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Hydronic Engineering Handbook Engineering GREAT Solutions · 2020. 9. 22. · primary system (see...
Transcript of Hydronic Engineering Handbook Engineering GREAT Solutions · 2020. 9. 22. · primary system (see...
EngineeringGREAT Solutions
Balancing & Control
Hydronic Engineering Handbook
Balancing of Control Loops
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© 2015 TA Hydronics SA, Eysins
www.imi-hydronics.com
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Balancing of Control Loops > 1. Why balance?
Contents
1. Why balance? ........................................................................................................................4
2. Control loops with two-way control valves ..........................................................................6
2.1 Variableprimaryandsecondaryflows ...........................................................................6
2.2 Variableprimaryflowandconstantsecondaryflow ....................................................13
2.3 Constantprimaryflowandvariablesecondaryflow ....................................................18
2.4 Constantprimaryandsecondaryflow ........................................................................20
3. Control loops with three-way control valves .....................................................................21
3.1 Variableprimaryflowandconstantsecondaryflow ....................................................21
3.2 Variableprimaryandsecondaryflows .........................................................................25
3.3 Constantprimaryandsecondaryflows .......................................................................26
3.4 Constantprimaryflowandvariablesecondaryflow ....................................................28
4. Control loops compared .....................................................................................................30
4.1 Activeprimarynetwork ...............................................................................................31
4.2 Passiveprimarynetwork .............................................................................................35
Appendix AThe authority of two-way control valves ................................................................................36
A.1 Theincompletedefinitionofvalveauthority .................................................................36
A.2 Thecorrectdefinitionofvalveauthorityβ’ ...................................................................38
A.3 Sizingofcontrolvalves ................................................................................................40
Appendix BThe authority of three-way control valves .............................................................................44
B.1 Inmixingfunction ........................................................................................................44
B.2 Indivertingfunction .....................................................................................................45
Appendix CHow to set the BPV to ensure the minimum pump flow ........................................................47
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1. Why balance?
Manypropertymanagersspendfortunesdealingwithcomplaintsabouttheindoorclimate.Thismaybethecaseeveninnewbuildingsusingthemostrecentcontroltechnology.Theseproblemsare widespread:
• Someroomsneverreachthedesiredtemperatures,particularlyafterloadchanges.
• Roomtemperatureskeepswinging,particularlyatlowandmediumloads,eventhoughtheterminalshavesophisticatedcontrollers.
• Althoughtheratedpoweroftheproductionunitsmaybesufficient,designpowercan’tbetransmitted,particularlyduringstartupafterweekendornightsetback.
Theseproblemsfrequentlyoccurbecauseincorrectflowskeepcontrollersfromdoingtheirjob.Controllerscancontrolefficientlyonlyifdesignflowsprevailintheplantwhenoperatingunderdesignconditions.
Theonlywaytogetdesignflowsistobalancetheplant.Balancingmeansadjustingtheflowbymeansofbalancingvalves.Thishastobedoneinthreerespects:
1. Theproductionunitsmustbebalancedtoobtaindesignflowineachboilerorchiller.Furthermoreinmostcases,theflowineachunithastobekeptconstantFluctuationsreducetheproductionefficiency,shortenthelifeoftheproductionunitsandmakeeffectivecontroldifficult.
2. Thedistributionsystemmustbebalancedtomakesureallterminalscanreceiveatleastdesignflow,regardlessofthetotalloadontheplant.
3. Thecontrolloopsmustbebalancedtobringabouttheproperworkingconditionsforthecontrolvalvesandtomakeprimaryandsecondaryflowscompatible.
4. Thishandbookdealswiththebalancingofcontrolloops.Ittellsyouhowtobalance23 commoncontrolloopsusing2-wayand3-waycontrolvalves.Forinformationaboutthebalancingofdistributionsystems,pleaseseeHandbookNo.2.Handbook3concernsBalancingofradiatorsystemswhilehandbook4examinesthestabilisationofthedifferentialpressure.
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Balancing of Control Loops > 1. Why balance?
Heating Cooling%45
35
25
15
5
20 21 22 23˚C
35
25
15
5
20 21 22 23˚C
%45
Whyistheaveragetemperaturehigherinaplantnotbalanced?Duringcoldweatheritwouldbetoohotclosetotheboilerandtoocoldonthetopfloors.Peoplewouldincreasethesupplytemperatureinthebuilding.Peopleonthetopfloorswouldstopcomplainingandpeopleclosetotheboilerwouldopenthewindows.Duringhotweatherthesameapplies.Itisjustthatitwouldbetoocoldclosetothechiller,andtoohotonthetopfloors.
Onedegreemoreorlessinasingleroomrarelymakesanydifferencetohumancomfortortoenergycosts.Butwhentheaveragetemperatureinthebuildingiswrong,itbecomescostly.
Onedegreeabove20degreesCincreasesheatingcostsbyatleast8percentinmidEurope(12%inthesouthofEurope).Onedegreebelow23degreesCincreasescoolingcostsby15percentinEurope.
Percentage increase in energy costs for every degree C too high, or too low, relative to average building temperature.
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2. Control loops with two-way control valves
2.1 Variable primary and secondary flows
STAD
∆H
C
V
q
ts
tr
∆pV
Fig. 1. Control of a variable flow terminal unit
InFig1thetwo-waycontrolvalvecontrolsthecoiloutputbyadaptingthewaterflow.
Theauthorityofthecontrolvalveβ’=∆pV/∆H.Theterm“authority”isexplainedindetailinAppendixA and B.
Thetwo-waycontrolvalveisselectedtocreate,fullyopenanddesignflow,apressuredrop∆pV=∆H-∆pC-3(kPa)
Moreoverthisvalue∆pVmustbehigherthan0.25x∆Hmax.
Balancing procedure fig 1
1. Openallcontrolvalvesfully.
2. AdjusttodesignflowwithSTAD.Dothisaspartofthebalancingprocedurefortheentireprimarysystem(seeTAHydronicsHandbookNo2).
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Balancing of Control Loops > 2. Control loops with two-way control valves
ts q
tr
∆pV
STAD
∆H
C
BPV
Fig. 2. A modulating relief valve reduces the differential pressure by a constant value regardless of the flow.
Whenthecontrolvalveisoversized,forinstanceduetothelimitedchoiceofKvvalues,theprimarydifferentialpressurecanbeindirectlyreducedbymeansofaBPVmodulatingreliefvalve.TheBPVreducesthedifferentialpressurebyaconstantvalueregardlessoftheflow.
Thecontrolvalveauthorityβ’=∆pV/(∆H-∆pBPV).
Balancing procedure fig 2
1. Openallcontrolvalvesfully.MakesurealltheBPVsareopen(minimumsetpoint).
2. AdjusttodesignflowwithSTAD.Dothisaspartofthebalancingprocedurefortheentireprimarysystem(seeHandbookNo2)andbeforeyouproceedtostep3.
3. FindoutwhichhandwheelsettingofSTADthatwillcreateapressurelossofatleast3kPainSTADfordesignflow.UsetheTA-SCOPEoraTAnomogramtofindthecorrectsetting.
4. ReadjustSTADaccordingtostep3.TheflowinSTADshouldnowbehigherthandesignvalue.
5. AdjustthesetpointoftheBPVuntilyougetbacktodesignflowinSTAD.MeasuretheflowinSTADasyouadjusttheBPV.
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STAD
∆HC
V
STAP
Fig. 3. A ∆p controller keeps constant the differential pressure a cross the control valve.
Dependingonthedesignoftheplant,thedifferentialpressureavailableonsomecircuitscanvarydramaticallywiththeload.Inthiscase,toobtainandmaintainthecorrectcontrolvalvescharacteristic,thedifferentialpressureacrossthecontrolvalvescanbemaintainedpracticallyconstantwitha∆pcontrollerasrepresentedonfigure 3.
Thedifferentialpressureacrossthecontrolvalve”V”isdetectedononesidebyconnectingthecapillarydownstreamthemeasuringvalveSTAD.ThepressureontheothersideisconnecteddirectlytotheactingmembranebyaninternalconnectionintheSTAP.
Whenthedifferentialpressureacrossthecontrolvalveincreases,theSTAPclosesproportionallytocompensate.
Thecontrolvalve”V”isneveroversizedasthedesignflowisalwaysobtainedforthevalvefullyopenanditsauthorityisandremainsclosetoone.
