Table of Contents
5.2 Technology of the components of modern gas turbines
The mirror finished lacquered bumpers are integrated with the autobody. May be the fuel consumption benefits thanks good aerodynamics? No annoying black rubber bulge. That’s elegance! But offers such a design also concrete advantages? A little „body contact“ with traffic partners leads to angrily damages. A nice exterior cries for a repair finishing or just an exchange. That has to be bought dearly.
Also the trend to an apparently unlimited increase in motor power is awesome. Even the fuel consumption could drop, if we don’t use completely the now offered performance. Naturally does this influence also the loads and the lifetime of other components, e.g., tires.
Also a gas turbine is subject to the necessity of further development. Thereby should, like for the car, besides trends which forces the law maker (e.b., emissions) also the fuel consumption decline. Naturally there is at once the desire to raise the performance. This leads to higher loads of the components and so is rather opposed to their robustness. How behave for example 3-D-profiles of modern compressor blades with typical thin edges and high surface finish over a long operation time when erosion and fouling play a role? How susceptible are hot parts against erosion by coke impact from the combustion chamber and against air contaminations? Questions which let the concerns of the car owner pale.
Modern industrial gas turbines exhibit already the typical technologies of aero engines. This chapter deals primarily with trends in the area of design, materials- and production technologies as well as the estimation of lifetime and monitoring techniques.
Compressor blades which are designed for supersonic flow. The required profiles in the compressor lead to very sharp edges. Thus, the FOD sensitivity increases. Through the constantly increasing pressure ratio at each stage and stage load, the allowed margin for operation conditional alterations at the blades, such as roughness increase or change in the geometry of the edges through erosion, becomes ever smaller. The high compressor end pressures demand small tip clearances of the corresponding small blades and react more sensitively to roughness, because of the thinner boundary layer thickness ( "Ill. 3.1.1-2"). In order to reduce the number of components (blades) for cost reasons, the stage number is decreased. This leads to wide chord blades which are susceptible to edge wise vibration ( lyra mode, "Ill. 3.1.2.1-8") and, because of the high flow velocity, to high rotary speeds. These, in comparison to older compressors, clearly heavier blades with extremely high centrifugal forces, load the blade root and the disk in a special measure and the problem related to long running use can be expected here.
In order to reduce the rotor weight, one goes over determinedly to so called blisks ( "Ill. 3.1.1-5"), where the blades are joined to the disk integrally, not held individually at the roots in the serrations of the rotor. Because this style of design does not possess a blade fixture, the notch effect is lacking ( "Ill. 3.1.2.1-2") and must not be compensated through a heavy type of construction. This leads to a light design with correspondingly small material use. Perhaps, improvements in production technique will lead to cost advantages also for new parts. How the question of single blade repair and the logistics are satisfactorily resolved must, however, show up in practical operation.
The demands for narrow tip clearances lead to measures with the goal of adjusting the expansions from rotor and casings to each other, e.g., ‘active clearance control’. One attains this goal through a regulated cooling or heating up of the casings with compressor air, through measures such as the bringing in of masses (thermal inertia) at the casings, or by means of local insulation through coatings. These coatings must often exhibit a sufficient cutting in behavior ( "Ill. 3.1.2.4-1") for the blade tips, demanding compromises. Coating disruptions and erosion failures are typical problems here, radically influencing the life of the engine.
If we take the excellent efficiency of a gas turbine, expecially of the compressor for granted, the problem exists how to conserve it over a long operation time. That means we have to minimise the Deterioration. So seals become to the central challenge. Brush seals ( "Ill. 3.1.2.4-8") are an alternative, in order to get a better grip on the unavoidable problem of the remaining clearance increase in labyrinth seals. These abide on the principle of the sealing of swing doors with a brush. Such a brush sealing has advantages, indeed, in the failure mechanism and the holding of the clearance even after bridging. Experiences, as of date, with long operation times, are not yet adequate to guarantee advantages in stationary operated engines (chapter.3.1.2.4.).
The problem of combustors based on the dry low NOx principle or those with water, respectively, steam injection was already reported in Chapter 3.2.2. These combustors are in the introductory phase and must first prove their mechanical and functional usefulness for industrial gas turbine operation. In this context it has to be pointed out again, that expensive decisions demand proofs and consent from the manufacturer. Modern combustors are, in the meantime, accomplished in tile design ( "Ill. 3.2.1-4"). The tile structure is on the side of the combustion chamber and fixed onto a supporting shell in such a manner that the individual tiles do not experience any unpermitted deformation hindrance.Such a construction is known to be advantageous (e.g., less thermal fatigue stress) in contrast to the conventional single shell design. One asks, however, how the principle conditioned stronger vibrations of the dry low NOx combustors ( "Ill. 3.2.2-5") can be controlled and if they do not lead to fatigue fractures of the tile mountings.
A technology, still in the development phase is catalytic combustion. Whether this will reach practical implementation maturity cannot be said at this point.From natural gas, at least, a failure of the catalyst through impurities is not to be expected.
As a consequence of the trend towards increasing turbine inlet temperatures (not seldom well over the softening range of the blades) to raise thermal efficiency, the design efforts for an efficient cooling is more extensive. That means it should facilitate greater temperature decrease with possibly little cooling air in the hot part. Ingenious, often very filigree cooling designs ( "Ill. 3.3-3" and "Ill. 3.3-6") that are, unfortunately, sensitive to operation influences like FOD, hot gas corrosion or blockages ( "Ill. 3.3-12") are, thus, necessary. The larger the temperature gradients, the more probable the problems with thermal fatigue. The lesser thermal expansion of the cooled interior cross sections leads to high tensile loads ( "Ill. 3.3-17"). Are thermal barrier coatings on the turbine rotor blades or vanes an element of the dimensioning, this must be seen much more critical then a merely additional certainty.
The relatively thin blade walls become weakened through the necessary, but brittle corrosion protection coatings. The layout takes this into consideration, indeed, but if it shows that special influences (e.g., oxidation protection instead of sulfidation protection) demand other coatings during operation, it can become a difficulty.
The constantly higher revolutions and temperatures can lead to life limitation, especially in some very highly stressed components greatly influencing the overhaul intervals and costs. The decisive load is the cyclical expansion of the disks during speed and load alterations (LCF, "Ill. 3.1.2.1-0"). It is also of special interest for the operator to know which components show such life limitations. Not always is the highest performance engine the most cost effective choice.
