Industrial Gas Turbines

"Illustration 3.3.3-1": (Lit.3.3-13 and Lit.3.3-14): Pyrometer can directly monitor the blade temperature of a visible turbine rotor stage. This is a precondition to determine the life consumption. It is highly important for the failure prevention and for the logistic.

A pyrometer (upper sketch) consists of a lense system that points direct at a predetermined blade area, here at the leading edge. In modern systems the light is guidet to a receiver (photo cell). The necessary bendable light cable consists of a multitude single fiber. This permit to mount the sensitive electronic in the colder region of the engine. Pyrometer which use the stroboscopic effect to determine the temperature respectively its distribution on individual rotor blades are already applied in industrial gas turbines and test beds ( "Ill. 3.3.3-2"). They can be used in multiple aspects (frame). Thereby the possibility exists to identify individual blades with elevated material temperature and, if necessary, to exchange those. Such an individual temperature rise can be based on a disturbance of the cooling guidance. For this, blockage and FOD (carbon impact) are typical ( "Ill. 3.3-12"). The "Ill. 3.6.2-3" deals with specific failures of pyrometers.

"Illustration 3.3.3-2": (Lit.3.3-14): This is an example of an equipment in use, to measure the individual surface temperatures of turbine rotor blades (lower diagram). This can be carried out on up to 30 spots at every blade. So, it is possible to establish temperature profiles.

The upper sketch shows a scheme of the configuration.

A control module uses the signals of a rotating phase recording for a ‘stroboscopic optical pyrometer’. With it individual rotating blades can be chosen for the measurement. The temperature data go from the data processing to the data analysis. The results are digital stored and shown for frequent questions with adapted screen displays. In critical cases an automatic alarm can be triggered.

Benefits of such an arrangement:

• Continued temperature measurement of in- dividual rotor blades.
• Optimization of the combustion and with this the efficiency of the engine.
• Identification of blades with poor cooled zones ( "Ill. 3.3-10"). Typical are constricted or blocked cooling channels/holes ( "Ill. 3.3-12").
• Ensuring temperature limits for the whole blade ring.
• Early warning of a blade failure / fracture through overheating.
• Continuous monitoring of the condition (thermal abrasion) of the protective oxidation protecting coating (diffusion coating, "Ill. 3.3-7"). Also unusual changings like failures on the oxidation protection coatings or thermal barrier coatings ( "Ill. 3.2.3-7" and "Ill. 3.2.3-8") can be identified. With this exists the possibility to exchange the blades in time in a repairable condition.

That can also be helpful for the logistic respectively the definition of overhaul intervals or ‘on condition’ measures (Chapter 5.1)

The individual blade monitoring can minimize time and effort/costs by changing or treating only the concerned parts.

"Illustration 3.3.3-3": (Lit.3.3-14): this display shows distinctive the alarming temperature increase of a single blade. The rise seems to begin in the last ten operation days. It may be due to an individual problem of this blade. This is in such an extent, that in a short operation time the life is consumed, respectively with an appreciable rest life can not be reckoned.

"Illustration 3.3.3-4": This longtime trend of the temperature from a turbine bucket set allows important conclusions:

• In about 6 months the temperature of all rotor blades of this stage (not one alone) increased. As cause, a rise of the gas temperature can be seen by which all blades of the stage are affected. The rise can be interpreted with an efficiency drop (deterioration) of the engine ( "Ill. 1.1-3"). For this normally effects in the compressor and the turbine are responsible like fouling (deposites), roughening and abrasion wear of the seals (blade tips, labyrinths).
• With an overhaul the temperature level of the hot gas and with this also of the turbine blades could be lowered. That speaks in favor of a rise of the efficiencies. Besides the equivalent fuel reduction there can be reckoned with a markedly smaller operation damage of the blades ( "Ill. 4.1-2"). With the drop of the surface temperature of about 100°C a considerable increase in life time of the costly hot parts can be expected. Surely, thereby the temperature of the load bearing blade walls did not drop in the same degree. Effects like an insolation by the thermal barrier coatings or the inner cooling may diminish the effect on the load bearing blade walls, because they dont allow an equal temperature drop like the hot gas ( "Ill. 2.3-1").

Is creep ( "Ill. 3.3-13") the life governing load a drop of the material temperature of about 12°C can mean, that the operation lifetime doubles ( "Ill. 2.3-2").

The extended lifetime leads to distinctive cost reductions like:

• Less costs of spare parts.
• Reduction of repair costs.
• Shorter dead times that would be needed for an exchange of parts.

"Illustration 3.3.3-5": (Lit. 3.3-14): Pyrometers have the big advantage over thermocouples that they can measure the temperature of rotating components like turbine blades contactless. Thereby it is possible to indicate overtemperatures that are not based on an increase of the gas temperature. This is, e.g,. the case if cooling passages in hot parts are blocked ( "Ill. 3.3-12"). But pyrometers have also weak points that mean an increased maintenance effort. To this belongs:

• („1“) Contamination of the front lense (engl. lens fouling) simulate a lower temperature niveau to the controll unit. This can strongly influence the hot part lifetime. About 15°C rise of the material temperature leads in the region of the operation temperatures to a bisectioning ot the hot parts life time. Besides the optical transmissibility of the lense the calibration of the pyrometer is changed. That reduces the required overhaul interval. So a clean front lense of the pyrometer is an important maintenance task. In return it is not allowed to exceed the overhaul intervals, scheduled by the OEM.
  The reasons of the lense contamination are particles in the hot gas stream that come from the combustion chamber and end up in the sight tube. To minimize this effect, pyrometers are impinged by cleaning air (purge air) from the compressor rear. It acts for the hot gas as sealing air and exits from the sight tube into the gas stream. However this air can, contrary to the planed effect, promote itself a contamination of the lenses. That is the case when particles with enough kinetic energy (speed, size) break through the stream of cleaning air by swirls in front and hit the lense.
• („2“) Fracture of glass fibers in the fiber-optic (frame below). Is the light transferred to the photo cell with a light cable then the danger of stress corrosion cracking (SCC) in the glass fibers exists. It was observed that apparently over the time in periods of shutdown condensate can collect in the fiber bundles. Underlie the glass fibers a critical tensile stress niveau they can breake with delayed crack growth in the humidity. Dangerous stresses, e.g., can occur in the fitting of the glass fiber bundle behind the lense and/or in front of the photo cell. Also a too narrow bending radius of the glass fiber bundle can release under the entrance of condensate and/or humid air the cracking of fibers. Then the measured data drift to a seemingly lower temperature. So the costly exchange of the system will be inevitable.
• („3“) The problem of a change in the emission behavior of the measuring zone at the part is not to explain with the pyrometer itself. However oxidation, contamination, erosion or FOD can change the radiation spectrum and the measurement data drift away. Also a falsification by glowing coke particles can not be ruled out.
• („4“) Till now a tarnishing of the lense as consequence of erosion was not reported but is a probable failure mode. This gets actuality with the use of hard particles in the compressor (hard facing of the blade tips) and ceramic thermal barrier coatings in the combustor and the turbine. The kinetic energy of such particles can suffice to hit the lense against the cleaning air stream. Even a very hard sapphire (alumina) lense could be damaged by this erosive effect.