AlladditionaldifferentialpressureisappliedtotheSTAP.Thecontrolofthedifferentialpressureisquiteeasyincomparisonwithatemperaturecontrolandasufficientproportionalbandcanbeusedtoavoidhunting.
Astheflowsarecorrectateachterminal,nootherbalancingprocedureisrequired.IfallcontrolvalvesarecombinedwithSTAP,thenbalancingvalvesinbranchesandrisersarenotnecessarybutfordiagnosispurposes.
Balancing procedure fig 3
1. Openfullythecontrolvalve”V”.
2. PresettheSTADtoobtainatleast3kPafordesignflow.
3. Adjustthesetpoint∆pLofthedifferentialpressurecontrollerSTAPtoobtaininSTADthedesignflow.
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Balancing valve on each terminal
qp
STAP
STAD
STADor
TBV
VV
C C
∆H
Fig. 4. A differential pressure controller STAP stabilises the differential pressure across a set of terminal units.
Whenseveralsmallterminalunits”C”areclosetoeachother,itcanbesufficienttostabilisethedifferentialpressureonthewholesetaccordingtofigure4.
ThesupplypressureistransmittedtotheSTAPbymeansofacapillarymountedonthebalancingvalveinthesupplypipe.
Whenthedifferentialpressure∆Hincreases,thevalveSTAPshutstocompensate.Eachcontrolvalves”V”ischosentocreate,fullyopenandatdesignflow,approximatelythesamepressuredropasitscoilunit.
Balancing procedure fig 4
1. OpentheSTAPfully. ThecontrolvalvesVarefullyopen.
2. BalancetheterminalsofthebranchaccordingtotheTABalancemethod,whichdoesnotdependonthedifferentialpressure∆Havailable.STADserveaspartnervalves.
3. AdjustthesetpointoftheSTAPtoobtaindesigntotalflowqpthroughtheSTAD.
Balancing of Control Loops > 2. Control loops with two-way control valves
Balancing valve on each terminal
10
tp
qb
qp qstr
∆H
STAD-1 STAD-2
BPV
ts
B
AD
V
Fig.5. A secondary pump creates a sufficient differential pressure. The secondary water temperature is necessarily different from that of the primary.
Ifthedifferentialpressure∆Histoolowtogiveareasonableauthorityforthecoilcontrolvalves,asecondarypumpcancreateasufficientdifferentialpressure.
ThesolutioninFig5canalsobeusedwhentheprimarydifferentialpressureistoohigh.
Thesecondarywatertemperaturetscanbeconstantorvariable,butisnecessarilydifferentfromtheprimarytemperaturetp.Inheatingts<tp,whileincoolingts>tp.
Atlowloads,thedifferentialpressureacrossthesecondarytendstoincrease.Whenthispressureexceedsacertainvalue,theBPVopenstoallowaminimumflowtoprotectthepump.Thisflowalsolimitsthetemperaturedropinthepipessothatthenecessarywatertemperatureisobtainedthroughoutthesecondarynetwork.
Balancing procedure fig 5
The secondary
1. Openallcontrolvalvesfully.ClosetheBPV.
2. BalancethecoilsinthesecondarysystemwithSTAD-2asthePartnervalve (seeHandbookNo2).
3. SettheBPVonthemaximumallowed∆pforthecoilcontrolvalves.
4. Closethecoilcontrolvalves.
5. SettheBPVtoobtaintheminimumpumpflow(seeAppendixC).
The primary
1. OpenthecontrolvalveV.
2. Iftheprimaryflowisunknown,calculateitusingtheformulaonpage15.
3. AdjusttheprimarydesignflowwithSTAD-1.Dothisaspartofthebalancing
4. procedurefortheentireprimarysystem(seeHandbookNo2).
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tp
qb
qp
qs
tr
∆H
STAD-1
STAD-2
BPV
ts
B
AD
∆T
t1t2VC
Fig.6. A differential temperature controller maintains a minimum flow qb in the bypass, such that ts = tp.
Ifthesecondarywatertemperaturemustbeequaltothatoftheprimary,thecircuitinFig6(heatingonly)orthecircuitinFig7(bothheatingandcooling)maybeused.
Toobtaints=tp,theflowqbthroughthebypassmustbegreaterthanzero.A∆TcontrolleractsontheprimarycontrolvalveVtoensureaminimumflowqbintherightdirection.The∆Tcontrollerkeepst2slightlyhigherthant1.Normally,thesetpointofthe∆Tcontrollerisbetween1and2degrees.
Balancing procedure fig 6
The secondary
1. Openallcontrolvalves.ClosetheBPV.
2. BalancethecoilsinthesecondarysystemwithSTAD-2asthePartnervalve (seeHandbookNo2).
3. SettheBPVonthemaximumallowed∆pforthecoilcontrolvalves.
4. Closethecoilcontrolvalves.
5. SettheBPVtoobtaintheminimumpumpflow(seeAppendixC).
The primary
1. OpenthecontrolvalveV.
2. Iftheprimaryflowisunknown,calculateitusingtheformulabelow.
3. AdjusttoprimarydesignflowwithSTAD-1.Dothisaspartofthebalancing procedurefortheentireprimarysystem(seeHandbookNo2).
qp=1.05qs
Balancing of Control Loops > 2. Control loops with two-way control valves
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tp
qb
qp
qs
tr
∆H
STAD-1
ts = tp
B
AD
C
STAP
STAD-2
Fig. 7. A differential pressure controller keeps the flow constant in the bypass, ensuring a constant differential pressure across this bypass.
ThecircuitinFig7maybeusedincoolingplantswhere∆Histoolowtogiveasufficientauthorityforthecoilcontrolvalves,andwhere∆Hvariesgreatly.
ThecontrolvalveSTAPmaintainsasmallandconstantflowinthebypass,regardlessofvariationsin∆H.ThissmallflowismeasuredbymeansofSTAD-2.When∆Hincreases,theSTAPclosescorrespondingly,ensuringaconstantdifferentialpressureacrossthebalancingvalveSTAD-2.
Balancing procedure fig 7
1. Openallcontrolvalves.
2. SetSTAD-2tocreate,for5%ofdesignflowqs,apressurelosscorrespondingtotheselectedsetpointofthe∆pcontroller.UsetheTA-SCOPE,oraTAnomogram,tofindthecorrectsettingforSTAD-2.
3. BalancethesecondarycircuitwhereSTAD-1isthePartnervalve (seeHandbookNo.2).
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2.2 Variable primary flow and constant secondary flow
qb
tpts
tr
qp
qs = ct
∆H
STAD-2STAD-1
V
B
A
C
tr
Fig. 8. Control of coil output for a coil supplied at constant flow.
Thiscircuitisfrequentlyusedbothinheatingandincooling.Thecoilsupplytemperaturetsisadaptedtothepowerneedthroughcontroloftheprimaryflow.
If,underdesignconditions,tsmustbeequaltotp,themaximumflowqpintheprimarymustbeequaltoorgreaterthanthesecondaryflowqs.Otherwise,theinstalledpowercannotbetransmittedtothesecondarysincethedesignvaluetsccannotbeobtained.Primaryandsecondaryflowsmustbecompatible.TheseflowsareadjustedbythebalancingvalvesSTAD-2andSTAD-1.
A floor heating example:Assumethattsc=50°C,whichiswellbelowthetp=80°C.Thecontrolvalvemustthenbeselectedforarelativelysmallflow.Forareturntemperaturetrc=45°C,theformulabelowshowsthattheprimaryflowwillbeonly14%ofthesecondaryflow.Ifthecontrolvalveisselectedforthisflow,itcanoperateoveritsentirerange.Thelimitof50°Cforthecircuitsupplytemperaturewillnotbeexceededatthemaximumvalveopening.Ifthesecondarypumpfails,theprimaryflowpassesthroughthebypass,preventingoverheatinginthecircuit.
Balancing procedure fig 8
1. Opencontrolvalvesfully.
2. AdjusttosecondarydesignflowwithSTAD-2.
3. Iftheprimaryflowisunknown,calculateitusingtheformulabelow.
4. AdjusttheprimaryflowwithSTAD-1.Dothisaspartofthebalancingprocedurefortheentireprimarysystem(seeHandbookNo2).
qp=qs =qs =0.14qs
ts-tr 50 - 45
tp-tr 80 - 45
Balancing of Control Loops > 2. Control loops with two-way control valves
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qb
tpts
tr
qp
qs = ct
∆H
STAD-2STAD-1
V
B
A
C
tr
Fig. 9. A check valve in the bypass allows a certain waterflow through the coil C even if the secondary pump fails.
