Francis & Pelt On Turbine

A. FRANCIS & KAPLAN TURBINE

1. INTRODUCTION :Francis Turbine are one of the most commonly used type of prime movers for driving Electric Generators. They are commonly used for heads varying from 30m to 600m.It has been generally said that hydraulic turbines have reached such a range of perfection that only minor changes are possible in way of improvements. However , contrary to this belief certain spectualar changes have taken place in the recent past, which could not have been envisaged & probably some more to come in future. More economic machines have been designed to increase the specified speed, which has led to the present day of Francis Turbine. The Kaplan Turbine is reaction type feathering propeller where both the kinetic & pressure energy in the water are converted into mechanical work in the Runner. This covers the specific speed range between 300 to 1000 rpm. The main feature of Kaplan Turbine being the simultaneous adjustability of its runner blade & guide vane opening to match the fluctuation of the load. This coordinated opening will result in a very flat efficiency curve & better part load operation over a wide range of load when compared with other reaction turbine. Individual units up to 225000 HP are running & trend is towards to go for bigger bigger units. The upper limit of head is being idly extended & Kaplan units of over 70 metres are now possible as more information about the problems associated with Turbine Thrust , Vane operating forces, bearing loads are now available. For operating range between 40 to 70 metres , Kaplan Turbines , are much competition with Francis Machines. The hub diameter and the number of blades increase with the head & this given rise to increasing with cavitation. In order to improve cavitation performance the setting of machine would have to lowered.

2. TURBINE EFFECIENCY :-

One of the most important consideration is that the Turbine must be efficient with a high peak efficiency & a sustained efficiency at both part loads & overloads as far as possible. Considerable improvements in efficiency have been achieved with a fairly flat efficiency. There is an amphasis on reducing the PCD of guide vanes & hence the size of the machine. Efficiencies has high as 94 to 95% has been achieved.

3. DISMANTLING METHODS :Power units dismantling provisions should given due provisions. Three dismantling methods have been adopted at various times depending on the size of power plant concerned. A. From Intermediate Floor. B. From Alternator Floor. C. Bottom Dismantling. Dismantling from Intermediate floor is not longer adopted as this result in longer Intermediate shaft which is connected with problem of critical speed. Other two methods of dismantling are now days adopted which have act their merits & demerits. 4. COMPONENT DESIGN TRENDS :Functions of various components required for smooth operations are discussed below.

A. DRAFT TUBE :A well design Draft Tube recovers a large portion of the kinetic energy in the discharged water from the Runner by de-accelerating its velocity to the outlet Tail Race with minimum loss. Large vertical sets, with few exceptions, are of the ELBOW types & this construction leads to considerable savings in the station excavation cost. The bottom of the Draft Tube leg is shaped upwards to the outlet to minimize excavation. The Draft Tube cone angle, its length & elbow forms are determined from the flow tests to evolve efficient deceleration & maximum energy conversion. Draft Tube is provided with door of sufficient size for entry for inspection of runner & embedded parts. A cock is provided at the bottom of door to ensure that there is no water before opening the door. Spider is provided in the Draft Tube to brake the vortex formed for smooth running of Turbine. Legs of spider are used for entry of atmospheric air at the bottom of runner through air valve. To facilitate De-Watering suitable connection is provided, at the side at the bottom most elevation connecting it to dewatering pit. B. SPIRAL CASING :This is provided in Reaction Turbine to distribute water uniformly through gates into the runner & to give tangential whirl component of velocity to the runner. This is normally weld fabricated of plate steel. While designing care is taken to achieve the uniform flow by gradually reducing the sections. Spiral Casing is made in parts to suit transport limitations. Smaller Spiral Casing cross section have been achieved by

