaturefortheboiler.

Generally,themaintenanceonoperatingandlimitingcontrolsonlyrequiresvisualinspectionofthe

deviceforevidenceofwear,corrosion,etc.Iftheswitchisamercurybulbtype,itshouldalsobe

checked.

LowAirPressureFuelCut-OffSwitch

Thisdeviceisusedtopreventburneroperationwhentheamountofcombustionairsuppliedtothe

furnaceisinsufficienttoprovidesafecombustionofthefuel.Abasicsketchofthisdeviceisshownin

Fig.6.Thedeviceconsistsofasealedhousingdividedintwobyaflexiblediaphragmwhichis

connectedtoaswitchcontrollingthepowersupplytothefuelvalve.One

sideofthehousingisopentoatmosphere,theothersideisconnectedto

thedischargesideoftheforceddraftfan.Whenthefanisnotinoperation

orwhenitdevelopsinsufficientairpressure,thespringactingonthe

diaphragmwillkeeptheswitchopenandpowersupplytothefuelvalveis

notpossible.

Whenthefanisstartedandcomesuptospeed,sufficientairshouldbe

forcedintothefurnacetobuildupenoughpressurefortheairenteringthe

controltomovethediaphragmagainsttheforceofthespringandtoclose

theswitch.Thiswillallowthefuelvalvetobeenergizedandtheburnerto

belit.Ifduringoperationtheamountofcombustionairsuppliedwould

dropoffbelowthesafeminimumrequiredduetoamalfunctionofthefan,

thepressureoftheairwilldropaswell.Thecut-offswitchwillopen,de­

energizingthefuelvalve,andtheburnerwillceasefiring.

TESTThetestingofthelowairpressurecut-offismorecomplicated.Itisnecessarytotakemanual

controloftheprogrammer,ifpossible.Atestlightwithpowersourceshouldbeattachedtotheterminals

ofthecontrol.Withthefaninoperationandthe

burnerdamperinwideopenpositionformaximum

airflowandminimumpressure,thecontactsofthe

cut-offshouldbeclosedandthelightburning.By

restrictingtheairintaketothefan,thecut-offpoint

ofthecontrolcanbefound.

LowGasPressureFuelCut-OffSwitch

Thissafetycontroldeviceworksonthesame

principleasthelowairpressurecut-off.If,forany

reason,thepressureofthegassuppliedtotheburner

dropsbelowaminimumvalue,whichindicates

insufficientgasfbrsafecombustion,theswitchopens

up,andthefuelsupplyiscutoff.TheswitchinFig.

7consistsofadiaphragmorbellowswithpressurefromthefuelactingonitslowersideandthe

opposingforceofthespringactingonitsupperside.Ifthepressureofthefueldropsbelowasafelevel,

theleafspringwilldrop,openingtheswitchtothesolenoidoperatingthegasvalve.Thevalvethen

closes,shuttingdowntheburner,preventinganexplosiveconditionintheboilerduetoanunstable

flame.

TESTThelowgaspressurecut-offcanbetestedduringtheoperationoftheboilerbyslowlyclosing

themaingascock.Observethepressuregageonthegasline.Whenthepressureinthelinedropstothe

cut-offpoint,theburnervalveshouldshutautomatically

PRINCIPLE:cutsofffuelsupplywhenthepressureisatorabovethesetpressure.Iftheoperating

rangewasbetween5.5and8andthepressureofthefuelwasrising:analarmwouldbesentwhengas

pressurereached8.25.Ifthepressurecontinuedtorise,thefuelwouldbeshutoffwhenpressurereached

8.75.Allthesemeasurementsareinpsig.

TEST:

b.Lowgaspressurecutoff.(5marks)

PRINCIPLE:apressureoperatedswitchwhichpreventstheburnerfromoperatingwhenthegas

pressureinthesupplylinedropsbelowthepressurenecessarytomaintainastableflameattheburner.

Worksthesameasthehighpressurecontrolbutsendsfirstanalarmandthenshutsfueloffaspressure

isdropping.Aswiththeabovenumbers:analarmwouldbesentwhenpressuredropsto5.25andfuelis

shutoffat5.0.Again,themeasurementsareinpsig.

TEST:

c.Lowairpressurecutoff.(5marks)

ANSWER

PRINCIPLE:-worksinsameprincipleasthelowgaspressurefuelcutoff.Iftheamountofair

suppliedtotheburnerdropsbelowtheminimumrequiredforsafecombustion,thesupplypressurein

theducttotheburnerdropsoffaccordingly,thentheswitchopensupandthefuelsupplyiscutoff.

