Doc - 03 Fundamental of Steam & Gas Turbine.pdf

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TRAINING MANUAL 388.5 MW Combined Cycle Power Plant DOC No. Fundamental of Steam & Gas Turbine Page No.

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IBDC/ L&T/ VCCPP/ 03 Page 1 of 50

Fundamental of Steam Turbine & Gas Turbine I. Steam turbine Introduction A steam turbine extracts the energy of pressurized superheated steam as mechanical movement. It has completely replaced the reciprocating piston steam engine primarily because of its greater thermal efficiency and higher power to weight ratio. Also, because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator as it doesn't require a linkage mechanism to convert reciprocating to rotary motion. The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency to the use of multiple stages in the expansion of the steam (as opposed to the one stage in the Watt engine), which results in a closer approach to the ideal reversible process. Turbines are classified according to principle action of steam i.e. Impulse and Reaction type. In impulse type blade steam is not making impact on blade but the steam is gliding over the smooth surface of blade and changing its direction. The rate of change in momentum at inlet and outlet of the blade will cause the impulse force on blade. Impulse blades are thicker at middle and thinner at both the end. The blade is having a symmetrical shape. The reaction type blade will act as convergent and divergent nozzle. The steam turbine blade is thicker at inlet and thinner at outlet. The steam expands across the blade and pressure energy is converted into kinetic energy, the high jet velocity is produced. The velocity of steam leaving the blade is very high and this leaving velocity will create propulsive force on blade which is known as reaction force. The first impulse steam turbine was built and tested by the Swede Carl Gustav de Laval in 1883. One year later, in 1884, Charles Algernon Parsons succeeded in developing a reaction steam turbine. Since both of the processes are of the same standard, they are still in use today in industrial and large-scale turbines. The following prerequisites had to be fulfilled for bringing the development of the steam turbine up to today's technology standard: the production of new heatresistant materials, and a highly sophisticated manufacturing technology. The steam turbine became the most important driving engine for the electric power generation process, since most of the generators are driven by steam turbines. It is the engine featuring the highest unit capacity. In conventional reheat turbines, there are single-shaft turbine generators with an output of 850 MW, in saturated steam turbines there are single-shaft turbine generators with an output of 1,360 MW.


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Design and function of a steam turbine General design of a steam turbine Figure 1 shows a longitudinal section of a single-casing condensing steam turbine designed for a speed of 3000 min-1 with small output which operates according to the reaction principle.

10. control valves 11. blade wheel 12. guide blade row 13. moving blade row 14. exhaust steam nozzle

1. turbine rotor 2. turbine casing 3. radial bearing 4. radial bearing 5. axial bearing 6. nozzles 7. front shaft seal 8. rear shaft seal 9. steam chest

15. extraction 16. main oil pump 17. speed governor 18. actuator 19. flange coupling 20. turning gear (hydraulic) 21. front bearing casing 22. casing drainage pipes

Fig 1 • Basically, it consists of the turbine rotor (1) rotating on its own axis, and the static turbine casing (2). The turbine casing encloses the rotor. • The rotor is mounted between two journal-type radial bearings (3 and 4). An axial bearing (thrust bearing) (5) fixes the rotor in an axial direction and takes up the axial thrust produced by the steam. • There are contactless shaft seals (7 and 8) at the outlet points of the shaft ends. • The steam is led through nozzles (6) and guide blades (12) through the turbine. • Coming from the admission casing, also referred to as "steam chest" (9), the live steam goes through the control valves (10) to the nozzles (6) and hits against the blade wheel (11). • Then the steam flows through various stages. One stage consists of a static guide blade row (12) and a moving blade row (13) mounted to the rotor. Length and width of the blades increase from stage to stage because the steam volume increases while the pressure of the steam decreases on its way through the turbine. The steam transfers a great part of its usable energy to the rotor, and flows then through the exhaust steam nozzle (14) into the condenser, where it is condensed


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as water. At the different extraction points (15), steam can be extracted at different pressure levels. • The front bearing casing (21) accommodates the main oil pump (16). This pump delivers the high-pressure oil for the bearing lubrication and the control systems. • The speed governor (17) is located in the front bearing casing, too. Through the actuator (18), the speed governor moves the control valves (10). • In the rear part of the turbine, the inductor of the generator is coupled with a rigid flange coupling (19) to the turbine rotor. The rotor can be slowly turned by the turning gear (20) after stopping and before starting the turbine, in order to cool down the rotor as steadily as possible and to prevent it from deforming during cooling down. Operating principle of the steam turbine Generally speaking, a turbine's purpose is to convert energy. In the turbine blades, the thermal energy (pressure and energy) of the steam is converted Ø first, into flow energy (kinetic energy) of the steam Ø then, into mechanical energy (kinetic energy) of the turbine. The conversion of the energy can take place by means of two different methods on the basis of which the turbines are also classified: 1. Impulse method 2. Reaction method Both methods have been in use for more than 100 years. They are absolutely of the same standard and often used simultaneously in one steam turbine. Impulse method In the impulse method, the whole thermal gradient of one stage is converted in a nozzle into flow energy and is then led to the moving blades. The steam jet leaves the nozzle at a high velocity and is deviated inside the moving blade channels. The velocity of the steam is reduced thereby. The steam jets hits the moving blade at full power. The blade wheel is turned by this impulse force. This is principle illustrated in Fig. 2.


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TRAINING MANUAL 388.5 MW Combined Cycle Power Plant DOC No. Fundamental of Steam & Gas Turbine Page No. 1


2

4 3

1. steam 2. acting force 3. nozzle (mounted in the casing) 4. moving blade

Fig 2. Principle of the impulse method • The moving blade channels have a constant cross-section. • That means the inlet cross-section is equal to the outlet cross-section. • Therefore, there is no expansion (decrease of pressure) of steam in the moving channels. In this process, the same pressure prevails upstream and downstream of the rotor blade. That is why it is often termed as "constant pressure method". Figure 3 shows a longitudinal section of the nozzle and moving blade part of an impulse turbine with a single-row impulse wheel (Laval wheel). The pressure and velocity characteristics of this turbine is diagrammatically shown in the top view.

nozzles

moving blades

p1 steam pressure upstream of the nozzle p2 steam pressure downstream of the moving blade c0 velocity of the incoming steam (e.g. in the live steam line) c1 velocity of the steam at the 1. step

steam flow

2. step

nozzle outlet c2 velocity of the steam downstream of the moving blade

Fig 3. Longitudinal section of the impulse stage with functions of pressure and velocity


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In an impulse turbine, the radial clearance between blade wheel and casing can be great since the steam pressure is the same upstream and downstream of the moving blade. Nevertheless, the leakage losses occurring at these points remain small. Theoretically, an axial thrust does not exist in a pure impulse turbine. In practice, however, larger impulse turbines are designed to feature a certain percentage of reaction too. Thus, an axial thrust exists which is actually a characteristic feature of the reaction method. The existing axial thrust must be compensated. Normally, the impulse wheel ("A-wheel") has only one moving blade row and is thus a single wheel. If one wants to strongly decrease the pressure and temperature of the live steam in the first stage, a higher thermal gradient must be processed in the nozzle, resulting in a higher steam velocity. This increased steam velocity is converted in a two-row impulse wheel. This impulse wheel is also referred to as "Curtis wheel" ("C-wheel") after the American Curtis, who, in 1896, was the first to introduce velocity staging through such an impulse wheel. In this process, the velocity of the steam is not fully exploited at the nozzle outlet in the first moving blade row. The steam is rather deflected through a static guide blade row and led to a second moving blade row. (Fig. 4 and Fig. 5).

1. turbine casing 2. nozzle 3. nozzle segm ent 4. calking material 5. guide blade 6. calking material 7. seal strip 8. moving blade 9. turbine shaft 10. calking piece 11. shaft sealing

Fig 4. Impulse stage with a multirim impulse wheel (Curtis wheel, C-wheel)


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n o z z le s

800

liv e s te a m p re s s u r e c h a r a c te r is tic o f v e lo c ity

600

400 c h a r a c te r is tic o f p re s s u re o u tle t p re s s u r e

200

0

Fig 5. Schematic showing the pressure and velocity characteristics in a Curtis wheel The use of a Curtis wheel has advantages and disadvantages in comparison with the single-row impulse wheel: • Advantage: stage. • Disadvantage: (friction losses).

Processing of a greater thermal gradient is possible in just one Worse efficiency since the steam is repeatedly deflected

The Curtis wheel is mainly used in industrial back-pressure turbines. Here, worse efficiency is less important, because the exhaust steam can be exploited in other steam consumers. The Curtis wheel is usually given a two-stage design, sometimes also a three-stage design. It can be fully or partially admitted with steam. Fig. 6 shows a so-called "Zoelly turbine". A Zoelly turbine is an impulse turbine with pressure staging, including several single-stage impulse wheels connected in series.



impulse chamber III, etc.

impulse chamber II


impulse chamber I

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p

steam pressure

c

steam velocity

Fig 6. Impulse method with pressure staging • •

•

Here, the rotor consists of several blade wheels. Between the blade wheels, there are the diaphragms (guide wheels) with the nozzles. In the nozzle of the individual stages, only a part of the thermal gradient is transformed into velocity. The transformation decreases proportionally to the number of the turbine stages.

•

From stage to stage, less of the thermal gradient is converted into velocity.

•

Reaction method In the reaction method, the thermal gradient is transformed into flow energy (i.e. kinetic energy) inside the fixed guide blade channel as well as inside the moving blade channel. The force at the moving blades is generated by: 1. the deflection of the steam jet inside the moving blade channel 2. the reaction effect of the outlet steam jet. Fig. 7 illustrates the principle of the reaction method.


