Turbine Systems

8/12/2019 Turbine Systems

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Turbine systems (from Rolls-Royce, The Jet Engine)

Design criteria:

Match the designRequiriments whileMinimizing: cost, weight,Fuel consumption,Emissions, delivery time.

Three-shaft turbineSystem by RR.


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Example of a turbofan engine


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Scheme of a turbofan engine


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Turbine dimensions

Civil turbine (aircraft): length < 1.4 m; max. diameter < 1.3 m.

Military turbine (aircraft): length < 0.4 m (two-stage); diameter < 0.75 m.

Helicopter turbines are smaller.

Engine architecture

For delivering shaft power to an external system (energy or marineapplications), driving torque is derived from a free-power turbine. Thisarchitecture allows the free-turbine to be run at its optimum speed as it ismechanical independent of both the gas generator turbine and

compressor shaft.

Energy applications : the free-turbine drives a compressor, a pump, or analternator.

Marine applications : the free-turbine drives a propeller or an alternator.


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Two types of power turbines: Heavyweight and aero-derivative

Heavyweight: custom designed, high speedThe gas-generator is not directly coupled to the power turbine. The gas generator

is removable for maintenance. The turbine is characterized by long life and highrotational speed.


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Two types of power turbines: Heavyweight and aero-derivative

Aero-derivative: based on the aero-engine LP turbine.Typical three-shaft configuration. The LP turbine rotational speed typically

matches a driven alternator.


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The importance of the flow swirl

The design of the turbine is such that the swirl of the flow will be removedby its operation and so the flow at the exit of the turbine will be substantiallymore axial as it flows into the exhaust system. Excessive residual swirlreduces the efficiency of the exhaust system and can also producevibrations. This also explains why each stage of a conventional turbinerequires a nozzle guide vane (NGV) to recondition the flow with appropriateswirl and axial velocity for the receiving downstream rotor.

Counter-rotating (statorless) turbines

This kind of turbine have been designed using the latest aerodynamicmethods. The upstream rotor exit velocities and remaining swirl are tailored

to suit the inlet requirements of the following rotor, which will counter-rotate tomaintain efficiency. The benefits of such a design are: weight reduction;minimized engine length; reduction in the number of components. In militaryengines, the HP turbine rotates counter to the IP one (in three-shaft engines)or to the LP one.


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Example of a counter-rotating turbine


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Turbine cooling

HP turbine may work at temperatures of about 2000 K , greater than themelting point of the leading nickel-based alloys from which they are cast.Therefore, the HP blades, NGVs, and seal segments are cooled internallyand externally using cooling air from the exit of the HP compressor at atemperature of about 1000 K (achieved through compression) at a pressurewhich is only about 2 bar greater than the inlet turbine pressure.The decision about cooling a blade row depends on: the material; the use of

a thermal barrier coating (TBC); performance requirements; engine cost.

Casting

Nickel alloys are an almost universal solution for high temperature turbineblades and NGVs due to their high temperature creep resistance and

strength retention. The turbine design and material selection is dependent onthe trade balance between temperature, life, and component cost.Three common turbine blade casting options with increasing performance(reducing cost): 1) equiax; 2) directionally solidified; 3) single crystal alloy


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EquiaxDirectionallysolidified Single crystal


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Creep limit


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HP turbine blade cooling flow


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HP NGV cooling flows


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Turbine components A typical turbine assembly may be broken up into five main componenttypes: 1) casings and structures, 2) discs, 3) shafts, 4) NGVs, 5) blades .


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1) Casing and structures

The casings form the outer structure of the turbine and enclose the hotgases exiting the combustor. They are normally constructed from forged steelor nickel alloys strong enough to contain the internal gas pressure. Thecasing must contain any debris if a component fail. Casing are designed totransmit and react to axial and torsional loads imposed by the turbineassembly. Structures are designed to connect these casings to internal shaft

bearing supports, transmitting the bearing loads into the case and stiffeningthe assembly. Air and oil systems, to lubricate and cool the bearings, passthrough casings and structures. Other static components fit into the casing:NGVs, seals, supporting rings . Seal segments typically form a peripheralring of abradable material around the blades rotating tips. It is essential tocontrol the thermal movement of the seals so that optimum blade tip running

clearance is maintained. The thermal expansion is controlled bycompressed cooling air fed into the case-mounted cooling manifold. Accessports are provided within the casing for monitoring devices such asthermocouples and borescopes.