increasing the flow velocities at spiral intake. The velocity coefficients are normally kept between 0.14 (for low Ns) & 0.20 (for high Ns). However the velocity should not exceed 9 to 10 m/sec (at max. flow). With the event of more advanced methods of stress analysis & computer technology considerable improvements have been achieved in plate thickness. For low head applications, customer some time prefer to go for concrete spiral which work out cheaper compared to steel spiral. During concreting the steel spiral, a reasonable gap is kept on top of spiral from centre line by lying a wool felt of required thickness. This gap will allow expansion of spiral during pressurizing & at the same time relives it from external loading due to concrete. Man hole is provided in suitable place for entry into the spiral for inspection of water path surface. C. SPEED RING :To resist the bursting forces due to pressure inside the spiral, the throat of the spiral outside the guide vanes is bridged by the ring of fixed stay vanes which resist the axial loads on the spiral. The ring is sectionalized as necessary to meet the shipping requirements. Weld fabricated & cast stay ring facilitate easier & better shop site erection in view of their inherent rigid construction & provides for better alignment. Well designed construction provides greater resistance against distortion during shop manufacture & at site against concrete working. To avoid distortion and to maintain dimensional stability , weld fabricated and cast stay rings are stress relieved to relieve the residual internal stresses of forming, welding & casting. Stay Ring made in parts is joined by studs which are slogged or heat tightened to achieve the required elongation. The inside profile of the contour is seal welded to prevent leakage of water during testing & normal working of Turbine. One or two stay vanes are made hollow to facilitate drainage of water from shaft seal by gravity in addition to the ejector. The ejector will come into operation as soon as the level in top cover reaches the predetermined level. D. GUIDE APPARATUS :Guide Vanes regulates the quantity and direction of the water to the Runner. Smaller and medium sized vanes are cast in mild steel or stainless steel or bronze. Relatively larger gates are of fabricated plate steel welded construction or dowelled to the vane trunnions. Longitudinal sealing edges, pivot ring & top cover adjoining faces are mild steel guide vanes may be protected with stainless steel against corrosion and abrasion. Pitting and wear on these faces is made good by weld overlay. Rubber sealing strips along the length of the vane and on the top cover and pivot ring faces may be provided to reduce water leakage during Shutdown or Synchronous Condenser Operation.

The ring of vanes are swivelled for regulation by lever & link arrangement from the regulating ring. The connecting link between lever and regulating ring is provided with a safety device braking link or shear pin to break in the event of obstruction between guide vanes preventing the gate closure. The regulating ring mounted on the top cover is rotated by one or two or more hydraulic Servomotors, through the connecting rods. The ring which is of cast iron, cast steel or plate fabricated according to sizes and loads to be carried, has pins bolted to the lower flange to which the guide vane links are attached. Turn buckles are used for adjustment of bedding clearances which is simple in design compared to eccentric pin and can cater for wider adjustments. E. TOP COVER :Top cover is the part of guide apparatus and the bearing required for guide vanes. It is bolted on speed ring & covers the top of Guide Vanes and Runner. Top labyrinth is also housed in top cover which reduces the leakage and thus saves the water which other wise have gone to waste without producing the energy. Loading on top cover is also reduced by introducing the runner labyrinth and relieving holes. Top Cover design depends on the turbine size, head and erection method. Weld fabrication is widely adopted for large sized covers. The structure is designed to provide adequate rigidity against cover deflection and slope at guide vane due to water load, servo reaction loading G.V. reaction and bearing loads. Generator stator bore diameter shall be kept slightly bigger clearing the O.D. top cover for taking out the top cover on house supports for guide bearing, shaft gland, regulating ring, air valve & piping. In the region adjoining the guide vanes in the closed position the top cover surface is provided with stainless steel linear plates. These linear plates can be of welded type or screwed type. Screwed type of linear plates are adopted in case customer wants it replaceable and spare linear plates are supplied for this purpose. F. PIVOT RING :Pivot Ring or bottom cover houses the lower bearing of the guide vane and is usually welded plate or cast steel construction. It is made in single piece or in parts to suit transportation. It is provided with linear plates in similar way as adopted for top cover. G. MAIN SHAFT :Forged steel shaft, with working stress of approximately 350 to 500 kg/cm2 at rated output having UTS of having 5200 kg/cm2 are quite conservative and assume reliable service. The shaft is coupled to

