Table of Contents
2.1 Principles
The gas turbine frequently finds itself as a portion of a so-called package with an extensive periphery ( "Ill. 2.1-1"). The gas turbine itself consists of three main components: the compressor, the combustor and the turbine ( "Ill. 2.1-2").The compressor compresses the inlet air to a pressure that can be above 40 bars in a modern engine. The air is heated up to 600°C, depending on the compression. Fuel is added to the air and burnt in the combustion chamber. Relative to ecology, this is the decisive process. In the turbine, the hot gas releases it’s energy as widely as possible. The turbine, in multiple spool engines - a part of the turbine - powers the compressor, which internally requires two thirds of the total performance ( "Ill. 2.1-3.1"). Except for a very small loss, the internally delivered, compressor energy is led back into the engine through the compressed air flow. Merely, approximately, one third of the total performance of the turbine can be used to mechanically empower an external aggregate. The unused, exhaust energy corresponds to two thirds of the total performance of the fed-in fuel energy. The exhaust temperature does not lie under, approximately, 500° C. Parts of the exhaust energy can be recycled in a combined-cycle plant (GCC) e.g., with the help of heat exchangers (recuperators), heating up the compressor air before the combustor, or steam injection ( "Ill. 2.1-3.2"), or removed through power- heat coupling ( "Ill. 2.1-3.4"). For the purposes of industrial processes, e.g., drying, exhaust energy lets itself still be used with a relatively low temperature niveau (Power-Heat Cogeneration . "Ill. 2.1-3.3").
The compressor has two basic forms of design, the axial compressor ( "Ill. 2.1-5") and the centrifugal compressor ( "Ill. 2.1-4"). These are also combined. Centrifugal compressors are more robust against erosion and FOD. However, because of the high mechanical rotor load, the construction is clearly limited in diameter and, therefore, typical for small performance engines (small gas turbines up to approximately 2 MW). This is similarly valid for radial gas turbines. The combustors can have different forms. ( "Ill. 2.1-6.1", "Ill. 2.1-6.2" and "Ill. 3.2.1-5.1"). Compressor and turbine are positioned at a single shaft ( "Ill. 2.1-7") which also releases power. In multiple spool engines ( "Ill. 2.1-7"), the low pressure turbines deliver the power. If the engine is used merely as a gas generator (e.g. as with pure thrust engines), a power turbine is added. Two cycle power plants usually deliver the power from the low pressure turbine towards the rear.
Constructions and specific operating conditions
Depending on the demands of operation two basic constructions ( "Ill. 2.1-7") come, with equal rights, into use today. This has to do with the so called heavy frame construction, that was specially developed for steady state use and the aero-derivate ( "Ill. 2.1-8"), derived from aero engines.
The technology of those constructions converges. E.g., from derivates typically derive thermal barrier coatings ( "Ill. 3.3.3-5"), complex interior cooling systems in turbine blades and vanes ( "Ill. 3.3-3") as well as single crystal materials ( "Ill. 3.3-4"). The durability of the derivates approximates the heavy frames. Therefore the application limits obliterate.
Gas turbines of the heavy frame type are especially equipped for continuous steady state operation. They normally belong to a higher performance class (up to over 100 MW). There are also small gas turbines of this type in the area of 10 MW. Their main use as two shaft engines is the generator drive. Small gas turbines use higher driving speeds. To get the exact operation speed a gear is interconnected ( "Ill. 3.7.2-1" and "Ill. 3.7.2-2"). The one shaft design serves mainly to drive pumps and compressors. Not seldom, the engines from european manufacturers show one or two radially- arranged, big, single combustion chambers in a tube design. There are also, however, manufacturers, especially in the anglosaxon region, who prefer several axially-arranged combustion chambers. The casings are very massive, with relatively thick wall size and axial splitting (horizontal split lines). The big disc cross sections of the compressor and turbine rotors give rise to appropriate masses. A connection of the entire rotor system through the axial tie bolts is a popular design principle ( "Ill. 2.1-7"). There are also compressor rotor constructions as a welding design. Even combinations of both connection types are used.
The advantages of heavy engines lie in their design layout for especially long operation times and their realization of high loads and performances. The gas turbine is already conceived and optimized as a shaft performance engine. This is valid for the necessary output speed as also for torque characteristic. By means of less strict limitations on weight, the components can be designed for lower loads, especially the rotors, whereby a potential for long life service is originated. In the choice of materials there is more space for operation specific demands like corrosion resistance of particular parts. Overhaul plans are already adapted in the concept of the steady state operation.