ThisisessentiallythesamecircuitasinFig8.However,acheckvalveisaddedtopreventcirculationindirectionBAinthebypass.
Ifthecircuitisusedindistrictheatingandtheprimarycontrolvalveisoversized,thecheckvalvepreventsheatingofthereturnwater.Ifthecircuitisusedforaheatingcoilincontactwithoutsideair,thecheckvalveeliminatestheriskoffreezingduetosecondarypumpfailure.
Notethatit’simpossibletoobtainaprimaryflowgreaterthanthesecondaryflow.
Balancing procedure fig 9
tsc equal to tp:
1. ClosethecontrolvalveV.
2. AdjusttosecondarydesignflowqscwithSTAD-2.
3. OpenthecontrolvalveV.
4. AdjusttheprimaryflowtothesameflowqscwithSTAD-1.Dothisaspartofthebalancingprocedurefortheentireprimarysystem(seeHandbookNo2).
tsc not equal to tp:
1. ClosethecontrolvalveV.
2. AdjusttosecondarydesignflowwithSTAD-2.
3. Ifprimaryflowisunknown,calculateitusingtheformulabelow.
4. Openthecontrolvalve.
5. AdjusttheprimarydesignflowwithSTAD-1.Dothisaspartofthebalancingprocedurefortheentireprimarysystem(seeHandbookNo2).
(ts-tr)
(tp-tr)
qp=qs
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qb
tp
tr
qp
qs = ct
∆H
STAD-2STAD-1
V
B
A
tr
C C
ts
Fig. 10. A primary constant flow distribution converted to primary variable flow.
It’scommontoconvertexistingconstantwaterflowdistributionsystemsintovariableflowinlargeplants.Therearethreereasons:1)Thesupplywatertemperaturecanthenbekeptconstantwithouthavingtokeepinserviceallproductionunitsatallloads.2)Avariabledistributionflowmeansreducedpumpingcosts.3)Theplantcanbedesignedwithadiversityfactor.
Normally,thesecondarysidecontinuestoworkwithconstantflow.
Afterconversion,wecannotworkwithts=tp.WhenthevalveViscompletelyopen,wecangetts=tpwithflowreversalinthebypass.Sincethedemandissatisfiedinthissituation,thereisnosignaltomakethetwo-wayvalveclose.Itremainsopenandwearebacktoaconstantflowdistributionsystem.Toavoidthis,tshastobeadjustedsothatts<tpinheatingandts>tp in cooling.
P
1 + (tsc -trc)
(tp -trc)
P
100( -1)
%qp=
Theprimaryflowwillvaryasafunctionoftheload:
Pistheloadinpercentofdesignpower.
Nowassumethattp=6°C,tsc=8°Candtrc=12°C.ForP=50%,wegetqp=75%.Thustheflowdemandis75%forapowerdemandof50%.
Beforeconvertingtheconstantflowsystemintoavariableflowsystem,theflowdemandwas100%forapowerdemandof50%.
Thisconversiondoesnotchangereallytheprimaryintoatruevariabledistributionastheflowin%remainshigherthanthepowerin%.
Balancing procedure fig 10
1. Balancethethree-wayvalvecircuits(seeHandbookNo2).STAD-2isthePartnervalve.
2. Iftheprimaryflowqpisunknown,calculateitusingtheformulabelow.
3. OpenthecontrolvalveV.
4. AdjusttheprimaryflowqpwithSTAD-1.Dothisaspartofthebalancingprocedurefortheentireprimarysystem(seeHandbookNo2).
Balancing of Control Loops > 2. Control loops with two-way control valves
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(ts -tr)
(tp -tr)
qp=qs
Fig 11a Fig 11b
≈ 0
qp
STAD-3
qb
STAD-2
ts
tr
qs
STAD-1V
CB
A
F+E-
≈ 0
BPV
qb
STAD-2
STAD-1V
C
F+E-
tptp
Fig. 11. Secondary pumps induce flow through the distribution network.
Ifthedistributionnetworkisalowpressurelosspassivecircuit,circulationmaybeinducedbysecondarypumps.
ThebalancingvalveSTAD-3createsacertaindifferentialpressurebetweenFandE.ThispressuregeneratestheprimaryflowqpinthecontrolvalveV,throughFCBandAE.Thedifferentialpressure∆pcEFisobtainedforqb=qsc-qpc.Thisimpliesthatqschastobegreaterthanqpc.WhenthecontrolvalveVisclosed,thebypassflowqb=qscand∆pEFisatitsmaximum.ThisisalsothedifferentialpressureappliedacrosstheclosedcontrolvalveV.Toobtainagoodauthorityforthisvalve,it’simportanttoavoidbigchangesin∆pEF.Thismeansthatqpchastobeassmallaspossiblecomparedtoqsc.Consequently,thissystemcanonlybeconsideredifthereisalargedifferencebetweents and tp,asforexampleinfloorheating.
Theflowthroughthebypassisgivenbythisformula:
(tp -ts)
(tp -tr)
qb=qs
Assumingthatthesecondaryflowqsismoreorlessconstant,thecontrolvalveauthorityβ’ = ∆pV/∆pEFmax.
Example: Floorheatingwithtp=80°C,ts=50°C,tr=45°Candqs=100.Atfullload,qb=100(80-50)/(80-45)=85.7.Atthisflow,thebalancingvalveSTAD-3inthebypassmustcreateadifferentialpressurethatcompensatesforthepressurelossofthetwo-wayvalve(8kPaforinstance)andoftheprimarycircuit(5kPa),atotalof13kPa.Whenthetwo-wayvalveisclosed,atzeroload,theflowqb changesto100(assumingthattheincreaseinthepressurelossEFhaslittleeffectontheflowqs)
17
andthepressurelossintheSTAD-3becomes∆pEFmax=13x(100/85.7)2=18 kPa.
Thecontrolvalveauthorityisthereforeβ’=8/18=0.44.
TheSTAD-3canbereplacedbyamodulatingreliefvalveBPV(Fig11b)whichkeepsaconstantdifferentialpressureEF.Inthefloorheatingexample,thisimprovescontrolvalveauthorityfrom0.44to0.61.
Balancing procedure fig 11
STAD-3 in the bypass (Fig 11a):
1. Openallcontrolvalvesfully.
2. SettheSTAD-3tocreateapressureloss∆pEF=∆pcV+pressuredropintheprimarycircuit(8+5=13kPainourexample)foraflowrateqb=(qsc-qpcinthebypass.UsetheTA-SCOPE,ortheTAnomogram,tofindthecorrecthandwheelsettingforSTAD-3.
3. SettheSTAD-1tocreateapressurelossof3kPaforprimarydesignflow.Usethe TA-SCOPE,oraTAnomogram,tofindthecorrecthandwheelsettingforSTAD-1
4. ClosethecontrolvalveV.AdjusttodesignflowwithSTAD-2.
5. Iftheprimaryflowqpcisunknown,calculateitusingtheformulabelow.
6. OpenthecontrolvalveV.ReadjustSTAD-3toobtainqp=qpcmeasuredinSTAD-1.
(tsc -trc)
(tp -trc)
qp=qs
BPV in the bypass (Fig 11b):
7. Openallcontrolvalvesfully.
8. SettheSTAD-1tocreateapressurelossof3kPaforqp=qpc.UsetheTA-SCOPE,oraTAnomogram,tofindthecorrecthandwheelsettingforSTAD-1.
9. OpenSTAD-2.AdjusttheBPVtoobtaindesignflowinSTAD-1.
10. AdjustSTAD-2toobtaindesignflowinthesecondary.
Balancing of Control Loops > 2. Control loops with two-way control valves
18
2.3 Constant primary flow and variable secondary flow
tp
qb
qp
qs
trSTAD
tp
BPV
A
B
∆H
Fig. 12. A modulating relief valve BPV stabilizes the differential pressure applied to small units.
Whentheavailabledifferentialpressureontheprimaryistoohighforthesecondary,thecircuitinFig12maybeused.
ThesetpointoftheBPVcanbeselectedwithinarangefrom8to60kPa.Thismakesitpossibletoensuregoodworkingconditionsforthecoilcontrolvalves(goodauthority)regardlessofvariationsinthedifferentialpressure∆H.TheBPVensuresaconstantdifferentialpressurebetweenAandB.STADcreatesapressurelossof(∆H-∆pBPV).
