september 2006 issue, vol. 27 no. 2

BHEL JOURNAL

Volume 27 No. 2 September 2006

Editorial Advisory Committee

Ramji RaiK. RavikumarD. IndranS.K. Goyal

Editor : R.K. Bhattacharya

Associate Editor : D. Roy

BHEL JOURNAL is published quarterly.All correspondence and enquiries are to beaddressed to :

Mr. R.K. BhattacharyaEditor, BHEL JournalBharat Heavy Electricals LimitedBHEL House, Siri Fort,New Delhi-110 049

The statements and views expressed in thisJournal are entirely those of the authors, andnot necessarily that of the Organisation.

Contents may be referred to or reproducedpartially with due acknowledgements.

Copyright reserved.

CONTENTS Page

ADVANCES IN MATERIALS FORADVANCED STEAM CYCLE POWER PLANTS 1

SELF-EXCITATION IN 3-PHASE SQUIRRELCAGE INDUCTION GENERATOR FORWINDMILL APPLICATION 20

TURBO-GENERATOR INDUCED VOLTAGEWAVEFORM COMPUTATION ANDTELEPHONE HARMONIC CAPABILITYPREDICTION 26

EFFECT OF PRELOAD FACTOR ANDWORN DEPTH ON THE DYNAMICCOEFFICIENTS AND STABILITY OF ALOADING ARC (WORN) TWO-LOBEBEARING USED IN TURBO-GENERATOR 35

COLLECTION, HANDLING ANDTREATMENT OF LIQUID EFFLUENTSIN THERMAL POWER PLANT 45

INNOVATIONS — FROM BHEL 58

RECENT MAJOR ACHIEVEMENTS OF BHEL(during March'06-August'06) 62

1

2

4

3 Cover Photographs

1. 600MW Western Mountain Gas Turbine Power Plant,Libya.

2. Srisailam Hydro-electric Plant (7x110 MW).

3. NALCO Captive Power Plant (960 MW).

4. Advanced Control Room at 2x500 MW (Stage-II) RihandSTPS.

1BHEL JOURNAL, September 2006

ADVANCES IN MATERIALS FOR ADVANCED STEAMCYCLE POWER PLANTS

Kulvir Singh

SYNOPSIS

The efficiency of conventional boiler or steam turbinefossil fuel fired power plants is strongly based on steamtemperature and pressure. Since the energy crisis of the1970s, there have been efforts worldwide to increaseboth : extensive research has been pursued worldwide.The need to reduce carbon dioxide emissions has providedfurther impetus to improve efficiency. Development ofstronger high-temperature materials is the primerequirement. EPRI and many other organizations haveextensively reviewed the materials technology for ultrasupercritical power plants. This article reviews the potentialbenefits, operational experiences, the present trend andthe advances in materials that require special attention,in respect of power plants with supercritical steamconditions. This will serve as a basis for defining material

issues for both the boilers and the turbines in next-generation ultra supercritical power plants.

Key Words:

Power Plants; Creep-Resistant Steels; Rotors; Casings;Boiler; Superheater.

1. INTRODUCTION

An enhanced ecological awareness in the industrialisedcountries prompted increased initiatives world overto reduce CO

2 emission levels in the power plants.

This is essentially achieved by improving the efficiencyof the plants. Figure-1 shows some possibilities ofincreasing power plant efficiency [1]. In conventional

FIG. 1 : EFFICIENCY IMPROVING MEASURES FOR STEAM POWER PLANTS [1]


FIG. 2 : DEVELOPMENT OF UNIT SIZES AND STEAM PARAMETERS IN JAPAN [1]

FIG. 3 : POWER PLANT EFFICIENCIES IN JAPAN [1]

power plants, a marked improvement of efficiencycan be achieved by advancing steam parameters. Theresulting developments of unit sizes and steamparameters are illustrated in Fig.2 [1]. The powerplant efficiencies achieved and planned for new

plants in Japan are shown in Fig.3. Steam conditionswere raised very rapidly during the 1950s and anumber of sets with supercritical steam conditionswere installed in 1950s and 60s. Subsequently, thetrend was reversed in respect of the steam conditions


but the capacity of individual sets, however, continuedto increase till a limit with the existing fabricationand handling technologies has been reached. Thereversal in steam conditions is primarily the result ofexperience in 1960s and 70s when several newlycommissioned plants with advanced steam conditionsdid not live up to expectation in respect of availabilityof sets caused by operational problems.

Initially, the performance of the supercritical plantswas so poor that many utilities experiencedconsiderable downtime and significant financial loss[2]. Consequently, it created misconception thatimproved efficiency sacrifices reliability and therewas rapid retrenchment to subcritical units on theassumption that they would be more reliable.Therefore, plants with operating temperature of538°C received a wide favour in late 1960s and 70s.However, the concerted efforts of designers in liaisonwith metallurgists and material scientists inunderstanding of initial problems and takingcorrective steps led to a great deal of improvementin plant performance. Analysis of the historical

records and the stock of accomplishments ofsupercritical plants show that their reliability iscomparable to the conventional units [3-5]. The twooil crises in 1973 and 1978 which caused a drasticincrease in fuel cost and the encouraging operationalresults now available from earlier supercritical unitsprompted a renewed interest in supercriticalconditions to make the best use of the heat rateadvantage provided by these advancements [6-9].

1.1 Potential Benefits

Material development work over the past twodecades has paved the way for large thermal powerplants to be built today with live steam temperaturesof 610°C, reheat temperatures of 625°C andsupercritical steam pressures. The likely potential forreducing the heat rate by increasing the pressure andtemperature of the steam admitted to the turbine onthe basis of single and double reheating is shown inFig.4 [10]. At live steam conditions of 600°C and300 bar with double reheating, for example, the heat

FIG. 4 : NET HEAT RATE IMPROVEMENT FOR SINGLE AND DOUBLE REHEAT CYCLES [10]


rate can be reduced by 8% compared with the heatrate of today's standard power stations featuringsteam parameters of 540°C/180 bar and singlereheat.

This improvement in thermal efficiency helpsconsiderably to conserve fuel resources and reduceCO

2 emission by 20%. This is a substantial

contribution on the part of power generatingindustries towards achieving the Germany's target oflowering CO

2 emission by 25-30% by 2005 [10].

This objective requires an ambitious developmentprogramme for advanced materials, which canwithstand such steam conditions. The researchprogramme has been undertaken simultaneously byUSA, Japan and European nations. It has focused ondeveloping further the existing high-temperature-resistant ferritic martensitic 12% CrMoV Steels forthe production of rotors, casings and chests, pipesand headers capable of operating at 593°C, as wellas further development of the existing high-temperature austenitic steels suitable for inlet steamtemperatures up to 649°C. For smaller highlystressed components such as first stage moving

blades and bolts, the objective was to employ andfurther develop existing high-temperature-resistantsuperalloys eg. Nim 80A and 90 etc.

Further development of ferritic steels geared to inletsteam temperatures up to approximately 625°C in thecontext of COST 501 European research programmewas spurred by research activities in USA and Japan.Figure-5 gives an overview of the international researchprogrammes aimed at developing power plantmaterials[10]. Today, 65 partners from 13 countriesare involved in the European programme backed bya research budget of some DM31 million.

In the case of pulverized-coal-fired boilers, even amarginal improvement in plant efficiency, say from34 to 37%, is reported to bring a savings of at least$5 million a year for each 1000 MW of capacity [2].However, in estimating the actual gains, the plant netheat rate gain should be weighed against the increasein total plant cost. Increase in steam parametersrequires more expensive materials of construction asthese advancements increase the severity of the serviceconditions the components must undergo.

FIG. 5 : INTERNATIONAL RESEARCH PROGRAMMES FOR DEVELOPING ADVANCED STEAM CYCLE PLANTS [10]


A study funded by Electric Power Research Institute(EPRI), USA, suggested 750 MW unit as optimalsize for the advanced steam conditions offering a10% improvement in thermal efficiency, comparedwith present commercial designs [2]. The technologypromises a drop in heat rate of as much as 865 Btu/kWh. Assuming a first year fuel cost of $1.38/865BTU/kWh, the study concluded that cost savings inoperation could go as high as $160 million (1978value) over the plant's life time. Capital costs, on theother hand, are estimated to be 3 to 5% more thanthose for conventional capacity ($800/kW versus$775/kW). Keeping in view the current high andescalating level of fuel and capital costs, the potentialfuel savings, increasing environmental consciousness,the supercritical steam conditions with advancedmaterials offer significant benefits that require seriousattention in selecting future capacity additions.However, it is necessary to consider local conditionssuch as grid size, expected annual utilisation period,cycling duty requirements, fuel cost etc. to work outoptimal unit capacity and operational conditionsincluding the number of reheats.

1.2 Service Experience

Based on the operating data from supercriticaldouble reheat units in the range from 600-825 MWplant sizes, Westinghouse and GEC reported [2] thatthey achieved average availability of 80%. This ishigher than the average availability of 600-825 MWunits as a whole and is comparable to that of smallunits. A VGB evaluation also shows that theoperational availability of their supercritical plants isapproximately the same as with sub-critical units [5].Average forced outage rates for the period 1970-83for ABB turbine operating with supercritical mainsteam pressures in several countries in Europe wasonly about 1% [8]. A 100 MW steam turbine hasbeen operating for several ten thousand hours inUSSR, as a test unit under steam conditions of 29.4MPa and 630-650°C and reheat steam conditions of3 MPa and 565°C. This unit was designed withcooling of many elements of HP housing and theoperating experience shows that it is highly reliable[9].

Environmental conditions in several cases haveproved to be important in the availability of theplant. Operational errors in the water treatment ledto stress corrosion cracking of austenitic stainlesssteel tubes in some power stations [5]. Due to widespread deterioration of the quality of coals in severalcountries, a number of power stations experiencedfire side corrosion of tubes. In such places, newsubcritical power units installed in the same periodalso showed similar reduced availability [2].Supercritical units under those conditions requireda quite uneconomic purification of basic fuels [13].In case of oil-fired or dual-fired boilers, austeniticstainless steel superheater tubes suffered from excessivecorrosion problem.

1.3 Present Trend

Encouraging operational history of the earliersupercritical units, availability of more versatilematerials at a reasonable cost, progress in designtools such as computer programmes, experiencegained in the designing and manufacture of largesteam turbines and the continuously increasing trendof fuel cost, all together prompted several leadingpower plant suppliers, in the recent past, to revivetheir interest in units with advanced steam conditions.To avoid any technical risk, the developmentprogrammes have been planned in a phased manner.Two such programmes were launched independentlyon similar lines by the EPRI [9] and ToshibaCorporation, Japan [7].

Since most of the conventional materials readilyavailable are expected to meet at least for the firsttwo phases, the developmental programmes mainlyconcentrated in rationalizing the designs throughcomputer-aided programmed and in perfecting themanufacturing technologies especially in the case oflarge-sized components. Having completed suchexercises for the units of 700 MW capacity withsteam conditions of 566/566/566°C and 32.2 MPa,both EPRI and Toshiba are expected to take up theirproduction, while the developmental activities tomeet the remaining two phases will continue toprogress. Mitsubishi Heavy Industries Ltd., Japan,


on the other hand, since they have already suppliedfive out of the ten 450 MW steam turbines now inoperation in Japan with steam temperatures of566°C, is already making proof tests of material andturbine elements, using an actual power plant, forsteam temperatures of 590-650°C. A 50 MWprototype unit manufactured by Mitsubishi withsteam temperature of 590-650°C was put intooperation in the year 1987 [6]. At present, many ofthe units up to the sizes of 1000MW are operatingin Europe, USA and Japan with the steam parametersof 600/610/610°C temperature and 300 bar pressure.

2. REQUIREMENTS OF MATERIALSFOR HIGH-TEMPERATURE APPLI-CATIONS

When a unit has to be designed and built to a highintegrity, one of starting points is concerned with thechoice of materials. As shown by the experiencegained, this assumes additional emphasis with thesupercritical sets as the equipment reliability andavailability otherwise may nullify the performancebenefits expected to accrue from the advancementsin technology. Generally speaking, the proper materialfor use at elevated temperatures is the one that bestmeets the following requirements at the lowest cost:

1. Adequate strength to resist deformation andrupture when exposed at the design conditionsfor the designed life, to the operatingenvironment.

2. Adequate fatigue strength at the designconditions and damping capacity whenvibratory stresses are involved.

3. Sufficient ductility to accommodatecumulative plastic strain and notch strengthto resist stress concentrations during theservice life.

4. Good resistance to service environment towithstand oxidation, corrosion and erosion.

5. Structural stability to resist damagingmetallurgical changes at operating conditions.

6. Ability to be fabricated with ease, as bymachining, forging, casting and welding.

7. Low coefficient of thermal expansion toresist the thermal stresses imposed bydifferential temperatures and thermal cyclingor shocks during heat treatment, weldingand operation.

8. Good thermal conductivity for efficient heattransfer and to minimise thermal gradientalong the wall thickness of the thick walledcomponents so as to reduce thermal stressesduring start-up or quenching due to carryover.

9. Low density to provide a high strength-to-weight ratio for applications such as the laststage blading of the large capacity steamturbines.

10. Availability in the desired size and shape.

11. Enough long-term test data to allow sufficienthigh-temperature analysis to validate thedesign to the satisfaction of the safetyregulations and licensing authorities.

The power plants have depended mainly on the low-alloy steels for metal temperatures up to about580°C. At temperatures above this level, theirresistance to creep is such that the resulting wallthickness becomes uneconomical. Also, theiroxidation resistance at higher temperatures is notsufficient. With increased metal temperatures above580°C, austenitic stainless steels were employed formost of the power plant components designed inearly 1950s. However, austenitic stainless steels,though they are superior in high-temperature strength,have problems such as steam oxidation, and high-temperature corrosion in oil-fired boilers. Further,they are very expensive, susceptible to stress corrosioncracking and give rise to weld problems. Ferriticsteels, on the other hand, offer several technologicaladvantages such as better workability, high thermalconductivity and lower coefficient of thermalexpansion as compared to austenitic stainless steels.As a result, to bridge the gap between the low-alloy


ferritic steels and austenitic stainless steels, severalhigh-alloy ferritic steels were developed during thelast three to four decades. These include 5CrMo,6CrMoVWTi, 7CrMoTi, 8CrMoTi, 9CrMo,9CrMoVNb, 9CrMoWVNb, 12CrMoV and12CrMoWVNb steels. Among these, 9Cr and 12Crclass of steels were extensively studied and successfullyemployed in several power stations. In fact, there areover two hundred grades of 12Cr steels withdifferent trade names cited in the literature, of whicha number of steels are generally used for differentapplications in gas turbines, and a few in thermal aswell as nuclear power plants [14,15]. A brief list ofmaterials is given in Tables-1 and 2.

With the operating temperature around and above

600°C, austenitic stainless steels are required to beused for high-temperature strength combined withresistance to environmental attack. Simple austeniticsteels of type AISI 304, 316 and 347 have beenextensively used for power plant components. Butseveral complex austenitics of the type Essehete 1250,Alloy 800 H, 17-14CuMo, A286 and NF709 havebeen developed to give improved service performancearound 650°C. Based on the actual service conditionsof a component, several higher alloys including nickelbase alloys are also being considered as candidatematerials for meeting exacting requirement to improvereliability. In view of the above, the candidate materialsfor advanced steam cycles are suggested, and theapproaches to meet the higher steam cycles arediscussed, in the following sections.

TABLE-1 : CANDIDATE MATERIALS FOR BOILER TUBES, PIPES AND HEADERS

Sl. No. MATERIALS FOR BOILER TUBES AND PIPES

Sub Critical Super Critical

1. C-Mn Steel HCM2S (T23)

2. ½Mo (T1) 7 CrMoV TiB 10 10 (T24)

3. 1¼Cr½Mo¾Si (T11) X20 CrMoV 12 1

4. 2¼Cr1Mo (T22) X10CrMoVNb 91 (T 91)

5. X20 CrMoV 12 1 X10CrMoWVNb 911 (E911)

6. X10CrMoVNb 91 (T 91) X10CrMoWVNb 92 (T 92-NF616)

7. AISI 304 X10CrMoVNb 12 1 (T 122)

8. AISI 310 X10CrNiMoTiB 15 15

9. AISI 316 X8CrNiMoVNb 16 13

10. AISI 321 X3CrNiMoNb 16 16

11. AISI 347 NF 709

12. E1250 Alloy 800

13. 17-14CuMo HR 3C

14. X10CrNiMoTiB 15 15 HR6W

15. X8CrNiMoVnb 16 13 AC66


3. BOILER MATERIALS

3.1 Superheater and Reheater Tubing

Superheater and reheater tubes operate in the creeprange since their main function is to provide heattransfer between the hot flue gas and the Pressurisedsteam carried within them. They are, therefore,designed primarily based on the maximum allowablestress to rupture in 100,000 hours, as specified bythe mandatory codes or standards. The other propertybases for their selection are a combination ofadequate corrosion resistance to both steam and theflue gas and ease of fabrication, particularly in regardto bending and welding. Reheaters receive partiallyexpanded steam from the turbine and serve to raiseits operating temperature to the required inlet levelof the Intermediate pressure turbine. Consequently,they operate at lower pressure and are made of largerdiameter but thinner walled tubes, as compared to

superheaters. Table-1 lists candidate materials forsuperheater and reheater tubing for power plantapplications. Figures-6 and 7 show their maximum

TABLE-2 : MATERIALS USED FOR THE ADVANCED STEAM TURBINES AT HIGH TEMPERATURES [47]

Component 566°C 620°C 700°C 760°C

Casings/shells Cr MoV (cast) 9-10%Cr (W) CF8C+ CCA617(Valves; steam 10Cr MoVNb 12CrW (Co) CCA617 Inconel 740chests; nozzle Inconel 625box; cylinders) IN 718

Nimonic 263

Bolting 422 9-12%CrMo V Nimonic105 U7009-12% CrMo V A286 Nimonic115 U710Nimonic80A IN 718 Waspaloy U720

IN 718 Nimonic 105Nimonic 115

Rotors/Discs 1 Cr MoV 9-12% CrWCo CCA617 CCA61712 CrMoVNbN 12CrMoWVNbN CCA617 Inconel 74026NiCrMoV11 5 Haynes 230

Inconel 740

Vanes/Blades 422 9-12% CrWCo Wrought Ni Base Wrought Ni Base10 CrMoVNbN

Piping P22, P91 P91, P92 CCA617 Inconel 740

FIG. 6 : MAXIMUM ALLOWABLE STRESSES FOR VARIOUSBOILER GRADE STEELS [20]


allowable stresses and 100,000h stress rupturestrength, respectively [14-20].

