Industrial Gas Turbines

"Illustration 3.2.3-1": Damages on the combustor can always influence the hot parts of the turbine which are in the gas flow. Those pictures show the damage mechanisms that affect the inner wall of a combustion chambers in the overhaul. They can help the interpretation of a borescope inspection ( "Ill. 4.1-6"). With this procedure usually failures can be recognized on time at the initial stage and, if required, remedies are provided. This picture should contribute to a better recognition and understanding of „where and what“ during borescope inspections. For this purpose the example of a perspectively shown annular combustion chamber (to the right), at the left in profile was choosen.

„1 and 9“ Distortion: Usually is the cause a nonuniform temperature distribution with high thermal stresses. Deformations of the combustion chamber walls accelerate though the cooling effect and/or the combustion process are unfavorable influenced. A typical result are „hot streaks“ in connection with a bulge of the cooling air guiding lips.

„2“ Buckling develops during local overheating. Releasing acts by colder material hampered heat strain. So high compression stresses develop and bulgings emerge. Thereby it is to reckon with crack formation. In an extreme case the wall of the combustion chamber burns out (heavy oxidation). Extreme secondary damage up to an explosion of the pressure bearing casing are to be expected.

„3“ Oxidation threatens especially the edges of walls in the gas stream but also overheated wall zones. A typical failure mode is the „orange peeling effect“( "Ill. 3.3-10"). That is a distinct roughened, mostly dark grey or greenish colored zone with flat, eroded crack fields.

„4“ Burnings are called oxidation zones that develop under extreme high material temperatures and fast oxidation. In the surrounding also signs of melting are possible. It’s to reckon with a throughout material damage.

„5 and 16“ Abrasion emerges as rubbing wear (fretting) at combustion chambers, preferential at contact surfaces of plug-in connections. Fretting is explained by relative movements. They arise in connection with heat expansion, frequent also with vibrations of the combustion chamber. The oxidation of the fresh metallic abraded surfaces acts accelerating. Especially flame instabilities of a NOx-combustion may aid to the damage development ( "Ill. 3.2.2-5").

„6“ Spalling of coatings concerns the thermal barrier coatings (TBC) that are frequently used in modern combustor types to reduce the wall temperature ( "Ill. 3.2.3-7" and "Ill. 3.2.3-8"). There is often a connection with a plastic deformation (distortion, bulge) of the wall. The edge of the walls is especially prone for chipping off. Is the edge impingned by the hot gas stream, the TBC heats up fast and high. This leads to pressure stresses in the coating which particularly, if a weakening of the bond coating by oxidation exists, causes chipping.

„7 and 11“ Crack formation by thermal fatigue ( "Ill. 3.3-16") is a typical problem of combustion chambers. High cyclic temperature gradiants during load changes, especially start and shut down lead to plastic deformations and fatigue cracks (LCF, "Ill. 3.1.2.1-0"). The propagation velocity of the cracks slows down at the beginning, often to a halt. Over a longer time and acting mechanical loads like by the gas pressure and vibrations an accelereating crack propagation can begin. In this situation a fast failure of the component is imminent.

„8 and 10“ Crack formation by HCF ( "Ill. 3.1.2.1-0") is normally triggered by high frequent vibrations. Those can be typically traced to gas fluctuations during the combustion process (flickering, "Ill. 2.6-2" and "Ill. 3.2.2-5"). Such cracks start at weak points like damage by thermal fatigue. Because of their fast propagation they are almost not containable by borescope inspection.

„12“ Blocking of fuel nozzles can be caused by foreign particles and chips (assembly, production). Also the formation of coke as result of an unnormal high nozzle temperature leads to blockage. So a non-uniform temperature distribution in the hot gas stream arises, with log time failures at the turbine blades and vanes. ( "Ill. 3.3-9"). Even more dangerous is a deflection of the fuel spray cone which over heats the combustor walls till bursting occurres.

„13“ Erosion in the fuel nozzle (injector) can be the result of a too high fuel temperature. Thus in the fuel, crack processes are going on which form tiny, very hard particles. They can erode the nozzle orifice and deflect the fuel jet so that the combustor wall is dangerous overheated.

„14“ Distortion of the nozzle respectively its attachement is possible in connection with heat strains of the whole structure of the combustion chamber. Once more a dangerous result is the overheating of the combustion chamber walls (see point 2 and 3).