Balancingprocedurefig12
1. Openallcontrolvalves.CloseallBPVs.
2. Balancethecoilsagainsteachother,thebranchagainstotherbranchesandtheriseragainstotherrisers(seeHandbookNo2).Dothisbeforeyouproceedtostep3.
3. Closethecontrolvalvesofthisbranch.
4. ReducetheBPVsetpointslowlyuntilyougetbackto2/3ofthedesignflowinSTAD.
5. (Seealsohandbook4-appendix5.5forcomplementaryexplanations).
19
tp
qb
qp
qs
tr
∆H
STAD-1 STAD-2
BPV
ts = tp
Fig. 13. Reducing or increasing the differential pressure applied across coils by means of a secondary pump.
Whentheprimarydifferentialpressureistoohighortoosmallforthesecondary,thecircuitinFig13presentsapossiblesolution.Inthiscircuit,areliefvalveisusedtoobtainaminimumflowtoprotectthepump.STAD-1isessentialtoavoidshort-circuitingtheprimarysystem.
Balancingprocedurefig13
1. Openallcontrolvalves.CloseallBPVs.
2. BalancethecoilsagainsteachotherwithSTAD-2asthePartnervalve(seeHandbookNo2).
3. SettheBPVonthemaximumpermitteddifferentialpressureforthecoilcontrolvalves.
4. Closethecoilcontrolvalvesofthisbranch.
5. Ifnecessary,reducetheBPVsetpointuntilyouobtaintheminimumpumpflow(seeAppendixC).
6. AdjusttoprimarydesignflowwithSTAD-1.Dothisaspartofthebalancingprocedurefortheentireprimarysystem(seeHandbookNo.2).
Balancing of Control Loops > 2. Control loops with two-way control valves
20
2.4 Constant primary and secondary flow
tp
qb
qp
qstr
STAD-1
tp
BPV
A
B
∆H
D
C
qv
ts
C
STAD-2
V
Fig. 14. Constant primary and secondary flows.
Acoilissuppliedatconstantflow.Thesupplywatertemperatureismodifiedbythetwo-waycontrolvalveV.Thistemperaturehastobeadjustedsothatts<tpinheatingandts>tp in cooling. The BPV keepsthedifferentialpressureCDconstant.ThisisthedesignpressurelossforthecontrolvalveV,whichhasanauthoritycloseto1afterbalancing.
Balancing procedure fig 14
1. Openallcontrolvalves.CloseallBPVs.
2. Iftheprimaryflowisunknown,calculateitusingtheformulabelow.
3. AdjusttheprimaryflowwithSTAD-1.Dothisaspartofthebalancingprocedurefortheentireprimarysystem(seeHandbookNo2)andbeforeyouproceedtostep4.
4. ClosethecontrolvalveV.
5. MeasuretheflowinSTAD-1.ReducetheBPVsetpointslowlyuntilyougetbackto2/3ofthedesignflowinSTAD-1.
6. AdjustthesecondarydesignflowwithSTAD-2.
(tsc -trc)
(tp -trc)
qp=qs
(Seealsohandbook4-appendix5.5forcomplementaryexplanations).
21
3. Control loops with three-way control valves
3.1 Variable primary flow and constant secondary flow
Passive primary network
Apassiveprimarynetworkisadistributionnetworkwithoutapump.Thesecondarypumppressurizesboththeprimaryandthesecondary.
tg ts
tr qs
E C
DSTAD
G
qg
HV
L
qb
tr
Fig. 15. Mixing circuit associated with a production unit.
Fig 15 shows a circuit controlled by a three-way mixing valve. The primary circuitconsists of an exchanger, a bypass line or a boiler that can either accept zero flowor be equipped with a bypass pump that generates a minimum flow. The three-wayvalve should be selected for a pressure loss at least equal to that in G, and at least 3 kPa.
Balancing procedure fig 15
1. Openthethree-wayvalvecompletely.
2. AdjusttodesignflowwithSTAD.
Balancing of Control Loops > 3. Control loops with three-way control valves
22
tg ts
tr qs
E
STAD-2
G
qg
HV
L
qb2
tr
C
D
qb1
B
A
tp
STAD-3
STAD-1
Fig. 16. Mixing circuit with intermediate bypass.
Whentheflowqsinthecircuitisgreaterthanthedesignflowthroughtheproductionunit,abypassABinsurescompatibilitybetweentheflows.
ThepressuredropcreatedbytheSTAD-3,forawaterflowqb1=qsc -qgc,isthenecessarydifferentialpressuretocompensatethepressuredropsintheSTAD-1+G+the3-wayvalve.
Thepressuredropcreatedbythethree-waycontrolvalveforthedesignflowqgcmustbeequalorhigherthanthedesignpressuredropinGandaccessorieswithaminimumof3kPa.
Balancing procedure fig 16
1. Openthethree-waycontrolvalve”V”.
2. Calculatethedesignflowqb1requiredintheSTAD-3andtheflowqgcinSTAD-1withformulabelow.
3. STAD-3andSTAD-1arebalancedaccordingtotheTABalancemethod(SeeHandbook2).
4. AdjusttheflowqswiththeSTAD-2.
(tsc-trc)
(tg -trc)
qgc =qsc qb1 =qsc -qgc
23
Active primary network
Anactiveprimarynetworkisadistributionsystemwithitsownpump.
Theprimarypumpcreatesadifferentialpressurewhichforcesthewaterflowthroughthesecondarycircuits.
L
tr
qs
qb C
∆H
STAD-2
V
E
tsqp tp
D
F
-
+H
tr
CSTAD-1
Fig. 17. Mixing valve with differential primary pressure and compensation balancing valve.
Thethree-wayvalveinFig17issuppliedbyaprimarydifferentialpressure∆H.Thispressuremaydisturbthefunctionofthethree-wayvalve.Thewaterflowqbinthebypassmayreverseandcancelthemixingfunctionofthecontrolvalve.
Topreventthis,thebalancingvalveSTAD-1hasbeeninstalled.ThepressurelossinSTAD-1shouldbe∆Hfordesignflowqpc.
Thedesignpressurelossacrossthethree-wayvalvemustbeatleastequalto∆Htogiveanauthorityof0.5.Thispressurelosshastobecoveredbythesecondarypump.
Balancing procedure fig 17
1. Closethethree-wayvalve.
2. AdjusttosecondarydesignflowwithSTAD-2
3. Openthethree-wayvalve.
4. ContinuetomeasuretheflowwithSTAD-2.AdjustSTAD-1toobtainthesameflowasinstep2.Dothisaspartofthebalancingprocedurefortheentireprimarysystem(seeHandbookNo2).
Balancing of Control Loops > 3. Control loops with three-way control valves
24
qb
qs
tp
tr
ts
qp
C
V2
∆p = min
STADV1
∆H
Fig. 18. Elimination of the primary differential pressure by means of a pressure controller.
Insomeplantsthethree-wayvalvesdonotworksatisfactorilybecauseofatoohighprimarydifferentialpressure.Sometimesadifferentialpressurecontrollerisinstalledtoeliminateorreducethispressuretoareasonablevalue,asshowninFig18.
Thisisanexpensivesolution.However,itmaybeconsideredifthedifferentialpressurecontrollerisusedtosupplyseveral3-wayvalvesandwhenavariableflowsystemisrequiredinthedistribution.Ifaconstantprimaryflowisaccepted,thedesignoffigure20isbetter.
Balancing procedure fig 18
1. Closethethree-wayvalve.
2. AdjusttosecondarydesignflowwithSTAD.
3. Adjustthesetpointofthedifferentialpressurecontrollertoavalueaslowaspossible.
25
3.2 Variable primary and secondary flows
Passive primary network
qb
qstp
tr
ts
qp
∆p≈ 0
+
BPV
STAD
-V
Fig. 19. The three-way valve prepares the water temperature in the distribution system.
Thethree-wayvalvecontrolsthesecondarywatertemperature.Thetwo-waycontrolvalvesmakethefinetuningoftheenergysupplybyadaptingtheflowtothedemands.
Thethree-wayvalvehasanauthoritycloseto1.Atlowloads,themodulatingreliefvalveBPVensuresaminimumpumpflow,andalsoreducesthetemperaturedropinthepipeline.
Note:Belowacertainflow,athree-wayvalvewillworkwithlaminarratherthanturbulentflow.Thenthethree-wayvalvetemporarilylosesitsbasiccharacteristicsandthecontrolloopbecomesdifficulttostabilize.Thus,theminimumflowcontrolledbytheBPVmustbehighenoughtocreateapressurelossofatleast1kPainthethreewayvalve.
