guide vane assembly mechanism & its function

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Guide Vane Assembly-Mechanism& its Function

By

Er NC Goyal

Former Chief Engineer,PSEB

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Guide Vanes

Guide Vanes arc installed in the turbine to regulate the quantity of water to the runner with change in load.

These are operated by two servomotors through guide vane operating mechanism via links & levers.

The servomotors get signals from Governor. The guide vanes are of aero flow section, which allows the

flow of water without formation of eddies in all positions. Depending upon silt flow, the guide vanes may be made of

mild steel or stainless steel with integral machined stems, which are drilled for grease lubrication of bushes.

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Guide Vanes

In case of mild steel castings, stainless steel liner is provided on the mating surface.

The guide vanes rotate in three bush bearings. The top two are located in the head cover&the

bottom is in the pivot ring slot. These bushes were earlier made of gun

metal/bronze& lubricated externally with grease through grease passage in the guide vane itself.

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Guide Vanes

The present trend is to make upper bushes grease lubricated & the bottom of ferobestos material, which is self lubricated.

During renovation of Bhakra right bank powerhouse, M/s. T.P.E. Russia has supplied Teflon coated bushes, which do not require any greasing.

Ferobestos self-lubricated bushes have not been successful in some silt -effected power plants & have been replaced with grease lubricated bushes at Chhibro of U.P.S.E.B.& Dehar of BBMB.

Each guide vane is provided with a thrust collar to carry weight of the vane & resist any upward thrust.

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Guide Vanes

Arrangement is also made for adjustment of vertical clearance between the vane & top cover / pivot ring.

A stuffing box is provided at middle &. lower stem to prevent leakage of water through clearance between bush & stem.

These days, generally chevron packing are provided on the middle stem & rubber cup sealing on the lower stem.

The present trend is to provide middle seal replaceable with out dismantling guide apparatus.

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Operating –Ring

Pit-Liner

GV

By-pass valve

Spiral

DT Cone

DT

Servo-motors

Head cover

Exit

Links & Levers

FrancisBlades

Francis Turbine

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Stay Vane

Spiral Case

Francis Runner

Wicket gate

Shaft

Draft Tube

Servomotor

Head Cover

Bottom Ring

Operating Ring

Francis Turbine

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Head Cover

Operating Ring

Servomotor

Wicket Gate

Bottom Ring

Lever

Frances Turbine Distributor

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Maintenance Of Guide Vanes

The maintenance of guide vanes is clubbed with the rest of the machine.

Over hauling may have to be done after 1-3 years in case of silt effected power plants & 5-7 years in other cases.

There may be power plants where arrangement exists for taking out guide vanes without dismantling machine. In such silt effected power plants, over hauling may be done every year. For all silt effected new power plants, provision should be made for removal of guide vanes without dismantling turbine.

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It can be done by lifting turbine cover & lowering the guide vanes with chain pulley blocks.

A good lathe & shaper machine for accurate machining of stem, face & mating surface of guide vanes is very necessary. Similarly a wooden / steel template made from a new guide vane profile is also very necessary for ensuring correct profile of vanes.

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MONTHLY MAINTENANCE:Check for free flow of grease to bushes.Check leakage from guide vane stem packing. If

excessive,then press/replace the packing. It may have to be replaced biannually in silt-

effected power plants.

12Diagram Of Guide Vane And Adjoining Turbine Parts

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Annual Maintenance:i) GREASING: - Normally, greasing of guide vane bushes is done daily

with the help of centralized greasing system from the top stem of each vane.

The grease should reach whole areas of the three bushes. During routine inspection, check from pivot ring that

grease is actually reaching the middle&lowest bushing. If not then dismantle the grease nipple & clean the grease

hole in the guide vane with a thin wire brush/GSL

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ii) MATING SURFACES: - First check mating surfaces with the help or straight

edge& carry out correction whenever due with the help of welding, grinding & polishing.

Check clearance between mating/bedding surfaces when in fully closed position without giving any pinch or servo motor oil pressure.

Normally there should be no gap.

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Maximum permissible is 0.1mm. Weld repair the same in case it is more than 0.1 mm.

After grinding, check with straight edge.Leakages through guide vane not only

waste water & hence energy but also makes the stopping of machine difficult.

III) Weld repair other surfaces also.

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IV) FREE MOVEMENT OF GUIDE VANES: -Measure upper& lower gaps(Fig).

These should be as per manufacturer's recommendations. Generally the minimum value is 0.1 mm & maximum value

is 0.4 mm. The values would increase due to erosion/corrosion. In silt-effected power plants, these would increase very much

but almost nothing can be done without taking these out. If vanes are not moving freely due to unequal upper & lower

gaps, adjust with adjusting bolts. It needs lot of care as other adjustments are also affected.

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v)STEM PACKING: - Check guide vane stem packing. Replace annually in silt

effected power plants. In silt free plants, take action based on experience so that no shut down on this score has to be taken during the year.

An idea of condition of stem packing can be had from leakages. If it is very less say less than 100 c.c. per minute, then packing is very tightly pressed. If it is more than 700-800 c.c. per minute, then the packing needs retightening/replacement.

Check tightness of all bolts & nuts. Since these are not easily approachable during annual maintenance, check by hammering.

