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NNERfiY INTIilA '90s. Assessing an uneertain

energy future. IIow to get the most

out of your boilersystems

Valvoda and t'anelltackle short-cireuitcalculations Page 70

At left: Energy considerations willplay a key role in mechanical/elec-trical designs for the '90s. Inset:Optimizing boiler efficiency.

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ENERGY TECHNOLOGY

0ptimize BoilerOperation Through

System DesignAduan ces'in equiprvtpnt an'd control

d,esign, combinnd ulith qn'irt creased focuson economics, prouifu th'e bankgrm'nd'

fw aptimized boiler system de,sign

HEMANT MEHTA, P.E.SenimVice Presid,entDirector, Central Utility GrouPERVIN KALMANSenim EngineerSyska & Hennessyand LEONARD PRITKIN, P.E'Senr.or EngiwerE lectronic Sy stems Associ,ates,

a subsil,iarg of Syska & HennessyNew York City

Designing a boiler plant is a complexprocess that star"ts with establishingpresent and future requirements,ionditions and constraints. Opportu-nities for optimization begin with theoverall schematic plant design andcontinue on through the design ofsubsystems and components.

Design elements invariably requirean initial capital outlay. However, theoperating efficiencies they provide re-sult in a payback and, ultimately, costsavings over the boiler's life. The pay-back period may.or may not be attrac-tive to a particular client. This be-comes a design parameter to be con-sidered by the engineer.

The main process in a boiler plantis to convert fuel heat input into us-able heat output, usually as steam orhot water. In addition to fuel, theboiler plant needs energy to driveauxiliary equipment and controls.

The energy balance of a Plant

(simplified) is shown in Figure 1,indicating that a significant part ofthe output represents loss due towaste heat. Minimizing losses and€nergy required by auxiliary equip-menf through design is the mainthrust of this article, and its im-portance to optimal boiler operationcannot be overemphasized.

Selecting the number of boilers andtheir individual capacities is based onload profiles derived from a site mas-ter plan or load studies. Selection isinfluenced by immediate and pro-jected needs. If one focuses strictlybn boiler performance, a single largeunit would be the choice due to bestefficiency, smallest space require-ment and simplest operation. How-ever, demand variations and reliabil-ity require plants with multiple boil-ers for maximum efficiency.

The next tendency is to select anumber of identical boilers to covermaximum demand and provide sparecapacity. However, depending on sea-sonal or daily demand variation, thismay have to be altered. For examPle,a heating-only boiler plant justifiesproviding a smaller summer boiler op-erating near peak efficiency. If sum-mer loads are high due to steam-operated chillers, low demand timesmay exist during spring and fall, jus-tifuins a smaller boiler.

bxc"ept for very large utility boil-

DDC-based, controls are iust one ofthe opti,mizati.on features of this boil'er instalLution.

ers, shop-fabricated package boilersare available in standard sizes andtypes from various rnanufacturers.Tliey usually are gas and/or oil fired'Based on the application, the keY toboiler selection is efficiency andequipment quality.-Main

boiler-plant parameters, heat-ing medium, pressure/temperaturelevels and capacities are determinedby user requirements. Howeve_r, -pa-rameter changes may be justified inmany cases. As a real-life examPle,one plant required 125-psig steam, so

a 250-psig boiler system was selected.This was justified with large steam-turbine-driven chillers which favor ahigher steam pressure level to opti-mize overall site operation' Highpressure increases turbine efficiencyand reduces the water flow rate.

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56/MARCH 1991, CONSULTING/SPECIFYING ENGINEER

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:ri? pcrtrlds 0f .ct,e&rn iler houyi:it1's,ep{}we1, v,/xth 125-psig

,nfi lleiieXop 198.2 irr:rsepcvren,,iiaft.. 'i'lrr oarne 1'-urbine ai 2b0;:,;t *" ryat,:1.-r'lalur rate cf 32:.,i- ite!,r.:l rex holtr tii:res hcrse-rirrir Oereicpg 252"5 hoi"se -

i:'ru:r, i.he waier".ficw rate de-i:l' -ri:i.Fta. ,nhi1+:. at lhe sarne

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t-r:.r-:e ii_*' ihl sa.;yie rneci:anieal;1-rJiraii';:r.

