1 Variable-Pitch Axial Flow Fans for Thermal Power Stations ...

1 Variable-Pitch Axial Flow Fans for Thermal Power Stations

Dipl.-Ing. (FH) Lothar Müller,Zweibrücken

Variable-Pitch AxialFlow Fans for ThermalPower Stations

Axial-flow fans with impeller bla-des adjustable under load havebeen designed and built for ther-mal power stations for about 30years. The decision to develop thisfan type was prompted not only byits easy design integration intooverall plant configurations but al-so and primarily by the operatingcost benefits it offers, specificallywhen compared with centrifugalfans with variable inlet vanes.

Since the magnitude of the economicbenefit obtained (reduced station po-wer consumption) depends on the si-ze of the generating station (blockoutput), the fan operating regime(part-load operation), overall plantdesign and fuel costs, it took a num-ber of years for axial-flow fans withvariable pitch (VP) impellers to beco-me established in thermal power sta-tion applications.

By now, this fan type has gainedworldwide acceptance in forced draft,induced-draft, pulverizer air fan andflue gas desulphurization (FGD) ser-vice. A large percentage of thesefans, specifically larger ones, goes toNorth American markets, where ap-prox. 450 axial-flow fans have beendeployed on power station blocks inthe up to 900 MW range since 1974.

Among the most interesting plant ty-pes are the so-called “mono-block”systems which comprise only a singleforced-draft, induced-draft, and pul-verizer air fan per boiler.

The two induced-draft fans at theWeiher and Bexbach power stations,with their outside impeller diametersof 5.0 and 5.3 m and input power ra-tings of 13500 and 11500 kW, res-pectively, are among the world’s lar-gest power station fans. They are al-so worth noting for the high tip speedof 162 m/s of their (nodular cast iron!)blades.

The decision to adopt a “mono” solu-tion for blocks of this size had beenpreceded by several years of satis-factory experience gathered with theinduced-draft fans of two coal-fired350 MW blocks which were likewiseoperating with only one induced-draftunit per boiler.

Axial-flow fans with variable bladepitch angle may be of single-stage ormulti-stage design. To our knowled-ge, only fans with up to two stagesare in use in power stations today -with the exception of the three-stageforced draft unit shown in Fig. 1which, in 1953, marked the start ofthis fan development at TLT (still na-med Dingler Werke at the time).

Comparison of axial andcentrifugal fancharacteristics

It is evident from Fig. 2 that the iso-ef-ficiency curve of variable-pitch axialflow run approximately parallel to thesystem resistance graph, implyinggood efficiencies throughout a broadoperating range. In the case of centri-fugal fans with variable inlet vanes,the iso-efficiency curves intersect thesystem resistance curves, meaningthat their efficiency under part-loadconditions is automatically lower thanwith axial-flow units.

Moreover, an axial-flow fan can beselected to ensure that the boiler de-sign point will be located above themaximum efficiency range in the fieldof characteristic curves, the operating

Fig. 1: Three-stage axial flow fresh-air fan

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Variable-Pitch Axial Flow Fans for Thermal Power Stations 2

points of maximum interest thereforefalling into the highest efficiencyspectrum.

Fields of application

Axial-flow fans in thermal power stati-ons are used as fresh-air (forced-draft), induced draft and pulverizer airfans; in recent years they have alsobecome more widespread in a flue-gas desulphurizing (FGD) context(Fig. 3). Their use is not contingent onthe fuel type employed (coal, oil, gas,peat), although fuel type is naturally adesign determinant, specifically withinduced-draft units.

Regarding the installation of fansdownstream of electrostatic precipita-tors, today’s flue gas desulphurizingplants support various circuit configu-rations and hence, different arrange-ment of induced draft and FGD fans.In recent years, axial fans operatingas “booster fans” on the wet-gas sidedownstream of the scrubber have gai-ned particular importance. With theseunits, the choice of material, surfaceprotection considerations and sealingtowards the conveyed-medium circuitrequire particular attention.

Disposition

Axial-flow boiler fans may be fittedhorizontally or vertically. Fresh-airand pulverizer air fans are preferablyinstalled horizontally, while induceddraft units are also known to performwell when fitted in an upright positionin the stack. In flue gas desulphu-rization systems, fans serving on thewet-gas side downstream of thescrubber are likewise designed forvertical operation and are even so-

metimes configured with integratedmotors.