„15“ Coke formation can take place in many regions in which a convenient temperature and chemical conditions prevail. On fuel nozzles coke attatchement can dangreously deflect the spray cone.

"Illustration 3.2.3-2": This picture shows the radial temperature distributions (RTD) in detail, in three, peripherally distributed areas in the annular ducts at the exit of the combustor of a 5 MW heavy frame engine (Lit. 0-2). It allows one to recognize, by means of the components, the magnitude of the temperature non uniformity of a normally operated engine devoid of particularities or defects. The maximum temperature difference lies at over 100%. The inner platform (root platform) is, therefore, around 75 °C colder than the outer shroud , which should benefit the disk area of the high pressure turbine ( "Ill. 3.3-11"). Depending on the type and condition of the engine, one can definitely reckon with bigger temperature differences.

The RTD is not balanced out by the rotation in the gas flow; the distribution at the periphery (OTD) is averaged in contrast. The high pressure turbine guide vanes see the temperature distribution, in comparison, without compensating effects, so individual guide vanes can be subject to very different temperature loads and overheating damages in size and distribution ( "Ill. 3.3-9").

"Illustration 3.2.3-3": Thermal spray coatings are used in many areas of gas turbines as:

• Wear protection.
• Thermal barrier coatings.
• Oxidation protection and protection against hot gas corrosion (HGC).
• Abradables, as platings.
• Hard coatings.

Typical characteristics and particularities should be familiar not only to the materials expert. There are a number of thermal spray processes (e.g., plasma and flame spray processes on air or under protection gas, with atmospheric pressure or low pressure). The typical spray coating thickness lies in the region of many tenths of millimeters.

The structure of one such coating is lamellar („A“) corresponding to the impacting soft or fluid particles.

With processes that are made on air, there are increasing oxides („B“) and particles with oxide layers. Noticeable oxidation of the bond coating through oxygen , during high process temperature, deteriorates the bonding of the coating. Premise for a good bonding is a sufficient roughening of the spray surface. This must provide the coating to clamp to the substrate. That is provided by abrasive blasting. It leaves minerally particles like Al2 O3 or Si O2 (“E”) sticking in the surface. Such a ‘charging effect’ can influence the bond coat.

Remarkable oxidation of the bond surface by oxygen access at high process temperatures degrade the adhesion of the TBC ( "Ill. 3.2.3-7").

One often recognizes oxidized particles that had formed in the spray in contact with oxygen („C“) and insufficiently molten particles („D“).

Remnants („F“) spray particles (beads) in bigger numbers on the substrate are an indication of bad contact adhesion (“bead problem”, a phenomenon that we observe on the adhesive film on a dusty surface, "Ill. 3.1.2.4-7.2").

Pores („G“) are typical in the spray coatings and are necessary to attain particular features (e.g., cutting in capacity and alternating thermal stability). The evaluation must, therefore, adjust to the specifications belonging to it. In case of doubt a comparison with the structure or approved coatings can help. Too large porosity in the bond coating influences the strength of the coating negatively. The porosity is essential affected by the coating process.

Non melted inclusions („H“) with a high melting point, recognized in the coating structure, show forbidden impurities of the spray powder on frequent emergence.

"Illustration 3.2.3-4": Thermal barrier coatings (TBS) also reduce hot part temperatures and/or minimize cooling air consumption (Lit.3.2-17). This is related to ceramic coatings that, in the main, consistent of zirconia (ZrO2 ). To get a coating structure for the desired operation behavior as heat insulation, resistance against thermal fatigue and long time stability (no change of the structure) yttria is added ( "Ill. 3.2.3-8").

Preferred coating processes are thermal spraying or vapor deposition (PVD, "Ill. 3.2.3-6"). In the beginning, these coatings were used in the combustor (A). Then the interior sides of the shroud (D) and later the air foils (B) of the high pressure turbine guide vanes ( "Ill. 3.3-6") followed. Today, coated turbine rotor blades have already been introduced (C). Here, PVD coatings are used especially because of the thermal fatigue favorable columnar structure ( "Ill. 3.2.3-6"). Also the erosion resistance is compared with the spray coatings superior.

