Hydro Electric Power Plants

4.2 Turbine Guide Bearings

 A number of turbine guide bearing designs are in use. These may be classified as follows:

i) Plain water cooled bearing

ii) Bath type with circular cooling turbines

iii) Bath type with cooling water tubes embedded in the pads.

iv) Rotating bath type.

v) Grease lubrication bearing.

In the case of plain water cooled bearings, either ferrobestos or rubber lined pads are used against a welded shaft sleeve. The ferrobestos lined bearing have given considerable trouble at one of the power station and these had to be replaced by rubber lined pads.

Small diameter cooling pipes embedded in bearing pads have a tendency to clog especially at the time of high silt contents resulting water starvation.

Complaints of excessive oil splashing have been received about the rotating bath type bearing. Grease lubrication bearings have a tendency to clog when in contact with the water and it is very essential to use grease with the right type of properties.

A number of cases of turbine guide bearing failures have come to notice. These are:

i) Starvation of oil in the bath

ii) Failure of cooling water due to clogging of pipe.

iii) Mal-functioning of instruments like RTDs, TSDs, oil level relay and flow relays etc.

To avoid failure of bearing due to cooling water tube, a new design of turbine guide bearing has been developed by BHEL, Hardwar. Two separate oil sumps are located in the top cover in diametrically opposite locations. These sumps, through pipe lines are connected to the oil bath of the bearing. The oil is circulated between sumps and the bath. Top cover being stationed just above the draft tube, separate to cooling water arrangement for bearing oil is not required. The bearing temperatures with this arrangement never go beyond 40oC to 42oC and this design is effectively working at Chila Power Station.

 4.3 Gland Seals

Normally two types of shaft gland seals are in use in different power stations:

i) Carbon or ferrobestos segment.

ii) Rubber flap type

 4.3.1 Carbon or Ferrobestos Segment Type

 The seal segments are housed in the stuffing box. Stuffing box being always in touch with the shaft is subjected to excessive wear and tear. The overhauling of the stuffing box becomes necessary when it is observed that consumption of cooling water has considerably increased or excessive water in top cover appears to be coming. In general maintenance of the seal is required to be done annually.

In the event of breakage or damage to a carbon segment it is advisable to replace the whole set of carbon segments. In very rare case only the damaged segment is replaced care must be taken to ensure that the axial thickness of the new segments falls within the limit size to ± 0.002” of the existing ring to which it is to be fitted.

All carbon segments and spacers are fitted to place and match marked on assembly. Whenever any part is replaced, matching marks / indents should be made after final assembly is satisfactorily completed. Whenever reassembly of the gland seal with existing gland ring or new ring is done it is important to ensure:

1. All carbon/ ferrobestos segments are carefully examined for any chipping or damage.

2. All stainless steel facings are flat and square with the gland sleeve and there are not steps at the facing joints.

3. Stainless steel facing and sleeve are completely free from grease.

4. Ensure proper bedding of segments with shaft sleeve.

5. All segments to segment and segment to stainless steel mating surface are completely free from grease.

6. All garter springs are assembled to obtain even tension all around.

7. Alignment of segments in the lower assembly is carefully checked with a hard wood peg for similar device before fitting retaining pins.

 4.3.2 Rubber flap type

Maintenance of rubber flap type gland seal is comparatively simpler and easier. Only precaution during assembly of rubber gland is jointing of the rubber seal in the proper way. The quality of rubber used plays a very important role for satisfactory performance of the rubber gland. In one of the recently commissioned power stations rubber gland seal used to fail very frequently. The cause of frequent failure was discussed and analysed to be lying in the quality of rubber. The problem after selection and use of proper quality of rubber is now over.

4.4 Guide vane servomotor

Normally main source of trouble is rubber seals which need to be replaced after a few years. Rubber seals should be replaced during annual maintenance. It is important that all the parts are match marked before dismantling so that reassembly is correctly done.

 4.5 Governor

Different types of governors are in use in different hydro power stations:

a) Mechanical governor can be classified as follows

i) Fly ball type

ii) Accelero technometric type

b) Governor employing magnetic amplifier.

c) Governor employing electro hydraulic amplifier.