t : i:,o,:-g*.ti fi *:nt rc*overy,..r sigp,:i,,.n1,r'l ,ipal. jiss r!i a

',:::iirs fi.i the pcini rvhere fluei r:j:: tj:ie boiier." iili:r. gasj cci:t-

rL)1, combus.ii,un-pr*duct ga.s,. anil ineri ciMt$orrenLs. Loss

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LOSSES 2AoiolA 25yaSTACK, I}\SULNTION,

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A sr'gtzifitant patt of the heat c,ut1tu.t .rept"esents loss riue to uLcsie heat

CONSULTING/SPEC]FYiNC ENGJNEER. },{AR,CH 1991/5?

oPTlMlzlNG BOILER @Fumnce draft, Wessure i"s mfrantnivrcd Ua 4M inlet uan'es,

drumpers a, aahnble-speed draft'-fan control

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oiii,-niinti Ll, not to be tn*eased. The percentage of ercess-air Lncrease uQrLes

subsiantially with the bu,mer mnnufactwer'

ALI bu.rners require excess a'ir to ensure com'plete.fue| coylustioy, CXy,bustion-air

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Blow-down sYstemContinuous and intermittent blow

downs of boiler water are requiredin steam boilers' Continuous blowdown maintains acceptable concentra-tion levels of dissolved solids andchemicals in the boiler water. Inter-mittent blow down removes PreciP!lated solids settled at the bottom ofthe boiler. Both of these representheat losses. The losses can be mini-mized by using sophisticated watertreatment and ihemical injection pro-

ETams. This can be exPensive, de-

iending on the raw. make-uP wateroualitv. A reasonable comPromtserirust 6e established.

Continuous blow down lends itselfto heat recovery' A good sYstem con-

sists of a flash tank with flash steamDressure maintained at deaeratorir"".sute. This saves steam from theboiler. In addition, hot water in theflash tank can be cooled with coldmake-up water in a blow-down heatexchanfer. This ailows heat recoverybv nroiidins warrner make-up whiletfie^blow-down is cooled to acceptabletemperatures for disPosal.

Due to its short duration, blow-down heat from intermittent blowdown is not easilY recoverable, butthe losses are very low'

Good plant layout is an imPortantfactor in'minimiZing losses' too. Lay-out should foliow the logic of process

flows closely. Combining Iayout withtieht design minimizes PiPingIeilEths. reducing investment costs,pr"i.ut" drops and heat losses. How-ever. a compromlse must be reached

between tight design and necessaryservice and maintenance sPaces' Agood layout also allows for expansion'

Control sYstemBoiler-control-sYstem design in

1991 should include-direct digital con-

trols. They use computer- Iogic .tomanase a process. Older electronicsand darlier'pneumatic systems do notmeet today-'s performance exp-ecta-tions. Sinc6 the introduction of directdigital control electronics into boiler.uit".nt, almost any conceivable logico"r control scheme can be implementedeasilv, accurately and safely. This isdone at three levels of authority: each

boiler master panel, the plant masternanel and the^site energy monitoringind control system (EMCS).---ivti.rooto.eisor intelligence is dis-tributed so that safe operation is pos-

Feed-waterThe feed-water sYstem for a steam

nlant consists of a deaerator and a

boiler feed-water pump. AIso ass-oci-

ated with the sYstem are a conden-sate-return (sur[e) tank and a set ofdeaerator feed-water PumPS.

The key selection parameter in thissvstem ii deaerator-pressure' It de-

t6rmines feed-water temperature'Entering feed-water cools the flue gas

in the e"conomizer. By creating localcold spots on the flue-gas-side, raPidcorrosion may occur. Selecting thenroDer watef temPerature dePendsbn ttt" amount of

-sulfur in the fuel'In boiler plants with multiple fuels,such as sis and oil with gas as Pri-mary fue-I, different feed-water tem-peratures are required for each fueltvoe. To avoid varYing deaeratoro"rLssures. a steam-heated feed-water'preheater usually is provided for thehrel with hieher sulfur (oil).