Carbon steels are suitable and economical up toabout 400 to 450°C metal temperature. Low-alloysteels ⏐ Mo (SA209 T1), 1...Cr ⏐ Mo—Si (SA213T11) and 2...Cr1Mo (SA213 T22) are used widelyfor metal temperatures up to 480, 550 and 580°Crespectively [21]. T22 steel which has been extensivelyused for the final superheater for conventional unitsoperating at a main steam temperature of 540°C hastoo low a creep strength to be accepted as a finalstage superheater material for use with a steamtemperature of 565°C. Though these steels can stillbe used for the tube banks operating up to theirexisting allowable temperatures in the boilers ofsupercritical units, with the increase in steam pressuretheir required wall thickness increases. There is astrong incentive in using improved carbon steels nowavailable with higher allowable yield strength atlower temperatures up to 450°C and in bringingdown the temperature range of application of thelow-alloy steels such that strong high-alloy ferritic

steels can be used to some extent even at steamtemperatures slightly below 540°C. This solutionpermits thickness of tubes less than those needed forthe common low-alloy steels for the same operatingconditions as shown in Fig. 8. This results in savingof base material and welding filler material, reducesthermal stresses and welding problems and alsoimproves the heat transfer efficiency.

FIG. 7 : 1,00,000H STRESS RUPTURE STRENGTH FOR BOILER MATERIALS [10]

FIG. 8 : COMPARISON OF THE SIZE OF THE WALL THICKNESSOF P22, X20, P91 AND NF616 STEEL PIPES

10CrMo9.10

NF616

X20 T91/P91


As the boiler steam advances, the metal temperatureof the last banks of the superheater tubes of the565°C unit exceeds the maximum allowable limit ofthe low-alloy steels. In these regions, high-alloyferritic steels are required to be used. 9Cr1MoVNb(SA213T91) steel has improved oxidation resistance.The allowable stresses for 9Cr1MoVNb (SA213T91)are about 100% higher than T22 steel at temperaturesabove 540°C and are even higher than X20 CrMoV12 1 (DIN17175 - 1.4922) steel in the range of 500to 650°C. The other two high-alloy ferritic steels9Cr1MoWVNb (SA213 T92) andX12CrMoWVNbN 10 11 (E911) developed in USAand Japan are much superior in their stress rupturestrength as compared to all other ferritic steelsincluding the 12Cr steel (X20 CrMoV 12 1) uptoabout 650°C. Their allowable stresses are also higherthan TP304H and TP321H at temperatures below600°C. 9Cr1MoVNb steel, due to its low carboncontent exhibits good weldability and workabilityand has been giving satisfactory service as superheatersand reheaters, for over two decades in Japan [22].X20 CrMoV 12 1 is also supplied with 0.4 to 0.6%tungsten, which is then designated as X20 CrMoWV12 1 (DIN 17 175-1.4935) [15]. Addition oftungsten is reported to give greater creep strengththan the steel without tungsten but, based on somelong-time investigations no effect of tungsten hasbeen found for additions up to 1% [14]. It is,however, reported to be beneficial for thick sections.Both X20 CrMoV 12 1 and X20 CrMoWV 12 1were developed and extensively used in Europe forsuperheater and reheater tubes. These steels beingmartensitic grade have a strong self-hardeningproperty. Due to the formation of martensite, theweld metal and the heat affected zone (HAZ)become very brittle on cooling. For better results,both preheat and post weld heat treatments aremandatory. 9Cr1MoVNb steel has been extensivelystudied for over two decades in USA and its tubesamples are currently in service in the USA & UK[18]. Of the two V and Nb bearing 9Cr steels, thisoffers better rupture strength. Among the varioushigh-alloy ferritic steels, X20 CrMoV 12 1,9Cr1MoVNb (T/P91) and 9Cr1MoWVNb (T/P92)are the three most promising candidates for tubing.

P92 has an edge over the other two due to its highrupture strength.

Austenitic steels would be required for finalsuperheaters of 593°C units and for most of thesuperheater and reheater of the 650°C units. Thedesign stress values derived by various boiler codesdiffer based on the approaches adopted by them.Similarly, ranking of austenitic steels 304, 316, 347,based on their allowable stresses varies depending onthe code. However, 304, 316 and 347 type of tubesare widely used for operation at higher steamtemperatures. TP347H type of tubes are extensivelyused in USA, Japan and Germany [5, 22] due to itssuperior properties such as resistance to fire-sidecorrosion, steam-side oxidation and higher thermalfatigue strength, as compared to 316 type of steel.Also, ASME Boilers code allows higher design stressfor TP 347H as compared to TP316H type [15],whereas in the case of BSI, the reverse is true [26].Type 347 and 321 steels are prone to strain-inducedembrittlement due to the formation of strongcarbides like NbC and TiC [26]. As a result, AISI316 steel is preferred in U.K. For superheatersoperating at the highest temperatures of the high-pressure units, stronger austenitic steels like Incoloy800H, 17-14 CuMo, Esshete 1250, NF616 and 15-15N, X8CrNiMoNb 16 13 and X3CrNiMoN 17 13would be required for reliable operation. For mostexacting conditions, materials such as Inconel 617which contains 12.5% Co, though very costly, mayalso have to be considered.

3.2 Steam Piping and Headers

Ferritic steels are preferred because of their higherthermal conductivities and lower coefficient of thermalexpansion coupled with their good fabricability.High-strength high-alloy ferritic steels such asX20CrMoV 12 1, X20CrMoWV 12 1, 9Cr1MoVNband 9Cr1MoWVNb are, therefore, employed fortemperatures up to 625°C. These steels are, however,subject to temper embrittlement in the temperaturerange of 540 to 595°C, where advanced supercriticalsteam power plants operate. Though the temperembrittlement behaviour is not likely to have any


effect on their use for piping, it can be controlledby maintaining low levels of manganese and siliconcontents [9]. Also low levels of sulphur should bemaintained, as the toughness of these steels is verysensitive to sulphur content.

At 650°C steam condition, it is necessary to usehigher-strength austenitic steels. Niobium stabilizedaustenitic steels of the type 347 are preferred inGermany because of their high fatigue strength andsuperior steam-side oxidation as compared to niobium-free steels [27]. Due to temperature-constrainedexpansion during and after welding, niobium steelsshould be used in the case of wall thickness up to30mm, whereas niobium-free austenitic steelsX8CrNiMoNb 16 13 and X3 CrNiMoN 17 13 aredesirable for thick walled pipes. In case of 316stainless steel, main steam pipe failure due to sigmaphase formation was reported [9]. Extensive databaseup to about 100,000 hours available on Esshete 1250,NF709, HR6W steels confirm their reliability forsteam pipe and header application [19, 46].

3.2.1 FIRE-SIDE CORROSION

In coal-fired boilers, the ash corrosion results fromthe formation of complex alkali iron sulphates in ashdeposits, which become aggressive in molten state.The severity of liquid ash corrosion varies withtemperature and follows a bell-shaped curve [29].Corrosion increases sharply from about 595°C to700°C. Below 595°C, the corrodents in the ashdeposits will be in a fairly dry state and therefore donot aggressively attack the tubes. As the temperatureis increased up to about 700°C, the corrodentsbecome molten and the corrosion rate increases.With further increase in temperature, the moltedsulphates begin to vaporize and become unstable,decreasing the corrosion rate.

Superheaters and reheaters of the supercritical plantsat 566°C operate close to the beginning of the bell-shaped curve. 9Cr and 12Cr tubes should be goodenough to serve without experiencing any significantfireside corrosion. But the final superheaters andreheaters of the plants at 593°C and 649°C wouldoperate at the apex of the curve, where corrosion is

most severe. Alkali sulphate corrosion rate decreaseswith the increase in chromium content and asuperior resistance can be obtained with chromiumcontents of over 25% (Fig.9) [30]. Simulated liquidash corrosion tests carried out on different superheatertubing alloys show that their resistance vary widelyas shown in Fig.10 [31]. Amongst the alloys tested,

FIG. 9 : COMPARISON OF FIRE-SIDE CORROSIONRESISTANCE OF VARIOUS ALLOYS [31]

FIG. 10 : HOT-CORROSION WEIGHT-LOSS wrt Cr CONTENTFOR VARIOUS ALLOYS [30]


the 17-14CuMo austenitic steel used for superheatertubing in Eddystone unit [9] experienced the highestcorrosion rate, while the Inconel 671 (50Cr-50Ni)alloy was practically immune to liquid ash corrosion.It can also be seen that the 17-14 Cu Mo alloy, onchromizing, exhibits liquid ash corrosion resistancealmost similar to Inconel 671. There are variouspotential means of overcoming the coal ash corrosionproblem of superheater tubing such as bandageshields of more corrosion-resistant materials, surfacecoatings, grain refinement and composite tubes.Bandage shielding decreases heat transfer efficiencybecause of the insulating air gap, and it may alsoresult in significant increase in corrosion rate of non-bandaged tubing [32]. Surface coatings such aschromizing [33], chromating [22], chromium plating[33] and calorizing [32] have been attempted. Thesemethods require further detailed study for theireffect on fabrication, ductility and high-temperaturecreep strength. However, studies carried out on achromized austenitic steel showed encouraging results[33]. Since both outside surfaces can be chromizedat the same time, it appears to be a promisingapproach to prevent corrosion of both the surfaces.Grain refinement promotes the grain boundarydiffusion of chromium to the surface [22]. Thisresults in improved corrosion resistance [20] but ithas an adverse effect on the rupture strength of thematerial. For most advanced supercritical steamconditions and under highly aggressive serviceconditions generated for combustion of coal withhigh chlorine contents where a single materialcannot provide an economically viable solution,technical advantage provided by two differentmaterials can be utilised by employing compositetubes. Composite tubes are produced by co-extrusionof two different materials comprising a high-strengthinner material such as Essehete 1250, alloy 800H,17-14 Cu Mo to provide the stress-bearing capacityand an outer casing of material of high chromiumcontent like TP310 or Inconel 671 for protectionagainst corrosion. The problems expected from suchtubing are thermal fatigue, welding and sigma phaseformation in outer casing [28]. The serviceperformance for several years in a number of powerstations, however, confirms the integrity and

economics of using composite tubes [19]. Fordemanding environmental applications, it maysometimes be necessary to select even nickel orcobalt base alloys, provided economics permit.

3.2.2 STEAM-SIDE OXIDATION

General experience indicates that the oxidationresistance of high-temperature steels in drysuperheated steam is almost the same as theiroxidation resistance to air at the correspondingtemperature [26]. The oxide scales formed on theinternal surface of superheater tubes, reheater tubes,headers and piping spall off, or exfoliate, when thethermal stresses due to differential thermal expansionbetween the oxide scale and the base metal exceedthe bond strength of oxide scale. 9Cr and 12Crsteels, due to their low coefficient of thermalexpansion, offer better resistance to oxide exfoliationas compared to austenitic steels, in the temperaturerange of their application. The spalled oxides can besevere enough to clog the bends in superheater andreheater tubing, eventually leading to overheat failures.At high steam pressures, these oxides from headersand steam pipes can be carried to the turbine at highvelocities and cause turbine erosion. Turbine erosiondamage not only causes a loss in efficiency but alsois expensive to repair and increases the time ofturbine overhaul outages [34]. It was reported thatthe erosion damage had led to the destruction of aturbine. Since the rate of steam oxidation variesexponentially with temperature [35], exfoliation canbe much more of a threat to advanced supercriticalsteam plants. Amongst the conventional austeniticstainless steels, TP347H provides better resistance toboth fire-side corrosion and steam oxidation,presumably due to its high chromium content.Steam oxidation, as in the case of fire-side corrosion,is a function of chromium content in the innersurface of the tube. To overcome steam oxidationproblem, several methods such as chromizing,chromating, grain size refinement and cold workingof the inner surface through shot peening/blasting,all of which increase the chromium content at thesurface, are applicable. Chromizing being a veryhigh-temperature process, is applicable to only new


and replacement components. Chromating, on theother hand, can be performed at lower temperaturesand even on assembled parts. Both grain refinementand surface shot peening methods aim at enhancingthe chromium diffusion rate to the surface, but theireffect is lost during the process of welding or whenstress relief annealing has to be carried out afterbending. Combination of chromizing and chromatingtechniques seems to be the best choice to overcomethe environmental problems.

4. TURBINE MATERIALS

In the case of turbine, the advancement in steamconditions mainly affects its high pressure (HP) andintermediate pressure (IP) sections. As a result, theassociated rotations as well as stationary parts ofthese sections experience more severe serviceconditions than that of conventional sets. Since theyoperate well within the creep range, their design isbased primarily on the long-term creep strength ofthe material, but the stress levels during steady andnon-steady operating conditions, particularly during

start-up and shut-down periods, must also be takeninto account. Figure-11 shows the temperature rangefor application of different grades of steels [4]. It isclear that the low-alloy ferritic steels are limited touse at temperatures up to about 550°C, and theirrange of application further decreases for rotatingcomponents. Table-2 gives a list of candidate materialsof interest, and Fig. 11 depicts their 100,000hrupture strength as a function of temperature [4, 5,36, 37, 38]. For the sake of comparison andcompleteness, some of the low-alloy ferritic steels arealso included.

4.1 HP/IP Rotors

As can be seen from Fig.11, 12Cr steel can beemployed for HP/IP turbine rotors at 566°C steamtemperatures. X22 CrMoV 121 has been successfullyused for rotors of the supercritical units for manyyears. With rotor cooling, it can also be used up to595°C. Both EPRI [9] and Toshiba [9] have chosen12Cr steel for HP and IP rotors of turbines foroperation at 566°C. Their advanced designs at

FIG. 11 : 1,00,000H CREEP STRENGTH FOR STEAM TURBINE APPLICATIONS [10]


593°C contemplate the use of 12Cr steel rotor withsteam cooling to bring the rotor temperature downto 566°C, where its creep strength is adequate tomeet the design pressure. However, presently severalsuper 12Cr steels with much superior creep resistanceare available and they should also be consideredbefore a final decision is taken. Above 593°C steamtemperature, X12CrMoWVNbN 10 11 and austeniticstainless steel must be considered. Amongst theaustenitic steels, A286 and X8CrNiMoBNb 16 16offer better creep strength for an HP rotor ofadvanced sets operating at 649°C.

One of the rotor-related problems is the maximumsize that can be produced from the 12Cr andaustenitic steels. Due to severe segregation inconventional ingots, the size of the austenitic steelrotors used in earlier supercritical units was limitedto small size, as a result of which, it becamenecessary to divide the HP turbine into two stages[9]. It is estimated that a large advanced plant wouldrequire a one-piece super-alloy HP rotor forgingweighing 11,300 kg with a barrel diameter of890mm. Similarly, a double-flow reheat rotor madeof 12Cr steel is expected to be about 1150mm indiameter and 31,750 kg in weight, which wouldrequire to start with an ingot size of 63,500 kg [9].Significant progress has been made, in recent years,in increasing the size as well as the quality of theforging by employing modern steel making techniquessuch as low sulfur silicon deoxidation (low S),vacuum oxygen decarburization (VOD), vacuumcarbon deoxidation (VCD), central zone refining(CZR), electro slag hot topping (ESHT) and electroslag remelting (ESR). By employing these techniques,either individually or in combination, productionexperience with low-alloy ferritic [39], 12Cr as wellas austenitic steels [4, 40] demonstrate that therotors of the candidates materials can be made to therequired size and quality without experiencing muchproblems.

4.2 Blading

Conventional 12CrMoV steel blades are adequate tomeet the steam temperature at 566°C. But a wide

variety of high-temperature blade materials withproven service performance in large gas turbines areavailable, and they should be considered for moreadvanced steam conditions. These include super12Cr steels, austenitic steels, Nimonic 80A, 90, 105,115, In718 and precision casting alloys such asUdimet 500 and IN 738LC.

4.3 LP Rotor

The principal requirements of material for low-pressure (LP) rotor are high yield strength towithstand the high stress imposed on it by longblades and high fracture toughness to minimize sub-critical flaw growth so as to avoid the possibility offast fracture. 3.5NiCrMoV steel is widely used forLP rotor throughout the power industry. To avoidtemper embrittlement, the maximum operatingtemperature of the LP rotor made of this steel isgenerally limited to about 350°C [9]. The inletsteam temperature to LP turbine of the supercriticalunits, on the other hand, is dictated by the exhauststeam from the second IP section. The IP-LPcrossover temperature from advanced supercriticalunits at steam temperatures of 593°C and abovewould be 400-455°C [9]. To maintain the inletsteam temperature of LP turbine at its presentmaximum allowable limit, it would be necessary tocool the steam either through cooling of the rotoror by adding an additional stage of expansion to theIP turbine. The latter approach would be a difficultdesign task, as it requires usage of long blades at hightemperature, whereas the former approach has tosacrifice a part of the thermal efficiency.

