Turbo Machinery Dynamics Part 3 ppt

It is assumed in the derivation that the pres-sure ratios in the compressor and the turbine remain the same, the fuel flow rate is consid-erably smaller than that for air, the specific h

In order to improve thermal efficiency, a turbine’s hot path components are exposed togases at extraordinarily high temperatures, sometimes beyond the capability of the mater-ial Sophisticated cooling methods are required in the combustor and the turbine toavoid degradation of the material’s strength and durability characteristics New breeds ofsuperalloys are constantly under development to deliver the required characteristics, andstill can be formed to the required component dimensions during manufacturing The dif-ferential thermal growth between mating rotating and static parts must be predicted with ahigh level of precision Besides normal operating conditions, dynamic loads arise fromrotating unbalance, fluid flow forces, and misalignment Manufacturing techniques havealso seen vast changes in the past 20 years to allow machining and fabrication of highlycontoured airfoils to close tolerances.

A gas turbine operates on the Brayton cycle The Brayton principle consists of two isobaricand two isentropic processes, as shown in Fig 3.1 The former take place in the gas tur-bine’s combustor and in the steam generator’s gas side, and the latter represents the com-pression of air and expansion of gases in the turbine Gas turbine cycle efficiency depends

on the compression ratio and turbine firing temperature, increasing with both parametersaccording to the relation shown in Eq (3.1) It is assumed in the derivation that the pres-sure ratios in the compressor and the turbine remain the same, the fuel flow rate is consid-erably smaller than that for air, the specific heat of the gases stays constant, and that allcomponents operate without incurring any losses:

(3.1)

where h is the ideal cycle efficiency, r represents the compression ratio, and g is the ratio of

specific heats at constant pressure and constant volume Turbine and compressor efficiencies

will have a moderating effect on the overall efficiency Figures 3.1 and 3.2 demonstrate theimpact of the two parameters Work performed per pound of air occurs at a lower pressureratio than the point of maximum efficiency for a given firing temperature The overall cycleefficiency is improved with the increased pressure ratio, cooler compressor inlet tempera-ture, and higher turbine inlet temperature (Fig 3.3) Evaporative cooling, direct water fog-ging, and refrigerated cooling are some of the methods used to cool the air at the inlet Theimpact of efficiencies in the compressor, combustor, and turbine, as also system-relatedpressure losses in a simple-cycle turbine is shown in Fig 3.4.

Since the gas temperature at the turbine exit is higher than that at the compressor exit,the insertion of a regenerator to preheat the air between the compressor and the combustor

with the turbine’s exhaust gases will reduce the fuel requirement If T1, T2, T3, and T4

rep-resent inlet temperatures of the compressor, regenerator, combustor, and turbine and T5isthe exit temperature from the turbine, ideal regenerator efficiency (assuming no pressureloss) may be expressed in the following form:

(3.2)ηRe g=T T3−−T T2

5 2

FIGURE 3.2 Compression ratio and thermal efficiency.

System cycle efficiency then takes the form

Power output from a gas turbine can be increased in a number of ways Intercooling ofair between the stages in a compressor may be used to reduce the work done to pressurizethe air Temperature reduction of the partially compressed air causes its volume to shrink,consequently less work is required to compress it to the next pressure level Thermal effi-ciency of a simple cycle is decreased by the addition of an intercooler, but the addition of

an intercooler to a regenerative gas turbine cycle increases thermal efficiency and poweroutput The explanation is that a larger portion of the heat required for regeneration nowcomes from the turbine exhaust instead of additional fuel consumption

When turbine expansion is split into two or more steps, with constant pressure heating

taking place before each expansion, the process is referred to as reheat cycle As in

inter-cooling, the thermal efficiency of the simple cycle is reduced by reheat, while work outputrises In combination with a regenerator, reheat can be made to increase thermal efficiency.Performance curves for a simple-cycle gas turbine that includes provision for intercooling,regeneration, and reheat are shown in Fig 3.5 Figure 3.6 provides details of a single-shaftgas turbine supported on two bearings

3.3 INDUSTRIAL COMBUSTION TURBINE

Gas turbines are a practical and economic way for utility and industrial service Advanceddesign units offer high firing temperatures, low NOx, and improved efficiency For example,the 60 Hz 200 MW class W501G engine has been jointly developed by WestinghouseElectric Corporation, Mitsubishi Heavy Industries, and FiatAvio (Southall and McQuiggan,1995) This machine continues a long line of large heavy-duty single-shaft combustion tur-bines by combining the proven efficiency and reliability concepts of the W501F with thelatest advances in the aero technology Designed for both simple- and combined-cycleapplications, the turbine can operate on all conventional turbine fuels and on coal-derivedlow-Btu gas produced in an integrated gasification plant