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steam


nozzle-shaped m oving blades reacting force

Fig 7. Illustration of the reaction method In the reaction method, the cross sections of the guide and moving blade channels are shaped like nozzles. The steam expands in the "nozzle". Therefore, there is just as different a pressure upstream and downstream of the moving blade as upstream and downstream of the guide blade. The blade shape as well as the pressure and the velocity characteristics of a reaction turbine are depicted in Fig. 8.

nozzles

moving blades

p1 steam pressure upstream of the guide blade p2 steam pressure downstream of the guide blade, i.e. upstream of the moving blade p3 steam pressure downstream of the moving blade c1 velocity upstream of the guide blade c2 velocity downstream of the guide blade, 1. step

2. step

i.e. upstream of the moving blade c2 velocity downstream of the moving blade

steam flow

Fig 8. Longitudinal section of a reaction stage with functions of pressure and velocity Compared to an impulse stage, a reaction stage can exploit only a small thermal gradient at a good efficiency factor, due to the sealing problems between moving blade and casing. Therefore, reaction turbines are provided with more stages than impulse turbines (compare Fig. 9).


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drum

p steam pressure c steam velocity

Fig 9. Reaction turbine with 5 stages (drum-type turbine) Although they have great number of stages, they are not bigger than reaction turbines because the blade rows can be arranged closely next to each other on the drum-type rotor. Since the pressure in reaction turbines is higher upstream than downstream of the moving blades, an axial thrust exists in the direction of the turbine outlet cross section. To compensate this axial thrust, a so-called "balance piston" (also referred to as "dummy piston") is necessary.

1 axial thrust by the reaction rotor 2 axial thrust by the balance piston 3 impulse wheel

Fig 10. Compensation of axial thrust through the balance piston • • •

The balance piston is solidly mounted on the turbine shaft. Its exterior side is connected to the low pressure side of the turbine. The steam pressure (2) acts on the interior surface of the piston in opposite direction of the axial thrust (1) and balances it to a large extent. The diameter of the balance piston, and thus the surface available for the pressure to act upon, must be dimensioned accordingly.


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If in double-casing reaction turbines the steam flows in the opposite direction, the axial thrusts neutralize each other to a large extent. In that case, smaller balance pistons mounted on the two turbine shafts are sufficient. In double-flow turbine sections, into which the steam enters in the middle of the turbine rotor, axial thrust compensation works similarly to the double-casing reaction turbines with opposed steam flow. Balance pistons are not necessary. This design is often used for low-pressure turbines (LP turbines). High-pressure turbines (HP turbines) and intermediate-pressure turbines (IP turbines) are designed as double-flow turbines, if they have a high output. Reaction turbines are fully admitted with steam That means the steam flows over the whole blade wheel from one stage to the other, flowing around all the blades. In partial steam admission, a pressure balance would be reached via the surface which is not admitted with steam. Reaction stages are usually provided with an upstream-connected impulse stage or a Curtis wheel. In these impulse stages partial admission is possible. This allows, moreover, to process a high thermal gradient over a few stages. In reaction turbines, the radial clearance between the rotating moving blades and the casing as well as between the fixed guide blades and the rotor must be as small as possible, in order to keep the radial leakage losses as small as possible (sealing!). The guide and moving blades of reaction turbines have smaller profiles than those of an impulse turbine because only a small thermal gradient is exploited in the individual stages; a smaller force acts on the blades. The following table gives an overview of the different characteristics of the impulse method and the reaction method: Characteristics Energy conversion inside the guide wheel of one stage

Impulse method the whole stage heat drop is transformed into kinetic energy

Moving blades

steam jet is only deflected (change of direction of the steam jet) = change of impulse = force acting on moving blades constant cross-section

Cross section of the moving blade channels Pressure on the moving blades Admission of the moving blades Exploitation of the thermal gradient in one stage Axial thrust Profile of the blades Loss of steam in the radial gap of the blade wheels

pressures upstream and downstream of the moving blades are equal fractional admission is possible great thermal gradient can be exploited, hence a small number of stages theoretically non-existing, practically not zero thick profile small

Reaction method only a part of the stage heat drop is converted into kinetic energy (the rest is converted in the moving blades) the steam jet is deflected and just a part of the pressure gradient is converted (change of direction and pressure) are deviated

nozzle-shaped cross-section pressure upstream of the moving blades is higher than downstream only full admission is possible only a small thermal gradient can be processed, hence a high number of stages existing; must be balanced slim profile high


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Design types and designation of steam turbines The characteristic features and names of the various types of steam turbines are standardized in the German DIN standards DIN 4304 and DIN 4305. This facilitates communication between steam turbine manufacturers and operators. This chapter gives an introduction into various characteristic features used for differentiating and classifying different steam turbine types. Steam turbines can be classified according to the following characteristics: • • • • • • • •

according according according according according according according according

to to to to to to to to

the the the the the the the the

flow direction of the steam, operating principle, nominal steam condition at the inlet, steam supply, steam exhaust, steam side configuration, structural design of the entire turbine, and intended use.

Differentiation according to the steam flow direction Depending on the flow direction of the steam, one differentiates between: • axial turbines • radial turbines Axial turbine The steam flows in one or several axial stages in the direction of the turbine axis through the blades. It streams alternately through the fixed guide blade rows and through the moving blade rows which rotate around the rotar. Axial turbines are designed as impulse and reaction turbines.

Fig 11. Section drawing of an axial turbine HP section

2. Radial turbine: The steam flows in one or several radial stages in an approximately radial direction through the turbine. Depending on the design, the steam flows either from the inside


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to the outside, or from the outside to the inside. Radial turbines are built very rarely today. A special design variant of the radial turbine is the Ljungström turbine. The Ljungström turbine is a counter-rotating radial turbine and was named after the Swedish brothers B. and F. Ljungström, who built the first turbine of this kind in 1910. Differentiation according to the operating principle As discussed earlier, depending on the operating principle, one differentiates between • impulse turbines and • reaction turbines Differentiation according to the nominal steam condition at the inlet The nominal steam condition is described by the thermodynamic steam parameters. Depending on the level of the steam pressure at this point, one can differentiate between: Low pressure turbines Intermediate pressure turbines High pressure turbines Maximum pressure turbines

inlet pressure inlet pressure inlet pressure inlet pressure > (=critical steam

< 10 bar 10 - 90 bar 90 - 221 bar 221 bar pressure)

Differentiation according to the steam supply Here, one differentiates between: • live steam turbines • exhaust steam turbines • multipressure turbines (also referred to as "mixed-pressure turbines") 1. Live steam turbines The steam necessary for energy conversion is directly supplied from the steam generator. This takes place at temperatures and pressures as high as possible (high efficiency). 2. Exhaust steam turbine Exhaust steam turbines exploit steam with low pressure and expand it down to levels in the negative pressure range. The steam is either exhaust steam from engines or originates from thermal processes. Exhaust steam turbines are low-pressure turbines with a small usable thermal gradient. They are virtually live steam turbines with a low inlet pressure. 3. Two-pressure or multipressure turbines The steam is supplied through separate steam supply systems. The steam pressure levels can be different but should be kept as constant as possible.


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Differentiation according to the steam exhaust Here, one differentiates between • codensation turbines • back-pressure turbines • bleed and extraction turbines 1. Condensing turbines In condensing turbines, the steam condenses in a downstream condenser, giving off heat. This condensation heat of the steam is normally not used any more, but is released into the environment via a cooling agent (normally cooling water). The mechanical energy of the steam (pressure) is supposed to be used as far as possible. Therefore, the escape pressure must be as low as possible. In modern plants, the escape pressure, which is equal to the pressure in the condenser, is about 0.05 bar; it is thus clearly below the ambient pressure of 1.013 bar. In condensing turbines, this difference between the energies contained in the steam at the inlet and at the outlet of the turbine is very great. The energy content of the steam is also referred to as "enthalpy". That is why this theorem is valid that condensing turbines exploit a great thermal gradient. Since the steam greatly increases its volume in the turbine, the blades in the final low pressure stages must have a great cross-section and must therefore be especially long. Due to the effect of centrifugal force, however, their length is limited. That is why the low pressure parts of condensing turbines are given a multiple-flow design, i.e. in practice there are several low pressure turbines, not just one. 2. Back-pressure turbines In back-pressure turbines, the steam is discharged into downstream-connected back-pressure steam system. The steam leaves the turbine at a certain positive pressure and is used in the downstream-connected industrial operations for heating and manufacturing purposes. This way, part of the steam energy can be used directly for electric power generation and the rest as process energy. Steam energy can so be used in a particularly economically efficient way. In this version, the steam quantity for the turbine - and hence the shaft power - is firmly coupled to the process steam quantity required for industrial production operations. Therefore, the generation of electric power depends on the demand for back-pressure steam (which is a disadvantage for demand-dependent electric power supply). 3. Bleed and extraction turbines Both turbine types are used in places where not only generation of electric power but also heating steam is needed. For this, steam can be extracted at one or several points. Typical of these turbines are the control valves at the bleed or extraction points. Larger condensing turbines have bleed points for preheating condensate and feedwater. Also back pressure turbines can be equipped with bleed points or controlled extraction points.