Another approach to the problem would be torender LP rotor material more resistant to temperembrittlement [41]. Efforts are, therefore, beingmade to improve the fracture toughness of the IProtor steel by improved steel making technology andcloser control of chemical composition. Theinteraction between Mn, Si, P and Sn was shown tohave promoted the degree of temper embrittlement[41]. Resistance to temper embrittlement of3.5NiCrMoV rotor steel with low Mn and low Sicontents was found to have greatly improved as


compared to conventional steel [9]. By utilising themodern steel making technologies, it is now possibleto decrease both Mn and Si contents to levels of0.001 - 0.002%.

4.4 Heavy Section Stationary Parts

High-pressure turbine requires a number of heavysection static components such as the inner and outercasings, static components such as the inner and outercasings, the steam valve, the nozzle box, and the inletpipe. Besides high temperature and pressure, theseparts are subject to thermal cycling. If the sectionsizes are very high, there is a danger of experiencingthermal cracking as a result of the heavy thermalstresses that might develop during start-up or carry-over. It is, therefore, desirable to minimize the sectionsize by using high-strength steel so as to reducethermal stresses. Depending on the stresses and therequired wall thicknesses, it might be advisable toemploy 12Cr steel at temperatures lower than 566°Cand austenitic steels at temperatures as low as 566°C.This could be advantageous especially in the case ofhigh-pressure units designed for 31 MPa and above.Given the choice, forgings are preferred as they allowthinner sections but it would be economical to usecastings. Toshiba [7] will be using a 12Cr cast steel(10CrMoVNbN) for these parts of the units at566°C, whereas EPRI [9] intends to give preferenceto forgings for the initial advanced units.

In order to minimize differential thermal expansion,it is desirable to make the rotor and the stationaryparts of the turbine of the same material. However,for 593°C units, it is likely that the rotor would bemade of 12Cr steel, while the inner casing would bemade of austenitic steel. Under such circumstances,shaft seals must be used to accommodate the greaterthermal expansion of the casing. Larger clearancesare required to be given, when austenitic steels areemployed for rotating and stationary parts, for mostadvanced steam conditions. Both the designrequirements, 12Cr rotor cooling at 593°C andlarger clearance to be provided with austenitic steelsat higher temperatures, adversely affect the cycleefficiency. This, in turn, partly reduces the net heat

rate gain achieved by elevating the steam parameters.In addition, rotor cooling complicates the design ofthe turbine and its external piping, and calls for anoverall economic justification in final selection of thecandidate materials.

Since the outer casing is subjected to cooler and low-pressure steam as compared to the other casing, thiscould be still made of the conventional low-alloyferritic steel.

4.5 Transition Weld Joints

In cases where main steam piping and the outercasing are made of austenitic and low-alloy ferriticsteels, respectively, the inlet piping to the turbinewill have to utilize transition joints. An approachsuggested to this problem is to make the joint inthree sections, utilising a material of intermediatethermal expansion coefficient such as Alloy 800H inbetween the pipe and the casing with nickel-basedfiller metal for welding. Nickel-based filler metalimproves the rupture life as much as five times morethan the austenitic filler metals. Due care must alsobe taken in design to minimize bending stresses, aslife of transition joints is greatly reduced if bendingstresses are superimposed upon stresses from thermalexpansion.

4.6 Bolting

Bolts and studs are used in many joints of theturbine, which need to be separated for maintenanceor repairs, as for example, castings and valves. Thebolts and studs differ from all other turbinecomponents in that they are notched, subjected tocold as well as hot stressing and a varying patternto stressing due to practice of tightening andretightening. The strain to which bolts are tightenedis based on both the properties of the materials andthe design practice. The usual strain applied in UKis 0.15% [44], whilst it is 0.2% in Germany [36].The elastic strain produced by the initial tighteningof the bolts is progressively converted to creep strain,thereby reducing the effective load on the joint.Bolts for turbine parts are required to possess


sufficient resistance to stress relaxation to maintainflange toughness against internal steam pressure atleast for the period between overhauls and should bere-usable throughout the life of the plant. Inaddition, bolting materials must have coefficient ofthermal expansion close to that of flange material,high proof strength, good notch rupture ductilityand resistance to embrittlement.

The stress relaxation behaviour of bolting materialsthat have already demonstrated their successfulservice performance in power stations is shown inFig.12 [36, 45]. 1Cr1Mo—VTiB steel, which isextensively used in the conventional unitsmanufactured in the country, can still be used forouter casing joints of the supercritical units, but itis desirable to use 12Cr steel for inner casing flangebolts. At steam temperature of 566°C, the differencein the stress relaxation behaviour of these two steels,as shown in Fig.12, is as a result of the differencein their initial strain. But, at the same initial strainlevel of 0.15%, 12Cr bolting steels possess betterresistance to stress relaxation. In some of the earlierunits, 12Cr bolts were also used at highertemperatures through steam cooling [13]. Since the12Cr bolts undergo extensive stress relaxation at593°C, consideration is being given by EPRI to anumber of high-strength nickel-based bolting

materials. For units at 649°C steam temperature,nickel-based bolting materials are required to beused, as the austenitic steel bolts are not strongenough to meet the requirement. Based on a criticalsurvey of worldwide experience and stress relaxationtests, Incoloy X750, Nimonic 80A, Nimonic 90,Refractaloy 26 and PER 2B have been identified asthe best candidates for use up to 650°C.

5. SUMMARY AND CONCLUSIONS

i) The quest for lower-cost power generationled to a rapid progress in the steam conditions,reheat cycles and output capacity of thepower plants during 1950s and 60s.Supercritical power plants offer considerablegain in heat rate.

ii) During the last two decades, ferritic-martensitic 9 to 12% Cr steels have beendeveloped under international researchprogrammes, which permit (live) inlet steamtemperatures for thermal power stations upto approximately 610°C, pressures of up toabout 300 bar and reheat temperatures up toabout 625°C. The results have beenimprovements in efficiency of around 8%versus conventional steam parameters.

iii) The newly developed 9-12% Cr steels arealready being used in 12 European and 34Japanese power stations with inlet steamtemperatures of up to about 610°C. Theexperience with the components made ofthese steels has been decidedly positive.

iv) Advancements in steam parameters increasethe severity of the service conditions thatmaterials must undergo. The presentlyavailable low and high alloy ferritic steelswith proven service experience can meet therequirements of both boiler and steam turbinecomponents of units designed for steamtemperature at 600°C.

v) For advanced steam plants of temperaturesFIG. 12 : COMPARISON OF 30,000H RELAXED STRESS FORBOLTING MATERIALS [20]


above 600°C, austenitic steels are required tobe employed for final stages of super heatersand static components of the steam turbine,such as inner casing, steam valve and nozzleblock. 12Cr steel can be used for HP/IProtors subject to reliable design for coolingthem so as to bring the material temperaturedown to 566°C.

vi) At steam temperatures of 649°C/621°C,high-strength martensitic steels are requiredfor HP/IP steam turbine rotors and stationarycomponents. Preventive measures such asapplication of composite or chromized tubesto withstand fireside corrosion of superheaterand reheaters as well as steps to minimizesteam-side oxidation should be taken.

vii) For steam turbine blading and bolting attemperatures above 593°C, materials such asNimonics and Refractaloy 26 etc. should beconsidered.

viii) To meet the IP-LP crossover temperature forunits at steam temperatures of 593°C andabove, it is necessary to improve the temperembrittlement of the existing 3.5NiCrMoVLP rotors steel, by employing modern steelmaking techniques to eliminate elementssuch as Mn, Si, P and Sn. Alternatively, thesteam should be cooled in the IP turbine tolimit the LP inlet steam temperature to theexisting maximum allowable level of about350°C.

References

1. Husemann, R.U., et.al., 'Processing andPractical Application of New Materials inPower Plant Constructions', VGBKraftwerkstechnik, 75(3), 1995, 241-255

2. EPRI Journal, September 1981, 22

3. Spencer, R.C., Proc. Amer. Power Conf. 42,1980, 225

4. Haas, H., et al., Proc. Amer. Power Conf., 44,1982, 330

5. Schneider, A., VGB Kraftwerkstechnilk, 58,1978, 168

6. Kawai, T., Turbomachinery International, 25,March 1984, 34

7. Akiba, M. and Aizawa, K., TurbomachineryInternational, 25, March 1984, 37

8. Muhlhauser, H., Brown Boveri Review, 71,1984, 120

9. Gold, M. and Jaffee, R.I., J. Materials forEnergy Systems, 6, 1984, 138

10. Mayer, K.H., et.al., 'New Materials forImproving the Efficiency of Fossil Fired ThermalPower Stations', VGB Power Tech, Jan 1998,22-27

11. Trojanowskij, B.M., VGB Kraftwerkstechnik,60 (1980) 525

12. Plastow, B., et al., Int. Conf. on Creep andFatigue in Elevated Temperature Applications,Instn. Mech. Engrs. Sheffield, 1974, PaperC49/ 74

13. Gemmill, M.G., ASTM Jl. Testing andEvaluation, 2, 1974, 3

14. Briggs, J.Z. and Parker, T.D., 'Super 12% CrSteels' Climax Molybdenum company, NewYork, NY, 1965

15. Section I, Power Boilers, ASME Boiler andPressure Vessel Code, ASME, New York, 1983SI Edition, 176-227

16. Characteristics of HCM9M Steel Tubings forBoiler application, Mitsubishi Heavy IndustriesLtd. (MHI) and Sumitomo Metal IndustriesLtd. (SMI). July 1979.

17. Sumitomo, High Strength Boiler Tubes, SMI,80-F-No.1081

18. Canonico, D.A., The factors that Influence theSelection of High Temperature Materials,presented at the National Symposium on Creep


Resistant Steels for Power Plants, BHEL R&D,Hyderabad (India), January 1983

19. Orr, J. and Nileswar, V.B., Stainless Steels-84,The Institute of Metals, London 1985, 533

20. Viswanathan, R., 'Damage Mechanisms andLife Assessment of High TemperatureComponents,' ASM Intl., 1989

21. Ranganathan, S., et al., National Symposiumon Creep Resistant Steels for Power Plants,BHEL R&D, Hyderabad (India), January1983, Paper No. 1.03

22. Inoue, M., et al., The Sumitomo Search no. 29Nov 1984, 64

23. Fricker, H. and Walser, B., Ferritic Steels forFast Reactor Steam Generators, BNES, London,1978,35

24. Properties of 9Cr steel tubes and pipes, SIM,804-f-No. 1194, February 1984

25. Caubo, M., Improved ferritic steels for superheater tubing, ASME paper No. 63-WA-246,1963

26. Gemmill, M.G., The technology and propertiesand ferrous alloys for high temperature use,George Newnes Ltd., London, 1966

27. Wyatt, L.M., Materials of construction forsteam power plants, applied science publisherLtd., London, 1976

28. Wyatt, L.M., Mat. Sci. and Engg., 1, 1971,273

29. Koopman, J.G., et al., Proc. American PowerConf., 21, 1959, 236

30. Sumitomo, High alloy composite tubes forpulverized coal fired boiler application, SMI,803-F-No. 1006

31. Ohtomo, A., et al., 'High temperature corrosioncharacteristicsof superheater tubes', IHI Engg.Rev., 16(4), October 1983

32. Flatly, T., and Lathom, E., Materials in powerplants, spring residential course Instn. ofMetallurgists, Chamelon press, London, 1975,63

33. Sumitomo, Chromized stainless steel tubes,SMI 803-F-No. 1079, Jun 1983

34. Haberman, J.A., and Keyton, H., Proc. Amer.Power conf., 44, 1982, 1970

35. Rehn, I.M., et al., NACE corrosion 80, Chicago,IL, March 1980, Paper No. 192

36. Warmfeste Hochwarmfeste Werkstoffe FurSchrauben and Mattern, Gutevorschriften, DIN17240, July 1976

37. Wegst, G.W., Stahlschlussel, Verlag StahlschlusselWegst GmbH, 1983

38. Warmfester Ferritischer Stahlgu?, TecchnischeLieferbedingungen, DIN17245, October, 1977

39. Swaminathan, V.P. and Jaffee, R.I., MetalProgress, 128, December 1985, 52

40. Manufacturing of Trial A 286 Rotor Forging,Kobe Steel, TKE 83-57, January 1984

41. Todeu, H., et al., Mitsubishi Power SystemsBulletin MBB-82113E, November 1982

42. Watanabe, J. and Murakami, Y., Proc. Amer.Petroleum Inst., 1981, 216

43. Viswanathan, R., et al., 'Dissimilar MetalWelds in Power Plants', Presented at AWS-EPRI Conf. on Joining Dissimilar Metals,Pittsburgh, PA, August 1982

44. Branch, G.D., et al., Int. Conf. on Creep andfracture in Elevated Temperature Applications,Sheffield, 1974, Paper C192/73

45. British Steel Corporation's Data Sheets onDurehete 900 and Durehete 1055 Steels

46. Oakey, J.E., Pinder, L.W., Vanstone, R.,Henderson, M. and Osgerby S, 'Review ofstatus of advanced materials for powergeneration', Report no. COAL R224, DTI/URN 02/1509, 2003

47. Wright, I.G., Maziasz, P.J., Ellis, F.V., Gibbons,T.B. and Woodford, D.A., 'Materials issues forturbines for operation in ultra supercriticalsteam', ORNL report, USA.


Mr. Kulvir Singh graduated in MetallurgicalEngineering from University of Roorkee, (nowIIT, Roorkee) in the year 1981. He completedM.Tech. (Metallurgy) from IIT, Kanpur, in1983.

Thereafter, Mr. Singh joined MetallurgyDepartment of Corporate R&D, BHEL,Hyderabad. Since the beginning, he was involvedin indigenization of creep-resistant steels for steamturbine and boiler applications. Subsequently, healso studied structure property correlation and

creep crack growth behaviour of power plantsteels. He has also carried out extensive studies onthe creep-rupture behaviour of P91 and X20steels, their weldments and simulated heat-affectedzones. He is actively involved in residual lifeassessment of steam and gas turbine components.He is also working in the area of indigenousdevelopment of gas turbine buckets and heattreatment of steels by microwaves. His otheractivities include many important failureinvestigations of boiler, steam and gas turbinecomponents and process industry equipment. Heis currently working as Dy. General Manager andheading Creep Lab of the Metallurgy Department.

Mr. Singh has published/presented over 50technical papers in various national andinternational journals and conferences. He hasalso received BHEL Excel Award in the bestTechnical Paper category for the year 2003.


SELF-EXCITATION IN 3-PHASE SQUIRREL CAGEINDUCTION GENERATOR FOR WINDMILL APPLICATION

P.K. Khanna

SYNOPSIS

In India, there are certain areas where plenty of energyis available but it remains untapped. Wind energy isone such area. Also, it is well known that InductionGenerators are best suited for power generation throughwindmill due to their simple construction. In this paper,an attempt has been made to describe how an InductionGenerator can be made to get self-excitation and thusbe used in any stand-alone situation, especially forwindmill application.

Key Words:

Induction Generator; Self-Excitation; Stand-Alone.

1. INTRODUCTION

With the advancement of technology, a need isalways felt to have as much simplification as ispossible but with high reliability of operation.Induction Generator is one such category of machine,which is most simple in its construction, as well asin operation. As the name implies, the InductionGenerator has a squirrel cage rotor. This obviatesbrush gear assembly, brushless excitation system orpermanent magnet etc. as is necessary for other kindof generators. For excitation of the inductiongenerator, a capacitor bank is used across the statorterminals. The main advantage of such generators issimple construction, low cost and high reliability.

2. TYPES OF SQUIRREL CAGEINDUCTION GENERATOR

These are of two types:

(i) In the first type, when a squirrel cage motoris run above its synchronous speed, it starts

functioning as a generator. In this type, it isalways necessary that the machine runs aboveits synchronous speed and it should remainconnected to power supply for excitation.

(ii) In the second type, when a squirrel cagemotor running near to its synchronous speedis switched off and simultaneously a capacitorbank is connected across the motor terminals,it starts functioning as a generator. This typeis characterised by self-excitation.

In this article, the second type of generator only(self-excitation type) has been discussed in detail.

3. PRINCIPLE OF WORKING OF THESELF-EXCITATION TYPE

When an Induction motor is running under steadystate condition, an e.m.f. (E) exists across its statorwinding. Now, if the speed of motor is maintainedthrough a prime mover and supply to inductionmotor is switched off, simultaneously connecting itacross a capacitor bank, there is a flow of excitationcurrent in the stator winding as per the load line ofcapacitor (Fig. 1). This current produces rotatingflux, which, in turn, induces e.m.f. in the statorwinding. Under steady state conditions, the e.m.f.induced in the winding has a magnitude anddirection the same as that of the original e.m.f.(E).Thus, the voltage E continues to be sustained.

4. PHENOMENON OF SELF-EXCITATION

The most important condition for self-excitation totake place is that there should be presence of residual


flux in the rotor. When a capacitor bank is connectedacross the stator winding after switching off itssupply, capacitive current flows in the stator winding.Then the flux produced by this current aids theresidual flux. The increase in flux increases theinduced e.m.f. in the winding and this cycle continuestill there is saturation. Under this condition, thesteady state e.m.f. is given by the intersection of themagnetizing curve of motor with the capacitive loadline. If the residual flux in the rotor is not sufficient,the self-excitation fails to occur and therefore, thevoltage at the terminals does not build up.