the Runner through keys or fitted bolts at the lower end and through fitted bolts to the generator shaft at the coupling end. H. GUIDE BEARING :Turbine Bearing can be lubricated by water, grease or oil. In small size machine rubber pads with water lubrication can be provided. Forced grease lubricated white metal lined bearings can be used for the smallest to largest sized shafts. But these have not met such favour because of the disadvantage or continuous wastage of grease. This has led the preference oil lubricated bearing of forced circulating type or oil immersed self pumping type. Self oil lubricating white metal bearings generally used for large size turbines, can be of shall or type with stationary or rotating sump. Titleling pad which have got advanced of achieving desired clearance at site by simply rotating the studs and locking it in desired position. Oil is cooled by circulating the water in cooler provided for this purpose.

I. SHAFT SEAL:The gland has a function that is it prevents leakage of water up the shaft by providing a positive seal and the entry of air into the turbine at low pressure. There are various design having two rubber flaps which are cooled and lubricating by clean water is how being adopted generally. J. AIR VALVE:The tendency towards lower pressure in the draft tube during sudden load rejection results in rough and noisy running of the machine. This condition is alleviated by vacuum is achieved due to sudden load rejection. In addition to these valves one valve is provided which is directly connected to atmosphere and admits air as soon as it is required in the draft tube through the spider. B.

PELTON TURBINE :-

1. PRINCIPLE :The Pelton Turbines operate on the principle of impulse, i.e. the potential energy of pressure water is converted into high velocity jets which hit the buckets of pelton wheel. The entire energy conversion takes place on the buckets of pelton wheel, which rotate in the condition of free atmospheric pressure. In general pelton turbine are used for heads from 200 m up to any available head say 2000 m, with the specific speed ranging from 12 to 35 per jet.

2. ARRANGEMENTS :-

PELTON TURBINES have two basic arrangements. 1. Vertical axis machines . 2. Horizontal axis machines . Horizontal Machines are used for small capacity units & Vertical Machines are used for all large capacity units. The horizontal units can have one or two jets per runner, with single runner or a pair of runners coupled to a common generator. The vertical axis units can have 2,3,4,5 or 6 jets depending upon head, power output, speed of rotation & other factors. Use of more jets on one runner increases the rotational speed & reduce the unit size.

3. CONSTRUCTIONAL FEATURES :3.1 The main elements of Pelton Turbine are i. ii. iii. iv.

Runner Nozzles Distributor Turbine Casing

3.2

RUNNER :-

The energy conversion in the buckets of Pelton Runner is the most complex phenomenon and is least understood theoretically. The pelton runner / bucket are evolved / improved mostly by experimentation. With the advancement in foundry technology the runner with bolted buckets are obsolate and the integrally cast runners are adopted. Pelton runners up to a weight of 42 Tonnes are operating & still larger sizes are in planning stage. The earlier runner materials of cast steel , cast steel with 2% Ni , Aluminium Bronze have been replaced by high strength stainless steel of 13/1 , 13/4 ,17/4 Chromium-Nickel composition. 13/4 & 17/4 are S.S. steels have the following advantages over classical 13/1 S.S. Steel . i. ii. iii.

Better Weldability Better Machinability Better cavitation , erosion & erosion resistant.