An overview of these installation prin-ciples is given in Fig. 4. The inlet boxopening may have any orientation, upto 360 deg., relative to the fan axis.

Vertical solutions may provide the fol-lowing benefits:

- simplified flue gas ducting;

- reduction of pressure losses due tofewer deflection points;

- no need for sound insulation or spe-cial silencer structures (with in-ductfans);

- no need for separate installationspace as unused space is available;

- easier assembly and disassemblythrough optional “lateral offset” ofactive components, i.e., the housingand rotor (i.e., these can be movedsideways without requiring anychange in the position of adjoiningcomponents).

Mounting configurations

Fig. 5 summarizes the main installati-on arrangements that have found tobe viable in practice. For fans moun-ted at floor level, buried concreteblock foundations were primarily em-ployed in former years (refer to sub-fi-gure a).

Fig. 2: Comparison between characteristic maps of axial-flow and centrifugal fans

Centrifugal fan

Volume flow �%�

100 % boiler load point

Boiler design point

Axial-flow fan

Boiler flow resistance line

Fig. 3: Axial-flow fans in thermal power plants

FL-V Fresh-air fan / SZ-V Induced draft fanML-V Pulverizer air fan / REA-V FGD fan

Boiler

Airpre-heat-

er

Airpre-heat-

er

StackDamper

REA-V

SZ-V

FL-V

ML-V

REA

Electro-static

precipi-tator


Solutions illustrated in sub-figures b)and c) are preferred nowadays sincethey are associated with a less com-plex oscillation behaviour. Put simply,a configuration of this type may beviewed as a two-mass oscillation sy-stem.

Mass 1: Rotor, consisting of the im-peller and main bearing assembly

Spring 1: Overall spring stiffness ofthe main shaft, bearing assembly andfan housing

Mass 2: Concrete block

Spring 2: Spring stiffness of the anti-vibration mountings

Given the mass ratio of approx. 20 : 1between the concrete block and therotor and the resulting low frequencyresponse of the foundation, the two-mass oscillation system may be con-sidered decoupled for the purposes ofoscillation modelling. At this mass ra-tio, the foundation’s influence on the Fig. 4: Arrangement of axial flow fans

a) Vertically in the stack

c) Horizontally at floor level

b) Vertically in a supportingsteel structure

Fig. 5: Axial flow fan installation configurations

a) Buried concrete block foundation b) Vibration-insulated concrete blockfoundation on buried concrete slab

c) Vibration-insulated concrete blockfoundation on ceiling slab

d) Vibration-insulated steel framefoundation on supporting steelstructure

e) Raised “table type“ slab foundationon supporting crossmembers

f) Vibration-insulated upright fan onsupporting steel structure

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natural bending frequency of the rotorsystem is negligible.

With isolated frame foundations of thetype illustrated in sub-figure d), thenatural vibration behaviour of the fra-me must be included in the analysis.Frequency criteria generally used forisolated foundations must be appliedto the natural frequencies of the fra-me as well.

When fans are placed on ceilingslabs, as shown in sub-figure c), caremust be taken to ensure that an ap-propriately sized girder extends un-der both the fan and the motor.

Raised slab foundations of the “table”type, illustrated in sub-figure d), haveto be supported by strong crossmem-bers under the motor and fan at themain force transmission points.

For fans erected directly on anti-vi-bration mounts (e.g., upright induc-

ted-draft units fitted in the stack orvertical FGD fans mounted on sup-porting steel structures) as depictedin sub-figure f), the natural oscillationbehaviour of the frame structuremust be taken into account, just aswith horizontal fans mounted on iso-lated foundations.

Computing models for the block andframe foundations are usually availa-ble today for both anti-vibrationmounting and direct floor installation.The natural oscillation frequenciescan be determined for such foundati-ons with up to 6 degrees of freedom,including translational motion and ro-tation about the three main spatialaxes, plus the most frequent coupledmodes.

A few other boiler fan installation me-thods exist but are of minor signifi-cance and shall therefore not be dis-cussed here.

