CONSTRUCTIONAL DETAIL AND WORKING PRINCIPLES OF TURBINE

1.PRINCIPLES OF OPERATION:

Hydraulic (water) turbines are machines that convert hydraulic energy into mechanical energy. This conversion is possible in a complex harnessing called hydro power plant (HPP).Usually, the mechanical energy is converted into electrical energy, in the same harnessing, using electrical machines called electrical generators. The most usual generators are synchronous generators, which are called hydro generators due to the fact that these are driven by hydraulic turbines.

The power capacity of a hydropower plant is primarily the function of two variables: (1) flow rate expressed in cubic meters per second (m3/s), and (2) the hydraulic head, which is the elevation difference the water falls in passing through the plant.

Fig.:- Schematic View of a Hydro Power Plant.

2.TYPES OF TURBINE:-

Water turbines may be classified in different ways. One way of classification is according to the method of functioning (impulse or reaction turbine).Water turbines may operate as turbines, as pump turbines or a combination of both. These may be of the single regulated or double regulated type. Turbines may also be classified according to their specific speed. Another way is according to the design (head and quantity of water available). According to the method of functioning turbine are classified as follows:

i) Impulse turbineii) Reaction turbine

2.1. PELTON TURBINE:-

In the impulse type of hydraulic turbine, the runner converts the kinetic energy in one or more jets of high-velocity water into torque in the turbine shaft. The runner consists of a definite number of buckets, each of double- bowl construction, located on the rim of a central disc.

The velocity of the water of the jet which enters each bucket is reduced nearly to zero in the bucket during operation of turbine. The kinetic energy in that portion is thereby converted into a force acting on the moving bucket, and producing a torque on the turbine shaft. The force of the jet is applied only during the time that the water is in the bucket. The impulse of a force is defined as the product of the force and the time during which it acts.

Pelton turbines are made either with vertical or with horizontal shafts. The vertical shaft turbine usually has only one runner and up to six jets. Most horizontal- shaft units have one or two jets per runner and one or two runners per turbine. Horizontal turbines usually cover those combinations of power and head for which vertical turbines are not economical.

Fig.:- Assembly view of Pelton Turbine.

.

(1) : Control frame work (2) : Distributor (3) : Brake jet nozzle

(4) : Turbine Chamber (5) : Nozzle body (6) : Turbine Shaft

(7) : Guide Bearing (8) : Runner Bucket (9) : Foundation Concrete Wall

Fig.-Typical section through Pelton turbine

2.1.1 Distributor:-

Distributor is the part of the Pelton turbine to facilitate pressurized water flow from MIV (Main Inlet Valve) and to feed to the nozzle-jet assembly, which in turn strikes the water jet to the runner buckets. In multi-jet turbine system, distributor is surrounding the runner.The overall dimensions and shape of the distributor are determined on the basis of following considerations:-

a) The water should enter in each nozzle-jet assembly at a uniform flow rate.b) The flow in the distributor should satisfy the equal area of cross-section through

each nozzle-pipe.

Therefore, the overall dimensions of the distributor depend on the discharge and head for which it is designed or depends on the area of its inlet section and the pressure of the working fluid.

The cross-sectional area of the distributor reduces after the bifurcation to connect each jet-assembly-feeding-nozzle. The areas of cross-section of a distributor at various sections are designed in such a manner that, the reduction of the area of cross-section is equivalent to the area of cross-section of the nozzle feeding cross-section.

Distributor of a Pelton turbine is the part in which the pressure energy is converted into a velocity head. In this process- change, distributor ensures smooth hydraulic during the operation. Where ever the cross section changes of the distributor are there, regular and continuous contour should be followed to minimize the losses in the distributor.

The foundation of the servomotors for each nozzles-assembly is provided on the body of the distributor and forms the integral part of the structure of the distributor.

For small capacity, distributor is made by casting whereas of big capacity it is made in parts by fabricating the steel plates. Appropriate safety margin of pressure is kept to bear the maximum water pressure which could be there during the water hammering under the situation of worst circumstances.

Bigger size of distributor is generally made in parts to facilitate its transportation, handling and erection.

2.1.2 Jet nozzle assembly:

It is placed between the distributor’s delivery-end and the runner. Jet nozzle assembly consists of a spear which is operated by a servomotor which is fixed on distributor body. Full forward movement of spear blocks the water jet and its backward movement allows the water jet from distributor to impinge on the runner bucket. Shape of spear and jet opening is so regulated that in its movement zone it regulates the water input to the runner.

In multi jet machines all the spears move in unison and controlled by governing system.

Tip of the spear is generally a conical precisely machined section and is subjected to wear from water and silt. This is a replaceable component generally threaded on the spear rod.

Similarly nozzle from where water jet emerges is also precisely machined component and subjected to wear from water and silt. This component is also made easily replaceable.

In modern Pelton turbines, spear of cone and nozzle are made from wear resistant 13/4 stainless steel. Diameter jet and profile of spear and nozzle mouth depends upon water head and discharge requirement.

Fig.:- jet nozzle cross section of Pelton Turbine

2.1.3 Deflector:-

As the name indicates purpose of deflector is to deflect water jet.When turbine needs to be stopped, spear should move forward to close the water jet. however, timing of spear servomotor is slow and for immediate closure of machine some fast acting mechanism is required which should deflect the water jet from runner. this is accomplished by deflector which is positioned in between water jet and runner.

Initially when starting, the turbine deflector moves away from path of the jet and allows the jet to impinge on runner. While shutting down the unit, spear servomotor allows movement of spear in forward direction to shut down the jet with inbuilt timing of spear servomotor. However, deflector moves much faster and blocks the water jet.

