Table of Contents
5.1.2 Sensors and methods for remote monitoring
Gas turbines with a multiplicity of probes and sensors are continually controlled (condition monitoring, Chapter 5.1.1, Lit. 5.1-1, Lit.5.1-2, Lit.5.1-3, Lit. 5.1-6, Lit.3.3-14), in order to recognize failures and faulty functioning, already in the stage of formation.
A typical example is the controlling of the bearings with vibration pickups. In laboratories and in practical test, there are numerous interesting monitoring techniques for further important components: The pyrometric monitoring of the temperatures of turbine rotor blades is in serial use. Here, an integral temperature measurement at the periphery is carried out. Many, very far reaching effects are possible with a clocked pyrometer (Lit.5.1-7 and 3.3-14) that the temperature of the individual blade detects. Up until now trials have been enormously encouraging and show temperature differences up to 30° C between the blades with a life deviation corresponding to factor four. Through such a process, especially creep endangered blades can be recognized, facilitating an early diagnosis.
Apparently still in the development and the test phase stands the monitoring of crack development in rotor parts ( "Ill. 5.1.2-1"). Apart from unbalances, rather promising seems the analysis of contactfree measurements of torsion vibrations of the rotor shaft.
Controlling the gas flow of particles carried along and their analysis (engine debris monitoring): sensors that react to the electrostatic charge are installed into the main flow. If the particle amount rises through a rub procedure, the electrostatic charge also increases, according to experience. Thus the time of events can be established as well as conclusions, involving the affected parts (e.g., blades or labyrinths) and the cause of failure detected.
To have, in all conditions of operation, a continuous monitoring, measurement and control of the radial clearances between blade tips and casing is an ancient dream, that clearly goes further than the employment of active clearance control, by which, merely, casings are warmed or cooled depending on the operation condition, according to fixed values. One has hopes of having found the necessary probe, e.g., through a laser beam led via a glass fiber optic that is reflected from the blade tip. Such a real clearance regulation could balance clearance increase on the grounds of erosion or unusual rotor deflection (rotor bow).
Monitoring of blade vibrations during operation. There are some approaches: With the laser Doppler velocity meter, the vibration from blade airfoils can be measured and controlled. In adherence to that, there is the possibility at least of a temporary limited monitoring of problematic parts. It will take time, however, till this is possible in practice.
With the acoustic Doppler method, the sound of vibrating blades can be recorded and analyzed. The necessary probes find themselves in the direction of flow behind the stage to be examined, outside the casings. This procedure assists recognition of dangerous resonance vibrations of the blades, as also already cracked blade airfoils (on their altered natural frequency). Even here, we find ourselves in the laboratory stage.
Recently, one makes efforts in engine design to find methods for safe early recognition of dangerous flow instability (e.g., compressor surges, "Ill. 5.1.2-2" and Lit. 5.1-9) and pressure fluctuations (e.g., of combustors). If this succeeds, the requirement for a regulated avoidance of such conditions is asserted, this also includes continuance avoidance of flow interruptions in the compressor. A promising approach in test stage is the measurement and analysis of pressure fluctuation.
"Illustration 5.1.2-1": (Lit. 5.1-9): Since a long time, it is tried to detect contact-free cracks in rotating systems in the phase of the stable propagation. Initially there was the approach to measure vibrations at bearing outer rings and/or casings. With it, unnormal vibrations, triggered by unbalances, should be identified. More promising seems, recording changes of the shafts torsion vibration resulting from a crack formation in the blading. For this, the effect that such cracks (thermal fatigue, blade vibrations, creep) preferential propagate in axial direction, is used. The closer the crack is located at the blade root the more ist affects the bending vibration. This markedly influences the frequency of the torsion vibration of the shaft.
The functional demonstration was provided by an OEM in rig tests with a high pressure rotor of a large aero engine. With this the question arises, if such a procedure is also suitable for the very rigid and massive shafts of a hevy frame gas turbine ( "Ill. 2.1-7").
Thereto a suitable located, equally disrupted reflecting surface (in this case 60 ‘teeth’) at the whole circumference of the shaft is used. Illuminated by a fibre glass bundle the reflected light impulses are transferred to an analysis device (sketch above). For the functional demonstration, three blades on a high pressure turbine disc have been suitable prepared. Then, at two rotation speeds, the trends of the torsion frequencies were measured and evaluated (diagram below). With the characteristic amplitude peaks and the related frequencies it was possible to identify the individual blades by a frequency shift. When this method, assumed the development is successful, comes into application is currently not foreseeable.
