Patent Publication Number: US-7584617-B2

Title: Monitoring health of a combustion dynamics sensing system

Description:
FIELD OF THE INVENTION 
     The invention relates to gas turbine engines, and more particularly, to a method for monitoring a health of a combustion dynamics sensing system. 
     BACKGROUND OF THE INVENTION 
     Gas turbines having can-annular combustors are known wherein individual cans, including a combustion zone within the can, feed hot combustion gas into respective individual portions of an arc of a turbine inlet. The individual cans may receive fuel and air for combustion and be disposed in a ring around a central region of a combustor of the engine. Combustion generated dynamic pressure fluctuations, or combustion dynamics, produced in gas turbine engines, and especially in gas turbine engines having Dry, Low NOx (DLN) combustion systems, need to be carefully monitored and controlled to achieve acceptable system durability and reliability. As DLN combustion systems are increasingly required to be operated more aggressively with regard to emissions and gas turbine cycling, the combustors tend to become less robust against these combustor dynamics. Consequently, system failures caused by excessive dynamics become possible. Typically, continuous monitoring of combustor dynamics with a combustion dynamics sensing system having internally mounted dynamic condition sensors are used to provide advance warning of excessive dynamics that may result in damage to combustion system. Such dynamic condition sensors tend to be expensive and typically require continuous maintenance monitoring to ensure that they are functioning properly. In addition, combustion dynamics sensing system problems, such as water in damping tube of the system or signal amplifier failures, may result in erroneous dynamic condition signals being generated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in following description in view of the drawings that show: 
         FIG. 1  is a schematic cross sectional diagram of a can annular combustor of a gas turbine engine including a system for monitoring a health of a combustion dynamics sensing system. 
         FIG. 2  shows an example frequency spectrum of a Fourier-transformed acoustic waveform signal for a conventional DLN-type can annular combustor. 
         FIG. 3  shows an example phase spectrum of a Fourier-transformed acoustic waveform signal corresponding to the frequency spectrum of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Individual cans of a can annular combustor of a gas turbine engine may exhibit amplitude spikes at certain acoustic frequencies during operation.  FIG. 2  shows an example frequency spectrum  32  of respective Fourier-transformed acoustic waveform signals  34 ,  35  for two adjacent cans of a conventional DLN-type can annular combustor. As shown in  FIG. 2 , amplitude spikes  36 ,  38 ,  40  typically occur at about 140 Hz, 190 Hz, and 440 Hz, respectively. In addition to exhibiting such amplitude spikes, adjacent cans of a can annular combustor may interact dynamically with one another at these acoustic frequencies. For example, adjacent cans may interact in a push-pull mode, wherein a phase signal corresponding to an amplitude spike for one of the cans of an adjacent pair is 180 degrees out of phase with respect to the other can of the pair. In the case of a can annular combustor having an even number of cans, it has been demonstrated that a phase angle difference between dynamic conditions of adjacent cans at certain acoustic frequencies consistently varies by about 180 degrees.  FIG. 3  shows an example difference phase spectrum  42  of a Fourier-transformed acoustic waveform signal  44  for the two adjacent cans corresponding to the frequency spectrum of  FIG. 2 . As shown in  FIG. 3 , a phase angle difference  46  between adjacent cans may be 180 degrees in a range of frequencies  48 ,  50  around the amplitude spike frequencies  36 ,  38 ,  40 . Conversely, at frequencies e.g.,  52 ,  54  away from the range of frequencies  48 ,  50  around the amplitude spike frequencies, the phase angle difference between adjacent cans may approach zero. 
     The inventors of the present invention have innovatively recognized that a variance of a dynamic condition of a can from a baseline dynamic condition relationship with another can may be indicative of an abnormal health condition of the sensing system. For example, if a monitored phase angle difference between adjacent cans varies less than a baseline phase angle difference value of about 180 degrees, the monitored phase angle difference may be indicative of a sensor system failure corresponding to one or both of the cans being monitored. Accordingly, a health of a combustion dynamics sensing system may be assessed by monitoring a sensed dynamic condition, such as a phase relationship between at least two cans of a can annular combustor, and identifying a variance from a baseline relationship indicative of a degraded signal quality provided by a dynamic condition sensor associated with at least one of the cans. By tracking phase relationships among sensed dynamic conditions of the cans over time, signal qualities corresponding to dynamic condition sensors associated with each of the cans may be identified as being degraded, for example, when a baseline phase relationship varies outside of predetermined limits. 
       FIG. 1  is a schematic cross sectional diagram of a can annular combustor  12  of a gas turbine engine (not shown) including a system  14  for monitoring a health of a combustion dynamics sensing system  10 . The combustor  12  includes a plurality of combustor cans  16  disposed in a ring about a central region  18  of the combustor  12 . Fuel and air are typically mixed and combusted in each of the combustor cans and hot combustion gases produced by each of the cans are fed into a downstream turbine (not shown) to extract power from the hot combustion gases. 