In a multi jet machine there is an individual deflector for each jet and all the deflectors are connected by lever link mechanism and are operated from a single servomotor placed in the above turbine casing. As in case of spear, deflectors also operate in unision by action of single servomotor. Commands for deflector operation are received from programmable governing system.

Deflector thickness and material depends upon the water head and jet diameter. It should be very robust construction as it has to take force of water jet.

2.1.4 Runner:-

Buckets are mounted on the periphery of a disc and the disc is coupled with the turbine-lower-flange. As the center of the shaft and the centre of the runner are arranged in line, the mechanical power generated by the runner is fed to the shaft for further transmitting. The blade’s /bucket’s profile looks like two concave-surfaces placed together side by side. At

the joint of the concave surfaces placed together side by side. At the joint of the concave – surfaces a projected ridge is formed and the outer end of the ridge is called tip of the blade. During the operation, the water jet is divided into two parts as it strikes the tip and ridge of the blade. The bucket’s surface is called the guide-surface. the jet enters the guide- surface, imparts the mechanical power to the runner and while leaving the surface its angle turned almost by 165°.

Diameter of the runner depends upon R.P.M. of the machine, head and discharge.

Selection of runner of buckets is calculated on the basis that all water particles are forced to give off all their kinetic energy to the runner wheel. In a wheel with a wider spacing of buckets, the energy of some water particles will not be utilized, and, on the other hand, when the spacing is too narrow, the jet will be superfluously distributed by interfering bucket.

Some impulse runners are made with individually bolted buckets and others are solid cast. Double- overhung installations are made with a generator in the center and the runner positioned on the two overhanging ends of a single shaft in the horizontal Pelton turbine of big capacity.

The Pelton runner is generally made from single casting. The material of the runner used in modern design is 13/4 stainless steel having anti abrasion and anti pitting properties. While manufacturing, very thorough inspection of bucket root is essential as this area faces maximum water impact. With advancement in manufacturing facilities now a days, many manufacturers are offering Pelton runners in which buckets profiles are machined from a integral forged disc.

Fig.:- Runner of Pelton Turbine2.1.5 Runner casing :-

Structures around the runner are called the runner casing. The runner casing for a vertical Pelton type of the turbine is discussed here. The distributor structure is supported on the vertical concrete walls foundation. The shape of the vertical concrete walls foundation is round or hexagonal and hollow at the center. Inspection path and maintenance approaches are also made through these foundation walls from the turbine floor level. The inspection

path and maintenance approaches remain closed during turbine floor level. The inspection path and maintenance approaches remain closed during turbine operation. At the foot-level of the concrete foundation, a passage is kept for water exit-flow leading to the tail race. The entry/passage at the foot of the foundation also facilitates the erection of the shaft, runner and other turbine parts. Some design provides the steel lining on the inner walls. At the top of the runner, casing (top cover) in sectors are provided. At the neck of the top cover female-groove is provided to match (with certain clearance) the dimensions of the sleeve provided on the shaft. The arrangement acts as a preventor of the water splashing beyond the top cover of the turbine.

2.1.6. Hydraulic (water) jet brake:-

At the time of tripping, water jets supplying power to the runner are immediately deflected by deflector and stopped by the jet and nozzle assemblies.

Due to moment of inertia, even after stopping the input, machine takes a long time to come to a safe speed when brakes can be applied. This is because Pelton runner rotating in air after water is stopped and where there is very less resistance. Hence for quickly reducing the speed to a safe level where generator brakes can be applied, a special feature called braking-jet is used in most machines.

Braking-jet is a single water jet tapped from the distributor such that it strikes on the back of the bucket and thus imparting retarding force.

Braking-jet is controlled by a suitable solenoid valve and gets command from programmable governor

2.2. FRANCIS TURBINE:-

In 1849, James B. Francis, An American engineer, set out to improve upon the design of the few hydraulic turbines operating at the time in France and the United States. Most of the earlier turbines were so constructed that the water entered their runners at the centre and flowed radially outward.

The turbine that francis built following his investigations allowed the water to enter the runner from the outside and to flow inward through the radial blades. The francis design was subsequently improved but changing the shape of the runner blades so that the water was turned from a radial to an axial path within the runner, rather than outside it. During all the changes made of this type of turbine, it was the name of francis that became, and remained, associated with it.

Reaction turbines are driven by a combination of velocity and the pressure of the water; the francis turbine is in this class. The water passages between the runner buckets are simultaneously and continuously filled with water.As the water flows through the runner, its velocity is changed both in magnitude and direction, which produces a force on the runner blades.

Important developments in francis turbines followed the introduction of thrust bearings of carrying very heavy loads. Up to that time, hydraulic turbines of any size had to have horizontal shafts with journal bearings and double runners to counteract each others hydraulic thrust. The large, high-Powered, vertical turbines that followed the advent of the new thrust bearing were made possible by its ability to carry the tremendous load of the generator rotor, shaft, turbine runner and hydraulic thrust.

The distinguishing feature of a francis runner are:

- A crown at the top,- Curved blades (usually from eleven to twenty-one in number) attached to it and, at

the bottom, a surrounding band connecting the lower ends of the blades. The proportions of the runner vary with the head, power, and speed for which it is designed.

Francis turbine runners are usually one-piece steel casting, although in certain cases runner are constructed by welding cast steel or plate steel blades to the crown and band. Very large runners may be made in sections, to conform to shipping restrictions or because of their weight.

Fig.:- Pictorial View of Francis Turbine.