The slope of the motor magnetizing curve is calledthe critical slope. The size of the capacitors in thecapacitor bank is chosen in such a way that itscapacitive reactance is less than the critical slope ofthe magnetizing curve, otherwise self-excitation willnot take place. Figure-2 shows the case in which the

load line of capacitance is tangent to the criticalslope and, thus, self-excitation does not take place.

This phenomenon can be observed in a laboratoryalso by connecting three-phase supply to an Inductionmachine driven by a prime mover, At the ratedspeed, when the supply to the motor is switched offand simultaneously a capacitor bank is connectedacross it, a steady state voltage is generated at themotor terminals.

The value of capacitance in the capacitor bankshould neither be too low nor too high. If the valueof capacitance is too low, self-excitation will not takeplace. If the value of capacitance is too high, therewill be inadequate build-up of steady state voltagedue to saturation of flux paths. Hence, there is aneed to use optimum size of capacitor for properbuild-up of voltage and also for keeping the cost ofcapacitor bank low.

5. CONFIGURATIONS OF STATORWINDING AND CAPACITOR BANK

There are various configurations of stator windingand capacitor bank, which are possible for thepurpose of self-excitation, e.g.:

(i) Star-connected stator winding and Star-connected capacitor bank (Fig. 3)

(ii) Star-connected stator winding and Delta-connected capacitor bank.(Fig. 4)

FIG. 1 : LOAD LINE OF CAPACITANCE CUTTINGTHE MAGNETISING CURVE

FIG. 2 : LOAD LINE OF CAPACITANCE TANGENTTO THE MAGNETISING CURVE

STAR-CONNECTED STAR-CONNECTEDSTATOR WINDING CAPACITOR BANK

FIG. 3


(iii) Delta-connected stator winding and Star-connected capacitor bank (Fig. 5)

(iv) Delta-connected stator winding and Delta-connected capacitor bank (Fig. 6)

In all the above four cases, whenever the capacitorsare connected in Delta, the value of capacitancerequired in each phase reduces to one third of itscorresponding value in Star, but the Peak InverseVoltage (PIV) of the capacitors required becomes √3times its corresponding value in Star connectedcapacitor bank. As far as performance of the InductionGenerator is concerned, both Star and Deltaconnected capacitor banks are equivalent for a givenconnection of stator winding, and give the sameperformance.

6. INDUCTION GENERATOR ONLOAD (Fig. 7)

Under no-load condition, since only the magnetizingcurrent is flowing through stator winding, voltagedrop in stator winding is very small, and thereforethe voltage at the generator terminals is almost equalto the induced e.m.f. in stator winding.

When the Induction Generator is connected to apure resistive load and current flows through statorwinding, there is a voltage drop in the statorwinding, which is usually less than 5% of theinduced e.m.f. Now, if the load current is increasedfurther, then at a certain point, where the criticalslope of magnetizing curve coincides with the loadline of capacitor, the Induction Generator stops

STAR-CONNECTED DELTA-CONNECTEDSTATOR WINDING CAPACITOR BANK

FIG. 4

DELTA-CONNECTED STAR-CONNECTEDSTATOR WINDING CAPACITOR BANK

FIG. 5

DELTA-CONNECTED DELTA-CONNECTEDSTATOR WINDING CAPACITOR BANK

FIG. 6 FIG. 7 : GENERATOR WITH LOAD


generating any voltage. Thus, in an InductionGenerator with a resistive load, there is no possibilityof overheating of the Induction Generator due toover-load / over-current.

When there is an inductive load put across thegenerator, a part of the capacitive current due tocapacitor bank will get neutralized by the inductiveload current. Thus, the magnetizing current availableto the generator will get reduced. Therefore, morecapacitance will be required to be added in thecapacitor bank for maintaining the terminal voltageof the Induction Generator.

7. DETERMINATION OF CAPACI-TANCE VALUE

The following procedure can be adopted fordetermining the value of capacitance required in thecapacitor bank under no-load condition -

Draw the no-load characteristic of Induction Motor.Calculate the critical slope of the magnetizingcurrent (Fig. 8). Draw a load line of capacitorshaving a slope less than the critical slope ofmagnetizing current. From this, back calculate thevalue of capacitance required under no-loadcondition.

To calculate the capacitance under full-load condition,the following procedure may be adopted:

There will be a voltage drop of approx. 5% interminal voltage due to resistive load. Inductive loadcurrent will have to be provided by capacitance.Hence, load line of capacitance with inductive loadis drawn accordingly, as shown in Fig. 8. From this,back calculate the value of capacitance requiredunder full-load condition.

Generators of this type are best suited for resistiveloads.

8. CIRCUIT TO ENSURE SELF-EXCITATION

Due to any reason, if there is no residual flux in themotor, the process of self-excitation can be initiatedby connecting a battery momentarily across one ofthe capacitors of the capacitor bank (Fig. 9). Withthis, the capacitor bank gets charged and when itdischarges through the stator winding, a flux isproduced in the rotor of Induction Machine.

FIG. 8 : DETERMINATION OF CAPACITANCE VALUE

FIG. 9 : SELF-EXCITATION WITH THE AID OF BATTERY

9. VOLTAGE REGULATION

Though in the above paragraphs, it has beenpresumed that at rated voltage the rotor core getsfully saturated, but practically the core is neversaturated fully and thus, it would always result inlarge variation of the terminal voltage with respectto load current. This problem of drop in voltage


with load can be overcome in the following twoways:

(i) Manual Voltage Regulator

In this case, the variable capacitors are usedin the capacitor bank in place of fixedcapacitors. By manually varying thecapacitance with respect to load, the terminalvoltage can be maintained. While selectingthe range for the variable capacitor, it shouldbe ensured that the lower value of capacitancecorresponds to load line of generator underno-load condition, and that the higher valueof capacitance in the range corresponds to

load line of generator under fully-loadedcondition. In this case, while starting, thegenerator operation must be started withlower value of capacitance across the terminals.

(ii) Automatic Voltage Regulator

In this case, there is a main capacitor bank,which is used for no-load operation of thegenerator. Then, a provision is kept foradding another capacitor bank in parallel tothe first one when there is a dip in voltageat the terminals. Connection of anothercapacitor bank in parallel is achieved throughan electronic circuit (Fig.10).

FIG. 10 : AUTOMATIC VOLTAGE REGULATION OF INDUCTION GENERATOR


10. CONCLUSION

The Induction Machine when operated as a Generatoralways requires its excitation from outside. Either itremains connected to the supply if it is to feed powerto grid or its excitation can be provided through acapacitor bank if it is feeding power to a stand-alonesystem. In stand-alone system like wind mill installedin a remote area having no grid power supply, thereare various options available for providing thenecessary excitation to Induction Generator and alsofor maintaining a constant terminal voltage atdifferent loads.

Acknowledgement

The author wishes to express his sincere gratitude toSh S.K. Goyal, GGM, Corp. R & D, BHEL,Hyderabad, for his continuous encouragement andguidance in writing this paper. Thanks are also dueto Sh M.S. Dhami, AGM (EME) & Sh S.C. Goel,SDGM (MM) for their support and help incompleting this paper.

Bibliography

1. S.S. Murthy, C.S. Jha and P.S. Nagendrarao,"Analysis of grid connected induction generatorsdriven by hydro/wind turbine under realisticsystem constraints," in IEEE Trans. EnergyConversion, vol. 5, pp. 1-7, Mar. 1990.

2. L. Shridhar, B. Singh, C.S. Jha and B.P.Singh, "Analysis of self-excited inductiongenerator feeding induction motor," in IEEEPower Eng. Soc., Summer Meetings, 1994, pp.1-7.

3. L. Shridhar, B.Singh, C.S. Jha, B.P. Singh andS.S. Murthy, "Selection of capacitors for the selfregulated short shunt self-excited inductiongenerator," in IEEE Trans. Energy Conversion,vol. 10, pp. 10-17, Mar. 1995.

4. S.P. Singh, Sanjay K. Jain and J. Sharma,"Voltage regulation optimization of compensatedself-excited induction generator with dynamicload," in IEEE Trans. Energy Conversion, vol.19, pp. 724-732, Dec. 2004.

Mr. P.K. Khanna graduated in ElectricalEngineering from Indian Institute of Technology,New Delhi, in the year 1979.

Mr. Khanna joined BHEL, Haridwar, as an EngineerTrainee in 1979 and was posted in AC MachinesEngineering Department. For more than 24 years,

he has been involved in the electrical & mechanicaldesign of various capacities of tailor-made ACmotors for Thermal Power Station, Irrigation,Cement, Petrochemical and other Industries. Hehas also undertaken retrofit jobs to develop completedesign of AC motors to replace non-BHEL-makeAC motors for various customers. At present, he isworking as Deputy General Manager in ElectricalMachines Engineering at Haridwar and is involvedin design and development of 300 MVA TARI AirCooled Generator.

Prior to this paper, Mr. Khanna has contributedone technical paper in an International Conferenceheld at IIT, Roorkee, recently.


TURBOGENERATOR INDUCED VOLTAGE WAVEFORMCOMPUTATION AND TELEPHONE HARMONIC

CAPABILITY PREDICTION

C. Prem Kumar

SYNOPSIS

The Finite Element method has enabled accurateestimation of the magnetic field in electrical machinesand devices. The approach has therefore made possiblethe accurate estimation of the various flux-relatedparameters — the induced voltage magnitude being oneamong them. Though the machine characteristic curveor the open-circuit characteristic is readily deducible,the induced voltage waveform, however, is not. Thispaper presents details of a method for the computationof turbogenerator induced voltage waveform, its harmoniccontent and machine Telephone Harmonic Factor or theTelephone Interference Factor — all at design stage.

Key Words:

Voltage Waveform; Harmonics; SynchronousMachines; Telephone Harmonic Capability.

1. INTRODUCTION

Accurate evaluation of the magnetic field distributionsin electrical machines has been made possible by theFinite Element method. Magnetic field-relatedmachine parameters such as inductances, inducedvoltage magnitudes & waveforms, useful & strayfluxes, leakage co-efficients, induction-related losses,saturation effects etc, in turn, stand accuratelyevaluated. Though the open-circuit characteristic isreadily deducible from the magnetic field mapping,the induced voltage waveform, however, is not. Thetime variation of the induced e.m.f in a conductorof the stator in a synchronous machine has the sameform as the space distribution of the flux density in

the air gap. Therefore, only a sinusoidal wave of theairgap flux density can result in a sine-wave inducedvoltage. Several factors such as rotor saturation,shape of the rotor core and the style of field coildisposition render realisation of a sinusoidal airgapflux wave impossible.

This article details an approach to the evaluation ofinduced voltage waveform in a turbogenerator,quantification of harmonic voltage magnitudes andcomputation of Telephone Harmonic capability ofthe machine — all at design stage.

2. FE ANALYSIS

Of the several approaches propagated, the FE methodhas found increasing acceptance from industry. Theformulation of the FEM and its application tomagnetic field analysis has been adequately detailedelsewhere[1 to 8]. The FE approach is the cheapest,fastest and certainly the surest way to the predictionof machine parameters at design stage. The fieldsolution is only the first step in the analysis process.More important and relevant in industry are themachine parameters derivable from the field solutions.The availability of affordable desk-top computingpower and the arrival of powerful menu-drivensoftware have largely contributed to the acceptanceof such methods in design offices[1,2].

2.1 Turbogenerator Magnetic Field at No-load

Prediction of the induced voltage waveform in aturbogenerator necessitates estimation of the no-loadmagnetic field distribution in the machine. At no-


load, the singly excited magnetic circuit of theturbogenerator presents a picture of symmetry bothalong the direct and quadrature axis. A symmetricquarter region of the machine cross-section extendingfrom the direct axis to the adjacent quadrature axis issufficient to evaluate the no-load parameters of a two-pole turbogenerator. However, in the case ofhydrogenerators, the region for analysis must alsoensure symmetry of the stator slots. Figure-1 shows theflux distribution in a symmetric quarter section of atwo-pole turbogenerator. The corresponding fluxdensity distribution is shown in Fig. 2.

3. AIR-GAP INDUCTION PROFILE

The airgap flux density profile can be extracted fromthe flux density plot by mapping the flux density onto an arc at the mean radius of the machine airgap.Figure-3 shows the radial component and the fluxdensity magnitude mapped along a mean airgap lineof the machine. As can be seen from the graph, thenormal component of the flux density is equal to themagnitude of the flux density for a large portion ofthe curve except at the quadrature axis where thetangential component contributes significantly tothe flux density magnitude. Of the two componentsof the airgap flux density, only the radial componentcontributes to the induced stator voltage while theperipheral component does not. In reality, the radialcomponent of the airgap induction is very nearlyequal to the total induction at every point in theairgap of the machine.

FIG. 1 : FLUX DISTRIBUTION AT NO-LOAD

FIG. 2 : FLUX DENSITY DISTRIBUTION AT NO-LOAD

FIG. 3 : AIRGAP FLUX DENSITY PROFILE AT NO-LOAD

3.1 Evaluating the Voltage Inducing Flux

The useful flux is defined as the flux linking thestator winding and causing the induced voltage. Thisflux is lesser than the total flux by the amount ofleakage flux. The useful flux in a turbogenerator canbe arrived at by integrating the radial component ofthe airgap induction along the afore-mentionedairgap line.


The expression for the useful flux per pole is givenby -

ϕu = {∫Β.dl}*L

I*S

f*M

f(wb) (1)

where

ϕu

is the useful flux per pole in webers

B is the radial component of the airgapinduction in Tesla

LI

is the nett length of iron in meters

Sf

is the stacking factor(typically around 0.94)

Mf

is the model factor

4. INDUCED E.M.F COMPUTATION

The induced phase voltage in a 3-phase synchronousmachine can readily be arrived at from the usefulflux per pole computed earlier, together with certainstator winding details.

The induced e.m.f per phase is given by [9,13,14]-

Eph

= 4.44*kd*k

p*f*T

ph*[ϕ

u] (Volts) (2)

where

kd

is the stator winding distribution factor

kp

is the stator winding pitch factor

f is the frequency in Hertz

Tph

is the number of series turns per phase of thestator winding

ϕu

is the useful flux per pole in webers.

5. HARMONIC ANALYSIS

The time variation of the induced e.m.f in aconductor of the stator winding in a turbogeneratorhas the same form as the space distribution of theflux density in the airgap. Therefore, only a sinusoidalwave of the airgap flux density can result in a sine-wave induced voltage. Several factors such as rotorsaturation, shape of the rotor core and the style offield coil disposition render realisation of a sinusoidalairgap flux wave impossible.

Non-sinusoidal airgap inductions such as the oneshown in Fig.3 can be resolved into a fundamentaland higher-order components using Fourier Analysis.The symmetric airgap flux density wave results inthe cancellation of even harmonics, leaving a spacedistribution comprising a fundamental and harmonicswhich are odd multiples of the fundamental[9,10,11]-

B = B1sin(θ) + B

3sin(3θ) + B

5sin(5θ) + …. …. +

Bnsin(nθ) (3)

where B1 is the fundamental component of the

airgap flux density and B3, B

5 etc. are the third

harmonic and fifth harmonic components respectively.Typical harmonic spectrum of the airgap inductionfor a turbogenerator is shown in Fig. 4.

The decomposed representation of the airgap fluxdensity distribution enables consideration of themachine as having 2p pairs of fundamentalpoles(fictitious), 6p pairs of poles contributing tothird harmonic, 10p pairs of poles contributing tofifth harmonic component and, in general, 2np polescontributing to the nth harmonic component of thefield form. The fundamental as well as the harmonicpole fluxes generate e.m.fs of corresponding frequencyin the conductors, but the proportion of harmonicsin the phase and line e.m.f waveform is reduced dueto grouping and factors related to the stator windingdisposition.

FIG. 4 : AIRGAP INDUCTION HARMONIC SPECTRUM


6. INDUCED VOLTAGE WAVEFORM

The r.m.s induced voltage per phase due to the nth

harmonic component of the flux density is given by[9,11,13,14,15] -

Eph n

= 4.44 kdn

kpn

fn φ

n T

ph (Volts) (4)

whereE

ph n= r.m.s induced phase voltage due to the nth

harmonick

dn= sin(nσ/2) / (nσ/2) is the distribution

factor for nth harmonick

pn= sin(nσ/2) is the pitch factor for nth

harmonicf

n= nth harmonic frequency

φn

= (Bn r.m.s

*Li* τ/n) is the nth harmonic flux

n = harmonic numberT

ph= number of series turns per phase

and

σ = phase-belt angular width in elec. radian

α = coil-pitch in elec. radian

Bn r.m.s

= r.m.s value of the nth harmonic fluxdensity

Li

= active length of iron

τ = pole-pitch in air-gap

The magnitude of the induced r.m.s voltage due toeach of the harmonics can be evaluated using theabove expression.

The variation in time of the fundamental, harmonicand cumulative voltage computed using the proceduredescribed above is shown in Fig. 5 for a synchronousgenerator whose line-to-line voltage is 11kV.

And finally, the r.m.s value of the resultant phasevoltage is given by [11,12,13] -

FIG. 5 : TYPICAL COMPUTED HARMONIC VOLTAGE PROFILES FOR A 11kV GENERATOR


Eph

= √[(Eph 1

)2 + (Eph 3

)2 + (Eph 5

)2 + … +(Eph n

)2]

(Volts)… (5)

Eph

= Eph 1

*√[1 + (Eph 3

/ Eph 1

)2 + … +(Eph n

/Eph 1

)2]

(Volts)… (5.a)

The value in the radical is very nearly unity, leadingto the phase voltage being equal to fundamentalalone since the harmonic magnitudes are small incomparison to the fundamental.

