Siemens LMV51.140C1 - Considerations When Using O Trim with FGR

LMVSeriesTechnicalInstructions 
DocumentNo.LV5‐1000
SCCInc. Page29 Section6
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ConsiderationswhenUsingO
2
TrimwithFGR
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SomeburnersuseahighpercentageofFGR(FGRflowcomparedtoairflow)tolowerNOxemissions.
Sincefluegaseschangein%O
2
andarebeingdrawnbackintotheblowerinsignificantquantities, these
typesofburnersareinherentlymoredifficulttotrimthe%O
2
inthestack.
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Thereasonbehindthisincreaseddifficultyistheoxygencontentoftheair/FGRmixtureisdependent
onthe%O
2
inthestack,whichaddsanotherdynamicvariableintothesystem.OnaburnerwithoutFGR,
theoxygencontentoftheairattheblowerintakeisalwaysaconstant20.9%O
2
.Naturally,alarger
percentageofFGR(20%)hasthepotentialtovarytheoxygencontentoftheair/FGRmixtureatthe
blowerintakemorethanasmallpercentageofFGR(5%)would.
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Moreover,alargepercentageofFGRcanchangetheoxygencontentoftheair/FGRmixtureatthe
blowerintakeinawaythatcansetupa"cycleofintensification".Anexampleofthiscycleisbelow:
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1. The%O
2
inthestackincreasesforsomereason.
2. The%O
2
intheFGRalsoincreases.
3. Theoxygencontentoftheair/FGRmixtureattheblowerintakeincreases,whichincreasesthe%O
2
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inthestackevenmore.
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Naturally,theexampleabovewouldalsoholdifthe%O
2
inthestackdecreased,exceptthecycleof
intensificationwouldservetopushtheburnerrichinsteadoflean.
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InadditiontothecycleofintensificationthatisinherenttomanyFGRburners,themechanicaldesignof
theburner/boilerinfluencestherepeatabilityoftheFGRflow.TherepeatabilityoftheFGRflowhasa
largeinfluenceonhowwelltheO
2
trimsystemcanwork.Obviously,non‐repeatabilityintheFGRflow
willcausenon‐repeatabilityinthe%O
2
readinthestackandthereforetheoperationoftheO
2
trim.
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TwodifferentmethodsofinducingFGRintoaburnerareillustratedandexplainedbelow,andtheir
behaviorsfromanFGRflowrepeatabilitystandpointarediscussed.
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MethodshowninFigure6‐12:
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PressureP1relativetopressureP2(differentialpressureacrosstheFGRdamper)changesdrastically
withfiringrate.VerylittledifferentialpressurewillbegeneratedacrosstheFGRdamperatlowfire,
requiringtheFGRdampertobemostlyopentoachieveevenminimalFGRflowatlowfire.Asfiringrate
increases,P1willdecreaseandP2willincrease,yieldingmuchmoredifferentialpressureacrosstheFGR
damper.Asaresult,theFGRdamperwillneed
toberampedclosedastheburnerisrampeduptohigh
fire.DuetothemuchhigherdifferentialpressureathighfiretheFGRdampermightbeoversizedfor
effectivecontrolathighfire
.Otherpointstoconsider:
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1. IfusingO
2
trim,trimmingwiththeairactuator(setto"airinfluenced")closedshouldserveto
decreaseP1anddecreasetheFGRflowslightly(dependingonthepositionofthestackdamper).
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2. AtlowfirewhentheFGRvalveismostlyopen,evensmallchangesinthedifferentialpressureacross
theFGRdamperwillcauselargechangesinFGRflow.
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