Design

Induced draft, forced draft, pulverizerair and FGD fans do not differ greatlyin terms of their basic design. The fo-cus of the present article is on axial-flow induced draft fans. The horizon-tal fan type shall be considered for thepurposes of our further comments.

In line with the design objective, va-riable-pitch axial flow fans were de-veloped with the following main crite-ria in mind:

- good access to rotating partsthrough an appropriate separationof housings and suitable arrange-ment of access doors;

- possible avoidance of inlet and out-let side duct displacement in theevent of a rotor change;

- minimum shut-down times, achie-ved through a replacement of entire

fig. 6: Axial-flow boiler fan

Fan housing / top part

Dual-stage rotor

Coupling half

Intermediate shaft

Compensator

Inlet box

Hydraulic adjusting mechanism

Duct angle unit

Noise insulation

Actuator for impellerblade pitch adjustment

Oil supply system

Vibration sensor

Bearing temperature indicator

Diffuser

Fan housing /bottom part


components (e.g., rotor, main bea-ring assembly, actuating mecha-nism);

- high availability and longevitythrough selection of appropriate ma-terials and sealing systems, in con-junction with rugged design;

- maximum standardization of com-ponents to speed up the accumula-tion of operating experience.

Fig. 6 clearly illustrates how the abo-ve design requirements are met inpractice. The rotor - consisting of theimpellers, the main bearing assemblyand the blade adjustment mechanism- can be installed and removed as acomplete subassembly on bothsingle-stage and dual-stage fan mo-dels.

The fan housing with its removabletop portion is connected to the diffu-ser and inlet box via a quickly remo-vable non-metallic bandage helddown by a steel strap.

With this design, a rotor replacementon the induced draft fan of a 600 MWblock can be accomplished in aboutthree shifts.

The induced draft fan shown in Fig. 7has a two-stage rotor whose bladesare adjusted simultaneously by theactuating mechanism provided on theimpeller outlet side.

The fan is powered by a constant-speed electric motor normally arran-ged outside the fan itself. The motoris connected to the rotor via a hollowshaft with a torsionally flexible cur-

ved-tooth or multiple spring disccoupling. Basically, an integration ofthe drive motor into the fan housinghub is likewise conceivable. This de-sign was adopted for the flue gas de-sulphurization fans in three NWK po-wer stations; these fans are all arran-ged on the wet-gas side.

Due to the temperature loads actingon induced draft fans, the interior ofthe hub is thermally insulated in orderto protect the rotating components.

Cooling air is supplied into the hubthrough the hollow bracing and bla-des by a set of separate externalfans. It is important that the coolingair-carrying ducts are insulated toprevent temperatures below the dewpoint.

Fig. 7: Dual-stage induced draft axial flow fan with bade pitch adjustment

14 13 2

10

Section B-B Section C-C Section A-A

3 1 5 4B

B

C

C 6 13

117

9

8

12

6

A

1. Rotor2. Inlet box3. Fan housing4. Diffuser

5. Hydraulic bladeadjusting system

6. Oil supply systems7. Actuating gear unit

8. Cooling air fan9. Brake10. Anti-vibration mounts11. Vibration sensor

12. Pumping limit indicator13. Compensator14. Drive motor

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If the rotor is supported in sliding bea-rings, a brake is fitted on the drive-si-de coupling to protect the bearingsagainst running in mixed-friction con-ditions and to prevent rotor spinningonce the motor has been de-energi-zed.

Lubricating oil for the main bearingand hydraulic oil for the hydraulic ac-tuating mechanism are supplied by oilsupply units mounted outside the fan(Fig. 8). These are normally equippedwith two pumps of approximatelyequal output, of which one is a stand-by pump brought on stream by apressure monitoring switch when thefirst pump fails. To prevent bearingdamage when the fan coasts to a stopafter a power failure, the secondpump is sometimes connected to anuninterruptible power supply, specifi-cally on fans with sliding bearings.

Forced lubrication oil is fed to thepoint of use via a dual filter and an air-oil or water-oil heat exchanger. If thebearings of the main drive motor arelikewise lubricated off this system, anaccurate distribution of the oil flow tothe various bearing points must beensured.