3.3

DEFLECTORS :-

DEFLECTORS can be “Under Jet” or “Over Jet” type. The Overjet type deflectors can deflect the full jet by entering about half the jet radius, whereas the underjet type deflectors have to travel fully into the jet stream for effecting full deflection. In modern machines the “Overjet” type of deflectors are used since apart from shorter deflector travel needed in this type, it has an inherent feature in which deflector once in the jet stream, is further assisted by the jet force in its movement. These advantages make the overjet deflectors highly efficient & can act much faster with reduced shocks. Deflectors are generally cast steel, and robust in construction for withstanding complete jet impact stainless steel overlay is generally adopted, particularly for high head jobs. For large size machines, the deflectors are carried on roller bearings for reducing the size of servomotor, and to take shock loads effectively. Unlike Francis or Kaplan runners the buckets of Pelton Runners are subjected to very severe service due to alternating stresses caused by direct hit of high velocity jets. In addition to centrifugal vibration stresses. 3.4

NOZZLES :Two designs are generally adopted.

i. ii.

Internal Servomotor Type – This is also called straight flow nozzle. External Servomotor Type – In this arrangement the servomotor is placed outside the water passage.

While the nozzles with external servomotor type are easy for maintenance, the other variant of straight flow type are better from hydraulic consideration through the sizes of distributor shall be more in the later case. Because of simplicity & other advantages the external servomotor type nozzles are generally adopted. The main parts of nozzles are always made of high strength stainless steel & are specially hardened particularly for high head projects for achieving long operating life. 3.5

DISTRIBUTOR :-

The Distributor can be either cast, fabricated or a composite cast fabricated construction. Cast distributor are better from hydraulic considerations as generally the intricate shapes of Wye branches can’t be achieved so easily in fabricated construction. From the cost consideration , sometimes fabricated or cast fabricated construction is adopted. For high head jobs the use of high tensile plates helps to achieve overall economy.

4.

SPECIAL FEATURES OF PELTON TURBINE :-

4.1

DUAL REGULATION :-

Pelton Turbines have two regulation elements i.e. deflectors and nozzles which can be independently controlled to achieve the minimum speed rise and pressure size of hydro unit. In case of load throw off, while the deflectors out away the water quickly (with in say 2 sec.) away from the runner to limit the speed rise; the needles close slowly to prevent high pressure rise in the distributor & penstock. This results in lesser thickness of penstock and makes the design of alternator easier by reducing the required flywheel effect. 4.2

BRAKE JET , DYNAMIC BRAKING , C.W. SYSTEM :-

The Pelton Runner rotates in the free air, it does not have water churning resistance, hence it takes sufficiently longer period for the unit to come to standstill condition. This time is of order of 30 to 50 minutes in large size unite. To bring the unit to standstill condition quickly, brake jet is used, which supplies the water jet hitting on the back side of the buckets. As due to large rotating inertia of big size units, the brake jet may not achieve required results , dynamic braking is generally recommended for these units. Also the use of brake jet has following disadvantages – i. It causes cavitation / damage on the buckets and may affect the life of runner. ii. It must be switched off before the machine comes to standstill to prevent danger of reverse rotation. iii. In modern automatic controls it may pose problems . Hence the brake jet is avoided in the modern machines and instead, dynamic braking is used. As the pelton units take longer time to shut down , any failure of cooling water system during this, shall pose a serious problem for the bearings, hence reliable C.W. system is a pre-requisite for large size units. Generally an overhead tank of sufficient capacity with quick acting valves & all connected controls a interlocks for safety of the unit is adopted in modern Pelton installations.

4.3

RUNNER SETTING & AERATION SYSTEM :-

The pelton runners are set at the maximum Tail Water Level (T.W.L.) & the minimum recommended clearance between the runner C.L. and maximum T.W.L. has to be maintained for ventilation effectively & vacuation of water and to prevent back flushing of tail water on the runner. The high T.W.L. may result in loss of power & efficiency. In some cases, it has been observed that units can’t deliver required output due to rise in T.W.L. because of faulty design of tail

race. Also a minimum recommended air cushion is required above Tail Race for effective ventilation of housing. Aeration pipes & ventilation system is also provided for preventing vacuum pockets in the housing. Insufficient ventilation may cause humming noise & vibration. 4.4