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In bleed turbines, the steam pressure at the bleed point is not controlled. The steam pressure changes depending on the turbine output (i.e. steam throughput). In extraction turbines, the steam pressure is initially partially expanded and then kept constant by a suitable control equipment at the extraction point. The electric output is controlled independently of the steam extraction. Differentiation according to the steam side process design Here, one differentiates between: • topping turbines • tailing turbines • reheat turbines • branch turbines 1. Topping turbines Topping turbines are back-pressure turbines with their own driven machine (generator). They are connected upstream of the main turbine and expand the pressure of the live steam as required for the main turbine. They discharge their steam also to back-pressure steam systems, which supply the downstream turbine generators. Sometimes they are used in older power stations to increase the output of such a power station by generating high-pressure heating steam without changing the existing turbine generators of the plant and to attain thus a better thermal efficiency. 2. Tailing turbines Generally, they are intermediate pressure turbines with their own driven machine (generator). They are connected downstream of the main turbine in order to exploit its exhaust steam. 3. Reheat turbines Reheat turbines are high-pressure or intermediate-pressure turbines, to which single-reheat or multiple-reheat steam is admitted. They are operated as backpressure turbines supplying their exhaust steam to downstream condensing turbines. The reason for reheating is to improve efficiency. For this, the exhaust steam of the high-pressure topping turbine is reheated in the boiler. The reheat steam should have approximately the same temperature as the live steam at the inlet of the reheat turbine (inlet pressure about 40 bar). 4. Branch turbines Branch turbines are counter-pressure or condensing turbines with their own driven machines (pumps, compressors, generators, etc.). They process partially expanded steam which is branched off or bled off from a main turbine. The inlet pressure (in back-pressure turbine also the exhaust steam pressure) varies with the load of the main turbine. Differentiation according to the structural design Here, one differentiates between: • type of power transmission • number of casings


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• number of parallel steam flow sections • number of shaft trains 1. Type of power transmission 1.1 Directly coupled turbines These are directly connected to the driven machine (generator, pump, compressor, etc.). 1.2 Geared turbines Geared turbines are connected to the driven machine by an interconnecting gear. The turbine speed is always higher than the speed of the power driven machine. The turbine speed is 6,000 - 20,000 min-1. Geared turbines have a relatively small output (150 kW - 50 MW) and are equipped with an A-wheel, 2-C-wheel, or are given a multiple-flow design, depending on the output. Geared turbines with an output of 50 MW are used for generation of traction current; here, the generators coupled to the gear are built for a speed of 1000 min-1 (corresponding to 16 2/3 Hz in 2-pole generators). For historical reasons, the alternating current frequency in the German traction system is 16 2/3 Hertz and not 50 Hertz as in the mains. In other countries both systems have the same frequency (e.g. in France approx. 250 Hz). In geared steam turbines with small output, turbine, gear, oil supply (pumps and cooler) and oil tanks are unit-mounted (package-design). 2. Number of casings 2.1 Single-casing turbines In single-casing turbines, the whole steam expansion takes place in just one turbine enclosed by a single casing. 2.2 Multiple-casing turbines In multiple-casing turbines, the steam expansion is distributed among several turbine units, which are connected in parallel or in series, each unit with its own casing. If the existing enthalpy gradient were converted in just one stage, the steam velocities and thus the turbine speed would get too high, causing problems in terms of material strength. In order to avoid this, turbines are designed with several stages arranged one downstream of the other. Since the length of turbine rotors is limited for mechanical reasons and rotors hence can accommodate only a limited number of moving blade rings, multiple-casing turbines are built.

3. Number of parallel steam flow sections 3.1. Single-flow turbines Each one of the single turbine sections (HP, IP, LP) consists of one turbine through which the entire steam flow passes. 3.2 Multiple-flow turbines The steam mass flow is distributed to several turbine sections configured in parallel. In high-output condensing turbines, for instance, the low-pressure section is given a double-flow, three-flow or four-flow design because the steam volume at the turbine


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outlet is very high. This requires a very large turbine outlet cross-section and hence very long exhaust blades. The practically feasible blade length, however, is limited by the strength of the material (centrifugal force increases proportionally to the blade length). Hence, "double-flow" or "multiple-flow" means that the steam flow is divided among two or several equal turbines. Saturated steam turbines and big reheat turbines may even have a six-flow design. 4. Number of shaft trains 4.1 Single-shaft turbines Single-casing or multiple-casing steam turbines which have just one shaft, or one shaft train formed by a rigid-connection mechanical couplings of single-shafts.

4.2 Multiple-shaft turbines Multiple-casing steam turbines which are distributed among two or several shafts or shaft trains. Each shaft train can drive a machine either separately or together with other shaft trains coupled by a gear. Differentiation according to the intended use Depending on the intended use one differentiates between: • • • • • •

power station turbines industrial turbines combined heat and power station turbines auxiliary turbines driving turbines for pumps/compressors marine turbines

Structural components of the steam turbine The main components of a Steam Turbine are: • turbine blading (nozzle blades, guide blades and moving blades) • turbine rotor • turbine casing • shaft seals • bearings and bearing casings • emergency stop valves • control valves for the nozzle segments Critical speed Critical speed is the coincidence of the natural vibration frequency and the speed of the turbine rotor (resonance). While the turbine speed is accelerated through the critical speed range, the turbine rotates un-smoothly. It is therefore necessary • to accelerate the turbine as rapidly as possible through the critical speed range (the critical speed must not coincide with the operating speed!), and • to keep a minimum clearance of +/- 20 % between the operating speed and the critical speed. The critical speed depends on the magnitude of the bending of the rotor (i.e. on its flexibility), due to his own weight.


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The higher the bending of the rotor, the lower the critical speed. Slim rotors feature a great bending. Therefore the operating speed is higher than the critical speed. In this case, it is a flexible, elastic rotor, also referred to as a "supercritical rotor". Thick rotors (e.g. drum rotors) feature a low bending (the critical speed is higher). In this case, the operating speed is lower than the critical speed. This stiff or rigid rotor is, thus, referred to as a "sub critical rotor". In a turbine consisting of several flexible-combined rotors, each rotor has its own critical speed. In rigid-combined rotors, combination influences occur in each rotor, resulting in changes of the individual critical speeds. The combined critical speeds differs from the individual critical speeds. The number of the combined critical speeds of a turbine is equal to the number of the rotors that are combined. Turbine bearings Generally, steam turbines have friction bearings with force-feed oil lubrication. Bearings which absorb vertically acting forces are called radial bearings or journal bearings Bearings which absorb axially acting forces are called axial bearings or thrust bearings. Friction bearing The effect of a friction bearing (hydro-dynamical bearing) is based on the fact that between the sliding surfaces a wedge-shaped gap exists, into which oil is delivered (by the sweeping effect of the shaft). This oil accumulates in the narrowest part of the gap and thereby causes a pressure, which keeps the sliding surfaces apart; the oil film bears the shaft. The oil (mineral oil or synthetic oil) which is used as lubrication material, In order to attain an oil film capable of load-bearing, • the shaft must have a certain peripheral speed, and • a certain wedge angle must exist. When the shaft rotates (at an adequate peripheral speed), the wedge angle is formed on its own by dislocation of the shaft center. During acceleration from standstill (while the peripheral speed is too low), a coherent oil film does not yet form. Therefore, devices were developed which lift the shaft in the bearings by means of high-pressure oil (shaft lift oil).


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TRAINING MANUAL 388.5 MW Combined Cycle Power Plant DOC No. Fundamental of Steam & Gas Turbine Page No. axial section


cross section

oil supply

pressure peak p

local pressure

D

shaft diameter

A

center point of the bearing shell

pmax

maximum local pressure

e

eccentricity of the shaft to the bearing shell

F

bearing force

hmin

smallest oil film thickness

C

center point of the shaft

B

width of the bearing

n

direction of rotation

Fig 13. Pressure distribution in the hydro-dynamical oil film of a friction bearing As a matter of principle, bearings are designed with a clearance between the shaft journal and the bearing shell. The size of the bearing clearance depends on the bearing geometry and on the bearing dimensions. The bearing clearance is necessary • for forming a wedge-shaped gap and, hence, for making up of oil film, • for making the bearing journal float on the oil film and, hence, for avoiding metalto-metal or combined friction while the turbine is run up, • for absorbing the heat expansion of the shaft journal due to creep and friction heat, and • for absorbing small tiltings and bendings without damaging the bearing. 1. Radial bearings (journal bearings) The following sketch shows the structure of a journal-type radial bearing with various possible oil supplies.

plain-sleeve bearing

lemon bearing

pocket bearing

MGF bearing

segmental bearing

Fig 14. Types of bearings used in turbine construction The bearing shells or bearing segments are babbitted with bearing metal, usually white metal (WM).


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Journal-type radial bearings are held with a spherical or cylindrical seat by a bearing ring in the bearing casing (eccentric arrangement of bearing and shaft). The oil is supplied from the side and reaches the oil grooves (also referred to as "oil pockets") of the bearing. An oil film is created between the shaft journals and the bearing, causing hydraulic friction. The clear diameter of the bearing shell must be larger than the shaft diameter (bearing clearance, approximately 1 - 2% of the shaft diameter).

1. saddle ring 2. fitting piece

6. oil supply on the left and on the right 7. orifice

3. bearing top shell 4. bearing stuts

8. bearing oil 9. shaft lift oil piping

5. bearing bottom shell

Fig 15. Radial bearing with oil jacking 2. Thrust bearing In turbines, thrust bearings have the purpose of fixing the rotor in a certain axial position and absorbing the axial thrust of the turbine (in the direction of the shaft axis) The magnitude and direction of the axial thrust depend on the load condition of the turbine. As a rule, thrust bearings consist of circularly arranged segments (thrust shoes), which are shaped such that a wedge-shaped gap is formed between the rotating and the stationary part. In single-casing turbines, the thrust bearing is located in the front bearing casing; in multiple-casing turbines, it is located either in the front bearing casing or between the HP and IP casings. It absorbs the thrusts in both directions and prevents, within the limits of the bearing clearance, any axial shifting of the shaft train from occurring (except for heat expansion). With regard to thrust bearings, one differentiates between "Kingsbury bearings" and Mitchell bearings", both named after their respective inventors. Figure 16 shows a thrust bearing of the Kingsbury-type.


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TRAINING MANUAL 388.5 MW Combined Cycle Power Plant DOC No. Fundamental of Steam & Gas Turbine Page No. A


throttle oil inner casing

adjusting ring

sealing ring oil

section A-A

A

section E-F bearing cage compensating elements thrust pads

thrust collar (shaft collar)

Fig 16.