Fig. 9 is a cross-sectional view of asingle-stage rotor. It consists of theimpeller with blades, the main bearingassembly, and the blade control me-chanism.

Impeller body

In this design the impeller body is ent-irely of welded construction. The cen-trifugal forces are absorbed by a ringarranged inside the hub.

This welded design has proven high-ly advantageous, particularly on indu-

ced-draft fans, since a cost-efficientcasting for the load levels encounte-red would be difficult to produce withany degree of reliability.

The welded hub design makes it pos-sible to select induced draft fans ofhigher speeds, which in turn permitsreduced fan sizes and the use ofsingle-stage instead of dual-stageunits (examples include the induced-draft fans in the Weiher, Bexbach andMannheim power stations).

Blade shaft bearing assembly

In a variable-pitch axial flow fan theblade shaft bearing assembly is oneof the most critical components.

In the design illustrated on page 7,centrifugal forces are absorbed byhermetically sealed deep-groove ballthrust bearing while the transverse

Fig. 8: Bearing lubricating oil circuit schematic

Motor oilreturn

Fan oil return

Fan leakageoil line

Thermostats Heating elements Pumps Level sensors

Dual filter withdifferential pres-sure indication

Motor oil supply

Water cooler

Flow monitoring switch

Fan oil supply

Pressuremonitoring

Mixer valve

Drive motor Fan


forces resulting from the adjustmentfunction are handled by an angular-contact ball bearing.

Anti-friction bearings are by definitionintended to rotate; however, in thepresent application they serve merelyto accommodate the blade pitchangle adjustment. Proper design, lu-brication and sealing of the bladeshaft bearing assembly are thereforeof outstanding importance.

The bearings may either be greasedor oil-lubricated. Operating tests anddevelopment trials have shown thatonly a few grease types will retaintheir lubricating properties over an ex-tended period under the prevailingtemperature loads and centrifugal for-ces. A fully enclosed design of thethrust bearing was therefore adopted;this has greatly increased the servicelife of the bearing assembly compa-red to the solutions used in previousyears. Since the anti-friction bearingswill not fail suddenly, the bearing sta-

tus can be monitored “on-line” fromoutside the fan by measuring the re-quisite oil pressure for the bladeangle adjustment.

Blade foot sealing

Tightness of the blade shaft passagethrough the hub casing is a major re-liability criterion, specifically in the de-sign of induced-draft axial flow fans.

Experience has shown that the sea-ling system employed ensures a100% tight shaft entry into the hubchamber.

Impeller blades

Impeller blades are screwed onto theblade shafts. Individual blades canthus be replaced without removingthe entire rotor. Proven blade materi-als include aluminium alloys for fresh-air (forced draft) and pulverizer airfans, and cast steel or nodular castiron for induced draft fans in coal-firedboiler duty.

Although the performance and sepa-rating efficiencies of today’s elec-trostatic precipitators are much im-proved and dust loads on the clean-gas side have dropped significantlyas a result, the accumulated experi-ence suggests that cast steel ornodular cast iron remain the materialsof choice.

Fig. 9: Rotor of a single-stage axial-flow fan

Multi-disc coupling

Radial bearing

Lubricating oil supply

Thrust bearing

Bearing housing

Lubricating oil return line

View X

Actuating lever Oil supply line

Oil return line

Leakage oil

Hydraulic controvalve

Actuating cylinder

Blade shaft bearing

Counterweight

Impeller blade shaft

Guide vaneImpeller blade

Blade foot gasket

Shaft

Fig. 10: Particle-size distribution of variousdust types

Overs R �%�

Gra

in s

ize

d �µ

m�

Undersize particles D �%�

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It has been found that short-time filterfailures give rise to high wear rates;moreover, in the case of an air-preheater failure, temperatures in thefan area may reach 300°C.

Since the endurance strength of al-uminium alloys drops very quickly attemperatures over 200°C, the use ofaluminium blades on induced draftfans in coal-fired boiler service im-plies significant operating reliabilityand safety hazards.

Impeller blade wear

Abrasive wear of the impeller bladesis a function of the following:

- relative speed of dust particles im-pinging on the blade surface

- impeller blade material

- angle of impact

- dust concentration

- grain size distribution

- dust load distribution

- hardness of dust particles

Only the first two parameters are con-trollable by the fan manufacturer.