The general configuration of the combustion turbine calls for a bolted construction rotorsupported in two bearings (Fig 3.7) The 18-in-diameter bearings have compounded form,with tilting pads in the lower half and a fixed arc element on the upper side The compres-sor segment of the rotor is assembled from spigotted disks bolted together by 12 throughbolts Alignment and torque transmission are assured by employing radial pins between thedisks The turbine rotor section is made up of disks provided with curvic clutches (seeFig 3.8), and are bolted together by 12 through bolts The curvic clutch is machined in theform of uniformly spaced teeth protruding axially from the flat face of the disk The teethengage and interlock with a similar pattern machined on the face of the adjacent disk, pro-viding a slippage-free joint under the action of the clamping load from the through bolts.The combustion system consists of 16 cans arranged in an annular pattern Stability ofthe flame and uniformity in the distribution of fuel flow between the combustors are mon-itored by thermocouples located downstream of the last turbine stage Malfunctions in thecombustor when at load and sensing of ignition during the startup mode can also be detectedwith the aid of ultraviolet sensors

The casings are split horizontally to permit maintenance with the rotor in place Theinlet and compressor casings are made of nodular cast iron and cast steel, while the com-bustor, turbine, and exhaust casings are of alloy steel The inlet end bearing housing is sup-ported from eight radial struts At the exhaust end the bearing is supported by tangentialstruts that respond gradually during transient conditions, and maintain alignment betweenthe rotor and the bearing through rotation of the housing about its axis to accommodatethermal growth The arrangement provides an additional benefit of reducing thermalstresses in the struts The fairings around the struts are configured in the form of an airfoil

to enhance aerodynamic performance Individual blade rings are employed for each pressor and turbine stage to control leakage past the blade tips The rings have a relativelyhigh thermal response independent of the outer casing, and are provided with features toobtain concentricity with the rotor to prevent rubbing from the blade tips, minimize radialclearance, and thus maximize performance

com-Since the turbine operates at extremely high inlet temperatures, cooling air for the rotor

is extracted from the compressor discharge, and is externally cooled and filtered beforereturning to the torque tube casing to cool the disks and the first-, second-, and third-stage

blades and vanes (Fig 3.9) Filtration is deemed essential for eliminating blockages by pended particles of the intricate cooling passages inside the blades Bleed air from the com-pressor is also used to cool the blade ring cavities and to cool and purge the interstage diskcavities to prevent ingestion of hot blade path gases.

sus-Compressor diaphragms are coated to improve aerodynamic performance and to obtainprotection from corrosion The stationary vanes and rotating blades for the first two stages

of the turbine are provided with a thermal barrier coating The operating firing temperaturelevel of 1425°C is selected to be commensurate with the capabilities of the superalloys forthe components in the hot path and with the cooling schemes The cycle pressure ratio ischosen to maximize power output during simple-cycle operation and efficiency in the com-bined cycle mode With a compression ratio of 19.2:1, the potential-combined cycle effi-ciency is 58 percent The cycle airflow rate is controlled through the annular flow area atthe turbine exit, since the last-stage blade stress level is directly proportional to it A flowrate of 1200 lb/s results in a conservatively stressed blade and higher power output.The 17-stage axial flow compressor is patterned from the proven design for the W501Fengine Flow and pressure coefficients are similar in the two designs, with the mean diam-eter of the stages increased to accommodate the 25 percent increase in flow Bleeds forstarting and cooling flows are located at the 6th and 11th stages, and the 14th-stage bleed

is used for the hot path components The compressor is also equipped with variable inletguide vanes (Fig 3.10) for improving the low-speed surge characteristics and to enhancepart-load performance in combined cycle applications The rotating blades are controlleddiffusion airfoils, made of multiple circular arc forms Stationary blades are fabricated intwo 180° segments for easy removal, and are provided with sealing at the inner shroud.Moderate aerodynamic loads are used for the four turbine stages operating at a higherperipheral speed than the W501F engine Aerodynamic airfoil shapes are obtained from afully three-dimensional viscous analysis code The third- and fourth-row blades areshrouded The first-row stationary vanes are individual precision cast of IN939 alloy, andcan be removed from access manways without lifting the cylinder cover Inner shrouds aresupported from the torque tube casing to limit flexural stresses and distortion Vane seg-ments for the other rows are supported in a separate inner ring