Part list of Francis Turbine drawing

1Turbine lower part arrangement 16

Runner/Turbine shaft coupling arrangement

2 Upper part arrangement 17Turbine guide bearing arrangement

3Turbine pit & upper part arrangement 18

Shaft seal arrangement

4Turbine Distributor arrangement 20

Turbine foundation

9Spiral casing arrangement 21

Connection for an eventual tail water depression system

11Draft tube manhole arrangement 22

Balancing pipe

12Fix labyrinth arrangement 29

Over speed detector

13Servomotor arrangement

15Head cover fix/runner rotating labyrinth arrangement

14Servomotor anchoring arrangement

Fig.:- Sectional view of Vertical Francis Turbine.

Fig.:- Assembly view of Francis Turbine.2.2.1. Scroll casing:-

The overall dimensions and shape of the scroll casing are determined on the basis of the following two considerations.

1. The water should enter the turbine at a uniform flow rate over the entire distributor circumference.

2. The flow in the scroll casing should satisfy the law of constant circulation, according to which the peripheral velocity at any point, multiplied by radius of this point, is constant.

The overall dimensions of the scroll casing depend on the area of its inlet section.

Scroll casing of medium and high head turbine are made of steel. These take up the water pressure and prevent seepage of water into the concrete. The scroll casings are connected in after positioning, so that these also form part of the concrete structure of the power house.

The scroll casing of medium head turbines have circular sections; these are fabricated from sheet steel. The wall thickness of a welded scroll casing is determined by allowing only for the load due to the internal water pressure. Sheets of different thickness are, therefore, used for the various sections. The sheets are thickest at the inlet section; their thickness thereafter decreases as the cross section of the scroll casing becomes smaller. The various parts of the scroll casing are made of steel sheet of standardized thicknesses according to the dimensions of the casing and transportation requirements.

2.2.2. Speed ring:-

The turbine speed ring forms the immediate part of the water passage between the scroll casing and the distributor. It transmits to the foundation the load due to the weight of stationary and rotating part of the generating set, the axial hydraulic force exerted by the water on the runner and the weight of concrete block above the generating set. The speed ring stay vanes are streamlined in the flow direction.

The speed rings used with steel scroll casings of large Francis turbines are cast and welded or fabricated. The upper and lower bands of such a speed ring have flanges on the outside, to which the scroll-casing lining is secured (usually by welding). The speed rings of francis turbines having runner diameters of less than 3.0 m are sometimes cast integrally.

The stay vanes and the upper and lower bands of a cast and welded speed ring are cast separately and then welded together to form a single structure.fabricated speed rings are welded from parts consisting of plane, bent, and stamped rolled sections. Speed rings of high head francis turbines with steel scroll casings are integral with the latter, forming their exits. The number and dimensions in plan of the stay vanes, as well as their configuration and disposition depends on the geometry of the scroll casing, the flow past the stay vanes, and strength requirements.

2.2.3. Draft tube:-

The draft tube of a reaction turbine serves to discharge the water from the runner into the tailrace with the minimum energy losses.

By using a draft tube it is possible to arrange the turbine runner above the tail water level without incurring a loss of head, and to utilize a considerable part of the kinectic energy of the water leaving the runner. The draft tube is of particular importance in a low-head hydraulic turbine where the water downstream of the runner still remains 40 to 50% of its total energy.

A vacuum is created in a draft tube arranged beneath the runner. The total head acting on the runner blades thus is the sum of the static head, equal to the difference between the headwater level and the elevation of the runner exit section, and of the negative head (vacuum) beneath the runner.

The kinetic energy of the water leaving the runner is utilized to create a vacuum beneath the runner. The draft tube, therefore, has the form of a conical divergent nozzle (diffuser). This design ensures that the flow velocity in the exit section of the draft tube is considerably less than in the inlet section, so that the energy losses are reduced. However, the magnitude of the draft head and the concavity of the draft tube are subjected to limitations by the need to prevent cavitations.

The optimum shape of the draft tube, which ensures minimum losses, is that of a straight cone. Such draft tubes, however, can be used only with horizontal turbines and low power

vertical turbines. The reason for this is that the necessary length L=(4.5 to 5.9) D 1 of a straight draft tube would require a very deep power house in the case of medium size and large vertical turbines. Bent (elbow) draft tubes are, therefore, used for large vertical turbines.

The elbow draft tube consists of conical divergent flare, elbow, and horizontal flare. The overall dimensions of an elbow draft tube depend on the height h and length L when the width of the generating set block is given.

2.3. Kaplan turbine:-

The Kaplan turbine is generally the most suitable type for low-head and medium-head installations where large variations of flow and head are encountered.

The distinguishing feature of the Kaplan turbine is the automatic adjustment of the angle or pitch of the runner blades as water flow or power output varies.

The blades, projecting from and supported in the hub of the runner, move simultaneously during a change of load. The runner blades are rotated on their trunnions by levers, links and crosshead, or by some other suitable mechanical linkage, operated by the vertical motion of a servomotor piston rod.

The blades adopt automatically the most efficient position for the power output of the turbine. Also, the angle of the blades may be adjusted automatically for head variations by adjusting the cams which determine the relative movement of, and the relationship between, the wicket gates of the turbine and the blades of the runner.

Besides the features already mentioned, a Kaplan turbine is equipped with an oil head and with vertical, rotating oil pipes housed within the shaft of the generator and turbine. The oil head conveys the oil from the stationary governor oil pipes to the rotating pipes which lead to the runner blade servomotor.

Where turbines are required to operate at part-load efficiently generally, Kaplan turbines are installed. This requirement is most often encountered in run-of-river plants with limited storage facilities. The Kaplan turbine also allows the runner blades to be opened to a greater angle during periods of high flow and low head, giving higher power outputs during those periods than could be had from a fixed blade turbine of the same size and rated output.

Fig.:- Sectional view of Vertical Kaplan Turbine.