Pump cafiacitY and number selec-

tion should foilow the PrinciPlesdeseribed earlier in boiler selec-tion. PumPing equiPment must be

canable of providing feed-water atali loads ahd should be selectedbased on the highest efficiency,.-reli-abilitv and maintainability availablefor the service parameters. Availa-bilitv of qualitY PumPS maY havean iinpact-on selecting the number

and capacity of PumPs,*'ir l".r,rti."-"'*i.ti that the boiler

olant will undergo future expansions,heaerator-tank iizing should reflectthat. However, oveisized units in-crease losses due to larger surfaces'Final size usuallY is a comPromisebetween conflicting parameters. Simi-lar principles apply to choosing deaer-atoi feed-wat6i-iumP and conden-sate-tank sizes.

Thermal distributionThe thermal distribution sYstem

should be designed for minimumlosses via ample pipe size, good.insu-lation, qua[fu traPs and minimumleakase.'This desidr must be comple-ment6d by good traP maintenanceand leak prevention.

Checking traP malfunctions andsteam leaki is not easY. However, a

modern leak-detection method existsthat enables testing without servicedisruption (providing a condensatechamber with a weir and conductiv-ity measurement)' A leak-detectionsvstem is recommended to Providethe means for a good testing Pro-sram. Leaks rePresent heat lossesfhroueh steam or hot water thatmust

"be rePlaced with cold water'In addition, water and treatmentcosts and chemical consumption costs

are incurred due to leaks.

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6O/MARCH 199T, CONSUI-'IING/SPECIFYING ENGINEER

OPTIMIZING BOILER OPERATION

Boiler-control-syste,m designers optimtze th,eir designs by - -

infrL,udilg integiatpd, d;treci dtglnl controls ushBreaer possible

sible without higherJevel authority.Maximum efficiency can be achieveddue to increased control accuracv andthe optimization of each element ofthe whole. By specifYing comPuterIosic. the enqineer ensures that theop"erator has" the flexibility to finetune the system based on his own fa-miliarity *ittr ttre particular relation-ships at his site. The operator has theneCessary data stored in computermemory, and they are all availablein printouts, reports and comPutergraphics. As all routine repetitive op-erations are perforrned by computer,the operator has more time for Pre-ventive maintenance and is free to re-spond more rapidly to emergencies'

A boiler masterFuel/air-The digital boiler master

controller is fully metered. Each typeof fuel flow is measured and fuelvalves are controlled. Airflow ismeasured and air damPers or vari-able-speed fans are controlled, whilethe fuel/air ratio is accurately main-tained to obtain the most efficientcombustion.

Crosslimiting control ensures safefiring during load changes. Lead/lagcontiol increases airflow during loadincreases and lags during load de-creases, maintaining excess air dur-ing load changes. The fueVair ratiois-changed precisely when anotherfuel is ielected. Dual fuels maY beburned and the optimum ratios main-tained. Fuel demand charges also can

be optimized. Computer calculationscan fimit or shut dbwn one fuel and

maintain the preferential fuel. Theseare typical of the strategies best im-plemented by digital controllers.-

Oxygen trim-Oxygen trim is afine-tuning adjustment of the fueVaircombustion controllers, depending onthe oxygen content of the flue gas.Availa6il-ity of good zirconium-oxidesensors has made this control techno-logically feasible. As oxygen contentdecreases, flue-gas heat loss de-creases up to a point where unburnedfuel losses start to increase. The oxy-gen trim digital controller provides aninput to the combustion controller toobtain peak efficiency.

Three-element feed'water con-trol-For steam boilers, the highestefficiencies are reached by preciselymeasuring drum level and Pressure,feed-water flow and steam flow. In-puts combine in complex control loopsio adiust the feed-water throttlingvalve. The drum-level control loop isslower acting and responds to inter-mittent losses such as blow down.

Furnace draft Pressure-Draftpressure is maintained by. either inlet-vanes,

dampers or variable-sPeeddraft-fan control. Fast response is re-quired and sometimes an overridecbntroller is used to prevent furnaceimplosion.

Steam-header pressure-For a sin-gle boiler, the steam-pressure con-[roller is a simple input to the com-bustion controller.