The cooling scheme maintains the NiCrMoV turbine disks under 400°C, within the creeprange for long life Row-1 vane cooling is done by the methods of impingement, convection,and film cooling, as shown in Fig 3.9 Impingement inserts are used in combination with an

FIGURE 3.8 Turbine rotor disks with curvic

coupling (Courtesy: Siemens Westinghouse)

Transition mouth seal

Shower head

69

array of film cooling holes and a pin fin at the trailing edge Pin fins help to increase bulence and the surface area Film cooling is provided at the leading edge on the pressureand suction sides This limits thermal gradients and external surface temperatures at thewalls of the vane Special attention is paid to the inner and outer shrouds because of the flattemperature profile from the dry low NOxcombustor The shrouds are cooled by impinge-ment plates, film cooling, and by convection through drilled holes.

tur-The cooling arrangement for the row-1 blades consists of serpentine passages withangled turbulators (Fig 3.11) Film cooling uses fan-shaped cooling holes, and is usedextensively at the tip to reduce the metal temperature of the squealer tip The airfoil iscoated with a vapor deposited thermal barrier coating For the row-3 blade the cooling isunique in that it positively cools the blade tip shroud (Fig 3.12) Because of the flat profilefrom the combustor and because of leakage past the tips of row-1 and row-2 blades, posi-tive cooling for the tip shroud is deemed essential

FIGURE 3.10 Compressor inlet variable guide vanes (Courtesy: Siemens Westinghouse)

The rotating blades are made from CM247 for all four rows The blades are providedwith long root extensions, or transitions, to reduce the stress concentration as the load trav-els through the airfoil into the shank The blade roots are of multiple serration type, withfour serrations on the first three rows and five on the last-stage blades.

The dry low NOxcombustor operates at 25 ppm NOxlevel at 1260°C turbine inlet perature Steam cooling is used for reduced emissions at higher firing temperatures Byeliminating the transition cooling air virtually all the combustion air is introduced into theprimary zone of the combustor to maintain the flame temperature at nearly the same level

tem-as in the W501F engine

OF STEAM TURBINES

Steam turbines remain the workhorse of power generation worldwide Hero of Alexandria

is credited with developing the first steam turbine 2000 years ago Dr de Laval strated driving of a paddle attached to a shaft by expanding steam through a trumpet-shapedsteam jet in the latter half of the nineteenth century In 1894, Sir Charles Parsons inventedthe multistaged steam turbine Today, steam turbines are the favored choice in the driving

demon-of electric generators, mostly due to the extensive availability demon-of coal

A steam turbine converts the thermal energy of steam into kinetic energy by expansion

in nozzles, the resulting jet then forcing rows of blades mounted on a rotor Steam turbine

power plants may be split into three groups: (1) heat sources such as boilers or steam erators, feed water pump, and heater; (2) power generation components that include turbine and generator; and (3) condensers and condensate pump Rankine cycle is most commonly

gen-employed, with water-steam as the working medium Water is pressurized isentropically bythe feed-water pump before entering the boiler, where it evaporates to steam and is even-tually superheated The cycle requires substantial amount of heat to raise the temperature

of water at pump discharge to steam temperature at turbine inlet Using high-temperature

FIGURE 3.12 Cooling for row-3 blade shroud (Southall and McQuiggan, 1995).

Shroud cooling feed hole

Shroud cooling hole

Airfoil cooling hole

exhaust gas for this purpose partially offsets the amount of external heat required by mizing the temperature difference between the two points This is the concept behindregenerative heating A more common practice calls for intermediate pressure steam to per-form the function of heating the feed water.

mini-Steam tends to increase in moisture content as it progresses through the stages of a bine Wet steam tends to affect the buckets, since the higher density of water impacting itsleading edges causes erosion A reheat of steam withdrawn after partial expansion is required

tur-to overcome the situation (Boyce, 2002)

A regenerative and reheat steam arrangement is shown in Fig 3.13 and the ing temperature-entropy diagram in Fig 3.14 The compressed liquid at D is heated to sat-uration point (D1), evaporated to steam (D2), and finally superheated (D3) After isentropic

FIGURE 3.13 Regenerative—reheat steam turbine plant schematic.

expansion in the ideal engine to point E (point E′ after accounting for losses), some of thesteam is extracted from an intermediate stage in the turbine to heat the pressurized con-densate in the feed-water heater The rest of the extracted steam is reheated and thenreturned to the turbine (point F) After further expansion to point G′ for the real case thesteam passes to the condenser, converting it to saturated liquid at point A.