The governor may require maintenance because of the following reasons:

i) Chocking of oil parts and throttles

ii) Wearing out of throttles due to which oil leakage becomes more and readjustment of governor becomes essential. In this case governor should be opened and all the throttles etc. should be cleared. Filters should also be cleaned, and after cleaning and reassembly governor parameters and characteristics should be readjusted so that there is no hunting of the governor.

4.6 Governing Oil System

The oil sump should be well cleaned and filled with filtered oil. The oil samples should be got tested for verification of the desired properties. Regular centrifuging of oil with the help of De-Laval type oil purifying machine would go a long way in enhancing the life of the oil. In certain cases oil retained its properties even up to 15 to 20 years of continuous use.

During annual overhauling OPU sump and pressure accumulator should be completely emptied and cleaned. The strainers should be inspected and repaired of necessary. The OPU pumps require maintenance when they develop excessive noise or vibrations. This may be due to some worn out bearing of the pump which would be replaced.

Another problem which has been faced in different power station is entry of water in the governing oil system. This problem was analysed in detail and remedial measures were taken at Chila Power Station. From following two sources the water can enter in the governing oil system:

1. From top cover, through oil leakage pumps which caters leakage of servo motor oil. Its sump being located well below the level of servo motors in the top cover may not be properly sealed, thus providing access to the top cover water which may ultimately be pumped in to the OPU sump.

2. In case of Kaplan turbine water may enter into the runner hub through cup seals. To eliminate first possibility the oil leakage unit delivery was isolated from the OPU sump and connected to a separate tank.

But for the second possibility there is no way except replacing blade seals if excessive water found in the Governor oil.  Daily check of the OPU sump oil sample and test of the sample is necessary to keep training of such possibilities.

4.7 Header

 In Kaplan turbine the oil header is required to supply governing oil to the runner servomotor and return oil to the OPU sump. Oil header has an oil guide connected with the rotating and servo tube. The servo tube has ports to receive return oil to the pipes coming from OPU sump. This tube is guided by three sets of bushes in the oil. Due to run out of the shaft these bushes had to press the servo tube. Failure of these had been very frequent in one of the recently commissioned power station.

Monitoring of wobbling of the servo tube with help of dial indicator may provide a guide line and save the bushes from further wearing. Remedial measures to reduce run out of the servo tube must be taken at this stage.

At the time of assembly of various parts of header proper match marking and dowelling is essential so that reassembly may be correctly done.

Due to failure of these bushes oil splashing occur which may drench the PMG, main & Pilot exciter and reduce the life expectancy of the windings and brush as such all out efforts should be made to prevent such happenings.

Turbine Auxiliaries

1. DPM

i) Inspect top cover drain system, overhaul the ejector and drainage pumps.

ii) Check pipe lines and valves. Replace gaskets and other parts, if necessary.

2. Oil Cooling Unit

i) Overhaul cooling pumps

ii) Attend all the valves and pipe lines for leakage.

3. Centralized Grease Lubrication System

i) Overhaul greasing pumps

ii) Check whole greasing lines. Replace worn out valves and gaskets etc.

iii) Check all the nylon pipes connected with the guide vane bushes. Replace damaged pipes.

iv) Check that all the guide vanes are receiving grease properly.

4. Oil Leakage Unit

i) Check the oil leakage unit overhaul the pumps.

ii) Clean tank and check that float is properly working.

iii) Checking all the pipe liens and valves for leakages.

 3 Requirement of effective maintenance

In addition to planning maintenance and implementing a suitable schedule (on the basis of seasonal water availability perhaps), the following items also require attention otherwise it may be difficult to keep to the schedules in practice:

1. Man Power Planning and arrangement is essential as without experienced and skilled staff any maintenance programme may fall.

2. Planning and arrangement of spares and consumable in advance so that time is not lost in re-commissioning the plant after the shut down.

3. The maintenance engineers should have in his possession all the erection and commissioning log sheets documents to establish a record of installed learances, parameters, alignment results, test characteristics of all the power plant equipment. These may be required at the time of diagnosis of the operational problems as well as defined maintenance purpose.

4. Log sheets of the previous maintenance exercise carried out on the machines. These may be required to compare with the clearances / settings / characteristics achieved during present maintenance.