Single thrust bearing according to the Kingsbury principle

The thrust shoes (also referred to as "thrust pads") are made of steel and have a slide surface made of white metal. On the back side, each thrust pad has a hardened insert with a universal ball joint, which enables the thrust pad to tilt slightly. Thereby, the bearing oil wedge formed by the oil film between the thrust pad and the thrust collar is able to adapt immediately to any kind of stress. The thrust pads rest with the hardened universal ball joint on a disk of intermeshed compensating elements. Each of these compensating elements works as a balancing arm. Through this, an elastic system is created that balances a possible uneven stress distribution among the thrust pads. Thrust pads and compensating elements are kept in their position by a bearing cage and are secured against twisting. The compensating elements are additionally fixed in the bearing cage by cylindrical holders arranged alternately in a radial and axial way. The lubrication oil is supplied under pressure to the thrust bearing. It enters through the drill hole of the casing from the bottom into the ring-shaped channel around the bearing cage and flows through radial channels on the back side to the inside and to the shaft. From here, it flows between the thrust pads and drains off over the edge of the thrust collar. The shaft collar runs against the individual thrust pads. The bearing body is fixed in the bearing block and absorbs the axial thrust of the turbine shaft.


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babbit lining


shaft collar drilling for cylinder pin

thrust bearing segment

shock absorber

pivot edge

bearing shell

Fig 17. Axial bearing segment of a Mitchell bearing The Mitchell design (see Fig. 17) enables the thrust pads to stand on a tripping edge somewhat diagonally so that a bearing oil film can form. Here, too, there is only hydraulic friction. As a rule, the moving surfaces of the thrust pads are coated with a white metal layer; they can, however, be made of a special bonze as well. The white metal layer must be thinner than the smallest axial clearance between the moving and the guide blades, or between the shaft seals. This prevents the rotor from rubbing axially against the blades or the shaft seals. section A-A

B

1 2 3 4 5 6 7 8

9 10 11 12 13

section F-F

detail A 24 8 9 23 section G-G

6

section E-E 20

21 7 9 22

Figure 18

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

adjusting screw bearing bracket transition piece spherical block shim upper bearing disk axial bearing block bearing bushing lower bearing disk turbine rotor oil drain casing spherical block spherical seat shim shim pan-head screw fitting piece oil piping bearing casing thermocouple (radial bearing) thermocouple (axial bearing) cylindrical pin sealing babbit linning

Combined radial-axial bearing

The contact surfaces for the thrust bearing, are machined from the solid shaft.


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Axial and journal type radial bearings must be monitored by measuring the • oil outlet temperature • temperature of the thrust pads (with thermocouples in the white metal) A change of the oil temperature indicates problems in the bearing! The temperature in the white metal must not exceed 120 °C! Thrust bearings are equipped with a safety device which trips the emergency stop of the turbine in case of an unacceptably high wear of the white metal. Shaft seals The shaft seals are used for the sealing between the rotating turbine rotor and the stationary turbine casing in order to prevent the steam from leaving the casing interior and to prevent air from entering into the casing along the shaft. One differentiates between axial and radial shaft seals, but today axial shaft seals are used almost without any exception. Radial shaft seals can only be found in older barrel-type turbines and radial turbines and in Ljungström turbines. Sealing is made by means of labyrinth seals, which consist of sealing strips connected in series as shown in Fig. 19. The sealing effect is created by converting pressure energy into flow energy with subsequent swirling of the flow (throttling effect). A small leak-off steam flow, however, is inevitable.

A = shaft B = casing C = sealing strip D = calking w ire

Fig 19. Labyrinth shaft seal Shaft seals can be divided into sealing segments and can be arranged flexibly in order to improve their capability of adapting to temperature changes. A labyrinth seal consists of sealing strips, which are inserted into the shaft and run through grooves and sealing lips machined into the casing. As shown in Figure 20, there are various possible design types of shaft seals. • calked sealing strips in the rotor and in the casing (20 a) • calked sealing strips only in the casing (20 b)


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A) calked sealing strips in the rotor as well as in the casing B) calked sealing strips in the casing. Rotor without sealing elements (straight shaft seal, no axial limitation, sealing strip with one or two seal points)

Fig 20. Designs of shaft seals Gland steam system In addition to a distinction by design, shaft seals are also differentiated according to Low-pressure shaft seals

High-pressure shaft seals

they prevent air from getting in (condensing turbines); gland steam must be admitted to this type of seals because a vacuum prevails on the lowpressure side they seal the turbine against the outgoing steam; gland steam arises at high-pressure shaft seals

For economical reasons, the gland steam arising at high pressure shaft seals is supplied to the low-pressure shaft seal. If the gland steam is not sufficient, live steam is added. The gland steam supply to shaft seals is controlled by valves. At the outside of each shaft seal, there is an exhaust pipe for removal of the steam vapors, allowing a small steam flow to escape and facilitating controlling of the steam supply. Since superheated steam is invisible, an indicator must be provided at each vapor escape pipe, e.g. in the form of a flap or disk, which is lifted by the outgoing steam. If the gland steam pressure is automatically controlled, escape pipes to extract visible steam vapors need not be provided. Figure 35 shows various configurations of gland steam supply and extraction for shaft seals. The arrows indicate the flow direction of the steam. Turning gears In order to avoid deformation of the turbine rotor the whole shaft train must • during the cooling down and after stopping • during the warming up before starting


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• during generating a vacuum be kept continuously at a low speed by a turning gear or be turned at certain time intervals. The continuous turning is necessary to • prevent temperature staging in the casings which would cause deformation of the turbine rotor and the casing, • provide for a good heat transfer at the inner wall of the casing through the blade ventilation and thereby to provide for a temperature balance between the top and bottom sections of the casing, • to prevent the shaft seal parts from local superheating (while gland steam is supplied during generation of a vacuum). Especially during short interruptions of turbine operation, a quick starting with a straight shaft is made possible by a turning gear. What to do if the turbine shaft is bent and it is not possible to turn it manually or with the turning gear: In any case, it should not be tried to break it away by admitting steam into the turbine or by means of a crane! This might deform blades and sealing strips. One should wait until the temperature which caused the deformation of the shaft or the casing has equalized! (This may last some hours). Mechanical turning gear With a mechanical turning gear, the rotor is turned continuously by an electrical motor via a drive to guarantee a steady cooling down. The mechanical turning gear works, depending on its design, with a speed of up to 80 min-1. The auxiliary oil pump must remain in operation while the electrically driven turning gear is being operated. To facilitate the start of the idle rotor and to overcome the break-away torque in the bearings, the shaft is pressed upwards by a high oil pressure in all bearings of the shaft train so that a bearing oil film can build up. The high-pressure oil for the pressure relief in the bearing is supplied by a positive displacement pump. In order to form a load-bearing oil film in the bearings, a minimum speed of 8 - 20 min-1 must be kept (depending on the oil temperature). After having stopped the cooling-down operation, the shaft turning gear must be switched off manually! Hydraulic turning gear Regarding hydraulic turning gears one differentiates between • continuously rotating turning gears • intermittently rotating turning gears Electrical turning gear Electrical turning gears consist of a shaft motor and a separate fan. The coupling of the turbine rotor containing the coupling studs is at the same time the rotor for the shaft motor. The stator with the magnetic coil is mounted above the coupling. The rotating field builds up in the magnetic fields as the turning gear is switched on.


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Turbine Lubrication System For the operation and operational safety of a turbine, especially for the safety of the bearings, a permanent oil supply must be ensured. In a steam turbine, oil is needed for the following purposes: for lubrication, heat removal and signal transmission • as high-pressure oil • as motive agent • for load-relieving in the bearing (shaft lift oil)

for control and governing operations • for actuators • in the hydraulic turning gear • while starting and accelerating the turbine

Lubrication and heat removal: In the turbine bearings, a wedge-shaped oil film is built up between the surface of the rotor and the bearing shell by continuously supplying high-pressure oil. This oil film serves for lubrication and is maintained by the rotation of the turbine shaft. The oil removes various types of heat from the bearings: • heat resulting from friction in the bearings, • heat creeping from the turbine rotor into the bearing, and • heat transmitted from the turbine casing to the bearing casing by thermal radiation. Control and governing operations: The oil serves as a power multiplier in the hydraulic control system and for transmitting signals between the individual controlling components. The oil conveys the control signals for the emergency stop valve and the safety systems. The usual working medium for governing and lubrication system of the turbine is the MOBIL DTE oil medium or Turbine Oil- 14 of INDIAN OIL COMPANY or servo prime 14 or Turbinol 47. OIL SPECIFICATION 1. 2. 3. 4. 5. 6. 7.

Specific gravity at 500 C Kinematic viscosity at 500 C Neutralization number Flash point Pour point Ash percentage by weight Mechanical impurities

0.852 28 centistokes 0.2 2010C (min) 6.60 C (max) 0.01% NIL

Oil circuit: The oil flows through the turbine in various circuits: it is extracted from the oil tank by a main oil pump and is pumped to the different points where it is needed. From there it is returned to the oil tank.


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Oil tank

Main oil pump Point in need

Inside a turbine, the following oil circuits are differentiated: • bearing-oil circuit • control-oil circuit • lift-oil circuit The oil supply system requires the following components: Main oil pump (usually driven by the turbine shaft, sometimes electrically) Oil tank Oil cooler

Control-oil throttle Bearing-oil throttle Oil strainers and filters Oil pipes (supply and drain pipes) Auxiliary oil pumps (steam-driven or motor-driven) Shaft lift oil pump

extracts the oil from the tank; increases pressure to control-oil pressure from which the oil is extracted and into which it returns removes the heat from the oil which was absorbed by the oil in the various circuits adjusts the control-oil pressure and keeps it constant adjusts the oil supply to the oil quantities needed by the individual bearings retain impurities connect the various stations supply the necessary oil during standstill, starting and stopping enables the building up of an oil film beneath the shaft journals during the turning facility operation; reduces the break-away torque

Oil pressures: The oil pressures are different in each type of circuit: Control-oil pressure Bearing-oil pressure Shaft-lift oil pressure

5 - 40 bar 1 - 3 bar 100 - 200 bar

The proper functioning of the oil supply system is rather evaluated by the constancy of the set pressure level than by the absolute level of the pressure. At the same speed, the once set pressure may only change insignificantly! Pressure changes may be caused by slight changes of the oil itself or by temperature changes.