Heat rate, a modified reciprocal of thermal efficiency, defines the heat chargeable inBtu/kW⋅h for a straight condensing or a noncondensing turbine, and may be expressedas

Heat rate= [(h1− h f 2) × 3415]/WnetBtu/kW⋅h (3.4)

where h1is the enthalpy of throttle steam, h f 2is the enthalpy of liquid water at exhaust

pres-sure, and Wnetis the available work output at the generator coupling, Btu/lb of steam atthrottle

Among the many different ways of classifying steam turbines, fluid flow direction ifies the engine type as axial, radial, or mixed flow In axial flow machines, steam flowsvirtually parallel to the machine axis; medium and large turbines are configured with thisarrangement Some small turbines have a radially outward flow among the blades Axialflow machines may be split into two categories—impulse and reaction The former uses asystem in which all steam expansion occurs in fixed nozzles; pressure drop is absent in thepassages between the moving blades The change in enthalpy in the nozzles increases thesteam’s kinetic energy, which is then imparted to the blades Reaction, or Parsons, turbinescall for the expansion of steam in both the nozzle vanes and in the moving blades Steamturbine manufacturers most commonly use a combination of the two methods The initialstages are usually of the impulse type, while the subsequent stages are based on the reac-tion principle The degree of reaction is defined as the ratio of change of enthalpy reduc-tion in the blades to the total enthalpy change in the stage

spec-Two forms of losses play a major role in the design of steam turbines For high flowMach numbers, the loss factor remains constant up to 1.0 To avoid choking of passagesand severe shock, the Mach number is limited to 1.15 The other source of loss arises fromthe arrangement of nozzles along the periphery at the turbine’s admission At the high-pressure (HP) end the nozzles are placed in selected locations around the circumference.This form of partial arc admission facilitates operation of the turbine at part load, but it alsocauses loss in energy As the flow progresses, the increased volume of steam necessitateslarger circumferential arcs to be occupied by the nozzles A balance between the flow Machnumber and the partial arc length determines the design layout in steam turbines.Most power plants employ turbine sections designated as high-pressure, intermediate-pressure (IP), and low-pressure (LP) The number of stages in each section is based on pres-sure, temperature, and the amount of steam available from the boiler, as also the stagepercentage reaction Basic rotor and casing configurations can be made in selected ways(see Fig 3.15) Steam enters at one end, flows through the nozzles and blades parallel tothe shaft axis, and exits into the condenser at the other end in a single casing machine.Tandem compound turbines call for steam expansion in two or more separate units, with asingle coupled shaft driving the generator Exhaust from the high pressure may be reheatedbefore entering the intermediate section Figure 3.16 provides a layout for a compound flowunit, with high and intermediate sections in one casing and LP turbine in the second casing.Exhaust from the intermediate unit is carried to one or more LP units by the crossover pipe.The path through the LP turbines is split into parallel flows because of limitations on theblade length and for the need to balance axial end thrust

Low volumetric flow in the HP and IP turbines causes their blades and vanes to beshorter in length Generally straight, HP, and IP turbine blades still tend to have some lean-ing and bowing, creating some three-dimensional characteristics Shrouds at the outer tiphelp in sealing steam flow radially, thus contributing to improved efficiency Structurally,

the shrouds also constrain tip motion in the axial and tangential directions, which reducesvibratory stresses and improves fatigue life by damping elastic motion of the blades LP tur-bine blades are considerably longer and may require one or two midspan interlocking con-strain mechanisms (such as tie-wires) in addition to the outer shroud LP turbine blades may

be connected in groups of two to eight blades, or all blades in a row may be connected tinuously At the blade tip a protruding tenon aids in attaching the shroud in the form of a

HP turbineinletThrust and

journal

bearings

Crossover piping

Low-pressurestagesJournalbearings

To condenserPedestal

Intermediatepressure stages

IP turbineinlet

Rotor

cover with a rivet Tie wires may take the form of integral forged stubs welded or brazedtogether, or may be rods inserted through a hole with a boss in the blade.