5. History registers of all plant should be kept with records of all the abnormalities observed on the machine and details of action taken. This data can be used to as a guideline for future maintenance work at the power station.

6. Logging of the performance characteristics of the power plants on daily basis recording all the abnormalities and misbehaviours (if any) of the total plant observed during its generation programme from one maintenance exercise to another.

 4 Major maintenance problems of water turbines

 Some of the major problems encountered in the hydro turbines are damage in runners due to erosion, cracking and cavitation due pressure pulsation in draft tube, instability of operation at partial gate opening. Other serious issues include failure of turbine guide bearings, leakages of water through turbine guide bearings, leakage of water through guide vane seals and turbine gland seals. These problems are discussed in detailed in the following section.
 4.1 Runner

 4.1.1 Erosion due to silt

Erosion of turbine runners, guide vanes and other under water parts is a serious service problem especially in run-of-river schemes. The rivers in the Northern region of India carry significant silt loads especially during monsoon period so much so that hydro power stations are often closed down to prevent serious damage to the turbines parts and water passage.

Greater attention should, therefore, be paid to effective de-silting arrangements. Excessive wear and damage often occurs on the runner labyrinths, seals, guide vanes, butterfly valves, shaft seals and draft tube cone. Wear due to silt occurs so fast that the turbine units have to be taken out for repair every few months in some stations. The solution lies in the original specification of effective de-silting civil works however, this is of little practical help in the service environment. In the service environment the use of turbine parts coated with or made from material with harder and erosion resistant properties is the most practical option to be pursued.

At Chilla Project following measures of de-silting have been taken:

i) Silt extruders have been provided near under sluice gates of the barrage.

ii) Half a kilometre downstream of the head regulator, a silt ejector has been constructed in the bed of the power channel.

 4.1.2 Cavitation related cracking and wear of the runner

The problem of cracks in turbine rotors and Pelton buckets has been reported in few power stations. This can be due to following reasons.

i) Faulty design

ii) Poor metallurgy

iii) Metal fatigue

The phenomenon of cavitation occurs due to the vaporization of water in a zone of excessive low pressure. Cavitation damage can occur if the turbine has to operate at part flow. Limits of structural wear (metal removal) are normally specified beyond which repair of the plant is needed. To minimize the effects of cavitation, the following steps are recommended:

i) Periodically (annual) inspection of the runner and other turbine parts to judge when repair is necessary.

ii) Operation of the turbine plant according to the manufacturers guidelines. Specifically, a turbine unit should not be run below the minimum load (discharge), recommended by the manufacturer.

iii) At design stage, ensure i) proper submergence of the turbine ii) correct specifications of cavitation resistant materials and

iii) specification of the runner profile based on model tests for cavitation onset.

iv) As a result of draft tube pulsation and surges at no load or part gate opening excessive noise, vibrations and cavitation is experienced. To minimise pulsations of draft tube following measures must be taken:

- Air admission through air admission/ vacuum breaking valve installed at top cover.

-Provision of fins or flow splitter in draft tube to break the vortex flow.

- Provision of a bypass arrangement for releasing the pressure built up below the top cover.

Normally the discharge side surface of buckets or blades, areas on the crown on the throat ring and the tip of the blades and the upper portion of the draft tube liner are affected by the action of cavitation. In rare cases, there may be pitting on the pressure faces of the buckets or blades due to an unusual amount of over hung of the guide vanes improper design or unusual operating conditions.

Hydro Power plant operators have, over the years, gained expertise of ways of repairing and welding of the runner at site, but it becomes a regular maintenance problem as wear and cracking occurs quickly.

4.1.3 Precaution in welding of runners

 Some general remarks about the welding repair of turbine runners are given below. It should be noted that different materials will require specific (often proprietary) welding processing.

1. Surface of the parent material should be prepared by chipping or grinding.

2. To locate cracks, inclusions and the like, a die penetration test must be carried.

3. Preheating of the blade to about 60oC is necessary

4. Avoid any localized excessive heating. This is achieved by welding for a short time in any one particular area and then moving to a diametrically opposite area to continue with the work.