Balancing procedure fig 19
1. Openallcontrolvalves.ClosetheBPV.
2. Balancethesecondarysystem(seeHandbookNo2)withSTADasthePartnervalve.
3. Closealltwo-waycontrolvalves.
4. SettheBPVtoobtaintheminimumpumpflow(seeAppendixC).
Balancing of Control Loops > 3. Control loops with three-way control valves
26
3.3 Constant primary and secondary flows
Active primary network
qs
trSTAD-2
A
∆H
B D
qp
tp
qb
ts
STAD-1
V
C
Fig. 20. The balancing valve STAD-1 and the bypass AB, eliminate the primary differential pressure across the three-way valve.
Iftheprimaryflowmaybeconstant,it’ssimpletoavoidatoohighdifferentialpressureontheprimaryofamixingthree-wayvalve.It’sjusttoinstallabypassABandtocompensatetheprimarydifferentialpressurewiththebalancingvalveSTAD-1.Theauthorityofthethree-wayvalvewillthenbecloseto1.
Balancing procedure fig 20
1. Openthethree-wayvalve.
2. AdjusttosecondarydesignflowwithSTAD-2.
3. Iftheprimaryflowqpisunknown,calculateitusingtheformulabelow.
4. AdjusttheprimaryflowwithSTAD-1.Dothisaspartofthebalancingprocedurefortheentireprimarysystem(seeHandbookNo.2).
(tsc -trc)
(tp -trc)qp=qs
27
qs
trSTAD-2A
∆H
B
D
F
qp
tp
qbp qbs
ts
STAD-1V
C
Fig. 21. When tsc is not equal to tp, it is better to place the bypass on the secondary side.
Whenthedesigntemperaturetscisnotequaltotp,thecircuitinFig21isoftenpreferabletothatinFig 20.
TheflowinthecontrolvalveislowerinFig21thaninFig20(qpinsteadqs),thusallowingtheuseofasmallerthree-wayvalve.
Theauthorityofthethree-wayvalveiscloseto1.
Balancing procedure fig 21
1. Openthethree-wayvalve.
2. AdjusttosecondarydesignflowwithSTAD-2.
3. Iftheprimaryflowqpisunknown,calculateitusingtheformulabelow.
4. AdjusttheprimaryflowwithSTAD-1.Dothisaspartofthebalancingprocedurefortheentireprimarysystem(seeHandbookNo.2).
(tsc -trc)
(tp -trc)qp=qs
Balancing of Control Loops > 3. Control loops with three-way control valves
28
3.4 Constant primary flow and variable secondary flow
tr
qs
qbSTAD-3 C
∆H
STAD-1V
tp
qp
tp
Fig. 22. A three-way mixing valve in a diverting circuit.
Thethree-wayvalveusedasamixingvalveinadivertingcircuitcansupplythecoilatvariableflowandconstantwatersupplytemperature,whilekeepingprimaryflowconstant.Thisway,thethree-wayvalveeliminatesinteractivitybetweencircuitsontheprimaryside.
Thethree-wayvalveshouldcreateadesignpressurelossequaltoorgreaterthanthepressurelossincircuitCtoensureanauthorityofatleast0,5.
Note: The most important balancing valve is the STAD-1. STAD-3 can be omitted if∆pC<0.25∆H.
Balancing procedure fig 22
1. Openallthree-wayvalves.
2. AdjusttodesignflowwithSTAD-1.Dothisaspartofthebalancingprocedurefortheentireprimarysystem(seeHandbookNo2)andbeforeyouproceedtostep3.
3. Closethethree-wayvalve.
4. MeasuretheflowinSTAD-1.AdjusttodesignflowwithSTAD-3.
29
tr
qs
qbSTAD-3 C
STAD-1V
tp
qp
tpBA
≈ 0
H
Fig. 23. Circuit in diversion on a passive distribution.
Ifthedistributionsystemispassive(noactivedifferentialpressure),thereisaneedforaseparatepump.Thispumpcanbecommontoseveralcircuits.
Note:ThemostimportantbalancingvalveisSTAD-1.STAD-3canbeomittedif∆pC<0.25H.
Balancing procedure fig 23
1. Openallthree-wayvalves.
2. AdjusttodesignflowwithSTAD-1.Dothisaspartofthebalancingprocedurefortheentireprimarysystem(seeHandbookNo2)andbeforeyouproceedtostep3.
3. Closeallthree-wayvalves.
4. MeasuretheflowinSTAD-1.AdjusttodesignflowwithSTAD-3.
Balancing of Control Loops > 3. Control loops with three-way control valves
30
4. Control loops compared
Variable primary water flow
Variablesecondarywaterflow Constantsecondarywaterflow
2-way 5 2-way 8 - 9 -10
3-way 19 3-way 17 - 18
Constant primary water flow
Variablesecondarywaterflow Constantsecondarywaterflow
2-way 12 - 13 2-way 14
3-way 22 3-way 20 - 21
Same functions obtained with two way or three way control valves.
31
4.1 Active primary network
STAD
��H
C
V
q
ts
tr
��pV
STAD
��HC
V
STAP
ts q
tr
��pV
STAD
��HC
BPV
tp
qb
qp qstr
STAD-1 STAD-2
BPV
ts
B
A
tp
qb
qp
qs
tr
STAD-1
STAD-2
BPV
ts
B
A
��T
t1t2VC
tp
qb
qp
qs
trSTAD-1
ts = tp
B
A
CSTAP
STAD-2
1
7
6
5
3
2
��pV > ��H/2 *
��pSTAD = ��H -��pV - ��pC
ßß' = ��pV / ��H
��pV > (��H - ��pBPV)/2 *
��pSTAD > 3 kPa
��pBPV = ��H - ��pV - ��pC - ��p STAD
ßß' = ��pV / ( ��H - ��pBPV)
��pV > Min STAP set point ≥10 kPa��pSTAD ≥3 kPa
ßß' close to one
qs > qp
��pV > ��H/2 *
��pSTAD-1 = ��H -�pV
ßß' = ��pV / ��H
ts = tp
qs < qp
��pV > ��H/2 *
��pSTAD-1 = ��H -�pV
ßß' = ��pV / ��H
ts = tp
��pV > ��H/2 *
��pSTAD-1 = ��H - ��pV - ��p STAD-2
ßß' = ��pV / ��H
Balancing of Control Loops > 4. Control loops compared
Variable primary and secondary water flows. Variables are represented by their design values -
Values recommended (*).
32
8��pV > ��H/2 *
ßß' = ��pV / ��H
ßß' = ��pV / ( ��pV + ��pH )
qs > qp
��pV > ��H *
��pSTAD-1 = ��H
ßß' = ��pV / ��H
��pV1 > ��H/2 *
��pSTAD-1 = ��H -��pV
ßß'V1 = ��pV1 / (��H -���p )
��pV2 > 3 kPa *
��pSTAD-1 = ��H - ��pV1
qb
tpts
tr
qp
qs = ctSTAD-2STAD-1
V
B
A
C
tr
qb
tpts
tr
qp
qs = ctSTAD-2STAD-1
V
B
A
C
tr
qb
qs
tp
tr
ts
qp
C
V2
�p= min
STAD-2V1
�H STAD-1
L
tr
qs
qb C
��H
STAD-2
V
E
tsqp tp
D
-
+H
tr
CSTAD-1
18
17
9
qs > qp
��pV > ��H/2 *
��pSTAD-1 = ��H -�pV
Variable primary water flow and constant secondary water flow. Variables are represented by their design values - Values recommended (*).
33
12
ßß' = ∆∆pV / ( ∆∆pV + ∆∆pC )
∆∆pSTAD-3 = ∆∆pC
∆∆pSTAD-1 = ∆∆H -∆∆pBPV
∆∆pSTAD-1 = ∆∆H - ∆∆pV - ∆∆pC
ts = tp
∆∆pV > ∆∆pC *
∆∆pSTAD-1 = ∆∆H
tp
qb
qp
qs
trSTAD
tp
BPV
A
B
tp
qb
qp
qs
trSTAD-1 STAD-2
BPV
ts = tp
tr
qs
qbSTAD-3 C
∆H
STAD-1V
tp
qp
tp
22
13
ts = tp
ts = tp
Balancing of Control Loops > 4. Control loops compared
Constant primary water flow and variable secondary water flow. Variables are represented by their design values - Values recommended (*).