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Oil pumps: Since each one of the oil circuits needs different oil quantities and oil pressures, different pumps must be installed as well. For operational safety, standby pumps must be available in the bearing-oil and control-oil circuits (redundancy)! 1. Main oil pump: The main oil pump supplies the bearings and the control units with oil during normal operation. The main oil pump can have its own drive or be coupled to the turbine rotor. In combined systems, the main oil pump is located in the front bearing casing and is driven directly by the turbine rotor, to which it is connected via a coupling. It • • •

supplies the entire turbine generator with oil: for bearing lubrication for shaft journal cooling as control-oil for the hydraulic control system

2. Full load auxiliary oil pumps: During starting and stopping of the turbine as well as during standstill, the oil supply is guaranteed by auxiliary pumps. They are located on the oil tank or in its immediate proximity. They can be driven by an electric motor or a steam turbine. Full load auxiliary pumps are auxiliary pumps which can supply the control oil and the lubrication-oil circuits with oil. Depending on their design, these pumps can also deliver oil to the intake connection of the main oil pump via an injector (which serves as suction support for the main oil pump). 3. Emergency oil pump: The emergency oil pump is used when all other oil pumps have failed. In this case it provides a sufficient quntity of lubrication-oil during turbine coast-down, in order to ensure the lubrication of the bearings. Therefore the auxiliary oil pump is equipped with a dependable drive, mostly a d.c. motor. If a separate power supply is available, a three-phase current motor can also be used as a drive. However, it may also be driven by steam. The auxiliary oil pump is installed on the cover of the main oil tank. Its oil delivery lines are directly connected to the lubrication-oil lines downstream of the filter and the oil cooler. Hence, the oil is neither cooled nor filtered. The full load auxiliary pump and the emergency oil pump are interlocked when the main oil pump is operating. Every turbine plant must be protected so that in case of failure of a pump another pump starts operation. In this context, it is assumed that the emergency oil pump never fails. 4. Lifting oil pump: The lifting oil pump has the purpose of building up a bearing oil film between shaft and bearing shells in the friction bearings during starting and stopping of the turbine. Generally one or several positive displacement pumps are used as lifting oil pump, which can be driven electrically. They can create an oil pressure of 100 - 200 bar. 5. Control oil pumps:


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Control-fluid pumps are used when the lubrication-oil and control-oil supply are operated separately. They are driven separately by three-phase current motors. Oil flow diagrams Separate lubricating oil and control oil supply The oil flow diagram of separate lubricating oil and control oil supply systems is shown in Figure 21. Here, the high-pressure oil system (for hydraulic control and actuation systems) and the low-pressure oil system (for lubrication) are supplied with oil from two separate oil tanks by separate pumps and separate pipes. Both mineral oil and synthetic fluids may be used. 1. turbine 2. bearing casing 3. control and m ain stop valves 4. bearing oil tank 5. control fluid tank 6. m ain pum p for bearing oil 7. em ergency oil pum p 8. auxiliary pum p for bearing oil 9. bearing oil system 10. m ain pum p control system 11. auxiliary pum p control system 12. control fluid system 13. jacking oil pum p 14. filter 15. cooler 16. hydraulic accum ulator 17. vapor extractor 18. oil and fluid drain

suction oil pressureless bearing-oil approxim ately 1 - 3 bar control-oil approxim ately 10 - 40 bar lifting-oil approxim ately 10 - 200 bar

Fig 21. Configuration of separate lubricating oil and control oil supply systems

The oil system is divided into three oil circuits. Each oil circuit has its specific purpose(s): Low-pressure (LP) oil circuit

High-pressure (HP) oil circuit Lifting oil circuit

lubrication and cooling of the bearings; operation of the control units and the hydraulic protection facilities; driven by hydraulic turning gears actuation of the control valves lifting of the shaft train to relieve the bearings

1. LP oil circuit: In normal operation of the turbine, the main oil pump sucks oil through the suction lines from the main oil tank. During starting and acceleration of the turbine, a full-


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load auxiliary pump is used to supply oil through a throttle into the suction oil line, to make suction for the main oil pump possible. Through the high-pressure oil pipes, the main oil pump delivers oil to the control units (and to hydraulic protection facilities) and lubricating oil to the bearings of the turbine generator unit. An emergency isolating gate valve (oil fire protection valve) is installed in the lines to the control units. The oil supply is immediately interrupted by this valve in case of an oil fire. Before the lubricating oil reaches the bearings, it is cooled down in the oil coolers, reduced to a low pressure by throttles and piped via bearing-oil throttles which adjust the flow to the needs of each specific bearing. The oil for the radial-axial bearings is additionally led through a two-stage oil filter, where it is filtered. In the last years, in some steam turbine generators, the entire oil has been filtered. For this purpose, a main filter has been integrated downstream of the oil coolers. 2. HP oil circuit: The high-pressure oil for the operation of the control valves is supplied by two HP control-oil pumps.

Oil vapor extraction unit To a great extent, the main oil tank is air-sealed. Oil vapor extraction blowers are used to create a slight vacuum of 0.2 mbar to 0.5 mbar • in the main oil tank, • in the oil return pipes, and • in the bearing casings. This allows extraction of oil vapors, and oil exit losses can be prevented in the area of the shaft seals at the bearing casings. The vacuum can be measured by a simple U-tube. Oil return: Having lubricated and cooled the bearings, the oil flows back through a collecting main line into the main oil tank. Before the lubricating oil leaving the exciter set flows back to the main oil tank, it is diverted via a loop into a degassing tank. Loops are integrated into the return pipes of the generator piping. They prevent hydrogen gas (cooling agent for the generator) from reaching the main oil tank in case of disturbances in the seal oil system. Shaft lift oil circuit: The shaft lift oil circuit has the purpose of lifting the shaft train to relieve the bearings. For this, high-pressure oil is pressed under the journals of the rotor bearing. In order to prevent damages to the bearings, an oil film must exist between the bearings and the turbine rotor. This oil film is created by the shaft lift oil (sometimes


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also referred to as "jacking oil"). The shaft floats on an oil film, which is approximately 20 to 30 mm thick. Bearing damages are caused by metal-to-metal friction resulting from low speed! During accelaration and coast-down of the turbine (low speeds), a hydraulic lifting device is necessary to create and maintain the oil film. Furthermore, the lifting device reduces the torque moment which must be overcome by the hydraulic or the manual turning gear when the turbine is started. The lifting device must be turned off when the accelarating turbine reaches a speed of 80 min-1 Figure 22 shows the lift oil circuit of a turbine.

a bearing-oil line b oil drain line

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

main oil tank oil cooler check valve shut-off valve magnetic filter jacking-oil pump collecting pipe check valve fine control valve bypass valve bypass valve

12. 13. 14. 15. 16. 17. 18. 19.

relief valve pressure gage HP turbine section IP turbine section LP turbine section 1 LP turbine section 2 generator exciter

Fig 22. Hydraulic shaft lifting system Main oil tank: Generally, the main oil tank is located on the same level as the turbine. If it has to be arranged at a lower level, an additional tank (suction oil tank) has to be installed on turbine level to get the necessary suction head for the pump. The main oil tank not only stores the oil for lubrication, cooling and regulation of the turbine generator unit but serves also for degassing of the oil. Moreover, aging products of the oil settle and accumulate in the tank. The capacity of the oil tank is dimensioned such that its entire content is circulated not more than eight times per hour. The oil circulation rate U (approximately 8 to 10 per hour) is calculated according to the following equation: U = circulated quantity of oil per hour / content of oil tank Hence, the dwell time of the oil in the tank is approximately 7 to 8 minutes. During this time, i.e. between the arrival of the oil at the tank and its extraction by the


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pumps, the oil can release the air that has been absorbed and the aging products can settle. dwell time = 60 / oil circulation rate (min) The configuration of the main oil tank is shown in Fig. 23. In large turbine generating units, the main oil tank is longitudinally divided by a partition wall. The oil is re-circulated in the oil tank as follows: Coming from the oil system, the oil flows into the tank through the oil inlet which is below the oil level, into the riser space of the tank. The oil rises and an initial deaeration takes place.


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1.

suction line for main oil pump

2. 3.

auxiliary oil pump main drain line

4. 5.

suction line for separator sludge outflow

6. 7.

air extraction

8. 9.

oil level indicator with float oil baffle for air and sludge separation

10. oil strainer 11. intake oil filter for oil pumps

Fig 23. Oil tank From the riser space, the oil passes through two oil strainers into the adjacent oil chamber, flows around the longitudinal partition wall and reaches, through the oil outlet connection via a check valve, the main oil pump in the front bearing casing. The control oil pumps, the auxiliary pumps and the emergency oil pumps are installed on the main oil tank. The driving units of the pumps are mounted on the mounting plates of the tank cover. The pump bodies are immersed in the oil. They extract the oil at the deepest point of the tank in order to extract the oil which is as free from air as possible. The check valves prevent the oil from flowing from the suction or delivery lines of the main pump or the auxiliary pumps back into the oil tank. Blowers are used to create a slight vacuum in the tanks, in the return pipes and in the spaces of the bearing casing so that arising oil vapors can be extracted. The main oil tanks are equipped with a level indicatior and a level monitoring system. They indicate the minimum or maximum oil level. The oil tank is not completely filled when the turbine is operated. The oil reaches just a certain operational oil level (which corresponds to the nominal content of the tank, i.e. there is a so-called reserve capacity between the roof of the tank and the actual oil level. This reserve capacity is sufficient to accomodate the oil of the whole oil system if the turbine generator is stopped. The bottom of the tank is sloped and provided with outflow devices at its lowest point. At this point, impurities and the condensate getting into the oil system in the area of the bearing casing can be extracted. The condensate must be analyzed to ensure that it is not untreated water. Untreated water in the oil circuit indicates a defect of the oil cooling system! Increased condensate quantities mean that there is a defect in the oil system! The oil sediments should be extracted from the tank on a daily basis; at any rate must they be extracted once per week!