5. The parent material should be about 70 to 75 mm from the weld and should not be allowed to get too hot to be touched with bare hand.

6. Plenty of time should be allowed for the welded area to cool down since forced cooling may cause distortion due to locked in stresses. Hot peening is also required.

7. A close check should be made at least two to three times per day during the repair for runner to runner chamber clearances.

8. After welding all the welded areas should be properly ground to match with the desired profiled.

9. Die penetration tests should once again be carried out to ensure crack free welding.

Rectification if necessary should be done.

If extensive welding on the runner is required, it will be necessary re-balance statically and possibly dynamically all rotating parts and stress relieving before recommissioning otherwise.

2.1.2. Weekly Maintenance Checks:

 1. Greasing of guide vanes and servomotor with centralized grease lubrication system and manually.

i)  Oil in the gear box shall be checked.

ii)  Check for any leakage

iii) Working of end pressure relay and solenoid valves, if defective, should be reported.

2. Cleaning of OPU filters

3. Cleaning of throttle filters in the governor mechanical cabinet.

4. Cleaning of governor compressor air filters and checking of oil levels.

5. Checking physically oil of OPU of the running machine after sample taking through the sampling cock and do the crackle test for detecting presence of water. Take remedial measures.

6. Check oil level of all the bearings.

Check wobbling of shaft at coupling flange and at oil header servo-tube.

2.1.3. Monthly Maintenance Checks

 All the checks covered as part of the weekly maintenance are also carried out as part of the monthly check. In addition to these checks, more attention is paid and short shutdowns, if required, for rectification are taken.

2.1.4. Annual Preventive maintenance of Hydro Turbines

 After successful running of plant for about one year, a few weeks are required to inspect rotating parts, control equipment and measuring instruments and to analyze the cause of changes in the performance characteristics, if any. Modify, repair or replace (wherever required) worn out parts in order to prevent unplanned outages of plant at later date.

After every five years it is necessary to inspect the machine more critically for abnormalities like fatigue defects or excessive wear and tear of some parts or any change in original parameters/clearances etc. This exercise becomes very essential in cases where performance level has been observed to have gone down in 5 years operation.

The checks for annual and five yearly maintenance specified for Chilla Power Station are enlisted below:

1. Foundation Parts:

i) Check condition of water path system. The damage due to cavitation and wear to be rectified.

ii) Check painting of spiral casing.

2. Runner:

Check the condition of the surfaces of the runner hub and the blades. The damage due to cavitations & wear to be rectified by welding and grinding.

ii) Check the runner blade seals by pressurizing the system. Change seals if necessary. No oil leakage is to be allowed.

iii) Check the runner sealing for hermetic tightness, leakages of water in the runner hub is not to be permitted.

3. Guide Apparatus:

i) Check the presence of rubber sealing cords and the tightness of the rubber sealing between the adjacent guide vanes in fully dosed position of guide apparatus.

ii) Change oil in the regulating ring.

iii) Replace damaged shear pins.

iv) Check cup sealing of guide vane journals and replace, if necessary.

v) Check the bushes of guide vanes and change the worn out bushes of guide vanes journals.

vi) Inspect the servomotor and change the seals, if these are worn out.

4. Guide Bearing:

i) Check the condition of rubbing surfaces of guide bearing. Clean the surface and polish it with the help of chalk powder.

ii) Adjust the clearances by moving the segments with the help of adjusting bolts.

iii) Thorough cleaning of housing if necessary.

iv) Check all the RTDs and TSDs replace damaged one.

5. Shaft Gland Seal and Air Seal:

i) Check the condition of rubbing surface of sealing rings. In case found damaged change the same.

ii) Check pipe lines and piping joints for any leakage if any, attend the same.

6. Emergency Slide Valve:

i) Check the functioning of emergency slide valve and the condition of inner surfaces.

ii) Swift return of the valve in its original position after emergency operation should also be checked.

7. Centralised Grease Lubrication System:

i) Check satisfactory working of CGLS system.

ii) Attend wherever fault is located.

8. OPU:

i) Check and attend leakage from any valve or flanged joints etc.

ii) Provide proper lubrication to the bearings of pump motor.

iii) Check filter and repair, if required.

iv) Clean oil sump, refill with centrifuged oil.

v) Check setting of the float relays for proper sequence of operation of pumps.