TEST:

7.Listthenecessarycontrolsforanautomaticallycontrolledlowpressuresteamheating

boileranddescribethepurposeandmethodofoperationofeachcontrol.(10marks)

ANSWER:4thclassc74

SAFETYDEVICESANDINTERLOCKS

Interlocksmaybedefinedasanumberofelectricalswitchesconnectedinseries,whosepurposeisto

protectapieceofequipment,orsystem,frombeingoperatedinanunsafecondition.Ifallconditionsor

measurementsrelevanttosafeoperationoftheequipment,suchasaboiler,aresatisfied,itwillkeep

operating.Ifoneormoreunsafeconditions,suchaslowdrumlevel,etc.,shouldoccur,therespective

relayorelectricalswitchwillopen,andtheprocessorequipmentwillshutdown.Someoftheelectrical

relaysorautomaticallyoperatedswitchesusedonboilersaredescribedbelow.Ineachcase,theswitchis

locatedintheelectricalcircuitsupplyingpowertoanelectricallyoperatedvalveinthemain(orpilot)

fuelgasline.Ifanyoftheseswitchesopensthevalvewillclose,thusputtingtheboilerintoasafe,non­

operatingcondition.

LowAirPressureFuelCutoffSwitch

Thelowairpressureswitchisasafetydevicemountedonboilerswithforceddraftfansorblowers.Itis

usedtoshutdowntheboilerfuelsupplywhenthesupplyofcombustionairisinsufficienttoprovidea

stableflame.Fig.1isabasicillustrationofthedevice.

Theswitchconsistsofasealedhousingdividedintotwosectionsbyadiaphragmwhichisconnectedto

aswitchcontrollingthepowersupplytothefuelvalve.Onesideofthehousing,containingthespring,is

opentotheatmosphere.Theothersideisconnectedtothedischargeoftheforceddraftfan.Ifthefanis

notinoperation,orwhenitdevelopsinsufficientdischargepressure,thespringactingontheleftsideof

thediaphragmwillkeeptheswitchopentothefuelvalvepowersupply.

Whenthefanisstarted,andtheforceduetotheairpressureexceedsthespringforce,thediaphragm

willmovetotheleftandclosetheswitch.Thiswillpermitthefuelvalvetobeenergizedandtheburner

tobelitiftheotherinterlocksareclosed.Shouldtheairsupplydropbelowasafeminimumdueto

malfunctionofafanduringboileroperation,thepressureoftheairwilldropandthespringwillopen

theelectricalcircuittothefuelvalvecausingtheburnertobeshutdown.

LowGasPressureFuelCutoffSwitch

Thissafetycontroldeviceoperatesonthesameprincipleasthelowairpressurefuelcutoffswitch.Ifthe

fuelgaspressuredropsbelowthevaluerequiredtomaintainastableflame,theswitchwillopenthe

electricalcircuittothefuelgasvalve.Asimilarlowoilpressureswitchcanbeusedonoil-firedboilers.

FlameFailureDetector

If,foranyreason,theburnerflameshouldgooutduringtheoperationofanoil-orgas-firedboiler,itis

necessarytoshutoffthefuelvalveimmediately.Thiswillpreventunbumedfuelfromcollectinginthe

furnaceandcausinganexplosion.Aflamefailuredetectorservesthepurpose.

Onetypeofflamefailuredevice,illustratedinFig.2usesaleadsulphidecellwhichissensitivetothe

infraredraysradiatingfromtheflame.Itsresistancetocurrentflowdecreasesastheintensityofinfrared

radiationincreases.Iftheflameshouldgooutthenthecellwillnolongersensetheradiationcausingits

resistancetocurrentflowtoincreaseandthefuelvalvetoclose.Theleadsulphidecell,commonly

calleda"scanner”mustbemountedonthefurnacefrontinsuchawaythatithasaclearviewofthe

flamethrougharestrictedorifice.Thetubeorrestrictionwillreducetheeffectsofradiationfromthehot

refractorytowhichthisdetectorisalsosensitive.

Theultravioletsensororscannerissensitivetoultravioletradiationfromtheflame'sprimary

combustionzone.Iftheflamegoesoutthepowersupplytothefuelvalvewillbeopenedandthevalve

willclose.

Mostpackagedboilersuseascanner