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Hydraulic accumulator Hydraulic oil accumulators are also referred to as "bladder-type accumulators". They are used as additional energy accumulators in hydraulic systems. Hydraulic accumulators are charged by oil pumps over a longer period of time and may, when required, release the accumulated quantity of oil without delay.

1. hydraulic accumulator 2. test pressure gage 3. shut-off valve 4. shut-off gate valve 5. drain valve 6. reducer

Fig 24. Arrangement of a hydraulic accumulator in a hydraulic system Hydraulic accumulators are used when an additional quantity of oil is needed for a short period of time. Their purpose is: • to maintain the operational pressure at a constant level during short-term maximum oil demand, • to attenuate (i.e. to absorb) occurring peak pressures and fluctuations (e.g. to compensate a sudden pressure drop in the oil system). Function: Through the high-pressure gas valve, the elastic accumulator bladder is filled with nitrogen up to a minimum pressure. From below, the oil flows into the steel cylinder via an oil valve and hereby compresses the gas up to the pump pressure of the hydraulic system. At a sudden pressure drop in the hydraulic system (e.g. during control operations), the following reaction takes place: The fluid pressure decreases. Ø The gas volume in the accumulator bladder expands, Ø until the gas pressure coincides again with the fluid pressure. Ø The oil volume corresponding to the increase of the gas volume, is pressed out of the tank and compensates the pressure drop in the oil system.


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Oil cooler: For cooling the circulating oil, two oil coolers (three in large turbine units) are generally connected in order to have at least one standby oil cooler available. The standby cooler is necessary when one oil cooler is cleaned, so that the turbine does not have to be stopped, and at a higher cooling water temperature (e.g. during summer) Design and function: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

s a fe ty v a lve v e n t v a lv e re tu rn b o x v e n t v a lv e u p p e r tu b e b o tto m d e a d -e n d tu b e in n e r s h e ll la rg e r o il tu rn p la te o il tu rn p la te o u te r s h e ll ro d s p a c e r tu b e th e rm o m e te r lo w e r tu b e b o tto m s e a lin g c o n n e c tio n p ie c e fin n e d c o o lin g tu b e s in s e rt s le e v e o il d ra in v a lve w a te r c h a m b e r

Fig 25. Oil cooler The oil cooler consists mainly of: • the cooling tubes (finned cooling tubes) (17) • the inner shell (7) • the outer shell (10) • the water chamber (20) and • the return box (3) The cooling tube bundle flown through by cooling water penetrates the outer shell and is immersed in the oil chamber. The oil to be cooled reaches the inner shell through the connecting pieces (16, 18), either from below or from the top, depending on the connection. The internal shell supports the large oil turn plates (8), which are provided with an oil passage in their middles. Between each two large oil turn plates, there is a small oil turn plate (9), which is held by the short spacer tubes (12) mounted onto short rods. The small oil turn plate does not reach the inner shell. By this design, a cross-flow arrangement is attained. The oil flowing to the outlet is forced to flow around the large oil turn plates in their middles and around the small oil turn plates at the side of the oil chamber. The inner oil shell with the oil


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turn plates is fixed at the lower tube bottom (14), into which the cooling pipes are rolled. Moreover, the water chamber (20), with a cooling water inlet socket and a cooling water outlet socket, is bolted onto the lower tube bottom. The cooling tubes can move freely upwards, allowing for thermal expansion. The cooling water flows in and out through the water chamber. The water chamber is divided by a transverse partition wall such that the cooling water must flow back to the water chamber through one half-bundle. In the area of the transverse partition wall, dead-ended tubes (6) are arranged in place of cooling pipes. The safety valve (1), which is mounted on the return box (3), prevents an impermissible pressure rise in the cooling water chamber. The inlet and outlet sockets of the water chamber are provided with thermometers (13). When filling up the oil chamber, the vent valve (4) must be opened. The oil chamber is emptied through the oil drain valve (19). The cooling water velocity in oil coolers is kept constant by an oil bypass temperature control. The required oil outlet temperature is attained by controlling the cooling water throughput. The lubricating oil temperature is adjusted by mixing cold oil which has passed through the oil cooler with uncooled oil. Even when changing cooling water temperatures it is possible to keep the same cooling water velocity by fully supplying the cooler with cooling water. This prevents to a great extent precipations on the tubes, which would cause corrosion. Thermometer wells are attached at the inlet and outlet sockets of oil and water, allowing monitoring of oil cooling and warming-up of cooling water. The oil temperature before admission into the bearings corresponds to the outlet temperature at the oil cooler. It must be approximately 40 °C to 50 °C! Oil filters: Oil filters (strainers) are integrated into the oil circuit at various points in order to remove impurities from the oil system, and thus to protect components from getting damaged. Oil filters are differentiated according to Filter class coarse filter fine filter superfine filter

Filter type e.g. oil strainers e.g. interchangeable double filters e.g. plate-type filters

Point of use oil tank axial and radial bearing, turbine gear unit control oil system

The filters must • be regularly checked, • be regularly cleaned, and • have their fabric inserts replaced regularly. Filters are protected against excessively high pressures by: • an automatic overpressure protection (short circuit valve) • differential pressure measurements (protecting filter inserts, enabling a changeover in time)


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II. Gas Turbine Gas turbine is a heat engine that uses high-temperature, high-pressure gas as the working fluid. Part of the heat supplied by the gas is converted directly into mechanical work. High-temperature, high-pressure gas rushes out of the combustor and pushes against the turbine blades, causing them to rotate. In most cases, hot gas is produced by burning a fuel in air. This is why gas turbines are often referred to as "combustion" turbines. In 1903, Aegidius Elling of Norway built the first successful gas turbine using both rotary compressors and turbines - the first gas turbine with excess power and in 1918, General Electric Company started a gas turbine division. Dr. Stanford A. Moss developed the GE turbosupercharger engine during W.W.I. It used hot exhaust gases from a reciprocating engine to drive a turbine wheel that in turn drove a centrifugal compressor used for supercharging. Gas Turbine Process A gas turboset converts the chemical energy contained in a fuel into electrical energy, which is then supplied to the grid or network. The energy conversion occurs in several steps. The chemical energy in the fuel is released by combustion and is transformed into heat energy in the combustion chamber. This heat energy is forwarded to the turbine. The heat of the combustion gas is converted initially into kinetic energy and later into mechanical energy. For this purpose the blading of the turbine is used. The mechanical energy is then transmitted to the generator via the coupled shaft. Using the induction principle, the generator transforms this mechanical energy into electrical energy, which it forwards to the network through its high voltage (HV) bushings, circuit breaker and main step-up transformer. Heat is converted into kinetic energy with the help of a nozzle. By this way the arrangement of the turbine blades creates multiple nozzles to convert the heat energy of the combustion gas into kinetic energy. The latter one then develops a force on the moving blades of the turbine providing a torque that represents the mechanical energy of the turbine. Heat energy is mainly represented by the quantities of pressure, temperature and volume. To include all quantities that represent the energy form of heat, the concept of enthalpy is used. The quantities of pressure, temperature and volume are interrelated. So, for example, if pressure of a fluid is reduced, its temperature decreases and its volume increases. On the other side, within a nozzle, the fluid pressure decreases and its velocity increases. So arranging the turbine blades in a proper sequence, it is possible to extract the maximum mechanical energy from the heat of the combustion gas. This energy conversion can be repeated until the fluid pressure reaches nearly the value of the atmospheric pressure. At that moment the fluid is discharged from the turbine into the atmospheric at low pressure and comparatively high temperature. To obtain a high energy conversion efficiency, a high turbine inlet temperature (IIT) is used, and the expansion process is completed at approximately atmospheric pressure and with a high exhaust gas temperature or temperature after turbine (TAT). To maximize the use of this high exhaust gas temperature, a heat recovery steam Generator (HRSG) is added. When the energy from heat is utilized, it is necessary to apply to it the quantity of entropy. The latter one represents simultaneously some important characteristics of heat, so it represents:


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The direction into which heat flows by itself, namely from a high to a low temperature level. The degree of convertibility from heat into other energy forms, namely that heat can never be converted into another energy form by 100% The irreversibility of different real processes, namely that energy losses in the form of heat appear in real processes.

The proper combination of the quantities of temperature and entropy allows to represent graphically the heat from a process that is available for conversion into another energy form: mainly into mechanical energy. The heat is represented by the area enclosed by the curves that represent the different changes of state of the fluid within the process.

Figure 1 shows a gas turbine process with standard combustion as two interrelated representations. Functional Description The gas turbine process can be described in terms of the work performed by the air intake system, turbine, compressor, combustor and generator-exciter. Air Intake System No mechanical energy can be added to the air until it arrives at the compressor’s first row of rotating blades. The compressor, therefore, removes air in front of the first stage to reduce local pressure. Atmospheric pressure then pushes intake air through the filter and manifold to replace the air removed. Pressure in the intake system is always falling as it moves from the outside toward the compressor intake section. This means that the journal bearing at the compressor inlet must be carefully designed to ensure that no oil is sucked into the air intake, where it would contaminate the blading. Axial Flow Turbine The axial flow turbine is called so, because the main flow direction of the combustion gas through the turbine is parallel to the main turning axis of the turbine. As explained previously, the turbine converts the heat energy of the combustion gas into mechanical energy. This energy conversion is made with the help of blades. There exist two kinds of blades:


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Fixed blades, stator blades or vanes – these are attached to the turbine casing and they guide or direct the combustion gas in an optimum way onto the moving blades. Moving blades, rotor blades or buckets – these are mounted on the periphery of the rotor and they deliver the mechanical energy to the rotor.