Figure 3.17 shows a typical LP turbine blade Attachment of the blades to rotor or diskcan be made in different ways The fir tree configuration is widely used for HP turbineblades since its side entry feature permits easy replacement But this form may result invibration modes with frequencies close to nozzle wake frequency Longer blades may beequipped with triple pins for attaching to the wheel Serrated and T-shaped roots allowinsertion into individual axial slots, or may be introduced tangentially at a gap in the disk toform a continuous assembly Male or female forms for the dovetail roots may be designed.Figure 3.18 provides some illustrations

Stationary nozzle vanes in steam turbine construction are of wheel and diaphragm type

or of the drum form Used in impulse stages, the diaphragm (Fig 3.19) consists of stationaryvanes, an outer ring to locate in the casing and a web reaching into the cavity between thediscs where the labyrinth seals are located In the first control stage, the nozzles are sepa-rated into segments, with each group in individual nozzle chest or box Control valves foreach box are used to get the desired engine power output Pressure variation causes the vanesand diaphragm to bend in a plane perpendicular to the turbine’s axis Pressure differential isgreatest for HP turbine blades, but they are shorter, resulting in lower bending stress.Varying operating conditions in the HP and LP sections demand appropriate materialrequirements for the blades Creep strength and associated fatigue are of special interest

in the higher-pressure regions, but corrosion resistance due to operating environmentgets greater premium for LP turbine blades Partial arc admission in the HP turbine alsocalls for increased material damping Erosion resistance from solid particle ingestion inthe HP section is desirable, while condensed droplets in the LP region impose a similarneed 12-chrome martensitic stainless steel is the preferred choice for HP and IP rotating

and stationary blades American Iron and Steel Institute (AISI) 422 for HP blades andAISI 403 and AISI 410 for LP blades find wide application Austenitic stainless steels (forexample, AISI 300) have higher tungsten and chromium content for higher temperatureenvironment, but are more prone to stress corrosion when operating in wet steam.Differences in thermal coefficient of expansion must be taken into account Thus, clear-ances at the attachment of an austenitic blade and a martensitic disk will be affected by the

FIGURE 3.18 Blade root forms for steam turbines.

FIGURE 3.19 Steam turbine

relative thermal growth HP and IP diaphragms are mostly made from stainless steel;carbon steel is an option if the steam temperature is below 650°F Titanium with 6 percentaluminum and 4 percent vanadium for LP blades offers some distinct advantages Lowerdensity, about half that of chrome steel, reduces centrifugal stress, permitting the use oflonger blades for increased flow area and performance The alloy also has favorablemechanical properties at low temperature, and corrosion and impact resistance from waterdroplets The last feature helps to eliminate the erosion shield Titanium alloys, however,are expensive, more difficult to work with in manufacturing, have low resistance to wearfrom sliding, and lower high-cycle fatigue endurance.

Steam turbines for electric power generation offer some unique advantages The workrequired to increase the fluid pressure in the liquid phase in the feed water pump is consid-erably less than that needed to compress gas Steam boilers have the ability to burn a widespectrum of fuels without the steam coming in direct contact with the products of combus-tion Fuels range from natural gas to heavy residual fuel and solid fuel such as coal andrefuse Consequently, production costs compare favorably against production by gas tur-bines However, a large amount of equipment is required, raising the initial capital invest-ment and cost per installed kilowatt of power Indirect heating in a furnace necessitateslarge heat exchangers to obtain pressurized and superheated steam The boiler requires sub-stantial land area for installation, involves many different auxiliary components, and takes

a long lead time for erection Finally, it takes several hours for a boiler to raise steam andput the turbines on line; the cool-down sequence for the boiler is also considerably long.The system is designed to operate continuously for long periods of time rather than formeeting short-duration surges in demand

The thermodynamic performance of a steam turbine is primarily determined by the design

of the steam path components such as valves, inlet, nozzles, buckets, steam leakage trol devices, and exhaust Because the efficiency of the entire power plant cycle is highlydependent on the efficiency of the steam turbine, it is important to minimize aerodynamicand steam leakage losses in the path of both the rotating and stationary components.Figure 3.20 shows the various losses experienced in a typical turbine stage Nozzle andbucket aerodynamic profile losses; secondary flow losses; and tip, root, and seal leakage

con-losses can be significant if the airfoil shapes are not optimized for the given set of operatingconditions Profile losses are driven by the surface finish, total blade surface area, airfoilshape, and velocity distribution Attention to adequate matching between the nozzle vanesand buckets is also necessary to control incidence losses (Cofer, 1995).