Extensive trials have shown that the-re exists an approximately squarecorrelation between the relativespeed of the dust particles and the ra-te of blade abrasion. Under otherwiseequal conditions, the granulometricdistribution of the particles also has asignificant influence on blade wear,as illustrated for three dust types inFig. 10. As will be appreciated fromFig. 11, the volumetric abrasion rate(cm3 of material removed per kg ofimpinging dust) is much higher with“F36” dust than with “S” type par-ticles. Knowing the dust particle sizedistribution is therefore a key prere-quisite for any correct advance eva-luation of impeller blade service lifeunder wear conditions.

Extensive wear tests conducted overmany years, supported by field expe-rience gathered with induced draftfans, have led to the development ofa computing method whereby the ser-vice life of impeller blades can be pro-jected if the values of the above para-meters are known.

Rotor main bearing

The illustration on page 9 shows the“compact design” bearing systemwhich has given good results insingle- and dual-stage axial flow fansfor years. This design approach mini-mizes the necessary removal, refit-ting and alignment work (particularlythe latter), since the flanges in thebearing mounting area are handledsimultaneously with the blade runningsurface of the outer fan housing shell.In addition, this bearing design allowsfor the selective use of sliding and an-ti-friction bearings without any chan-ge in exterior diameter.

Both systems have proven their valuein many installations for years. Theanti-friction bearings are oil-lubrica-ted, with an external oil-supply unit re-circulating the oil sump in the bearinghousing. Moreover, this oil sumpallows the fan to remain in operationfor quite a while if the forced circulati-on system should fail.

Sliding bearings

The sliding bearing assembly (Fig.12) consists of tilting-pad radial bea-

Fig. 11: Volumetric steel abrasion as a functionof dust particle size

Impact angle �

Vol

umet

ric a

bras

ion

rate

Fig. 12: Sliding bearing assembly

Impeller

Radial bearing

Radial bearing

Oil supply

Oil return

Bearing housing

Section A-A Section B-B

Shaft

Gap pump for emer-gency operation

Oil level

Thrust bearing

Thrust bearing

Temperaturesensor

A B

BA Oil supply


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rings and thrust bearings with self-ad-justing circular sliding pads arrangedcircumferentially on both sides of ashaft collar. The bearing housing issplit horizontally, allowing bearingparts to be inspected or replacedwithout having to remove the impel-lers from the shaft.

Bearings are lubricated by an oil sup-ply system mounted outside the fan.Emergency lubrication after an oil-supply failure is ensured by a “gappump” fitted directly onto the mainshaft. This pump draws oil from thesump in the bearing housing andfeeds it to the point of use. The sy-stem is effective only briefly under fullload, but permits an extended coast-down cycle.

The decision between sliding and an-ti-friction bearings is often a philoso-phical one, at least in part, since bothbearing types have proven their meritover the years. It may be observedthat split-type sliding bearings offeradvantages with large and thereforeheavy rotors, and may yield an un-limited service life when combinedwith a reliable lubricating system.

The tilting pads of the radial bearingsare adjustable both longitudinally andtransversely and will therefore adaptto possible shaft deflections.

A properly rated sliding bearing, unli-ke an anti-friction bearing, is not a“wearing” part requiring periodic re-placement if used with appropriateoil-quality.

Moreover, bearing failures will deve-lop over much longer time spans andcan thus be forecast, and hence avoi-ded, via temperature and oscillationmonitoring.

Anti-friction bearings, on the otherhand, provide superior emergencyoperating characteristics due to theexisting oil sump in the bearing hou-sing. Nevertheless, a premature failu-re of such bearings can never be ru-led out.

Hydraulic blade pitch adjustment

For controlling the impeller bladepitch setting and hence, the fan’s vo-lumetric throughput and outlet pres-

sure, the following actuator systemsare available:

- pneumatic

- electromechanical

- mechanical

- oil hydraulic

Pneumatic and electromechanical sy-stems play virtually no role in powerplant fan engineering, while mechani-cal blade pitch control systems usedto be employed specifically on smal-ler units. Oil-hydraulic control sy-stems have emerged as the most sui-table solution for this purpose. Theyoperate with less hysteresis sincethey use fewer mechanical powertransmission elements; in addition,they are capable of transmitting hig-her actuating forces of the magnituderequired in over 300 kW blocks.