Part list of Kaplan Turbine drawing1a Runner cone 10 Turbine guide bearing1e Runner hub 11 Guide vane servomotor1f Runner blades 12 Servomotor connecting rod3 Top & bottom plates 13 Guide vane regulating ring

4 Spiral case 14 Guide vane link5 Stay vane 15 Guide vane arm6 Guide vane 16 Shaft seal7 Draft tube 17 Head cover8 Discharge ring 18 Runner blade servomotor9 Turbine shaft

Designs of components such as spiral casing, speed ring, guide apparatus system, bearing shaft and draft tube are similar to that in francis turbine. Main design features of the following components which are different from francis turbine are described below:

2.3.1 Runner:-

The runner of a Kaplan turbine differs from that of a francis turbine in that the flow has an axial direction at both blade inlet and exit. The runner also has no lower band: the number of blades is less and they can be rotated about their axes. The runner consists of hub, blades and hub extension.

The number of blades, and thus the diameter of the runner hub, increases with the head. Four blades are ordinarily used at heads of up to 20 m. while eight blades are used at heads of between 40 and 80 m. The hub diameter dhub depends on the number of blades and varies between 0.3 D1 (for 4 blades) and 0.6D1(for 8 blades).

Fig: Runner of Kaplan turbine

The runner blades of Kaplan turbines are usually made from cavitation resistant stainless steel, the parts of the blade actuating mechanisms are made of high strength steel.

The turbine runner is connected to the turbine shaft via an intermediate part of the hub. This is the runner cover to which the flange of the turbine shaft is connected. The duties of the runner cover are in some turbine designs performed by the expanded bottom flange of the turbine shaft.

The blade-actuating mechanism consists of the servomotor (driving motor) and a lever system connecting the piston rod or directly the servomotor piston with cranks arranged on the blade pivots. The servomotor piston moves up or down under the action of the oil pressure and thus actuates the lever system by which the blades are turned.

Runners may be divided into two groups according to the manner in which the lever system is connected with the piston, viz. those with crosshead and those without. The lever system of a runner with crosshead servomotor is connected with the servomotor via a crosshead fixed to the piston rod. The lever system of a runner without crosshead is directly connected to the servomotor piston.

A runner with crosshead designed for a large Kaplan turbine. The runner has eight blades. The outer surface of hub body is machined spherical. This reduces the end clearances between the blades and the hub body at the various blade-setting angles. Blade pivot is carried in bronze bushings and lever is mounted on the pivot inside the hub body, while the blade is secured to the outside of the pivot. Blade flange, pivot, and lever are held together by bolts and fixed relative to one another by cylindrical keys.

The centrifugal force acting on the blades, which appear during rotation of the runner, are taken up by bronze rings. The eccentricity arranged lever pins are connected with crosshead via links and eyes. The crosshead is secured to piston rod. The piston rod is guided in bronze bushings respectively inserted into internal bosses in the hub and in the shaft flange. The upper end of the piston rod is connected to rod through which oil flows to the runner servomotor. The servomotor cylinder is formed by the upper part of the hub. The runner (cylinder) cover is formed by flange of the turbine shaft. Piston moves inside the cylinder. The piston is secured to piston rod by split locking ring. Rotation of the crosshead when the links are inclined is prevented by two sliding keys in the hub body, which engage matching keyways in the crosshead.

Oil is supplied to the servomotor under pressure through rod which consists of two concentric pipes. Delivery of oil to the cylinder space above the piston causes the latter to move down. This motion is transmitted to the blades via piston rod, crosshead and links, and causes the blades to move into the full load position. Delivery of oil to the cylinder space beneath the piston causes the latter to move up, so that the blades move into the no load position. The lower part of the hub is always filled with oil which leaks along the rod. Excess oil is forced into the hollow shaft through central pipe. The hub body is closed from below by bottom which prevents leakage of oil from the hub. Leakage of oil along the blades and entry of water into the hub between the latter and the blade flanges is prevented by seals. Hub extension is secured to the hub bottom; it serves to direct the flow downstream of the runner.

The peripheral blade edges are machined so that the clearances between them and the throat ring do not exceed one thousandth of the runner diameter.

The runner-blades seals of modern hydraulic turbines are mostly detachable so that they can be repaired without removing the runner blades. The seal consists of blade ring bolted to blade flange springs are located in cylindrical recesses in the blade ring. These springs rest against clamping ring secured to blade ring with screws. The clamping ring can move vertically. Leakage of oil through the clearance between the blade ring and the clamping ring is prevented by one piece rubber diaphragm forced by two split rings and against blade ring and clamping ring, respectively. All these part move together with the blade when it is rotated about its axis. Rubber sealing ring is forced by supporting ring against the end gap in the opening of runner hub. Clamping ring is pressed against the protruding parent of sealing ring hub. Clamping ring is pressed against the protruding part of sealing ring by the oil pressure inside the runner hub and by springs which move together with the blade. The seal is closed by cover. All parts of the seal can be dismantled, except the blade ring, the clamping ring, the rubber diaphragm, and the sealing ring.

2.3.2. Runner chamber/throat ring:-

The throat ring of a Kaplan turbine is usually cylindrical above the runner-blade axes and spherical beneath them. The overall height Hth of a standardized throat ring is (0.5 to 0.53) D1.The throat ring usually consists of lower distributor ring and several intermediate cylindrical sections bolted together. The throat ring is connected to speed ring by its upper flange, while the lower flange rests on foundation beam and is connected with lining of the draft- tube cone via joining section. The throat ring has ribs on the out side in order to ensure better fixation in the concrete and greater rigidity, a removable non concreted segment is usually provided in the central part of the throat ring opposite the runner blade axis. This makes it possible to remove the runner blades for repairs or exchange.