7. TELEPHONE HARMONIC FACTOR

Telephone/Communication lines running parallel topower grid lines can experience severe interferenceby induction, resulting in hum and high pitch noisebecause of the presence of harmful frequencies in thegrid. It is, therefore, necessary to limit the harmoniccontent in the output voltage waveform of everygenerator likely to be connected to the grid.

The IEC test procedure and recommendations onthe tolerable limits of telephone harmonic factor(THF) for synchronous machines is reproducedbelow -

8.9.2 Limits : When tested on open circuit and at ratedspeed and voltage, the telephone harmonic factor(THF)of the line-to-line terminal voltage as measured accordingto the methods laid down in 8.9.3 shall not exceed thefollowing values:

Rated output of the machine % THF

300kW(orKVA)< PN < 1000kW(orkVA) 5.0%

1000kW(orKVA)< PN < 5000kW(orkVA) 3.0%

5000kW(orKVA)< PN

1.5%

The section 8.9.3 of IEC details the tests and theapproach to be adopted for the measurement ofsynchronous machine THF. The THF capability ofa machine is given by -

THF(%) = 100.√(E12.λ2+E

22.λ2+E

32.λ2+…… E

n2.λ2)

U

where

En

is the r.m.s value of the nth harmonic of theline-to-line terminal voltage

U is the r.m.s value of the line-to-line terminalvoltage of the machine

λ is the weighting factor for frequencycorresponding to nth harmonic

8. "WAVE"—THE SPREAD-SHEETCODE

The procedures detailed in the article have beencoded into a design office utility package by name"WAVE". Exclusively developed for synchronousmachines, the Microsoft Excel spread-sheet utilitycode computes and displays graphically theinformation on harmonic voltage magnitudes/waveforms, cumulative voltage waveform and machineTelephone Harmonic Factor (THF) for each harmonicand cumulative value up to the 100th harmonic.The program has built-in logic to account for triplenharmonics and even harmonics from the computedvalues for line and phase quantities.

The code is structured in two levels and spread over12 sheets and can be tailored to suit individualdesign office requirements. An overview of the codefollows :

The Machine ID Section of the utility is anidentifier section for design office documentation/records and contains information on customer name,order number and machine nominal rating particulars.The program uses colour coding to distinguishunlocked input cells from locked coded cells.

The Inputs Section seeks machine dimensionalinformation such as gross length, number of radialventilating ducts & their width, rotor diameter,stator inner diameter, coil-throw, bars per slot etc.These input details are made use of in thecomputation through formulae embedded in thecells (Fig. 6).

The Computation Section uses the input data tocompute harmonic winding factors, pole-pitch valuefor each harmonic, harmonic flux magnitudes,


FIG. 6 : THE INPUTS SHEET IN "WAVE"—SPREAD-SHEET UTILITY


induced phase e.m.f due to each harmonic etc forthe fundamental and harmonics up to the 100th. Theprogram can detect and display the type of windingemployed. The program can tackle both integral-slotand fractional-slot chorded winding which are verycommon with practical synchronous machines withtwo-pole cylindrical rotor constructions and multiplesalient-pole low-speed hydrogenerators.

The THF Section uses the embedded harmonicweighting factors to compute the individual andcumulative THF due to each harmonic up topredefined significant harmonic. This section alsogenerates a plot of the %THF versus harmonicnumber, as shown in Fig. 7.

Other Features : Graphical comparison of therelative magnitudes of fundamental and significantharmonics (up to13th) and its variation in time are

provided together with a zoom of significantharmonics. The IEC:1996 recommendations on theweighting factor for various frequencies to be usedfor the computation of Telephone Harmonic Factor(THF) have been built into the program. The airgapflux density harmonic magnitudes are used toreconstruct and compare with the original airgapinduction curve.

9. CONCLUSION

The Finite Element approach has been used for theaccurate estimation of the magnetic field in aturbogenerator at no-load. Harmonic components ofthe airgap induction computed from the airgapinduction profile have been used to arrive at theharmonic voltage magnitudes and the cumulativeinduced voltage waveform using a code specifically

FIG. 7 : THF CONTRIBUTION FROM INDIVIDUAL HARMONICS AND CUMULATIVE VARIATION WITH INCREASING HARMONIC NUMBER


developed. The analysis indicates that for a "n" slotmachine, the (n-1)th harmonic and the (n+1)th

harmonic are predominant[11]. The TelephoneHarmonic Factor or the Telephone InterferenceFactor have been computed for the machine analysed.The harmonic levels of the airgap induction aredictated by the airgap induction profile which, inturn, reflects the radial airgap permeance presentedby the magnetic boundaries constituted by the statorand rotor iron. A separate study investigating thedependance of THF on saturation in the machineis under way.

References

(1) M.V.K. Chari, P.Silvester, "Analysis ofTurboalternator Magnetic Fields by FiniteElements", IEEE Trans. on PAS, Vol-PAS-90,No:2, March-April 1971, pp.454 to 464.

(2) M.V.K. Chari "Finite Element Analysis ofElectrical Machinery and Devices", IEEE Trans.on Magnetics, Vol.MAG-16, No.5, Sept.1980,pp. 1014 to 1019.

(3) M.V.K. Chari, "Nonlinear Finite ElementSolution of Electrical Machines Under No-loadand Full-Load Conditions", pp.686 to 689.

(4) P.Silvester and M.V.K. Chari, "Finite Elementsolution of Saturable Magnetic Field Problems",IEEE. Trans. on PAS, Vol.PAS-89, No.7, Sept-Oct 1970, pp.1642 to 1651.

(5) P. Silvester, H.S. Cabayan and B.T. Browne,"Efficient Techniques for Finite Element analysisof Electric Machines", IEEE Trans. on PAS,Vol.PAS-92 1971,pp.1274 to 1281.

(6) Parviz Rafinejad et al,"Finite ElementComputer Programs in Design of

Electromagnetic Devices", IEEE Trans. onMagnetics, Vol. MAG-12, No.5, Sept1976,pp.575 to 578.

(7) Mulukutla S.Sarma, "Magnetostatic FieldComputation by Finite Element Formulation",IEEE. Trans. on Magnetics, Vol.MAG-12, No.6,Nov1976, pp1050 to 1052.

(8) J.R. Brauer, E.A. Aronson et al,"ThreeDimensional Finite Element Calculation ofSaturable Magnetic Fluxes and Torques of anActuator", IEEE. Trans. on Magnetics,Vol.MAG-24, No.1, January 1988, pp455 to458.

(9) M.G. Say, "The Performance and Design ofAlternating Current Machines", Sir IsaacPitman & Sons, London.

(10) Ralph R.Lawrence & Henry E. Richards,"Principles of Alternating Current Machinery",McGraw-Hill Book Company, USA.

(11) Alexander S. Langsdorf, "Theory of Alternating-Current Machinery", McGraw-Hill BookCompany, USA.

(12) Robert L.Ames, "A.C. Generators: Design andApplication",John Wiley & Sons, USA.

(13) Mulukutla S. Sarma, "Synchronous Machines- Their Theory, Stability and ExcitationSystems", Gordon & Breach Science Publishers,New York.

(14) Essam S. Hamdi, "Design of Small ElectricalMachines", John Wiley & Sons, USA.

(15) Brian Chalmers & Alan Williamson, "A.C.Machines - Electromagnetics and Design", JohnWiley & Sons Inc, USA.


Mr. C. Prem Kumar obtained his Engineeringdegree from Bangalore University in the year1975.

Mr. Prem Kumar joined BHEL at the Corporate

R&D Division, Hyderabad, after a brief stint withM/s Oblum Electrical Industries, Hyderabad.Currently, he is working at the Electrical MachinesLab in the R&D Complex, as Senior DeputyGeneral Manager. He specializes in the modellingand analysis of large rotating electrical machinesusing the Finite Element approach.

Mr. Prem Kumar has several papers to his credit,published & presented in national & internationalforums.


EFFECT OF PRELOAD FACTOR AND WORN DEPTH ONTHE DYNAMIC COEFFICIENTS AND STABILITY OF ALOADING ARC (WORN) TWO-LOBE BEARING USED

IN TURBO-GENERATOR

K. Dargaiah and P. Kamalam

SYNOPSIS

This paper is the extension of an earlier paper publishedin BHEL Journal, (Vol. 26 No.1 February 2005),wherein it was concluded that the load capacity of aloading arc (worn) two-lobe bearing used in turbo-generator is found higher and that the stability zone islimited when compared to a normal two-lobe bearingwithout loading arc (δ

o=0.0) used in steam turbine. In

the present paper, the stability (whirl onset/thresholdspeed) of a worn two-lobe bearing L/D=0.82 ofSiemens design used in turbo-generator was studied atdifferent preload factors and worn depths. Bearingperformance data was obtained at preload factors(delta) = 0.5, 0.6 and 0.75 and non-dimensional worn

depths (Aδo) = 0.0, 0.1 and 0.2, which are of practicalimportance in the design of turbo-generator bearing.The results presented in this paper helps the designer inselecting a suitable combination of preload factor andworn depth in order to obtain the desired bearingperformance in terms of Sommerfeld number (1/loadcapacity), dynamic coefficients and stability. The bearingdynamic coefficients are useful for rotor dynamicanalysis.

Key Words:

Two-lobe Bearing; Loading Arc (worn region); WearDepth (wear dent); Whirl Onset (Threshold) Speed.

NOMENCLATURE

Ch

= side clearance (m) or Cp = pad radial clearance

Cv

= top clearance/2 (m) or Cb = bearing radial clearance

D = bearing diameter (m)

Delta = Pre load factor = (Ch-C

v)/C

h

e = eccentricity (m)

g = gravitational constant (m/sec2)

h = film thickness (m)

L = bearing length (m)

N = rotating speed (rps)

R = D/2 (m)

W = load on the bearing (N)

punit

= W/LD, unit pressure on the bearing (Mpa)

ω = Angular speed of journal (rad/sec)


ε = eccentricity ratio (e/Ch)

δ = wear depth (mm)

δo = maximum wear depth (mm)

μ = lubricant viscosity (N sec/m2)

Kij, where i=x,y & j=x,y = stiffness coefficients (N/m)

Cij, where i=x,y & j=x,y = damping coefficients (N sec/m)

Non-dimensional quantities:

S = Sommerfeld number = (μNLD/W)(R/Ch)2

Aδo = non-dimensional maximum wear depth = δo/Ch

γ = non-dimensional whirl onset (threshold) speed = ω(Ch/g)1/2

AKij…,

= non-dimensional stiffness coefficients = Kij (C

h/W)

ACij…,

= non-dimensional damping coefficients = Cij (C

hω/W)

1. INTRODUCTION

Loading arc (worn) two-lobe journal bearings areused in supporting the heavy rotors of turbo-generators. These bearings are originally designedby Siemens. Load per unit area on this type ofbearing is high (around 2.5 MPa) when comparedto a normal two-lobe bearing without loading arc

used in steam turbine. The loading arc of a two-lobe bearing is similar to lemon type bearingwith a cylindrical surface 60 deg. arc machinedat the bottom half of the bearing (Fig.1). Themaximum depth (wear depth) of a loading arc isalso a critical parameter in addition to preloadfactor, which affects the bearing performanceconsiderably.

FIG. 1 : NOMENCLATURE OF A LOADING ARC (WORN) TWO-LOBE BEARING


Dufrane et al [1] have established wear profilemodels from actual measured data and modifiedthe film thickness equation as a function of weardepth (Fig.1). Hashimoto et al [2,3] studied thesteady state and dynamic characteristics of worncylindrical journal bearing (L/D=1) in both laminarand turbulent regimes. They concluded that thethreshold speed (whirl onset) for worn journalbearing ofAδo < 0.3 is lower, and that forAδo > 0.3is higher, than that of non-worn bearing. Tanakaand Hori [4] and Tanaka and Suzuki [5] madetheoretical and experimental studies respectively ona lightly loaded high-speed two-lobe journal bearingwith wear dent, and concluded that the linearstability is found to worsen drastically due to weardent. The authors of this paper earlier presented [6]the analysis and performance data of a heavilyloaded worn two-lobe bearing (L/D=0.82, preloadfactor = 0.673) at different non-dimensional weardepths from 0.0 to 0.2. In the present paper, thebearing non-dimensional data, which include steadystate, dynamic coefficients and whirl onset(threshold) speed of a rigid rotor, is presented ingraphical form at different wear depths from 0 to0.2. Effect of preload factor and wear depth on thebearing performance is discussed. This paper is theextension of an earlier work [6] and it helps thedesigner in selecting a suitable combination ofpreload factor and worn depth in order to obtainthe desired bearing performance in terms ofSommerfeld number (1/load capacity), dynamiccoefficients and stability.

2. HYDRODYNAMIC ANALYSIS

The non-dimensional hydrodynamic equation forbearing lubrication [7] is given by:

∂/∂θ (Ah≥∂Ap/∂θ) + r"∂/∂z (Ah≥∂Ap/∂z)= -3ε sin θ + 6 (Au sin α

1 +Av cos α

1) (1)

Film thickness equation is given by:

Ah(θ)

= 1+ε cosθ, for non-worn region (2)

Ah(θ)

=1+εcosθ +Aδ(θ)

, for worn region (3)

where the worn profile [1] is governed by:

Aδ(θ)

= Aδo - (1 + cos θ)Aδ > 0= 0, otherwise (4)

Equation (1) is solved for p (pressure) by finiteelement method (FEM) with Reynolds boundaryconditions for non-worn and worn region [7]. Twocomponents of oil film force, Fx and Fy, areobtained by integrating the calculated pressuredistribution. The equilibrium position of the journalcentre is obtained by making F

x = 0 by changing the

attitude angle. The load capacity of the bearing is Fy

at the equilibrium position. S, Sommerfeld number,is 1/F

y. The steady state performance characteristics,

like oil flow rate, power loss, temperature-rise andminimum oil film thickness, is obtained atequilibrium position for different eccentricity ratios.Stiffness and damping coefficients are obtained usingthe method given by Shang and Dien [6]. Thismethod helps to obtain eight dynamic coefficientsby solving Reynolds equation only once. In thepresent paper, the finite element method used is thesame as in [7]. Four stiffness and four dampingcoefficients are used for computing the whirl onset(threshold) speed. Routh-Hurwitz criterion was usedfor stability analysis assuming a rigid rotor supportedon two identical symmetrically aligned bearing [3].In the present paper, the non-dimensional results offour stiffness and four damping coefficients andthreshold speed are presented as a function ofSommerfeld number at different preload factors andworn depths.

3. RESULTS AND DISCUSSION

Figures 2-4 show the variation of oil film stiffnesscoefficients (AKii) at worn depths (Aδo=0.0-0.2) andpreload factors (delta) 0.5, 0.6 and 0,75 withSommerfeld number (S). It can be seen from thegraph thatAKyy i.e. vertical stiffness is increasingwith increase in delta at all S. However, thesecoefficients are non-linear w.r.t S. AKxx, i.e. horizontalstiffness, is decreasing with increase in delta at allS for non-worn bearing (Aδo=0.0), whereas for otherworn bearings (δo=0.1 and 0.2), it is slightly


FIG. 2 : VARIATION OF STIFFNESS COEFFICIENT (Kii) WITH SOMMERFELD NUMBER(S) AT DIFFERENT PRELOADFACTORS (DELTA) FOR L/D = 0.82,Aδδδδδo=0.0




increasing. Figures 5-7 show the variation of thecross stiffness coefficients (AKij) at worn depths(Aδo=0.0-0.2) and preload factors (delta) 0.5, 0.6and 0.75 with Sommerfeld number (S). It isobserved that these coefficients are non-linear. AKxyis negative for all values of S for worn bearings andsome values of S for non-worn bearing. Figures 8-10 show the variation of damping coefficients (ACij)with Sommerfeld number (S), at worn depths(Aδo=0.0-0.2) and preload factors (delta) 0.5, 0,6and 0.75. It can be seen from the graphs that ACxxand ACyy are increasing with increase in delta, andcross damping coefficients (ACxy=ACyx) are foundto be negative after certain value of S, and dampingcoefficients are also higly non-linear. Figures 11-13 show the whirl onset (threshold) speed (γ) withS at preload factors (0.5, 0.6 and 0.75) and worndepths (0.0-0.2). For non-worn bearing (Fig. 11),the bearing is always stable at delta=0.5 fromS=0.0217 to 0.3586, at delta=0.6 from S=0.05309to 0.2556, and at delta=0.75 from S=0.0345 to0.1369. It can be observed from results that stabilityparameter is decreasing with increase in preloadfactor for non-worn bearing (Aδo=0.0). Similartrends can be seen for worn bearings (Figs. 12-13)with different stable regions of operation.

4. CONCLUSIONS

A theoretical study has been carried out at differentpreload factors (0.5,0.6 and 0.75) and worn depths(0.0, 0.1, and 0.2) on the loading arc (worn) two-lobe bearing (L/D =0.82)used in turbo generator of

Siemens design, and the following conclusions aredrawn.

I. Preload Factor:

Stiffness coefficients: Vertical oil film stiffness (AKyy)is increasing and horizontal oil film stiffness (AKxx)is decreasing with increase in preload factor for non-worn bearing (Aδo=0.0), whereas vertical stiffness isincreasing at higher rate than horizontal stiffness forworn bearings (Aδo=0.1and 0.2) at all Sommerfeldnumbers. Cross stiffness coefficients (AKyx) isincreasing and (AKxy) is decreasing with increase indelta at all values of S. These coefficients are non-linear.