"Illustration 3.2.3-5": Thermally sprayed ceramic thermal barrier coatings, with their typical laminar texture ( "Ill. 3.2.3-3"), can balance out the heat expansion differences of the metallic base material (substrate) only through isolated crack formation, so-called segmentation: the coating „ breathes“. This segmentation should form already through the temperature control during the production process, or should be so influenced (internal stresses) so as to occur in the first operation cycles in the desired form. In the heating up period (right detail) the insulating ceramic coating expands because of the quick heating up, despite a relatively small expansion coefficient, more strongly than the base material. The cracks close and there are controllable compressive stresses. In steady state operation, a residual stress condition builds up, depending on the temperature gradients in coating and base material, tolerable over a long period of time. On the cooled hot parts, the described effect in the heating up phase can lead to a higher compressive stress level. In the cooling down phase (left detail), the process is reversed: the cracks open again. In the cold condition a crack can, because the higher heat contraction of the metallic base material, be closed. This should not be hampered by the operation influences, so that the inner strength of the coating and to the bonding surface is excessed and chips off ( "Ill. 3.2.3-7").

"Illustration 3.2.3-6": The structure of ceramic thermal barrier coatings (compare "Ill. 3.2.3-5") is strongly process dependent. This induces a very different coating behavior that can be optimized for various operation loads.

Where an especial thermal fatigue strength and particularly high quality is demanded, e.g., on the airfoils of turbine rotor blades, vapor deposited coatings (PVD coatings) are used, showing a columnar structure, vertical to the surface (detail), implying a very fine segmentation. These coatings have an additional advantage in the erosion resistance as opposed to coatings with lamellar structures.

"Illustration 3.2.3-7": The ceramic, thermal barrier coatings made from ZrO2 should serve as a typical example of newer technologies and their problems, recognized through experience. Their multiple use is shown in "Ill. 3.2.3-3".

„A“: Erosion through particles and/or gas flow ( "Ill. 3.2.3-8"). Typical are damages on the high pressure turbine liners. These are casing side sealing surfaces opposite the HPT rotor blade tips. Drops of injected water can also erode TBC. Erosion, can be triggered at other non coated parts, by tiny outbreaks of the TBC-surface that will always occur under thermal stresses and gas stream forces ( "Ill. 3.2.3-8").

„B“ : Melting of the dust deposits penetrating into the segmentation cracks, inducing a splitting effect during cooling when shut down (Lit. 3.3-9).

„C“ : Chemical reaction with deposits. To this belong residues of fuels and contaminated injection water ( "Ill. 3.2.2-3").

„D“: Spalling of the coating, as a result of bad adhesion (e.g. through production problems).

„E“: Oxidation of the contact surfaces, e.g., the bond coating due to conduction/diffusion of oxygen ions through the hot ceramic coating. This failure mechanism has a relatively long incubation time and determines the life time of the coated parts (up to several 1000 operation hours). This is especially problematic for the long operation time of gas turbines. With highly oxidation resistent bond coats (e.g., of the MCrAlY-Type) guarantee periods of 104 operation hours can be reached today .

„F“: Thermal stresses from production and operation can lead to spalling, especially in convex radii and edges ( "Ill. 3.2.3-1").

„G“: Over longer operation times, micro structural changes with an alteration of thermal conductivity can arise (Fig.3.2.3-8, Lit. 3.2-3). Thus, the thermal conductivity is increased up to three times, which, by the same cooling , allows one to expect a clear temperature rise, linked with a „painful“ life limitation of the affected hot parts ( "Ill. 2.3-2").

"Illustration 3.2.3-8": Failures during operation (Lit. 3.3- 9) of atmospheric plasma sprayed YSZ (Yttrium stabilized zirconia) TBCs (thermal barrier coatings) are primarily dependent from the surface temperature of the coating. Just the desired isolation effect causes an intense heating of the coating. This counts especially for the typical long operation times of stationary gas turbines. Damages in the left area of the diagram that lead to an early failure are ascribed to production problems. The graphs “1”,”2“,”3” show the failure behavior during dust exposure ( "Ill. 3.2.3-7"). Such deposits are e.g., FeO+NiO from seal abrasion and MgO+ CaO, Al2 O3 , SiO2 from extern ingested dusts. At lower operation temperatures erosion is increased noticeable ( "Ill. 3.2.3-7"). During very long operation times even despite yttrium-stabilization effects get active that are based on a change in the coating structure:

• Phase transformation: This form of aging of the TBC can rise the thermal conductivity up to three times, that means a comparable decrease of the thermal isolation effect (Lit 3.1.2.3-1). Thereby the temperature of the cooled hot parts rises, especially of the high pressure turbine blades and vanes. So it is possible that the lift of those costly parts shortens drastic ( "Ill. 2.3-2").
• Sintering effects the segmentation (stress releasing crack formation, "Ill. 3.2.3-5") and with this influencing the thermal fatigue behavior
• Oxidation of the bond coat at the transition of the adhesion zone to the TBC ( "Ill. 3.2.3-7").
• Increase of the roughness by erosion of the TBC on the turbine segments (vanes), hot gas path and blades. That can lead to a drop in the performance of the gas turbine in the percent region respectively to a corresponding efficiency drop (Lit 3.1.2.3-1).

"Illustration 3.2.3-9": (Lit. 3.2-26): With the help of online measurements and a computer aided analysis (Chapter 5.1) the continuous monitoring of the "Ill. 3.2.3-9" combustion chamber is possible. Typical problems in this connection are

• Fault of the fuel injection system.
• Failures like crack formation and distortion.
• Irregularities of the combustion.

This example shows the display on the monitoring screen of a multishaft engine (detail above right, "Ill. 3.1-2"). With thermalcouples a certain temperature distribution at the exit of the gas generator (EGT) was measured.

The characteristic indicator (fault index) results from percental aberration of the EGT. The polygons correlate the distribution of the thermalcouples over the circumference. They show the absolute temperature difference to the mean EGT

"Illustration 3.2.3-10": (Lit. 3.2-26): The minimization of emissions is today a central demand for a gas turbine plant. Unfortunately the tendency of the two main emissions NOx and CO because the influence on the combustion is in the direction of low gas/ flame temperatures ( "Ill. 3.2.1-1") against each other ( "Ill. 3.2.2-1" and "Ill. 3.2.3-11" ).

The NOx formation rises with the turbine inlet temperature, the fuel-air-ratio and the pressure in the combustion chamber ( "Ill. 3.2.1-3"). They reinforce each other with increasing performance of the engine.

The measurement of the emissions in the exhaust gas can be technically very demanding and costly. With the help of parametric models statements on the basis of computer aided calculations are possible. The required measurements of the parameters like the air flow in the combustion chamber and its exit temperature (turbine inlet temperature) are not practicable. For example because such heat resistent thermocouples for long time use are not available.

Those measurements can be avoided with derived data from other, easier to measure parameters that anyway accumulate in line with a gas path analysis (Chapter 5.1, "Ill. 5.1-2"). This is a feasible and low cost method. This approach was approved for different gas turbine types and is widespread in use. Its accuracy is impressive. The diagram shows NOx in the exhaust gas for the example of a middle performance class derivate engine in dependence from the power output.

Only minimal aberrations can be seen between the parametric method and the calculations from the gas path analysis. A temperature rise due to the efficiency drop (deterioration) over the operation time of the gas turbine, highly affects the NOx production. During full load the CO formation drops only slight ( "Ill. 3.2.3-11").

Even a seemingly small rise of NOx can have far-reaching consequences for the operator. A contaminated compressor (fouling, "Ill. 4.2-1.1" and "Ill. 4.2-1.2") for example can aggravate the NOx emissions by 5 %. Thereby the legislator defined limits can be already passed. For this reson is the efficiency monitoring of the engine and its components of great importance for a limitation of the emissions.

For the determination of the emissions with the help of a computer aided calculation the following parameters are measured at a two shaft engine:

• Compressor:
  • intake temperature,
  • intake pressure,
  • discharge pressure.
• Low pressure turbine / gas generator:
  • endtemperature,
  • exit pressure,
  • rotary speed.
• Fuel:
  • flow,
  • lower limit of the calorific fuel value.

It is interresting, that neither the performance of the engine nor the mass flow of the air at the compressor intake are required.

"Illustration 3.2.3-11": (Lit. 3.2-26): CO as very noxious stands besides NOx for the prevention in the foreground ( "Ill. 3.2.3-10"). If we lower for the NOx reduction the combustion temperature, more CO is produced ( "Ill. 3.2.1-3"). To find an optimum compromise the CO content in the exhaust gas is to determine. For this the parametric model is applied. It uses in line with the gas path analysis needed direct measurements in the combustion chamber like the temperature in the primary zone, pressure and pressure drop.