MECTHERMAL SPRAY FACILITIES

Tech. Bulletin#06/103.00

For a developing county like India the need ofthe hour is to generate Power. This wouldaccelerate the rate of development.Simultaneously the search for clean power is amustandinthisregardHydropoweristheoneofthe feasible option. However there is a still a lotof untapped Hydro power and technologicallimitation should not come in between forrealizing this option and exploiting the same tothefullest.

A Hydro Power plant is a national asset and it isour duty to see that the components that aretherein should last its design life. Thisnecessitates us to look for the solutions. Themajor problem faced by this industry is that offcorrosionandwear.

Theproblemof erosionisperpetuatedduetosiltin India and has been found to be quite seriousin a number of hydropower stations, especiallyin those located in the Himalayan region. Thisproblem isaggravated due to increased silt sizeand contentandisresponsiblefor quick erosionand corrosion. Some of the power stationsaffected by silt-erosion damage in India aregiveninTable1

The components prone to erosion of thesepower stations are categorizedforlowandhighparticle impact energy. Since hydro-abrasion isa function ofmanydifferentparameterssuchassand-particle size, hardness, concentration,quantity, shape and velocity, in most cases, thisisminimizedbycontrollingtheseparameters.

COATING SYSTEMS FOR HVOF, PLASMA, FLAME SPRAY, ARC SPRAY & ROD SPRAY

The size of the particles is generally restricted to250 µm and velocity below 75 m/s to haveproper control on high-energy impact wear. Inspite of this, significant surface damages tohydroelectric machines and structures occur,thereby causing scratching and materialremoval. The surfaces exposed to abrasion aredivided into two main categories viz., lowenergy impact, such as profile portion of guidevanes and high-energy impact, such as runner,labyrinth & guard rings. Most of these are madefrom corrosion-resistant martensitic stainlesssteel&areconsideredlessresistanttoabrasion.

Protection of these components with suitablecoating is essential to minimise their damagedue to abrasion. Hydro power stations in Indiafacing regular damage due to excessive silt inwater.Legendfortheextentofdamage.A) Very severe; requires repair every two

monsoons, particle impact energy>10µJ.

B) Severe; requires repairs every 3 - 4monsoons, particle impact energy510 µJ.

C) Considerable; requires repair everyseven to eight monsoons, particleimpact energy<5µJ

In recent years, with thedevelopment of thermalspraying technology andin particular High velocityoxy fuel (HVOF) systemhas revolutionized thecoating industry. Thecoatings generated bythis system are proven tobe efficient against harshe n v i r o n m e n t a lconditions.

Since the coatings showadvantages in densityand bond strength itmakes it attractive form a n y w e a r a n dcorros ion res i s tanceapplications. Its highcoating quality resultsfrom the use of a hotcombustion-driven high-speed gas jet for thermalspraying.

These coatings haveenvironmental advantages compared tochemical ly/ electro-c h e m i c a l l y f o r m e dcoatings. depictso n e s u c h c o a t i n gprocess in progresswherein a hydro guidevane is being coatedusing HVOLF system usingtungstencarbidealloy.

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Fig 1

State Unit X MwHead(mm)

Speed(rpm)

Type ofTurbine

Extent ofdamageProject

Baira Siul

Maneri

Jaldhaka

Bhaba

Chilla

Dehar

Khatima

Lagyap

Machkund

Gamerbal

Kosi

Perriyar

N. Jhakri

Salal

Shanan

Ghanvi

Chibro

Pathri

Dhakrani

Dhalipur

UBDC

Kundah-II

Kundah-III

Kundah-IV

Umtru

Andanpur

H.P.

U.P.

W.B.

H.P.

U.P.

H.P.

U.P.

Sikkim

A.P.

J&K

Bihar

T.N.

H.P.

J&K

Punjab

H.P.

U.P.

U.P.

U.P.

U.P.

Punjab

T.N.

T.N.

T.N.