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Seal segment


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2) Discs

The main function of the turbine discs is to locate and retain the rotating bladesenabling the circumferential force to be transmitted through the central shaft.Each row of blades is retained in the rim of a disc via a root fixing, commonly offir-tree design , designed to withstand the enormous centrifugal loads. Discs aretypically formed from nickel alloy forgings, selected and inspected for lack ofdefects to mitigate the risk of disc failure. Design criteria on ultimate tensilestress, proof stress, creep, and fatigue all have to be satisfied.


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3) Shafts

The turbine shafts have three main functions: transmitting torque ; transmittingaxial loads to the compressor and location bearings; supporting the discs andblade assemblies . They are carried on oil-cooled and lubricated bearingsmounted within the structure. On a modern three-spool engine the three shaftseach rotate concentrically within one another at their optimum speed. Typically attake off the LP shaft rotate at 3000 rpm, the IP shaft at 6000 rpm, and the HPdrum at 10000 rpm. Military engine are smaller and rotate much faster.

4) NGVs

NGVs are shaped to swirl the gas flow in the direction of rotors rotation. They arestatic components mounted into the casings, designed both to with stand the axialtorque loads imparted form the gas stream and to react thermally without inducinghigh internal stresses within the assembly. NGVs are designed to achieve

optimum stage efficiency and for compatibility with compressor and combustordesign. They are of an increasingly complex curved aerorfoil shape. In modernsystems, HP and IP NGVs tend to be cooled, whereas LP NGVs are often rununcooled. Cooling air is flowed into the vane aerofoils (and sometimes thevane platform) at a higher pressure than that of the surrounding gas path. Thispressure differential flows the cooling air through rows of machine cooling holes,


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Bathing the components gas-washed exterior in a film of cool air ( film cooling ).Without this film, the vane temperature would quickly exceed the melting point ofthe alloy. To minimize the amount of cooling air, modern HP and IP NGVs are castusing single crystal nickel alloy and are typically coated in a ceramic TBC.

5) Blades

Turbine blades are designed to generate power by translating circumferentialaerodynamic forces on the aerofoils to the rotating disc. The blades rotate with atypical tip speed of 460 m/s . The power output of a single HP blade for civil

applications is ten times higher than the one of a small car and the centrifugalforce transmitted into the disc is about 18 tonnes with a centripetal accelerationof about 60000g . The hottest blades are cast in high-temperature nickel alloys using the lost wax casting method and are often coated in a ceramic TBC ontheir aerofoils and platforms. Operating temperature dictate the need of internallycooling the HP blades. Cooling flow is detrimental to turbine performance and is

regulated very carefully. The blades glow red-hot during engine running . Theymust resist also to fatigue, thermal shock, corrosion, and oxidation. Blades mayincorporate a shroud at tip forming an outer annulus ring when assembled.Shroudless blades can be run at higher rotational speed due to their lower massbut suffer from a larger overtip leakage and resultant performance effects. Dueto creep, blade length increases in time. Useful time limit for civil applications

is 35000 hours (for an airliner: 14 hours flight per day, means 6 years).


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HP turbine blades


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Major seal types


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Seals and bearings


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Non-isentropic adiabatic flow in a duct (Fanno line)

(from Sandrolini, Naldi, Macchine, I vol., Pitagora Ed.)


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Labyrinth seals (from Sandrolini, Naldi, Macchine, I vol.)