9. Oil Header:

i) Measure clearances of upper and lower bushes, if found increased get the bushes replaced.

ii) Clean the oil bath.

iii) Check the rubber cord fixed below the guide to check any oil dipping on the exciter winding.

10. Oil Leakage Unit:

i) Check satisfactory working on Auto as well as manual.

ii) Clean the tank.

iii) Check the pipeline joints and valves for leakage, attend wherever necessary.

11. Oil Cooling Unit:

i) Check all the oil and water pipe lines for leakage and attend if necessary.

ii) Check satisfactory working of all cooling unit.

12. Governor Mechanical Cabinet:

i) Check filter and throttle if found damaged replace the same.

ii) Attend leakage of oil through pipe line joints and valves.

iii) Check auto rod setting, if found disturbed; set the same.

iv) Check (Alpha – Beta) characteristic, if found disturbed set the same.

Alignment of feed back wire rope pulleys.

 2.1.5. Capital Maintenance

Overhauling or capital maintenance of hydro plant is usually recommended after about 10 years of operation services. The whole unit is to be stripped off during capital maintenance and all the defective/worn out parts/components repaired/replaced with new ones. Then the unit is re-commissioned as per originally established commissioning practice of the power station.

After capital maintenance the units are subjected to all maintenance exercise outlined above before it reach the next cycle of capital maintenance.

Following checks are to be exercised during capital maintenance of a hydro set:

1. Turbine Bearing:

i) Dismantling, inspection, cleaning, measurement of clearances, polishing of guide pads, centering of shaft, reassembly, setting of clearances, filling of oil sump with filtered water.

ii) Check the temperature sensing device, if necessary, replace with new ones.

2. Gland Seals and Isolating Air Inflated Seals:

Dismantling, inspection, cleaning and reassembly. Replacing of worn out rubber flaps or carbon segments, if necessary.

3. Clean Water System:

Clean water pipes are dismantled, cleaned, reassembled with new gasket all the valves are attended for any leakage etc.

4. Guide Vane Servomotor:

Dismantling for inspection and cleaning. Reassembling and replacing the seals with new ones, if necessary.

5. Guide Vanes Bush Housing:

i) Removing, cleaning and inspecting for wear and tear replacing with new ones if found necessary. Replace seals, if necessary.

ii) Guide vanes are reconditioned.

6. Governor:

i) Cleaning and checking OPU pumps. Replace bushes, bearings etc. if found worn out. Also attend pump motors.

ii) Cleaning OPU sump and pressure accumulator and refill with filtered oil.

iii) Attend oil pipeline flanges and valves for leakages.

iv) Check setting of pressure switches installed for Auto Operation for OPU pumps.

v) Attend Governor Mechanical cabinet for leakages, loose links. Clean main and pilot slide valves. Set Auto rod as per designs Alpha Beta setting may also be checked.

vi) Check electrical circuit. Tightening of all the connections should be done.

7. Submerged Parts

i) Cleaning and painting of spiral casing and draft tube liner.

ii) Overhauling of spiral drain valve and draft tube drain valve.

8. Runner

i) De-watering of draft tube and fabrication of platform in the draft tube for inspection of runner.

ii) If it is a Kaplan Runner test the same after applying full governor pressure for leakage of oil.

iii) Replace blade seals, if necessary.

iv) Inspect blades of the runner and make up profile of the blades by welding. Due to erosion, abrasion and cavitations, material of the blade washes away with passage of time.

v) In case the runner is found to be irreparable arrange to replace the same with new one.

Experience of running hydro power stations in India has shown that even after careful project planning and good quality control measures from construction to commissioning, unforeseen problems do occur in service resulting in unplanned outages / low generation and load shedding etc. This causes disruption to consumers and reduced cash generation for the operator. A contributing factor to these operational problems is the fact that hydro power equipment and plant is custom built. The equipment cannot be fully assembled or tested in a factory before sending it to site. Maintenance activities at predetermined time intervals are necessary to ensure the following:

1. Quality and reliable operation of equipment in the service environment through planned, periodic inspection and checking of components and systems. Together with replacement or rectification of parts wherever required.

