Siemens LMV52.240B1 - Pre-Control; O 2 Trim Terminology

TechnicalInstructions  LMVSeries
DocumentNo.LV5‐1000
Section6 Page4 SCCInc.
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Formanynozzlemixingburners(traditionalboilerburners),theO
2
setpointwillbeahighervalueatlow
fireandalowervalueathighfire.Thisistruesincemostnozzlemixingburnersneedmoreexcessair
(higherO
2
)atlowfiretoachievecompletecombustion.ForthistypeofburneranO
2
setpointof5%O
2

(wet)atlowfireandanO
2
setpointof2%O
2
(wet)athighfireisnotuncommon.
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Pre‐control
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Asmentionedabove,allO
2
trimsystemswithanO
2
sensorintheboiler'sexhaustmustuse"old"O
2

readingsduetophysicalrealitiesoftheboiler.Thisfact,combinedwithanO
2
setpointthattypically
variesfromlowfiretohighfiremeansthattheO
2
trimsystemcannotactivelytrimwhentheburner
makesalargechangeinfiringrate.ThisbegsthequestionofwhathappenstotheO
2
trimwhenthe
burnertransitionsfrom20%firingrateto,say,80%firingrate.TheansweristheO
2
trimchangesmodes 
fromactivetrimtowhatisreferredtoas"pre‐control".

Pre‐controlispossiblebecausetheLMV52learnsthecharacteristicoftheburnerateachpointonthe
curveduringtheO
2
trimcommissioning.ThespecificcharacteristicthattheLMV52learnsishowmuch
ofachangeinairflow(airrate)isnecessarytoachieveacertainchangeinthe%O
2
.Thisspecific
characteristicisknownastheLambdaFactor,andisessentiallytheburner'ssignaturefromanO
2
trim
standpoint.Justlikethedelaytime(Tautime)thatwasmentionedearlier,itisnecessaryforthefuel
flowtomatchtheloadnumbersothattheLambdaFactoriscorrectandaccurateforeachpoint.
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Pre‐controlusestheLambdaFactorsothattheburnercanbemodulatedwhilekeepingthe%O
2
closeto
theO
2
setpoint,eventhoughthesetpointchangeswiththefiringrateandtheO
2
trimisusing"old"O
2

readings.Oncetheburnerstopsmodulatingoronlymodulatesasmallamount(+/‐5%),theO
2
trimwill
changemodesbackto"active"andperformactiveO
2
trimonceagain.

AproperlycommissionedO
2
trimsystemwilltypicallymaintainthe%O
2
setpointwithadeviationof+/‐
0.1%O
2
whentheburnerisnotmodulatingasignificantamount(+/‐5%loadchange).Withthissmall
loadchange,theO
2
trimwillstayinthe"active"mode.Whensignificantmodulationisencountered(+/‐
15%loadchange),thepre‐controlwillengageandthedeviationfromsetpointwillincrease.Deviations
of+/‐0.3%orlessaretypicalduringmodulationwhenthepre‐controlisengaged.

O
2
TrimTerminology
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1. %O
2
Wet–In‐situO
2
sensors,liketheQGO20sensorthatisusedwiththeLMV52,readthe%O
2
ona
wetbasis.Thisisincontrasttothevastmajorityofportablecombustionanalyzersthatread%O
2
on
adrybasis.%O
2
wetshouldalwaysbealowervaluethan%O
2
dry.Figure6‐11(locatedlaterin this 
section)givestheapproximaterelationshipbe tween%O
2
wetand%O
2
dry.
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2. TauTime‐Thisisthedelaytimebetweenwhenanadjustmentismadeattheburner(movingthe
airdamper,etc.)andwhenthatadjustmentisreadattheO
2
sensor.Foragivenburner/boiler,this
timewillbeshorterathighfireandlongeratlowfire.
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3. O
2
RatioControl‐Thisisthemeasured%O
2
wetwhentheactuators/VSDareontheFuel‐AirRatio
ControlCurveandarenotbeingtrimmed.ThisisrecordedforeachpointonthecurveexceptPoint
1.Thisisalsocommonlyreferredtoasthe“leancurve”.
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