The throat ring is subjected to variable forces during rotation of the runner, due to alternating pressure rises and drops. This sometimes results in the throat ring becoming loosened. Such pressure fluctuations are particularly dangerous in the case of turbine where, because of transportation requirements, the throat ring consists of several parts bolted together along flanges in the horizontal and vertical planes. The bolts may become slack as a result of the alternating loads, and the joints may open. Better fixation of the throat ring to the concrete foundation is ensured by tie rods and braces (spacer jacks).

2.4. BULB TURBINE:-

These types of turbines are similar to Kaplan turbines, except their shaft position which is not vertical, but either horizontal or inclined. Also there is no spiral case, and the discharge tube is straight.

Between Kaplan and bulb turbine some differences are given below. A Bulb turbine is suitable for lower heads (under 15 meters or 45 feet, down to 2 meters or 6 feet) than a Kaplan turbine.

The maintenance for Kaplan unit is easier because there is more space around the turbine and the generator. The Bulb unit is like a reversed U-boat (submarine), where everything is at lower limit.

At Bulb units, the generator is into the same case with the turbine, meaning that they are both under the water, with a small access to the generator. At a Kaplan unit, around the generator is plenty of space, and the operator can observe certain parts of the generator.

A more recent development is the bulb type turbine in which generator is placed in a bulb shaped water tight steel housing located in the centre of the water passage. The generator may be driven by the runner directly or through gears. The bulb shaped generator housing period in the centre of the water passages way, requires an enlarged water passage around the bulb. These special turbines are meant for low head developments of power from tidal waves.

Fig.:- Cross-Sectional view of Bulb Turbine.

2.5. PROPELLER TURBINE:-

Propeller Turbine consists of a hub with blades fixed to it and a nose piece which is generally separately cast & bolted to the hub. Development of this type of turbine has been the result of search for high specific speed machine for low heads which cuts cost of machine per horse power and in low head developments; cost of machinery is proportionately higher.

Fig.:- Pictorial View of Propellor Turbine.

2.6 DERIAZ TURBINE:-

It is a diagonal flow type turbine. The approx. head range (M) is 50-150.The head variation (% of rated head) is 125-65. The specific speed range (M-MHP) is 200-400 & Peak efficiency (Percentage) is 95.

Fig.:- Pictorial View of Deriaz Turbine.

3. COMPONENTS OF TURBINES :-

3.1 Turbine shaft:-

Turbine shaft made of forged carbon steel of appropriate hollow section should be able to deliver turbine torque to generator. Shaft is provided with two coupling runner at the lower flange and to couple generator-shaft with the upper flange. The forged journals for turbine guide bearing are provided around the shaft near the lower end. The outer diameter of the journal is decided to bring the required range of the surface velocity suitable to the guide bearing system. Below turbine guide bearing journal, a sleeve is provided on the shaft and at the neck of the top cover of the turbine-enclosure, grooved collar is provided. The arrangement stops splashing of water which intends to come out form the neck of the top cover during operation of the turbine.

Through hole in center of the shaft is usually of 100 to 150 mm diameter. The hole is used for air admission while unit is under operation and to facilitate assembly of shaft, other components and to conduct Non Destructive Testing on the shaft during manufacturing.

Fig.:- Shaft which connects turbine with generator.

3.2. Bearing :-

The bearing of vertical generating sets serve mainly as guides since these are ordinarily subjected only to loads caused by the dynamic imbalance of the rotating parts and by asymmetric flow of the water through the turbine. Journal bearings with lubrication by oil or water are usually employed in large hydraulic turbines.

A design of an oil lubricated bearing of a vertical hydraulic turbine. Bearing housing is mounted on the turbine cover plate. Split babbitted shells are inserted into housing. Bearing cover is located on top of the housing; it has a gap seal for preventing upward leakage of oil along shaft. Oil is slipped to the bearing through pipe. Oil thrower is secured to the shaft beneath the bearing shells. It diverts used oil escaping from the bearing into receptacle and from there it is pumped into pipe. Penetration of water into the bearing from the interior of the runner is prevented by seal beneath the bearing.

The babbitted shells are usually not longer than 0.8 to 1.0 times the shaft diameter. Data on the clearances between shafts and bearing shells are presented in table for turbines in operation.

Shaft diameter,

mm

Clearance, mm Shaft diameter, mm

Clearance, mm

Shaft diameter,

mm

Clearance,mm

80-120 0.08 – 0.12 360-500 0.17-0.25 1000-1250 0.29-0.45

120-180 0.10-0.16 500-630 0.20-0.31 1250-1600 0.32-0.55

180-260 0.12-0.18 630-800 0.23-0.35 > 1600 0.40-0.60

260-360 0.14-0.21 800-1000 0.26-0.41

Water-lubricated guise bearings with rubber shells are now widely used in the USSR for large hydraulic turbines. Shells of laminated-wood plastic (Lignofol) are also used in small and medium- size turbines. Such bearings are simple in design and more convenient in service, since there is no need for intricate seals beneath the bearings. It is, therefore possible to locate the runner nearer to the bearing and increase the operating reliability of the turbine. There is also no need for auxiliary equipment for bearing lubrication.

3.3 Distributor/guide apparatus system:-

The distributor of a reaction turbine imparts to the water the required direction at the entry to the runner blades, and regulates the discharge according to the load and the rotational speed of the generating set. When closed, the distributor completely stops the admission of water into the runner and thus forms a shut-off gate ahead of the turbine.

The turbine discharge and the runner blade inlet angle are varied by rotating the guide vanes. Shock less entry of the water into the runner of a Kaplan turbine at all operating conditions is ensured through simultaneous rotation of runner blades and guide vanes.

Fig : Guide Vane

The guide vane positions during the regulation process are determined by the distributor operating, whose magnitude represents the minimum distance between the trailing edge of one vane and the surface of the adjacent one.