2. Maximum availability of equipment and a minimum of unplanned shut downs by using planned / periodic shutdowns to inspect all equipment (serviceable and non-servicable).

3. Eradication of operational problems by a timely analysis of the cause of faults / problems and replacement of short term solutions by long lasting and permanent ones.

2 Preventive maintenance of Hydro turbine

In order to achieve above objectives of maintenance, time has to be allotted every year for each machine. Normally the periodicity and the procedure for maintenance is recommended by the manufacturer of the equipment. However, experience of operators of Hydro Power Stations in India has shown that the maintenance is required according to the following guidelines.

Routine Maintenance

Normally there will be daily, weekly, monthly and quarterly checks as per the maintenance schedules are done. These checks are necessary for controlling any change in the installed clearances, commissioning characteristics etc. connected with the performance of equipment. Rectification and adjustment wherever required should be carried out in order to arrest any deterioration of the equipment. The daily, weekly and monthly check schedules designed for Chilla Hydro Power Station are used to illustrate the kind of maintenance schedule that will be needed:

2.1.1. Daily Maintenance Checks

 1. Foundation Parts and Expansion Joints:

Check for any leakage in draft tube manholes, spiral casing manhole, expansion joint.

2. Vacuum Breaking Valve:

Check the working of both vacuum breaking valve and see that there is no abnormality in the springs, seats etc.

3. Water Seal and Air Seal:

• Check the position of water leakage around the water seal and check that there is no

• excessive splashing and water level do not rise in top cover.

• Note water pressure of water sealing/under sealing.

•  4. Turbine Guide Bearing:

•  Check the oil level (stand still machine/running machine).

•  Note the temperature of bearing and check that the temperature of oil and guide bearing pads are within limits.

•  Note the maximum and minimum temperature of the previous day.

•  Check for any oil leakage from the bearing housing and check that oil is flowing above the bearing pads.

5. Guide Apparatus:

Check any leakage from GV servomotor and its piping.

Oil Leakage Unit:

• Check any leakage from pipe line joints.

• Check its satisfactory running on `Auto’.

Top Cover Drain System:

•  Main supply of `ON’ for DPM.

• Vibration noise in the pump motor.

• Any leakage from the water piping.

• Working and water pressure of the ejector.

Centralised Grease Lubrication System:

•  Check for any leakage from grease pipes, unions and nipples.

•  Check grease container and fill grease, if required.

Oil Header:

• Check from perspex sheet manhole any splashing of oil from top and bottom bush.

•  Check any oil leakage from the joints.

•  Note the pressure difference of opening and closing side of runner.

Oil Pressure System:

• Check if there is any abnormal sound in the running of the motor and pump unit of OPU.

•  Check the oil level in pressure accumulator.

• Check any oil leakage from oil piping and its valve.

• Check for over heating of motor.

•  Note the timing of OPU pumps running.

Mechanical Cabinet of Governor:

• Pressure in transducer.
• Check any oil leakage from joints of piping.

Effect of oil pressure

 At low sliding speeds a bearing performs satisfactorily when immersed in an unpressurized oil bath, but at higher speeds the centrifugal effects of the collar on the surrounding oil become substantial. Recirculation slots are provided at the back of the carrier ring to equalize the pressure gradients created by the collar, but their success is only partial.

Tests were carried out at constant speeds to measure the static pressures at the outer and inner peripheries of the pads for given oil supply pressures. Fig. 13.7 shows the results for 9000 rev/min, which was the maximum possible test rig speed when the tests were performed. It will be seen that pressures at the outer periphery of the pads were substantially less than the supply pressure, and pressures below atmospheric were measured at the inner periphery of the pads with low oil supply pressures. This was particularly noticeable at high speeds where higher temperature readings indicated oil starvation at the pad inlet. These sub-atmospheric pressures can be avoided by increasing the supply pressure. Other workers (4) have shown that the safety factor of a tilting pad thrust bearing

is increased by raising the housing pressure to relatively high values (10 bar),  but from tests performed it is concluded that supply pressures in the range of  1-1.5 bar are sufficient for the majority of modern applications.