Systems embodying the principle illu-strated in Fig. 13 have been built withonly minor changes for more than 30years.

An actuator system of this type com-prises the following main elements:

- an actuating cylinder moving axiallyalong the fan axis and turning withthe rotor;

- a piston within the actuating cylinderwhich is axially fixed and rotateswith the same speed as the cylinder;

- a feedback rod

- a stationary control valve which re-ceives the command to change theblade angle via an actuating gearunit outside of the fan housing andconverts it into a hydraulic signal.Pressurized oil will thus be directedto the appropriate cylinder side, im-parting an axial movement to the cy-linder. This axial displacement cau-ses the impeller blade to turn, due tothe geometry of the levers attachedto the end of the blade shafts whichengage the actuating disk. The mo-vement is carried out simultaneous-ly, even on multi-stage fans.

The actual position of the impellerblades is indicated outside the fanhousing and can be transmitted to acontrol center.

Effective sealing in the joint areasbetween stationary and rotary com-ponents is an essential requirementwith such actuator systems. Sealsmay consist of plastic or metal ele-ments. The control delays for the re-levant pitch adjustment range usuallyvary between 30 and 45 seconds. Ho-

Impeller blade pitch indication(Actual position)

Impeller blade pitch(Setpoint command)

Feedback rod

Control valve

Leakage oilOil return

Oil supply

Actuating stroke

Actuating cylinder

Actuating lever

Piston

Fig. 13: Schematic view of the hydraulic blade pitch control system

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wever, faster responses can beachieved through appropriate dimen-sioning of the actuating system.

Instrumentation

The choice of instruments and moni-toring devices are major factors in thedesign of variable-pitch axial flowfangs.

The complexity of the instrumentationsystem is increased primarily by the

frequent request for a “2v3” solutionto be implemented in the fan monito-ring system for integration into the au-tomatic operating control environ-ment.

Fig. 14 summarizes the main instru-ments provided on a variable-pitchaxial flow fan in induced-draft serviceand its peripheral equipment.

Fan protection

To ensure the safe and reliable ope-ration of an axial flow fan, the relevantkey parameter values (readings)must be continuously known.

By continuously recording all chan-ges in fan operating behaviour, speci-fically oscillations and current opera-ting point positions (pumping limit mo-nitoring), it is possible to ensure an

Fig. 14: Schematic instrumentation diagramm

Pressureswitch

Brake r. p. m.measurement

Pump monitoring unit

Pressure and flow indication

Blade pitch position indicator

Actuating pressure indicator

Vibration measuring devices

Temperature monitoring

Cooling air fan

Hydraulic impeller blade pitch actuating systemBearing lubrication

Pumps

Dual filter

Water cooler

Mixer valve

Thermocouples

Levelmeasuringdevices

Heatingelements


advance detection of dangerous ope-rating states and imminent failures.

In addition, reliable monitoring of thefan allows the appropriate mainten-ance and overhaul steps to be sche-duled so as to be carried out upon at-tainment of defined limits, instead ofupon completion of a defined numberof operating hours.

Figs. 15 to 17 show examples of theswitchgear and control schematicswith protection system criteria for in-duced-draft axial flow fans.

Operating experience

From the experience gathered to da-te, it emerges that operating cam-paigns of six years and more are de-finitely realistic with variable-pitch axi-al flow units representing state-of-the-art technology.

However, extensive prior develop-ment work was necessary to achievethis outstanding performance. Impro-vements at the level of fan monitoringand control equipment were a neces-sary part of this effort.

The following paragraphs give a des-cription of the operating experiencegained with key fan components.

Impeller blades

The problem of blade wear had longbeen a priority issue in induced-draftfan engineering. Through the selec-tion of improved blade materials(steel, nodular cast iron) and higherfilter efficiencies it has been possibleto reduce wear rates substantially.