Profile and dimensions of the guide vanes are chosen in accordance with the overall dimensions and type of the scroll casing and the runner type.

Three very widely employed guide-vane profiles are:-i) Convex, ii) concave, and iii) Symmetric.

Additional vorticity is imparted to the flow ahead of the runner when convex guide vanes are used. Such guide vanes are fitted in turbines with open flumes.

However, concave guide vanes reduce the vorticity at the runner inlet. Such guide vanes are used in francis turbines with scroll casings.Kaplan turbines usually have guide vanes of the simplest, symmetric profile.

The forces act on a vane when the distributor is closed. The resultant P of the pressure forces acts at a point located at a distance є (eccentricity), downstream of the axis of rotation of the guide vane. The magnitude of the eccentricity is usually (.03 to .05) L. The eccentricity is considered to be positive if the hydraulic moment M tends to open the distributor. The distributor tends to spontaneous closing if the eccentricity is negative.

Mostly widely used arrangement in conventional francis/ Kaplan turbine is cylindrical distributor with guide vanes whose axes are parallel to one another and to the turbine axis, and which are located on a cylindrical surface.

Guide vanes are arranged with their axes on a circle of diameter D0 directly downstream of the speed ring at the turbine inlet. The lower pivot of each guide vane is located in bushing, inserted into lower distributor ring, while the upper pivot turns in bearing, having two bushings; the bearing is mounted in upper distributor ring whose outside flange rests on the upper flange of the speed ring.The inner flange of the upper distributor ring carries turbine cover plate. The top of the guide vane is connected with gate ring via a lever and link which is adjusted during fitting. The gate ring rests on support and is connected with the distributor servomotors by rods. Displacements of the servomotor pistons causes rotation of the gate ring, so that the guide vanes are actuated via the links and levers, thus opening or closing the distributor.

A braking element is provided in each guide-vane actuating mechanism in order to permit closing of the distributor and shut-down of the turbine when a foreign object has become wedge between two vanes. The distributor can then be closed, except for the duct in which the foreign object is wedged.

The braking element in most modern designs is shearing pin inserted into lever and lever bushing. The lever is secured to the upper guide-vane pivot by dowel. Vane cover is placed on top of the lever and fixed to the upper guide-vane pivot by a screw. The guide vane is suspended from this screw and thus rests on the bearing end face via the lever bushing. This makes it possible to adjust the vane and equalize the clearances at top and bottom.

Leakage of water from the turbine past the guide-vane pivots is presented by seals which consist of rubber or leather glands. Any water seeping through the upper pivot gland is collected in an annular groove through the bearing and discharged into the turbine cover plate through pipe.

The supporting bushings of bearing and lower pivot are made of bronze or gun metal. The latter is a laminated wood plastic made of birch plywood; it has good mechanical and antifriction properties, but tends to swell in water. Both bronze and lignofol bushings should be lubricated with thick oil or grease.

The guide vanes are hollow steel castings integral with their pivots. Welded guide vanes recently been introduced in large hydraulic turbines. These guide vanes consist of forged pivots and vane bodies stamped from sheet metal.

Leakage through the closed distributor of a low-or medium-head turbine is prevented by means of rubber seals. Such a seal consists of a grooved planed into the front of the vane. The groove has a dovetail cross-section into which a rubber cord is inserted. This rubber cord can be secured in addition by a spring strip. Such cords are inserted to annular grooves machined into the lower distributor ring. The leading edge of each guide vane is forced against the rubber cord in the neighboring vane when the distributor is closed. The chords in the upper and lower rings seal the clearances at the vane end faces.

The guide vanes of high-head turbines are usually sealed by fitting them accurately against one another, so that there is no clearance between them when the distributor is closed.

The total end clearance between the guide vane and upper and lower distributor rings is 0.5 to 0.6 mm in medium- size turbines and up to 1.5 to 2.0 mm in large ones. When adjusting the required clearances in a turbine having the thrust bearing on the cover plate it should be remembered that operation of the generating set causes a certain settlement of the upper distributor ring as a result of the load transmitted to it by the axial hydraulic force and the weight of the rotating parts. This reduces the end clearances, so that guide vanes may jam if the gaps are too small.

Faultless and safe operation of the generating set require that leakage of water through the closed distributor should not cause the rotating parts to move when no brake is applied, and that the hydraulic turbine can be stopped without braking (by letting it coast to rest).

The distributor of a francis turbine does not differ in design from that of a Kaplan turbine. The only difference is that the upper distributor ring is integral with the turbine cover plate. Usually no provision is made in a francis turbine for removing the guide vanes without lifting the turbine the cover plate.

The design of the upper distributor ring and of the turbine cover plate depends on the dimensions, type, and general layout of the turbine considered. Usually no provision is made in a francis turbine for removing the guide vanes without lifting the turbine cover plate.

The design of the upper distributor ring and the turbine cover plate of recent designs are usually fabricated from parts having box sections. These components are made in several parts because of transportation requirements, and are then bolted together.

The gate ring transmits the forces from the servomotors through a lever system simultaneously to all guide vanes. One or two eyes are provided on top of the gate ring for the pins of cylindrical hinges connecting it to the servomotor rods. Eyes for the link, hinges are provided at the bottom the gate ring. The gate ring is made in several parts if it cannot be transported as a whole; these parts are then bolted together. The hollow space in the gate-ring support is filled with oil or thick grease in order to reduce the force which the servomotors have to exert in order to rotate the gate ring.

3.3.1. Servomotors:-

Servomotors are simple piston cylinder assembly which operates by oil pressure and get command from governing system for opening or closing the guide vanes for starting, loading and closing the unit as per system requirement.

Fig.:- Speed Control by Servomechanism.