Equally significant losses can be caused by the complex secondary flows that develop

as the viscous boundary layers along the inner and outer sidewalls of the steam path areturned through the rows of blades Steam leakage through the seals between the stationaryand rotating components does not contribute to the work output of the stage, and the lossescan be quite significant, especially at the bucket tips In the shorter HP stages the tip leak-age loss is driven by the pressure level and the relatively larger clearance when comparedwith the blade radial height In the taller IP and LP stages, this loss is controlled by thehigher level of reaction at the bucket tips, which increases the pressure drop across the tip.Steam leakage through the diaphragm shaft packing and the shaft end packing also causeslosses, but generally not as severe as at the outer tip

Development programs to better understand and reduce the losses have focused on fourkey elements:

1 Better computational fluid dynamics (CFD) computer codes for an accurate prediction

of the complex behavior of the steam flow throughout the turbine

2 Refined design concepts to integrate improvements in component configuration with

manufacturing capabilities

3 Extensive laboratory tests to validate the predicted results from the CFD codes.

4 A suite of powerful design automation and optimization tools to permit the

implemen-tation of advanced aerodynamic design features on a custom basis to maximize ciency for specific applications

effi-The available energy of the steam can be more completely used by simulating the dimensional flow field with CFD tools In particular, an improved understanding of the effects

three-of wet steam, viscosity, and unsteady rotor–stator interactions is needed to check the losses.Unique flow visualization tools are developed to aid in understanding the physics of secondaryflows One method to obtain low-particle trajectories is by shining a strobe light on helium-filled zero buoyancy soap bubbles injected into the flow Figure 3.21 shows one leg of the

“horseshoe” vortex spiraling around the nose of a nozzle cascade with parallel sidewalls.The viscous Euler code is widely used for a variety of steam turbine design problems

A calculation grid can be developed for a blade-to-blade plane, with a similar one for thenozzle passage A complete stage solution is obtained by automatically iterating betweenthe nozzle and bucket solutions as they run in parallel on separate workstations Up to250,000 grid points may be required for each blade passage to resolve the secondary flowdetails The code permits accurate solutions of the fully three-dimensional viscous Navier-Stokes flow equations, independent of the free vortex restrictions Some of the outstandingfeatures of this concept are as follows:

• Radial flow distribution is tailored to maximize efficiency based on individual stagegeometry and operating conditions

• Nozzle surface area is reduced by using fewer vanes with aerodynamic profile shapesaround the circumference without affecting the mechanical strength

• Variable tangential or compound lean is used in the nozzles to obtain larger gain in all stage efficiency

over-• Root reaction is increased in varying degrees to improve bucket root performance, andtip reaction is generally decreased relative to reduced tip leakage

The last-stage bucket is perhaps the most important contributor to the performanceand reliability of the steam turbine, ranging in size from 20 in for 3000 rpm applica-tions to 48-in titanium for 3600 applications Some salient features of the blades are asfollows:

• Improved vane profile, including a convergent–divergent supersonic passage, and highsolidity

• Enhanced mass flow distribution

• Improved tip leakage control for moisture extraction

• Continuously coupled covers, with side or over and under entry

• Self-shielding erosion protection

With the last-stage buckets typically developing 10 percent of the total unit output, and

up to 15 percent in combined cycle applications, improvements in their efficiency can siderably impact the total power generated by the unit A bladed disk using 40-in titaniumlast-stage buckets for a 3600-rpm turbine is shown in Fig 3.22

con-Contouring of the sidewalls must aim for reduction in secondary flow losses by ing the cross-channel pressure gradients, thereby reducing the strength of the flow Thecontour also reduces the size of the loss region near the inner sidewall, and also diminishesnozzle profile losses due to the lower velocity at its inlet Contoured sidewalls are com-monly used in the first stage of the turbine

reduc-Bucket tip leakage control mechanisms typically rely on a single spill strip mounted onthe upstream side of the bucket, or two spill strips attached on either side of the bucketcover tenon Leakage along the shaft is controlled by the packing, which contains multipleteeth located between the diaphragms and the shaft Flow past the teeth is controlled by thereduced radial clearance and by the torturous path between the rotating and stationary com-ponents Last-stage buckets may be provided with a radial spill band on the cover to main-tain a tight clearance

FIGURE 3.21 Secondary flow in turbine nozzle cascade (Cofer, 1995).