More recently, increased blade wearhas been reported only where unitswere operated significantly above the

Fig. 15: Start-up program of an induced-draft axial flow fan

Fan motor ON

Shutoff-damper OPEN

Automatic operating control

Command

Start-up trigger signal

Lubricating pump ON

Hydraulic pump ON

Brake oil pump OFF

Impeller blades closed

Brake disengaged

Control system OFF

Shutoff damper closed

Clearance criteria

Lubricating oil level � min.

Hydraulic oil level � min.

Brake disengaged

Oil temperature � min.

Bearing temperature � limit

Impeller blades closed

Bearing oil flow � min.

Oil pressure � min.

Cooling water present

Shut-off damper closed

Operation monitoring ON

Fan ON

Air / flue duct unobstructed

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load levels assumed at the designand rating stage.

Blade shaft bearings

The difficulties observed in this res-pect in previous years were attributa-ble to unsuitable lubricants and ina-dequate sealing of the bearing as-sembly.

Service life deficiencies have beenvastly improved through selective de-sign improvement in conjunction withlaboratory and field trials.

Today, service lives permitting a boi-ler campaign of more than 4 years’duration are no longer uncommon. Inindividual cases, service periods inexcess of 60,000 operating hours ha-ve been reached.

Hydraulic blade-pitch adjustment

An analysis of past failures of thissubassembly has revealed that theseals in the joint area between its sta-tionary and rotating componentsused to constitute a “weak link”. The-se problems have been overcomethrough dedicated design optimizati-on (use of metal sealing elements)supported by laboratory and opera-ting trials. As a result of these efforts,

the campaign durations now com-monly expected will be reliably rea-ched.

Rotor main bearing

Anti-friction bearingsExceedingly frequent operation at ze-ro load with thrust reversals (frequentstart-ups) or exceeding the pumpinglimit may reduce the service life of thebearings. Fretting corrosion associa-ted with the anti-friction bearings andindividual failures due to alternatingstress situations that could not be an-ticipated at the time of design havebeen ruled out through new bearingdesign approaches and expandedcalculation methods.

Sliding bearings

No serious problems have occurredto date with the sliding bearing confi-guration outlined above. The tilting-pad bearings employed accommoda-te operating deflections of the fanshaft, thus avoiding edge loading ef-fects.

A reliable distribution and monitoringof oil flows to the bearing points andadvanced anti-seizure features (en-suring performance after a failure of

the lubricant supply) ensure a longservice life.

Only minor improvements have beenmade to the shaft seal system.

Summary

Summing up, it may be stated thatthese variable-pitch axial flow fanshave performed well in thermal powerstation service. A further intense in-formation-sharing process betweenthe operator and fan manufacturerand ongoing product development fo-cused on critical components willyield further improved results in thefuture.

Fig. 16: Automatic controlscheme of an induced draftaxial flow fan

Hydraulic oil temperature � 40°C

Heater OFF

Hydraulic oil temperature � bar

Hydraulic oil pump 2 ON

Lubricating oil pressure � bar

Lubricating oil pump 2 ON


Heater ON

Lubricating oil temperature � 20°C

Heater ON

Hydraulic oil temp. at constant 50°C

Automatic oil flow control via mixer valve


Heater OFF

Bearing ambient temp. � 60°C

Cooling air fan ON

Bearing ambient temp. � 30°C

Cooling air fan OFF

Lubricating oil temp. at constant 50°C

Automatic oil flow control viamixer valve


Fig. 17: Operation monitoring and emergency shutdown program for an induced-draft axial flow fan

Lubricating oil pump 2 ON

Hydraulic oil pump 2 ON

Bearing temperatures � 75 °C

Lubricating oil level � min.

Hydraulic oil level � min.

Fan lubricating oil flow � 1/min.

Motor lubricating oil flow � 1/min.

Lubricating oil filter �p � bar

Hydraulic oil filter �p � bar

Fan at stall limit

Oscillation amplitude � 100 µm

Lubricating oil level � max.

Hydraulic oil level � max.



Volume measurement with advancepumping limit alarm

�pabsolute

�pinlet box

�ptotal

Medium temperature

Processor

Alarm Warning to control room

Emergency shutdown

Bearing temperature � 85°C

Lubricating oil pressure � min.

Oscillation amplitude � 250 µm

Fan at stall limit

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