The guide vanes are rotated by means of one or two servomotors. Activation by a single imposes a non-uniform load on the gate ring and is therefore employed only in small hydraulic turbines. Two servomotors apply a couple to the gate ring, so that the load acting on it is uniform and the force transmission is simpler.

Actuation by two servomotors is used in modern medium-size and large hydraulic turbines. The servomotors are in this case usually secured on flanges in recesses in the steel lining of the turbine pit. One of the servomotors is equipped with a locking device so that the turbine can be blocked when the distributor is closed.

A piston-type servomotor mounted on a supporting plate outside the gate ring. Servomotor cylinder is closed by rear cover and front cover. Piston moves inside the cylinder. Guide-sleeve passes through the front cover and carries the piston at its end. The piston is connected to servomotor rod by means of piston pin. The servomotor rod forms the connection to the gate ring. Locking device is mounted on the front cylinder cover. Stuffing box is mounted on the front cover in order to prevent leakage of oil along the guide sleeve.

The servomotor is locked by stop which descends into the gap between the guide sleeve and the locking device when the piston is in its extreme left position. The stop is moved by auxiliary servomotor. The position of the piston of the main servomotor is shown by indicator.

The servomotor has two flanges to which the oil pipes are connected. Oil is delivered into the cylinder on either side of the piston and thus moves the latter from one position to another. The oil is discharged from the cylinder through ports into pipes and from there into a tank where it is collected and returned to the oil installation. The piston is broken at the end of its closing stroke. This prevents impact of the piston against the rear cylinder cover when the distributor is closed suddenly. Braking is effected through the closing of a

bypass by the piston before it reaches its end position. The oil can then escape only through a throttling orifice of small diameter. This increases the oil pressure on the left of the piston and stops its motion.

3.3.2 Guide vane links, lever & bush housing :-

Individual guide vane is having bottom trunnions and top trunnions and a shaft. Bottom trunnions of guide vane are housed in individual bottom bush which is located in bottom ring at specified P.C.D. (Pitch Circle Diameter).Top shaft of individual guide vane is having a bush housing fixed in top cover. Bush housing is having adequate sealing arrangement so that water does not come in the top cover. Individual guide vane at top also have a guide vane connected to it which in turn is connected to regulating ring by means of suitable link which is also individually adjustable. 3.4 Shaft sealing system:-

3.4.1 Service shaft seal -

The rotating part and the stationary part of the generating unit do have some clearances. Runner and shaft (both moving parts) and the turbine cover (stationary part) also has clearance. the high pressure water comes from penstock and falls on runner to cause rotation. Due to high pressure, water tends to move in all the directions where opening (clearance) is found. The water also tends to enter in top cover from runner chamber through clearance through clearance between top cover and turbine shaft. To prevent this water to come out from the top cover/neck, a sealing arrangement is made between the runner and the turbine top cover. This is generally called shaft sealing arrangement. Because this seal works during operation, the seal is also referred as service seal.

3.4.2 Maintenance seal -

In some of the shaft seal system, a seal, below the shaft is provided. This is affected when service-seal is required to be opened for the maintenance/replacement. Therefore, this seal is called maintenance seal. The maintenance seal can be engaged or disengaged during the non rotation of the shaft. By engaging the maintenance seal, the shaft seal (also can be referred as service seal) can be dismantled/repaired under the pressurized water created by the tail race water level.

The maintenance seal can be engaged by jacking the rotating parts-assembly of the generating set and keeps on providing the sealing effect in the jacks’ position.

This type of maintenance seal is use at Salal power station.

Fig.:- Aerial view of maintenance seal.

There are various types of shaft sealing arrangements:-

i) Gland type arrangement -

For small machine with low head, simple gland packing is provided between the moving shaft surface and gland housing provided/supported at the top cover neck. the simplest type of gland packing is made of asbestos rope or ribbon of suitable cross section. gland sealing is self lubricating in nature. The life of gland sealing is not very long. therefore, it requires frequent replacement.

ii) Carbon segment type -

In this type of sealing arrangement, soft carbon sectors matching to the shaft circular profile are provided and housed on the sealing supporting arrangement. The assembly is placed on the top cover. During the rotation of the shaft, the soft-carbon-sector parts keep on touching the shaft with the help of the spring whose tension can be adjusted as per the requirement. This way the pressurized water is stopped to come out from the top cover and the runner shaft clearance. When the working pressure of the turbine is very high more than one such soft carbon sectors are used one above another to achieve the sealing effect. Clean water is used as coolant for lubrication purposes.

This type of arrangement is shown in use at Chamera-I HEP. It is interesting to note that replaceable liner is fixed on shaft coupling and sealing arrangement is done at this point. In some other arrangements replaceable liner is in space between journal and flange.

iii) Rubber (neoprene) sheet type -

In this kind of sealing arrangement a seal drum (rotating along with shaft) is mounted on the shaft. At the top cover neck a supporting structure is provided where annular (the diameter of the inner annular rubber seal is less than the diameter of the seal drums) rubber seal can be accommodated/fixed. The similar arrangement is again provided above the previous rubberized seal which forms the integral part of the sealing arrangement. Between these two rubber seals, the pressurized water (1 to 2 kg/cm2) as clean as possible is provided. At the working pressure, the pressurized water starts flowing between the labyrinths provided at upper portion of the runner and at the top cover. The water looses its pressure by the action of the labyrinth and a residual pressure of .5 to 1 kg/cm2 is supposed to act on the rubber seal. Therefore, the pressure on outside of sealing annular rubber sheet is more in magnitude which prevents leakage of water from inside turbine chamber system. Hence the sealing effect is achieved. The pure water is taken from the filtration plant and this water acts as lubricant and the heat sink for the labyrinth sealing sheet when rotation of the shaft takes place during operation.