34
14
ßß' = ∆∆pV / ∆∆pBPV
∆∆pSTAD-1 = ∆∆H -∆pBPV
∆∆pSTAD-1 = ∆∆H - ∆pV
∆∆pV > 8 kPa
∆∆pSTAD-1 = ∆∆H
21
20
qs > qp
tp
qb
qp
qstr
STAD-1
tp
BPV
A
B
∆H
D
C
qv
ts
C
STAD-2
V
qs
trSTAD-2
A
∆H
B D
qg
tg
qb
ts
STAD-1
V
C
qs
trSTAD-2A
∆H
B
D
qg
tg
qbp qbs
ts
STAD-1V
C
∆∆pV > 3 kPa *
ßß' = 1
∆∆pV > 3 kPa *
ßß' = 1
Constant primary and secondary water flows. Variables are represented by their design values - Values recommended (*).
35
4.2 Passive primary network
∆pV > ∆p1 *
ß' = pV / (∆pV + ∆p1)
qp < q s
∆pSTAD-3 =
∆p1 + ∆pV + ∆pSTAD-1
∆pSTAD-1 ≥ 3 kPa *
∆pV ≥ ∆pSTAD-3 / 2 *
ß' = ∆pV /∆pSTAD-3 max
(11a)
V
qp
t p
qs
t g
STAD-1 STAD-2
STAD-3
L>10 dt r
qs
t s
t p
qb
qp
STAD-1 STAD-2
STAD-3
L>10 dt r
qs
t st p qb
qp
STAD-1 STAD-2
BPV
V
L>10 dt r
qs
t st p
qb
qp
STAD
V
L>10 d
t r
t s
qb2
qg
V
t r
qb1
(11b)
(16)
(15)
∆p= 0
qp < q s
∆pBPV =
∆p1 + ∆pV + ∆pSTAD-1
∆pSTAD-1 ≥ 3 kPa *
∆pV ≥ ∆pSTAD-3 / 2 *
ß' = ∆pV /∆pBPV
- +∆p
qp < q s
∆pV > ∆p1 *
ß' = ∆pV / (∆pV + ∆p1)
Balancing of Control Loops > 4. Control loops compared
Variable primary water flow and constant secondary water flow. Variables are represented by their design values - Values recommended (*).
36
Appendix A
The authority of two-way control valves
A.1 The incomplete definition of valve authority
Thestaticcharacteristicofacontrolvalveisdefinedforaconstantdifferentialpressureacrossthevalve.Butthispressureisrarelyconstantinaplant.Therefore,therealcharacteristicofacontrolvalveisnotthesameasthetheoreticalone.
Whenthecontrolvalveisfullyopen,thedifferentialpressure∆pminisequaltotheavailabledifferentialpressureminuspressurelossesinterminalunit,pipesandaccessories.
Whenthecontrolvalveisclosed,pressurelossesintheotherelementsdisappearsincetheflowiszero.Theentireavailabledifferentialpressure∆Hmax=∆pmaxisthenappliedacrossthecontrolvalve.
Butthecontrolvalveissizedbasedon∆pminsinceitisatthispressurelossthedesignflowistobeobtained(fullyopenvalve).
Whenthevalveisnearitsclosedposition,therealflowishigherthanthetheoreticalsincethedifferentialpressureisgreaterthan∆pmin.Thetheoreticalcharacteristicisdistorted.Thedegreeofthisdistortiondependsontheratio∆pmin/∆pmax.
Thisratioisthecontrolvalveauthority.
β =Dpmin
Dpmax
Coil
STADSupply Return
∆pmax∆pmin
pressures
Thedifferentialpressureappliedtothecontrolvalvedependsonitsdegreeofopening.
37
Distortion of a linear valve characteristic as a function of its authority.
Distortion of the “EQM” characteristic of a valve as a function of its authority.
100
0 10 20 30 40 50 60 70 80 90 100
90
80
70
60
50
40
30
20
10
0h %
q %
β = 0
.1
0 .25
0 .5
1
100
90
80
70
60
50
40
30
20
10
0
q %
0 10 20 30 40 50 60 70 80 90 100
h %
0.25
0.5 1
β = 0
.1
Thelowertheauthority,thebiggerthedistortionofthetheoreticalvalvecharacteristic.
Consideravalvewithalinearcharacteristic,designedtoobtainthedesignflowexactlyatfullopening,butwithafairlylowauthorityof0.1.Ata10%valvelift,theflowinthecircuitisabout30%.
Supposethattheterminalunitisaheatingcoilwithadesigntemperaturedropof10K.Then30%ofthewaterflowproduces80%ofthedesignpower.
Thefinalresultisthatthecoiloutputis80%ofthedesignpoweratacontrolvalveliftofonly10%.Undertheseconditionsthereislittlehopetoobtainstablecontrol.Thesituationwouldbeevenworseif,forthesameauthority,thecontrolvalvewasoversized!
Anauthorityof0.5isacceptablesinceitdoesnotgreatlydeformthevalvecharacteristic.Inotherwords,thepressurelossatdesignflowinafullyopencontrolvalvemustbeequivalenttoatleasthalftheavailabledifferentialpressure.
Notethatthedesignflowdoesnotappearinthedefinitionofvalveauthority.
Thecurvesinthefiguresaboveareplottedassumingdesignflowwhenthecontrolvalveisfullyopen.Butthatisrarelythecaseinpractice,sinceitisdifficulttoavoidacertaindegreeofoversizing.
Whenacontrolvalveisoversized,∆pminisreduced,assumingconstant∆pmax.Thus,thecontrolvalveauthorityisalsoreduced.Thetheoreticalvalvecharacteristicwillbeheavilydistortedandcontrolbecomesdifficultatsmallloads.
However,anoversizedcontrolvalvecanhaveagoodauthority.Ifthedifferentialpressureappliedtoacircuitisdoubled,∆pminand∆pmaxincreaseinthesameproportionandtheauthorityremainsunchanged,althoughthere’snowanoverflowinthecircuit.
What,then,willhappentothevalveauthorityinacircuitexposedtoavariabledifferentialpressure?
Then∆pmaxand∆pminwillvarysimultaneouslyinthesameproportions.Thevalveauthorityβthusremainsconstant.
However,thevalvecharacteristicisdeformed,despitethefactthattheauthorityβisthesame.
Therefore,theauthorityasdefinedabove,doesnotgiveenoughinformationabouttherealdistortionofthevalvecharacteristic.
Balancing of Control Loops > Appendix A. The authority of two-way control valves
38
A.2 The correct definition of valve authority β’
Wegetamorecoherentdefinitionoftheauthorityifwerelatethepressurelossinthecontrolvalvefordesignflowtothemaximumpressurelossinthevalve:
Dpacrossthefullyopencontrolvalveanddesignflow
Dpvalveshutβ’ =
0 10 20 30 40 50
h %
q %
60 70 80 90 100
90
100
110
120
130
140
80
70
60
50
40
30
20
10
0
β = 0.5, β' = 0.5
β = 0.5, β' = 0.32
β = 0.5, β' = 0.22β = 0.5, β' = 0.13
Theoretical
Flow as a function of valve lift when the circuit supply pressure varies with constant authority β.
Thefigureshowsthattheauthorityβ’takesintoaccountthedistortionofthevalvecharacteristic.Thisisnottruefortheauthorityβaccordingtotheconventionaldefinition.
Thetwoauthorityfactorsrelatetoeachotheraccordingtotheformulabelow
(Sqistheoverflowfactor):
β=(Sq)2 . β’
Sq≥1withthevalveopen.Whenthemaximumflowisequaltodesignflow,β= β’.
39
140
120
90 1000
20
40
60
80
100
20 30 40 50 60 70 80100
q : flow in %
b- with STAD
a- theoretical
h: lift in %
c- without STAD
Influence on the control valve characteristic through limiting the maximum flow by a balancing valve.
Can a balancing valve be put in series with a control valve?
AcontrolvalvewithexactlythecalculatedKv-valueisusuallynotavailableonthemarket.Consequently,theinstalledvalveismoreorlessoversized.Duringstartupafternightsetback,whenmostcontrolvalvesareopen,theoverflowinfavouredunitscreatesunderflowsinothers.It’sthereforeessentialthattheflowthroughthecontrolvalveislimitedbyabalancingvalve.