3.6 COMBINED CYCLE MODE

The Brayton-Rankine cycle combines a gas turbine with a steam turbine and has many efits for electric utilities and for process industries requiring steam The hot gases from theexhaust of the gas turbine are employed in a fired boiler to generate superheated steam todrive a steam turbine (Fig 3.23)

FIGURE 3.22 Forty-inch titanium last-stage buckets (Cofer, 1995).

From the first law of thermodynamics, work done by the compressor and gas turbine is

(3.5)

(3.6)

The net work done is in the same range as the work done in a steam injection cycle but

less fuel is required, which increases thermal efficiency The cost of a heat recovery steam generator (HRSG), steam turbine, condenser, and pump tends to considerably increase the

first cost associated with this cycle The NOxcontent of the exhaust gases depends on thegas turbine The main attraction of this cycle is the substantial advantages it offers in oper-ating performance and fuel costs Figure 3.24 provides performance curves for a typicalcombined cycle plant (Boyce, 2002)

ηC C W Q

yc yc

The power produced by a gas turbine can also be increased by injecting humidified andheated compressed air or steam into the compressor discharge or directly into the com-bustor, and by water injection in the midcompressor stages Heating of air or water may

be accomplished in a heat recovery steam generator, together with a separate compressor

or pump Flashing of water in the form of a spray mist in the middle rows of the pressor to cool the air aids in obtaining an isothermal compression process Evaporation

com-of the water through the consumption com-of latent heat lowers the temperature com-of the airstream as it enters the succeeding stage, and lowers the work done during compression.The technique has been successfully applied in many HP gas turbines (for example,General Electric’s LM6000 gas turbine engine), and may be combined with other methodsdescribed earlier A more elaborate system is required to inject the external humidifiedand heated air at a point upstream of the combustor A supplementary compressor for airpressurization, a saturation column for humidification and preheating, a water heater, and

a balance of plant equipment in the form of pipes, valves, and controls are required for thesetup

Water or steam may also be injected at the compressor discharge to augment the ated power For example, if the injected steam is 12 percent of the airflow, power increasewill be in the neighborhood of 25 percent If steam can be generated from the gas turbine’sexhaust, the procedure also yields higher efficiency

gener-When a gas turbine is equipped with nozzles to burn dual fuels, steam is injected intothe combustor with the primary intent of controlling NOxformation Combustion concernswill limit the amount of steam injected, but 3 percent of the airflow will provide a boost of

4 percent in the power output Steam would come from the HRSG

A number of power plant operators are faced with inadequate power-generation ity during the summer, especially in the daytime when air conditioning and industrialrequirements reach a peak Peaking units designed to operate for only a few hours in theday and started and put on line quickly have to be installed expressly for the purpose Butthe investment is not particularly attractive since the return is not high To alleviate the sit-uation, Alabama Electric Cooperative (Brown, 2000) operates a plant based on a cycleinvolving storage of compressed air During off-peak hours excess power drives a train ofcompressors, and stores compressed air into solution-mined underground caverns Thecompressor train includes a combination motor and generator with clutch mechanisms.When compression is required the motor is engaged to pressurize and store air in thebunker, while also disengaging the expansion mechanism To boost capacity, the compres-sor may be split into two or more modules with intercoolers in between them Conversionfrom compressed air to electric power takes place in high- and low-pressure air expandersand generators For this mode of operation, the clutch disengages the compressor train Airreturning from the bunkers may be heated regeneratively in a recuperator using the exhaustgas from the LP expander and additionally burned in combustors before entering the HPexpander From the HP expander, the air is further reheated in combustors before enteringthe LP expander Can combustors are used With the aid of two combustors the HP expanderproduces a quarter of the power, the remaining three quarters coming from the LP expanderwith its eight combustors The plant has dual fuel capacity, natural gas and No 2 distillatefuel oil, and is capable of producing between 100 and 110 MW

capac-Many configurations for combined cycle plants are designed In most applications thegas turbine cycle is the topping cycle, and the steam cycle is the bottoming cycle Thermalefficiencies reach close to 60 percent in the combined cycle, with the gas turbine producing

60 percent of the power and the steam turbine contributing 40 percent at design point

At off-design conditions the inlet guide vanes in the compressor control the amount of airentering to maintain temperature levels in the HRSG Steam may be formed at one or atthree different pressure levels Steam generation at multiple pressure levels decreaseslosses at the exhaust stack