Damping coefficients: Vertical (ACyy) and horizontal(ACxx) damping coefficients are increasing and crossdamping coefficients (ACxy=ACyx) are decreasingwith increase in delta at all values of S and they arenon-linear.

Whirl onset (threshold) speed (γγγγγ): Stability parameteris decreasing with increase in preload factor for non-worn and worn bearings (Aδo=0.0 to 0.2), Similartrends can be seen for worn bearings (Figs. 12-13)with different stable regions of operation.

II. Worn Depth: Stability range of Sommerfeldnumbers for different preload factors andworn depths is given in Table-1.

The above Table is very useful for checking thestability of existing bearing and for designing a newbearing of L/D = 0.82, heavily loaded TG bearings.

TABLE-I : STABILITY RANGE OF SOMMERFELD NUMBERS

Preload Factors (delta)

Worn Depth (Aδδδδδo) 0.5 0.6 0.75

0.00 0.0217 - 0.3586 0.0531 - 0.2556 0.0345 - 0.1369

0.10 0.0215 - 0.1526 0.0181 - 0.1291 0.0173 - 0.0373

0.20 0.0663 - 0.1494 0.0452 - 0.1204 0.0199 - 0.0798


FIG. 5 : VARIATION OF CROSS STIFFNESS COEFFICIENT (Kij) WITH SOMMERFELD NUMBER(S) AT DIFFERENTPRELOAD FACTORS (DELTA) FOR L/D = 0.82,Aδδδδδo=0.0




FIG. 8 : VARIATION OF DAMPING COEFFICIENT (Cij) WITH SOMMERFELD NUMBER(S) AT DIFFERENT PRELOADFACTORS (DELTA) FOR L/D = 0.82,Aδδδδδo=0.0




FIG. 11 : VARIATION OF THRESHOLD SPEED (WHIRL ONSET) WITH SOMMERFELD NUMBER(S) AT DIFFERENTPRELOAD FACTORS (DELTA) FOR L/D = 0.82,Aδδδδδo=0.00




The designer should select an optimum wear depthand preload factor which should satisfy both load-carrying capacity (1/S) and stability limit speedwhile designing a loading arc two-lobe journalbearing. The results i.e., steady state, dynamiccoefficients and threshold speed data, presented inthis paper, can be used for performance evaluationof loading arc two-lobe bearing for different load,speeds and lubricant conditions. Also, the bearingdynamic coefficients data can be used for rotordynamic analysis.

References

1. Dufrane, K.F., Kannel, J.W., and McCloskey,T.H., "Wear of Steam Turbine Journal Bearingsat Low Operating Speeds", ASME Trans.,Journal of Lubrication Technology, Vol. 105,July 1983, pp. 313-317.

2. Hashimoto , H., Wada , S.,and Nojima, K.,"Performance Characteristics of Worn JournalBearings in Both Laminar and TurbulentRegimes". Part I: Steady State characteristics",ASLE Trans. Vol. 29,1986, 4, 565-571.

3. Hashimoto , H, Wada , S., and Nojima, K.,"Performance characteristics of Worn JournalBearings in Both Laminar and Turbulent

Regimes. Part II: Dynamic Characteristics",ASLE Vol. 29,1986, 4, 572-577.

4. Tanaka, M., and Hori, Y., "StabilityCharacteristics of Worn Journal Bearings",Proc. 3rd IFTOMM Rotor dynamic conference,Lyon,1996,93-97.

5. Tanaka, M., and Suzuki, K., "ExperimentalVerification of Stability Characteristics of Two-Lobe Journal Bearings with Surface WearDent", ImechE, 1996, pp. 143-150.

6. Dargaiah,K., and Kamalam,P., "Analysis andPerformance Data of a Loading Arc(worn)two-lobe bearing used in Turbo-Generator",BHEL journal, vol. 25 No.2, February 2005.

7. Shang, L., and Dien, I.K., " A matrixMethod for Computing the Stiffness andDamping Coefficients of Multi-Arc JournalBearings", STLE Tribology Trans., Vol.32,1989,pp.397-404

8. Dargaiah, K., Kamalam, P., and Prabhu, B.S.,"A Finite Element Method for ComputingDynamic Coefficients of Multi-Lobe JournalBearings", STLE, Tribology Transactions, Vol.36,1993, No.1., pp. 73-83.


Mr. K. Dargaiah graduated in MechanicalEngineering from the Osmania EngineeringCollege, Hyderabad, in 1976, and completedM.S. from the Jawaharlal Nehru TechnologicalUniversity, Hyderabad, in 1984. He obtained hisPh.D. degree from IIT-Madras in 1994.

Dr. Dargaiah joined the Corporate R&D Division

of BHEL, Hyderabad, in 1977. He was activelyinvolved in setting up the Tribology Laboratory inthis R&D Complex. He has developed a journalbearing test rig and carried out a number ofexperimental and analytical investigations oncylindrical, ring-lubricated, multi-lobe and tiltingpad hydrodynamic journal bearings. At present,he is working as Deputy General Manager in theMachine Dynamics Laboratory of the R&DComplex.

Dr. Dargaiah has presented & published a numberof papers in the area of hydrodynamic journalbearings, in national as well as internationalconferences and journals on Tribology.

Ms. P. Kamalam did her Post-graduation inMathematics from the Madras University andcompleted her doctorate from Indian Institute ofScience, Bangalore, in 1975.

Dr. Kamalam joined Corporate R&D Division of

BHEL, Hyderabad, in 1977. Here, she has beenactively involved in research in the fields of HeatTransfer, Electro Magnetism and Tribology. Shedeveloped FEM program in the areas of cylindrical,multi-lobe and tilting pad bearings. She alsodeveloped FEM program in the field ofelectromagnetics for transformer. At present, sheis working as Senior Deputy General Manager inElectrical Machines Laboratory of the R&DComplex.

Dr. Kamalam has presented & published a numberof papers in the area of bearings, in internationalconferences and journals on Tribology.


COLLECTION, HANDLING AND TREATMENT OFLIQUID EFFLUENTS IN THERMAL POWER PLANT

P. Shandilya, S.S. Phogat, G.S. Mahal, S. Balaji and Sudhir Bhartiya

SYNOPSIS

Power plants use fuel, air and water inputs andgenerate electric power. Liquid, solid and gaseouseffluents are also generated in the process of powergeneration.

Ash is the major solid waste. It is generally pumped toash pond, as ash slurry. As per the current pollutionnorms, ash is to be 100% utilized and hence disposedof as dry ash.

Gaseous emissions are SOX, NO

X, CO

2, and particulate

matter in the flue gases. These are currently controlledby ESP and provision of tall chimneys to meet pollutionnorms.

The liquid effluents may contain one or more of thecontaminants like high suspended solids, oil, highdissolved solids, high residual chlorine, pH value outsidethe permitted range etc. The acceptable limits of thesepollutants are specified by the pollution controlauthorities.

Generation of effluents depends on the input quantities,process used and nature of plant operation, which maybe continuous, intermittent or very infrequent.

The provisions for collection, treatment and disposal ofliquid effluents needs to be viewed in the context ofpractical aspects of liquid handling, including equipmentselection for rated flow and frequency / duration ofequipment / system operation.

This paper deals with the nature and frequency ofliquid effluents generated in coal-fired / combined-cycleprojects, and their handling & treatment, giving dueconsideration to the operating practices, installation costand code requirements.

Key Words:

Effluents; Pollution; Emissions; pH Value; Blow-down; Combined-cycle; Turbidity; Dissolved Solids;Alkaline; COC; Sludge; Make-up; Thickener;Centrifuge; HVAC; Back-wash; Zero-discharge;CCPP; ppm.

1. INTRODUCTION

Thermal power plants can be broadly classified intwo groups viz. Coal-fired power plants andCombined-cycle power plants. Both these types ofPower plants use fuel (coal, oil and gas), air andwater inputs, and generate electric power. Lubricatingoil and chemicals are also used in the power plants.

Solid, gaseous and liquid effluents are generatedalong with power generation. Ash, produced fromcoal combustion, is the major solid waste. It ishandled and disposed of in the Ash handling system.The other solid effluent can be sludge from centrifuge.ESP, APH and chimney design are major provisionsfor handling flue gas (gaseous effluent).

This paper deals with the liquid effluents only.Liquid effluents, from power plants, can be classifiedin the following three categories:

● High-turbidity effluents.

● High-dissolved-solids effluents, includingacidic / alkaline effluents.

● Oily effluents.

The distribution of water supply, in the plant, isshown in Schematic-1 "Plant water system". Thisscheme also shows the effluents generated fromvarious processes.


Schematic-2 "Waste water management system basedon zero-discharge philosophy" shows only theeffluents generated from different processes, andtheir handling / disposal systems.

2. GENERATION OF LIQUIDEFFLUENTS DUE TO NATURE OFPLANT OPERATION

The major liquid effluents generated due to natureof plant operation, are given below:

● Seasonal changes in raw water quality affectthe following:

� Pre-treatment plant sludge flow rate,

� Cooling water system blow-down flowrate,

� Cooling water system blow-own quality.

● Storm water drain is also of seasonal nature.Its flow rate is very large with short duration.

● Intermittent operations produce intermittenteffluents. Following are the common examplesof such effluents:

� DM plant regeneration leads tointermittent flow rate from theneutralizing pit.

� Service water from floor washingoperation is generated in a short spanof 2 to 3 hours.

● Plant mal-operation or failure may result ineffluents with flow rates of variable nature.Time span for such flows is failure-specific.Alarm system, with or without auto operation,is generally provided for such effluents.

● Maintenance effluents can be handled in aplanned way since their quantities andqualities are fairly well known in advance.

● Sewage effluent is mostly discharged duringdaytime. It is treated and disposed ofindependent of the other process effluents.

3. HIGH-TURBIDITY LIQUIDEFFLUENTS

There is no well-accepted borderline of high turbidity.We can consider high turbidity referring to thoseeffluent streams which have suspended solids ofapproximately 100 ppm or greater, since the effluentturbidity above 100 ppm is not permitted. Thiscategory will typically include the following effluents:

● Sludge from raw water Pre-Treatment Plant(PTP) and from Effluent Treatment Plant(ETP) equipment such as lamella clarifiersand thickeners.

● CW blow-down generally falls in this category,depending on the make-up water suspendedsolids content and the operating Cycles ofConcentration (COC).

● Filter backwashes (from gravity / pressure /side stream filters).

● Wastewater from routine floor washing.

● Ash slurry water.

● Coal pile run-off.

3.1 Pre-Treatment Plant (PTP) and ETPSludge

Sludge is produced from clarifiers, from the turbidityof raw water supply to the plant. The raw water supplyto the plant is primarily for meeting the requirementsof clarified water and direct raw-water make-up toconsumers like fire water system, ash handling plant,service water system etc. Raw water requirementgenerally does not vary much during plant operation.In the event of ash water recovery from the ash pond,which may take 2-3 years after initial plantcommissioning, there may be a need to cut down onraw water requirement. Reduced raw water requirementleads to reduction in sludge generation also.

Raw water turbidity is a major variable (in additionto raw water flow rate), which decides the quantumof sludge generation. Turbidity in the source of rawwater supply is likely to have the variations as givenin Table-I.


The variations in turbidity from reservoir and river/canal source, is site-specific and depends on the sizeof reservoir, source of river, time / period related torainy season etc.

PTP sludge up to approximately 60 m3/h can beeffectively discharged from the plant through ashdisposal system, and this practice is prevalent in theexisting plants.

It is recommended to adopt this practice, as a firstpreference, for PTP sludge disposal. It may beappreciated that the normal ash consistency, asdischarged from ash slurry sump, is about 20-30%.The PTP sludge, with 2-3 % consistency, practicallybehaves like make-up water to ash slurry sump,where ash slurry disposal system is envisaged. Incertain plants, where slurry disposal system is notenvisaged and only dry disposal system is specified,separate sludge disposal / treatment would have tobe considered.

The turbidity levels are not uniform throughout theyear. In the event of the sludge generation rateexceeding the maximum limit acceptable to ashhandling system, the following methods can be usedfor its disposal.

i) Disposal of sludge (during high-turbidityperiod) through plant drainage system. Thismay be acceptable to pollution control

authorities as the water streams which receivethis effluent also have very high suspendedsolids in monsoon, and since the suspendedsolids generally consist of clay, they are nota health hazard.

ii) Processing of the total sludge in thickenerand centrifuge. This alternative has theadvantages of water recovery (and reuse)from the sludge. However, it is an expensiveprovision, if intended to be used in monsoonseason only.

3.2 Cooling Water (CW) System Blow-Down

CW system temperature rises by 8-10°C. It is cooledby the same extent in cooling tower. The coolingeffect is achieved by loss of CW, by evaporation.About 1.8% of CW flow rate is lost by evaporation,for every 100 C cooling effect.

Cooling water system blow-down is practised tomaintain concentration of suspended / dissolvedsolids in the circulating water, so that the scaling /corrosion of the wetted surfaces is minimized. Theblow-down quantity is a function of circulating flowrate and the cycles of concentration (COC) adopted.

Acid dosing and side stream filtration can bejudiciously used for the selected COC and make-up

TABLE-I : TURBIDITY VARIATIONS IN THE RIVER WATER SUPPLY TO PLANTS

Source of Raw Water Supply Typical Turbidity Levels

Raw water supply from sea. Turbidity is low (typically 10-50 ppm).

Raw water supply from bore-well. Turbidity is low (typically 5-10 ppm).

Raw water supply from reservoir. Turbidity is generally low (typically 10-100 ppm). Dependingon site location and reservoir size, it can however rise to about200 ppm in monsoon months.

Raw water supply from rivers/canals. Generally lower than supply from reservoirs, butvariations during monsoon months can be 200-500 ppm.


water quality, to maintain the permitted limits ofscale and corrosion.

The quality of cooling tower blow-down waterdepends on the water chemistry in the CW system,CW treatment provided, COC and rate of blow-down. Typical details are given in Table-II.

TABLE-II: TYPICAL COOLING TOWER BLOW–DOWN QUALITY

Constituent Quantity

Total dissolved solids 350-800 mg/l

pH 7.8-8.8

Suspended solids 100 mg/l

Oil and grease 5 mg/l

Free available Chlorine 0.3 mg/l

Permissible limits of Cooling tower blow-downeffluents, if disposed of separately, are given inTable-III.

TABLE-III: PERMISSIBLE LIMITS OFCOOLING TOWER BLOW-DOWNAS PER CPCB NORMS


Total Chromium 0.2 mg/l

Zinc 1 mg/l

Phosphate 5.0 mg/l

Free available Chlorine 0.5 mg/l

Other corrosion Limits to beinhibiting materials established on

case-to-case basis

3.2.1 PERMISSIBLE LIMITS FOR TEMPERATURE-RISE (OF COOLING WATER IN THECONDENSER) AS PER CPCB NORMS

● New thermal power plants commissionedafter June 1, 1999:New thermal power plants, which will beusing water from rivers / lakes / reservoirs,

shall install cooling towers irrespective oflocation and capacity. For thermal powerplants which will use seawater for coolingpurposes, the condition below will apply.

● New projects in coastal areas using seawater:The thermal power plants using sea watershould adopt suitable system to reduce watertemperature at the final discharge point sothat the resultant rise in the temperature ofreceiving water does not exceed 7°C over andabove the ambient temperature of thereceiving water bodies.

● Existing thermal power plants:Rise in temperature of condenser coolingwater from inlet to the outlet of condensershall not be more than 10°C.

● Discharge point guidelines:The discharge point shall preferably be locatedat the bottom of the water body at mid-stream for proper dispersion of thermaldischarge. In case of discharge of coolingwater into sea, proper marine outfall shall bedesigned to achieve the prescribed standards.The point of discharge may be selected inconsultation with the concerned StateAuthorities. No cooling water discharge shallbe permitted in estuaries or near ecologicallysensitive areas such as mangroves, coral reefs/spanning and breeding grounds of aquaticflora and fauna.

3.2.2 DISPOSAL OF COOLING WATER

● CW blow-down should preferably be usedfor ash disposal in coal-fired plants. Its usefor ash disposal will lead to reduced make-up water requirement for AHP andconsequently lower demand of raw water.

● Cooling tower blow-down is normallycontaminated with treatment chemicals. Itgenerally has high hardness / dissolved solids.


The flow quantities being very high, it canbe effectively used to dilute othercontaminants like oil and pH, in CentralMonitoring Basin (CMB).

● In case wet ash disposal is not practised (andalso in case of combined-cycle plants, whereash disposal is not applicable), the CW blow-down, being of huge quantity, cannot befully utilized without treatment. In such asituation, CW blow-down needs to bedisposed of directly from the plant.

● Alternatively (if zero-discharge is envisagedin the strict sense), CW blow-down water(after mixing with other effluents in theCMB) needs to be treated for recycle. Thefollowing treatment is required:

Clarification and subsequent use of clarifiedwater for the following applications:

� For dust suppression in coal handlingplant.

� Service water applications.

� Boiler blow-down cooling.

� Supply to other consumers of theclarified water (except DM Plant, Potablewater).

� Fire protection system make-up.

Treatment of clarified water in RO systemand further use of the permeate in thefollowing systems:

� Supply to DM Plant

� CW make-up

� HVAC system make-up

Since the CW blow-down flow rate is veryhigh, its treatment by RO is a very costlyproposal, and should be avoided as far aspossible, unless the additional cost can bejustified.