Thefigureaboveshowshowthistypeoflimitationinfluencesthecontrolvalvecharacteristic.Withoutabalancingvalve,theoverflowinthefullyopencontrolvalvewillbe22%anditsauthorityβ = 0.5 accordingtotheconventionaldefinitionofvalveauthority.Butthisisarathermisleadinginformationabouttheauthority,sincetheflowiswrong.
Theauthorityβ’=0.34indicatestherealdistortionofthevalvecharacteristic.
Theauthorityβ’isthesamewithorwithoutthebalancingvalve,anddependsmainlyontheinitialchoiceofcontrolvalve.
Byinstallingabalancingvalve,wecanobtainthecorrectwaterflowunderdesignconditionsandimprove thecontrolvalvecharacteristic.
Balancing of Control Loops > Appendix A. The authority of two-way control valves
40
A.3 Sizing of control valves
The Kv factor
Acontrolvalvecreatesacomplementarypressurelossinthecircuittolimitthewaterflowtotherequiredvalue.Thewaterflowdependsonthedifferentialpressureappliedtothevalve:
(F3.10) 1000p
Kvq ⋅ρ
∆ ⋅=
Kvisthevalveflowfactor.
ρ isthedensity.Forwater,ρ =1000kg/m3at4°Candρ=970at80°C.
qistheliquidflowinm3/h.
∆pisthedifferentialpressureexpressedinbars.
ThemaximumKvvalue(Kvs)isobtainedwhenthevalveisfullyopen.Thisvaluecorrespondstothewaterflowexpressedinm3/h,foradifferentialpressureof1bar.
ThecontrolvalveisselectedsothatitsKvsvaluewillgivedesignflowforthedifferentialpressureavailablewhenthevalveisworkingunderdesignconditions.
ItisnoteasytodeterminetheKvsneededforacontrolvalvesincetheavailabledifferentialpressureforthevalvedependsonmanyfactors:
• Theactualpumphead.
• Pressurelossesinpipesandaccessories.
• Pressurelossesinterminalunits.
Thesepressurelossesalsodependontheaccuracywithwhichbalancingisdone.
Duringthedesignoftheplant,thetheoreticallycorrectvaluesforpressurelossesandflowsforvariouscomponentsarecalculated.Butcomponentswithexactlythespecifiedpropertiesarerarelyavailable.Theinstallermustnormallyselectstandardvaluesforpumps,controlvalvesandterminalunits.
Controlvalves,forinstance,areavailablewithKvsvalueswhichincreaseinageometricprogression,called a Reynard series:
Kvs:1.01.62.54.06.31016.....
Eachvalueisabout60%greaterthanthepreviousvalue.
Itisunusualtofindacontrolvalvethatcreatesexactlythedesiredpressurelossfordesignflow.If,forinstance,acontrolvalvethatcreatesapressurelossof10kPaforthedesignflowisneeded,itmayoccurthatthevalvewiththenearesthigherKvsvaluecreatesapressurelossofonly4kPa,whilethevalvewiththenearestlowerKvsvaluecreatesapressurelossof26kPafordesignflow.
41
∆p (bar), q (m3/h) ∆p (kPa), q (l/s) ∆p (mm WG), q (l/h) ∆p (kPa), q (l/h)
pKvq ∆⋅= p36Kv
q ∆= pKv10q ∆= pKv100q ∆=2
Kvq
p
=∆
2
Kvq
36p
=∆
2
Kvq
0.01p
=∆
2
Kvq
0.1p
=∆
pq
Kv∆
=p
q36Kv
∆=
pq
0.1Kv∆
=p
q0.01Kv
∆=
Some formulas involving the flow, Kv and ∆p (ρ = 1000 kg/m3).
Inaddition,pumpsandterminalunitsareoftenoversizedforthesamereason.Thismeansthatcontrolvalvesfrequentlyhavetoworkneartheirclosedposition,resultinginunstablecontrol.Itisalsopossiblethatthesevalvesperiodicallyopentoamaximum,definitelyduringstartup,creatinganoverflowintheirunitandunderflowsinotherunits.Weshouldthereforeaskthequestion:
What to do if the control valve is oversized?
Wehavealreadyseenthatweusuallycan’tfindexactlythecontrolvalvethatwewant.
Takethecaseofa2000wattcoil,designedforatemperaturedropof20K.Itspressurelossis6kPaforthedesignflowof2000x0.86/20=86l/h.Iftheavailabledifferentialpressureis32kPaandpressurelossesinpipesandaccessoriesare4kPa,thedifferenceis32-6-4=22kPawhichmustbeappliedacrossthecontrolvalve.
TherequiredKvsvalueis0.183.
IfthelowestavailableKvsvalueis0.25,forexample,theflowwillbe104l/hinsteadofthedesired86l/h,anincreaseby21%.
Invariable-flowsystems,differentialpressuresappliedtoterminalunitsarevariablesincepressurelossesinpipesdependontheflow.Controlvalvesareselectedfordesignconditions.Atlowloads,themaximumpotentialflowinallunitsisincreasedandthereisnoriskofcreatingunderflowsinsomeunits.Underdesignconditions,andwhenmaximumloadisrequired,it’sessentialtoavoidoverflows.
a- Flow limitation by means of a balancing valve in series
Iftheflowintheopencontrolvalveunderdesignconditionsisgreaterthantherequiredvalue,abalancingvalveinseriescanbeusedtolimitthisflow.Thisdoesnotchangetherealauthorityofthecontrolvalve,andweevenimproveitscharacteristic(seefigurepage41).Thebalancingvalveisalsoadiagnostictoolandashut-offvalve.
Balancing of Control Loops > Appendix A. The authority of two-way control valves
42
STAD
∆H
C
V
q
ts
tr
∆pV
A balancing valve limits the flow in the control valve without changing its authority β’
b - Reduction of the maximum valve lift
Tocompensateforoversizingofacontrolvalve,thevalveopeningmaybelimited.ThissolutioncanbeconsideredforEqualpercentagecharacteristicvalves,sincethemaximumKvvaluecanbesignificantlyreducedwithareasonablereductioninthemaximumopening.Ifthedegreeofopeningisreducedby20%,themaximumKvvalueisreducedby50%.
Inpractice,balancingiscarriedoutbymeansofbalancingvalvesinserieswithfullyopencontrolvalves.Thebalancingvalvesarethenadjustedineachcircuitsuccessivelyinordertogiveapressurelossof3kPaatthedesignflow.
Thecontrolvalveliftisthenlimitedtocreate3kPainthebalancingvalve.Sincetheplantisandremainsbalanced,theflowisthereforeactuallyobtainedunderdesignconditions.
c- Flow reduction using a ∆p control valve in series
Thedifferentialpressureacrossthecontrolvalvecanbestabilizedaccordingtofigurebelow.
STAD
∆HC
V
STAP
A ∆p controller keeps the differential pressure across the control valve constant.
ThesetpointofthedifferentialpressurecontrolvalveSTAPischosentoobtainthedesignflowforthecontrolvalvefullyopen.Inthiscase,thecontrolvalveisneveroversizedanditsauthorityiskeptclosetoone.Balancingprocedureisdescribedonpage10.
43
Some rules of thumb
Whentwo-waycontrolvalvesareusedonterminalunits,mostcontrolvalveswillbeclosedoralmostclosedatlowloads.Sincewaterflowsaresmall,pressurelossesinpipesandaccessoriesarenegligible.Theentirepumpheadisappliedacrossthecontrolvalves,whichmustthenbeabletoresistthispressure.Thisincreaseinthedifferentialpressuremakescontroldifficultatsmallflows,sincetherealvalve authorityβ’isreducedgreatly.
Assumethatacontrolvalveisdesignedforapressurelossof4%ofthepumphead.Iftheplantworksatsmallflow,thedifferentialpressureacrossthiscontrolvalveincreasesfrom4toalmost100%.Thedifferentialpressureisthusmultipliedby25.Forthesamevalveopening,allflowsarethenmultipliedby5(√25=5).
Thevalveisforcedtoworknearitsclosedposition.Thismayresultinnoiseandhunting(underthesenewworkingconditions,thevalveisoversizedfivetimes).
Thisiswhysomeauthorsrecommendthattheplantisdesignedsothatthedesignpressurelossinthecontrolvalvesisatleast25%ofthepumphead.Then,atlowloads,flowoversizingofcontrolvalvesdoesnotexceedafactor2.