LARSEN & TOUBRO LIMITED TRAINING MANUAL 388.5 MW Combined Cycle Power Plant DOC No. Fundamental of Steam & Gas Turbine Page No.

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The blades are arranged in rows one behind the other so that the combustion gas can cross them all. The blades conform channels with the shape of a convergent nozzle. The combination of a row of vanes and a row of buckets is called a turbine stage. It is in the stage where the energy conversion from heat to mechanical energy takes place. Figure 2 shows such a turbine stage as follows:

In the upper part a perspective representation In the lower part a top view, including the velocity triangles or diagrams.

Axial Flow Turbine The combustion gas enters the vane row with a given absolute velocity c0. Due to the nozzle shape of the channels between blades, the combustion gas absolute velocity increases to c1 and enters the row of buckets with this velocity. The acceleration of the fluid (combustion gas) happens at the cost of:

A decrease in fluid pressure. A decrease in fluid temperature. An increase in fluid volume.

This absolute velocity c1 at the inlet of the bucket row, decomposes into a:

Relative velocity w1 Tangential velocity u1.

Because of the nozzle shape of the bucket row, at its outlet the relative fluid velocity increases to w2 (w2 > w1). The outlet tangential velocity U2 remains the same due to the constant radius of the buckets (r1 = r2). So at the outlet of a bucket row the absolute velocity c2 gets practically the same direction and size again as at the inlet of the vane row (c2 ˜ c1). By this way the fluid enters the rest stages and the velocity decomposition happens again. The direction of the absolute velocity c is aligned approximately axially. The benefit from the velocity diagrams is double. They are used to:

Give the shape of the foil of the blade. Calculate the mechanical energy developed by the turbine.

The latter point is basically developed now further. From mechanics: M=Fxr Where, M: torque [N.m] F: turning force [N] r: turning radius [m] From Newton’s second law: F = m x ?c/?t = ?I/?t, Where F: force [N] m: mass [kg] ?c: speed difference [m/s] ?t: time span [s] ?I: impulse difference [kg.m/s] or [N.s]


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It follows: M: ?I x r/?t = ?D/?t, Where ?D: angular momentum change [Nms] or [Js] It also follows: M = m x r x ?c/?t And, ?c = c2 – c1 c2: outlet speed [m/s] c1: inlet speed [m/s] therefore, M = m x r x (c2-c1)/?t = m’ x r (c2-c1) [m’: fluid throughput (kg/s)] Within a turbine stage the following considerations apply to the developed torque: M = m (r1 x c1u – r2 x.c2u) Where, r1: inlet turning radius [m] r2: outlet turning radius [m] c1u: inlet tangential component of c [m/s] c2u: outlet tangential component of c [m/s] The decomposition of the absolute speed into its components can be seen in Figure below. Here the flow path of a fluid is represented schematically at the inlet and outlet sections of a rotor. The inlet radius is r1 and the outer radius is r2.

1 = inlet 2 = outlet C = absolute velocity Cm = Axial or meridian component

Cr = radial component Cu = tangential component r = turning radius

The inlet and outlet absolute speeds are c1 and c2 respectively. Each of these velocities is decomposed in the following way:


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C2 is the axial or meridian component. This velocity component has no influence on energy conversion because it posses no turning radius. Ct is the radial component. This velocity component again has no influence on energy conversion because it possesses no turning radius. Cu is the tangential component. This velocity component influences the energy conversion because it possess the turning radius r.

For an axial flow machine, the following conditions apply:

r1 = r2 = r u1 = u2 = u

From mechanics: Pmech = M x ω Where, Pmech: mechanical power [W] M: torque [N.m] ω: angular velocity [1/s] It follows: Pmech = m (r1 x ω x c1u – r2 x ω x c2u) Or, Pmech = m x (u1 x c1u – u2 x c2u) [since, u = r x ω] The above formula shows the decisive importance of the kinetic energy in developing the mechanical power of the turbine. The kinetic energy is represented by the product of the velocities u x cu. For the special case of the axial flow turbine, it follows: Pmech = m x c x (c1u – c2u) For a driver, like a gas turbine, it is valid: c2 < c1. Consequently:

c2u < c1u The developed mechanical power is positive - It is given off from the driver.

To complete the picture about the axial flow turbine, it is necessary to look at the forces in the blading.


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Above figure shows the force acting on a single bucket in a turbine stage. The resulting force acting on the bucket arises from the combined action of the combustion gas throughput and the change in direction and size of the relative velocity. Said with a formula:


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FR = m x ∆w = m x (w2 – w1) Where, FR: resulting force acting on the bucket [N] m: combustion gas throughput [kg/s] ∆w: relative velocity change [m/s] w1: inlet relative velocity [m/s] w2: outlet relative velocity [m/s] The resulting force decomposes into three components:

Fax is the axial component in direction of the main turning axis of the rotor it develops the thrust force of the turbine rotor. Ft, which is Not shown in Figure is the radial component in direction of the radius of the rotor - it develops the traction force onto the bucket. Fu is the tangential component in direction of tangential speed it develops the turning force for the torque of the rotor.

Since w2 > w1, the resulting force is positive. This means FR of the turbine is a driving or motive force. Compressor In addition to adding heat to the air flow, the gas turbine process requires pressure, which is provided by the compressor. The compressor is fixed to the same shaft as the turbine and works similar to but opposite from it. In the compressor the rotating shaft moves the moving blades through the air. The moving blades add mechanical energy to the air by increasing its velocity and then the vanes change that kinetic energy into pressure energy. Since it is more difficult to compress a working medium than to expand it with axial blading, more stages are needed to compress the air and fewer are needed to expand the combustion gas. It is difficult to exactly match the mass flow of a compressor and a turbine at all rotational speeds. To obtain the highest efficiency, we ensure that matching the mass flows is good at rated rotational speed. This means, however, that they are not well mass flow matched at lower rotational speeds and the compressor wills urge if no adjustments are made. To overcome this problem the compressor is equipped with blow-off valves, which open to expel air during start-up and shutdown when the rotor rotational speed is low. The blow-off valves close when the shaft reaches rated rotational speed, where mass flow matching is good. As with the turbine, the compressor blades are arranged in rows one behind the other so that the air can cross them all. The blades again conform channels with the shape of a divergent nozzle. The combination of a row of buckets and a row of vanes is called a compressor stage. It is in the stage where the energy conversion from mechanical energy to pneumatic energy takes place. Figure 5 shows such a compressor stage as follows:


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In the upper part there is a perspective representation. In the lower part there is a top view, including the velocity triangles or diagrams.

As stated previously, the compressor blading is arranged opposed to that of the turbine. Consequently, it also works in the opposed way: that is, it converts the mechanical energy of the rotor into pneumatic energy. The mechanical energy of


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the rotor is available as kinetic energy that is absorbed by the air. Within the compressor blading the fluid (air) is decelerated at the cost of:

An increase in fluid pressure An increase in fluid temperature A decrease in fluid volume

As within the turbine, for the special case of the axial flow compressor, the mechanical power is given by the formula: Pmech = m.u (c1u – c2u) For a driven machine, like a compressor, it is valid: c2 > c1. Consequently: c2u > c1u Which means, the developed mechanical power is negative and it is absorbed by the driven machine. Experience shows that the following relation is valid: PmechC ˜ 2/3 PmechT Where, PmechC : mechanical power absorbed by the compressor PmechT : mechanical power supplied by the turbine. Combustion Chamber To increase turbine efficiency, the turbine inlet temperature must be increased. In the combustion chamber the chemical energy of the fuel is released as heat into the combustion air. By this action the air:

Increases its specific energy (enthalpy). Raises its temperature to peak values. Converts to combustion gas.

The pressurized combustion air is delivered by the compressor and the combustion process takes place under constant pressure. This is called an isobaric thermal process. Within an environmentally friendly combustion, the process is characterized by the use of special burners, namely the environmental (EV) burners. These ones apply to following combustor types: The silo combustor The annular combustor. The Annular combustor has higher gas turbine efficiency and lower exhaust gas emission concentration, namely NOx emission concentration. The environmental combustion with EV burners: Creates an extremely homogenous fuel/air mixture. Leads to thorough combustion Effectively inhibits NOx emission. Environmental combustion works in the following steps: Compressed air is fed into the double-cone EV burners, creating a homogenous, lean fuel/air mixture.


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A vortex flow is induced by the shape of the burners, which breaks down at the EV burners exit into the combustion chamber, forming a recirculation zone. The fuel/air mixture ignites into a single, low temperature flame ring. The recirculation zone stabilizes the flame in free space within the combustion chamber, avoiding contact with the combustor wall. The hot combustion gas exits the combustion chamber and after moving through the turbine, discharges as exhaust gas.