     As a result of combustion, the cans  16  are subjected to a variety of combustion effects. For example, the cans  16  may be subject to combustion dynamics that may be detrimental to operation of the combustor  12 . Each can  16  may be fitted with a damping tube  19  to help damp combustor dynamics. In addition, combustor dynamic sensing systems  10  are typically used to monitor dynamic conditions of the combustor  12 , such as the dynamic conditions of each of the cans  16  of a can annular combustor  12 . A combustor dynamics sensing system  10  may include a plurality of dynamic condition sensors  20  disposed proximate the cans  16  to sense respective dynamic operating conditions of the cans  20 . In one embodiment, dynamic condition sensors  20  may include a pressure sensor, an acoustic sensor, an electromagnetic energy sensor, an optical sensor, or other type of sensor known in the art for sensing a combustion dynamic parameter responsive to combustion dynamics in the cans  16  of the combustor  12 . The sensors  20  may provide raw signals  26  responsive to the respective combustion dynamics to a processor  24 . Processor  24  may take any form known in the art, for example an analog or digital microprocessor or computer, and it may be integrated into or combined with one or more controllers used for other functions related to an operation of the gas turbine engine. In an example embodiment, the raw signals  26  may be conditioned by signal processing elements, such as amplifiers  28  amplifying the signals  26 , before being provided to the processor  24 . 
     The processor  24  may perform signal processing of the received signals  26 , such as by executing a Fast Fourier Transform (FFT) on the received signals  26  to generate amplitude and phase information in the frequency domain, such as shown in  FIGS. 2 and 3 , from which combustion dynamics of the respective cans  16  may be determined. As described previously, a phase angle difference between adjacent cans  16  of the can annular combustor  12  may differ by about 180 degrees in a frequency range region around an amplitude spike. Such phase angle difference information may be readily discerned from FFT transformed data as shown in  FIGS. 2 and 3 . 
     In an embodiment of the invention, the processor  24  may be configured for monitoring a health of the combustion dynamics sensing system  10 . For example, the processor  24  may be configured to use a dynamic condition relationship responsive to combustion in respective cans  16  to identify a degraded signal quality of one of the signals  26 . The steps necessary for such processes may be embodied in programmable logic  30  accessible by the processor  24 . The logic  30  may be embodied in hardware, software and/or firmware in any form that is accessible and executable by processor  24  and may be stored on any medium that is convenient for a particular application. 
     The steps may include monitoring respective dynamic conditions of at least two combustor cans of the can annular combustor, such as two adjacent cans. In an embodiment, the dynamic conditions may be monitored within a frequency range associated with a spiked, or peak, dynamic frequency response condition. For example, frequency ranges of about 120 Hz to about 220 Hz and about 400 Hz to about 500 Hz may be monitored. Other frequencies and/or frequencies ranges may be monitored as desired. Monitoring may include obtaining raw signals responsive to combustion in a plurality of the cans, and then performing a transformation operation, such as an FFT on the raw signals to generate respective phase angle information corresponding to each signal. 
     The steps may also include establishing a baseline relationship between the respective dynamic conditions. For example, the baseline relationship may include phase relationships between phase angle values of the respective dynamic conditions at common frequencies. The baseline relationship may include an out of phase relationship between cans comprising a phase angle difference of about 180 degrees at a certain frequency. In an aspect of the invention, the baseline relationship may be continually monitored and an average value for the relationship may be calculated. Once a baseline relationship is established, a variance from the baseline relationship may be identified as being indicative of an anomalous dynamic condition reading. For example, a variance away from a baseline relationship may include a sensed phase angle difference between adjacent cans being less than about one hundred and eighty degrees. In an embodiment of the invention, a phase angle variance indicative of an anomalous dynamic condition may be in the range of more than about 10 degrees, and preferably more that about 20 degrees, and even more preferably about 30 degrees, +/−, away from 180 degrees. In an embodiment, occurrence of a variance and/or a time period associated with an occurrence of a variances may serve as a criteria for sending notification of an anomalous dynamic condition reading. 
     It has been demonstrated by the inventors that a phase angle difference variance may be indicative of a degraded signal quality. In an aspect of the invention, a phase angle difference variance provided by a dynamic condition sensor associated with at least one of the cans of a pair of adjacent cans away from a baseline relationship may indicate a problem with the health of the dynamics condition sensing system. For example, the variance in the phase angle difference may be a result of a damping tube associated with one of the cans being contaminated with water, or failure of a signal amplifier associated with one of the cans. The phase information may be analyzed for variances by comparing the phase information for each signal at a desired frequency and/or in a selected range of frequencies to evaluate a signal reliability of the raw signals. When phase variances are identified, notification may be provided to indicate presence of an anomaly in the dynamic condition sensing system that may require further investigation, and/or servicing of the dynamic condition sensing system. It may also be possible to correlate an identified variance with a specific component and/or specific degraded condition of the dynamic condition sensing system. 
     While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.