CAVITATION IN PELTON BUCKETS :-

As the pelton runner operates in the atmospheric conditions, theoretically there should be no cavitation on the buckets of all the conditions are ideal. However in the practical cases some cavitation do occur in some critical areas of buckets due to following reasons :i. Minor undulations in hydraulic profiles . ii. Eroded buckets due to sand silt . iii. Operation at heads higher than design . An absolute control of cavitation is impossible, hence minor cavitation damage on pelton buckets is an accepted phenomenon. However with careful & regular inspection, grinding and polishing of bucket profiles, the extent of cavitation damage can be minimized & runner life can be extended. 4.5

FALAISE PHENEMENON & HEAD VARIATION :-

Falaise phenomenon is characterized by the interference of jets due to incomplete evacuation of water from the bucket when the next jet takes over. This happens due to lower jet velocities because of operation at heads lower than design values for the multi jet particularly with 5 & 6 jets. Falaise phenomenon results in noisier machine followed by the vibration off runner buckets and is harmful to the runner in the long run. While on the higher head side the cavitation damage may pose problem; on lower head side falaise phenomenon limits the operation particularly for 5 to 6 jet machines. In view of this a limited head variation of ±5% can be considered as tolerable for pelton turbines.

5. PELTON TURBINE vs FRANCIS TURBINE :BHEL manufacture three types of Hydro Turbines – 1. Pelton 2. Francis 3. Kaplan Here is an analysis of merits and demerits of Pelton Turbines over High head Francis Turbines. I.

II.

III.

IV.

V.

Pelton Turbine is the simplest form of a prime mover where buckets are struck by a jet or jets of water emerging from a nozzle/nozzles controlled by a spear/spears. In Francis Turbine, water is admitted through a number of openings controlled by a carefully shaped guide vanes. To improve efficiency, the clearance between the end covers and runner are required to be kept extremely fine. Pelton wheel rotate in air at atmosphere pressure and hence there is no need of a gland seal on the shaft. Whereas in Francis Turbine, a shaft seal / gland is a must. To reduce the shaft wear when silt is present in water, a protected sleeve is fitted to the shaft. Replacement of sleeve requires major strip-down of the unit. Pelton Nozzles, spear tips & ultimately runners get eroded by the presence of solid particles in water. The nozzles and spears are easily replaced / repaired. Even Runners, when required , can be repaired without removing the same from shaft. This repair has little effect on the efficiency of the Turbine. In FRANCIS TURBINE, the solid particles in the water cause very rapid wear in passing through the fine clearances between guide vanes and end covers and worst still between the Runner & Labyrinths. This reduces the overall efficiency of the turbine considerably. These parts are not easily repairable and replacement needs major strip-down of the machine. PELTON TURBINE working parts are easily accessible through Tailrace and suitably placed manholes. FRANCIS TURBINE inspection and overhaul is much more difficult unless the physical size of the machine is sufficiently large. GOVERNING:Governing of Pelton Turbine is done by et deflector and operation of this is virtually instantaneous on load rejection, thereby controlling the speed rise within acceptable limits and cutting down the required flywheel effect to a minimum the operation of spear. The rate of closure is arranged to suit pipeline profile and to keep the pressure rise to a nominal figure. FRANCIS TURBINE GOVERNING is arranged by the opening and closing of the guide vanes also. As such on load rejection the problem of speed rise pipeline pressure rise both become critical. Pressure rise can be kept within favourable limits by having a

VI.

VII.

relatively sluggish governor but this would increase the speed rise limits meaning an increase in the flywheel effect. Alternatively, speed rise can be kept within favourable limits by a fast operating speed governor but this would require provision of a relief valve to keep the pressure rise within reasonable limits. Pelton Runners are almost free from the danger of cavitation and have much better efficiency on part loads whereas in Francis Turbine it is an ever persistent phenomenon. In case of Pelton Turbines the net head is reduced as Runner is always above maximum Tail water level. They also have a lower efficiency at full load. Francis Turbine with fine clearances have a high efficiency from full to half load but below half load cavitation problems become serious.