The disadvantage of heavy engines is that the engine concept cannot optimally take into account the starting speed and the number of starts. The thick discs and the casing cross sections follow temperature changes only sluggishly. This reduces the possibility of quicker starts, as either the component loads become very high because of thermal stress or the danger of bridging clearances exists, due to thermal expansion. The high weight can limit the applications if, for example, no sufficiently stable foundations are possible. The advantages of the „big number“, as with aero-derivates, cannot be used.
Aero engine derivatives ( light construction, aircraft derivatives) have the typical design characteristics of the aero engines ( "Ill. 2.1-9"). They are mostly lightly modified for industrial implementation and cover, preferably the lower performance range of about 100KW up to 40MW.The convenient, starting attributes predestine this concept for the covering of peak load, during relatively short operation, and as an emergency aggregate. Through further development, impressively long running times, however, (several 104 h) are attained. In engines that have been deviated from thrust engines, propulsion only through the exhaust gas , the engine works as a gas generator that delivers the propulsion gas for the power turbine ( "Ill. 2.1-7"). This turbine is preferably designed in a manner that it has exact the needed rotation speed. So no intermediate gaer is necessary. The shaft of turboshaft engines (for turboprops and helicopters) leads mostly from the compressor spool to the front. Single shaft engines give off their performance through the only one rotor shaft. Multiple shaft engines, like modern fan engines, use the low pressure turbine, which drives the fan as a base for the power turbine. The output shaft is, however, in contrast to aero engines, leads towards the rear. On grounds of their special operation demands, such as more frequent and short starts, load changes, engines indicate high performance concentration and relatively short overhaul periods. This demands special design features that have a creative multiplicity, as a result. The casings are thin and, not seldom, equipped with abradable systems ( "Ill. 3.1.2.4-1"). The rotors are light and are adapted in their expansion function to the casings ( "Ill. 3.1.2.4-3"). The engines are mostly two shaft. Because the combustion chamber length contributes strongly to the engine weight, the combustors of modern engines are very short. One moved over from tube combustors ( "Ill. 3.1.2.4-2"), of old engines, to annular combustion chambers ( "Ill. 3.2.3-1"). In contrast to engines of heavy design, there are, frequently, annular radial flanges, vertical split lines, used by manufacturer dependent aero engines. The relatively short overhaul intervals have favored the so called convenient module design, by which the entire component group can be exchanged ( "Ill. 4.2-4").
An advantage of the aero engine derivative is it’s behavior during quick and frequent starts. The lower weight is beneficial, it expands the spectrum of use, e.g., of weight limited platforms or in architectural limited buildings. The advantages of the „ big number“ can be widely utilized, as here there are favorable component costs, statistically secured technical maturity, availability of spare parts, recognition of weak areas and a higher development budget, in the framework of the development of the aero engines. The modular design ( "Ill. 4.2-4") has the potential of convenient repair and overhaul processes.
The disadvantages of the aero engine derivates are based, in the first place, on the high component loads by reason of engine specific demands for higher performance by lower weight. Thus, the long life due to the weight decrease of high loaded components, especially of the rotors, does not have the same priority as in the case of heavy engines during the engine design. Technologies are, therefore, more advanced in the derivatives. The rotational speed of the aero engines, from where the derivates were diverted, are not necessarily optimally adequate to the layout of the powering aggregates. That is why, either an interconnected gearbox is necessary or a newly developed power turbine must be used. The short combustion chambers of the aero engines become a special task for designers, relative to temperature profile and distribution, as well as exhaust gas purity. This is often considerably altered for steady state use ( "Ill. 3.2.2-2" and "Ill. 3.2.2-3"), which includes the problem of adjustment on the remaining part of the engine and necessitates the appropriate trials and verifications.
"Illustration 2.1-1": A gas turbine package includes the gas turbine with the actuated auxiliaries, as well as the entire periphery (Chapter 3.7) necessary to the operation. Mostly this are generators, compressors or pumps. Such packages are delivered in their entirety from the manufacture of the engine and /or from so called „packagers“. They concern the following components:
(A) the gas turbine, (B) inlet filter, (C) inlet noise suppressor, (D) plenum chamber, (E) exhaust manifold, (F) exhaust heat exchanger (e.g. recuperator or regenerator), (G) exhaust noise suppressor, (H) driving agreggate (e.g. generator, compressor,pump ), (K) shaft and coupling.
"Illustration 2.1-2": The main components of a gas turbine are the compressor, for the compressing of air, the combustor, which supplies the fuel energy through burning and the turbine, which takes out the energy of the hot gases. It drives the compressor and releases the mechanical collectible energy. In aero derivates, the compressor frequently consists of a low pressure and a high pressure section. They are driven with separate shaft systems by the high pressure turbine, respectively, the low pressure turbine.
