Inhaltsverzeichnis
2.5.1 The fall in the degree of efficiency during running time
2.5.1.1 Influence of the seals on engine performance
The operation performance of a gas turbine and it’s components depends, to a high degree, on the air and air/oil sealing systems between rotor and structural parts. The deterioration of the output during operation time shows itself in the form of increasing fuel consumption (SFC= specific fuel consumption) or reduced performance ( "Ill. 2.5-2"). This can be traced back, to a large extent, to wear through a rub between the rotating, e.g., rotor blades, intermediate rings, labyrinth carrier and static components, e.g., casings, diffusers, and liner segments.
During transient conditions, e.g., during acceleration of an engine, the rotating seal elements expand differently to static components, as a result of the centrifugal force and thermal expansion. The static sealing parts are usually supported by the casings and expand differently. This can give rise to bridging of clearances and rubbing at the seals. The heating up by the rubbing process increases the thermal expansion and the abrasion. A rub can therefore lead to a significant amount of abrasion that signifies a correspondingly big labyrinth clearance for the steady state operation. That a new rub takes place after a restart is now highly improbable. A similar process is also conceivable during shut down if the casing shrinks quicker than the rotor (see "Ill. 3.1.2.4-2" and "Ill. 3.1.2.4-3"). Clearances of the new engines are comparatively narrow. During running in at the first start/shut down- cycles of a gas turbine, relatively intensive rub is to be expected. One realizes that a careful and conforming to rules running in can optimize the current operation costs. The quicker the load changes occur, the bigger are the expected clearance enlargements.
So it is understandable that one expects a stronger and quicker drop in the degree of efficiency from engines with frequent and instantaneous start/shut down cycles like those necessary for peak loads than for engines for base load operation.
Besides the apparent accompaniment of the performance drop there are a plurality of potentially damage- resulting effects:
- Surge in the compressor through the reduction of the surge margin ( "Ill. 3.1.1-1"and "Ill. 3.1.1-2") or over temperatures in the turbine (chapter3.3). The main causes are air leaks at blade tips (rotating blades and guide vanes) and/or labyrinth failure.
- During acceleration of reduced performance.
- Over heating of discs and shafts through hot gas ingestions, e.g., in the area of the sealings in the turbine.
- Deterioration of the efficiency of the hot part cooling, e.g., cooling of the high pressure turbine blades. For example by the blockage of the cooling air passages by abrasion during rub. This leads to shortening of lifetime and /or failure of the components through high temperatures.
- Erosion in the compressor and turbine by abrasion particles from a blade tip rubbing.• Foreign object damage (FOD) on the blades through the breaking away of larger coating chunks from the casings..
- Shaft fractures and/or through- separation of rotors through wear and high abrasion temperatures.
- Main bearing damages as a consequence of changed thrust load, too high or unloaded, ( "Ill. 2.5-1") as a result of pressure changes in the area of the rotor discs. Endangered are as well overloaded as unloaded bearings.
- Oil leakage and, in extreme cases, oil fire by failure of air back pressure in bearing chambers.
- Ignition of an oil fire in the bearing chamber or of a titanium fire (only blades of titanium alloys) through sparks and local heating up.
- Fatigue fractures on blades as a consequence of excitation due to rub.
- Crack formation on labyrinth and blade tips, an effect of hot tears, thermal fatigue and/or mechanical fatigue (HCF) by vibrations. There are also combinations. For example when high friction temperatures damaged fatigue loaded zones These can be a structure change with embrittlement and/ or strength reduction.
2.5.1.2 Increase of blade-roughness and profile changes
Erosion, corrosion and pollution ( "Ill. 3.1.2.3-2" and "Ill. 3.1.2.4-3") show up especially on the aerodynamic effective surfaces, blade airfoils and inner casing walls. They increase the roughness, change the profile and enlarge the tip clearance to the casings. Their influence depends on the operation profile (e.g., stand still times) and the surrounding conditions (e.g., air pollution and filter system).
"Illustration 2.5-2": This picture was transmitted from statements on aero engines, with derivates for industry gas turbines. The data refers more to peak load use. The representation is also only valid in tendency, relative to an industrial gas turbine.
The left diagram shows the deterioration (see also "Ill. 4.2-1.1" and "Ill. 4.2-1.2") in form of an increase of the SFC (specific fuel consumption, "Ill. 1.1-3") in the steady state operation of individual parts. It is clearly recognizable that a deterioration of the engine performance enters around approximately 0,5% through tip clearance enlargement and is to be allocated to the lower pressure compressor and especially the high pressure part, compressor and turbine. After about 1000 operation cycles notably the compressor degraded. In the high pressure turbine even more remarkable thermal deformations of the casings and the guide vanes (nozzles) ,which influence the clearances as well, contribute to the SFC rise.
The diagram at the right (Lit. 2-7 and Lit. 2.8) shows that the increased fuel consumption (here for aero engins) obviously enters, in the first place, through the rub of the blade tips and labyrinth seals during the first cycles of the pass of test. The drop of the efficiency takes place relatively slow over a long operation time. Alterations like the fouling of the blades, erosion of the blades, in case the filter does not do justice to the dust loaded intake air, and thermal deformations of the hot parts, e.g., casings and turbine guide vanes, play a part here.
This performance shows the suitability of the pass of test. The latter are often not comparable in their influence to the clearances of normal operation and are, therefore, problematic.