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Labyrinth seals


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Labyrinth seals


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Labyrinth seals

Fig. A 33


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Labyrinth seals

* Infatti, gli stati cherappresentano lecondizioni del fluidonei vani hanno lastessa entalpia etemperatura, uguale

alla temperatura totale.


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Labyrinth seals


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Labyrinth seals


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Steam turbine (from GE Report 3646D)

Modern steam turbine designs for electrical power generation are the result ofmore than 90 years of engineering development. The product line of fossil-fueled, reheat steam turbines for both 50Hz and 60Hz applications extends from125-1100 MW and is based on a design philosophy and common characteristicfeatures that ensure high reliability, sustained high operating efficency and caseof maintainance.

Historically, increases in steam turbine ratings have been accompanied by longerlast-stage buckets in order to maintain an economical unit size. Longer last-stagebuckets (LSB) can accommodate larger steam flows and loadings at relativelythe same performance level by maintaining or reducing exhaust losses, withoutincreasing the number of low pressure turbine flows.

The longer LSBs include a 40-inch/1016 mm, 60 Hz titanium LSB and a 42-inch/1067 mm, 50 Hz LSB.

The benefit of the increased annulus area associated with the longer last stagebuckets is demonstrated by a more compact steam turbine configuration.

S bi (f GE R 3646D)


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St t bi (f GE R t 3646D)


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Peak efficiency is obtained in an impulse stage with more work per stage than ina reaction design (Figure 15), assuming the same diameter. For this reason,

impulse bladings are mounted at early stages to lower very fast pressure andtemperature inside the turbine.


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Minimizing stage leakage flow is important to stage efficiency. With less pressuredrop across the buckets, the loss due to leakage at the bucket tip is obviously

much less for an impulse design than for a reaction design.

Greater pressure drop exists across the stationary nozzles in an impulse designthan in a reaction design. However, the leakage diameter is typically 25% lessand, therefore, the cross-sectional area for leakage is less. Also, with fewerstages there is sufficient space between wheels to mount seals packings.

On a relative basis, however, the leakage losses on a reaction stage will alwaysbe greater than those on an impulse stage designed for comparable application.

With more energy per stage, steam velocities in an impulse stage are higherthan in a reaction stage. These higher velocities have the potential of resulting inprofile losses that could offset the effects of reduced leakage loss if poor nozzleand bucket profiles were used. This was a legitimate concern in the early days ofsteam turbine development with only very simple bucket profiles used. Profilelosses, however, are very amenable to reduction with increased sophistication ofnozzle and bucket profiles. With current computer analysis methods andaerodynamic testing techniques, significant gains continue to be made inreducing profile and other secondary losses.


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Because of the relatively large pressure drop that exists across the movingblades in the reaction design, a very high thrust force would exist on therotor if the blades were mounted on wheels with faces exposed to thepressure differential. A drum type rotor, as shown in Figure 17, is used inreaction-type turbines to avoid excessive thrust.

This solution reduces leakageflow losses and increase

efficiency.

This solution reduces thrust.


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Steam turbine: off design (from Wikipedia)

Speed regulation

The control of a turbine with a governor is essential since some applications,such as the generation of alternating current electricity, require precise speedcontrol. Uncontrolled acceleration of the turbine rotor can lead to an overspeedtrip, which causes the nozzle valves that control the flow of steam to theturbine to close.During normal operation in synchronization with the electricity network, power

plants are governed with a five percent droop speed control . This means thefull load speed is 100% and the no-load speed is 105%. This is required for thestable operation of the network without hunting and drop-outs of power plants.Normally the changes in speed are minor. Adjustments in power output are madeby slowly raising the droop curve by increasing the spring pressure on acentrifugal governor . Generally this is a basic system requirement for all power

plants because the older and newer plants have to be compatible in response tothe instantaneous changes in frequency without depending on outsidecommunication.


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Steam turbine: off design (from Weekipedia)

Speed regulation by lamination valve (reduction of total pressure).


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Steam turbine: off design (from Wikipedia)

Partial admission of steam in the impulse turbine at the HP stage.

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Turbine Systems