The rotational speed of the high pressure system is clearly higher than that of the low pressure system. The division of the two separate shaft systems, that are solely aerodynamically coupled, is necessary in derivates, because the low pressure part also normally drives the relatively slow revolving fan (see "Ill. 2.1-8") on the same shaft. This low rotation is too slow for the compressor. The useful, collectible energy is removed by the low pressure shaft in derivates ( effective turbine, power turbine). In contrast, heavy frame engines ( "Ill. 2.1-7") have mostly only a one shaft system that drives the compressor and releases the useful energy ( "Ill. 2.1-6.1" and "Ill. 2.1-6.2"). A gas generator is a gas turbine that produces the propulsion hot gas for the susequently connected power turbine.
"Illustration 2.1-3.1": Only a small portion of approximately one third of the energy fed in with the fuel is available as removable, mechanical energy for the actuation of generators, pumps etc. The largest part, approximately two thirds, is contained in the exhaust gas. It is therefore obvious that one should use this energy likewise. This can happen in different ways: through heat exchangers for heating up gaseous and fluid media. e.g. for the purpose of heating ( "Ill. 2.1-3.2", "Ill. 2.1-3.3" and "Ill. 2.1-4"), for drier or process energy for production procedures. It is further possible to heat up the air before the combustor and thus recycle exhaust heat with one part of the exhaust gas energy through a heat exchanger. This is complicated indeed, but can clearly save fuel. Although today not only the lower direct fuel costs are to be considered, but also possibly lower exhaust emissions.
"Illustration 2.1-3.2": Power-Heat Coupling: Of course, one tries to use the remaining heat energy in the exhaust gas (Fig.2.1-3). This can happen in various ways. The pictures show schematic presentations of three such relatively frequently used systems.
In the simplest case in a so called ‘waste heat boiler’ steam is produced. It can be used in many ways for energy production (see combined-cycle plant, "Ill. 2.1-3.3") and/or as process steam. Beyond that a heat exchanger for hot water or heating can be added. The overall efficiency of such a plant lays in the region of 80%.
"Illustration 2.1-3.3": In a combined gas and steam turbine process, (GaS) an especially high portion of the fuel energy is converted into mechanical, actuation energy. This means an appropriately high current figure. The electrical coefficient of such plants lies by 40 %.
"Illustration 2.1-3.4": Newest developments use advantages through the injection of the steam in the air flow before and in the combustor ( "Ill. 3.2.2-3"). The steam is produced in an exhaust heat boiler. If need be, additional energy is inserted here with a further burner. Such plants adapt extraordinarily flexibly to fluctuating current and heat demands. This technology is offered under such titles as „Cheng Cycle“ (after the inventor) or STIG- process (steam injected gas turbine).This permits the attainment of lowering the NOx part in the exhaust gas, a performance increase of the engine up to around 50% and an increase of the degree of efficiency around above 20%. Still, where light is we may assume shade. So there are not to be overlooked hints of specific engine problems in specialist literature with regard to the technology of steam injection. Dundas mentions (Lit. 0-1), that in the space of time between 1985 to 1990 the most expensive damages on derivates, (see "Ill. 0-1") often with compressor surge ( "Ill. 3.1.1-6") , took place in connection with the steam injection.
"Illustration 2.1-4": Centrifugal compressors, with regard to engines in the lower performance range, are mostly applied up t a few MW output performance.
The air flow is deviated often, according to the number of steps. This is also a reason for the somewhat poor efficiency of this design in comparison to axial compressors.
The design of a centrifugal compressor is very robust against influences like FOD and erosion. A high pressure ratio is reached on a short axial length. The diameter of the engine is relatively big, through the radial arrangement of the collector and diffuser, which is not necessarily a disadvantage for steady state use.
The inlet of centrifugal compressors is normally sidewards and therefore less good visually inspected without aid (mirror). Along with this there is a higher potential danger that e.g. things stay unnoticed in the compressor entrance during installation, a situation which can lead to considerable damage when starting the engine. There was a case where, in the test stand, a little plastic sack with screws remained unnoticed in the inlet canal. This got into the compressor when starting the engine and caused considerable damage.
"Illustration 2.1-5": Axial compressors come into use, especially in big engines, with a high air flow rate. The single stage pressure ratio of axial compressors lies around 1,2 up to 1,4, between 2 and 5 by centrifugal compressors, the multiplication of the single stage pressure ratio gives the entire pressure ratio of the compressor. The area of operation ( "Ill. 3.1.1-1") is narrower in centrifugal compressors as in axial compressors, that means they allow no such wide variation of pressure ratio and flow rate as do axial compressors. By big diameters or rather flow rates,centrifugal compressors reach mechanical limits of load of the impeller earlier than axial compressor rotors. Axial compressors are axially longer in comparison than radial engines. On the other hand, they are smaller in diameter.
Some engines in a lower performance range show (down to approximately 400 KW) an axial compressor combined with a centrifugal compressor.