The plant masterMultiple boilers on a common

header are controlled by a plant mas-

ter panel. In its simPlest form, thesteam-pressure input distributes theload equally to equally sized boilers.If the boilers are unequal or if, forexample, the other boilers are usingother fuels, loading the boilers for op-timum efficiency is best handled bYa computer. The boiler with the low-est incremental cost should be loadedto the maximum. Sometimes the deci-sion to start up or shut down boilersmay be influenced by the increasedeleitrical cost of operating additionalpumps. The computerized plant mas-ter k-eeps track of all variables, calcu-lates "what if' scenarios and then is-sues advisory messages to the opera-tor to start or stop the lag boiler'

Advanced computer color graPhicsnow available can "window" into com-puter-simulated control loops to ob-ierve the system at all levels: over-view, group and detail. Flow dia-grams have dynamic digital readingsthat are updated every few seconds.Graphic-diiplay trend logs with indi-viduall.y selected ranges have re-placed the unreliable ink recorderswhose pens always seem to go drY.Dedicated printers for alarms andlogs are essential.

Other plant master functions are tokeep track of and alarm all accessoryanilauxiliary systems, such as:. Burner management. Deaerator and surge tanks. Fuel distribution. Preheaters and economizers. Instrument air compressors. Fuel viscosity control. NOx emission control. Uninterruptible power suPPlies. Gas- and steam-pressure regulators. Burner steam or air atomization. Water chemical treatment.

'Although it is current practice forindividual local controllers' such as

tanklevel control, to be conventionalpneumatic types, the designer shouldionsider the advantages ofdirect digi-tal control and move toward provid-ing totally integrated plant systems.

Using a site EMCSThe boiler is a component of the

plant, which is, in turn, a comPonentof the site control system' Boiler op-eration is affected by major userssuch as steam turbines, absorPtionchillers, electric generators and build-ing and process loads.

Now that integrated networking ofcomputer systems is becoming more

switchgear for the central utility ptant helps keep euerything running srnoothly.

62/MARCH 1991, CONSULTING/SPECIFYING ENGINEER

OPTIMIZING BOILER OPERATION

Thn keA selectt'on Pa,rarneter in a

feed+uater system is d'eaerator Wessure

available, it is important to considerallowing local loop microprocessors to"talk" to each other and to computersat every level. Global requirements,such as-demand energy charges, day/night rates, energy storage, occu-na"ncv and nrocess loads also shouldbe consider6d. By reviewing these is-sues, it will be more likely that plantmanagers will develop a global viewof the system and help speed the proc-ess tow-ard complete centralized moni-toring and control.

Logs and accountingTodav's computer systems are ex-

pected" to repiace the dailY hand-written logs that require operators toread gauges hourly. Data are not oniyprinted but stored in disk memory foriecords. Some of the automatic logsthat are available include:

-Each Boiler, DailY SummarY Re-

port, hourly data

-Plant Daily SummarY RePort,hourly data

-Plant Monthly Totals Report, daily

data

-Alarm Report, automatic

-Each Boiler, Current Status, on

demand

-Plant Master, Current Status, on

demand

-Boiler Optimization SummarY, ondemand.

The impact of comPuters on Plantsupervision is maximum in its energy-ac6ounting capabilities, which includemanagement reports, historical-datafiles. utilitv usage and demand, steamproduction, evaloration rate, fuel oiland gas, water, chemicals and aver-age, maximum, minimum and totals."Modern boiler-control-system de-signers optimize their designs by in-cluding integrated direct digital con-trols wherever possible. This is a

difficult task in an industry that isnecessarily inherently conservativeand extremely safety conscious' To-day's economic, energY conserva-tioi and environmental requirements

demand that we do no less'Optimizing boiler-plant operation

through design features requires athorough knowledge and understand-ing oflhe processes and equiPmentiniolved, including the relationshipsof various parameters. Design fea-tures that improve safety, reliability,-maintainability, longevity, ease ofservice and control also are part ofoptimization. Trying to improve someparameters sometimes also improves-other

parameters. In cases where im-provement in one area creates dete-iioration in others, a good compro-mise is required. Good judgment'therefore, ii a prerequisite in opti-mized boiler-plant design. f]

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64/MARCH 1991, CONSULTING/SPECIFYING ENGINEERFor information circle 236