The RO concentrate, in this option of zero-discharge, requires to be evaporated inevaporation pond, to be specifically providedfor this purpose.

3.3 Wash Water Effluents

Some of the utilities have a practice of using servicewater for washing dust from boiler area and TG hallto improve house keeping. The following areas ofthe plants are generally provided with washingfacilities:

i) Boiler area

The wash water discharge is likely to beapplicable for a period of 2-3 hours in thegeneral shift. This wash water is contaminatedwith ash. Wash water is collected in localsump and subsequently pumped to ETPsump for further treatment in Lamella / Tubetype clarifiers. The clarifier supernatant isdischarged to the CMB.

ii) Turbine hall ground floor

The wash water discharge is likely to beapplicable for a period of 2-3 hours in thegeneral shift. This wash water is contaminatedwith suspended matter. Oil content in thewash water is minor depending on the areato be washed. It is reduced to negligible valueafter dilution effect of other effluents.

Wash water is collected in local sumps andsubsequently pumped to Lamella / Tube typeclarifiers. The clarifier supernatant isdischarged to the CMB.

The sludge generated in the lamella, providedto remove suspended matter from washwater effluent, is very little. Layout permitting,it is recommended to drain it to the pre-treatment plant sludge sump, to simplify itsdisposal. Alternatively, the sludge quantitybeing little, it can be disposed of manuallyin an acceptable way.

iii) Wash water from buildings

It is of minor nature. It drains to respectivebuilding drain system and does not join theEffluent Treatment Plant (ETP) for theprocesses described in this paper.


3.4 Filter Back-Wash

Back-wash water from the gravity or pressure filtershas up to 1500 ppm suspended solids. The back-wash water may be recirculated back to the clarifierto minimize wastage of water.

3.5 Ash Slurry Water

Ash slurry water is generally disposed of from the ashpond. About 80% of water supply to ash slurrysump may also be recovered from ash pond, for reusein the plant Ash disposal system. Ash water recoveryis generally delayed, after plant commissioning. Thedelay period can be from 6 months to 2 years. Therecovered water has significant quantity of suspendedand dissolved solids. It can be, however, used formake-up to the ash water sump for ash disposal

If cooling water blow-down has been used for make-up to the ash handling system, the situation changesafter AHP recovery starts. CW treatment systemcapability permitting, the COC can be furtherincreased, leading to reduced raw water demand forthe plant. The discharge standards of ash pondeffluents are given in Table-IV.

TABLE-IV: DISCHARGE STANDARDS OF ASH PONDEFFLUENT AS PER CPCB NORMS

Parameter Limit

pH Valve 6.5-8.5, preferably > 7Suspended Solids 100 mg/lOil & Greases 20 mg/l

3.6 Coal Pile Run-off

Coal pile run-off quantity is site-specific, primarilydepending on the rainfall and the coal yardtopography. If site conditions suggest significant coalpile run-off, it needs to be collected in a below-gradepond. Coal pile run-off may have significant quantityof suspended matter. After a reasonable period forsettlement of the suspended matter, the effluent(consequent to the rain) will be discharged to theplant drain system.

4. LIQUID-EFFLUENTS HAVINGHIGH-DISSOLVED-SOLIDS

High-dissolved-solids-content liquid effluents referto effluent streams having high ionic loads. In powerplants, this category of effluents will typically includethe following:

● Waste from DM / CPU neutralization pit.

● Boiler / HRSG blow-down

● CW blow-down.

4.1 Recommendations for DM Plant / CPUneutralization waste / chemical waste

DM Plant regeneration waste (as well as CPUregeneration wastes if CPU has been provided) iscollected in the neutralization pit (NP). It is normallyself-neutralized. Dosing system is also provided forNP, to neutralize the regeneration waste, in case itis not self-neutralized. The NP effluent is pumpedto CMB. Typical analysis of DM-regeneration wasteis given in Table-V.

TABLE-V: TYPICAL ANALYSIS OF DM-REGENERATION WASTE


Total Dissolved Solids 4400-6000 ppmpH 6.5-7.5Suspended solids 100 ppm

5. OILY EFFLUENTS

In normal operations, oily effluents are not expectedfrom areas of the power plant other than the fuel oilhandling area. Oily water effluents with oil contentgreater than 20 ppm may arise from transformerareas during abnormal conditions such as fires /accidents etc.

The major sources of oily effluents are:

i) Oily Effluent from fuel oil storage area.This effluent should be collected in local


sump. Oil is separated (up to 10 ppm ) inthe oil/water separator. The treated water canbe pumped to CMB for further disposal.

ii) Floor wash water effluent from fuel oilunloading, storage and pumping areas.

iii) Oil leakage / spillage from the turbine / BFPlube oil system and workshop.This effluent, if any, is insignificant andrequires to be collected in local trays / wipedwith cotton waste.

iv) Generator Transformer area effluent.The effluent from the generator transformerarea originates either from rain or consequentto the operation of fire protection system. Asump is provided to collect this water fromtransformer area. Experience shows thatnormally there is no oil leakage from thetransformers.

In case of fire, water is sprayed in the transformerarea. This water drains to the sump. In the event ofoil leakage during fire, both oil and water drain tothe sump. The following is recommended for disposalof the effluents from the generator transformersump:

a) If there is no oil leakage, pump the sumpwater to plant drain system.

b) In case of oil discharge consequent to thefailure of the oil system of the transformer,the water is first discharged to the plantdrain system. When the oil starts coming, oilbeing above water, it should be collected inthe drums.

6. BOILER BLOW-DOWN

Boiler blow-down rate is a function of boiler feedwater system water chemistry program. Blow-downrates of 1-3% (of main steam flow rate) have beenused. A value of ½ % for modern high-pressureboilers is not unusual these days.

The quality of boiler blow-down water depends onthe water chemistry of power cycle system and the

rate of boiler blow-down. Typical details are given inTable-VI.

TABLE-VI: TYPICAL DETAILS OF BOILER BLOW-DOWN FOR 250 MW UNIT


Suspended solids 166 ppm

TDS 100 ppm

pH 8.9

Oil and grease 0.2 ppm

Silica 0.2 ppm as SiO2

Permissible limits of boiler blow-down effluents, ifdisposed of separately, are given in Table-VII.

TABLE-VII: PERMISSIBLE LIMITS OF BOILERBLOW-DOWN EFFLUENTS IFDISPOSED OF SEPARATELY


Suspended solids 100 mg/l

Oil and grease 20 mg/l

Copper total 1.0 mg/l

Iron total 1.0 mg/l

Free available chlorine 0.5 mg/l

Boiler blow-down does not require any treatment,and it can be directly pumped to CMB.

7. SEWAGE / CANTEEN WASTE

The sewage from the plant buildings and canteen isgenerally led to sewage treatment plant. The treatedwater can be used for horticulture. It is recommendedthat this effluent not be mixed with process effluents.

8. EFFLUENTS GENERATED DURINGCOMMISSIONING AND MAINT-ENANCE ACTIVITIES

Cleaning effluents from boiler auxiliaries are likelyduring commissioning and maintenance stages. This


is a planned commissioning/maintenance activity,and it is done only when the plant is under shut-down.

The maximum frequency of washing for Air-pre-heater and Electrostatic precipitator could be once ayear. The acid cleaning activity and alkali-boil-outoperation are commissioning activities.

It is not advisable to provide permanent installationsfor collection of these effluents. The effluentcharacteristics for the above operations being uncertain,predefined treatment, if provided, may not workefficiently. Temporary arrangement for effluentcollection and disposal is considered appropriate andjustifiable. It is recommended that temporaryarrangements be provided for disposal of these effluents.

9. COLLECTION AND FINALDISPOSAL OF LIQUID EFFLUENTS

Each category of effluents is generally treatedindividually for removing oil, suspended solids andpH correction, if these parameters cannot be managedto meet the final disposal limits. All effluents arecollected in the Central Monitoring Basin (CMB)before disposal from the plant.

The CMB is generally in two compartments. Whileeffluents of one compartment are under disposal,after final pH correction, the other compartment isunder filling. Each compartment is generally sizedfor 2 hours inflow capacity. Discharge pumps areprovided in the common compartment.

Acceptable limiting parameters of specific effluentshave been specified above for CW blow-down, boilerblow-down and ash-pond effluents. However for theeffluents collected in the CMB, no specific effluentlimits have been given for power plants. Thefollowing limits are generally specified by plantowners for coal-fired power plants:

Parameter Limit

Suspended solids : 100 ppmOil and grease : 10 ppmResidual chlorine : 0.5 ppmpH : 6.5 to 8.5

The suspended solids, oil / grease and residualchlorine parameters of the CMB effluents, aregenerally stable, whereas pH may have marginalfluctuation, thereby requiring its correction. To takecare of this aspect, on-line pH indicator is providedin the CMB discharge piping. Other parameters viz.suspended solids, conductivity, oil / grease andresidual chlorine, are tested in the laboratory.

10. EFFLUENTS FROM COMBINED-CYCLE POWER PLANTS (CCPP)

The pollution control requirement for CCPP issimilar to that for coal-fired plant except for thefollowing:

i) Since coal handling and ash handling packagesare not applicable, the following effluents arenot generated in CCPPs:

● Coal pile run-off.

● Ash-pond effluent.

● Boiler-area floor washing effluent (sincethe washing requirement is insignificant).

ii) In the absence of Ash Handling plant, thesludge from clarifiers cannot be disposed ofwith the ash slurry. Thickener and centrifugewill be required if the sludge quantity issignificant.

iii) The permissible limits for effluents:The following limits have been specified byCentral Pollution Control Board (CPCB) forGas / Naphtha based power plants:

Parameter Limit

pH = 6.5-8.5Temperature = As applicable to

coal-fired plantsCopper total = 1 ppmIron total = 1 ppmZinc = 1 ppmChromium total = 0.2 ppmPhosphate = 5 ppmTotal suspended solids = 100 ppm maxFree chlorine = 0.5 ppm max


Oil and grease = For discharge toinland surfacewater: notexceeding 10 ppm.For discharge tomarine coastalareas: notexceeding 20 ppm

11. ZERO-DISCHARGE ETP

11.1 Classification of Effluents

The effluents from the plant can be classified in thefollowing two broad categories:

i) Process effluents, as from CW system, boilerblow-down, water treatment systems, floorwashing, building effluents etc. These effluentshave bearing on the size and operatingconditions of the systems concerned.

ii) Effluents such as storm water drain. Thistype of effluent is site-specific. It is generatedin large flow volumes in a short time.

11.2 Considerations for Applying Zero-Discharge Concept

The following aspect of practical nature needs to beconsidered when applying this concept to the powerplants:

In case of CCPPs and coal-fired plants with dry ashdisposal (and also in case of coal-fired plants witha low value of condenser cooling water COC), theliquid effluents from the plant become surplus evenafter all justifiable reuse in the plant. In such asituation, the zero discharge can be attempted onlyat a very high cost.

In view of the above, it is the considered view of theauthors that the need for zero discharge should bespecified with the considerations in respect of returnon investment.

12. CONCLUSION

The provision of ETP for power plants is arelatively new requirement. For earlier plants, thegeneral requirement was plant design to meetlimiting parameters of the liquid effluents fromthe plant. Recent statutory changes include revisedlimitations on the condenser cooling water returntemperature.

Permanent provisions for handling and treatmentof effluents, which are maintenance /commissioning related, is not economically justified.It is our view that such facilities should be oftemporary nature and activity-specific. Also if thequantity of some effluent is insignificant, permanentinstallations for treatment and disposal of sucheffluents will only increase the initial cost of theplant, and may not be available for use (due toinadequate maintenance) when really needed. Forsuch cases, mobile tankers could be provided tocollect the wastewater and treat in the ETPlocated elsewhere.

The supply of raw water to power plants, isbecoming more and more restricted. In view of this,some of the projects are being cleared based onprovision of zero liquid discharge from the plant.This requirement needs a re-look in view of the needfor consequential provisions, if it is to be followedliterally.

Bibliography

1. htpp://www.cpcb.nic.in/standard63.htm

2. http://envfor.nic.in/legis/legis.html

3. Minimal National Standards (MINAS).

4. P. Shandilya and S S Phogat, "Environmentmanagement in power plants", 3rd InternationalConference, World Council of Power Utilities,New Delhi, 21-24 Nov. 2001.


Mr. Prabhakar Shandilya obtained his B.Tech(Chemical Engg.) degree in 1966 from I.I.TKanpur.

Mr. Shandilya worked with BIRD & Co. Ltd intheir sales, design, installation departments forwater treatment equipment for 8 years beforejoining JK Synthetics Kota in their Acrylic Fibre

Plant as a process engineer for 1 year. He joinedBHEL in 1976 and has been responsible fordesign & execution of BOP (Balance of Plants)for various thermal power stations. His corecompetence is in the area of water & wastewatertreatment for power stations. He was trainedabroad with KWU Germany for one year. Heretired as General Manager & Head (MAX) fromPEM, New Delhi, in May 2005.

Mr. Shandilya is currently working as VicePresident (Engg) with LMZEIL, Delhi, and isresponsible for the execution of BOP for NTPCSipat / Barh 3 X 660 MW Power Stations, andcontract closing of HIL Dahej 60 MW CaptivePower Station.

Mr. S.S. Phogat obtained his B.Sc. (Engg) degreewith Honours in Mechanical Engg., from thePunjab University.

Mr. Phogat joined BHEL-Bhopal, in Power StationEngg Dept., in the year 1970 and was transferredto erstwhile Consultancy Services Division (now

PEM) in 1974. He underwent on-the-job trainingwith M/s Siemens, Germany, for one year.Presently, he is working as Addl. General Managerand Department Head of Mechanical AuxiliaryDepartment in PEM, and is responsible forProject Engineering of Balance of Plants.

Mr. Phogat has been actively involved in theProject Engineering of Power Cycle Systems,Plant Layout and BOPs. He has authored severaltechnical papers on Air-Cooled Condesers,Thermal Insulation, Tube-Cleaning and DebrisSystems, Cathode Protection, Piping and PumpingSystems, Effluent Treatment Plants, WaterTreatment etc.


Mr. G.S. Mahal obtained his B.Sc. (Engg.) degreewith Ist Class Honours in Mechanical Engg fromthe Punjab Engineering College, Chandigarh in1972. Thereafter he obtained his M.E degree fromthe Birla Institute of Technology & Science, Pilaniin the year 1974.

Mr. Mahal joined BHEL in 1975 as E.T in theerstwhile Consultancy Services Division (nowPEM). He has been responsible for Design &Engineering of Material Handling Systems ofThermal Power Stations and has more than threedecades of experience in the related fields ofMaterial Handling Systems. He is presentlyworking as Addl. General Manager in PEM.

Mr. Mahal is a member of the national level BISsubcommittee ME 7 on Material HandlingSystems.

Mr. S. Balaji obtained his Engineering degree inMetallurgy from the Indian Institute of Science,Bangalore, in 1983, winning the Prof. BrahmPrakash Medal. Earlier he had graduated in B.Sc.

(Chemsitry) from the Univ. of Madras in 1980.He was trained abroad under the Colombo planto acquire a P. G. Diploma in Management ofEnvrionment from the Maastricht School ofManagement, The Netherlands in the year 2000.

Mr. Balaji joined BHEL as E.T in 1983 at Trichy.Since then he has been working in variousfunctions such as Lab, QC, Quality Management,Health, Safety & Environment. At present, he isworking as Dy. General Manager in the Enggfunction in Water Chemistry section of PEM.

Mr. Sudhir Bhartiya graduated in MechanicalEngineering from MM Engg College, Gorakhpur,in the year 1995, and went on to obtain a P.G.Diploma in Thermal Power Plant Engg fromNTPI Delhi.

Before joining BHEL in 1998 as E.T, Mr.Bhartiya worked in the field of power plantoperations in Bihar State Electricity Board. Since1998, he has been working in the field of waterchemistry and has been mainly dealing withpretreatment plant, effluent treatment plant,condensate polishing unit and Demineralizationplant for different power plants. He is currentlyworking as Sr. Engineer in Engg function in theWater Chemistry section of PEM.


INNOVATIONS — FROM BHEL

DESIGN AUTOMATION OF 500 MWCONDENSER USING KNOWLEDGE BASEDENGINEERING — AN MOU PROJECT

Knowledge Management has been identified as oneof the key corporate strategies to automate totalprocess of product design including release ofmanufacturing drawings and all engineeringdocuments,with a view to meeting the demand forshorter deliveries and tight commissioning schedulesof power projects. In order to meet this objective,an MOU project "Design Automation of 500 MWCondenser Using Knowledge-Based Engineering"was taken up by BHEL- HEEP, Haridwar, to beginwith. The project has now been successfullycompleted.

Steam surface condenser is one of the major keyequipment in a thermal power plant. The designsolely depends on the site conditions vis-à-vis theplant layout, cooling water chemistry and the typeof cooling water system.

In this project,the total automation of condenserdesign has been done capturing design inputs rightfrom the tendering stage up to the contract executionstage. A software tool kit has been developed basedon custom-made program logics and KnowledgeFusion tools to help design condenser in an automatedfashion. A wizard has been developed incorporatingdeveloped Graphic User Interfaces (GUIs), integratedlegacy software to perform thermo-hydraulic designand for generation of performance curves, detaileddesign calculations, 3-D parametric models ofcondenser with 30 manufacturing groups comprisingnearly 1000 parts, preparation of engineeringdocuments comprising 2-D manufacturing drawingsfor the 30 manufacturing groups, 14 erectiondrawings, 3 customer drawings and 1 tender drawing(equivalent to nearly 3000 A4 drawings). All 2-Ddrawings are in AutoCAD format. Weightcalculations, generation of BOM, generation of

Datasheets and Cost Estimations have also beencovered.