"Illustration 2.1-6.1": Typical arrangements are single tangential combustors in smaller performance engines (Lit.1.1-2) and, not rarely, several angular radial combustors in big engines ( "Ill. 3.2.1-2"). This construction offers for modern engines with low-NOx-feature advantages using special devices. They use the greater length to avoid combustion instabilities ( "Ill. 3.2.2-5").
An advantage of the schown combustor arrangement as opposed to axial tube and annular combustors, is the especially good accessibility for inspections and repairs, few constructions in size and few individual parts. When damages occur the number of affected components is at a minimum.
It is to be expected that the temperature distribution in the hot gas flow in front of the turbine ( "Ill. 3.2.3-2") with several individual combustors, depending on their place and number, is more clearly influenced than in annular combustion chambers. The causes are differences in fuel injection, cooling in the flame tubs an the combustion within.
On the other hand, a relatively long gas flow passage between combustor outlet and turbine inlet can take care of temperature balance. This is easier to realise as construction element in heavy-Frame engines.
"Illustration 2.1-6.2": This picture shows an angular annular combustion chamber. The advantage compared with axial tubes or annular combustion chambers is the especially good accessibility for inspections and repair. There are fewer limitations in size (axial length) and smaller number of parts. Therefore in cases of failures there are fewer parts involved. The designer has more room for special devices to minimize emissions.
From an annular combustion chamber a more even temperature distribution in the hot gas before the turbine ( "Ill. 3.2.3-2") can be expected.
"Illustration 2.1-7": Aero derivates (above, "Ill. 2.1-8") and heavy frame engines, do not differ, above all, in the performance niveau but in characteristics that touch upon two different philosophies of design. The derivate engine has the typical features of the aviation engine, at least in the area of the gas generator ( "Ill. 2.1-2"). It has a comparatively filigree structure with thin material cross sections of the static and rotating components. On that depends a rapid heating up and suitable short time for the attainment of a stable temperature distribution, good clearance stability ( "Ill. 3.1.2.4-3") even at transient operation and good acceleration conditions. That makes them especially appropriate where quikker and numerous starts are required. Their light weight facilitates transportable use and fixtures with limited foundation load capacity, e.g., oil bore platforms, and limited space, e.g., retrofitting for industrial purposes.
The heavy frame machine is identifiable through an especially robust construction, its heavy construction promotes long running operation with few starts.
The trend lies towards combinations with the advantages of both systems (Lit. 2-11). Thus manufacturers of heavy frame machines increasingly take over technologies that come out of aero engine design. Frequently, they have an appropriate development partner. The manufacturer of derivates use, e.g., in the downstream power turbine, heavy frame design features.
Typical design differences, that can, however, appear individually by all means distinctly different, are:
- „A“: Casing wall thickness.
- „B“: Disc cross-sections.
- „C“: Shaft system and rotor bolt connections. Derivates frequently show, also in the area of gas generator, multiple shaft systems. The damping of the rotor occurs through a central tie rod. In derivates, at least with big engines, the individual components of the rotor are bolted together via flanges.
- „D“: Anti friction bearings/roller bearings (chapter 3.5.2.1) in the derivate, sleeve plain bearing (Chapter 3.5.2.2) in the heavy frame version.
- „E“: Delivery of power through a free running power turbine at the derivate. In the heavy construction there is, frequently, output directly from the engine shaft.
"Illustration 2.1-8": For example is shown a gas turbine of the OEM General Electric (G.E). The upper half correlates with the industry gas turbine. Beneath is the version of the corresponding aero engine. The common components are marked (grey field).
"Illustration 2.1-9": Modular constructions ( "Ill. 4.2-3" and Lit 4.2-1) are found primarily at aero-engine derivates.
A module is an assembly that can be completely exchanged without disassembling itself or the rest ot the engine. To meet the benefits of a modular construction there must be a corresponding conceptual design. The usability of a modul change assumes special features of the assembly. Including, e.g., a sufficient balancing of the individual rotor components. After the fitting of the module the balancing of the whole system should not be necessary. The example shows schematic an engine like "Ill. 2.1-8". For a derivate it’s a relative big engine with modules that are not all found in smaller engines. The modul design facilitates repair changes and improvements like power and/or efficiency increase. With this concept installation work and hold times can be minimized as well as logistical problems can be avoided.
“A” is the low pressure compressor, “B” the high pressure compressor, “C” the combustion chamber with the high pressure turbine,”D” the "Ill. 2.1-9" low pressure turbine to drive the low pressure compressor. “E” is the power turbine ( "Ill. 2.1-7") which delivers the effective mechanical power and “F” auxiliary aggregates (Chapter 3.6) like gears, control units, starter- generator and pumps.