It’snotalwayspossibletofindcontrolvalvescapableofresistingsuchlargedifferentialpressurewithoutgeneratingnoise.Itisalsodifficulttofindvalvessmallenoughtosatisfytheabovecriterionwhenlowpowerterminalunitsareused.Then,differentialpressurevariationsintheplantshouldbelimited,forexamplebytheuseofsecondarypumps.
Takingthisadditionalconceptintoconsideration,thesizingofatwo-waycontrolvalvemustsatisfythefollowingconditions:
1. Whentheplantoperatesundernormalconditions,theflowinthefullyopencontrolvalvemustbethecalculatedflow.Iftheflowisgreaterthanthis,abalancingvalveinserieswilllimittheflow.Anauthorityof0.30isthenacceptableforaPItypecontroller.Iftheauthorityislower,thecontrolvalveshouldbereplacedforasmallerone.
2. Thepumpheadshouldbesuchthatthepressurelossinthetwo-waycontrolvalvescanbeselectedtobeatleast25%ofthispumphead.
ForOn-Offcontrollers,theauthorityconceptismeaninglesssincethecontrolvalveiseitheropenorclosed.Itscharacteristicisthereforenotveryimportant.Inthiscasetheflowislimited,withnofundamentalrestriction,byabalancingvalveinseries.
Balancing of Control Loops > Appendix A. The authority of two-way control valves
44
Appendix B
The authority of three-way control valves
B.1 In mixing function
Athree-wayvalveusedinmixingcansupplyacircuitatconstantflowandvariableinletwatertemperature.
Theprimarywaterattemperaturetpismixedwiththereturnwaterattemperaturetrinthenecessaryproportiontoobtaintherequiredmixtemperaturets.
tp ts
tr qs
E C
DSTAD-2
G
qp
HV
L
STAD-3qb
tr
Three way valve in mixing function.
WhenportEopens,portLclosesinthesameproportion.Thethirdcommonportremainsopen.WhenportEisclosed,thethree-wayvalveisclosedandnoenergycanbeextractedfromtheprimary.Thetemperaturetsisthenequaltotrwhichgraduallyreachesthemeantemperatureoftheroom.
ThebalancingvalveSTAD-2canadjusttheflowtotherequiredvalue.Inprinciple,ahydraulicresistanceequaltothatofGmustbecreatedinthebypassbymeansofSTAD-3inordertogivethesamewaterflowqsregardlessofwhetherthethree-wayvalveisopenorclosed.Inthiscase,thethree-wayvalveisbalanced.
The authority of the three-way valve
Wewillreplacethethree-wayvalvebytwotwo-wayvalvesworkinginopposition.Wewillthenobtainthesamemixingfunction.
ts
qs
VE
STAD-2
G
qp
H
VL
tr
C
D
qb
tr
tp C qs
A three-way valve can be represented by two two-way valves working in opposition.
45
ThevalveVErepresentsthecontrolport.Itspressuredropforthedesignflow=∆pV.Ifthecircuitflowqsisconstant,thepumppressureheadHisconstant,asarepressurelossesinthecircuit.Theresultisthatthepressuredifference∆pDCisconstant.ThispressuredifferenceisappliedtovalveVEwhenitisclosed.Bydefinition,thevalveauthorityisgivenbytheratio∆p(valveopen)to∆p(valveclosed).Thus:
∆pV ∆pV
∆pDC ∆pV+∆pGβ’ = =
Thisauthorityisequalto0.5ormoreif∆pV>or=∆pG.Thismeansthatthepressurelossacrossthethree-wayvalvemustbeatleastequaltothepressurelossinthevariableflowcircuitG,includingthepipes.
Thecircuitbelowgivesaconstantflowintheproductionunitandthethree-wayvalveauthorityiscloseto1.
ts
qs
E
STAD-2
G
qp
HV
L
tr
STAD-1
B
A
C
D
qb
qg
tr
A bypass AB and a primary pump can give a constant production flow and a three-way valve authority close to 1.
Infact,thethree-wayvalvedrawsfromanddischargesintothebypassABwhichactuallyformsavirtualproductionunitwithnopressureloss.Inthiscasetheauthorityofthethree-wayvalveis:
DpV
DpV+∆pDBAEβ’ =
Since∆pDBAEislow,theauthorityofthecontrolvalveiscloseto1.
B.2 In diverting function
Whenusedindivertingfunction,athree-wayvalvecansupplyacircuitatvariableflowandconstantwater supply temperaturewhile keeping theprimaryflowpracticallyconstant.
Diverting valve mounted in a diverting circuit.
L
tr
qs
qbSTAD-3 C
∆H
STAD-1
V
E ts
qp
ts
Balancing of Control Loops > Appendix B. The authority of three-way control valves
46
TheprimaryflowistransferredthroughportEorbypassedthroughportL.Inprinciple,it’sconstant.ThebalancingvalveSTAD-1locatedintheconstant-flowpipe,limitstheflowbycreatingaconstantpressureloss.
Sincethethree-wayvalveinadistributioncircuitisusedtokeeptheprimaryflowconstanttoavoidinteractivitybetweencircuits,itwouldbelogicaltotakewhateveractionisnecessarytoensurethatthispurposeisactuallysatisfied.
ThisisdonebyplacingabalancingvalveSTAD-3inthebypassinordertocreateapressuredropequivalenttothatofCforthesameflow.Inthisway,theprimaryflowisunchangedifportEorportLisfullyopensincethehydraulicresistancesinserieswiththeseportshavethesamevalue.
ThemostimportantbalancingvalveisthisSTAD-1.STAD-3canbeomittedif∆pCislowerthan0.25times∆H.
Note:
Three-wayvalvesareusuallydesignedtoperformamixingfunction:twoinputsandoneoutput.Theiruseindivertingfunctionwithoneinputandtwooutputswillgeneratewatercirculationthroughthevalveinthedirectionoppositetotheplanned.Forsomevalves,thisreversalmayleadtoasignificantincreaseinthenoiselevelandavalvechatteringphenomenon.
tr
qs
qbSTAD-3 C
∆H
STAD-1V
tp
qp
tp
Diverting circuit using a three way mixing valve
Thisiswhyadivertingfunctionwithathree-waymixingvalveisobtainedbyplacingthevalveinthereturncircuitasshowninthefigure.Thisthenprovidesthesamefunction,whilerespectingthewatercirculationdirectionthroughthevalve.
Inbothcasesthevalveauthorityis:
∆pV
∆pV+∆pCβ’ =
Toobtainanauthorityofatleast0.5,thepressurelossinthethree-wayvalvemustbeequaltoorgreaterthanthepressurelossintheterminalunitC.
47
Appendix C
How to set the BPV to ensure the minimum pump flow
InsomecasesaBPVmodulatingreliefvalveisinstalledtoensureaminimumflowtoprotectthepump,asinthefigure.
Ifthisminimumflowisforinstance10%ofdesignflow,thepressurelossinthebalancingvalveSTAD-2isonly1%ofthepressurelossatdesignflow.Normally,thisisafartoolowvaluetoallowprecisemeasurement.Sohowcanwemeasuresuchasmallflowasqsmin?
Thefollowingmethodcanbeused:
a) FindoutwhichhandwheelsettingofSTAD-2thatwillcreate3kPafortheminimumpumpflowqsmin,forinstance10%ofdesignflow.UsetheTA-SCOPE,oraTAnomogram,tofindthecorrectsetting.
b) AdjustSTAD-2tothissettingtemporarily.Closethetwo-waycontrolvalves.
c) OpentheBPVslowlyuntilyouobtaintheminimumpumpflowqsmininSTAD-2.
d) ReopenSTAD-2toitspresetposition.
Whenthecoilcontrolvalvescloseandtheflowqsgoesbelowthespecifiedminimumflowqsmin,theBPVopens.TheBPVthenbypassesaflowqsminforaslongastheflowqsinthecoilcontrolvalvesremainsbelowqsmin.
tp
qb
qp
qs
tr
∆H
STAD-1 STAD-2
BPV
ts = tp
Thismethodisonlyapplicableiftheflowmeasurementdeviceisofthevariableorificetype,astheSTADbalancingvalve.
Balancing of Control Loops > Appendix C. How to set the BPV to ensure the minimum pump flow
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Balancing of Control LoopsHydronicEngineeringHandbookN°1
“Balancingcontrolloops”isthefirsthandbookinTAHydronicsseriesofpublicationsabouthydronicdesignand balancing. The second handbook dealswithbalancingofdistributionsystems,thethirdwithbalancingofradiatorsystemsandthefourthwithhydronicbalancingwithdifferentialpressurecontrollers.