Megha

Punjab

3x60

3x30

3x9

3X40

4x36

6x165

2x13.8

2x6.3

4x17

3x3

4x4.8

4x35

6x250

3x115

1x50

1x11.6

4x60

3x6.8

3x11.2

3x17

3x15

5x35

3x60

2x50

4x2.8

4x33

240

180

148

900

32.5

282

18

530.5

245.4

137

6.1

374

428

95

488

374

110

11i

19.8

30.5

17.1

713

412

64

53.3

187.5

375

428.6

600

500

187.5

300

166.7

1000

600

1600

93.8

750

300

187.5

375

600

250

136.3

187.5

150

150

428.6

333.3

157.9

500

187.5

Francis

Francis

Francis

Pelton

Kaplan

Francis

Kaplan

Pelton

Francis

Francis

Bulb

Francis

Francis

Francis

Pelton

Pelton

Francis

Kaplan

Kaplan

Francis

Kaplan

Pelton

Pelton

Francis

Francis

Kaplan

A

A

A

A

B

B

B

B

B

B

B

B

B

B

B

C

C

C

C

C

C

C

C

C

C

C

Page 2

Tungsten carbide powders are widely used inthe HVOF spraying system. These are used toproduce dense, high hardness and excellentwear resistance coatings generally to combattheerosion and corrosion occurringinhydro power plants and pumps. Inapplications where abrasive orerosive wear resistance is of primaryimportance, WCCo with and withoutnickel or chrome is used. HoweverWCCoCr powders are preferredwhen high corrosion resistance isneeded.

Further it is very important to be verycareful in selecting appropriatecoating system as well as correctcomposition of coating material.Improper selection again will lead topremature failure.

Fig 1 : Coating of Guide Vane and Pelton Wheelusing HVOLF System & allied by Robotics forefficient coating processes.

Fig 2 : Un-coated Hydro Turbine Component (Runner)

Page 3

Hence it becomes very important to select aproper coating as the requirement changeswith different types of wear as the mechanismof wear involved is differentineachcase. Asanexample erosive wear is caused by plasticdeformation or brittle fracture, but bothmechanisms can also operate simultaneously.Erosion of metals usually involves plastic flowand the predicted wear rate, W, is inverselyproportional to H, the hardness of the surface.Hence a hard coating is preferred. Whilecavitation can be addressed by having acoating which is not only corrosion resistant butalso pseudo elastic in nature. For theseproperties Nickel based Tungsten carbidecoatingscanbeused.

Experiments carried by researchers haveshownthat Thermally sprayed Tungsten carbide withNickel binder have reduced cavitation as wellas slurry erosion and especially coatings donebyHighVelocityOxy-LiquidFuel(HVOLF)systemhave fared better. In general HVOLF coatingshaveimprovedthelifeofbarematerialasmuchasby86-143times.

1 2

WC-17Co WC-10Co-4Cr

Fig 3 : Salt Spray Tests carried out on coatings

Inadditiontotheturbinemachinery,preventionof corrosion in steel structure by atmosphericcorrosion is also of very importance and theyalso need to be suitably coated as the stakesinvolved are very high. The thermal coatingsystems developed by us to coat Zinc andAluminum have proven applicationstocombatthis very problem. The dam depicted in Fig 4 isatestimonytothatclaim.

There are many different types of corrosion aswell as many types of wear. Hydro industry isfacing all of them. One of the viable andeconomic options is to thermally coat suchcomponents and improve the life, because it isour collective responsibility to protect ournationalassts.

Fig 3 compares between two commonly usedTungsten carbide coatings while coating (1)has corroded while coating (2) containing Crhasnot.

PREVENTION OF CORROSION

Fig 4 : Three gates of 3000 m2 each, was Powder Flamesprayed at 80 microns, using 11tonne of Zinc Powder

27 yearsago&stillinexcellentcondition today

Fig 5 : A Rail Bridge protectedbyZn-Al Coating

Page 4

METALLIZING EQUIPMENT CO. PVT. LTD.E-101, M.I.A., Phase-II, Basni, Jodhpur-342005 (India)

Ph. : +91 291 2747601 Fax : 2746359E-Mail : mecpl@sancharnet.in / mecpl@vsnl.com

Web : www.mecpl.com

Delhi Mumbai Bangalore Secunderabad09312890541 022-24934370 0 040-278137609343861755mecdly@ndf.vsnl.net.in mecbby@bom5.vsnl.net.in mecpl.bgl@gmail.com mecsec@satyam.net.in

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