Main characteristics of the annular combustor are: The burners are evenly distributed in an annular ring. Gas flow and temperature increase does not increase the metal temperature of the turbine blading. The hot gas path is considerably shorter than in comparable machines. There is lower demand for cooling air There is more air available for flame dilution Less formation of NOx emission concentration. There is less material to be cooled The EV burner: Is a dual fuel burner for gaseous and/or liquid fuels Functions on an elementary design principle: • The burner is shaped like 2 half-cones. • These are slightly offset sideways to form 2 slots of constant width • The slots run the component’s full length Is based on the vortex breakdown principle: • The lean fuel/air mixture leaves the cone and is ignited • At the exit of the burner the vortex breaks down, forming a recirculation zone which stabilizes the flame in free space within the combustion chamber. The 2 slots between both half-cones: Allow combustion air to enter the burner. Are equipped with a series of fine holes along their edges. • Across these holes, the gaseous fuel is injected radial into the burner. Allow an arrangement where fuel and air intensively mixed. Excess air is a characteristic of the EV burner design that results in a: Lower flame temperature of around 500ºC (278ºF) than that of a conventional burner. Very low NOx emission concentration. • The formation of NOx depends mainly on high temperature and long residence time in the combustor. Generator-Exciter The generator is that component of the gas turboset that converts the mechanical energy of the gas turbine into electrical energy and feeds it into the electrical grid or net. This energy conversion is based on the principle of electromagnetic induction. The latter one requires the simultaneous action of: A magnetic field, created by a strong magnet. An electrical coil that envelops the magnet. A relative movement between the magnet and the coil.


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When the electromagnetic induction principle applies, an induced voltage appears at the terminals of the electric coil. A generator is the execution of the electromagnetic principle. The rotor of the generator is the magnet. The coil, enveloping the magnet, is the stator winding. The relative movement between magnet and coil is provided by the rotor of the driver (gas turbine). To reinforce the effect of the magnetic field the rotor has its own winding, called excitation winding. Through this excitation winding direct current flows, called excitation current. The variation of the excitation current changes the value of the induced voltage. By this way the voltage can be stabilized, just by varying the excitation current. The excitation current is delivered by an independent electric energy source, called exciter. There are basically two types of exciters that fulfill the same task, these are: The static exciter. The brushless exciter with rotating diodes. The brushless exciter: Is also a generator that includes: • A magnetic field, where the magnet is fixed and • An enveloping electric coil, which rotates. The excitation current for the exciter is supplied from the voltage regulator. The latter can be: A manual voltage regulator An automatic voltage regulator (AVR). When the generator functions:

Its outlet voltage is measured and fed back to the AVR. • Here it is compared against the set value. • Any difference between these two values triggers a command to change the excitation current to the exciter and consequently also the excitation current to the generator. • The adjustment continues until the set value and actual value difference reaches a pre-established value. The excitation current for the exciter is rectified by the AVR from an alternating current input from the main generator. • The excitation current for the generator is rectified by rotating diodes from the alternating current outlet of the exciter.

A battery or a permanent magnet supplies the AVR with direct current to flash the field. Being a combustion machine, the gas turbine needs help from outside to start working. Here something similar happens as with a car engine. That it can start working, requires the engagement of a starting motor. With the gas turbine the starting motor is the generator. An electric motor:

Has the same components as a generator Functions in opposed direction. It converts electric energy into mechanical energy.


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That means by applying a voltage on the stator winding and excitation current to the magnet the rotor is turned.

This principle is applied within the gas turboset.

A static starting device (SSD) is fed with electric energy from the grid. • One outlet of the SSD supplies voltage with variable frequency to the stator winding. • The other outlet of the SSD delivers directly excitation current to the rotor. • By varying the frequency of the applied voltage, the rotational speed of the rotor can be changed correspondingly.

When the requirement for the starting motor is not more needed: The SSD is disconnected and The gas turbine accelerates by itself under combustion.

Increasing the Gas Turbine performance The performance of a gas turbine viz. the power output, and the heat rate (measure of efficiency, i.e. the amount of energy consumed per kWh of electricity produced) depends on the following major factors: • Site altitude i.e. atmospheric pressure • Inlet pressure drop in the filters and intake system • Outlet pressure drop in the HRSG. • Site design temperature. • Site design relative humidity corresponding to site design temperature. A gas turbine is a constant volume machine i.e. the volume of air compressed is fixed, irrespective of ambient temperature. Hence, as the temperature of air rises, the density of air decreases and the mass flow rate of compressed air is reduced. As the power output of the gas turbine is proportional to the mass flow rate of air, power output reduces as the ambient temperature increases. Further, the efficiency of the gas turbine also falls as more power is required to compress warmer air. For a given site and the configuration of the plant, the first three parameters are fixed and cannot be changed. However it is possible to change the other two parameters and obtain a higher output and improved efficiency by cooling the air before it is admitted in the gas turbine compressor section. Thus, cooling of the inlet air gives the following advantages: • Improves the power output and efficiency (heat rate) of the turbine • The output of the gas turbine is independent of the ambient temperature and does not decrease with increase in the ambient temperature. • By careful selection of the temperature, to which he inlet air is cooled it is possible to ensure that the gas turbine operates at its highest efficiency through-out the year irrespective of ambient temperature. The design inlet temperature at the gas turbine is also affected by the capabilities of the equipment available. The minimum chilled water temperature available from lithium bromide absorption chillers or mechanical chillers is around 5°C. Thus, typical air temperatures at the outlet of the cooling coil and the inlet of gas turbine compressor will be around 10°C. Furthers the inlet air cooling system must be designed to avoid icing at the compressor inlet or anywhere in the air intake system. Ice fragments a sucked into the compressor can cause serious structural damage. Icing is a potential problem, inlet Air Cooling, any time the ambient temperature drops to near the freezing


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mark. The problem is exacerbated for inlet air cooling systems because warm ambient air will almost always be saturated after passing through the inlet air cooling coils. When the air is drawn into the mouth of the compressor its velocity increases and its temperature drops further as air enthalpy is transformed into kinetic energy in an adiabatic process. Methods of Inlet Air Cooling Inlet air cooling can be achieved by any of the following methods: Indirect Cooling Using chilled water - In this case, the air is cooled by circulating chilled water through cooling coils. The cooling coils are installed in the intake air path and chilled water is produced using a vapor compression refrigeration cycle or absorption cycle. Disadvantages of this method - There is penalty on the turbine performance because of pressure drop in the air stream. Also this being an indirect method of cooling, the temperature of air leaving the coil will be approximately 3 to 5°C more than the outlet chilled water temperature. The advantage of this type of system is that it uses standard, proven, factory tested equipment such as centrifugal / screw chillers or absorption machines. Direct expansion of refrigerant - In this case the air is cooled by direct expansion of refrigerant such as ammonia or R134a, in cooling coils. The type of refrigeration system can be single stage / cascaded vapor compression system with liquid overfed air cooling coils. It is also possible to have multi stage cooling thereby consuming lesser power consumption per ton of refrigeration. The advantages of this system over mechanical chillers / absorption machines are: • Eliminating the auxiliary power consumption in circulating the chilled water, as the power consumption in circulating refrigerant is substantially lower. • The heat rejection duty in this case is substantially lower than absorption machines, thereby saving on cooling tower and cooling water pumping costs. The disadvantages are, in case of accidental leakage of ammonia it could affect the down stream equipment. The compressor systems also require electric power to drive the compressor performance because of pressure drop in the cooling coils. Direct Cooling: Evaporative Cooling Systems - Evaporative cooling works on the principle of reducing the temperature of an air stream through water evaporation. The process of converting water from a liquid to a vapor state requires energy. This energy is drawn from the air stream, the result being cooler and more humid air. The effectiveness of an evaporative cooling system depends on the surface area of the water exposed to the air stream and the residence time. Conventional media type evaporative coolers use a wetted honeycomb like medium to maximize the evaporative surface area and cooling potential. However this has several drawbacks, such as the media cause a pressure drop in the inlet air duct as well as the installation requires substantial inlet air ducting modifications and the amount of cooling that can be achieved can be fairly small in humid climates. High pressure fogging systems - It is a more recent technology employed in inlet aircooling. It is similar to evaporative cooling but instead of using water as an evaporative medium, the water is atomized into billions of super-small droplets thereby creating a large evaporative surface area. In these systems, the evaporative surfaces are the frog droplets themselves. For this reason the size of a droplet generated by the fog system is a critical factor. For instance water atomized


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into 10 micron droplets yields ten times more surface area than the same volume atomized into 100 micron droplets. A water droplet less than 40 microns is a fog and over 40 microns it is called mist. Fog systems use high pressure water pumps to pressure demineralised water to between 70 to 210 bar. The water then flows through a network of stainless tubes to fog nozzle manifolds that are installed in the air steam. In order to make droplets small enough to create the fog, impaction pin nozzles are normally used. These nozzles atomize the water into micro-cine fog droplets which evaporate quickly thereby cooling the inlet air. Other factors being equal, the speed of evaporation of water depends on the surface area of water exposed to the air. Another interesting development is "overcooling". In overcooling more fog is injected into the air stream than can be evaporated. Un-evaporated fog droplets are carried into the first stages of the turbine compressor section, where the air is hot due to the work of compression. Higher temperatures increase the moisture holding capacity of air so the fog droplets that would not evaporate in the inlet air duct, do so in the compressor. Once the fog evaporates in the compressor, it cools and makes the air denser. This accelerates the total mass flow of air through the turbine giving an additional power boost. Chilled water air washer cooling systems - In this case the air is cooled by bringing it in direct contact with chilled water in an air washer. As the pressure drop in the air stream is minimal, there is no significant penalty on the performance of the GTG. Further as the air is in direct contact with chilled water, the temperature of air leaving the air washer is very close to the outlet chilled water temperature. Here also the chilled water is produced using a vapor compression refrigeration cycle or absorption cycle. In all types of direct cooling the quality of water, an regards contaminants, needs to be controlled very accurately e.g. the total maximum limit on Na + K ions which can be tolerated, from all sources, for aero-derivative gas turbine is of the order of 0.1 ppm. Hence extremely pure DM (demineralised) water is required. There is also the danger of carry-over of bigger water droplets / moisture in the compressor section, which could cause damage to the compressor section of the gas turbine. Larger droplets could have enough mass to damage the compressor blading due to liquid impaction caused by impaction of water droplets.

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