The life of this sealing sheet depends upon the wear and tear as it keeps on pressing the rotating drum. Hence, during the rainy season when some silt is present in the filtered water, the seal needs frequent replacement/repair.

This type of arrangement is used at Salal power station.

iv) Hydro dynamic type -

In this type of sealing arrangement, water jets are directed to the clearance between shaft (moving) and turbine top cover (stationary). The pressure of this water jets is so adjusted that it counters the water pressure coming out through the clearances of shaft surface and the top cover neck. This type of shaft sealing arrangement does need a high pressure water pump and homogeneous providing the jet-water around the place where sealing effect is required. For high head turbine system this method is more suitable for shaft sealing arrangement. This type is used at Loktak power station

4. CRITERIA FOR SELECTION OF HYDRULIC TURBINES:-

4.1 General guideline for selection of type of turbine:i) low head up to 20M, discharge and load vary widely - Bulb turbineii) Medium and lower range of heads (up to 80M ) constant discharge – Fixed blade

propeller turbineiii) Medium and lower range of heads (up to 80M) when discharge and load vary

widely - Kaplan

iv) Medium and high range of 30-550M, turbine discharge varying in acceptance limits – Francis turbine

v) Head greater than 400M, varying head discharge and load conditions- Pelton turbine

Type of turbine Approx. head range (Meters)

Head variation (% of rated head)

Specific speed range (MHP)

Peak efficiency ( percentage)

Pelton Above 300 120-80 15-65 92Francis 30-400 125-65 60-400 96Deriaz 50-150 125-65 200-400 95Kaplan 10-60 125-65 300-800 95

Propeller 10-60 110-90 300-800 93

Selection of type of turbine

Relation between head and specific speed

4.2. Criteria for selection of hydraulic turbine in over lapping zone:-

i) Minimum output for continuous operation

Type of turbine Minimum output for continuous operation Pelton 30 Francis 50 Kaplan/ Bulb 30 Propeller 85 Deriaz 40

ii) Turbine setting and Excavation requirementsiii) Transport considerations (Runner)iv) Pressure rise and speed rise considerations:

Type of turbine Pressure rise (%) Speed rise (%) Pelton 15 -20 20-25 Francis 30-35 35-55 Kaplan/ Bulb & Propeller 30-50 30-65 Deriaz 20-45 35-65

v) Turbine efficiency vi) Susceptibility to silt

5.CAVITATION IN HYDRAULIC TURBINES:-

The surfaces of the water passages of hydraulic turbines very often undergo a peculiar kind of sponge-like destruction during operation. This damage is caused by cavitations, which is a very complicated physical phenomenon appearing in a rapidly flowing liquid.

Damage due to cavitation is especially severe at the backs of the runner blades and at the throat rings of axial turbines, at the runners and foundation rings of francis turbines, and at the buckets and nozzles of Pelton turbines.

Cavitation is accompanied by noise, shocks, and strong vibrations of the set. The efficiency decreases abruptly, as do the discharge capacity and the power developed by the turbine.

One of the main causes of cavitation are considered to be sharp local pressure pulsations in the water. The continuity of the flow breaks down at very high velocities. Voids or cavities are formed in the zone of maximum velocity; these voids are filled with vapour of the liquid, whose pressure depends on the temperature of the surroundings. These voids and cavities are entertained by the current to regions of higher pressure, where the vapour condenses in the voids and latter collapse. A surface adjacent to such collapsing voids (bubbles) is gradually destroyed. Physical phenomena are observed during cavitation and these cause luminescence of the cavities; chemical reactions also commence and cause oxidation (corrosion) of the metal.

One method of countering the destructive action of cavitation is to use cavitation resistant materials for the parts of the turbine water passages. At present only stainless steel containing chromium is used.

The most promising way of preventing such damage is to ensure cavitation –free turbine operation. Such conditions can be created by matching the turbine type to the head, by correctly choosing the vertical position of the turbine in relation to the tailrace, and through restrictions imposed on the operating conditions of the generating set.

Cavitation in reaction turbine can be prevented or reduced by arranging the runner above the tail water level or below it at a vertical distance not exceeding the permissible value for cavitation-free turbine operation.

A criterion of the suitability of a certain turbine for operation for operation at a given head is the turbine cavitation coefficient σT. It depends on the form and dimensions of the water passages and on the turbine operating conditions. The turbine cavitation coefficient is determined by means of test of model turbines. It is a uniquely defined magnitude for every runner and is plotted on the universal turbine characteristic.

The cavitation coefficient σT varies in practice between 0.4 and 2.0 for modern axial turbines and between 0.3 and 0.35 for modern francis turbines.

For cavitation to be prevented it must be borne in mind during design of the hydropower plant that the turbine coefficient must be slightly less than the cavitation coefficient of the power plant. This can be achieved by employing turbines with a low σT – values or by increasing σpl. The latter procedure involves a reduction of the draft head HS. This, however, usually leads to a considerable increase of the turbine depth and thus to a greater volume of construction work. One, therefore, mostly tries to use with a turbine with a low σT – value, i.e., with good cavitation properties.

The draft head of a vertical propeller or Kaplan turbine is measured from the axes of rotation of the runner blades to the tail water level HS. the corresponding magnitude of a francis turbine is reckoned to be the vertical distance between the plane of the lower speed ring and the tail water level. The draft head of a horizontal turbine (with scroll casing or of bulb type) is the distance between the highest point of the runner blades and the tail water level. The draft head is considered to be positive if the tail water level is located beneath the above-mentioned reference elevations, while it is considered to be negative if the runner is located below the tail water level.

Fig. 1:-Spiral form cavitation.

Fig.2:-Damaged blade of Kaplan turbine due to cavitation.