With the completion of this project, the knowledgeof design codes, handbooks, knowledge assimilatedthrough collaborators, in-house R&D developments,design improvements, competitor's practices includingrich experience of work-force in condenser area—allthese have been integrated using proper logistics soas to avoid (or to keep at bare minimum) humanintervention during total process of design, in orderto evolve various design alternatives and selectoptimum design.

The effective use of this project will yield optimizedcondenser designs with reduction in design cycletime & cost and improvement in quality & deliveryby way of use of modern IT tools.

DEVELOPMENT OF HYDRO TURBINEBLADE/RUNNER MODEL THROUGH RAPIDPROTOTYPING TECHNIQUE

BHEL is the largest manufacturer of hydro turbinesin the country. At present, scaled models of each andevery new machine is made to demonstrate the

3-D Model of 500MW Condenser Assembly


hydraulic efficiency and related parameters, prior tothe manufacturing process. These models are beingmade through a lengthy and time-consuming process.Rapid Prototyping technique is widely being adoptedinternationally by major manufacturers for cycletime reduction in engineering especially when newproducts are introduced. In line with this trend, thefeasibility of adopting the Rapid Prototyping (RP)technique for making metallic models of HydroTurbines Blades/Runners was takenup in BHEL.

The Runner blade being the most critical component,was modeled through the RP technique. Crown &lower ring were made by conventional machining.Section profiles and solid model of the hydro turbineblade were generated from the drawings usingIDEAS software. Solid models of the matchingcrown and the lower ring were also generated forassembly checks.

Using the solid model of the Hydro Turbine blade,RP master of the blade was made by theStereolithography technique. This was inspected byprofile checking templates and also by 3D co-ordinate measurement equipment. A silicone rubbermould was made, using the RP master as thepattern, and wax patterns were made using thismould. Stainless steel blades were made using thesewax patterns by investment casting.

By adopting Rapid Prototyping technique for makingthe blades and using conventional method for

making crown and lower ring, savings in time andcost were achieved. A common 3D solid model canbe used for manufacturing and CFD analysis,thereby minimizing discrepancy in the theoreticaland experimental results.

11kV / 750 MVA (0.25 SECONDS)WITHSTANDING EPOXY TERMINALBUSHINGS FOR HIGH-VOLTAGE ACMOTORS DEVELOPED

Epoxy terminal bushings are one of the critical itemsof a high-voltage ac motor, as they carry high voltageand high current. Current level withstandingrequirements of these high-voltage bushings areincreasing day by day from customers. BHEL, aleading supplier of high-voltage ac motors in thecountry, has been, hitherto, importing these bushings.In order to indigenise these bushings; BHEL hasnow developed 750 MVA (0.25 seconds) fault levelwithstanding epoxy terminal bushings for 11kVhigh-voltage ac motors so as to meet the marketdemands.

A number of designs were made with differentvariants, and finally two designs, viz. (a) circularcollar type and (b) elliptical collar type, werefinalized to meet the requirements of existing terminalbox manufactured at BHEL. An optimum processcycle was established to process and manufacturesample bushings.

Routine and type tests were conducted on thesebushings at ERDA, Vadodara ,and Corporate R&DDivision of BHEL. The bushings successfully

Circular Collar Type Bushing


withstood the following specified tests to meet therequirements of 11 kV insulation system—impulse,partial discharge, power frequency, breakdown voltage,tan delta, capacitance and short-time current rating(at 44 kA for 0.25 second).

With this indigenous development of expoxy-basedterminal bushings, BHEL would be able to use thesebushings in all the non-flameproof high-voltage acmotors of 11 kV/1200 A rating, and thereby save inforeign exchange.

DISCHARGE CURRENT TEST ON HV SERIESCAPACITOR — FACILITY ESTABLISHED INBHEL

BHEL has developed HV series capacitors for 400kV series compensation project, for the first time inthe country. IEC 60143 standard calls for somespecial tests like Discharge Current Test for HVseries capacitors used in long transmission lines.This test facility is not available indigenously, andeven getting this test conducted at reputedInternational Testing agencies is beset with problems.In view of this, BHEL undertook and implementedestablishing of this test facility in-house for the firsttime in the country, to meet Discharge Current Testrequirement of our esteemed customer, M/s PowerGrid Corporation. This test facility was establishedwith internal resources mobilized within BHEL-Bhopal.

The testing was completed successfully in the presenceof the representative of the Customer.

Thus, BHEL has added one more new test facilityto the series of new test facilities it has been

developing for conducting special tests in-house, inaccordance with International Standards.

EVALUATION OF INHIBITORS FOR SS 304AND ADMIRALTY BRASS CONDENSER TUBEMATERIALS IN CONTACT WITH RIVERWATER

Due to increase in industrialization, there isdeterioration in the quality of surface water, but thereis no alternative except to use the naturally availablewater for cooling purposes. Natural water containsdissolved solids, gases and sometimes colloidal orsuspended matter. All these impurities affect thescaling / corrosion properties of the water in relationto the metals with which it is in contact. This leadsto many operational problems like corrosion, erosion,scaling, fouling etc. Where the available source ofcooling water is from a river, either SS 304 or copperalloys is used as tube materials in power plantCondenser. However, due to bad quality of water,these materials, though resistant to corrosion, are alsoprone to corrosion attack. In order to minimize thecorrosion, commercially available inhibitors are dosedinto the cooling water. Addition of these inhibitorssubstantially reduces the corrosion rate of the tubematerials, thus increasing the life of the condensers.Performance evaluation of various inhibitors for SS304 and admiralty brass was recently taken up atCorporate R&D Division of BHEL.

Elliptical Collar Type Bushing

Capacitor undergoing Discharge Current Test


Three commercial inhibitors were evaluated foradmiralty brass and cupro-nickel (90/10) in rivermedium, in static as well as dynamic conditions.Electrochemical experiments were conducted to assessthe corrosion rates of these tube materials with andwithout the addition of inhibitors. The optimumconcentration of the inhibitor was found to be0.1%. In order to carry out the corrosion experimentsunder dynamic conditions, special probes were usedin the dynamic corrosion test rig to evaluate theperformance of the inhibitors through linearpolarization technique. The corrosion current wascontinuously measured and recorded in the dataacquisition system.

Results :

All the inhibitors performed well under dynamiccondition and reduced the corrosion current largely,exhibiting an efficiency of nearly 99% on Copperalloys. In river water medium, SS 304 has highcorrosion resistance compared to copper alloys, andhence addition of inhibitors is not required. Forcopper alloys, any one of the three inhibitors testedcan be used, based on the economic considerations.

The studies conducted and the data collected, shallbe useful to diagnose, analyse and interpret variouscooling water chemistry related problems incondensers. Appropriate inhibitor can also berecommended to minimize the corrosion of condensertube materials.

250 LPD SOLAR WATER HEATING SYSTEMWITH HEAT PIPE COLLECTORS DEVELOPED

Based on the successful development of 100 LPDSolar Geyers (as already reported in the December2004 issue), and also from the market study, therewas a need to develop higher-capacity DomesticSolar Water Heating System to cater to the differentsegments of customers. Hence, development of 250LPD Solar Water Heating System with heat pipecollectors was taken up. As a result, two prototypeshave been developed and installed for long-termtesting at the Corporate R&D Division. The systemdelivers 250 litres of hot water per day at atemperature of 60 to 700C, depending on the solarintensity.

The 250 LPD system based on heat pipe collectoris much more efficient as compared to a standardthermo-syphon system, as there is no heat loss dueto reverse circulation during the night, since the heatpipe acts as a thermal diode,. Further, the systemdeveloped does not require interconnecting insulatedpipeline between storage tank and the collector array.

The 250 LPD System developed will meet thedemand of the higher-end domestic customers aswell as Large Solar Water Heating Systems. Thesesystems can be connected in series and parallelcombination to meet the required capacity of LargeSolar Water Heating System, i.e. four systems of250 LPD can be supplied for a 1000 LPD system.The system can be installed close to the utility point,thereby reducing hot water loss in the pipelines.

Dynamic Corrosion Test Rig 250 LPD Solar Water Heating System


RECENT MAJOR ACHIEVEMENTS OF BHEL(during March'06-August'06)

ORDERS BAGGED

Overseas

● Achieved yet another breakthrough in theinternational market by winning its secondconsecutive order from Ethiopia for 230 kVsubstations on EPC basis from EthiopianElectric Power Corporation (EEPCO). Lastyear, BHEL won a similar contract, fundedby the World Bank, for substations, makinginroads in Ethiopia. BHEL has won thecontract outbidding Chinese and othermultinational companies. The project isfunded by the Kuwait Fund. The contractagreement for this project, signed betweenBHEL and EEPCO, is part of anelectrification programme initiated by theEthiopian Government in the Afar State ofEthiopia. BHEL's scope of work in theproject includes design, supply, erection, civilconstruction and commissioning of 230 kVSemera and Dichoto substations. Thesubstations are to be completed in a scheduleof 18 months.

● Won a prestigious contract for setting up 230kV substations on EPC basis in Bangladesh.Outbidding Chinese, Malaysian and IndianCompanies, BHEL has won the contract forsupply and installation of a 230 kV substationat Baghabari Power Plant and extension of a230 kV substation at Ishurdi. The AsianDevelopment Bank (ADB) funded contracthas been placed on BHEL by Power GridCompany of Bangladesh. This is the firstorder for BHEL for substations in Bangladesh.BHEL's scope of work in the project includesdesign, supply, erection, civil constructionand commissioning of a 230 kV substation atBaghabari and expansion of Ishurdi substation.

● Secured two prestigious contracts inAfghanistan as part of the company'saggressive plans for increasing business inSouth Asia. This also marks BHEL's foray inAfghanistan, which is a potential market.The two turnkey contracts have been securedby BHEL from Power Grid Corporation ofIndia for setting up a 220 kV substation atKabul, and from Water & Power ConsultancyServices (WAPCOS), India, for supply andinstallation of Electromechanical Packagesfor 42 MW Salma Hydroelectric Power Plantin Afghanistan.

● Achieved a breakthrough in the internationalmarket for transformers by securing aprestigious export order from Egypt. Won inthe face of stiff international competition,the order has been placed on BHEL by theEgyptian Electricity Transmission Co.(EETC). This is the single-largest order fortransformers ever received by BHEL and themaiden order for transformers from Egypt.This order is expected to give a fillip toBHEL's efforts at playing a more prominentrole in the large Power Projects planned inEgypt — one of the largest countries inAfrican continent. A wholly ownedGovernment organization under the Ministryof Electricity and Energy, Egypt, EETC isresponsible for Management, Operation andMaintenance of High and Ultra high VoltageElectric Power Transmission grids, all overthe country.

Domestic

● Secured contract for setting up a 500 MWthermal power plant (2x250 MW) in UttarPradesh. Reposing confidence in BHEL'scapability and technological excellence, Uttar


Pradesh Rajya Vidyut Utpadan NigamLimited (UPRVUNL) has placed orders forsetting up 2 units of 250 MW each (Units5&6) at Parichha TPS Extn. The project isslated for commissioning in fiscal 2009-10.With this, BHEL has maingtained its trackrecord of bagging most of the orders placedby UPRUVNL for power generatingequipment in Uttar Pradesh. So far, BHELhas commissioned over 9,000 MW of powergenerating sets in the state. These includethermal, gas-based, nuclear and hydro unitsof various ratings.

● Secured contract from India's largest PowerUtility, NTPC, for setting up the first unitof 490MW capacity, at its National CapitalThermal Power Project (NCTPP) Stage-II, atDadri in Uttar Pradesh. With this, NTPChas once again reposed confidence in BHEL'sproven technological excellence and capabilityin executing projects of this magnitude. Theproject is slated for commissioning in fiscal2009-10.

● Achieved a major success in the form of aprestigious contract for a 250 MW ThermalPower Station (TPS), bagged in the face ofstiff competition from Chinese and Koreanequipment suppliers. The contract has beenwon from Tata Power Company (TPC)under competitive bidding. Notably, this isthe first order secured by BHEL where theboiler will be designed to suit firing ofimported coal. TPC has reposed its confidencein BHEL by placing the order for design,engineering, manufacture, supply, erection,testing and commissioning of main plantpackage for Unit-8 of Trombay TPS inMaharashtra.

● Secured contracts for setting up two thermalpower projects in Rajasthan. Reposingconfidence in BHEL's technological excellenceand project execution capabilities, RajasthanRajya Vidyut Utpadan Limited (RVUNL)has placed orders, cumulatively valued at

Rs.842 Crore, for setting up a 250 MW unitat Suratgarh TPS and a 195 MW unit atKota TPS. The units are slated forcommissioning in fiscal 2008-09, followingwhich power situation in the state will easeconsiderably. With these contracts, BHELhas maintained its track record of bagging allthe orders placed by RVUNL for powergenerating equipment in Rajasthan. BHELhas so far commissioned over 2400 MW ofpower generating sets in Rajasthan.

● Secured contracts for the supply and erectionof Electromechanical equipment for twoseparate Hydro Electric Projects (HEP) inAndhra Pradesh, won in the face of intensecompetition from European and Chinesemultinational companies. The order wasbagged by BHEL under InternationalCompetitive Bidding (ICB), as its offer wasfound techno-economically the best.Cumulatively valued at nearly Rs.82 Crore,the orders for Nagarjunasagar Tail PondDam (2x25 MW) and Sriramsagar HEPExtn. (1x9 MW), have been placed onBHEL by Andhra Pradesh Power GenerationCorporation (APGenco) and are a measureof the customer's confidence in BHEL'sproven capabilities in execution of hydelpower projects.

● Secured order for setting up a Lift IrrigationScheme in Andhra Pradesh. The scheme willbenefit thousands of farmers of the state byirrigating 2.75 Lakh hectares of parched landand making it arable. Won against stiffcompetition, the order for the 5x30 MWKalwakurthy Lift Irrigation Scheme, hasbeen placed on BHEL by Gammon India,who secured the EPC contract for the projectfrom the AP Govt. BHEL is already executingStage-I of the Scheme, the order for whichwas placed on the company by PatelEngineering.

● BHEL is to set up on turnkey basis, anotherlignite-based power project in Gujarat for


GIPCL. The power plant will be equippedwith an eco-friendly, state-of-the-artCirculating Fluidised Bed Combustion(CFBC) Boiler, specifically designed to utilizelow-grade fuels like lignite, high ash coal,washery rejects etc. To this effect, GIPCL hasonce again reposed confidence in BHEL'sproven technological excellence and capabilityby placing an order valued at Rs.1200 Crorefor setting up 2 units of 125 MW each(Units 3&4) for Surat Lignite Thermal PowerStation Expansion project. The two unitswill be commissioned in a tight schedule of33 and 37 months respectively.

● Secured order for a captive power plant to beinstalled in a steel plant in Gujarat. Reposingconfidence in BHEL's capabilities, the EssarGroup has placed an order for a 110.6 MWGas Turbine Generator Set for an open-cyclecaptive power plant for the expansion of theexisting captive power plant at Essar Group'sHazira project. The supplies for the projectare slated for completion in a very tightschedule of just 14 months. For Essar, BHELis already executing an order for a captivepower plant of similar rating at Hazira. Theproject is in an advanced stage and isexpected to be commissioned shortly. Thesynchronization of these units will ensurecontinuous supply of 216 MW of qualitypower to the steel plant, which will be highlycost-effective for the customer, making theend product economical.

COMMISSIONING HIGHLIGHTS

● The first 500 MW unit at NTPC'sVindhyachal Super Thermal Power Station

(STPS) Stage-III, in Madhya Pradesh, hasbeen commissioned. With the synchronizationof the unit, 12 million units of electricitywill be added to the grid of the power-deficitstate, every day. Valued at Rs.2125 Crore,NTPC had placed the Main Plant Packagecontract for the 2x500MW VindhyachalSTPS - Stage III on BHEL, once againreposing confidence in BHEL's technologicalexcellence & capability in executing projectsof this magnitude. With the commissioningof this unit, the cumulative generatingcapacity of the power station has gone up to2760 MW. The second unit is also targetedfor commissioning by BHEL in fiscal 2006-07. On completion of Stage-III, thegenerating capacity of Vindhyachal STPSwill be enhanced to 3260 MW, making itIndia's largest power generating station.

AWARDS

● For outstanding export performance, BHELhas won the Engineering Export PromotionCouncil (EEPC)'s award, for the 16th yearin succession. Conferred on BHEL in thecategory 'Star Performer in 2004-05: PowerGeneration Equipment and Parts — LargeEnterprises', the award was presented by theHon'ble Union Minister for Commerce &Industry, Mr. Kamal Nath, on August 10,2006. Overseas business has been identifiedas a major thrust area by the company. Inthis direction, short-term and long-termplans have been chalked out which areyielding rich dividends. During the lastfiscal, BHEL booked the highest-ever physicalexport orders of Rs.3,348 Crore — a six-foldincrease over the previous year.

Rihand STPS—BHEL has supplied 2x500 MW sets.

Kayamkulam CCPP (350 MW).

Registration No. RN-27700/76

Bharat Heavy Electricals LimitedBHEL House, Siri Fort, New Delhi-110 049

Visit our Website at : http://www.bhel.com

Published by Bharat Heavy Electricals Ltd., and printed at VIBA PRESS PVT. LTD.,C-66/3, Okhla Indl. Area, Phase-II, New Delhi-110020. Tel. : 41611300, Telefax : 26386500

september 2006 issue, vol. 27 no. 2

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