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Vl) WEAROF STEMS AND BUSHES:- Check wear of stems&bushes by pushing some

guide vanes at random by a 5 ton jack in two directions & measure movement by dial indicator.

Though no remedial measure can be taken during annual maintenance, yet their condition would be known.

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Capital Maintenance/Over Hauling:-

Carry out all checking as described under annual maintenance thoroughly at appropriate stage of over hauling.

Greasing: Clean all grease passage & pipes with brush and kerosene oil thoroughly& ensure that no dried grease flake is sticking. Fresh grease should flow freely to all require places.

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Weld Repairs: Weld repair whole of guide vane. Machine, grind &

polish it to exact manufacturer's drawling. Check mating surfaces with a straight edge. The clearance between adjacent mating surfaces should

be nil. The profile should also be as per original. For this, check with template. Check up proper machining & dimensions of guide

vane stems.

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IF THE STEMS ARE NOT EXACTLY STRAIGHT AND OF CORRECT DIMENSION, THEN LOT OF DIFFICULTY WOULD ARISE AT THE TIME OF ASSEMBLY.

If lathe machine is not accurate, then check deformation as per Fig

If provision for S.S. liners has been made, then replace the same as the opportunity for next replacement would come only on next capital maintenance

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If provision does not exist, then for silt-effected power plants, consult manufacturers for making the provision at site.

At the time of final assembly, ensure that upper & bottom clearances are as per manufacturer's drawing/design.

If S.S.liners are provided then after replacement exact dimensions would be achieved.

Keep a record of damage to the guide vanes.

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Guide Vane Bushes

ANNUAL MAINTENANCE: Practically no annual maintenance is possible as these are in accessible without dismantlement of guide vanes. However ensure free flow of grease.

CAPITAL MAINTENANCE: - i) Check the play of guide vane stem in the bushes by

measuring outside dia. Of stem & inside dia. of bush. If the gap is more than permissible (Generally less

than0.2mm), replace bushes. Even if the gap is within limits & from experience it is

observed that by the next capital maintenance, it would exceed normal values,these should be replaced.

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Guide Vane Bushes

ENSURE ACCURATE DIMENSIONS AND CIRCULARITY 0F THE BUSHES OTHERWISE GREAT DIFFICULTY WOULD BE EXPERIENCED DURING REASSEMBLY.

A LOT OF GRINDING OF THE BUSHES ANDHOUSING HAVE TO BE DONE TO FIT THE BUSHES AND ENSURE FREE MOVEMENT OF GUIDE VANES.

IT IS BETTER TO SPEND SOME MORE TIME AT THIS STAGE THAN WASTE TIME LATTER ON DURING ASSEMBLY.

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Guide Vane Bushes

• ii) FITTING OF THE BUSHES: -The bushes are fitted in the bush housing provided in the head cover & pivot ring.

Different methods are adopted in different power plants& some of these are as follows:

a) At some places,these are shrunk by dipping them in liquid nitrogen & are then placed inside the housing. This process takes5-10 minutes.

b) At some places, these are shrunk by using dry ice. c) At some places these are press fitted with the help of

jacks screws,

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Guide Vane Bushes

While using method a & b exact dimensions may be ensured.

If outer dimensions are more, then due to shrinking, the bush would fit in the housing but great difficultly would be encountered while taking these out on next over hauling.

A very tight bush would unnecessarily cause high local stresses on housing & bush body.

In general ensure that proper interfacing is achieved.

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Guide Vane Mechanism

CONSTRUCTION IN BRIEF: The guide ring is fabricated or casted & is supported on

top cover. The rings, its links & levers are strong enough to

transmit power from servomotor to guide vane for normal & emergency closing without any distortion.

The simultaneous movement of all guide vanes during opening & closing is achieved with the help of guide vane operating mechanism.

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The top portion of each stem of guide vanes is coupled to regulating ring by means of links & levers.

The servomotor actuates the regulating ring, which in turn operates all the guide vanes through those links & levers.

The moving parts of the turning mechanism are grease lubricated.

The present day trend is to provide self lubricating PTFE bushes.

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Stops are provided to prevent movement of guide vanes beyond fully open & close position.

Shear pins are provided & designed to fail, if any foreign body gets entrapped between vanes during closing operation.

Stops are provided in the bush housing to limit angular movement of guide vane ,whose pin has broken to prevent its interference with other guide vanes.

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Similarly a lever stopper is fixed to the upper bush housing to prevent lifting of guide vane due to water pressure.

A cover is provided on guide vane lever through which vane is suspended.

This suspension enables accurate adjustment of guide vane top & bottom clearances.

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i) MONTHLY MAINTENANCE: Inspect for free flow of grease to all bushes, guide

rings, links & pins, etc.

ii) QUARTERLY MAINTENANCE: Examine the check nuts on guide vane connecting

links for tightness. Check the fixing keys of all the pins. Clean all links thoroughly.

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iii) CAPITAL MAINTENANCE: Disassemble all levers, bushes etc. & wash thoroughly. Check for any grooves or damage to journal surface of

levers. Repair by welding, grinding & polishing to nearest bore

dia. Check looseness of bushes, wear of pins etc. & set these

right. Finally clean & paint wherever required.

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Thanks for

Patient Hearing

guide vane assembly mechanism & its function

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