"Illustration 5.1.2-2": (Lit. 5.1-10): Flow instabilities, especially compressor surge can play an important roll in gas turbines, used for electricity generation and must be avoided. That is especally true, if gas turbines must guarantee a sufficient frequence accuracy/stability in the power grid. If an unacceptable drop of the frequency in the power grit occures, the gas turbine has to deliver within seconds increased power (backup power) at constant (rotation) speed. This increased performance demands more fuel in the combustion chamber. That leads to a rise of the pressure in the combustion chamber respectively at the compressor exit. The operating curve closes thereby to the surge line ( "Ill. 3.1.1-2"). So the risk of a compressor surge with extensive damages rises ( "Ill. 3.1.1-6"). There are some strategies to avoid compressor surges:
Precautionary actions: An operation below the limits of the engine. This can mean that efficiency of the engine will be given respectively a higher fuel consumption will be accepted. Premature measures are carried out in save distance to the surge line. This can be a demanding task if the efficiencies of the components decrease during operation (deterioration). To this belongs the so called fouling of the compressor ( "Ill. 4.2-1.1") and increased gap losses at the blade tips and labyrinths ( "Ill. 3.1.1-2").
Acute actions are required if there are already pre-stages of the compressor surge. They occur even a fraction of seconds (some milliseconds) before the surge line is reached. Primarily so called ‘rotating stalls’ are concerned, a flow separation at separate blades or blade clusters ( "Ill. 3.1.1-5"). Thereby develop characteristic high frequency pressure fluctuations in the concerned compressor stage. Those can be identified with fast pressure sensors at the casing wall above the blade tips.
Besides the rotating stall an other triggering mechanism for a stall was recognised. We speak about so called modal waves, periodical changes of the axial flow velocity. Like the rotating stall, they are slower then the rotor speed in the annular space. Both effects, rotating stall and modal waves seem obviously not to be separated. Besides such a demanding measuring system a confident analysis of the impulses/data is important. Thereby it is necessary to trigger the stabililizing actions in the extreme short residual time (some milliseconds). To such actions belong:
Fast movements of adjustable vanes: this is very short-term and differs from a preventative adjustment.
Pulsed changes in the fuel supply (fuel spiking, fuel blipping): a short-term reset of the fuel can equal decrease the pressure in the combustion chamber. But here is the sluggishness of the mechanical admeasurement (control unit, fuel injection, valves) a problem.
Air injection: this is carried out with the help of discrete air jets at the compressor stage, affected by the rotating stall. Possibly they have to be synchronised with the rotating flow. For this, very fast pressure pick-ups, computer supported analysis and control of the air injection is necessary.
Literature of chapter 5.1
5.1-1 R.Burkel, J.Murphy, “Infrared Imaging Systems Automate Aircraft Engine Inspection at General Electric“, IE, April 1989, Page 28-32.
5.1-2 C.B.Meher-Himji, „Detect, troubleshoot gasturbine blade failures“, Zeitschrift Power, December 1995, Page 35-38
5.1-3 R.Swanekamp,“Maintain top performance from gas-turbine-based systems“, Zeitschrift Power, February 1996, Page 13-22.
5.1-4 P.Smith, „Gas path analysis“, Aircraft Engineering and Aerospace Technology, Volume 68,Number 2 (1996), Page 3.-9.
5.1-5 M.P.Boyce, C.B. Mehr -Homji, B. Wooldridge, „Condition Monitoring of Aeroderivative Gasturbines“, ASME Paper 89-GT-36.
5.1-6 N.Bolt, „Kosteneffiziente Forschungsergebnisse für Gasturbinenbetreiber“, VGB Kraftwerkstechnik 76 (1996), Heft 6, Page 471-475.
5.1-7 Fa. Gas Path Analysis Limited (GPAL) „ State of the Art Performance Monitoring Systems for Gas Turbines, Process Compressors & CHP Systems“, www.gpal.co.uk, Mai 2008.
5.1-8 I.E.Traeger, „ Aircraft Gas Turbine Engine Technology“, second Edition, Glencoe/ Macmillan/ McGraw-Hill, ISBN 0-07-065158-2, 1979, Page 343 - 351.
5.1-9 K.Maynard, M.Trethewey, R.Gill, B.Resor, „ Gas Turbine Blade and Disk Crack Detection Using Torsional Vibration Monitoring: A Feasibility Study“, SCS Contract Number C-98- 001172, 1998, Page 1-7.(4647)
5.1-10 H.-G. Uhlmann, „ Früherkennung aerodynamischer Verdichterinstabilitäten mittels Wavelet- Transformationsregeln“, Dissertation 2003, Page 16-14.